Gas-phase velocity estimation in practical sprays by Phase-Doppler technique

Abstract

Practical sprays are characterized by a two-way coupling between droplets and the surrounding gas. The effect of sprays on process performance is critical in numerous applications; hence, the gas-phase velocity is often estimated via two methods, using Phase Doppler measurement data. The first one is using a rule of thumb, i.e., estimating the gas-phase velocity by analyzing droplet sizes below a few micrometers. The second way of estimating the Stokes number, Stk, is using a threshold well below unity, such as 0.1. There are numerous definitions available in the literature for Stk, resulting in several magnitudes difference. These complex problems are resolved in this paper in the following way. A new definition for Stk is provided, which is sufficiently robust for, e.g., pressure and twin-fluid atomizers. According to the results, the Stk < 0.1 threshold means droplet sizes between 2 and 10 micrometers above 10 m/s gas-phase velocities. Filtering for too small droplets could lead to biased characteristics, especially in estimating the turbulent properties. Hence, the gas-phase velocity estimation is a function of the measurement setup and has a significant spatial dependence. The reader can find the software code online and the algorithm in Appendix A.