Everything on MEC203 Flashcards
Gain a good understanding of the basic concepts to enable you to answer the questions.
What is Beta?
It is the volume expansion coefficient. Variation of fluid density with temperature at constant pressure.
What does the Grashof (Gr) number represent?
The natural convection effects in the momentum equation.
What does the Biot number represent?
The ratio of convection at the surface of a body to conduction within the body.
What kind of bodies are most likely to satisfy the condition for lumped system analysis?
Small bodies with large thermal conductivities and low convection coefficients.
What is a thermal boundary layer?
The flow region over the surface in which the temperature difference in the direction normal to the surface is significant.
What does the Nusselt number represent?
The enhancement of heat transfer through a fluid layer as a result of convection relative to conduction across the SAME fluid layer.
True or false, the larger the Nusselt number, the more effective the conduction?
FALSE. The larger the Nusselt number, the more effective the convection.
What is the Fourier number?
The ratio:
The rate at which heat is conducted across a body of length L and volume L^3.
—————————–DIVIDE—————————————–
The rate at which heat is stored in a body of L^3.
What does a large Fourier number suggest?
A large propagation of heat through a body.
What is the Biot number?
The ratio of external convection resistance to internal diffusion resistance.
What is FILM temperature?
Tf=0.5*(Ts+Tinfinity)
What is the Rayleigh number?
A dimensionless parameter that measures the instability of a layer of fluid due to differences in temperature and density at the top and bottom of the flow.
Describe the no-slip condition.
It occurs when a fluid in direct contact with a surface sticks to the surface and there isn’t any slip. It is due to the fluid’s viscosity.
What does the friction coefficient represent in fluid flow over a flat plate?
Represents the resistance to fluid flow over a flat plate.
What is turbulent thermal conductivity (kt)?
It accounts for thermal energy transport by turbulent eddies.
Why nondimensionalise the convection equation?
- Reduction in parameters. You go from six to two (Re and Pr)
- Allows you to group a large number of experiments and compare data
Define isothermal.
No change in temperature.
What is the definition of heat flux (q)?
q=-k(dt/dy)
True or false, for an ideal gas, kinematic viscosity is inversely proportional to pressure?
TRUE
True or false, for an ideal gas, thermal conductivity and Prandtl number are independent of pressure.
TRUE
What is the Prantdl Number?
It is a dimensionless number, representing the ratio of momentum diffusivity to thermal diffusivity.
What is the Nusselt number?
The Nusselt number is the ratio of convective to conductive heat transfer across a boundary.
What is the general formula for Nusselt number to find h?
Nu=(hD/k) where k is thermal conductivity and D is diameter.
What is kinematic viscosity?
Dynamic viscosity PER UNIT DENSITY
What is transient?
Time dependant
What is the Biot Number?
It is a dimensionless parameter (heat transfer coefficient). Bi=hL/k.
What is the time constant in lumped system analysis?
b=(hA)/(ρV*c,p)
What is characteristic length?
Lc=V/A
What does fin effectiveness tell you?
It tells your whether more or less heat is extracted from the surface when the fin is present.
What does fin efficiency tell you?
It compares the heat transfer of the actual fin to the heat transfer of the fin if all the fin was at the base temperature.
What does thermal diffusivity represent?
How fast heat diffuses through a material.
What is the formula for thermal diffusivity?
α=k/ρ(c,p)
What is absorptivity?
The fraction of radiation energy incident on a surface that is absorbed by the surface.
What is emissivity?
The ratio of radiation emitted from a surface compared to that emitted from a blackbody.
What is a blackbody?
The idealised surface that emits radiation at the maximum rate
What is the equation for the similarity variable η? When is it used?
η=x/(4αt)^0.5
Used in transient heat conduction in semi-infinite solids.
What is a formula for Prandtl number?
Pr=(kinematic viscosity(ν))/(thermal diffusivity(α))
What is the temperature of a 100% efficient fin?
It is the fin’s base temperature.
What equation do you use for a fully efficient fin?
Qfinmax=hAfin(tb-tinf)
You assume the entirety of the fin is at the same temperature.
Transient lumped system analysis equation?
(T-Tinf)/(Ti-Tinf)=e^(-bt)
What is Rcond and Rconv (convection and conduction resistance)?
Rcond=L/KA
R conv=1/hA
What is Rcyl and Rsph for conduction?
Rcyl=((lnr2/r1)/(2piLk))
Rsph=(r2-r1)/(4pir1r2k)
Define thermal entry length.
The length of the region of flow in which the thermal boundary layer develops and reaches the tube centre.
Equation for average velocity in internal forced convection.
Vavg = (2/R^2)∫u(r)rdr
between 0 and R
Equation for mean temperature in internal forced convection.
Tm = (2/VavgR^2)∫T(r)u(r)rdr
between 0 and R
True or false: The shear stress at the surface of a tube during fully developed turbulent flow is minimum since the shear stress is proportional to the velocity gradient, which is minimum at the tube surface.
FALSE: The shear stress at the surface of a tube during fully developed turbulent flow is maximum since the shear stress is proportional to the velocity gradient, which is maximum at the tube surface.
What is the equation for maximum velocity for fully developed internal laminar flow?
Vmax=2Vavg
What is the actual definition of the Grashof Number?
The ratio of buoyancy forces to viscous forces acting on a fluid.
What force of the Reynolds Number is replaced by buoyancy in the Grashof Number?
Inertial is replaced by buoyancy.
Ideal gas equation?
P=ρRT
In cylinders, what does To and Ti mean?
To = the temperature along the centreline of the cylinder Ti = The initial temperature of the cylinder (spatially uniform)
What is the name of the transient temperature chart?
Heisler chart.