Effect of Boundary Conditions on Propagation and Morphology of Premixed Flames in Narrow Conduits

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 \u27smooth\u27 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 \u27smooth\u27 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

Thermal analysis of tilted roofs composed of two separated surfaces

Due to the rising power costs and lack of nonrenewable energy sources, the cooling of houses is becoming more expensive. Looking for alternative methods applicable to this process is becoming not only an option, but also a necessity. Changes in the roof structure of buildings can be applied in order to achieve a more favorable thermal transmission behavior. The utilization of a tilted roof, composed of two separated surfaces, generates natural convection currents in the channel between them. These currents, after driving off part of the transferred heat, decrease the temperature of the lower surfaces and consequently, the heat flux through the ceiling into the living areas.;The natural convection phenomenon is treated by numerical means, and the influence of the dimensions of the proposed design on the ventilation rates is analyzed in order to determine the most efficient geometry. The comparison of thermal performances between the proposed roof and a typical unventilated design is also established in order to realize the quantitative advantage of the proposed model.;Results show that the separation between surfaces strongly influences the process within certain values; i.e. a reduction in the heat flux through the ceiling achieved by the system of 32.9% can be raised to 45.4% by increasing the width of the channel from 0.05m to 0.15m, and keeping the other dimensions constant. Moreover, higher tilt angles also improve natural ventilation rates. For example, a 32.8% reduction obtained by the system at a 30 degree tilt angle grows up to 41.6% by raising the tilt angle to 65 degrees. A vertical extension or exhaust channel on the top increases the reduction of heat flux too, but with less intensity. In this sense, the heat flux reduction achieved by the system, when the vertical exhaust length is 12.5% of the length of the roof, increases from 32.9% to 45.5% when a considerably bigger vertical extension is used, 60% of the roof length