UCLA

High-temperature supercritical CO2 Brayton cycles are promising candidates for future stationary power generation and hybrid electric propulsion applications. Supercritical CO2 thermal cycles potentially achieve higher energy density and thermal efficiency by operating at elevated temperatures and pressures. Heat exchangers are indispensable components of aerospace systems and improve efficiency of operation by providing necessary heat input, recovery, and dissipation. Tubular heat exchangers with unconventionally small tube sizes (tube diameters less than 5 mm) are promising components for supercritical CO2 cycles and provide excellent structural stability. Accurate and computationally efficient estimation of heat exchanger performance metrics at elevated temperatures and pressures is important for the design and optimization of sCO2 systems and thermal cycles. In this study, new Colburn and friction factor correlations are developed to quantify shell-side heat transfer and friction characteristics of flow within heat exchangers in the shell-and-tube configuration. Using experimental and CFD data sets from existing literature, multivariate regression analysis is conducted to achieve correlations that capture the effects of multiple critical geometric parameters. These correlations offer superior accuracy and versatility as compared to previous studies and predict the thermohydraulic performance of about 90% of the existing experimental and CFD data within �15%. Supplementary thermohydraulic performance data is acquired from CFD simulations with sCO2 as the working fluid to validate the developed correlations and to demonstrate application to sCO2 heat exchangers. A computationally efficient and accurate numerical model is developed to predict the performance of STHXs. The highly accurate correlations are utilized to improve the accuracy of performance pre- dictions, and the concept of volume averaging is used to abstract the geometry for reduced computation time. The numerical model is validated by comparison with CFD simulations and provides high accuracy and significantly lower computation time compared to exist- ing numerical models. A preliminary optimization study is conducted, and the advantage of using supercritical CO2 as a working fluid for energy systems is demonstrated. A microtube heat exchanger is fabricated, and essential design and fabrication guidelines of a compact shell-and-tube heat exchanger with microtubes (with inner diameters of 1.75 mm) are provided. A heat exchanger test rig is used to evaluate the thermohydraulic performance of this heat exchanger with supercritical CO2 and air as working fluids. Thermohydraulic data are reported for more than forty sets of experiments with varying Reynolds numbers for shell and tube flows. Critical performance metrics are calculated from the data and compared with predictions from the numerical model. The average deviations between the experimental and model results fall within 10% for all critical metrics. This excellent agreement validates the numerical model for supercritical CO2 heat exchanger optimization and scale-up. A generalized costing model is developed to estimate the capital costs incurred to manufacture microtube shell-and-tube heat exchangers. This model is utilized in conjunction with an accurate and efficient 2D numerical shell-and-tube heat exchanger performance prediction model to conduct optimization studies with two key objectives - minimization of cost and maximization of heat exchanger power density - on supercritical CO2 microtube heat exchangers utilizing superalloy Haynes 282 as the solid material. A methodology is then demonstrated to optimize these heat exchangers for aerospace applications, and highly compact and cost-effective optimal designs with power density around 20 kW/kg and cost per conductance less than 5 $ � K/W are obtained.