Numerical study on laminar flame velocity of hydrogen-air combustion under water spray effects

https://doi.org/10.1016/j.ijhydene.2019.04.225Get rights and content

Highlights

  • Application of sprays for mitigation of Hydrogen explosion effects involving deflagration waves.

  • Development of a new predictive model for hydrogen/air laminar flame in presence of water droplets.

  • Physical analysis of the main factors influencing laminar flame velocity interacting with water droplets.

  • Validation of the model using available experimental and numerical data.

Abstract

In the context of hydrogen safety and explosions in hydrogen-oxygen systems, numerical simulations of laminar, premixed, hydrogen/air flames propagating freely into a spray of liquid water are carried out. The effects on the flame velocity of hydrogen/air flames of droplet size, liquid-water volume fraction, and mixture composition are numerically investigated. In particular, an effective reduction of the flame velocity is shown to occur through the influence of water spray.

To complement and extend the numerical results and the only scarcely available experimental results, a “Laminar Flame Velocity under Droplet Evaporation Model” (LVDEM) based on an energy balance of the overall spray-flame system is developed and proposed. It is shown that the estimation of laminar flame velocities obtained using the LVDEM model generally agrees well with the experimental and numerical data.

Introduction

Spray systems are used as emergency devices for the mitigation of effects of explosions involving deflagration waves. Such systems are installed, for example, inside industrial buildings or on offshore facilities. Spray nozzles are also present inside some nuclear reactor buildings, and they are designed for preserving the containment integrity in case of a severe accident [1], [2]. In case of an explosion, for a spray system to act successfully upon unwanted premixed-flame propagation, an understanding of, (i), the dynamics of the water spray exposed to the explosion-induced flow field, and, (ii), the ability of the spray to mitigate the explosion, is needed.

The droplets generated by industrial water-spray systems have a Sauter mean diameter of the order of 100 μm. For example, the spray systems usually installed on offshore platforms generate droplets of Sauter mean diameters in the range 200–700 μm [3] while those installed inside reactor buildings produce droplets of a Sauter mean diameter in the range 280–340 μm [1]. Numerous investigations have demonstrated [4], [5], [6] that, if certain conditions are met, large droplets might break up and cascade down into a large number of small droplets, i.e., droplets of a volume mean diameter of approximately 10 μm. These small droplets have the capability to evaporate fully, or almost fully, inside a laminar flame thus modifying the flame structure. Experimental results devoted to the interaction of a laminar flame with small water droplets are scarce. Laboratory-scale tests reported in [7] showed that water droplets with diameters of the order of 10 μm have a similar influence on the structure of inert methane-air mixtures as water vapor. Early small scale experiments [8] as well as recent small and medium scale experiments using hydrogen [9], [10] have revealed that sprays containing small-size droplets can be effective against premixed combustion. The experiments performed in [11] were devoted to hydrogen-air laminar flame velocity measurements in the presence of water mist.

In the context of spray-decelerated or spray-retarded deflagration waves that have originated from explosions, laminar-flame velocity – occasionally also termed “laminar flame speed” – is an important physical quantity. In particular, most of the combustion models used for simulation of large-scale, turbulent premixed combustion – see, e.g. [12], [13], [14], [15], [16], – contain the laminar-flame velocity as input parameter which has to be procured by some means such as suitable numerical simulation or suitable experiments. In the literature several correlations exist [17], [18].

characterizing the flame speed of purely gaseous laminar hydrogen/air flames as a function of the mixture equivalence ratio. However, the small water droplets of a water spray modify the internal structure of the laminar flame and hence reduce its velocity. Thus a model is needed which takes into account the effect of water spray on flame structure and burning velocity.

In this paper, a “Laminar Flame Velocity under Droplet Evaporation Model” – abbreviated LVDEM – for hydrogen/air mixtures is proposed. This model has been constructed using the idea of Ballal and Lefebvre [19] who considered the energy balance inside the flame zone. The most crucial step is the model validation. For this purpose, the results obtained with the dedicated code Cosilab [20] and the experimental results of [11] are used. The results obtained using the LVDEM model generally agree well with the experimental and numerical data.

Section snippets

Phenomenology of computed flame structures

In this section, a description of the main phenomena related to the interaction of laminar hydrogen/air premixed, freely propagating flames with small droplets of a liquid water spray is given. The “small droplets” means droplets typically having a volume mean diameter of the order of 10 μm or smaller. For the numerical simulations, the Cosilab code [20] has been our main tool. This code can compute the internal structure of a laminar steady flame, with or without the presence of a liquid-water

LVDEM model for SL under droplets evaporation

In this section, the LVDEM numerical model of laminar flame velocity based on the energy balance is described. The comparison between the LVDEM model and the results of the Cosilab code is presented. Experimental results are used to validate the two methods.

Conclusions

In this paper, a “Laminar Flame Velocity under Droplet Evaporation Model” (LVDEM) for hydrogen/air mixtures has been developed and validated using the results of the Cosilab code [20] and the experimental results of [11]. Initially, the hydrogen-air mixture is supposed to be at normal ambient conditions and the water droplet diameter of the order of O(10) μm.

A key ingredient of the LVDEM-model is the droplet evaporation model of [29]. Application of the latter model is necessary in order to

Acknowledgement

This work has been performed with a financial support of the Electricité de France (EDF) in the framework of the Generation II&III reactor program, which is gratefully acknowledged.

References (38)

  • R. Grosseuvres et al.

    Combustion properties of H2/N2/O2/steam mixtures

    Proc Combust Inst

    (2019)
  • A. Satija et al.

    Vibrational CARS thermometry and one-dimensional simulations in laminar H2/air counter-flow diffusion flames

    Int J Hydrogen Energy

    (2015)
  • G.W. Koroll et al.

    Burning velocities of hydrogen-air mixtures

    Combust Flame

    (1993)
  • J.R. Travis

    A heat, mass, and momentum transport model for hydrogen diffusion flames in nuclear reactor containments

    Nucl Eng Des

    (1987)
  • J. Yanez et al.

    An analysis of flame instabilities for hydrogen–air mixtures based on Sivashinsky equation

    Phys Lett

    (2016)
  • A. Foissac et al.

    Droplet size and velocity measurements at the outlet of a hollow cone spray nozzle

    Atomization Sprays

    (2011)
  • C. Joseph-Augustea et al.

    On the use of spray systems: an example of R&D work in hydrogen safety for nuclear applications

    Int J Hydrogen Energy

    (2009)
  • K.V. Wingerden et al.

    The influence of water sprays on gas explosions. Part 2: mitigation

    J Loss Prev Process Ind

    (1995)
  • J.C. Meng et al.

    Numerical simulation of the aerobreakup of a water droplet

    J Fluid Mech

    (2018)
  • Cited by (16)

    • On the understanding of a cryogenic two-phase LOX/GH2 flame: Parametric sensitivity, characteristic scaling and phase instability

      2023, International Journal of Hydrogen Energy
      Citation Excerpt :

      Depending on the chamber pressure, either subcritical or supercritical, previous research shows that the cryogenic combustion process exhibits distinct characteristics. At subcritical pressure, the mixing and reaction are controlled by the two-phase effects involving a series of complex physical processes, such as the liquid breakup, droplet formulation, and spray flame [2,3]. On the other hand, at supercritical pressure, the repulsive inter-molecular forces become dominant and the surface tension vanishes.

    • Experimental research on combined effect of obstacle and local spraying water fog on hydrogen/air premixed explosion

      2022, International Journal of Hydrogen Energy
      Citation Excerpt :

      The current researches show that the explosion overpressure and flame velocity can be obviously suppressed by the water fog of 2–35 μm. And the smaller the diameter of water fog, the better the suppression effect [2,20–24]. The mechanism of water fog inhibiting hydrogen/air explosion includes both physical and chemical effects, diluting oxygen concentration, lowering flame temperature, blocking thermal radiation belongs to physical suppression, reducing active groups and terminating chain reaction.

    • Investigating the trend of hydrogen's flame velocity profile in relation to pressure and temperature above and below the adiabatic point

      2021, International Journal of Hydrogen Energy
      Citation Excerpt :

      Higher speed causes a rapid release of heat associated with little overpressure and steamy combustion product, and when a hydrogen cloud is combusted, all of the energy of the cloud will be released [24]. G. Gai et al. [25], conducted numerical simulations of laminar, premixed hydrogen flame propagating freely into a spray of liquid water. The authors determined in their investigation that the flame velocity reduced when it traveled through the influence of water spray.

    View all citing articles on Scopus
    View full text