Numerical study on laminar flame velocity of hydrogen-air combustion under water spray effects
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 . For example, the spray systems usually installed on offshore platforms generate droplets of Sauter mean diameters in the range 200–700 [3] while those installed inside reactor buildings produce droplets of a Sauter mean diameter in the range 280–340 [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 . 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 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 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 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 .
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.
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