Thermal-hydraulic performance assessment of a micro shell-and-tube heat exchanger operating under part-load conditions in the NET Power cycle recuperator

Abstract

This paper numerically investigates the thermal-hydraulic performance of a micro shell-and-tube heat exchanger (MSTHE) for application in the thermal recuperator of the innovative oxy-combustion-based NET Power cycle, operating under cycle-relevant part-load conditions. The aim is to support the technological transition from the established printed circuit heat exchangers (PCHE) to MSTHE, which offer a lower inertia, cost-effective, and maintenance-friendly high-performance alternative. To this end, a thermal-hydraulic computational model of the MSTHE was developed, capable of capturing the rapid variation of the supercritical CO2 (scCO2) properties and the partial filmwise condensation of the turbine exhaust gases. Results show that the MSTHE must contain at least 60,000 tubes so that the pressure drop on the tube-side is lower than 1 bar at nominal conditions. The MSTHE effectiveness decreases from 89.2% to 65.1% as the cycle load is reduced from 100% to 20%. The overall heat transfer coefficient decreases gradually between 100% and 40% cycle load, drops sharply between 40% and 30%, and then stabilizes between 30% and 20% cycle load. This stabilization is attributed to the abrupt local increase of the heat capacity on the scCO2-side during the pseudo-critical phase transition, which also enhances local condensation heat release and thickens the condensate film on the shell-side. However, it was found that this phenomenon induces strong axial temperature gradients that may induce thermal stresses, representing a trade-off to the proposed compact design. While the floating microtube bundle of MSTHEs can accommodate these thermal stresses, the rigid compact block structure of PCHE is more prone to damage, revealing an addi- tional key advantage of MSTHEs.