| Alternative refrigerants has become a common problem to the world, due to thenature of binary mixed refrigerant may have complementary advantages, and thus themixed with HFCs refrigerant has been widely used. R134a as a transitional refrigeranthas been widely applied in refrigerating system. But because of its high globalwarming Potential-GWP is about1430, so it can only to a certain extent, meet people’sneeds in certain period.Will be facing elimination. R125as typical HFCs refrigerant, iswidely used in many mixed refrigerants.Evaporator as one of the important components of the air conditioning system, It isparticularly important to improve its performance improvement and thermal efficiencyfor refrigeration units. In the field of refrigeration and air conditioning, nucleate poolboiling occurs within a flooded evaporator. To strengthen the nucleate pool boiling heattransfer and low temperature difference of boiling heat transfer, In the rational use ofexisting energy and development of new energy has great economic significance.The pool boiling heat transfer data in the pure R134a and three different massconcentration ratio of R134a/R125mixed refrigerant (the concentration ratio of94%/6%、88%/12%、82%/18%) on single horizontal smooth tube and seven enhancedtubes were measured at saturation temperature of10±0.05℃. Theprimary influencefactors were analyzed in this thesis. The main results were as follows:(1) In the pure R134a and three different mass concentration of R134a/R125mixedrefrigerant, The trend is consistent with smooth tube and enhanced tubes. Experimental results showed that the test tubes could effectively enhance heat transfer. The boilingheat transfer coefficients of enhanced tubes are with the average values of4.33times asthat of smooth tube, within pool boiling of pure R134a. In the three different massconcentration of R134a/R125mixed refrigerant(the concentration ratio of94%/6%、88%/12%、82%/18%), The boiling heat transfer coefficients of enhanced tubes arewith the average values of3.49、2.94and2.71times as that of smooth tube,respectively.The results indicate that with the increase of R125mass concentration, in R134a/R125mixed refrigerants, the strengthening of enhanced tubes weakened gradually.(2) The boiling heat transfer coefficient of enhanced tubes with the increase of feetper inch (fpi) is not increase (or decrease). The boiling heat transfer outside a singlehorizontal tube to the parameters of tube is selective. When the heat flux is93kW/m2,tube E35with46fpi is optimal.(3) With pure R134a have almost the same change trend, in the R134a/R125mixedrefrigerant, the boiling heat transfer coefficient increases with the heat flux. With theincrease of mass concentration of R125, the boiling heat transfer coefficient of smoothtube and enhanced tubes were reduced in turn. When the heat flux is equal, in the R134a/R125mixed refrigerant with the concentration ratio of94%/6%、88%/12%、82%/18%,the boiling heat transfer coefficient of smooth tube than pure R134a has a decreaseby5%,8%and13%,respectively; and the boiling heat transfer coefficient of enhancedtubes than pure R134a has a decrease by17%,29%and37%in turn. The boiling heattransfer coefficient of enhanced tube in the reduction with the mixed refrigerant is muchbigger than the smooth tube.(4) The range of Reynolds number is:3104~5.6104. The thermal resistances ofthe water and refrigerant side of the enhanced tubes have almost the same change trendwith the smooth tube. In the R134a/R125mixed refrigerant, with the increase of massconcentration of R125, the thermal resistances of the refrigerant side was increases inturn, up to70%. So the tube outside surface needs to be further strengthened.(5) Combining the experimental data, A new correlation is presented to calculatenucleate boiling heat transfer coefficients of binary mixtures. The maximum deviationbetween all calculated values by the new correlation and the experimental data is lessthan±20%．It’s can satisfy the need of engineering application. |
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