A Calculation Method for the Optimized Thermal-Fluid Dynamic Sizing of Heat Exchangers

Abstract

A thermal-fluid dynamic calculation model has been developed for sizing some compact heat exchangers. In particular, the exhaust heat recovery systems are considered, as the internal regeneration considerably improves the turbogas power cycles efficiency. The calculation model is designed for selecting the best heat transfer surfaces and optimizing some objective-functions. For almost 60 plate-fin heat transfer surfaces, analytical correlations are derived in a database-like form, to calculate the Colburn j factor and Fanning f factor as functions of Reynolds number. A model is developed also for innovative PCHEs, using a simplified approach. The sizing procedure, based on the core mass velocity equation, is implemented on an electronic worksheet. If the minimum heat exchanger effectiveness and the maximum pressure drops are input, the model gives the heat exchanger core sizing for the possible combinations of the heat transfer surfaces. For each combination, the calculation method minimizes (or maximizes) the selected objective-function by means of an optimization procedure, performed by a solver through the Newton or the conjugate gradients algorithm. The thermal-fluid dynamic calculation method is applied to a high-temperature recuperator with a high-streams pressure ratio. The results show the difficulty of arranging streams with highly different volumetric flows (i.e., same massive flow rates but very different pressures) on the two sides of the heat exchanger, so that very unbalanced aspect ratios arise for the cores, due to the limits imposed to the maximum allowable pressure drops.