HE Xi, HUANG Wenshen, TANG Yongzhi, LU Lin, HUANG Yichen, DING Zhonggen, FENG Qing

(Department of Energy and Power Engineering, Jingdezhen Ceramic University, Jingdezhen, 333403, Jiangxi, China)

Extended abstract:[Background and purposes] As an emerging heat storage technology in recent years, molten salt phase change heat storage technology has the advantages of high heat storage density, long cycle stability, energy conservation, environmental protection, safety and reliability, etc. It can be used to well realize the recovery, storage and time-space utilization of high temperature flue gas waste heat in the intermittent ceramic kiln, so as to alter the current situation of high energy consumption, high pollution and high carbon emission of intermittent kiln. However, the heat conduction performance of molten salt heat storage materials is still too low, while the heat storage efficiency of molten salt is not fully utilized in the existing industries. It is still necessary to to enhance the heat storage performance of molten salt. The most common method is to add fins, while most of existing researches on strengthen phase change heat storage with fins focus on low temperature application scenarios, such as electric vehicle thermal management, chip temperature control and heat dissipation, etc. Although high temperature molten salt heat storage has been widely used in recent years, it is mainly used for heat energy storage from the new energy surplus power electric or the photo-thermal. No matter the application scenario, heat source form or heat transfer mode, the heat storage of high temperature flue gas is quite different from the existing ones, while the heat transfer characteristics and the influence law of fins structural parameters of the heat storage process are different.[Methods] In this study, a heat transfer enhancement scheme with fins was proposed for molten salt heat transfer pipe to achieve efficient recovery of the waste heat from the high temperature flue gas, while a mathematical physics model was constructed to simulate the high temperature flue gas molten salt heat storage process. More attentions were paid on the influences of the fins number, length and tilt angle on the liquid phase change, flow field distribution and heat storage performance. In this way, the influence laws of the fin structural parameters on the heat transfer characteristics and heat storage performance are reached.[Results] Although adding fins will play a certain inhibitory role in the flow of molten salt near the wall in the direction of buoyancy to some extent in the initial stage, it can accelerate the melting rate of molten salt on the near wall side, prompting the molten salt system to enter a thermal storage mode dominated by convective heat transfer earlier and hence improving the overall thermal storage performance. There exists an optimal set of fin structural parameters that enable to achieve desired heat storage performance, i.e., the fins length L=52.5 mm, fins number n=4 and fins angle θ=45°. In this case, the liquid phase mass fraction f and the molten salt average temperature T both achieve the maximum value during the heat storage process. However, any one of structural parameter of the fins exceeds its optimal value, the flow resistance of the molten salt thermal fluid near the wall along buoyancy direction increases and the flow path increases, whereas the local vortices between the fins are strengthened evidently, thus suppressing the global heat transfer and deteriorating the thermal storage performance.[Conclusions] In this study, the fundamental link between the structural parameters of heat transfer tube among the heat transfer characteristics and the heat storage performance is established for the melting salt heat storage scenario of high temperature flue gas. The results have important guiding value for the efficient recovery, storage, conversion and utilization of the medium-high grade industrial waste heat. On this basis, the subsequent research on structural innovation and multi-parameter collaborative optimization can be further expanded.

Key words: high temperature flue gas; molten salt thermal storage; heat exchange tube ribbing; structural optimization; enhanced heat transfer