overview Flashcards
What is heat transfer?
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.
What are the three modes of heat transfer?
Conduction: Heat transfer through direct contact.
Convection: Heat transfer through fluid motion.
Radiation: Heat transfer through electromagnetic waves.
What is Fourier’s Law of Heat Conduction?
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.
What is the formula for thermal resistance in conduction for a plane wall?
Rcond = L / (kA), where L is the wall thickness, k is the thermal conductivity, and A is the area.
What is convection?
Convection is heat transfer between a solid surface and a moving fluid, involving both conduction at the surface and fluid motion.
What is the formula for convective heat transfer rate?
Q = hAΔT, where h is the convective heat transfer coefficient, A is the area, and ΔT is the temperature difference.
What is the Nusselt number (Nu)?
The Nusselt number is a dimensionless number representing the ratio of convective to conductive heat transfer across a boundary.
What is the formula for the convective heat transfer coefficient (h)?
h = (Nu * k) / L, where Nu is the Nusselt number, k is the thermal conductivity, and L is the characteristic length.
What is the Reynolds number (Re)?
The Reynolds number is a dimensionless number that indicates whether fluid flow is laminar or turbulent.
What is the formula for the Reynolds number?
Re = (ρvL) / μ, where ρ is density, v is velocity, L is the characteristic length, and μ is dynamic viscosity.
What is the Prandtl number (Pr)?
The Prandtl number is a dimensionless number comparing momentum diffusivity to thermal diffusivity of a fluid.
What is the formula for the Prandtl number?
Pr = (cpμ) / k, where cp is specific heat at constant pressure, μ is dynamic viscosity, and k is thermal conductivity.
What is the Grashof number (Gr)?
The Grashof number is a dimensionless number representing the ratio of buoyancy to viscous forces in natural convection.
What is the formula for the Grashof number?
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.
What is radiation?
Radiation is heat transfer through electromagnetic waves, requiring no medium.
What is the Stefan-Boltzmann Law?
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.
What is the formula for heat transfer rate by radiation between two surfaces?
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.
What is emissivity (ε)?
Emissivity is a measure of a surface’s efficiency in emitting thermal radiation, relative to a perfect blackbody.
What is a heat exchanger?
A heat exchanger transfers heat between two fluids at different temperatures without mixing them.
What is the Log Mean Temperature Difference (LMTD) method?
The LMTD method calculates heat exchanger performance using the temperature differences of fluids at inlet and outlet.
What is the formula for LMTD?
ΔTlm = (ΔT₁ - ΔT₂) / ln(ΔT₁ / ΔT₂), where ΔT₁ and ΔT₂ are the temperature differences at each end of the heat exchanger.
What is the Number of Transfer Units (NTU) method?
The NTU method analyzes heat exchangers when outlet temperatures are unknown, using effectiveness and the exchanger’s size.
What is the formula for NTU?
NTU = (U A) / Cmin, where U is overall heat transfer coefficient, A is area, and Cmin is the minimum heat capacity rate.
What is the overall heat transfer coefficient (U)?
U represents the total thermal conductance, accounting for all modes of heat transfer through a series of layers.
What is the formula for the overall heat transfer coefficient for a composite wall?
1/U = 1/h₁ + L/k + 1/h₂, where h₁ and h₂ are convective coefficients, L is thickness, and k is thermal conductivity.
What is a thermal resistance network?
A thermal resistance network models heat transfer analogously to electrical circuits, simplifying complex systems.
What is the lumped capacitance method?
The lumped capacitance method assumes uniform internal temperature in a body for transient conduction, valid if Bi < 0.1.
What is the Biot number (Bi)?
Bi is a dimensionless number comparing internal conduction resistance to external convection resistance.
What is the formula for the Biot number?
Bi = (h Lc) / k, where h is convective coefficient, Lc is characteristic length, and k is thermal conductivity.
What is the critical heat flux (CHF)?
CHF is the maximum heat flux in nucleate boiling before transitioning to film boiling, where heat transfer efficiency decreases.
What is thermal conductivity (k)?
Thermal conductivity is a material property that measures its ability to conduct heat, indicating how easily heat travels through the material.
What are the units of thermal conductivity?
Watts per meter per Kelvin (W/m·K).
What factors affect thermal conductivity?
Temperature, material structure, density, and moisture content all influence thermal conductivity.
What is a thermal insulator?
A material that resists heat flow and has a low thermal conductivity.
What are some common examples of thermal insulators?
Fiberglass, mineral wool, polystyrene foam, and aerogel.
What is a thermal conductor?
A material that allows heat to pass through it easily, having a high thermal conductivity.
What are some common examples of thermal conductors?
Metals like copper and aluminum, diamond, and graphite.
What is film condensation?
Condensation that forms a continuous liquid film on a cool surface, reducing heat transfer compared to dropwise condensation.
What is dropwise condensation?
Condensation occurring as discrete droplets on a surface, enhancing heat transfer due to easier removal of liquid.
What are some factors affecting the condensation heat transfer coefficient?
Surface properties, vapor properties, and fluid properties all influence the condensation heat transfer coefficient.
What is a heat pipe?
A passive device that uses a working fluid’s evaporation and condensation to transfer heat efficiently.
How does a heat pipe work?
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.
What are the advantages of using a heat pipe?
High thermal conductivity, passive operation, and compactness.
What are the applications of heat pipes?
Used in electronics cooling, aerospace thermal management, and solar energy systems.
What is the boiling point of a liquid?
The temperature at which its vapor pressure equals the surrounding pressure, allowing the liquid to boil throughout its volume.
What factors affect the boiling point of a liquid?
Pressure, impurities, and intermolecular forces.
What is nucleate boiling?
A boiling regime where vapor bubbles form at nucleation sites on a heated surface, enhancing heat transfer.
What is film boiling?
A high heat flux regime where a vapor film forms between the surface and liquid, reducing heat transfer.
What is the Leidenfrost effect?
A phenomenon where a liquid droplet levitates on a hot surface due to a vapor layer, slowing evaporation.
What is the capillary limit in heat pipes?
The maximum heat transport capacity determined by the wick’s ability to return liquid to the heated section.
What is the difference between steady-state and transient heat transfer?
Steady-state involves no change in temperature over time, while transient involves temperature changes until equilibrium is reached.
