Thermocline dynamics in a thermally stratified water tank under different operation modes

The mantled solar heat storage (MSHS) tank can separate water utilization and heat collection, and its structure is not only conducive to the formation of temperature stratification, but also easy to be combined with the building, which has a wide range of application value. However, when it is in the simultaneous charging/discharging mode (CD-Mode), the mixing in the inner tank is affected by the heat transfer of the mantle exchanger, and the mixing mechanism and the development law need to be further explored. In this paper, the mixing mechanism of negative buoyancy jet at the inlet and the variation trend of thermocline in the tank under different operating modes were explored numerically and experimentally. As well as its influence on the overall thermal performance of the tank, a simple and feasible way to optimize the diffuser is discussed as well. It is found that in the CD-Mode, the mixing inside the water tank becomes more severe in the early stage of water use, and the thermocline thickness shows a trend of thickening first and then thinning over time. However, the mixing state is relatively stable during the whole water use period, and the change rate of thermocline thickness is smaller. But it is more affected by the inlet negative buoyancy jet than that in the discharging mode (D-mode), especially when the Rei number is large. Under the two operation modes, when the dimensionless time t* = 0.4-0.8, it enters the stable water use process, and the thermocline is in the stable development stage. The thickness of thermocline changes less in the CD-mode, which makes it easier for users to obtain longer hot water service time, reflecting the advantages of mantled heat exchange water tank in maintaining thermal stratification. For the working conditions studied, after optimized the diffuser, the hot water output rate can be 109.9% higher than the straight tube with the same tube length, and the hot water service time can be prolonged by 90.2%, which has practical application value.

Automated summary

This study discusses the mixing mechanism and the variation trend of thermocline in the mantled solar heat storage tank in the simultaneous charging/discharging mode. The research reveals that the mixing is more severe in the early stage of water use and the thermocline thickness increases and then decreases over time. However, the mixing state is relatively stable during the whole water use period and the change rate of thermocline thickness is smaller.