The improvement of flow conditions in a whirlpool with a modified bottom: An experimental study based on particle image velocimetry (PIV)

Experiment and Numerical Simulation on Hydraulic Loss and Flow Pattern of Low Hump Outlet Conduit with Different Inlet Water Rotation Speeds

The rotation speed of water at the inlet of the low hump outlet conduit has a great effect on its hydraulic performance. Therefore, the influence of different inlet water rotation speeds on hydraulic loss and flow pattern of low hump outlet conduit is studied in this paper. By solving RANS equations and the RNG k-ε turbulence model, the hydraulic loss and 3D flow field of the low hump outlet conduit were calculated under different inlet water rotation speeds. To verify the numerical results, the model tests of low hump outlet conduit with different guide vanes were conducted. The results show that along with the growth of inlet water rotation speed, the hydraulic loss of outlet conduit will firstly decrease by degrees and then increase dramatically, the vortex location moves from the whole bottom of the descent segment to the right bottom of descent segment and the vortex area becomes smaller, the flow pattern of the whole conduit is improved obviously. The hydraulic loss and flow field of numerical simulation are consistent with those of the model test. Because of its great influence on hydraulic performance, inlet water rotation speed must be taken into consideration in the hydraulic optimization design of guide vane and low hump outlet conduit.

Visualization and Measurement of Swirling Flow of Dry Ice Particles in Cyclone Separator-Sublimator

The dry ice sublimation process of carbon dioxide (CO2) is a unique, environmentally friendly technology that can achieve a temperature of −56 °C or lower, which is a triple point of CO2 in CO2 refrigeration systems. In this study, a cyclone separator-evaporator was proposed to separate dry ice particles in an evaporator. As an initial step before introducing the cyclone separator-evaporator into an actual refrigeration system, a prototype cyclone separator-evaporator was constructed to visualize dry ice particles in a separation chamber. A high-speed camera was used to visualize the non-uniform flow of dry ice particles that repeatedly coalescence and collision in a swirl section. Consequently, the dry ice particle size and the circumferential and axial velocities of dry ice were measured. The results show that the equivalent diameter of the most abundant dry ice particles in the cyclone separation chamber is 2.0 mm. As the inner diameter of the separation section decreases, dry ice particles coalesce and grow from an equivalent diameter of 4 mm to a maximum of 40 mm. In addition, the comparison of the experimental and simulation results shows that the drag force due to CO2 gas flow is dominant in the circumferential velocity of dry ice particles.

Enhancing Flow Field Performance of a Small Circulating Water Channel Based on Porous Grid Plate

Low-cost and high-efficiency circulating water channels are widely used in the hydrodynamic tests of an underwater device. The current research mainly focuses on obtaining a better velocity uniformity of the test section by optimizing the curve function of the boundary in the contraction section. While, for small underwater device, their hydrodynamic characteristics are sensitive to turbulence. Thus, the circulating water channel, which can obtain the required turbulence characteristics, is urgently needed. A small circulating water channel, which can reduce the turbulence intensity based on a porous grid plate and can be used to test the hydrodynamic characteristics of a small underwater device, is designed. The relationships between porosities and resistance coefficients of a porous grid plate are established. The effects of the honeycomb (porosity and thickness); screen (porosity, number of layers, and spacing); and pumping flow rate on the turbulent characteristics of the test section are studied. The relationships between the parameters and the turbulent characteristics of the test section are established, and the methods to achieve the required flow characteristics of the test section are proposed. Experiments are carried out, and the validity of the obtained results is verified. In this work, the turbulence intensity of the fluid field in the test section can be restrained to 0.0491, which is enough to meet the turbulence requirements for the hydrodynamic test of a small underwater device. This work can provide references for the construction of a hydrodynamic test platform for small underwater devices.