Numerical and experimental studies of extinguishment of cup-burner flames by C6F12O
Introduction
Research for the last two decades to find a replacement for the effective but ozone-destroying fire suppressant Halon 1301 (bromotrifluromethane, CF3Br) used in aircraft cargo-bays has made good progress; however, a replacement agent with all of the properties of CF3Br has not yet been identified. Unlike CF3Br, some replacement agents failed a mandated Federal Aviation Administration (FAA) Aerosol Can Explosion Test [1,2] as they caused overpressures due to unwanted combustion enhancement, as has been reported in the literature [3], [4], [5], [6], [7], [8], [9], [10]. Based on thermodynamic equilibrium and perfectly stirred-reactor calculations for premixed systems, it was postulated [11], [12], [13] that higher overpressures in the FAA aerosol can tests might be due to higher heat release from reaction of the inhibitor itself. Nonetheless, the agents should still reduce the overall reaction rate and inhibit the reaction [14,15] in these stirred-reactor systems since all reactants, intermediates, and products exist simultaneously. In diffusion flames and most fires, however, the flame structure, i.e., the distributions of the temperature, species concentrations, and velocity, play decisive roles in the interactions between the inhibitor and the flame. In turn, such interactions change the flame structure. Therefore, there is an urgent need to study the effects of a potential halon replacement agent on the chemical structure and extinguishment mechanisms of diffusion flames.
In previous papers [16], [17], [18], the results of full-chemistry computations of cup-burner flames with CF3Br, C2HF5 (pentafluoroethane, HFC-125), C2HF3Cl2 (2,2-dichloro-1,1,1-trifluoroethane, HCFC-123), and C3H2F3Br (2-bromo-3,3,3-trifluoropropene, 2-BTP) added to the coflowing air were reported. Additional numbers of carbon and fluorine atoms in the halon-replacement-agent molecules, compared to CF3Br, represent potential energy contributions at a fixed concentration if they burn completely to COF2 and HF. The combustion enhancement was evident in the computation, particularly with C2HF5 or C2HF3Cl2 added [18]. For C2HF3Cl2 or C3H2F3Br added, however, the calculation was unable to obtain the converged solution near the extinguishment limit. An attempt was unsuccessful even at low concentrations of one other agent C6F12O (dodecafluoro-2-methylpentan-3-one, FK-5–1–12, Novec 1230). C6F12O is a good agent on a mole basis but (like most other replacements) still quite inferior to CF3Br on a mass basis. In fact, the mass-basis extinguishing concentration of C6F12O is comparable to chemically passive gases such as N2, CO2, CF4, and C4F10, and C6F12O maybe acts like an inert fire-extinguishing agent. However, the physical and chemical effects of C6F12O on the diffusion flame structure and extinguishment processes have not yet been fully studied.
This paper extends the prior effort [18] on halon-replacement agents to C6F12O by overcoming the convergence difficulties in the numerical simulation and by conducting the cup-burner flame extinguishment experiments using propane as the fuel.
Section snippets
Experimental procedures
The standard cup burner [19] consists of a cylindrical glass cup (26 mm i.d., 30 mm o.d.) positioned inside a glass chimney (8.6 cm inner diameter, 45.7 cm height). To provide uniform flow, 6 mm glass beads fill the base of the chimney, and 3 mm glass beads (with a 15.8 mesh/cm screen on top) fill the fuel cup. The gas flow rates are measured by mass flow meters (Hastings HFM-2001
Computational method
The numerical simulation of coflow diffusion flames stabilized on the cup burner is performed using a time-dependent, axisymmetric numerical code (UNICORN) [20,21]. The code solves the axial and radial (z and r) full Navier-Stokes momentum equations, continuity equation, and enthalpy- and species-conservation equations on a staggered-grid system. The buoyancy effect is included in the momentum equation. A clustered mesh system is employed to trace the gradients in flow variables near the flame
Extinguishment experiment
The flame base anchors at the burner rim, supports a trailing diffusion flame, and controls the flame attachment, detachment, and oscillation processes [32,34]. As the C6F12O is added to the coflowing oxidizing stream, the flame height increases (flares up) substantially and the flame base oscillates inward and outward before blowoff extinguishment. The measured MEC value is 4.17±0.30% from 29 extinguishment experiments.
Flame structure
In the computation, as the agent volume fraction in the coflowing oxidizing
Conclusions
The interactions of the fire-extinguishing agent C6F12O and the cup-burner diffusion flame structure, leading to extinguishment, are studied numerically and experimentally. The C6F12O added to the coflowing air weakens the flame-holding reaction kernel in the flame base. The reaction kernel detaches from the burner rim and moves to a location where a ratio of the residence time and the reaction time (i.e., Damkhöler number) is maintained. As the flame base moves away and oscillates in the
Declaration of Competing Interest
None.
Acknowledgments
The work at Innovative Scientific Solutions and Case Western Reserve University was supported in part by the Air Force Small Business Initiative Research program (Contract Number: FA9101-17-P-0092) and the National Science Foundation (Grant Number: 1842067). Assistance in conducting the experiment by Wilson Fisher and Samuel Faulk is acknowledged.
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2023, International Journal of Hydrogen EnergyCitation Excerpt :Under these circumstances, the ozone layer cannot be damaged as C3H2F3Br would have already been completely consumed before reaching the stratosphere [7]. Although it is a suitable fire extinguishing agent on a molar basis, C6F12O is significantly different from CF3Br on a mass basis [8]. The C6F12O mass-based extinguishing concentrations are comparable to those of inert gases, such as N2, CO2, CF4, and C4F10.