This study investigated the transient natural convection phenomenon in an underground water pit thermal storage with heat losses through the surrounding walls. An experimental test rig was built up, and a numerical model was developed to obtain the characteristics of the thermal stratification in the water pit thermal storage. Fluid properties are assumed to be constant, except for the density changes with temperature which is treated using the Boussinesq approximation. The simulations of temperature profiles are reasonably proved by experiments with the maximum relative errors of ±9.77%. The results show that water temperature decreases close to the walls due to the heat losses, which leads to a downward flow along the inclined sidewalls. A slight upward flow occurs at the center of the water pit thermal storage, which lifts the warmer water to a higher level. As a result, the buoyancy-driven flow gradually builds up the thermal stratification in the water pit thermal storage. The modelling results also show that the values of the average Nusselt numbers on the inner surface of the inclined sidewalls and the bottom of the tank are much higher than that of the top thermal insulation layer. The thermal energy storage efficiency decreases rapidly in the first five minutes from 100% to 83.19% due to the intense heat transfer, and then tends to level off at the end of 40 min. The maximum velocity of the natural convection appears close to the upper part of the inclined sidewalls, and it decreases with the cooling process evolved. The present work has a valuable attempt to fill the gap in the existing studies and is useful for guiding the water pit thermal storage design.

这项研究调查了地下水坑蓄热中的瞬时自然对流现象，热量通过围墙散失。建立了一个实验测试台，并建立了一个数值模型，以获取水坑蓄热器中热分层的特征。除了密度随温度的变化（使用Boussinesq逼近法处理）外，假定流体特性是恒定的。通过实验合理地证明了温度曲线的仿真，最大相对误差为±9.77％。结果表明，由于热损失，靠近壁的水温下降，这导致沿倾斜的侧壁向下流动。在水坑蓄热器的中心发生轻微的向上流动，这将温暖的水提升到更高的水平。结果，浮力驱动的流逐渐在水坑蓄热器中建立了热分层。建模结果还表明，倾斜侧壁的内表面和罐底部的平均Nusselt值比顶部隔热层的平均Nusselt值高得多。由于强烈的热传递，热能存储效率在前五分钟迅速从100％降低到83.19％，然后在40分钟结束时趋于平稳。自然对流的最大速度出现在靠近倾斜侧壁的上部，并且随着冷却过程的发展而降低。