Composite-Matrix Regenerators for Stirling Engines

One can exploit the properties of composites to reduce thermal and flow losses.

A prototype Stirling-engine regenerator containing a matrix made of carbon-fiber-based composite materials has been developed. The concept underlying this development effort is one of exploiting the properties of composite materials (e.g., the anisotropy of thermal conductivity of carbon fibers and the tailorability of composite materials and structures) to reduce thermal and flow losses below those of previously developed regenerators containing metal matrices.

The regenerator in a Stirling engine is an internal heat exchanger for transferring heat between a working fluid and a flow-channel wall (which is also part of the regenerator). The fluid can be helium or another gas that has suitable thermodynamic properties and that does not react chemically with engine components. A typical regenerator is cylindrical in overall shape and includes one or more axial passage(s) containing a matrix -- an open, thermally conductive structure with many flow paths and large surface area for transfer of heat to and from the working fluid. ("Matrix" as used here is meant to be distinguished from "matrix" as used elsewhere to designate the nonfibrous or nonparticulate component of a composite material. Hereafter in this article, the terms "regenerator matrix" and "matrix material" will be used to avoid ambiguity.) Stated somewhat differently, the matrix provides a thermal connection between the gas and the heat capacity of the wall.

Problems associated with making effective regenerators stem from limitations of materials of which they are made. Regenerator matrices are subjected to hot, oscillating gas flows and high temperature gradients. For high performance, a regenerator should be thermally insulating in the axial direction (along which a substantial thermal gradient can exist) and should exchange heat rapidly with the working fluid. The regenerator should contain minimum dead volume because dead volume reduces the engine compression ratio. The flow of gas through the regenerator matrix introduces drag and viscous losses, which should be minimized in order to maximize performance. Efforts at reducing some loss mechanisms tend to aggravate others.