Modeling non-adiabatic counter flow microchannel heat exchangers

• Thermal model of a counter flow microchannel heat exchanger with non-adiabatic wall is presented.
• Equations for determining fluid temperatures, heat transfer between entities and LMTD are provided.
• Model depends on NTU, heat capacity ratio, surrounding temperatures and thermal resistances.
• Effectiveness can either improve or degrade depending on surrounding temperature.
• For unbalanced flow operating condition best performance is when hot fluid has lowest heat capacity.

This article presents a thermal model of counter flow microchannel heat exchangers that are subjected to thermal interaction with its surroundings (ambient or substrate or neighboring microdevices) due to its non-adiabatic outer wall. The thermal model consists of two governing equations, one for each of the fluids, describing the variation of temperature in the axial direction. Analytical solutions of the model can be used for determining the temperature at any axial location; it is used primarily for calculating the effectiveness with respect to each fluid. The effectiveness with respect to each fluid is found to depend on NTU, heat capacity ratios, surrounding temperatures and the thermal resistance between each of the fluid and the respective surrounding. When heat transfer is from the surrounding to the fluids the effectiveness based on the hot and cold fluid decreased and increased, respectively. For this scenario an optimal effectiveness exists for hot fluid but none exists for the cold fluid. The opposite is true in all aspects when heat transfer is from the fluids to the surroundings. Among the two operating possibilities associated with unbalanced flow condition of the counter flow microchannel heat exchanger the operating condition in which the cold fluid has the higher heat capacity, among the fluids, is found to be the best in terms of effectiveness of the fluids.