Photographic study and modeling of critical heat flux in horizontal flow and vertical upflow boiling with inlet vapor void

This study explores the mechanism of flow boiling critical heat flux (CHF) in a 2.5 mm x 5 mm rectangular channel that is heated along one of its walls for horizontal and vertical upflow configurations. Using FC-72 as working fluid, experiments were performed with mass velocities ranging from 185-1600 kg/m2s. 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-sped video motion analysis of interfacial features at heat fluxes up to CHF as well as during the CHF transient. For horizontal configuration 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%. For vertical upflow configuration the flow is observed to enter the heated portion of the channel separated into two regions, with vapor residing between the liquid. At CHF-, the flow consists of a large central vapor core with liquid flowing near the walls. In the inlet region, a wavy vapor layer develops along the heated wall between a thin liquid layer and the heated wall. This vapor layer evolves immediately at the leading edge of the heated wall. The wall layers undergo gradual thinning along the channel due to increases in flow velocities and shear stresses. CHF increases monotonically with increases in mass velocity, inlet quality and outlet quality. With a MAE of 24.28%, the two-phase viscosity relation by Owens [38] provides the best most accurate predictions of pressure drop among the different two-phase friction factor and viscosity methods tested in conjunction with HEM. Among four rectangular channel and two circular channel CHF correlations tested, a correlation by Mishima & Ishii [44] provides the best predictions of the present data, with a MAE of 20.78%