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

2018-05-14 02:59:11

temperature transfer heat plate 壁温



非均匀壁温平板自然对流传热广泛存在于工业生产及日常生活中。以往的研究工作大都是在等壁温或等热流密度条件下进行的,在有关这类问题的计算中也只是提出了等壁温及等热流密度条件下的Nu数实验关联式。对于像冰箱板管式冷凝器这种非均匀壁温竖板的自然对流传热量计算问题,还没有公认的关联式可用,一般的计算方法是将其视为等壁温或者等热流情况。由于非均匀竖板本身温度场分布的不均匀,并且存在空气侧自然对流传热与平板导热的耦合的问题,所以在许多实际场合下,对于这种非均匀壁温竖板的自然对流传热问题,若按等壁温边界条件和计算方法进行计算,往往产生较大的计算误差。为了解决冰箱板管式冷凝器的自然对流传热量计算问题,本文设计并建立了相应的实验装置,进行关于非等温壁面的表面传热系数的研究;建立了数学模型,分区对非等温壁面的自然对流传热量进行了计算研究。
实验中采用一块900mm×500mm的磨光钢板作为实验平板,将加热条等间距布置在钢板背面,作为平板的加热源,这样就在平板上形成了非均匀的温度场。在控制加热热流密度和加热壁温两种条件下,测量了不同加热条间距平板的自然对流传热量和温度分布。计算中将整块平板按热源数分为相同数量的区间,对于各区间分别应用本文建立的直肋模型进行求解,采用各区间的温度积分平均值作为该部分的特征温度。根据实验及计算结果在Ra=5×108~5×109的范围内对Nu数实验关联式进行了修正,拟合出了关联式系数与热源间距的关系公式。文中还计算出了各种边界条件下平板及平板各区间的传热量与辐射传热量,计算结果与实验值比较相对误差均在±15%以内,证明本文数学模型及计算方法有较高的可靠性。
冰箱板管式冷凝器传热问题属于典型的带离散热源非均匀壁温竖板自然对流传热问题。本文针对一冰箱板管式冷凝器进行了传热实验研究,调查了在8种实验条件下的该冷凝器传热量和温度分布;并且根据文中提出的数学模型和自然对流表面传热系数关联式,对该冷凝器的传热量进行了计算。计算结果表明,采用本文提出的表面传热系数关联式和计算方法,冰箱冷凝器换热量计算结果与实验结果最大相对误差为9.19%,均方根误差为5.68W,说明了本文提出的表面传热系数关联式和数学模型有较好的可靠性,可以用于冰箱冷凝器的传热计算。通过与按等壁温的边界条件和计算方法计算的结果进行比较,说明了采用本文提出的冷凝器换热量计算方法,计算误差较小,有着较高的计算精度。

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.