Fernando F. Fachini

Bio: Fernando F. Fachini is an academic researcher from National Institute for Space Research. The author has contributed to research in topics: Diffusion flame & Combustion. The author has an hindex of 7, co-authored 39 publications receiving 205 citations.

Asymptotic analysis of stationary adiabatic premixed flames in porous inert media

Abstract: The structure of adiabatic premixed flames within porous inert media is investigated using the asymptotic expansion method. For this, the flame structure is divided into three characteristic length scales. The two innermost length scales, the gas-phase diffusion length scale and the reaction length scale, are the same scales defined in the classical premixed flame structure analysis. The outermost length scale, the solid-phase diffusion length scale, is related to the heat conduction in the porous matrix. The differences among these three characteristic length-scales result in large temperature differences between the phases and justify the application of asymptotic expansions to determine an approximate (analytical) solution. Since the main focus of this work is the examination of the processes in the outer and the first inner regions, the simplest kinetic mechanism of one global step is adopted to represent the fuel and oxygen consumption. Then, the description of the reaction zone is obtained using the large activation energy asymptotic method. The description of the problem of the order of the gas-phase length scale is obtained using the boundary layer expansion. This work evaluates the influence of the equivalence ratio, the ratio of the solid to the gas thermal conductivities, the porosity of the medium and the fuel Lewis number on such flames. A parameter that universalizes the flame properties is then identified and discussed.

Theoretical analysis of ultra-lean premixed flames in porous inert media

Abstract: The structure of stationary adiabatic premixed flames within porous inert media under intense interphase heat transfer is investigated using the asymptotic expansion method. For the pore sizes of interest for combustion in porous inert media, this condition is reached for extremely lean mixtures where lower flame velocities are found. The flame structure is analysed in three distinct regions. In the outer region (the solid-phase diffusion length scale), both phases are in local thermal equilibrium and the problem formulation is reduced to the one-equation model for the energy conservation. In the first inner region (the gas-phase diffusion length scale), there is local thermal non-equilibrium and two equations for the energy conservation are required. In this region, the gas-phase temperature at the flame is limited by the interphase heat transfer. In the second inner region (the reaction length scale), the chemical reaction occurs in a very thin zone where the highest gas-phase temperature is found. The results showed that superadiabatic effects are reduced for leaner mixtures, smaller pore sizes and smaller fuel Lewis numbers. The results also show that there is a minimum superadiabatic temperature for the flame propagation to be possible, which corresponds to the lean flammability limit for the premixed combustion in porous inert media. A parameter that universalizes the leading-order flame properties is identified and discussed.

Theory of Flame Histories in Droplet Combustion at Small Stoichiometric Fuel-Air Ratios

Abstract: Motivated by observations on the spherically symmetrical burning of single, free heptane droplets in quiescent helium-oxygen atmospheres, made in Spacelab aboard the Space Shuttle, the theory of droplet burning is extended for conditions under which the Burke-Schumann flame sheet lies in the outer transient zone. Account is taken of Lewis numbers different from unity and of radiant energy loss that sometimes leads to extinction. The asymptotic analysis employs the square root of the ratio of the gas to liquid density as a small parameter. In suitably scaled outer variables, the droplet then appears as a time-varying point source, and the flame radius first grows then decreases in size. Numerical integrations of the outer parabolic partial differential equations provide flame histories and histories of the outer temperature and concentration fields. An analytical criterion for flame extinction enables the extinction point along the flame history to be determined, when extinction is caused by radiative energy loss. The theoretical results can be useful for comparison with experiment and for predicting whether radiative extinction will occur in droplet combustion.

Maximum superadiabatic temperature for stabilized flames within porous inert media

Abstract: This work analyzes the superadiabatic temperature for laminar stationary lean premixed flames within porous inert media. The analysis is based on the excess enthalpy function applied to the one-dimensional volume-averaged equations. This formulation, with results obtained in a previous work, allows for the construction of an analytical solution valid over a large range of equivalence ratios. The model reveals the existence of a maximum non-dimensional superadiabatic temperature at a precisely determined equivalence ratio and connects previous works for near-stoichiometric and ultra-lean mixtures.

Transient effects in droplet ignition phenomenon

Abstract: We discuss the transient effects of mass and energy accumulation processes and of an applied acoustic field interaction on the droplet ignition phenomenon. In order to observe simultaneously the influence of those processes on the ignition, we assume that the chemical reaction occurs in the droplet unsteady far field, where the transient accumulation processes and the acoustic field are as important as the other processes (i.e., the conduction process and the chemical reaction). We also consider the Lewis number as being constant and nonunity. The results show that the ignition time is modified by the acoustic field.

Boundary Layer Theory

Abstract: The boundary layer equations for plane, incompressible, and steady flow are $$\matrix{ {u{{\partial u} \over {\partial x}} + v{{\partial u} \over {\partial y}} = - {1 \over \varrho }{{\partial p} \over {\partial x}} + v{{{\partial ^2}u} \over {\partial {y^2}}},} \cr {0 = {{\partial p} \over {\partial y}},} \cr {{{\partial u} \over {\partial x}} + {{\partial v} \over {\partial y}} = 0.} \cr }$$

Progress in knowledge of flamelet structure and extinction

Abstract: In the past 25 years there has been a considerable amount of progress in studying flamelets, their structures and their responses to various perturbations. The term “flamelet” as used here really would mean “laminar flame” to most readers and is employed only because a major motivation is for ultimate use in connection with more complex flows, mainly turbulent. There is, however, no consideration here of how the knowledge reviewed may be employed in flamelet modeling of turbulent combustion. Not even time-dependent flamelets are addressed, although a few related references are provided. The focus is narrower, namely on quasisteady flamelets, and therefore the considerations of flamelet extinction that are presented concern quasisteady extinction, that is, not the dynamics of extinction. Even in this narrow context, it will be seen that a great deal has been accomplished. When such a long-term view is taken, it is found remarkable how much progress has been made. The progress is addressed separately for premixed, nonpremixed and partially premixed systems. Suggested directions of future research also are indicated. Despite the limited scope of the topic and the extensive advancement that has occurred, much more research remains to be done.

Field dependent transition to the non-linear regime in magnetic hyperthermia experiments: Comparison between maghemite, copper, zinc, nickel and cobalt ferrite nanoparticles of similar sizes

Abstract: Further advances in magnetic hyperthermia might be limited by biological constraints, such as using sufficiently low frequencies and low field amplitudes to inhibit harmful eddy currents inside the patient's body. These incite the need to optimize the heating efficiency of the nanoparticles, referred to as the specific absorption rate (SAR). Among the several properties currently under research, one of particular importance is the transition from the linear to the non-linear regime that takes place as the field amplitude is increased, an aspect where the magnetic anisotropy is expected to play a fundamental role. In this paper we investigate the heating properties of cobalt ferrite and maghemite nanoparticles under the influence of a 500 kHz sinusoidal magnetic field with varying amplitude, up to 134 Oe. The particles were characterized by TEM, XRD, FMR and VSM, from which most relevant morphological, structural and magnetic properties were inferred. Both materials have similar size distributions and satu...

Microgravity n-Heptane Droplet Combustion in Oxygen-Helium Mixtures at Atmospheric Pressure

Abstract: Results are presented from experiments on the combustion of freely floated n-heptane droplets in helium-oxygen environments conducted in Spacelab onboard the Space Shuttle Columbia during the first launch (STS-83) of the Microgravity Science Laboratory mission in April 1997. During this shortened flight, a total of eight droplets were burned successfully in nominally 300 K oxygen-helium atmospheres having oxygen mole fractions of 25, 30, and 35% at a total pressure of 1 atm. Initial droplet sizes ranged from about 2 to 4 mm. The results demonstrated both radiative and diffusive flame extinction during burning, whereas droplet surface regression followed the d-square law. The full range of possible droplet-burning behaviors was thus observed. The results provide information for testing future theoretical and computational predictions of burning rates, soot and flame characteristics, and extinction conditions.

Adiabatic premixed combustion in a gaseous fuel porous inert media under high pressure and temperature: Novel flame stabilization technique

Abstract: This work presents an experimental investigation to study the characteristics of combustion using a premixed methane–air mixture within a non-homogeneous porous inert medium (PIM) under high pressure and temperature. In order to obtain a stable flame under these operating conditions within PIM, a novel flame stabilization technique in porous inert media (PIM) combustion under high pressure and temperature has been developed and evaluated. The proposed technique avoids the draw backs of the hitherto developed techniques by properly matching the flow and flame speeds and, consequently, ensuring a stable combustion, for a wide range of operating pressure and temperature. The success of this technique permits the extension of PIM combustion to new applications such as gas turbines. The validity of this new technique has been assessed experimentally in detail by analyzing combustion inside a prototype burner. The effects of various operating conditions, such as initial preheating temperature and elevated pressure, have been examined for an output power range between 5 and 40 kW. The experiments covered a broad spectrum of operating conditions ranging from a mixture inlet temperature of 20 °C and pressure ratio of 1 up to a temperature of 400 °C and a pressure ratio of 9. Evaluation of the results revealed excellent flame stability with respect to both flashback and blow-out limits throughout all the operating conditions studied, including relative air ratios far beyond the normal lean limit. While the blow-out stability showed no significant dependence on pressure, it was strongly determined by the preheating mixture inlet temperature. A remarkable broadening of the stability range from 0.6 to 1.0 on preheating to 400 °C was observed. This reveals the potential of pre-heat temperature to improve the dynamic modularity of the burner.