Experimental Study On Heat Transfer Performance Of Top Row Pipes

As one of the commonly used cooling equipment in the refrigeration storage, The top row pipes has many advantages in the cooling process in cooling storage. However, due to the complexity of natural convection process, the whole heat transfer process consists of three heat transfer forms such as heat conduction, convection and radiation. At present, in the calculation of the design and selection of top row pipes, Heat transfer coefficient is generally determined by experience or some relevant standard. However, heat transfer coefficient obtained by these methods can’t maintain the practical design requirement, so it is necessary to conduct a thorough research on it to get a further understanding of its heat transfer characteristics.In this paper, by referring to the standard of the performance test of the cold air machine, the preliminary design of the performance test of the top row pipes evaporator was completed. At first, the heat leak of the structure of the cold storage maintenance was tested, and the real heat transfer coefficient of the building envelope was obtained, then the performance test of the top row pipes under the condition of refrigeration was carried out by using the coefficient value. Then, the cooling experiment of top row pipes was operated under 12 kinds of cooling conditions as follows: the cold storage temperature were set to be –10℃, and-18℃,-25℃and-30℃, and the temperature differences between the temperature of the cold storage and the evaporating temperature of the top row pipes were set to be 10℃、7℃ and 5℃, The actual cooling capacity of the top discharge tube was obtained using method of heat balance on the air side and method of enthalpy difference on the refrigerant side under different conditions. Finally, conclusions were obtained according to data analysis of experiment based on experimental conditions as follows:(1) Under the condition of natural convection, changing regulations of the distribution and volatility of temperature in the cold storage were obtained: The temperature fluctuations in the cold storage was very small within 0.3℃ when the temperature of the cold storage was steady whether in heating or cooling process. In cooling conditions, when electric heating arrangement was not uniform, small temperature difference existed in cold storage and a certain small temperature difference appeared in the same position for the difference of height, and the temperature difference gradually decreased down-top at the same position.(2) A small difference of the cooling capacity of top row pipes existed within 12% between solutions measured by refrigerant enthalpy difference method and air side heat balance method. Value obtained by refrigerant enthalpy difference method was higher than that of air side heat balance method. And the cooling capacity increases with the increasing of temperature difference(3) Changing of heat transfer coefficients under different conditions was compared: The heat transfer coefficient increased with the increasing of temperature difference of heat transfer, but it changed just a little within 0.3W/m2·K; The maximum heat transfer coefficient were obtained at-25℃ with 10℃ heat transfer temperature difference and the minimum were at-30℃ with 5℃ heat transfer temperature difference.(4) Solutions of experiment and theory were well coincide by comparision of the heat transfer coefficient of experimental and theoretical calculation values. The heat transfer coefficient increased with the increasing of the temperature difference of heat transfer. Solution of theory 2 coincided with that of experiment better.