# Aerodynamics of jets

Fuel is almost always injected into a combustion chamber as a liquid or gaseous jet, so there exists a clear interest in investigating the structure and dynamics of free and confined jets. Much of our work has been devoted to the analysis of steady, nonreacting, constant-density jets at moderately large Reynolds numbers, including fundamental contributions to the understanding of influences of confinement, coflow, and swirl. Effects of density variations on the structure of the jet were considered in later work. Transient jets were also investigated because of their relevance in accidental ignition scenarios. The knowledge generated was applied to the analysis of reactive configurations, including triple fronts in lifted diffusion flames. To provide a quantitatively accurate description of reacting jets, reduced mechanisms are currently being incorporated into the computations. Applications include the assessment of critical radii for hot-jet ignition and the investigation of influences of aerodynamics on the combustion-stability characteristics of and pollutant emissions from flameless-combustion systems.

### Related publications

**Critical radius for hot-jet ignition of hydrogen-air mixtures**

*J. Carpio, I. Iglesias, M. Vera, A. L. Sánchez, A. Liñán, Int. J. Hydrogen Energy, 38 3105–3109 (2013).*[DOI]**Numerical analyses of deflagration initiation by a hot jet**

*I. Iglesias, M. Vera, A. L. Sánchez, A. Liñán, Combust. Theory Modelling, 16 994–1010 (2012).*[DOI]**Variable-density jet flows induced by concentrated sources of momentum and energy**

*M. Sánchez-Sanz, A. L. Sánchez, A. Liñán, Theor. Comput. Fluid Dyn., 25 281-292 (2011)*. [**DOI**]**The Hydrogen Laminar Jet**

*M. Sánchez-Sanz, M. Rosales, A. L. Sánchez, Int. J. Hydrogen Energy, 35 39193927 (2010).*[DOI]**Fronts in High-Temperature Laminar Gas Jets**

*M. Sánchez-Sanz, A. L. Sánchez, A. Liñán, J. Fluid Mec., 547 257-266 (2006).*[DOI]**Simulations of Starting Gas Jets at Low Mach Numbers**

*I. Iglesias, M. Vera, A. L. Sánchez, A. Liñán, Phys. Fluids, 17 038105 (2005).*[DOI]**Laminar Craya-Curtet Jets**

*A. Revuelta, C. Martínez-Bazán, A. L. Sánchez and A. Liñán, Phys. Fluids, 16, 208–211 (2004).*[DOI]**The Quasi-Cylindrical Description of Submerged Laminar Swirling Jets**

*A. Revuelta, A. L. Sánchez and A. Liñán, Phys. Fluids, 16, 848–851 (2004).*[DOI]**Confined Swirling Jets with Large Expansion Ratios**

*A. Revuelta, A. L. Sánchez and A. Liñán, J. Fluid Mec., 508 89–98 (2004).*[DOI]**Confined Axisymmetric Laminar Jets with Large Expansion Ratios**

*A. Revuelta, A. L. Sánchez and A. Liñán, J. Fluid Mec., 456, 319–352 (2002).*[DOI]**The Virtual Origin as a First-Order Correction for the Far-Field Description of Laminar Jets**

*A. Revuelta, A. L. Sánchez and A. Liñán, Phys. Fluids, 14, 1821–1824 (2002).*[DOI]

# Ignition, Liftoff, and Extinction of Gaseous Diffusion Flames

In many combustion processes, the fuel and oxygen are initially separated. After ignition, we find that in regions of high temperature, in which the reaction time is very short, the reactants coexist only, with small concentrations, in thin reaction layers, first termed “diffusion flames” by Burke & Schumann (1928). In these flames, the fuel and oxygen, after arriving from opposite sides, are completely consumed by the reaction while crossing the flame by diffusion.

Diffusion flames are found in fireplaces as well as in wildland and urban fires. They are ubiquitous in engineering systems for propulsion and energy production, including diesel engines, gas turbines, rocket engines, and power-plant furnaces, and also in heating devices for domestic applications and in the process industry. Diffusion flames have also played a central role in the history of humankind, with the earliest applications involving the burning of wood for heating, illumination, cooking, and use as a dissuasive means against insects and ferocious beasts. Humans soon mastered the rudiments of ignition, initially starting a fire with a local increase of temperature by friction. They learned that they could enhance the burning rate by blowing and could promote flame spread from a locally ignited tinder and also that excessive blowing leads to flame extinction. Our work addresses these processes of ignition, propagation, and extinction of diffusion flames with a view to their relevance for technological applications.

### Related publications

**Ignition, liftoff, and extinction of gaseous diffusion flames**

*A. Liñán, M. Vera, A. L. Sánchez, Ann. Rev. Fluid Mech., 47 293–314 (2015).*[**DOI**]**Lifted Laminar Jet Diffusion Flames**

*A. Liñán, E. Fernández-Tarrazo, M. Vera and A. L. Sánchez, Combust. Sci. Tech., 177 933-953 (2005).*[**DOI**]**Laminar Mixing in Diluted and Undiluted Fuel Jets Upstream from Lifted Flames**

*A. Revuelta, A. L. Sánchez and A. Liñán, Combust. Flame, 128, 199–210 (2002).*[**DOI**]

# Stability of variable-density flows

Because of their relevance in combustion applications, there is interest in understanding the stability characteristics of low-Mach number gaseous flows with significant density changes for moderately large values of the relevant Reynolds number. Sometimes, quasi-parallel linear, spatiotemporal stability analyses suffice to clarify the stability properties of the resulting flows, including their convective/absolute instability character. In other occasions, a global linear stability analysis is required to ascertain the global stability characteristics of the flow.

In recent work, we have applied the latter technique to the analysis of “flame flicker”, the periodic flow state that characterizes buoyancy-dominated jet diffusion flames. While early theoretical work assumed a convective instability, later experimental observations suggested that the flame flickering phenomenon was associated instead with a globally excited oscillation forced by a region of absolutely unstable flow near the base of the jet exit. In our work we employ, for the first time, a linear global instability analysis to examine buoyancy-induced flickering of axisymmetric laminar jet diffusion flames. The method determines directly, without invoking weakly nonparallel assumptions, the critical conditions at the onset of the linear global instability as well as the Strouhal number of the associated oscillations in terms of the governing parameters of the problem, thereby circumventing the need for analyzing the local convective/absolute stability character of the flow.

### Related publications

**Global Linear Instability Analysis of Diffusion-Flame Flickering**

*D. Moreno, W. Coenen, A. Sevilla, J. Carpio, A. Liñán, A. L. Sánchez, 25th ICDERS August 2-7 (2015)***Viscous stability analysis of parallel flows with discontinuous base profiles**

*W. Coenen, A. Sevilla, A. L. Sánchez, Eur. J. Mech. B/Fluids., 36 128–138 (2012).*[**DOI**]**Absolute Instability of Light Jets Emerging from Circular Injector Tubes**

*W. Coenen, A. Sevilla, A. L. Sánchez, Phys. Fluids, 20 074104 (2008).*[**DOI**]

# Effects of buoyancy on thermal ignition processes

The seminal theory of thermal explosions developed by Frank-Kamenetskii (1939) examined the onset of thermal ignition of reactive mixtures enclosed in vessels with isothermal walls by investigating the existence of steady weakly reactive solutions. The corresponding critical conditions for ignition, which have important safety implications for storage of reactive materials, are seen to be determined by a critical Damkohler number measuring the competition between the heat release by chemical reaction, which accelerates the temperature-sensitive reaction rate, and the heat losses to the container wall.

Although Frank-Kamenetskii addressed initially stagnant systems in which heat transfer occurred solely by thermal conduction, it was soon acknowledged that in gaseous (and also liquid) reactive systems under normal gravity conditions the density differences associated with the temperature increase induced by the chemical reaction, although small in thermal-explosion events, suffice to generate significant convection, thereby invalidating the assumption of stagnant fluid present in the original development. The influence of the resulting motion can be measured through a Grashof number based on the characteristic induced velocity associated with the Frank-Kamenetskii temperature increase. This influence is being investigated in our recent work by numerical integrations of the conservation equations in the Boussinesq approximation complemented by asymptotic solutions for extreme values of the Grashof number that serve to explain the observed behaviors and clarify the structure of the resulting flow.

### Related publications

**Effects of natural convection on thermal explosions in spherical vessels**

*I. Iglesias, A. L. Sánchez, F. A. Williams, A. Liñán, 25th ICDERS August 2-7 (2015)*