Many theoretical and experimental studies have been conducted to investigate elements of swirl injector hydrodynamics, such as variations in liquid film thickness or air core diameter. From these studies, some theoretical relationships have been established through an approximate analytical solution of flow hydrodynamics in a swirl nozzle. However, experimental studies on elements such as the measurement of liquid film thickness have not produced conclusive results. In a swirl injector, the atomization process is significantly influenced by the liquid film thickness. Thus, it is possible to investigate the effects of various geometric parameters on spray characteristics through the measurement of liquid film thickness. We used a specially designed injector based on the electrical conductance method to measure the liquid film thickness accurately. The liquid film thickness was measured through precise calibration, and the accuracy of this measurement was demonstrated in comparison with previous theories and experiments. From these results, we present an empirical relation for the liquid film thickness by adding orifice length to an existing analytical equation. The variations and stability of the air core were also examined by visualizing the formation of the air core in the swirl chamber with a high-speed camera system. This study confirms that air core shape and liquid film thickness are directly related. Thus, study of the fluctuations of liquid film thickness under various geometric conditions can be applied to the analysis of internal flow.

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When a heat release and acoustic pressure fluctuations are generated in a combustor by irregular and local combustions, these fluctuations affect the mass flow rate of the propellants injected through the injectors. In addition, variations of the mass flow rate caused by these fluctuations bring about irregular combustion, which is associated with combustion instability, so it is very important to identify a mass variation through the pressure fluctuation on the injector and to investigate its transfer function. Therefore, quantification of the variation of the mass flow rate generated in a simplex swirl injector via the injection pressure fluctuation was the subject of an initial study. To acquire the transient mass flow rate in the orifice with time, the axial velocity of flows and the liquid film thickness in the orifice were measured. The axial velocity was acquired through a theoretical approach after measuring the pressure in the orifice. In an effort to understand the flow area in the orifice, the liquid film thickness was measured by an electric conductance method. In the results, the mass flow rate calculated from the axial velocity and the liquid film thickness measured by the electric conductance method in the orifice was in good agreement with the mass flow rate acquired by the direct measuring method in a small error range within 1% in the steady state and within 4% for the average mass flow rate in a pulsated state. Also, the amplitude (gain) of the mass flow rate acquired by the proposed direct measuring method was confirmed using the PLLIF technique in the low pressure fluctuation frequency ranges with an error under 6%. This study shows that our proposed method can be used to measure the mass flow rate not only in the steady state but also in the unsteady state (or the pulsated state). Moreover, this method shows very high accuracy based on the experimental results.