Numerical simulation of drop impact and evaporation on superheated surfaces at low and high ambient pressures

Abstract

This thesis presents the numerical simulation of fluid dynamics, as well as heat and mass transfer for drop impingement on a hot solid surface for low and high ambient pressures. The technical application ranges from effective thermal management strategies using spray cooling, safety aspects in high pressure nuclear reactors to process technology in chemical or food industry. It is reported in literature that wetting characteristics depend on the ambient pressure. Drop splash is suppressed at low ambient pressure. High ambient pressure encourages compressibility effects. The compressibility of both the liquid and vapour phase increases with increasing pressure. Thereby, the effects of compressibility on drop impingement is of interest. Up to now, no attempt has been made to investigate a full pressure range for the evaporative drop impingement process. In order to provide insights into evaporative drop impingement processes under various ambient pressures, numerical simulations are performed. CFD simulations are conducted using a finite volume discretisation method solving the Navier-Stokes equations. The volume of fluid method is utilised to resolve two-phase flow. The solver accounts for compressible fluid flow, heat and mass transfer due to evaporation across the free liquid-vapour interface, evaporation in the vicinity of the three-phase contact line, as well as for heat conduction within the solid substrate. The dynamic contact angle is implemented using a subscale model. Effects of low and high ambient pressure on the three-phase contact line are investigated in the well established so-called micro region model. The focus is the non-splashing drop-wall collision in a non-boiling, single-component evaporation regime. Ambient pressure ratios ranging between p/pcr = [8*10^{-3} ... 0.5], Reynolds and Weber numbers ranging between Re = [600 ... 1300] and We = [10 ... 50] are investigated. The wall temperature is above saturation but below Leidenfrost temperature. The wall superheat is in the order of 10 K. Different parameter studies are dedicated to investigate the influence of low and high ambient pressures on the evaporative drop impact processes. Within one parameter study, dimensional drop impact parameters are kept constant, such as drop diameter, impact velocity and wall superheat. Caused by the variation in ambient pressure, material properties of the fluid change. Consequently, non-dimensional groups are changing, indicating a shift in dominant forces. Another parameter study keeps non-dimensional groups constant. Further parameter studies focus on the influence of the vapour phase on the drop impact outcome, especially for high ambient pressure. Within this work, results are presented for different length scales. The modelling of the vicinity of an evaporating three-phase contact line indicates a strong influence of the ambient pressure on the apparent contact angle and the heat being transferred in the micro region. For increasing pressure, the contact angle increases whereas the transferred heat has a local maximum within the investigated pressure range. For the macro-scale drop impingement process, strong influence on the fluid dynamics and heat transfer is identified. In summary, numerical simulations of the evaporative drop impact and the modelling of micro-scale thermodynamic effects for low and high ambient pressure are investigated in the present thesis. The results increase the understanding of the influence of pressure on the fluid dynamics, as well as the heat and mass transfer. Correlations for the maximum spreading ratio, spreading duration, as well as transferred energy and mass are reported. The findings are expected to improve design concepts for technical applications within the investigated parameter range.