Flow boiling occurs when all the phases are in bulk flow together in a channel; e.g., vapor and liquid flow in a pipe. The multiphase flow may be classified as adiabatic or diabatic, i.e., without or with heat addition at the channel wall (Figure 1.3). An example of adiabatic flow would be oil/gas flow in a pipeline, or air/water flow. In these cases the flow patterns would change as the inlet mass flow rates of the gas or liquid are altered or as the velocity and void distributions develop along the channel. Boiling would not take place and phase change would only occur if in a one component multiphase flow (e.g., steam-water) the pressure decreases and flashing occurs. Examples of diabatic flow are to be found in the riser tubes of steam generators and boiler tubes in power plants or in the coolant channels between nuclear fuel elements in a boiling water reactor. Boiling occurs on the walls of the channels and the flow patterns change due to vapor production as one observes the flow downstream in the channel due to vapor production. This is an important difference between pool boiling and flow boiling; i.e., that the forced flow of the multiphase system causes flow pattern transitions at a given wall heat flux (or temperature) as the integral power deposited in the fluid increases as it flows along the channel.
In all cases of multiphase flow the velocities of the phases are usually not equal. For example in a riser tube of a steam generator the vapor rises faster than the liquid due to buoyancy effects. One may term this velocity inequality as "slip" between the vapor and the liquid. The ratio of these velocities is called the "slip ratio". A better description of the phenomena is to consider it as a relative velocity difference between the phases, , and experimentally investigate these differences. Flow boiling heat transfer can occur under two different boundary conditions, either a specified wall heat flux or a specified wall temperature. The former case is an idealized example of a boiler tube in a fossil fuel boiler and the latter case is an idealized example of a riser tube in a steam generator.