Multi-stage phase-resolved conjugate heat-transfer analysis of an isobutane (R600a) refrigeration evaporator with material sensitivity assessment

This work presents a multi-stage Computational Fluid Dynamics methodology to analyse the thermal behaviour of a didactic isobutane (R600a) evaporator and support the optimisation of natural-refrigerant cooling systems. Improving evaporator performance with environmentally friendly refrigerants requires quantifying the relative influence of solid material properties and two-phase heat-transfer mechanisms, which remains insufficiently addressed in the literature. The framework combines three sequential simulations to resolve airflow distribution, conjugate heat transfer in the tube–fin structure, and refrigerant phase-change dynamics inside the coil. Simulations were performed under defined steady-state operating conditions representative of typical refrigeration systems. Temperature-dependent thermophysical properties for R600a were implemented to ensure physical consistency, and the mean tube–fin heat flux obtained from the conjugate simulation was imposed as a boundary condition for the two-phase model to enhance numerical stability within OpenFOAM. Validation against 24 experimental outlet temperature measurements showed good agreement: 84% of predictions were within ±10% of the mean, with a mean absolute deviation of 0.82 °C and a maximum discrepancy of 1.4 °C. For conventional engineering materials, the mean wall heat flux ranged from 4305 to 4817 W/m 2 (10.6% variation), resulting in only a 3.07 K (approximately 1.1%) difference in outlet air temperature. Under constant compressor power, this corresponds to a similar potential variation in cooling capacity and coefficient of performance. The novelty lies in the structured multi-stage framework and the adoption of a global heat-flux metric enabling consistent material comparison in strongly non-uniform multiphase conditions. Results indicate that evaporator performance is primarily constrained by air-side and two-phase resistances rather than solid conductivity alone, providing quantitative guidance for performance-oriented optimisation and energy-efficient refrigeration system design. • Three-stage CFD couples CHT and two-phase R600a boiling. • Global wall-heat-flux metric enables material comparison in CHT flow. • 84% of predictions within ± 10%; mean error 0.82 °C. • Solid conductivity alters COP by 10.6% under fixed power. • Nb₃Sn reduces wall heat flux by 82% vs Cu–Al.