Ultrasonic scattering measurements of jet gas–liquid interface fluctuations in confined spaces

Determining jet gas–liquid interfaces in liquid rocket engines is crucial for understanding the mechanisms underlying combustion instabilities. While optical diagnostic methods are commonly employed, they become ineffective when optical access is restricted. In such cases, ultrasonic techniques provide a viable alternative. The present study develops an ultrasonic method to detect jet gas–liquid interfaces in a confined chamber based on ultrasonic scattering principles. The fluctuations of the water jet's gas–liquid interface are precisely captured using this method and validated against high-speed optical imaging. For the optical method, sub-pixel boundary extraction is used to obtain the jet interfaces, while for ultrasonic signal processing, we extract the pulsed scattered waves generated by the jet in the confined space. The experimental results align well with theoretical predictions. Additionally, to facilitate accurate measurement of gas–liquid interface fluctuations, we create a database of ultrasonic scattering results using a set of metal rods to model the liquid jet. This enables high-frequency, high-precision measurements of jet interface fluctuations using ultrasonics. A comparison between the ultrasonic and optical methods reveals a mean measurement error of 70 μ m (3.45% mean relative error) and a maximum error of 250 μ m (12.6% maximum relative error), with the ultrasonic method offering a temporal resolution of 1 kHz. This proposed method provides a novel solution for measuring two-phase flow parameters in confined environments where optical access is limited.