Jet Prop

Unit IV Jet Propulsion Jt is the propulsion of jet aircraft or other missiles by the reaction of jet coming out with a high velocity. The term jet propulsion is used where the oxygen is obtained from the surrounding atmosphere. It consists of air plus combustion products. The principle of jet propu is o rom he app th Newton’ s ‘ law o motion. h is nothing but reaction principle. Since all the air craft engines breaths air from the surrounding atmosphere hence it is called air-breathing engines. The air breathing engines are classified as (1) Turbojet engine (ii) Turbo prop engine iiii Pulse jet (or) flying bomb and (iv) Ram jet engine Turbo Jet Engine It is a most common type of air breathing engine whose essential features are shown

Components of Turbo Jet Engines This engine consists of inlet diffuser, compressor, combustion chamber, turbine and an exhaust nozzle. The function of the diffuser is to convert the kinetic energy of the entering air into a static pressure rise. After this air enters to the compressor. (axial or centrifugal) which further compresses the air to a very high pressure and delivers it to the combustion chamber. Then fuel nozzle supplies fuel continuously and continuous combustion takes place at constant pressure. The high pressure and high temperature gases then enters the turbine. where they expand partially to provide drive power for the turbine. The turbine is directly connected to the compressor and all the power developed by the turbine is to drive the compressor and the auxiliary devices. After the gases leave the turbine, they expand further in the exhaust nozzle and are ejected with a very high velocity than the flight velocity to produce a thrust for propulsion.

Advantages I. Lower frontal area due to the absence of fan. Therefore the drag is less. 2. Suitable for long distance flights at higher altitudes and speeds. 3. Since this engine has a compressor, it is capable of operating under static conditions. 4. Reheat can be possible to increase the thrust. 5. Lower weight per unit thrust at design speed and altitude. 6. Since a diffuser is at the inlet, part of the compression is done by it without any work input. Disadvantages I. Propulsive efficiency and thrust are lower at lower speeds. 2. Thrust specific fuel consumption is high at low speeds and altitudes. 3. It is not economical for short distance flights. 4. Long runway is required due to slower acceleration. 5. Sudden decrease of speed is difficult to achieve. Applications Turbojet engines are used in military aircrafts, guided missiles and piloted aircrafts, etc. Turbo Prop Engine It is very similar to turbojet engine, the major difference being that the turbine is designed so that it develops shaft power for driving a propeller to provide most of the propulsive thrust (90%), and only a small amount jet thrust is produced in the nozzle is shown in the figure.

Components of Turbo-Prop Engine The engine consists of a diffuser, compressor, combustion chamber, turbine, exhaust nozzle, reduction gear and a propeller. The diffuser, compressor and combustion chamber functions are as same as the turbo jet engine. However, in the turbo prop engine, the turbine extracts much more power than the turbo-jet engine, because the turbine provides power for both the compressor and the propeller. When all of this energy is extracted from the high temperature gases, only little energy is left out for producing jet thrust. Thus the turbo-prop engine drives most of its propulsive thrust from the propeller and drives only a small portion (I 0 to 25%) from the exhaust nozzle. Since the shaft rotation speed of gas turbine engine is very high, a reduction gear must be placed between the turbine shaft and the propeller to enable the propeller to operate efficiently. Advantages I. Propulsive efficiency is very high. 2. The TSFC based on thrust is low. 3. High acceleration at lower speed enables to a shorter run way. 4. Thrust reversal is possible by varying the blade angle, this gives the advantage of decreasing the speed drastically. S. Used for shorter distance travels. (C <600 Kmph) Disadvantages 1. Heavier propeller, compressor and turbine decreases pay load capacity. 2. A reduction gear is required to transmit the power from the turbine shaft to the propeller shaft. 3. If the speed of the engine increases above 600 Kmph. the efficiency drastically decreases. 4. The frontal area is being blocked on account of large diameter propeller which increases the co-efficient of drag. 5. Engine is heavier and more complicated. Ram Jet Engine The simplest types of air-breathing engine is the Ram jet engine which is shown in

6.4. The engine consists of a supersonic diffuser, subsonic diffuser section, combustion chamber and a discharge nozzle section. The function of a supersonic and subsonic diffuser is to convert the kinetic energy of the entering air into a pressure rise, called the “ram pressure”. Air from the atmosphere enters the supersonic diffuser where in its static pressure increased and the velocity of air is reduced. Then the air enters the subsonic diffuser it is compressed further. The air then flows into the combustion chamber, where the fuel burners are located and here the air is heated to a high temperature (1600°C to 2000°C) by the continuous combustion of fuel. The highly heated products of combustion are then allowed to expand in the exhaust nozzle section and are discharged from the engine with a speed greater than that of entering air. Because of the rate of increase in momentum of the working fluid flowing through the engine, a thrust F’ is developed in the direction of flight.

Components of Ram Jet Engine The cycle pressure ratio of ram jet engine depends upon its flight velocity; the higher the velocity, the larger the ram pressure and consequently larger the thrust. Since the flight is very high. the pressure rise in the diffuser (ram pressure) is very high and this eliminates the compressor. Consequently the turbine is also eliminated, because, the function turbine is just to run the compressor. Since the ram jet engine cannot operate under static conditions as there will be no pressure rise in the diffuser, it is not self-operating at zero flight velocity. Therefore to attain the required flight speed some kind of starting device must be required such as launching rockets. Advantages I. Pay load capacity is very high due to the absence of fan, compressor and turbine.

2. Its fuel consumption decreases with flight speed and approaches reasonable values when the flight Mach number is between 2 to 4, and therefore, it is suitable for propelling supersonic missiles. 3. Since the frontal area is less, the co-efficient of drag is low. 4. It increases the mechanical efficiency due to the absence of sliding and moving parts. 5. High temperature and pressure can be employed. Disadvantages I. A starting device is required to propel ram jet upto supersonic speed. 2. Altitude limitation is there. 3. It has ow thermal efficiency and high TSFC. 4. Due to high temperature of gas coming out from the nozzle, erosion occurs at the exit of the nozzle. Applications Used as guided missiles and high supersonic speed aircrafts. Pulse Jet or Flying Bomb Fig. 6.5 shows a pulse jet engine which consists of a inlet diffuser, valve grid (contains springs that close on their own spring pressure), combustion chamber, spark plug and a discharge nozzle.

Components of Pulse Jet Engine The function of a diffuser is to change the kinetic energy of the entering air into static pressure rise by slowing down the air velocity. When a certain pressure drop exits across the valve grid, the valves will open and allow the fresh air to enter the combustion chamber, where fuel is injected and mixed with air. Hence combustion takes place with

spark ignition. There is a rapid increase in pressure, which causes the valve to close rapidly and surges the products of combustion rearward into the nozzle, where they expand and escape with higher velocity than the entrance velocity. Thus the thrust is produced at the nozzle exit. Since firing in the combustor is intermittent and therefore intermittent thrust is produced. The pulse-jet engine is a simple, cheap for subsonic flights and well adopted to pilotless aircraft. Advantages I. It gives higher pay load capacity due to the absence of compressor. propeller and turbine. 2. It is simple in construction and cheap. It is suitable for subsonic flights. 3. Drag co-efficient is less due to smaller frontal area. 4. Due to the absence of sliding and moving parts mechanical efficiency is very high. Disadvantages I. Limited flight speed and altitude. 2. Severe vibrations and high intensity of noise due to intermittent combustion. 3. Nozzle erosion occurs, due to the high temperature of gases coming out from the nozzle. Energy Relations and Efficiencies in a Turbo Jet Engine The energy equations and efficiencies for various processes are written in this section. Refer the T-S diagram (Fig. 6.2) for ideal and actual Brayton cycle. Inlet Diffuser The isentropic and adiabatic compression of air through the inlet diffuser (i —I) and the compressor (1 —2) is shown in the figure.

Compression of Air through the Inlet Diffuser and the Compressor I —‘ =Isentropic diffusion 1 —1 = Adiabatic diffusion — 2’ = Isentropic compression —2 = Adiabatic compression.

Ambient air enters into the inlet diffuser at a temperature T, pressure P and velocity C respectively. In the diffuser, pressure increases and velocity decreases. Since energy transformation takes place in the diffuser, the stagnation enthalpy is constant i.e., h = / The diffuser efficiency can be considered in two ways. (a) For Small Pressure Rise The efficiency for a small pressure rise is defined as

For Large Pressure Rise

The efficiency for a large pressure rise is defined as

The above equation can be expressed as a function of Mach number at the diffuser inlet (or) flight Mach number.

Assuming T1 = T01 = T02

Compressor Air enters the compressor at a pressure P reduced velocity C and Mach number M is compressed to a pressure P temperature T and velocity C respectively. The increased static enthalpy and the stagnation enthalpies for actual and isentropic compression are shown in the figure. The actual work done by the compressor on the air Isentropic work transfer

because of friction and irreversibilities.

Combustion Chamber The compressed air from the compressor enters the combustion chamber at a pressure P temperature T and velocity C During combustion, the enthalpy of the air-fuel mixture increases. The mass of air-fuel mixture coming out from the combustion chamber is

The efficiency of the combustion chamber.

Since /1 very small compared to ñla. therefore, the above equation can be written as

The combustion efficiency can also be defined as

Turbine The high temperature combustion products enter the turbine at a temperature T pressure P and velocity C-i. The expansion of gases through the turbine and nozzle is shown in Fig. 6.7. Since, there is a shaft work in the turbine, therefore the stagnation enthalpy is not constant.

Expansion of Gases through the Turbine and Noz

Total-to-total efficiency of the turbine

Exhaust Nozzle Exhaust gases (after expansion) from the turbine enter the propelling nozzle at a temperature T pressure P and velocity C Since energy transformation takes place in the nozzle, the stagnation enthalpy is constant. The gases expand adiabatically.in the nozzle to a pressure P and velocity Ce.

From adiabatic energy equation.

:. The exit velocity of gases Ce

Thrust The force which propels the aircraft forward at a given speed is called propulsive force or thrust. This propulsive force is mainly depends on the velocity of gases at the exit of the nozzle in turbojet engines and from the propeller in turbo prop engines.

(a) Jet Thrust (Turbo Jet Engine) The two sections 1 —I and 2 —2 of an imaginary control surface for a turbo jet engine is shown in Fig. 6.8. The flow of air (internal and external) is separated by the solid boundaries of the engine casing.

Flow of Gases in Turbo Jet Engine

Mass flov rate at inlet of the engine is ñ and the mass flow rate at exit is (, + Kg / sec. Part of the air flow at section 1 I is swallowed by the jet engine and experiences change in momentum flux, the remaining flows through the engine without any change in the momentum flux.

The net thrust on the engine = momentum thrust + pressure thrust F = Fmom + Fpr

(b Propeller Thrust Fig. 6.9 shows the air flow takes place across the propeller of a turbo prop engine. The air flow pattern before and after the propeller is shown in fig. A flow boundary similar to the walls of a duct which separates the fluid at rest and fluid in motion.

The pressure at section 1 —I and outside the boundary is ambient. Therefore, the thrust on the propeller and the aircraft is due to the change in momentum flux between inlet and outlet section. The thrust on the propeller F Where C = jet velocity and Cj = flight speed The flight to jet velocity ratio or effective speed ratio

Propulsive, Thermal and Overall Efficiencies The performance of an aircraft propulsion system can he analysed h various efficiencies. Fig. 6.10 shows the utilization of power of the fuel in a turbojet engine.

Utilization of Power in Aircraft Propulsion Power input to the engine (Fuel power) =

Propulsive power (or) Thrust power F x u =

Propulsive Efficiency

Divide both Nr. and Dr. by ‘ C,’ the above equation becomes

Case (a) : When the speed of air craft u = 0, the propulsive efficiency ri = 0. but the specific thrust is maximum. Maximum thrust is needed during take-off period. Case (b) When the speed of aircraft equals to the speed of jet i.e., u C,, r = l00%, but the specific thrust is zero. Therefore ‘ C,’ must be always greater than ‘ u’ when the aircraft is flying. In normal conditions when the speed ratio (ci) increases, the propulsive efficiency (ii will also increases. The propulsive efficiency can be increased by increasing the jet velocity close to the flight speed where as the thrust power can be increased by increasing the mass flow rate of air or gas through the propulsive device. The propulsive efficiency versus speed ratio for turbojet and turbo prop engine is shown in the figure.

Comparison of Turbo Prop and Turbo Jet Engines

Thermal Efficiency

Overall Efficiency

Specific Fuel Consumption It is the ratio between fuel consumption rate per unit thrust. Since the output is in the form of thrust, a thrust fuel consumption is

It is an important parameter to compare the engine performance of different types of aircraft propulsion systems.

Specific Thrust It is defined as the thrust produced per unit mass flow rate through the propulsive device.

It is an another useful parameter for comparing the different types of propulsion devices. Specific Impulse It is defined as the thrust produced per unit weight flow rate through the propulsive device. It is also an another useful performance parameter in aircraft propulsion devices.

Effect of Forward Speed The forward speed of the aircraft affects the compressor inlet pressure and temperature. As flight velocity increases, inlet drag will be more and the net specific thrust is reduced using the normal values of cycle variables. Therefore, propulsive efficiency is decreased. Effect of Altitude At higher altitude, the ambient temperature and pressure is very less. This ambient air is not sufficient to propel the aircraft engines. Therefore, the flight must fly at a designed altitude. Thrust Augmentation To achieve better take-off performance, higher rates of climb and increased performance at altitude during combat manoeuvres, there has been a demand for increasing the thrust utput of aircraft power plant for short intervals of time. The following methods of thrust a’ for turbojet engines are: (a) After-burning Burning additional fuel in the tail pipe between the turbine exhaust section and entrance section of the exhaust nozzle is shown in Fig. 6.12.

After Burner

This method of thrust augmentation increases the enthaipy of air entering the nozzle. Thus the jet velocity at the nozzle exit is increased, resulting in increased thrust. (b) Injecting Refrigerants Injecting refrigerants, water or water-alcohol mixture at some point between inlet and exit sections of the air compressor. This method of thrust augmentation increases the mass flow rate of air and decreases the compressor work. 1.A turbo-jet plane has two jets of 250 mm diameter and net power of the turbine is 3000 KW. The fuel consumption per Kwhr is 0.42 kg with a fuel of calorific value 49 MJ/Kg. When flying at a speed of 300 m /sec in atmosphere having a density of 0.168 Kg / m the air fuel ratio is 53. Ca/cu/cite (1) the absolute velocity of jet, ii) the resistance or drag of the plane, (iii) the overall efficiency of the plane iv)the efficiency of the turbine.

2 .A turbo jet engine takes in 50 Kg / sec of air and propels an aircraft with uniform flight speed 880 Km/hr. Isentropic enthalpy change for nozzle is 188 KJ/Kg and its velocity coefficient is 0.96. The fuel air ratio is 1.2%. Combustion efficiency is 95%, calorific value of fuel is 44000 KJ/Kg. Find out (1) Thermal efficiency of the engine. (ii,) Fuel flow in Kg/hr (‘ iii,) Propulsive efficiency. (iv) overall efficiency.

3. A turbo jet has a speed of 750 Km/hr while flying at an altitude of 10000 m. The propulsive efficiency of the jet is 50% and the overall efficiency of the turbine plant is 16%. The density of the air at 10000 m altitude is 0.173 Kg / m The drag on the plane is 6250 N. Calorific value of the fuel is 48000 KJ/Kg. Calculate (i) Absolute velocity of the jet (ii) Diameter of the jet and (iii) Power output of the unit in KW.

3. For a turbo jet with a flight velocity of 800 Km/h at an ambient of 60 KPa, the properties of gas entering the nozzle are 300 KPa and 200°C. The mass flow rate of air is 20 Kg/sec. Assuming air (r = 1.4 and R = 287 f/Kg °K) as working fluid,flnd (a) thrust developed, (b) thrust power and (c) propulsive efficiency.

4. The diameter of the propeller of an aircraft is 2.5 m. It flies at a speed of 500 Kmph at an altitude of 8000 m for flight to jet speed ratio = 0.75. Determine (i) the flow of air through the propeller, (ii) thrust produced, (iii) specific thrust,(iv) specific impulse, (v) thrust power.

5. A turbo jet engine is travelling at a speed of 236.11 m / sec under conditions 288°K and 1.013 bar. The ram efficiency is 85%. Calculate total pressure and total temperature of air after inlet dfffuser. Take Cpa = 1.005, ra = 1.4 For the above engine, the total temperature and total pressure at the inlet to the nozzle is 806°K and 2.124 bar. The flow through nozzle is adiabatic with jet efficiency (total-to static) 95%. Calculate the net specific thrust and thrust specific fuel consumption (TSFC) fuel air ratio is 0.0122. Take

6.A simple turbo jet unit operates with a turbine inlet temperature of 1 050°C. The following data refer to this unit when tested at ground level Assume and r for gases and air as same as 1.005 and 1.4 respectively. Compressor pressure ratio 7.5; Process through compressor and turbine is isentropic, nozzle efficiency 96% ; Ambient pressure 100 KPa, temperature 27°C; mass of air 25 Kg / sec. Neglect mass of fuel for calculation of thrust and neglect pressure losses. Calculate thrust, velocity and Mach number of the jet and exit of nozzle and SF C Given Data: The various processes in a simple turbo-jet engine is shown in figure (T-Sdiagram).

QUESTIONS AND PROBLEMS 1. What is jet propulsion? Give two examples of early and modem jet propulsion devices. 2. How is forward motion of an aircraft achieved by propeller action? How does the aircraft lift off the ground? Explain with the help of illustrative sketches. 3. What are the main components of a gas turbine engine used for turbojet aircrafts? Show the various processes occurring in the engine on a T-s diagram. 4. What is “after burning” in turbojet engines? Explain briefly with the aid of a diagram. 5. What is a by pass engine? What are its main advantages and disadvantages? 6. Describe the working of a ramjet engine. Depict the various thermodynamic processes occurring in it on h-s diagram. What is the effect of flight Mach number on its efficiency? 7. Draw the sketch of a pulse jet engine. Write down its main advantages and disadvantages. 8. Describe the working of a scram jet engine. What is its advantage over the ramjet? 9.(a) Draw a neat sketch of a supersonic diffuser for a ramjet engine with oblique and normal shocks. (b) Depict variation of the static pressure through the diffuser from its entry to exit. 10. Derive an expression. for the air-standard efficiency of a turbojet engine in terms of the pressure ratio. Depict graphically its variation with the temperature ratio. 11. How are efficiencies of the inlet diffuser and the propelling nozzle of a turbojet engine defined? Write down their formulas. 12. Derive the following relations for aircraft engines: (a) Combustion chamber efficiency = ratio of the theoretical to actual fuel —air ratio. (b) Flight to jet speed ratio

(b) Thrust in a turbojet engine

State the assumptions used in the derivation of the above expressions. 13. Diameter of an aircraft propeller is 4.0 metres. The speed ratio is 0.8 at a flight speed of 450 kmph. If the ambient conditions of air at the flight altitude are T= 256 K and p = 0.54 bar determine (a) propulsive efficiency (b) thrust and (c) thrust power. 14. A turbojet aircraft files at 875 kmph at an altitude of 10,000 m above mean sea level. Calculate (a) air flow rate through the engine (b) thrust (c) specific thrust (d) specific impulse (e) thrust power and (1) TSFC from the following data: diameter of the air inlet section = 0.75 m, diameter of jet pipe at exit = 0.5 m, velocity of the gases at the exit of the jet pipe = 500 m/s, pressure at the exit of the jet pipe = 0.30 bar, air to fuel ratio = 40. 15.. A turbojet engine operates at a flight speed of 903.312 kmph and an altitude of 12 km (T = 216.65 K, p = 0.193 N/m, p = 0.311 N/sqm , a = 295.2 m/s). The engine has the following data: Stagnation temperature at the turbine inlet = 1500 K, temperature drop in the turbine = 200°C. calorific value of the fuel = 43 MJ/kg, turbine efficiency = 0.91, compressor efficiency = 0.75, combustion chamber efficiency = 0.96, exhaust nozzle efficiency = 0.94, 16. A ramjet engine has the following data: Altitude = 6.5 km. flight Mach number = 4.0, Air fuel ratio = 50, calorific value of the fuel used = 44.18 MJ/kg. Diffuser inlet diameter = 0.5 m, ‘ y = 1.4, I? = 287 kJ/kg K for both air and the products of combustion. Efficiencies of the diffuser, combustor and the nozzle are 0.85, 0.98 and 0.95 respectively.