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Molten salt is popularly applied in solar thermal power generation as high-temperature heat transfer medium. The thermophysical properties of nitrate molten salt nanofluids can be improved compared with that of pure molten salt. In order to improve the heat transfer performance of molten salt nanofluids, a new heat transfer enhanced tube with converging–diverging bulge is proposed. In this study, the convective heat transfer characteristics of molten salt nanofluids in the enhanced tube with converging–diverging bulge are studied numerically. The effects of the height-width ratio (J) and length–width ratio (K) on heat transfer and flow resistance are investigated. The results show that Nussel number (Nu) increases with the increase of J value, and Nu decreases with the increase of K value. The range of Reynolds number (Re) is 8000–24,000, the range of PEC in enhanced tube is 1.15–1.48. When J = 0.27, K = 2.08, and Re = 8000, the PEC of the enhanced tube is the largest 1.48. The mechanism of heat transfer enhancement is analyzed based on the field synergy principle, and the simulation results are analyzed based on the improved entropy theory. The change trend of heat transfer efficiency and heat transfer entropy production is opposite. With the increase of J, the heat transfer exergy loss number (NHT) decrease, the heat transfer effectiveness (ε), flow resistance entropy number (Np) and pressure drop (Δp) increase. With the increase of K, the heat transfer exergy loss number (NHT) increase, the heat transfer effectiveness (ε), flow resistance entropy number (Np) and pressure drop (Δp) decrease. The new enhanced tube could be used to promote the further development of molten salt heat exchanger. [ABSTRACT FROM AUTHOR]

Molten salt as heat transfer and thermal storage medium is wildly used in concentrating solar power plant (CSP). At present, the traditional shell-and-tube heat exchanger is commonly used in CSP. So far, the research on shutter baffle heat exchanger with twisted tube (SBHX-TT) of molten salt based nanofluids is relatively few. In this work, the flow and heat transfer characteristics of molten salt based nanofluids in SBHX-TT with different geometric parameters are studied. The heat transfer coefficient, pressure drop, comprehensive evaluation index and synergistic field are comparetively analyzed. The results show that when Re is between 3487 and 20,924, the convective heat transfer coefficient increases with the decrease of the inclination angle and the ratio of baffle pitch to grille sheet width (Y), the pressure drop also concurrently increases. When Y = 4, the baffle inclination angle is 30°, the convective heat transfer maximally increases by 104.4% and the pressure drop increases by 174.2% compared with that without baffle. Through response surface analysis, the obtained highest comprehensive evaluation index is 1.51, when the grille sheet inclination angle is 36° and baffle pitch to grille sheet width is 5. [ABSTRACT FROM AUTHOR]

The performance of a heat exchanger is directly related to the energy conversion efficiency of the thermal storage system, and its optimal design is an important method to improve the performance of the heat exchanger. This paper uses the distributed parameter method to analyze the effect of the structural parameters and operating parameters of a heat exchanger on the entransy dissipation rate, the entransy dissipation number, the entransy dissipation heat resistance, entropy production rate, and entropy production number in a molten salt–supercritical CO2 concentric tube heat exchanger. The results show that the entransy dissipation rate and entropy production rate have the same trend, with the structural parameters and operating parameters, as well as the changes in the entransy dissipation number and entransy dissipation thermal resistance, jointly affected by the entransy dissipation rate and the heat exchange. Based on the above indicators, single-objective and multi-objective optimization calculations were carried out. The results show that taking the minimum entropy dissipation number, entransy dissipation heat resistance, and improved entropy production number as the objective functions, and using the heat transfer effectiveness as the evaluation index, the optimization effect is better. The ε value is increased by 41.2%, 39.5%, and 40.3% compared with the reference individual. In the multi-objective optimization, taking the minimum number of entransy dissipation and entropy production as the objective function, and using the efficiency of heat transfer and the pressure drop of the working fluid as the evaluation indicators, the optimization effect is better. Compared with the reference individual, the ε value increased by 23.5%, and Δ P h and Δ P c decreased by 51.9% and 32.5%, respectively. This study provides a reference for the optimization of supercritical CO2 heat exchangers by utilizing parameters such as entransy and entropy, which reflect the irreversible loss of the heat transfer process. [ABSTRACT FROM AUTHOR]

Summary: The single thermal energy storage tank (STEST) has been extensively investigated because of the advantage of cost and efficiency. This research presents heat discharging performance of STEST using coil heat exchanger (CHE) with annular baffle at different working conditions. The heat discharging performance, flow field and temperature distribution of different parameters of the annular baffle are presented and discussed. The results show that the heat discharging time decreases with an increase inlet air velocity of the CHE. The heat discharging efficiency is up to 95.7% in this study, and the heat discharging time can reach to 18.7 hours. In addition, optimal dimensionless diameters of annular baffle are Db/Dt = 0.6, Hs/Ht = 0.075 and Hx/Ht = 0.050 (Hs is the height from the top of baffle to the top of tank. Ht is the height of the tank. Hx is the height from bottom of the baffle to the bottom of the tank). The efficiency of heat discharging process can be enhanced with optimal dimensions of the annular baffle. This study presents a direct practicable guide to design the STEST. [ABSTRACT FROM AUTHOR]

Heat transfer enhancement techniques are popularly used in various engineering applications such as heat exchanger, air conditioning, chemical reactors and refrigeration systems. Twisted tape is one of the most important passive techniques, which has been proved to disturb the flow fluid in tube and then strengthen the heat transfer efficiency. This paper conducted comprehensive and comparative review on experimental and numerical works taken by researchers on self-rotating and stationary twisted tapes. It is found that the heat transfer enhancement and the function of online anti-scaling and descaling in tube can be obtained with the self-rotating twisted tapes. Meanwhile, the tube with self-rotating twisted tapes gives the lower pressure drop. Both heat transfer coefficient and friction factor increase with the decreasing of twisted ratio. The twisted tapes combined with different modified techniques are the new development directions of heat transfer enhancement. The appropriate twisted tape modification is necessary for heat transfer enhancement with pressure loss penalty at a reasonable level. [ABSTRACT FROM AUTHOR]

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 N u = 0.3897 R e 0.4370 Pr 1 / 3 , Re = 4296–8348, Pr = 13–23, 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 f = 4.6729 R e − 0.4855 , Re = 3536–17684, Pr = 13–23 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. [ABSTRACT FROM AUTHOR]