Combined PLIF-IR thermal measurements of wavy film flows undergoing forced harmonic excitation

A combined PLIF/IR thermography technique was developed and employed towards the measurement of unsteady and conjugate heat transfer in thin, gravity-driven falling liquid film flows (with and without flow pulsation) over an inclined heated metal foil. Simultaneous, local film thickness, film and substrate temperature, heat flux exchanged with a heated foil and heat transfer coefficient results are reported for a range of electrically applied heat input values, flow Reynolds (Re) numbers and flow pulsation frequencies. Moreover, interfacial wave velocities were calculated from cross-correlations across successive thickness profiles. Results concerning the instantaneous and local heat transfer coefficient variation and how this is correlated with the instantaneous and local film thickness variation (waves) suggest that the heat transfer coefficient experiences an enhancement in thinner films. The particular observation is most probably attributed to a number of unsteady flow phenomena within the wavy fluid films that are not captured by the steady analysis. At low flow Re number values the mean Nusselt (Nu) was around 2.5, in agreement with laminar flow theory, while at higher Re values, higher Nu were observed. Finally, lower wave amplitude intensities were associated with higher heat transfer coefficient fluctuation intensities