Gas/surface heat transfer in spray deposition processes

A numerical investigation of heat transfer dynamics between gas and solid surfaces during droplet spray impingement is presented. Aim of the work is to derive knowledge for control of spray deposition processes like spray painting or spray forming, analysing how the heat exchanged from the surface to the flowing gas is affected by the presence of impinging droplets. The investigation is carried on a macro- and a micro-scale, analysing velocity and temperature profiles close to a surface cooled by a spray on a scale of the whole spray and on a scale comparable to the droplet diameter, respectively. In the former case an Euler-Lagrange approach is used to reproduce the multiphase jet/spray for different nozzle geometries, gas conditions and droplets properties, as drop diameter and concentration. In the latter case, the gas flow close to the surface is studied during the collision of single and multiple droplets for different impact velocities superposed by different perpendicular gas boundary layer configurations. The "volume of fluid" (VOF) technique is utilized for the determination of the transient shape of the gas-liquid interface during droplet impact. From the data of the numerical case studies, a quantitative consideration about the global increase of surface/gas heat transfer in impinging dilute sprays as a function of the number flux of particles approaching the wall is derive