Influence of strut on cavity at subsonic speeds: Ignition characteristics

In high-speed airflow, the use of cavity and struts in combination can improve fuel distribution and flame-stabilization, but may weaken the ignition performance. Herein, the lean ignition characteristics of several cavity–strut flame holders in a tandem turbine-based combined cycles combustor are experimentally investigated with the flow fields by using particle image velocimetry and high-speed chemiluminescence imaging techniques. Additionally, the effects of the strut structure parameters on the lean ignition performance in the cavity are studied. Experimental results indicate that changes in structural parameters have the opposite effects on the ignition performance and the flame-propagation performance. Reducing the strut inclination angle has a contrary function with the decrease in the cavity–strut space, which also transforms the flame-stabilizing mechanism between strut-stabilizing and cavity-stabilizing, accompanied by the flame morphology behind strut changes from no-flame to intermittent-flame, and finally continuous-flame. The lean ignition limit changes with the structure parameters, mainly due to the inverse change in the mass exchange rate and cavity residence time. Compared with the single cavity, the proper cavity–strut combined structure has a wider lean ignition limit at high subsonic speeds due to the advantage of simultaneously increasing the mass exchange rate and cavity residence time.

Critical autoignition conditions for arbitrary reaction order

In this paper, the theoretical analysis of the critical autoignition conditions for exothermically reacting systems at any value of the reaction order was conducted. The calculated and approximate analytical dependencies for the relationship between the parameters at the ignition limit were obtained. On the basis of the obtained diagrams of critical parameters, the conditions of thermal explosion (TE) degeneration for reactions of arbitrary order were determined. It was established that the existing theory of TE gives the correct estimates of ignition temperatures for a given condition of exothermic reaction realization (heat transfer coefficient, specific surface area, initial concentration). However, the theory gives unsatisfactory predictions for the mentioned critical TE conditions at a given ambient temperature. Moreover, the classical theory cannot be applied in the intermediate case when the effect of reactant consumption is already significant but the reaction still proceeds with all the signs of a TE.

Influence of high ethane content on natural gas ignition

The effect of ethane on combustion and natural gas autoignition was studied in the present paper. Two fuel mixture of natural gas with high ethane content were considered, 75% CH4 – 25% C2H6 (mixture 1), and 50% CH4 – 50% C2H6 (mixture 2). Natural gas combustion incidence was analyzed through the calculation of energy properties and the ignition delay time numerical calculations along with an ignition mode analysis. Specifically, the strong ignition limit was calculated to determine the effect of ethane on natural gas autoignition. According to the results, ignition delay time decreases for both mixtures in comparison with pure methane. The strong ignition limit shifts to lower temperatures when ethane is present in natural gas chemical composition.

Experimental investigation on ignition limits of plasma-assisted ignition in the propane–air mixture

In recent past, the plasma-assisted ignition has been explored for applications on a variety of engines. The plasma ignition has been shown to possess special advantages such as reducing the ignition delay time, improving the reliability, and reducing the NOx emissions. By using a plasma jet ignition experimental system, the plasma jet ignition of argon-discharge arc has been investigated. Owing to the characteristics of high temperature, the mixture can be easily ignited by the plasma jet. Through the propane–air mixture ignition experiments, the ignition limits of the plasma jet and spark ignition are investigated. The results show that the plasma jet ignition could extend the ignition limits of propane–air mixture obviously. The ignition limit extends with the increase in the air flow rates. The average ignition limit (the gap between rich and lean limit) of spark ignition and plasma jet ignition are 2.34 and 2.57, respectively. The average ignition limit of the propane–air mixture extends by 9.8%. The plasma jet ignition limit extends with increasing arc current, and the degree of extending plasma jet ignition limit increases with increasing air flow rates. The average ignition limits of 5.7 A and 20.3 A are 2.57 and 2.79, respectively. The average ignition limit of the propane–air mixture extends by 8.5%. The plasma jet ignition limit extends with increasing argon flow rates. The average ignition limits of 200 L/h and 250 L/h are 2.79 and 3.08, respectively. The average ignition limit of the propane–air mixture extends by 10.4%.

Experimental Investigations of Spark Ignition in a Model Combustor With Synthesis Gas

The components of syngas derived from coal, biomass, and waste are significantly different from those of typical gas turbine fuels, such as natural gas and fuel oils. The variations of hydrogen and inert gases can modify both the fluid and the combustion dynamics in the combustor. In particular, the characteristics of spark ignition can be profoundly affected. To understand the correlation between the varying fuel components and the reliability of ignition, a test system for spark ignition was established. The model combustor with a partial-premixed swirl burner was employed. The blending fuel with five components, hydrogen, carbon monoxide, methane, carbon dioxide and nitrogen, was used to model the synthesis gas used in industry. The ignition energy and the number of sparks leading to successful ignition were recorded. By varying the fuel components, the synthesis gases altered from medium to lower heat value fuels. The ignition time, ignition limit, and subsequent flame developments with variations of air mass flow rates and fuel components were systematically investigated. With the increase of airflow, the syngas with a lower hydrogen content has a shorter ignition time compared with higher hydrogen syngas in the lean condition, whereas in the rich condition, syngas with a higher hydrogen content has a shorter ignition time. The effects of the hydrogen content, inlet air Reynolds number and spark energy on the ignition limit were investigated. The ignition limit was enlarged with the increase in the hydrogen content and the spark energy. Meanwhile, three distinct flame patterns after ignition were investigated. Finally, a map for the characteristics of the ignition and subsequent flame development was obtained. The results are expected to provide valuable information for the design and operation of stable syngas combustion systems and also provide experimental data for the validations of theoretical modeling and numerical computations.