Experimental Investigation of Heat Transfer and Evaporation Characteristics of a Falling Liquid Film on a Vertical Plate

Falling film evaporation over heated vertical surfaces is a well-established heat and mass transfer mechanism with applications in evaporators, thermal management systems, desalination units, and chemical process equipment. While numerous experimental and numerical studies have addressed this configuration, further high-resolution experimental data remain necessary to improve the understanding of local thermal behavior and to support the validation of predictive models. The present work provides a detailed experimental investigation of heat transfer and evaporation characteristics of a falling water film flowing over a vertically oriented plate subjected to a uniform heat flux.

The experimental setup is based on a metallic vertical plate of practical dimensions, uniformly heated by a copper heating element coated with a silicone layer to ensure homogeneous heat flux distribution. A distinctive feature of this study lies in the high spatial resolution of surface temperature measurements, achieved using an array of 32 negative temperature coefficient (NTC) thermistors distributed along the plate. This measurement approach enables precise monitoring of local temperature variations associated with film thinning, flow development, and evaporation processes. The system is carefully insulated, and heat losses from the rear surface and plate edges are minimized to ensure that the supplied thermal power is predominantly transferred to the liquid film.

Experiments are conducted for a range of liquid mass flow rates and imposed heat flux densities representative of practical operating conditions. The results demonstrate that increasing the imposed heat flux density significantly intensifies the evaporation process. In particular, at an imposed heat flux density of q = 2200 W·m⁻², the evaporation efficiency of the system reaches 54% for a liquid mass flow rate of Γ₀ = 2 kg·h⁻¹·m⁻¹, while it decreases to 39% when the mass flow rate is increased to Γ₀ = 3 kg·h⁻¹·m⁻¹. These quantitative results clearly indicate that lower mass flow rates promote thinner liquid films, leading to enhanced heat transfer and higher evaporation efficiency. Furthermore, the combined effect of low mass flow rates and high heat flux densities results in a substantial improvement in the overall thermal and evaporative performance of the system.

The originality of this work resides in the combination of a well-controlled uniform heat flux boundary condition with high-resolution surface temperature measurements over a plate of practical dimensions, providing reliable experimental data that remain limited in the existing literature. The findings offer new insights into local heat transfer and evaporation mechanisms in falling film flows and provide valuable reference data for the design, optimization, and validation of thermal systems and numerical models involving falling film evaporation.