ME 3304 - Ch. 1

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

PROPERTY

Diffusion of energy due to random molecular motion

• Fourier's Law
• Heat flux, q'' =

Same as conduction + energy transfer due to bulk motion

Newton's Law of Cooling

q'' = h ( T_surface - T_fluid )

Convection Heat Transfer Coefficient

W/(m² * K)

NOT A PROPERTY

energy transfer due to bulk motion

NEGATIVE heat transfer TO the surface

POSITIVE heat transfer FROM the surface

Flow is caused by external means, such as by a fan, pump, or atmospheric winds.

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

associated with a phase change between the liquid and vapor states of the fluid

Energy transfer by electromagnetic waves (or photons)

• q_rad'' = q / A =
• q_rad'' =

emissivity variable

PROPERTY

Blackbody: emits radiation at the maximum possible rate

= 1

Emissive Power

= 5.67 x 10^-8 W/(m²K⁴)

E_b =

may originate from a special source, such as the sun, or from other surfaces to which the surface of interest is exposed.

IRRADIATION

=

Irradiation energy = (reflected + transmitted + absorbed) energy

ABSORBTIVITY

radiation heat transfer coefficient

• 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

E_{dot stored} = 0

Q_dot = q = m_dot x C_p x ( Tout - Tin )

• 1. ideal gas with KE, PE, W = 0
• 2. incompressible liquid with KE, PE, W = 0
• OR
• ideal gas/incompressible fluid with negligible viscous dissipation

E_{dot in} - E_{dot out} = 0

q''_cond - q''_conv - q''_rad = 0