Impacts of Boundary Conditions on Premixed Combustion in Obstructed Conduits: A Computational Analysis

Premixed flame acceleration in channels or pipes has various practical applications, starting with fire safety problems and ending with advanced technologies such as pulse-detonation engines. In particular, a flame accelerates extremely fast when propagating through a comb-shaped array of narrow, tightly-spaced obstacles in a so-called "Bychkov tube". In the present thesis, the role of boundary conditions in such geometry is studied by means of the comprehensive computational simulations of combustion equations, with a fully-compressible hydrodynamics and an Arrhenius chemical kinetics. Specifically, the mechanistic (slip/nonslip) and thermal (adiabatic/isothermal) conditions at the walls/obstacles' surfaces, as well as the boundaries at the conduits' extremes (open/closed) are considered. The parametric study includes: the thermal expansion ratio in the range 5≤theta≤10; the wall temperature Tw being 298K≤Tw≤1000K; the pipe radius R exceeding the thermal flame thickness Lƒ by a factor of 12~48; the obstacles blockage ratio in the range 1/3≤alpha≤2/3; and the spacing between the obstacles Deltaz being 0.25R≤Deltaz≤2R.;It is shown that the impacts of both mechanistic and thermal surface conditions on flame propagation are minor and can be omitted. This is because the flame dynamics if mainly driven by flame spreading in an unobstructed portion of an obstructed pipe, i.e. far from the walls. With a fact that real walls are neither slip nor nonslip; neither adiabatic nor isothermal, but in between these categories, the minor role of surface conditions, identified here, validates the Bychkov model, which employs a number of simplifying assumptions, including slip and adiabatic surfaces.;In contrast, the role of the conditions at a pipe extreme is shown to be substantial. While in a semi-open pipe (one end is closed; a flame is ignited at this end and propagates towards the open end), the entire flame-generated jet-flow is pushed towards a single exit, in a pipe with two ends open, this flow is distributed between the upstream and downstream flows, thereby moderating flame propagation. As a result, in this geometry, a flame either accelerates much weaker (in a relatively wide pipe), with a possibility of blowout, or oscillates (in a narrow pipe). The oscillations appear nonlinear in all the situations when they are observed, and the present thesis quantifies the oscillation period and amplitude as well as the average flame velocity in the theta- Tw-alpha-Deltaz-R space.;Since these flame oscillations can be treated as fluctuations around a quasi-steady solution, the present thesis qualitatively supports the recent experiments, modeling and theory of flames in obstructed pipes with both ends open, which all yielded steady or quasi-steady flame propagation prior to an onset of spontaneous flame acceleration and deflagration-to-detonation transition.