| Developments in manufacturing have led to progressively smaller and more complex micro-electro-mechanical systems (MEMS), many of which employ heat exchangers to enhance performance. Due to the size constraints on these devices, adiabatic surfaces are difficult to create, and thus, ambient thermal interaction becomes an import factor in heat exchanger performance. Similarly, end-wall boundary conditions also become a concern at this scale. A unique closed form mathematical solution is presented for single-pass, two-fluid, parallel and counter flow microscale heat exchangers. The model includes the effects of axial wall conduction, ambient thermal interaction at the axial exterior surface, and general end-wall boundary conditions.Heat exchanger effectiveness above unity is found to be possible depending on the magnitude of the ambient thermal interaction and the objective of the heat exchanger. For an objective of heating the cold fluid, end-wall boundary conditions that introduce energy into the system such as isoflux (with heat flux into the system), convection (when the ambient temperature is greater than the end-wall temperature), and isothermal (where the temperature gradients introduce energy into the system) enhance performance. The heat capacity rate ratio is found to have potential for enhancing performance and mitigating the negative effects of ambient thermal interaction. When the ambient temperature is lower than the cold fluid inlet temperature and the heat exchanger objective is to heat the cold fluid, reducing the heat capacity rate ratio results in more energy transfer from the hot fluid to the cold fluid, thus enhancing performance. This is an important finding, as the heat capacity rate ratio can be manipulated to provide thermal control of the system. |
