Heat Transfer Flashcards
The second law tells us that energy has quality as well as…?
…quantity
The direction of heat flow is governed by the…?
…2nd Law of Thermodynamics
Heat can be transferred by…?
…conduction, convection, and radiation
The rate at which heat transfers is measured in…?
Joules per second (J/s), or Watts (W)
The rate of heat transfer is given the symbol…
In fluids, conduction is due to…?
…collision and diffusion of the molecules
In solids, conduction is due to the…
…vibration of the molecules in a lattice
The rate at which conduction occurs in a medium depends on…(3 things)
- Geometry of the medium
- Thickness of the medium
- Temperature difference across the medium
Give a property of an insulator?
Low rates of heat conduction
Steady-state heat conduction is governed by Fourier’s Law
What is his law? (Equation)
“Rate of heat conduction is proportional to…”
What is k?
- Rate of heat conduction is proportional to the area of the body, the temperature difference across it, and its thickness
- k is the thermal conductivity
What is Natural convection (or free convection)
Natural convection is when the fluid motion is not generated by any external source (like a pump, fan, suction device, etc.) but only by density differences in the fluid occurring due to temperature gradients
What is Forced convection?
Fluid is forced to flow over the solid surface by external forces (i.e. a fan, the wind)
Convection processes are expressed through Newton’s Law of Cooling
What is the equation for this?
What is this called?
And what does each symbol stand for?
Newton’s Law of Cooling
- h is the convection heat transfer coefficient
- As is the surface area of the body
- Ts is the surface temperature
- T∞ is the temperature of the fluid far from the surface
Radiation is a volumetric phenomenon
Define this
all solids, liquids and gases absorb and emit radiation to some degree
An idealised body which emits radiation at this maximum rate is called a…?
…blackbody
Radiation heat transfer is expressed by the Stefan-Boltzmann Law
What is the equation for this?
What is σ (sigma) in this equation?
σ is the Stefan-Boltzmann constant 5.67x10-8Wm-2K-4
What form of heat transfer does
Opaque, Gas and Vacuum do?
The rate of heat conduction is…
…inversely proportional to the thickness of the layer
A high value of k implies that the material is a ____ ____
A low value of k implies ____ ____ ____ - i.e. an ____
A high value of k implies that the material is a good conductor
A low value** of k implies **poor heat conduction - i.e. an insulator
When you intergrate and rearrange what do you get?
Rwall = ?
And what is Rwall ?
Rwall = L/kA
Rwall is called the thermal resistance of the wall
- Rate of heat transfer corresponds to the ____ ____
- Thermal resistance corresponds to the ____ ____
- Temperature difference corresponds to the ____ ____
- Rate of heat transfer corresponds to the electric current
- Thermal resistance corresponds to the electrical resistance
- Temperature difference corresponds to the potential difference
What does this correspond to in terms of an electric current? (Equation)
Qconv = ?
Rconv = ?
Under steady conditions, the rate of heat convection into the wall… = ? = ?
Under steady conditions, the rate of heat convection into the wall = rate of heat conduction through the wall = rate of heat convection from the wall
Rcond = ?
Rconv = ?
Rcond = L / kA
Rconv = 1 / hAs
Area of a sphere A = ?
A = 4πr2
For a hollow cylinder (or sphere) undergoing heat transfer by conduction with convection, the total thermal resistance can be denoted as
Rtotal = ?
Rtotal = Rconv,1 + Rcyl + Rconv,2
Plot the variation of heat transfer with insulating material radius, r2
- The variation is not linear
- There is a critical radius at which heat transfer is a maximum
For a cylinder, the critical radius is…
For a sphere, the critical radius is…
<span>r</span>cr,cylinder = k/h
<span>r</span>cr,sphere = 2k/h
For common insulating materials, the critical radius is of the order of…?
Insulation on electrical wires is of the order of…?
…1 cm
…1 mm thick
The convective heat transfer coefficient depends on several flow properties and also depends on whether the flow is…
…laminar or turbulent in nature
What is the no-temperature-jump condition
That if a surface has some temperature T, it follows from the no-slip condition that the temperature of the fluid one wall should also be T
Place a fluid layer between two plates of differing temperature;
if the fluid is motionless, the heat transfer will be by ____ only,
if the fluid is in motion, heat transfer will be by ____
Place a fluid layer between two plates of differing temperature;
if the fluid is motionless, the heat transfer will be by conduction only,
if the fluid is in motion, heat transfer will be by convection
Define heat flux
Rate of heat transfer per unit time per unit surface area
Heat flux equation for conduction
Heat flux equation for convection
What is the equation for the Nusselt number?
Define Prandtl’s number and the equation
The Prandtl number is defined as the ratio of momentum diffusivity to heat diffusivity (i.e. the relative thickness of the velocity and thermal boundary layers)
A low Prandtl number means that…
…the thermal boundary layer is thicker than the velocity boundary layer (high heat diffusivity rate)
For a flat plate, the critical Reynolds number (i.e. the Reynolds number at the x-location where transition occurs)… = ?
For Re number,
Laminar < ? < Turbulent
Laminar < 5x105 < Turbulent
Fluid properties are evaluated at the…?
(Name and equation)
Film temperature
- µs is the flow dynamic viscosity at the ____ of the ____
- µ∞ is the dynamic viscosity in the ____ ____
- µs is the flow dynamic viscosity at the surface of the sphere
- µ∞ is the dynamic viscosity in the freestream flow
As with flat plate flows, the flow in a pipe depends on the…(Equation as well)
…Reynolds number
- For Re < 2300, the flow is ____
- For Re > 4000, the flow is ____
- For Re < 2300, the flow is laminar
- For Re > 4000, the flow is turbulent
For the thermal boundary layer, the entrance length for laminar flow can be obtained from is…(equation)
For turbulent flow, the entrance length for the thermal boundary layer is approximately…
…10 pipe diameters
For a rough pipe, the ____ Chart can be used to determine the ____ ____
For a rough pipe, the Moody Chart can be used to determine the Nusselt number
High temperature fluids have a ____ ____ than colder fluids
High temperature fluids have a lower density than colder fluids
What is the equation of state for an ideal gas?
The Grashof number represents the ratio of…?
…the buoyant force to the viscous force
What is the equation for the Grashof Number?
(We are expected to remember this)
What does every symbol in this equation stand for?
What is the equation for the Rayleigh number of the flow?
But can alos be obtained by…?
…
Natural convection from a vertical plate - flow pattern is different when the flow is…?
…laminar or turbulent
When is the Nusselt number vertically and horizontally laminar?
Vertically when
104 < Ra < 109
Horizontally when
104 < Ra <<sup> </sup>107
When is the Nusselt number vertically and horizontally turbulent?
Vertically when
Ra > 109
Horizontally when
Ra > 107
Nusselt number when Laminar flow, horizontal plate face down
105 < Ra < 1011
c (speed of light) = ?
And define all symbols
c is the speed of light
funny thing is the wavelength
v is the frequency
e (energy) = ? = ?
And what is h?
h is Planck’s constant (6.6256x10-34 Js)
The shorter the wavelength of the photon, the higher its…?
…energy
Define a blackbody and give an example
A blackbody is defined as a perfect emittier and absorber of radiation
A good blackbody is the sun (or every star) because sthe surface termperature barely changes
What is the Stefan-Boltzmann Law (equation)
A radiator works by…?
…convection current
What is ε?
ε is the emissivitiy of the surface
The Stefan-Boltzmann Law tells us the…
…heat transfer rate over all wavelengths
The colour of room temperature object is due to selective reflection of radiation by the surface
(try to remember picture/diagram)
Kirchoff’s law for absorbtions states…
…that the emissivity and absorptivity for a body are the same
If a smaller body is enclosed in a larger body, the net rate of heat transfer is given by…(equation)
(Not sure how important this is)
What is the solar flux at the sun, S0, using the Stegan Boltzmann law? (Equation)
Solar flux decreases with the square of distance from the object so S = ?
The Earth is 1 Astronomical Unit (AU) away from the Sun - 1AU = ?
1.49x1011m
Tln = ?
Q = __ - __ / _____
Q = T1 - T∞ / Rtotal
