title: Experimental investigation of fluid dynamics and heat transport during single and multiple drop impingement onto a superheated wall
Creators: name: Gholijani, Alireza
Publisher: name: UNSPECIFIED
Year: 2023

description: This thesis is devoted to experimental investigation of hydrodynamics and heat transport during the impingement of a single drop and multiple drops onto a wall, whose temperature is above the liquid saturation temperature and below the onset of nucleate boiling. The drop impingement onto heated surfaces occurs in many industrial applications, for instance, spray cooling, which is considered among the most efficient cooling methods. By considering the fact that spray systems are comprised of an enormous number of interacting drops, a detailed characterization of fluid dynamics and heat transport mechanisms during the impact of individual drops throughout spraying process is complicated. Thus, many studies concentrate on fluid dynamics and heat transport mechanisms of a single drop or a group of drops impacting onto a surface under well-controlled conditions to obtain detailed knowledge of underlying physics of the spray cooling process. Although the drop dynamics during the isothermal drop impingement (non-heated surface) has been widely studied in the past decades, the fluid dynamics and heat transfer to the drop during the impingement process in the non-isothermal case (heated surface), in which evaporation plays a crucial role is not fully understood yet. Numerous studies on pool boiling and meniscus evaporation have reported a temperature minimum and accordingly, a huge evaporation rate in proximity of the three-phase contact line, where solid, liquid, and gas phases meet each other. The evaporation in this region constitutes a significant fraction of the overall heat transfer. Hence, any alteration in physical or thermodynamical parameter on the three-phase contact line could strongly affect the overall heat transfer. Some examples of influencing parameters are wall superheat, impact velocity, drop size, system pressure, and surface morphology. These parameters also affect the convection, considered to be the main heat transfer mechanism at the early stages of drop impingement. Therefore, the effects of aforementioned parameters on fluid dynamics and heat transport are investigated in the scope of this thesis. This study employs a high-resolution temperature measurement technique, allowing high temporal and spatial resolution of the heat flux. It is an accurate and detailed approach to investigate the drop hydrodynamics and heat transport mechanism during non-isothermal drop impact. The experimental results reveal that higher wall superheats, higher impact velocities, larger drop diameters, and lower system pressures each result in increasing heat flow to the drop after the impingement. The maximum spreading radius after impingement increases with the increase of impact velocity and impact diameter, and decreases with the increase of wall superheat and system pressure. Besides, the impact of a drop onto a porous surface is accompanied by lower heat flow at the early stages of impact, while it enhances significantly at the late stages of impact. The heat flow enhancement is due to the large solid–liquid contact area caused by the drop pinning effect. The last part of the thesis focuses on impingement of multiple drops onto a superheated wall as it represents a next step in modeling of spray systems compared to a single drop impact. Multiple drop impingement is far less investigated than the single drop impingement due to its complexity of the fluid mechanic and heat transfer mechanisms. In this thesis, hydrodynamics and heat transport during vertical and horizontal coalescence of multiple drops (successive and simultaneous drop impact) over a heated surface are addressed, as well. The investigations reveal that the solid–liquid contact area and accordingly heat flow rise after successive impacts. However, drop coalescence during simultaneous drop impingement delivers lower heat flow in comparison with non-coalescence cases. The experimental results of single drop impact onto a bare heater and at atmospheric pressure are compared against the numerical model developed in the author’s institute. A good agreement between measurements and model predictions has been observed. The simulation results are used to present a more accurate analysis and interpretation of the experimental results.