Fluids Final Exam
In a circular tube with uniform surface temperature and laminar, fully developed conditions, the Nusselt number is independent of Reynolds number and and axial location, but increases with increasing Prandtl number.
False
In the hydrodynamic entrance region a velocity boundary layer develops in the surface of the tube and thins with increasing x.
False
In the turbulent boundary layer, the fluid flow is highly ordered, and it is possible to identify streamlines along which fluid particles move
False
Neglecting temperature gradients within a solid allows you to consider the problem from within the framework of the heat equation.
False
Pipe flow can refers to an enclosed system that can be either partially or completely full of the flowing fluid
False
Reynolds transport theorem can only be used for a steady state system.
False
Specific weight is defined as the ratio of fluid density over the density of water at a given temperature and is not dependent on the unit system used.
False
The actual functional relationship, ϕ, between pi terms can be determined from dimensional analysis.
False
The amount of mass in a system will change with time
False
The biot number is the ratio of the boundary layer thermal resistance to the internal thermal resistance of a solid.
False
The dependent variable can be a repeating variable.
False
The entrance length cannot exceed the pipe length
False
The heat diffusion equation is a differential equation that provides a temperature distribution in a moving medium.
False
The heat transfer rate for a composite plane wall will vary for each material, when assuming steady state, 1D conduction, with no energy generation.
False
The lumped capacitance method is the ONLY way to solve transient conduction problems.
False
The lumped capacitance method may be used If there is a significant temperature gradient within a solid
False
The overall heat transfer coefficient for a system, U, is found using the average resistance for the system
False
The overall heat transfer coefficient for the hot fluid (Uh) will ALWAYS be equal to that of the cold fluid (Uc)
False
The system is characterized by steady-state conditions if the temperature at each point is dependent on time
False
The transport property of a material is independent of its state of matter
False
The use of fins ALWAYS increases the heat transfer rate.
False
The velocity is constant as we move radially outwards from the centerline of the pipe to the pipe wall, for fully developed flow.
False
There is usually only one option for a set of repeating variables.
False
Thermal conductivity (k) is based on the temperature difference within the material.
False
Turbulent flow shear stress is smaller than laminar flow shear stress because of the irregular, random motion.
False
When using the ideal gas law absolute temperatures do not need to be used
False
for laminar flow the friction factor, f, is dependent of both the Reynolds number and relative roughness
False
for smooth pipes the head loss is zero
False
What term corresponds to the definition: The ratio of the fin heat transfer rate to the heat transfer rate that would exist without the fin.
Fin effectiveness
What are ways the assess fin performance? Choose ALL correct answers.
Fin efficency Fin effectiveness
What term corresponds to the definition: the actual fin heat transfer rate, divided by the maximum possible heat transfer rate
Fin efficiency
Which of the following is NOT possible for laminar flow:
Flow is thermally developed but not hydrodynamically developed.
The nusselt number requires knowledge of the thermal conductivity of the:
Fluid
When solving for reaction forces, we use:
Absolute pressures
What is the primary mode of heat transfer for cooling yourself with a fan?
Forced Convection
what law governs conduction heat transfer?
Fourier's law
What is the primary mode of heat transfer for hot air rising, cooling and falling?
Free Convection
Which number is a measure of the ratio of the inertia force on an element of fluid to the weight of the element.
Froude number
The analogy between electrical and thermal circuits is current flow is analogous to
Heat Transfer
A heat exchanger design problem requires determining the:
Heat transfer surface area
The Reynolds Transport Theorem:
All of these are correct: Applies only to conserved properties Applies to steady and unsteady flow situations Allows us to convert the basic equations from a system to a control volume
Complete similarity requires:
All of these are required: Geometric Similarity Dynamic Similarity Kinematic Similarity
What is not a step in internal convection analysis?
All of these are steps for internal convection analysis.
Determine the maximum power output of the turbine when the fluid is water with a density of 1000 kg/m3.
2.23 MW
Exhaust, assumed to have properties of air (density = 0.00238 slugs/ft3), leaves the 4ft diameter chimney shown with a speed of 6 ft/s. Due to the wind, after a few diameters downstream the exhaust flows in a horizontal direction with the speed of the wind, 15 ft/s. Determine the horizontal component of the force the blowing wind exerts on the exhaust gases assuming incompressible flow.
2.7 lb
Determine W1, the relative velocity of water relative to the moving control volume at point 1 (entrance of CV). Vo = 10 m/s and V1 = 30 m/s.
20 m/s right
Ethyl alcohol (specific gravity = 0.79, reference density = 1000 kg/m3 and dynamic viscosity = 1.19 x 10-3 N-s/m2) is flowing through a 25 cm diameter horizontal pipeline. When the velocity is 0.17 m/s, what is the Reynolds Number?
28214
The roof of an electrically heated house is 7 m long, 10 m wide, and 0.25 m thick. It is made of a flat layer of concrete whose thermal conductivity is 0.92 W/m-degree C. During a certain winter night, the temperatures of the inner and outer surfaces of the roof are measured to be 15 degrees C and 4 degrees C, respectively. The average rate of heat loss through the roof that night was
2834 W
A systems behavior is described by 6 variables, and these 6 variables encompass 3 reference dimensions. How many pi terms are needed?
3
Consider a system where we have 4 pi terms. How many similarity requirement(s) are there?
3
A 3-m2 hot black surface (emissivity = 1) at 80 degrees C is losing heat to the surrounding air at 25 degrees C by convection with a convection heat transfer coefficient of 12 W/m2-degree C, and by radiation to the surrounding surfaces at 15 degree C. The total rate of heat loss from the surface is:
3451 W
For a plane wall, steady state, no energy generation, the heat transfer rate and heat flux:
Are constant with x
For the equation which term represents the Dynamic pressure?
B - 1/2pV^2
What is the maximum value of biot number for which the lump capacitance method can be used with minimal errors?
Bi<0.1
In case 1 what is the value of the Biot number?
Bi<<1
In case 2 what is the value of the Biot number?
Bi=1
What number(s) characterize transient conduction problems?
Biot Number and Fourier Number
Fluid properties for Nusselt number correlations should be evaluated at:
Bulk temperature
Properties used in evaluating convective heat transfer coefficients from Nusselt number correlations are evaluated at the:
Bulk temperature
A two-fluid heat exchanger has inlet and outlet temperatures of 65 and 40℃ for the hot fluid and 15 and 30℃ for the cold fluid. What is the configuration of the heat exchanger?
Counterflow or parallel flow - both are possible
A heat exchanger where fluid of different temperatures flow perpendicular to each other is called a _______ heat exchanger
Crossflow
A 1.5-m2 black surface (emissivity = 1) at 120C is losing heat to the surrounding air at 30C by convection with a convection heat transfer coefficient of 18 W/m2.C, and by radiation to the surrounding surfaces at 10C. The total rate of heat loss from the surface is :
3913 W
The wind blows through a 7 ft x 10 ft garage door opening with a speed of 5 ft/s as shown. Determine the average speed V of the air through the two 3 ft x 4 ft openings in the window. Assume steady and incompressible flow
5 ft/s
Water flows steadily into and out of the configuration with four pipes shown below. The CV is the entire pipe configuration. The mass flowrate through three of the pipes in kg/s is indicated. The mass flowrate m ̇ in the fourth pipe is:
5 kg/s
Crude oil of specific gravity 0.82 is flowing at a velocity of 8.15 m/s through a pipe of diameter 1.25 m with a friction factor of 0.02. The head loss (m) due to viscous effects over 100 m of the pipe is most nearly
5.4 m
A 1/6th scale model of a new automobile design is tested in a wind tunnel. The Reynolds number of the model is the same as that of the full-scale prototype. Assuming the model and the prototype are exposed to the same air conditions, the velocity in the wind tunnel is then:
6 times that of the full-scale vehicle
Heat always flows from
Hot to cold
A long, circular aluminum rod is attached at one end to a heated wall and transfers heat by convection to a cold fluid. Assume we can treat this as an infinitely long fin. If the diameter of the rod is tripled, would the heat transfer rate:
Increase
If the radius of insulation is LESS than the critical radius of insulation, heat transfer rate is _______________ due to the insulation.
Increased
For fully developed laminar flow, as the Reynolds number decreases, the friction factor:
Increases
There is a steady laminar flow of water in a horizontal pipe of length l. As the volume flowrate increases, the pressure drop over the length l:
Increases
For an bulk isotropic material, the thermal conductivity (k) is [ Select ] of the direction of heat transfer
Independent
Pressure at a point in a fluid at rest is ________________ as long as there are no shearing stresses present.
Independent of direction
Baffles can be used to (Choose ALL correct answers)
Induce cross-flow velocities Increase turbulence Increase convection coefficients Reduce tube vibrations
Consider the general Reynolds Transport Theorem. The second term on the right hand side represents:
Integral over control surface, represents net flowrate of parameter B across entire control surface
For a cylindrical tube wall, steady state, no energy generation, the heat transfer rate:
Is a constant
For a spherical shell, steady state, no energy generation, the heat transfer rate:
Is a constant
The conservation of linear momentum for a control volume is: The term, B:
Is zero if there is no linear momentum flow out of the CV
The conservation of linear momentum for a control volume is: The term, A:
Is zero under steady-state conditions
Contact resistance does not depend on all of the following, except:
It depends on all these parameters: Materials Surface finishes Contact Pressure Trapped Fluid Interstitial conditions
We want to determine the friction force exerted by the pipe wall on the air flowing between sections (1) and (2). What should we use as a CV?
Just air in the pipe
Which of the following is an example of a non-Newtonian fluid? Choose ALL correct answers.
Ketchup, Oobleck (corn starch and water), Blood, Mayonnaise
The magnitude of the pressure gradient, is _____________ in the entrance region than in the fully developed region,
Larger
What is not an example of a Newtonian fluid?
Latex Paint
A 1/20th scale model of an airplane is used to determine forces on the actual airplane. The 1/20th scale refers to
Lengths
For a plane wall, steady state, 1D conduction, no energy generation, the temperature varies _____________ with x.
Linearly
For a material to be a good insulator, it should have a ____________ thermal conductivity.
Low
To have the highest heat transfer through a two-layer material, a (choose one) thermal contact resistance is desired:
Low
Place these common items, maple syrup, milk, toothpaste, peanut butter in the correct position for their relative dynamic viscosity. What is at position 2?
Maple Syrup
The correct statement for the definition of density is
Mass per unit volume
Which of the following are considered primary quantities
Mass, Time, Length, Temperature
Consider the steady state temperature distributions shown within a composite wall composed of Material A and Material B. The conduction is 1D, and there is no internal generation. Which material has the higher thermal conductivity?
Material A
What is an example of a Bingham Plastic?
Mayonaise
What method did we use to determine Pi Terms?
Method of Repeating Variables
Place these common items, maple syrup, milk, toothpaste, peanut butter in the correct position for their relative dynamic viscosity. What is at position 1?
Milk
In order to have fully developed flow, we need the following to be true:
Must be thermally and hydrodynamically fully developed
which of the following are units of Pressure
N/m^2 Lb/ft^2 Psf Pa Psi
What is an example of a shear thinning fluid?
Nail Polish
Using the coordinate system shown, at section 1, is the mass flow rate:
Negative
Using the coordinate system shown, at section 1, is the x-direction linear momentum flow rate:
Negative
Using the coordinate system shown, at section 2, is the x-direction linear momentum flow rate:
Negative
For __________________ fluids the apparent viscosity is the same as the viscosity and is independent of the shear rate
Newtonian
A fluid for which shear stress and rate of shearing strain are linearly related with a slope of dynamic viscosity is a __________
Newtonian Fluid
Can we use thermal circuit analysis in a section of composite wall where we have thermal energy generation?
No
Consider the steady state temperature distributions shown within a composite wall composed of Material A and Material B. The conduction is 1D, and there is no internal generation. Does the thermal conductivity, for both Material A and B, vary significantly with temperature?
No
Consider a system where radiative and convective heat transfer are occurring simultaneously from a surface. This represents what type of thermal circuit?
Parallel
Tc,o can not exceed Th,o for a [ Select ] flow heat exchanger but can do so for a [ Select ] flow heat exchanger
Parallel Counter
The graph shows the temperature change for what type of heat exchanger:
Parallel flow
We want to determine the anchoring force to hold this pipe in place. What should we use as a CV?
Pipe + Air
When we have two pi terms, and we want to determine the functional relationship between them, we should:
Plot Pi 1 on the y axis, Pi 2 on the x axis, and fit the data
Using the coordinate system shown, at section 1, is the x-direction linear momentum flow rate:
Positive
Using the coordinate system shown, at section 2, is the mass flow rate:
Positive
Using the coordinate system shown, at section 2, is the x-direction linear momentum flow rate:
Positive
For fully developed viscous flow in a horizontal pipe, which of the following is true?
Pressure forces are balanced by shear forces
For fully developed viscous flow in a vertical pipe, which of the following is true?
Pressure, shear and weight forces all balance
At normal temperature and pressure what matter has the highest thermal conductivity?
Pure metals
Air flows out of a ventilation duct as shown. The flow can be assumed to be incompressible and steady. The relation between the volume flow rate Q1 at section 1 and Q2 at section 2 is
Q2 = Q1
What is an example of a shear thickening fluid?
Quicksand
For conduction across a plane wall what is the equation for thermal resistance?
R = L/kA
What is the equation for thermal resistance for convection?
R=1/hA
What is the equation for thermal resistance for radiation?
R=1/hrA
For conduction across a cylindrical shell what is the equation for thermal resistance?
R=ln(r2/r1)/(2piLk)
Consider the general Reynolds Transport Theorem. The first term on the right hand side represents:
Rate of change of B within CV as fluid flows through it
The fourier number is:
Rate of thermal energy storage
For flow in a pipe, the Reynolds number is defined as ρVDμ . The Reynolds number can be interpreted as:
Ratio of inertial to viscous forces
for what range of Reynolds number will flow in a round pipe be laminar
Re<2100
When measuring the average velocity of the flow within the pipe system shown below, which regions of the pipe system are the most optimal locations for the placement of a flow measurement device? Choose ALL correct answers.
Regions 2-3 Regions 5-6
This definition describes what dimensionless number: a measure of the ratio of the inertia force on an element of fluid to the viscous force on an element.
Reynolds number
A fluid continuously moves and deforms due to ___________.
Shear stress
Which of the following are units for density
Slugs/ft^3 Kg/m^3
under what conditions is heat flux constant, independent of the direction of heat flow?
Steady state One-dimension No heat generation Plane wall
The intensity of the molecular attraction per unit length along any line in the surface is called the
Surface tension
A ________________ is a collection of matter of a fixed identity which may move, flow, and interact with its surroundings
System
What is the equation for the thermal time constant?
T = pVc/hAs
A Rankine is a unit of what?
Temperature
For one-dimensional problems for a spherical, plane or cylindrical wall, using the heat equation you should be able to derive equations for (choose ALL correct answers)
Temperature Distribution Heat rate Heat flux
The driving force for heat transfer is _________________.
Temperature difference
Which of the following are considered primary quantities? Choose ALL correct answers.
Temperature, Time, Length, Mass
For thermally fully developed flow, choose the correct statement(s):
The dimensionless temperature profile remains unchanged. The temperature profile can vary with x in the flow direction.
For steady uniform flow of an incompressible liquid that enters a pipe, which of the following is true for the entrance region?
The maximum velocity increases with distance form the entrance
A U-tube manometer filled with water connects two tanks of air as shown below. The height of the water in the U-tube is as shown. The correct statement about the pressure of the two tanks is:
The pressure in Tank A is higher than that in Tank B.
A 1/5th scale model of a new automobile design is tested in a wind tunnel at the same Reynolds number as that of the full scale prototype. The force coefficient: (FA)12ρV2 of the model is the same as that of the prototype. Assuming the model and prototype are both tested in air, the force on the scale model, Fm is then:
The same as that of the full scale vehicle
The stagnation pressure for a fluid is
The total pressure when the fluid is brought to rest
Considering fully developed laminar flow, shear stress is a maximum at:
The wall
________________ is a measure of the materials ability to transfer thermal energy by conduction.
Thermal Conductivity
A region of flow characterized by temperature gradients and heat fluxes
Thermal boundary layer
The temperature drop across the interface between materials is attributed to
Thermal contact resistance
________________ is a measure of a material's ability to conduct thermal energy relative to storing thermal energy.
Thermal diffusivity
For fins, in order to receive the maximum possible heat transfer enhancement, it is desired that the temperature along the length of the fin is:
Uniform
What is NOT an example of a boundary condition we discussed for the Heat Diffusion Equation?
Uniform Energy Generation
The overall heat transfer coefficient for the hot fluid (Uh) will be equal to that of the cold fluid (Uc) under what conditions?
Thin walled tube - negligible conductive resistance
Consider the general Reynolds Transport Theorem. The term on the Left Side represents:
Time rate of change of extensive parameter of system
What type of fin is most commonly used to enhance heat transfer to or from circular tubes?
annular fins with a rectangular profile
The compressibility of a fluid is characterized by the
bulk modulus
The properties and flow rates for the hot and cold fluids of a heat exchanger are shown in the following table. Which fluid limits the heat transfer rate of the exchanger?
cold fluid
For a circular tube characterized by constant heat flux, fully developed laminar flow the Nusselt number (NuD) is
constant Nu=4.36
for a circular tube characterized by constant surface temperature, fully developed laminar flow the Nusselt number (NuD) is
constant, Nu=3.66
A _________________ is a volume in space through which fluid may flow
control volume
The graph shows the temperature change for what type of heat exchanger
counter flow
Fluid flows from a fire extinguisher tank. What is the relationship between , where mass is the extensive property B for time t > 0.
dBsys/dt > dBcv/dt
Temperature distribution within a 1D plane wall at an initial time, at steady state, and at several intermediate times are as shown. If there is volumetric generation, it is uniform throughout the wall, assume constant properties. What is the appropriate boundary condition at x = 0?
dT/dx |x=0
If the heat capacity rate of the cold fluid in a heat exchanger greatly exceeds that of the hot fluid (CC>>Ch) the temperature in the hot fluid will [ Select ] while that of the cold fluid will [ Select ] .
decrease remain constant
In the entrance region, for laminar flow, the local convective heat transfer coefficient ____________.
decreases with increasing x
Fin effectiveness increases with _______ convection coefficients
decreasing
an incompressible fluid is a fluid with a constant
density
for a smooth circular tube characterized by fully developed turbulent flow the Nusselt number (NuD) is
dependent on both Re and Pr
For a prescribed temperature gradient, the conduction heat flux is [ Select ] to thermal conductivity. So as k increases q'' [ Select ]
directly proportional Increases
The most frequent use of extended surfaces is to [blank] heat transfer between a solid and adjoining fluid
enhance
For steady flow the mass flow in is______________ mass flow rate out
equal to
constant on a streamline For the form of Bernoulli's equation what does term D represent
Total head
Consider constant altitude, steady flow along a streamline with a flow that satisfies the assumptions necessary for Bernoulli's equation. Which of the following have a constant value along the streamline?
Total pressure
A FBD must be drawn when applying conservation of linear momentum.
True
A major objective of conduction analysis is to determine the temperature distribution in a medium
True
A repeating variable must be dimensionally independent and cannot combine to form a dimensionless product.
True
A streamline is a line tangent to the velocity vector through out the flow field
True
A transient condition is initiated when a system experiences changes to its operating conditions and continues until the system reaches a new thermal equilibrium.
True
All theoretically derived equations should be dimensionally homogenous meaning Both sides use the same units
True
Energy generation causes the heat rate to vary with location.
True
Fin effectiveness is enhanced by the choice of a material of high thermal conductivity.
True
Fins are generally placed on the gas side, which is the side of the lower convection coefficient.
True
Flow in the fully turbulent boundary layer is, in general, highly irregular and is characterized by random, three-dimensional motion of relatively large parcels of fluid
True
For a one-dimensional system, temperature gradients exist along only a single coordinate direction
True
For a thermal circuit analysis, the heat transfer rate must be a constant through the circuit.
True
For fully developed laminar flow, shear stress varies linearly with the radial position in the pipe, r, from 0 at the centerline to a max at the wall.
True
For small elevation changes, the pressure of gaseous fluids does not change significantly.
True
For turbulent flow, the thermal boundary layer is nearly independent of Pr.
True
Fourier's law implies that heat flux is directional, specifically that qx'' is perpendicular to the cross-sectional area A
True
Heat transfer is in the direction of decreasing temperature.
True
If there is negligible heat loss to the surroundings, and negligible changes in kinetic and potential energies, the heat transfer rate of the hot fluid of a heat exchanger will be equal to that of the cold fluid
True
In a circular tube with uniform surface heat flux and laminar, fully developed conditions, the Nusselt number is a constant, independent of Reynolds number, Prandtl number and axial location.
True
If the Biot number is ~1 the conductive heat transfer rate is ________ the convective heat transfer rate.
equal to
An extended surface used to enhance heat transfer between a solid and adjoining fluid is called a [blank}
fin
a _____is defined as a substance that deforms continuously when acted on by a shearing stress of any magnitude
fluid
when pressure is measured relative to the local atmospheric pressure it is called the ----- pressure
gauge
For the same inlet and outlet temperatures the log mean temperature difference for counter flow is [blank] that of parallel flow
greater than
If the biot number is <<1 the conductive heat transfer rate is ______________ the convective heat transfer rate
greater than
What is the equation for Nusselt's number?
hL/k
For 1D steady state conduction for a cylindrical or spherical wall which of the following is independent of radius
heat rate
For uphill fully developed laminar flow, gravity _________________ the flow, and therefore a _________________ pressure drop is required.
hinders, larger
Water is flowing in a pipe. Which is the correct statement about the effect of an increase in the Reynolds number of the flow:
if the flow is laminar it could become turbulent.
The nozzle on a fire hose is connected to the hose via a coupling. When the fire hose is in use with water flowing through it and the hose is stationary, the coupling is
in Tension
The heat transfer rate for a heat exchanger increases, as the NTU _______________.
increases
in the thermally fully developed flow of a fluid with constant properties, the local convection coefficient is a constant, ________ of x.
independent
A system:
is a fixed amount of matter and a control volume is a volume in space
in the fully developed region of a pipe the velocity profile
is equal across any section of the pipe
Using Fourier's law it is implicit that the medium in which the conduction occurs is
isotropic
The transport properties for conduction include:
kinematic viscosity diffusion rate coefficients thermal conductivity
turbulent mixing promotes [blank] temperature and species concentration gradients adjacent to the solid surface
large
In turbulent flow the larger the turbulence intensity the __________ the fluctuations in velocity will be
larger
In the entrance region, local laminar convective heat transfer coefficients are ________ local turbulent convective heat transfer coefficients.
less than
The specific weight of a gas is typically _________________ the specific weight of liquids.
less than
For constant surface heat flux the mean temperature (Tm) varies ________________ with x
linearly
Thermal insulations consist of [ Select ] thermal conductivity materials combined to achieve an even [ Select ] system thermal conductivity.
low lower
head loss due to viscous effects for flow in a straight pipe is termed:
major loss
head loss due to pipe components is termed:
minor loss
A [ Select ] is a representation of a physical system that may be used to predict the behavior of the system in some desired respect.
model
The prandtl number is the ratio of:
momentum diffusivity / thermal diffusivity
In the Buckingham pi theorem the number of dimensionless products it equal to the number variables minus the minimum number of reference dimensions.
True
In the entrance region of the pipe, the velocity profile changes with location along the pipe.
True
In the fully developed flow region, the pressure decreases linearly with x (length along the pipe).
True
In the non-fully developed flow regions i.e. entrance region - Fluid accelerates or decelerates as it flows
True
It is impossible to simultaneously impose the conditions of constant surface heat flux and constant surface temperature.
True
It is required that the loss in available energy be positive.
True
Kinematic viscosity is defined as the ratio of absolute viscosity to the fluid density of a substance
True
Pascals law states that the pressure at a point in a fluid at rest, or in motion, is independent of direction as long as there are no shearing stresses present.
True
Pi terms can be formed by inspection by simply making use of the fact that each pi term must be dimensionless.
True
Resistances in series add directly to determine the total resistance.
True
Specific gravity is defined as the ratio of fluid density over the density of water at a given temperature, and is not dependent on the unit system used
True
The boundary layer velocity profile refers to the way velocity (u) varies with height (y)through the boundary layer
True
The exit pressure for a fluid jet is equal to that of the atmosphere, (P= 0 gauge)
True
The first pi term, Pi 1, must have the dependent variable in the numerator.
True
The friction factor is non-zero for a smooth (ε = 0) pipe.
True
The lumped capacitance method is the assumption that the temperature of the solid is spatially uniform at any instant during the transient process.
True
The need for fins is stronger when the fluid is a gas rather than a liquid.
True
The overall heat transfer coefficient is a modified form of thermal resistance to encompass multiple resistances to heat transfer.
True
The pressure drop for laminar pipe flow is found to be independent of the roughness of the pipe.
True
The thermal boundary layer is a region of flow characterized by temperature gradients and heat fluxes.
True
The velocity boundary layer is a consequence of viscous effects associated with the relative motion between a fluid and a surface.
True
The velocity boundary layer is a region of flow characterized by shear stresses and velocity gradients.
True
There is no rigorous theoretical treatment of turbulent flow
True
There is not a unique set of pi terms which arise from dimensional analysis, but the required number of pi terms is fixed
True
Thermal contact resistance is cause by surface roughness creating gaps and contact spots between two materials
True
We can assume one dimensional conduction in a fin in the longitudinal direction because in practice fins are very thin, so the temperature changes in the transverse direction is negligible relative to those between the fin and the environment.
True
We can ensure that each pi term is independent of those preceding it by incorporating a new variable in each pi term
True
a thermal boundary layer must develop if the fluid free stream and surface temperatures differ.
True
a vacuum pressure would be a negative gage pressure
True
For a given flow, the Reynolds number is 13,276. What type of flow is this?
Turbulent
for radiation, liquids can be considered
opaque
For the form of Bernoulli's equation what does term A represent
p/y
Water flows steadily without friction into a tank of water open to the atmosphere from a pipe at location 1 and then along a streamline to location 2 where it discharges into the atmosphere. With pressures measured in gage pressure and the datum for elevation as shown, the Bernoulli equation reduces to:
p1+1/2pV^2+yz1=1/2pV^2
Two tubes connected to two water reservoirs are shown. The tube on the left is straight, that on the right is a cone with the top area four times the base. The base of the cone has an area the same of that as the straight tube. The height of the water is the same for both cases. The relation between the pressures p1 and p2 at the base of the tubes is:
p2 = p1
fin effectiveness can be enhanced by increasing the ratio of [ Select ] to [ Select ]
perimeter cross sectional area
In general, where fluid flows through the control surface, it is advisable to make the control surface _____________ to the flow.
perpendicular to the
If the heat capacity rate of the hot fluid in a heat exchanger greatly exceeds that of the cold fluid, (Ch>>Cc) the temperature in the hot fluid will [ Select ] while that of the cold fluid will [ Select ] .
remain constant increase
The energy equation for a CV is given as: Where e is stored energy and u is the internal energy. For a system with an adiabatic control surface and no net mass flows in and out, the energy equation reduces to:
v
the problem of convection depends on:
viscosity specific heat thermal conductivity flow conditions density surface geometry
In the [layer], transport is dominated by diffusion and the velocity profile is nearly linear
viscous sublayer
p/y+v^2/2g+z For the form of Bernoulli's equation what does term C represent
z - elevation head
A U-tube manometer filled with water (gray) connects two tanks of air at different pressures as shown below. The height of the water in the U-tube is as shown. The water specific weight is γ. The pressure difference (pA - pB) is:
𝑝_𝑎−𝑝_𝑏= 𝛾(ℎ_3−ℎ_2 )
What is the SG for a substance whose density is 1500 kg/m3?
1.5
Heat is transferred steadily through a 0.2-m thick 8 m by 4 m wall at a rate of 2.4 kW. The inner and outer surface temperatures of the wall are measured to be 15 degrees C to 5 degrees C. The average thermal conductivity of the wall is
1.5 W/m-deg C
A 1/10th scale model of an airplane is tested in a wind tunnel at the same Reynolds number (ρVD/μ) as that of the full scale airplane. Properties of the air are the same. The velocity in the wind tunnel is then:
10 times that of the full scale airplane
The design of a river model is to be based on Froude number similarity, and a river depth of 3 m is to correspond to a model depth of 100 mm. Under these conditions, what is the prototype velocity corresponding to a model velocity of 2 m/s? The model and prototype operate in the same gravitational field. Froude number is important for flows with free surfaces and is the ratio of inertial to gravitational forces. The Froude number is: Fr=Vg⋅l
11 m/s
If the absolute pressure is 220 kPa, what is approximately the gage pressure?
120 kPa
Air flows steadily along a streamline from point (1) to point (2) with negligible viscous effects. The following is measured: Point (1): z = 2 m, p = 0 kPa Point (2): z = 10 m, p = 20 Pa, V = 0 m/s Determine velocity at point 1. Assume air has a density of 1.23 kg/m3.
13.8 m/s
For conduction across a spherical shell what is the equation for thermal resistance?
(1/r1-1/r2)/4pik
What is the primary mode of heat transfer for using an electric blanket to get warm?
Conduction
What is the primary mode of heat transfer into your hands as you hold a hot cup of coffee?
Conduction
The first law of thermodynamics is a statement of
Conservation of energy
For a plane wall, steady state, 1D conduction, no energy generation, the heat transfer rate is __________ with x:
Constant
You put a wet shoe on a floor heat vent to dry faster. Define the primary mode of heat transfer:
Convection
Consider a person standing in a breezy room at 20°C. Determine the total rate of heat transfer from this person (assumed to have an emissivity of 0.95) if the exposed surface area and the average outer surface temperature of the person are 1.6 m2 and 29°C, respectively, and the convective heat transfer coefficient is 6 W/m2-K. What mode(s) of heat transfer do we need to consider if the man is the Control Volume.
Convection Radiation
Which of the following contributes to the overall heat transfer coefficient for a system (U), when considering Heat Exchangers? Choose ALL correct answers.
Convection resistance Overall surface efficiency Fouling Conduction Resistance Fin efficiency
NTU is a function of (Choose ALL correct answers)
Effectiveness Cr
Heat exchanger effectiveness can exceed 1.
False
What is the approximate specific weight for a substance whose density is 1500 kg/m3?
What is the approximate specific weight for a substance whose density is 1500 kg/m3?
Using the coordinate system shown, at section 1, is the y-direction linear momentum flow rate:
Zero
Using the coordinate system shown, at section 2, is the y-direction linear momentum flow rate:
Zero
The Reynolds Transport Theorem provides the relationship between:
the time rate of change of an extensive property of the system and the control volume.
Unsteady or transient heat transfer are systems where heat transfer is dependent on ________
time
for water flowing through a smooth round pipe Re is found to be 38214. is the flow:
turbulent
How many boundary conditions are necessary for each spatial coordinate needed to describe the system?
two
Temperature distribution within a 1D plane wall at an initial time, at steady state, and at several intermediate times are as shown. If there is volumetric generation, it is uniform throughout the wall, assume constant properties. What is the appropriate boundary condition at x = L?
-k dT/dx |x=L = h(T(x=L)-Tinf)
To determine max possible power output of a turbine, what is the loss?
0 N-m/kg
Water enters the horizontal, circular, cross sectional sudden contraction nozzle at section (1) with a uniform velocity of 25 ft/s and a pressure of 75 psi. The water exits from the nozzle into the atmosphere at section (2) where the uniform velocity is 100 ft/s. Determine the y- component of the anchoring force required to hold the contraction in place.
0 lb
A Newtonian fluid (density of 1031 kg/m3) is placed between parallel plates which are separated by 1.27 cm. A force of 0.9 N is applied to the upper plate, causing a shear stress of 4.8 N/m2 to develop. The upper plate is moving with a velocity of 0.031 m/s and the bottom plate is fixed. The velocity distribution in the gap is linear. The kinematic viscosity of the fluid is:
0.0019 m2/s
An inclined-tube manometer is being used as shown to measure the pressure in a pressurized tank. The tank is partially filled to a depth of 20 cm with a fluid of specific weight 7.65 kN/m3. The specific weight of the manometric gage fluid is 34.3 kN/m3. The gage pressure in the headspace (kPa) when h = 8 cm is most nearly:
0.3 kPa
Consider a system where we have 4 pi terms. How many prediction equation(s) are there?
1
Temperature distribution within a 1D plane wall at an initial time, at steady state, and at several intermediate times are as shown. If there is volumetric generation, it is uniform throughout the wall, assume constant properties. How many initial condition(s) are needed to solve for the temperature distribution within the wall?
1
Select the order of steps in the method of repeating variables
1. List all the variables involved in the problem 2. Express each variable in terms of basic dimensions 3. Determine the number of required pi terms 4. Select the number of repeating variable where the number is equal to the number of reference dimensions 5. Form a pi term by multiplying one of the nonrepeating variables by the product of the repeating variables, each raised to an exponent that will make the combination dimensionless. 6. Repeat Step 5 for each of the remaining nonrepeating variables. 7. Check all the resulting pi terms to make sure they are dimensionless. 8. Express the final form as a relationship among the pi terms, and think about what it means.
A small sphere of reference grade iron with a specific heat capacity of 447 J/kg-K and mass of 0.515 kg is suddenly immersed in a water-ice mixture. Fine thermocouple wires suspend the sphere and the temperature is overserved to change from 15℃ to 14 ℃ in 6.35 s. The experiment is repeated with a metallic sphere of the same diameter, but of unknown composition with a mass of 1.263 kg. If the same observed temperature change occurs in 4.59 s, what is the specific heat of the unknown material.
132 J/kg-K
A 5-cm diameter pipeline is delivering water from a storage tank to an open canal. The water level in the tank can be assumed to be at a constant height of 12 m above the discharge point. Ignoring all losses, and assuming incompressible, inviscid flow, the velocity of the fluid exiting the pipeline is:
15.3
Heat is transferred steadily through a 0.15-m thick 3 m by 5 m wall whose thermal conductivity is 1.2 W/m.C. The inner and outer surface temperatures of the wall are measured to be 18C to 4C. The rate of heat conduction through the wall is:
1680 W
Consider the heat diffusion equation applied to a cylindrical coordinate system, where properties only change in the radial direction, with steady heat transfer. How many initial conditions and boundary conditions are needed to solve for the temperature distribution in the medium?
2 BC's; 0 IC's
What value of Reynolds number (ReD) corresponds with the onset of turbulent flow?
2300
Suppose we have a fluid flowing through a pipe with a constant surface temperature, Re = 500 and Pr = 2. The pipe is 100 m long and has a Diameter of 1 m. The thermal entrance length is 50 m and the hydrodynamic entrance length is 25 m. What is the nusselt number 30 m from the entrance of the pipe?
4
Consider the heat diffusion equation applied to a 2D cartesian system, which is unsteady. How many initial conditions and boundary conditions are needed to solve for the temperature distribution within the medium?
4 BC's; 1 IC
For a certain fluid flow problem, it is known that both the Froude number and the Weber number are important dimensionless parameters. If the problem is to be studied by using a 1:15 scale model, determine the required surface tension scale if the density scale is equal to 1. The model and prototype operate in the same gravitational field.
4.44 x 10-3
A 50-cm-long, 0.2-cm-diameter electric resistance wire submerged in water is used to determine the boiling heat transfer coefficient in water at 1 atm, 100 deg C, experimentally. The surface temperature of the wire is measured to be 130 degrees C when a wattmeter indicates the electric power consumption to be 4.1 kW. Then the convective heat transfer coefficient is:
43,500 W/m^2 - deg C
The figure shows a system for pumping water from a canal to an overhead tank. The pipe line is 0.1 m in diameter and the total length of the pipeline is 60 m. When the flow velocity is 3 m/s, the friction factor is 0.02. Ignoring minor losses, the energy added by the pump (m) under these conditions is most nearly
45.5 m
A 40-cm-long, 0.6-cm-diameter electric resistance wire is used to determine the convection heat transfer coefficient in air at 25 degrees C experimentally. The surface temperature of the wire is measured to be 150 degrees C when the electric power consumption is 90 W. If the radiation heat loss from the wire is calculated to be 30 W, the convection heat transfer coefficient is
63.7 W/m^2- deg C
A 10-cm high and 20-cm wide circuit board houses on its surface 100 closely spaced chips, each generating heat at a rate of 0.08 W and transferring it by convection to the surrounding air at 25degrees C. Heat transfer from the back surface of the board is negligible. If the convectionheat transfer coefficient on the surface of the board is 10 W/m2-Degree C and radiation heat transfer is negligible, the average surface temperature of the chips is:
65 deg C
If the biot number is >>1 the conductive heat transfer rate is [ Select ] the convective heat transfer rate
<
When determining the maximum possible heat transfer rate, we can write this as: qmax=Cc(Th,i-Tc,i), only if Cc is _________________ Ch
<
When determining the maximum possible heat transfer rate, we can write this as qmax=Ch(Th,i-Tc,i), only if Cc is ____________________ Ch
>
in general the use of fins may be rarely justified unless the efficiency is:
>60%
A tank is filled with a liquid and the surface is exposed to the atmosphere. Which of the following accurately represents the gage pressure distribution on the right-hand side of the tank?
A
All three control volumes shown below are valid. Based on our discussion, what is the best choice for a control volume to determine the pressure at point 1?
A
p For the equation which term represents the static pressure as the fluid flows along a streamline?
A
The Reynolds Transport Theorem is an analytical tool to convert conservation laws for A to B. Choose the answers for A and B
A = Systems B = Control Volumes
Consider the conservation of mass equation: The physical meaning of terms A and B are:
A represents the rate of change of mass inside the control volume and B represents the mass flows crossing the boundary of the control volume
What is not a step in internal convection analysis?
All of these are steps for internal convection analysis: Determine flow type (laminar or turbulent) Define method of heating and cooling Determine if fully developed (thermally, hydrodynamically) Determine appropriate Nusselt Number correlation
Bernoulli's equation is applied along a streamline, when there is no mechanical work. What of these assumptions must NOT be satisfied for its application to be valid?
All of these must be satiisfied: Steady None of these are correct Incompressible Inviscid No Shaft Work
The no slip condition is:
An experimental observation that the velocity of a fluid in contact with a solid surface is equal to the velocity of the surface
The laminar velocity profile for a Newtonian Fluid is shown. What figure best describes the variation of shear stress with distance from the plate?
C ________________________ straight across
For the equation which term represents the hydrostatic pressure? p+1/2pV^2+yz=constant along the streamline
C - yz
Chose the correct statement. The local convective coefficient:
Can vary point to point along the surface
Suppose we have a fluid flowing through a pipe with a constant surface temperature, Re = 500 and Pr = 2. The pipe is 100 m long. The thermal entrance length is 50 m and the hydrodynamic entrance length is 25 m. What is the nusselt number 10 m from the entrance of the pipe? DETERMINE USING THIS FIGURE.
Cannot determine using the figure - conditions don't match
The [ Select ] number and Mach number are important dimensionless groups in problems in which fluid compressibility is a significant factor.
Cauchy
Consider two metal plates pressed against each other. Everything else being equal, which of the measures below will cause the thermal contact resistance to increase?
Cleaning the surfaces to make them shinier
The four devices shown rest on frictionless wheels, are restricted to move in the x direction only, and are initially held stationary. The pressure at the inlets and outlets of each is atmospheric, and the flow is steady and incompressible. The contents of each device is not known. When released, which device will move to the left?
D
Water flows steadily through a horizontal circular pipe. Which curve most correctly describes the pressure change through the pipe as the length is increased?
D
A cylinder with diameter of D floats upright in a liquid as shown. When the cylinder is displaced slightly along its vertical axis it will oscillate about its equilibrium position with a frequency, ω. Assume that this frequency is a function of the diameter, D, the mass of the cylinder, m, and the specific weight of the liquid, γ. Using dimensional analysis, it is found that there is 1 pi term, ∏1=ωDmγ . If the mass of the cylinder is increased, the frequency would:
Decrease
As we more upwards in a fluid at rest, pressure will:
Decrease
Consider a case where the pump efficiency is 90%. Now, if the efficiency of a pump is 100%, meaning the loss is 0 ft-lb/lbm, for the same flow conditions, what would happen to the required pump power output?
Decrease
Consider a thin-walled, concentric tube heat exchanger. The overall heat transfer coefficient WITH fouling will be ______________________ compared to the overall heat transfer coefficient without fouling.
Decrease
To increase the effectiveness of a fin, it is necessary to _____________ the conduction / convection resistance of the fin.
Decrease
if Ts is fixed what could be done to increase the heat transfer rate? Choose ALL correct answers
Decrease the fluid temperature (T∞ ) Increase convection coefficient h Increase the fluid velocity Increase the surface area(A)
For Laminar developing flow situations, the Nusselt number __________ with increasing x.
Decreases
In a low speed flow, with a horizontal diffuser, the fluid velocity:
Decreases in the direction of the flow
heat flux is a vector in the direction of [ Select ] temperature
Decreasing
what are the dimensions of Biot number?
Dimensionless
When using the repeating variables method, the number of repeating variables that are selected should be what compared to the number of reference dimensions:
Equal to
For a problem with one pi term, this pi term is:
Equal to a constant
What is not a step in a conduction analysis?
Evaluate the convection heat transfer coefficient
A heat exchanger performance problem requires determining the:
Exit temperatures of the heat exchanger
Two fluid jets are pointed at surfaces as shown. The fluids are incompressible, and effects of gravity can be neglected. The mass flowrates and the velocities of the jets are identical. The cross sectional areas of the jets do not change significantly as the fluid flows. For steady flow, the correct statement regarding the horizontal forces is:
F1 = F2 / 2
A system filled with a liquid is shown below. On the left there is a piston in a tube of cross sectional area, A1, with a force F1 applied to it; and on the right there is a piston in a tube of cross sectional area A2, which is twice A1 and a force F2. The pistons are weightless and the two liquid levels are the same. Determine the relation between the force F1 and F2:
F2 = 2*F1
An extensive property is independent of the amount of mass.
False
Any interstitial substance that fills the gap between contacting surfaces and whose thermal conductivity exceeds that of air will increase the contact resistance.
False
Because the intermolecular spacing is much larger and the motion of the molecules is more random for the fluid state, thermal energy transport is more effective than for the solid state.
False
Bernoulli's equation can be used under compressible flow conditions.
False
Conservation of volume flowrate is a fundamental conservation law of fluid mechanics.
False
Control Volumes must be fixed and nondeforming.
False
During the entrance region, the pressure drop decreases linearly with x (distance along the pipe).
False
Energy generation is due to conversion of energy from one form to another in a convecting medium.
False
For one dimensional, steady-state conduction in a plane wall with no heat generation, the heat flux will vary with across the plane wall.
False
For turbulent flow the entry length is dependent on Reynolds number
False
For turbulent flow, the friction factor is independent of the relative roughness.
False
Fouling is advantageous for heat exchanger performance.
False
Fourier's Law can only be applied to solids.
False
Fully developed flow requires EITHER the velocity or temperature profile to be fully developed.
False
When solving for anchoring forces, we use
Gage pressures
When choosing variables for Dimensional Analysis, what is not something to consider?
Geometry Material Properties External Effects Constants
For a fluid element at rest, the forces acting on the element are:
Gravity and Normal Forces
Consider a thin-walled, concentric tube heat exchanger. The total resistance WITH fouling will be ______________________ compared to the total resistance without fouling.
Greater than
During the entrance region, the boundary layer
Grows
Water flows steadily through a smooth pipe. For turbulent flow, before wholly turbulent flow, if the velocity is increased, in general:
The friction fractor decreases
Dimensional analysis is useful for all of the following, except for:
Useful for all of these
A fluid flows steadily through a pipe with unifrom cross-sectional area. The density of the fluid decreases to half its initial value as it flows through the pipe. The correct statement about the average velocity V is:
V2 = 2V1
For the form of Bernoulli's equation what does term B represent
V^2/2g+z Represents velocity head
For a cylindrical tube wall, steady state, no energy generation, the heat flux rate:
Varies inversely with r
For a plane wall, steady state, no energy generation, the temperature:
Varies linearly with x
For a plane wall, steady state, with constant energy generation, the temperature:
Varies quadratically with x
Pitot tubes are placed in two ducts in which air is flowing and the static pressures are qual. The stagnation pressure tap and the static pressure tap are connected to manometers with readings as shown. The relation between Va and Vb is
Vb = sqrt(2)*Va
Pitot-static tubes measure fluid velocity by converting [ Select ] to [ Select ]
Velocity Pressure
What causes fluid layers to stick to the wall - i.e. the no-slip condition?
Viscous Effects
_______________ is a measure of a material's ability to store thermal energy.
Volumetric Heat capacity
What are the units for thermal resistance?
W/K
Basic horizontal pipe flow is governed by: (Choose ALL correct answers)
pressure forces viscous forces
what is the main driving force for pipe flow
pressure gradients
The surface area required to achieve a prescribed heat transfer rate q is ____________ for a counter flow versus parallel flow for the same over all heat transfer coefficient (U)
smaller
a horizontal pipe is tilted so that now the flow is at a downward angle, the pressure change needed to achieve the same flow rate would be:
smaller
Transient conductive heat transfer can be induced by changes in:
surface temperature surface convection conditions surface radiation conditions heat flux internal energy generation Correct Answer all of the above
