Dynamic Large-Eddy Simulation of Droplet Effects on a Reacting Plume in Countercurrent Configuration

Abstract

The effects of evaporating droplets on a reacting plume have been investigated using large-eddy simulation (LES) with dynamic subgrid flow models. A countercurrent configuration, in which droplets are discharged downward toward a rising buoyant reacting plume, is used to mimic an idealized small-scale, water-based fire suppression system. Parametric studies have been conducted by varying the initial Stokes number (St ₀) or nondimensional droplet size, volumetric flow rate of the spray nozzle or equivalent mass loading ratio (MLR ₀), and initial droplet speed (), independently. The interactions between the two phases are studied in both instantaneous and statistical means. The thermal and dynamic effects of droplets on the reacting plume are scrutinized using the transport equations for the filtered (reduced) internal energy and filtered kinetic energy of the gas phase. New insights on the droplet effects have been gained by rearranging the droplet source terms in the transport equations into physically meaningful terms which clearly represent various contributions due to droplets. Specifically, it was found that only the heat exchange between the phases tends to reduce the gas temperature. All the other droplet-related terms, including the interphase drag and evaporation contributions, are source terms to the internal energy and thus tend to warm up the plume. From the budget analysis of the grid-scale kinetic energy, it was found that the droplet effects arising from both the interphase drag and evaporation tend to decrease the grid-scale kinetic energy, except in regions close to the spray nozzle.

Keywords:

• Droplets
• Fire suppression
• LES
• Reacting plume
• Water spray

ACKNOWLEDGMENTS

Financial support from the BRE Trust and the EPSRC grant No. EP/E011640/1 are gratefully acknowledged. Supercomputing resources are from (a) the UK Consortium on Computational Combustion for Engineering Applications (COCCFEA) under the EPSRC grant No. EP/D080223/1 and (b) the UK Turbulence Consortium (UKTC) under the EPSRC grant No. EP/G069581/1.

Notes

Note: The common parameters include (a) Flow: Re = 4000, Fr = 10, S = 0.76; (b) Combustion: Da = 80, Ze = 8.5, Q _h = 250; (c) Droplets: ρ_d = 828, h _fg = 250; (d) Spray: θ₀ = 50^○. (e) Domain: ; (f) Grids: n _x  × n _y  × n _z  = 41 × 160 × 200.

^a Special case labels are used to distinguish different cases. RP designates the Reacting Plume. The names of all the droplet cases start with the letter “D,” and the second letter illustrates the initial droplet size, “s,” “m”, and “b” for “S t ₀ = 6.25 (small),” “S t ₀ = 25 (medium),” and “S t ₀ = 100 (big),” respectively. “M” and “v” indicate the initial mass loading ratio (ML R ₀) and initial velocity magnitude of the droplets.