FIG_2.4 // HEX_PRISM
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THERMODYNAMICS &
HEAT TRANSFER

Fundamental concepts, energy balance, and the relationship between thermodynamics and heat transfer mechanisms.

01 // BASIC CONCEPTS

Thermodynamics vs. Heat Transfer

Thermodynamics

Concerned with the amount of heat transfer as a system undergoes a process from one equilibrium state to another. Deals with equilibrium states.

Heat Transfer

Deals with the determination of the rates of thermal energy transfer. How fast energy moves.

Key Principles

  • Heat transfer is always from higher temperature to lower temperature.
  • Heat transfer stops when the two mediums reach the same temperature (Thermal Equilibrium).
  • Energy can exist in thermal, mechanical, kinetic, potential, electrical, magnetic, chemical, and nuclear forms.

02 // ENERGY & SPECIFIC HEATS

Internal Energy (U)

Sum of all microscopic forms of energy (kinetic and potential energies of molecules).

Sensible HeatKinetic energy of molecules
Latent HeatPhase change energy
Chemical/NuclearAtomic bonds

Enthalpy (h)

Combination property convenient for open systems (fluid flow).

h=u+Pvh = u + Pv

Pv = Flow work (energy to push fluid)

Specific Heats

CvConstant Volume
CpConstant Pressure
Cp > Cv (includes expansion work)

Ideal Gas Relations

Pv=RTPv = RT
Cp=Cv+RC_p = C_v + R
du=CvdT,dh=CpdTdu = C_v dT, \quad dh = C_p dT

03 // FIRST LAW OF THERMODYNAMICS

The conservation of energy principle: Energy can neither be created nor destroyed; it can only change forms.

Ein−Eout=ΔEsystemE_{\text{in}} - E_{\text{out}} = \Delta E_{\text{system}}

Closed System (Fixed Mass)

Q−W=ΔUQ - W = \Delta U

For stationary systems (negligible KE, PE changes).

Steady Flow System (Control Volume)

E˙in=E˙out\dot{E}_{\text{in}} = \dot{E}_{\text{out}}
Q˙−W˙=m˙(hout−hin)\dot{Q} - \dot{W} = \dot{m}(h_{\text{out}} - h_{\text{in}})

04 // HEAT TRANSFER MODES

01

Conduction

Transfer of energy from more energetic particles to less energetic ones due to interactions (collisions/diffusion).

Q˙=−kAdTdx\dot{Q} = -kA \frac{dT}{dx}
02

Convection

Energy transfer between a solid surface and the adjacent liquid or gas that is in motion.

Q˙=hA(Ts−T∞)\dot{Q} = hA(T_s - T_\infty)
03

Radiation

Energy emitted by matter in the form of electromagnetic waves (or photons). No medium required.

Q˙=εσA(Ts4−Tsurr4)\dot{Q} = \varepsilon \sigma A (T_s^4 - T_{\text{surr}}^4)

05 // FOURIER'S LAW OF CONDUCTION

The Rate Equation

The rate of heat conduction is proportional to the area normal to the direction of heat transfer and the temperature gradient in that direction.

Q˙cond=−kAdTdx\dot{Q}_{\text{cond}} = -kA \frac{dT}{dx}
k: Thermal conductivity [W/m·K]
dT/dx: Temperature gradient
Negative sign: Ensures positive heat flow in direction of decreasing temperature.
[DIAGRAM: Heat Flow Through Plane Wall]
THERMO_VER: 2.0.0
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