In the use of helical type agitator, the mixing process is usually restricted to the laminar flow regime. Common examples of laminar mixing are found where the fluid has a very high viscosity, i.e., pseudoplastic fluids. It can be indicated that a helical type agitator is sufficiently suited to the creeping flow mixing. The pumping characteristic of a Helical Screw Agitator with a draught tube (HSA) is required to evaluate its capacity for the optimal configuration of the mixing chamber. It could be executed by changing some parameters such as the number of helix, the angular velocity and the rotating direction and so on. In this study, the numerical simulation was carried out with the Eulerian multiphase mixture model and the moving mesh approximation. Some of the optimum design parameters have been developed with the aid of numerical data from the Computational Fluid Dynamics (CFD) analysis. Using the commercial code, Fluent, the pumping characteristics in the HSA are investigated from the rheological properties, and the results are graphically depicted.
The series of flows around the piers in draft tubes of bulb turbines were investigated. The flow fields were assumed to be steady incompressible turbulent flows. The primary factor responsible for total pressure loss in draft tubes was investigated based on the data obtained. Well-known horseshoe-shaped vortices were observed at the leading edges of piers as a result of simulations. These vortex structures were found to greatly contribute to total pressure loss in the present cases. Simulations on several variously shaped draft tubes were carried out and these suggested slight modifications to draft tubes would effectively curtail total pressure loss around a pier.
The difference of pump characteristics between two kinds of mixed flow pumps with low specific speed of 350 (min-1, m3/min, m), which have the same impeller and the different diffuser vanes, is presented in the present paper. It was confirmed from the previous study that a diffuser rotating stall (DRS) occurs in the original type of mixed flow pump at about 65% flow rate of best efficiency point (BEP) and there is an abrupt drop of the total head characteristic. The relationship between pump characteristic instability and internal flow is investigated in detail by using a dynamic PIV measurement system (DPIV) and a commercial CFD code. As a result, the cause of characteristic instability is supposed for the original type as follows. The flow on the vaned diffuser hub-side becomes unstable due to adverse pressure gradient and strong backflow occurs at partial flow rate. Then it impinges against downstream flow from the impeller and the secondary flow from hub- to casing-sides occurs. This secondary flow blocks the downstream flow from the impeller and the inlet flow angle at the leading edge of adjacent diffuser vane decreases. Therefore, the flow separates on the suction surface of the adjacent diffuser vane inlet and a strong vortex is generated. After that, it develops and becomes a stall core. Next, the modified type of pump, where only diffuser vanes are modified, is tested. As a result, the flow rate, at which characteristic instability occurs, is shifted to lower one and the pump operating range becomes widened. It is clarified upon above considerations that the secondary flow has been restricted and diffuser performance has been improved in comparison with the original type.
The attachment of inducer in front of main impeller is a powerful method to improve cavitation performance. In operating condition at partial flow rate and extremely low suction pressure, cavitation surge oscillation occurs and the stable operating range becomes narrower. It has been found from previous experimental study of authors that the cavitation surge oscillation with low frequency occurs with close relation between the inlet back-flow cavitation and the growth of blade cavity to the throat section of blade passage. One method, which is to install an axi-asymmetrical plate upstream of inducer, has been proposed to suppress the cavitation surge oscillation. It is considered conceptually to be the suppression mechanism of oscillation that the inlet flow distortion by suction axi-asymmetric plate avoids the flow at all throat sections getting unstable simultaneously. In the present study, in order to clarify the suppression mechanism experimentally observation of rotating behavior of cavitation in the inducer is performed with distributing multi cameras circumferentially, recording simultaneously and arranging the pictures to show the entire view of the flow around the linear cascade. The observed cavitation behavior is utilized for discussion on the suppression mechanism of oscillation with other measuring results such as casing wall pressure distribution and velocity distributions with LDV. Then the suppression mechanism of oscillation by installing the axi-asymmetrical plate will be clarified in more details.
Shape optimization of heat transfer augmentation device employed in turbine blade internal cooling passage has been performed numerically using single as well as multi-objective optimization procedures. Polynomial response surface approximation method and multi-objective genetic algorithm are used for single and multi-objective optimizations, respectively. Problem to enhance heat transfer rate considering staggered dimples on single surface of cooling passage has been formulated, and Reynolds-averaged Navier-Stokes equations are solved to analyze the flow field and the heat transfer. Three design variables defining channel and dimple dimensions, and two objective functions related to Nusselt number and friction drag are employed. Latin hypercube sampling is used to generate sampling points in design space, and the evaluated objectives are used to generate a set of optimal designs. Optimal shapes show higher heat transfer rates in the case of lower channel height, higher dimple depth and higher dimple spacing, but these geometries also produce higher pressure drops.
This paper is concerned with the numerical prediction of impeller blade vibration due to excitation by the wake of nozzle guide vanes in turbochargers. This problem is particularly important as turbochargers are used in a broad range of operation conditions, leading to some unavoidable resonant frequencies. In the paper, first the unsteady pressure distributions on the rotor blades are analyzed and structural response analysis is performed for the excitation component with largest contribution to pressure fluctuation. In a parametric survey, pressure expansion ratio and inlet flow temperature are varied in order to investigate the impact on vibration response. Unsteady CFD+FEM prediction well explains the qualitative trends that appeared in experimental survey. The result also points out that a fluctuation in natural frequency among the blades of only a few percent may cause large magnification factors. Finally, adjustment of disk thickness is suggested as a measure to reduce the magnification factor and its effectiveness demonstrated numerically as well as experimentally.
In vortex generator jets (VGJs), the beneficial effect of separation control is obtained only if the jets are pitched to the lower wall and skewed with respect to the freestream direction. In particular, in the case of VGJs with circular orifices, the jets are pitched at an angle of 45 deg or less to the wall to achieve effective suppression, due to the generation of strong dominant vortices. On the other hand, it was confirmed that the vortices for the VGJs with the rectangular orifices are stronger and appear to provide more effective momentum transfer across the wall boundary layer, in contrast to the vortices for the VGJs with circular orifices in the previous study. In the present study, the suppression effect is investigated for VGJs with T-shaped orifices combined into two rectangular orifices in the case of a large pitch angle (60 deg). The VGJs with T-shaped orifices were practically applied to the flow separation control of a two-dimensional diffuser, and the suppression effect for the T-shaped orifices was compared to that for the circular orifices. The behavior and the decay of longitudinal vortices in the downstream direction are also described. The T-shaped orifices produce a counter-rotating vortex pair that has different strengths between positive and negative vortices, and longitudinal vortices exist near the lower wall. Therefore, the VGJs with T-shaped orifices provide more effective momentum transfer to the boundary layer and make the separation control effective, in contrast to the VGJs with circular orifices. The formation and behavior of the counter-rotating vortex pair produced by the interaction of the jets and the freestream in the downstream direction were strongly affected by the jet orifice shape.
The effect of simple spiral fin around a bare tube on Karman vortex shedding was experimentally investigated. We measured the distributions of the mean velocity, the intensity of velocity fluctuation, the spectrum of velocity fluctuation and the coherence distribution of Karman vortex in the spanwise direction. And the flow around a fin tube was visualized by using smoke wire technique. As a result it was made clear that the periodic velocity fluctuation caused by Karman vortex was remarkably observed in the wake of the fin tube although the fin was mounted around a bare tube with a large pitch along the tube axis. The vortices in the near-wake of the fin tube make cells structure with the scale of the pitch length of the fins. The rotation axis of each vortex was inclined against the tube axis. The large scale vortex was formed by the coalescence of the vortex cells. Therefore, the spanwise scale of the vortex in the wake of the fin tube was larger than one pitch of the fins.
In this study, special attention is directed to static pressure fluctuation in a sirocco fan for a car air-conditioning system, because it is expected that there is a close connection between the fluid noise and the pressure fluctuation. The final purpose of this study is to clarify the relationship between the static pressure fluctuation between fan blades and the sound noise emitted to the outside of the fan, and to develop an air-conditioning system with highly low noise level. For this purpose, first of all, a new micro probe for the measurement of static pressure fluctuation has been developed. This new micro probe is composed of an L-type static pressure tube (the outer diameter is 0.5 mm and the inner diameter is 0.34 mm) and a very small pressure transducer. This probe exhibits a flat frequency response until approximately 2,000 Hz, and it is set between the blades of the fan rotating at 1,500 rpm. The measurements of the static pressure fluctuation between the blades have been performed, and the intensity of sound source was quantified from the second derivative of the phase-averaged static pressure fluctuation signals on the basis of Ribner's formula (Ribner 1962). The experiments have been made in two different modes, i.e., the cooling mode (FACE MODE) and the heating mode (FOOT MODE). It is shown that the static pressure increases rapidly as the blade approaches to the nose of the casing. It is also found that the sound source for FACE MODE shows the larger value than that for FOOT MODE as a whole. In particular, the largest intensity of sound source is observed when the blade approaches to the nose. From these results, it is confirmed that the present new static pressure probe is useful to specify the distributions of sound source in a sirocco fan.
This paper focuses on the relationship between acoustic resonance and vortex shedding from the tube banks of a boiler plant. We have built a model similar to the actual boiler plant to clarify the characteristics of acoustic resonance phenomena and vortex shedding. The model used in-line tube banks with a small tube pitch ratio. We examined the relationship between the acoustic resonance of the actual plant and that of the model, and measured the sound pressure level, acoustic pressure mode shape, spectrum of velocity fluctuation, and gap velocity. Gap velocity was defined as the mean velocity in the smallest gaps between two neighboring tubes in the transverse direction. As a result, the resonant frequencies and mode shapes of the acoustic resonances in the actual boiler plant agreed well with those in the similar model. We found many peak frequencies in the sound pressure level spectrum when acoustic resonances occurred. The typical Strouhal numbers at the onset velocity of acoustic resonances were about 0.19, 0.26 and 0.52. Periodic velocity fluctuation caused by vortex shedding was observed inside the tube banks without acoustic resonance. The Strouhal number measured for vortex shedding was 0.15. Acoustic resonances of higher-order modes were generated in this plant.
In this study, the wake characteristics of an arc blade were measured by the wind tunnel experiment; the characteristics were defined as the width of the wake, diameter of the vortex, ratio of the vortex scale, and the local lift. The influence of the angle of attack on the aerodynamic noise of the blade was quantitatively predicted by using these characteristics. It was clarified experimentally that the sound pressure of the aerodynamic noise becomes small since the gradient of the differential of the lift fluctuation was reduced according to the increase of the angle of attack. The wake characteristics were applied to the prediction of the broadband noise generated from a multiblade fan; the fan noise level distribution was estimated with high accuracy to be in the range from 1000-3000 Hz and was used to analyze the broadband noise of the fan. From the analysis of the fan noise level, it was found that the difference in the relative velocity caused by the biased internal flow was related to the noise levels.
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