Accurate prediction of critical heat flux (CHF) in microchannels and small diameter tubes is of great interest in estimating the safe operational limits of cooling systems employing flow boiling. Scale analysis is applied to identify the relevant forces leading to the CHF condition. Using these forces, a local parameter model is developed to predict the flow boiling CHF. The theoretical model is an extension of an earlier pool boiling CHF model incorporating a force balance among the evaporation momentum, surface tension, inertia, and viscous forces. Weber number, capillary number, and a new non-dimensional group K2, representing the ratio of evaporation momentum to surface tension forces, emerged as main groups in quantifying the narrow channel effects on CHF. The constants in the model were calculated from the available experimental data. The mean error with ten data sets is 19.7 percent, with 76 percent data falling within ±30% error band, and 93 percent within ±50% error band. Evaluating individualized set of constants for each data set resulted in mean errors of less than 10 percent for all data sets. The success of the model indicates that flow boiling CHF can be modeled as a local phenomenon and the scale analysis is able to reveal important information regarding fundamental mechanisms leading to the CHF condition. The final equations resulting from this model are given by Eqs. (18–22) along with the transition criteria given by Eq. (28).

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