Date of Graduation

2015

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

V'yacheslav Akkerman

Committee Co-Chair

Vitaly V Bychkov

Committee Member

Donald Ferguson

Committee Member

John M Kuhlman

Committee Member

Hailin Li

Committee Member

Gregory J Thompson

Abstract

Boundary conditions play a key role in the evolution and morphology of flame fronts, especially when combustion occurs in narrow chambers. The burning intensity and the flame-generated flow can be significantly modified by the momentum and energy transferred at the walls, which are further modified by the exothermal nature of the process. In this work, the effect of the wall roughness and thermal conditions on the flame propagation is explored. Specifically, conduits with and without obstacles, having adiabatic or isothermal walls, are investigated.;Wall friction constitutes one of the main reasons of spontaneous flame acceleration in narrow pipes. Although this phenomenon has been intensely studied, the researchers have focused on the mechanistic scenario of the combustion intensification, induced by the wall friction, putting less emphasis on the heat exchanged at the walls. In this study, besides the adiabatic condition, the surfaces have been kept at multiple constant temperatures in order to explore the wall thermal effects on the burning process, recognizing its potential to diminish or even quench the reaction.;Moreover, the inclusion of solid obstacles at the pipe walls provides a mechanism of extremely fast flame acceleration, which is driven by an intense jet-flow generated by the delayed combustion occurring between obstacles. In this work, the flame dynamics promoted in the obstructed configuration is analyzed, comparing the attained acceleration rates to other mechanisms such as that generated by the wall friction and the so-called finger flame evolution.;For this purpose, a parametric study provided by extensive fully-compressible numerical simulations of the combustion and hydrodynamic equations is performed. The geometry is primary given by 2D channels, although cylindrical 'smooth' tubes have been also considered. The wall conditions include non-slip walls and slip walls with obstacles; adiabatic and isothermal, with the fuel characterized by the thermal expansion coefficient. Four regimes of flame propagation in isothermal 'smooth' channels have been identified, for flames propagating a distance around 100-150 times the flame thickness: (i) no flame propagation or extinction; (ii) linear flame velocity; (iii) almost-constant flame propagation speed; and (iv) oscillating flame velocity. In the obstructed configuration, the developing of turbulent and laminar combustion regimes at the early stages of the process have been identified in relation to the obstacles size and spacing, including a finger flame-like limit when small enough obstacles are in place.

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