Heat transfer characteristics and particle behavior around circular tubes immersed in fluidized beds

In this study, the particle velocity, particle volume fraction (attenuation of transmission light), and heat transfer coefficient were simultaneously measured for a horizontal tube bundle immersed in a fluidized bed of 0.42 mm diameter glass beads. To improve the accuracy of the particle velocity measurements, an optical fiber probe composed of a combination of transmission and reflection types was designed. The particle velocities were calculated by using the cross-correlation method and the particle passing-time method, respectively. It was shown by comparison of particle velocities obtained by the two methods that the particle passing-time method gives more stable results. The particle velocities at the bottom and at the side of the tube were relatively large and appeared to have periodicity while those near the top of the tube were low. This particle contacting feature can be used to explain the heat transfer characteristics. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(5): 306–318, 1997     

A study on heat transfer for immersed tube in internally circulating fluidized bed

INTRODUCTIONThemethodhowtodealwiththedomesticandindustrialwasteswithoutfurthercontaminationisoneofthemostimportantenvironmentalissues.Fluidizedbedcombustor(FBC)hasadvantagesofhighcombustionefficiency)lowpollution,convenienceinpreprocessingbeforefedin...     

Analysis of particle motion and heat transfer in circulating fluidized beds

In this study, the forces affecting the motion of particle clusters near the wall of a CFB were theoretically analysed. The motion trajectory and the contact time of clusters were determined from the proposed model for two cases, steel ball having density of 6980 kg m^?3 and sand having density of 2500 kg m^?3. Computational results showed that the construction and operational parameters such as the bed equivalent diameter, the gas velocity and the bed temperature have great influence on the contact time of clusters. Based on analysis of the contact time of clusters, a theoretical model was developed for predicting the particle–gas convection heat transfer coefficient. The results were compared with experiments and were a quite agreement with the measured data in the open literature which suggests that the theoretical analysis conducted in this work can very well describe the convection heat transfer in a CFB. Copyright © 2004 John Wiley & Sons, Ltd.     

管外冷凝强化换热管的结构及发展趋势

介绍了各种类型的管外冷凝强化换热管,分析了其强化机理及结构特点,并总结得出:管外冷凝强化管的换热系数与管型有关,且各管型的结构参数对强化传热具有重要的作用.对国内外管外冷凝强化技术研究工作进行分析,结果表明,目前管外冷凝强化换热管的研究主要集中于翅片形状、翅片密度、翅片高度等结构参数对换热性能的影响.强化换热管的冷凝传热性能不仅与翅片结构参数有关,而且也与管材的表面特性和导热系数有关.管外冷凝强化换热管的研究重点是开发新型三维结构翅片的双侧强化管并研究其传热关联式,以及研究不锈钢等低成本材料制造的强化管换热管的传热性能和强化结构的优化.     

Turbulent heat transfer in a horizontal helically coiled tube

An experiment has been conducted in detail to study the turbulent heat transfer in horizontal helically coiled tubes over a wide range of experimental parameters. We found that the enhancement of heat transfer in the coils results from the effects of turbulent and secondary flows. With Reynolds number increasing to a high level, the contribution of the secondary flow becomes less to enhance heat transfer, and the average heat transfer coefficient of the coil is closer to that in straight tubes under the same conditions. The local heat transfer coefficients are not evenly distributed along both the tube axis and the periphery on the cross section. The local heat transfer coefficients on the outside are three or four times those on the inside, which is half of the average heat transfer. A correlation is proposed to describe the distribution of the heat transfer coefficients at a cross section. The average cross-section heat transfer coefficients are distributed along the tube axis. The average value at the outlet section should not be taken as the average heat transfer coefficient. © 1999 Scripta Technica, Heat Trans Asian Res, 28(5): 395–403, 1999     

Heat transfer on tube bundles embedded vertically in liquid‐fluidized beds

Bed voidage was measured in liquid‐fluidized beds having tube bundles embedded vertically in beds, and the heat transfer coefficient was measured on the outer surface of the tube. There were six kinds of test channels used, and a total of nine types of particles of glass and ceramics were tested. The measured bed voidage agreed well with those developed for in‐column fluidization, when the hydraulic equivalent diameter was used. Measured heat transfer coefficients on the vertically embedded tube bundles were higher than those on the vertically embedded single tubes, the calculated values for the in‐column fluidization, and the calculated values for the horizontally embedded tube bundles. Correlations for predicting the heat transfer coefficient were derived for the vertically embedded tube bundles and single tubes. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20267     

Study of local condensing heat transfer coefficients for stratified flow inside horizontal three‐dimensional inner microfin tubes

This paper presents a new type of three‐dimensional inner microfin tube. The flow patterns, the flow pattern criteria, and the heat transfer enhancement performances for horizontal condensation inside these tubes were obtained by experiment. The correlation of the local condensing heat transfer coefficient for stratified flow inside a two‐dimensional inner microfin tube was obtained on the basis of analysis and experimental data. The calculated results with the correlation are consistent with the experimental data within ±30%. © 2000 Scripta Technica, Heat Trans Asian Res, 29(8): 623–633, 2000     

Numerical investigation for improved heat transfer characteristics in micro‐fin tubes

Generally, internal micro‐fin tubes are used for increasing the life and performance of electronic devices. The micro‐fins enhance the heat transfer rate by increasing the surface area with an increase of the pressure drop. In this study, heat transfer and pressure drop are analyzed by varying Reynolds number with the increase in the number of fins in tubes. Heat transfer and pressure drop, together with turbulence kinetic energy of micro‐fin tubes (helical and straight) and a smooth tube, have been evaluated for different Reynolds numbers (60 000, 40 000, 20 000, and 2000) at a constant temperature of 350 K, which clearly establishes laminar to turbulent flow. It is observed that the helical micro‐fin tube has a better result compared with the straight micro‐fin tube and smooth tube at Reynolds numbers 60 000, 40 000, and 20 000 at velocity 2, 1, and 0.5 m/s, respectively. This study is an attempt to establish a comparison of different micro‐fin geometries with varying Reynolds numbers, concluding that a high Reynolds number is suitable for the same.     

Heat transfer in liquid‐fluidized beds affected by column wall

Heat transfer from a column wall to liquid‐fluidized beds was investigated experimentally. The diameter ratio of particle to column was changed from 0.089 up to 0.332, using 11 sizes of particles and two sizes of columns. It was found from the experiment that the critical diameter ratio of particle to column is 0.2; below this value a good fluidizing condition is maintained for any flow rate and the heat transfer coefficient changes smoothly from the minimum fluidization to the liquid single‐phase flow. Based on the experimental data, a correlation was derived to predict heat transfer coefficients for the case affected by a column wall. © 2000 Scripta Technica, Heat Trans Asian Res, 29(7): 598–608, 2000     

Reverse heat transfer and re-condensation phenomena in non-adiabatic capillary tubes

A numerical simulation model for lateral capillary tube-suction line heat exchangers is presented here to analyze its performance characteristics in small vapour compression refrigeration systems (e.g. domestic refrigerators). Appropriate heat transfer correlations have been used to illustrate the reverse heat transfer and re-condensation phenomena inside the tubes. Some convergence problems were encountered during the execution of the model when lower vapour temperature inside the suction line caused the two-phase refrigerant inside the capillary tube to re-condense within the heat exchange region. Therefore, a relationship between the re-condensation phenomenon and the divergence problem has been analyzed in the paper. The modelling was performed with two refrigerants, namely HFC-134a and HC-600a. Further, a simple theoretical equation has been developed to express the re-condensation phenomenon in non-adiabatic capillary tubes.     

Analogy between pressure drop and heat transfer in liquid–solid circulating fluidized beds

An analogy was found between the frictional pressure drop and the heat transfer in liquid–solid circulating fluidized beds. This investigation is based on the predicting correlations for the particle holdup, the heat transfer coefficient, and the pressure drop, which were all developed by the authors. When the heat transfer coefficients were expressed in terms of the modified j‐factor, then a close mutual relationship was observed between the modified j‐factor and the friction factor of the pressure drop due to liquid and particle flow. A correlation to express this mutual relationship was derived, which consists of the density ratio of particle to liquid and the non‐dimensional riser diameter. The heat transfer coefficient predicted from the derived correlation agreed well with the experimental data by the authors, and with existing data. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20246     

Simple power-type heat transfer correlations for turbulent pipe flow in tubes

Heat transfer in the turbulent flow of fluid in a pipe is analyzed. Nusselt number as a function of the Reynolds and Prandtl number is given. Power-type correlations were proposed within a wide range of Reynolds and Prandtl number. Relationships for the Nusselt number compare well with experimental data. The paper presents three power-type correlations of a simple form, which are valid for Reynolds numbers range from 3·103 ≤ Re ≤ 106, and for three different ranges of Prandtl number: 0.1 ≤ Pr ≤ 1.0, 1.0 Pr ≤ 3.0, and 3.0 Pr ≤ 103. Heat transfer correlations developed in the paper were compared with experimental results available in the literature. The comparisons performed in the paper confirm the good accuracy of the proposed correlations. They are also much simpler compared with the relationship of Gnielinski, which is also widely used in the heat transfer calculations.     

Oscillation-controlled heat-transport tube (heat transfer coefficient in tubes in heating and cooling regions)

In the present study, heat transfer coefficients of oscillatory flow at inner surfaces of the heating and cooling regions of oscillation-controlled heat-transport tubes (OCHTs) are investigated numerically. The numerical simulation is conducted under the following three conditions for the tube walls at the heating and cooling regions: isothermal, extremely thin, and actual wall systems. Based on the numerical results and Hausen's correlating equation for laminar flow heat transfer in tubes, a correlating equation of the heat transfer coefficient is developed which can be generally applied to these three conditions. Next, using this correlating equation and the authors' simplified model of overall thermal resistance in OCHTs, heat-transport rates are predicted, and it is found that the predicted results are in good agreement with the numerical results. Finally, numerical simulation is conducted also to compare the heat-transport rates of OCHTs with those of the conventional forced circulation type under the same pumping power. The results indicate that there exist oscillatory flow regions in which the heat-transport rate of the phase-shifted OCHTs is larger than that of the conventional circulation type. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(6): 415–430, 1998     

混合气体横向冲刷水平管时对流冷凝换热的机理研究

采用修正的膜模型与Nusselt凝结理论相结合的方法，对含湿混合气体以一定速度冲刷水平管外时对流冷凝换热进行研究，在考虑气相边界层分离的情况下讨论了液膜流动和换热的情况，同时研究了气体来流冲刷角度对总体换热的影响。结果表明，冷凝液膜是一个相当薄的膜层，液相导热热阻在整个换热的过程中基本可以忽略。     

Theoretical research of convective condensation heat transfer for mixture gas on a horizontal tube

The mechanism of convective condensation heat transfer of moist mixed gas across a horizontal tube was studied in this paper. The models referring to how the liquid film flows and the heat transfers on the tube are set up by combining modified film model and Nusselt condensation theory. The effects of Re number, wall temperature, and water vapor concentration on condensation heat transfer are discussed. Results predict that the film thickness profile on the tube is influenced greatly by vapor shear force on liquid film. Local Nusselt number depends remarkably on gas phase heat resistance, which is different from pure vapor and very similar to single‐phase gas. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(6): 324–333, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20169     

Effect of outline curvature degree on heat transfer and flow characteristics inside dimpled tubes and spirally grooved tubes

The flow behaviors and heat transfer characteristics have been studied inside the dimpled tubes and spirally grooved tubes with different curvature degrees, which is considered for the first time within the influence factors. A three-dimensional numerical simulation by periodic boundary conditions is performed to model the fully developed flow of dimpled and grooved sections to acquire the finer mesh and more accurate results. In addition, the dimple and groove outlines with different curvature degrees are generated by polynomial functions. Effects of curvature degrees for dimpled tubes and spirally grooved tubes on flow, heat transfer, and comprehensive performances are discussed. The results indicate that the influence of curvature degrees for dimpled tubes exhibit an opposite behavior when compared with those for spirally grooved tubes. On the whole, all performance factors increase with the growing curvature degree for dimpled tubes but decrease with the increasing curvature degree for spirally grooved tubes. By comparing different curvature degrees, the maximum ranges of heat transfer enhancement are 1.50–2.22 and 2.54–2.82, respectively, for dimpled and grooved tubes with respect to Re. Thermal and hydraulic fields are considered to analyze the mechanism of heat transfer enhancement. The analysis shows that the way the dimple changes thermohydraulic properties differs from the way the groove changes the properties.     

水平单管内换热实验研究

利用隔膜泵作为系统动力输出源,搭建了单管内传热和流动测试实验台,对制冷剂R22在水平单管内的换热性能进行了实验研究,考察了不同蒸发温度和不同冷凝温度对总传热系数、制冷剂表面换热系数和管内压降的影响.实验结果表明:总传热系数和制冷剂表面换热系数均随着蒸发温度和冷凝温度的上升而增大;管内压降随着蒸发温度的上升而减小,随着冷凝温度的上升而增大;对于同一根实验管,在相同的冷却水流量和制冷剂质量流量下,最佳蒸发工况为10℃;冷凝实验中,总传热系数和制冷剂表面换热系数在40℃时高于其他两种冷凝温度时的值,但35℃冷凝时,管内压降高于其他两种工况.     

Mechanism research of convective condensation heat transfer for a gas mixture flowing over a horizontal tube

The modified film model combined with Nusselt's condensation theory are used for the study of convective condensation heat transfer on a horizontal tube with moist mixed horizontal gas flows at a given speed. A theoretical model considering gas boundary layer separation was set up. The liquid film flows and the heat transfer on the tube are presented. The effects of the flow direction on condensation heat transfer are discussed. The results predict that the condensate film is so thin that the liquid phase heat resistance can be ignored. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20238     

Condensation heat transfer on tubes in actual flue gas

In order to improve boiler efficiency, latent heat recovery from flue gas is a very important concept. Condensation heat transfer on horizontal stainless‐steel tubes was investigated experimentally by using an actual flue gas from a natural gas boiler. The experiment was conducted at different air ratios of the flue gas and a wide range of tube wall temperatures. The condensation pattern was similar to a dropwise condensation near the dew point. By decreasing the wall temperature, the wall region covered with a thin liquid film increased. The heat and mass transfer behavior was well predicted with the analogy correlation at the high‐wall‐temperature region. At the low‐wall‐temperature region, the total heat transfer was higher than that predicted by the analogy correlation. © 2001 Scripta Technica, Heat Trans Asian Res, 30(2): 139–151, 2001     

椭圆横管外降膜流动和换热特性分析

本研究基于VOF算法编写用户UDF(自定义函数),采用FLUENT软件建立了椭圆横管外降膜流动和换热的计算模型。根据CFD(计算流体力学)模型计算和分析了在不同长短轴比下管外降膜速度分布、压力分布、液膜内温度分布和管外换热分布的变化规律。研究结果表明:长短轴比的变化影响了管外液膜速度分布、压力分布和膜内温度分布;相比圆管,椭圆管的管外膜内液体流速更快。壁面压力沿周向逐渐减少并在X=0.9附近快速回升;随长短轴比e的增加,周向压力最小值位置逐渐向后推移。局部Nu数分布与压力分布在趋势上存在一致性。当e=1.65附近时,椭圆的换热性能最优;最后,通过对管形的研究分析,提出横管的传热分区模型。