Combustion at microscale - Grupo de modelización de procesos de combustión
Combustion at Microscale
The recent development of nano and micro-fabrication technologies have accelerated the minimization of multiple electro-mechanical devices. The combustion at microscale has increased in importance has a way to provide the instant power requeriments of such portable devices which nowadays relies on batteries. It is estimated than a micro-power generator using hydrocarbon fuels with 3% of energy conversion efficiency should compete with the most advanced lithium ion batteries.
As the combustor size is reduced, the surface to volume ratio increases dramatically, creating strong thermal and chemical couplings between flame and solid wall interfaces, which affect the combustion stability. Heat recirculation, catalytic combustion or preheat of the fresh reactants are usually actions taken to avoid flame quenching. The Group develops fundamental studies in the dynamics of flame propagating in channels and ducts of smal size (micro and meso scale) or confined flames between plates. These studies are significant to the advance of our understanding of the physicochemical processes in microscale combustion and have a great importance on the design of efficient microscale combustors
Flame propagation in microchannels
Numerical simulations in the frame of the constant density approximation have helped in identifying the effect of the differential diffusion on the stability of flames propagating in narrow channels. In particular, symmetry-breaking bifurcations or oscillating and rotating modes were demonstrated. The effect of thermal expansion and detailed transport and chemistry is also being investigated.
The figure below shows the flame shape obtained in a DNS computation of a lean hydrogen-air flame mixture with detailed transport and chemistry. The flame propagates throughout 1 mm channel width and fueled with a flow rate of 2·10-4 m2/s (per unit of length). Notice that the non-symmetric solution (upper figure) is the stable one, and therefore the solution that arises in the experiments. The symmetric solution (lower figure) was obtained forzing the symmetry of the problem. The fuel consumption (and therefore the propagation velocity) per unit of time is larger in non-symmetric flames than in the symmetric counterpart. Non-symmetric flames release more power and can better sustain the combustion in confined geometries. Understanding the physical break-of-symmetry process of those flames has an enourmous importance on the correct prediction of the parametric range of stability in microcombustion devices. The condition of the upstream flame propagation in safety hazards, known as the flashback effect, can only be anticipated with a thorough understanding of the phenomenon.
Flame propagation in Hele-Shaw cells
The flame propagation between closely spaced parallel plates is the simplest configuration where multidimensional effects are present. This configuration is sometimes employed by experimentails to better observe the characteristic flame instabilities that appear during the flame propagation which is usually difficult to analyze in 3D geometries. The proposed configuration is known as Hele-Shaw cell, which employs two transparent plates to facilitate the visualization of the flame instabilities. In the images below we show an example of the different flame wrinklings and cell structures found by different authors and for different fuel-air mixtures
In particular, our Group studies the effects of the thermal expansion, the differential diffusion, the buoyancy, the viscosity contrast and the heat losses on the dynamics and the stability of those flames. A two-dimensional/quasi-2D model has been recently proposed to simplify this study, assuming Darcy's law for the flow field. The figure below (right) shows the isocontour of temperature calculated for a very diffusive flame front (Lewis number of 0.3), typically found in lean hydrogen-air mixtures. Note the characteristics arrow-shaped cells also found in recent experiments (left). The image is courtesy of the group of Prof. Paul D. Ronney at CPL laboratory, University of Southern Californa. The numerical model shows good qualitative results when compared with experiments.
Flame acceleration in narrow channels
In long narrow channels opened at both ends, the flame self-acceleration arises from the combined effects of the gas expansion and the lateral confinement. Due to the frictional forces at the walls and, since the pressure at both ends is maintained constant, the gas motion that develops in the burnt gas sets a pressure gradient that also drives the fresh unburnt gas towards the other end of the channel. In these circunstances the flame front suffers from an additional stretching that accelerates the flame reaching velocities that are ten-to-twenty times larger than the laminar flame speed. The figure below depicts the flame stretching and the flow field of a flame propagating in a channel of height 10 times the thermal flame thickness.
