Filtered Rayleigh Scattering-Based Concentration Measurements for a Horizontal Buoyant Jet

Abstract

Filtered Rayleigh scattering (FRS) is a non-intrusive technique which is used to study a variety of flowfields. It utilizes a narrow linewidth laser, an absorption filter, and a detector. The laser illuminates the flow field and molecular scattering provides the signal to the camera. The difference in molecular cross section can be used to discriminate between unmixed gaseous components. The focus of this research is to document the behavior of a horizontal buoyant jet of gaseous helium using FRS. Despite its simplicity, the behavior of the near field of horizontal buoyant jets is sparsely described in the literature compared to vertical buoyant jets. Gross features, such as jet core trajectory and gross mixing rate are generally characterized using Reynolds number (Re) and Grashof number (Gr), and are presented herein as well. The FRS system for this research consists of a continuous coherent argon Sabre R series ion laser, an iodine absorption filter, and a PCO.4000 camera. A helium jet of precisely controlled mass flow rate is flows through a tube into the surrounding ambient laboratory with a very low velocity co-flow of particle free air. The behavior of jets with Reynolds numbers under 1000 are described in this report. Our results of our study show a clear expulsion of core fluid in the form of side jets, consistent with helical mode excitation. The pattern of core fluid expulsion is striking and apparently is Reynolds number dependent. Additionally, the trajectory and mixing characteristics of the buoyant jet were analyzed as a function of downstream distance. A significant portion of this study is dedicated to measuring the rate at which FRS data can be acquired with a continuous wave laser. Exposure times as small as 0.01 seconds provided some data, though most of the analysis was performed for longer time exposures to improve the signal to noise ratio.