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A finite volume numerical code has been developed to numerically approximate the rate of ice crystal growth in a laminar falling film flowing down a cooled vertical plate. The governing energy equation contains the phase energy as the source term. Enhancement of heat transfer as a result of suspended ice crystals is accounted for in the use of effective values of thermal conductivity, viscosity, thermal diffusivity, and specific heat as function of volumetric concentration of ice crystals in the falling film. Nusselt number, overall heat transfer coefficients between the fluid and cooled plate, and ice crystal growth rate were calculated for different film thicknesses with and without axial diffusion. Nusselt number and ice crystal growth rates were found to be dependent on film thickness. Axial diffusion effects were found to be negligible for larger film thickness (large flowrate). 相似文献
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One of the authors has proposed a novel transport/storage system for the waste cold from the gasification process of liquefied natural gas (LNG), which consists of an evaporator, a cold trap, and a pipeline. In order to estimate the performance of this system, one should know the pressure in the evaporator, in which evaporation–freezing of a PCM occurs, and in the cold trap, as well as the pressure drop of the pipeline due to the flow of low pressure vapor of the PCM. In this paper, the cooling/freezing phenomena of a water droplet due to evaporation in an evacuated chamber was experimentally examined, and the heat transfer dominating the evaporation-freezing phenomena was investigated in order to estimate the pressure in the evaporator. From the results, it was shown that the water droplet in the evacuated cell is effectively cooled by the evaporation of water itself, and is frozen within a few seconds through a remarkable supercooling state, and that the cooling rate of the water droplets were dominated by heat transfer within the droplet under the abrupt evacuation condition. The later result means that, in order to obtain an ice particle by evaporation–freezing, the surroundings of the water droplet should be evacuated at the pressure as low as the saturate pressure of water at the maximum supercooling temperature of the droplet. 相似文献
56.
以Saint-Venant方程组为基础,介绍一种线性代数方程组的追赶法模型在汀江上杭河段洪水演算中应用,获得了满意成果。该算法简单明了,易编通用程序,舍入误差小和数值稳定性好。 相似文献
57.
This paper presents an overview of the issues and new results for in-tube condensation of ammonia in horizontal round tubes. A new empirical correlation is presented based on measured NH3 in-tube condensation heat transfer and pressure drop by Komandiwirya et al. [Komandiwirya, H.B., Hrnjak, P.S., Newell, T.A., 2005. An experimental investigation of pressure drop and heat transfer in an in-tube condensation system of ammonia with and without miscible oil in smooth and enhanced tubes. ACRC CR-54, University of Illinois at Urbana-Champaign] in an 8.1 mm aluminum tube at a saturation temperature of 35 °C, and for a mass flux range of 20–270 kg m−2 s−1. Most correlations overpredict these measured NH3 heat transfer coefficients, up to 300%. The reasons are attributed to difference in thermophysical properties of ammonia compared to other refrigerants used in generation and validation of the correlations. Based on the conventional correlations, thermophysical properties of ammonia, and measured heat transfer coefficients, a new correlation was developed which can predict most of the measured values within ±20%. Measured NH3 pressure drop is shown and discussed. Two separated flow models are shown to predict the pressure drop relatively well at pressure drop higher than 1 kPa m−1, while a homogeneous model yields acceptable values at pressure drop less than 1 kPa m−1. The pressure drop mechanism and prediction accuracy are explained though the use of flow patterns. 相似文献
58.
In this report we present a new instrument (a droplet nucleation analyzer) to be used in the study of the influence of ultrasonic waves on the freezing of pure water. This influence can be of great interest in the cryopreservation of biological material. Two different types of experiments have been carried out. In the first set of experiments, ultrasound waves were used during the cooling process. In the second set, ultrasound was applied prior to the cooling process, trying to quantitatively reproduce some experiments in which the ice nucleation temperature of water was successfully decreased. A theoretical discussion of the results is also presented. 相似文献
59.
Seung Woo Yang Jinhee Jeong Yong Tae Kang 《International Journal of Refrigeration》2008,31(1):130-137
The objectives of this paper are to study the heat transfer characteristics for enhanced surface tubes in the pool boiling and to provide a guideline for the design conditions for the evaporator using HFC134a. The shape of tube surfaces, the wall superheat, and the saturation temperature are considered as the key parameters. Copper tubes (do = 19.05 mm) are treated with different helix angles and the saturation temperatures are controlled from 3 to 16 °C. It is found that the pool boiling heat transfer coefficient decreases with increasing the wall superheat. It is also found that boiling heat transfer coefficients for Turbo-II and Turbo-III are 1.5–3.0 times and 1.2–2.0 times higher than that for Turbo-I without the helix angle, respectively. The higher heat transfer performance from Turbo-II and Turbo-III can be explained by the “bubble detention” phenomenon on the surface without the helix angle for the Turbo-I. The experimental correlations for the pool boiling heat transfer on the present enhanced tubes without (Type I) and with the helix angle (Type II and Type III) are developed with the error bands of ±30%, respectively. 相似文献
60.
The objectives of this paper are to develop experimental correlations of heat transfer for enhanced tubes used in a falling film condenser, and to provide a guideline for optimum design of the falling film condenser with a high condensing temperature of 59.8 °C. Tests are performed for four different enhanced tubes; a low-fin and three Turbo-C tubes. The working fluid is HFC134a, and the system pressure is 16.0 bar. The results show that the heat transfer enhancement of low-fin tube, Turbo-C (1), Turbo-C (2) and Turbo-C (3) ranges 2.8–3.4 times, 3.5–3.8 times, 3.8–4.0 times and 3.6–3.9 times, respectively, compared with the theoretical Nusselt correlation. It was found that the condensation heat transfer coefficient decreased with increasing the falling film Reynolds number and the wall subcooling temperature. It was also found that the enhanced tubes became more effective in the high wall subcooling temperature region than in the low wall subcooling temperature region. This study developed an experimental correlation of the falling film condensation with an error band of ±5%. 相似文献