Research On The Flow Mechanism And Structure Optimization Of Annular Jet Pumps

The jet pump has many outstanding advantages such as simple construction, high reliability, low operation and maintenance costs and so on. Compared with the conventional central jet pump (CJP), the annular jet pump (AJP) has no obstruction in its suction passage and the direction of the suction flow needs not change. The geometry of the AJP makes it well suitable for applications in the hydraulic transport of large solids. Its typical usage includes the transporting of live fish, mineral, large cylindrical capsules, industrial waste, etc. Therefore, it has been used widely in many engineering fields. However, few investigations have been done on the flow mechainism inside the AJP and its structure optimization. The flow field inside the AJP is confined annular wall jet developing in circumstance of adverse pressure gradient. The adverse pressure gradrient, jet shear layer, wall boundary layer and possible backflow region make the flow field highly complicated. Hence, this thesis devotes to analyzing the flow mechainism inside the AJP based on large-eddy simulation (LES) results with turbulence statistical theory and coherent structure theory and adopting unconventional structures to improve the AJP performance. The results and conclusions are described briefly as follows:(1) The experiment of the AJP with three different area ratios of the throat to the annular nozzle (m for short and m=1.72,2.26and3.33respectively) was carried out for LES validation. The experiment data shows that the self-similarity exists inside the flow field of the AJP as that of the CJP and the high-efficiency region of the AJP is wider than that of the CJP. It also shows that the AJP structure corresponding to best performance should change with m. The performance curve moves to larger flow ratio as m increases.(2)The AJPs with m=1.72and m=3.33under different working conditions are calculated through LES and the effect of different grid numbers and distributions and subgrid models is analyzed. The experiment data was utilized to vailidate the LES results. The LES results agree well with the experiment data. The vortex shedding frequency and Strouhal number St are obtained by FFT to pressure coefficient. The characterized vortex shedding frequency St of the AJP with m=1.72decreases with the flow ratio among0.2-0.22while that of the AJP with m=3.33is unchanged equaling0.23. (3) The time-averaged flow field is analyzed and it is found that the potential core lengths of the primary and secondary flows increase linearly. The half-width increases linearly inside the suction chamber, and the smaller the flow ratio is, the more greatly the half-width increases. The increase rate of the boundary-layer has no corelation with the flow ratio inside the suction chamber, while its increase rate rises with the flow ratio inside the throat. A power surplus coeffiecient is defined in this thesis to describe the power change along the AJP. The coeffiecient decreases downstream and the smaller the flow ratio is, the more rapidly the coeffiecient drops. The backflow region has great difference between time-averaged and instantaneous simulation result. The shape and distribution of the instantaneous backflow region is distorted and irregular, even discontinuous. The backflow region moves downstream and its size reduces as the flow ratio rises. Comparing with the attachment point, the separate point moves more downstream.(4) The pressure-criterion, vorticity and the Q-criterion have proved to be effective to educe vortices in the AJP flow field. Compared with the Q-criterion, the pressure-criterion has a lower identification degree for the smaller coherent structures, especially for the ribs which exist wildely in the vortex braid region. The coherent structures are mainly found in the mixing layer, the boundary layer and the backflow region and have reciprocal actions. The increasing of the vortex in the mixing layer forces the near-wall boundary layer, leading to the production of the vortex rings in the boundary layer. The vortices in the two layers have contrary rotation directions and the contact region has the same flow direction, resulting in weak interaction. The forming mechainism of the streamwise vortex is different from that of the spanwise vortex, leading to different strengths, sizes and evolving processes. The vorticity value of the streamwise vortex is much lower than that of the spanwise vortex, but the streamwise vortex promotes the distortion and dissipation of the spanwise vortex, and then plays a major role in the entrainment and mixing. The time-averaged evolution rules are explained from the point of vortex motion.(5) Due to the large cost of LES, the RANS is adopted to validate the effect of the new structure on the AJP performance instead. Six turbulence models frequently used in the jet pump simulations are validated. The RNG k-εmodel is proved to give more accurate results of the performance and time-averaged flow field of the AJP. The standard k-εmodel underestimates the AJP performance while predicts well the variation trend of the performance and wall pressure coefficient. (6)The constant rate of velocity or pressure change method is utilized to design the diffuser of the AJP to lower the power loss due to the non-uniform change of velocity or pressure inside the conical diffuser. The brief and practical design formulas are derived from the methods. Both the methods can produce uniform velocity and pressure change inside the diffuser and improve the AJP performance in deed, especially when the diffuser has larger angle or shorter length. Hence the methods are especially suitable for situations with limited space or weight. The diffuser designed with the constant rate of pressure change method has better performace than that with the constant rate of velocity change method.(7) A new type sandwich annular nozzle is proposed to improve the AJP performance. This new nozzle produces high speed annular primary flow surrounded both inside and outside by the secondary flows. The wall friction loss is decreased and the interface between the two flows is increased, leading to the peak efficiency raise greatly from35.8%to45.1%. Both the distance between the annular nozzle and the wall and the velocity ratio between the two secondary flows have great influence on the performance of the AJP with the new type nozzle, and the best velocity ratio changes with the distance. When the best velocity ratio is unknown, the velocity ratio of1/1could always obtain better results.