Flow boiling is considerably more complicated than pool boiling, owing to the coupling between hydrodynamics and boiling heat transfer processes. A sequence of two-phase and boiling heat transfer regimes takes place along the heated channels during flow boiling, as a result of the increasing quality. The two-phase flow regimes in a boiling channel are therefore “developing” everywhere and are morphologically different than their namesakes in adiabatic two-phase flows.

Forced-Flow Boiling Regimes

The preferred configuration for boiling channels is vertical upflow. In this configuration buoyancy helps the mixture flow, and the slip velocity between the two phases that is caused by their density difference actually improves the heat transfer. However, flow boiling in horizontal and even vertical channels with downflow are also of interest. Horizontal boiling channels are not uncommon, and flow boiling in a vertical, downward configuration may occur under accident conditions in systems that have otherwise been designed to operate in liquid forced convection heat transfer conditions.

Figure 12.1 displays schematically the heat transfer, two-phase flow, and boiling regimes that take place in a vertical tube with upward flow that operates in steady state and is subject to a uniform and moderate heat flux. The mass flow rate is assumed constant. When the fluid at the inlet is a highly subcooled liquid, at a very low heat flux, the flow field in the entire channel remains subcooled liquid [Fig. 12.1(a)].