Drop size distribution evolution after continuous or intermittent injection of butane or propane in a confined air flow

Drop size distributions and velocities have been measured of n-butane and propane sprays, rapidly evaporating in air flowing at constant velocity, 15 m/s typically. The inlet air temperature has been found to be of main importance in the evaporation process. After a period of the order of the relaxation time, τ, of the relative velocity, drop temperatures become almost constant at temperatures as low as −40°C (n-butane), nearly independent of initial droplet radius and temperature. At inlet temperatures 0°C, or less, of air and −18°C of n-butane evaporation effectively stops after τ s, which is explained by the occurrence of hydrates at the drop surface. The Sauter mean diameter (SMD) increases downstream with continuous injection. If the evaporation rate would not depend on the drop radius, r, this increase would not have been found in the experiments. If τ would not depend on r, the SMD would not decrease in the tip and tail of each spray pulse during intermittent injection. This decrease is measured and explained. Directly following injection, conductive heat transfer inside the droplets is important. In combination with a multigroup approach, the measured history of an SMD has successfully been reproduced. To obtain a homogeneous spray pulse, injection should preferably take place in a converging channel.