Thermal energy transport:
cause by temperature difference, high T -> low T
Conduction
Heat transferring by solid medium
Convection
Transferring energy between solid surface and fluid
Mass transport
Natural (free) convection
Forced convection
Radiation
Heat transferring by electromagnetic waves
Conduction
Fourier’s Law
Q= -KA ΔT/ΔL
Q: heat transfer rate
A: cross-sectional area of heat flux
ΔT/ΔL: temperature gradient
K: thermal conductivity (W/mk)
Ex. Al = 230
Cu = 380
Mylar = 1.8
Convection
Newtonian cooling Law
Qc = hc As (Ts – Ta)
Qc: convection heat transfer rate
As: surface area
Ts: surface temperature of solid
Ta: tmperature of ambient
hc: heat transfer coefficient, f(flow type, body geometry, physical property, temperature, velocity, viscosity…)
Natural convection & Forced convction
hc of air, natural convection: 0.0015~0.015 W/in2℃
forced convection: 0.015~0.15 W/in2℃
Radiation
Qa = εσAF1-2( Ts4 – Ta4)
Qa: radiation heat transfer rate
ε: emissivity, 0 ≦ ε ≦ 1
σ: Stefan-Boltzmann constant
A: surface area
F1-2: view factor
Ts: temperature of body s
Ta: temperature of body a
Thermal Resistance
R = V / I
V: voltage = ΔT: temperature difference
I: current = Q: heat
Conduction
Rk = ΔL/KAk
Convection
Rs = 1 / hcAs
Radiation
Ra = (Ts – Ta) /εσAF1-2( Ts4 – Ta4)
Important Components For Thermal Design ..
Heat Sink
Heat Pipe
Fan
TIM ( Thermal Interface Material)
Combination of aforementioned components