Fluid-dynamic origins of energy performance degradation in large-scale air-source heat pumps Array: Vortex transport, cold-air recirculation, and layout control

Air-source heat pumps (ASHPs) continuously discharge cold air during winter heating, which may form a cold island around the units, reduce evaporator inlet temperature, and degrade system performance. Existing studies mainly focus on temperature drop and efficiency loss, while the coupling of multiple negatively buoyant jets, vortex transport, and cold-air retention in large ASHPs array remains unclear. To clarify the fluid-dynamic nature of the array cold island effect, this study develops a numerical model based on actual unit dimensions and investigates the effects of unit number, layout, spacing, ambient wind direction, and wind speed on airflow organization and thermal distribution. The Q -criterion is used to identify three-dimensional vortices, and the cold-pool escape coefficient C _escape and virtual unit load ( VUL ) are introduced to quantify cold-air escape difficulty and ineffective thermal load caused by recirculation. Results show that the array cold island effect is not a linear superposition of single-unit exhaust recirculation, but an integrated cold-pool phenomenon governed by multi-jet interaction, array blockage, and coupled internal–external vortices. With increasing unit number, cold air accumulates in the array center, and the average recirculation rate increases from 0.553 to 0.811, showing a clear scale effect. Compact layouts tend to form a central low-temperature core, while elongated layouts reduce recirculation. Larger spacing weakens jet coupling and cold-pool confinement, but with diminishing benefits. Ambient wind does not always suppress the cold island effect; at low wind speeds, it may shift cold air downstream and induce local re-ingestion. These findings support layout optimization, ventilation corridor design, and operational control of large ASHP clusters.