Micro heat exchangers

Micro heat exchangers were designed, fabricated, and tested. Two types of heat exchangers were studied: those consisting of uniform channels and those consisting of pin arrays. The operational characteristics of the heat exchangers were evaluated using both analyses and three-dimensional, numerical simulations. In particular, the thermal resistance was computed as a function of various geometric parameters and flow conditions. The geometric dimensions which minimize the thermal resistance under given operational constraints such as pressure drop were identified. In the first part of the thesis, the generalized, conjugate Graetz problem with axial conduction was investigated. The analysis led to a nonlinear eigenvalue problem. Quasi-analytic solutions were generated, and the various observations were made on the mathematical properties of the eigenvalues. The role of axial conduction on the heat transfer process was clarified. The quasi-analytic solutions were used later as benchmarks to verify numerical coded. Then, using the SIMPLER procedure, the conjugate heat transfer problem was solved for uniform channel and pin array heat exchangers. The performance of these two types of heat exchangers was compared. For the special case of microchannels operating with liquid nitrogen, a simple analytic model was developed. The model allows one to easily predict the optimal dimensions of the heat exchanger. Microfabrication techniques were developed and samples of micro heat exchangers were fabricated on silicon wafers. An experimental procedure was developed and preliminary experiments were conducted to test the heat exchanger performance. The results of the measurements were compared with theoretical predictions. Finally, means of controlling the structure of the flow in channels were investigated theoretically. The feedback control system sensed the deviations of the wall shear from their laminar values and directed actuators to impose velocity normal to the solid boundary in such a way as to amplify or suppress naturally occurring disturbances in the fluid. The control system was capable of advancing and delaying the loss of stability of the planar Poiseuille flow