ME 3304 - Ch. 1
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Heat Transfer (Heat)
Thermal energy in transit due to a SPATIAL temperature difference.
The rate of heat transfer in the x direction per unit area PERPENDICULAR to the direction of transfer
q'' = q / A
Conduction Heat Transfer Coefficient
Mechanism, Who?, Equation
Diffusion of energy due to random molecular motion
- Fourier's Law
- Heat flux, q'' =
Mechanism, Who?, Equation
Same as conduction + energy transfer due to bulk motion
Newton's Law of Cooling
q'' = h ( Tsurface - Tfluid )
Convection Heat Transfer Coefficient
W/(m2 * K)
NOT A PROPERTY
energy transfer due to bulk motion
Convection heat transfer is_______ if transferred TO the surface
--( Tsurface < Tfluid )
NEGATIVE heat transfer TO the surface
Convective heat transfer is________ if heat is transferred FROM the surface
--( Tsurface > Tfluid )
POSITIVE heat transfer FROM the surface
Flow is caused by external means, such as by a fan, pump, or atmospheric winds.
Free (natural) Convection
Flow is induced by buoyancy forces, which are due to density differences caused by temperature variations in the fluid.
Internal thermal energy of the fluid
Latent heat exchange
associated with a phase change between the liquid and vapor states of the fluid
**occurs most efficiently in a vacuum**
Energy transfer by electromagnetic waves (or photons)
- qrad'' = q / A =
- qrad'' =
Blackbody: emits radiation at the maximum possible rate
= 5.67 x 10-8
may originate from a special source, such as the sun, or from other surfaces to which the surface of interest is exposed.
Irradiation energy = (reflected + transmitted + absorbed) energy
radiation heat transfer coefficient
Thermal and Mechanical Energy Equation
Internal energy consists of a:
1. Sensible component
2. Latent component
3. Chemical component
4. Nuclear component
- 1. accounts for translational, rotational, and/or vibrational motion of the atoms composing the matter.
- 2. relates intermolecular forces influencing phase change
- 3. accounts for energy stored in the chemical bonds
- 4. accounts for binding forces in the nucleus
Simplify the TME Equation for STEADY STATE conditions.
Edot stored = 0
Simplified STEADY-FLOW thermal energy equation
= q = mdot
x Cp x ( Tout - Tin )
- 1. ideal gas with KE, PE, W = 0
- 2. incompressible liquid with KE, PE, W = 0
- ideal gas/incompressible fluid with negligible viscous dissipation
Surface Energy Balance
Edot in - Edot out = 0
q''cond - q''conv - q''rad = 0
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