Experimental laminar falling film heat transfer coefficients on a large diameter horizontal tube

A test rig was designed and fabricated to measure local heat transfer coefficients from a large-diameter, steam-heated horizontal tube to thin falling films of water. The results of the experiments generally support the theory developed by Rogers for laminar film conditions, which were found to persist up to film Reynolds numbers of about 2000. Agreement between theoretical predictions of heat transfer coefficients and the measured values is reasonable for the thermal boundary layer development region, but measured values are about 20% lower than predicted values in the developed region. The lower values in the developed region are attributed mainly to measured film thicknesses being greater than predicted values.     

Heat transfer in turbulent flow on a horizontal tube falling film evaporator. a theoretical approach

A simplified theoretical approach for the prediction of evaporation (no-nucleate boiling) heat transfer coefficient in a horizontal tube falling film is proposed. The correlation is derived from an analysis of the thermal boundary layer under the assumption of turbulent flow regime and taking into account the thermal developing region. A diagram to evaluate $\text{h}$(μ²/ρ²gk³)^{$\text{1}\text{3}$} as a function of Re and Pr is proposed.A regression analysis of the numerical computations allows us to provide a dimensionless formula $\text{h}$(μ²/ρ²/gk³)^{$\text{1}\text{3}$} = 0.046 Re^0.18Pr^0.47 valid in the ranges Re = 1500–5000 and Pr = 1–5.     

Distribution characteristics of falling film thickness around a horizontal tube

An experimental investigation of a liquid film falling around a horizontal tube was performed using a displacement micrometer to determine the distribution characteristics of the film thickness. Measurements were carried out for circumferential angles ranging from 15° to 165°, outside diameters varying from 20 to 32 mm, intertube spacings ranging from 10 to 40 mm, and film Reynolds numbers varying from 150 to 800. It is found that the distribution characteristics of the film thickness are mainly affected by circumferential angle, intertube spacing and film Reynolds number. However, the outside diameter has little influence on the film thickness. The experimental results also validate the Nusselt's theory giving a relatively reasonable prediction of the film thickness around the upper perimeter of the tube but a poor prediction around the lower perimeter. The minimal values of the film thickness locate at different circumferential angles in the range of 90°–115° rather than at the single point of β = 90° predicted by Nusselt. Moreover, the liquid film gets thinner around the scaled tube, and the thickness distributions for pure water agree well with those for seawater. Based on the experimental data, a new correlation has been suggested to predict the film thickness.     

On the mass transfer into a falling laminar film with dissolution

Exact solution of the convective diffusion equation for the case of dissolution of a solid wall into a falling laminar liquid film is obtained in terms of confluent hypergeometric functions. By the aid of this solution the applicability of the concentration boundary layer theory is checked and the accuracy of some approximate solutions is estimated.Experimental data for the dissolution of gypsum into water under the conditions of a wavy film flow indicate that waves cause a slight effect on the mass transfer rate within a wide range of Reynolds numbers.     

水平蛋形管降膜蒸发器管外液膜铺展数值模拟

水平异形管与圆管相比,具有管外液膜成膜排数多、液膜流动稳定和传热性能好等优点.对柱状流下水平管降膜蒸发器管外液膜流动进行三维数值模拟,分析椭圆率、喷淋高度和横截面尺寸对蛋形管管外液膜铺展的影响.结果表明:蛋形管管外液膜铺展分为喷淋区、平稳区、叠加区与脱落区.液膜厚度随轴向位移Z增大而增大;当Z=0-6 mm时,液膜厚度...     

Heat transfer from a horizontal tube to a fluidized bed in the presence of unheated tubes

An experimental study of heat transfer from a horizontal tube to a fluidized bed was carried out in the presence of nearby unheated tubes. The effect of the number of tubes, their size, configuration, and the type of fluidized materials was studied. Certain configurations caused as much as a 56% reduction in heat transfer over the most favorable case. The top of the tube was covered by stagnant solids, the bottom partially by a gas film, and the sides by a small region of vigorously agitated solids. Most of the heat transfer occurred in the side region. An overall heat transfer coefficient of 35 Btu/hr-ft²-°F. was found at the best conditions.     

木质素悬浮液管式降膜蒸发传热研究

管式降膜蒸发器由于具有传热系数高、传热温差小等优点,已应用于多个工业领域,对管式降膜蒸发器的研究也成为近年来传热技术领域的一项重要课题.文中设计了一套管式降膜蒸发装置,以木质素悬浮液为工质,对影响降膜蒸发传热的因素进行了实验研究,通过冷膜实验测试了所设计的筛板分布器的性能,得出筛板分布器能较好地使物料在换热管内壁上成膜...     

Laminar falling film flow and heat transfer characteristics on horizontal tubes

Knowledge of falling film flow and heat transfer characteristics on horizontal tubes is required in the assessment of certain CANDU reactor accident sequences for those CANDU reactors which use moderator dump as one of the shut-down mechanisms. In these reactors, subsequent cooling of the calandria tubes is provided by falling films produced by sprays. This paper describes an analysis of falling film flow and heat transfer characteristics on horizontal tubes using integral methods. The application of the results to an assessment of the stability of the films on the calandria tubes following a loss-of-coolant accident with impaired emergency coolant injection flow is discussed.     

Experimental study of falling film evaporation heat transfer outside horizontal tubes

The heat transfer process of falling film horizontal evaporation includes evaporation outside tubes and condensation inside tubes, the heat transfer coefficient of the former is about 50% of that of the latter. So the overall heat transfer coefficient is influenced mainly by the falling film evaporation outside tubes. An experimental study of falling film heat transfer outside horizontal tubes was carried out in order to show how the heat transfer coefficient is affected by different parameters such as flow density evaporation temperatures, temperature difference between wall and saturation water, and mass concentration of the seawater. Experiments were conducted using 14 mm outer diameter Al-brass tubes heated by internal electric heaters so that a uniform heat flux was generated on the outside surface of tubes. The results show that when flow density Γ varies between 0.013 kg/ms < Γ< 0.062 kg/ms, the heat stransfer coefficient of falling film evaporation outside horizontal tubes h increases with the increase in liquid feeding, evaporation boiling temperature and heat flux. h also increases with an increase in distributor height, however there is a maximum height in which any height above this. Besides, the amount of non-condensing gas has significant effect on h. The difference of heat transfer coefficient between freshwater and seawater is small. These results contribute to further improving the performance of heat transfer process and developing new evaporator.     

切应力作用下层流降膜传热特性的数学分析

利用合流超几何函数求解了切应力作用下降膜过程的能量方程,其中液膜内速度分布由气液界面切应力决定,并解得努塞尔数的计算表达式,分析了液膜温度和努塞尔数的变化趋势,讨论了同向与反向切应力、贝克来数对层流降膜传热特性的影响。分析表明,同向切应力作用下液膜最大速度恒位于气液界面处,对液膜具有拉薄效应,因此换热效果优于反向切应力作用,且努塞尔数随着切应力的增大而增大,而反向切应力则相反;同时努塞尔数在发展段随贝克来数的增大而增大,在充分发展阶段趋于一致。     

电解质溶液的受热降膜传热与Marangoni效应

采用红外热成像技术研究了电解质Na2SO4-H2O溶液受热降膜传热过程中的Marangoni效应。实验结果表明,降膜表面的流向和径向上存在分别由温度和浓度分布不均引起的热和溶质Marangoni效应。降膜径向热Marangoni效应引起了液膜的收缩,使液膜厚度增加,增大了液膜热阻;流向热Marangoni效应加剧了液膜的湍动,减小了液膜滞流层厚度,降低了液膜热阻;径向溶质Marangoni效应则减弱了热Marangoni效应对液膜的收缩作用,促进了液膜的扩展。分析发现在本实验条件下,表征Marangoni效应大小的Marangoni数Ma不能准确地反映不同雷诺数Re下Marangoni效应对降膜流动的影响,为此引入判断Marangoni效应对降膜流动影响程度的Marangoni效应影响因子ε,ε与实验结果具有较好的一致性。     

Gas-side mass transfer coefficients in a falling film microreactor

Gas–liquid mass transfer in a falling film microreactor (FFMR) with 29 microchannels (0.6 mm width each) was investigated. CO₂ was absorbed from a CO₂/N₂ gaseous mixture into a NaOH aqueous solution and the liquid-side mass transfer coefficient and the gas-side mass transfer coefficient were measured. The influence of gas concentration on the value of gas-side mass transfer coefficient has been discussed.     

Heat transfer from a vibrating vertical array of horizontal cylinders

Results are presented for experiments performed on a vibrating array of five identical heated cylinders. The cylinders are arranged with their axes horizontal and in the same vertical plane. They are held by a U-shaped yoke. The spacing between cylinders was set at two diameters for most of the work. The array yoke is clamped to a resonant beam which is driven by a magnetic vibrator. The range of vibration intensities from zero to 0.61 has been studied. The vibrating array showed an increase in heat transfer rate of up to 54% for the conditions covered in this report. The trend of the data indicates greater increases at higher vibration intensities.     

ZrO_2-水纳米流体管内层流的传热特性

制备了粒径为50 nm的ZrO2-水纳米流体,并通过添加分散剂NH4PAA改善纳米流体的稳定性。测量了4种不同质量分数(0.2%,0.4%,0.8%,1.2%)的ZrO2-水纳米流体在层流状态下的对流换热系数。实验结果表明:在相同雷诺数下,纳米流体的换热系数要比纯水的有所提高,并随着ZrO2纳米颗粒质量分数的增加而增大。当纳米流体的质量分数为0.2%,0.4%,0.8%,1.2%时,其平均换热系数比纯水分别提高了1.9%,2.4%,5.2%和8.8%。实验管道内的不同位置也影响纳米流体换热系数的提高,入口段的换热系数要比充分发展段提高得更明显,其主要原因是纳米颗粒对流体边界层的干扰。     

Heat transfer between very shallow fluidized beds and an immersed horizontal tube

A new correlation is proposed for the heat transfer coefficient between an immersed horizontal tube and very shallow fluidized beds (static bed heights of 10-40 mm). The correlation is based upon experimental data obtained in this work for a horizontal tube with an outside diameter of 13.1 mm, immersed in beds of spherical alumina particles with mean particle sizes of 335-1261 microns. The maximum bed pressure drop was 92.5 mm water. The effects of tube elevation, static bed height and distributor design were investigated. Nine different distributors were used, with maximum pressure drops ranging from 3 to 800 mm water and open areas from 2.2 to 36%. A comparison between the proposed correlation and data reported in the literature showed an agreement of approximately ±10%.