Naoya Fukushima

Effect of flow-geometry on turbulence-scalar interaction in premixed flames

Turbulent combustion of stoichiometric hydrogen-air mixture is simulated using direct numerical simulation methodology, employing complex chemical kinetics. Two flame configurations, freely propagating and V-flames stabilized behind a hot rod, are simulated. The results are analyzed to study the influence of flame configuration on the turbulence-scalar interaction, which is critical for the scalar gradient generation processes. The result suggests that this interaction process is not influenced by the flame configuration and the flame normal is found to align with the most extensive strain in the region of intense heat release. The combustion in the rod stabilized flame is found to be flamelet like in an average sense and the growth of flame-brush thickness with the downstream distance is represented well by Taylor theory of turbulent diffusion, when the flame-brushes are non-interacting. The thickness is observed to saturate when the flame-brushes interact, which is found to occur in the simulated rod st...

Statistical properties of the local structure of homogeneous isotropic turbulence and turbulent channel flows

Statistical properties of the local structure are investigated using the data of direct numerical simulation of homogeneous isotropic turbulence for Re λ=60.1∼287.6 and turbulent channel flows for Re τ = 180, 400, 800, and 1270. The dimensionless second invariant is used as the representative local structure, and is defined as the second invariant of a velocity gradient tensor normalized by the magnitude of the velocity gradient tensor. In homogeneous isotropic turbulence, the probability density function (pdf) profiles of the dimensionless second invariant show good agreement for different values of Reynolds number. The volume fraction, the average, and the variance of the dimensionless second invariant converge to certain values at high Reynolds number, respectively. For the turbulent channel flows, the pdf profiles of the dimensionless second invariant for different values of Reynolds number coincide very well for the distance from the wall in wall units. The probability density and the volume fraction of the dimensionless second invariant at the center of the channel are in good agreement with those of homogeneous isotropic turbulence.

Bioelectrochemical analysis of a hyperthermophilic microbial fuel cell generating electricity at temperatures above 80 °C.

We examined whether a hyperthermophilic microbial fuel cell (MFC) would be technically feasible. Two-chamber MFC reactors were inoculated with subsurface microorganisms indigenous to formation water from a petroleum reservoir and were started up at operating temperature 80 °C. The MFC generated a maximum current of 1.3 mA 45 h after the inoculation. Performance of the MFC improved with an increase in the operating temperature; the best performance was achieved at 95 °C with the maximum power density of 165 mWm−2, which was approximately fourfold higher than that at 75 °C. Thus, to our knowledge, our study is the first to demonstrate generation of electricity in a hyperthermophilic MFC (operating temperature as high as 95 °C). Scanning electron microscopy showed that filamentous microbial cells were attached on the anode surface. The anodic microbial consortium showed limited phylogenetic diversity and primarily consisted of hyperthermophilic bacteria closely related to Caldanaerobacter subterraneus and Thermodesulfobacterium commune. Hyperthermophilic microorganisms were for the first time proved to be capable of transferring electrons to anode and acting biocatalysts in anodes of microbial fuel cells.

A Fractal Dynamic SGS Combustion Model for Large Eddy Simulation of Turbulent Premixed Flames

ABSTRACT Direct numerical simulation of a turbulent hydrogen-air premixed plane jet flame is performed to investigate fractal characteristics and to evaluate the fractal dynamic subgrid scale (FDSGS) combustion model. The DNS results show that the fractal dimension of flame surfaces increases with the downstream distance, and the fractal dimension computed using a 3D box-counting method reaches about 2.54 in the region where turbulence is developed by mean shear. An inner cutoff representation employed in the FDSGS combustion model could be used in large eddy simulations (LES) of complicated combustion problems. Static tests show that the procedure applied in the FDSGS combustion model adequately predicts the fractal dimension, Kolmogorov length scale, and flame surface area despite the presence of strong mean shear. Dynamic model evaluations are also carried out by conducting a series of LES using the FDSGS and other combustion models. In the dynamic tests, the mean temperature distributions and peak positions of variations of a reaction progress variable fluctuation in the transverse direction obtained from the LES with the FDSGS combustion model show good agreement with the filtered DNS fields. The present evaluation also revealed that one of the strengths in the FDSGS combustion modeling approach is that the model does not require SGS turbulent velocity fluctuation, since modeling of this quantity is straightforward for neither homogeneous turbulence nor the turbulent shear flows.

Numerical study on dynamics of local flame elements in turbulent jet premixed flames

Direct numerical simulations (DNSs) of turbulent jet premixed flames have been conducted to investigate dynamics of local flame elements. Behaviors of turbulent jet flames are discussed by considering characteristics of local flame elements such as flame displacement speed, local turbulent burning velocity, curvature and tangential strain rate. The peak of probability density function (pdf) of the local turbulent burning velocity locates at laminar burning velocity, whereas the maximum turbulent burning velocity reaches to about 5 times laminar one and the minimum burning velocity is negative. The result indicates that dynamics of turbulent flame cannot be explained by the flamelet concept. The concept of flame stretch is evaluated by growth rate of the flame area, which is a function of flame displacement speed, flame curvature and tangential strain rate. In the concept of flame stretch, the time-averaged growth rate must be zero because flame front should stay in a certain area statistically. However, in DNS, the mean growth rate is not zero, which suggests that flame does not exist as a statistically steady state. The present results indicate a possibility of the existence of flame elements which cannot be explained by the flamelet concept and the concept of flame stretch.

Flow-Geometry and Reynolds-Number Effects on Flame-Turbulence Interactions

Three-dimensional direct numerical simulation (DNS) with a detailed kinetic mechanism has been conducted for statistically-planar turbulent flame and turbulent V-flame of hydrogen–air mixture to clarify the effects of mean flow velocity on principal strain rates at flame front and on flame geometry. Reynolds numbers based on Taylor micro scale and turbulent intensity are selected to 60.8 and 97.1, and mean flow velocities for V-flame are 10 and 20 times laminar burning velocity. From results of DNS, eigenvalues and eigenvectors of strain tensor are evaluated to investigate characteristics of strain field near flame and flame normal alignments with the principal axes of strain in detail. It has been revealed that Reynolds number affects both magnitude of strain rates and alignment between flame normal and principal axis of strain, and that the magnitude of mean flow velocity affects flame normal alignments in turbulent V-flame.Copyright

DNS of Turbulent Premixed Flames and Heat Transfer in a Constant Volume Vessel

Three-dimensional direct numerical simulation (DNS) of turbulent hydrogen-air premixed flames in a constant volume vessel at relatively high Reynolds numbers have been conducted considering detailed kinetic mechanism and temperature dependence of the transport and thermal properties. The flame behavior and heat transfer characteristics are investigated in the vessel. The flame is strongly affected by the growth of the internal pressure which is caused by the temperature rise in the vessel. Since the pressure increase makes the flame thickness thin, the heat release rate of each flame element is augmented. The local pressure rise due to the dilatation also enhances turbulence and finer scale vortices appear, which make the flame surface more complicated and result in an increase of the flame surface area. Due to the increase of the mean pressure in the vessel, the maximum wall heat flux induced by the flame front is enhanced during the combustion.Copyright

DNS on Autoignition and Flame Propagation of Inhomogeneous Methane–Air Mixtures in a Closed Vessel

Direct numerical simulations (DNSs) on autoignition and flame propagation of inhomogeneous methane–air mixtures in a closed vessel are conducted with considering detailed kinetic mechanism and temperature dependence of transport and thermal properties. The mixtures with spatial inhomogeneity of temperature or equivalence ratio are investigated. Periodic condition for non-heatloss cases or isothermal wall condition for heatloss cases is imposed on the boundaries. From the DNS results without heatloss, effects of spatial inhomogeneity of temperature and equivalence ratio on mean heat release rate are clarified. Increase of spatial variations of temperature or equivalence ratio suppresses drastic rise of mean heat release rate and reduces its maximum value. Autoignition process is affected by temperature more strongly than equivalence ratio. In the cases with heatloss, ignition delay increases and the maximum mean heat release rate decreases. After autoignition process, propagating flame is formed along walls. Heat transfer characteristics in a closed vessel are also discussed with combustion mechanisms.Copyright

Flame Brush Structure of H2-Air Turbulent Premixed Flames at High Reynolds Numbers

Three-dimensional direct numerical simulations (DNSs) of turbulent premixed planar, jet and V flames of hydrogen-air mixture have been conducted to investigate the flame brush and the local flame structures at high Reynolds number turbulences. The detail kinetic mechanism including 12 reactive species and 27 elementary reactions was used to represent the hydrogen-air reaction. For planar flame, flame front is highly fluctuating, and multi-layer structure, multiply-folded flame front and unburned mixture island which lead to corresponding increase of the flame brush thickness can be observed. The flame brush thickness of the planar flame is relatively uniform along the flame front, and is about 2∼3 times the integral length scale (l), which is defined from an energy spectrum. For the jet and V flames, the flame brush thicknesses grow with the streamwise direction from about 0.5∼1 times the integral length scale (l) to about 2∼3 times the integral length scale (l) due to the highly fluctuating flame front at the downstream region.Copyright

DNS of Flame-Wall Interaction and Heat Transfer in a Constant Volume Vessel

Direct numerical simulations of turbulent hydrogen/air and methane/air premixed flames in a rectangular constant volume vessel have been conducted with considering detailed kinetic mechanism to investigate flame behaviors and heat losses. For the hydrogen cases, since heat release rate increases with pressure rise due to dilatation during combustion in the constant vessel, heat flux on a wall also increases. For the methane cases, the pressure increase does not raise wall heat flux significantly because of the decrescence of heat release rate caused by thermo-chemical reaction near a wall. Pressure waves caused by wall reflection fluctuate flame propagation for the hydrogen flames. Flame displacement speed decreases remarkably at the moment when the pressure wave passes through flame fronts from unburnt side to burnt side. However, the turbulent burning velocity at that time does not decrease because of increases of fluid velocity normal to the flame fronts.Copyright