A Study on the Horizontal Wall-Jet Diffusion for Stable Stratification in Horizontal Buffer Tanks

Buffer tanks in data center cooling systems need to maintain thermal stratification to ensure that chilled water remains available during disturbances. We examined two diffuser configurations through transient three-dimensional simulations: Case 1 features a single 360° annular discharge near the upper wall, while Case 2 employs an upper loop manifold with 20 nozzles to distribute the same total flow. We quantified thermal stratification using the mixing index (MI) and the cold-layer volume fraction (𝜙𝑐), and we tracked tank and outlet average temperatures over a duration of 600 seconds. Case 2 consistently maintained a sharper and longer-lasting thermocline compared to Case 1, with a lower minimum MI (~0.31) occurring later in the process. Additionally, 𝜙𝑐 decreased more slowly in Case 2, retaining approximately 10% of the cold volume at around 400 seconds, while Case 1 was nearly depleted. The rise in outlet temperature was correspondingly more gradual in Case 2, indicating a stronger buffering effect. The multi-nozzle manifold in Case 2 disperses jet momentum into a horizontal wall-jet that attaches to the upper surface, effectively suppressing entrainment across the thermocline. In contrast, the concentrated annular jet of Case 1 promotes wall impingement and substantial recirculation, which increases vertical entrainment and accelerates mixing. In conclusion, for applications where maintaining stratification is crucial, we recommend using an upper multi-port (dispersive) diffuser that is aligned for horizontal discharge and designed to distribute jet momentum. This approach helps preserve the low-temperature layer, delays outlet warming, and enhances overall performance.