Raymond L. Speth

Economic and environmental benefits of higher-octane gasoline.

We quantify the economic and environmental benefits of designing U.S. light-duty vehicles (LDVs) to attain higher fuel economy by utilizing higher octane (98 RON) gasoline. We use engine simulations, a review of experimental data, and drive cycle simulations to estimate the reduction in fuel consumption associated with using higher-RON gasoline in individual vehicles. Lifecycle CO2 emissions and economic impacts for the U.S. LDV fleet are estimated based on a linear-programming refinery model, a historically calibrated fleet model, and a well-to-wheels emissions analysis. We find that greater use of high-RON gasoline in appropriately tuned vehicles could reduce annual gasoline consumption in the U.S. by 3.0-4.4%. Accounting for the increase in refinery emissions from production of additional high-RON gasoline, net CO2 emissions are reduced by 19-35 Mt/y in 2040 (2.5-4.7% of total direct LDV CO2 emissions). For the strategies studied, the annual direct economic benefit is estimated to be

Two-dimensional simulations of steady perforated-plate stabilized premixed flames

0.4-6.4 billion in 2040, and the annual net societal benefit including the social cost of carbon is estimated to be

Capillary instability on a hydrophilic stripe

1.7-8.8 billion in 2040. Adoption of a RON standard in the U.S. in place of the current antiknock index (AKI) may enable refineries to produce larger quantities of high-RON gasoline.

Impact of Hydrogen Addition on Flame Response to Stretch and Curvature

The objective of this work is to examine the impact of the operating conditions and the perforated-plate design on the steady, lean premixed flame characteristics. We perform two-dimensional simulations of laminar flames using a reduced chemical kinetics mechanism for methane–air combustion, consisting of 20 species and 79 reactions. We solve the heat conduction problem within the plate, allowing heat exchange between the gas mixture and the solid plate. The physical model is based on a zero-Mach-number formulation of the axisymmetric compressible conservation equations. The results suggest that the flame consumption speed, the flame structure, and the flame surface area depend significantly on the equivalence ratio, mean inlet velocity, the distance between the perforated-plate holes and the plate thermal conductivity. In the case of an adiabatic plate, a conical flame is formed, anchored near the corner of the hole. When the heat exchange between the mixture and the plate is finite, the flame acquires a Gaussian shape stabilizing at a stand-off distance, that grows with the plate conductivity. The flame tip is negatively curved; i.e. concave with respect to the reactants. Downstream of the plate, the flame base is positively curved; i.e. convex with respect to the reactants, stabilizing above a stagnation region established between neighboring holes. As the plates thermal conductivity increases, the heat flux to the plate decreases, lowering its top surface temperature. As the equivalence ratio increases, the flame moves closer to the plate, raising its temperature, and lowering the flame stand-off distance. As the mean inlet velocity increases, the flame stabilizes further downstream, the flame tip becomes sharper, hence raising the burning rate at that location. The curvature of the flame base depends on the distance between the neighboring holes; and the flame there is characterized by high concentration of intermediates, like carbon monoxide.

Impact of microjet actuation on stability of a backward-facing step combustor

A recent experiment showed that cylindrical segments of water filling a hydrophilic stripe on an otherwise hydrophobic surface display a capillary instability when their volume is increased beyond the critical volume at which their apparent contact angle on the surface reaches 90 (Gau et al 1999 Science 283 46-9). Surprisingly, the fluid segments did not break up into droplets—as would be expected for a classical Rayleigh-Plateau instability—but instead displayed a long-wavelength instability where all excess fluid gathered in a single bulge along each stripe. We consider here the dynamics of the flow instability associated with this setup. We perform a linear stability analysis of the capillary flow problem in the inviscid limit. We first confirm previous work showing that all cylindrical segments are linearly unstable if (and only if) their apparent contact angle is larger than 90 . We then demonstrate that the most unstable wavenumber for the surface perturbation decreases to zero as the apparent contact angle of the fluid on the surface approaches 90 , allowing us to re-interpret the creation of bulges in the experiment as a zero-wavenumber capillary instability. A variation of the stability calculation is also considered for the case of a hydrophilic stripe located on a wedge-like geometry.

Mode Selection in Flame-Vortex driven Combustion Instabilities

We present a formulation of a cylindrical laminar flame stabilized in an axisymmetric stagnation flow. The formulation is described by a one-dimensional set of governing equations incorporating detailed kinetics and transport, including thermal diffusion and Dufour effects. Numerical solution with a preconditioned inexact Newton-Krylov method ensures efficient and robust convergence. The formulation is extended to model the opposed-flow laminar twin-flame as well. The impact of hydrogen addition on the response of a lean premixed methaine–air flame to strain and curvature is examined.

Dynamics and Stability Limits of Syngas Combustion in a Swirl-Stabilized Combustor

In this paper, we investigate the effect of using steady air injection in the cross-stream direction near the step to stabilize combustion in a backward-facing step combustor. Air is injected from a 2 mm wide slot extending across the entire span of the combustor, or from twelve 0.5 mm diameter evenly spaced microjets. The microjet flow is choked, while the slot flow is not and may couple with the pressure oscill ations in the combustor. The combustor dynamics are examined when the fuel bar is located 35 cm or 95 cm upstream of the step, without or with hydrogen addition to the primary fuel, in this case propane. The combustor dynamics showed significant sensitivity to the fuel bar location. When the fuel bar was located closer to the step, both the microjets and the slot were able to suppress the instability. However, to produce equivalent reducti ons in the sound pressure level, the slot requires 3 times as much air flow as the microjets. The effect of hydrogen addition is not significant in this case. When the fuel bar was loc ated further upstream of the step, the combustor was more unstable, the microjets were less effective, and the hydrogen enrichment had stronger impact on the combustion dynamics.

Instability Suppression in a Swirl-Stabilized Combustor Using Microjet Air Injection

In this paper, we investigate flame-vortex interaction in a lean premixed, laboratory scale, backward-facing step combustor. Two series of tests were conducted, using propane/hydrogen mixtures and carbon monoxide/hydrogen mixtures as fuels, respectively. Pressure measurements and high speed particle imaging velocimetry (PIV) were employed to generate pressure response curves as well as the images of the velocity field and the flame brush. We demonstrate that the step combustor exhibits several operating modes depending on the inlet conditions and fuel composition, characterized by the amplitude and frequency of pressure oscillations along with distinct dynamic flame shapes. We propose a model in which the combustors selection of the acoustic mode is governed by a combustion-related time delay inversely proportional to the flame speed. Our model predicts the transition between distinct operating modes. We introduce non-dimensional parameters characterizing the flame speed and stretch rate, and develop a relationship between these quantities at the operating conditions corresponding to each mode transition. Based on this relationship, we show that numerically-calculated density-weighted strained flame speed can be used to collapse the combustion dynamics data over the full range of conditions (inlet temperature, fuel composition, and equivalence ratio). Finally, we validate our strain flame based model by measuring the strain rate using the flame image and the velocity field from the PIV measurement. Our results show that the measured strain rates lie in the same range as the critical values at the transitions among distinct modes as those predicted by our model.

Dynamics and Stability Limits of Syngas Combustion in a Backward-Facing Step Combustor

The combustion dynamics, stability bands and flame structure of syngas flames under different operating conditions are investigated in an atmospheric pressure swirl-stabilized combustor. Pressure measurements and high-speed video data are used to distinguish several operating modes. Increasing the equivalence ratio makes the flame more compact, and in general increases the overall sound pressure level. Very close to the lean blowout limit, a long stable flame anchored to the inner recirculation zone is observed. At higher equivalence ratios, a low frequency, low amplitude pulsing mode associated with the fluid dynamic instabilities of axial swirling flows is present. Further increasing the equivalence ratio produces unstable flames oscillating at frequencies coupled with the acoustic eigenmodes. Additionally, a second unstable mode, coupled with a lower eigen-mode of the system, is observed for flames with CO concentration higher than 50%. As the amount of hydrogen in the fuel is increased, the lean flammability limit is extended and transitions between operating regimes move to lower equivalence ratios.Copyright

Flame–vortex interaction driven combustion dynamics in a backward-facing step combustor

In this study, we examine the effectiveness of microjet air injection as a means of suppressing thermoacoustic instabilities in a swirl-stabilized, lean-premixed propane/air combustor. High-speed stereo PIV measurements, taken to explore the mechanism of combustion instability, reveal that the inner recirculation zone plays a dominant role in the coupling of acoustics and heat release that leads to combustion instability. Six microjet injector configurations were designed to modify the inner and outer recirculation zones with the intent of decoupling the mechanism leading to instability. Microjets that injected air into the inner recirculation zone, swirling in the opposite sense to the primary swirl were effective in suppressing combustion instability, reducing the overall sound pressure level by up to 17 dB within a certain window of operating conditions. Stabilization was achieved near an equivalence ratio of 0.65, corresponding to the region where the combustor transitions from a 40 Hz instability mode to a 110 Hz instability mode. PIV measurements made of the stabilized flow revealed significant modification of the inner recirculation zone and substantial weakening of the outer recirculation zone.