Heat Pipe

A Project Report On

HEAT PIPE EXPERIMENT

Babaria Institute of Technology

Department of Mechanical Engineering BABARIA INSTITUTE OF TECHNOLOGY VARNAMA, VADODARA GUJARAT UNIVERSITY

(2007-08)

Sandeep P Prajapati 04ME39

A Project Report On

HEAT PIPE EXPERIMENT

Guided by:

Prepared by:

Prof. RAMESH MEVADA

SANDEEP P. PRAJAPATI (04ME39)

Asst. Professor

BHAVIN I. PRAJAPATI (04ME36)

Dept. of Mechanical Engineering

HARDIK V. SHAH (04ME43)

Babaria Institute of Technology Department of Mechanical Engineering BABARIA INSTITUTE OF TECHNOLOGY VARNAMA, VADODARA GUJARAT UNIVERSITY (2007-08)

Babaria Institute of Technology

CERTIFICATE This to certify that Mr. Sandeep Prajapati (04ME39), Mr. Bhavin Prajapati (04ME36), Mr. Hardik Shah (04ME43), satisfactorily completed their Project on “HEAT PIPE EXPERIMENT” towards the partial requirement of VIIIth semester, BE Mechanical Degree curriculum. The project work is a bonafied record carried out by them during academic year 2007-08 in Department of Mechanical Engineering, Babaria Institute of Technology, Varnama, Vadodara391240.

DATE:

Guide (Prof. Ramesh Mevada) Dept. of Mechanical Engg.

HOD (Prof. P. H. Agarwal) Dept. of Mechanical Engg.

ACKNOWLEDGEMENT This project work has been the most exciting part of our learning experience, which would be an asset for our future carrier. It is a great pleasure for us to express our thanks and heartiest gratitude to all those who have helped us during the preparation of this project. We would like to take opportunity to thank them all. Our most sincere thanks to our project Guide Prof. RAMESH N. MEVADA (Mechanical Engineering Department) for his kind co-operation and who has always been guiding, encouraging and motivating us throughout the project with his experience and knowledge, while preparing this project. We are also greatly thankful to the entire Mechanical Department Staff who have helped us in completion of this project directly or indirectly.

SANDEEP P. PRAJAPATI (04ME39) BHAVIN I. PRAJAPATI (04ME36) HARDIK V. SHAH (04ME43)

ABSTRACT The heat pipe is widely used because of its amazing heat transfer rate. Since last decade heat pipes became more popular because of its wide applications in cooling systems. Earlier heat pipes were mostly used for temperature equalization in spacecrafts and satellites. Nowadays, we commonly find heat pipes in notebook computers, game consoles, and even integrated into normal PC CPU coolers. One reason for the rise in popularity is the fact that prices have dropped dramatically, since high-volume cooling product manufacturers like Asia Vital Components now have their own heat pipe manufacturing facilities, and heat pipe manufacturing is no longer reserved to a few specialized companies. When it comes to cooling, "heat pipe" has become a buzzword; but still, few people understand how heat pipes work, and what factors must be considered when using a heat pipe-based cooling system. This project work should provide some knowledge of heat pipe. By performing this experiment, we see the practical and physical characteristic of heat pipe. So there is better idea about the operation and working of heat pipe.

CONTENTS 1. THEORY OF HEAT PIPE ........................................................................................................................................................ 1      

1.1 INTRODUCTION .................................................................................................................................................... 1 1.2 PRINCIPLE OF HEAT PIPE ....................................................................................................................................... 2 1.3 BASIC THERMODYNAMIC CYCLE ............................................................................................................................ 2 1.4 WORKING ............................................................................................................................................................. 3 1.5 APPLICATION OF HEAT PIPE .......................................................................................................................................... 5 1.6 LIMITATIONS ............................................................................................................................................................ 6

2. EXPERIMENTAL DETAILS..................................................................................................................................................... 7 3. APPARATUS ....................................................................................................................................................................... 9 4. HEAT PIPE DESIGN ............................................................................................................................................................ 10      

4.1 INTERNAL HEAT PIPE STRESS ....................................................................................................................................... 10 4.2 DESIGN CONSTRAINTS .............................................................................................................................................. 11 4.3 THE WORKING FLUID ................................................................................................................................................ 12 4.4 THE WICK OR CAPILLARY STRUCTURE ........................................................................................................................... 14 4.5 HEAT PIPE DYNAMIC ................................................................................................................................................ 17 4.6 SINTERED POWDER WICK CALCULATIONS ...................................................................................................................... 21

5. SELECTION OF HEATER ..................................................................................................................................................... 24  

5.1 TYPES OF ELECTRIC HEATERS ...................................................................................................................................... 24 5.2 DESIGN OF HEATER .................................................................................................................................................. 26

6. THERMOCOUPLES ............................................................................................................................................................ 28         

6.1 K TYPE THERMOCOUPLE ............................................................................................................................................ 28 6.2 E TYPE THERMOCOUPLE ............................................................................................................................................ 29 6.3 J TYPE THERMOCOUPLE ............................................................................................................................................. 29 6.4 N TYPE THERMOCOUPLE ............................................................................................................................................ 29 6.5 B, R, AND S TYPE THERMOCOUPLE ............................................................................................................................... 29 6.6 T TYPE THERMOCOUPLE ............................................................................................................................................ 30 6.7 C TYPE THERMOCOUPLE ............................................................................................................................................ 30 6.8 M TYPE THERMOCOUPLE ........................................................................................................................................... 31 6.9 THERMOCOUPLE COMPARISON ................................................................................................................................... 31

7. CONTROLLER .................................................................................................................................................................... 35 

7.1 CONFIGURATION ..................................................................................................................................................... 36

8. TEMPERATURE SCANNER ................................................................................................................................................. 38 9. EXPERIMENT SETUP ......................................................................................................................................................... 39 10. PROCEDURE ................................................................................................................................................................... 41 11. OBSERVATION ................................................................................................................................................................ 42 12. CALCULATION................................................................................................................................................................. 43 13. RESULT TABLE ................................................................................................................................................................ 44

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14. CONCLUSION .................................................................................................................................................................. 45 15. FURTHER SCOPE ............................................................................................................................................................. 47 16. BIBILOGRAPHY ............................................................................................................................................................... 51  

16.1 BOOKS.............................................................................................................................................................. 51 16.2 LINKS ................................................................................................................................................................ 51

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LIST OF FIGURES Figure 1: comparison of metal and heat pipe ......................................................................................................... 1 Figure 2: Thermodynamic cycle of heat pipe .......................................................................................................... 2 Figure 3: Typical heat pipe ..................................................................................................................................... 3 Figure 4: Heat pipe Demonstration Unit ................................................................................................................. 7 Figure 5: Known geometrical parameter ...............................................................................................................11 Figure 6: Merit number comparison for various liquid ...........................................................................................13 Figure 7: Several types of wick structures ..............................................................................................................15 Figure 8: Sintered powder wick .............................................................................................................................15 Figure 9: Cross section of wire screen and grooved wicks ......................................................................................15 Figure 10: Microscopic picture of sintered particles ..............................................................................................16 Figure 11: Experimental drag coefficient data for laminar flow..............................................................................19 Figure 12: K type ...................................................................................................................................................28 Figure 13: compare data .......................................................................................................................................35 Figure 14: temperature controller diagram ...........................................................................................................35 Figure 15: Universal controller ..............................................................................................................................36 Figure 16 temperature scanner .............................................................................................................................38 Figure 17: Circuit diagram .....................................................................................................................................39 Figure 18: Length vs. temperature graph for cu rod...............................................................................................45 Figure 19: Length vs. temperature graph for heat pipe ..........................................................................................46 Figure 20: The temperature rise along the heat pipe vs. the amount of heat conducted by the heat pipe .............47 Figure 21 :Three-dimensional plot of data such as that in Figure 20 ......................................................................48 Figure 22: Projection of Figure 21 onto the P-T plane ...........................................................................................49 Figure 23 Dependence of the maximum stable heat flow on orientation for Tavg = 95 C .......................................50

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1. THEORY OF HEAT PIPE  1.1 INTRODUCTION

The term “heat pipe “as the name implies, is a device for transferring heat from a source to sink by means of evaporation and condensation of a fluid in a sealed system. The transfer of thermal energy by conduction using solid material is essentially limited by the thermal conductivity of the material structure. The best obtainable thermal conductors are metallic and tend to be high cost material. Because the thermal energy is transported by evaporation-condensation process rather than by conduction, the heat pipe can transfer heat much more effectively than a solid conductor of the same cross section. In practice, the thermal conductance of heat pipe may be well several hundred (500) times than best available metal conductor.

Figure 1: comparison of metal and heat pipe

The heat pipe is a very simple device and very efficient heat transfer device. Basically, it can be considered a super-conductor that transmits heat by the evaporation and condensation of a working fluid. It can transmit 500 times heat transmitted by best metal conductor and with a temperature drop less than 5c per meter length of heat pipe.

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What is more amazing is that heat pipe has no moving parts, requires no external energy, and is reversible in operation and completely silent. It is rugged like any piece of tube or pipe and can withstand a lot of abuse.

 1.2 PRINCIPLE OF HEAT PIPE The operation of heat pipe is based on the following physical principal:  At a specified pressure, a liquid will vaporize or vapor will condense at certain temperature, called the saturation temperature. Thus, fixing the pressure inside a heat pipe fixes the temperature at which phase change will occur.  At a specified pressure or temperature the amount of heat absorbed as a unit mass of liquid vaporizes is equal to amount of heat rejected as that vapor condense.  The capillary presser developed in a wick will move a liquid in the wick even against the gravitational field as a result of the capillary effect.  A fluid in channel flows in the direction of decreasing pressure

 1.3 BASIC THERMODYNAMIC CYCLE

Figure 2: Thermodynamic cycle of heat pipe

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1-2 Heat applied to evaporator through external sources vaporizes working fluid to a saturated (2’) or superheated (2) vapor. 2-3 Vapor pressure drives vapor through adiabatic section to condenser. 3-4 Vapors condenses, releasing heat to a heat sink. 4-1 Capillary pressure created by menisci in wick pumps condensed fluid into evaporator section. Process starts over.

 1.4 WORKING

Figure 3: Typical heat pipe

Diagram showing components and mechanism for a heat pipe containing a wick. Heat pipes employ evaporative cooling to transfer thermal energy from one point to another by the evaporation and condensation of a working fluid or coolant. Heat pipes rely on a temperature difference between the ends of the pipe, and cannot lower temperatures at either end beyond the ambient temperature (hence they tend to equalize the temperature within the pipe). 3

When one end of the heat pipe is heated the working fluid inside the pipe at that end evaporates and increases the vapor pressure inside the cavity of the heat pipe. The latent heat of evaporation absorbed by the vaporizations of the working fluid reduces the temperature at the hot end of the pipe. The vapor pressure over the hot liquid working fluid at the hot end of the pipe is higher than the equilibrium vapor pressure over condensing working fluid at the cooler end of the pipe, and this pressure difference drives a rapid mass transfer to the condensing end where the excess vapor condenses, releases its latent heat, and warms the cool end of the pipe. Non-condensing gases (caused by contamination for instance) in the vapor impede the gas flow and reduce the effectiveness of the heat pipe, particularly at low temperatures, where vapor pressures are low. The velocity of vibrating molecules in a gas is approximately the speed of sound and in the absence of non condensing gases; this is the upper velocity with which they could travel in the heat pipe. In practice, the speed of the vapor through the heat pipe is dependent on the rate of condensation at the cold end. The condensed working fluid then flows back to the hot end of the pipe. In the case of verticallyoriented heat pipes the fluid may be moved by the force of gravity. In the case of heat pipes containing wicks, the fluid is returned by capillary action. When making heat pipes, there is no need to create a vacuum in the pipe. One simply boils the working fluid in the heat pipe until the resulting vapor has purged the non condensing gases from the pipe and then seals the end. An interesting property of heat pipes is the temperature over which they are effective. On first glance, it might be suspected that water charged heat pipe would only start to work when the hot end reached 100 °C and the water boils resulting in the mass transfer which is the secret of a heat pipe. However, the boiling point of water is dependent on the pressure under which it is held. In an evacuated pipe, water will boil right down to 0 °C. Heat transfer will start, therefore, when the hot end is warmer than the cold end. Similarly, a heat pipe with water as a working fluid can work well above 100 °C.

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The main reason for the effectiveness of heat pipes is due to the evaporation and condensation of the working fluid, which requires/releases far more energy than simple temperature change. Using water as an example, the energy needed to evaporate one gram of water is equivalent to the amount of energy needed to raise the temperature of that same gram of water by 540 °C. Almost all of that energy is rapidly transferred to the "cold" end when the fluid condenses there, making a very effective heat transfer system with no moving parts. The heat transfer or transport capacity of a heat pipe is specified by the “Axial power Rating” (ARP) which is the energy moving along the pipe. The larger diameter of the pipe has the greater the ARP for a given length and greater the length the smaller the ARP for a given diameter. A 5mm diameter and 15cm long pipe has an ARP of 75watts which increases 500watts if the diameter is increased to 19mm. The other hand, the ARP rating of a 19mm pipe 2m in length is reduced to about 350 watts. The highest length used till today is 15m but the usual maximum length is between three to four meters.

 1.5 Application of heat pipe Heat pipes have since been used extensively in space craft as a means for managing internal temperature conditions. Heat pipes are extensively used in many modern computer systems, where increased power requirements and subsequent increases in heat emission have resulted in greater demands on cooling systems. Heat pipes are typically used to move heat away from components such as CPUs and GPUs to heat sinks where thermal energy may be dissipated into the environment. Heat pipes are also being widely used in solar thermal water heating applications in combination with evacuated tube solar collector arrays. In these applications, distilled water is commonly used as the heat transfer fluid inside a sealed length of copper tubing that is located within an evacuated glass tube and oriented towards the Sun. Heat pipes are used to dissipate heat on the Alaska Oil Pipeline. Heat from the friction of the oil against the wall of the pipe and from the turbulence of the oil would conduct down the legs of the

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pipe to melt the permafrost into which they penetrate. Heat pipes with radiators at the top are used on each leg to keep them cold so they won't melt the permafrost and let the pipeline collapse. In solar thermal water heating applications, an evacuated tube collector can deliver up to 40% more efficiency compared to more traditional "flat plate" solar water heaters. Evacuated tube collectors eliminate the need for anti-freeze additives to be added as the vacuum helps prevent heat loss - these types of solar thermal water heaters are frost protected down to more than 35 °C and are being used in Antarctica to heat water.

 1.6 Limitations Heat pipes must be tuned to particular cooling conditions. The choice of pipe material, size and coolant all have an effect on the optimal temperatures in which heat pipes work. When heated above a certain temperature, all of the working fluid in the heat pipe will vaporize and the condensation process will cease to occur; in such conditions, the heat pipe's thermal conductivity is effectively reduced to the heat conduction properties of its solid metal casing alone. As most heat pipes are constructed of copper (a metal with high heat conductivity); an overheated heat pipe will generally continue to conduct heat at around 1/80th of the original conductivity.

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2. EXPERIMENTAL DETAILS The performance of a heat pipe as a superconducting device can be studied by measuring the temperature distribution along its length and it can be compared with similar metallic pipes. It comprises two geometrically identical elements. One of them is heat pipe with pure distilled water as working fluid. The other is copper rod with same size. One end of these pipes is heated by electrical heaters while small capacity tanks acting as heat sinks are provided at the other end. These tanks are filled with water up to a certain level. Three thermocouples are embedded along the length of each pipe to measure the temperature distribution. Heat transfer rate can be calculated from the rise in temperature of water in the heat sinks. Power input to the heaters is varied through dimmer stats and can be measured by ammeter and voltmeter. The heat pipe used in these measurements was made of copper with sintered Cu powder as the wick and water as the working fluid. The outside dimensions were 8mm dia. by 250 mm long. The goal of the measurements was to establish steady state heat flow at various power levels and average temperatures in an effort to map out the thermal characteristics of the heat pipe. A number of precautions were taken to measure these parameters accurately and to prevent loss of heat from the system. In order to measure the temperature profile along the heat pipe, 4 thermocouples (0.25 mm dia., Type K) were attached with epoxy at equally spaced intervals. Heater wire (0.125 mm dia. Nichrom 600 ) was wound and epoxy directly on a 75 mm section at one end of the heat pipe, defining it as the evaporator section. None of the heater windings touched the thermocouples.

Figure 4: Heat pipe Demonstration Unit

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A condensing portion was formed at the other end by a water jacket that consisted of a 75 mm length. The cooling water was thus in direct contact with the copper heat pipe except in the vicinity of each thermocouple, where the epoxy served as a thermal barrier between the water and the bead of the thermocouple junction, which was pressed against the heat pipe.

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3. APPARATUS  Heat pipe (8 mm dia, 250 mm)  Heater  Thermocouples  Controller  Temperature scanner

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4. HEAT PIPE DESIGN For this experimental set up we required heat pipe which operated temperature range 40°C to 100°C and transmitted 100w power. For that we design heat pipe.

 4.1 Internal heat pipe stress The operating pressure inside the heat pipe gives rise to hoop stress in the heat pipe walls. This stress can be calculated using the following hoop stress equation:

Where: σ Hoop= hoop stress r0=outer radius of the heat pipe

ns= safe factor tw= heat pipe wall thickness The hoop stress in equation 1 is set to be one third of the ultimate tensile stress value for copper. To ensure pipe rigidity and reliability a safety factor of 5 is include .rearranging and solving for tw, the following heat pipe wall thickness value is obtained:

Evidently, this is much less than can be reasonably manufacturing. Therefore a more common size of 2mm is assigned for the heat pipe wall thickness.

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 4.2 Design constraints The following layout for the pipe shows some of the constraints that can be used in defining some of the physical parameters:

Figure 5: Known geometrical parameter For the 2mm thick, 100w power we assume the following dimension;  dv =6mm(diameter of evaporating area)  di=7mm(inside diameter )  do=8mm(outside diameter)  Qtotal=100w  Qactual=80w Evaporation area is:

The axial heat flux in pipe is:

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 4.3 The working fluid The suitable working fluid must meet the following guidelines for optimum heat transfer capacity of a heat pipe:  Compatibility with wick and wall materials  Good thermal stability  Wet ability of wick and wall materials  Vapor pressures not too high or low over the operating temperature range  High latent heat  Low liquid and vapor viscosity  Acceptable freezing The combination of above stated properties of a fluid in accordance with optimum thermodynamics considerations is further used for calculating viscosity, sonic, capillary, entrainment and nucleate boiling limitation of the heat pipe. Material compatibility is of high importance. Most of the problems associated with long life heat operation are a direct consequence of material incompatibility. A good thermal stability is therefore a necessary feature of the working fluid. The tension in the liquid surface is independent of surface area. All over the surface area of a liquid there is a pull due to the attraction of the molecules tending to prevent their escape. Thus, the surface tension is directly governed through temperature and pressure, with pressure variations being rather small. Therefore, in a heat pipe design it is desired to have a high value of surface tension in order for the liquid in the pipe to be able to operate the gravity and to generate a high capillary force. In order to reduce high temperature gradients and avoid high vapor velocities, it is necessary to choose a liquid with sufficiently great vapor pressure. Likewise, high Latent heat is desired in order to transfer large amounts of heat with a minimum heat flow, hence low pressure drops are 12

maintained at the same time. Moreover, high thermal conductivity is desired for minimum radial temperature gradient and reduced possibility of nucleate boiling at the wick/wall interface1. Choosing fluids with low values of vapor and liquid viscosity minimizes the resistance to fluid flow. A very convenient means for quick comparison of working fluids is provided by the Merit number defined as:

Figure 6: Merit number comparison for various liquid

A figure 6 show Merit number for various liquids and helps us determine the right liquid for our desired temperature range. Likewise, table 1 compares melting, boiling and critical temperatures of various liquids and their recommended operational range. Since the temperature range for our system is between room temperature and approximately 115 degrees, several fluids are available: Water, Methanol and Ammonia. The following table shows extracted values for each liquid at 100°C, which can further be used to finalize our fluid selection for this heat pipe application:

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Table 1: Properties of considered liquids

Therefore, we can logically conclude that the water would be the most appropriate fluid to select for this application. Even though it might be noted that the vapor pressure is much lower than in case of ammonia and methanol, but this will also greatly reduce the necessity for a thick-walled container, which provides additional weight that should be minimized due to other electronic equipment being unable to support excessive weight. The primary factors, which make water our primary choice, are: suitable operating temperature range, high latent and thermal conductivity, and slightly higher surface tension. It should also be noted that water is a much more economically favorable fluid and it does not require any special material from which wick and container structures will be made.

 4.4 The Wick or Capillary Structure The main purpose of the wick structure is to generate capillary pressure to transport the working fluid from the condenser to the evaporator section. These functions require wicks of different form, particularly, if the liquid is to be returned over a larger distance. As seen in figures 7 and 8, wick structures are placed against the inner wall of the heat pipes. They serve as both liquid and heat transporters and can be wire-mash or sintered powder type.

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Figure 7: Several types of wick structures

Figure 8: Sintered powder wick

Figure 9: Cross section of wire screen and grooved wicks

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In order to increase the operational efficiency, the wick should have the following properties:  Good thermal conductivity  High permeability  Sufficient driving/capillary pressure  High effective thermal conductivity As could be expected, the maximum capillary head generated by a wick, increases with decreasing pore size. The wick permeability, which is highly desirable, increases with increasing pore size. Figure 10 shows the microscope picture of sintered powder material and the average particle arrangement. For homogenous wicks, it should be noted that there exists an optimum pore size, which could impose a compromising factor on our design. Another important feature of the wick is its thickness. Thicker wicks provide higher heat transport capability. However, increase in thickness provides greater thermal resistance or the wick, which further decreases the evaporator heat flux, which is also a combination of both wick thermal resistivity and working fluid thermal conductance.

Figure 10: Microscopic picture of sintered particles

The final factor to be considered in wick selection is material compatibility with water and easy manufacturability. For this application, we will assume a homogenous wick structure, which will serve as a reference and temporary solution to the problem. However, it should be noted that a 16

possible improvement could be achieved by designing a new non-homogenous wick structure that will have more promising specifications. In practical applications, fibrous wicks were found to be less stable due to continuous support required. Research shows that sintered powder wicks are the best for the application in electronics industry and cooling of electronic devices. However, sintered powder metal wicks have small pore radii and relatively low permeability. The advantage of sintered powder wicks is the high capillary pumping pressure and multi-directional orientation of the heat pipe during operation. Other wick structures do not work as well in horizontal orientation because they cannot lift the returning working fluid the length of the heat pipe against gravity. It was found that the flow behavior of the working fluid depends on the wicking structures and the number of wick layers. The heat transfer characteristics and the effective thermal conductivity are related directly to the flow behavior. Increasing the number of wick mash layers (up to 16 layers) increases the heat flux with smaller temperature differences. The flattening phenomenon of the thermal resistance was observed after 16 wicks layers due to the entrainment limit. Therefore, both wire mash (16 layers of mash) and sintered copper wicks are the most suitable choices to make for this purpose while still retaining material compatibility with water. However, the physical layout, as shown in the introduction, shows that heat pipes, attached to the vertically positioned chip array would have to operate in horizontal position. Therefore, we can safely exclude wire mash wick due to its poor performance in horizontal operating position. The remaining single choice is the sintered powder wick, which will be used in proceeding calculations.

 4.5 Heat pipe Dynamic Since the sintered wick is the key mechanism for liquid evaporation cycle, which furthermore limits the total heat transport capacity of the pipe, the calculations of it require careful consideration. The overall heat transport capacity of the heat pipe is dependent on several factors such as: evaporator and condenser lengths, wick permeability, sintered powder particle size, evaporating area, for the sintering material. Maximum driving or capillary pressure is one of the main requirements in wick design, balanced with sufficient wick permeability. 17

The capillary force, being the main driving force for the heat pipe system is found by subtracting normal and axial pressure from the effective capillary pressure as seen in following equation Maximum driving pressure is given by:

Where

Normal hydrostatic pressure along the pipe:

Axial hydrostatic pressure along the pipe

Vapor and liquid friction coefficient play the key role in heat pipe transport since they are directly related to the fluid velocity, which is a function of heat flux and fluid latent heat which is desired to be as high as possible:

The velocity equation yields the expression for the liquid pressure gradient:

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The Cd is the laminar flow drag coefficient and can be obtained from figure of experimental laminar flow drag coefficient data.

Figure 11: Experimental drag coefficient data for laminar flow

Furthermore, the liquid friction factor is given by the following equation:

……………………….. (11) Where K is the permeability of the wick:

Similarly the vapor friction coefficient can be found through the pressure gradient equation for the vapor state:

……………………………. (12)

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Thus, the vapor frictional coefficient is inversely proportional to the diameter of the evaporation area, and is given by the following equation1:

………… (13)

Each wick, having its own hydrodynamic properties, will further have its own separate frictional coefficients, which can then be used in calculating the capillary heat transport factor given by the following equation1

………. (14)

Finally capillary heat transports limit Qc, max is1:

…………… (15) Capillary limit, being of primary interest, however, is not the only limit imposed in heat pipe operation. To complete the operational range of this heat pipe system, the remaining limits, discussed in heat transport limitations section, are as follows:

Sonic limit (watts):

…………….. (16) Entrainment limits (Watts): 20

……………………. (17) Boiling limit (watts);

…………………… (18) Therefore, the heat transport to be achieved should be within the four given heat transport limits.

 4.6 Sintered Powder Wick Calculations Table 4 shows various combinations of wicks and working fluids, and was used as a sizeavailability reference for further calculations. The sintering powder grain size is selected from some of the available combinations of sintered copper powder wick found in the table.

Table 2: Available sintered copper grain size

The effective conductivity of the sintered powder wick is given by the following equation: 21

……….. (19) Where: kl =thermal conductivity of the liquid (water) kw = is the thermal conductivity of the wick material (copper). The permeability of the sintered powder wick is1:

………. (20) Where, the rc (effective capillary radius) is given by1 : ……………. (21) Based on the commonly-found1 wick porosity, assumed wick porosity (which can be significantly adjusted during the process of manufacturing) wick porosity plays an important role since its relation in the porosity equation is of cubic order, thus every slight change in wick porosity varies the final heat transport result significantly. Using the governing equations described above, the sintered copper powder wick calculations are performed referring to following nomenclature1:

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From above governing equation we find this basic dimension Pipe outer dimension=8mm Pipe inner dimension=7mm Vapor core dimension=6.5mm Thickness of pipe=1.5mm Adiabatic length=100mm Evaporating length=75mm Condenser length=75mm Porosity=0.36mm

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5. SELECTION OF HEATER Electric heaters and electric water heaters can be any heating device that is powered by electricity and creates heat. Electric heaters are used to heat a variety of materials in domestic, commercial and industrial settings. Electric heaters can also be used to heat a specific area, shape or melt materials or even preserve the molten state of a substance. Some heaters use Peltier modules to produce heat, and some use light or other methods. Electric water heaters are available in many unique sizes, shapes and heating configurations. An electric heater may heat an object from room temperature up to over 1300°F. Various grades and alloys of metal are the material of choice for the heating element itself. Electric heaters can utilize a variety of methods to move and transfer heat. A standard hard wired electric heater can have a central heating element and then use fans to force the hot air throughout a larger system of ductwork, such as duct heaters. A ceramic heater or cartridge heater has heating elements that come into direct or near-direct contact with the area or substance needing heat. An example of a ceramic heater would be a heater used to keep prepared hot food products warm for consumption. The heating element directly warms the substance rather than indirectly via fans or ductwork.

 5.1 Types of Electric Heaters  Air heaters use electricity to warm air.  Band heaters are o-shaped heating devices that secure around an element. They can clamp around the outside of a cylindrical element and heat from the outside or clamp around the inside.  Cartridge heaters are compact cylindrically shaped heaters which are used primarily for immersion applications. They also have a protective sleeve or sheath protecting the heating element from the immersion liquid.

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 Cable heaters and coil heaters are formed from straight pieces of heating cable. These heating elements are formed into coils, spiral, simulated, star wound or other patterns.  Ceramic fiber heaters contain a layer of ceramic fiber insulation combined with a heating element. It is usually an industrial heater and available in cylindrical and flat configurations.  Circulation heaters are used primarily to heat fluid streams in motion. Fluid runs through the heater, which increases the stream temperature; any liquid or gas is generally suitable for use with a circulation heater.  Drum heaters are used to heat drums or their contents. Most drum heaters can accommodate various sizes of drums and many different substances.  Duct heaters can heat moving gas streams and heat air as it moves through the heater. It is also sometimes used to intensely heat an object at the end of a stream of gas.  Electric down flow heaters blow hot air down into the area needing heat and rapidly heat it to a desired level.  Flexible heaters are devices that may be formed to fit a variety of items. Flexible heaters are made from pliable materials such as rubber or neoprene so they can be formed to fit a variety of circumstances.  Foil heaters are made of flexible heater wire bonded to a thin aluminum substrate. The wire can be bended into a variety of shapes and act as the transport for the heat used.  Immersion heaters are used when it is necessary to immerse a heater in the material being heated. Examples of such materials can be water or liquid polymers.  Infrared heaters use a shield to reflect radiant heat onto a surface that is heated. Types of infrared heaters include metal-sheathed tubular heaters, quartz tubes, quartz lamps, gas fired catalytic, flat-faced panels and ceramic emitters.  Over-the-side heaters are the same as immersion heaters except that they hang over the side of a tank into the heated material. 25

 Radiant heaters diffuse energy heat rays in a 160 degree arc, and deliver heat evenly. They can maintain an almost uniform area temperature so that there is not more than 2 degrees variation in the space; many radiant heaters are so exact that heat can be directed to specific locations.  Strip heaters are electric heaters that require minimal space.  Thermoelectric heaters convert electric energy into heat. This is an irreversible conversion of electricity into heat; these heaters are often used for water and other fluids.  Tubular Heaters are used to heat air, solids or liquids generally for custom heating purposes. These can sometimes be designed for mobile jobs in various fields.

 5.2 Design of heater For our requirement ceramic wire type heater is used. A first selection criterion for choosing heater wire is resistance of heater wire. So we calculate the resistance of heater wire: According to Joule's Law, the heat power produced by a resistor is: P = IV Where P is the power in watts I is the current in amperes, and V is the potential difference in volts, And according to Ohm's Law I and V are related as follows: V = IR Where R is the resistance of the heating element, in ohms, is given by ρl/A

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We can combine these two formulae to obtain the heat output from the heating element in terms of either current or voltage:

For heaters powered by AC mains, I and V are the root mean square (RMS) values of current and voltage. For 100w and 230v heater resistance is 530Ω, can be achieving. There are different types of heating wire available and their electrical properties are shown in table AW G

Phosph or Bronze

32

Resistance (/m) 4.2 K

77 K

305 K

3.34

3.45

4.02

Diamet er (mm)

Fuse curren t air (A)

Fuse curren t vacuu m (A)

Numb er of leads

Nam e

Insulat ed diamet er (mm)

Insulatio n type

Insulati on thermal rating (K)

Insulatio n breakdo wn voltage (VDC)

0.203

4.2

3.1

1

SL32 DT32 QL32 SL36 DT36 QT36 QL36 NC32 HD30 CT34 MW30 MW32 MW36

0.241

Polyimide

493

400

0.241

Polyimide

0.241

Polyimide

0.152

Formvar®

368

250

0.152

Polyimide

493

400

0.152

Formvar®

368

250

0.152

Polyimide

493

400

0.241

Polyimide

493

400

0.635

Teflon®

473

250

0.254

Teflon®

473

100

0.295

Heavy Formvar® Heavy Formvar® Heavy Formvar®

378

400

2 4 36

8.56

8.83

10.3

0.127

2.6

1.4

1 2 4

Nichro me Copper

32

33.2

33.4

34

0.203

2.5

1.8

1

30

0.04

0.32

0.254

10.2

8.8

1

0.10 1 9.13

0.81

0.160

5.1

4.4

2

30

0.00 3 0.00 76 8.64

9.69

0.254

4.6

4.3

1

32

13.5

14.3

15.1

0.203

3.8

3.5

1

36

34.6

36.5

38.8

0.127

2.6

2.5

1

34 Mangan in

0.241 0.152

400 250

Table 3

This heater wire wounded 8mm diameter and 75mm length. So we required approximate 1m heater wire. From the above table we selecting Nichrom wire. So dimension for heater wire is Length =1.35m Wire diameter =0.055mm 27

6. THERMOCOUPLES In electronics and in electrical engineering, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal potential difference into electric potential difference. They are cheap and interchangeable, have standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy; system errors of less than one degree Celsius (°C) can be difficult to achieve. A variety of thermocouples are available, suitable for different measuring applications. They are usually selected based on the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria include the inertness of the thermocouple material, and whether or not it is magnetic. The thermocouple types are listed below with the positive electrode first, followed by the negative electrode.

Figure 12: K type

 6.1 K type thermocouple Type K (chromel–alumel) is the most commonly used general purpose thermocouple. It is inexpensive and, owing to its popularity, available in a wide variety of probes. They are available in the −200 °C to +1350 °C range. The type K was specified at a time when metallurgy was less advanced than it is today and, consequently, characteristics vary considerably between examples. Another potential problem arises in some situations since one of the constituent metals, nickel, is 28

magnetic. The characteristic of the thermocouple undergoes a step change when a magnetic material reaches its Curie point. This occurs for this thermocouple at 354°C. Sensitivity is a 41 µV/°C.

 6.2 E type thermocouple Type E (chromel–constantan) has a high output (68 µV/°C) which makes it well suited to cryogenic use. Additionally, it is non-magnetic.

 6.3 J type thermocouple Type J (iron–constantan) is less popular than type K due to its limited range (−40 to +750 °C). The main application is with old equipment that cannot accept modern thermocouples. J types cannot be used above 760 °C as an abrupt magnetic transformation causes permanent decalibration. The magnetic properties also prevent use in some applications. Type J thermocouples have a sensitivity of about 50 µV/°C.

 6.4 N type thermocouple Type N (nicrosil–nisil) thermocouples are suitable for use at high temperatures, exceeding 1200 °C, due to their stability and ability to resist high temperature oxidation. Sensitivity is 39 µV/°C, at 900°C , which is slightly lower than type K. Designed to be an improved type K, it is becoming more popular.

 6.5 B, R, and S type thermocouple Types B, R, and S thermocouples use platinum or a platinum–rhodium alloy for each conductor. These

are

among

the

most

stable

thermocouples,

but

have

lower

sensitivity,

approximately10 µV/°C, than other types. The high cost of these thermocouple types makes them 29

unsuitable for general use. Generally, type B, R, and S thermocouples are used only for high temperature measurements. Type B thermocouples use a platinum–rhodium alloy for each conductor. One conductor contains 30% rhodium while the other conductor contains 6% rhodium. These thermocouples are suited for use at up to 1800 °C. Type B thermocouples produce the same output at 0 °C and 42 °C, limiting their use below about 50 °C. Type R thermocouples use a platinum–rhodium alloy containing 13% rhodium for one conductor and pure platinum for the other conductor. Type R thermocouples are used up to 1600 °C. Type S thermocouples use a platinum–rhodium alloy containing 10% rhodium for one conductor and pure platinum for the other conductor. Like type R, type S thermocouples are used up to 1600 °C. In particular, type S is used as the standard of calibration for the melting point of gold (1064.43 °C).

 6.6 T type thermocouple Type T (copper–constantan) thermocouples are suited for measurements in the −200 to 350 °C range. Often used as a differential measurement since only copper wire touches the probes. As both conductors are non-magnetic, type T thermocouples are a popular choice for applications such as electrical generators which contain strong magnetic fields. Type T thermocouples have a sensitivity of about 43 µV/°C.

 6.7 C type thermocouple Type C (tungsten 5% rhenium – tungsten 26% rhenium) thermocouples are suited for measurements in the 0 °C to 2320 °C range. This thermocouple is well-suited for vacuum furnaces at extremely high temperatures and must never be used in the presence of oxygen at temperatures above 260 °C.

30

 6.8 M type thermocouple Type M thermocouples use a nickel alloy for each wire. The positive wire contains 18% molybdenum while the negative wire contains 0.8% cobalt. These thermocouples are used in the vacuum furnaces for the same reasons as with type C. Upper temperature is limited to 1400 °C. Though it is a less common type of thermocouple, look-up tables to correlate temperature to EMF (milli-volt output) are available.

 6.9 Thermocouple comparison The table below describes properties of several different thermocouple types. Within the tolerance columns, T represents the temperature of the hot junction, in degrees Celsius. For example, a thermocouple with a tolerance of ±0.0025×T would have a tolerance of ±2.5 °C at 1000 °C.

Temperature Temperature Tolerance Tolerance IEC Type range

°C range

°C class

(continuous) (short term)

one class

two Color

(°C)

(°C)

±1.5

±2.5

between

between

code

−40 °C and −40 °C and K

0 to +1100

−180 to +1300

375

°C 333

°C

±0.004×T

±0.0075×T

between

between

375 °C and 333 °C and 1000 °C

J

0 to +700

−180 to +800 ±1.5 between 31

1200 °C ±2.5 between

BS

ANSI

Color

Color

code

code

−40 °C and −40 °C and 375

°C 333

°C

±0.004×T

±0.0075×T

between

between

375 °C and 333 °C and 750 °C

750 °C

±1.5

±2.5

between

between

−40 °C and −40 °C and N

0 to +1100

−270 to +1300

375

°C 333

°C

±0.004×T

±0.0075×T

between

between

375 °C and 333 °C and 1000 °C

1200 °C

±1.0 between 0 °C

and

1100

°C

±[1 R

0 to +1600

+

−50 to +1700 0.003×(T − 1100)]

and

between 0 °C and 600 °C

Not

±0.0025×T

defined.

between

between 1100

±1.5

°C 1600

600 °C and 1600 °C

°C

S

0 to 1600

±1.5 −50 to +1750 ±1.0 between 0 between 0 32

Not defined.

°C

and °C and 600

1100

°C °C

±[1

+ ±0.0025×T

0.003×(T − between 1100)]

600 °C and

between

1600 °C

1100 and

°C 1600

°C

±0.0025×T B

+200 to +1700 0 to +1820

Not

between

Available

600 °C and 1700 °C

±0.5

±1.0

between

between

−40 °C and −40 °C and T

−185 to +300 −250 to +400

125

°C 133

°C

±0.004×T

±0.0075×T

between

between

125 °C and 133 °C and 350 °C

350 °C

33

No

No

standard standard use

use

copper

copper

wire

wire

Not defined.

±1.5

±2.5

between

between

−40 °C and −40 °C and E

0 to +800

−40 to +900

375

°C 333

°C

±0.004×T

±0.0075×T

between

between

375 °C and 333 °C and 800 °C

900 °C

Table 4 Thermocouple comparison

From above table, we conclude that K type thermocouple is best suitable for our experiment. So we purchase 8 thermocouples which have 0.25mm dia.

34

7. CONTROLLER There are two universal controllers we can use. In this experiment set-up the heater is operated with high power density. So there is chance for the overheating of heater. Therefore heater safety we required temperature controller. This controller is based on PLC circuit which explains below. In PLC controller, to compare the data, the program instruction will contain comparison instruction, the sources of data and the destination address. This is shown by fig.

Figure 13: compare data

Figure 14: temperature controller diagram

35

Such a comparison might be used when the signals from sensors are to be compared by the PLC before the action is taken. For example the on/off switch might be required to open when the temperature goes above 100°C. (PLC is assumed to be set at 95°C) and remains as it is till temperature falls below the 90°C. The input temperature data is inputted to source address and the destination address contains the set value. When the temperature falls below 90°C or lower, the data value in the source address becomes ≤ destination address value and there is an output to the switches which latches the input. Similarly when the temperature rises to 100°C or higher, the data value in the source address becomes the ≥ destination address value and there is an output to the relay which open and so switches off it.

 7.1 Configuration So we should purchase the ADT5002 Microprocessor Based Universal Controller which has the following configuration.

Figure 15: Universal controller

 Input Type J, K, T, R, S Thermocouples,  Voltage 0-10 V DC, Current 4-20 mA or RTD Type Pt100, 36

 Programmable through from key,  Polynomial Linearization  Display 5 Digit Seven segment Red LED Display 12.5 mm Size’  Resolution 0.1°C up to 999.9 and 1°C above 1000°C, Programmable through from key  Operates on 230 V AC/110 V AC supply. Optionally 90 to 270 V AC, 24 VDC  Accuracy better than ±0.25% Rdg ± 1 Count  Available in Wall mounting / Panel mounting  Control action ON/OFF, PD

37

8. TEMPERATURE SCANNER We required the temperature indicator for measure the various temperatures along the length. So we purchase the 16 channel temperature scanner which has following configuration. ADT3005 Digital Temperature Indicating Controllers Single/Dual Set Points

Figure 16 temperature scanner

 Input Type J, K, T, E, R, S, B Thermocouples, RTD Type Pt-100, 4-20 mA  Display 3½, 4½ Digit Seven segment Red LED Display, 12.5/25 mm size  Automatic cold junction compensation for Thermocouples  Operates on 230 V AC/110 V AC supply.  Accuracy better than ±1% FS  Control action ON/OFF, PD  1 C/O Output contacts per set point, rated for 5Amps. @ 230 V AC

38

9. EXPERIMENT SETUP

Figure 17: Circuit diagram

39

1) AC supplier 2) Temperature scanner 3) Water jacket 4) Thermocouples 5) 80 watts bulb 6) Heater 7) Controller 8) Heat pipe

To Join the circuit, as shown in figure and completed the experiment setup.

40

10. PROCEDURE 1. Note the diameter and length of two pipes, viz copper rod and heat pipe. 2. Pore water into the heat sinks of the three pipes and measure their quantities. 3. Measure the temperature of water in the sinks with a thermometer. 4. Using controller give the same power input to the heating elements of the two pipes. 5. Under steady state condition note the readings of thermocouples. 6. Measure the final temperature of water in the sinks. 7. Repeat the experiment for different heat inputs.

41

11. OBSERVATION Mass of the water=300gram Power =80watts Initial temperature of water= 26°C

SR NO. 1 2 3 4 5 6 7 8 9 10 11 12 13

T set°C 40 45 50 55 60 65 70 75 80 85 90 95 100

T Cu°C 28 28.5 30 31.5 32 33 34.5 35.5 36.5 37.5 38 40 41

T Heat°C 29 30 32.5 36 38 40 43 46 49 52 54 58 60

T1°C 32 34 37 39 41 44 47 50 52 55 59 60 63

T set =heater temperature in °C T Cu =water temperature at the cu pipe sink in °C T heat = water temperature at the Heat pipe sink in °C T1=temperature of thermocouple 1 on cu pipe in °C T2=temperature of thermocouple 2 on cu pipe in °C T3=temperature of thermocouple 3 on Heat pipe in ° C T4=temperature of thermocouple 4 on Heat pipe in °C

42

T2°C 28 30 33 37 38 40 43 45 46 46 48 49 51

T3°C 28 32 35 38 40 44 47 50 52 55 60 62 64

T4°C 28 32 35 38 40 44 47 50 52 55 60 62 64

12. CALCULATION A) Copper pipe

1. Power p= 80w 2. T Set= 80°C, T1=50°C, T2=43°C , T cu=36.5°C 3. M=300gram 4. Initial temperature T0=35.5°C 5. Final temperature Tf=T Cu=36.5°C

B) Heat pipe

1. Power p= 80w 2. T Set= 80°C, T3=52°C, T4=52°C , T heat=49°C 3. M=300gram 4. Initial temperature T0=46°C 5. Final temperature Tf=T Cu=49°C

43

13. RESULT TABLE SR NO. 1 2 3 4 5 6 7 8 9 10 11 12 13

Q Cu (watts) 2.252 0.63 1.89 1.89 0.63 1.26 1.89 1.26 1.26 1.26 0.63 2.52 1.26

Q Heat (watts) 3.78 1.26 3.15 4.14 2.52 2.52 3.78 3.78 3.78 3.78 2.52 5.04 2.50

44

K heat pipe/ K cu 1.09 1.1 1.14 1.23 1.33 1.28 1.31 1.36 1.4 1.43 1.44 1.48 1.47

14. CONCLUSION  From result table we conclude that thermal conductivity of heat pipe 1.5 times of copper rod.  We also see that the heat gained of water by heat pipe is more than the heat gained by copper rod. So we conclude that heat transfer by heat pipe is more than the copper.

Figure 18: Length vs. temperature graph for cu rod

45

Figure 19: Length vs. temperature graph for heat pipe

 From the above graph, we see the temperature profile on heat pipe and heat pipe and we conclude that at the adiabatic section of heat pipe, there is no temperature gradient while the copper there is temperature gradient along whole length.  There is small difference in practical thermal conductivity and experimentally thermal conductivity. The main reasons are:      

Conduction loss Convection loss in air Sensitivity of thermocouple Calibration of temperature scanner Accuracy of controller Efficiency of heater

 For minimize the error we provide the more insulator and fine-tuning controller.

46

15. FURTHER SCOPE By this experiment, we can also find the thermal resistivity of heat pipe. For that, the inlet water temperature adjusted at each power level to maintain roughly constant temperature at the cool end of the heat pipe. At each power level, average temperatures can defined for the evaporator section and the condenser. (The rise in temperature along the condenser at high power levels may be due to supersonic flow). The difference

T between evaporator and condenser see to rise gradually in the power range.

For some power range, the end thermocouple on the evaporator clearly shows runaway performance and for higher powers, the temperature profile is unstable. This can illustrated in Figure 1, show liner relationship between temperature difference and power and slop of the line gives the thermal resistivity of heat pipe. So by this procedure we can also find the thermal resistivity of heat pipe.

Figure 20: The temperature rise along the heat pipe vs. the amount of heat conducted by the heat pipe

47

ducted by the heat pipe Such data can be obtained at many different average temperatures and plotted individually, but we find it helpful to combine them in a 3-D plot of T, P, and shown in Figure 5 for the case of

T, as

=0 (horizontal). This data has been interpolated at 10oC

intervals in T and 5W intervals in P for ease of plotting. (The data are limited to temperatures less than ~100oC by the use of water as a cooling agent at the evaporator end, not by the operation of the heat pipe.)

Figure 21 :Three-dimensional plot of data such as that in Figure 20

One advantage of the 3-D plot is that it indicates clearly the range of T and P over which linear behavior is observed. At higher power levels, the heat pipe is unstable and loses its very low thermal resistance. Without the help of the working fluid, the thermal resistance of the heat pipe rises to that of the copper tube alone. For comparison, the dashed lines show the thermal behavior of a solid copper rod of the same outside diameter and of a length equal to the distance between centers of the evaporator and 48

condenser. The dashed lines represent a constant thermal resistance (The resistance of a copper tube and wick will be even higher.) We point out that while the heat pipe characteristics in Figure 21 are expected to be roughly independent of heat pipe length, the resistance of the copper rod (dashed lines) will increase directly with the rod length. Manufacturers of heat pipes typically provide plots of maximum power vs. T. For direct comparison with the data in Figure 21, we show in Figure 22 a projection onto the P-T plane of the data in Figure 22. The dashed line is the maximum P (T) recommended by the manufacturer and is seen to lie well below the onset of instability for this particular heat pipe.

Figure 22: Projection of Figure 21 onto the P-T plane

49

Figure 23 Dependence of the maximum stable heat flow on orientation for Tavg = 95 C

The dependence of the maximum stable heat-carrying capacity on angle of inclination is shown in Figure 22 for a temperature of 95oC. The maximum power is clearly highest when the condensed working fluid is returned to the evaporator by the force of gravity ( > 0). The fact that the heat pipe works at all in the "upside-down" orientation ( < 0) is due, of course, to the wicking of the condensed fluid against gravity. Again, the manufacturer's recommended maximum power is found to be conservative for this particular heat pipe. For performing above the experiment, we required the more accurate thermocouples and fine tuning controller as well as dimmer-stats.

50

16. BIBILOGRAPHY  16.1 BOOKS     

“Principles Of Heat Transfer” “ Thomson Learning Publishers” “By Frank Kreith & Mark S. Bohn” Page[673683] “Fundamentals Of Heat & Mass Transfer” “New Age International Publishers” “By R.C.Sachdeva” Page[301-304] “Heat Transfer” “TATA Mc Graw Hll” “By Yunus A. Cengel” Page[683-691] “A Course In Heat & Mass Transfer” “Dhanpat Rai” “By Arora Domkundwar” Page[24.1-24.6] “Heat Transfer” “TATA Mc Graw Hill” “By P.K.Nag” Page[565-578]

 16.2 LINKS             

www.electronics-cooling.com/articles/1996/sep/sep96_02.php www.enertron-inc.com/enertron-products/heat-pipes.php www.engr.sjsu.edu/ndejong/ME%20146%20files/FundamentalsofHeatPipesII. www.rpi.edu/tphtl/research/scms/imece-111902.ppt www-physics.lbl.gov/~gilg/Pixel2002Talks/miller.ppt www.engr.sjsu.edu/ndejong/ME%20146%20files/FundamentalsofHeatPipesII. www.edata-center.com/journals/IHTC13,339f209755c1e2ca,19ae360575444448.html www.springerlink.com/index/J23J1151G061J24N.pdf www.freepatentsonline.com/4047198.html www.wickipedia.com www.anser.com’ www.science.com www.benchtest.com

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