Flow of steam is different from other gas flows and involves droplet generation in the flow field. This phase-transition affects not only flow aspects, but also machine performance in a negative way. These days, CFD is widely used in machine design and optimization processes, so phase-transition phenomena in steam flows should be considered in CFD to predict the internal flows precisely. In the past, non-equilibrium wet-steam model was implemented on in house code T-Flow and it was applied to steady calculations of a steam turbine model with changing stator/rotor interface. The results showed that mixing plane method is not appropriate to simulate steam condensing flow and frozen rotor method can be affected by relative position between a stator and a rotor. Therefore, steady wet-steam flows with non-equilibrium phase-transition were simulated for a steam turbine model in this study with 4 different stator-rotor positions and their effects on the wet-steam flow fields were investigated in detail.

View full abstract

Based on the gas-liquid phase flow theory, an indirect method is proposed to on-line measure the effective bulk modulus by constructing the mathematical model which reveals the relationships of the effective bulk modulus, bubble volume fraction, pressure and natural frequency. The natural frequency of hydraulic fluid is a key parameter for measuring effective bulk modulus utilizing the model, so it is online measured by the proposed pseudo-excitation method. The frequency response function is obtained by solving the amplitude spectrum functions of pseudo-excitation signal and the response signal, and the natural frequency is obtained. The numerical simulation and experiment are carried out, and we can deduce from the analysis of simulation and experimental results that the effective bulk modulus in hydraulic system can be easily online measured by the proposed indirect method.

View full abstract

Small hydropower is one of potential energy resource existing in our living environment and yet not been widely used. Small hydro-turbine is suitable in these energy resources while it still has some shortages, like performing low efficiency and easy to be out of control when there are foreign materials in the water. Thus we designed this contra-rotating small hydro-turbine to increase its efficiency and enhance its ability of keeping stable operation. The experimental apparatus was designed and assembled, and some experiments were conducted. A new kind of cylinder spoke, which supports the front and rear rotor, was adopted and the numerical analysis was carried out in this paper. As the numerical analysis results shown, the efficiency of test turbine with cylinder spoke increased in a wide range of flow rate. The maximum efficiency increased about 2.2%, reaching to 66.4%, and it was obtained at 1.25Qd. The internal flow condition, especially at the areas behind the spoke, was improved by the cylinder spoke.

View full abstract

To study the cylinder motion characteristics, cylinder and shaft sleeve stress characteristics of cylinder stroke and return process influenced by different shaft sleeve structure, the cylinders with non-chamfering shaft sleeve, C0.3 chamfering shaft sleeve and R0.3 round corner shaft sleeve were used for simulation of cylinder stroke and return process. The results show that the piston velocity curves of three kinds of shaft sleeve are identical. Maximum stress of different shaft sleeves fluctuates and maximum stress elements of C0.3 and R0.3 shaft sleeve are similar. When cylinder strokes, maximum stress of cylinder occurs at the end of piston rod connected to the load; the stress of C0.3 shaft sleeve is the lowest. When cylinder returns, maximum stress of cylinder occurs at the shaft sleeve; the stress of C0.3 and R0.3 shaft sleeve differ little and are both lower than that of non-chamfering shaft sleeve. It provides a reference for structure optimization and service life improvement of the cylinder shaft sleeve.

View full abstract

To improve the design of the splitter blade in the centrifugal pump impeller, inner flow numerical simulations are performed on the centrifugal pump impeller without splitter blades firstly, inner flow diagnoses are performed based on the theory of the boundary vorticity dynamics, and distributions of the boundary vorticity flux (BVF), friction force as well as vorticity on the inner walls of the impeller are carefully analyzed to find the location of bad flows and their dynamic sources. Later, according to the inner flow diagnosis results, splitter blades are designed and reformed for the original impeller. The inner flow filed in the impellers equipped with the preliminary splitter blade and reformed splitter blade are numerically simulated and diagnosed. Finally, comparisons are made among the three impellers, and it is found that, compared to the original impeller (impeller 0), the BVF, friction force and vorticity distribution in the impeller equipped with the preliminary splitter blade (impeller 1) has no obvious improvement, although the pump head increases, the efficiency decreases; the BVF, friction force and vorticity distribution in the impeller equipped with the reformed blade (impeller 2) has improved obviously, BVF peak decreases evidently and BVF distributes more uniformly, flow separation is inhibited, the hydraulic force acting on the impeller increases, with the pump head and efficiency increasing dramatically. It is proven that, flow diagnosis based on the theory of the boundary vorticity dynamics is an effective supplemental way for the design of the splitter blade in the centrifugal pump impeller, and will provide reference for the design of other turbo machinery as well.

View full abstract

An improvement of the half-ducted axial flow fan design by applying the forced vortex design to the rotor blade was conducted by CFD. The quasi three-dimensional flow theory was applied for the half-ducted fan design, and the meridional flow was calculated by the method of streamline curvature. When the radial balance equations from hub to tip was solved, the vortex design was changed from the constant-whirl-velocity design to the forced vortex design in reference to our previous experimental results of the half-ducted fan's internal flow. The analysis of the three-dimensional internal flow fields of the half-ducted fan designed by adopting the forced vortex design was conducted. As a result of CFD, the efficiency η of the fan was about 1-3 % larger at the flow rate coefficient φ of 0.158-0.343 than the constant-whirl-velocity design.

View full abstract

The present paper investigates the leading edge (LE) undulations of a Wells turbine blade through numerical analysis. The aspiration for this modification came from humpback whales, which have uneven protrusions at the LE of their pectoral flippers. The flippers help whales to maneuver during swimming. The work is performed by using three-dimensional steady, incompressible Reynolds Averaged Navier-Stokes (RANS) equations with turbulent closer model. The LE of the turbine blades is modified with undulations of three different amplitudes: 1mm, 2.5mm, and 4mm. The results show that the undulation changes the turbine performance. The amplitude 2.5mm gives the peak performance. The comparison between blades with different amplitudes and the reference blade has been discussed throughout this study.

View full abstract

Computational fluid dynamics (CFD) is a highly efficient paradigm that is used extensively in marine renewable energy research studies and commercial applications. The CFD paradigm is ideal for simulating the complex dynamics of Fluid-Structure Interactions (FSI) and can capture all kinds of nonlinear fluid motions. While nonlinear simulations are considered more expensive and resource intensive compared to the frequency domain approaches, they are much more accurate and ideal for commercial applications. This review study presents a comprehensive overview of the computation fluid dynamics paradigm in context of wave energy converter (WEC) and highlights different CFD tools that are available today for commercial and research applications. State-of-the-art CFD codes such as ANSYS CFX that are highly ideal for WEC simulation problems are highlighted and aspects such as time and frequency domains are also thoroughly discussed along with efficacy of the nonlinear simulations compared to the linear models. The paper presents a comparative evaluation of different WEC modelling codes available today and illustrates the code framework of different CFD simulation software suites.

View full abstract

Rotating stall phenomenon limits the operation range of turbomachines, therefore it is important to understand the crucial parameters of this phenomenon. In the present study, the diffuser rotating stall in a three-stage centrifugal pump was experimentally studied. Examined main parameter was an axial offset of rotor against the stationary part, which might be unavoidable due to accumulation of geometrical tolerances and assembling errors. The effect of leakage flow rate at the balance drum section employed as a thrust balancing device, which increases the thru-flow rate at the first and second stage diffusers, was also studied. The effect of rotor axial offset was clearly observed and, with the rotor axial offset to the suction side, the rotating stall appeared only at the third stage diffuser. By setting the balance flow rate set to zero, the onset range of rotating stall became wider in the first and second stage diffusers, which was well explained by the decrease of the thru-flow rate.

View full abstract

A numerical method for the calculation of turbulent cavitating flow over submerged objects is proposed in present work. Cavitation is modeled via a single-fluid cavitation model which is derived based on a truncated form of the Rayleigh-Plesset equation and the mixture multiphase theory. The approach has been implemented by user-define function which is widely used in ANSYS FLUENT. Detailed results are presented for sheet cavitation over a submerged hemispherical object in a wide range of cavitation numbers and the cloud cavitation around a Clark-Y hydrofoil. In particular, for the hemispherical body, we compared the surface pressure distribution with experimental data which was available in literature. Later the cloud cavitation structure and its effect on the forces of the hydrofoil were studied. The comparisons between the simulating and experimental results show that present numerical approach has good capability to predict the surface pressure coefficient and the pulsation frequency at cavitation number σ=0.4, 0.55 and 0.65 of the hemispherical body under cavitation conditions. Meanwhile, for the hydrofoil, the proposed approach is sufficiently robust to predict the characteristics of the time-averaged lift and drag coefficients and the evolution of the cloud cavity with time.

View full abstract

This paper is aimed for suggesting the best combination of loss models for a radial inflow turbine among various empirical models from the open literatures. The 1D mean streamline analysis procedure is utilized to confirm the suitability of loss models and the recommended combinations. The performance of loss models are evaluated by comparing to the well-documented NASA turbine test data. To suggest a more acceptable combination of loss models, each loss model is analyzed and assessed in accordance with the proven loss mechanisms. This study especially focused on the rotor passage loss and the tip clearance loss, and the tip clearance loss model modification was suggested. The agreement of the newly suggested loss model combination with test data is significantly improved in comparison to the existing combinations. The design procedure presented in this paper can be used as a 1D design tool of a radial inflow turbine for the preliminary design phase.

View full abstract

Wells turbine, widely used to harness the wave energy, has high efficiency at a low flow rate. However, it has severe stall problem. In order to alleviate this problem, a dual-turbine system of Wells turbine with a booster impulse turbine was proposed. In the present study, the effect of impulse turbine geometries on the performance of combined turbine system was investigated by CFD analysis. As a result, the boosting effect of impulse turbine depends on the pressure differences that caused by the setting angles at a high flow rate. Moreover, the decrease of flow rate with the setting angle has a significant influence on the efficiency.

View full abstract

Guide vane was added to pump as turbine (PAT), the transient flow characteristics of PAT were calculated and experimented under different guide vane numbers. The results show that after the PAT was added guide vane, the distribution of speed streamline and blade load is more uniform than before, and the unsteady radial force on impeller decreased greatly, which is good for the stability of PAT's operation. When the guide vane number is equal to 9, both the maximum radial force difference and dominant frequency amplitude of radial force are the minimum. So there is an optimal guide vane number, for the given geometric parameters of PAT.

View full abstract