CFD evaluation of performance of Gravitational Water Vortex Turbine at different runner positions

A free vortex is a region in which flow revolves around an axis line that requires a small head to form, about 0.7 m-2 m. In Gravitational water vortex turbine, water assuming to be non-rotational and inviscid passes through an open channel and enters the basin tangentially where it forms a powerful vortex. Then, the dynamic force of water is transmitted by the vortex to the turbine via mixed flow, impulse and reaction, phenomena. It can be a promising cheap and effective solution to compliment recent strives for renewable energy technologies. This paper deals with design and development of a prototype Gravitational water vortex turbine and analysis with computational and experimental methods. Initially, the computational method focuses on determining maximum tangential velocity of water achievable in the setup without turbine. And further computational analysis is carried out for the setup with turbine to determine performance characteristics by adjusting the runner heights in three positions. This computational study is validated by performing experimental testing by fabricating test rig. It showed best efficiency when runner was at lowermost portion of the conical basin.

Development of a Misaligned Tropical Cyclone

A cloud-resolving model is used to examine the virtually shear-free evolution of incipient tropical cyclones initialized with different degrees of misalignment between the lower- and middle-tropospheric centers of rotation. Increasing the initial displacement of rotational centers (the tilt) from a negligible value to several hundred kilometers extends the time scale of hurricane formation from 1 to 10 days. Hindered amplification of the maximum tangential velocity υm at the surface of a strongly perturbed system is related to an extended duration of misalignment resulting from incomplete early decay and subsequent transient growth of the tilt magnitude. The prolonged misalignment coincides with a prolonged period of asymmetric convection peaked far from the surface center of the vortex. A Sawyer–Eliassen model is used to analyze the disparity between azimuthal velocity tendencies of selected pre–tropical storm vortices with low and high degrees of misalignment. Although no single factor completely explains the difference of intensification rates, greater misalignment is linked to weaker positive azimuthal velocity forcing near υm by the component of the mean secondary circulation attributed to heating by microphysical cloud processes. Of note regarding the dynamics, enhanced tilt only modestly affects the growth rate of kinetic energy outside the core of the surface vortex while severely hindering intensification of υm within the core for at least several days. The processes controlling the evolution of the misalignment associated with inefficient development are examined in detail for a selected simulation. It is found that adiabatic mechanisms are capable of driving the transient amplification of tilt, whereas diabatic processes are essential to ultimate alignment of the tropical cyclone.

A Triggering Mechanism for Rapid Intensification of Tropical Cyclones

Triggering processes for the rapidly intensifying phase of a tropical cyclone (TC) were investigated on the basis of numerical experiments using a three-dimensional nonhydrostatic model. The results revealed that the rapid intensification of the simulated TC commenced following the formation of a circular cloud, which occurred about 12 h after the TC became essentially axisymmetric. The circular cloud (eyewall) evolved from a cloudy convective cell that was originally generated near the radius of maximum wind speed (RMW). The development of the convective cell in the eyewall was closely related to the radial location of the strong boundary layer convergence of axisymmetric flow. The radius of maximum convergence (RMC) was small relative to the RMW when the TC vortex was weak, which is consistent with the boundary layer theory for a rotating fluid system on a frictional surface. As the TC intensified, the RMC approached the RMW. An eyewall was very likely to form in the simulated TC when the RMC approached the RMW. Because the RMC is theoretically determined by a Rossby number defined by the maximum tangential velocity, RMW, and Coriolis parameter, a series of numerical experiments was conducted by changing the three parameters. The results were consistent with the hypothesis that intensification occurs earlier for larger Rossby numbers. This finding indicates that initial TC vortices with larger Rossby numbers are more likely to experience rapid intensification and, hence, to evolve into strong hurricanes.

An Analytical Model for Decaying Swirl Flow and Heat Transfer Inside a Tube

Decaying swirl flow can enhance the heat transfer inside a tube. For the decaying swirl flow of which maximum tangential velocity is located in the immediate vicinity of the wall, an analytical model based on the fluid theorem about the moment of momentum is proposed for the local maximum tangential velocity, local friction factor, and local Nusselt number in this paper. The analytical solutions compare favorably with the experimental data. Influences of the Reynolds number, wall roughness and initial tangential-to-axial velocity ratio on the decaying characteristics of the friction factor and Nusselt number have been analyzed. The analytical results show that the swirl flow decays more rapidly at the initial segment; for same conditions, the friction factor decays more severely than the Nusselt number; relative to the values of the nonswirl flow, the friction factor increases more intensely than the Nusselt number. [S0022-1481(00)70401-4]

The Limit Load of Transonic Turbine Blading

Limit load represents the maximum tangential velocity or blade force that can be generated within a transonic turbine blade. Its accurate prediction is therefore extremely important if the expected thermal efficiency and power output from the turbine is to be obtained. This report presents methods of calculating limit load for typical transonic turbine blading. Comparisons are made between predictions and test data from: (a) two-dimensional stationary cascade and (b) model low pressure turbine with dry steam. Satisfactory agreement is shown between the predictive techniques and the test values of maximum tangential velocity.