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.

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.

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.

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.

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.

Plate-fin heat exchanger with reduced axial heat conduction

1985 ◽  
Vol 21 (8) ◽  
pp. 398-401
Author(s):  
V. P. El'chinov ◽  
V. A. Kirpikov ◽  
A. I. Smorodin
Keyword(s):  
Heat Conduction ◽  
Heat Exchanger ◽  
Axial Heat ◽  
Axial Heat Conduction

Axial Heat Conduction in Counter Flow Microchannel Heat Exchangers

2008 ◽  
Author(s):  
B. Mathew ◽  
H. Hegab
Keyword(s):  
Boundary Condition ◽  
Heat Conduction ◽  
Heat Exchanger ◽  
Biot Number ◽  
Mixed Boundary Condition ◽  
Mixed Boundary ◽  
Counter Flow ◽  
Axial Heat ◽  
Biot Numbers ◽  
Axial Heat Conduction

This paper analyzes the effect of axial heat conduction on the thermal performance of a balanced counter flow microchannel heat exchanger. The ends of the wall separating the coolants are subjected to the mixed-boundary condition. Analytical equations were developed for predicting the axial temperature of the fluids and the wall as well as the effectiveness of the fluids. Moreover, equations for determining the heat transferred between the heat exchanger and its surroundings have been provided in this paper. The effectiveness of the fluids depended on the NTU, axial heat conduction parameter, manifold fluid temperatures and Biot numbers (of the manifolds). By varying the Biot number the model presented here can be used for designing a MCHXCF with Dirichlet, Neumann or mixed boundary condition at the ends of the wall separating the coolants. At very low values of Biot number the end walls act as if they are insulated. At these values of Biot numbers the effectiveness of the fluids degraded with increase in axial heat conduction parameter for a particular NTU. At very high values of Biot number the end walls assume a temperature that is close to the temperature in the manifold. At high values of Biot number the effectiveness of the fluids can either improve or degrade depending on the manifold temperatures. Moreover, the model developed in this paper has been verified using existing models that consider either adiabatic or isothermal condition at the end walls.

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