Simultaneous Vapor and Droplet Measurement Using Wavelength -Multiplexed Laser Extinction 

This research focuses on the continuous detection of vapor and droplet properties in sprays simultaneously with rapid time response (better than 1 ms). Such properties include vapor concentration, vapor temperature, droplet number density, droplet size distribution, etc. The goal of this research is provide diagnostics for the research of spray combustion and droplet evaporation, and also to provide sensors for the control of spray generation and spray combustion.

This research need to overcome several challenges. First, in most applications, the signal (Mie scattering) from droplets greatly exceeds that from the vapor (usually LIF or absorption), therefore rendering the discrimination of vapor signal difficult. Second, the size of droplets frequently spans a wide range from sub-micron to tens of microns in many applications, requiring the droplet diagnostic to have a wide dynamics range. We developed an approached based on Wavelength-Multiplexed Laser Extinction (WMLE) to overcome these challenges. A schematic of the WMLE concept is shown below.

Here, the WMLE scheme consists of laser sources spanning from visible to mid-IR. Among these wavelengths, some wavelengths (the first, second, and fourth shown above), are off the absorption features of the vapor. Therefore, they are used to measure the droplet size, droplet volume fraction, and distribution function of the droplets based on Mie scattering without interference from vapor. Other wavelengths (the third, fourth, and firth) are on the absorption feature of the vapor and are used to monitor vapor concentration and temperature. Here, a differential absorption technique is used at these wavelengths to minimize the interference from droplet scattering.