| With its inexhaustible huge advantages,solar energy makes the use of all-glass vacuum tube solar water heaters an effective way for rural areas to realize renewable energy instead of traditional energy heating and reduce environmental pollution.All-glass vacuum tube solar water heaters used in the building heating process due to the presence of forced convection will affect the temperature field and flow field of the water heater,resulting in unstable energy supply and other issues.In order to study the flow field and temperature field in solar water heaters,the first group of all-glass vacuum tube solar water heaters in a 99m~2 single-unit building heating system in Weiling Township,Qilihe District,Lanzhou City,Gansu Province was studied.The temperature distribution inside the storage tank and vacuum tube was measured and analyzed.The heat loss coefficient of the storage tank was also anslysed.The numerical simulation method was used to study the inlet flow of 1kg/s,2.85kg/s,4kg/s and the inlet temperature of 25°C and 35°C under forced circulation for influence on the flow field and temperature of the storage tank of all-glass vacuum tube water heaters.The main findings of this paper are as follows:(1)Use the Reynolds number Re and Rayleigh number Ra to judge the flow state of the water tank part and the vacuum tube part of the all-glass vacuum tube solar water heater.The Reynolds number Re=98622.7>4000,and the flow state in the water tank is turbulent,Ra=5.32×106?T,and the temperature difference within the vacuum tube can not be greater than 50°C,so Ra?10~9,whenever the vacuum tube flow state must be laminar.(2)The experimental results show that the February 6th,2018 is sunny and the ambient wind speed is less than 1m/s,the ambient temperature is between-12.7°C and2.7°C,the installation angle of the all-glass evacuated tube solar water heater is 45°,and the average mass flow rate of the inlet water is 2.85kg/s,the axial temperature in the vacuum tube is lower than the temperature of the water tank.In this case,the temperature of the water at 1.7m from the axial outlet end of the vacuum tube has been changing with time,the temperature is basically lower than the water temperature of0.9m and 0.4m from the axial outlet end of the vacuum tube in the 24 hours.But in the12:00-18:00 time period,due to solar radiation enhanced convection heat transfer in the vacuum tube,the water temperature at 1.7m is very similar to 0.9m and 0.4m,while in other time period,less than temperature at 0.9m and 0.4m.Due to the solar radiation enhanced the convection heat transfer between the vacuum tube and the water in the hot water storage tank,the temperature in the water tank is stratified,and temperature at the 0.1m from the top of the tank is close to 0.18m,and at 9:00-16:00 period,the temperature inside the vacuum tube is similar the temperature at the 0.10m and 0.18m measurement points of the tank,while the temperature at 0.28m is lower.In the absence of solar radiation,due to forced convection within the tank,and convection heat transfer between the tank and the vacuum tube weakened,the temperature distribution within the tank is relatively uniform.(3)On February 6th,2018,the vacuum tube temper ature stratification mainly occurs at 20:00-00:00 and 00:00-9:00,the maximum temperature difference is 3.8°C,and the temperature stratification within the heat storage tank mainly occurs at 9:00-18:00,with the maximum temperature difference.7.9°C,the two parts of the temperature stratification time is basically the opposite.For example,at 17:20-18:00on February 07,2018,the axial temperature distribution of the vacuum tube is relatively uniform during the period of solar radiation and forced circul ation,the maximum difference is 1.0°C.With the increase of the inlet temperature,the water temperature of the water tank gradually increased and there was a clear delamination phenomenon,the maximum temperature difference reached 8.1°C.(4)By calculation,the natural convection mass flow rate in the vacuum tube without solar radiation is 0,the average value of the heat loss coefficient of the solar water heater under the non-mandatory cycle condition is 2.14W/(m~2?°C),and the average value of the heat loss coefficient of the solar water heater under the forced circulation is 29.08W/(m~2?°C),the difference between the two is about 26.94/(m~2?°C),the latter is 12.6 times bigger than the former.(5)By comparing and analyzing the corresponding test data and simulation results under forced circulation without the solar radiation at 17:20-18:00,on February 06,2018,it is proved that the simulation results and methods are correct,and then the overall temperature field and flow field in the all-glass vacuum tube water heater are performed.The analysis shows that under the experimental conditions,when the inlet flow of the water heater is 2.85kg/s,a vortex will form at the shoulder of the solar water heater hot water storage tank and at the junction of the vacuum tube and the storage tank,due to the mainstream inertia at the upper shoulder Diverting occurs,forming a relatively large vortex,and driving the vortex to rotate under the action of viscous forces,and the strength of the vortex at the junction of the tank is weakened in the axial direction before it increases.(6)When the inlet temperature is 25°C and the inlet flow rate is 1kg/s,2.85kg/s,4kg/s respectively,the greater the incoming mass flow,the slowe r the speed drop,and the faster the temperature drop in the storage tank,the more intense the blending.The greater the mass flow,the greater the eddy current at the shoulder of the storage tank and the greater the energy loss.(7)The mass flow rate is 2.85kg/s,the inlet temperature is 25°C and 35°C respectively,the fluid with lower temperature can be mixed with the fluid in the hot water storage tank more quickly,but it also makes the fluid temperature in the tank falling faster,thus increasing the amount of heat released to the environment and reducing heating efficiency.The innovations of this subject are as follows:(1)The temperature field of the all-glass vacuum tube solar water heater with forced circulation strategy was analyzed within 24 h ours a day,and the heat loss coefficient and non-forced convection of the storage tank under forced convection were compared.(2)Through the simulation analysis of the effect of inlet flow and temperature on the temperature field and flow field of all-glass vacuum tube solar water heater under the conditions of no solar radiation and forced circulation when the vacuum tube is inserted into the interior of the water tank,the internal mixing mechanism of the storage tank is revealed. |
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