Investigation on transient cooling process in a water heat storage tank with inclined sidewalls

The transient cooling process of an underground water pit thermal storage with inclined sidewalls is investigated in this paper. An experimental device was designed in order to validate the mathematical model proposed. Materials properties have been assumed constant with temperature, except for the water's density that has been treated using the Boussinesq approximation. The simulations of temperature distributions are proved well by comparison with the experimental results. Results show that the water temperature decreasing next to the tank walls by the heat losses from the top and sidewalls of the tank, which creates a downward flow along the tank wall. At the center of the tank, a slight upward flow is generated, which lifts the warmer water at the bulk of the tank to a higher level. In this way, the buoyancy-driven flow gradually builds up the thermal stratification in the tank. The Nusselt number values show that comparing with the upper surface of the storage tank, there is more radical heat exchange at the bottom and inclined sidewalls. The maximum velocity appears near the top part of the inclined sidewalls, and its value decreases as the cooling continues.