Study On Heat Transfer And Flow Characteristics Of TiO₂-H₂O Nanofluids Flowing Through Spiral Tubes

“Energy shortage”and“low efficiency”are two major factors which restrict industrial development at present.Then heat transfer enhancement technology is introduced to solve this problem,and it can be divided into active type and passive type according to whether the external energy is consumed.In the field of passive heat transfer enhancement technology,including two methods:one is to improve the thermal properties of heat transfer working media and another is to improve the heat exchanger surface.On the one hand,the thermal conductivity of traditional heat transfer working medium?water,alcohol?can't meet the heat transfer load;On the other hand,the space utilization rate of traditional heat transfer equipment?straight pipe,square pipe,etc.?is low,and the heat transfer effect of per unit area is limited.In this paper,combined with two passive enhanced heat transfer technologies,TiO₂-H₂O nanofluids are used to replace water and alcohol,and spiral tubes are used as heat transfer tubes instead of straight tubes.The heat transfer and flow characteristics of nanofluids with different mass fractions in spiral tubes considering the effects of different structures and inclination angles under different flow modes are studied by combining theoretical analysis and experimental investigation.The heat transfer and flow characteristics of nanofluids and deionized water in spiral tubes and horizontal straight tubes are compared.The main conclusions are as follows:?1?TiO₂-H₂O nanofluids with different mass fractions?0.1wt%,0.3wt%,0.5wt%?are prepared for the experiment,and the microscopic properties of TiO₂ nanoparticles as well as the physical parameters of TiO₂-H₂O nanofluids are studied respectively.Compared with deionized water,the heat transfer performance of nanofluids in the spiral tube can be improved by 17.0%at the maximum extent,and its resistance coefficient can be increased by 3.5%and 7.6%in the downward and upward flow modes respectively.?2?Effects of different screw pitches on heat transfer and flow characteristics of spiral tubes are studied.The Nusselt number and the resistance coefficient increase with the decreasing screw pitch of spiral tube.Compared with horizontal straight tube,the Nusselt number can be increased by 49.8-62.0%and 47-57.8%at the maximum extent in downward and upward flow modes respectively and its resistance coefficient can be increased by 180.1-264.4%and 179.6-204.1%respectively.?3?Effects of different inclination angles on heat transfer and flow characteristics of spiral tubes are researched.The best heat transfer performance and the maximum resisitance coefficient all appear at?=45°,the worst heat transfer performance occurs in spiral pipe with?=90°and the smallest resisitance coefficient occurs in spiral pipe with?=0°.In downward and upward flow modes,heat transfer performance of spiral tube with?=45°can be improved by 8.8-9.3%and 8.7-9.7%when compared with spiral tube with?=90°respectively;resistance coefficient of spiral tube with?=45°can be increased by 5.0-9.4%and 5.1-9.7%when compared with spiral tube under?=0°respectively.?4?Comprehensive performance index and comprehensive thermal efficiency are applied to evaluate the heat transfer-flow resistance of the experimental system.In the downward flow mode,considering the effects of screw pitches and inclination angles,the comprehensive performance indexes of the experimental system can reach 1.25and 1.24 respectively.In the upward flow mode,comprehensive thermal efficiency shows that data from experimental system almost locate in Region I and Region II,moreover,the experiment system will get the best heat transfer-flow resistance performance at the working condition:s=10 cm+?=45°+Re>8000.