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The slit wall effect on the flow instability and heat transfer characteristics in Taylor-Couette flow was numerically studied by changing the rotating Reynolds number and applying the negative temperature gradient. The concentric cylinders with slit wall are seen in many rotating machineries. Six different models with the slit number 0, 6, 9, 12, 15 and 18 were investigated in this study. The results show the axial slit wall enhances the Taylor vortex flow and suppresses the azimuthal variation of wavy Taylor vortex flow. When negative temperature gradient exists, the results show that the heat transfer augmentation appears from laminar Taylor vortex to turbulent Taylor flow regime. The heat transfer enhancement become stronger as increasing the Reynolds number and slit number. The larger slit number model also accelerates the flow transition regardless of the negative temperature gradient or isothermal condition.

Numerical simulation was applied to investigate the Taylor vortex flow inside the concentric cylinders with a constant radial temperature gradient. The reliability of numerical simulation method was verified by the experimental results of PIV. The radial velocity and temperature distribution in plain and 12-slit model at different axial locations were compared, and the heat flux distributions along the inner cylinder wall at different work conditions were obtained. In the plain model, the average surface heat flux of inner cylinder increased with the inner cylinder rotation speed. In slit model, the slit wall significantly changed the distribution of flow field and temperature in the annulus gap, and the radial flow was strengthen obviously , whichpromote d the heat transfer process at the same working condition. Keywords : Taylor vortex flow, Slit wall, temperature gradient, heat transfer, numerical simulation 1. Introduction In 1923, Taylor[1] discovered the secondary flow existed in the annulus gap between two relatively rotating concentric cylinders, which is called Taylor-Couette flow. Since then, a lot of scholars carried out in-depth research on it. Yang[2] studied the transition process of Taylor-Couette flow by flow visualization method, the aluminum particles were added to investigate the stability of flow field, they found as the rotation speed increased, the aluminum particles have the axial motion before the transition to Taylor vortex flow. Kedia et al.[3] studied the effects of heat transfer on Taylor-Couette flow, and found the heat transfer decreased with Grashof number in axisymmetric Taylor vortex flow regime, and increased with Grashof number after the flow becomes non-axisymmetric. Lee and Minkowycz [4] studied heat transfer characteristics by using the naphthalene sublimation technique in the annular gap between two short concentric cylinders which had either two walls or one plain plain and one axially slit wall, and yielded qualitative information regarding heat transfer but did not address the flow phenomena inside the annular gap. Liu[5] studied the axial wall slits effect on the Taylor vortex flow in the gap between two concentric cylinders by Particle Image Velocimetry, and found the existence of slit wall accelerated the transition process.

In order to study the internal flow characteristics of roto-jet pump, the standard turbulence model and SIMPLEC algorithm were used to calculate the three-dimensional turbulence flow field under design condition. The simulation results reveal the internal flow phenomena of roto-jet pump, which shows the pressure near the suction surface and the inlet edge of impeller is minimum, and the cavitations is easy to occur . At the same time, the circumfluence and flow separation phenomenon is not found in the vicinity of the short blade, which is used to shunt the flow. The pressure gradient changed little in the rotor cavity. Is easy to form vortex at manifold import corners, there is no vortex generated on the second corner when the diffusion tube is used. The results obtained in this research provide a theoretical foundation for improving pump performance and reduce the hydraulic loss.

The wavy vortex flow in the plain model was studied by experimental measurement; the preliminary feature of wavy vortex flow was obtained. This flow field in the plain model was also studied by numerical simulation. The reliability of numerical simulation was verified by comparing with the experimental and numerical simulation results. To study the slit wall effect on the wavy vortex flow regime, another two models with different slit number were considered; the slit number was 6 and 12. By comparing the wavy vortex flow field in different models, the axial fluctuation of Taylor vortices was found to be different, which was increased with the increasing of slit number. The maximum radial velocity from the inner cylinder to the outer one in the 6-slit number was increased by 12.7% compared to that of plain model. From the results of different circumferential position in the same slit model, it can be found that the maximum radial velocity in slit plane is significantly greater than that in other planes. The size of Taylor vortices in different models was also calculated, which was found to be increased in the 6-slit model but was not changed as the slit number increased further.