- Ignition by Under-expanded Hot Jets
We developed a novel rapid compression machine that used impact on a plunger to compress and ignite a rich hexane-oxygen mixture. A locking mechanism was used to prevent the plunger from rebounding and the jet was created by rupturing a diaphragm
covering a nozzle with an exit diameter between 0.25 and 1 mm. The jet development and ignition processes
in the main chamber filled with hexane-air mixture were visualized using high-speed schlieren and OH*
chemiluminescence imaging. Unlike the case of subsonic jets
in which ignition occurs at the shear layer near the nozzle exit, ignition of combustion in the main chamber was
found to take place downstream of the Mach disk terminating the supersonic expansion and within the turbulent
mixing region created by the startup of the supersonic jet. The results are interpreted using a constant-pressure,
well-stirred reactor model simulating the mixing between the hot jet and cold ambient gas.
Yunliang Qi and J.E. Shepherd. Ignition of Hexane-air Mixtures by Highly
Under-expanded Hot Jets. 39th International Symposium on Combustion, Vancouver, BC Canada, 28 July 2022.
- Pressure and Dilution Effects on Thermal Ignition
Follow up studies on thermal ignition by hot, vertical cylinders have examined the effects of fuel type, ambient pressure and nitrogen dilution. An experimental study using the fixture developed for previous scaling study was used to determine ignition thresholds for n-hexane, hydrogen and ethylene in
oxygen/nitrogen atmospheres. A range of initial pressures (1, 0.7, 0.466, 0.238 atm) and nitrogen dilution levels (N2/O2 = β = 3.76, 5.64, and 7.52, equivalent to mole fractions of O2 = 20.6, 14.8, 11.6%) were explored for n-hexane. A
modest increase of ignition temperature threshold was observed for both decreasing pressure and
increasing nitrogen concentration at fixed stoichiometry for hexane-air mixtures. A semi-empirical
correlation based on the work of Ono et al. (1976) was found to be a reasonable representation of
the pressure dependence for each β.
C. Martin and J.E. Shepherd. Thermal Ignition: Effects of Fuel, Ambient Pressure and Nitrogen Dilution. 14th International Symposium on Hazards, Prevention, and Mitigation of Industrial Hazards, (ISHPMIE), Braunschweig, Germany, July 11 – 15, 2022.
- Scaling of Thermal Ignition
The threshold for thermal ignition from a hot surface has been correlated by previous researchers with the size of the hot surface; a dramatic decrease in threshold temperature is found with an increase in surface area above 100 cm2. These studies are confounded by using data from very different geometrical configurations and there is significant bias in the data with all studies reporting low-temperature thresholds being carried out with internal, recirculating flows and the high-temperature thresholds being exclusively from external flow generated by natural or forced convection. We have re-examined the dependence of ignition temperature threshold on surface area for a single geometry, a vertical cylinder creating a natural convection flow with no recirculation. We find that for this configuration, ignition temperature thresholds do not show a dramatic change with increasing surface area and the ignition thresholds are up to 500 K higher than previously reported at surface areas of 200 cm2.
S. Jones and J.E. Shepherd. Thermal ignition of n-hexane air mixtures by vertical cylinders, 13th International Symposium on Hazards, Prevention, and Mitigation of Industrial Hazards, (ISHPMIE), Braunschweig, Germany, July 27 – 31, 2020.
S. M. Jones and J. E. Shepherd. Thermal ignition by vertical cylinders. Combustion and Flame, 232:111499, 2021. Preprint, see https://doi.org/10.1016/j.combustflame.2021.111499 for the published version.
Silken Jones. Thermal Ignition by Vertical Cylinders. PhD thesis, California Institute of Technology, Pasadena, CA, January 2021. Permanent link at Caltech Electronic Thesis Distribution.
- Low-Temperature Ignition of Aviation Kerosene and Surrogates
Extensive studies of autoignition have been carried on aviation kerosene and surrogate fuels for high-temperature and high-pressure conditions typical of gas turbine combustor conditions. Our laboratory has carried out complementary studies of the low-temperature and low-pressure autoignition using the ASTM-E659 standardized test method that is more relevant to safety evaluations in the lower-temperature, un-pressurized regions of an aircraft. We tested both standardized references formulations of aviation kerosene (Jet A) and surrogate fuels as well as the individual molecular components of the surrogates. Ignition tests were carried out for a wide range of fuel concentrations and temperatures. Results of autoignition tests exhibited complex,non-binary ignition behavior including both luminous and non-luminous cool flames.
C. Martin and J.E. Shepherd. Low Temperature Autoignition of Jet A and
Surrogate Jet Fuels. Journal of Loss Prevention in the Process Industries, 71:104454, July 2021. Preprint, see
for published version.
C. Martin and J.E. Shepherd. Low Temperature Autoignition of Jet A and
Surrogate Jet Fuels. 13th International Symposium on Hazards, Prevention, and Mitigation of Industrial Hazards, (ISHPMIE), Braunschweig, Germany, July 27 – 31, 2020.
C.D.Martin and J.E. Shepherd. Autoignition Testing Of Hydrocarbon Fuels Using the ASTM-E659 Method. Graduate Aeronautical Laboratories, California Institute of Technology, Technical Report EDL2020.001, March 2020.
- Ignition by Water Hammer
The potential of water hammer events for igniting hydrogen-oxygen mixtures was examined in an experimental
study. Compression waves simulating water-hammer events were created by projectile impact on a piston in a waterfilled
pipe terminated by a test section filled with gas. The gas layer was compressed in volume by up to a factor of 50 and the gas pressures increased to as high as 20 MPa within 2 to 4 ms. The distortion of the water surface (Richtmyer-Meshkov and Rayleigh-Taylor instabilities) during compression resulted in a significant increase in interfacial area and ultimately, creation of a two-phase mixture of water and compressed gas. Some ignition events were observed, but the dispersion and mixing of water with the gas almost completely suppressed the pressure rise during the ignition transient. Only by eliminating the instability of the water interface with a solid disk between the water and gas were we able to observe consistent ignition with significant pressure rises associated with the combustion.
S.A. Coronel, J.C. Veilleux, J.E. Shepherd. Ignition of stoichiometric hydrogen-oxygen by water hammer. Proceedings of the Combustion Institute, 38, 2020 (in press).
J.-C. Veilleux, S.A. Coronel, J.E. Shepherd. Ignition by Water Hammer, GALCIT Report EDL2019.001
J.E. Shepherd. Ignition modeling and the critical decay rate concept, GALCIT Report EDL2019.002
- Optics of the Human Eye and Intraocular Gas
Optical effects associated with refraction created by intraocular gas tamponade following retina surgery are reported and analyzed. Observations of dramatic change in both near point and magnification are explained using ray tracing and simplified models of the eye as an optical system. The gas bubble shapes are computed using the Young-Laplace law and compared to previous clinical measurements and analyses. The effect of bubble shape on refraction is determined by including the curvature of the gas-liquid interface on the optical axis of a downward facing eye. A possible clinical application is the estimation of gas volume from near point measurements.
Joseph E. Shepherd. Optical Effects of Intraocular Gas Tamponade. Unpublished, March 23, 2019.
Note A cautionary tale on the vastness of the scientific
literature and dangers of wandering far afield from your area of expertise. Despite my efforts at searching the literature, when I wrote up my work in 2018, I was unaware that Russell, Gehrs and Hess at the University of Iowa had learned of similar observations by their patients and published (Retina, 20(3), 282-288, 2000) ray tracing analyses to explain these visual acuity effects. My conclusions are consistent with Russell et al. and my analyses are an extension to include the optical effect of the curvature of the liquid-gas interface. I thank Drs. Jon and Stephen Russell for bringing the prior study to my attention. (October 18, 2022)
- Autoinjector mechanics (Sponsor: Amgen)
Spring-actuated autoinjectors delivering viscous drug solutions resulting from large drug concentrations require large spring forces which can create high peak pressures and stresses within syringes. The high peak pressures and stresses can lead to device failure. Measurements with a suite of novel instrumentation and analysis using numerical simulation explain the peak pressures and peak stresses as originating from mechanical impacts between moving components, the large acceleration of the components, and surprisingly, the production of tension waves in the liquid resulting in cavitation.
Jean-Christophe Veilleux and Joseph E. Shepherd. Pressure and stress transients in autoinjector devices. Drug Delivery and Translational Research, 8:1238-1253, 2018.
Jean-Christophe Veilleux, Kazuki Maeda, Tim Colonius, Joseph E. Shepherd. Transient Cavitation in Pre-Filled Syringes During Autoinjector Actuation. 10th International Symposium on Cavitation (CAV2018). Baltimore, MD, May 14-16, 2018.
Jean-Christophe Veilleux and Joseph E. Shepherd. Impulsive Motion in a Cylindrical Fluid-Filled
Tube Terminated by a Converging Section. Journal of Pressure Vessel Technology, April 2019, 021301- 1, 021302-11. DOI: 10.1115/1.4042799
- Ignition and flame propagation (Sponsor: Boeing)
A critical issue for transportation and industrial systems is the prevention and mitigation of fire and explosion events under a range of normal operating conditions as well as possible equipment failures. To support industry efforts in the design and certification of engineering systems, we carry out laboratory experiments on ignition and flame propagation. The goals of these experiments and companion numerical simulations are to develop a scientific understanding that is the basis of a first-principles predictive capability for evaluating and mitigating explosion hazards.
Quenching of Laminar Flames in Arrestors
Flame arrestors are widely used in the chemical process and transportation industry as a safety measure
to prevent flames from propagating through piping systems or into vessels containing flammable gases. The
principle of operation of most arrestors is to quench the flame and cool the following flow by introducing
a large surface area of cold metal surrounding small flow passageways. If the gas flow emerging from the
arrestor is sufficiently cold and deficient in reactive species, the flame will not be transmitted downstream. Numerical simulation was used to investigate the quenching of laminar hydrogen-air flames by a fine-wire metal-mesh
flame arrestor. The unsteady propagation of a flame through a periodic array of
wires was investigated using the reactive Navier-Stokes equations with a detailed model of the chemical
kinetics, realistic thermochemistry and models for diffusive transport.
H. Saitoh, J. Melguizo-Gavilanes, and J. E. Shepherd. Simulation of Quenching Laminar Hydrogen-Air Flames in a Mesh Arrestor, November 30, 2017. Unpublished.
Thermal ignition by hot moving spheres
Temperature fields and ignition thresholds for flammable mixtures were experimentally and numerically determined using a moving hot spheres 2-6 mm in diameter. Interferometry was used to obtain spatially and temporally resolved temperature fields before and during ignition. Numerical simulations of the transient development of the 2-D axisymmetric motion and ignition were performed using the reactive Navier-Stokes equations and detailed models of the chemical reaction mechanisms for hexane and hydrogen fuels. At the ignition threshold, the critical location for ignition kernel development was at the flow separation point. Fuel decomposition within the the boundary layer is found to be an important process for hydrocarbon fuels.
S. Jones, J. Melguizo-Gavilanes, and J. E. Shepherd.
Ignition by moving hot spheres in H2-O2-N2 environments.
Proceedings of the Combustion Institute, in press, 37, 2018.
S. Coronel, J. Melguizo-Gavilanes, R. Mével, and J. E Shepherd.
Experimental and numerical study on moving hot particle ignition.
Combustion and Flame, 192:495-506, 2018.
S. Coronel, J. Melguizo-Gavilanes, S. Jones and J. E Shepherd.
Temperature field measurements of thermal boundary layer and wake of moving hot spheres using interferometry. Experimental Fluid and Thermal Science, 90:76-83, 2018.
J. Melguizo-Gavilanes, S. Coronel, R. Mével, and J. E. Shepherd. Dynamics of ignition of stoichiometric hydrogen-air mixtures by moving heated particles. International Journal of Hydrogen Energy, 42(11):7380-7392, 2017.
J. Melguizo-Gavilanes, R. Mével, S. Coronel, and J. E. Shepherd.
Effects of differential diffusion on hot surface ignition of stoichiometric hydrogen-air.
Proceedings of the Combustion Institute, 36(1):1155-1163, 2017.
R. Mével, U. Niedzielska, J. Melguizo-Gavilanes, S. Coronel, and J. E. Shepherd.
Chemical kinetics of n-hexane-air atmospheres in the boundary layer of a moving hot sphere.
Combustion Science and Technology, 188(11-12):2267-2283, 2016.
Thermal ignition from small hot cylinders
Ignition of hydrogen-air, ethylene-air and n-hexane-air mixtures from horizontally and vertically oriented heated circular cylinders were studied experimentally over a wide range of mixture compositions. The threshold ignition temperature is relatively insensitive to the composition away from the flammability limits. For vertically-oriented cylinders, a unique periodic puffing combustion mode is observed near the flammability limits with a limiting state of a single puff.
Lorenz Boeck, Josue Melguizo-Gavilanes, and Joseph E. Shepherd. Hot surface ignition dynamics in premixed hydrogen-air near thelean flammability limit. Combustion and Flame 210 (2019) 467-478. See also
Hot surface ignition dynamics in hydrogen-air mixtures near the flammability limits.Paper No. 1100, 26th International Colloquium on the Dynamics of Explosions and Reactive Systems, Boston, MA, 30 July – 4 August 2017, 2017.
Lorenz Boeck, Maxime Meijers, Andreas Kink, Remy Mével, and Joseph E Shepherd. Ignition of fuel-air mixtures from a hot circular cylinder. Combustion and Flame, Vol. 185, November 2017, 265-277.
J. Melguizo-Gavilanes, L.R. Boeck, R. Mével and J.E. Shepherd. Hot surface ignition of stoichiometric hydrogen-air mixtures. International Journal of Hydrogen Energy, 42(11), 7393-7403, 2017.
A. Nové-Josserand, Y. Kishita, J. Melguizo-Gavilanes, S. Coronel, L. Boeck, R. Mével, and J. E. Shepherd. Ignition of hydrogen-air mixtures by a concentrated stationary hot surface. International Symposium on Hazards, Prevention, and Mitigation of Industrial Explosion (ISHPMIE), July 24-29 2016, Dalian, China, 2016.
P. A. Boettcher, S. K. Menon, B.L. Ventura, G. Blanquart, and J. E. Shepherd. Cyclic Flame Propagation in Premixed Combustion. J. Fluid Mechanics, Volume 735, November 2013, pp 176 - 202.
Thermal ignition near the Autoignition Limit
The oxidation of hexane-air mixtures in heated vessels was examined for a range of heating rates. A transition between slow (non-explosive) and fast (explosive) oxidation was discovered experimentally and explained using analytical and numerical simulations of the reaction process. Measurements of species during slow, low-temperature oxidation reveal a multi-stage oxidation process with the initial rapid production of CO2, CO and carbonyls, identified as hydroperoxy-ketones; followed by a period of slower production of CO2 and H2O and consumption of hydroperoxy-ketones.
J. Melguizo-Gavilanes, P.A. Boettcher, R. Mével, and J.E. Shepherd. Numerical study of the transition between slow reaction and ignition in a cylindrical vessel. Combustion and Flame, 204:116-136, 2019
R. Mével, F. Rostand, D. Lemarié, L. Breyton and J.E. Shepherd. Oxidation of n-Hexane in the Vicinity of the Auto-Ignition Temperature. Fuel 236, 373-381, 2019.
R. Mével, K. Chatelain, P.A. Boettcher, G. Dayma, and J. E. Shepherd.
Low temperature oxidation of n-hexane in a flow reactor.
Fuel, 126:282-293, 2014.
P. A. Boettcher, R. Mével, V. Thomas and J. E. Shepherd. The effect of heating rates on low temperature hexane air combustion. Fuel 96:392-403 2012.
Experimental and numerical studies were performed to examine ignition of jet fuel, surrogates, and certification test mixtures by electrostatic discharge. Capacitive discharge systems were developed to produce very low-energy (50 microJoule to 1 milliJoule) sparks for a range of spark lengths. A well defined threshold (Minimum Ignition Energy) energy value does not exist, but ignition is statistical in nature and highly dependent on mixture composition and spark length. Experimental results are analyzed to obtain a probability distribution for ignition versus the spark energy per unit spark length. Mixtures previously used in FAA certification tests are examined in comparison to kerosene air and significant issues with mixture specification are identified.
S.P.M. Bane, J.L. Zeigler, and J.E. Shepherd.
Investigation of the effect of electrode geometry on spark ignition.
Combust. Flame, 162:462-469, 2015.
S. P. M. Bane, R. Ziegler, S. Coronel, and J. E. Shepherd. Experimental investigation of spark ignition energy in kerosene, hexane, and hydrogen. Journal of Loss Prevention in the Process Industries, Volume 26, Issue 2, Pages 290-294 (March 2013)
S.P.M. Bane, J.E. Shepherd, E. Kwon and A. C. Day. Statistical Analysis of Electrostatic Spark Ignition of Lean H2/O2/Ar Mixtures. International Journal of Hydrogen Energy, 36:2344-2350, 2011.
S. P. M. Bane, S. A. Coronel, P. A. Boettcher, and J.E. Shepherd.
Statistical analysis of spark ignition of kerosene-air mixtures.
2011 Fall Meeting of the Western States Section of the Combustion Institute, Riverside, CA October 17-18, Paper 0271C-0201, 2011.
S.P. M. Bane and J.E. Shepherd. Statistical analysis of electrostatic spark ignition.
2009 Fall Meeting of the Western States Section of the Combustion Institute University of California at Irvine, Irvine, CA October 26 & 27, Paper 09F-64, 2009
S. Gallier and F. Le Palud and F. Pintgen and R. Mével and J.E. Shepherd. Detonation Wave Diffraction in H2-O2-Ar Mixtures. Proceedings of the Combustion Institute, Vol. 36, No. 2, 2781-2789, 2017.
S. I. Jackson, B. J. Lee and J. E. Shepherd. Detonation Mode and Frequency Analysis Under High Loss Conditions for Stoichiometric Propane-Oxygen. Combustion and Flame, Vol. 167, 24-38, 2016.
G. Bechon, R. Mével, D. Davidenko and J.E. Shepherd. Modeling of Rayleigh scattering imaging of detonation waves: Quantum computation of Rayleigh cross-sections and real diagnostic effects. Combustion and Flame 162(5):2191-2199, 2015.
R. Mével, D. Davidenko, J. M. Austin, F. Pintgen and J. E. Shepherd. Application of a laser induced fluorescence model to the numerical simulation of detonation waves in hydrogen-oxygen-diluent mixtures. International J of Hydrogen Energy, Vol. 30, 6044-6060, 2014.
J. E. Shepherd. Detonation in Gases. Proceedings of the Combustion Institute, Vol. 32, 83-98, 2009.
- Shock Waves and Chemical Kinetics
R. Mével, K. Chatelain, G. Blanquart, and J. E Shepherd.
An updated reaction model for the high-temperature pyrolysis and oxidation of acetaldehyde.
Fuel, 217:226-229, 2018.
R. Mével and J.E. Shepherd. Ignition delay-time behind reflected shock waves of small hydrocarbons-nitrous oxide(-oxygen) mixtures. Shock Waves 25(3):217-229, 2015.
K. Chatelain, R. Mével, S. Menon, G. Blanquart, and J. E. Shepherd. Ignition and chemical kinetics of acrolein-oxygen mixtures behind reflected shock waves.
Fuel, 135:498-508, 2014.
R. Mével, S. Pichon, L. Catoire, N. Chaumeix, C.-E. Paillard, and J.E. Shepherd. Dynamics of excited hydroxyl radicals in hydrogen-based mixtures behind reflected shock waves. Proceedings of the Combustion Institute 34:677-684, 2012.
- Detonation-Structure Interaction
Damazo, J. and J.E. Shepherd. Observations on the normal reflection of gaseous detonations. Shock Waves, Vol. 27, September 2017, 795-810.
J. Karnesky, J. S. Damazo, K. Chow-Yee, A. Rusinek, and J. E. Shepherd.
Plastic deformation due to reflected detonation.
International Journal of Solids and Structures, 50(1):97-110, 2013.
J. E. Shepherd. Structural response of piping to internal gas detonation.
Journal of Pressure Vessel Technology, 131(3):031204, 2009.
T.-W. Chao and J. E. Shepherd. Fracture response of externally flawed aluminum cylindrical shells under internal gaseous detonation loading. International Journal of Fracture, 134(1):59-90, July 2005.
W.M. Beltman and J.E. Shepherd. Linear elastic response of tubes to internal detonation loading. Journal of Sound and Vibration, 252(4):617-655, 2002.