Elsevier

Combustion and Flame

Volume 222, December 2020, Pages 383-391
Combustion and Flame

Pressure effects on the soot production and radiative heat transfer of non-buoyant laminar diffusion flames spreading in opposed flow over insulated wires

https://doi.org/10.1016/j.combustflame.2020.09.003Get rights and content

Abstract

This paper investigates experimentally and numerically pressure effects on soot production and radiative heat transfer in non-buoyant opposed-flow flames spreading over wires coated by Low Density PolyEthylene (LDPE). Experiments, conducted in parabolic flights, consider pressure levels ranging from 50.7 kPa to 121.6 kPa and an oxidizer flowing parallel to the wire's axis at a velocity of 150 mm/s and composed of 20% O2/80% N2 in volume. The numerical model includes a detailed chemistry, a two-equation smoke-point based soot production model, a radiation model coupling the Full-Spectrum correlated-k method with the finite volume method and a simple degradation model for LDPE. An analysis of the experimental data shows that the spread rate, the pyrolysis mass flow rate, and the residence time for soot formation are independent of pressure whereas the soot formation rate is third-order in pressure. The model reproduces quantitatively the effects of pressure on soot production and captures the transition from non-smoking to smoking flames. The radiant fraction increases with pressure because of an enhancement in soot radiation whereas the contribution of radiating gases remains approximately constant over the range of pressures considered. In addition, gas radiation dominates at pressure lower than 75 kPa whereas soot radiation prevails at higher-pressure levels. Consistently with the data obtained at normal gravity, the smoke-point transition is found to occur for a radiant fraction of about 0.3 and the soot oxidation freezing temperature is estimated in the range 1350–1450 K. Eventually, whatever the pressure considered, the surface re-radiation from the wire is higher than the incident radiative flux from the flame to the surface along the entire wire. This shows that radiative heat transfer contributes negatively to the heating of the unburnt LDPE and to the heat balance along the pyrolysing surface.

Introduction

Unexpected overheating of wires by electrical current overshoots has been identified as a primary cause of fire initiation and growth in a space vehicle. This has motivated a significant amount of experimental [1], [2], [3], [4] and numerical [5,6], studies related to laminar flame spread over thin electrical wires in microgravity. This motivation has been reinforced by the inherent differences between non-buoyant and buoyant laminar diffusion flame structures [7] that do not allow the conclusions drawn at earth gravity to be extended to microgravity. Ambient conditions, such as pressure, oxygen content, and slow forced flow due to ventilation are expected to affect flame spread characteristics. Among these parameters, oxygen content and ambient pressure are intimately related in space exploration applications.

Ranging from conservative Earth-like conditions to ambitious low-pressure and high oxygen content environment, past spacecraft designs have incorporated one of the following three sets of pressure and oxygen content: (i) pure molecular oxygen (xO2=1) at a reduced pressure, P, of 34.5 kPa (Mercury, Gemini, and Apollo missions), (ii) nominal mixture composed by 21% O2 and 79% N2 (xO2=0.21 and xN2=0.79) and at sea-level pressure P=101.3kPa (Vostok, Voskhod, Orbiter, Spacelab, MIR, International Space Station, Soyuz, and Shenzhou missions), and (iii) oxygen-enriched low pressure mixture with xO2=0.72 and xN2=0.28 at P=34.5kPa (Skylab Space Station) [8]. In the near future, exploration atmosphere is likely to be an oxygen-enriched mixture at low pressure, with current target for the Orion Multi-Purpose Crew Vehicle of xO2=0.34 and xN2=0.66 at P=56.5kPa [7]. It should be pointed out that none of these atmospheric conditions are fire safe since the flammability limit of most of hydrocarbons is about 15% in terms of O2 mole fraction regardless of pressure. Fire incidents have been documented aboard Apollo 13 [9], Salyut-1 [10,11], Salyut-6 [12], the Orbiter fleet [13], and Mir [14]. In addition, the vast majority of incidents report surprising large amounts smoke [15], which raises doubts regarding the future safety of astronauts onboard the Orion spacecraft in the context of long-range missions. Consequently, understanding how ambient parameters affect the onset of smoke release is crucial towards the development of a safe space exploration framework.

In order to precisely break down the mechanisms responsible for the transition from non-smoking to smoking-flames, a special emphasis must be put on soot production and the related radiative heat transfer processes. Previous studies considering non-buoyant axisymmetric laminar diffusion flames fueled by gaseous hydrocarbons showed that, as observed at normal gravity [16], [17], [18], the soot production and therefore soot radiation increase with pressure [19,20]. A consequence of this increase in fuel sooting propensity with pressure is that the laminar smoke point (LSP) flame length is inversely proportional to pressure [20]. The mechanisms leading to the SP were found similar at both normal and microgravity and were identified as the quenching of the soot oxidation process at the flame tip owing to the radiative cooling mainly caused by soot radiation. However, the SP characteristics in microgravity were found different from those observed at earth gravity. First, non-buoyant SP flame lengths are significantly shorter for comparable conditions [20]. Moreover, the SP radiant fraction and the soot oxidation freezing temperature for microgravity were reported to be in the range 0.4–0.6 [19] and 1000 K [20], respectively, as compared to 0.3 [21] and the range 1300–1450 K [22], [23], [24] at normal gravity.

In addition to these gas-phase phenomenological discrepancies, alterations in flame spread rate are well documented in various configurations in the absence of buoyancy [2,7,25]. Since flame spread controls the fuel pyrolysis and, in turn, the flame geometry and residence time, these modifications increased doubts in the ability of standard tests conducted at normal gravity to successfully forecast smoking properties of a given material. Investigations of SP mechanisms over a spreading flame thus combine both effects to provide a holistic perception of the definite effects of flow conditions.

The present paper focuses on the soot production, the SP mechanisms and radiative structures of laminar diffusion flames spreading in an opposed flow configuration over idealized electrical wires. The main advantage of the wire configuration is that its 2D axisymmetric geometry allows the BMAE (Broadband Modulated Absorption Emission) technique, cautiously developed by the authors [4,26], to be implemented, which leads to the concomitant measurements of soot volume fraction and temperature in the spreading flame. The wires consist of a Nickel-Chrome (NiCr) metallic core coated with Low Density PolyEthylene (LDPE) and differ from the non-flammable polyimide wire insulation (MIL-W-81381) mostly employed in spacecraft [27]. However, although LDPE coated wires are not employed in space vehicles as such, their consideration is relevant for several reasons. First, it corresponds to an international target configuration to investigate fundamentally the flammability properties of electrical wires. Their use has emerged with the work of Bakham et al. [28] and, over the last twenty years, a significant amount of experimental studies, mainly related to fire safety in spacecraft, have adopted the PE coated wire configuration [3,4,[29], [30], [31], [32], [33], [34]]. Second, it produces soot volume fraction levels above the detection threshold of the BMAE over the range of pressure considered in the present study. The lower limit of the pressure range was selected to be above this threshold and the range was calibrated to observe the transition from a non-smoking flame to a smoking one. Finally, PE is a potential spacecraft material considered for space radiation shielding [35], and hence requires a careful evaluation of its flammability properties. The present work is organized as follows. The second section presents the experimental setup and optical diagnostics. The numerical model is presented in Section 3. The experimental and numerical results are discussed in Section 4 whereas Section 5 summarizes the main conclusions of the paper.

Section snippets

Experiments

Experiments were conducted in parabolic flights. The experimental setup and the optical diagnostics are detailed in Ref. [4] and are only briefly described here. The experimental setup consists of a cylindrical combustion chamber with an inner diameter of 190 mm. Cylindrical wires of length 150 mm, consisting of a 0.5 mm diameter NiCr core coated by a 0.3 mm thick LDPE insulation, are placed along the central axis of the chamber. Laminar oxidizer flows of velocity 150 mm/s, parallel to the

Governing equations

The model solves the steady-state governing equations of both gaseous and solid phases in a flame-fixed axisymmetric coordinate system [6].

A detailed description of the solid phase degradation model can be found in Ref. [6] and only a summary is given here. The study focuses on gas phase processes with a special emphasis on soot production and radiative heat transfer. Consequently, the solution of the conjugated heat transfer problem at the gas/solid interface is only needed to specify proper

Data analysis

The experimental data are analyzed to determine n in Eq. (1). The analysis assumes that the flame spreads in an opposed flow of free stream velocity, u, over a thermally-thin cylindrical material. The surface temperature of the material, TP, remains constant during the pyrolysis process and the gas phase conductivity, λfl, dynamic viscosity, μfl, and flame temperature, Tfl, are independent of pressure. A unit Schmidt number is assumed. The gaseous fuel, F, reacts with the oxidizer, Ox,

Conclusions

The effects of pressure on soot production and radiative heat transfer in the non-buoyant opposed-flow flame spread along NiCr wires coated with LDPE are investigated experimentally and numerically. The following conclusions can be drawn:

  • (1)

    Experimental data show that spread rate, pyrolysis rate, and residence time for soot formation are independent of pressure whereas the soot formation rate is third-order in pressure.

  • (2)

    A two-equation SP-based soot model is proposed and implemented in a CFD model

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors feel grateful to the Centre National d'Etudes Spatiales for its financial support under Contract No. 130615.

References (58)

  • A. Guibaud et al.

    Broadband modulated absorption/emission technique to probe sooting flames: implementation, validation, and limitations

    Proc. Combust. Inst.

    (2019)
  • N.N. Bakhman et al.

    Burning of polymeric coatings on copper wires and glass threads: I. Flame propagation velocity

    Combust. Flame

    (1981)
  • X. Huang et al.

    Ignition-to-spread transition of externally heated electrical wire

    Proc. Combust. Inst.

    (2013)
  • J.M. Citerne et al.

    Fire safety in space – investigating flame spread interaction over wires

    Acta Astronaut.

    (2016)
  • L. Hu et al.

    Limiting oxygen concentration for extinction of upward spreading flames over inclined thin polyethylene-insulated NiCr electrical wires with opposed-flow under normal- and micro-gravity

    Proc. Combust. Inst.

    (2017)
  • M. Nagachi et al.

    Can a spreading flame over electric wire insulation in concurrent flow achieve steady propagation in microgravity?

    Proc. Combust. Inst.

    (2019)
  • H. Guo et al.

    Numerical study on the influence of hydrogen addition on soot formation in a laminar ethylene–air diffusion flame

    Combust. Flame

    (2006)
  • Y Zhang et al.

    A burner to emulate condensed phase fuels

    Exp. Thermal Fluid Sci

    (2016)
  • A. Markan et al.

    A Burning Rate Emulator (BRE) for study of condensed fuel burning in microgravity

    Combust. Flame

    (2018)
  • Z. Qin et al.

    Combustion chemistry of propane: a case study of detailed reaction mechanism optimization

    Proc. Combust. Inst.

    (2000)
  • C.W. Lautenberger et al.

    A simplified model for soot formation and oxidation in CFD simulation of non-premixed hydrocarbon flames

    Fire Safety J.

    (2005)
  • J.L. de Ris et al.

    Radiation Fire Modelling

    Proc. Combust. Inst.

    (2000)
  • W.L. Flower et al.

    Soot production in axisymmetric laminar diffusion flames at pressures from one to ten atmospheres

    Proc. Combust. Inst.

    (1988)
  • H. Guo et al.

    Optimized rate expressions for soot oxidation by OH and O2

    Fuel

    (2016)
  • M.F. Modest et al.

    Assembly full spectrum k-distribution from a narrow band database: effects of mixing gases, gases and non-gray absorbing particles and non-gray scatters in non-gray enclosures

    J. Quant. Spectrosc. Radiat. Transf.

    (2005)
  • C. Wang et al.

    Full-spectrum k-distribution look-up table for non homogeneous gas–soot mixtures

    J. Quant. Spectrosc. Radiat. Transf.

    (2016)
  • H. Bedir et al.

    A computational study of flame radiation in PMMA diffusion flames including fuel vapour participation

    Proc. Combust. Inst.

    (1998)
  • J.H. Kent et al.

    Temperature and fuel effects in sooting diffusion flames

    Proc. Combust. Inst.

    (1985)
  • P.S. Greenberg et al.

    The USML-1 wire insulation flammability glovebox experiment

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