Materials and dimensions are chosen to optimize performance at microscale.
The device illustrated in Figure 1 is designed primarily for use as a regenerative heat exchanger in a miniature Stirling engine or Stirling-cycle heat pump. A regenerative heat exchanger (sometimes called, simply, a “regenerator” in the Stirling-engine art) is basically a thermal capacitor: Its role in the Stirling cycle is to alternately accept heat from, then deliver heat to, an oscillating flow of a working fluid between compression and expansion volumes, without introducing an excessive pressure drop. These volumes are at different temperatures, and conduction of heat between these volumes is undesirable because it reduces the energy-conversion efficiency of the Stirling cycle. Hence, among the desired characteristics of a regenerative heat exchanger are low pressure drop and low thermal conductivity along the flow axis.
As shown in the enlarged views in Figure 1, the device features a multilayer grating structure in which each layer is offset from the adjacent layer by half a cell opening along both axes perpendicular to the flow axis. In addition, each grating layer is a composite of a high thermal conductivity (in this case, nickel) sublayer (see Figure 2) and a low-thermal- conductivity (in this case, photoresist) sublayer. As a hot fluid flows through from, say, top to bottom, heat from the fluid is transferred to, and stored in, the cell walls of this device. Next, when cold fluid flows from bottom to top, heat is transferred from the cell walls to the fluid.
Axial thermal conduction in such a device can be minimized by constructing it of many layers containing low-thermal conductivity sublayers. The offset of adjacent layers creates reduced-size, partially isolated cross sections for thermal conduction, thereby further reducing the overall axial thermal conductance of the device.
Once the device has been installed in its intended operational setting (e.g., as a regenerative heat exchanger in a Stirling machine), some delamination of the layers is permissible, provided that the half-cell offset between adjacent layers is maintained and no debris is produced. Indeed, delamination enhances performance by reducing axial thermal conduction.
The reasonably high porosity of the device helps to keep the pressure drop low. The layer-to-layer offset disrupts the formation of boundary layers in the flow, thereby contributing to the maintenance of low pressure drop. Disruption of boundary layers also increases the coefficient of transfer of heat between the fluid and the cell walls.
This work was done by Matthew E. Moran of Glenn Research Center, and Stephan Stelter and Manfred Stelter of Polar Thermal Technologies. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4-8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17526-1.