Patent Application
20080295521
This invention relates generally to rotary machines and more particularly, to methods and apparatus for operating gas turbine engines.
At least some known gas turbine engines combust a fuel and air mixture to release heat energy from the mixture to form a high temperature combustion gas stream that is channeled to a turbine via a hot gas path. The turbine converts thermal energy from the combustion gas stream to mechanical energy that rotates a turbine shaft. The output of the turbine may be used to power a machine, such as, for example, an electric generator or a pump.
At least one by-product of the combustion reaction may be subject to regulatory limitations. For example, within thermally-driven reactions, nitrogen oxide (NOx) may be formed by reactions between nitrogen and oxygen in the air initiated by the high temperatures during the combustion process. Moreover, carbon monoxide (CO) may be formed by reactions between carbon and oxygen in the air and fuel. Generally, engine efficiency increases as the temperature of the combustion gas stream entering a turbine section of the engine increases. Such increases in efficiency facilitate mitigating CO formation. However, increasing the combustion gas temperature may undesirably increase the formation of NOx.
To control NOx emissions during turbine engine operation, at least some known gas turbine engines use combustors that operate with a lean fuel/air ratio and with fuel that is premixed with air prior to being supplied into the combustor. Premixing may facilitate reducing combustion temperatures and subsequently reducing NOx formation. However, there may be limiting parameters associated with decreasing combustion temperatures, such as, for example, CO formation and lean blow-out.
In one aspect, a method of assembling a turbine engine is provided. The method includes defining a first chamber and defining a second chamber. The method also includes forming at least one venturi device oriented with a predetermined venturi step angle greater than approximately 48°. The method further includes coupling the first chamber in flow communication with the second chamber via the venturi device therebetween.
In another aspect, a combustor assembly is provided. The combustor assembly includes a first chamber and a second chamber coupled in flow communication with the first chamber. The assembly also includes at least one venturi device between the first and second chambers, wherein the venturi device is oriented with a predetermined venturi step angle greater than approximately 48°.
In a further aspect, a turbine engine is provided. The engine includes at least one air source, at least one fuel source, and at least one cooling fluid source. The engine also includes a combustor assembly coupled in flow communication with the at least one air source, the at least one fuel source and the at least one cooling fluid source. The combustor assembly includes a first chamber and a second chamber coupled in flow communication with said first chamber. The assembly also includes at least one venturi device between the first and second chambers, wherein the venturi device is oriented with a predetermined venturi step angle greater than approximately 48°.
In operation, air flows through compressor 102 and a substantial amount of compressed air is supplied to combustor assembly 104. Assembly 104 is also in flow communication with a fuel source (not shown in
Combustor assembly 104 receives air from compressor 102 (shown in
Device 140 also includes a second outer wall 154 and a second inner wall 156. In the exemplary embodiment, walls 154 and 156 are substantially parallel to each other and at least partially define a second cooling fluid passage 158. Walls 154 and 156 are coupled to and extend from, walls 146 and 148, respectively. Moreover, in the exemplary embodiment, second passage 158 is coupled in flow communication with first passage 150 and wall 154 includes at least one cooling fluid opening 160 extending therethrough.
In the exemplary embodiment, device 140 also includes a third outer wall 162 and a third inner wall 164. Moreover, in the exemplary embodiment, walls 162 and 164 are substantially parallel to each other and at least partially define a third cooling fluid passage 166. Walls 162 and 164 are coupled to walls 154 and 156, respectively. Moreover, third passage 166 is coupled in flow communication with second passage 158. Wall 162 includes at least one cooling fluid opening 160 extending therethrough.
Inner walls 162 and 154 at least partially define a cooling fluid plenum 168. In the exemplary embodiment, plenum 168 extends substantially circumferentially about device 140 and is coupled in flow communication with compressor 102. Plenum 168 is also coupled in flow communication with passages 158 and 166 via openings 160. In the exemplary embodiment, wall 156 defines a venturi step angle 170 with respect to a line 172 extending substantially parallel to centerline 107. Moreover, walls 164 and 156 form a substantially annular apex 174 that at least partially defines a throat region 176. More specifically, throat region 176 separates chambers 106 and 144.
An exemplary method of assembling a turbine engine includes defining chamber 144 and defining chamber 106. The method also includes positioning at least one venturi device 140 to be oriented with a predetermined venturi step angle 170 greater than approximately 48°. The method further includes coupling chamber 144 in flow communication with chamber 106 such that venturi device 140 is therebetween.
In operation, referring to
Also, during operation, cooling fluid stream 142 is channeled into plenum 168. In the exemplary embodiment, the cooling fluid used in stream 142 is air channeled from compressor 102. Alternatively, the cooling fluid may be any fluid that facilitates operation of combustor 104 as defined herein, including, but not limited to, steam, water, and ethylene glycol. Fluid stream 142 is channeled from plenum 168 into third and second fluid passages 166 and 158, respectively, via openings 160. Fluid stream 142 is also channeled into first fluid passage 150 and into chamber 106 via port 152. As such, at least a portion of heat released by combustion of the fuel-air mixture within combustion chamber 106 is removed by fluid stream 142.
Thermal NOx is typically defined as NOx formed during combustion of fuel and air through the high temperature oxidation of nitrogen found in air. Specifically, the NOx formation rate is a function of the ratio of air as referenced to fuel, a temperature associated with the combustion of fuel and air within a pre-defined region, and the residence time of nitrogen at that temperature and in the combustor. Therefore, in general, as any of the percentage of fuel in the fuel-air mixture, the temperature of combustion, and/or the residence time increases, a rate of NOx generation increases as well. In contrast, decreasing the concentration of fuel in the fuel-air mixture towards limits of lean-flammability facilitates mitigating NOx generation. Moreover, optimizing residence times and temperatures facilitates complete combustion and facilitates the mitigation of NOx generation.
In the exemplary embodiment, during operation, lean premixed injection is used. Such injection methods include mixing air and fuel prior to injection within combustion chamber 106. Mixing air and fuel prior to injection facilitates attaining uniformity within fuel-air mixtures, which facilitates optimizing residence times and temperatures associated with combustion. Moreover, such lean premixed combustion methods are typically characterized by lower flame temperatures than those typically characterized by traditional non-premixed, or diffusion, methods of combustion. The lower combustion temperatures associated with the lean premixed combustion facilitates reducing in the rate and magnitude of formation of NOx, however, the lower temperatures may undesirably facilitate increased carbon monoxide (CO) formation due to a reduction in combustion efficiency. Moreover, potentials for lean-blow out, or flame-out (conditions wherein the flame cannot be maintained) and high frequency dynamic pressure oscillations are increased. Improved flame stabilization facilitates decreasing potentials for CO formation, lean-blow out and high frequency dynamics.
Flame stability, completeness of combustion, and NOx production may be affected by turbulence of the fuel-air mixture prior to combustion. Specifically, increasing turbulence may facilitate decreasing the residence times and the peak and local temperatures of combustion of fuel and air, thereby facilitating a decrease in NOx production. Other factors such as, but not limited to, fuel-air mixture flow velocities and mass flow rates, facilitate forming predetermined vortices (not shown) that include at least one localized flow field (not shown) that is defined within a predetermined volume and with a predetermined set of characteristics, such as, but not limited to, a predetermined turbulence, residence time and temperature.
In addition, flame-holding is facilitated when a residence time of a mixture of fuel and air in a pre-defined volume is greater than the fuel-air mixture's reaction time within the same volume, and a resultant flame as a result of combustion of fuel and air is realized. Specifically, when a flame speed is substantially similar to a fuel-air mixture flow speed, a resultant flame may be characterized as stable.
As is known in the art, a venturi device orientation similar to device 140 may be used to stabilize a combustion flame (not shown) downstream of device 140 within chamber 106. Venturi device 140 facilitates flame stabilization by receiving the premixed fuel-air mixture from premix chamber 144 and accelerating the mixture into combustion chamber 106 through throat region 176. Such acceleration into combustion chamber 106 facilitates the formation of vortices and recirculation zones downstream of venturi device 140 within chamber 106, as discussed further below.
Venturi device 140 facilitates forming vortices that include multiple localized flow fields (not shown). Specifically, venturi device 140 acts as a bluff-body that facilitates flame-holding. More specifically, device 140 includes a non-streamlined shape that induces sufficient resistance into the flow of the air-fuel mixture into chamber 106, thereby forming a wake region (not shown) in radially outboard regions (not shown) of chamber 106. As such, vortex formation is facilitated downstream of device 140. Moreover, vortex formation also facilitates vortex breakdown wherein at least one recirculation zone (not shown) between the bulk flow field and wall 148 forms and the fuel-air mixture exits the bulk flow field into the recirculation zone. The fuel-air mixture is then re-injected back into the bulk flow field, thereby facilitating increasing bulk flow field turbulence, and subsequently decreasing fuel and air residence time, combustion temperatures within the bulk flow field, and NOx formation. Therefore, such recirculation zones facilitate flame stabilization.
In the exemplary embodiment, venturi step angle 170 is greater than approximately 48°. Because angle 170 exceeds approximately 48°, flame-holding properties within combustion chamber 106, downstream from venturi device 140, are further facilitated, thereby further facilitating flame stability. Moreover, such values for angle 170 facilitate optimizing residence times and temperatures, thereby facilitating complete combustion and reduced NOx formation. In the exemplary embodiment, NOx concentrations within combustion gas stream 138 are below about 3 parts per million, volumetric dry (ppmvd). Furthermore, such improved flame stabilization facilitates decreasing potentials for CO formation, lean-blow out, and high frequency dynamics. Moreover, in the exemplary embodiment, CO concentrations within combustion gas stream 138 are below about 25 ppmvd. As such, the reduction in combustion by-products facilitates reducing a need for exhaust gas scrubbing apparatus.
The gas turbine engine and combustor assembly described herein facilitates mitigating combustion product emissions while facilitating a pre-determined heat release rate per unit volume. Specifically, the engine includes a lean premixed injection combustor assembly that facilitates thorough and rapid fuel and air mixing and combustion. More specifically, such combustor assembly includes a venturi device with a venturi step angle greater than approximately 48°. Such step angle facilitates a reduction in NOx, increased flame stability, increased combustion efficiency as measured by CO formation, and mitigation of undesirable combustion dynamics. As a result, the operating efficiency of such engines may be increased and the engine's operational costs may be reduced. Moreover, such engines' combustion by-products are reduced, thereby reducing a need for expenditures of capital and operating funds associated with exhaust gas scrubbing apparatus.
Exemplary embodiments of combustor assemblies as associated with gas turbine engines are described above in detail. The methods, apparatus and systems are not limited to the specific embodiments described herein nor to the specific illustrated gas turbine engines and combustor assemblies.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
