Hybrid Model for Micro channel Heat Exchangers used in sCO2 Brayton Power Blocks

Abstract

Studies have shown that Micro-channel Heat Exchangers (MCHEs) commonly referred to as Printed Circuit Heat Exchangers (PCHEs) are suitable candidates for recuperators and gas coolers used in sCO2 power blocks. CFD and 1-d unit-cell-based models have been proposed in the literature for the design and analysis of MCHEs. To estimate the heat exchanger size and arrive at an optimum channel configuration, CFD models are found to be computationally expensive and time-consuming, especially when full-scale MCHEs are to be modelled. On the other hand, 1-d models are inadequate for correctly predicting the performance. The thesis aims to address this gap by proposing a hybrid model. The hybrid model incorporates a Thermal Resistance Network (TRN) framework combined with a unit-cell CFD model to investigate the thermo-hydraulic performance of the complete MCHE stack. CFD-based unit-cell models are developed for straight and non-straight flow paths to obtain the variation of local heat transfer coefficient and Fanning factor along the channel length. A stack optimization scheme based on the rate of heat loss from the external surfaces of the MCHE core is proposed and incorporated in the hybrid model to arrive at the optimum stack width, height, and number of rows. Additionally, the hybrid model also includes a model for the inlet and exit manifolds utilizing the concept of flow resistance balance to obtain optimum pressure drop across the entry and exit manifolds. The proposed manifold pressure balance scheme facilitates uniform flow distribution among the channels in the MCHE stack. The efficacy of the hybrid model is presented for MCHE based recuperator and a gas cooler used in a 1-MW scale sCO2 Brayton power block. For the recuperator, optimum stack volume and corresponding pressure drop are presented for a 5and#730;C pinch temperature differential. The improvement over the straight channel is demonstrated by using sinusoidal and zigzag flow path configurations...