Effect of counter current airflow on film dispersion and heat transfer of evaporative falling film over a horizontal elliptical tube

Abstract

This study explores the influence of counter current airflow on evaporative heat transfer performance of falling films on a horizontal elliptical tube. The numerical analysis was based on the volume-of-fluid simulations to catch the dispersal evolution of descending films in conjunction with user defined function to model the evaporation effect at the water surface to the ambiance. The calculated results of heat transfer coefficients were in reasonably good agreement with the measured data from the literature for validation of the computational model. Calculations were performed to elaborate the interaction mechanism of counter current airflows with the formation process and heat transfer of evaporative liquid films on the tubes. In essence, upward air streams can induce the shear stresses at the water-air interface to form thicker wavy films. The counter current airflow has a more pronounced effect on heat transfer enhancement in the lower part of the tube than the upper part. Numerical experiments were also extended to assess the thermal performance of water film evaporators by systematically varying the inlet liquid mass flow rate of 0.093–0.186 kg/m-s and counter current airflow velocity of 0–3 m/s, respectively. The simulations results reveal steeper temperature gradients at the wall to enhance heat transfer outcomes, achieving an averaged heat transfer coefficient of 4.15 kW/m2 K for Γ = 0.149 kg/m-s and Vair = 3 m/s.