Radulescu Gasdynamics Laboratory

Experimental Investigation of Detonation Re-initiation Mechanisms following a Mach Reflection of a Quenched Detonation by Rohit Ranjan Bhattacharjee Detonation waves are supersonic combustion waves that have a multi-shock front structure followed by a spatially non-uniform reaction zone. During propagation, a decoupled shock-flame complex is periodically re-initiated into an overdriven detonation following a transient Mach reflection process. Past researchers have identified mechanisms that can increase combustion rates and cause localized hotspot re-ignition behind the Mach shock. But due to the small length scales and stochastic behaviour of detonation waves, the important mechanisms that can lead to re-initiation into a detonation requires further clarification.  If a detonation is allowed to diffract behind an obstacle, it can quench to form a decoupled shock-flame complex and if allowed to form a Mach reflection, re-initiation of a detonation can occur. The use of t...

The sequence of images illustrates the dynamics of cellular detonations.  The mixture is stoichiometric methane-oxygen, at an initial pressure of 3.5 kPa.  The detonation wave runs from left to right.  After the collision of triple points, the non-reacted gas accumulated behind the weak lead shock seperates from the lead front as a large pocket.  This pocket contains non-reacted gas.  The pocket is thinned out from the edges within a cell cycle.  The absence of notable gas dynamic events upon the pocket reaction (strong pressure waves) is indicative of diffusion assisted combustion. The slip lines emanating from the triple points, separating burnt and unburnt gas, develop Kelvin-Helmholtz instability.  This is characterized by the formation of whirls and resulting filamentary structure of the reaction zone interface.  These slip lines terminate with a jet pointing towards the Mach shock. Further analysis of the flow-field can be found in ...

On May 31, Philippe Julien from McGill's Alternative Fuels Laboratory came to our lab with a few toy balloons and a balloon inflation rig equipped with a retractable ignition sting. We put to the test our large scale 2m-by-2m shadowgraph system to visualize methane and hydrogen-air flames.  The flow evolution is visualized with a Phantom V1210 camera. In hydrogen-air, the flame exhibited the characteristic cellular structure. Batman begins! In methane, the larger flame thickness prevents hydrodynamic instabilities for curved flames.   Hats off, methane!

For the second consecutive year, Logan Maley, Jonathan Armstrong and myself did the grueling Rideau Lakes Cycle Tour.  Ottawa - Kingston - Ottawa, 344 km, in 2 days. A few days before, we managed to schedule a second and last training ride after technical presentations at the Annual H2CAN meeting in Mont Tremblant. and went out rolling towards St-Donat. Three days later, we were ready to attack the RLCT tour . Loaded up our back pockets with energy bars and some sort of special salts Jon brought. took a team picture, and left for Kingston on an epic ride.  After an hour, Logan's bike broke.  Luckily, team car Shane Maley brought a spanking full Record replacement bike.  With some help from Mike the mechanic, off we went.  After some 344 km, with a help of a slight tail wind and the hard work of Calin Focsa for a few hours on the second day, the sweet taste of victory!

and gets nominated for another one of those fancy awards...i guess it's a trend by now. photo courtesy of A. Bellerive Justin Tang thesis now shows that Fickett's model, i.e., a reactive form of the Burgers' equation, reproduces the same type of pulsating instabilities as real detonations! It also recovers the route to chaos via period doubling bifurcations. Space-time diagram showing the modulated amplification of forward facing pressure waves by the location of the reaction zone (controlled by the lead shock strength).  In-phase and out-of-phase passage of pressure waves through the reaction zone modulate the leading shock strength. Figure adapted from his Masters thesis. More reading available in our publications: Radulescu M.I. and Tang, J., Non-linear dynamics of self-sustained supersonic reaction waves: Fickett’s detonation analogue. Physical Review Letters 107(16):164503 (2011) DOI:10.1103/PhysRevLett.107.164503 Tang, J. and Radulescu, M...

The NSERC awards have been announced .  Both Nick Sirmas and Shem Lau-Chapdelaine received the top-of-the-line NSERC Alexander Graham Bell CGS-D awards for their PhD programs.   They follow in Brian Maxwell's footsteps. Congratulations to both for these awards ! Out of the 10 CGS PhD scholarships awarded at UOttawa (only 6 for the full 3 yrs), 2 of them are from our group, the only ones from engineering. 

Influence of Porosity on the Flame Speed inGasless Bimetallic Reactive Systems by Hesam Akbarnejad Self-propagating High-temperature Synthesis ( SHS ) is the synthesis of solid materials by a reaction wave propagating into the initial reactants, typically two metals, which can alloy exothermically . Typically, experiments are performed with the reactants in powder form, with relatively low density. Recent experiments by Bacciochini et al. revealed much larger flame speeds in densified powders near TMD, obtained by the cold spray process. The present thesis investigates why the flame speed increases dramatically with an increase in density of the powders. The investigation rests on the analytical model formulated by Makino by controlling how the variables are affected by changes in density. Flame speed measurements were performed in mixtures of nickel (Ni) and aluminum (Al) at different initial densities. The density was varied by controlling the cold-pressing of the s...

.. and onto a PhD. Numerical Simulations of Detonation Re-Initiation Behind an Obstacle by She-Ming Lau-Chapdelaine  This numerical study explored the mechanisms responsible for the re-initiation of a detonation, which quenched while diffracting over a half-cylinder obstacle. The purpose of the study was to make accurate predictions of when detonation re-initiation would occur, determine the role various re-initiation mechanisms, and compare the effect of different chemical models. The problem was modelled using the reactive Euler equations with either the one-step Arrhenius or two-step chain-branching chemical models, calibrated to post-shock conditions in order to properly reproduce the ignition delay. The simulations were validated using the stoichiometric methane-oxygen experiments of Bhattacharjee et al. Detonation failure and separation into shock-flame structure The numerical model was able to accurately predict detonation re-init...

Nick was also nominated for some fancy award - en route for the Sirmas instability and a promising PhD.   Shock Instability in Gases Characterized by Inelastic Collisions  by Nick Sirmas The current study addresses the stability of shock waves propagating through dissipative media, analogous to both granular media and molecular gases undergoing endothermic reactions. In order to investigate the stability, a simple molecular dynamics model was developed to observe shock waves and their structures with the inclusion of energy dissipation. For this, an Event Driven Molecular Dynamics model was implemented in a 2D environment, where a molecule is represented by a disk. The simulations addressed the formation of a shock wave in a gas by the sudden acceleration of a piston. Inelastic collisions were assumed to occur only if an impact velocity threshold is surpassed, representing the activation energy of the dissipative reactions. Parametric studies ...

Using the soap bubble technique, we visualize the dynamics of unconfined hydrogen-air flames using high speed schlieren video - a technique that permits to see density gradients. For sufficiently weak mixtures, i.e., low flame speeds, buoyancy effects become important, as shown in the following two photographs. As the flame begins to grow inside the soap bubble, the light products of combustion inside the flame ball begin to rise, forming a mushroom-shaped flame. The critical dimension of flame balls that will begin to rise due to buoyant forces is given by equating the time scale for flame propagation to the time scale of convection by buoyant forces. More details can be found in our arXiv paper and video , which has won one of the three Van Dyke awards for the Gallery of Fluid Motion at the 65th Annual Meeting   of the American Physical Society - Division of Fluid Dynamics . UPDATE 19-9-2013 :  The article is now available in Physics of Fl...