Natural convection systems

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Chapter 7 Natural convection systems
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7-1 Introduction

• natural or free convection the motion of the
  fluid due to
• density changes arising from heating process
• Buoyancy forces that given rise to the
  free-convection
• currents are body forces
• Examples radiator, refrigerator and electronic
  components
• In infinite spaces (???????)
• No body have influence on the natural
  convection
• in enclosed spaces (????????)
• The natural convection is affected by
  somebody

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7-2 Free-convection heat transfer on a vertical
flat plate

• Velocity profile is different from forced
  convection
• Temperature profile is similar to forced
  convection
• Out of boundary layer, the Tconst. u0
• The flow may be laminar flow, or turbulent flow

Solutions Differential equation Integral equation
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7-3 Empirical relations for free-convection
Where
? volume coefficient of expansion
For ideal (perfect) gas pvRT
Reference temperature
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• Characteristic dimensions

• the length L for vertical surface
• Diameter d for horizontal cylinder

• The values of the constants C and m are listed in
  table

• Rayleigh number

• A vertical cylinder may be treated as a vertical
  flat plate when

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7-4 Free convection from vertical planes and
cylinders 7-5 Free convection from horizontal
cylinders 7-6 Free convection from horizontal
plates 7-7 Free convection from inclined
surfaces 7-8 Nonnewtonian fluids 7-9 Simplified
equations for air 7-10 Free convection from
spheres canceled
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7-11 Free convection in enclosed spaces
Grashof number based on the distance ?
Correlations (7-54), (7-55) and (7-60)
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7-12 Combined free and forced convection
Momentum equation
Criterion for free or forced convection
Free convection effects should be considered
Free convection is of primary importance
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