Bubble formation and growth in two-phase mixtures within nozzles have a significant impact on the atomisation process and spray dynamics. Depending on the vaporisation rate and conditions the flow pattern might be bubbly, slug or annular. The regime of the flow has a significant impact on the flow characteristics and the mass flow rate. Calculation of the critical mass flow rate is extremely important in industry typical applications of which, require prior knowledge of the mass flow rate for safety and design purposes. Critical flow for a single-phase gas occurs when the flow is sonic (Mach number is one) at the smallest cross-section. The molecular relaxation phenomena are rapid enough so that the thermodynamic equilibrium is achieved. In cases of two-phase flows, the relaxation time-scales of heat and mass transfer are comparable to the residence time of the fluid at the choking region.