The Hydrogen Laminar Jet

Numerical and asymptotic methods are used to investigate the structure of the hydrogen jet discharging into a quiescent air atmosphere. The analysis accounts in particular for the variation of the density and transport properties with composition. The Reynolds number of the flow "Rj", based on the initial jet radius "a", the density and viscosity of the jet and the characteristic jet velocity, is assumed to take moderately large values, so that the jet remains slender and stable, and can be correspondingly described by numerical integration of the continuity, momentum and species conservation equations written in the boundary-layer approximation. The solution for the velocity and composition in the jet-development region of planar and round jets, corresponding to streamwise distances of order "Rj a", is computed numerically, along with the solutions that emerge both in the near field and in the far field. The small value of the hydrogen-to-airmolecular weight ratio is used to simplify the solution by considering the asymptotic limit of vanishing jet density. The development provides at leading order explicit analytical expressions for the far-field velocity and hydrogen mass fraction that describe accurately the hydrogen jet near the axis. The information provided can be useful in particular to characterize hydrogen discharge processes from holes and cracks.This work was supported by the SpanishMICINN under Project # ENE2008-06515-C04 and by the Comunidad de Madrid under Project S2009/ENE-1597 (HYSYCOMB)Publicad

Y yo quiero ser...ingeniero en fluidomecánica

Un ingeniero en Fluidomecánica se encarga de estudiar y aprovechar, en beneficio propio, el movimiento de los fluidos, las fuerzas inducidas por ese desplazamiento y el intercambio de energía asociado a él. La rama de la física que fundamenta esa rama de la ingeniería es la Mecánica de Fluidos y engloba tanto a líquidos como a gases

Laminar gas jets in high-temperature atmospheres

Numerical and asymptotic methods are used to describe the structure of low-temperature laminar gas jets discharging into a hot atmosphere of the same gas in the limit of small jet-to-ambient temperature ratios " = Tj/To 1. In the limit " ! 0, heat conduction cannot modify significantly the temperature in the cold gas, leading to a two-region flow structure consisting of a neatly defined unperturbed cold jet for r < rf (x), where T = T0/To = " and u = U0/Uj = 1, surrounded by a hot gas. These two region are separated by a transition layer where T − " " and 1 − u 1. In planar jets the front thickens with distance achieving thickness of order unity at axial distances x "−(1+)Rea forcing the near-axis fluid to change slowly its velocity and temperature, being necessary distances x "−2Rea to reach values of T and 1−u of order unity. In round jets the front remains at r = a up to distances x "−(1−)(log "−1)2 where the front is forced to move radially towards the axis of the jet, reaching r = 0 at x "−1 log "−1Rea. The arrival of the front forces the change of the velocity and temperature in the near-axis region, reaching values of order unity in a far field region of characteristic length x "−1 log "−1Rea, distance comparable to that needed by the front to achieve the axis. In both geometries, the distance necessary for the fully development of the cold jet is considerably longer than that required by the isothermal jet x Rea

Fronts in high-temperature laminar gas jets

This paper addresses the slender laminar flow resulting from the discharge of a low-Mach-number hot gas jet of radius a and moderately large Reynolds number Rj into a cold atmosphere of the same gas. We give the boundary-layer solution for plane and round jets with very small values of the ambient-to-jet temperature ratio ε accounting for the temperature dependence of the viscosity and conductivity typical of real gases. It is seen that the leading-order description of the jet in the limit ε → 0 exhibits a front-like structure, including a precisely defined separating boundary at which heat conduction and viscous shear stresses vanish in the first approximation, so that the temperature and axial velocity remain unperturbed outside the jet. Separate analyses are given for the jet discharging into a stagnant atmosphere, when the jet boundary is a conductive front, and for the jet discharging into a coflowing stream, when the jet boundary appears as a contact surface. We provide in particular the numerical description of the jet development region corresponding to axial distances of order Rja for buoyant and non-buoyant jets, as well as the self-similar solutions that emerge both in the near field and in the far field. In all cases considered, comparisons with numerical integrations of the boundary-layer problem for moderately small values of ε indicate that these front descriptions give excellent predictions for the temperature and velocity fields in the near-axis region

Variable-density jet flows induced by concentrated sources of momentum and energy

The planar and axisymmetric variable-density flows induced in a quiescent gas by a concentrated source of momentum that is simultaneously either a source or a sink of energy are investigated for application to the description of the velocity and temperature far fields in laminar gaseous jets with either large or small values of the initial jet-to-ambient temperature ratio. The source fluxes of momentum and heat are used to construct the characteristic scales of velocity and length in the region where the density differences are of the order of the ambient density, which is slender for the large values of the Reynolds number considered herein. The problem reduces to the integration of the dimensionless boundary-layer conservation equations, giving a solution that depends on the gas transport properties but is otherwise free of parameters. The boundary conditions at the jet exit for integration are obtained by analysing the self-similar flow that appears near the heat source in planar and axisymmetric configurations and also near the heat sink in the planar case. Numerical integrations of the boundary-layer equations with these conditions give solutions that describe accurately the velocity and temperature fields of very hot planar and round jets and also of very cold plane jets in the far field region where the density and temperature differences are comparable to the ambient values. Simple scaling arguments indicate that the point source description does not apply, however, to cold round jets, whose far field region is not large compared with the jet development region, as verified by numerical integrations.This collaborative research was supported by the Spanish MICINN under Project # ENE2008-06515-C04 and by the Comunidad de Madrid under Project# S2009/ENE-1597 (HYSYCOMB

Regimes of boundary-layer ignition by heat release from a localized energy source

This paper investigates the initiation of a deflagration in a premixed boundary-layer stream by continuous heat deposition from a line energy source placed perpendicular to the flow on the wall surface, a planar flow configuration relevant for small-scale combustion applications, including portable rotary engines. Ignition is investigated in the constant density approximation with a one-step irreversible reaction with large activation energy adopted for the chemistry description. The ratio of the characteristic strain time, given by the inverse of the wall velocity gradient, to the characteristic deflagration residence time defines the relevant controlling Damkhler number D. The time-dependent evolution following the activation of the heat source is obtained by numerical integration of the energy and fuel conservation equations. For sufficiently small values of D, the solution evolves towards a steady flow in which the chemical reaction remains confined to a finite nearsource reactive kernel. This becomes increasingly slender for increasing values of D, corresponding to smaller near-wall velocities, until a critical value D(c)1 is reached at which the confined kernel is replaced by a steady anchored deflagration, assisted by the source heating rate, which develops indefinitely downstream. As the boundary-layer velocity gradient is further decreased, a second critical Damkhler number D-c2 > D-c1 is reached at which the energy deposition results in a flashback deflagration propagating upstream against the incoming flow along the base of the boundary layer. The computations investigate the dependence of D-c1 and D-c2 on the fuel diffusivity and the dependence of D(c 1)on the source heating rate, delineating the boundaries that define the relevant regime diagram for these combustion systems.This work was supported by the Spanish MCINN through projects #CSD2010-00011, ENE2012-33213 and ENE2015-65852-C2-1-R

An experimental and numerical study of flames in narrow channels with electric fields

The proceeding at: 14th International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS 2014). Took at 2014, November, 18-21, in Awaji Island, Hyogo Japan. The event Web site in: http://powermems2014.org/The advancement of microscale combustion has been limited by quenching effects as flames cease to be much smaller than combustors. The long studied sensitivity of flames to electrical effects may provide means to overcome this issue. Here we experimentally and numerically investigate the potential of electric field effects to enhance combustion. The results demonstrate that, under specific conditions, externally electric fields will sustain combustion in structures smaller than the quenching distance. The analysis proposes a reduced mechanism to model this result and provides a study of the governing parameters. We find good qualitative agreement between the model and experiments. Specifically, the model is found to successfully capture the capacity to increase and decrease flame speed according to electric field magnitude and direction. Further, in both experiments and computations the sensitivity to electrical enhancement increases for more energetic mixtures. We do find that the model underpredicts the maximum achievable speed enhancement observed, suggesting that additional phenomena should be included to expand the range of conditions that can be studied.Publicad

Effect of an external electric field on the propagation velocity of premixed flames

There have been many experimental investigations into the ability of electric fields to enhance combustion by acting upon ion species present in flames. In this work, we examine this phenomenon using a one-dimensional model of a lean premixed flame under the influence of a longitudinal electric field. We expand upon prior two-step chain-branching reaction laminar models with reactions to model the creation and consumption of both a positively-charged radical species and free electrons. Also included are the electromotive force in the conservation equation for ion species and the electrostatic form of the Maxwell equations in order to resolve ion transport by externally applied and internally induced electric fields. The numerical solution of these equations allows us to compute changes in flame speed due to electric fields. Further, the variation of key kinetic and transport parameters modifies the electrical sensitivity of the flame. From changes in flame speed and reactant profiles we are able to gain novel, valuable insight into how and why combustion can be controlled by electric fields.This collaborative research was supported by the Spanish MCINN under Project #ENE2012–33213 and by King Abdullah University of Science and Technology (KAUST), Cooperative Agreement # 025478 entitled, electromagnetically Enhanced Combustion: Electric Flames.Publicad

The role of non-thermal electrons in flame acceleration

We examine in this work the effect of an external electric field on the propagation velocity of a laminar, one-dimensional and lean premixed flame, with the final goal of clarifying the relative importance of each of the three different mechanisms postulated in the literature to explain the effect of electric fields on flames: ionic wind, kinetic enhancement by non-thermal electrons and ohmic heating. The onedimensional model proposed here expands the four-reactions scheme previously presented by SanchezSanz, et al. (2015) to include the effect of non-thermal electrons and activated neutral molecules on flame acceleration. Two additional reactions are included in the model to complete a minimum set of six elementary reaction capable of qualitatively reproduce the results observed in classical Uaggers, and Von Engel, (1971).) and recent (Volkov et al., 2013; Murphy, et al., 2014,) experiments. The limit of weakly ionized plasmas is used to integrate the Boltzmann equation and to derive an explicit expression for the electron temperature proportional to the square of the electric field. The numerical integration of the conservation equations gives the flame propagation velocity for a given set of parameters. The results reveal the importance of the electric field polarity on flame acceleration, finding faster flames for positive electric fields than for equally intense negative fields. At low-intensity fields, our results indicate that the ionic wind, and the associated redistribution of the charged particles, is the main mechanism inducing flame acceleration. In more intense fields, the combined effect of the ionic wind and the heat transfer from the high-temperature electrons to the background gas induces a significant increase in the temperature field upstream and downstream of the flame front.This work was supported by the Spanish MCINN through projects ENE2012-33213 and ENE2015-65852-C2-1-R

A framework for compliant physical interaction : the grasp meets the task

Although the grasp-task interplay in our daily life is unquestionable, very little research has addressed this problem in robotics. In order to fill the gap between the grasp and the task, we adopt the most successful approaches to grasp and task specification, and extend them with additional elements that allow to define a grasp-task link. We propose a global sensor-based framework for the specification and robust control of physical interaction tasks, where the grasp and the task are jointly considered on the basis of the task frame formalism and the knowledge-based approach to grasping. A physical interaction task planner is also presented, based on the new concept of task-oriented hand pre-shapes. The planner focuses on manipulation of articulated parts in home environments, and is able to specify automatically all the elements of a physical interaction task required by the proposed framework. Finally, several applications are described, showing the versatility of the proposed approach, and its suitability for the fast implementation of robust physical interaction tasks in very different robotic systems