Heat Transfer and Exchangers Flashcards
What is the unit of the overall heat transfer coefficient (๐)?
W/mยฒยทK
The Nusselt number (๐๐ข) represents what?
Ratio of convective to conductive heat transfer
The Biot number (๐ต๐) is used to determine which type of heat transfer condition?
Unsteady-state heat transfer
What does the Fourier number (๐น๐) represent in unsteady-state heat transfer?
Ratio of conduction to heat storage
A material with a high thermal diffusivity (๐ผ) will:
Conduct heat quickly relative to its ability to store heat
When the Biot number (๐ต๐) is less than 0.1, which heat transfer method can be applied?
Lumped capacitance method
What is the primary reason for using the logarithmic mean temperature difference instead of a simple average temperature difference in heat exchanger calculations?
It accounts for varying temperature differences along the exchanger
The overall heat transfer coefficient (๐) depends on what factors?
Convection heat transfer coefficients of the fluids
Thermal conductivity of the heat exchanger material
Thickness of the heat exchanger walls
What is the key advantage of a counter-current heat exchanger over a co-current heat exchanger?
Higher heat transfer efficiency
In which type of heat exchanger configuration can the outlet temperature of the cold fluid exceed the outlet temperature of the hot fluid?
Counter-current flow
When Bi is large, _____ ______ is negligible
Convection Resistance
When Bi is small, _____ _____ is negligible
Conduction Resistance
When Bi is less than 0.1:
Lumped Capacitance method is used
When Bi is greater than 40
The surface temperature is nearly equal to the fluid temperature
Heisler charts are used when:
Bi falls between 0.1 and 40
What are the units of h?
W/mยฒยทK
What are the units of k?
W/mยทK
What are the units of thermal resistance?
mยฒยทK/W
What are the units of ฮฑ?
mยฒ/s
What is ฮฑ?
Thermal diffusivity
Thermal diffusivity
how quickly heat spreads through a substance
Heat Conduction Through a Cylindrical Pipe
Q = 2ฯkL(T1โT2) / ln (r2/r1)
โ
