Investigation into the deformation process of water droplets bag breakup in airflow with elevated temperatures

• Experimental and theoretical studies of droplet deformation in the presence of a temperature difference between the airflow and the droplet are carried out. • Based on the existing DDB model, an improved model considering windward-side heat transfer and evaporation is proposed, which greatly reduces the prediction errors of the droplet deformation diameter with time. • The droplet deformation mechanism in the presence of heat exchange is analyzed based on the energy method. Experimental and theoretical studies of droplet deformation under airflow-droplet temperature differentials remain scarce. This investigation examines the deformation process of water droplet bag breakup at five different temperatures (278 K, 303 K, 323 K, 348 K, and 368 K) in the airflow temperature range from 300 K to 493 K. Results demonstrate that both droplet and airflow temperatures significantly influence deformation dynamics. The effect of heat exchange between the airflow and droplet on the droplet deformation process arises from the combined effect of convective heat transfer and evaporation. It is found that the results of existing empirical and theoretical models for droplet deformation, such as the DDB (Drop Deformation Breakup) model, do not agree well with experimental data in this investigation due to unaccounted heat exchange effects. Based on the DDB model, an improved model considering windward-side heat transfer and evaporation is proposed, which significantly reduces the prediction errors of the droplet deformation diameter with time. Energy analysis further quantifies the contributions of aerodynamic forces and heat exchange to droplet energy evolution during deformation, validating the droplet deformation mechanism with heat exchange. In this investigation, the timescale of droplet deformation is sufficiently short to render heat exchange effects on droplet average temperature variations and evaporation-induced mass loss negligible. Both theoretical and experimental results confirm that heat exchange primarily modulates droplet surface energy, thereby influencing deformation dynamics.