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.