Simulation of Low-Temperature Oxidation and Combustion of N-Dodecane Droplets under Microgravity Conditions

Abstract

Fires are considered among the most dangerous accidents on manned spacecraft. That is why several programs of combustion experiments were implemented at the International Space Station (ISS) since 2008. In the experiments with n-heptane and n-dodecane droplet combustion, a new phenomenon was discovered, namely, the phenomenon of the radiative extinction of a burning droplet with subsequent multiple flashes of flame. In this paper, n-dodecane droplet ignition, combustion, radiative extinction, and subsequent low-temperature oxidation with multiple flashes of cool, blue, and hot flames under microgravity conditions are studied computationally. The mathematical model takes into account multiple elementary chemical reactions in the vicinity of a droplet in combination with heat and mass transfer in liquid and gas, heat release, convection, soot formation, and heat removal by radiation. The model is based on the non-stationary one-dimensional differential equations of the conservation of mass and energy in liquid and gas phases with variable thermophysical properties within the multicomponent diffusion concept in the gas phase. Calculations confirm the important role of the soot shell formed around the droplet and low-temperature reactions in the phenomenon of droplet radiative extinction with multiple flame flashes in the space experiment at the ISS. Calculations reveal the decisive role of the blue flame, arising due to the decomposition of hydrogen peroxide, in the multiple flame flashes. Calculations with forced ignition of the droplet reveal the effect of the ignition procedure on droplet evolution in terms of the timing and the number of cool, blue, and hot flame flashes, as well as in terms of the combustion rate constant of the droplet. Calculations with droplet self-ignition reveal the possible existence of new modes of low-temperature oxidation of droplets with the main reaction zone located very close to the droplet surface and with only partial conversion of fuel vapor in it.

The Effects of Multistage Fuel-Oxidation Chemistry, Soot Radiation, and Real Gas Properties on the Operation Process of Compression Ignition Engines

“…Such flame flashes and extinctions can repeatedly occur during the droplet's lifetime. The phenomenology of radiative flame quenching with subsequent flashes of cool and blue flames was computationally reproduced in [33,34] for large n-heptane and n-dodecane droplets. The possibility of the radiative quenching of small (submillimeter-size) droplets is still questionable.…”

“…The model of droplet autoignition was reported in [33,34]. The model is built on the concept of multicomponent diffusion in the gas, the DKM of hydrocarbon fuel oxidation, and the constant pressure condition in the gas-droplet system.…”

Spherical Diffusion Flames of Ethylene in Microgravity: Multidimensional Effects

“…Fourth, as soot is prone to appear on the fuel side [17,18], it forms inside the normal SDF and outside the inverse SDF (see Figure 1a,c) [19][20][21]. According to [21], soot forms in the region where the local C/O atomic ratio and local temperature satisfy the conditions 0.32 < C/O < 0.44 and 𝑇 > 1300-1500 K. The phenomenon of radiative flame extinction was also studied for droplet flames [22][23][24][25][26]. Contrary to the SDFs with controlled steady-state fluid supply to the PB, the fuel supply in droplet flames is determined by the rate of liquid vaporization, which is a function of time as well as the properties of liquid and surrounding gas.…”

“…The phenomenon of radiative flame extinction was also studied for droplet flames [22][23][24][25][26]. Contrary to the SDFs with controlled steady-state fluid supply to the PB, the fuel supply in droplet flames is determined by the rate of liquid vaporization, which is a function of time as well as the properties of liquid and surrounding gas.…”

Crankcase Explosions in Marine Diesel Engines: A Computational Study of Unvented and Vented Explosions of Lubricating Oil Mist

“…The ignition and combustion of fuel mist is a complex physicochemical phenomenon [31][32][33]. Contrary to the ignition and combustion of an isolated fuel droplet in an unconfined oxidizing atmosphere [34][35][36], the processes of ignition and combustion of fuel mist depend on the local instantaneous droplet size, liquid vapor concentration, and distance between droplets [37]. Both experimental and computational studies indicate that evaporating fuel droplets affect differently fuel-lean and fuel-rich laminar flames: they enhance the flames under fuel-lean conditions and suppress the flames under fuel-rich conditions [38].…”