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Convective heat exchange within a compact heat exchanger

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Title: Convective heat exchange within a compact heat exchanger


1
Convective heat exchange within a compact heat
exchanger
  • EGEE 520
  • Instructor Dr. Derek Elsworth
  • Student Ana Nedeljkovic-Davidovic
  • 2005

2
1. Introduction
  • Characterised mainly by a high heat transfer area
    per unit volume
  • Optimization between heat exchange and pressure
    drop
  • Parallel flow compact heat exchangers
  • d2mm

3
2.1 Governing Equations
  • Analytical expression describing parabolic
    velocity distribution
  • u16Umax(y-y0) (y1-y) (x-x0) (x1-x0) /
    (y1-y0)2(x1-x0)2
  • Energy balance equation
  • Boundary condition
  • Twall500K
  • T inlet300K Convective flow-outlet

4
2.2 Solution using FEMLABTemperature distribution
  • Air
  • k0.0505 (w/m K)
  • c 1529 (J/kg K)
  • ? 0.8824 (kg/m3)
  • Velocity
  • U max 2.2 (m/s)
  • Twall500K
  • Tinlet300K
  • Aluminum
  • k155 (w/m K)
  • c 895 (J/kg K)
  • ? 2730 (kg/m3)

5
3.1 Validation
  • FEMLAB results
  • ?T2dA0.001528 Km2 ?WdA3.168e-6 m/s m2
  • Mass and heat flow rate

Average heat transfer coefficient
? a89.21 W/m2K
Average value of the Nusselt number Nu
aD/k3.18 Thermally fully developed flow with
constant wall temperature
Nu2.976
( A.F. Mills, 1999, Heat transfer)
6
3.2 Validation
  • Re 68 lt2300
  • Tm400K
  • Thermally developing, hydraulically
    developed flow for Re lt2300 and constant wall
    temperature
  • (Housen)

7
4. Parametric study
Table1 Parametric study with variable velocity Table1 Parametric study with variable velocity Table1 Parametric study with variable velocity Table1 Parametric study with variable velocity Table1 Parametric study with variable velocity
v Tz T in T o a hot
m/s K K K W/m2K
2.2 500 300 471.6 89.2
2 500 300 475.3 84.21
1.8 500 300 479.0 78.69
1.6 500 300 482.7 72.61
1.4 500 300 484.8 64.93
Table2 Parametric study with variable wall temperature Table2 Parametric study with variable wall temperature Table2 Parametric study with variable wall temperature Table2 Parametric study with variable wall temperature Table2 Parametric study with variable wall temperature
v Tw Tin T o a hot
m/s K K K W/m2K
2 400 300 386.4 82.124
2 500 300 475.3 84.213
2 600 300 564.8 85.274
2 700 300 657.4 87.161
8
5.Section of the heat exchanger
9
6. Conclusion
  • Average value of the Nusselt number
  • Nu aD/k3.18
  • 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.
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