Effects of backpressure on self-pulsation characteristics of liquid-centered swirl coaxial injectors

A combined experimental and theoretical investigation is conducted to explore the self-pulsation characteristics generated by a liquid-centered swirl coaxial injector (LCSC) under various chamber backpressures. Filtered water and dried air were employed as simulant mediums. Data were obtained at pressures from 0.1 to 3.01 MPa and the momentum flux ratios from 0.004 to 15.99. The back-lighting photography technique was employed to capture the instantaneous self-pulsated spray and stable spray images with a high speed camera. A new empirical equation was presented to calculate the liquid film thickness at the pressure swirl injector exit without trumpet, and the flow characteristics in the trumpet of the inner injector have been calculated. Based on these researches, a theoretical analysis model has been proposed to analyze the flow pattern by calculating the liquid film angle in recess chamber. With the increase of backpressure, the flow in the LCSC injector changes from the outer mixing flow to the critical mixing flow, and develops into the inner mixing flow ultimately. Meanwhile, self-pulsation is intensified initially and then suppressed, while self-pulsation frequency fluctuates within a certain range. Self-pulsation is the strongest when liquid film angle is around the recess angle. Both variations of backpressure and injection conditions can change the flow pattern in recess chamber of the LCSC injector. When the flow pattern deviates from the critical mixing flow, self-pulsation is suppressed and the spray angle decreases accordingly. Backpressure affects self-pulsation by transforming the flow pattern in recess chamber and then strengthening or weakening the blocking actions of the conical liquid sheet. (C) 2019 Elsevier Ltd. All rights reserved.