Cavitation Instability Mechanism and Thermal Effects in a Turbopump Inducer

Abstract

An experimental investigation has been conducted on the cause and pulsation mechanism of rotating cavitation and thermal effects on the onset of rotating cavitation. To achieve the research objectives, the test facility has been designed and constructed in Seoul National University.

Based on the previous research, the author hypothesized that the incidence angle variation leads to the onset of rotating cavitation. To confirm the hypothesis, the incidence angle has been measured near the tip region of the leading edge with and without rotating cavitation through PIV (Particle Image Velocimetry) measurement method. Under the rotating cavitation conditions, large and small tip leakage vortex cavitation regions are formed, respectively, and the cavitation region at each blade becomes uneven. The large tip leakage vortex cavitation reduces the following blade incidence angle to the negative value and suppresses the cavitation region. On the other hand, the small tip leakage vortex cavitation increases the following blade incidence angle to the positive value and promotes the cavitation region. Reduction of the incidence angle due to the cavitation region leads to the onset of rotating cavitation. Based on the suppression and promotion mechanism, cavitation region at each blade pulsates in sequence at the measured rotating cavitation frequency. The propagation of rotating cavitation has also been confirmed by high-speed camera visualization. Through the time-resolved static pressure measured at the inlet of the turbopump inducer, the onset cavitation number of rotating cavitation has been determined for varying Reynolds number and non-dimensional thermal parameter values. Increasing non-dimensional thermal parameter suppresses rotating cavitation and causes a monotonic decrease in the rotating cavitation onset cavitation number. At low non-dimensional thermal parameter values (e.g., 0.0125), the onset cavitation number is independent of the Reynolds number. However, at higher values of the non-dimensional thermal parameter (e.g. higher than 0.0537), the onset cavitation number increases with increasing Reynolds number. Thus, the Reynolds number promotes rotating cavitation onset.

This study provides the first experimental results of the cause and mechanism of rotating cavitation. The first assessment of the individual effects of the non-dimensional thermal parameter and Reynolds number is also presented.