Droplet concentration in water spray using short-range elastic backscatter lidar

To our knowledge, a novel experimental method is proposed to remotely measure the droplet concentration in a water spray using a short-range elastic backscatter lidar. A specific calibration technique is proposed to determine the lidar radiometric constant, enabling the conversion of lidar signals into attenuated backscatter signals and, ultimately, into aerosol properties, including average volume concentration. To our knowledge, a new formulation for the lidar constant is proposed, using the total spray transmittance and the particle size distribution of the water droplets, measured using the established laser diffraction technique. The lidar constant value is then compared and discussed in relation to an alternative method based on a Lambertian surface. Ultimately, the attenuated backscatter signals, calculated using the calibration constant, enable the estimation of the average volume concentrations of droplets in the spray under various injection conditions of a full-cone pneumatic atomizer. The retrieved concentrations range between 10⁸ and 10⁹ droplets per cubic meter, which are comparable to those obtained from laser diffraction measurements, especially for sprays with large droplets, i.e., with a Sauter mean diameter greater than 50 µm.