| How to improve the COP of the refrigeration system and avoid polluting the environment is the most popular topic in the refrigeration field.Application of high efficient heat exchanger and environment-friendly refrigerant is the most fundamental means to solve the above problem.As the most potential alternative refrigerant to R22,R290 has not only excellent physical and chemical characteristics,but also the ozone depletion potential(ODP)and global warming potential(GWP)which are all close to zero.But now the investigation about boiling heat transfer characteristics in horizontal mini-channel is not more,Mechanism study of R290 flow boiling heat transfer in mini-channel is not enough,and relevant knowledge is also limited,many observed phenomenon cannot be explained very well,so through the theoretical and experimental aspects to investigate is necessary.The objective of this study is to investigate boiling heat transfer characteristics of R290 in mini-channel,the dry-out characteristics during heat transfer process and critical heat flux(CHF)characteristics when dry-out happens through experiment and theoretical analysis,which is used to provide theoretical basis and data support for mini-channel heat exchanger design.In order to achieve above aims,this paper mainly works for the following aspects:(1)Study the model for flow boiling heat transfer in mini-channel.Systematically analyze the mechanism of flow boiling heat transfer and dry-out phenomenon,and introduce the mathematical model of heat transfer coefficient,dry-out and flow pattern transition is introduced.(2)The experimental research about R290 flow boiling heat transfer characteristics in horizontal mini-channel is carried out.The R290 flow boiling heat transfer coefficient in mini-channel is tested at different heat flux,saturation temperature,flow rate,diameter,and then analyze its changing rules.(3)The visual researches about R290 flow pattern and tube surface temperature are carried out and the dry-out mechanism and effects are investigated by comparative study of theoretical analysis and experimental evidence.The relationship between flow pattern and heat transfer coefficient is discussed through flow observation in the heat transfer process.(4)The experimental research about R290 flow boiling heat transfer CHFcharacteristics in horizontal mini-channel is carried out.The R290 flow boiling heat transfer CHF in mini-channel is tested at different saturation temperatures,flow rates,diameters,entrance dry-out qualities and then analysis its changing rules.The main results obtained in present work are as follows:(1)For R290 in the mini-channel,heat flux has significant influence on intensifying nucleate boiling heat transfer;and flow rate has little influence on heat transfer;the traditional diameter tube heat transfer model no longer fit the micro tube diameter;the heat transfer coefficient increases with the monotonic increase of saturation temperature.(2)The results show a obvious corresponding relationship between heat transfer coefficient and flow pattern through visual studies of dynamic flow patterns and the analysis of its transition characteristics.In addition results also indicate that dry-out mainly occurs during annual-mist flow,wavy-mist transition process and unstable annual flow.(3)The studies for R290 in mini-channel shows that the CHF increases with the increase of flow rate and heat pipe diameter,and the CHF decreases with the monotonic increase of saturation temperature and entrance dry-out quality.Concluding from the work done in this thesis,the study conducted some degree of innovation on the basis of previous study.Effect of heat flux,flow rate,saturation temperature on flow patterns and flow patter transition is analyzed by flow pattern visualization research,while test tube surface temperature is monitored with infrared imaging.Combined with flow patterns and infrared imaging,The R290 flow boiling heat transfer characteristics in mini-channel and dry-out characteristics during heat transfer process are eventually concluded,which is used to provide theoretical basis and data support for mini-channel heat exchanger design. |
PDF Full Text Request