Research On Heat Transfer Enhancement Mechanism And Heat Transfer Characteristics Of Spiral Fine Ribs In Plate Channel

In facing of the tensions of energy shortage and environment pollution, the en-ergy conservation and emission reduction has become a focal issue concerned by the world. Consequently, the development of energy efficient heat transfer enhancement technologies has a very important significance to energy conservation and energy ef-ficiency improvement. Based on the analysis of heat transfer enhancement for low Reynolds number turbulent flows, a new method for heat transfer enhancement by using spiral fine ribs (SFRs) in plate channel is proposed, and detailed research has been conducted numerically and experimentally to investigate the heat transfer en-hancement mechanism.From the analysis of low Reynolds number convection heat transfer, it can be found that the thermal resistance not only exists in the near wall region but also in the mainstream because the temperature drop appears on the whole section of the channel. Therefore, just keep disturbance to the fluid in the near wall region is not enough. It needs more disturbances to the mainstream, so as to make the temperature distribution more even in it. The equally spaced SFRs in the channel can form a packing layer which resemble a kind of quasi-porous media with large porosity, and can produce efficient disturbance both to the boundary layer and the mainstream.Based on the above analysis, the mechanism of heat transfer enhancement caused by SFRs is studied numerically. The results show that the spiral surface of the SFRs can produce an effective guidance to the fluid that flow around its rib wires and in-duce a series of longitudinal vortices in the channel including leading eager longitu-dinal vortex, mainstream longitudinal vortices, near wall longitudinal vortices and rear edge central longitudinal vortex. The multi-longitudinal vortices induced by SFRs can significantly increase the tangential velocity components in the cross sec-tion to reach above20%of the average velocity of the mainstream, which is helpful to promote the micro-fluctuation in the fluid. Besides, the transport action caused by the longitudinal vortices can improve the mass exchange between the boundary layer and the mainstream. The above factors not only speed up the heat migration from the channel walls, but also enhance the heat diffusion in the mainstream. This improves the temperature distribution uniformity in the channel. As the near wall longitudinal vortices can transport the mainstream fluid directly to the wall, they produce effec-tively disturbance to the boundary layer and lead to sharp increase of temperature gradients in the near wall region, which can be an order of magnitude higher than that of the smooth channel.The analysis based on field synergy principle shows that the areas with higher absolute cosine value of field synergy angle are greatly affected by the longitudinal vortices, therefore they not only appears in the near wall region but also in the central region of channel. It indicates that the multi-longitudinal vortices induced by SFRs can improve the field synergy performance in the whole channel that leads to a great increase of face average of field synergy angle cosine value, so it is favorable for the improvement of overall heat transfer performance of the channel.Experimental system for flow visualization and particle image velocimetry (PIV) test is set up. On this platform, the PIV experiments are performed to investigate the flow structure induced by the SFRs in the channel. Through the analysis it can be concluded that multi-longitudinal vortices are induced downstream of the transversely placed SFRs and form a symmetrical vortex array along the horizontal central line of the transverse section. This can induce fierce turbulent diffusion in the channel at rel-atively low Reynolds numbers.As developing along the mainstream, the longitudinal vortices effectively inten-sify the continuous disturbance to the boundary layer, which significantly improve the velocity profile downstream of the SFRs. While the Reynolds number ranges between225and2022it leads to an increase of near wall velocity more than60%compared with the smooth channel and the velocity components in wall normal direction can reach to16%-20%of the mainstream average velocity. With the increase of Reynolds number, the induced longitudinal vortices gain strength and become straighter and closer to the channel walls, and consequently increase the turbulence intensity in the near wall region. From comparison under the same Reynolds numbers it can be found that the simulation results agree well with the experimental data.In the uniformly heated plate channel that filled with SFRs the convention heat transfer performance of air is investigate. The experimental results show that the heat transfer can be enhanced significantly with SFRs filled in the channel, and the heat transfer coefficient has increased by1～1.5times compared with the smooth channel. The heat transfer decreases with the increase of space intervals, but changes little with the variation of spiral pitches that involved in the study. The tests of flow resistance show that the friction factor will decrease with the increase of the porosity in the channel. Additionally, at high Reynolds numbers and approximate porosities the con-dition with lager space interval gets a smaller friction factor.The SFRs plate heat exchanger is designed and heat transfer experiments are car-ried out under the conditions of water-to-water and water-to-oil respectively on the comprehensive heat transfer experimental platform. Filling SFRs in plate channel is experimentally proved to be an effective way for heat transfer enhancement. Intensive fluid mixture can be achieved in the channel under relatively low Reynolds numbers and eliminates the outlet temperature fluctuation of the heat exchange that caused by the nonuniformity of heat transfer in the channel. When water is used as the working medium, heat transfer rate can be raised by50～190%with the SFRs filled in the channel. Considering flow resistance the total heat transfer performance can improve35～130%than that of the smooth channel within the Reynolds number range of800to13800and shows higher improvement at low Reynolds numbers. While oil is used as the working medium, a approximate improvement of heat transfer rate and total heat transfer performance are achieved at a much lower Reynolds number range of50to800, which are40～160%and20～150%respectively. Therefore, it confirms that the SFRs can effectively enhance the convection heat transfer at lower Reynolds numbers, especially for high viscosity fluid. Based on the experimental data, the cor-relation to estimate Nusselt number and friction factor has been proposed and can serve as references for the design and calculation of this kind of plate heat exchanger.