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Waylon Hernandez
Waylon Hernandez

Variable Geometry Turbine Technology For Marine... __EXCLUSIVE__



This chapter provides an overview of marine gas turbines, marine gas turbine variable geometry turbines, and variable geometry turbine design features and requirements. The development of modern advanced marine gas turbines must meet the military needs of ship power and the special requirements of the marine working environment. The variable geometry turbine is a technical method for effectively improving the acceleration and deceleration characteristics and low working condition performance of marine gas turbines.




Variable Geometry Turbine Technology for Marine...


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This book starts from the design requirements of variable geometry turbines for marine gas turbines. It systematically and comprehensively introduces the flow mechanism and characteristics of variable geometry turbines, aerodynamic design methods, variable vane turning design methods, structural design technology of the variable vane system, aerodynamic characteristics and reliability test technology for variable geometry turbines, and so on.


Gao Jie is Professor of Harbin Engineering University, Ph.D., and his main research interest is the aerodynamic thermodynamics of marine gas turbines. He has presided over more than 20 scientific research projects, won a 1-second prize of national defense technology invention, and published more than 100 academic papers, including 42 SCI/EI papers as the first author.


Zheng Qun is Professor of Harbin Engineering University, Ph.D., and his main research interest is the aerodynamic thermodynamics of marine gas turbines. He has presided over more than 30 scientific research projects, won 2-second prizes and 3-third prizes of provincial and ministerial science and technology awards, and published more than 100 academic papers.


NREL is investigating load-shedding capabilities when designing WECs with variable-geometry control surfaces. These control surfaces, like pitching blades in wind turbines, will add significant load-shedding capabilities in larger wave environments. Rendering by Josh Bauer, NREL


Because of the additional control they offer, variable geometry WECs can be optimized for greater energy capture, improved efficiency of operation, and ultimately, a more cost-competitive wave energy.


Artist rendering of the first-generation variable-geometry oscillating surge wave energy converter (VG-OSWEC) mounted on a raised foundation with the power take-offs shown in yellow. Rendering by Josh Bauer, NREL, and Jason Cotrell, RCAM Technologies


A novel control option for WEC design is the use of control surfaces that allow for changing or variable geometries. The novelty of the proposed design is the ability to alter WEC surfaces normal to the principal degree of freedom for energy capture, thereby reducing the wave pressure and corresponding loads. In current practice, the PTO is commonly the only control knob used to maximize power and limit peak loads. However, NREL is suggesting that an additional control knob be added that uses the WEC geometry for advanced load-shedding.


Furthermore, the extreme loads the device must withstand cannot be limited when the WEC geometry is fixed, thereby limiting the effectiveness of the PTO to minimize loads. Without the ability to shed greater hydrodynamic loads, the WEC must be placed in survival mode, and power production will not just be decreased but halted, reducing availability, and limiting the number of operational sea states. The reduced availability has negative impacts on technology acceptability as a result of reduced capacity factors and increased intermittency on the grid.


In addition, these fluctuations will have important implications for the stability of voltage and frequency to the grid and can be a problem for sensitive equipment. Therefore, it is essential to reduce the peak-to-average power ratio while trying to maximize, or at least maintain, the power output from the WEC by implementing energy storage/relief and advanced load-shedding methods such as WEC variable geometry control.


A significant reduction in the peak-to-average power will reduce the size, weight, and peak power of the entire PTO system. The peak structural loads can be set by the designer, who can balance energy production against structural costs and generator size and cost by designing the variable geometry control parameters for the site wave conditions. Possibly the most important impact of variable geometry load control is the ability to greatly reduce the cyclic fatigue loads on all of the system components.


The idea of using a variable geometry turbine in a turbocharger dates back at least to the 1950s [2646]. Since that time, a number of different designs have appeared. Two of the more common ones are the pivoting vane and moving wall types, Figure 1 [427][686]. Others include the variable area type, variable flow type and the sliding ring designs. These designs will be discussed in more detail in the following sections.


In many designs, a variable geometry turbine does not include a bypass so the turbine must be capable of handling all of the exhaust flow from the engine while avoiding overboost and overspeeding the turbocharger. For a given engine power rating, this would imply a larger turbine swallowing capacity than that required by a wastegated fixed geometry turbine and comparable with that used for a fixed geometry turbocharger with no bypass.


The fundamental difference between a fixed geometry turbine and a variable geometry turbine is illustrated in Figure 2 [2640]. Compared to a fixed geometry turbine, the variable geometry turbine allows significant flexibility over the pressure ratio/flow relationship across the turbine and by extension, the engine ΔP. This flexibility can be used for improving low speed torque characteristics, reducing turbocharger lag and in diesel engines, driving EGR flow.


The peak efficiency of a variable geometry turbine occurs at about 60% nozzle opening. It is usually comparable to or a few percent lower than that for a fixed geometry turbine. However, efficiency drops off rather quickly as nozzle opening is reduced or increased from a mid-vane opening position, Figure 3 [2641].


Variable-geometry turbochargers (VGTs), occasionally known as variable-nozzle turbines (VNTs), are a type of turbochargers, usually designed to allow the effective aspect ratio of the turbocharger to be altered as conditions change. This is done with the use of adjustable vanes located inside the turbine housing between the inlet and turbine, these vanes affect flow of gases towards the turbine. The benefit of the VGT is that the optimum aspect ratio at low engine speeds is very different from that at high engine speeds.


If the aspect ratio is too large, the turbo will fail to create boost at low speeds; if the aspect ratio is too small, the turbo will choke the engine at high speeds, leading to high exhaust manifold pressures, high pumping losses, and ultimately lower power output. By altering the geometry of the turbine housing as the engine accelerates, the turbo's aspect ratio can be maintained at its optimum. Because of this, VGTs have a minimal amount of lag, a low boost threshold, and high efficiency at higher engine speeds.


For heavy-duty engines, the vanes do not rotate, but instead, their effective width is changed. This is usually done by moving the turbine along its axis, partially retracting the vanes within the housing. Alternatively, a partition within the housing may slide back and forth. The area between the edges of the vanes changes, leading to a variable-aspect-ratio system with fewer moving parts.[3]


Several companies manufacture and supply rotating-vane variable-geometry turbochargers, including Garrett, BorgWarner, and Mitsubishi Heavy Industries. This design is mostly limited to small engines and light-duty applications (passenger cars, race cars and light commercial vehicles).


Twin-scroll turbochargers use two separate exhaust gas inlets to optimize exhaust gas flow, while variable-geometry turbochargers adjust the turbine housing geometry to maintain the optimum aspect ratio. Electrically-assisted turbochargers combine exhaust-powered turbines with electric motors to reduce turbo lag. The center hub rotating assembly connects the turbine to the compressor and may be water-cooled to protect the turbocharger's lubricating oil from overheating.


Variable-geometry turbochargers (also known as variable-nozzle turbochargers) are used to alter the effective aspect ratio of the turbocharger as operating conditions change. This is done with the use of adjustable vanes located inside the turbine housing between the inlet and turbine, these vanes affect flow of gases towards the turbine. Some variable-geometry turbochargers use a rotary electric actuator to open and close the vanes,[36] while others use a pneumatic actuator.


If the turbine's aspect ratio is too large, the turbo will fail to create boost at low speeds; if the aspect ratio is too small, the turbo will choke the engine at high speeds, leading to high exhaust manifold pressures, high pumping losses, and ultimately lower power output. By altering the geometry of the turbine housing as the engine accelerates, the turbo's aspect ratio can be maintained at its optimum. Because of this, variable-geometry turbochargers often have reduced lag, a lower boost threshold, and greater efficiency at higher engine speeds.[29][30] The benefit of variable-geometry turbochargers is that the optimum aspect ratio at low engine speeds is very different from that at high engine speeds.


In this cut-through diagram, you can see the direction of exhaust flow when the variable vanes are in an almost closed angle. The narrow passage of which the exhaust gas has to flow through accelerates the exhaust gas towards the turbine blades, making them spin faster. The angle of the vanes also directs the gas to hit the blades at the proper angle.


This cut-through diagram shows the exhaust gas flow when the variable turbine vanes are fully open. The high exhaust flow at high engine speeds are fully directed onto the turbine blades by the variable vanes. 041b061a72


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