Elliot Sullivan-Lewis

Flashback Propensity of Turbulent Hydrogen–Air Jet Flames at Gas Turbine Premixer Conditions

The flashback propensity of a premixed jet flame has been studied experimentally. Boundary layer flashback has been investigated under turbulent flow conditions at elevated pressures and temperatures (i.e., 3–8 atm and 300–500 K). The data presented in this study are for hydrogen fuel at various Reynolds numbers, which are representative of practical gas turbine premixer conditions, and are significantly higher than results currently available in the literature. Three burner heads constructed of different materials (stainless steel, copper, and zirconia ceramic) were used to evaluate the effect of tip temperature, a parameter found previously to be an important factor in triggering flashback. This study characterizes flashback systematically by developing a comprehensive nondimensional model which takes into account all effective parameters in boundary layer flashback propensity. The model was optimized for new data and captures the behavior of the new results well. Further, comparison of the model with the single existing study of high-pressure jet flame flashback also indicates good agreement. For a given equivalence ratio, the critical velocity gradient and bulk velocity at flashback vary exponentially with pressure. The pressure exponent of the critical velocity gradient was found to be close to 1.1 at fuel-lean conditions and becomes higher as equivalence ratio is increased. The developed dimensionless correlation is Da=Const·Le1.68·Pef1.91·(Tu/T0)2.57·(Ttip/To)−0.49·(Pu/P0)−2.1, which can be used to predict the boundary layer flashback propensity for given parameters.

Predicting Flameholding for Hydrogen and Natural Gas Flames at Gas Turbine Premixer Conditions

Lean premixed gas turbines are one of the most common methods for stationary power generation. By creating a homogeneous mixture of fuel and air upstream of the combustion chamber, temperature variations are reduced within the combustor, which reduces emissions of nitrogen oxides. However, by premixing fuel and air, a potentially flammable mixture is established in a part of the engine not designed to contain a flame. If the flame propagates upstream from the combustor (flashback), significant engine damage can result. While significant effort has been put into developing flashback resistant combustors, these combustors are only capable of preventing flashback during steady operation of the engine. Transient events (e.g. auto ignition within the premixer, pressure spikes during ignition) can trigger flashback that cannot be prevented with even the best combustor design. In these cases, preventing engine damage requires designing premixers that will not allow a flame to be sustained. Experimental studies were conducted to determine under what conditions premixed flames of hydrogen and natural gas can be anchored in a simulated gas turbine premixer. Tests have been conducted at pressures up to 9 atm, temperatures up to 750 K, and free stream velocities between 20 and 100 m/s. Flames were anchored in the wakes of features typical of premixer passageways, including cylinders, steps and airfoils. The results of this study have been used to develop tools that predict when a flame remains anchored to a particular feature.Copyright

Assessment of a Rich-Burn, Quick-Mix, Lean-Burn–Based Supplemental Burner System in a Vitiated Air Stream

ABSTRACT In an effort to increase overall efficiency of distributed power generation systems, strategies to optimize the combination of cooling or heating with power (CCHP) are desired. One significant issue in implementing a CCHP system is that electrical loads and heating/cooling loads are rarely synchronized. Duct burners are frequently used in large systems to provide additional heat when the waste heat available from the prime mover does not meet the needs of the heat recovery device. Duct burners must operate on vitiated oxidizer streams, which can result in poor stability, elevated emissions, or both. Rich-burn, quick-mix, lean-burn (RQL) style combustors have been shown to provide low emission and high stability in lean burn gas turbine applications, but have not been considered for duct burner applications. To this point, the current work carries out an experimental investigation to assess the merits of using an RQL style combustor in a duct burner application. A systematic evaluation of several RQL burner configurations revealed that a lean-zone-to-rich-zone air mass ratio of 2.5 produced the lowest emissions of NOx and CO at a fixed fuel-to-air ratio. However, this reduction in emissions was accompanied with a decrease in the stability range of the burner. Furthermore, decreasing the amount of swirl in the rich zone was found to decrease NOx emissions. It was observed that oxygen concentration of the oxidizer had a more significant effect on the emission of CO than any configuration of the burner.

Flameholding Tendencies of Natural Gas and Hydrogen Flames at Gas Turbine Premixer Conditions

Ground based gas turbines are responsible for generating a significant amount of electric power as well as providing mechanical power for a variety of applications. This is due to their high efficiency, high power density, high reliability, and ability to operate on a wide range of fuels. Due to increasingly stringent air quality requirements, stationary power gas turbines have moved to lean-premixed operation. Lean-premixed operation maintains low combustion temperatures for a given turbine inlet temperature, resulting in low NOx emissions while minimizing emissions of CO and hydrocarbons. In addition, to increase overall cycle efficiency, engines are being operated at higher pressure ratios and/or higher combustor inlet temperatures. Increasing combustor inlet temperatures and pressures in combination with lean-premixed operation leads to increased reactivity of the fuel/air mixture, leading to increased risk of potentially damaging flashback. Curtailing flashback on engines operated on hydrocarbon fuels requires care in design of the premixer. Curtailing flashback becomes more challenging when fuels with reactive components such as hydrogen are considered. Such fuels are gaining interest because they can be generated from both conventional and renewable sources and can be blended with natural gas as a means for storage of renewably generated hydrogen. The two main approaches for coping with flashback are either to design a combustor that is resistant to flashback, or to design one that will not anchor a flame if a flashback occurs. An experiment was constructed to determine the flameholding tendencies of various fuels on typical features found in premixer passage ways (spokes, steps, etc.) at conditions representative of a gas turbine premixer passage way. In the present work tests were conducted for natural gas and hydrogen between 3 and 9 atm, between 530 K and 650K, and free stream velocities from 40 to 100 m/s. Features considered in the present study include a spoke in the center of the channel and a step at the wall. The results are used in conjunction with existing blowoff correlations to evaluate flameholding propensity of these physical features over the range of conditions studied. The results illustrate that correlations that collapse data obtained at atmospheric pressure do not capture trends observed for spoke and wall step features at elevated pressure conditions. Also, a notable fuel compositional effect is observed.Copyright

Performance Assessment of a Gas Fired RQL Combustion System Operated in a Vitiated Air Stream

Increased interest in distributed power generation has led to increased interest in combined heating cooling and power (CCHP). One significant issue in implementing a CCHP is that electrical loads and heating or cooling loads are rarely synchronized. Duct burners are frequently used to provide additional heat when the waste heat available from the prime mover does not meet the needs of the heat recovery device. Duct burners are required to operate on vitiated oxidizer streams which can result in poor stability, poor emissions, or both. Rich-burn, quick-mix, lean-burn (RQL) style combustors have been shown to provide low emission and high stability in lean gas turbine applications. An experiment was performed to assess the merits of using a RQL style combustor in a duct burner application. Several burner configurations were tested and it was found that an RQL combustor showed greatly improved stability over a single stage combustor when operating under vitiated conditions without adversely effecting emissions.