Spray cooling provides a means of removing high heat flux in many applications. It involves dynamic processes characterized by forced convection, surface nucleation and evaporation mechanisms. To this end, analysis of the spray cooling mechanism is mainly dependent on the behavior of the liquid film flow interface, which directly influence the heat transfer process. Spray cooling has been extensively studied, with most studies focusing on the heat transfer characteristics as well as the effects of factors including spray parameters, control strategy and coolants.
Nevertheless, despite the significant improvement in the understanding of spray cooling, the associated heat transfer mechanism is still poorly understood. The challenges associated with studies on heat transfer mechanisms of spray cooling mainly emanate from the difficulty in capturing the quantitative data due to the high spatial resolution. Thus, it is important to carry out more quantitative studies on heat transfer and liquid film flow to provide a better understanding of the spray cooling process complexities. Unfortunately, there are still limited experimental studies on liquid film flow and existing models are far from analytical and are separated from the physical phenomena.
Despite the detailed investigation of the morphology of the liquid film, little is known about the evolution of liquid films on the surface. This has been attributed to the strong disturbance and interactions between the thin films and the atomized droplets with increased turbulence. Additionally, theoretical and numerical models concentrate on single-phase heat transfer and their associated applications, which are highly empirical, especially in the two-phase regime. Therefore, overcoming these limitations requires adequate visualization technology to resolve the film flow dynamics and related interactions.
Herein, Dr. Xiao Zhao, Dr. Haifeng Zhang, Professor Bo Zhang and Dr. Xuehu Ma from Dalian University of Technology conducted a quantitative experiment to capture the heat transfer characteristics and dynamics of the liquid film in HFE-7000 spray cooling using high-speed camera technology. Also captured under different heat fluxes and inlet pressure was the film morphology and velocity, contact line length and wetted area. The work is currently published in the International Journal of Heat and Mass Transfer.
The research team showed that an increase in the heat flux toward the critical heat flux regime was characterized by gradual domination of the thermally induced agglomeration and the dispersion of the continuous liquid film into an isolated state. Based on the critical wetted area interval, the isolated state was called the ‘isolated film regime’. Under various conditions, the velocity of the isolated film was largely influenced by the velocity of the atomized surface conditions, droplets and surface temperature of the gradient. While the atomized droplet provided the initial momentum, the film was slowed down by the surface friction force.
The interactions among the isolated films in the isolated liquid regime were relatively weak and the isolated structure could be merely simplified as the coexistence of the liquid film flow and the impact of the droplet. Importantly, statistical models were proposed to predict the velocity distribution of the isolated films, and about 80.4% of the data were reportedly within the prediction limit of and a mean relative error of 7.9%, indicating the feasibility of the prediction models.
In summary, the dynamics of the isolated liquid film in spray colling were quantitatively studied. The concept enabled a quantitative experimental analysis of the liquid film flow to provide a better understanding of the dynamics of the isolated film flow, velocity, wetted area, contact length, as well as the relationship between the heat transfer and liquid film characteristics. In a statement to Advances in Engineering, the authors explained that their study provides a comprehensive understanding of film flow dynamics and would contribute to establishing advanced numerical and theoretical heat transfer models.
Zhao, X., Zhang, H., Zhang, B., & Ma, X. (2022). Dynamics and heat transfer characteristics of isolated liquid film in spray cooling. International Journal of Heat and Mass Transfer, 183, 122037.