Diapositiva 1

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Numerical Simulation of Glass Door Defogging in
Refrigerated Display Cabinets P. DAgaro, G.
Croce, G. Cortella Department of Energy
Technologies University of Udine, Italy
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Fogging Problems in Refrigerated Display Cabinets
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Glass Door Fogging Problem
Problem Every time the door of a refrigerated
display cabinet is opened, condensation of the
atmospheric humidity occurs on the cold surface
? reduced product visibility

• Defogging via
• air curtain along the glass surface
• electric heaters
• anti-fog coatings

Numerical procedure of the fogging- defogging
phenomena ? performance evaluation of the
glass-air curtain system at an early stage
of the design process
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Fogging and Defogging Phenomena
Conjugate fluid-solid heat transfer problems in
presence of a phase changing fluid on one side
of the solid surface.
? fluid dynamic commercial code (CFX)
? in-house routines for the unsteady simulation
of the water layer evolution
? in-house FVM code for structured meshes
? empirical correlations
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Conjugate Heat Transfer
The conjugate heat transfer coupling between
solid and fluid domains is obtained via an
exchange of boundary conditions
water layer
5) Update of Dirichlet b.c. T? on ? (CHT3D)
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Interface Between CFXand User Code
The exchange of boundary conditions between fluid
and solid solvers is reduced to
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Fogging/defogging Model
Continuous film or collection of closely packed
tiny droplets

• Water-layer energy balance

• Water layer time depending mass balance

H cell-averaged water layer height
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Physical Assumptions

• Droplet shape spherical segment

• Evaporation
• 1st stage Awet constant J ? (t1-4)
• 2nd stage Awet ? J constant (t5,6)

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Sample Display Cabinet
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The Multi-Glazed Glass Door
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Simulation Sequence
1) Dry simulation (door is closed glass is
dry) CFXsolid solver
3) Demist simulation (door is shut
again) CFXsolid solver
defogging routines
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1) Dry simulation
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2) Condensation
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3) Demist Simulation
Water layer evolution
Experimental defogging time ? 140s
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Heating Coating On
Experimental defogging time ? 50s
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Conclusions

• A numerical procedure for fogging/defogging
  simulation has been presented
• Since the interaction between physical domains is
  achieved through an exchange of boundary
  conditions, a high level of flexibility is
  granted and the procedure can be and has been
  easily applied to different combination of
  commercial and in-house codes
• The procedure has been validated for a full
  fogging-defogging cycle of a spectacle lens and a
  sample application for refrigeration industry has
  been shown
• The results show how the proposed numerical
  approach is a reliable and cost-effective tool
  for the design of defogging systems, allowing
  quick analysis of several parameters both of the
  refrigerated cabinet (air velocity and
  temperature, geometry, etc.) and of the glass
  door (heating coating, surface wettability, etc)

Acknowledgements The authors are indebted to
Pierluigi Schiesaro, RD Laboratory, Arneg
S.p.A., for his help and assistance. This work
has been performed in the framework of a PhD
fellowship supported by Arneg S.p.A., Italy.
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• Thank you very much for your attention