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)
Second law of thermodynamics
- The entropy of an isolated system never decreases
- Heat flows from a high to a low temperature
- Not all heat can be converted into work
Second law thermo equation
ηc = 1 - (T2/T1)
adiabatic
no heat transfer with the surroundings
Q = 0
isotherm
tempreature stays constant
ΔT = 0
Isobaric
pressure stays constant
ΔP = 0
Isochoric
volume stays constant
ΔV = 0
enthalpy is useful for describing
1) heat transfer at constant pressure
2) Adiabatic compression or expansion
formula for heat transfer at constant pressure
h1 - h2 = Q
Adiabatic compression equation
W = h1 - h2
Work in thermodynamics
W = p * v
When there is adiabatic compression or expansion
there is no change in entropy
entropy equation
Δs = ΔQrev/T
Two types of closed cycle power plants
- carnot cycle
- rankine cycle
The carnot cycle
Work goes in at the compressor and leaves at the turbine
Heat comes in at the boiler and leaves at the condensor
Carnot cycle power plant
Q1 - Q2 = Wt - Wcomp
