![]() As the flowing fluid is heated further in the heated tube, the bubbles start growing in size and slugs of vapor are formed. This region is referred to as the bubbly flow region. As the liquid proceeds further in the tube, bubbles are formed on the inner tube surface and a few bubbles detach from the surface and become a part of the mainstream. When the fluid enters the tube, initially it is in the subcooled condition and the transfer of heat occurs by forced convection. Consider a case of a fluid flowing in a heated tube. As the fluid flows through a restricted heated channel, the ratio of vapor to liquid phase varies. As the flow is restricted within a certain channel, the vapor and liquid phases are forced to flow together within the channel. Internal flow boiling is also known as two-phase flow. Furthermore, the heat fluxes increase with increasing flow velocity. An external flow boiling case over a cylinder or a plate is similar to the pool-boiling phenomenon however, the additional induced motion from the external source increases the nucleate boiling heat flux as well as maximum heat flux significantly. If the fluid flows inside a heated channel, it is classified as internal flow boiling, whereas when a fluid flows over a heated surface, it is referred to as external flow boiling. In addition, flow boiling can be further classified into internal flow or external flow boiling. The process of boiling in such cases includes effects of both pool and convection boiling. ![]() In case of flow boiling, bulk fluid motion is caused by an external means such as a pump as the phase change process occurs. To develop an understanding of the unique and significant effects of interfacial surfaces on the interaction of two bulk systems, a thermodynamic analysis will be performed here. Any mass exchange between the two bulk substances also requires consideration of momentum and energy exchange.Ī single substance undergoing a phase change is the simplest case of mass transport across an interfacial surface. When mass transport occurs between the two bulk substances, the interface problem becomes significantly more complicated. When analyzing such systems, the interface is a unique region that requires special attention. This very general description can be used to consider, for example, the cases of two immiscible liquids in contact with each other a single substance in two phases or a mixture of gases in contact with a solid, with a chemical reaction occurring on the surface of the solid. Two bulk fluids of large extent, separated by an interfacial region, constitute a system in equilibrium. Amir Faghri, Yuwen Zhang, in Transport Phenomena in Multiphase Systems, 2006 2.6.1 Equilibrium at the Interface
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