The growth of a vapor bubble in a superheated liquid is controlled by three factors: the inertia of the liquid, the surface tension, and the vapor pressure. As the bubble grows, evaporation takes place at the bubble boundary, and the temperature and vapor pressure in the bubble are thereby decreased. The heat inflow requirement of evaporation, however, depends on the rate of bubble growth, so that the dynamic problem is linked with a heat diffusion problem. Since the heat diffusion problem has been solved, a quantitative formulation of the dynamic problem can be given. A solution for the radius of the vapor bubble as a function of time is obtained which is valid for sufficiently large radius. This asymptotic solution covers the range of physical interest since the radius at which it becomes valid is near the lower limit of experimental observation. It shows the strong effect of heat diffusion on the rate of bubble growth. Comparison of the predicted radius‐time behavior is made with experimental observations in superheated water, and very good agreement is found.

The growth of vapor bubbles in superheated liquids. report no. 26-6

Effects of large computational time steps on the computed turbulence were investigated using a fully implicit method. In turbulent channel flow computations the largest computational time step in wall units which led to accurate prediction of turbulence statistics was determined. Turbulence fluctuations could not be sustained if the computational time step was near or larger than the Kolmogorov time scale.

Effects of the computational time step on numerical solutions for turbulent flow

Tidal range technology has seen much development and interest in recent years. The times when a barrage scheme would be rejected due to environmental and cost concerns is coming to an end. A large variety of new lower cost and less invasive methods have since emerged in the forms of tidal lagoons, reefs and fences. Since the construction of La Rance in 1967, advancements in turbine technologies and design has since resulted in a plethora of new, exciting turbine designs for tidal energy. A selection of new turbines with possible tidal range applications includes the modified bulb turbine with two sets of guide vanes, a counter-rotating turbine, Archimedes screw and a gyro device. However, the same design is continuously being chosen – the Kaplan bulb turbine. Through the use of a marking criterion covering key aspects that should be considered when choosing a turbine a variety of the new designs available are investigated. The key aspects researched include, environmental effects, the two-way efficiency, initial costs and maintenance costs/difficulty.

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Tidal range technologies and state of the art in review

In recent years, distributed renewable energy-generation technologies, such as wind and solar, have developed rapidly. Nevertheless, the utilization of ultra-low-head (ULH) water energy (i.e., situations where the hydraulic head is less than 3 m or the water flow is more than 0.5 m/s with zero head) has received little attention. We believe that, through technological innovations and cost reductions, ULH hydropower has the potential to become an attractive, renewable, and sustainable resource. This paper investigates potential sites for ULH energy resources, the selection of relevant turbines and generators, simplification of civil works, and project costs. This review introduces the current achievements on ULH hydroelectric technology to stimulate discussions and participation of stakeholders to develop related technologies for further expanding its utilization as an important form of renewable energy.

Ultra-low-head hydroelectric technology: A review

Transient characteristics during power-off process in a shaft extension tubular pump by using a suitable numerical model

Numerical simulation of transient flow in a shaft extension tubular pump unit during runaway process caused by power failure

Energy loss mechanisms of transition from pump mode to turbine mode of an axial-flow pump under bidirectional conditions

With the increasing adoption of renewable energy sources globally, hydropower contributes significantly to energy generation through various schemes ranging from big to small-scale plants. In small-scale hydropower plants, the preference for reverse-operated pumps (known as pump as turbines or PATs) over small-scale hydroturbines has increased. However, apart from the associated economic advantages, PATs, like any other hydraulic machinery, are not free from common problems such as cavitation. Cavitation is a phenomenon in which air bubbles are formed within the fluid medium due to substantial local pressure drop and their eventual collapse causes material erosion and degrades the overall machine efficiency. Several studies have focused on PAT conventional operating mode, while its reverse mode just begun to gain research interest. Nevertheless, cavitation remains a common problem in PATs at various hydro-sites. Therefore, to analyze PAT cavitation performance and highlight the differences between its two operating modes in terms of their development mechanisms, this article presents a thorough review of PAT cavitation dynamics and influencing parameters, as well as the future research directions. It is found that PAT reverse mode is more prone to cavitation, but more damages would occur in the conventional mode. Nevertheless, modifying the PAT geometric design parameters can considerably improve its cavitation performance. However, this approach has not been sufficiently investigated for PAT reverse operating mode and hence requires further research. Note that the terms “PAT conventional mode,” “PAT pumping mode,” and “pump” are equally used throughout this paper. • Studies on cavitation performance in hydraulic pumps as turbines (PATs) are reviewed. • Both pump and turbine modes are considered in the review. • PATs are found to have gained importance due to their applicability in remote areas. • PAT reverse mode operations require high heads and flows; prone to cavitation. • Geometric design modification is widely used for PAT cavitation performance improvement. 

Daqing Zhou

Papers

Transient characteristics during power-off process in a shaft extension tubular pump by using a suitable numerical model

Numerical simulation of transient flow in a shaft extension tubular pump unit during runaway process caused by power failure

Energy loss mechanisms of transition from pump mode to turbine mode of an axial-flow pump under bidirectional conditions

Pump as turbine cavitation performance for both conventional and reverse operating modes: A review

Large Eddy simulation of ventilated cavitation with an insight on the correlation mechanism between ventilation and vortex evolutions