运行工况对圆形楞涡流发生器CaSO4污垢特性的影响

为了研究运行工况对圆形楞涡流发生器CaSO4污垢特性的影响。采用数值模拟方法研究了布置圆形楞涡流发生器矩形通道内壁面CaSO4析晶污垢的沉积过程。主要分析了CaSO4溶液的浓度、壁面温度、入口速度和入口温度对污垢沉积率、剥蚀率和污垢热阻的影响。结果表明,随入口速度的增大沉积率和剥蚀率均增大,而污垢热阻值降低。随着壁面温度的增大沉积率、剥蚀率和污垢热阻均增大。随工质浓度的增大沉积率、剥蚀率和污垢热阻也是均增大。随入口温度的增大沉积率、剥蚀率和污垢热阻却基本不变。     

不同工况下冲孔矩形翼涡流发生器的纳米氧化镁颗粒污垢特性

为了研究不同工况情况下冲孔矩形翼涡流发生器的纳米氧化镁颗粒的污垢特性,通过实验对比了冲孔矩形翼涡流发生器和未冲孔矩形翼涡流发生器的污垢特性,探讨了水浴温度、工质质量浓度及工质流速对颗粒污垢的影响。实验结果表明:相同工况下,冲孔矩形翼涡流发生器较未冲孔涡流发生器具有更优的抑垢效果;随着水浴温度的升高,污垢热阻渐近值增加,而且结垢速率也增大;污垢热阻渐近值随着工质质量浓度的增加而增大,结垢速率有略微提升;随着工质流速的增大,污垢热阻渐近值和结垢速率均降低。     

涡流发生器表面参量与析晶污垢热阻间的关联分析

为研究换热表面上涡流发生器的流动参数和结构参量对析晶污垢的影响,并比较各类参数的关联程度。通过数值计算的方式,得出在不同入口速度、壁面温度和工质质量浓度下污垢热阻的变化情况。并得出在不同楞长、排列间距和半径下污垢热阻的变化情况。并根据灰色关联原理分析三个流动参数和三个结构参数与污垢热阻的关联程度。经分析其中两种流动参数对污垢热阻的影响高于所研究的结构参数的影响,但污垢热阻与楞长的关联度高于工质质量浓度的关联度。     

矩形通道中不同楞型涡流发生器的传热与阻力特性

在雷诺数Re为8900~9900的范围内,通过数值模拟方法对安装有相同迎流截面积的圆形楞、矩形楞和三角楞等5种涡流发生器的矩形通道进行传热和流阻特性的研究。结果表明:相同Re下,顺直三角楞的强化传热效果最好,传热强度比矩形光通道高40.65%~75.74%。但是相应的其流动阻力也是最大的,比矩形光通道高263.03%~376.04%。当以R=(j/j_0)/(f/f_0)作为综合特性评价标准时,圆形楞的综合换热性能最好,且最多可比顺直三角楞高52.11%。综合考虑,可以认为圆形楞涡流发生器是一种低阻高效的新型涡流发生器。     

圆形楞涡流发生器对矩形通道传热和流阻特性的影响

提出一种应用于太阳能换热器的新型圆形楞型涡流发生器,采用数值模拟的方法对安装有圆形楞涡流发生器的矩形通道进行传热和流阻特性的研究。分析在不同Re下,圆形楞涡流发生器的楞长、布置方式、半径的大小和排列间距等几何参数对矩形通道的传热和流阻特性的影响。研究结果表明,圆形楞涡流发生器顺排靠边布置时,其综合特性R值最大;随着楞长的增加圆形楞涡流发生器的综合特性R值先增后减,且在4H/8楞长处出现最大值;随着半径的增加,圆形楞涡流发生器的综合特性R值逐渐减小,半径为2.5 mm时综合特性R值最大;通过对综合特性R值以及出口温度和进出口总压降的分析,可认为圆形楞涡流发生器的最佳间距为30~40 mm。     

圆管内三角翼涡流发生器CaCO3污垢特性

采用CFD软件模拟研究管内三角翼涡流发生器CaCO_3污垢特性,考察涡流发生器排列间距、攻角、翼长及夹角对污垢热阻的影响。结果表明,在涡流发生器翼片两侧产生2个对称的运动方向相反的涡旋,且装有涡流发生器的强化管的污垢热阻小于光管的污垢热阻,说明涡流发生器对壁面边界层的扰动可有效抑制污垢的沉积。渐近污垢热阻随间距的增加而增大,且增速逐渐变缓;渐近污垢热阻在攻角小于90°时,随攻角的增加而增大,在攻角大于90°时,随攻角的增加而减小;渐近污垢热阻随翼长的增加而减小,且减小的速度逐渐增大;渐进污垢热阻随夹角的增加而减小,且减小趋势逐渐变缓。     

两种柱形涡流发生器CaSO_4析晶污垢特性实验研究

为得到涡流发生器污垢规律,采用质量浓度为2 100mg/L的硫酸钙过饱和溶液进行了传热过程中的污垢生成实验,并通过离线称重法得到了装有涡流发生器试片表面单位面积污垢沉积量生长曲线.结果表明：涡流发生器直径和排列间距对壁面污垢沉积量有显著影响.当涡流发生器直径（4mm）一定时,试片表面单位面积污垢沉积量均随着涡流发生器排列间距的增大而增加;在排列间距（10mm）一定时,污垢沉积量随着涡流发生器尺寸的增大而减少.     

板式换热器颗粒污垢特性的实验研究

以纳米氧化镁颗粒溶液为实验工质,进行了板式换热器颗粒污垢特性的实验研究,分析了颗粒质量浓度、颗粒粒径、流速和低温介质温度对颗粒污垢热阻的影响.结果表明:板式换热器颗粒污垢无明显诱导期存在,结垢速率和污垢热阻渐进值均随颗粒质量浓度的增大而增大,且增大幅度逐渐减小;颗粒粒径对污垢热阻的影响较明显,在相同质量浓度下,颗粒粒径越小,结垢速率越快,且污垢热阻越大;流速对污垢热阻的影响较为复杂,高流速下的结垢速率略大于低流速下,且高流速下达到稳定时的污垢热阻渐进值小于低流速下;低温介质温度对颗粒污垢热阻的影响不明显.     

矩形通道内冲孔矩形小翼涡流发生器的流动换热特性

雷诺数Re=214～10 703时,通过数值模拟方法对布置有冲孔和无孔的两种矩形小翼涡流发生器的矩形通道进行了传热和流阻特性的研究。计算结果表明:在低雷诺数下,冲孔矩形小翼涡流发生器的传热因子j值与无孔矩形小翼涡流发生器相差不大,而在高雷诺数下,冲孔涡流发生器的传热因子j值略低于无孔涡流发生器,大约低1.03%～3.05%。在相同的雷诺数下,无孔矩形小翼涡流发生器的阻力因子f大于冲孔涡流发生器,而且随着雷诺数的增大二者的差距也越来越大。通过对比综合性能指标可知,两种通道的综合性能指标均随着雷诺数的增加而减小,而且冲孔矩形小翼涡流发生器的综合性能要优于无孔矩形小翼涡流发生器。     

矩形通道中低肋涡流发生器抑垢效果分析研究

为了研究矩形通道内低肋涡流发生器抑垢效果特性,本文选用粒径为50 nm的MgO颗粒进行实验分析。通过安装低肋涡流发生器前后污垢热阻随时间的变化对比,改变低肋涡流发生器的高度、排列间距以及前后排列布置方式,对低肋涡流发生器的抑垢效果进行研究分析。实验结果表明:低肋涡流发生器以较小的压力损失作为代价,换取了较好的抑垢效果;低肋涡流发生器越高,抑垢效果越好;减小涡流发生器的间距,能够有效的抑制污垢沉积;前排列布置有更好的抑垢效果。     

Numerical investigation and analysis of heat transfer enhancement in channel by longitudinal vortex based on field synergy principle

3-D numerical simulations were presented for laminar flow and heat transfer characteristics in a rectangular channel with vortex generators. The effects of Reynolds number (from 800 to 3 000), the attack angle of vortex generator (from 15° to 90°) and the shape of vortex generator were examined. The numerical results were analyzed based on the field synergy principle. It is found that the inherent mechanism of the heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, that is, the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. The longitudinal vortex improves the field synergy of the large downstream region of longitudinal vortex generator (LVG) and the region near (LVG); however, transverse vortex only improves the synergy of the region near vortex generator. Thus, longitudinal vortex can enhance the integral heat transfer of the flow field, while transverse vortex can only enhance the local heat transfer. The synergy angle decreases with the increase of Reynolds number for the channel with LVG to differ from the result obtained from the plain channel, and the triangle winglet performs better than the rectanglar one under the same surface area condition.     

Numerical investigation and analysis of heat transfer enhancement in channel by longitudinal vortex based on field synergy principle

3-D numerical simulations were presented for laminar flow and heat transfer characteristics in a rectangular channel with vortex generators. The effects of Reynolds number (from 800 to 3 000), the attack angle of vortex generator (from 15° to 90°) and the shape of vortex generator were examined. The numerical results were analyzed based on the field synergy principle. It is found that the inherent mechanism of the heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, that is, the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. The longitudinal vortex improves the field synergy of the large downstream region of longitudinal vortex generator (LVG) and the region near (LVG); however, transverse vortex only improves the synergy of the region near vortex generator. Thus, longitudinal vortex can enhance the integral heat transfer of the flow field, while transverse vortex can only enhance the local heat transfer. The synergy angle decreases with the increase of Reynolds number for the channel with LVG to differ from the result obtained from the plain channel, and the triangle winglet performs better than the rectanglar one under the same surface area condition. __________ Translated from Journal of Xi’an Jiaotong University, 2006, 40(9): 996–1000 [译自: 西安交通大学学报]     

不同纵向涡发生器流动传热的数值模拟

利用三维数值模拟的方法对带有3种异形纵向涡发生器的H型翅片椭圆管换热器的空气侧流动传热特性进行研究。基于H型翅片椭圆管束,讨论了在不同雷诺数下,纵向涡发生器的摆放位置、摆放攻角和形状对空气侧流动传热的影响。研究表明:纵向涡发生器能够将高能量的流体引向流速较低的壁面区域,使冷热流体之间的混合加剧,增强流体的湍流动能,进而达到强化传热的效果;与无纵向涡发生器的管束相比,带纵向涡发生器管束的传热效果有明显的提高;当纵向涡发生器后置时,换热器的传热效果最优;在雷诺数相同,攻角为30°时,流体的传热性能和阻力特性均达到最优;相同攻角摆放时,椭圆角矩形发生器的传热性能和阻力因子均优于其他两种形式的发生器。研究结果为烟气余热回收系统换热器传热性能强化提供理论依据。     

运行工况对多向扰流强化管CaSO4污垢特性的影响

为更深入并准确研究运行工况条件对多向扰流强化管CaSO_4污垢特性的影响,基于FLUENT软件的UDF功能构建了恒壁温条件下结垢传质过程与温度场的耦合作用关系,进一步采用田口法对运行工况致垢的贡献率进行了模拟比较,分析了贡献率较大的运行工况对污垢特性的影响。结果表明:溶液溶度致垢的贡献率占53.2%,而壁面温度、进口流速和进口温度的贡献率分别为22.2%、19.3%和5.3%;溶液溶度在4.0～2.5 kg/m~3,污垢热阻降低达90.47%,并且随溶度降低其相邻溶度间降低比例基本不变;壁面温度在340.0～315.0 K时,污垢热阻降低了65.22%,在前一阶段相邻温度间降低比例基本上不变,当达到320.0 K后降低明显;流速在1.0～2.5 m/s时,随流速的增加,污垢热阻降低68.65%,且随流速的增加,相邻流速间降低的速度明显减缓。     

基于格子Boltzmann方法的微通道受热表面析晶污垢模拟

针对现有的污垢析晶沉积模型不能有效模拟真实污垢生长的问题,建立了一种引入析晶沉积动力学模型的多物理场耦合数值模型。模型基于格子Boltzmann方法和有限差分方法,模拟了微通道非等温热表面上近壁面处的沉积物溶质质量浓度分布和污垢生长过程,研究了流速、壁温和沉积物溶质质量浓度对微通道热表面污垢析晶沉积的影响。结果表明：沉积初始时刻流速和壁温对近壁面沉积物溶质质量浓度分布具有不同程度的影响,随着污垢不断生长,污垢-流体界面处的析晶沉积速率减小;相比于流速,沉积物溶质质量浓度对污垢热阻的影响更为显著。     

两种波纹通道内周期性充分发展对流换热的数值研究

应用数值方法研究了流体在正弦型和三角型两种波纹通道内周期性充分发展的层流流动与换热特性,分析了恒壁温条件下,雷诺数Re对流动与换热性能的影响,并对两种通道的摩擦阻力系数厂与努谢尔特数Nu分别进行了比较。结果表明：两种通道流动阻力的相对大小因雷诺数Re的大小而异;正弦型波纹通道的换热速率比三角型通道的换热速率大,因此换热器通道表面的选择应综合考虑。     

Numerical study on laminar convection heat transfer in a channel with longitudinal vortex generator. Part B: Parametric study of major influence factors

This paper presents the influences of main parameters of longitudinal vortex generator (LVG) on the heat transfer enhancement and flow resistance in a rectangular channel. The parameters include the location of LVG in the channel, geometric sizes and shape of LVG. Numerical results show that the overall Nusselt number of channel will decrease with the LVGs’ location away from the inlet of the channel, and decrease too with the space between the LVG pair decreased. The location of LVG has no significant influence on the total pressure drop of channel. With the area of LVG increased, the average Nusselt number and the flow loss penalty of channel, especially when β = 45° will increase. With the area of LVG fixed, increasing the length of rectangular winglet pair vortex generator will bring about more heat transfer enhancement and less flow loss increase than that increasing the height of rectangular winglet pair vortex generator. With the same area of LVG, delta winglet pair is more effective than rectangular winglet pair on heat transfer enhancement of channel, and delta winglet pair-b is more effective than delta winglet pair-a. Delta winglet pair-a results in a higher pressure drop, the next is rectangular winglet pair and the last is delta winglet-b. The increase of heat transfer enhancement is always accompanied with the decrease of field synergy angle between the velocity and temperature gradient when the parameters of LVG are changed. This confirms again that the field synergy is the fundamental mechanism of heat transfer by longitudinal vortex. The laminar heat transfer of the channel with punched delta winglet pair is experimentally and numerically studied in the present paper. The numerical result for the average heat transfer coefficient of the channel agrees well with the experimental result, indicating the reliability of the present numerical predictions.     

Numerical study on laminar convection heat transfer in a rectangular channel with longitudinal vortex generator. Part A: Verification of field synergy principle

This study presents numerical computation results on laminar convection heat transfer in a rectangular channel with a pair of rectangular winglets longitudinal vortex generator punched out from the lower wall of the channel. The effect of the punched holes and the thickness of the rectangular winglet pair to the fluid flow and heat transfer are numerically studied. It is found that the case with punched holes has more heat transfer enhancement in the region near to the vortex generator and lower average flow frictional coefficient compared with the case without punched holes. The thickness of rectangular winglet can cause less heat transfer enhancement in the region near to the vortex generator and almost has no significant effect on the total pressure drop of the channel. The effects of Reynolds number (from 800 to 3000), the attack angle of vortex generator (15°, 30°, 45°, 60° and 90°) were examined. The numerical results were analyzed from the viewpoint of field synergy principle. It was found that the essence of heat transfer enhancement by longitudinal vortex can be explained very well by the field synergy principle, i.e., when the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient, the heat transfer in the present channels will be enhanced. Longitudinal vortices (LVs) improve the synergy between velocity and temperature field not only in the region near LVG but also in the large downstream region of longitudinal vortex generator. So LVs enable to enhance the global heat transfer of channel. Transverse vortices (TVs) only improve the synergy in the region near VG. So TVs can only enhance the local heat transfer of channel.