EHD强化传热研究中的重要问题——电场特性的影响规律

在文献综述的基础上，分析了EHD强化传热中，电场特性的重要性和研究现状，指出了当前研究中的不足之处及存在的矛盾，为在EHD强化传热研究中，进一步进行电场特性影响规律的试验和理论探讨提出了建议。     

电场作用下气泡的行为研究

为进一步揭示EHD强化沸腾换热机理，分析了电场作用下气泡的受力情况，综述了国内外对电场作用下气泡的行为研究成果，系统总结了电场对气泡行为及其动力学特性的影响，并对该领域的进一步研究提出了建议。     

EHD_脉动流混合强化传热换热器的传热与阻力特性实验研究

以水为工质,实验研究了EHD(电场)与脉动流对单管换热器内的传热与阻力特性影响。实验中,电压值设定为0-40 kV,脉动流频率f=1、2、3 Hz,脉动流幅度A=1,脉动流条件下的管内时均流量q=0.1-0.5 m3/h。实验结果表明:管程脉动流单独作用时,同一流量条件下,f增加时传热系数α无明显变化,即脉动流单独作用对圆管传热强化作用不显著;EHD单独作用能显著强化传热,电压相对较小时,α增长缓慢,当电压U30 kV时α较快增长,但随着电压的继续增加渐趋平缓,α最大提高0.12倍;脉动流和EHD具有一定的复合作用,f对有电场强化条件下的α有较大影响,尤其是当U30 kV时,f越大,相同电场强度下管程α越大,对应的α最高可提高约0.25倍;EHD/脉动流混合作用时,随着Re的增大,管程阻力系数λ逐渐降低,并且在相同的Re情况下,脉动强化措施下的λ要明显高于无脉动情况;同一Re条件下,同频率工况时,电压的增大与否,对λ的改变并无影响,即阻力损耗主要来源于脉动流的作用,而电场对此的影响较小。     

以水为工质的EHD强化管内对流换热实验研究

以水作为工质,实验研究了EHD(Electrohydrodynamics)对水套管式换热器内对流换热过程的强化作用机理。实验中,在水套管换热器换热管中心设置一直流式高压电极,电极电压DC设置范围为0～40 kV,分别进行了5组不同流量下不同电压值的组合强化实验。实验结果表明:不同管内流量条件下电场对管内传热过程均有不同程度的强化作用,当流量为0.1 m3/h时,其电场强化系数θ最大,为1.224;流量为1.0 m3/h时,电场强化系数θ最小。实验证实了电场对于以水为工质的对流传热过程具有强化作用,但电场强化效果具有对流量变化敏感性的特点,同流量下存在最佳强化电压值而非电压值越高强化效果越大。     

管内复合强化传热技术及机理分析

文中对管内强化传热及复合强化传热技术进行了在紊流流动下的阻力和传热特性分析，并给出了几种复合强化传热技术的试验研究结果。提出了螺旋槽带中插入旋向相反的部分管长扭带是行之有效，效果明显的复合强化传热技术。     

强化管内沸腾换热实验研究

主要研究在低过热度下微槽对流动沸腾换热特性的影响，分别以单工质甲醇和甲醇与甲苯的混合物为工质对不同流量情况下光管、直槽管和螺旋槽管的流动沸腾换热特性进行了实验研究。研究结果表明：对单工质甲醇来说，螺旋槽管可以明显起到强化传热作用，而且流量越低，强化传热效果越明显。对混合工质来说，当流量较低时，螺旋槽管强化传热效果不明显，而在流量较高时，强化传热效果比较明显。无论是单工质还是混合工质，直槽管在实验所能达到的壁面温度条件下不能起到明显的强化传热效果。还给出了螺旋槽管强化传热的定性解释。     

管式电滤装备内多场耦合数值研究

为了研究管式电滤装备中内部流场特性及除尘效率,建立气流场、电场和颗粒运动场相互耦合的数学模型,利用Fluent软件,采用Rosin-Rammler颗粒分布模型,讨论多场耦合作用下的EHD流、不同放电电压和气流流速对管式电滤装备内部流场及除尘效率的影响。结果表明:随着气流流速的增大,EHD流对气流场的影响逐渐减小;EHD流的存在对细微颗粒的去除有微弱的影响,对整个管式电滤装备除尘效率的影响可以忽略;除尘效率随着放电电压的增大而增大,但随着气流流速的增大而减小。     

电场对乙醇扩散小火焰的影响

实验研究了各种角度下针形电场和平板电场对乙醇扩散小火焰的结构特性和熄灭特性的影响。结果表明，外加电场的角度对火焰特性有十分重要的作用，针形电场通过离子风的物理过程影响火焰特性，而平板电场通过电场力对火焰中离子的作用来影响火焰，两者有本质区别。与平板电场对比，针形电场产生的离子风对火焰的影响更大。实验结果为掌握电场对液体燃料微尺度扩散火焰的影响规律，用适当的电场来强化微燃烧以节省燃料提供了有益的参考。     

电场作用下冷气泡行为试验研究

气泡的生成、成长、脱离和上升等行为均是决定电场强化传热机理的主要过程,为获得电场作用下气泡生长的动态过程,研究了电场作用下冷态氮气泡的行为特性,利用高速摄像机拍摄了不同电场强度下的气泡生长试验图像,并对气泡脱离形态、周期和速度及加速度进行对比分析。试验结果表明,冷态氮气泡沿着场强方向拉长,呈圆柱体形状脱离壁面,气泡的脱离周期、速度与场强成正比,而加速度与场强成反比。     

管程组合转子传热及阻力特性的实验研究

借助于实验手段,分别以油和水为管程工质,研究了管程组合转子的传热及阻力特性,同时将转子自身结构参数对传热及阻力特性的影响进行了比较分析,分析结果表明,Re数在500～8000的层流区和转捩区内,增大转子螺旋角度对传热强化的贡献显著;Re数在1×104～1×105的旺盛湍流区内,增大转子外径对传热强化的贡献明显,从而为转子组合式强化传热装置的工业应用奠定了基础。     

电晕放电强化气体对流换热研究进展

在气体中施加高压电场后，引起气体放电，产生的电晕风可以强化空气的对流换热。综述了国内外电晕风强化气体对流换热方面的研究进展，概括了以往研究的特点，指出了目前研究所存在的问题，对今后在该领域的研究方向及重点提出了建议。     

Interrelation between configuration of electrodes for induction electric charging and collector current wind EHD power generator

This work aims to realize a wind electrohydro- dynamical (EHD) power generator using induction electric charging with a water jet. The electrodes for induction electric charging are composed of an orifice of which inner radius is 1.5mm and a ring made of a copper wire of which diameter is 1.5mm. It is important to design the electrodes that maximize induction electric charge. In this paper, it is experimentaly clarified that the ring electrode of inner diameter of 3.7cm gives the collector short current of the wind EHD power generator more than that of 4.7cm but has dispersion of the collector short current more than that of 4.7 cm.     

330 kV输电线路均压环优化研究

通过有限元法建立均压环仿真计算模型,对4种不同管径以及2种不同曲率半径330kV交流输电线路均压环表面场强进行计算分析,在特高压户外试验场以及特高压交流试验基地环境气候实验室开展均压环可见电晕试验研究,获得不同结构型式均压环表面电场分布规律、结构优化试验对比结果以及不同海拔条件下均压环电晕起始电压特性曲线。研究结果表明,通过优化均压环管径以及曲率半径方法,可有效降低均压环表面电场强度,提高电晕起始和熄灭电压,推荐校正算式校正误差小于CB311．1—1997和IEC60071—2：1996所提方法,可为高海拔地区输电线路均压环选型及优化设计提供参考依据。     

EHD Augmented Convective Boiling: Flow Regimes and Enhanced Heat Transfer

This work investigates the influence of electrohydrodynamics (EHD) on the flow and heat transfer during convective boiling of HFE7000. A unique tube-and-shell heat exchanger has been constructed with heated water flowing on the shell side and a saturated mixture of refrigerant flowing within the tube side. The heat exchanger is novel in that it allows full visual access to the flow in the inner tube while being both thermally and electrically conductive. This permits observation of the two-phase flow regimes, which is not possible with metallic test sections. In this work the influence of EHD on the flow regimes and subsequent overall heat transfer is investigated for fixed inlet refrigerant mass flux of 100 kg/m²-s, inlet quality of 3%, and wall superheat of approximately 11.5°C. For these conditions the applied voltage across a concentric inner electrode and the outer wall of the tube was varied between 0 kV and 10 kV at 60 Hz AC. In particular, this work focuses on quantifying the level of overall enhancement that is achievable with EHD for this heat exchanger. This is done in the context of the additional heat extracted by the working fluid in the heat exchanger compared with the field-free case and the additional power penalties required to do so. Heat transfer enhancements of up to 1.8 -fold were realized in this heat exchanger. Even so, there were hydraulic power increases as well as electrical power required to achieve the heat transfer enhancement. It was found that the electrical power was the dominant penalty and that an overall enhancement of 40 times more heat power extracted than input required was achieved. Finally, a proportional–integral–derivative (PID) control system has been utilized in conjunction with a high-voltage amplifier in order to accurately control the heat transfer rate of the heat exchanger. To our knowledge this is the first solid-state control system of this type for a two-phase heat exchanger.     

Enhancement of condensation heat transfer by means of EHD condensate drainage

The problem of electrohydrodynamic (EHD) condensation enhancement has been studied very recently. The electric field is commonly generated in such case by means of rod and mesh electrodes. The EHD method is suitable for dielectric media used in refrigeration and heat pump devices. The strong positive EHD effect has been obtained so far for vertical and horizontal bare tubes. However it was found that this effect for horizontal integral-fin tubes, commonly used in heat exchangers, is negligible small. The mechanism of the EHD condensation enhancement is discussed in the paper and a novel arrangement of the tube-electrode system is proposed. It consists of a horizontal finned tube with a rod type electrode placed beneath the tube. The experimental investigations have been carried out for this tube-electrode arrangement using HCFC-123 as a working medium. The obtained results confirmed the expectations since the application of EHD method for the new tube-electrode configuration increased the heat transfer coefficient from 27% to 110%, depending on the electrode potential. A simple model of the EHD condensate drainage has been proposed for the new tube-electrode arrangement.     

Recent progress in electric-field assisted combustion: a brief review

The control of combustion is a hot and classical topic. Among the combustion technologies, electric-field assisted combustion is an advanced techno-logy that enjoys major advantages such as fast response and low power consumption compared with thermal power. However, its fundamental principle and impacts on the flames are complicated due to the coupling between physics, chemistry, and electromagnetics. In the last two decades, tremendous efforts have been made to understand electric-field assisted combustion. New observations have been reported based on different combustion systems and improved diagnostics. The main impacts, including flame stabilization, emission reduction, and flame propagation, have been revealed by both simulative and experimental studies. These findings significantly facilitate the application of electric-field assisted combustion. This brief review is intended to provide a comprehensive overview of the recent progress of this combustion technology and further point out research opportunities worth investigation.     

NUMERICAL MODELING OF ELECTROHYDRODYNAMIC (EHD) EFFECT ON NATURAL CONVECTION IN AN ENCLOSURE

Mathematical and numerical modeling of electrohydrodynamic (EHD) enhancement of natural convection in enclosures is carried out. An electric current in dielectric liquid is modeled as a directed motion of electrically charged particles injected into a neutral fluid; the electric body force and Joule heat are added to the momentum and energy equations, respectively. Based on this, numerical studies are carried out for EHD effects on natural convection in enclosures. It is found that, at the same electric field intensity, the EHD enhancement of heat transfer is different for different electric density injections; applying a nonuniform electric field offers better EHD enhancement of heat transfer than applying a uniform electric field.     

Experimental research on a novel cold storage defrost method based on air bypass circulation and electric heater

Frost accumulation on evaporator decreased cooling capacity and COP (coefficient of performance) of cold storage refrigeration system, so timely and effective defrost was significant to cold storage energy-saving operation. The EHD (electric heat defrosting) method was commonly used in cold storages, however, defrost efficiency of the traditional EHD is rather low, and defrost operation usually caused an unfavorable storage temperature fluctuation, which is harmful to storage quality and shelf-life of stored products. In order to solve the problems existing in the traditional EHD method, a novel defrost method with air bypass circulation and electric heater was proposed for the first time in this paper. Five practical cases of this new method with different defrost heaters and air circulation modes were comparatively studied. The results showed that the case with heater embedded in evaporator fins and air circulating through bypass channel was the optimum implementation way of the new method. Compared with the traditional EHD method, the defrost time of this new method was shortened by 62.1%, defrost energy consumption was reduced by 61.0%, and storage temperature fluctuation was decreased by 70.1%. In addition, the defrost efficiency was up to 77.6%, which was 2.93 times of the traditional EHD method.