Numerical Simulation of the Effect of Air Distribution on Turbulent Flow and Combustion in a Tubular Heating Furnace

A three-dimension full-size numerical simulation of the effect of air distribution on turbulent flow and combustion in a tubular heating furnace was carried out. A standard k –ε turbulent model, a simplified PDF combustion model and a discrete ordinate transfer radiation model were used. The hybrid grid combining a structured and a non-structured grid was generated without any simplification of the complicated geometric configuration around the burner. It was found that the multistage combustion could reduce and control the peak value of temperature. At the same time, it was concluded that the amount of primary air had little effect on the global distribution of velocity and temperature in the furnace, but a great effect on that around the burner. It is recommended that 45% - 65% of the total amount of air be taken in in primary air inlets in the furnace. All the results are important to optimize the combustion progress.

Experimental Research on Flow Maldistribution in Plate-Fin Heat Exchangers

The flow maldistribution and the effect of different inlet configuration on the flow distribution in platefin heat exchangers were studied experimentally. It is found that the flow maldistribution is serious because of the defects of inlet configurations, while the inlet configuration and Reynolds number are the main factors affecting the flow distribution. The improved inlet configurations, which are the header with a two-stage distributing configuration and the guide vane with a fluid complementary cavity were proposed and tested in this paper. The experimental results show that the improved inlet configurations can effectively improve the performance of flow distribution in heat exchangers.

NUMERICAL STUDY ON FLOW DISTRIBUTION IN PLATE-FIN HEAT EXCHANGERS

Objective To investigate the flow distribution in plate fin heat exchangers and optimize the design of header configuration for plate fin heat exchangers. Methods A mathematical model of header was proposed. The effects of the header configuration on the flow distribution in plate fin heat exchangers were investigated by CFD. The second header configuration with a two stage distributing structure was brought forward to improve the performance of flow distribution. Results It is found that the flow maldistribution is very serious in the direction of header length for the conventional header used in industry. The numerical predictions indicate that the improved header configurations can effectively improve the performance of flow distribution in plate fin heat exchangers. Conclusion The numerical simulation confirms that CFD should be a suitable tool for predicting the flow distribution. The method has a wide variety of applications in the design of plate fin heat exchangers.

Distribution performance of gas–liquid mixture in the shell side of spiral-wound heat exchangers

The non-uniformity of gas–liquid mixture is a critical issue which leads to the heat transfer deterioration of spiralwound heat exchangers(SWHEs).Two-phase mass flow rate and the content of gas are important parameters as well as structural parameters which have prominent influences on flow distribution uniformity of SWHE shell side.In order to investigate the influences of these parameters,an experimental test system was built using water and air as mediums and a novel distributor named"tubes distributor"was designed.The effects of mass flow rate and the content of gas on two-phase distribution performance were analyzed,where the mass flow rate ranged from 28.4 to 171.9 kg·h-1 and the content of gas changed from 0.2 to 0.8,respectively.The results showed that the mixture mass flow rate considerably influenced the liquid distribution than that of gas phase and the larger mass flow rate exhibited the better distribution uniformity of two-phase flow.It was also found that the tubes distributor had the better two-phase uniformity when the content of gas was around 0.4.Tube diameter played an important role in the distribution of gas phase and slit width was more significant for the uniformity of liquid phase.

基于热负荷均匀分配原则的直接空冷系统轴流风机转速灵活调节策略

在直接空冷电站中,轴流风机功耗占厂用电比例较高。为获得更为高效的风机运行调整策略,以某典型2´600 MW直接空冷机组为例,基于热负荷均匀分配原则,采用数值模拟方法,研究轴流风机转速灵活调节策略。获得不同环境温度和风速条件下,各轴流风机转速分布规律以及通过空冷凝汽器单元的冷却空气质量流量分布规律。最终,计算得到不同环境气象条件下不同轴流风机转速调节策略风机总功耗变化规律。结果表明,基于热负荷均匀分配原则的轴流风机转速灵活调整策略明显优于轴流风机转速整体调节策略,结果可为直接空冷机组优化运行和深层次节能提供参考。

竖直铯热管传热特性的实验和数值模拟

近年来,有关中温铯热管的研究较少,主要因为铯热管造价昂贵且铯金属化学性质活泼,开展实验难度较大,因此通过数值模拟方法对铯热管启动特性与内部相变等过程进行分析预测具有重要意义。首先,本文开展铯热管冷启动实验,并基于目前的热管模拟模型,利用UDF建立蒸发与冷凝的自适应模型,对比实验热管稳态壁温与数值模拟计算壁温,最大温差小于20K,且较好地呈现了热管末端的温度突降,证明了模型的准确性。其次,基于竖直铯热管变功率冷态启动实验,模拟获得铯热管启动过程中热管壁面温度及内部工质相态分布的演变过程;并利用壁面温度的平均差数值评价有效长度上的均温性。最后,通过模拟研究改变热管蒸发段长度(200mm和120mm)、加热功率(1028.2W和844.4W)及充液率(8.8%、12%和15%),对比分析其相态分布、温度分布及压力分布;综合热管有效工作长度、壁面均温性、蒸发段液相堆积与干烧现象,分析蒸发段长度与充液率的最佳配比,为热管的优化设计提供基础。

两相并联换热通道间流量分配特性实验研究

为提高两相流动换热通道的换热能力及系统性能,探究其并联换热通道中流量分配不均的特征,开展了以水为工质的两并联通道换热实验,对流量分配不均所带来的参数变化进行研究。实验参数变化主要包括进口温度、热流密度、进口流量,以及外部扰动、集管结构带来的流量分配差异。将实验结果与模型计算结果进行了对比,结果表明:并联换热通道间流量分配不均时,优先沸腾通道中流量较少,未沸腾通道出口温度明显降低;增加进口温度、增加热流密度、降低进口流量会导致流量分配不均的出现,其产生和消失对应的进口温度、热流密度、进口流量分别变化了24.8℃、175.6 W/m、19.8%,并存在滞后现象;扰动会加速流量分配不均的出现,对应进口温度相差3.2℃;集管结构会影响优先沸腾通道的位置,单相状态下流量较少的通道倾向于优先沸腾。研究结果可为提高并联换热通道流量分配的均匀性提供参考。

高速飞行器边界层质量引射降热减阻技术流量分区优化研究

边界层质量引射被认为是解决临近空间高超声速飞行器关键部位热防护问题和减小其飞行阻力的有效途径之一.然而,已有研究主要关注边界层质量引射对热流或阻力的单独影响,少有研究对边界层质量引射能够带来的降热减阻效果进行综合评估.针对高空高超声速层流条件下钝楔与钝锥外形的边界层质量引射降热减阻问题开展数值模拟研究,并采用多目标优化方法开展了边界层质量引射的分区流量优化设计,在满足减阻性能的同时,提高了整体的热防护效果,实现了给定冷却剂流量条件下的降热减阻综合性能优化.研究结果表明,层流条件下,边界层质量引射通过改变速度边界层和温度边界层能够显著降低壁面摩阻和热流.在单位面积引射质量流量为28.028 g/(m^(2)·s)时,对于钝楔外形,采用均匀引射方案能够减阻3.60%,同时峰值热流下降12.06%,而采用分区优化方案能够将减阻效果提高到4.30%,同时峰值热流下降91.01%;对于钝锥外形,采用均匀引射方案会造成阻力增大2.60%,同时峰值热流仅下降8.57%,而采用分区优化方案能够在减阻19.75%的情况下同时将峰值热流降低99.95%.

旋流式微通道热沉传热特性研究

随着微电子设备高度集成化和小型化发展,电子器件发热量增加,其热管理问题是目前的研究热点。本文针对旋流式微通道热沉传热和流动特性开展了数值模拟的研究,讨论了冷却剂流量和均热板材料导热系数的影响。研究结果表明,与常规微通道不同,随冷却剂流量增加,旋流式微通道热沉内冷却剂旋律程度增强,流程增加,导致高温向热沉中心出口位置迁移,高低温差降低。热沉底部均热材料导热系数增加时,加热面上方径向和周向导热增强,导致加热面平均温度和高低温差降低。

气侧均流装置对冷却三角单元流动传热特性影响的实验研究

对于间接空冷系统冷端基本传热元件−冷却三角单元,通过在其加装及相邻未加装气侧均流装置冷却三角单元内的两侧冷却柱沿散热管束横向布置风速、风温、壁温测点,进一步在左右两侧出水柱布置壁温测点,对比分析两侧冷却柱沿散热管束横向迎风风速、风温、壁温及出水壁温的变化规律,研究气侧均流装置对冷却三角单元流动传热特性及出水壁温分布的影响机制。研究结果表明:加装气侧均流装置后冷却三角单元两侧冷却柱沿散热管束横向迎风风温、壁温分布更加均匀,两侧出水柱壁温均值更低,且降低了两侧出水柱壁温差,气侧均流装置优化了两侧冷却柱的换热均匀性。

混合板换热器板片结构对流道内触点分布及流动换热性能的影响

混合板式换热器作为余热利用系统中高效节能设备,其换热性能对系统效率及稳定性有重要影响。该研究基于RNG k-?湍流模型,对M型和H型板式换热器流道内流动及传热过程展开数值模拟,研究板片结构对板间触点分布的影响,及雷诺数Re和板片组合对横向流道内流动及换热特性的影响规律。结果表明:M型板间触点呈“方格”,H型呈“菱形”,且H型流流体速度较大、温度场分布较均匀。随Re数增大,压降△P及平均努塞尔数■均增大;Re数较低时,H型的■明显提高,且△P增幅较小;而高Re数(Re=6000)时,与M型相比,H型■的增幅仅为△P的25%左右。随波纹间距s减小,触点数明显增多,△P和■均增大;s为12~16 mm较合适。研究结果为混合板式换热器的设计及优化提供理论依据。

大型环盘折流板反应器壳程流动模拟及结构设计

环盘结构是大型换热式反应器中最常用的壳程导流结构之一,针对环盘折流板结构参数的选择影响反应器壳程流场特性的情况,分析了环盘型折流板的结构特性,发现大型反应器折流板相对板距d*的取值通常为0.15~0.20;在此基础上建立了不同相对缺口面积S^(*)条件下的模型进行流场模拟;详细分析折流板结构参数对壳程流体流动形态、停留时间分布和压降特性的影响,综合考虑后提出S^(*)=0.15~0.30是大型反应器相对缺口面积的合理设计范围;最后,根据现有研究中单弓型折流板换热器的数据,在模拟的结构条件范围内对比了环盘和单弓结构的性能,为工程设计提供指导。

集成方位跟踪太阳能塔式接收器热流密度分布特性研究

为研究接收器与竖直方向夹角和射线集中点的位置对热流密度值与分布的影响,我们以集成方位跟踪太阳能集热实验装置中集热单元为研究对象,基于SolTrace软件和射线追踪技术搭建了定日镜和接收器光学模型。以光学模型为基础,通过改变接收器的角度和位置来改变热流密度值与分布,得到接收器在不同位置时的热流密度值与分布状态。结果表明:接收器上热流密度值在接收器与竖直方向夹角为0°~90°时,随夹角的增大先增后减,夹角为40°时其值达到最大;射线焦点在接收器开口焦点2位置附近时热流密度分布较为均匀,射线可全部进入接收器内,热流密度峰值只有70 kW/m^(2)左右。建模仿真结果对集热单元中接收器安装和位置的调节提供了有益参考。

PCHE细小多通道内天然气沸腾两相流分配特性数值分析

印刷电路板式换热器具有体积小、结构紧凑、换热效率高等特点,在天然气生产工艺中应用前景极为广阔,在PCHE换热过程中,冷、热侧通道内工质往往发生沸腾相变,各子通道间普遍存在汽液两相流分配不均问题,严重降低换热效率。对此,以换热器封头和12个细小通道为建模对象,采用天然气汽液两相混合物为工质,通过Aspen Plus软件获得其物性数据,基于VOF模型追踪相界面运动位置,构建适用于预测分析PCHE内天然气两相流分配特性的三维非稳态数值模型,并开展了相关数值研究。结果表明,相比于单相液态天然气,当换热器入口混入汽相工质变为汽液两相混合物时,各子通道间工质的质量流量相对标准偏差由0.0004增加到0.036,两者相差近100倍,分配不均现象明显加剧。导致该现象的主要原因在于来流汽相工质明显倾向于进入距离换热器入口最近的若干通道,使得这些通道的汽相流量显高于其他通道,各通道汽相工质流量分配特性整体上呈现明显的“峰值分布”,但是,随着换热器入口汽相工质流速的不断增大,各通道间汽相工质的流量分配不均现象逐渐缓和,而液相工质的流量分配特性发生明显恶化。

Residence time distribution and heat/mass transfer performance of a millimeter scale butterfly-shaped reactor

A millimeter scale butterfly-shaped reactor was proposed based on sizing-up strategy and fabricated via femtosecond laser engraving. An improvement of mixing performance and residence time distribution was realized by means of contraction and expansion of the reaction channel. The liquid holdup was greatly increased through connection of multiple mixing units. Structure optimization of the reactor was carried out by computational fluid dynamics simulation, from which the effect of reactor internals on mixing and the influence of parallel branching structure on heat transfer were discussed. The UV–vis absorption spectroscopy was used to determine the residence time distribution in the reactor, and characteristic parameters such as skewness and dimensionless variance were obtained. Further, a chained stagnant flow model was proposed to precisely describe the trailing phenomenon caused by fluid stagnation and laminar flow in small scale reactors, which enables a better fit for the experimental results of the asymmetric residence time distribution. In addition, the heat transfer performance of the reactor was investigated, and the overall heat transfer coefficient was 110–600 W m^(-2)K-1in the flow rate range of 10–40 m L/min.

ANALYTICAL AND EXPERIMENTAL INVESTIGATION OF FLOW DISTRIBUTION IN MANIFOLDS FOR HEAT EXCHANGERS

The prediction of flow distribution in flow manifolds is important for the design of heat exchangers. The pressure drop along the flow in the header is the most influential factor in flow distribution. Various continuous models available in literature have failed to satisfactorily predict the pressure distribution in the headers of the flow manifolds. In this article, a discrete model matching the real physical phenomena has been proposed, to predict the pressure distribution in headers. An experimental evaluation of relevant flow characteristic parameters has been carried out to support the discrete model calculations. The validity of the theoretical discrete model has been performed with experimental results, under specific conditions. Refined experimental probes, for pressure heads with ultrasonic measuring devices, have been used to obtain accurate results. The experimental results fully substantiate the soundness of the theoretical prediction. In addition, the advantage of the ability to accommodate local disturbances in the discrete model has been pointed out. The effect of some local disturbances may be substantial. As a result of the analysis presented in this article, improved designs of flow manifolds in heat exchangers can be realized, to assure operation safety under severe operating conditions.

EXPERIMENTAL AND NUMERICAL ANALYSIS OF LAMINAR AND LOW TURBULENT FLOW DISTRIBUTIONS IN INLET DIVIDING HEADER OF SHELL AND TUBE HEAT EXCHANGER

Flow distribution headers play a major role in heat exchangers.The selection of header diameter,branch pipe diameter,branch pipe spacing etc.is based on the designer＇s experience and general guide lines.The proper selection of the header dimensions will yield uniform flow distribution in heat exchangers,which in turn will enhance the heat exchanger efficiency.In this work,the flow distribution in branch pipes and the pressure variation across the branch pipes in laminar and low turbulence region is studied with two models of the inlet dividing headers.When the numerical analysis has been applied,its inability to predict the no flow condition through the branch pipes is revealed.The results are presented in the form of flow rate ratio through branch pipes and nondimensional coefficients across branch pipes which are useful to apply the existing mathematical models for the present experimental setup.

Golmud-Ejin heat flow profile and straightforward calculation of crust-mantle temperature

The Golmud-Ejin heat flow profile consists of 18 observed heat flow values. They arelimited in data number, distributed unevenly and vary from 55 mW·m-2 to 79 mW·m-2

Computational analysis of the flow of pseudoplastic power-law fluids in a microchannel plate

The flow of pseudoplastic power-law fluids with different flow indexes at a microchannel plate was studied using computational fluid dynamic simulation.The velocity distribution along the microchannel plate and especially in the microchannel slits,flow pattern along the outlet arc and the pressure drop through the whole of microchannel plate were investigated at different power-law flow indexes.The results showed that the velocity profile in the microchannel slits for low flow index fluids was similar to the plug flow and had uniform pattern.Also the power-law fluids with lower flow indexes had lower stagnation zones near the outlet of the microchannel plate.The pressure drop through the microchannel plate showed huge differences between the fluids.The most interesting result was that the pressure drops for power-law fluids were very smaller than that of Newtonian fluids.In addition,the heat transfer of the fluids through the microchannel with different channel numbers in a wide range of Reynolds number was investigated.For power-law fluid with flow index(n=0.4),the Nusselt number increases continuously as the number of channels increases.The results highlight the potential use of using pseudoplastic fluids in the microheat exchangers which can lower the pressure drop and increase the heat transfer efficiency.