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.
Edited and published by : Turbomachinery Society of Japan, Korean Fluid Machinery Association, Chinese Society of Engineering Thermophysics, IAHR Produced and listed by : Turbomachinery Society of Japan, Korean Fluid Machinery Association, Chinese Society of Engineering Thermophysics, IAHR