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4B4 - Heat Transfer > overview > Flashcards

overview Flashcards

1
Q

What is heat transfer?

A

Heat transfer is the movement of thermal energy due to a temperature difference, occurring from a region of higher temperature to a region of lower temperature.

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2
Q

What are the three modes of heat transfer?

A

Conduction: Heat transfer through direct contact.
Convection: Heat transfer through fluid motion.
Radiation: Heat transfer through electromagnetic waves.

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3
Q

What is Fourier’s Law of Heat Conduction?

A

Fourier’s law states that heat flux is proportional to the temperature gradient: q = -k(dT/dx), where q is the heat flux, k is the thermal conductivity, and dT/dx is the temperature gradient.

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4
Q

What is the formula for thermal resistance in conduction for a plane wall?

A

Rcond = L / (kA), where L is the wall thickness, k is the thermal conductivity, and A is the area.

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5
Q

What is convection?

A

Convection is heat transfer between a solid surface and a moving fluid, involving both conduction at the surface and fluid motion.

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6
Q

What is the formula for convective heat transfer rate?

A

Q = hAΔT, where h is the convective heat transfer coefficient, A is the area, and ΔT is the temperature difference.

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7
Q

What is the Nusselt number (Nu)?

A

The Nusselt number is a dimensionless number representing the ratio of convective to conductive heat transfer across a boundary.

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8
Q

What is the formula for the convective heat transfer coefficient (h)?

A

h = (Nu * k) / L, where Nu is the Nusselt number, k is the thermal conductivity, and L is the characteristic length.

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9
Q

What is the Reynolds number (Re)?

A

The Reynolds number is a dimensionless number that indicates whether fluid flow is laminar or turbulent.

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10
Q

What is the formula for the Reynolds number?

A

Re = (ρvL) / μ, where ρ is density, v is velocity, L is the characteristic length, and μ is dynamic viscosity.

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11
Q

What is the Prandtl number (Pr)?

A

The Prandtl number is a dimensionless number comparing momentum diffusivity to thermal diffusivity of a fluid.

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12
Q

What is the formula for the Prandtl number?

A

Pr = (cpμ) / k, where cp is specific heat at constant pressure, μ is dynamic viscosity, and k is thermal conductivity.

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13
Q

What is the Grashof number (Gr)?

A

The Grashof number is a dimensionless number representing the ratio of buoyancy to viscous forces in natural convection.

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14
Q

What is the formula for the Grashof number?

A

Gr = (gβΔTL³) / ν², where g is gravitational acceleration, β is thermal expansion coefficient, ΔT is the temperature difference, L is characteristic length, and ν is kinematic viscosity.

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15
Q

What is radiation?

A

Radiation is heat transfer through electromagnetic waves, requiring no medium.

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16
Q

What is the Stefan-Boltzmann Law?

A

The Stefan-Boltzmann Law states that the total radiant energy emitted by a blackbody per unit area is proportional to the fourth power of its absolute temperature.

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17
Q

What is the formula for heat transfer rate by radiation between two surfaces?

A

Q = σ(T₁⁴ - T₂⁴) / (1/ε₁ + 1/ε₂ - 1), where σ is the Stefan-Boltzmann constant, T₁ and T₂ are absolute temperatures, and ε₁ and ε₂ are emissivities.

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18
Q

What is emissivity (ε)?

A

Emissivity is a measure of a surface’s efficiency in emitting thermal radiation, relative to a perfect blackbody.

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19
Q

What is a heat exchanger?

A

A heat exchanger transfers heat between two fluids at different temperatures without mixing them.

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20
Q

What is the Log Mean Temperature Difference (LMTD) method?

A

The LMTD method calculates heat exchanger performance using the temperature differences of fluids at inlet and outlet.

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21
Q

What is the formula for LMTD?

A

ΔTlm = (ΔT₁ - ΔT₂) / ln(ΔT₁ / ΔT₂), where ΔT₁ and ΔT₂ are the temperature differences at each end of the heat exchanger.

22
Q

What is the Number of Transfer Units (NTU) method?

A

The NTU method analyzes heat exchangers when outlet temperatures are unknown, using effectiveness and the exchanger’s size.

23
Q

What is the formula for NTU?

A

NTU = (U A) / Cmin, where U is overall heat transfer coefficient, A is area, and Cmin is the minimum heat capacity rate.

24
Q

What is the overall heat transfer coefficient (U)?

A

U represents the total thermal conductance, accounting for all modes of heat transfer through a series of layers.

25
Q

What is the formula for the overall heat transfer coefficient for a composite wall?

A

1/U = 1/h₁ + L/k + 1/h₂, where h₁ and h₂ are convective coefficients, L is thickness, and k is thermal conductivity.

26
Q

What is a thermal resistance network?

A

A thermal resistance network models heat transfer analogously to electrical circuits, simplifying complex systems.

27
Q

What is the lumped capacitance method?

A

The lumped capacitance method assumes uniform internal temperature in a body for transient conduction, valid if Bi < 0.1.

28
Q

What is the Biot number (Bi)?

A

Bi is a dimensionless number comparing internal conduction resistance to external convection resistance.

29
Q

What is the formula for the Biot number?

A

Bi = (h Lc) / k, where h is convective coefficient, Lc is characteristic length, and k is thermal conductivity.

30
Q

What is the critical heat flux (CHF)?

A

CHF is the maximum heat flux in nucleate boiling before transitioning to film boiling, where heat transfer efficiency decreases.

31
Q

What is thermal conductivity (k)?

A

Thermal conductivity is a material property that measures its ability to conduct heat, indicating how easily heat travels through the material.

32
Q

What are the units of thermal conductivity?

A

Watts per meter per Kelvin (W/m·K).

33
Q

What factors affect thermal conductivity?

A

Temperature, material structure, density, and moisture content all influence thermal conductivity.

34
Q

What is a thermal insulator?

A

A material that resists heat flow and has a low thermal conductivity.

35
Q

What are some common examples of thermal insulators?

A

Fiberglass, mineral wool, polystyrene foam, and aerogel.

36
Q

What is a thermal conductor?

A

A material that allows heat to pass through it easily, having a high thermal conductivity.

37
Q

What are some common examples of thermal conductors?

A

Metals like copper and aluminum, diamond, and graphite.

38
Q

What is film condensation?

A

Condensation that forms a continuous liquid film on a cool surface, reducing heat transfer compared to dropwise condensation.

39
Q

What is dropwise condensation?

A

Condensation occurring as discrete droplets on a surface, enhancing heat transfer due to easier removal of liquid.

40
Q

What are some factors affecting the condensation heat transfer coefficient?

A

Surface properties, vapor properties, and fluid properties all influence the condensation heat transfer coefficient.

41
Q

What is a heat pipe?

A

A passive device that uses a working fluid’s evaporation and condensation to transfer heat efficiently.

42
Q

How does a heat pipe work?

A

Heat evaporates the fluid at the hot end, the vapor moves to the cool end and condenses, then the fluid returns via a wick to repeat the cycle.

43
Q

What are the advantages of using a heat pipe?

A

High thermal conductivity, passive operation, and compactness.

44
Q

What are the applications of heat pipes?

A

Used in electronics cooling, aerospace thermal management, and solar energy systems.

45
Q

What is the boiling point of a liquid?

A

The temperature at which its vapor pressure equals the surrounding pressure, allowing the liquid to boil throughout its volume.

46
Q

What factors affect the boiling point of a liquid?

A

Pressure, impurities, and intermolecular forces.

47
Q

What is nucleate boiling?

A

A boiling regime where vapor bubbles form at nucleation sites on a heated surface, enhancing heat transfer.

48
Q

What is film boiling?

A

A high heat flux regime where a vapor film forms between the surface and liquid, reducing heat transfer.

49
Q

What is the Leidenfrost effect?

A

A phenomenon where a liquid droplet levitates on a hot surface due to a vapor layer, slowing evaporation.

50
Q

What is the capillary limit in heat pipes?

A

The maximum heat transport capacity determined by the wick’s ability to return liquid to the heated section.

51
Q

What is the difference between steady-state and transient heat transfer?

A

Steady-state involves no change in temperature over time, while transient involves temperature changes until equilibrium is reached.

4B4 - Heat Transfer (2 decks)

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