Measurement of Turbulent Flame Speeds of Hydrogen and Natural Gas Blends (C1-C5 Alkanes) using a Newly Developed Fan-Stirred Vessel

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2014-05-06

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A fan-stirred flame speed vessel was developed at Texas A&M University to conduct turbulent combustion studies. Four high-speed impellers were installed in a central-symmetric pattern at the central circumference of an existing cylindrical laminar flame bomb. The fans generated homogeneous and isotropic turbulence with negligible mean flow (< 10% u?) at the vessel center, and flames up to 12.7 cm in diameter can be measured. The fan designs were optimized using particle image velocimetry inside a Plexiglas model of the vessel. The uniformity of the flow fields was verified using spatial uniformity maps, two-point correlations, and the energy spectra. Additionally, the capability to independently vary the intensity level and the integral length scale was developed. Where the former changed with fan speeds, increasing the blade pitch angle of the impeller decreased the integral length scale.

Turbulent flame speeds of fuels that are of topical interest to gas turbines were measured in the fan-stirred bomb. Schlieren photography was used to visualize the flame growth under constant-pressure conditions, and the captured images were processed using an edge-detection code developed in-house. The equivalent-circle-area method was used to determine the flame radii. The shot-to-shot variability was minimal, which resulted in a low experimental scatter close to 10 cm/s. The flame speeds increased with radius due to flame acceleration. Effective turbulent intensity levels were estimated which increased progressively with flame radius.

A systematic approach was followed to determine the effects of hydrogen addition on the turbulent displacement speeds of alkanes (C1-C3). Particularly, a natural gas surrogate (NG2) containing large amounts of C2+ hydrocarbons (>20%) was studied. Turbulent displacements were higher for alkane mixtures with Lewis number below unity than those with Le>1. NG2 and methane gave near-identical turbulent displacement speeds consistent with the laminar flame speed trends. Similar trends in displacement speeds were observed for blends of NG2/H_(2) and CH_(4)/H_(2), thus validating the newly established experimental technique. Additionally, turbulent flame speeds of hydrogen and a generic, high-hydrogen-content syngas blend (50:50 H_(2):CO) were studied. The wide range of laminar flame speeds explored herein revealed significantly different flame surface features between the various regimes of turbulent combustion.

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