BOILING HEAT TRANSFER IN A FINNED TUBE BUNDLE

Abstract

A boiling is a two phase phenomenon as it consists of a mixture of liquid and vapour. Since the heat transfer rate in boiling is usually very high, boiling has been used to- cool devices requiring high heat transfer rate. Finned surfaces further enhances the rate of heat transfer. Use of tube bundles in pool and cross flow .condition is found in many boiling equipment. Examples are kettle reboilers and flooded evaporators used in power plant, refrigeration and air conditioning systems and in many chemical and petroleum process systems. Also in nuclear power plant, the use of finned surface to cool reactors is gaining importance due to high heat transfer rate associated with it. An accurate knowledge of boiling characteristics in tube bundles is therefore- a must for the efficient design of these equipment. A review of literature on boiling heat transfer in tube bundles specially on finned tubes, reveals that very limited research work has so far been published and certainly not enough for widely applicable correlations to be made. In view of this, an experimental investigation was undertaken to study the boiling heat transfer in a finned tube bundle (P/do = 1.5) consisting of five tubes placed horizontally one above the other. Resistive heating of tubes connected in series was done-and data were collected for both pool and cross flow conditions. The effect of heat flux, mass flux, tube location and circumferential variation on. local boiling heat transfer in tube bundle was investigated. Investigation is done for following operating condition: Working fluid - : Distilled water System pressure : Near atmospheric U Inlet fluid temperature Tube material Outer tube diameter Inner tube diameter Pitch circle(root) diameter Tube geometry Heat flux Mass flux Water temperature for single phase forced convection heat transfer Saturation temperature Stainless steel (AISI 304) 18.93 mm :16 mm 18.59 mm 1x5 in-line helical finned (24TP1) tube bundle (P/do = 1.5) :9.88-42.41 kW/m2 0-102.69 kg/m2s : 60°C, 78°C, 86°C On the basis of experimental results it has been found that nucleate boiling heat transfer coefficient increases with heat flux and mass flux values. The enhancement is found to be more at low heat flux values and decrease with the increase of heat flux. The heat transfer coefficient of the finned tube is higher than plain tube specially at low heat flux values and the difference decreases with the increase of heat flux. The heat transfer coefficient on a tube increases in the direction of fluid flow i.e. from bottom tube toward the top tube. Circumferential variation of local heat transfer coefficient show that in all cases the maximum and minimum heat transfer coefficient occurred near the bottom and top positions of tube surface respectively. Average bundle heat transfer coefficient is higher than on a bottom tube, which behaves like a single tube. Boiling heat transfer coefficient of a tube is much higher than that of single phase forced convection heat transfer coefficient.