INVESTIGATION OF BOILING OVER CYLINDRICAL SURFACES AND INTERACTION BETWEEN NEIGHBORING SITES

Abstract

Boiling heat transfer is well known in engineering applications as a superior mode of heat removal technology from a hot body than conventional single phase means. In boiling, major transfer mechanisms of heat depend on interface distribution. Hence, understanding the bubble dynamics during boiling heat transfer is utmost important. Nucleation, growth, departure, sliding and merging are important features of bubble life cycle and controlling parameter for heat transfer. Complexity increases manifold when two or more evolving bubbles come under influence of each other and interact amongst themselves. Engineering surfaces, promoting high rate of boiling heat transfer coefficient, are designed to accommodate closely spaced nucleation sites. Interacting bubbles are inevitable during application of these surfaces. Numerical investigations have been made to study interacting film boiling on infinite horizontal cylinders in 2-D framework. Three different cases are tried namely, horizontal and vertical inline cylinder arrangement and staggered test matrix. Interaction parameters are studied based on pattern of film boiling around unconstrained single horizontal cylinder. Departure from lateral symmetry in case of horizontal array and suppression from unconstrained pinch off in vertical cylinder stack is noticed. Staggered array of cylinders has led to chaotic interfacial interactions leading towards complete dry out. Numerical analyses have been also performed to study site-site interaction on flat horizontal surfaces with distinct nucleation sites. Diffused interface concept is used for treating the interface between gas and liquid. Different nucleation shape and sizes and their effect of the neighboring on nucleation sites on flat structured surfaces have been studied. Site interaction on inclined surface has also been analyzed keeping bubble merging into consideration for a wide variety of heat flux. Multi bubble interaction on cylindrical surface having diameter larger than departure size is studied through combined experimental and numerical analyses. From in house experimental study, it has been established that heat transfer coefficient varies based on azimuthal location of large cylinder. Studies from inclined cylinder showed azimuthally bottom most point of top end has faster bubble release and highest heat transfer coefficient.Spiral like bubble motion has been noticed after detachment from nucleation sites over the surface. Equivalent numerical simulation has also depicted complex bubble dynamics which helps in obtaining spatial variation of heat flux. Study of different interfacial dynamics during phase change heat transfer will help in design of efficient heaters for industrial applications and cooling of hot spots.