Janet L. Ellzey

The interaction of a shock with a vortex: Shock distortion and the production of acoustic waves

Numerical simulations of a shock interacting with a compressible vortex are presented for shocks and vortices of various relative strengths. The simulations show the effects of the vortex on the shock structure and the structure of the acoustic field generated by the shock–vortex interaction. A relatively weak vortex perturbs the transmitted shock only slightly, whereas a strong vortex leaves the transmitted shock with a structure corresponding to either a regular or Mach reflection. The acoustic wave generated by the interaction consists of two components: a ‘‘quadrupolar’’ component produced by the initial shock–vortex interaction and the complex reflected shock system. When these waves merge, they form the asymmetric structure seen in experiments.

Measurements of Emissions and Radiation for Methane Combustion within a Porous Medium Burner

Abstract In this paper, we report the results of an experimental investigation of methane-air combustion within a porous medium burner for various equivalence ratios and flow rates. Measurements of emissions and radiant output are presented. The results indicate that CO and NOx, emissions increase with flame speed. For a given equivalence ratio, however, NOx, emissions are fairly constant over the range of flame speed studied. The radiant output increases but the radiant thermal efficiency decreases with flame speed.

Flame stabilization, operating range, and emissions for a methane/air porous burner

A porous burner in which a premixed flame is stabilized within the solid matrix is a promising technology for various applications due to its low emissions. In this article, experimental measurements of flame stabilization, operating range, and emissions are reported for several porous burners. The burner consisted of two sections of reticulated porous ceramic: a section of 23.6 pores per centimeter (ppcm) followed by a section of 3.9 ppcm. Two different porous media materials, yttria-stabilized zirconia/alumina composite (YZA) and zirconia-toughened mullite (ZTM), were used in two separate, but identical, burners. Results reveal that the interface between the two sections of porous ceramic stabilized the flame effectively over a range of conditions in the YZA burner. In the ZTM burner, however, theflamepropagated through the interface and into the upstream section. A third burner was constructed of YZA in which the length of the upstream section was reduced. The operating range for this burner was determined for a range of equivalence ratios. Firing rates ranged from 675 to 3951 kW/m 2 . Unburned hydrocarbons were elevated at low firing rates but were low at higher firing rates. Carbon monoxide (CO) emissions were less than 15 ppm. Nitric oxide (NO x ) emissions were below 10 ppm for all cases.

COMPUTATIONAL AND EXPERIMENTAL STUDY OF A TWO-SECTION POROUS BURNER

In this study, experiments and computations were conducted on a two-section porous burner operated on propane/air and methane/air mixtures. The burner consisted of an upstream section of reticulated yttria-stabilized zirconia with 23.6 pores per centimeter (ppc) and a downstream section of 3.9 ppc. Measurements of axial and radial temperatures, pressure drop, and emissions levels were recorded. The predictions from a one-dimensional transient mathematical model with full chemistry were compared to experimental results. Both computations and experiments showed that the stable operating range increases with equivalence ratio. The predicted upper limit agrees well with experiments but the lower limit is somewhat overpredicted. The average temperature in the exhaust stream increases with both inlet velocity and equivalence ratio and is relatively uniform across the burner. Pressure drop is much greater for reacting flows than cold flow and generally increases with inlet velocity. Measured levels of unburned hydrocarbons, oxides of nitrogen, and carbon monoxide are low.

Effects of thermal boundary conditions on flame shape and quenching in ducts

Near quenching laminar flames in parallel plate and cylindrical ducts are investigated computationally using one-step chemistry and a two-dimensional finite volume formulation. The effects of varying the heat transfer boundary conditions on the flame shape and propagation speed are examined. Two flame shapes are shown to arise, depending on the channel width and wall heat losses. A quenching criterion is developed for cases of restricted conductive or convective heat loss through the duct walls, and results are compared to the existing theory. As expected, the quenching Peclet number is found to be proportional to the square root of the overall heat loss coefficient. The importance of internal wall radiation and through-wall heat losses to the flame shape and quenching process is also examined and discussed. Radiation inside the channel is shown to inhibit quenching.

Dynamics of a strongly radiating unsteady ethylene jet diffusion flame

This work was sponsored by the Naval Research Laboratory through the Office of Naval Research. Computing time was provided by Numerical Aerodynamic Simulator (NAS) and the Naval Research Laboratory.

SUBADIABATIC AND SUPERADIABATIC PERFORMANCE OF A TWO-SECTION POROUS BURNER

ABSTRACT Flames may be stabilized in porous media at velocities either above or below the laminar flame speed. These two regimes are often called superadiabatic and subadiabatic, respectively. In this paper, several burners are investigated experimentally at both superadiabatic and subadiabatic conditions. The upper and lower velocity limits of stable combustion are reported. For equivalence ratios of 0.70 and below, both sub- and superadiabatic performance was seen. For both sub- and superadiabatic performance, the flame was stabilized at or near the interface between the upstream and downstream sections of porous media. The lower velocity limit for superadiabatic performance was extinction of the flame. The upper limit for superadiabatic performance was blowoff, which occurred for all burners at a flow velocity several times the adiabatic laminar flame speed, a phenomenon which is attributed to heat recirculation within the porous media. For equivalence ratios above 0.70, only subadiabatic performance was observed. The lower velocity limit occurred when the flame reached extinction. The upper velocity limit occurred when heat recirculation enhanced the laminar flame speed to a point at which the flame front propagated through the upstream section of porous media and flashed back upstream of the burner. Both the upper and lower velocity limits minimized at or near an equivalence ratio of 1.1. For equivalence ratios between 1.3 and 1.7 and flow velocities between 4 and 9 cm/s, oscillations of the flame front within the downstream section of porous media were observed.

The shock-vortex interaction:The origins of the acoustic wave

In this paper we discuss the mechanisms responsible for the formation of the acoustic wave when a shock interacts with a vortex. Experimental measurements have shown that this interaction produces a primarily quadrupolar acoustic wave with a strong compression attached to the shock front. We review earlier work which shows that this strong compression is due to the distortion of the shock. The origin of the quadrupolar component is examined by comparing two-dimensional computations of the shock-vortex interaction to those of an isolated elliptical vortex. The elliptical vortex is similar to the compressed vortex produced when a shock interacts with an initially circular vortex. We concentrate on interactions in which the shock transit time is short. The pressure field of the shock-vortex interaction is compared to that of an analogous isolated elliptical vortex for three cases: a weak shock interacting with a weak vortex, a strong shock interacting with a weak vortex, and a strong shock interacting with a strong vortex. Our results indicate that both shock distortion and vortex compression are important to the formation of the acoustic wave.

Effect of porous reactor design on conversion of methane to hydrogen

ABSTRACT In this study, two different designs for a reactor to reform methane into hydrogen are examined. The two reactors were identical except the porous media in one was yttria stabilized zirconia (YZA) reticulated ceramic with 3.9 pores per centimeter (ppc), and in the other it was a packed bed consisting of 3 mm diameter aluminum oxide pellets. Exhaust gas concentrations and temperatures were measured for experiments over an equivalence ratio range of 2 to 5. Hydrogen production and percent conversion were significantly better for the reticulated ceramic reactor than the packed bed reactor when compared based on interstitial velocity. Material properties of the porous media were responsible for the greater percent conversion in the reticulated ceramic reactor. The largest effect was due to the lower volumetric heat transfer coefficient in the reticulated ceramic.

Emissions of CO and NO from a Two Stage Porous Media Burner

ABSTRACT In this paper, we present measurements of CO and NO emissions and the fraction of the heat of reaction emitted as radiation for a two stage burner. The fuel-air mixtures of different equivalence ratios were burned in two separate porous ceramic sections. The primary radiating surface in this burner is the outer cylindrical surface of the porous media. We compared the two stage results to those obtained by burning the entire mixture in a single stage. The emissions of NO and CO from the two stage burner were lower than those from the single stage burner. In addition, the emissions of the two stage burner could be minimized by proper choice of the equivalence ratios in the two stages without significantly affecting the radiant fraction. Typical values of NO and CO emissions from the two stage burner were l7-30 ppm and 10-75 ppm (corrected to 0% O2), respectively, for an overall equivalence ratio of 0.75-0.95. Typical radiant fractions were 35-45% for an energy input of 3.9-7.1 kW. For the two stage...