Session P01: Reacting Flows: General and Experiments (3:10pm - 3:55pm CST)

Heat release characteristics of ammonia/hydrogen flames in MILD conditions

The utilization of ammonia as a fuel is a pragmatic approach to pave the way towards a low-carbon economy. Ammonia contains almost 18$\%$ of hydrogen by mass and is accepted as one of the hydrogen combustion enablers with existing infrastructure for transportation and storage. To provide fundamental insights into the heat release characteristics of ammonia and ammonia-hydrogen flames at various conditions, different levels of hydrogen addition in moderate or intense low oxygen dilution (MILD) conditions were investigated numerically. In particular, the heat release characteristics and dominant reaction pathways were analyzed in one-dimensional laminar premixed configurations. The analysis reveals that the peak of heat release for the ammonia flame occurs near the burned gas. The dominant heat release reactions and significance of NH$_{2}$ and OH, which serve as precursors for characterization of the reaction zone, are highlighted. With increasing hydrogen in the mixture, the heat release is enhanced and the peak shifts toward the unburned gas, which is due to dominance of the reaction H$_{2}$ + OH $<=>$ H + H$_{2}$O at higher hydrogen levels.   

Ignition and Combustion of TNT-Dispersed Aluminum Powder

A multidimensional numerical study was conducted to explore the ignition and combustion of aluminum powder dispersed by a TNT charge. The simulations used a high-order numerical method for a compressible reactive gas that is coupled to an Eulerian kinetic-theory-based granular multiphase model. The model for the particles accounts for compaction waves, particle collisions, etc. and is valid up to the packing limit. Scenarios where an annular shell of highly packed monodisperse Al powder surrounding the TNT charge were considered. The results show the formation of particle fingers as the particles are radially dispersed by the expanding TNT detonation products. Particle inertia initially separate the Al particles from the TNT fireball, which prohibits ignition. Al particles under 10 $\mu$m-diameter ignite when they interact with the afterburning TNT fireball, which is the only location that exceeds the Al ignition temperature. The inner edge of the Al dust cloud comes into contact with the fireball during the reshock phase of the blast when the flow reverses. Particles greater than 30 $\mu$m-diameter Al particles did not ignite because their inertia launched them too far from the TNT fireball.   

Dissociation and recombination reactions for diatomic Nitrogen in Argon-Nitrogen mixed gaseous thermal plasma

The dissociation and recombination reactions for diatomic Nitrogen in Argon-Nitrogen mixed gaseous thermal plasma with initial state composition 75 mol{\%} of Argon and 25 mole{\%} of Nitrogen, having two rotational degrees of freedom for Nitrogen molecules and with no internal degrees of freedom for Argon and electron, is studied using Direct Simulation Monte Carlo (DSMC) simulations The dissociation of a molecule is considered to take place when the Larsen-Borgnakke selection of energy into the vibrational mode leads to a level beyond the maximum level. During the dissociating collisions it is thought that the rotational mode of the molecules have disappeared, while the recombination reactions are considered to be based on the equilibrium collision theory, and the ratio of the recombination cross-section to the elastic cross-section in an atom-atom collision is determined through $\sigma_{R\thinspace }/\sigma_{T} =$\textit{ 2C (1}$+ ( E_{d\thinspace }/((? +$\textit{3/2 - ?)k}$_{B}$\textit{ T))) ((n}$_{T} V Q^{A2})/(Q^{A})^{2})$ The equilibrium degree of dissociation, which corresponds to the rate of recombination same as rate of dissociation, is computed for initial number densities in the range \textit{10}$^{22\thinspace }$\textit{\textless n}$_{O\thinspace }$\textit{\textless 10}$^{25}, $with equilibrium temperatures \textit{3000 K\textless T}$_{eq\thinspace }$\textit{\textless 20,000 K, }and \quad compared with the theoretical equilibrium state, and found excellent agreement (error within 2{\%}) An important finding is that the extent of recombination process decreases linearly with the initial number density, and under rarefied condition they can be ignored.   

Flame stabilization mechanisms in hydrogen enriched methane-air low-swirl flames

A newly designed low-swirl combustor by the Korea Institute of Machinery and Materials was tested for fully premixed H$_{\mathrm{2}}$-CH$_{\mathrm{4}}$/air reactant mixtures with equivalence ratios 0.6 to 0.9 and H$_{\mathrm{2}}$ fractions 0{\%} to 80{\%}. Using simultaneous OH/CH$_{\mathrm{2}}$O planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (S-PIV), flame stabilization mechanisms and flame shape transitions were observed for three distinct flame shapes, namely lifted bowl-, lifted W-, and attached crown-shapes. The three flame shapes were stabilized through the dynamic balance between flame speed and flow velocity in the non-swirling, central flow region. Bowl-flames were generally shrouded by the inner shear layer, while the W- and crown-shaped flames involved flame segments reaching the outer shear layer of the flow, and were stabilized by shed eddies in the former and were hardware-stabilized in the latter. In the effective operating window of the burner, the flame could transition from one shape to another based on the equivalence ratio, H$_{\mathrm{2}}$ fraction, and bulk flow velocity. The transition from W- to bowl-flames also led to an unusual internal blow-out zone within the global flame stability limits, where the existence of this zone was related to the laminar-flame extinction strain rate.   

Effect of microgravity on the formation and geometry of whirling flames

Fire Whirls (FWs) are structures that frequently occur in wildfires and are formed when a buoyant plume is subjected to ambient circulation. The two primary physical processes controlling FW structure are circulation and buoyancy. Here, we describe experimental investigations performed at the NASA Glenn Research Center's Zero Gravity Research Facility drop tower, which provides 5.18 s of microgravity time to study the effects of normal (1g) and micro gravity ($\mu $g) on FW geometry. The FWs were formed in both 1g and $\mu $g, using a paraffin wax wick in an enclosure formed by two offset half cylinders. Ambient circulation was controlled using a vertical bank of fans at each inlet. Results show that in $\mu $g, where the effects of circulation are higher, the flame height reduces dramatically, and the flame width increases moderately. The burning rate also reduces in $\mu $g, sometimes leading to short-lived blue flames, attributed to the increased importance of diffusion. Elevated gravity (\textasciitilde 35g), due to deceleration at the end of the drop, resulted in a brief transition to a blue-whirl-like regime, that is, a state in which a recirculation zone exists. Finally, a scaling approach to analyze whirling flames in $\mu $g is presented.