This study explores the mechanism of flow boiling critical heat flux (CHF) in a 2.5 mm x 5 mm horizontal channel that is heated along its bottom 2.5 mm wall. Using FC-72 as working fluid, experiments were performed with mass velocities ranging from 185-1600 kg/m(2)s. A key objective of this study is to assess the influence of inlet vapor void on CHF. This influence is examined with the aid of high-speed video motion analysis of interfacial features at heat fluxes up to CHF as well as during the CHF transient. The flow is observed to enter the heated portion of the channel separated into two layers, with vapor residing above liquid. Just prior to CHF, a third vapor layer begins to develop at the leading edge of the heated wall beneath the liquid layer. Because of buoyancy effects and mixing between the three layers, the flow is less discernible in the downstream region of the heated wall, especially at high mass velocities. The observed behavior is used to construct a new separated three-layer model that facilitates the prediction of individual layer velocities and thicknesses. Combining the predictions of the new three-layer model with the interfacial lift-off CHF model provides good CHF predictions for all mass velocities, evidenced by a MAE of 11.63%. (C) 2012 Elsevier Ltd. All rights reserved.