| China’s low-grade energy accounts of more than half of the world’s total heat production.and its recycling is an important way to achieve enerty conservation and emission reduction.At present,the organic Rankine cycle is one of th enew power cycle technologies that is ideal for recovering low-grade energy,and improving the energy effieieney of its thermal system is an effedive way to achieve the goal of "dual carbon".Heat exchanger is key component of organic Rankine system,small channel heat exchanger will improve heat exchange efficiency and heat transfer rate compared with conventional size heat exchanger,thereby effectively improving the performance and thermal efficiency of the system,has been a hot spot in academic research in recent years.Therefore,this paper builds a micro-channel flow heat transfer experimental platform,studies the boiling heat transfer and pressure drop characteristies of R134a in 1mm and2 mm horizontal amooth round tubes in the subcritical state,and uses FLUENT to carry out numerieal simulation,and further studies its heat transfer characteristics,the main research content of this paper is as follows:(1)The experimental study of R134a flow heat transfer in the horizontal smooth round tube was carried out,and the working conditions were:mass flow rate 650-2400kg/(m2·s),heat flux density 20-65 kW/m2,saturation pressure 0.5-3.5MFa,inlet temperature of 20～50℃,dryness 0.05-0.98.The results show that the heat transfer coefficient increases with the increase of mass flow,heat flus density and inlet temperature,and deereases with the inerease of saturation pressure and pipe diameter.The eritical dryness deereases with the increase of mass flow,heat flux density and inlet temperature,and increases with the increaqse of asturatin pressure,and the change trend of heat transfer coeffieient in the diameter of the two sizes in consistent,but the heat transfer coefficient in 1 mm straight pipe is higher than that of 2 mm pipe under various working conditions.(2)The frictional pressure drop of the two phases increases with the increase of mass flow,heat flux density,saturation pressure and inlet temperature,among which the pressure drop is most affected by the mass flow,with a maximum increase of 2.8 times,and the heat flux density is the least,only increasing by 0.125 times.The heat transfer correlation of four horizontal tubes was analyzed and compared with experimental data.Finally,on the basis of Gungor&Winterton’s(1987)heat transfer correlation formula,combined with experimental data,it was modified to obtain a flow boiling heat transfer correlation formula suitable for medium and high temperatures in horizontal tubes.(3)The two-phase heat transfer characteristics of R134a in the microchannel were simulated and calculated by FLUENT,the boiling heat transfer process of gas-liquid two-phase flow in the tube was simulated and calculated by using the VOF multiphase flow model and turbulence model,and the influence of different influencing factors on the heat transfer characteristics of flow boiling was compared and analyzed,and the gas phase volume,temperature,flow velocity and pressure distribution clouds of different cross-sectional positions of the pipeline were obtained,and the law of the simulation results was consistent with the experimental results,and the average deviation between the experimental results and the numerical simulation was only 5.09%. |
