Matthew R. Johnson

Efficiencies of low-momentum jet diffusion flames in crosswinds

Abstract Low-momentum propane, natural gas, and propane/CO 2 diffusion flames in a crosswind were studied experimentally using a closed-loop wind tunnel. Flames were established at the exit of a burner tube mounted vertically in the wind tunnel and perpendicular to the airflow, a configuration that is relevant to continuous gas flaring in the atmosphere. Analysis of the products of combustion showed that inefficiencies result from fuel stripping, and photographic images link this process to changes in mean and instantaneous flame structure. Flame images also show qualitatively that a maximum mean flame length and the onset of downwash for different fuels are related to momentum flux ratio ( R ) of the two streams. Other features of the flame, such as burning in detached pockets and the disappearance of the flame tail, do not coincide at fixed values of R for different fuels. The measured combustion efficiency data show that increased crosswind speed ( U ∞ ) adversely affects the efficiency, while increased jet exit velocity ( V j ) makes the flame less susceptible to the effects of crosswind. Consideration of buoyancy and momentum forces as defined by a Richardson number successfully predicted the velocity dependency of the combustion inefficiency as being U ∞ / V j 1/3 and not R .

Black carbon particulate matter emission factors for buoyancy-driven associated gas flares.

Flaring is a technique used extensively in the oil and gas industry to burn unwanted flammable gases. Oxidation of the gas can preclude emissions of methane (a potent greenhouse gas); however, flaring creates other pollutant emissions such as particulate matter (PM) in the form of soot or black carbon (BC). Currently available PM emission factors for flares were reviewed and found to be questionably accurate, or based on measurements not directly relevant to open-atmosphere flares. In addition, most previous studies of soot emissions from turbulent diffusion flames considered alkene or alkyne based gaseous fuels, and few considered mixed fuels in detail and/or lower sooting propensity fuels such as methane, which is the predominant constituent of gas flared in the upstream oil and gas industry. Quantitative emission measurements were performed on laboratory-scale flares for a range of burner diameters, exit velocities, and fuel compositions. Drawing from established standards, a sampling protocol was developed that employed both gravimetric analysis of filter samples and real-time measurements of soot volume fraction using a laser-induced incandescence (LII) system. For the full range of conditions tested (burner inner diameter [ID] of 12.7–76.2 mm, exit velocity 0.1–2.2 m/sec, 4- and 6-component methane-based fuel mixtures representative of associated gas in the upstream oil industry), measured soot emission factors were less than 0.84 kg soot/103 m3 fuel. A simple empirical relationship is presented to estimate the PM emission factor as a function of the fuel heating value for a range of conditions, which, although still limited, is an improvement over currently available emission factors. Implications Despite the very significant volumes of gas flared globally and the requirement to report associated emissions in many jurisdictions of the world, a review of the very few existing particulate matter emission factors has revealed serious shortcomings sufficient to suggest that estimates of soot production from flares based on current emission factors should be interpreted with caution. New BC emissions data are presented for laboratory-scale flares in what are believed to be the first such experiments to consider fuel mixtures relevant to associated gas compositions. The empirical model developed from these data is an important step toward being able to better predict and manage BC emissions from flaring.

Diffuse-light two-dimensional line-of-sight attenuation for soot concentration measurements

A technique of diffuse-light two-dimensional line-of-sight attenuation (diffuse 2D-LOSA) is described and demonstrated that achieves very high levels of sensitivity in transmissivity measurements (optical thicknesses down to 0.001) while effectively mitigating interferences due to beam steering. An optical system is described in which an arc lamp coupled with an integrating sphere is used as a source of diffuse light that is imaged to the center of the particulate laden medium. The center of the medium is then imaged onto a CCD detector with 1:1 magnification. Comparative measurements with collimated 2D-LOSA in nonpremixed flames demonstrate the accuracy and improved optical noise rejection of the technique. Tests in weakly sooting, nonpremixed methane-air flames, and in high pressure methane-air flames, reveal the excellent sensitivity of diffuse 2D-LOSA, which is primarily limited by the shot noise of the lamp and CCD detector.

Creation of a standardized geometry of the human nasal cavity

A novel, standardized geometry of the human nasal cavity was created by aligning and processing 30 sets of computed tomography (CT) scans of nasal airways of healthy subjects. Digital three-dimensional (3-D) geometries of the 60 single human nasal cavities (30 right and 30 mirrored left cavities) were generated from the CT scans and measurements of physical parameters of each single nasal cavity were performed. A methodology was developed to scale, orient, and align the nasal geometries, after which 2-D digital coronal cross-sectional slices were generated. With the use of an innovative image processing algorithm, median cross-sectional geometries were created to match median physical parameters while retaining the unique geometric features of the human nasal cavity. From these idealized 2-D images, an original 3-D standardized median human nasal cavity was created. This new standardized geometry was compared against the original geometries of all subjects as well as limited existing data from the literature. The new model has potential for use as a geometric standard in future experimental and numerical studies of deposition of inhaled aerosols, as well as for use as a reference during diagnosis of unhealthy patients. The specific procedure developed could also be applied to build standard nasal geometries for different identifiable groups within the larger population.

Quantitative Field Measurement of Soot Emission from a Large Gas Flare Using Sky-LOSA

Particulate matter emissions from unconfined sources such as gas flares are extremely difficult to quantify, yet there is a significant need for this measurement capability due to the prevalence and magnitude of gas flaring worldwide. Current estimates for soot emissions from flares are rarely, if ever, based on any form of direct data. A newly developed method to quantify the mass emission rate of soot from flares is demonstrated on a large-scale flare at a gas plant in Uzbekistan, in what is believed to be the first in situ quantitative measurement of soot emission rate from a gas flare under field conditions. The technique, named sky-LOSA, is based on line-of-sight attenuation of skylight through a flare plume coupled with image correlation velocimetry. Monochromatic plume transmissivities were measured using a thermoelectrically cooled scientific-grade CCD camera. Plume velocities were separately calculated using image correlation velocimetry on high-speed movie data. For the flare considered, the mean soot emission rate was determined to be 2.0 g/s at a calculated uncertainty of 33%. This emission rate is approximately equivalent to that of 500 buses driving continuously and equates to approximately 275 trillion particles per second. The environmental impact of large, visibly sooting flares can be quite significant.

Characterization of the Spray Velocities from a Pressurized Metered-Dose Inhaler

BACKGROUND Pressurized metered dose inhalers (pMDIs) are widely used to deliver aerosolized medications to the lungs, most often to relieve the symptoms of asthma. Over the past decade, pMDIs have been modified in several ways to eliminate the use of chlorofluorocarbons in their manufacture while increasing efficacy. Numerical simulations are being used more frequently to predict the flow and deposition of particles at various locations, both inside the respiratory tract as well as in pMDIs and add-on devices. These simulations require detailed information about the spray generated by a pMDI to ensure the validity of their results. METHODS This paper presents detailed, spatially resolved velocity measurements of the spray emitted from salbutamol sulfate pMDIs obtained using optically triggered particle image velocimetry (PIV). Instantaneous planar velocity measurements were taken and ensemble-averaged at nine different times during the spray event ranging from 1.3 to 100 msec after a pneumatically controlled actuation. RESULTS AND CONCLUSIONS The mean spray velocities were shown to be bimodal in time, with two velocity peaks and velocity magnitudes found to be much lower than published data from instantaneous single point measurements. Planar velocity data at each time step were analyzed to produce prescriptive velocity profiles suitable for use in numerical simulations. Spray geometry data are also reported. Statistical comparisons from several thousand individual spray events indicate that there is no significant difference in measured velocity among (1) two brands of pMDI canisters, (2) two pMDIs of the same brand but having different lot numbers, and (3) a full pMDI versus an almost empty pMDI. The addition of a secondary air flow of 30 SLPM (to represent simultaneous inhalation and spray actuation) deflected the spray downward but did not have a significant effect on flow velocity. Further experiments with an added cylindrical spacer revealed that within the spacer, the spray direction and cone angle were altered, although the peak velocities remained similar.

A FUEL STRIPPING MECHANISM FOR WAKE-STABILIZED JET DIFFUSION FLAMES IN CROSSFLOW

Experiments were conducted to reveal the origins of measured inefficiencies in low-momentum jet diffusion flames in crossflow. Natural gas flames were established at the exit of a burner tube mounted vertically in a crossflow of air in a wind tunnel, a configuration that is relevant to gas flaring in the atmosphere. At low momentum flux ratios, the flame exists in a wake-stabilized mode and exhibits combustion inefficiencies primarily in the form of emitted unburned fuel. Measurements with a fast flame ionization detector were used to determine the path of the unburned fuel being emitted from the flame. Analysis of the hydrocarbon concentration time series data show that hydrocarbons are ejected from the underside of the flame in a highly intermittent process. A mechanism is proposed in which the mean flow induced by the standing vortex that exists on the leeward side of the stack transports and stretches the ring vortices from the upper shear layer and ejects them on the underside of the flame.

The Use of a Closed-Loop Wind Tunnel for Measuring the Combustion Efficiency of Flames in a Cross Flow

This paper describes and validates a new experimental technique for measuring the combustion efficiency of flames in a cross flow. This technique has been developed for use in a systematic investigation of the effects of wind speed, flaring rate, and fuel composition on the performance of continuously operating flares used in the energy industry. The impact of cross flow on the overall combustion efficiency of these flames is not well understood because of the shortcomings associated with previous measurement techniques (e.g., single point aspirating probes). The proposed methodology uses a closed-loop wind tunnel to create the cross flow and to capture the products of combustion. A multiple species mass balance based on rates of accumulation of combustion products in the tunnel allows the combustion efficiency to be calculated as the fraction of carbon in the fuel being converted to CO2. Problems of leakage from the tunnel and reburning due to the recirculation of gases are accounted for in the mass balances. Two sample results of methane jet diffusion flames with combustion efficiencies of 97% and 91% are presented. These two tests are considered in detail to examine the expected statistical uncertainty and measurement sensitivities that result from this methodology. The expected uncertainty is shown to be less than 0.1% for these two cases. A sensitivity analysis shows that the most important measurements involve tracking the accumulation of CO2 in the tunnel and knowing its concentration as part of the reactant gas stream. Errors of 5% in either of these measurements produce an error in the calculated efficiency of the order of 0.4%. Hence, this methodology is shown to be an accurate and robust approach to measuring the combustion efficiency in these flows.

An Analysis of Flaring and Venting Activity in the Alberta Upstream Oil and Gas Industry

ABSTRACT Alberta, Canada, is an important global producer of petroleum resources. In association with this production, large amounts of gas (1.14 billion m3 in 2008) are flared or vented. Although the amount of flaring and venting has been measurably reduced since 2002, data from 2005 reveal sharp increases in venting, which have important implications in terms of resource conservation and greenhouse gas emissions (which exceeded 8 million tonnes of carbon dioxide equivalent in 2008). With use of extensive monthly production data for 18,203 active batteries spanning the years 2002–2008 obtained in close cooperation with the Alberta Energy Resources Conservation Board, a detailed analysis has been completed to examine activity patterns of flaring and venting and reasons behind these trends in the Alberta upstream oil and gas industry. In any given year, ∼6000 batteries reported flaring and/or venting, but the distribution of volumes flared and vented at individual sites was highly skewed, such that small numbers of sites handled large fractions of the total gas flaring and venting in the Province. Examination of month-to-month volume variability at individual sites, cast in terms of a nominal turndown ratio that would be required for a compressor to capture that gas and direct it into a pipeline, further revealed that volumes at a majority of sites were reasonably stable and there was no evidence that larger or more stable sites had been preferentially reduced, leaving potential barriers to future mitigation. Through linking of geospatial data with production data coupled with additional statistical analysis, the 31.2% increase in venting volumes since 2005 was revealed to be predominantly associated with increased production of heavier oils and bitumen in the Lloydminster region of the Province. Overall, the data suggest that quite significant reductions in flaring and venting could be realized by seeking mitigation solutions for only the largest batteries in the Province. IMPLICATIONS Global flaring and venting of substantial volumes of waste flammable gases is a significant environmental concern. Alberta, Canada, is an example of a jurisdiction that has achieved significant reductions in flare and vent volumes from historic highs; however, these efforts have stalled in recent years. Through unprecedented access to several years of site-by-site production data for the entire upstream oil and gas industry in this region, a detailed analysis of current practices and activity trends in flaring and venting in Alberta has been made possible for the first time. The results of this analysis, including a specific investigation of potential barriers to mitigation of flaring and venting in a mature oil- and gas-producing region, have important implications for flaring and venting mitigation throughout the world.

Scaling of wake-stabilized jet diffusion flames in a transverse air stream

This experimentally based study considers the scaling of the flame length, the cross-stream dimension of the plume of combustion products, and the overall combustion efficiency of wake-stabilized jet diffusion flames. The study focuses on scaling with respect to the burner tube diameter ( d s ), but relationships include scaling with the jet exit velocity ( V j ) and the transverse air velocity ( U ∞ ). Phenomenological models are used to correlate the data presented. The flame length ( L f ) is shown to correlate well by relating the time-scale for the flow to advect the length of the flame ( L f / U ∞ ) to the time scale of the external flow with respect to the fuel jet dimension ( d s / U ∞ ). A consequence of this model is that L f ∞ d s , and L f either increases or decreases depending on whether the operating conditions are greater or less than a critical value of d s / U ∞ . The maximum flame length occurs at the critical value of d s / U ∞ . The proposed model for flame length predicts that L f actually scales with ( p j V j ) 1/2 d s / U ∞ , but the ( p j V j ) 1/2 term has yet to be validated. The characteristic cross-stream dimension of the plume ( d p * ), defined by the square root of its cross-sectional area, is shown to correlate with non-reacting plume models that use buoyancy forces and air stream momentum to give d p * ∝ g 1 3 V j 1 3 U ∞ x 2 3 d s 2 3 where g is the gravitational constant and x is the downstream position. The combustion inefficiency is shown empirically to scale inversely with d s 1/3 .