Effect of a Non-Perforated Double-Plate Diffuser’s Geometry on Thermocline Thickness in a Stratified Chilled-Water Storage Tank: A CFD-Based Parametric Optimization Study

Abstract

Stratified chilled-water storage tanks are an important form of cool thermal energy storage for large air-conditioning systems. They are used to reduce peak electricity demand and shift chiller operation from high-tariff daytime hours to nighttime periods, when system loads are lower and electricity is generally cheaper. Their effectiveness depends on how sharply the thermocline is maintained. A thin, stable thermocline increases the usable volume of chilled water and limits mixing between the cold and warm layers. In this context, diffuser geometry is a governing factor because it controls the internal flow structure, the level of mixing, and the persistence of the thermocline. While many previous studies have investigated perforated, radial, or hemispherical diffuser concepts, the geometric effect of a non-perforated double-plate diffuser on thermocline thickness in chilled-water storage tanks remains insufficiently documented. This study therefore investigates the influence of the geometric parameters of a non-perforated double-plate diffuser on thermocline thickness during discharge.

Two-dimensional unsteady CFD simulations were conducted in ANSYS Fluent for a cylindrical tank with a diameter of 0.8 m and a height of 1.2 m. Identical nonperforated double plate diffusers were installed at the top inlet and bottom outlet. Two geometric ratios were investigated: dp/D = 0.50, 0.75, and 0.875, and hp/di ≈ 0.303, 0.455, and 0.758, giving nine cases in total under a constant flow rate of 2.5 L/min. Thermocline thickness was obtained from vertical temperature profiles within the height range 0.40 to 0.77 m measured from the tank top. Reynolds number (Re) and Richardson number (Ri) were also calculated to support interpretation of the flow mechanisms.

The simulations show that diffuser geometry has a marked effect on thermocline thickness. Within the evaluation range, thermocline thickness varied from 0.199 to 0.260 m. The minimum thickness was obtained in Case 5, with dp/D = 0.75 and hp/di = 0.455, while the maximum thickness occurred in Case 9, with dp/D = 0.875 and hp/di ≈ 0.758. Across the nine cases, Re ranged from 12.48 to 21.84 and Ri from 0.0614 to 2.938. The results indicate that thermocline thickness cannot be attributed to Re or Ri alone; instead, it reflects the flow structure imposed by diffuser geometry. Within the investigated parameter range, dp/D = 0.75 and hp/di = 0.455 provides the most favorable outcome and may be used as a practical reference geometry for subsequent diffuser design and further study under different operating conditions.