Heat exchangers have a fundamental role in aviation engineering. Continuous efforts are made to achieve even greater compactness, less weight, and higher performance. Heat exchanger's operation relies on the use of refrigerant fluids with high global warming potential. Although the F-gas II regulation does not yet apply to the aeronautical field, actions are being taken by the entire industry to reduce the environmental footprint of air traffic and investigate the use of low GWP refrigerants. Currently, multiphasic heat exchanger headers have mainly been designed and optimized with an empirical approach and an experimental validation. And the multiphasic heat exchanger core is up-to-now designed with experimental-based correlations, all obtained with the high GWP fluids.
The EU-funded PANTTHER project aims to optimize the performance of multiphase heat exchangers with low GWP fluids using both experimental and numerical strategies. Experimentally, the objectives are to develop innovative diphasic measurement techniques as well as assess the effect of a large series of parameters on the flow maldistribution in the heat exchanger header, which is the cause of up to 30% loss of performances. Numerically, the objectives were to develop an innovative 3D CFD porous media model that takes phase change phenomena and fin structures into account, coupled with a two-phase flow model of the header, that allows to model the complete heat exchanger, but also to investigate how to optimize the performances, considering the flow is diphasic inside the heat exchanger.
Experimentally, two experimental facilities were designed and built. An innovative inline measurement technique for diphasic mass flow rate was developed, as well as assess the effect of many parameters on the flow distribution through a Design of Experiment methodology. A vapor cycle system was designed and built, using R1234ze as fluid (GWP of 7), that allowed first to build and experimental database on heat transfer coefficient and pressure losses for R1234ze in a 10mm horizontal pipe, but also allowed to confirm the conclusions drawn from the adiabatic test setup on the flow distribution, but this time applied on an actual evaporator prototype. Numerically, the phase change in 3D CFD porous media model was developed, using as a database for the source term of phase change, VOF simulations on a simplified finned channel. In terms of optimization, two approaches were developed. First, a topological optimization framework was developed for 3D, thermal, hydraulic, and diphasic optimization, but only in laminar conditions. Second, the same Design of Experiment methodology was applied to numerical simulations, to identify the cases with the worst flow distribution, that would be then the starting point for an optimization based on the presence of a perforated plate.
