Measured and Modeled Charging of a Stratified Chilled Water Thermal Storage Tank with Slotted Pipe Diffusers

Abstract

As part of ASHRAE Research Project 1185, field data from the constant flow rate charging of a stratified chilled water storage tank with double-ring octagonal slotted-pipe diffusers serving a university chilled water system were compared with results of a transient axisymmetric computational fluid dynamics (CFD) model. Charge processes at flows near design flow rate and 50% of design were modeled. Laminar and turbulent simulations were performed for a range of slot dimensions. Performance of the model was assessed by direct comparison of temperature profiles and by comparison of common thermal performance metrics, including thermocline thickness, lost capacity, and equivalent lost height. The inlet Richardson number based on slot hydraulic radius was found to have the strongest effect on model results. The model Richardson number giving best agreement with the lost height of the field data for near design flow rate was roughly an order of magnitude greater than the Richardson number of the actual diffuser. It is conjectured that this is due to inherent differences between the flow produced by multiple three-dimensional jets in the actual diffuser and the continuous slot of the axisymmetric model. The effect of turbulence varied from negligible to moderate as the Richardson number decreased.