Theoretical analysis of the condensation of combustion products in thin gaseous layers

In this paper, a theoretical analysis of the condensation of combustion products in narrow gaps between planar plates is performed. The investigation is motivated by the empirical results shown by Veiga-López [“Flame propagation in narrow channels,” Ph.D. thesis (Carlos III University of Madrid, 2020)] and the lack of a theoretical description directly applicable to them. In these experiments, he describes how discontinuous condensed water films appeared on the walls of the combustion chamber, forming dry/wet stripes parallel to the flame front at the products region. The formulation developed here is derived from a general approach for condensation, which is simplified considering the conditions of high-temperature combustion products. Notably, the liquid phase disappears from the system of equations, which exclusively contains the gaseous phase. The expressions resulting are analytical, simple, and easy to interpret. They allow us to understand qualitatively the effects of the main physical phenomena of the process, which is described by the interaction between heat exchange, mass transfer, the thermodynamic conditions, and the velocity of the combustion products. The construct is subsequently utilized to perform the numerical parametric studies, to analyze the influence of two main parameters of the problem: gap thickness and flame velocity. Despite the relative simplicity of the model, it predicts similar condensation–vaporization–condensation cycles to those observed at the laboratory

Characterization of unconventional hydrogen flame propagation in narrow gaps

The physical limits of the unconventional flame propagation regimes recently discovered [Veiga-Lopez, Phys. Rev. Lett. 124, 174501 (2020)PRLTAO0031-900710.1103/PhysRevLett.124.174501] are analyzed. These regimes appear in combustible gaseous mixtures approaching the lean quenching limit of hydrogen-air flames in narrow gaps. They are characterized by a split of the flame front into a dendritic and a bifurcating set of flame cells separated by nonburned material. A feature selection analysis utilizing dimensionless numbers is applied to reveal the most significant parameters governing the separation between unconventional and traditional flame propagation regimes. It is concluded that (a) the outbreak of unconventional propagation is mostly due to heat losses, (b) the phenomenon is governed by the Peclet number and only appears in thin channels, and (c) the Lewis number does not determine the propagation regime. Additionally, an equation describing the optimal border of the unconventional regime is derived from experiments

Thermoacoustic analysis of lean premixed hydrogen flames in narrow vertical channels

Thermoacoustic instabilities arise for lean hydrogen-air flames propagating in narrow channels. We provide here a detailed experimental analysis of such phenomena in a semi-confined vessel, analyzing the effect of the mixture composition, geometry and gravity on the onset of acoustic-driven flame vibrations. Downward-propagating flames leaner than a critical value vibrate smoothly and transit to the secondary oscillating instability, which develops strong variations of pressure that couple with the propagation dynamics. The transition threshold changes during the propagation along very narrow channels, where heat losses are no longer negligible. The parametric region of equivalence ratio for the secondary thermoacoustic instability diminishes, showing an additional transition for very lean flames. There, the front breaks into several structures and the flame-wave feedback becomes weaker. The influence of gravity is studied by comparing upward and downward propagating flames, where the Rayleigh–Taylor instability arises for sufficiently small values of the Froude number in slow-propagating lean flames. For a constant mixture, buoyancy-driven upward-propagating flames develop less wrinkled fronts than those propagating downwards, and remain unresponsive to acoustic-front interaction. We show here a direct relation between front shape and thermoacoustics. In agreement with previous studies [1], [2], [3], curvature and strain effects on conduction and diffusion characterize the response of the flame to pressure perturbations, with the Markstein number controlling the aforementioned transition. Nevertheless, the theoretical analyses found in the literature can only be used on nearly equidiffusional mixtures, and are not accurate enough to describe the highly diffusive fuel mixtures (i.e. lean hydrogen-air flames) considered in our experiments.This work was supported by projects ENE2015-65852-C2-1-R (MINECO/ FEDER, UE), BYNV-ua37crdy (Fundación Iberdrola España) and KIT. The authors want to thank the technical support of ProScience GmbH. in the construction and operation of the experimental setup. D. Martnez-Ruiz would like to acknowledge F. Higuera for fruitful discussions.Publicad

A benchmarking of commercial small fixed-wing electric UAVs and RGB cameras for photogrammetry monitoring in intertidal multi-regions

Small fixed-wing electric Unmanned Aerial Vehicles (UAVs) are perfect candidates to perform tasks in wide areas, such as photogrammetry, surveillance, monitoring, or search and rescue, among others. They are easy to transport and assemble, have much greater range and autonomy, and reach higher speeds than rotatory-wing UAVs. Aiming to contribute towards their future implementation, the objective of this article is to benchmark commercial, small, fixed-wing, electric UAVs and compatible RGB cameras to find the best combination for photogrammetry and data acquisition of mussel seeds and goose barnacles in a multi-region intertidal zone of the south coast of Galicia (NW of Spain). To compare all the options, a Coverage Path Planning (CPP) algorithm enhanced for fixed-wing UAVs to cover long areas with sharp corners was posed, followed by a Traveling Salesman Problem (TSP) to find the best route between regions. Results show that two options stand out from the rest: the Delair DT26 Open Payload with a PhaseOne iXM-100 camera (shortest path, minimum number of pictures and turns) and the Heliplane LRS 340 PRO with the Sony Alpha 7R IV sensor, finishing the task in the minimum time.Agencia Estatal de Investigación | Ref. PID2021-125060OB-I00Agencia Estatal de Investigación | Ref. TED2021-129756B-C31Ministerio de Universidades | Ref. FPU21/01176Universidade de Vig

Una nueva escuela. Un nuevo modelo de dirección

Following proposals and principles are reflected in the Organic Law 8/2013 to improve educational quality (LOMCE), we find in Article 122, with a basic idea that we can consider when looking for significant changes and the pursuit of quality improvement in schools. Shares of educational quality will leave a comprehensive consideration of the center, which can take as reference models recognized at European level management, and must contain all the necessary for the realization of an educational project quality tools.Siguiendo propuestas y principios que encontramos reflejados en la Ley Orgánica 8/2013 para la mejora de la calidad educativa (LOMCE), nos encontramos en su artículo 122, con una idea que podemos considerar básica  a la hora de la búsqueda de cambios significativos  y de la procura de la mejora de la calidad en los centros educativos. Las acciones de calidad educativa partirán de una consideración integral del centro, que podrá tomar como referencia modelos de gestión reconocidos en el ámbito europeo, y habrán de contener la totalidad de las herramientas necesarias para la realización de un proyecto educativo de calidad

Experimental analysis of oscillatory premixed flames in a Hele-Shaw cell propagating towards a closed end

An experimental study of methane, propane and dimethyl ether (DME) premixed flames propagating in a quasi-two-dimensional Hele-Shaw cell placed horizontally is presented in this paper. The flames are ignited at the open end of the combustion chamber and propagate towards the closed end. Our experiments revealed two distinct propagation regimes depending on the equivalence ratio of the mixture as a consequence of the coupling between the heat-release rate and the acoustic waves. The primary acoustic instability induces a small-amplitude, of around 8 mm, oscillatory motion across the chamber that is observed for lean propane, lean DME, and rich methane flames. Eventually, a secondary acoustic instability emerges for sufficiently rich (lean) propane and DME (methane) flames, inducing large-amplitude oscillations in the direction of propagation of the flame. The amplitude of these oscillations can be as large as 30 mm and drastically changes the outline of the flame. The front then forms pulsating finger-shaped structures that characterize the flame propagation under the secondary acoustic instability. The experimental setup allows the recording of the flame propagation from two different points of view. The top view is used to obtain accurate quantitative information about the flame propagation, while the lateral view offered a novel three dimensional perspective of the flame that gives relevant information on the transition between the two oscillatory regimes. The influence of the geometry of the Hele-Shaw cell and of the equivalence ratio on the transition between the two acoustic-instability regimes is analyzed. In particular, we find that the transition to the secondary instability occurs for values of the equivalence ratio phi above (below) a critical value phi(c) for propane and DME (methane) flames. In all the tested fuels, the transition to the secondary instability emerges for values of the Markstein number M below a critical value M-c. The critical MarkstPublicad

The role of conductive heat losses on the formation of isolated flame cells in Hele-Shaw chambers

The propagation of low-Lewis-number premixed flames is analyzed in a partially confined Hele-Shaw chamber formed by two parallel plates separated a distance h apart. An asymptotic-numerical study can be performed for small gaps compared to the flame thickness deltaT . In this narrow-channel limit, the prob- lem formulation simplifies to a quasi-2D description in which the velocity field is controlled by domi- nant viscous effects. After accounting for conductive heat losses through the plates in our formulation, we found that the reaction front breaks into one or several isolated flame cells where the temperature is large enough to sustain the reaction, both in absence and in presence of buoyancy effects. Under these near-limit conditions, the isolated flame cells either travel steadily or undergo a slow random walk over the chamber in which the reacting front splits successively to form a tree-like pathway, burning only a small fraction of the fuel before reaching the end of the chamber. The production of quasi-2D circular or comet-like flames under specific favorable conditions is demonstrated in this paper, with convection, conductive heat losses and differential diffusion playing an essential role in the formation of the isolated one and two-headed flame cells.This work was supported by the project ENE2015-65852-C2-1-R (FV,MSS,DMR) and ENE2015-65852-C2-2-R (DFG,VK) (MINECO/FEDER, UE). Daniel Martínez-Ruiz would like to thank Amable Liñán for fruitful discussions

On-Line Monitoring of Blind Fastener Installation Process

Blind fasteners are of special interest for aircraft construction since they allow working on joints where only one side is accessible, as is the case in many common aerospace box-type structures, such as stabilizers and flaps. This paper aims to deliver an online monitoring method for the detection of incorrect installed blind fasteners. In this type of fastener, the back side of the assembly is not accessible, so monitoring the process installation is suitable as a system to assess the formed head at the back side with no access. The solution proposed consists of an on-line monitoring system that is based on sensor signals acquired during the installation. The signals are conveniently analyzed in order to provide an evaluation outcome on how the fastener was installed. This new method will help production to decrease/eliminate time and cost-intensive inspections and fasteners over installation in structures. The decrease of the number of installed fasteners will also contribute to weight savings and will reduce the use of resources.This research is part of the BLINDFAST: INNOVATIVE BLIND FASTENER MONITORING TECHNOLOGY FOR QUALITY CONTROL project, which has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement 686827

Suppression of thermoacoustic instabilities by flame-structure interaction

We present here an experimental study of the influence of the aeroelastic coupling between the combustion chamber walls and the acoustic fluid field on the onset and development of thermoacoustic instabilitiesin stoichiometric propane-air premixed flames. A horizontal quasi-two-dimensional Hele-Shaw chamber formed by two parallel plates separated a small distance h is used. The flames are ignited at the open end, in contact with the atmosphere, and propagate towards the opposite closed end. The experiments reveal three distinct propagation regimes determined by the stiffness of the plates and the evolution of the pressure perturbation generated during ignition: (i) for sufficiently rigid plates, we observed secondary acoustic instabilities with large amplitude oscillations in the direction of propagation of the flame; for flexible enough walls to be compliant with ignition-related pressure changes, (ii) the propagation of the flame undergoes small-amplitude oscillations (primary acoustic instabilities) along the channel or (iii) it is smooth with no oscillations whatsoever. The flexural rigidity of the plate is modified experimentally by changing both the widthW and thickness hw of the top plate of the Hele-Shaw cell. The data recorded by the pressure transducer and the accelerometer is used to plot a stability map in the W - hw parametric space to define the combination of structural parameters that triggers the onset of thermoacoustic instabilities. Our experimental measurements, supplemented with results from a theoretical analysis of the walls vibration modes, indicated that deformation-induced volume changes of around 0.1% of the volume of the Hele-Shaw cell are sufficient to suppress thermoacoustic instabilities.This work was funded by the Agencia Estatal de Investigación of Spain under grants PID2019-108592RA-C43 and PID2019-108592RB-C41, and by the Regional Government of Madrid (Comunidad de Madrid-Spain) under the Multiannual Agreement with UC3M (H2SFE-CM-UC3M). The authors wish to thank the technical knowledge and assistance of David Díaz, Israel Pina and Manuel Santos in the design, construction and operation of the experimental setup. The assistance of Rubén Palomeque in the preparation of figure 6 is greatly acknowledged. We would also like to acknowledge Raúl San Miguel and Ramón Zaera for providing the values of E and v of the PMMA plates. Funding for APC: Universidad Carlos III de Madrid Read & Publish Agreement CRUE-CSIC 2022

On-Line Monitoring of Blind Fastener Installation Process

Blind fasteners are of special interest for aircraft construction since they allow working on joints where only one side is accessible, as is the case in many common aerospace box-type structures, such as stabilizers and flaps. This paper aims to deliver an online monitoring method for the detection of incorrect installed blind fasteners. In this type of fastener, the back side of the assembly is not accessible, so monitoring the process installation is suitable as a system to assess the formed head at the back side with no access. The solution proposed consists of an on-line monitoring system that is based on sensor signals acquired during the installation. The signals are conveniently analyzed in order to provide an evaluation outcome on how the fastener was installed. This new method will help production to decrease/eliminate time and cost-intensive inspections and fasteners over installation in structures. The decrease of the number of installed fasteners will also contribute to weight savings and will reduce the use of resources.This research is part of the BLINDFAST: INNOVATIVE BLIND FASTENER MONITORING TECHNOLOGY FOR QUALITY CONTROL project, which has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement 686827