08-17-2017, 12:39 AM
MICRO-SCALE REGENERATIVE HEAT EXCHANGER
A micro-scale regenerative heat exchanger has been designed, optimized and fabricated for use in a micro-Stirling device. Novel design and fabrication techniques enabled the minimization of axial heat conduction losses and pressure drop, while maximizing thermal regenerative performance. The fabricated prototype is comprised of ten separate assembled layers of alternating metal-dielectric composite. Each layer is offset to minimize conduction losses and maximize heat transfer by boundary layer disruption. A grating pattern of 100 micron square non-contiguous flow passages were formed with a nominal 20 micron wall thickness, and an overall assembled ten layer thickness of 900 microns. Application of the micro heat exchanger is envisioned in the areas of micro-refrigerators/coolers,micro-power, and devices micro-fluidic devices.
Meso and/or micro-scale devices operating on the Stirling cycle are an attractive concept based on the high efficiencies realized for Stirling machines at traditional scales. A further attraction of micro-scale Stirling devices is the ability of the Stirling cycle to be used for generating power where a temperature difference is maintained; or, in the reverse cycle, producing refrigeration with power input. However, two critical performance issues become highly problematic at the micro-scale for Stirling devices: axial thermal conduction and pressure drop.
Pressure drop in a regenerator is exacerbated by the large surface area to volume ratios inherent in fluid passages at the micro-scale. Viscous forces in such small passages enable the growth of large boundary layers (relative to the passage diameter) and correspondingly large pressure drops and lower heat transfer film coefficients. Micro-scale regenerators must address this pressure drop issue without unduly compromising the heat transfer performance of the device.
Furthermore, for all the regenerators except a reticulated open-cell ceramic concept, the flow passages are continuous and unbroken allowing significant boundary layers to develop. This configuration eliminates the contiguous single-material structural thermal path from the hot end to the cold end.
Submitted by:
SHRUTI BHATNAGAR
MECHANICAL FINAL YEAR