z. yang

The double-suction centrifugal fan is typically installed in the ventilation unit and driven by a motor beside it. One of the two inlets of the double-suction fan is partially blocked by the motor, and the flow in the fan becomes asymmetric and non-uniform. This work numerically investigated the effect of the motor blockage on the transient characteristics of the asymmetric flow in a centrifugal fan. The distance between the motor and the adjacent collector is typically 20mm or 40mm. Numerical results reveal that compared with the baseline motor-free model, the motor blockage of the two models decreases the flow rate by 30.4% and 20.8%, respectively, at the obstructed inlet of the fan, and the inflow is non-uniform and presents a local reversed flow. The motor blockage decreases the static pressure efficiency by 9.45% and 6.04%, respectively, while the static pressure rise is hardly affected. The flow fluctuation is notably asymmetric and non-uniform due to the non-axisymmetric geometry of the volute and the motor blockage. The blade passages are occupied by strong reversed flow, and a low-pressure region exists in the impeller. This work also performed a comparative study on the correctness and applicability of the boundary condition. The type of boundary condition of constant pressure at the outlet and a flow rate at the inlet, which is a common choice for fans without considering the obstacles, is analyzed. It was found that this type of boundary condition underestimates the efficiency of the fan with motor blockage, and the pressure field at the fan inlet is considerably different.

The hydraulic turbine has been used extensively in the field of energy conservation. For turbines that have low heads and large discharge, improving recovery efficiency and stability is crucial due to their significant hydraulic impact. This paper provides a detailed analysis of the correlation between the influence of radial guide vanes on the stability of low-head, large-discharge turbines focusing on hydraulic performance and energy dissipation before and after the implementation of guide vanes. Moreover, in this paper, two types of turbines, with and without guide vanes, were designed considering the desulfurization scenario. Hydraulic efficiency, radial force, and internal flow field mechanics were numerically studied, and validated through experiments. The results reveal that the working range of the hydraulic turbine could be widened and the energy recovery efficiency improved by a maximum of 3.11% in the small flow rate under the action of guide vanes. Furthermore, it results in a substantial reduction in the radial force of the impeller. Subsequently, the variation in entropy production of different components under full flow rate conditions was compared between the models with and without guide vanes. The total energy consumption decreases sharply under overall working conditions due to the flow control ability of guide vanes affecting the flow state. The entropy production rate of the impeller remains the largest regardless of the presence of a guide vane in the turbine. The vortices inside the guide vanes increase obviously with the flow rate increase.

Low-frequency buffeting is a common problem in automobile wind tunnels, it induces pulsations of pressure and velocity in the test section. A 1:15 3/4 open-jet return-type scaled wind tunnel was used for this research, and numerical simulations and tests were implemented to study the flow characteristics of the jet shear layer in a model wind tunnel. The results show that guide devices on the inner wall of the nozzle can effectively reduce the low-frequency buffeting, but the presence of the devices deteriorated the axial static pressure gradient of the flow in the test section. The shape of the guide devices was optimized through the Explorative Gradient Method, and numerical simulations were carried out. An optimal shape can effectively reduce the low-frequency buffeting and ensure flow field uniformity in the test section. Finally, the reliability of the numerical simulation and the practicability of the optimal case were verified through a hot wire test and a microphone test.

We perform a thorough numerical analysis of the impact of inflow conditions on the aerodynamic performance of a tandem cascade. In particular, we investigate the effects of the incidence angle and the inlet boundary layer (IBL) thickness on the three-dimensional flow field structure and aerodynamic performance. Our results show that the gap flow strength of the tandem cascade decreases with the increase of incidence angle, and it can effectively reduce the mixing of the wakes of the forward blade (FB) and rear blade (RB). In turn, this prevents the passage vortex (PV) in the RB passage from developing along the circumferential direction. The occurrence of IBL does not modify the effects of the incidence angle on the tandem cascade, however, it reduces the load of the RB and the gap flow strength near the endwall. Under all incidence angles, IBL increases the total pressure loss of the tandem cascade, and decreases the static pressure rise (except for an incidence angle equal to -6°). The maximum loss increment is at 2° incidence angle, and the maximum static pressure rise decrement is at 6° incidence angle (Thick-IBL condition) or 7° incidence angle (Thin-IBL condition). Furthermore, we found that the presence of IBL changes the minimum loss condition from 0° (design condition) to -2° incidence angle. Our results thus indicate that in the practical engineering application of the tandem cascade, the reality that IBL degrades the tandem cascade performance in the full incidence angle range should be considered. And the strong endwall secondary flow effect caused by IBL should be considered in the tandem cascade three-dimensional design, so that the tandem cascade two-dimensional performance advantage can be better played.

When check valve works in long distance or high lift liquid pipeline system, it is often subjected to water hammer. In this study, the UDF program was used to simulate the closing process of an axial flow check valve at the moment of pump shutdown, and the porous media model was applied to simulate the complete closing of the valve disc. It was found that there was local vacuum at the end of the valve disc at the moment when the valve was completely closed. The water hammer and characteristics of the force acting on the valve disc in the whole closing process were also obtained. In order to reduce the pressure surges on the valve disc and seat, a built-in critical damping was designed and added to the to the valve disc drive system. Since the spring force is directly proportional to the movement displacement of the valve disc, the elastic force and the speed of the valve disc reach the peak value when the valve is fully closed, while the damping force is directly proportional to the speed of the valve disc, therefore, the damping force increases gradually with the speed of the valve disc, which only produces the maximum damping force at the moment of fully closing, so as to reduce the slam shut, but has little effect on the closing time, thus adding damping is more effective than reducing the elastic force of spring. The current study provides a possible approach to protect the valve disc and seat of check valves in liquid supply and drainage systems.