"THERE'S AN OPPORTUNITY TO SIGNIFICANTLY REDUCE THE COST OF AN ENGINE AND TO REDUCE THE COST OF ELECTRICITY."
Light, heat, kitchen appliances, television and stereo, air conditioners, computing life as we know it runs on electricity, and electricity comes from turbines. Think of them as jet engines bolted to the floor. Rather than thrust to lift an aircraft off the ground, these turbines produce powerful rotation that drives generators to produce megawatts of electricity, which flows through wires into your home.
The process starts with fossil fuel, still the raw energy source for nearly 90 percent of electrical power worldwide. Two kinds of turbines, steam and gas, share the load. For steam turbines, coal and fuel oil heat boilers from which pressurized steam turns the windmill-like turbine blades. In gas turbines, combustors ignite the fuel and blast hot, pressurized exhaust gas to do the turning work.
As a cleaner-burning fuel, natural gas is favored for low-emission, nearly pollutant-free turbines of the present and foreseeable future. High efficiency as complete as possible conversion of raw energy into turbine rotation is the key not only to low CO2 emission, but also to the cost of electricity. Small gains in efficiency that slightly reduce cost per megawatt translate to huge savings overall, and turbine engineers measure efficiency in tenths of a percent.
One new way of thinking about gas turbines is to let combustion carry over from the combustor into the turbine and even to inject additional fuel into the turbine for an extra kick of power, roughly analogous to a jet engine afterburner. This potentially would allow more complete burning and more overall work from the fuel as well as a healthy power boost. The idea is called a "turbine-combustor" and actually, says Paul Cizmas of Texas A&M, it's an old idea revived for new times.
"The idea started in the 1960s, but we didn't have the technology to test it," says Cizmas, a former senior scientist at Westinghouse in Pittsburgh, "and until two or three years ago, it was considered bad to have flame in the turbine, mainly because of the problem of cooling the blades. Then we realized we already design the first row of blades, the vanes, as if the flame is there. The problems come in the rows after that, and recent studies have shown we need to worry about how to cool these rows anyway, even if we don't have turbine combustion."
In September 2000, the U.S. Department of Energy (DOE) Strategic Center for Natural Gas selected Siemens-Westinghouse Power Corp. to study the turbine-combustor idea, with Texas A&M on board to focus on computational simulation. "There's an opportunity to significantly reduce the cost of an engine," says Tom Lippert, manager of science and technology center combustion programs at Siemens-Westinghouse, "and to reduce the cost of electricity."
A necessary step out of the starting blocks is to develop a reliable way to simulate the physical processes. "This could be a different combustion process altogether from what we're used to," says Lippert. "You have to have analytical tools that allow you to combine combustion kinetics with aerodynamics. This hasn't been done before, at least in this fashion."
In work a few years ago at the Pittsburgh Supercomputing Center, Cizmas developed software that simulates the complex flows involved in turbine blade design. By relying on the computing power of massively parallel systems, Cizmas's approach proved to be both faster and more accurate than prior approaches. In 2001, through the Super Computing Science Consortium, a southwest Pennsylvania-West Virginia regional partnership that links DOE's National Energy Technology Laboratory with PSC, Cizmas used the 512-processor CRAY T3E at Pittsburgh to develop and test a computational method for the much more complex flow problems involved in a turbine-combustor.
As they began this project, Cizmas and his colleague, Dragos Isvoranu of Politehnica University Bucharest, soon realized they had an interesting opportunity. A literature search revealed no prior data on combustion within a turbine.
The first challenge was not a new one simulating flow in the alternating rows of stationary and rotating blades, vanes and rotors, that characterize modern power turbines. The passageway between vanes is like a nozzle that speeds up the hot gas and shoots it toward the spinning rotors. Each vane-rotor series is called a stage, with many stages, hundreds of blades, in operational turbines. Although these flows are complicated, Cizmas had already tackled this problem with success.
The added complication is combustion, which changes everything. By itself, combustion has been modeled, but not when the flow is this complicated. "Flow in the turbine," says Cizmas, "is like a nightmare because of the rotor-stator interaction. Now on top of this, we want to have a stable flame. Flow in the combustor is almost steady, if not steady, but in this case, it's very unsteady. So you have the problem not only of correctly simulating the combustion, but also unsteadiness in the combustion a very challenging problem."
Added heat not only increases temperature, it also can radically change the flow patterns. Flow from the combustor alone induces "hot streaks" downstream in the turbine, which affect blade life and blade design. With the added stress of combustion within the turbine, these problems are exacerbated.
As a practical matter, computer simulation is the only hope for gaining enough understanding to arrive at a design concept. "The question is how should we inject fuel," says Cizmas, "at what location, what pressure, what quantity? Should it be continuous or pulsed? The possible variations are infinite. Experimental investigation with scale models would take years."
When the Fuel Hits the Rotor
The numerical challenge was to develop an algorithm that coupled the equations for fluid flow with the "species equations" for each chemical constituent of the combustion reaction natural gas (methane), oxygen, carbon-dioxide, carbon-monoxide, water. Cizmas and Isvoranu used an efficient approach a fully implicit, finite-difference method that employs a moving set of mesh-like grids to subdivide the space around the turbine blades. In these initial computations, the blades are represented as a uniform cross-section extended from the turbine hub.
Using the CRAY T3E to put their software through its paces, the researchers modeled a one-stage turbine-combustor with 32 vanes and 49 blades a relatively simple configuration that corresponds to an existing turbine. The work is shared among processors, one for each blade. For this initial test, they modeled fuel injection with basic parameters through a single small hole at the trailing edge of the vanes, at low velocity, and relatively high temperature and pressure.
As Cizmas expected, the simulations show delayed ignition. In the space between blades, the fuel rapidly gains heat and mixes with oxygen and then ignites as it hits the rotating blades. Ignition occurs with unexpected intensity, however, almost like an explosion, says Cizmas. "At the moment when the wake of fuel from the stator hits the rotor, you see rapid acceleration of the reaction. We were expecting strong coupling between the flow and combustion, but we didn't know we'd see this quite important increase in reaction speed."
To assess accuracy, Cizmas ran the simulations with three different grid resolutions coarse, medium and fine with nearly 50,000 processor hours on the T3E. Analysis showed good agreement between the high and medium resolution grids, a strong indication that the modeling is accurately representing the physical process. An innovative correction algorithm for the species computation, says Cizmas, appears to work well. Coupling between the equations for flow and combustion raised a theoretical question about reaction thermodynamics, a specialty of Isvoranu, which the researchers are looking at and which may necessitate a revised approach.
Cizmas expects that the Siemens-Westinghouse research program will lead to operational turbine-combustors within five years with implementation in power plants in ten years. One possibility envisioned by this research is a turbine-combustor unit that eliminates the combustor as a separate component. "This is an opportunity," says Lippert, "to significantly reduce the cost of an engine. If you integrate the combustion system into the blade path, you eliminate a big piece of hardware."
ADDING TURBINE-COMBUSTION TO EXISTING TURBINES WILL CREATE A FLEXIBLE POWER RESERVE.
A more immediate use, though, is likely to come from adding turbine-combustion to existing turbines, creating a flexible power reserve. "If there's a point in the day that the plant needs to produce more power," says Cizmas, "it can be done by turning on combustion in the turbine." The only current option for a sudden power crunch is to bring an additional turbine on-line, potentially producing more power than required, which gluts the bidding market for power and lowers profitability.
Cizmas is preparing his turbine-combustor software for a more comprehensive series of simulations on LeMieux, PSC's 3,000-processor terascale system. These studies will incorporate a fully 3D representation of the blades, with one processor assigned to each of 50 cross-sectional slices per blade to capture the finely detailed aerodynamics of blade curvature.
Though results are preliminary, the first round of simulations offers new understanding to help point the way ahead. The accelerated ignition at the rotors suggests the fuel may need to be pulsed in time with rotor frequency. "Other issues," says Cizmas, "include what angle to inject. We're looking at a variation of angles to increase turbulence, which helps mixing. And at what velocity should we inject? What temperature and pressure? How many injection points per blade? How should they be spaced? We're also looking at fuel composition. Should we use pure methane, or methane mixed with air? And what about using hydrogen? We're going to be busy for awhile."
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Revised: October 21, 2002
The Pittsburgh Supercomputing Center is a joint effort of Carnegie Mellon University and the University of Pittsburgh together with the Westinghouse Electric Company. It was established in 1986 and is supported by several federal agencies, the Commonwealth of Pennsylvania and private industry.
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