Translated Abstract
Phase change heat transfer is an important heat transfer mode in refrigeration and air-conditioning. In this dissertation, different enhanced surfaces are studied in pool boiling, condensation and falling film evaporation of R134a with the application of water cooled evaporator and condenser in air conditioning system being as background the influencing factors on enhanced phase change heat transfer are analyzed. The prediction of forced turbulent convection heat transfer of helically ribbed tubes is also studied. In the central air-conditioning system, shell and tube heat exchanger is one of the most important heat transfer equipment, three water cooled condensers are re-designed and tested. The major findings are as follows:1. The influence of lubricant PVE on the refrigerant R134a pool boiling heat transfer is studied at different mass fraction, being 0.25%, 0.5%, 1%, 2%, 3%, 5%, 7% and 10%，respectively. The mean ratio of the boiling heat transfer coefficient of the polluted refrigerant over that of pure medium is proposed to characterize the effects of oil mass concentration, and the general variation trends of the non-dimensional quantities are obtained, which are convenient to engineering application.2. The pool boiling heat transfer of porous media of open celled copper foam with different parameters is studied better parameters are obtained the porous media of open celled copper foam has comparatively higher heat transfer performance at lower heat flux(less than 30kW/m2), with the increase in heat flux, the enhanced ratio of pool boiling heat transfer decreases and the performance of the enhanced tube gradually approaches that of the plain tube.3. It is found that the Cooper’s correlation has higher prediction accuracy within the heat flux region of 10-100 kW/m2, and when the heat flux region is greater than 200 kW/m2, the deviation between experimental and predicted results gradually increases, indicating that it can not be applied to the vapor block region. For the different enhanced structures of pool boiling, their enhanced ratios gradually decrease as the increment of heat flux, and finally their heat transfer coefficients are even less than that of the smooth tube.4. High fin-density integral-finned tubes are designed the overall heat transfer coefficient of the tube is about 6% higher than two high performance three dimensional enhanced tubes at the same in-tube water velocity. The condensate has no negative row effect on the integral finned tubes, and it even has positive effect on the condensing heat transfer of low-finned tubes because of the impact effect of the falling liquid drops. While negative row effect is always observed for all the three dimensional enhanced tubes studied So it is recommended that integrally-finned tubes are used in the shell and tube condensers at the bottom rows.. 5. Three water cooled condensers of one centrifugal compressor unit and two screw compressor units are re-designed and tested through rearranging tube bundle and replacing the three dimensional enhanced tubes by high performance integral-finned tubes. The tube numbers of the redesigned condensers are reduced by 15%, 10% and 15% ,respectively. The condenser cost is greatly reduced, with a saving of more than 30 thousand Yuan for the centrifugal compressor unit. 6. Condensing heat transfer performance of four enhanced tubes of low thermal conductivity materials (titanium, B10,B30 and stainless steel) are studied. It is found that the condensation heat transfer coefficient of these tubes are somewhat enhanced, with the heat transfer coefficients being in the range of 10 to 20 kW/m2 at the heat flux range of 10 to 80 kW/m2. For tubes with the same enhanced geometries but different materials, the condensation heat transfer coefficients are much different, while the index nin the forms of is basically the same.7. Based on the test results and the theory of analogy, a correlation to predict the turbulent convection heat transfer of internal helically ribbed enhanced tubes is proposed. Compared with totally 440 data, among whom 152 data of 16 enhanced tubes were obtained by author himself, the correlation agrees with 72% data with a deviation within ±10%, 93% data with a deviation within ±20%, and more than 99% data within ±40%.8. An experimental apparatus for falling film evaporation heat transfer outside horizontal tube bundles is designed and established, seemingly first in domestic refrigeration field for middle and high temperature refrigerants. The heat balance is mostly within 3%. Compared with Nusselt analytical solution of condensation, the deviation is within ±10%. 9. The falling film evaporation heat transfer of smooth and two enhanced tubes are studied. The resulted falling film evaporation heat transfer coefficients are higher than those of pool boiling heat transfer coefficients, with the highest one being 60% higher than that of the best pool boiling heat transfer tubes in this dissertation.
Translated Keyword
[Refrigerant Condenser Porous media pool boiling condensation Falling film evaporation]
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