Oscillating-Flow Heat-Transfer and Pressure-Drop Test Rig

Test specimens are plugs of material representative of regenerator matrices of Stirling engines.

The figure illustrates an apparatus for measuring heat-transfer and pressure-drop characteristics of porous plug specimens in oscillating flows. The apparatus is built around an oscillating-flow test rig that was originally designed for pressure-drop (but not heat-transfer) measurements and has since been modified and refined. The flows and specimens are chosen to be representative of those encountered in the regenerators of Stirling engines.

The apparatus includes an assembly of a piston cylinder, cooling section, specimen holder, and heating section aligned sequentially from bottom to top along a vertical flow path. The foregoing assembly is contained in a pressure vessel to enable testing at specified elevated pressures. The oscillating flow is generated by the piston, which fits closely in the cylinder and is driven by a variable-stroke, variable-frequency linear motor.

The bottom end of the heating section opens to the top of the specimen holder, while the top end of the heater opens to a relatively large, fixed, thermally insulated buffer volume. A capillary tube vents the buffer volume to the surrounding space within the pressure vessel.

The heating section comprises a copper cylinder with drilled flow passages and with electrical band heaters clamped to its outer surface. The electrical power to the heaters is regulated by a commercial temperature controller. The cooling section is a shell-and-tube heat exchanger, with circulating water at controlled temperature serving as the coolant.

Five fine-wire thermocouples are installed on each face of a specimen. The specimen with thermocouples attached is sandwiched between flow-diffuser disks. The sandwich is installed in the specimen holder.

Because of the large volume and other aspects of the design, the pressure swing in the piston cylinder is attributable mostly to frictional pressure drop in the specimen and in the heating and cooling sections, rather than to compression effects. Therefore, the mass flow rate is very nearly sinusoidal and spatially uniform in the specimen and in the heating and cooling sections.

In its heat-transfer mode, the apparatus is used to measure the net thermal-energy flux, which is the quantity of "bottom-line" importance in a Stirling engine. During operation, the piston is actuated, while the heating section is maintained at a temperature about 200 °C above that of the cooling section, giving rise to a temperature gradient in the specimen. The resulting axial conduction and imperfect heat transfer give rise to a net thermal flux along the specimen; this flux is ultimately rejected to the cooler, where it is measured. Static-conduction losses through cylinder walls and flanges and piston work due to pressure drop also contribute to cooler heat rejection, but these quantities can be calibrated out, so that it is possible to infer the thermal performance of the specimen in isolation.

Specimens tested thus far have been made of woven metal screens and metal felts, with a wide range of porosities, all representative of Stirling-engine regenerator matrices. Experiments on these specimens have yielded generic correlations for friction factors, Nusselt numbers, axial-conductivity-enhancement ratios, and overall-heat-flux ratios.

This work was done by Diane M. Chapman of Lewis Research Center and D. Gedeon of Gedeon Associates and J. G. Wood of Wood Experimental. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Physical Sciences category, or circle no. 130 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Lewis Research Center, Commercial Technology Office, Attn: Tech Brief Patent Status, Mail Stop 7–3, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16448.