Heat Transfer

Question 1

Conduction, Fourier’s Law

Answer 1

Q/A = - K dT/dx

• Heat transferred (Q)
• Area through which the heat is transferred (A)
• Thermal conductivity of material (K)
• Temperature (T)

Question 2

Convection, Newton’s Law

Answer 2

Q = Ah(Tw - Tf)

• Heat transferred (Q)
• Area through which the heat is transferred (A)
• Temperature wall/surface (Ts)
• Temperature fluid (Tf)

Question 3

Radition, Stephan-Boltzmann

Answer 3

Q = σAT^4
- Heat transferred (Q)
- Stephan-Boltzmann Constant 5.67 x 10-8[W/m2K4]
- Area through which the heat is transferred (A)
- Temperature (T)

Question 4

what is Steady-state conduction

Answer 4

Heat transfer where the temperature distribution does not change with time.

• Q is constant.
• No heat accumulation in the material.

Question 5

How does thermal conductivity (K) vary with temperature for metals vs. non-metals?

Answer 5

Metals: K decreases as T increases (except non-ferrous metals, where K increases).

Non-metals: K generally increases with T.

Question 6

What is superinsulation, and where is it used?

Answer 6

• Multiple reflective layers with insulating spacers, evacuated to minimize air conduction.
• Used for cryogenic liquid storage (e.g., hydrogen at -250°C).

Question 7

What is the difference between natural and forced convection?

Answer 7

• Natural: Fluid movement due to density differences (e.g., hot air rising).
• Forced: Fluid movement due to external forces (e.g., fans, pumps).

Question 8

What is the key assumption in steady-state conduction problems?

Answer 8

• No variation of Q with time or in directions other than the primary axis (e.g., x).