Zhenwei Zhao

SPONTANEOUS IGNITION OF PRESSURIZED RELEASES OF HYDROGEN AND NATURAL GAS INTO AIR

Abstract This paper demonstrates the “spontaneous ignition” (autoignition/inflammation and sustained diffusive combustion) from sudden compressed hydrogen releases that is not well documented in the present literature, for which little fundamental explanation, discussion or research foundation exists, and which is apparently not encompassed in recent formulations of safety codes and standards for piping, storage, and use of high pressure compressed gas systems handling hydrogen. Accidental or intended, rapid failure of a pressure boundary separating sufficiently compressed hydrogen from air can result in multi-dimensional transient flows involving shock formation, reflection, and interactions such that reactant mixtures are rapidly formed and achieve chemical ignition, inflammation, and transition to turbulent jet diffusive combustion, fed by the continuing discharge of hydrogen. Both experiments and simple transient shock theory along with chemical kinetic ignition calculations are used to support interpretation of observations and qualitatively identify controlling gas properties and geometrical parameters. Although the phenomenon is demonstrated for pressurized hydrogen burst disk failures with different internal flow geometries, similar phenomena apparently do not necessarily occur for sudden boundary failures of storage vessel or transmission piping into open air that have no downstream obstruction. However, subsequent reflection of the resulting transient shock from surrounding surfaces through mixing layers of hydrogen and air may have the potential for producing ignition and continuing combustion. Much more experimental and computational work is required to quantitatively determine the envelope of parameter combinations that mitigate or enhance spontaneous ignition characteristics of compressed hydrogen as a result of sudden release, particularly if hydrogen is to become a major energy carrier interfaced with consumer use. Similar considerations for compressed methane, for mixtures of light hydrocarbons and methane (simulating natural gas), and for larger carbon number hydrocarbons show similar autoignition phenomena may occur with highly compressed methane or natural gas, but are unlikely with higher carbon number cases, unless the compressed source and/or surrounding air is sufficiently pre-heated above ambient temperature. Spontaneous ignition of compressed hydrocarbon gases is also generally less likely, given the much lower turbulent blow-off velocity of hydrocarbons in comparison to that for hydrogen.

BURNING VELOCITIES AND A HIGH-TEMPERATURE SKELETAL KINETIC MODEL FOR n-DECANE

ABSTRACT Laminar flame speeds of n-decane/air mixtures were determined experimentally over an extensive range of equivalence ratios at 500 K and at atmospheric pressure. The effect of N2 dilution on the laminar flame speed was also studied at these same conditions. The experiments employed the stagnation jet–wall flame configuration with the flow velocity field determined by particle image velocimetry. Reference laminar flame speeds were obtained using linear extrapolation from low to zero stretch rate. The determined flame speeds are significantly different from those predicted using existing published kinetic models, including a model validated previously against high-temperature data from flow reactor, jet-stirred reactor, shock tube ignition delay, and burner-stabilized flame experiments. A significant update of this model is described that continues to predict the earlier validation experiments as well as the newly acquired laminar flame speed data and other recently published shock-tube ignition delay measurements.

THE INITIAL TEMPERATURE AND N2 DILUTION EFFECT ON THE LAMINAR FLAME SPEED OF PROPANE/AIR

Laminar flame speeds of propane/air mixtures were determined experimentally over an extensive range of equivalence ratios at room temperature, 500 K, 650 K, and atmospheric pressure. Nitrogen addition to simulate effects of exhaust gas dilution on the laminar flame speed was also studied at these conditions for selected equivalence ratios. The experiments employed the stagnation jet-wall flame configuration in which the flow velocity was obtained by using particle image velocimetry. The laminar flame speed was obtained using linear extrapolation to zero stretch rate. The measured flame speeds were compared with literature data and numerical predictions using a published detailed kinetic model (Qin, Z., Lissianski, V., Yang, H., Gardiner, W. C., Jr., Davis, S. G., and Wang, H., Proc. Combust. Inst., vol. 28, pp. 1663–1669, 2000). The predictions generally agree well with the experimental data. Both the data and model predictions reveal a quasi-linear relationship between the laminar flame speed and the dilution ratio, contrary to the nonlinear correlations commonly suggested in the literature. The linear dependence issue is numerically extended to include hydrogen and methane flame systems.

Burning Velocities of Real Gasoline Fuel at 353 K and 500 K

Burning velocities for unleaded conventional gasoline (CR-87) and air mixtures were determined experimentally over an extensive range of equivalence ratios at 353 K and 500 K and at atmospheric pressure. Nitrogen dilution effects on the laminar flame speed were also studied for selected equivalence ratios at these same conditions. Experimental measurements employed the stagnation jet-wall flame configuration and Particle Image Velocimetry (PIV). The laminar burning velocity was obtained using linear extrapolation of stretched flame data to zero stretch rate. The measured flame speeds were compared with numerical predictions using a minimized detailed kinetic model for primary reference fuel (PRF) mixtures, which was developed based on stirred reactor, shock tube and flow reactor data.

Measurements of Hydrocarbon Flame Speed Enhancement in High-Q Microwave Cavity

In this work we demonstrate that a small amount of microwave power below its breakdown threshold can be locally absorbed into a flame combustion zone. The absorbed microwave power can significantly change the flame speed of both laminar and turbulent flames. PIV technique was employed to measure the laminar flame speed. It was found that microwave assisted flame speed enhancement was greatly dependent on Q of the microwave cavity. Due to the unsteady nature of interaction, microwave assisted flame speed measurements were difficult to make, however, preliminary observations of the flame luminosity indicated that there was energy addition occurring without microwave breakdown and the flame speed was increased.