TWO PHASE CROSS FLOW ROILING HEAT TRANSFER IN HORIZONTAL TUBE BUNDLES

Abstract

Boiling heat transfer is he phenomenon of occurring a change of phase from liquid to 7 1. vapour. It has. two ,basic types of boiling he. pool boiling and flow boiling. Pool boiling is boiling on a heating surface submerged in a, pool of initially quiescent liquid and the flow boiling is boiling in a flowing stream of fluid: Since the heat transfer rate in boiling is usually very high, boiling has been used to cool devices requiring high heat transfer rate. Finned surface further enhances the rate of heat transfer. There are a number of industrial important applications in which boiling occurs in cross-flow over a heated tube or bundle of tubes. This type of system is important in some kinds of steam generators, and in horizontal and kettle type re-boilers where the cross flow is induced by circulation patterns within the shell. The use of finned surface in nuclear power plants to cool reactor is gaining importance due to high heat transfer rate associated with it. Finned tubes are widely used in modem tube bundle evaporator, especially those in refrigeration machinery. However, for the required range of parameters, there are comparatively few experimental results for funned ube bundles and certainly not enough to recommend widely applicable co-relations to design such evaporators. An experimental investigation has been done to study the results from pool and flow boiling on a finned tube bundle having five stainless steel tubes (AISI 304) with an outer diameter of 18.93 nun and inner diameter of 16 mm placed horizontally one above the other. Fin's (24 TPI), in the form of continuous helical thread. is .cut on the outer surface of each tube. Tubes are connected in series, submerged in distilled water in a test vessel and are resistively heated. Data are collected at various Mass flux, Heat flux, tube location and circumferential variation on local boiling heat transfer in tube bundle. After experimental investigation it has been found that. nucleate boiling heat transfer coefficient increases with mass flux, and heat flux values. Experimentally it has been found that the enhancement of upper tubes in tube bundle is more at low heat flux values and decreases with increase of heat flux. The heat transfer coefficient on a tube increases in the direction of fluid flow. Obviously the average bundle heat transfer coefficient is found to be higher than on'a bottom tube, which behaves like a single tube. Maximum and minimum heat transfer coefficients are occurred near the bottom and top positions of tube surface respectively. (