An instability criterion for viscoelastic flow past a confined cylinder

Dou, Hua-Shu,Phan-Thien, Nhan The Korean Society of Rheology 2008 Korea-Australia rheology journal Vol.20 No.1

It has been known that there is a viscoelastic instability in the channel flow past a cylinder at high Deborah (De) number. Some of our numerical simulations and a boundary layer analysis indicated that this instability is related to the shear flow in the gap between the cylinder and the channel walls in our previous work. The critical condition for instability initiation may be related to an inflection velocity profile generated by the normal stress near the cylinder surface. At high De, the elastic normal stress coupling with the streamline curvature is responsible for the shear instability, which has been recognized by the community. In this study, an instability criterion for the flow problem is proposed based on the analysis on the pressure gradient and some supporting numerical simulations. The critical De number for various model fluids is given. It increases with the geometrical aspect ratio h/R (half channel width/cylinder radius) and depends on a viscosity ratio ${\beta}$(polymer viscosity/total viscosity) of the model. A shear thinning first normal stress coefficient will delay the instability. An excellent agreement between the predicted critical Deborah number and reported experiments is obtained.

An instability criterion for viscoelastic flow past a confined cylinder

Hua-Shu Dou,Nhan Phan-Thien 한국유변학회 2008 Korea-Australia rheology journal Vol.20 No.1

It has been known that there is a viscoelastic instability in the channel flow past a cylinder at high Deborah (De) number. Some of our numerical simulations and a boundary layer analysis indicated that this instability is related to the shear flow in the gap between the cylinder and the channel walls in our previous work. The critical condition for instability initiation may be related to an inflection velocity profile generated by the normal stress near the cylinder surface. At high De, the elastic normal stress coupling with the streamline curvature is responsible for the shear instability, which has been recognized by the community. In this study, an instability criterion for the flow problem is proposed based on the analysis on the pressure gradient and some supporting numerical simulations. The critical De number for various model fluids is given. It increases with the geometrical aspect ratio h/R (half channel width/cylinder radius) and depends on a viscosity ratio β (polymer viscosity/total viscosity) of the model. A shear thinning first normal stress coefficient will delay the instability. An excellent agreement between the predicted critical Deborah number and reported experiments is obtained.

Theory and Prediction of Turbulent Transition

Dou, Hua-Shu,Khoo, Boo-Cheong Korean Society for Fluid machinery 2011 International journal of fluid machinery and syste Vol.4 No.1

We have proposed a new approach based on energy gradient concept for the study of flow instability and turbulent transition in parallel flows in our previous works. It was shown that the disturbance amplitude required for turbulent transition is inversely proportional to Re, which is in agreement with the experiments for imposed transverse disturbance. In present study, the energy gradient theory is extended to the generalized curved flows which have much application in turbomachinery and other fluid delivery devices. Within the frame of the new theory, basic theorems for flow instability in general cases are provided in details. Examples of applications of the theory are given from our previous studies which show comparison of the theory with available experimental data. It is shown that excellent agreement has been achieved for several configurations. Finally, various prediction methods for turbulent transition are reviewed and commented.

Employing rotating vaneless diffuser to enhance the performance of plenum fan

Dou, Hua-Shu,Wu, Lin,Wei, Yikun,Chen, Yongning,Cao, Wenbin,Ying, Cunlie Korean Society for Fluid machinery 2017 International journal of fluid machinery and syste Vol.10 No.1

Numerical simulation is carried out for flow characteristics in a plenum fan and the influence of the diameter ratio of the rotating vaneless diffuser on the performance of plenum fan is analyzed. The diameter ratio of the rotating vaneless diffuser employed is from 1.03 to 1.3. The research results show that the rotating vaneless diffuser is able to enhance the performance of plenum fan. It is found that there is significant improvement in static pressure and efficiency at the diameter ratio of 1.05 at high flow coefficients, while the optimal diameter ratio is 1.2 at rated and low flow coefficient.

Employing rotating vaneless diffuser to enhance the performance of plenum fan

Hua-Shu Dou,Lin Wu,Yikun Wei,Yongning Chen,Wenbin Cao,Cunlie Ying 한국유체기계학회 2017 International journal of fluid machinery and syste Vol.10 No.1

Numerical simulation is carried out for flow characteristics in a plenum fan and the influence of the diameter ratio of the rotating vaneless diffuser on the performance of plenum fan is analyzed. The diameter ratio of the rotating vaneless diffuser employed is from 1.03 to 1.3. The research results show that the rotating vaneless diffuser is able to enhance the performance of plenum fan. It is found that there is significant improvement in static pressure and efficiency at the diameter ratio of 1.05 at high flow coefficients, while the optimal diameter ratio is 1.2 at rated and low flow coefficient.

Optimization of Blade Profile of a Plenum Fan

Wu, Lin,Dou, Hua-Shu,Wei, Yikun,Chen, Yongning,Cao, Wenbin,Ying, Cunlie Korean Society for Fluid machinery 2016 International journal of fluid machinery and syste Vol.9 No.1

A method of optimization design for the blade profile of a centrifugal impeller by controlling velocity distribution is presented, and a plenum fan is successfully designed. This method is based on the inner flow calculation inside the centrifugal impeller, and is related to the distribution of relative velocity. The results show that after optimization, the boundary layer separation on the suction surface has been inhibited and the stability of plenum fan is improved. The flow at the impeller outlet is also studied, and the jet-wake pattern at the impeller outlet is improved obviously by optimization. The calculation result shows that the static pressure and static pressure efficiency can be increased by 15.4% and 21.4% respectively.

Optimization of Blade Profile of a Plenum Fan

Lin Wu,Hua-Shu Dou,Yikun Wei,Yongning Chen,Wenbin Cao,Cunlie Ying 한국유체기계학회 2016 International journal of fluid machinery and syste Vol.9 No.1

A method of optimization design for the blade profile of a centrifugal impeller by controlling velocity distribution is presented, and a plenum fan is successfully designed. This method is based on the inner flow calculation inside the centrifugal impeller, and is related to the distribution of relative velocity. The results show that after optimization, the boundary layer separation on the suction surface has been inhibited and the stability of plenum fan is improved. The flow at the impeller outlet is also studied, and the jet-wake pattern at the impeller outlet is improved obviously by optimization. The calculation result shows that the static pressure and static pressure efficiency can be increased by 15.4% and 21.4% respectively.

Effect of Blade Profile on the Performance of a Centrifugal Fan with Different Velocity Distribution Functions

Yifan Zhang,Hua-Shu Dou 한국유체기계학회 2020 International journal of fluid machinery and syste Vol.13 No.3

The velocity distribution method is a way to design the blade profile according to the average relative velocity in the impeller passage. Present research is aimed to investigate the effect of blade profile on the performance of a centrifugal fan with different velocity distribution functions through numerical simulations. The results show that compared with common arc blade, the centrifugal fan with new blade profile designed by velocity distribution method gets a higher efficiency, the relative velocity distribution along impeller passages of the new model is more uniform, and the boundary layer separation is delayed than that in the original model.

Study of flow instability in a centrifugal fan based on energy gradient theory

Meina Xiao,Qing Xiao,Hua-Shu Dou,Xiaoyang Ma,Yongning Chen,Haijiang He,Xinxue Ye 대한기계학회 2016 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.30 No.2

Flow instability in a centrifugal fan was studied using energy gradient theory. Numerical simulation was performed for the threedimensional turbulent flow field in a centrifugal fan. The flow is governed by the three-dimensional incompressible Navier-Stokes equations coupled with the RNG k-ε turbulent model. The finite volume method was used to discretize the governing equations and the Semiimplicit method for pressure linked equation (SIMPLE) algorithm is employed to iterate the system of the equations. The interior flow field in the centrifugal fan and the distribution of the energy gradient function K are obtained at different flow rates. According to the energy gradient method, the area with larger value of K is the place where the flow loses stability easier. The results show that instability is easier to generate in the regions of impeller outlet and volute tongue. The air flow near the hub is more stable than that near the shroud. That is due to the influences of variations of the velocity and the inlet angle along the axial direction. With the decrease of the flow rate, instability zone in a blade channel moves to the impeller inlet from the outlet and the unstable regions in different channels develop in opposite direction to the rotation of impeller.

Numerical simulation and analysis of flow characteristics in the front chamber of a centrifugal pump

Yang Wu,Xiaoping Chen,Hua-Shu Dou,Lulu Zheng,ZuChao Zhu,BaoLing Cui,Boo Cheong Khoo 대한기계학회 2017 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.31 No.11

We performed numerical simulations to study the flow characteristic in a centrifugal pump based on the RANS equations and the RNG k-ε turbulent model. The flow field, including the front and back pump chambers, the impeller wear-ring, the impeller passage, the volute casing, the inlet section and outlet section was calculated to obtain accurate numerical results of fluid flow in a centrifugal pump. The flow characteristic was studied from the internal flow structure in pump chambers, the radial velocity at impeller outlet as well as the pressure inside of the pump, the circumferential velocity and the radial velocity in front pump chamber. The variation of flow parameters in internal flow versus flow rate in the centrifugal pump was analyzed. The results show that the overall performance of the pump is in good agreement with the experimental data. The simulation results show that the distribution of flow field in the front pump chamber is axial asymmetry. The energy dissipation at the impeller outlet is larger than other areas. The distribution of the circumferential velocity and that of radial velocity are similar along the axial direction in the front pump chamber, but the distribution of flow is different along the circumferential and the radial directions. It was also found that the vorticity is large at the impeller inlet compared with other areas.