Optimization of counterflow heat exchanger geometry through minimization of entropy generation

A counterflow heat exchanger (CFHX) is an essential element for recuperative cooling cycles. The performance of the CFHX strongly influences the overall performance of the cryocooler. In the design of a heat exchanger, different loss mechanisms like pressure drop and parasitic heat flows are often treated separately. Acceptable values for the pressure drop and total heat leakage are estimated and thus a CFHX geometry is more or less arbitrarily chosen. This article applies another, less familiar design strategy where these losses are all treated as a production of entropy. It is thus possible to compare and sum them. In this way, a CFHX configuration can be found that is optimal for a certain application, producing a minimum of entropy and thus has minimum losses. As an example, the design steps of a CFHX for the micro cooling project at the University of Twente are given. Also a generalization of micro CFHX dimensions for cooling powers between 10 and 120 mW is presented.