A new methodology to predict the onset of flow instability (OFI) in single horizontal microtube with inlet orifice is proposed. The predictive methodology states that OFI occurs as the pumping power under no heating condition is equal to the pumping power under heating condition in the microtube at the same volume rate. Since the pumping power can be simply described as the product of volume rate and pressure drop cross the microtube, the heat flux at OFI is determined as the two-phase pressure drop under heating condition is equal to the single-phase pressure drop under no heating condition at same flow rate. The addition of inlet orifice increases the pumping power under no heating condition. The increased pumping power by orifice delays the onset of flow instability. The predictive methodology is validated by comparing the predicted heat flux at OFI with our previous experimental data in the microtubes with three different inlet restriction ratios. The result shows that the proposed method is capable of prediction of heat flux at OFI with a deviation of 30% and mean absolute error of 13% at mass flux less than 2000 kg/m2·s.

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