Dynamic Vapour Fraction Measurement in Turbopump Inducers by X-Rays

This work intended to evaluate the instantaneous vapour fraction in the turbo-pump inducer of a liquid propellant rocket engine. Experimentations held on an experimental pump test facility and cavitation was attained by reducing the inlet pressure in the machine while maintaining constant the inducer rotational speed. Measurements of vapour fraction through the rotating inducer were achieved by means of an x-ray-based system. The system exerted an industrial x-ray generator and 10 collimated scintillation detectors. Detectors were functioning in current mode thus permitting an acquisition at 5 kHz for each detector. A reference x-ray detector situated between the x-ray generator and the machine permitted the treatment of x-ray beam energy fluctuations related to industrial generators [1, 2, 3]. Acquisitions were performed in three axial positions on the inducer. For each measurement position, three cavitation sequences with different flow rate conditions (Q/Qn = 0.9, 1, 1.1, where Qn is the nominal flow rate) were accomplished. Each cycle is performed by decreasing gradually the pressure while maintaining an imposed rotational speed of 4000 rpm. Each test is constituted of 10 pressure points varying from 2.40 to 0.48 bars representing a complete cavitation sequence. X-ray acquisition was performed for each pressure point and it was carried out for 10 seconds thus corresponding to 667 tours of the inducer. Vapour fraction was determined instantaneously thus showing the applicability and the precision of the method in such measurements despite of the geometry and rotation speed constraints. Consequently a quantitative and qualitative evaluation of the vapour fraction is presented. Results show that the vapour distribution is well related to cavitation development on the blades of the inducer for steady cavitation condition.