| The spiral deformation tube (SDT) that made of a circular tube by plastic deformation can induce spiral flow and generate a secondary flow in the direction normal to main stream in tube, which enhances the flow mixing. Meanwhile, fluid outside tube mainly flows longitudinally accompanied by disturbance of flow rotation, separation and mixing. The SDT is an efficient heat transfer element, however, the complexity of the flow and heat transfer process in tube and outside tube leads to the investigation is still not perfect. For this reason, combining theoretical analysis, numerical simulation and experimental method, this paper studies the heat transfer and flow resistance characteristics of SDTs and analyzes the heat transfer enhancement mechanism inside tube and the flow resistance reduction mechanism outside tube, and also evaluates the energy saving effect. The main work and results are as follows:(1) Field synergy principle was employed to analyze the heat transfer enhancement mechanism inside a SDT and the flow resistance reduction mechanism in the SDT bundle under single phase convective heat transfer condition. The longitudinal vortex in a SDT caused by the special tube wall, had higher vortex intensity than that in a circular tube by almost four orders and didn’t decay along the flow direction. The longitudinal vortex made the fluid have larger velocity component in direction normal to the heat transfer surface, which improve the synergy degree between the velocity field and temperature field to enhance the heat transfer. On the other hand, fluid in the SDT bundle flowed longitudinally mainly, the synergy performance of velocity field and pressure field was better than the common circular tube segmental baffle bundle, the flow direction and the driving force direction were more consistent, thus the flow resistance decreased obviously.(2) Combining numerical simulation and experiment, the heat transfer and flow resistance properties in SDTs were researched. Turbulence was generated in a SDT at lower Reynolds number and therefore the laminar-turbulent transition was not obvious. When the Reynolds number was less than10000, the heat transfer enhancement performance of SDTs was better. The Nusselt number could be3.8times of the circular tube, but the friction factor f was also increased largely and could be3.7times of the circular tube. There was also a maximum comprehensive performance evaluation factor η larger than2.0, which prove the SDT was more suitable for the low flow velocity operation. With the increase of Prandtl number, the Nusselt number increased and the heat transfer enhancement effect increase, the friction factor did not change and the comprehensive performance evaluation factor increased, which prove the SDT was more suitable for the mediums with larger Prandtl numbers. With the reduction of spiral pitch S or the increase of aspect ratio A/B, both the Nusselt number and friction factor increased and the later was more obvious. The comprehensive performance evaluation factor was less affected by the spiral pitch, but improved slightly with increase of the aspect ratio. According to the research results, the structure parameters of SDT were optimized, and the correlations for the calculation of Nusselt number and friction factor were derived.(3) The heat transfer and flow resistance properties of the SDT bundles were researched using numerical method. The flow resistance of the SDT bundle was significantly less than that of the circular tube segmental baffle bundle, and the heat transfer coefficient per unit pressure drop of the SDT bundle could be20times of the latter. With the increase of Prandtl number, the heat transfer ability strengthened but the friction factor did not change and the comprehensive performance of the SDT bundle weakened. When the Prandtl number was less than30, the comprehensive performance enhancement of the SDT bundle was more apparent relative to the circular tube bundle. Decreasing the spiral pitch, the increase of Nusselt number (less than10%) was far less than the increase of friction factor (about80%) for a SDT bundle. The SDT bundle with lager spiral pitch had better comprehensive performance. If the diameter of the bundle shell was reduced while the aspect ratio A/B decreased, the compactness of the bundle could be ensured to improve the heat transfer ability of the bundle. According to the research results, the structure parameters of SDT bundle were optimized, and the correlations for the calculation of Nusselt number and friction factor were derived.(4) The condensate film thickness and heat transfer coefficient computing models for the steam condensation outside an elliptical tube with an arbitrary inclined long axis were developed, and the correlation for the heat transfer coefficient for steam condensation outside the SDT was obtained. The theoretical analysis found that the ellipticity e, spiral pitch S and surface tension factor σ of the condensate could influence the condensation heat transfer performance of the SDT, and ellipticity was the main influencing factor. Ellipticity and spiral pitch increased, the average condensation heat transfer coefficient increased. With the increase of surface tension factor, the condensation heat transfer enhancement performance of the SDT degraded, and the influence of surface tension factor was more obvious when the ellipticity had larger value. The experimental results also proved ellipticity was the key influencing factor of the condensation heat transfer performance. Among the tested tube, the SDT with a ellipticity of0.86and a spiral pitch of192mm had the largest ellipticity and also the best condensation transfer performance, whose condensation heat transfer coefficient was about1.34times of the circular tube. For SDTs with a spiral pitch of192mm, there was a minimum ellipticty of0.77that determined whether the SDT could enhance condensation heat transfer. The SDTs had the advantage of double side heat transfer enhancement; its total heat transfer coefficient under condensation condition could be comparable to three dimensionally finned tubes those commonly used for condensation heat transfer enhancement. |
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