• Conduction and convection require the presence of temperature variations in a material medium.

• Although radiation originates from matter, its transport does not require a material medium and occurs most efficiently in a vacuum.

Heat Rate (W) qx = q''x . A

(1.3a)

h: Convection Heat transfer coefficient W/m2 K

Radiation

E = ε Eb = ε σ T4s (1.5)

E: Emissive power (W/m2)ε: Surface emissivity (0 ≤ ε ≤ 1)Eb: Emissive power of a black body (the perfect emitter σ: Stefan-Boltzmann constant (5,67X10-8 W/m2.K4) Energy absorption due to irradiation Gabs = α GGabs: Absorbed incident radiation (W/m2)α: Surface absorptivity (0 ≤ α ≤ 1)G: Irradiation (W/m2)

Heat Transfer RatesIrradiation: Special case of surface exposed to large

surroundings of uniform temperature, Tsur

G = Gsur = σ Tsur4

If α = ε, the net radiation heat flux from the surface due to the exchange with the surroundings is :q''rad= ε Eb (Ts) – α G = ε σ (Ts

4 – Tsur4) (1.7)

Problem 1.73(a): Process identification for single-and double-pane windows

qconv,1 Convection from room air to inner surface of first pane

qrad,1 Net radiation exchange between room walls and inner surface of first pane

qcond,1 conduction through first pan

qconv,s convection through airspace between pans

qrad,s Net radiation exchange between outer surface of first pane and inner

surface of the second pane (across airspace)

qcond,2 conduction through second pane

qconv,2 convection from the outer surface of single pane (or second) pane

to ambient air

qrad,2 Net radiation exchange between outer surface of single

pane (or second) pane and surroundings such as the ground

qs Incident solar radiation during day; fraction transmitted to room

is smaller for double pane

Problem 1.31: Power dissipation from chips operating at a surface temperatureof 85oC and in an enclosure whose walls and air are at 25oC for (a) free convection and (b)

forced convection.

Assumptions: (1) Steady-state conditions, (2) Radiation exchange between a small surface and a large enclosure, (3) Negligible heat transfer from sides of chip or from back of chip by conduction through the substrate.

Thermal conductivityTo use Fourier’s law, the thermal conductivity of the material must be known. This property, which is referred to as a transport property, provides an indication of the rate at which energy is transferred by the diffusion process. It depends on the physical structure of matter, atomic and molecular, which is related to the state of the matter.

FIGURE 2.4 Range of thermal conductivity for various states of matter at normal temperatures and pressure.

FIGURE 2.6 Electron or phonon trajectories in (a) a relatively thick film and (b) a relatively thin film with boundary effects.

FIGURE 2.8 The temperature dependence of the thermal conductivity of selected gases at normal pressures. Molecular diameters (d) are in nm [10]. Molecular weights () of the gases are also shown.

FIGURE 2.9 The temperature dependence of the thermal conductivity of selected nonmetallic liquids under saturated conditions.