ENTC 489: Announcements

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ENTC 489 Announcements

• Homework No. 7
• 11.7, 11.8, 11.14, 11.19, 11.31, 11.57
• Due Thursday, November 5th before 1050 am
• For more information, go to
• http//etidweb.tamu.edu/classes/entc489/502/Index.
  htm
• Exam II
• Thursday, 19 November
• Homeworks 6 8

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Free food!!!
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Contact Resistance

• Perfect contact No temperature drop (difference)
  or thermal resistance
• Actual contact There is a temperature drop
  (difference)

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Contact Resistance

• Thermal contact resistance

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Thermal Constant Resistance
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Convection Heat Transfer
Forced convection
q
at Tf
at Ts
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Natural Convection
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Forced Convection in External Flows
Local fluid velocity is about 99 of free stream
(V8) velocity
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Forced Convection in External Flows
? thermal boundary thickness
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Effect of thermal boundary thickness on
convective heat transfer coefficient
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Physical Properties that Affect Convective Heat
Transfer

• cp Specific heat (ability to store heat)
• m fluid viscosity (resistance to flow which
  affects velocity profile)
• k thermal conductivity of the fluid
• Pr cpm/k (Prandtl Number)

? kinematic viscosity
? thermal diffusivity
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Prandtl Number

• Ratio of momentum transport (represented by
  viscosity) to thermal transport (represented by
  thermal diffusivity)

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Conduction vs. Convection in Fluids
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Nusselt Number

• Nusselt Number
• Dimensionless heat transfer coefficient
• Nu depends on Reynolds Number (Re) and Prandtl
  (Pr) Number
• So, to increase heat transfer by convection, Re
  should increase, Pr should increase, or both
  should increase simultaneously

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Convection in a isothermal plate
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Conventions

• Evaluate fluid properties at Tfilm
• Nux ? local Nusselt number at (x) yielding hx
• Average heat transfer coefficient (h)

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Average Nusselt Number

• Average heat transfer coefficient (h)

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Things to consider in convection problems

• Reynolds (Re) number
• Prandtl (Pr) number
• Boundary conditions
• Isothermal (Ts is prescribed)
• Constant heat flux (q is known at the surface)
• Local vs. Average convective (Nu ) heat transfer
  correlations
• Local Nu depends on xcritical
• Average Nu depends on entire Lcharacterisitic

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Convection Heat Transfer Example

• A flat surface is at 65 C and is being cooled by
  air (Tf 20 C) flowing at 3.4 m/sec, determine
  the amount of heat that is removed by convection.