Monofiber optical probe using Doppler signals detection for Drop Size and Velocity measurement in air assisted atomization

TL;DR

This paper introduces a novel monofiber optical Doppler probe for measuring droplet size and velocity in dense sprays, demonstrating high accuracy without calibration in assisted atomization conditions.

Contribution

It presents a new optical probe combining phase detection and Doppler signals, capable of reliable measurements without calibration in complex spray flows.

Findings

Probe achieves 92-99% of injected liquid flow rate in tests.

Reliable drop velocity and size statistics obtained.

Effective signal processing routine developed for complex conditions.

Abstract

Reliable measurement of droplet/bubble size and velocity distributions in dense flows is desired in a variety of research fields, both for laboratory and industrial use. A new type of single-mode monofiber optical probe manufactured by A2 Photonic Sensors is introduced in this paper: it combines traditional phase detection with the collection of a Doppler signal returned by an incoming gas-liquid interface to provide information on residence times, drop concentration and velocity, which afford then drop chords and liquid flux measurements. Compared with classical optical probes, that new sensor does not require any calibration. The purpose of the present work is to test this technique in assisted atomization in order to provide a mean for spray characterization and ultimately to improve our understanding of atomization mechanisms. The probe has been tested downstream of a coaxial air-assisted atomizer operated at liquid velocity =. to. / , and gas velocity from = to about /. We first analyzed raw signals in various flow conditions. It happens that, when increasing the gas velocity and the number density of drops, the signal experiences very strong fluctuations of the gas level, making the identification of individual droplets more difficult. That leads us to develop a new signal processing routine specifically adapted to such complex working conditions. At = / and for three liquid flow rates, the spatial integration of local liquid fluxes represents 92 to 99% of the injected liquid flow rate. These good results demonstrate that the Doppler probe provides reliable statistics on drops velocity and size.