Thermal Performance of Counter Flow Microchannel Heat Exchangers Subjected to Axial Heat Conduction and External Heat Transfer

2009 ◽  
Author(s):  
B. Mathew ◽  
H. Hegab
Keyword(s):  
Heat Transfer ◽  
Heat Conduction ◽  
External Heat ◽  
Governing Equations ◽  
Counter Flow ◽  
External Heat Transfer ◽  
Microchannel Heat Exchanger ◽  
Two Fluids ◽  
Axial Heat ◽  
Axial Heat Conduction

The thermal model of a balance counter flow microchannel heat exchanger subjected to external heat transfer and axial heat conduction is modeled in this paper. Three governing equations are developed, one for each of the two fluids and the third for the wall separating the fluids. The ends of the wall separating the fluids are assumed to be insulated. The equations are solved numerically using finite difference method. The model developed in this paper is verified using the conventional effectiveness-NTU equations and existing models that consider each of these effects individually. The combined effect of axial heat conduction and external heating always degraded the hot fluid effectiveness for all values of NTU. Irrespective of NTU the cold fluid effectiveness either increased or decreased depending on whether the degradation in heat gain due to axial heat conduction was compensated by external heat transfer.

Modeling a Non-Adiabatic Counter Flow Microchannel Heat Exchanger With Axial Heat Conduction

2009 ◽  
Author(s):  
A. Kunjumon ◽  
B. Mathew ◽  
T. J. John ◽  
H. Hegab
Keyword(s):  
Heat Conduction ◽  
Heat Exchanger ◽  
Boundary Conditions ◽  
Ambient Temperature ◽  
Inlet Temperature ◽  
Governing Equations ◽  
Counter Flow ◽  
Microchannel Heat Exchanger ◽  
Axial Heat ◽  
Axial Heat Conduction

In this paper the effect of axial heat conduction in a non-adiabatic counter flow microchannel heat exchanger is analyzed. The non-adiabatic nature of the heat exchanger causes fluids to exchange heat with the ambient which is at a constant temperature. There are three governing energy equations, one for each fluid and one for the wall separating the fluids. Two of the boundary conditions are the inlet temperature of the fluids. Insulated boundary conditions are used for the wall separating the fluids. The temperature of the fluids and the wall at several points between the inlets and outlets of the MCHXCF are obtained by solving the governing equations using finite difference method. Second order difference schemes are used for discretizing the governing equations. The effectiveness of the fluids depends on the NTU, axial heat conduction parameter, the ambient temperature and the ratio of the thermal resistance between the fluids to that between the ambient and the individual fluids. There is a decrease in the effectiveness of the fluids due to axial heat conduction alone. In the presence of just external heat transfer, increase in ambient temperature reduces the effectiveness of the hot fluid while increasing that of the cold fluid and the opposite trends occur if the ambient temperature is decreased. The combined effect of these two phenomena on the effectiveness of the fluids will depend on the net heat gained/lost by them.

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.

Axial Heat Conduction in Parallel Flow Microchannel Heat Exchangers

2009 ◽  
Author(s):  
B. Mathew ◽  
H. Hegab
Keyword(s):  
Heat Conduction ◽  
Heat Exchanger ◽  
Parallel Flow ◽  
Operating Conditions ◽  
Inlet Temperature ◽  
Microchannel Heat Exchanger ◽  
Cold Fluid ◽  
Axial Heat ◽  
End Wall ◽  
Axial Heat Conduction

This paper deals with the effect of axial heat conduction on the hot and cold fluid effectiveness of a balanced parallel flow microchannel heat exchanger. The ends of wall separating the fluids are subjected to Dirichlet boundary condition. This leads to heat transfer between the microscale heat exchanger and its surroundings and thereby leading to axial heat conduction through the wall separating the fluids. Three one dimensional energy equations were formulated, one for each of the fluids and one for the wall. These equations were solved using finite difference method. The effectiveness of the fluids depends on the NTU, axial heat conduction parameter, and the temperature of the ends of the wall separating the fluids. With decrease in temperature of the end wall at the inlet section of the fluids, while keeping the temperature of the other end wall constant, the effectiveness of the hot and cold fluid increased and decreased, respectively. When the temperature at the ends of the wall separating the heat exchanger is average of the inlet temperature of the fluids then there is no axial heat conduction through the heat exchanger. The effectiveness of a counter flow microchannel heat exchanger is better than that of a parallel flow microchannel heat exchanger subjected to similar operating conditions, i.e. axial heat conduction parameter and end wall temperatures.

scholarly journals Constant-Wall-Temperature Nusselt Number in Micro and Nano-Channels1

2001 ◽  
Vol 124 (2) ◽  
pp. 356-364 ◽  
Author(s):  
Nicolas G. Hadjiconstantinou ◽  
Olga Simek
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Heat Conduction ◽  
Flow Regime ◽  
Wall Temperature ◽  
Slip Flow ◽  
Constant Wall Temperature ◽  
Slip Flow Regime ◽  
Axial Heat ◽  
Axial Heat Conduction

We investigate the constant-wall-temperature convective heat-transfer characteristics of a model gaseous flow in two-dimensional micro and nano-channels under hydrodynamically and thermally fully developed conditions. Our investigation covers both the slip-flow regime 0⩽Kn⩽0.1, and most of the transition regime 0.1<Kn⩽10, where Kn, the Knudsen number, is defined as the ratio between the molecular mean free path and the channel height. We use slip-flow theory in the presence of axial heat conduction to calculate the Nusselt number in the range 0⩽Kn⩽0.2, and a stochastic molecular simulation technique known as the direct simulation Monte Carlo (DSMC) to calculate the Nusselt number in the range 0.02<Kn<2. Inclusion of the effects of axial heat conduction in the continuum model is necessary since small-scale internal flows are typically characterized by finite Peclet numbers. Our results show that the slip-flow prediction is in good agreement with the DSMC results for Kn⩽0.1, but also remains a good approximation beyond its expected range of applicability. We also show that the Nusselt number decreases monotonically with increasing Knudsen number in the fully accommodating case, both in the slip-flow and transition regimes. In the slip-flow regime, axial heat conduction is found to increase the Nusselt number; this effect is largest at Kn=0 and is of the order of 10 percent. Qualitatively similar results are obtained for slip-flow heat transfer in circular tubes.

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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