基于场协同理论的超声波对强化换热影响

本研究分别对圆管、波节管和横纹管在Fluent软件中进行数值模拟,模拟了3种管型在紊流工况下的换热效果并对数值模拟所得到的结果用场协同的理论分析。结果表明:从场协同理论得出加入超声波会增强场的协同程度,增强换热管的换热效果;圆管、波节管、横纹管的场协同数则随着雷诺数的增加而减小;而努塞尔数和表面传热系数随着雷诺数的增加而增加,综合性能系数随着雷诺数的增加而增加,而效能评价系数会随着雷诺数的增加而减小。     

The application of field synergy number in shell-and-tube heat exchanger optimization design

In the present work the field synergy principle is applied to the optimization design of the shell-and-tube heat exchanger with segmental baffles. The field synergy number which is defined as the indicator of the synergy between the velocity field and the heat flow is taken as the objective function. The genetic algorithm is employed to solve the heat exchanger optimization problems with multiple design variables. The field synergy number maximization approach for heat exchanger optimization design is thus formulated. In comparison with the initial design, the optimal design leads to a significant cost cut on the one hand and an improvement of the heat exchanger performance on the other hand. The comparison with the traditional heat exchanger optimization design approach with the total cost as the objective function shows that the field synergy number maximization approach is more advantageous.     

Numerical Study of Natural Convection in an Enclosure with an Internal Heat Source at Higher Rayleigh Numbers

Natural convection is extensively used in cooling of large scale electrical and electronic equipments. This work involves study of flow and heat transfer characteristics in enclosures with partial openings having an internal heat source at higher Rayleigh number (Ra_h > 10⁶). It involves the numerical simulation of 2D steady state natural convection in enclosures of different aspect ratios (H/W = 2 and 3) for five Rayleigh numbers (Ra_h = 10⁷, 10⁸, 10⁹, 10¹⁰, and 10¹¹). Two different configurations have been considered based on the number and position of vents—diagonal side (DS) and two inlets one outlet (2I1O). The time dependent nature of the flow is characterized by performing a Fast Fourier Transform (FFT) analysis of temperature and velocity at a characteristic location in the enclosure. The global parameters considered are the mass flow rate driven through the cavity by the heater and the average Nu defined over the heater surface. It is seen that with increase in Ra_h, flow becomes more fluctuating and moves towards chaotic regime and this transition is quicker at lower H/W. For the given configuration both the global parameters increases with increase in Ra_h and decrease in H/W.     

A numerical investigation on effects of ceiling and floor surface temperatures and room dimensions on the Nusselt number for a floor heating system

In this study, the effect of ceiling and floor surface temperatures and room dimensions on the Nusselt number over the floor of a floor heating system has been investigated numerically. The variation of the Nusselt number with Rayleigh number has been analyzed under constant wall temperature condition for different ceiling temperatures (10–25 °C) and room dimensions. It has been seen that when the room dimensions and temperature difference between the ceiling and interior air are increased, the Nusselt number over the floor increases as well. The numerical results have been compared with the correlations given in the literature. It has been seen that the correlations available in the literature are valid only for given thermal conditions and room dimensions. The results calculated from the correlations which do not consider the effects of ceiling and floor surface temperatures deviate up to 35% than the results of this numerical study carried out for different ceiling and floor surface temperatures and room dimensions. Therefore, a new correlation for Nusselt number over the floor, which contain the influence of thermal conditions and all of room dimensions must be discovered.     

Analysis of solidification in the presence of high rayleigh number convection in an enclosure

A simple and convenient analysis of the process of time-dependent solidification in an enclosed liquid cooled from the side in the presence of natural convection is presented, the influence of each parameter on the process of solidification is carried out systematically. The accuracy of this model is justified by comparing its predicting values with the previous results.     

基于场协同理论的热空气吹掠型太阳能蒸馏器对流传热性能优化分析

以热空气吹掠型太阳能蒸馏器为研究对象,利用数值模拟结果和均匀设计法分析蒸馏器内流动与传热场协同角的变化规律和影响因素。结果表明,影响场协同角的主要因素是热空气进口速度,热空气进口温度和水温的影响不显著。在蒸馏水温30～70℃、进口空气温度40～90℃,进口空气速度为8 m/s时平均传热场协同角最小,为73.14°,传热系数最大,可达到同等条件下自然对流传热系数的4倍。     

壳管式相变储能换热器性能研究与场协同效应分析

建立壳管式相变储能换热器仿真模型,利用Fluent软件对蓄能、释能工况分别进行模拟。从温度场、速度场、固液交界面3个方面分析逐时换热强度的变化原因;探讨在蓄热过程中热源管壁温、热管口径规格对传热效率的影响。研究结果表明:蓄热工况下管间热扰及自然对流作用明显,10000 s时液相分数达到95%。放热工况以导热为主,60000 s时相变材料仅凝固了75%。从场协同效应分析,采用小尺寸热源管能缩短时间,但单位面积换热强度有明显下降。换热温差≤20℃时,其强化效果开始衰弱。     

含加热圆管方腔内自然对流的数值研究

采用数值计算方法对含不同直径圆管以及相同直径圆管位置不同方腔内的层流自然对流进行了研究。以冷热壁面温度差为基准的瑞利数Rn为10^6,以圆管壁面热流密度为基准的Ra为10^8。计算结果表明,当圆管处于方腔中间位置时,随着圆管直径的增大,圆管表面局部努塞尔数呈减小趋势。当圆管直径不变时,由于在不同位置处浮力作用的强弱不同,随着圆管在方腔内位置的改变,方腔内流场结构和温度场分布也会发生变化。整个计算结果可为工程设计提供参考。     

基于场协同原理的异型管强化石蜡蓄放热性能分析

将1.5%石墨烯/石蜡复合相变材料填充到内管形状不同、换热面积相同的套管换热器内,采用数值模拟的方法分析内管形状对石蜡类复合相变材料蓄放热性能的影响。结果表明,异型管能有效提升石蜡复合相变材料的熔化及凝固速率,滴型管外石蜡复合相变材料的熔化速率比椭圆管及圆管分别提高53%、62%,滴型管外石蜡复合相变材料的凝固速率比椭圆管及圆管分别提高6.7%、9.8%。基于场协同原理分析异型管的强化石蜡类复合相变材料的传热机理,由于滴型管能使石蜡类复合相变材料在相变过程中温度场与速度场协同性更高,因此能更有效地提升其相变速率。     

关于管内单相对流换热强化的极限问题

从场协同理论出发,分析了通道内表面全部为射流冲击换热表面时的极限换热率;将全射流冲击管内换热与普通流动管内换热进行了比较。给出了层流和紊流工况下全射流冲击换热可能达到的最大强化比。针对相同R e,分析得出:在层流充分发展段,全射流冲击通道的强化极限是16.9倍;在紊流充分发展段是3.5倍。综合现有各种通道内强化换热的研究结果进行比较,其换热率均低于全射流冲击管内换热率,其中层流工况以折流翅片式通道和交叉缩放椭圆管的换热率与极限换热率最为接近;紊流工况以内插螺旋丝强化管最为接近。     

Experimental study of natural convection heat transfer of air layers in vertical annuli under high rayleigh number conditions

In this paper experimental investigations of natural convection heat transfer of air layers in vertical annuli are presented. In these experiments, the surface of the inner cylinder is maintained at a constant heat flux condition and the outer cylinder is cooled in the atmosphere. In order to obtain the convective contribution, the overall heat transfer data are corrected for thermal radiation and axial conduction losses from the end plates in the annuli. Special emphasis, in these investigations, was placed on the high Rayleigh number region where no experimental data are available in the literature. Data were obtained for Rayleigh numbers greater than 10⁹. The radius ratios studied were 2.03 and 3.92, and the aspect ratios studied were 23.94 and 66.67. Heat transfer correlations for average Nusselt numbers were developed for different Rayleigh number regions. For the low Rayleigh number region the results of this paper agree with the correlations reported in the literature. Much needed data and correlations for the high Rayleigh number region are obtained for the first time. These results improve the predictive ability for the heat transfer characteristics in the high Rayleigh number region. ©1999 Scripta Technica, Heat Trans Asian Res, 28(1): 50–57, 1999     

基于多场协同理论的炉膛燃烧传热分析

为了探究炉膛传热性能,利用场协同理论对不同条件下的炉膛燃烧流场进行了分析。结果表明：在炉膛燃烧区壁面附近及中心部分区域,协同角α较小,而介于这两部分的大块区域,协同角α较大。协同角β大的区域占比相对较大。在整个燃烧区域,尤其是喷口附近,协同角γ较大,且分布比较均匀。协同角φ分布也比较均匀,大角度φ主要集中在壁面附近。在炉膛中心大部分区域以及炉膛顶部及折焰角处协同角θ较大,但是在喷口附近,协同角θ较小。