Pseudo-Critical Inlet State Mapping of an sCO₂-Water Printed Circuit Heat Exchanger Pre-Cooler for Brayton Cycles

Abstract

A design-oriented inlet state mapping methodology is presented for a printed circuit heat exchanger (PCHE) operating as a supercritical carbon dioxide (sCO₂) water pre-cooler in a Brayton-cycle application. The objective is to quantify how the thermal and hydraulic response of a fixed straight-channel PCHE core changes as the sCO₂ inlet state moves relative to the pseudo-critical region. A one-dimensional distributed counterflow model is formulated for a representative hot-side and cold-side channel pair. Real-fluid thermophysical properties are evaluated locally from pressure and specific enthalpy, and local heat-transfer and friction models are coupled with a wall conduction resistance represented through a two-dimensional conduction shape factor. The boundary-value problem is solved using a shooting method.

The model is assessed against straight channel sCO₂-water PCHE data from the literature. For four outlet temperature comparison cases, the mean absolute error is approximately 0.6 K for the CO₂ outlet temperature and 1.28 K for the water outlet temperature. An additional comparison with digitized near critical axial temperature profiles is treated as a profile shape consistency check.

The assessed model is applied to a fixed semicircular channel PCHE geometry over maps of hot side inlet pressure and temperature for three cooling water inlet temperatures. The maps show heat duty, effectiveness, straight core pressure drop, mean overall heat-transfer coefficient, minimum temperature approach, and hot outlet temperature. The results demonstrate the competing influence of pseudo-critical heat transfer enhancement, heat sink temperature, pinch margin, and hydraulic penalty, providing a compact preliminary design tool for sCO₂ Brayton cycle pre-cooler assessment.