Effect of Axial Heat Conduction and Internal Heat Generation on the Effectiveness of Counter Flow Microchannel Heat Exchangers

2010 ◽  
Author(s):  
Bobby Mathew ◽  
Hisham Hegab
Keyword(s):  
Heat Conduction ◽  
Heat Generation ◽  
Heat Exchangers ◽  
Internal Heat ◽  
Internal Heat Generation ◽  
Counter Flow ◽  
Axial Heat ◽  
Axial Heat Conduction

The Effectiveness-NTU Relationship of Microchannel Counter Flow Heat Exchangers With Axial Heat Conduction

2007 ◽  
Author(s):  
B. Mathew ◽  
H. Hegab
Keyword(s):  
Heat Transfer ◽  
Heat Conduction ◽  
Heat Exchanger ◽  
Heat Exchangers ◽  
Microchannel Heat Exchanger ◽  
Axial Temperature ◽  
Axial Heat ◽  
Relationship Of ◽  
End Wall ◽  
Axial Heat Conduction

In this paper the effect of axial heat conduction on the thermal performance of a microchannel heat exchanger with non-adiabatic end walls is studied. The two ends of the wall separating the coolant are assumed to be subjected to boundary condition of the first kind. As the end walls are not insulated heat transfer between the microchannel heat exchanger and its surroundings occur. Analytical equations have been formulated for predicting the axial temperature of the coolants and the wall as well as for determining the effectiveness of both fluids. The effectiveness of the fluids has been found to depend on the NTU, axial heat conduction parameter and end wall temperatures. The heat transfer through the end walls have been expressed in nondimensional terms. The nondimensional heat transfer from both ends of the wall also depends on the axial heat conduction parameter and temperature gradient at the end walls. A new parameter, performance factor, has been proposed for comparing the variation in effectiveness due to axial heat conduction coupled with heat transfer with the effectiveness without axial heat conduction. The effectiveness of both the hot and cold fluid for several cases of end wall temperatures and axial heat conduction parameter are analyzed in this paper for better understanding of heat transfer dynamics of microchannel heat exchangers.

Simulation of Nanoscale Multidimensional Transient Heat Conduction Problems Using Ballistic-Diffusive Equations and Phonon Boltzmann Equation

2005 ◽  
Vol 127 (3) ◽  
pp. 298-306 ◽  
Author(s):  
Ronggui Yang ◽  
Gang Chen ◽  
Marine Laroche ◽  
Yuan Taur
Keyword(s):  
Heat Conduction ◽  
Boltzmann Equation ◽  
Boundary Conditions ◽  
Heat Generation ◽  
Internal Heat ◽  
Internal Heat Generation ◽  
Computational Time ◽  
Discrete Ordinates ◽  
Fourier Law ◽  
Phonon Boltzmann Equation

Heat conduction in micro- and nanoscale and in ultrafast processes may deviate from the predictions of the Fourier law, due to boundary and interface scattering, the ballistic nature of the transport, and the finite relaxation time of heat carriers. The transient ballistic-diffusive heat conduction equations (BDE) were developed as an approximation to the phonon Boltzmann equation (BTE) for nanoscale heat conduction problems. In this paper, we further develop BDE for multidimensional heat conduction, including nanoscale heat source term and different boundary conditions, and compare the simulation results with those obtained from the phonon BTE and the Fourier law. The numerical solution strategies for multidimensional nanoscale heat conduction using BDE are presented. Several two-dimensional cases are simulated and compared to the results of the transient phonon BTE and the Fourier heat conduction theory. The transient BTE is solved using the discrete ordinates method with a two Gauss-Legendre quadratures. Special attention has been paid to the boundary conditions. Compared to the cases without internal heat generation, the difference between the BTE and BDE is larger for the case studied with internal heat generation due to the nature of the ballistic-diffusive approximation, but the results from BDE are still significantly better than those from the Fourier law. Thus we conclude that BDE captures the characteristics of the phonon BTE with much shorter computational time.

Performance of Cross Flow Microchannel Heat Exchangers Subjected to Viscous Dissipation

2010 ◽  
Author(s):  
B. Mathew ◽  
T. J. John ◽  
W. Dai ◽  
H. Hegab
Keyword(s):  
Heat Conduction ◽  
Temperature Profile ◽  
Viscous Dissipation ◽  
Heat Exchangers ◽  
Heat Dissipation ◽  
Cross Flow ◽  
Combined Effect ◽  
Cold Fluid ◽  
Axial Heat ◽  
Axial Heat Conduction

This paper analyzes the effect of viscous dissipation on the thermal performance of balanced flow cross flow microchannel heat exchangers. The cross flow microchannel heat exchanger analyzed in this paper is one that is subjected to axial heat conduction. Governing equations are developed for each of the fluids and the wall separating the fluids. The equations are solved simultaneously using the numerical technique of finite difference method to obtain the temperature profile. The effectiveness of each fluid is determined using the temperature profile thus obtained. The effectiveness and the temperature of the fluids are found to depend on NTU, axial heat conduction parameters and the viscous dissipation parameter. In the presence of axial heat conduction the effectiveness of the fluid decreases for a specific NTU. In addition, the effectiveness of the fluids decreases with increase in axial heat conduction parameters at a particular NTU. The effectiveness of the hot fluid in the presence of viscous heat dissipation alone decreased at a particular NTU. On the other hand the effectiveness of the cold fluid for the same amount of viscous heating improved at a specific NTU. The combined effect of axial heat conduction and viscous dissipation on the hot fluid is to decrease its effectiveness. With regard to the cold fluid effectiveness it can either increase or decrease due to the combined effect of axial heat conduction parameter and viscous dissipation.

Effectiveness of Counter Flow Microchannel Heat Exchangers Subjected to External Heat Transfer and Internal Heat Generation

2009 ◽  
Author(s):  
B. Mathew ◽  
T. J. John ◽  
H. Hegab
Keyword(s):  
Heat Transfer ◽  
Heat Exchanger ◽  
Ambient Temperature ◽  
Heat Generation ◽  
Internal Heat ◽  
External Heat ◽  
Internal Heat Generation ◽  
Counter Flow ◽  
External Heat Transfer ◽  
The Individual

The effect of external heat transfer and internal heat generation on the thermal performance of a balanced counter flow microchannel heat exchanger is theoretically analyzed in this paper. External heat transfer occurs due to the thermal interaction between ambient and the fluids. Internal heat generation takes into account the heat generated inside the channels due to the conversion of pumping power into heat. One-dimensional governing equations for both fluids were developed and solved to obtain the axial temperatures. The governing equations were solved using a 2nd order finite difference scheme. The effectiveness of the fluids is dependent on NTU, the ambient temperature, the thermal resistance between the individual fluids and the ambient and the pumping power. With increase in ambient temperature the effectiveness of the hot and cold fluid decreased and improved, respectively. On the other hand, reductions in the ambient temperature always lead to the improvement and degradation of the hot and cold fluid effectiveness, respectively. Depending on the ambient temperature, the thermal resistance between the individual fluids and the ambient increased or decreased the effectiveness of the fluids. Internal heat generation always reduced and improved the hot and cold fluid effectiveness, respectively. The combined effect of external heat transfer and internal heat generation on the effectiveness of the fluids depends on the net amount of heat gained/lost by the individual fluids. The effectiveness of a microchannel counter flow heat exchanger is found to be better than of a parallel flow heat exchanger subjected to the same set of external conditions. The model developed in this paper has been verified using existing models that consider each of these effects individually.

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