Convective heat exchange within a compact heat exchanger

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Convective heat exchange within a compact heat exchanger EGEE 520 Instructor: Dr. Derek Elsworth

Convective heat exchange within a compact heat exchanger EGEE 520 Instructor: Dr. Derek Elsworth Student: Ana Nedeljkovic-Davidovic 2005

1. Introduction q q q Characterised mainly by a high heat transfer area per

1. Introduction q q q Characterised mainly by a high heat transfer area per unit volume; Optimization between heat exchange and pressure drop; Parallel flow compact heat exchangers d=2[mm]

2. 1 Governing Equations q Analytical expression describing parabolic velocity distribution u=16 Umax(y-y 0)

2. 1 Governing Equations q Analytical expression describing parabolic velocity distribution u=16 Umax(y-y 0) (y 1 -y) (x-x 0) (x 1 -x 0) / [(y 1 -y 0)2(x 1 -x 0)2] q q Energy balance equation Boundary condition Twall=500[K] T inlet=300[K]; Convective flow-outlet;

2. 2 Solution using FEMLAB Temperature distribution q Air: k=0. 0505 (w/m K) c=

2. 2 Solution using FEMLAB Temperature distribution q Air: k=0. 0505 (w/m K) c= 1529 (J/kg K) ρ= 0. 8824 (kg/m 3) Velocity: U max = 2. 2 (m/s) Twall=500[K] Tinlet=300[K] q Aluminum: k=155 (w/m K) c= 895 (J/kg K) ρ= 2730 (kg/m 3)

3. 1 Validation FEMLAB results: ∫T 2 d. A=0. 001528 [Km 2]; ∫Wd. A=3.

3. 1 Validation FEMLAB results: ∫T 2 d. A=0. 001528 [Km 2]; ∫Wd. A=3. 168 e-6 [m/s m 2] Mass and heat flow rate: Average heat transfer coefficient: a=89. 21 [W/m 2 K] Average value of the Nusselt number: Nu= a. D/k=3. 18 Thermally fully developed flow with constant wall temperature Nu=2. 976 ( A. F. Mills, 1999, Heat transfer)

3. 2 Validation q q Re= 68 <2300 Tm=400[K] Thermally developing, hydraulically developed flow

3. 2 Validation q q Re= 68 <2300 Tm=400[K] Thermally developing, hydraulically developed flow for Re <2300 and constant wall temperature (Housen) q

4. Parametric study Table 1: Parametric study with variable velocity Table 2: Parametric study

4. Parametric study Table 1: Parametric study with variable velocity Table 2: Parametric study with variable wall temperature v Tz T in To a hot [m/s] [K] [K] [W/m 2 K] 2. 2 500 300 471. 6 89. 2 2 500 300 475. 3 84. 21 1. 8 1. 6 1. 4 500 300 479. 0 482. 7 484. 8 78. 69 72. 61 64. 93 v Tw Tin [m/s] [K] To a hot [K] [W/m 2 K] 2 400 386. 4 82. 124 2 500 300 475. 3 84. 213 2 600 300 564. 8 85. 274 2 700 300 657. 4 87. 161

5. Section of the heat exchanger

5. Section of the heat exchanger

6. Conclusion q q q Average value of the Nusselt number Nu= a. D/k=3.

6. Conclusion q q q Average value of the Nusselt number Nu= a. D/k=3. 18 Convective heat transfer coefficient increases with an increase in velocity and with an increase in wall temperature To calculate more precise value of a and Nu , local heat transfer coefficient is necessary to be determined.