Time-Dependent Experimental and Trefftz–HPM Analysis of Flow Boiling in Parallel Minichannels

This study investigates flow resistance and heat transfer during flow boiling of three low-boiling refrigerants (HFE-649, HFE-7100, and HFE-7200) in a system of five parallel horizontal minichannels (length 43 mm, depth 1 mm, width 6 mm). The research combines time-resolved experimental measurements with numerical reconstruction of transient heat transfer processes based on Trefftz-type methods. The working fluid flowed over a 0.1-mm-thick Haynes-230 alloy foil, which formed the heated channel wall and served as the electrical heat source. The experimental procedure provided both input and validation data for the numerical analysis. Measurements acquired at 1 Hz included inlet and outlet pressures and temperatures, infrared thermography of the external wall-temperature field, electrical parameters of the heated foil, and mass flow rate. Experiments at mass flow rates of 20, 40, and 60 kg/h (measured for the entire module) were analysed for the central minichannel, assuming equal flow distribution among the five parallel channels. The measured thermal and flow data were used to reconstruct transient wall and fluid temperature fields by solving an inverse heat transfer problem governed by the transient energy equation with prescribed boundary conditions. The numerical analysis employed two Trefftz-based approaches: the classical Trefftz method and a hybrid method combining the homotopy perturbation method with time-dependent Trefftz functions. Reconstructed foil temperatures agreed well with infrared measurements, with a mean relative difference below 0.5%, confirming the reliability of the numerical procedure. Pressure-signal stability was evaluated using statistical time-series metrics. Frictional pressure losses were predicted using a separated-flow approach, and a new correlation for the Fanning friction factor was proposed and evaluated against experimental data, with more than 75% of predictions falling within ±35% of the measurements. The study provides time-dependent distributions of wall temperature, pressure drop, and Nusselt number for flow boiling in parallel minichannel systems.