In two-phase flow, local convective heat transfer coefficients are evaluated from the channel local thermal hydraulic conditions and the heater rod surface temperature. In theory all the main two-phase flow patterns such as bubble flow, slug flow, churn flow and annular flow are likely to exhibit different heat transfer characteristics. It is generally recognized that more accurate prediction of flow boiling can be obtained by adopting a flow pattern specific method for each individual flow regime. However, practically all of the existing correlations correlate flow boiling data using a single equation covering all the different flow patterns. General boiling curve is used to define the wall heat transfer regimes (convective, pool boiling, flow boiling, film boiling, etc.) and the related convective heat transfer coefficient. In addition, distinction must be made for condensation and evaporation. Further, well-established models for the critical heat flux (CHF) are required for the calculation of pre-CHF and post-CHF wall heat transfer.