STUDY OF POOL BOILING HEAT TRANSFER OVER FINNED TUBES ARRANGED IN A VERTICAL PLANE

Abstract

Wide variety of heat exchange equipment involve boiling of a liquid on the outside surface of metallic tubes. Efficient desjgn of such equipment requires accurate knowledge of the boiling characteristics of saturated liquids and their dependence on flow velocity. Comparatively few studies of crossflow boiling of water, as it occurs in devices such as f~c2<. tube boilers, are available in the literature.' The investigation of boiling on single tube can provide a tool for attacking more complex problems because this simple system forms a basic building block for actual multiple systems. So, the present study has been undertaken to investigate the effect of crossflow in boiling across a single and two horizontal tubes of OD 19.05 mm in a vertical plane. In this work the nucleate boiling of saturated water during pool and superimposed low velocity forced convection (crossflow) was investigated. Heat transfer coefficients for the following arrangements were determined . 1. Single horizontal plain tube. 2. Single horizontal finned tube (28TPI, continuous wolverine fins). 3. Twin horizontal finned tube arranged in ver=tical plane for four.. different pitch distances such that s/do = 5.88 = 4.41 2.94 = 1.47 -iv- The experiments were performed for the following operating conditions Heat Flux 9.00, 13.00, 18.00, 22.00, 27.00,. Mass Velocity Water Temperature . 30.00, 35.00, 43.00 kW/m2. 0, 1.83,.3.65, 6.08 kg/m2s. For Saturated Pool Boiling 100°C at Saturation Pressure For Convective Heat Transfer 55°C, 65°C, 75°C Heat transfer coefficients for plain and finned tubes provided data for the study of effects after using fins on the tube. The flow geometry considered was a horizontally placed electrically heated stainless steel tube in a pool of saturated distilled water. On the basis of the experimental data it has been found that for free convection and forced convection heat transfer processes, the heat transfer coefficient for finned tube is higher than plain tube; but for boiling heat transfer process, the heat transfer coefficient for plain tube is more than finned tube. Increase in mass velocity increases the boiling heat transfer coefficient for single plain tube; but in finned tube case, as the mass velocity is increased from 0 to 1.83 kg/m2s., the boiling heat transfer coefficient decreases. Further increase in mass velocity results in increasing of heat transfer coefficient, but it is still lower than the pool boiling case upto the mass velocity of 6.08 kg/m2s. Decrease in pitch distance (centre to centre distance between the tube axis) results in increase of boiling heat transfer coefficient for the upper tube and there is small variation in lower tube values.