Thermal Analysis of the Contra-Flow Regenerative Heat Exchanger

Abstract

The regenerative heat exchanger was first proposed and constructed by Dr. Robert Stirling in 1816, in connexion with a regenerative hot-air engine. Since then its use has been mainly confined to furnace air preheaters. With the advent of the gas turbine, however, the need has arisen for a compact heat exchanger of high thermal ratio and low pressure loss. This, of “necessity, tends to the use of gas passages of small hydraulic diameter. In the recuperator, a limit to the size of the gas passages is imposed by the difficulties of producing and supporting in position long thin passages which must perform the dual function of transferring heat from one gas to another and of keeping separate the two gases at different pressures. Small gas passages in a recuperator also tend to become blocked with the deposits from the hot gases of a gas-turbine unit. The renewed interest recently taken in the regenerative heat exchanger is due to the promise which it shows of meeting these objections. The requirement that the gas passages must keep the gases separate is removed, so that these passages may, for instance, be merely the voids in any porous material or in any aggregate of elements. The manufacture and task of supporting in position such a “matrix” is comparatively simple. Information to date suggests that the tendency to blockage of the gas passages in a gas-turbine regenerator should be very much reduced because of the blasts of clean air which alternate with every gas discharge. So far as the author is aware, however, no regenerative heat exchanger has yet been set to work in conjunction with a gas-turbine installation. Little appears to have been written in the English language on the subject of regenerator design, but the converse is true of the German language. This paper has been written after a fairly extensive research into German publications. The paper makes no attempt to deal with pressure losses or the application of the regenerator to the gas turbine. The treatment of these aspects of the problem follows fairly conventional lines, once the thermal behaviour of the regenerator has been investigated.