HEAT TRANSFER ENHANCEMENT FOR THE BOILING OF OZONE-SAFE REFRIGERANTS

Abstract

The nucleate pool boiling phenomenon has immense heat transfer applications due to the ability to remove a significant quantity of heat from the heating surface with a limited temperature difference. This suggests a reduced size of heat exchangers by improving the efficiency of equipment used in many industries such as refrigeration and air conditioning industries, thermal power plants, process industries, and many other allied industries. A flooded evaporator is widely used as a heat exchanger in the refrigeration and air-conditioning industry. In this type of equipment, the refrigerant boils on the outer surface of the single tube or a bundle of tubes. Industrial demands for more efficient heating surfaces and economic incentives have spurred the development of methods to enhance the boiling heat transfer coefficients by applying the lower wall superheats. The concern over environmental impacts such as ozone depletion in the stratosphere and global warming has resulted in an international commitment to discontinue the use of CFCs and HCFCs refrigerants, which led to the development of a number of alternative refrigerants. Out of a number of new environmentally safe refrigerants, widely used alternative refrigerants such as pure refrigerants (R-134a, R-32, and R-600a), quasi azeotropic refrigerant (R-410A), and zeotropic refrigerant (R-407C) have been used for the present study. The present investigation deals with the experimental studies related to nucleate boiling heat transfer from re-entrant cavity tubes to pools of R-134a, R-32, R-600a, R- 410A, and R-407C. The plain and the re-entrant cavity heating tubes were fabricated from the same lot of extruded copper rods. All the re-entrant cavity tubes were identical in their dimensions except for cavity mouth size and height of fins. A total of five different re entrant cavities with different widths and depths were manufactured by rolling square fin and triangular fin tubes on the copper test section. Each test section tube had a diameter of (25.41 mm OD and 16.5 mm ID) and a tube length of 208 mm. The rolling of square finned tubes resulted in a T-shaped fin mouth, and the rolling of triangular finned tubes resulted in it just like a Turbo B-shaped fin. The rolling resulted in cavity mouth sizes of 0.51,0.41, 0.31, 0.25, and 0.21 mm, respectively. It is emphasized that the interior dimensions of the cavities were the same irrespective of cavity mouth size and height of the fin. The experimental data were obtained to increase heat flux in sixteen steps from 6.9 kW/m2 to 79.7 kW/m2 at saturation temperatures ranging from 70C- 160C. As the experiments were conducted on re-entrant cavity tubes for the boiling of the number of refrigerants with different physico-thermal properties, the conclusions are of a more generalized nature than those presented by the earlier investigators. Another important aspect of the present investigation is that the effect of cavity mouth size for the nucleate pool boiling heat transfer of different refrigerants has been studied. Thus, the conclusions emerging from the present investigation would form a sound base for the furtherance of knowledge and research in this area of interest.