Empirical correlation for performance evaluation of electric/corona wind on natural convection

The effect of the corona wind on the natural convection at a rectangular channel was investigated experimentally. The results indicate that the natural convection in the absence of electric/corona wind at obtuse angles outperforms than acute angles and keeps improving by increasing the angle. However, the efficiency of the electric/corona wind at acute angles is higher than obtuse angles. Generally, in the presence of electric/corona wind, heat transfer coefficient was increased. The effect of the electric/corona wind was decreased by raising heat flux. This mainly stems from the fact that the temperature gradient raises the thermal boundary layer and reduces the secondary flow power. Eventually, empirical correlation for the estimation of Nusselt number was achieved.     

Targeted heat transfer augmentation in circular tubes using a corona jet

Natural convection heat transfer can be noticeably enhanced by corona wind in tubes and channels. A corona-induced secondary flow may be generated in tubes with no major changes in the geometry, or causing any noise or vibration. In this investigation, it is shown that the eccentric configuration of a wire electrode inside a tube forms a local jet along the eccentricity direction, which impinges on the tube wall and improves the local heat transfer. Since the direction of the corona jet is determined by the eccentricity direction of the electrode, the jet may be oriented properly to target the desired spots.     

ELECTROHYDRODYNAMIC ENHANCEMENT OF HEAT TRANSFER IN A SHELL-AND TUBE HEAT EXCHANGER

Enhancement of heat transfer in a heat exchanger via a DC corona discharge was studied experimentally using a single-tube shell-and-tube heat exchanger. Air was the working fluid in both the tube and shell sides. Excitation of the tube side was via a single wire electrode, while that of the shell side was via four rod electrodes oriented symmetrically at 90^° intervals. Three series of experiments were performed: (1) excitation of the tube side only, (2) excitation of the shell side only, and (3) simultaneous excitation of the tube and shell sides. Both heat transfer and pressure drop measurements were performed, with Reynolds number and electric field potential as parametric quantities in the tube and shell sides. It was found that highest enhancements take place when the tube and shell sides are excited simultaneously, yielding a 322% increase in the overall heat transfer coefficient. Study of the heat transfer enhancements per unit pumping power indicates that for the range of parameters studied, the technique is most efficient at moderate Reynolds numbers and at electrode potentials in the midrange between threshold and sparkover limits.     

A Study on Nucleate Boiling Heat Transfer Characteristics of Acetone on Smooth and Indented Surfaces

This article presents the nucleate boiling heat transfer characteristics of acetone at one bar on smooth and enhanced circular stainless steel surfaces (SS 316) of 20 mm diameter for heat flux between 1 and 4 W cm^? 2, which mimic the operating condition of a typical immersion electronic cooling system. The experimental heat transfer coefficient from the smooth surface is validated against Borishanski correlation [1] within acceptable limits of ± 5%. The steel smooth surface is enhanced by providing 100 equally spaced indents of 0.5 mm diameter and 0.05 mm depth. The experimental results indicate that the enhanced surface shows a good shift in the boiling curve and thus, enhancing the nucleate boiling heat transfer at a lesser wall super heat when compared to the smooth surface by around 35% for tested condition. The effect of subcooling on nucleate boiling in enhanced surface reveal that the heat transfer coefficient degrade by 40 to 55% for a sub cooling of 5 to 10 K. The influence of material is studied by a similar enhanced surface made of brass and compared for the same working condition. The brass enhanced surface showed an improved of around 50% against the steel-enhanced surface. Also, the influence of fluid is studied by comparing acetone and n-pentane, which showed that the latter an enhancement in heat transfer coefficient of 50% over the former.     

Effects of corona discharge on the surface structure, morphology and properties of multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) were modified by corona discharge and then heat treated in the air. The influences of corona discharge parameters such as treatment time and processing power were investigated. The results of energy dispersive X-ray analysis (EDX) and thermogravimetric analysis (TGA) indicated the introduction of oxygen-containing functional groups onto the surface of the MWCNTs after heat treatment. The water contact angle tests showed that the hydrophobicity of the MWCNTs was decreased to some extent. The static water contact angle was reduced from 146° to 122° when the time of the corona discharge treatment reached 3 min at the processing power of 200 W. The enhanced thermomechanical and mechanical properties of epoxy nanocomposites filled with the corona discharge treated MWCNTs were observed. The modified MWCNTs conferred better properties on the composites than the pristine MWCNTs because of the improved dispersion of MWCNTs in matrix and the enhanced interfacial interaction between the treated MWCNTs and epoxy.     

Experimental investigation of cooling of a plate by ionic wind from a corona-forming wire electrode

We report on the results of experimental investigations of the kinematic structure of ionic wind from a wire electrode placed near a heated plate, which plays the role of the earthed electrode. Experiments are carried out in a wide range of voltages for different polarities of the wire for several values of the electrode gap. We compare the structures of the flows emerging as a result of natural convection in open air for different positions of the plate and in the presence of a fast ionic wind jet that considerably intensifies heat transfer in the boundary layer at the heated planar electrode. Local temperature distributions over the plate surface are obtained, as well as the integral dependences of the effective heat removal on the electric parameters of the corona discharge. The velocity of air flows with ionic wind reaches 4 m/s, and the heat power removed from the plate for fixed overheating increases ninefold compared to the situation with natural convection.     

半干旱草原下垫面动量和感热总体输送系数参数化关系研究

本文利用锡林郭勒草原2008年春季近地层涡旋相关系统和铁塔的风、 温平均梯度观测资料, 分析了总体输送系数随梯度Richardson数的变化特征, 建立了动量总体输送系数随大气稳定度、近地层风速以及感热总体输送系数随大气稳定度和近地层气温的关系. 中性条件下, 半干旱草原植被下垫面动量总体输送系数与近地层大气动力状态之间存在明显的相互作用, 总体输送系数与近地面层风速之间满足二次曲线拟合关系; 风速较小时, 大气动力特征对地表粗糙度长度的改变不是很明显, 动量总体输送系数随气流增强而增大; 而当风速较大时, 强风速会使植被高度发生改变, 动量总体输送系数随气流增强而减小. 另外, 感热总体输送系数与近地层气温之间也存在二次曲线关系. 动量总体输送系数与近地层风速之间的关系、感热总体输送系数与近地层气温之间关系的建立为总体输送系数参数化提供了重要途径, 同时该方案避免了对动力学粗糙度长度和热力学粗糙度长度的求解. 关键词： 总体输送系数 参数化 湍流通量 相似性函数     

斜翅型冷凝强化换热管传热性能的实验研究

对一种斜翅型外翅片带内螺纹的冷凝强化换热管进行传热性能的实验研究。管外冷凝换热的制冷剂为R134a,管内对流换热的介质为水。分别在定热流密度与定水流速的条件下进行一系列工况的实验,得到相应的实验数据。在定热流密度条件下,利用Wilson图解法得到管内的换热系数数据及相应的计算关联式。在定水流速的条件下,利用分离方法得到管外冷凝换热系数数据及相应的计算关联式。将强化管换热系数数据与光管换热系数的理论计算值进行了比较,结果表明:冷凝强化换热管管内对流换热的强化倍率为2.4,管外凝结换热系数随壁面过冷度的增加而增大,管外凝结换热的强化倍率为:1.78～3.92。     

扁平管交错蛇形短翅片强化传热的研究

针对火电空冷凝汽器采用的扁平管蛇形翅片长度较大,空气在翅片间流动对强化传热的效果受到边界层发展抑制的缺陷,根据锯齿翅片通过破坏边界层发展强化传热的思想,提出一种扁平管交错蛇形短翅片结构。实验结果表明,扁平管交错蛇形短翅片的传热性能优于原有结构,在不同雷诺数Re范围,努塞尔数Nu增加了1.4%～16%;同时空气侧流动阻力也明显增加,摩擦系数f增加了18%～45%。由综合评价指标PEC也可以得到,扁平管交错蛇形翅片有效地强化了空气侧的换热。     

Study of Automatic Transmission Fluid in a Serpentine Minichannel Heat Exchanger: An Experimental Approach

An experimental investigation was conducted on automatic transmission fluid cooling in a minichannel heat exchanger using a closed-loop integrated thermal wind tunnel test facility. Effects of automatic transmission fluid Reynolds number (Re_L) on heat transfer coefficient and Nusselt number were examined within the Re_L of 3–30 for air-flow Re of 1,450–5,200. Effects of serpentine on heat transfer enhancement and flow characteristics were evaluated through Dean number analysis. The analysis of Eckert number and Brinkman number showed a contribution to the viscous heating even for a low Re_L in the minichannel. The study showed enhanced heat transfer characterizations of the multi-port minichannel heat exchanger.     

RESEARCH ON SYSTEM IDENTIFICATION OF WALL SURFACE HEAT TRANSFER PROCESSES

A new experimental procedure is proposed based on an instrumental variable method [1] for analyzing wall surface heat transfer processes and deducing wall surface heat transfer coefficients under actual conditions. Tests were carried out under a wide range of conditions, and the heat transfer coefficient was found to vary from 8 to 26 W/m 2 ” K while wind speed ranged from 0 to 7.5 m/s, and correlations were obtained in terms of wind speed. In addition, the experiments indicated that wind direction does not have a significant effect on the heat transfer coefficient of wall surfaces.     

Numerical study of EHD-enhanced water evaporation

A numerical model has been developed for the investigation of water evaporation enhanced by corona wind. The corona wind is generated by a wire electrode charged at a high dc voltage. Only positive corona discharge is considered in the present study. The effect of cross-flow on EHD-enhanced water evaporation is also examined. The results show that water evaporation can be greatly enhanced by corona wind. However, a cross-flow with a high velocity may diminish the effect of corona wind. The numerical results are also compared with experimental data reported in the literature. A satisfactory agreement is found between these results.     

Pulsed power corona discharges for air pollution control

Successful introduction of pulsed corona for industrial purposes very much depends on the reliability of high-voltage and pulsed power technology and on the efficiency of energy transfer. In addition, it is of the utmost importance that adequate electromagnetic compatibility (EMC) is achieved between the high-voltage pulse source and the surrounding equipment. Pulsed corona is generated in a pilot unit that produces narrow 50 MW pulses at 1000 pps (net average corona power 1.5 kW). The pilot unit can run continuously for use in industrial applications such as cleaning of gases (100 m³/h) containing NO or volatile organic compounds (VOCs) or fluids (e.g., waste water). Simultaneous removal of NO and ethylene to obtain clean CO₂ from the exhaust of a combustion engine was tested at an industrial site. Various chemical processes, such as removal of toluene or styrene from an airflow are tested in the laboratory. We developed a model to analyze the conversion of these pollutants. To examine the discharges in the reactor we use current, voltage, and E-field sensors as well as a fast charge-coupled device (CCD) camera. Detailed energy input measurements are compared with CCD movies to investigate the efficiency of different streamer phases. EMC techniques incorporated in the pilot unit are based on the successful concept of constructing a low transfer impedance between common mode currents induced by pulsed power and differential mode voltages in signal lines and external main lines     

COMPARISON OF HEAT AND MASS TRANSFER IN FALLING FILM AND BUBBLE ABSORBERS OF AMMONIA-WATER

An experimental analysis of ammonia-water absorption process was performed for the falling film and bubble modes in a plate-type absorber. The experiments were made to examine the effects of solution flow rate and gas flow rate on the performance of the absorber. It was found that the bubble mode is superior to the falling film mode for mass transfer performance, and more heat was generated in the bubble mode. Increase of solution flow rate rarely affected the mass transfer, but improved the heat transfer. As the gas flow rate increased, fluidization occurred in the bubble mode and influenced the thermal boundary layer. However, channeling appeared in the falling film mode and decreased the heat transfer area. Increase of the gas flow rate greatly enhanced the performance of heat transfer in the bubble mode but made it worse in the falling film mode. Finally, the results were converted into dimensionless numbers to elucidate physical phenomena and we plotted Sherwood number versus Reynolds number for mass transfer performance and Nusselt number versus Reynolds number for heat transfer performance.     

板式离子风散热系统性能研究与优化

为探究板式离子风散热系统的性能和提供系统优化的新思路,提出一种针-平行板电极的板式离子风发生器,讨论针的位置、板间距、板厚等参数对风速和对流换热系数的影响,利用热红外成像仪分析板电极产生离子风时的温度分布以及焦耳热效应对热传递的影响。结果表明针电极距板外10 mm、板间距10 mm和板厚1.5 mm为优选方案,最大传热系数达25 W·m^-2·K^-1。     

新型沟槽道微热管散热性能实验研究

微热管以其效率高、响应快且无能耗,在高功率集成微电子散热方面应用广泛。针对电子器件的小型化、高能耗发展趋势,本文提出一种新型沟槽道微热管结构,对该沟槽道微热管进行稳态和瞬态热性能实验研究,研究了风速、角度、加热功率等因素对该新型热管的热性能影响规律。结果表明,该微热管在整个散热器传热上起主导作用,性能比达到0.88,冷凝端温差为0.8℃,具有良好的均温性,该微热管加热功率为140 W,空气流速1.5 m/s时,换热系数可达2 359 W/(m²·℃),热阻为0.27℃/W;高功率状态下可保持良好的热扩散性能,有效避免微热管的热应力集中,有望高效解决集成电子器件的散热问题。     

基于电晕放电的大功率LED散热研究

针对大功率LED芯片的散热问题,提出了一种基于电晕放电原理的离子风散热方案。通过试验,研究了电晕放电的电学性能,同时探寻了放电电压对制冷效果的影响以及温降随电晕放电功率的变化规律。结果表明,放电间距相同时,对发生器施加负电晕能够在较低的电压下产生离子风,降温效果显著。电晕电流平方根与放电电压呈线性关系。电晕放电功率为1.5 W、放电间距为10 mm时,散热效果最好。     

Experimental investigation of heat transfer and friction coefficient of the water/graphene oxide nanofluid in a pipe containing twisted tape inserts under air cross-flow

The water/graphene oxide nanofluid effect in a pipe equipped by twisted tape inserts under air cross-flow is investigated and the optimal tape geometry is determined. The range of internal and external Reynolds numbers are: 3800<Re_o<21500 and 550<Re_i<2000. Heat transfer and pressure drop increase by increasing Re and inserts width and heat transfer performance coefficient increased up to 1.4, indicating enhanced heat transfer compared to undesirable pressure drop. On the other hand, the heat transfer coefficient is 26% higher when compared with water in a plain tube. According to the results, this method is a good alternative in heat exchangers.     

环路热管式光子吸收器的换热可行性

利用环路热管换热技术对光子吸收器进行换热可以提高光子吸收器的换热效率、减小其结构尺寸,而且运行时无振动,是未来高性能加速器中设计光子吸收器的重要技术储备。分析环路热管在光子吸收器上应用时的传热性能,发现目前环路热管的换热能力完全满足光子吸收器的换热需求,但热沉的结构、特别是导热距离需要严格优化。利用航天五院C18型号环路热管,优化设计了一台环路热管式光子吸收器样机,数值模拟其运行时的温度分布,并实验测试了光子吸收器样机的总体换热能力。     

空气源热泵机组结霜问题的实验分析

基于空气源热泵在低温寒冷地区运行中遇到的结霜问题,对不同风速工况下,结霜过程中设备性能的变化进行分析,以换热量、换热系数为指标对不同翅型换热器的换热特性进行研究.实验结果显示:换热器结霜过程中,换热过程主要分为初始增加段、换热平稳段、缓慢衰减段、后期平稳等四段,结晶体在增加空气湍流度强化换热的同时,也增加了换热热阻使换...