Undercurrent: The Alden Blog

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Industrial fluid dynamics insights


Energy Dissipating Devices: Total Dissolved Gas Production at High Head Dams - Part 3
In Parts 1 and 2 of this series we talked about production of total dissolved gas (TDG) at high head dams, and the use of air supply systems to mitigate cavitation damage to spillways and spillway modifications. In this part, we’ll discuss the structural designs used to reduce TDG during flow release from spillways. Super-cavitating roughness elements, or baffle blocks for short, are designed to break up flow as water travels down a spillway. Depending on the placement of the blocks on the spillway surface, they can also help spread out the jet of water laterally as it exits the spillway, resulting in a larger impact area on the tailrace. Why does this help reduce TDG production? As we discussed in Part 1, dissolved gas supersaturation in the tailrace is a function of how long it takes the bubbles from the aerated flow to reach the water surface. By spreading out the impact zone of the spillway jet, and decreasing the amount of energy the jet has, plunge depth of the jet is reduced and the aerated flow can rise ...

Air Supply Ramps and Cavitation: Total Dissolved Gas Production at High Head Dams – Part 2
Part 1 of this series outlined how high concentrations of total dissolved gas (TDG) can occur downstream from high head dams when their spillways are open, and how this TDG can be harmful or even fatal to fish. Alden has been involved in several recent projects for which the objective was to reduce TDG downstream of high head dams. Alden performed the hydraulic and structural design of roughness elements that break up the high velocity jet of flow discharged from the spillway. TDG production is reduced by these roughness elements because they cause the jet to spread out and thereby reduce the plunge depth in the receiving water, which reduces TDG. The roughness elements work very well at reducing plunge depth, but they can cause cavitation, which can damage the spillway surface and the blocks themselves. The design and implementation of the roughness elements will be topic of another article. The present article focuses on reducing the potential for cavitation on the roughness elements. Alden designed roughness elements have been installed on spillways at Cabinet Gorge and Boundary Dams. Cabinet ...

The Economics of Hydro Turbine Performance Testing
Today’s entry comes from a guest blogger, Jim Walsh, President of Rennasonic, a small consulting firm specializing in turbine and pump performance testing and optimization of multi-unit hydroelectric power plants using ultra-sonic multi-path flow meters.  Alden has partnered with Rennasonic for numerous turbine performance tests, providing supplemental flow measurements using dye dilution and current meter profiling. Power output, flow, and head measurements being taken during performance testing As a hydropower electric power generating utility, how do I know when I should, or should not, invest in my equipment?  The answer can be complicated due to many factors, including but not limited to: the current price of power, generating capacity, equipment age, and government regulations.  To determine the performance of an installed hydropower turbine, the measurements of water flow, head, and power must be made within a reasonable amount of uncertainty.  Generally speaking, flow is the most difficult parameter to measure in the field and, consequently, is the most expensive.  The cost of measuring flow can seem unsurmountable for small hydropower owners, so the question becomes when testing expenditures yield a ...

Improving Water Quality: Total Dissolved Gas Production at High Head Dams – Part I
During spill season at hydroelectric dams, more water flows into the upstream reservoir than can be used to generate electricity in the powerhouses. This excess flow must pass through a number of different flow release structures in order to bypass the dam and powerhouse. Spillways, diversion tunnels, and low-level sluice gates are commonly used to route flow past dams. Open channel spillways are one of the most common flow release structures at high head dams, and create a highly aerated, turbulent jet of water that exits the spillway up to 150 feet above the river downstream of the dam. This waterfall of aerated flow can plunge to the bottom of the tailwater pool, where the bubbles of atmospheric gases are slowly dissolved into solution with the water. The deeper the jet plunges, the more pressure is exerted by the water on the bubbles, dissolving them faster and preventing them from rising to the surface. This is why we see a frothy white plume of flow that can stretch up to half a mile downstream of a dam when flow ...

Pumped Energy Storage: Old Technology, New Demand
There is an interesting past and new future for the world’s largest batteries.  For decades pumped storage plants have helped stabilize large power grids by supplying peak power support to base loaded nuclear and fossil fuel power plants across the United States and the world.  Today, as many nations make a conscious move away from these generating sources toward renewable energy technology, such as solar and wind, and smaller micro grids, these behemoth batteries are finding new purpose.  Keeping in the spirit of this green power revolution, it is imperative to keep the environmental impact of pumped power plants, and other power storage technologies, as low as possible. Base loaded power plants, nuclear or fossil fuel, operate at full production 24 hours a day because they are difficult and slow to start and stop; however society’s power demand fluctuates dramatically throughout the day.  In the morning and during business hours when most people are running home appliances or working with tools and equipment, the base load power may not be able to keep up.  In contrast, at night when most ...