Koichi Yonezawa

One-Dimensional Analysis of Full Load Draft Tube Surge

One-dimensional stability analysis of a hydraulic system composed of a penstock, a runner and a draft tube was carried out to determine the cause of the full load draft tube surge. It is assumed that the cavity volume at the runner exit is a function of the pressure at the vortex core evaluated from the instantaneous local pressure at the runner exit and an additional pressure decrease due to the centrifugal force on the swirling flow. It was found that the diffuser effect of the draft tube has a destabilizing effect over all flow rates while the swirl effects stabilize/destabilize the system at larger/smaller flow rates than the swirl free flow rate. Explanations of the destabilizing mechanism are given for the diffuser and swirl flow effects. The effect of finiteness of sound velocity in the penstock is also discussed

Cause of Cavitation Instabilities in Three Dimensional Inducer

Alternate blade cavitation, rotating cavitation and cavitation surge in rocket turbopump inducers were simulated by a three dimensional commercial CFD code. In order to clarify the cause of cavitation instabilities, the velocity disturbance caused by cavitation was obtained by subtracting the velocity vector under non-cavitating condition from that under cavitating condition. It was found that there exists a disturbance flow towards the trailing edge of the tip cavity. This flow has an axial flow component towards downstream which reduces the incidence angle to the next blade. It was found that all of the cavitation instabilities start to occur when this flow starts to interact with the leading edge of the next blade. The existence of the disturbance flow was validated by experiments.

Three Dimensional Unsteady Flow Simulation of Compressed Truncated Perfect Nozzles

Recently, a large side load was observed during the startup and shutdown transients of the Japanese LE-7A rocket engine nozzle. The objective of the present paper is to clarify the mechanism producing the large side loads during the startup transient. The unsteady three-dimensional Navier-Stokes equations are solved for the startup transients of the three types of nozzle contours – the Truncated Perfect (TP) nozzle, the 86% Compressed Truncated Perfect (CTP) nozzle and the CTP50-R5-L nozzle. The contour of the TP nozzle is the LE-7 nozzle and the contour of the 86% CTP nozzle is the LE-7A nozzle. The CTP50-R5-L nozzle was tested by Tomita et al and Takahashi et al . The asymmetric flow patterns and the large side loads are observed in LE-7A and CTP50-R5-L nozzle. There are two types of flow patterns that cause the large side loads. The first is the simultaneous occurrence of the Free Shock Separation (FSS) and the Restricted Shock Separation (RSS). The second is the oscillation of shock wave under RSS condition . The interaction between the internal shock and the Mach disk causes the cap shock and the trapped vortex. For both flow patterns, the trapped vortex oscillates with the cap shock. The trapped vortex is developed with oscillation as the Nozzle Pressure Ratio (NPR) increases. At a certain range of the NPR, oscillations of the cap shock and the trapped vortex become larger, and then the supersonic jet on the both sides of the cap shock reattaches to the nozzle wall partially –the FSS and the RSS occur simultaneously. The second flow pattern is observed in CTP50-R5-L nozzle. With the RSS, the cap shock oscillates with the trapped vortex. These flow patterns produce the asymmetric wall pressure distribution and the large side load. Nomenclature p : Pressure R : Gas constant γ : Specific heat ratio M : Mach number NPR : Nozzle Pressure Ratio, a c p p / Subscript c : combustion chamber a : ambient Introduction Several new rocket engines in the world have the new-shaped nozzle instead of the conventional Truncated Perfect (TP) nozzle. For example, the thrust-optimized nozzles are applied for the American Space Shuttle Main Engine (SSME) and the European Vulcain. On the other hand, the Japanese LE-7A nozzle is designed as the Compressed Truncated Perfect (CTP) nozzle. In these nozzles, the large side loads are observed during the startup and shutdown transients at the sea level firing test. The side loads are undesirable since it may damage the engine support system. The side loads in rocket nozzles have been the subjects of various studies, but the detailed flow structures and the mechanisms of the side loads have not been understood completely. The flow separates from the nozzle wall under the low chamber pressure condition during the startup and shutdown transients. It is considered that the behavior of the separated flow influences on the side load. *Graduate Student, Graduate School of Engineering Science, Osaka University †Associate Professor, Department of Mechanical Engineering, Nagoya Institute of Technology, Member AIAA ‡Professor, Graduate School of Engineering Science, Osaka University, Member AIAA §Senior Engineer, NASDA, Member, AIAA ¶Associate Senior Engineer, NASDA, Member AIAA 38th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit 7-10 July 2002, Indianapolis, Indiana AIAA 2002-3991 Copyright

Suppression of Cavitation Instabilities in an Inducer by Circumferential Groove and Explanation of Higher Frequency Components

The purpose of the present research is to suppress cavitation instabilities by using a circumferential groove. The circumferential groove was designed based on CFD so that the tip leakage vortex is trapped by the groove and does not interact with the next blade. Experimental results show that the groove can suppress rotating cavitation, asymmetric cavitation and cavitation surge. However, weak instabilities with higher frequency could not be suppressed by the groove. From the analysis of pressure pattern similar to that for rotor-stator interaction, it was found that the higher frequency components are caused by the interaction of backflow vortices with the inducer blades.

One-Dimensional Analysis of Full Load Draft Tube Surge Considering the Finite Sound Velocity in the Penstock

The effects of acoustic modes in the penstock on the self-excited oscillation in hydraulic power system were studied by assuming a finite sound velocity in the penstock. The flow in the draft tube is considered to be incompressible assuming that the length of the draft tube is smaller than the wavelength of the oscillation. It was found that various acoustic modes in the penstock can become unstable (amplified) by the diffuser effect of the draft tube or the effect of swirl flow from the runner. Their effects on each mode are discussed.

Experimental method for the evaluation of the dynamic transfer matrix using pressure transducers

ABSTRACT This paper introduces an experimental method for the evaluation of dynamic transfer matrices using only pressure transducers. The discharge fluctuations are evaluated from the fluctuation of the pressure difference at different streamwise locations. The transfer matrices of the resistance, the inertance and the compliance elements are determined by using simple flow configurations. This method is then validated by comparing the transfer matrix components to theoretical values. The results show that the direct measurement of the transfer matrices produces good results below the first structural eigenfrequency of the system. Furthermore, a deviation from the mass continuity in the amplitude ratio of the fluctuating upstream and downstream discharges is investigated. This behaviour can be explained with a simple model taking into account the compliance in the system.

Control of Operation Mode Transition in Dual-Bell Nozzles with Film Cooling

In addition to aspiration drag, the advantages gained by dual-bell nozzle utilization in practice are circumvented with a dip of thrust during operation mode transitions that occur outside of the optimal conditions. Practical transition from lowto high-altitude operationmode occurs at lower nozzle pressure ratios (NPRs) than the optimum transition point. The influence of the film-cooling flow on nozzle operationmode transition is investigated. It is found that the operationmode transitionNPR is increased, and the thrust dip during the transition is decreased through the injection of the film-cooling flow with an appropriate inlet pressure and flow rate.

Cavitation Surge in a Small Model Test Facility Simulating a Hydraulic Power Plant

Model tests and CFD were carried out to find out the cause of cavitation surge in hydraulic power plants. In experiments the cavitation surge was observed at flow rate, both with and without a surge tank placed just upstream of the inlet volute. The surge frequency at smaller flow rate was much smaller than the swirl mode frequency caused by the whirl of vortex rope. An unsteady CFD was carried out with two boundary conditions: (1) the flow rate is fixed to be constant at the volute inlet, (2) the total pressure is kept constant at the volute inlet, corresponding to the experiments without/with the surge tank. The surge was observed with both boundary conditions at both higher and lower flow rates. Discussions as to the cause of the surge are made based on additional tests with an orifice at the diffuser exit, and with the diffuser replaced with a straight pipe.

Inducer Design to Avoid Cavitation Instabilities

Three inducers were designed to avoid cavitation instabilities. This was accomplished by avoiding the interaction of tip cavity with the leading edge of the next blade. The first one was designed with extremely larger leading edge sweep, the second and third ones were designed with smaller incidence angle by reducing the inlet blade angle or increasing the design flow rate, respectively. The inducer with larger design flow rate has larger outlet blade angle to obtain sufficient pressure rise. The inducer with larger sweep could suppress the cavitation instabilities in higher flow rates more than 95% of design flow coefficient, owing to weaker tip leakage vortex cavity with stronger disturbance by backflow vortices. The inducer with larger outlet blade angle could avoid the cavitation instabilities at higher flow rates, owing to the extension of the tip cavity along the suction surface of the blade. The inducer with smaller inlet blade angle could avoid the cavitation instabilities at higher flow rates, owing to the occurrence of the cavity first in the blade passage and its extension upstream. The cavity shape and suction performance were reasonably simulated by three dimensional CFD computations under the steady cavitating condition, except for the backflow vortex cavity. The difference in the growth of cavity for each inducer is explained from the difference of the pressure distribution on the suction side of the blades.

LE-7A Engine Nozzle Flow Separation Phenomenon and the Possibility of RSS Suppression by the Step inside the Nozzle

LE-7A engine was encountered two nozzle extension troubles under the early development. These troubles are the two kinds of large side loads and the some nozzle tubes damage during the start up and shut down transients. The Restricted Shock Separation caused a kind of large side loads and regenerative cooling tube damage. The largest side loads was caused by the separation point stagnation and jump on the film cooling step inside the nozzle. The LE-7A original nozzle phenomenon such as ‘the separation stagnation and jump’ and ‘the RSS annihilation and occurrence’ indicates a significant suggestion to suppress the RSS flow by the small step inside the nozzle. This paper presents a focus on the LE-7A engine specific separation phenomenon during the startup and shutdown transients and the possibility of RSS suppression by the step inside the nozzle.