Study On Heat Dissipation Characteristics Of All-glass Vacuum Tube Solar Water Heater At Night

All-glass vacuum tube solar water heater has been widely used in China due to its simple structure,excellent performance and high performance-price ratio.The amount of heat loss at night not only affects the stability of supplying heat energy to users,but also affects the ability of the water heater to resist cold and freeze at night.At present,there are many researches on the heat collecting performance of water heaters at home and abroad,and the quantitative analysis of the effects of multiple factors on nighttime heat loss and the characteristics of flow and heat transfer in water heaters at night have not yet been studied.In this thesis,the all-glass vacuum tube solar water heater is taken as the research object.Firstly,the nighttime heat dissipation process is analyzed theoretically,and the key factors affecting the nighttime heat loss of the water heater are found out.In order to reveal the influence of external factors such as initial temperature,ambient temperature and wind speed on the heat loss of the water heater at night,a heat loss test platform for the all-glass vacuum tube solar water heater was built,and the nighttime heat loss test was carried out for 14 consecutive days.According to the test results,the convective heat loss of the hot water tank,the radiant heat loss of the vacuum tube and the convective heat loss accounted for the total heat loss of the water heater were obtained.The multiple linear regression equation of external factors and night heat loss of the water heater was obtained.In order to analyze the influence of the thermal conductivity of the thermal storage material of the water heater,the thickness of the insulation layer and the emissivity of the vacuum tube coating on the heat loss of water heater at night,the CFD software was used to simulate the heat dissipation process of the water heater.While obtaining the influence of the structural parameters of the water heater on the nighttime heat loss,it also reveals the law of flow and heat transfer during the heat dissipation process of the water heater.The main contents and conclusions are as follows:(1)During the nighttime heat dissipation of the water heater,the water storage tank is cooled to the surrounding environment,and the temperature of the fluid close to the wall surface drops first,the density becomes larger,the flow along the tank wall faces downward,the speed is large,and the flow rate of the remaining parts is small.Similarly,the fluid near the wall of the vacuum tube also flows downward,and the fluid having a higher temperature in the tube flows upward under the action of the lifting force,and forces a part of the cold fluid that has not reached the bottom of the vacuum tube to flow back into the tank.At this point,a random vortex appears at the junction of the tank and the vacuum tube.As the heat dissipation progresses,the temperature stratification in the entire water heater becomes more and more obvious,the flow velocity of the fluid becomes smaller and smaller,and the static region in the tube gradually expands.(2)The heat loss of the nighttime water heater mainly includes the convective heat loss of the tank,the convective and radiant heat loss of the vacuum tube.Among them,the radiant heat loss of the vacuum tube is the largest,accounting for 57.8% of the total heat loss of the water heater.Followed by the convective heat loss of the water tank,accounting for about 32.1%.The convective heat loss of the vacuum tube is the smallest,accounting for about 3.7%.The remaining heat loss is the heat loss caused by other factors such as the connection between the water tank and vacuum tube.(3)Under the same condition of the structural parameters of the water heater,the effects of initial temperature,ambient temperature and wind speed on the nighttime temperature drop of tank and vacuum tube and the nighttime heat loss of the water heater were analyzed by multiple linear regression method.The specific results are as follows: For every 1 ? increase of the initial temperature of the water tank,the nighttime temperature drop of tank increases by 0.15 ?,the heat loss of the water heater increases by 0.13 MJ;For every 1 ? increase of the initial temperature of the vacuum tube,the nighttime temperature drop of tube increases by 0.53 ?,the nighttime heat loss of the water heater increases by 0.22 MJ;For every 1 ? increase in the average ambient temperature,the nighttime temperature drop of the tank decreases by 0.08 ?,and the nighttime temperature drop of the vacuum tube decreases by 0.26 ?,the nighttime heat loss of the water heater is reduced by 0.18MJ;When the temperature is increased by 1 m/s,the temperature drop of the water tank increases by 0.11 ?,and the temperature drop of the vacuum tube increases by0.01 ?,the nighttime heat loss of the water heater increases by 0.10 MJ.(4)For the water heater with thermal conductivity of 0.035 W/(m.K),insulation thickness of 50 mm and coating emissivity of 0.06,under the same other conditions,when the thermal conductivity of insulation material was 0.020 W/(m.K),the nighttime heat loss of solar water heater could be reduced by 8.5%;when thethickness of insulation material was 60 mm,the nighttime heat loss of solar water heater could be reduced by 5.0%;when the coating emissivity of vacuum tube was0.05,the nighttime heat loss of solar water heater could be reduced by 4.0%.