|M.Sc Student||Parahovnik Anatoly|
|Subject||Gas to Gas Microfluidic Heat Exchanger: Axial|
Conduction and Compressibility Effects
|Department||Department of Mechanical Engineering||Supervisor||Professor Gilad Yossifon|
|Full Thesis text|
Micro-heat exchangers (µHE) are of particular interest due to their higher heat exchange and energy effectiveness compared to regular sized heat exchangers and reduced weight and size. A unique experimental setup was designed for studying counter-flow heat-exchangers over large inlet temperature (80-340 K) and elevated pressures (up to 40 MPa) ranges, using nitrogen as the fluid. A counter-flow heat exchanger, constructed of an array of multiple microchannels, was designed, fabricated and tested. The studied HE was fabricated using standard photolithography techniques and consisted of a combination of silicon and glass wafers. Divergence from classical theoretical heat exchanger behavior was observed due to discernible axial conduction effects. Hence, device performance was evaluated using a modified NTU approach that accounts for axial conduction. In light of the findings presented herein, it can be concluded that in order to reduce axial conduction and increase HE efficiency, heat exchangers should be made from materials with a moderate thermal conductivity, e.g., glass, or that display anisotropic properties. Another aspect that was studied in the presented work are different boundary conditions for the analytical analysis of the axial conduction effects. It is commonly assumed that the HE is insulated and there is no interaction with the surrounding. However in the presented study the mass flow rate through the µHE is very small, hence, heat losses from the HE can’t be ignored and are accounted for using temperature boundary conditions. This results in better agreement between the experiments and the theoretical calculations. An additional study involved a chip consisting of a single glass microchannel. Choking behavior due to compressibility effects, which limited the maximal possible flow rate, was observed. Commonly, the choking behavior in micro channels is related to rarefied gas flow. However, in the presented application we used pressurized gas. A comparison between experimental results and classical compressible flow theory was made. In particular, its coupling to the heat transfer problem by varying the inlet temperature was studied for the first time.