Abstract

Molten salt is one of the most promising heat transfer and thermal energy storage medium for the third-generation solar thermal power generation, and the spiral-wound tube heat exchanger can improve the heat transfer performance of molten salt. In this paper, the flow and heat transfer characteristics of molten salt nanofluids in spiral-wound tubes are experimentally and numerically studied. Molten salt nanofluids and synthetic oil as the heat transfer fluids, the heat transfer performance of them in the spiral-wound tube heat exchanger were studied. The Wilson separation method was used to process the experimental data, and the heat transfer correlation of molten salt nanofluids is obtained through experiments. The heat transfer correlation can be in good agreement with the experimental data, the heat transfer correlation on the molten salt nanofluids side is  , Re = 4296–۸۳۴۸, Pr = 13–۲۳, and the maximum error is 17.9%. A three-dimensional model of the spiral-wound tube heat exchanger is used to explore the distribution of the pressure, temperature and velocity fields of the molten salt nanofluids. The friction factor correlation on the molten salt nanofluids side is  , Re = 3536–۱۷۶۸۴, Pr = 13–۲۳ and the maximum error is 12.65%. The heat transfer correlation and resistance correlation of molten salt nanofluids obtained in this paper can contribute to the design of heat exchanger for the third-generation solar thermal power.

Introduction

Solar thermal power generation technology has received extensive attention due to high efficiency and sustainability. Energy storage technology is an effective measure to solve the problem of intermittent solar power generation [[1], [2], [3]]. Molten salt can be used in the field of energy storage because of its large heat capacity, low viscosity, low vapor pressure, scholars around the world have paid extensive attention and carried out related research [[4], [5], [6]].

Liu et al. [7] conducted an experimental study on the turbulent heat transfer of molten salt in a circular tube, and obtained the heat transfer correlation of molten salt in the circular tube. Chen et al. [8] conducted an experimental study on the convective heat transfer of Hitec salt in three kinds of transversely corrugated tubes. He et al. [9] explored the convective heat transfer characteristics of ternary salts in shell and tube heat exchangers through experiments. Yang et al. [10] studied the flow and heat transfer characteristics of molten salt in trough solar collectors by numerical simulation. Chen et al. [11] carried out numerical simulation on the convective heat transfer of molten salt mixed in a unilaterally heated horizontal square tube. The results show that under the condition of non-uniform heating, the buoyancy force causes the core area of molten salt flow to be close to the heating surface to form eddy currents, which can enhance the heat transfer effect. Yang et al. [12] studied the heat transfer and flow performance of molten salt in the annular channel with helical coils by numerical simulation. The results show that adding helical coils can effectively enhance the heat transfer of molten salt in the annular channel, but the flow resistance also increased. Du et al. [13] designed a U-shaped tube for the experimental test of the heat transfer characteristics of molten salt in a shell-and-tube heat exchanger in transition flow, and fitted the heat transfer correlation of molten salt. Qiu et al. [14] proposed a baffle rod shell-and-tube heat exchanger configuration applied in the field of concentrated solar thermal power generation, using ternary salt and heat transfer oil as shell-side working fluids for experimental research. He et al. [15] studied the turbulent heat transfer characteristics of molten salt in shell-and-tube heat exchangers with Reynolds numbers ranging from 10,000 to 91,000 and 11,000 to 27,000 on the tube side and shell side, respectively. The comparison of available correlations for molten salt is in Table 1.

Conclusions

In this paper, the spiral-wound tube heat exchanger is taken as the research object, and the molten salt nanofluids and synthetic oil are used as heat transfer fluid to carry out experiments and numerical simulation studies respectively. The experiment was carried out with the salt-side temperature ranging from 573 K to 613 K, mass flow rate ranging from 0.6 to 1 kg/s. The corresponding Reynold’s number range is Re = 4296–۸۳۴۸٫ The flow heat transfer characteristics inside the heat exchanger, the following results are obtained: