Recent Volume
Vol14 , No 4 (in press)
Cross-Scale Flow Field Analysis of Sealing Chamber and End Face Considering the CO2 Real Gas Effect
Pages : 979-991
Authors : XUEJIAN SUN,  Pengyun Song,  Xiangping Hu,  Wenyuan Mao,  Qiangguo Deng,  Hengjie Xu, 
The dry gas seal (DGS) is a non-contacting, gas-lubricated mechanical face seal commonly used in rotating machinery. Traditional analyses of DGSs treat the end face as an independent factor by setting the end-face inlet as boundary conditions, but limited attention is focused on the sealing chamber of the DGS. Using the finite volume method and the shear stress transport (SST) k-ω model, the coupling between the millimeter-scale sealing chamber and the micrometer-scale end face are simulated with regard to the real gas effect of CO2. The three-dimensional distributions of velocity, pressure and temperature in the cross-scale flow field are investigated under different working conditions. Moreover, the boundary parameters of the end-face inlet are modified by response surface methodology with a central composite rotatable design. The results demonstrate that the real gas effect of CO2 leads to an increased total inlet flow. When the pressure reaches 10.3 MPa, the relative difference is 51.90% compared to ideal gas. Minor temperature and pressure changes occur in the sealing chamber when the dry gas seal is in operation. However, the inlet temperature of the end face Tf increases and the inlet pressure of the end face pf decreases. These research results provide a reliable reference for engineering practice.

Fluid Dynamics Associated with Body Impact
Pages : 993-1002
Authors : B. Skews,  Bright Ndebele, 
Experiments and numerical simulations were conducted in order to examine the flow field surrounding a flat-faced body impacting on a flat surface. In the experiments impact velocities ranged up to nearly 5 m/s. Visualisation was with a standard z-format schlieren system using a high-speed camera. The associated flow field exhibited ejected gas jets, shed vortices and weak compression waves in the external flow, as well as in the gap depending on pressure differences between the gap and the external field. A computational fluid dynamic simulation (CFD) was undertaken, enabling detailed evaluation of: the flow in the gap, the flow of the emerging jet near the impacting surface, and the development of the wave system and flow on the upper and lower surfaces of the impactor during its descent. It was found that very high pressures are generated in the gap between the impactor and impacting surface and that the jet emerging from the periphery of the impactor can reach supersonic velocities.

Numerical Study on the Closing Characteristics of a Check Valve with Built-in Damping System
Pages : 1003-1014
Authors : HaiMin Wang,  Si Chen,  KeLiang Li,  HuanQi Li,  Zhen Yang, 
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.

Investigation on the Agent Precise Quantitative Control for Foam Technology with Cavitation Jet
Pages : 1015-1030
Authors : Xin-xiao Lu,  Yu Han,  Cong Shen,  Yun Xing,  Guo-yu Shi, 
A reliable agent addition control is crucial for the foam technology that is prevalent in many industrial fields. The objective of this paper is to reveal the precise quantitative control mechanism and distinctive performance of cavitation jet. The cavitation evolution suction process is analyzed by the vapor appearance order defined. A 5-6 mm vapor-liquid transition interface is found in the cavitation jet with a remarkable mutation in fluid pressure, density and velocity. The vapor region in the jet device decreases and the maximum vapor volume fraction declines from 96.4% to 0 as the pressure ratio increases. The precise quantitative control is realized by the cavitation jet at the negative pressure less than -87 kPa in the suction port. The absorption amount decreases with the absorbed liquid viscosity increasing and a various level of precise quantitative control is achieved by the orifice plate area. The relation between the absorption amount and plate area is quadratic curve. Furthermore, the dust suppression practical was successfully conducted in a coal bunker to verify the effectiveness of foam technology using cavitation jet. Based on the above contribution, it is believed that the proposed precise quantitative control method has a strong applicability and popularization in industrial control field.

Effects of Bionic Volute Tongue Bioinspired by Leading Edge of Owl Wing and Its Installation Angle on Performance of Multi-Blade Centrifugal Fan
Pages : 1031-1043
Authors : Ximing Dong,  Hua-Shu Dou, 
Effect of the volute tongue of the multi-blade centrifugal fan on the performance of the machines is significance. The shape and installation angle of the volute tongue affect the circulating internal flow behavior of the volute as well as the energy loss around the volute tongue. In this study, the profile of the leading edge of the owl wing is applied to the volute tongue of a multi-blade centrifugal fan to improve the aerodynamic performance of the fan. The fan models with different volute tongue installation angles are numerically simulated under different flow conditions. The research results show that the proposed design is able to improve the aerodynamic performance of the fan at different flow rate conditions. In addition, an improved method for quantitatively evaluating the level of impeller-volute tongue interaction based on the unsteady simulation result is proposed and it is verified to be effective. Furthermore, the two parameters for evaluating the internal flow circulation which are influenced by the installation angle of the bionic volute tongue are analyzed, namely the recirculated flow coefficient and the reversed flow coefficient. Combined with the analysis of energy loss around the volute tongue, the mechanism of variation of the aerodynamic performance of the multi-blade centrifugal fan with different volute tongue installation angles is explained.

Numerical Study of Liquid Film Formation around Tubes of Horizontal Falling Film Evaporator
Pages : 1045-1052
Authors : Awdhesh Poddar,  N. K. Singh, 
Falling film evaporative heat exchangers are extensively used in processing industries; broad areas of application being refrigeration, desalination and food processing industries. The fundamental aspect of this type of heat transfer process is to extract the process heat in the form of latent heat by liquid which is sprayed over the surface of the process tubes. Formation of liquid film over a fully wetted horizontal round tube of falling film evaporator has been numerically simulated here. Two numerical approaches, Volume of Fluid (VOF) technique and the Eulerian multiphase model are applied to compare their results. The effect of varying flow and geometrical parameters on the film thickness is investigated. Two horizontal tubes of diameter 19.05mm and 25.04mm with three different uniform spacing have been selected for simulation. Film Reynolds numbers 650, 950 and 1250 are considered for the above set of parameters. Ii is observed that the geometrical and flow parameters considerably influence the film thickness. Transient analysis of the film formation has been carried out and parameters like pathline of liquid film and the velocity profile have been obtained for understanding the flow behavior in a better manner. All the simulated results agree well with the published data.

Research on the Characteristics of Bluff Body Wake Field Induced by Synthetic Jet with PANS Model
Pages : 1053-1063
Authors : Zongrui Hao,  Gang Liu,  Wanlong Ren,  Yue Wang,  Haiyan Bie, 
Based on the active control theory, the synthetic jet behind the blunt body is explored by considering the control of flow around the blunt body in this paper. The Partially Averaged Navier-Stokes model was carried out for flow around circle cylinder at two subcritical Reynolds numbers (Re=1000, 3900), whose results have a good agreement with the experimental data. The results indicate that synthetic jet behind the circle cylinder has essential effects on the vortex shedding of flow around circle cylinder. The analysis of the Vorticity and Q vortex shows that the increasing velocity of synthetic jet has a strong effect on the vortex shedding of the original flow field. It is noted that the information including the coherence data and the directivity pattern with the existence of synthetic jet is different from that without synthetic jet. These results imply that the synthetic jet in the tail of the blunt body could control the flow fields around the blunt body.

Numerical Study on Natural Ventilation around a Burning Car inside a Tunnel
Pages : 1065-1076
Authors : HeeChang LIM, 
This study aimed to analyse the flow and temperature fields around a car on fire inside a medium-size tunnel under natural ventilation. The study used the fire dynamics simulator (FDS), which is an open-source software package, to simulate the thermo-fluidic characteristics inside the tunnel. A constant heat release rate (HRR) was assumed to ensure the release of consistent heat from the car while varying the speed of the mass flux natural ventilation. A medium-size tunnel and scaled car were modelled to simulate a realistic field environment; the car was assumed to have its primary components, including the body frame, tyres, seats, and other flammable materials. The constant HRR was set at 3.8 , which was measured in a field experiment using a cone calorimeter. The closed car windows were set to break when the temperature around them reached 600°C. Additionally, natural ventilation was one of the parameters used in the calculation; it was assigned as an inlet condition, referred to as the steady longitudinal inlet velocity, and ranged from 1.8 to 3.0 m/s. Based on the ventilation velocities, the variations of the flow characteristics, temperature field, turbulence kinetic energy levels, and vortex structure inside the tunnel were investigated. Additionally, the critical longitudinal velocity for natural ventilation was estimated by performing the current simulation under different ventilation velocities. Given that the thermo-fluidic parameters and geometry information are all similar, the FDS results were compared to those from three semi-empirical models for predicting critical ventilation velocities from previous studies.