INVESTIGATION OF VOID FRACTION AND BOILING HEAT TRANSFER IN A TUBE BUNDLE

Abstract

Boiling heat transfer coefficient plays an important role in transferring heat from heating element (tube) to working fluid esp. water. High value of heat transfer coefficient is always desirable for heat transfer. Heat transfer coefficient depends upon the surface condition and properties of material and fluid properties. When tubes are kept in a bundle, it has different value of boiling heat transfer coefficient as compare to a .single tube. Heat transfer coefficient mainly depends upon flow pattern whether it is laminar or turbulent. Presence of turbulent flow provide high value of heat transfer coefficient as compare to laminar flow. Rate of bubbles generation from other tubes make flow more turbulent. It also depends upon spacing between tubes, tube diameter, length of tube, tube surface etc. Along with heat transfer coefficient void fraction is also observed. Void fraction is ratio of volume occupied by gas to the total volume under consideration. Void fraction is function of pressure as density is function of pressure. Void fraction is measured with the help of a single point conductivity probe. Void fraction gives the idea about flow pattern near tube. Size of bubble depends upon pressure. So when it is near water surface its size is larger as compare to when it is much below water surface due to pressure exerted by water column. Also steam is added along length of tube. Hence it is expected that at the top of tube void fraction will be higher as compare to lower portion.