Optimization of Airflow Profiles and Thermal Uniformity in a Cold Room at Negative Temperature

This study presents a CFD-based optimization of airflow and thermal uniformity in a negative-temperature cold storage room (6.8 × 2.4 × 3 m), simulated using ANSYS 19.4 with the Realizable k-ε turbulence model. Four pallet loading configurations were evaluated on a computational mesh of 470,780 elements, with convergence residuals ≤ 10<sup>-5</sup>. Boundary conditions included a supply air temperature of 248.15 K (−25°C), an inlet velocity of 4 m/s, and an ambient external temperature of 303.15 K (30°C). Baseline simulations showed pallet surface temperatures ranging from 250.4 K to 256.4 K, resulting in a maximum thermal non-uniformity of ΔT = 6 K. Stagnant zones exhibited velocities below 0.5 m/s, with longitudinal velocity dropping to 0.2 m/s at z = 6 m. In Case 2, a thermal gradient of approximately 6 K was observed between the top and center of the storage zone. Peak temperatures reached 257.3 K in low-velocity regions, where airflow between pallets fell below 0.25 m/s. In Case 3, jet velocities reached up to 10 m/s at the evaporator outlet but decayed to below 1 m/s upon entering the storage zone. Product temperatures subsequently rose to 264 K at z > 4 m. The optimized configuration (Case 4) featured a stepped pallet arrangement with 0.1 m inter-pallet spacing and an increased supply velocity of 12.4 m/s. This reduced the maximum temperature difference to ΔT = 2.4 K (249.3 K – 251.7 K), representing a 60% improvement in thermal homogeneity. Longitudinal velocity at the chamber bottom improved from 0.2 m/s to 1.2 m/s (+500%), and vertical thermal stratification decreased from 5.6 K to 2.0 K (−64%). Critically, iso-clip analysis confirmed that hot zones exceeding 255 K were virtually eliminated in the optimized case.