Yoshiyuki Nakase

Quasi-Three-Dimensional Analysis of Cavitation in an Inducer

A method for the prediction of steady cavitation in turbopumps is proposed on the assumption that the fluid is inviscid and the stream surface is rotationally symmetric. The analysis in the meridian plane is combined with that in a blade-to-blade stream surface where a singularity method based on a closed cavity model is used. The present method is applied to a helical inducer

Structure of a Turbulence Wedge Developed from a Single Roughness Element on a Flat Plate

A turbulence wedge which develops downstream from a single roughness element placed in a laminar boundary layer on a flat plate is experimentally investigated. Mean and fluctuating velocities and the intermittency factor were measured. Contours of the wedge and its development in the normal and spanwise directions were clarified by drawing intermittency contour lines. The mean properties inside of the wedge just behind the roughness did not reach fully developed turbulence, and they gradually developed in the downstream direction. The contour of the wedge is not formed by straight lines very near or far from the roughness, and the extent of the wedge in which the lines are straight is limited. Just behind the roughness, distributions of the mean and fluctuating velocities are complicatedly distorted, so the existence of several vortices is presumed. The intermittency contour Lines in the section normal to the flow are also presented.

Quasi Three-Dimensional Analysis of Cavitation in an Inducer

A quasi three-dimensional analysis of steady cavitation in a herical inducer is carried out by a method based on the assumption that the fluid is inviscid and the stream surface is rotationally symmetric. The analysis in the meridian plane are combined with that in blade-to-blade stream surface where a singularity method is used based on a closed cavity model. It was found that the influence of the three-dimensionality of the flow on cavitation mainly appears as the change of angle of attack associated with the change of meridian velocity caused by the movement of meridian streamline in radial direction.Copyright

Properties of a Relaminarizing Turbulent Boundary Layer under a Favorable Pressure Gradient.(Analysis of Bursting Structure with VITA Technique).

The bursting phenomena of the relaminarizing turbulent boundary layer on a flat plate are experimentally investigated using the VITA technique. First, an attempt is made to obtain a normalized bursting frequency universally applicable to the development process of the turbulent boundary layer, the relaminarization process and the retransition process. The ensemble averages of the streamwise fluctuating velocities are drawn and the characteristics of each process are investigated. No time scales can render the non-dimensional bursting frequencies universally constant. The tendency of the distribution of the bursting frequency in the relaminarization process is different from that in the development and retransition process of the turbulent boundary layer. This presumably supports the previous visualization result, according to which the relaminarization process begins from the outer region. In the ensemble averages of the fluctating velocity, it is considered that the relaminarization changes the ejection and sweep, though it does not particularly attenuate the bursting in the inner layer. This phenomenon is qualitatively explained with the aid of the mean velocity profiles and the mixing length concept.

Study on Reverse Running Pump Turbine (Radial Thrust and Runaway Speed Characteristics).

Directly applying power generation technlogy for large-scale hydraulic turbines to a small hydraulic power generation system inevitably results in high costs. Consequently, technology specific to small hydraulic power generation must be developed. For example, a technology to use reverse running pump turbines, wherein mass-produced centrifugal pumps are operated in reverse rotation to obtain power, has been gaining popularity. The 1 st report gave the detailed test results on grasping the turbine characteristics and clarifying the internal flow. On this research, the radial thrust in the turbine mode and at the runaway speed condition was measured and this was evaluated comparing with the measuring data of distributions of circumferential velocity and static pressure at runner inlet and outlet, then the radial thrust tactor Kr was obtained. In addition to this, detailed experiments on the internal flow at the runaway speed condition were carried out to clarify the runaway speed characteristics, further the possibility of predicting the runaway speed characteristics was studied.

Visualization of Flow toward the Exit Duct in a Centrifugal Blower Casing

Connection between low flow rate characteristics and internal-flow condition in a centrifugal blower casing of commercial design was investigated by static pressure measurement and flow visualization using oil-film and depth-tuft methods. Inward flow oil-film pattern depicted on the top and bottom plates in scroll was basically unchanged over the whole flow rate. At the inlet region of exit duct, where sudden pressure rise occurred, reverse flow region was already observed on the top plate and the upper part of the side wall at flow rate φ/φ d=0.61. This separation zone grew with decreasing flow rate. It was suggested that condition of flow from scroll toward the exit duct was closely connected to the low flow rate characteristics of blower.

Study on the Draft Tube of a Cross-Flow Turbine.

The draft tube of a cross-flow turbine uses the difference in level between runner and tail water, especially in the case of low heads, We intented to obtain the desirable profile of a draft tube of a cross-flow turbine and present the effects of water level in the draft tube on the turbine performance. The main results are as follows. (1) The flow velocity in the draft tube is high near both side walls and low at the central part, because the flow which is directed radially outward at the runner exit is restrained by both side walls of the draft tube. (2) In the case of underwater operation of the runner, the vortex flow accompanying the re-entry flow to the runner must be prevented. (3) The optimum ratio of the width of draft tube to the length of nozzle exit arc is nearly 1.6.

Unsteady Fluid Forces on a Blade in a Cross-Flow Turbine.

The internal flow in a cross-flow turbine is nonuniform because the water passes through only part of the runner. Therefore, the unsteady fluid forces act on a blade through rotation. The experimental and theoretical studies for determination of fluid forces on the blade in a cross-flow turbine are conducted. In the experiment, the tangential and radial forces are measured on a test blade using the strain gauges and slip rings. On the other hand, in the theoretical study, they are calculated numerically using the unsteady momentum theory. The calculated results are compared with experimental data and good agreement between them is demonstrated. Furthermore, the maximum forces are found to occur immediately before the blade leaves the nozzle exit in both the experimental and theoretical results.

A numerical method of free jet from a cross-flow turbine nozzle.

The nozzle of cross-flow turbine has to give a certain circumferential velocity and an optimum angle to the flow at the nozzle exit (runner inlet). Therefore the nozzle shape has an important influence upon the turbine performance. Its shape is asymmetric and complicated, and the exit flow from it has the free boundaries. The flow from such a nozzle has not enough analyzed until now because of complications of its flow. In this report, the flow from a nozzle with arbitrary asymmetric curved surfaces was calculated numerically with a Schwarz-Christoffel method, In order to estimate the accuracy of this method, the calculation results of the flow a cross-flow turbine nozzle are compared with experimental ones. Furthermore, the effects of nozzle shape on the exit flow are investigated.

A Quasi-Three Dimensional Analysis of Choking Flow for Radial Gas Turbines

A quasi-three dimensional.flow analysis has previously been reported for a mixed flow impeller by one of the present authors. In the analysis, the velocity gradient method has been used in meridional plane and the rotating annular cascade theory has been used for blade-to-blade solution. In this report, the analysis is generalized to allow prediction and analysis of choking flow for a radial inflow gas turbine. Moreover, this analysis is corrected to include passage contraction effects and passage loss effects due to boundary-layer growth. The efficiency and choking flow rate of gas turbine may be obtained in a single computer run without the complicated throat area estimation. Some numerical examples for a burst furnace gas energy recovery turbine are presented.Copyright