Experimental and theoretical studies of spray impingement on a hot surface in reacting stagnation flows

摘要

This paper reports experimental and theoretical results concerning spray impingement on a hot wall in a laminar stagnating flow, both with and without a flat premixed flame present near the solid surface. Water sprays with drop diameters up to 200 mum are transported in a fuel-lean methane-air stream from a circular duct toward a stainless-steel plate. When the flame is present it heats the wall to a temperature around 1250 K and causes the smaller droplets to disappear as they approach it while the larger droplets cross the flame and impinge on the hot wall. Influences of the wall temperature on droplet impingement are studied by extinguishing the flame and allowing the plate to cool gradually to room temperature. Trajectories of the water droplets in the spray, illuminated by a Ar~+-laser, are recorded by a camera. Measurements of droplet diameter and of two components of velocity are made by a phase doppler particle analyzer (PDPA). Velocity profiles of the gas without spray are also measured with the PDPA by seeding the flow with MgO particles. The experimental results provide fundamental information on droplet vaporization in the flame, on the relationshipo between arrival and departure velocities of droplets striking the wall and on the influence of wall temperature on droplet breakup. Simple theoretical models are developed to describe droplet collision dynamics at the wall and droplet trajectories in the stagnation flow. Comparisons between theory and experiment reveal how droplets respond to the flame and to the hot wall.