非均匀壁温竖板自然对流传热研究及冰箱冷凝器换热量的计算/Study on Natural Convection from Vertical Plate with
Natural convection heat transfer from a vertical plate with uneven temperature distribution exists widely in industry and daily life appliance. Most of calculations for the heat transfer are under the constant temperature or heat flux conditions, and the Nusselt number correlations come mostly from constant temperature or heat flux conditions. Besides these, it is difficult to find a Nusselt number correlation for heat transfer of such heat exchanger with a vertical plate with uneven temperature distribution as a wrapped-type heat exchanger used in refrigerator. It may produce larger error that the calculation method of heat transfer of natural convection from vertical plate with even temperature is used for the calculation of heat transfer in the wrapped-type heat exchanger. In order to solve the problem, an experimental set-up is established to investigate the heat transfer coefficient of natural convection from vertical plate with uneven temperature distribution. And a mathematical model was developed to calculate the heat transfer.
A vertical steel plate with the dimension 900 mm×500 mm was used in the experiment. The strip heaters were mounted discretely on the plate back to heat the plate which would form the uneven temperature distribution on the plate. The heat transfer and temperature distribution of the plate were measured with different heating space under different temperature and heat flux conditions. In the procession of calculation, the plate was divided into several parts according to the number of heat sources. Then by defining the integral average temperature as the characteristic temperature, the mathematical model was used to calculate the heat transfer of correlative part on the plate. The Nusselt number correlation was modified based on the experimental and calculation data in the range of Ra=5×108～5×109 . The expressions for the constant c used in Nusselt number correlation were proposed as the function of the heating space. By using this new correlation, the comparison of calculation results and the experimental data shows that the deviation is less than 15%, which demonstrates both the mathematical model and new correlations are reliable.
The wrapped-type condenser of a refrigerator can be considered as a typical vertical plate with discrete heat sources and uneven temperature distribution when the pipe temperatures are known. Experiments for the heat transfer of a wrapped-type condenser were done under 8 different heating cases, in order to investigate the heat transfer capacity of the condenser and temperature distributions on the condenser. The mathematical model and new correlation were also used to calculate the heat transfer capacity of the wrapped-type condenser. Compared with the experimental data, the calculation results show that the deviation is less than 9.19% and the standard error is 5.68W, which demonstrates that the mathematical model and new correlation can reliably be used for the heat transfer of the wrapped-type condenser of the refrigerator. By comparing with the calculation results obtained in the case that the temperature of the vertical plate is considered as even, the calculation results based the model are more reasonable and reliable.