Leqin Wang

Theoretical and experimental analysis for leakage rate and dynamic characteristics of herringbone-grooved liquid seals

In this paper, a theoretical analysis method for leakage rate and dynamic characteristics of herringbone-grooved liquid seals is proposed. Steady-state velocities and leakage rate are figured out first with the inertia term of the fluid combined with mass conservation within the seals. Subsequently, the rotordynamic coefficients are solved based on the investigation of Iwatsubo and Childs. Laboratory tests were performed on a specially designed test rig. Detailed comparisons between the experimental results and theoretical predictions show a good agreement on the leakage rate and hydraulic forces for five model seals, which provide an evidence of validation of the analysis method developed in this paper. Moreover, the influence of rotational speed and pressure difference on the leakage rate and rotordynamic coefficients of herringbone-grooved seals is discussed. The results show that the operating conditions and seal structures greatly influence the sealing and rotordynamic performances, which is helpful for designing herringbone-grooved seals.

Numerical Simulation of the Transient Process of Power Failure in a Mixed Pump

A hydraulic-force coupling method was used to simulate the transient process of power failure condition. Computational fluid dynamics (CFD) was used to study the three-dimensional (3D), unsteady, incompressible viscous flows in a mixed flow pump in power failure accident. The dynamic mesh (DM) method with nonconformal grid boundaries was applied to simulate the variation of rotational speed of the field around the impeller. User-defined function (UDF) was used to obtain the rotational speed by solving the momentum conservation equation. External characteristics, such as rotational speed, head, flow rate, and hydraulic torque, were obtained during the transient process. Numerical speed and flow rate were compared with results calculated by semiempirical equation and they were in good agreement. The differences between transient and quasisteady results were also studied. Transient head and quasisteady head did not differ too much. The reason that caused this deviation was theoretically analyzed. The difference was explained to be caused by the inertia effect of the fluid contained in the pump and the pipeline. Internal flow field was also shown. Relative velocity vectors showed that the stall form and existence time in transient simulation were different from those in the quasisteady simulation. It is suspected to be one reason for head deviation.

Fluid-Structure Interaction Analysis on Turbulent Annular Seals of Centrifugal Pumps during Transient Process

The current paper studies the influence of annular seal flow on the transient response of centrifugal pump rotors during the start-up period. A single rotor system and three states of annular seal flow were modeled. These models were solved using numerical integration and finite difference methods. A fluid-structure interaction method was developed. In each time step one of the three annular seal models was chosen to simulate the annular seal flow according to the state of rotor systems. The objective was to obtain a transient response of rotor systems under the influence of fluid-induced forces generated by annular seal flow. This method overcomes some shortcomings of the traditional FSI method by improving the data transfer process between two domains. Calculated results were in good agreement with the experimental results. The annular seal was shown to have a supportive effect on rotor systems. Furthermore, decreasing the seal clearance would enhance this supportive effect. In the transient process, vibration amplitude and critical speed largely changed when the acceleration of the rotor system increased.

Research on cavitation characteristic of inducer

The inducer has significant effect on improving the cavitation characteristic of a centrifugal pump. The fact which can not be neglected is that the inducer itself is a kind of axial pump. Research on inducers cavitation characteristic is very important. Several inducers were designed and modeled by Pro/E software. The mesh of flow field was done by ICEM and imported to ANSYS CFX to analyze the inducers cavitation characteristic. The relationship between cavity length and head breakdown was discussed. With the decrease of NPSH, there is a slight increase in the head just prior to the decrease associated with head breakdown. This conclusion coincides with experimental results. The influence of backflow eddy on the inducers cavitation characteristic was analyzed, and the change of backflow eddy in the process of cavitation was illustrated. It can be concluded that the correlation between the inducer head breakdown and the relative cavity length is very close which agrees well with the theoretical and experimental results. As the inlet pressure is decreased, inception almost always occurs in the tip vortex generated by the corner where the leading edge meets the tip. And backflow vortex gradually disappears in the process of cavitation.

Instability Analysis of a Model Pump-Turbine with MGV Based on Nonlinear Partially Averaged Navier-Stokes Methods

Pump-turbines were always running at partial condition with the power grid changing. Flow separations and stall phenomena were obvious in the pump-turbine. Most of the RANS turbulence models solved the shear stress by linear difference scheme and isotropic models, so they could not capture all kinds of vortexes in the pump-turbine well. At present, partially-averaged Navier-Stokes (PANS) model has been found to be better than LES in simulating flow regions especially those with less discretized grid. In this paper, a new nonlinear PANS turbulence model was proposed, which was modified from RNG k-ε turbulence model and the shear stresses were solved by Ehrhardts nonlinear methods. The nonlinear PANS model was used to study the instability of “S” region of a model pump-turbine with misaligned guide vanes (MGV). The opening of preopened guide vanes had great influence on the “S” characteristics. The optimal relative opening of the preopened guide vanes was 50% for the improvement of the “S” characteristics. Pressure fluctuations in the vaneless space were analyzed. It is found that the dominant frequency at the vaneless space was twice the blade passing frequency, while the second dominant frequency decreased as the preopening increased.

The flow and acoustic characteristics of underwater gas jets from large vertical exhaust nozzles

The flow and acoustic characteristics of underwater gas jets exhausted from large vertical nozzles are experimentally investigated in this work with gas flow rates of 30–150 m3/h, nozzle widths of d = 10 mm, 20 mm, 30 mm, and 40 mm. A high-speed digital video camera is used to examine bubble behavior and flow regimes. Sound pressure is measured by two hydrophones and recorded by a digital audio tape recorder. The audio and video signals are synchronized to find out the relationship between sound and gas behavior. Experimental results indicate that the general behavior of gas exhausted into water is of periodical necking and expansion. Sound pressure peaks are mostly excited by necking in two ways: pinch-off and redial expansion. Necking itself is a kind of low frequency behavior, corresponding to strong low frequency sounds. Moreover, necking can force the growing bubble to oscillate and emit broadband sound. As the gas velocity increases, necking would happen more frequently, and gas jets would grow into larger volume in shorter time, and then the sound radiated from the gas jets would have higher frequency and larger amplitude.

Experimental and Numerical Studies of Pump Transient Characteristics During Stopping Period

Experimental and numerical studies were carried out to study the transient characteristics of the centrifugal pump during stopping periods. Different stopping schemes were realized by changing the rotational inertia of the flywheels. Transient revolution rotational speed, flow-rate, total pressure rise and torque were measured under different rotational inertia. Experimental results of different operating conditions were compared, and transient hydrodynamics performances of the centrifugal pump model were analyzed. In order to provide boundary conditions for numerical simulations, the revolution curves of the flow-rate and the rotational speed were polynomial fitted. Three dimensional unsteady incompressible viscous flows during stopping periods were studied by using DES model in FLUENT. Results show that the hydrodynamic characteristics of numeric and experiment agree well. The transient effect is not so evident, and the quasi-steady assumption is acceptable during most part of coastdown process. The pump characteristics are further explained by analyzing the relative velocity on the middle stream surfaces. At the end of the stopping period, the transient vortex evolution between blades is the main reason why the transient curves deviates the steady curve. The studies can help understand the operating characteristics of centrifugal pump when power failure accident occurs.Copyright