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Combustion noise and thermo-acoustic instabilities are of primary importance in highly critical applications such as rocket propulsion systems, power generation, and jet propulsion engines. Mechanisms for combustion instabilities are extremely complex because they often involve interactions among several different physical phenomena such as unsteady flame propagation leading to unsteady flow field, acoustic wave propagation, natural and forced hydrodynamic instabilities, etc. In the past, we have utilized porous inert media (PIM) to mitigate combustion noise and thermo-acoustic instabilities in both lean premixed (LPM) and lean direct injection (LDI) combustion systems. While these studies demonstrated the efficacy of the PIM concept to mitigate noise and thermoacoustic instabilities, the actual mechanisms involved have not been understood. The present study utilizes time-resolved particle image velocimetry (PIV) to measure the turbulent flow field in a nonreacting swirl-stabilized combustor without and with PIM. Although the flow field inside the annulus of the PIM cannot be observed, measurements immediately downstream of the PIM provide insight into the turbulent structures. Results are analyzed using the proper orthogonal decomposition (POD) method and show that the PIM alters the flow field in an advantageous manner by modifying the turbulence structures and eliminating the corner recirculation zones and precessing vortex core (PVC), which would ultimately affect the acoustic behavior in a favorable manner. [ABSTRACT FROM AUTHOR]

A counter-flow annular heat recirculating burner was designed for lean prevaporized; premixed combustion. Prior to entering the combustor, the reactants are passed through a porous media-filled preheating annulus surrounding the combustor. Kerosene is dripped by gravity onto the porous media and vaporized by the heat conducted through the combustor wall. Experiments were conducted to evaluate heat transfer and combustion performance at various equivalence ratios, heat release rates, and inlet air temperatures. Results show low CO emissions over a range of equivalence ratios. NOx emissions were high at high heat release rates, indicating inadequate prevaporization and premixing of fuel with air. Heat recirculation and heat loss characteristics are presented at various operating conditions. [ABSTRACT FROM AUTHOR]

In this study, heat recirculation in an annulus around the combustor is utilized together with the flame stabilized on the surface of a porous inert media (PIM) made of silicon-carbide coated carbon foam. The effectiveness of the proposed concept is demonstrated experimentally for methane combustion in chamber volume of 0.364 cm3 and overall system volume of 1.5 cm3. Experiments were conducted for reactant flow velocities varying from 0.25 to 1.0 m/s in the equivalence ratio range of 0.50 to 0.80. Measurements include carbon monoxide and nitric oxides concentrations and product gas temperatures at the combustor exit, and temperature profiles on the exterior surface. A computational fluid dynamics (CFD) model incorporating the physics of conjugate heat transfer, radiation heat transfer, flow and heat transfer in the PIM, and heat release by combustion is developed to predict the thermal performance. Results show excellent agreement between measured and computed temperature profiles at different reactant flow rates. The CFD analysis is used to identify important thermal pathways within the system. Finally, a modified design is presented and analyzed computationally. The modified combustion system design achieves a significant reduction in the heat loss as compared to the baseline design tested experimentally. [ABSTRACT FROM AUTHOR]

• We introduce Peripheral Fuel Injection (PeFI) using multiple injectors to replace Conventional diesel combustion (CDC). • In this study, detailed 3D CFD of reacting flow is used to compare thermal and thermodynamic performance of CDC and PeFI. • PeFI is shown to eliminate detrimental jet-wall and jet-to-jet interactions. • PeFI reduces heat loss to the chamber wall by up to 5 percentage points as compared to CDC. • PeFI allows faster heat release rate and provides greater access to ambient air during the non-premixed combustion mode. • PeFI benefits are expected to increase when differences in the ROI profiles for CDC and PeFI are taken into consideration. A centrally located multi-hole injector supplying fuel towards the chamber wall is the most common fuel injection strategy for conventional diesel combustion (CDC) today. CDC strategy is prone to flame-wall and liquid-wall interactions with adverse effects on heat transfer, combustion and emissions performance. In the present study, peripheral fuel injection (PeFI) supplying fuel inwards from the chamber wall using multiple single-hole injectors is explored to achieve higher performance. The two methods of fuel injection are compared numerically in a constant volume combustion chamber. Simulations use n-dodecane with the same rate of injection profile for both injection schemes to obtain a conservative estimate of differences. Six configurations are trialed including a six-hole central injector and six, single-hole peripheral injectors with layout angles of θ = 0, 15, 22.5, 30 and 37.5 degrees relative to the radius of the chamber. Quantitative analysis shows that a properly configured PeFI eliminates flame-wall interactions, reduces heat loss, and improves heat release rate when compared to an equivalent CDC system. PeFI could also provide thermal efficiency and emissions benefits although these were not addressed explicitly in the present work. [ABSTRACT FROM AUTHOR]

Recent increases in fuel costs, concerns for global warming, and limited supplies of fossil fuels have prompted wide spread research on renewable liquid biofuels produced domestically from agricultural feedstock. In this study, two types of biodiesels and vegetable oil (VO) are investigated as potential fuels for gas turbines to generate power. Biodiesels produced from VO and animal fat were considered in this study. The problems of high viscosity and poor volatility of VO (soybean oil) were addressed by using diesel-VO blends with up to 30% VO by volume. Gas chromatography/mass spectrometry, thermogravimetric analysis, and density, kinematic viscosity, surface tension, and water content measurements were used to characterize the fuel properties. The combustion performance of different fuels was compared experimentally in an atmospheric pressure burner with an air-assist injector and swirling primary air around it. For different fuels, the effect of the atomizing airflow rate on Sauter mean diameter was determined from a correlation for air-assist atomizers. Profiles of nitric oxides (NOx) and carbon monoxide (CO) emissions were obtained for different atomizing airflow rates, while the total airflow rate was kept constant. The results show that despite the compositional differences, the physical properties and emissions of the two biodiesel fuels are similar. Diesel-VO fuel blends resulted in slightly higher CO emissions compared with diesel, while the NOx emissions correlated well with the flame temperature. The results show that the CO and NOx emissions are determined mainly by fuel atomization and fuel/air mixing processes, and that the fuel composition effects are of secondary importance for fuels and operating conditions of the present study. [ABSTRACT FROM AUTHOR]

High strength porous inert media (PIM) placed in the reaction zone of a swirl-stabilized lean-premixed combustor is a passive method of controlling combustion noise and instabilities. In this study, the effect of swirler location and swirl number on combustion without and with PIM has been investigated experimentally, using a methane-fueled quartz combustor at atmospheric pressure. Three axial swirlers were designed with eight vanes, a solid centerbody, and vane angles of 30, 45, and 55 deg to yield calculated swirl numbers o f 0.45, 0.78, and 1.10, respectively. Swirler location was varied to obtain recess depth in the premixer tube of 0.0 cm, 2.5 cm, and 5.0 cm. A downstream bluff body was used with the recessed swirlers to stabilize the flame at the dump plane. Experiments were conducted at constant air flow rate of 300 SLPM and equivalence ratios of 0.70, 0.75, and 0.80. PIM annular rings with increasing and decreasing cross-sectional area in the flow direction were tested, referred to as diverging and converging PIM. The performance of each test case is compared by observing the flame behavior and measuring sound pressure level (SPL) with a microphone probe. Results include total SPL and SPL in one-third octave bands. PIM proved effective in mitigating combustion noise and instability for all flush-mounted swirlers with total SPL reductions of up to 7.6 dBA. The effectiveness of the PIM generally improved with increasing equivalence ratio. Combustion instability that occurred within the frequency band centered about 630 Hz was suppressed with both PIM configurations. These results confirm that PIM is an effective method to control combustion noise and instabilities in swirl-stabilized LPM combustion. [ABSTRACT FROM AUTHOR]