Thermal Energy Flashcards
Change in thermal energy
ΔQ = CΔT
or
ΔQ = mcΔT
Heat capacity
C= mc
with c = specific heat in J/K per kg
Thermal energy during phase change
During a phase change the temperature stays constant
Thermal energy during phase change equation
ΔQ = mL, where L = latent heat of evaporation or fusion
Three ways of thermal energy and heat transfer
- Conduction
- Convection
- Radiation
Conduction
transfer of heat and energy through an object
Parameters that play a role in conductivity
- Temperature difference (T1-T2)
- Area (cross-section) (A)
Thickness or length (d or l) - Type of material
-> Thermal conductivity k (W/m*K)
Fourier’s law of heat conduction:
P (W) = kA(T1-T2)/d
thermal conductivity k:
Is high for for highly conductive materials
Low for not so conductive materials
k<0.2 for insulating materials (wood, paper, glass)
Convection
Movement of energy through a fluid based on density differences due to temperature gradients
Convection equation
(Heat transfer due to convection at boundary of body
)
P/A = Nu*k(T1-T2)/L
nu =
Nusselt number
Radiation
Heat transfer by electromagnetic waves
Radiation equation
Pe/A = εσT^4
ε = emissivity
σ ~ 5.67 x 10-8 W/(m2K4) (Stefan-Boltzmann constant)
Example of forced convection
an oven
Main factors influencing heat conductivity
- thermal conductivity
- thickness
- area
Ways to improve heat transfer
- use high emessivity materials
- use high thermal conductvity materials
- thin walls, large area
- optimize design to stimulate convection
reduce heat transfer
- low emissivity
- apply insulation to reduce conduction
- thick walls, small area
- limit or control ventilation
Thermal insulation equation
Q = UAΔT
Thermal conductance equation
U = 1/R = k/d
Thermal mass
use the thermal capacity of the building to maintain steady interior atmosphere
Example: During the day a large concrete floor absorbs extra heat and radiates it during the night/ cools down during the night.
First law of thermodynamics
The heat input in a system (Q) and the work output of a system (W) are equal to the change in internal energy ΔU
(mass conservation)
Firts law of thermodynamics eqaution
ΔU = Q-W
Specific enthalpy equation
h = u + p*v = u + (p/ρ)
