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Numerical Modelling of Internal Flow in Water Mist Injectors: Effect of Nozzle Geometry and Operating Conditions

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Abstract

Water mist fire fighting has achieved a well established position in fire protection for industrial and civil applications. The performances of water mist nozzles, largely used in water mist production, are deeply influenced by the internal nozzle flow characteristics and CFD modelling is a powerful tool to analyse them in details. The paper focuses on the numerical investigation of the internal flow in pressure swirl injectors for water mist applications. 3D large eddy simulations based on the Volume-Of-Fluid methodology have been implemented. The flow is assumed to be incompressible and under isothermal non-reacting conditions. Validation of the model against available experimental data is performed with satisfactory results. The effect of internal nozzle geometry on the injector behaviour is investigated by modifying the inclined swirling channels (five configurations) and the injection pressure in the range 5 to 320 bar. The effect of the size of cylindrical and conical swirl chambers are also independently investigated. Three different flow regimes can be distinguished as a function of imposed swirl and injection pressure and a map is reported to clarify the effect of these parameters. When a stable hollow cone spray is formed the mass flow rate fluctuations are < 5%, corresponding to a discharge coefficient between 0.34 and 0.50. When no air core is present in the discharge hole, mass flow rate fluctuations as high as 16% are observed, which correspond to discharge coefficient as high as 0.7. A detailed quantification of in-nozzle characteristics, like swirl number and momentum flux distribution along the nozzle and lamella thickness in the discharge hole, is reported and discussed, with particular emphasis on stability and transient behaviour of the atomiser internal flow, which represents the main novelty of the present study.

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Abbreviations

CFD:

Computational Fluid Dynamics

LDV:

Laser Doppler Velocimetry

LES:

Large Eddy Simulation

PDF:

Probability Distribution Function

PSA:

Pressure Swirl Atomiser

PIV:

Particle Image Velocimetry

SIMPLE:

Semi-Implicit Method for Pressure Linked Equations

VOF:

Volume Of Fluid

A :

Area (\({\mathrm{m}^{2}}\))

d :

Diameter of injector sections (m)

\(G_{z}\) :

Axial flux of momentum (\({\mathrm{kg}\;\mathrm{m/s}^{2}}\))

\(G_{\theta }\) :

Axial flux of angular momentum (\({\mathrm{kg}\;\mathrm{m}^{2}\mathrm{/s}^{2}}\))

h :

Lamella thickness (m)

l :

Length of nozzle sections (m)

\(m_{l}\) :

Mass flowrate (kg/s)

\({P_{inj}}\) :

Injection pressure (bar)

r :

Radial distance from injector main axis (m)

\({{R_{0}}}\) :

Nozzle discharge hole radius, \(R_{0}=\frac{d_{0}}{2}\) (m)

\({{\mathrm{Re}_{0}}}\) :

Nominal Reynolds number at hole exit (–)

SN :

Swirl number (–)

\({{V_{z}}}\) :

Axial velocity (m)

\({{V_{\theta }}}\) :

Tangential velocity (m)

z :

Coordinate along the injector (m)

\({{\alpha }}\) :

Liquid fraction over a hole section (–)

\({{\beta }}\) :

Inclination of the swirling channels (\(^\circ\))

\({{\theta }}\) :

Nozzle element angle (\(^\circ\))

\({{\mu _{l}}}\) :

Liquid viscosity (kg/ms)

\({{\rho _{l}}}\) :

Liquid density (kg/m\(^{3}\))

\({{\sigma \%}}\) :

Percentage relative root mean square (–)

A :

At section A (Fig. 13)

con :

Conical swirling chamber

cyl :

Cylindrical swirling chamber

D :

At section D (Fig. 13)

e :

External

p :

Inlet slot

sc :

Swirling channels

0:

Discharge hole

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Authors and Affiliations

  1. Department of Engineering and Applied Sciences, University of Bergamo, Viale Marconi 6, 24044, Dalmine, BG, Italy

    S. Tonini, P. Conti & G. E. Cossali

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  1. S. Tonini
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  2. P. Conti
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  3. G. E. Cossali
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Correspondence to S. Tonini.

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Tonini, S., Conti, P. & Cossali, G.E. Numerical Modelling of Internal Flow in Water Mist Injectors: Effect of Nozzle Geometry and Operating Conditions. Fire Technol 55, 2395–2417 (2019). https://doi.org/10.1007/s10694-019-00871-3

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  • DOI: https://doi.org/10.1007/s10694-019-00871-3

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