| 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. |
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