Interfacial wave effects on heat transfer to a falling liquid film

Tae-Hwan Lyu, Purdue University

Abstract

Experiments were performed to provide insight into the heat transfer aspects of a wavy liquid film. Local film thickness and liquid temperature were measured simultaneously using a thermal conductance thickness probe and fast response thermocouples as the wavy film fell over an electrically heated wall. Liquid temperature at a fixed distance from the wall generally increased in the thin substrate portion of the film and decreased within the large waves. Temperature showed complex excursions within the large waves due to turbulent eddies and the relative liquid motion between the large wave and surrounding substrate. The measurements were examined with the aid of statistical tools to investigate the relationship between film thickness and liquid temperature at various film Reynolds numbers and heat fluxes. The correlative between the two variables was stronger at higher heat fluxes than at lower fluxes. A cross-spectrum analysis showed a distinct band of dominant frequencies in the relationship between the two variables. A technique using hot-wire method for local thickness measurements of dielectric liquid films is presented. The thickness probe was made from 0.0254 mm diameter platinum-10% rhodium wire which was stretched across the liquid-gas interface. A constant d.c. current was applied through the probe wire, and film thickness was determined from variations in the probe voltage drop. The technique allows simultaneous measurements of film thicknesses and liquid temperature for direct contact resistive heating of deionized water. A clear and simple wave profile is directly available without filtering signals. Variations of the wall temperature and convection heat transfer coefficient during a wave period were studied numerically using boundary conditions for liquid temperature obtained from experimental measurements. The energy equation in the heater wall was solved assuming periodic boundary conditions in the moving coordinates. Results show significant heat flux and convection heat transfer coefficient variation over a wave period. But the wall temperature fluctuation was so small that an assumption of uniform wall temperature over a wave period resulted in a small error in the heat transfer calculations for Re $\geq$ 3000 and q $\leq$ 75,000 W/m$\sp2$.

Degree

Ph.D.

Advisors

Mudawar, Purdue University.

Subject Area

Mechanical engineering|Fluid dynamics|Gases

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