November 9th: Another Graduate Seminar Double Header

Coming up Friday November 9th:  another graduate seminar double header!  Rohit Bhattacharjee will give a talk titled “Experimental investigation of detonation re-initiation mechanisms following a mach reflection of a quenched detonation.”   Also presenting is Hesam Akbarnejad on the subject of the “Influence of Porosity on the Flame Speed in Bi-metallic Reactive Systems.”

The seminar abstracts follow below.

Date: Friday  9th November

Time: 2:30pm

Room: SITE B0138

Experimental investigation of detonation re-initiation mechanisms following a mach reflection of a quenched detonation

Rohit R. Bhattacharjee, Advisor – Dr. Matei Radulescu

Abstract
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 periodically re-initiates into an overdriven detonation following a transient Mach reflection process. While past researchers have identified mechanisms that can increase combustion rates following a Mach reflection, due to the small length scales and stochastic behaviour of propagating detonation waves, the important mechanisms that can cause detonation re-initiation have yet to be elucidated.

If a detonation wave diffracts behind an obstacle, it can quench to form a decoupled shock-flame complex and if allowed to form a Mach reflection, detonation re-initiation can occur. This technique permits the study of re-initiation mechanisms reproducibly with larger spatial scales. Therefore, the objective of this study is to experimentally investigate and clarify the key mechanisms that can increase chemical reaction rates and sequentially lead to re-initiation of a decoupled shock-flame complex, behind a cylindrical obstacle, into an overdriven detonation wave following a Mach reflection.

All experiments were carried out in a thin rectangular channel using a stoichiometric mixture of methane-oxygen. One of the obstacles used was a half-cylinder obstacle. Schlieren visualization was achieved by using a Z-configuration setup, high speed camera and high intensity light source.

Results indicate that forward jetting of the slipline behind the Mach stem can potentially increase combustion rates by entraining hot burned gas with unburned gas. Following ignition and jet entrainment, a detonation wave first appears along the Mach stem. The transverse wave can form a detonation wave due to rapid combustion of unburned gas which may be attributed to – a) shock interaction with the unburned gas; b) turbulent mixing by the Kelvin-Helmholtz instability.

Influence of Porosity on the Flame Speed in Bi-metallic Reactive Systems

By: Hesam Akbarnejad.  Supervisor: Matei Radulescu

Abstract

The topic of SHS process (Self-propagating High-temperature Synthesis) in bi-metallic reactive systems has been the focus of much attention over the past two decades. The effect of initial porosity in such systems is shown to influence the reaction behavior; however, this phenomena has, as of yet, not been accurately modeled. This thesis documents and studies the effect of porosity on the flame speed in Bimetallic Reactive Systems.

The analytical approach is based on the Makino’s model which predicts the heterogeneous behavior of SHS process using Spray-Combustion method. An attempt has been made to modify this model as a function of material porosity by relating the flame speed to the thermal conductivity of the mixture. To do so, thermal conductivity of porous materials is modeled and experimentally measured as a function of the density ratio. To precisely model the thermal conductivity, the micro-structure of the mixture is also studied. Consequently, bimetallic reactive systems are categorized into the low and high porosity materials and for each case, a specific model has been suggested. Finally, using empirical adjustments, an equation relating the flame speed to the density ratio of the mixture is obtained.

Comparisons of the model with the experimental results are satisfactory. Using these results, it can be concluded that the flame speed in SHS processes is related to the initial porosity of the mixture mainly by its thermal conductivity.

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