Experimental Study On The Boiling Heat Transfer Characteristics Of R32 In Horizontal Tubes

As is known to us all,the refrigeration technology seems to be playing a very important role in modern economic development and daily social life.However,there is a widespread issue over the HCFCs refrigerants' effects on the environment,which mainly include the ozone depletion and the greenhouse effect.R32,namely the main component of R410 A,is thought to be a kind of transitional alternative refrigerant in developing countries.Therefore,it should be necessary for researchers to explore the boiling heat transfer characteristics of R32 in horizontal tubes,thus being of beneficial reference to further optimization research and applications on boiling heat transfer of R32.Firstly,this thesis presented the alternative performances of R32,such as thermophysical property,environmental protection,favorable cycle performance,safety performance and availability.Then,a set of experimental apparatus of boiling heat transfer of R32 in horizontal tubes was built up based on the present research work.Last of all,experimental study on the boiling heat transfer charateristics was conducted and effects of channel geometry and flow conditions on heat transfer coefficients were investigated.The results show that heat transfer coefficients increase with mass flux,saturated evaporation temperature and heat flux,among which heat flux is playing a dominant role.By comparing the experimental data,it is obvious that the boiling heat transfer coefficients of R32 in the micro-fin tube are larger than in the smooth tube while the boiling heat transfer coefficients decrease with the hydraulic diameter of tubes.Further research shows that the boiling heat transfer coefficients of R32 in the micro-fin tubes with the hydraulic diameter of 5 mm and 7 mm are respectively,1.19 and 1.23 times as large as that in the smooth tube.Vapor quality seems to have a complicated influence on the boiling heat transfer coefficients of R32 due to the emerging dry-out point.The boiling heat transfer coefficients increase with vapor quality before reaching the dry-out point and turn to decreasing with vapor quality beyond the dry-out.The dry-out values are experimentally tested to range from 0.41~0.57.However,the decrease of mass flux effectively helps to delay the arrival of the dry-out point.Besides,the increasement of heat flux and decrease of hydraulic diameters of tubes are also conducive to postpone the coming dry-out.