Spray Combustion

Homogenized description of spray combustion

In liquid-fueled combustion devices, the liquid fuel jet injected into the combustion chamber breaks up into droplets to generate a spray. Although individual droplet combustion may occur, fuel sprays typically evaporate and burn as a group. Numerical computations of spray combustion are hindered by many complicating factors, including disparate length and time scales associated with the fast chemistry and with the multiphase nature of the flow, which is highly turbulent in most applications. Clearly, modeling strategies aimed at removing the numerical stiffness associated with these disparities can be instrumental in enabling more efficient computations to be performed.

Our work has addressed in particular the Burke-Schumann limit of infinitely fast reaction, in which the flame appears as a sheet, separating an internal oxygen-free region from an external region where no gaseous fuel is present in significant amounts. The resulting formulation can be used in direct numerical simulations of spray diffusion flames and may also serve as a starting point in modeling strategies of turbulent flows.

ketch of fundamental spray-combustion processes in typical liquid-fueled continuous-combustion systems (from Proc. Combust. Institute, 35 1549–1577, 2015).

ketch of fundamental spray-combustion processes in typical liquid-fueled continuous-combustion systems (from Proc. Combust. Institute, 35 1549–1577, 2015).

Related publications

  1. The role of separation of scales in the description of spray combustion
    A. L. Sánchez, J. Urzay, A. Liñán, Proc. Combust. Institute, 35 1549–1577 (2015). [DOI]
  2. Coupling-function formulation for monodisperse spray diffusion flames with infinitely fast chemistry
    J. Arrieta-Sanagustín, A. L. Sánchez, A. Liñán, F. A. Williams, Fuel Process. Technol., 107 81–92 (2013). [DOI]
  3. Sheath vaporization of a monodisperse fuel-spray jet
    J. Arrieta-Sanagustín, A. L. Sánchez, A. Liñán, F. A. Williams, J. Fluid Mec., 675 435–464 (2011). [DOI]

Ignition of spray flames

Fuel sprays can be ignited using external sources such as electric sparks, torches or plasma jets, as is needed during the start and relight of jet engines and in the operation of gasoline direct-injection engines. Forced ignition is not needed during the normal steady operation of continuous-combustion systems, such as gas turbines or industrial furnaces. In many of these systems, the injection velocities are much higher than the characteristic deflagration speed, thereby precluding upstream triple-flame propagation. In that case, combustion stabilization must rely on the autoignition of the fuel–air mixture, which is facilitated by the high temperature of the surrounding gas, with ignition often occurring near the edge of the spray jet, where the temperatures are higher.

Figure4.2a

Canonical model problems can be instrumental in providing understanding of these complex spray-ignition phenomena, an example being the laminar mixing layer separating a hot-air stream from a monodisperse spray carried by either an inert gas or air. The numerical integrations unveil two different types of ignition behavior depending on the fuel availability in the reaction kernel, which in turn depends on the rates of droplet vaporization and fuel-vapor diffusion. When sufficient fuel is available near the hot boundary, as occurs when the thermochemical properties of heptane are employed for the fuel in the integrations, combustion is established through a precipitous temperature increase at a well-defined thermal-runaway location, a phenomenon that is amenable to a theoretical analysis based on activation-energy asymptotics. By way of contrast, when the amount of fuel vapor reaching the hot boundary is small, as is observed in the computations employing the thermochemical properties of methanol, the incipient chemical reaction gives rise to a slowly developing lean deflagration that consumes the available fuel as it propagates across the mixing layer towards the spray. The flame structure that develops downstream from the ignition point depends on the fuel considered and also on the spray carrier gas, with fuel sprays carried by air displaying either a lean deflagration bounding a region of distributed reaction or a distinct double-flame structure with a rich premixed flame on the spray side and a diffusion flame on the air side.

Ignition kernels of (a,c) heptane and (b,d) methanol sprays as obtained from numerical integrations of the model problem.

Ignition kernels of (a,c) heptane and (b,d) methanol sprays as obtained from numerical integrations of the model problem.

Related publications

  1. Dynamics of thermal ignition of fuel sprays in mixing layers
    D. Martínez-Ruiz, J. Urzay, A. L. Sánchez, A. Liñán, F. A. Williams, J. Fluid Mec., 734 387–423 (2013). [DOI]

Counterflow spray diffusion flames

For the high Reynolds numbers typically encountered in combustion applications the flow is turbulent and the flames appear embedded in thin mixing layers that are locally distorted and strained by the turbulent motion. In applications involving spray combustion, the interactions of the flame with the flow are also dependent on the presence of the fuel droplets.

Schematic view of the typical experimental arrangement employed in experimental studies of counterflow spray flames.

Schematic view of the typical experimental arrangement employed in experimental studies of counterflow spray flames.

These interactions can be investigated by consideration of simple laminar problems, a renown example being the counterflow mixing layer, which has been widely used as a cartoon to represent local flow conditions in strained mixing layers. The evolution of the droplets in their feed stream from the injection location is seen to depend fundamentally on the value of the droplet Stokes number, St, defined as the ratio of the droplet acceleration time to the mixing-layer strain time close to the stagnation point. Two different regimes of spray vaporization and combustion can be identified depending on the value of St. For values of St below a critical value, equal to 1/4 for dilute sprays with small values of the spray liquid mass-loading ratio, the droplets decelerate to approach the gas stagnation plane with a vanishing axial velocity. In this case, the droplets located initially near the axis reach the mixing layer, where they can vaporize due to the heat received from the hot air, producing fuel vapor that can burn with the oxygen in a diffusion flame located on the air side of the mixing layer.

The character of the spray combustion is different for values of St of order unity, because the droplets cross the stagnation plane and move into the opposing air stream, reaching distances that are much larger than the mixing-layer thickness before they turn around. The vaporization of these crossing droplets -and also the combustion of the fuel vapor generated by them- occurs in the hot air stream, without significant effects of molecular diffusion, generating a vaporization-assisted nonpremixed flame that stands on the air side outside the mixing layer. The separate treatments developed for these two regimes of combustion display a reduced number of controlling parameters that effectively embody dependences of the structure of the spray flame on spray dilution, droplet inertia, and fuel preferential diffusion. Besides enabling investigations in the limiting cases of pure vaporization and of infinitely fast chemistry, the formulations can be used to examine extinction conditions for spray diffusion flames.

The droplet velocity as a function of the distance to the stagnation plane for for St = 1.0 (upper plot) and for St = 0.2 (lower plot).

Related publications

  1. Regimes of spray vaporization and combustion in counterflow configurations
    A. Liñán, D. Martínez-Ruiz, A. L. Sánchez, J. Urzay, Combust. Sci. Tech., 187 103–131 (2015). [DOI]