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BE 350 Final Exam Review
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Gravity
Terms in this set (83)
A system experiencing steady-state heat transfer must be in thermal equilibrium. (T/F)
False
Heat transfer between two bodies occurs only when they are not in equilibrium. (T/F)
True
Which temperature best describes the limit above which most high-order organisms cannot survive?
50 deg C
Which material typically has the higher thermal conductivity?
biological materials (frozen)
What are the units of thermal conductivity?
Btu/(h ft F)
Heat conduction can be described by...
Fourier's Law.
Fourier's Law states that heat flux through a material is equal to:
the negative of the temperature gradient times the thermal conductivity of the material.
Heat convection at a surface can be described by an equation that states that the rate of heat flux is equal to:
the temperature difference between the surface and the surrounding fluid times the convection coefficient.
The Stefan-Boltzman Law states the the maximum radiative heat flux from a body is equal to:
a universal constant times the body temperature to the fourth power.
Which expression best describes a general thermal energy balance on a control volume, based on the first law of thermodynamics?
Ein - Eout + Egen = Estored
∂T/∂t
This expression is best described as what term in the governing equation
storage
Which type of boundary condition would be used to describe a situation that is nearly perfectly insulated?
surface flux specified (Type 2)
Which type of boundary condition would be used to describe a situation with air blowing over the surface?
convection at the surface (Type 3)
In following a good process for solving heat transfer problems, which of the following steps should you do first?
drop irrelevant terms from the general governing equation
If a system is at steady-state, heat can still flow. (T/F?)
True
For steady-state heat conduction in a homogeneous, symmetrical slab with internal heat generation, the temperature profile inside the slab is...
quadratic
For steady-state heat conduction in a non-homogeneous (composite, multi-layer) slab without heat generation, the temperature profile inside the slab is...
multi-step linear
In describing steady-state heat conduction as an analogy to electrical current flow, the term L1/(kA) would best be described as...
resistance
The overall heat transfer coefficient through a composite slab is equal to the sum of the thermal resistances. (T/F?)
False
How many boundary conditions are needed to solve a steady-state heat conduction problem in a slab or cylinder?
2
It is possible for a heat transfer extended fin to reduce heat transfer from the base surface. (T/F?)
True
The lumped parameter method for solving unsteady-state heat conduction problems involves which assumption?
negligible internal thermal resistance
When using the lumped parameter method for solving unsteady-state heat conduction problems, which of the following statements is most true?
Increased surface area to mass ratio decreases time to achieve a given temperature.
For transient heat conduction problems in a symmetrical, 1-D slab, with non-negligible internal resistance and fixed surface temperature, the governing equation reduces to...
a partial differential equation, second-order in position and first-order in time.
The Fourier number (Fo) is a representation of...
dimensionless time.
For "long times", it is acceptable to use only the first term of the infinite series solution for transient heat conduction problems in 1-D slabs. (T/F?)
True
The Heisler charts are graphical solutions for...
the truncated (first term) series solution for transient, 1-D heat conduction.
Which term is not a value represented in the Heisler chart for a symmetrical, infinite slab?
Re (= rho
v
d/mu)
Which statement is most true, regarding solution for transient heat conduction problems in finite geometries?
The dimensionless temperature solution for a finite cylinder is the product of the dimensionless temperatures of an infinite slab and infinite cylinder.
The semi-infinite approximation for solving transient heat conduction problems is acceptable when...
the thickness (L) is larger than 4
(alpha
t)^1/2
The solution to a heat conduction problem in a semi-infinite geometry is...
an expression containing the error function, erf().
The thermal boundary layer is defined as the distance from the surface where the fluid temperature is 0.99 times the fluid bulk temperature. (T/F?)
False
The definition of the convective heat transfer coefficient (h) is derived from which of the following analyses conducted at the surface between a solid and fluid phase?
Conduction in the fluid at the surface is equal to convection at the surface.
The Nusselt number (Nu) is not equal to:
(viscosity)x(specific heat)/(thermal conductivity)
The Grashof number (Gr) is equal to:
the ratio of buoyancy force to viscous force.
The convection coefficient h varies with location across a surface. (T/F?)
True
The "film temperature" is...
the average of the surface and bulk fluid temperatures.
All things being equal, which yields the larger convection coefficient?
natural convection at a horizontal surface, with the hot side facing up.
For forced convection over a flat plate, the Nusselt number is not directly related to...
the Raleigh number
Which is greater?
latent heat of fusion or latent heat of vaporization
latent heat of vaporization
Which of the following occurs when solid water changes to liquid water?
specific heat increases
For a dilute solution, ...
the freezing point decreases linearly with increasing solute molality.
Which is not an assumption of Plank's equation for predicting freezing time?
The latent heat of freezing is negligible.
A transfer medium (e.g., air or water) is required for radiative heat transfer. (T/F?)
False
Which wavelength range best describes thermal radiation?
10^-1 to 10^2 microns
For radiative energy incident on a surface, reflectivity + absorptivity + transmissivity = 1. (T/F?)
True
Radiative flux through a biological material...
decreases exponentially as the depth below the surface increases, due to absorption by the material.
Which of the following electromagnetic wave types has the greatest penetration depth in biological materials?
radio-frequency
The power (W per area per wavelength) emitted by a black body at temperature T is equal at all wavelengths within the infrared range. (T/F?)
False
When calculating radiative flux between two bodies, which factor is not lumped into the view factor?
temperature
It is possible for a plant surrounded by night air at 4degC to freeze (i.e., have its tissue temperature reduced below 0degC). (T/F?)
True
In a gaseous mixture (assumed ideal), the concentration of any one species is inversely proportional to its partial pressure. (T/F?)
False
If a container of water containing N2 in solution is in equilibrium with the surroundings, but then is heated, what will happen?
N2 will leave the solution.
When a gas (e.g., moist air) and a solid are in equilibrium, the concentration of water in the solid is equal to the concentration of water in the air (kg/kg). (T/F?)
False
Are most biological materials hygroscopic or non-hygroscopic?
hygroscopic
If the disappearance of a pollutant in water follows first-order reaction kinetics, which is true?
The concentration decreases as an exponential decay with time.
The Arrhenius law states that the rate of a chemical reaction...
increases non-linearly with temperature.
The empirical equation describing the movement of a liquid in a porous material is...
Darcy's Law
The matric potential for liquid flow in a porous medium (e.g., soil) encompasses the effects of liquid-to-solid adhesion, capillarity, and osmotic potential. (T/F?)
True
For a given problem with liquid flow through a porous medium, which value is greater?
true average fluid velocity
The hydraulic conductivity (K) of a porous medium is a function of both fluid properties and the physical characteristics of the matrix. (T/F?)
True
Which driving force for mass flow is directly affected by temperature, as reflected by T showing up in the governing law/equation?
osmotic pressure
Fick's law of diffusion is analogous to which heat transfer law?
Fourier's Law
Dispersion of a mass species in a fluid system is a result primarily of...
turbulent flow.
What are the units of the convective mass transfer coefficient (hm)?
m/s
If the general governing equation for 1-D mass transfer, each term has what units?
kg/(m3s)
When a convective mass transfer boundary condition occurs, the concentration of species A in the solid at the surface is equal to the concentration of species A in the surrounding fluid. (T/F?)
False
Steady-state mass transfer occurs when a system is at equilibrium. (T/F?)
False
In diffusion of a mass species "A" through a solid slab (with a liquid on either side of the slab), the concentration of "A" in the slab, right at the slab surface is equal to...
the concentration of 'A' in the liquid right at the surface times a distribution coefficient (K*).
In the electrical resistance analogy for mass transfer, 1/hm represents which type of resistance?
convective
If steady-state diffusion of species "A" is occurring through a slab, which of the following is true?
Greater flux of 'A' occurs at the higher-concentration surface when there is first-order decay of 'A' in the slab then when there is no reaction.
The lumped parameter method for mass transfer of water out of a solid is most typically valid for which situation?
very wet solids
For transient diffusion of species "A" in a solid with non-negligible internal resistance, the concentration of species "A" at any point in the solid is a linear function of time. (T/F?)
False
The Heisler charts for unsteady-state mass transfer are based on a truncated series solution of the governing equation for mass transfer. (T/F?)
True
For unsteady-state diffusion of species "A" in a solid with non-negligible internal resistance and a non-negligible convective boundary condition, the Heisler charts can...
be used to solve the problem, given changes in the formulas (relative to the heat transfer solutions) for the dimensionless concentration and 'm'.
An unsteady-state mass transfer problem can be treated as a semi-infinite problem when the thickness of the material is...
>4(DAt)1/2
Dispersion of a mass species "A" during fluid flow is primarily and uniquely caused by what driving force?
turbulence
In convective-diffusion of water vapor through stagnant air (e.g.,
vertically out of a flask, as shown in the figure), the net flux of
water vapor is...
directly proportional to the diffusivity of water vapor in air.
If the moisture content of a stream of air flowing over a flat plate of biological material is in equilibrium with the moisture content of the plate, how thick is the concentration boundary layer in the air? (Hint: This is a "slightly tricky" question, so think about it carefully, in the context of the reading material on boundary layers.
0
The Schmidt number is defined as:
μ/ρDAB
The Sherwood number is defined as:
hmL/DAB
The equations used for Nusselt number correlations in heat transfer, where Nu=f(Re,Pr), can be used to calculate hm in mass transfer problems, given which of the following sets of substitutions.
Substitute Sh for Nu and Sc for Pr
The primary assumption in the classic wet-bulb thermometer analysis (of combined heat and mass transfer) is "Heat gained by convection equals heat loss due to evaporation." (T/F?)
True
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