Numerical Study On Streamwise Vorticity And Entrainment Enhancement Of A Round Jet

The jet pump has many outstanding advantages such as simple structure, high reliability, and low operation and maintenance costs and so on. Therefore, it has been widely used in nuclear industry fields. The entrainment ability of the jet pump is the core performance. It is generally recognized that, although the vorticity value of streamwise vortex is smaller than that of spanwise, it plays an important role in the entrainment process. Thus, a numerical simulation of streamwise vortex influence on the entrainment ability of a round jet is carried out. To enhance the entrainment of the round jet, tabs (streamwise vortex generator) and vibrator (perturbation) are set at the exit of nozzle. Tabs are set of number as 0, 2, 4 and 6, and the frequency of vibrator is set as natural frequency and its half. Based on the above configuration, numerical simulations of different nozzle types were carried out. Based on above results, firstly the influence of tabs on the entrainment ability was analyzed. The vortex intensity of the jet flow is obviously enhanced under introducing tabs. Similarly, the intensity of streamwise vortex downstream also increase significantly, thus enhance the entrainment ability. From the figure of local entrainment flow rate, the jet of nozzle with 6 tabs has the strongest entrainment ability; its local entrainment flow rate is 1.5 times of the round jet. Then, the influence of perturbation on the entrainment ability is analyzed. After introducing velocity perturbation, the evolution of the mixing layer evidently accelerated. Under natural frequency perturbation, mixing layer evolution is the most rapid. The mixing layer roll into a vortex ring in less than one diameter distance downstream allowing to 1.75 times the amount of fluid entrained in comparison with the round jet. Finally, integrating the influence of tabs and perturbation, the variation of the jet entrainment ability is analyzed. Due to the combined effect of tabs enhancing vortex intensity and natural frequency accelerating the evolution process, local entrainment ability of the jet is further improved which is more than three times that of the round jet. Taking the nozzle with 6 tabs and perturbation of natural frequency as an example, the streamwise vorticity increases rapidly to a maximum. Local entrainment flow rate also has a corresponding maximum value, which is one-to-one correspondence with the intensity distribution of streamwise vortex, indicating that streamwise vortex plays a predominant role in the entrainment ability of the jet.