Theoritical Analysis Of Micro Channel Heat Exchanger

THEORITICAL ANALYSIS OF MICRO CHANNEL HEAT EXCHANGER

GUIDE Dr. JAHAR SARKAR

SUBMITTED BY AMIT SHRIVASTAVA SECOND YEAR THERMAL AND FLUID ROLL NO. 07030615

MICRO CHANNEL HEAT EXCHANGER STRUCTURAL DETAILS IN THE RANGE OF MICRONS

HEAT SINK HEAT EXCHANGER    

ELECTRONIC CIRCUITS CHIPS VLSI LAPTOPS

CLASSIFICATION SURFACE AREA DENSITY OR COMPACTNESS

ABOUT 10,000 m2 /m3

CHANNEL DIMENSION • Micro scale: 1-100 µm • Meso scale: 100 µm- 1 mm • Macro scale: > 6 mm

CHANNEL SIZE AND SURFACE AREA DENSITY ARE INTER RELATED

DEPARTURE FROM CONVENTIONAL THEORY

•

A CHANGE FROM CONVENTIONAL ASSUMPTION SUCH AS CONTINUUM APPROACH

•

INCREASED INFLUENCE OF SOME FORCES LIKE ELECTROKINETIC OR ELECTRO OSMOTIC FORCE

•

UNCERTAINITY REGARDING EMPERICAL FACTORS DERIVED AT LARGE SCALE. EG: ENTRANCE LOSS, EXIT LOSS

METHODS OF OPTIMIZATION EXERGY ANALYSIS

MINIMUM IRREVERSIBILITY CONCEPT

THERMOECONOMICS

ENTROPY GENERATION MINIMIZATION

MINIMUM RESISTANCE TO HEAT FLOW

IRREVERSIBILITY MINIMIZATION • Itotal = Ithermal +Ipressure Ithermal

Ipressure

DEPENDS ON

DEPENDS ON

ENTROPY

VISCOUS

GENERATION

DISSIPATION

TO ACHIEVE TOTAL MINIMUM IRREVERSIBILITY

HEAT SINK HEAT EXCHANGER Q Width W Height of channel Dd Length of channel L Thickness of metal saperating heat source from fluid t

t Dd W L

Heat rate Q Channel width Dc

tf

Dc

Fin thickness t f SKETCH DIAGRAM OF HEAT SINK

BASIC DIMENSIONS OF THE TWO HEAT EXCHANGERS HEAT EXCHANGER 1 Width 10mm Thickness of base plate 0.2mm Height of channel 0.5mm Thermal conductivity Of the heat exchanger material 170W/mC

HEAT EXCHANGER 2 Width 76.2mm Thickness of base plate 0.2mm Height of channel 12.7mm Thermal conductivity Of the heat exchanger material 170W/mC N, Dc, tf, L are variable

N, Dc, tf, L are variable

RESULTS AND DISCUSSIONS

HEAT EXCHANGER 1 WITH WATER AS THE COOLING MEDIUM

IRREVERSIBILITY VARIATION W.R.T. HEAT TRANSFER RATE With increase in Q Irre_t

IRREVERSIBILITY VARIATION W.R.T. HEAT TRANSFER

200

25

180

Irre_p 160

20

140 120

15

100 80

10

60 40

5

20 0 0

50

100 HEAT TRANSFER (W)

150

0 200

DEL_T (K)

IRRE_P, IRRE_T, IRRE_TOTAL (W)

flow

IRRE_P IRRE_T irre_total DEL_T

IRREVERSIBILITY VARIATION W.R.T. NO. OF PASSES (Tf constant)

Dc decreases

Irre_t

Higher 120 del_t rise 100 IRRE_P, IRRE_T, IRRE_TOTAL (W)

flow

IRREVERSIBILITY VARIATION W.R.T. NO. OF PASSES 20 18 16 14

Irre_p first reducing and then increasing 80

60

12

irre_p irre_t

10

irre_total del_t

8

Irre_p = To[m*Δ p 40]

6

Tavg*ρ 20

4 2

0 0

20

At Dc below 0.105 mm or no. of passes above 80 (optimum) irre_p rises again

40

60 NO. OF PASSES

80

100

0 120

IRREVERSIBILITY VARIATION W.R.T. NO. OF PASSES (Dc constant) IRREVERSIBILIY VARIATION W.R.T. NO. OF PASSES

small 120 increase in del_t

Irre_p

18 16

100 14

So 80 irre_t

12 10

60 8 40

6 4

20 2 0

Overall decrease0in total irreversibility but huge pressure drop and limit to fin thickness

20

40

60

NO. OF PASSES

80

0 100

DEL_T (K)

Del_p

IRRE_P, IRRE_T, IRRE_TOTAL (W)

flow

irre_p irre_t irre_total del_t

IRREVERSIBILITY VARIATION W.R.T. LENGTH

Irre_t

slightly

120

30

100

25

80

20

60

15

40

10

20

5

0 Large length is restricted by huge 0 pressure drop

20

40

60 LENGTH (MM)

80

100

0 120

DEL_T (K)

Irre_p and del_p

IRRE_P, IRRE_T, IRRE_TOTAL (W)

As length increases mass IRREVERSIBILITY VARIATION W.R.T. LENGTH flow required reduces

irre_p irre_t irre_total del_t

HEAT EXCHANGER 2 WITH WATER AS THE COOLING MEDIUM

IRREVERSIBILITY VARIATION W.R.T. HEAT TRANSFER RATE

IRRE_P, IRRE_T, IRRE_TOTAL (W)

With increase in flow rate 160 both irre_t and 140 irre_p 120 increases.

IRREVERSIBILITY VARIATION W.R.T. HEAT TRANSFER 20 18 16 14

100

12

80

10 8

60

6 40 4 20

2

0 0

20

40

60

80

100

HEAT TRANSFER (W)

120

140

0 160

IRRE_P IRRE_T irre_total DEL_T

IRREVERSIBILITY VARIATION W.R.T. NO. OF PASSES (Tf constant) Dc reduces IRREVERSIBILITY VARIATION W.R.T. NO. OF PASSES 25

But del_p increases so 100 irre_p increases.

20

80 15 60 10 40 5

20

0

0 0

2

4

6

8

10

NO. OF PASSES

12

14

16

DEL_T (K)

In this case overall effect is the reduction of total irreversibility

IRRE_P, IRRE_T, IRRE_TOTAL (W)

120 So irre_t reduces

irre_p irre_t irre_total del_t

IRREVERSIBILITY VARIATION W.R.T. NO. OF PASSES (Dc constant) Flow Irre_t

IRREVERSIBILITY VARIATION W.R.T. NO. OF PASSES 120

25

100

24 23.5

Irre_p 80

23

60

22.5 22

DEL_T (K)

IRRE_P, IRRE_T, IRRE_TOTAL (W)

24.5

irre_p irre_t irre_total del_t

40

21.5 21

20

20.5 0

20 0

10

20

30

NO. OF PASSES

40

50

60

IRREVERSIBILITY VARIATION W.R.T. LENGTH

IRRE_P, IRRE_T, IRRE_TOTAL (W)

irre_t reduces

and irre_p rises due to rise in pressure drop

IRREVERSIBILITY VARIATION W.R.T. LENGTH 120

25

100

20

80 15 60 10 40 5

20

0 0

100

200

300 LENGTH (MM)

400

500

0 600

DEL_T (K)

With increase in length,

irre_p irre_t irre_total del_t

HEAT EXCHANGER 1 WITH AIR AS COOLING MEDIUM

IRREVERSIBILITY VARIATION W.R.T. HEAT TRANSFER RATE FLOW IRREVERSIBILITY VARIATION W.R.T. HEAT TRANSFER 1200 Irre_t

25

IRRE_P. IRRE_T IRRE_TOTAL (W)

1000

20

Irre_p

15 600 10 400

5

200

0

0 0

2

4

6

8

HEAT TRANSFER (W)

10

12

14

DEL_T (K)

800

irre_p irre_t irre_total del_t

IRREVERSIBILITY VARIATION W.R.T. NO. OF PASSES (Tf constant) Dc decreases IRREVERSIBILITY VARIATION W.R.T. NO. OF PASSES 50

20

Irre_t

IRRE_P, IRRE_T, IRRE_TOTAL (W)

45

18

40

16

35

14

30

12

Irre_p

25

10

20 First del_p and irre_p reduces and then15starts increasing after n10= 80

8 6 4

5

2

0

0 0

20

40

60 NO. OF PASSES

80

100

120

DEL_T (K)

Flow decreases

irre_p irre_t irre_total del_t

IRREVERSIBILITY VARIATION W.R.T. NO. OF PASSES (Dc constant) flow IRREVERSIBILITY VARIATION W.R.T. NO. OF PASSES

Irre_p 200

14

180 160

140 Irre_t

10

slightly

120

8

100 6

80 60

4

40 2 20 0

0 0

20

40

60

NO. OF PASSES

80

100

DEL_T (K)

IRRE_P, IRRE_T, IRRE_TOTAL (W)

12

irre_p irre_t irre_total del_t

IRREVERSIBILITY VARIATION W.R.T. LENGTH With Increase in length, irre_p increases and irre_t IRREVERSIBILITY VARIATION W.R.T. LENGTH decreases 25

50

20

40 15 30 10 20 5

10

0

0 0

10

20

30 LENGTH (MM)

40

50

60

DEL_T (K)

IRRE_P, IRRE_T, IRRE_TOTAL (W)

60

irre_p irre_t irre_total del_t

HEAT EXCHANGER 2 WITH AIR AS COOLING MEDIUM

IRREVERSIBILITY VARIATION W.R.T. HEAT TRANSFER RATE flow IRREVERSIBILITY VARIATION W.R.T. HEAT TRANSFER

Irre_p and irre_t IRRE_T, IRRE_P, IRRE_TOTAL (W)

300

9 8

250

7

200

6

150 100

irre_t

4

irre_total

3 2

50

1

0

0 0

20

40

60

HEAT TRANSFER (W)

80

irre_p

5

100

del_t

IRREVERSIBILITY VARIATION W.R.T. NO. OF PASSES (tf constant) Dc decreases IRREVERSIBILITY VARRIATION W.R.T NO. OF PASSES

IRRE_P, IRRE_T IRRE_TOTAL (W)

500

25

450 400

20

350 Irre_p first decreases and 300 then increases

15

250 200

10

150 100

In this case 21 no. of 50 passes and Dc 0.605 0 comes out to be 19 the optimum value.

5

0 20

21

22 NO. OF PASSES

23

24

25

DEL_T (K)

flow

irre_p irre_t irre_total del_t

HEAT EXCHANGER 1 WITH AIR AS THE COOLING MEDIUM Graph clearly shows that if height of channel is increased than lower length can be employed MINIMUM POSSIBLE LENGTH FOR GIVEN HEIGHT 9 8 LENGTH (MM)

7 6 5

length

4 3 2 1 0 0

0.2

0.4

0.6

0.8

1

1.2

HEIGHT (MM)

Graph a

CONCLUSION •

HIGHER “h” CAN BE OBTAINED BY REDUCING Dh up to certain limit due to rapid increase of pressure drop.

•

Large length reduces irre_t but del_p and irre_p rises. So restricted.

•

With increase in height of the channel, length of the channel can be reduced

•

When channel width is decrease then thermal irreversibility may increase or decrease depending on the extent of change in del_t but pressure irreversibility decreases and then increases. This may give rise to optimum value.6.3.2

•

With an increase in load, both thermal and viscous irreversibility increases.

IRREVERSIBILITY VARIATION W.R.T. NO. OF PASSES (Dc constant)

450

8

400

7

350

6

300

5

250 4 200 3

150

2

100 50

1

0

0 0

5

10 NO. OF PASSES

15

20

DEL_T (K)

IRRE_P, IRRE_T IRRE_TOTAL (W)

IRREVERSIBILITY VARIATION W.R.T. NO. OF PASSES

irre_p irre_t irre_total del_t

IRREVERSIBILITY VARIATION W.R.T. LENGTH 160

8

140

7

120

6

100

5

80

4

60

3

40

2

20

1

0

0 0

200

400

600

LENGTH (MM)

800

1000

DEL_T (K)

IRRE_P, IRRE_T IRRE_TOTAL (W)

IRREVERSIBILITY VARIATION W.R.T. LENGTH

irre_p irre_t irre_total del_t