Heat-transfer Performance Of U-tube Filled Vacuum Tube

The building energy consumption accounts for27%of the total energy consumption and the HVAC energy consumption accounts for65%of the building energy consumption. The use of solar energy and the building integrated with solar energy systems have became a hot topic of research and focus of the industry in energy saving building and new energy field, which can reduce the building consumption or supply the heat source for HVAC system. The performance of solar collector is the key for solar energy utilization, solving the problem of solar cooling technology, spreading solar heating and cooling system and decreasing the energy consumption of fossil fuel.The vacuum tube collector with U-tube, which has simple structure and better transfer properties, is one of the best collecting and heating equipment to supply hot water to solar heating and cooling system. However, in the actual research and analysis of the vacuum tube collector with U-tube, we found that the internal heat transfer model, heat transfer mechanism and the heat transfer efficiency of the vacuum tube collector with U-tube need to study still further. There little research for the thermal performance evaluation and test method of the vacuum tube. Therefore, the main object of this research are the study the internal heat transfer performance of the vacuum tube collector with U-tube and the increase of heat transfer performance. The research of this paper is summarized as follows:Firstly, the U-tube vacuum tube with copper fin was taken as the research object. The steady state model of the vacuum tube has been established and heat transfer performance has been studied by analytic solutions. The results show that the heat efficiency factor will be subject to the influence of air layer between absorber tube and the copper fin. When the synthetical conductance improved from to5W/(m-K) to40W/(m-K), the solar collector efficiency decrease10%, and the outlet fluid temperature decrease16%. And the surface temperature of the absorbing coating increases30℃due to the effect of air thermal resistance. The boost of the absorbing coating temperature can lead to the increase of heat loss coefficient and decrease of collector efficiency.Secondly, based on the quantitative analysis of the U-tube vacuum tube with copper fin, the filled-type vacuum tube with one U-tube and the multi U-tube filled-type vacuum tube have been developed in this paper. The structure and thermal resistance network diagram of the filled-type vacuum tubes with U-tube have been given and the heat transfer of filled-type vacuum tubes with U-tube has been analyzed in uniform boundary condition. The internal fluid temperature model and its dimensionless form of U-tube filled-type vacuum tube have been established. The energy balance equations have been solved. Based on linear superposition principle, the thermal resistance and the tube wall temperature of the vacuum tube have been obtained. And then the temperature of absorption tube and the heat loss coefficient have been studied. So the temperature variation along flowing direction of U-tube filled-type vacuum tube has been acquired.Thirdly, the accuracy of the theoretical model for U-tube filled type vacuum tube had been proved by experimental method. Based on the analysis of the problems of the solar collector thermal performance evaluation method, the evaluation and test of the thermal performance of vacuum tube have been proposed, which can evaluate the thermal performance scientifically. On that basis, the thermal performance testing platform has been established, and the test difficulty, error analysis and accuracy problem have been given.Fourthly, based on the study of the heat transfer model of the U-tube filled-type vacuum tube in uniform boundary condition, the factors influencing the thermal performance of U-tube filled-type vacuum tube, which include the heat transfer coefficient of filled layer, structural style of filled layer and heat loss coefficient and so on, are analyzed. The results show that the thermal performance can enhance by adjusting the thermal resistance of the vacuum tube and optimizing the structural style of the filled layer. Base on the analysis to the effect of structural style of filled layer on the thermal performance of U-tube filled-type vacuum tube, the hollow U-tube filled layer collector is proposed and the performance of the hollow U-tube filled layer collector are studied and discussed.In this paper, taken the U-tube vacuum tube with copper fin as a key point and developed the filled-type vacuum tube with one U-tube and the multi U-tube. The internal heat transfer model and analytic solutions has been established. The evaluation and test of the thermal performance of vacuum tube have been proposed, and the thermal performance testing platform has been established. The thermal performance of the filled-type vacuum tube with one U-tube and the multi U-tube has been studied by theory and test method. The study of the heat transfer mechanism and the increase of collecting efficiency have important research meaning. The development of this vacuum tube collector has significance theoretical and practical significance for the development of solar collector and use of solar energy in solar building integrated field.