Single-Phase Flow And Heat Transfer Inside Horizontal Tubes With Multi-Start Internal Helical Fins

As more attention is paid to reducing the energy consumption of buildings,the energy saving of cooling and heating equipment in the building has become the focus of the research.The tube with multi-starts internal helical fins,developed to improve the energy efficiency,has been widely studied and applied for its excellent heat transfer performance.However,the single phase flow and heat transfer problem in the internal helical finned tube remains to be clarified,which greatly limits the development and application of this technology.The purpose of this study is thus to obtain heat transfer and friction factor data in whole flow regions of developing flow inside enhanced tubes,using water-ethylene glycol mixture as the working fluid,and to develop correlations from these results.Firstly,to solve tube-side heat transfer coefficient,the wall temperature measurement method of the bilateral enhanced tube was proposed.Stability and feasibility of the wall layout of thermocouples under constant heat flux were validated and studied.Secondly,characteristics of the single-phase flow inside internal helical finned tubes under the adiabatic and uniform heat flux boundary were investigated.On the basis of the theoretical analysis of previous and experimental results,semi-empirical models of the friction factor in whole flow regions were established.The results show that:1)Friction factors of single-phase flow in the internal helical finned tube were higher than the counterpart of the plain tube.Perceived discrepancy was seen between enhanced tubes and equivalent roughness tubes used in the Nikuradse experiment before the friction factor reached the extremum value in the turbulent,however,results could not be accurately predicted by the existing semi-empirical models;2)Influence of the hydraulic developing section on the friction factor under the uniform heat flux is within-3.5%-0.32%,which could be neglected;3)The semi-empirical model of the adiabatic friction factor in laminar region was established in the range of 110<Re<2060,and the prediction relative deviation was-10.3%-9.4%;the one under the uniform heat flux was established in laminar region between 450 and 2580,and the prediction relative deviation was-30.2%-71.1%;the one of the adiabatic friction factor was established in transition region between 1900 and 19900,and the prediction relative deviation was-36.1%-41.2%;the one under the uniform heat flux was established in transition region between 2400 and 15100,and predication relative deviation was-19.6%-66.9%between 8600 and 97600;the one was established in fully turbulent region,and the prediction relative deviation was-36.0%-56.9%.Thirdly,the heat transfer inside the internal helical finned tube was investigated under the uniform heat flux.Semi-empirical models of the heat transfer were established results.The results are as follows:1)Convection heat transfer coefficients were 4.2 to 7.6 times higher than the theoretical value(Nu=4.36)for Hagen-Poiseuille flow under the uniform heat flux,which was attributed to the buoyancy-induced secondary flow;2)The enhancement was mainly due to the swirling flow induced by helical fins and the secondary flow,which had a great influence on mass transfer and heat transfer near the wall;3)The influence of the thermal developing section under the uniform heat flux on the heat transfer was less than 4%,which could be neglected;4)The semi-empirical model of heat transfer in laminar region was established in the range of 450<Re<2580,and the prediction relative deviation was-21.3%-22.8%;the one of heat transfer was established in fully turbulent region between 8100-31000,and the prediction relative deviation was-23.8%-22.1%.Fourthly,the influence of thermal boundary conditions on the single-phase flow and heat transfer inside the internal helical finned tube was investigated.The results show that:1)Heating boundary affected more in the laminar and transition flow regions,compared with the adiabatic boundary and the difference was between-5.8%-15.8%;2)The secondary flow advanced the flow stability,and the greater the heat flux is,the later the transition appeared.In addition,the transition region was shortened.In turbulent region,the difference of friction factors under different thermal boundary conditions was between 0 to 15.8%.Finally,four criteria were performed to evaluate these tested tubes.These criteria are related to the amount of enhancement gained by employing an enhanced tube instead of a plain tube.The results show that:in all cases,the enhanced tubes only became a viable option when the to-be-replaced plain tubes were operating at a Reynolds number greater than approximately 4000.The study results could be used for the thermal design of compact heat exchangers with tubes with multi-starts internal helical fins,and provide a reference for further perfecting the theory of single-flow and heat transfer in the tube with multi-starts internal helical fins.