【作者】 宋克伟；

【导师】 王良璧；

【作者基本信息】 兰州交通大学 ， 车辆工程， 2014， 博士

【摘要】 不断改进和发展新的强化传热技术不仅有助于提高紧凑式换热器的性能而且是节能的必然要求。含有纵向涡的流动是流体动力学和传热中的一个重要物理现象,是一种有效的强化换热流动结构。纵向涡是指涡的旋转轴与流动方向或主流方向平行的涡,因其能以相对较小的阻力损失获得较大的强化传热效果,在管翅换热器翅片强化传热技术中被广泛应用。纵向涡的强化传热效果与纵向涡的强度有关,由于纵向涡在传输过程中强度不断衰减,为提高换热器通道内纵向涡的强度,通常要按一定规律布置多个纵向涡产生器。涡产生器布置方式不同时,在通道内形成纵向涡的旋转方向也不同。当通道内纵向涡间距离较近时,相同旋转方向和相反旋转方向纵向涡之间就存在不同程度的相互干涉,干涉影响到纵向涡的强度及其强化传热效果。由于纵向涡受到涡产生器的形状、参数、布置方式、攻击角、翅片间距等较多参数的影响,因此纵向涡运动较为复杂。一直以来对纵向涡的研究主要集中在应用上,而对纵向涡干涉流动结构、纵向涡干涉对纵向涡强度和强化传热的定量研究较少。较早研究中由于缺乏纵向涡强度描述参数,关于纵向涡应用的研究大多忽略了纵向涡之间的干涉,使得对涡干涉的研究主要停留在实验观察和定性分析层面上。因此,研究纵向涡干涉对流动与传热的影响有助于对涡干涉及其强化传热机理的深入认识,对进一步提高换热器的性能具有重要意义。本文首先对二次流强度无量纲数Se进行了定义并分析了Se描述二次流强度的普遍适用性,定义了用于描述纵向涡间干涉的参数--干涉系数Ir;然后以机车车辆中采用涡产生器强化传热技术的换热器为研究对象,通过Se定量的研究了换热通道内旋转方向相反、旋转方向相同的两个纵向涡之间的干涉以及扁管换热通道内多个旋转方向相同、相反纵向涡之间的干涉现象;对相同、相反旋转方向纵向涡间干涉对流动结构、纵向涡强度、强化传热、流动阻力的影响以及纵向涡干涉与Ir之间的关系进行了详细的分析,对纵向涡间干涉现象有了新的认识。本文研究结果表明:(1)来流中两个纵向涡相互干涉时,旋转方向相反纵向涡的稳定性较好,纵向涡干涉时未发生融合。而旋转方向相同的两个纵向涡相互干涉时,纵向涡稳定性较差,在纵向涡间干涉时发生融合现象,干涉后纵向涡融合为一个纵向涡。(2)涡产生器间距相同时,旋转方向相同、相反纵向涡之间的干涉程度不同,而旋转方向相反与旋转方向相同纵向涡间完全干涉时所对应的涡产生器间距以及纵向涡间干涉对纵向涡强度以及换热强度的影响也不同。(3)旋转方向相反纵向涡发生部分干涉并形成明显的向壁型流场时,纵向涡间干涉有利于翅片表面传热。旋转方向相反纵向涡干涉后形成的流场结构不同时,流场结构对翅片表面强化传热的影响也不相同。相反旋转方向纵向涡干涉后诱导纵向涡间流体流向下翅片表面时,有利于下翅片表面换热,而当相反旋转方向纵向涡干涉后诱导流体离开下翅片表面时,不利于下翅片表面换热但有利于上翅片表面换热。旋转方向相同的纵向涡间干涉形成的流场结构不利于翅片表面换热。(4)纵向涡间发生完全干涉时,纵向涡干涉不利于翅片换热。相同、相反旋转方向纵向涡间完全干涉时对Se,Nu和f的影响程度不同。在旋转方向相反的两个纵向涡间完全干涉时,计算区域内平均纵向涡强度Se最大衰减超过40%,Nu和f分别减小6%和2%。而旋转方向相同的两个纵向涡间完全干涉时,计算区域内平均纵向涡强度Se最大减小18%,平均Nu减小不足2.5%,而f基本不变。(5)旋转方向相反的纵向涡间干涉对等流量、等压降和等泵功条件下的综合评价准则JF的影响最大值分别达到5.0%,6.0%和6.3%。而旋转方向相同纵向涡干涉对等流量、等压降和等泵功条件下的综合评价因子JF的影响很小,最大变化量分别为2.0%,2.3%和2.0%。(6)对于所研究的扁管换热器结构,扁管同侧涡产生器产生的纵向涡完全干涉后融合为一个纵向涡,而不同管排周围涡产生器产生的旋转方向相反的纵向涡干涉后形成不同流场结构。涡产生器间距变化对不同管排周围旋转方向相反的纵向涡间干涉的影响最大。在旋转方向相反纵向涡间完全干涉时,Se衰减13%,Nu减小10%,f减小7.6%(7)扁管换热器通道内旋转方向相反纵向涡间完全干涉时,对应的涡产生器间距随着涡产生器攻击角的增大而减小。通道内换热强度与纵向涡强度有关,在翅片间距、涡产生器高度、攻击角、涡产生器间距变化时,Nu与Se之间存在线性对应关系,Nu与拟合式之间的最大误差小于5%。扁管换热器通道内二次流强度决定换热强度。(8)纵向涡干涉系数Ir可用于描述纵向涡间的干涉程度。Ir=0时,纵向涡间完全干涉,Se,Nu和f都达到最小值;在|Ir|<2时,纵向涡间的干涉比较显著;在|Ir|大于2时,纵向涡间的干涉比较小。而在1<|Ir|<3时,相反旋转方向纵向涡干涉对应的Nu要大于相同旋转方向纵向涡干涉对应的Nu值,且在Ir=±2时,Nu的值最大。在所研究扁管换热器通道内,纵向涡干涉系数均小于2,涡产生器间距变化对纵向涡间干涉的影响比较大。更多还原

【Abstract】 Developing and innovating new techniques to enhance the heat transfer of a new compact heat exchanger is not only useful but also necessary for energy saving. The flow with longitudinal velocity components is an important phenomenon in fluid dynamics and heat transfer. The longitudinal vortex has the rotating axis parallel to the main flow direction and is the typical presentation of secondary flow. Longitudinal vortices are generated by flow separation along the side edges of the VGs due to the pressure differences between the upstream and downstream sides, and are perpendicular to the main flow direction. The flow with longitudinal velocity components is an important phenomenon in fluid dynamics and heat transfer. The longitudinal vortices can cause bulk fluid mixing, boundary–layer modification, flow destabilization, and thereby potentially enhance convective heat transfer with small pressure loss penalty. Setting protrusions that can generate longitudinal vortices on the fin surface is a promising technique to enhance the airside heat transfer. There are many protrusions that can generate longitudinal vortices. Vortex generators(VGs) are among the most popular actuators for the fin-side heat transfer enhancement. In order to obtain a better heat transfer performance, researchers always try to punch lots of vortex generators out of the fin surface. But the increasing of the number of VGs is not necessarily linked with the rise in heat transfer performance augmentation. This is because the vortices not only change the boundary layer structure but also can interact with each other when they met in the flow channel and the interaction of vortices affects the intensity of vortices and their effect on heat transfer enhancement.As the longitudinal vortices are affected by many factors, such as the fin spacing, the shape, the attack angle and the arrangement of the VGs, thus the longitudinal vortex flow is much complicated. The application of longitudinal vortices for heat transfer enhancement has been carried out by many researchers, but seldom works consider the interaction between the longitudinal vortices and their effect on heat transfer. Although few studies on the interaction of the longitudinal vortices has been carried out, these few studies mainly stay in the level of experimental observation and qualitative analysis, quantitative study of the interaction of longitudinal vortices was seldom reported due to the lack of parameter which can define the intensity of the longitudinal vortices. Therefore, the study about the interaction between the longitudinal vortices will contribute to the understanding of the mechanism of interaction of longitudinal vortices and their effect on heat transfer and fluid flow, and has great significance for improving the performance of the heat exchangers.For the plate-fin and tube-fin heat exchangers with VGs, there always many rows of VGs on the fin surfaces, interaction between these vortices that generated by different VGs will be a common physics phenomenon. This thesis focuses on the quantitative study of interaction between two counter-rotating and co-rotating longitudinal vortices with different transversal pitches, the effects of interaction of longitudinal vortices on the intensity of vortices, the flow structure, the heat transfer enhancement and pressure losing are also carried out. The main results are as following:(1) The stability of the longitudinal vortices with counter-rotating directions is well when they interact with each other, and the counter-rotating vortices keep rotating around their axes respectively. But the co-rotating longitudinal vortices have bad stability when they interact with each other, the longitudinal vortices merge into a new vortex when the longitudinal vortices have overlapping flow area.(2) Close proximity of other vortices strongly affects the spreading of the vorticity. The degree of interaction between the counter-rotating longitudinal vortices is different from that between the co-rotating longitudinal vortices even though under the same transversal pitch of VGs.(3) The interaction between counter-rotating vortices not necessarily decreases the heat transfer performance of the longitudinal vortices. The flow field structure formed by the interaction of longitudinal vortices affects the heat transfer performance of longitudinal vortices. The partial interaction of longitudinal vortices is benefit to the heat transfer enhancement when there has common flow region between the vortices. In the region where two neighboring vortices induced flow toward the heat transfer surface, local heat transfer was locally enhanced. Conversely, in the regions where neighboring vortices induced outflow departs the heat transfer surface, the local heat transfer was decreased. The flow field formed by the co-rotating longitudinal vortices is not benefit to the heat transfer of the fin surface.(4) The interactions between the counter-rotating longitudinal vortices and the co-rotating longitudinal vortices have different effects on the intensity of longitudinal vortices(Se), Nusselt number(Nu) and friction factor(f). When the counter-rotating longitudinal vortices are fully interacted, the maximum decreasing percentage of Se is about 40%, and for Nu and f it is about 6% and 2%, respectively. But for the case that the co-rotating longitudinal vortices are fully interacted, the maximum decreasing percentage of Se is 18%, and for Nu it is only about 2.5%, the value of f remains essentially unchanged.(5) Considering the effects of interaction of counter-rotating longitudinal vortices on the heat transfer enhancement evaluation criteria JF under conditions that with equal flow rate, pressure losing and pump power, the maximum decreasing percentage of JF are 5.0%, 6.0% and 6.3%, respectively. But the effect of the interaction of co-rotating longitudinal vortices on the criteria JF is small, and the maximum decreasing values are 2.0%, 2.3% and 2.0%, respectively.(6) For the flat tube bank fin heat exchangers, the longitudinal vortices generated by the VGs located on the same side of the tube have the same rotating directions and merged into a new longitudinal vortex when they fully interacted with each other. Different flow field structures are formed by the interaction of counter-rotating longitudinal vortices which are generated by the VGs around different tube rows. The changing of the transversal pitches of VGs mainly affects the interaction of counter-rotating longitudinal vortices around different tube rows. The maximum decreasing percentage of Se is 13%, for Nu it is about 10% and for f the value is 7.6%.(7) The heat transfer performance not only depend on the intensity of the vortices but also dependent on the structure of the vortices. But the flow structure has a relatively small effect compared with the intensity of the longitudinal vortices. For the flat tube bank fin heat exchangers, the heat transfer modification produced by the vortex was strongly depended on interaction of vortices. For the flat tube bank fin heat exchangers with VGs, good linear relationship exists between averaged values of Se and Nu. The maximum difference between the numerical results and the linear fitting formula is less than 5%, the intensity of secondary flow determines the intensity of heat transfer.(8) An interaction factor Ir is defined to evaluate the interaction between the vortices. When Ir=0, no matter the counter-rotating vortices or the co-rotating vortices, the longitudinal vortices fully interacted and the values of Se, Nu and f all reach the minimum values. When Ir>0, there has a common-flow-up region between the counter-rotating vortices and there has a common-flow-down region between the vortices when Ir<0. When the absolute value of Ir is less than 2, |Ir|<2, the interaction between the longitudinal vortices is obvious, and the interaction between the longitudinal vortices is very weak when |Ir|>2. When 1<|Ir|<3, the value of Nu for the counter-rotating longitudinal vortices is larger than the value of Nu for the co-rotating longitudinal vortices. For the flat tube bank fin heat exchanger studied, the value of Ir is less than 2 and the changing of pitches between vortex generators has obvious effect on the interaction between the longitudinal vortices.更多还原