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Droplet cooling in atomization sprays

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Abstract

Transport between droplets/particles and a gas phase plays an important role in numerous material processing operations. These include rapid solidification operations such as gas atomization and spray forming, as well as chemical systems such as flash furnaces. Chemical reaction rates and solidification are dependent on the rate of gas-particle or gas-droplet transport mechanisms. These gas-based processes are difficult to analyze due to their complexity which include particle and droplet distribution and the flow in a gas field having variations in temperature and velocity both in the jet cross-section and in the axial distance away from the jet source. Thus to study and properly identify the important variables in transport, these gas and droplet variations must be eliminated or controlled. This is done in this work using models based on a single fluid atomization system. Using a heat transport model (referred to as thermal model) validated using single fluid atomization of molten droplets and a microsegregation model, the effect of process variables on heat losses from droplets was examined. In this work, the effect of type of gas, droplet size, gas temperature, gas-droplet relative velocity on the heat transport from AA6061 droplets was examined. It is shown that for a given gas type, the most critical process variable is the gas temperature particularly as affected by two-way thermal coupling and the droplet size. The results are generalized and applied to explain the difference in droplet cooling rate from different atomization processes.

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Abbreviations

A :

Surface area of the droplet

Cp :

Heat capacity of the droplet

Cp m :

Integral heat capacity

f s :

Fraction solid

Fo*:

Modified Fourier number for droplet-gas thermal transport

h total :

Total heat transfer coefficient

h conv :

Convective heat transfer coefficient

H :

Droplet enthalpy

k s :

Gas thermal conductivity

ΔLatent:

Solidification enthalpy

m and (m + 1):

Indices for the numerical calculation

Nu :

Droplet Nusselt number

Pr :

Prandltl number

Re :

Droplet Reynolds number

t :

Time

t*:

Reference time

t f :

Solidification time

T liq :

Liquidus temperature of AA6061

T m :

Droplet temperature during solidification

T nuc :

Nucleation temperature for simulation

T sol :

Solidus temperature of AA6061

Tu :

User-defined primary undercooled temperature

T ∞ :

Gas temperature in the free stream

V :

Volume of the droplet

ρ :

Droplet density

μbulk :

Gas viscosities at the temperature of the free flow gas

μsurf :

Gas viscosities at the temperature of the droplet

θ :

Dimensionless temperature

τ :

Dimensionless time

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Acknowledgements

The author wishes to acknowledge funding for this work from the Natural Science and Engineering Research Council of Canada (NSERC) and from the Canada Council for the Arts for award of a Killam Research Fellowship. The contribution of Karine Navell in making the cell spacing measurements presented in Fig. 12 are also acknowledged.

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Authors and Affiliations

  1. Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada, T6G 2G6

    Arvind Prasad & Hani Henein

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  1. Arvind Prasad
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  2. Hani Henein
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Correspondence to Hani Henein.

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Prasad, A., Henein, H. Droplet cooling in atomization sprays. J Mater Sci 43, 5930–5941 (2008). https://doi.org/10.1007/s10853-008-2860-2

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  • DOI: https://doi.org/10.1007/s10853-008-2860-2

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