Flow fields in 2-blade and 4-blade half-ducted propeller fans for the outdoor units of air-conditioners were calculated with large eddy simulation based on finite element method with the aim of investigating the influence of blade number on aerodynamic noise. We confirmed that the tip vortex had a great influence on aerodynamic noise in half-ducted propeller fans. The length of the tip vortex trajectory and the blade pitch for the 2-blade propeller fan were longer than those for the 4-blade propeller fan. These were suppressed the interaction between the tip vortex and the adjacent blade in the 2-blade propeller fan. The 2-blade propeller fan was therefore more silent than the 4-blade propeller fan.
The servo valve is the key component in the electrical-hydraulic servo system. Status for the servo valve temperature drift characteristics of the test system is studied in this paper firstly. Since Research on temperature servo valve drift characteristics has not formed systematic theory. As for Position closed-loop test method, the theory and calculation algorithms of servo valve temperature drift are analyzed. Simulation and experiments both show the feasibility of the theory test, and by a large number of experiments, the servo valve drift results are put into classification. Finally, corresponding phenomenon is elaborated, pointing out its causes and appropriate preventive measures, hoping to reduce drift value influenced by the temperature on the servo valve.
The destruction of ozone layer due to the use of refrigerants is a matter of concern in the present world. This has been addressed through various platforms and several guidelines have been framed for their usage to prevent its further degradation. Since the existing chlorofluorocarbon (CFC) and hydrofluorocarbon (HCFC) refrigerants are at the verge of phasing out because of their higher environmental impacts, this has raised various questions about the systems which employ these refrigerants. One such question is: Will the existing systems work efficiently when the existing refrigerant is replaced by a viable environment friendly substitute? The present work makes an effort to answer this question.
CFD has played a significant role to investigate the performance and improve the design of hydraulic turbines; however, the improvement of CFD method demands powerful computer resources including time, CPU, memory, and commercial licenses. In present work, both global and local parameters of a high head Francis turbine were studied using several geometrical and interface modelling approaches. The aim of the work is to find suitable strategy for designers to simulate the hydraulic turbines to balance the numerical accuracy and the requirement of computational resources. The geometrical modelling approaches include combinations of turbine components such as, spiral casing, distributor, runner and draft tube. The interface modelling approaches includes, stage, Frozen rotor and transient rotorstator types. The study showed that the proper combinations of both approaches can effectively reduce the numerical error.
The flow instabilities and pressure pulsations can be generated during the pump's operation. However, it is even more serious under the part-load condition. Currently, the links between flow instabilities and pressure pulsations were still not fully understood. In the present study, the experimental investigation was performed on pressure pulsations by utilizing the dynamic pressure transducers and the internal unsteady flow structures were measured by PIV. The pressure pulsations were extracted at 6 different locations around the volute under different flow rates conditions ranging from 10% to 120% of the nominal flow rate. The study allowed relating the pressure pulsations and unsteady flow structures in a pump. It was noted that higher intensive broadband pressure pulsations can be found at the small flow rate together with the fully developed flow instabilities. This led to the preliminary conclusion that the broadband pressure pulsations are exclusively induced by the flow instabilities, especially the vortices in the flow passages of impeller.
Frequencies of deep surges and their behaviors in axial flow compressors were surveyed numerically. Relative surge frequencies, normalized by the basic acoustical resonance frequencies, are seen to tend to lower together with increases in the stalling pressure ratios, i.e. increases in the number of stages and the compressor tip speed, and also together with increases in the flow-path sectional area ratios. However, it appears difficult to express simply the general behaviors of the relative frequencies affected by the various factors. In order to know the essential behaviors, a modified reduced surge frequency is proposed, which is a dimensionless number comparing the mass flow filling and emptying the plenum volume in surge and the mass flow provided by the compressor. The modified reduced surge frequencies are found to have or approach a definite and nearly constant value in conditions of deep surges. The parameter suggests the fundamental mechanism of deep surges and could be used to determine approximate frequencies of deep-surges in various conditions of compressors and flow-paths.
In consideration of the particle features and behaviors, the Computational Fluid Dynamics (CFD)-Discrete Element Method (DEM) coupled method has been applied to simulate the liquid-solid flows in the centrifugal pump with crystallization phenomenon. The crystal particles tend to distribute more uniformly in the inlet section and enter the impeller along the pressure sides of the blades with a moderate rise in velocity. Particle number density is different at different regions in the impeller passages with the characteristics of small density near suction sides and large density near pressure sides. In addition, large crystal particles are mainly located near the pressure sides and small crystal particles predominantly appear in the region near suction sides. The relative velocity magnitude of flow near the impeller inlet tends to be higher than that of crystal particles, while the velocities of the solid particles are substantially higher than liquid phase at the outlet.
Small hydropower generation is one of important alternative energy, and potential of small hydropower is great. Efficiency of small hydro-turbine is lower than that of large one, and these small hydro-turbine's common problems are out of operation by foreign materials. Therefore, we adopted contra-rotating rotors, which can be expected to achieve high performance and enable to use low-solidity rotors to achieve stable operation. Experimental apparatus of the contra-rotating small hydro-turbine with 60mm casing diameter was manufactured and some experiments were conducted. In this research, the internal flow with foreign vegetable materials, i.e. leaves and grasses, were investigated by a high speed camera, and the performance experiments when two pieces of cudweeds were attached on the blade of front rotor were conducted. As the results shown, the passing rate of small leaves is 41.2% and all of the middle leaves can't pass through the contra-rotating small hydro-turbine. The highest efficiency also decreased about 13%, reaching to 51.2% at 1.4Qd, because of the foreign vegetable materials.
Electric motors are used as the main power sources in many industrial equipments and household appliances. The miniaturization and weight reduction of electric motors generally increase the internal heat generation. Therefore, it is important to understand the flow characteristics of motor cooling fans and to improve their performance. The present study aimed at systematically investigating the effects of the inlet vent and blade shapes on the aerodynamic performance of a low-voltage electric motor cooling fan. The flow characteristics of the low-voltage electric motor cooling fan was numerically analyzed using three-dimensional Reynolds-averaged Navier-Stokes equations. The k-ε turbulence model was selected for the analysis of turbulence using a turbulence model test. An optimal grid system in the computational domain was selected through a grid dependency test. The mass flow coefficient and torque coefficient were considered as the performance parameters of the cooling fan. Eleven inlet vent shapes and eleven blade shapes of the cooling fan were tested by evaluating the aerodynamic performance of the cooling fan. The mass flow coefficient and torque coefficient were considered as the performance parameters of the motor cooling fan. Eleven inlet vent shapes on the fan cover and eleven blade shapes were tested to evaluate their effects on the mass flow coefficient and torque coefficient. The maximum mass flow coefficient of 0.0908 and the minimum torque coefficient of 0.0089, were achieved using different combinations of vent and blade shapes.
To analyze the dynamic evolutions between the draft tube pressure pulsations and vortex ropes of a Francis turbine, the runaway transient process of a hydropower system is simulated by coupling a one-dimensional model of the water conveyance system and a three-dimensional model of the Francis turbine. The results show that the annular-distributed pressure pattern at the entrance of the draft tube breaks and induces small vortex ropes, which then merge into an eccentric-distributed helical one with the transient operating point moving away from the rating region. In this process, low frequency pressure pulsations form and continue to strengthen. When the operating point moves to the runaway point, the vortex ropes keep dividing and merging irregularly, causing random-like pressure pulsations.
In order to investigate the effect of solid particles on the cavitation characteristics and lubricating properties of the micro-gap liquid film in Upstream pumping mechanical seals,the Eulerian multiphase flow model was used to simulate the liquid film with different diameter and volume fraction of solid particles to analyze the influence of the particles on the distribution of vacuole , opening force and friction torque of the film under different working conditions. The results showed that the particles have an inhibitory effect on the cavitation, and the cavitation area and the volume fraction of the bubbles were both decreased.The cavitation area increased with the increase of particle diameter, which indicated that the inhibition of cavitation was weakened with the increase of particle diameter. The cavitation area decreased with the increase of the particle volume fraction, and the volume fraction increased the cavitation inhibition effect. The presence of particles improved the opening force of liquid film to a certain extent and increased with the increase of particle volume fraction , but the effect of particle diameter on opening force was different under different rotating speed and different medium pressure.The friction torque did not change obviously with the particle diameter,and decreased only slightly with the increase of the particle volume fraction.In the working condition range, the cavitation degree is not related to the pressure of the medium, but increases with the increase of the rotational speed, and the cavitation area and volume fraction of bubbles Were significantly decreased when there were solid particles.
In the present study, two mesh smoothing techniques, Laplace and Winslow smoothing techniques, for unstructured grids on turbomachinery application are investigated. These operators are based on the solution of elliptic equations. In the first case, Laplace`s equations are solved using a barycentric averaging procedure. Solution of Winslow`s equations has been a challenging work for unstructured grids because of existence of cross derivative terms in the equations. This issue is addressed devising a local control volume. Both methods are compared using different grid quality criteria. Finally, these operators have been applied to turbomachinery configurations and the advantages and disadvantages are discussed.
Fluid machineries for fluid food have been used in wide variety of field i.e. transportation, filling, and improvement of quality of fluid food. The flow condition of these fluid machineries is quite complicated because the fluid food is different from water. Therefore, a design method based on the internal flow condition is not conducted. In this research, a turbo-pump having small number of blades was used to decrease shear loss and keep wide flow passage. In previous studies, it was found that internal flow condition was complex in the test pump, but those flow phenomena were not clarified in detail. In order to investigate the complex internal flow condition, the unsteady numerical analysis using low viscous fluid was conducted. In this paper, the relation between the blade geometry and the performance was investigated. In addition to that, the internal flow condition of the centrifugal pump using low viscous fluid was clarified by the numerical analysis results.
The aim of the present study is to use the artificial roughness geometries in order to investigate the thermal and hydrodynamic behaviors in a rough microchannel using the most effective alternative method in micro-flows namely Lattice-Boltzmann Method (LBM). The rough surfaces are configured with triangular, trapezoidal and rectangular roughness elements with a relative roughness height 0~8% of the channel height. To analyze the roughness effects, the friction coefficients in terms of Poiseuille number (Pn), the rate of heat transfer in terms of Nusselt number (Nu) and the mass flow rate have been discussed at the slip flow regime, 0.01 ≤ Kn ≤ 0.10, where Kn is the Knudsen number. Finally, the overall performance has been studied numerically. All of these numerical results for all kind of roughness geometries are compared with smooth microchannel.
In reality, the flow condition inside meridional channel may vary from hub to tip, therefore meridional analysis will affect to the blade geometry whether is twisted or tapered. The prediction of maximum surface velocity is not easy, but Lieblein was postulated the alternative form of diffusion factor (DF) on airfoil which can be applied for the cascade analysis. Diffusion factor on cascade is an important design parameter to avoid cavitation in turbomachinery due to the pressure distribution and velocity variation on the surface. The purpose of present study is to know the influence of tangential velocity distribution and constant DF on the blade shape and performance of the axial hydro turbine. There are four criteria of tangential velocity distribution or usually is called as swirl velocity that discussed in this paper, they are mixed vortex, free vortex, forced vortex and constant vortex. Constant DF was applied to the all tangential velocity distribution to obtain pitch chord ratio calculation. This will affect to the pitch chord ratio value and the blade shape of rotor. Performance prediction of axial turbine is performed through Computational Fluid Dynamic (CFD) to validate global parameter design. The result of numerical simulation can be used as basic consideration for manufacturing and experimental testing.
Numerical simulations of vortex-induced vibration of a three-dimensional flexible Cylinder under uniform turbulent flow are calculated when Reynolds number is 1.35×104. In order to achieve the vortex-induced vibration, the three-dimensional unsteady, viscous, incompressible Navier-Stokes equation and LES turbulence model are solved with the finite volume approach, the Cylinder is discretized according to the finite element theory, and its dynamic equilibrium equations are solved by the Newmark method. The fluid-Cylinder interaction is realized by utilizing the diffusion-based smooth dynamic mesh method. Considering VIV system, the variety trends of lift coefficient, drag coefficient, displacement, vertex shedding frequency, phase difference angle of Cylinder are analyzed under different frequency ratios. The nonlinear phenomena of locked-in, phase-switch are captured successfully. Meanwhile, the limit cycle and bifurcation of lift coefficient and displacement are analyzed using trajectory, phase portrait and Poincare sections. The results reveal that: when drag coefficient reaches its minimum value, the transverse amplitude reaches its maximum and the "lock-in" begins simultaneously. In the range of "lock-in", amplitude decreases gradually with increasing of frequency ratio. When lift coefficient reaches its minimum value, the phase difference, between lift coefficient and lateral displacement, undergoes a suddenly change from the "out-of-phase" to the "in-phase" mode. There is no bifurcation of lift coefficient and lateral displacement occurred in three dimensional flexible Cylinder submitted to uniform turbulent flow.