Diapositiva 1

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Investigation on Air Curtains Behaviour in
Display Cabinets by means of 3-D Computational
Fluid Dynamics
P. DAgaro, G. Croce, G. Cortella Dept. of Energy
Technologies, University of Udine P. Schiesaro R
D Department, Arneg S.p.A.
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Vertical open display cabinets
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Vertical open display cabinets

• Needs
• Open design for the best product display
• Wide display area (height of the opening)
• Fulfilment of correct product storage
  temperature
• Difficulties
• Ambient air infiltration through the air curtain
• Radiative heat transfer on the product surface
• Product temperature fluctuations (quality loss)
• High energy consumption

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Air curtain

• The air curtain plays a key-role in the operation
  of an open display cabinet
• air infiltration through the front opening of
  the cabinet can be up to 60-70 of the total
  heat exchange from the ambient
• air vorticity may cause a significant
  entrainment of warm ambient air

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Modelling

• Air curtains
• Environment

• Heat
• exchanger

• Air ducts

• Load

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Modelling - steady state or transient state ?

• Food temperature distribution
• The steady state model can be successfully
  employed
• Average air curtain flow conditions and average
  convective heat transfer coefficient are
  sufficient
• Radiative heat transfer must be considered
• Frosting and defrosting of the evaporator are not
  considered

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Modelling - steady state or transient state ?

• Air curtain
• The transient state model is desirable not to
  miss important features of the flow
• Transient fluctuations with time and length
  scale of the order of the residence time and of
  the curtain length
• The turbulence steady state model accounts only
  for smaller scale eddies

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Modelling - steady state or transient state ?
Air curtain steady state transient state
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Object of the investigation

• Display cabinets of reduced length
• air curtain optimisation through a 3-D model
• Evaluation of the impact of 3-D flow structures
  and endwall vortices on the cabinet performance
• 3-D effects showed to be significant during
  testing (air flow visualisation and product
  temperature)
• Longitudinal ambient air movement
• people moving
• required in test conditions

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The CFD code

• Commercial CFD code
• k-e turbulence model - incompressible flow
• transient state simulation with 0.01 s time step
  

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Object of the investigation

• Vertical open cabinet for frozen food
• length 2438 mm, height 2109 mm, depth 1138 mm
• internal air curtain, t -24.1 C, v 1.74 m/s
• central air curtain, t 1.0 C, v 1. 42 m/s
• external air curtain, t 25.0 C, v 0.8 m/s
• no air from the back of the shelves

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Object of the investigation

• Assumptions
• product surface temperature constant
• radiative heat transfer neglected
• mass transfer neglected

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The CFD code
Latent heat balance Induction factor (Rigot,
1990) for the evaluation of ambient air
infiltration rate
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Simulation results
Still air in the ambient - reference
case Qualitative description of air entrainment
through streamlines (side view)
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Simulation results
Still air in the ambient - reference
case Secondary flows isothermal surface at 25 C

• Curtain overturning in the side wall boundary
  layer
• Cold air overspills the sides
• Warm air entrainment from the third curtain in
  the midspan section

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Simulation results
Still air in the ambient - reference case
Secondary flows - bottom of the cabinet
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Simulation results
Still air in the ambient - reference case
Secondary flows - temporary structure at the
bottom
...
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Simulation results
Still air in the ambient - reference case
Secondary flows - temporary structure at the
bottom temperature distribution at the return
grill
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Simulation results
Still air in the ambient - reference
case Influence of the length of the cabinet
Vortex amplitude does not scale with the length
of the cabinet Extremity effects have a growing
influence as the cabinet length is reduced
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Simulation results
Still air in the ambient - third curtain
deactivated 16 ambient air entrained and
captured at the return grill
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Simulation results
Still air in the ambient - third curtain at 28
C a portion of the third curtain tends to rise
the front opening
Originating from openings third curtain at 28 C
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Simulation results
0.2 m/s transversal air movement in the ambient
Secondary vortices translated in the stream
direction Asymmetric air entrainment - effect on
product temperature
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Simulation results
0.2 m/s transversal air movement in the ambient
Still air Transversal air movement
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Simulation results
Influence of 3-D effects on the cabinet
performance
Reference case, still air in the ambient. X
induction factor
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Conclusions

• CFD simulations can help to understand the
  physical mechanisms of 3-D effects in open
  display cabinets
• In medium and short cabinets the endwall
  secondary vortices can be the main mechanism for
  hot air entrainment
• In the particular case
• The third curtain is effective against ambient
  air entrainment, however its temperature is
  crucial (11 refrigerating power for a 3 C
  increase)
• Devices such as small end baffles can be simple
  but powerful tools for the reduction of the
  effects especially in the short length cabinets