AUGMENTATION OF HEAT TRANSFER DURING CONDENSATION OF REFRIGERANTS INSIDE A HORIZONTAL TUBE

Abstract

The present research work has been carried out to study the heat transfer augmentation and associated pressure drop during condensation of refrigerants pure vapor inside.a horizontal tube with different inserts. The plain tube data have also been acquired for the sake of comparison. The experimental set-up has two different loops. The refrigerant loop consists of two test-condensers, visual sections, _ post-condenser, drier-cum filter, Coriolis effect mass flow meter, gear pumps and stainless steel tube evaporator. The test-condenser consisted of two identical lengths of 9.4 mm inner diameter, 12.76 outer diameter and 1.0 meter long hard drawn copper tubes. These test-section tubes were located concentrically inside an outer copper tube of 43 mm inner diameter. Thus, the two concentric tubes formed the counter flow test section. These test sections were placed in series while the refrigerant flowed inside the inner tube. Cooling water loop consisted of arrangement to flow the fresh cooling water in the outer annular space of both test-condensers. The cooling water flow rate was measured by a turbine flow meter for each test-condenser. Temperature of cooling water was measured at inlet and outlet of test-condenser by using a thermopile. The outer wall temperatures of the inner tube were measured at four axial locations for each test-condenser. At each of these four locations, four, copper-constantan thermocouples were brazed at the top, both sides and bottom positions. These thermocouples were accommodated through flanged joints in outer tube at each axial location. The temperature and the pressure of refrigerant were measured at the inlet of each test-section, and at inlet and outlet of steel tube evaporator. Pressure transducers were also placed at inlet header and outlet header of the geared pumps to monitor the pressure in the refrigerant loop. Refrigerant mass flow rate was varied by speed regulation of pumps through frequency controllers. Heat input to evaporator was varied through auto-transformer to get vapor quality ranging from 0.9 to 0.1 at inlet to test-condenser. National Instruments Inc. make data acquisition system with embedded controller was used to acquire the data of 36 thermocouples, 4 thermopiles, 4 pressure transducers, 2 differential pressure transducers and 3 flow meters. There were 10 test-runs at each mass flux to cover the entire vapor quality range. The data were acquired for 13 mass fluxes and each test run was made for both the refrigerant. The same numbers of test runs were made for each insert. A total of 7 coiled wire spring of different pitch, 7 twisted tape of different twist ratio and 4 perforated twisted tape of different pitch of perforation were used during the investigation. iii Abstract