Enhancing Two-phase Flow And Heat Transfer In Double Enhanced 3-D Surface Tubes

Two-phase flow is more than a joint phenomenon of single phase flow and the process of latent heat transfer,but also includes many other complicated thermodynamic phenomena,such as bubble generation and bubble merging.Heat transfer enhancement technologies in marco-scale always lead to an increase in total surface area,disruption and sufficient mixation near the flow and thermal boundary layer,while technologies in mirco-scale and nano-scale target at the mechanism of two phase flow and heat transfer,for example,bubble generation,bubble merging,departure from the surface,drainage effect of the condensate and the surface wetting characteristics.The purpose of this research is to make a breakthrough for two phase heat transfer in horizontal channels by using marco-scale and mirco/nano-scale enahcned methods,simultaneously.In this study,an experimental set-up was designed and constructed to test flow boiling and convective condensation heat and pressure drop characteristics in horizontal smooth,2EHT-1 and 2EHT-2 enhanced tubes by using refrigerant R410A.Effects of mass flux,vapor quality,gravity force,shear force between vapor phase and liquid phase,local wall temperature and critical hea flux were discussed elaborately in each chapter.Besides,parameters that affect the flow pattern transition were also analysed.At the end of experiment,the author proposed and fabricated a micro/nano-structure surface coating by sintering metal powders with different particle diameters on a smooth surface,which can drastically enhance flow boiling and convective condensation heat transfer coefficient.The author provided a comprehensive review on two phase flow enhancement techniques and issues occurred in the thermal design field.Based on the analysis of current application and research,the most important and urgent case is to improve the technology of surface design so as to meet the higher and higher heat flux request.Due to the results of high HTC and smooth temperature distribution,two phase flow is widely involved in a variety of fields,but there are still many problems in two phase flow.Thus,a two phase flow set up was built for evaporation and condensation in horizontal tubes,and a self-developed DAQ software was programed to get real-time monitor during two phase flow.R410A was used as working fluid both for condensation and evaporation,with vapor quality ranging from 0.2 to 0.9 and mass fulx ranging from 80~260 kg/(m~2 s).For condensation,the saturate temperature was maintained at 40℃,and 10℃for evaporation.The results showed that performance factor for condensation increased with increasing mass flux at low mass flux region,and tended to 1.16 when mass flux was higher than 150 kg/(m~2 s),while for evaporation,performance factor linearly increased with mass flux and the PF of2EHT-2 tube was higher than 2EHT-1 tube with a maximum value of 1.44.Local heat transfer characteristics of evaporation and condensation were also examined.Effects of local vapor quality,mass flux,heat flux and local wall temperature were discussed in detail.Experimental results were compared with the predictions of twelve heat transfer and pressure drop correlations,and new correlation was proposed for both local evaporation and condensation HTC.The results showed that the surface enhancements of micro-structure in the macro-channels had a great enhancement on HTC and critical heat flux with an acceptable increasement in frictional pressure drop,especially for saturated evaporation and condensation.Local measurements indicated that the wall temperature decreased with local vapor quality for condensation and the maximum degree was found to be 4.6℃compared with the smooth tube,while for evaporation the superheat degree of EHT tubes was about 2℃lower than smooth tube.Besides,the condensation HTC was 1.58 times of the smooth tube at high mass flux and vapor quality region.Additionally,the results of heat flux effect on flow boiling showed that the flow boiling HTC increase with increasing heat flux when the working heat flux was smaller than critical heat flux;while the flow boiling HTC decreased with increasing heat flux when the working heat flux was larger than critical heat flux.Based on this research,two conclusions were verified for enhancing two-phase flow heat transfer.For condensation,acceleration of vapor condensation increased the thermal resistance between the vapor phase and liquid film.Thus,the mechanism way of enhancing saturate condensation is to decrease the liquid film and to increase the hydrophobic properties of the surface.While the mechanism way of enhancing saturate flow boiling is to increase the nucleate boiling density at high mass flux and to increase the hydrophilic properties of the surface by using micro or nano surface modifications.