ASEN 3113 Final True/False

Consider a long cylindrical layer (such as a circular pipe) of inner radius r1, outer radius r2, length L, and average thermal conductivity k. The two surfaces of the cylindrical layer are maintained at constant temperature T1 and T2. There is no heat generation in the layer and the thermal conductivity is constant. For one-dimensional heat conduction through the cylindrical layer, we have T(r). Then Fourier's law of heat conduction for heat transfer through the cylindrical layer can be expressed as qcond, cyl = -kA dT/dr where A=2πrL is the heat transfer surface area at location r. If we write the heat transfer equation using electrical analogy as qcond, cyl = (T1 - T2)/Rcyl, you should be able to obtain the expression of Rcyl in terms of the known parameters (r1, r2, k, and L). If r1=1.5mm, r2=3.5mm, L=5m, k=0.15[W/m.K], determine the value of Rcyl in terms of [K/W].

0.1798 (use equation from Ch. 17)

T/F - If the Prandtl number is equal to 0.71 and the Grashof number equal to 2x10^6, what is the corresponding Rayleigh number? Please answer in scientific notation in the form ##.##x10^#.

1.42x10^6 [multiply the numbers]

The refrigerator in a kitchen shown below receives an electric input power of 150 W to drive the system, and it rejects 400 W to the kitchen air. Find the rate of energy taken out of the cold space

250W

TL =20C and TH =160C, what is the thermal efficiency of a Carnot engine, or any reversible heat engine?

32.3%

Earth's orbit around the Sun has an eccentricity 0.0167, which means that the farthest distance from the Sun is r_a = 1AU(1+e) and the closest distance from the Sun is r_p = 1AU(1-e), where e = 0.0167. Calculate the largest variation of the total solar irradiation along Earth's orbit in terms of percentage of the solar constant. (Please give answer as percentage with two decimal places (e.g. XX.XX%)

6.68%

T/F - At any distance x from the leading edge, the local heat flux may be obtained at y=0 in terms of the thermal conductivity of the fluid, k, and the temperature gradient at the surface: qs= qcond = - k (∂T/∂y)|y=0. Recognize that the surface heat flux is equal to the convective flux: qs= qconv =h (Ts - T∞). If k = 0.607 [W/m.K], Ts = 35 oC, T∞ = 25C, and (∂T/∂y)|y=0 = -10C/cm, what is the value of h in [W/m2.K]? (Do not include the units in your typed response, round to the nearest tenth. Answer in the form xx.x)

60.7

One Astronomical Unit (AU), in terms of Solar Radius (RS), is about A) 215 B) 209 C) 119

A) 215

The Earth's radius at equator is about A) 6371 km B) 5378 km C) 6162 km

A) 6371 km

What is the solar flux in terms of W/m^2 at the closest encounter of the Parker Solar Probe given the following information? Maximum Aphelion - 1.018 AU Minimum Aphelion - 0.73 AU Maximum Perihelion - 35.7 Rs Minimum Perihelion - 9.8 Rs Maximum solar flux at solar encounter (9.86 Rs) Minimum solar flux at aphelion (1.02 AU) Maximum solar flux during communication slew (0.7 AU) A) 647115 B) 669238 C) 536113 D) 29677

A) 647115 Use conservation of energy and solve for G1: 4pi x r1^2 x G1 = 4 x pi x r2^2 x G2 4pi x (9.86Rs)^2 x G1 = 4 x pi x (215Rs)^2 x (1361) **1AU = 215Rs G1 = (215Rs/9.86Rs)^2 x (1361) G1 = (215/9.86)^2 x (1361) G1 = 647115

With regards to thermal boundary layer, a greater delta_t means A) Greater heat transfer between the fluid and the plate B) Less heat transfer between the fluid and the plate

A) Greater heat transfer between the fluid and the plate

For a geometry with two concentric spheres with r1 = 10cm and r2 = 20cm, what is the view factor F_22? A) 1 B) 0.75 C) 0.5 D) 0.25

B) 0.75

Suppose our Earth has an average temperature of 17 degC and can be treated as a blackbody, at about what wavelength does the peak of the Earth's radiation occur? A) 1 micrometer B) 10 micrometers C) 100 micrometers

B) 10 micrometers (see diagram on slide 19 from Lecture 19)

Solar radius (RS), in terms of Earth Radii (RE), is about A) 200 B) 109 C) 209

B) 109

Will GOES-R (a s/c on geostationary orbit) have a solar eclipse during Winter and Summer Solstices? A) Yes B) No

B) No

The actual distance between the Earth and Sun is the closest during our A) Summer Solstice B) Winter Solstice C) Equinoxes

B) Winter Solstice

Geostationary orbit has an inclination of A) 90 deg B) 0 deg C) 52 deg

B) [polar is 90 deg, GPS is 52 deg to cover most of the Earth]

Earth's spin axis tilts away from the normal to the ecliptic plane by about A) 11.5 deg B) 0 deg C) 23.5 deg

C) 23.5 deg [tilt away during Northern Hemisphere winter, toward during NH summer]

The geocentric distance of Geostationary orbit, in terms of Earth Radius (RE), is about A) 4.2 B) 5.62 C) 6.62

C) 6.62

Archimedes figured out the crown was not made of pure gold because: A) The color of the crown doesn't look like the color of pure gold B) The crown weighs less than the pure gold given to the goldsmith C) The volume of the crown is greater than the volume of pure gold of the same weight & pure gold is the heaviest material readily available

C) The volume of the crown is greater than the volume of pure gold of the same weight & pure gold is the heaviest material readily available

For a geometry with two concentric spheres with r1 = 10cm and r2 = 20cm, what is the view factor F_21? A) 1 B) 0.75 C) 0.5 D) 0.25

D) 0.25

A cycle for which the cyclic integral of δQis greater than zero violates the Clausius inequality

False

A cycle for which the cyclic integral of δQis greater than zero violates the Clausius inequality.

False

A process that satisfies the first law of thermodynamics is ensured to take place if the process is restricted in a closed system.

False

An adiabatic process refers to a process for which the temperature remains constant, thus there is no heat transfer.

False

An alloy made of two metals of thermal conductivities k1 and k2 usually has a thermal conductivity k between k1 and k2.

False

Bodies at a temperature below 0 oC do not emit thermal radiation.

False

Both energy and entropy cannot be created and destroyed.

False

Change in entropy of a fluid having undergone a complete cycle in a Carnot heat engine is positive.

False

Consider a room filled with warm air. Some cold air leaks into the room and the air temperature drops somewhat. No air leaks out during the process. Now the room contains less energy.

False

Consider two solid blocks, one hot and the other cold, brought into contact in an adiabatic container. After a while, thermal equilibrium is established in the container as a result of heat transfer. The first law requires that the amount of energy lost by the hot body be equal to the energy gained by the cold one. The second law requires that the decrease in entropy of the hot body be equal to the increase in entropy of the cold one.

False

Considering steady one-dimensional heat flow through a plane wall of thickness L and thermal conductivity k that is exposed to convection on both sides to fluids at temperature T∞1 and T∞2 with heat transfer coefficients h1 and h2, respectively, we need to know the surface temperatures of the wall in order to evaluate the rate of heat transfer through it.

False

For a (cylindrical) wire with higher temperature than the surrounding to be insulated with an insulating material that has a thermal conductivity k, the thicker is the insulation layer, the smaller is the heat loss.

False

Heat transfer during internally reversible processes is negative when entropy increases and positive when entropy decreases.

False

In SI units, thermal conductivity, k, has the unit of [W/(m2·K)].

False

In a closed system, any energy transfer driven by a temperature difference is called heat transfer— otherwise it is work.

False

It is impossible for any device that operates to receive heat from a single reservoir and produce a net amount of work.

False

It is possible to compress an ideal gas isothermally in an adiabatic piston-cylinder device.

False

It is possible to have a 100% thermal efficiency (h=Wnet, out/QH) of a power cycle (heat engine) IF all friction and any other dissipative effects can be removed.

False

Like other ideal cycles discussed before, the ideal vapor-compression refrigeration cycle is also regarded as an internally reversible cycle.

False

Neither mass nor energy may cross the boundary of a closed system. Both mass and energy may cross the boundary of an open system.

False

Silicon oils and gaskets are commonly used in the packaging of electronic components because they provide both good thermal contact and good electrical conduction.

False

Since energy reservoirs are characterized by their temperatures, the thermal efficiency of heat engines is a function of the reservoir temperatures only.

False

T/F - A fluid boundary layer will be developed adjacent to a heated vertical plate (Ts>T∞). Both of the fluid temperature and velocity decrease with increasing distance away from the plate.

False

T/F - A small copper ball can be modeled as a lumped system, the same is true for a large chunk of roast beef.

False

T/F - An incandescent lightbulb operating at 2800 K emits about a quarter of its radiation energy in visible light.

False

T/F - For a flat plate, the correlations among Nusselt number, Rayleigh number, and Prandtl number are the same regardless whether the plate is in vertical or horizontal position.

False

T/F - Generally speaking, heat transfer rates associated with free convection are greater than those associated with forced convection.

False

T/F - In contrast to forced convection, free convection flows don't experience instabilities that lead to transition from laminar to turbulent flow.

False

T/F - In free convection, the hydrodynamic and thermal boundary layer flows are decoupled: thermal effects induce flow, which in turn affects the temperature distribution.

False

T/F - It is generally accepted that lumped system analysis is applicable if the Biot number is greater or equal to 0.1.

False

T/F - Surfaces that are intended to remain cool under the Sun, such as the surface of a radiator on a spacecraft, are desired to have high solar absorptivity and low emissivity in the infrared region.

False

T/F - The Nusselt number is defined as Nu= hL_c/k, where L_c is the characteristic length. The Nusselt number provides a measure of the convection coefficient. The larger the Nusselt number, the more effective is the conduction.

False

T/F - The heat transfer between a surface and an adjacent fluid with a different temperature is greater in the laminar flow region than in the turbulent region.

False

T/F - The hydrodynamic boundary layer is the thin region next to the surface in which the velocity of the fluid, V, changes from zero at the surface (no slip condition) to the free stream velocity, V∞, some distance from the surface. The boundary layer thickness is defined as the distance from the surface to that distance at which V=0.5V∞.

False

T/F - The solar constant (total solar irradiation) at 1 astronomical unit (AU) is 1361 W/m^2. Then the value of total solar radiation at 0.5 AU will be 2722 W/m^2.

False

T/F - Two bodies that are made of the same material (with the same amount of materials as well) have different shapes: a slender one and a well-rounded one. The lumped system analysis is more likely applicable to the well-rounded one.

False

The back work ratio for a gas turbine is much smaller than that for a steam power plant.

False

The cold-air standard assumptions include the air-standard assumptions while employing constant specific heats whose values are determined at freezing temperatures.

False

The equality in the Clausius inequality holds for irreversible cycles and the inequality holds for reversible cycles.

False

The fraction of the heat input that is converted to net work output is a measure of the performance of a heat engine and is called the thermal efficiency ηth.

False

The integral of δQ/T along a reversible or irreversible path would give us the same value of entropy change between two specified states.

False

The saturation temperature of water steam at 20 kPa is about 60oC. It is not possible to maintain a pressure of 20 kPa in a condenser that is being cooled by river water entering at 20oC.

False

The specific internal energy and enthalpy of ideal gases vary with temperature only, so does the entropy.

False

The thermal conductivity of Aluminum is almost twice as large as the thermal conductivity of Copper at room temperature.

False

The thermal contact resistance is greater for smooth surfaces than rough surfaces.

False

The thermal efficiencies of two reversible heat engines operating between the same two reservoirs can be different.

False

The thermal efficiency of a diesel engine is greater than that of an Otto engine for the same compression and specific heat ratio.

False

The thermal efficiency of any heat engine, reversible or irreversible, is given as ηth= 1 -TL/ TH, where TH and TL are the temperatures of the high-and low-temperature reservoirs, respectively.

False

The thermal resistance for conduction through a plane wall is proportional to the area of the wall and inversely proportional to thickness of the wall.

False

Thermodynamic cycles always involve a phase transition during the cycle.

False

When a temperature difference exists between two surfaces, a net heat transfer cannot occur in the absence of any intervening medium.

False

With respect to an air-standard analysis, one of the assumptions is that the combustion process is replaced by a heat-rejection process that restores the working fluid to its initial state.

False

Work can be converted to heat directly and completely, and vice versa.

False

T/F - The spectral irradiation is defined as the rate at which radiation of wavelength l is incident on a surface per unit area of the surface and per unit wavelength interval dl about l. For an opaque medium, the spectral irradiation is partially absorbed and partially transmitted.

False - for opaque mediums, the portion of spectral irradiation partially transmitted ALWAYS equals 0.

T/F - The view factor Fij is defined as the fraction of the radiation leaving surface i that is intercepted by surface j. Thus the view factor from a surface to itself will be zero, regardless of the shape of the surface.

False - the view factor depends on the shape of the surface (it is not always zero from itself to itself -- ex: flat surface, convex, concave)

A material that has a high thermal conductivity or a low heat capacity will have a large thermal diffusivity. The larger the thermal conductivity, the faster the propagation of heat into the medium.

True

A property of a system has fixed values at fixed states. Therefore, the property change between two specified states is the same no matter what path is followed during a process.

True

A reversible process is defined as a process that can be reversed without leaving any trace on the surroundings. However, reversible processes actually do not occur in nature. They are merely idealizations of actual processes.

True

A temperature scale that is independent of the properties of the substances that are used to measure temperature is called a thermodynamic temperature scale.

True

All four components associated with the Rankine cycle (the pump, boiler, turbine, and condenser) are steady-flow devices, and thus all four processes that make up the Rankine cycle can be analyzed as steady-flow processes.

True

An ideal gas undergoes a process at constant pressure between two specified temperatures. Both of the following equations will give the same entropy change (assuming the specific heats don't vary with the temperature).

True

An ideal heat pump will have a coefficient of performance (COP_HP) always greater than one.

True

Any excessive moisture at the final stages of the turbine should be avoided in a vapor power plant (a Rankine cycle), one way to reduce the moisture level is to expand the steam in the turbine in two stages, and reheat it in between.

True

Any physical body whose thermal energy capacity is large relative to the amount of energy it supplies or absorbs can be modeled as a heat reservoir.

True

Entropy can be transferred to or from a system in the forms of heat transfer and mass flow.

True

Entropy has fixed values at fixed states. Therefore, the entropy change ΔS between two specified states is the same no matter what path, reversible or irreversible, is followed during a process between these two states.

True

Equations, Tds=du + Pdv & Tds=dh-vdP, relate entropy changes of a system to the changes of other properties, regardless if the processes are reversible and irreversible.

True

Finned surfaces are commonly used in practice to enhance heat transfer.

True

For an isolated system (or just an adiabatic & closed system), the heat transfer is zero, so is the entropy transfer associated with heat transfer.

True

Forced convection inside the International Space Station is required.

True

Friction is a familiar form of irreversibility associated with bodies in motion and it does not always involve two solid bodies in contact.

True

Generally speaking, for both ideal and actual cycles, thermal efficiency increases with an increase in the average temperature at which heat is supplied to the system or with a decrease in the average temperature at which heat is rejected from the system.

True

Heat transfer is energy in transit due to a temperature difference.

True

If a process is internally reversible and adiabatic, it must be an isentropic process.

True

In gas-turbine engines, the temperature of the exhaust gas leaving the turbine is often considerably higher than the temperature of the air leaving the compressor.

True

Irreversibility's include friction, non-quasi-equilibrium expansion and compression, and heat transfer (with finite temperature difference).

True

It is a common experience to feel 'chilly' in winter and 'warmer' in summer in our homes even when the thermostat is kept the same. This is due to the so called 'radiation effect' resulting from radiation heat exchange between our bodies and the surrounding surfaces of the wall and the ceiling.

True

It is impossible to construct a device that operates in a cycle that produce no effect other than the transfer of heat from a lower-temperature body to a higher-temperature body.

True

It is possible for the entropy change of a closed system to be zero during an irreversible process.

True

Kirchhoff's law of radiation states that the emissivity and the absorptivity of a surface at a given temperature and wavelength are equal.

True

On a T-S diagram, the area under the process curve represents heat transfer for processes that are internally reversible. The area has no meaning regarding the amount of heat transfer for irreversible processes.

True

On a T-s diagram, a heat-addition process proceeds in the direction of increasing entropy, a heat-rejection process proceeds in the direction of deceasing entropy.

True

T/F - A Nusselt number of Nu=1 for a fluid layer represents heat transfer across the layer by pure conduction (excluding radiation here).

True

T/F - CO2 and water vapor in the atmosphere transmit the bulk of solar radiation but absorbs the infrared radiation emitted by the surface of the Earth.

True

T/F - Consider a plane wall of thickness 2L initially at a uniform temperature Ti. At time t = 0, the plane wall is placed in a large medium that is at a constant temperature T∞ and kept in that medium for t > 0. An analytical solution of transient conduction problem can be obtained and involves infinite series. However, the error involved in the one-term approximation (keeping the first term and neglecting all the remaining terms in the series) is less than 2 percent when the Fourier number τ (or Fo) is greater than 0.2.

True

T/F - Convection heat transfer from a solid surface to a fluid is merely the conduction heat transfer from the solid surface to the fluid layer adjacent to the surface when the no-slip condition is met.

True

T/F - For heat transfer between a surface and an adjacent fluid when they are at different temperature, the dominant contribution is generally made by the bulk or fluid motion of the fluid particles, although molecular motion contributes to this transfer as well.

True

T/F - For ideal gas, the volume expansion coefficient, b, is positive and always less than 1.

True

T/F - Fourier number τ (or Fo), τ= αt/L^2, is a measure of heat conducted through a body relative to heat stored. Thus a large value of the Fourier number indicates faster propagation of heat through a body.

True

T/F - General speaking, for forced convection the Nusselt number is a function of the Reynolds number and the Prandtl number.

True

T/F - In many practical applications, the surface temperature and the temperature of the source of incident radiation are of the same order of magnitude, and the average absorptivity of a surface is taken to be equal to its average emissivity.

True

T/F - Inside the International Space Station (ISS), there is no free (or natural) convection.

True

T/F - The Biot number can be viewed as the ratio of the convection at the surface to conduction within the body.

True

T/F - The Grashof number, GrL, represents the ratio of the buoyancy force to the viscous force, thus a greater GrL corresponds to a greater heat transfer by free convection.

True

T/F - The network representation of the diffuse-gray surface serves as a useful tool for visualizing and calculating radiation exchange.

True

T/F - Unlike emissivity, the total absorptivity of a material is practically independent of surface temperature but depends strongly on the temperature of the source at which the incident radiation is originating.

True

The COP relations for reversible refrigerators and heat pumps are given as COPR, rev= 1/(TH/TL -1) and COPHP, rev = 1/(1 -TL/ TH).These are the highest coefficients of performance that a refrigerator or a heat pump operating between the temperature limits of TL and TH can have.

True

The Carnot cycle is composed of four reversible processes -two isothermal and two adiabatic.

True

The Clausius inequality holds for all kinds of cycles, reversible ones and irreversible ones

True

The average fluid temperature should be as low as possible during heat rejection in order to increase the thermal efficiency of a steam power plant. The condensers of steam power plants usually operate well below the atmospheric pressure.

True

The back work ratio for a gas turbine power plant is defined as the ratio of the compressor work to the turbine work.

True

The compression process in an ideal cycle (e.g., Otto, Diesel, and Brayton) is internally reversible and adiabatic, and thus isentropic. The actual compression process, however, involves frictional effects, which increase the entropy, and heat transfer, which may increase of decrease the entropy, depending on the direction.

True

The displacement volume is the volume displaced by the piston as it moves between TDC and BDC.

True

The entropy of an isolated system remains the same during a reversible process and will always increase during an irreversible process. The irreversible effects are always to increase the entropy.

True

The ideal Rankine cycle does not involve any internal irreversibilities and consists of the following four processes: 1-2 Isentropic compression in a pump, 2-3 Constant pressure heat addition in a boiler, 3-4 Isentropic expansion in a turbine, 4-1 Constant pressure heat rejection in a condenser.

True

The reversible steady-flow work is closely associated with the specific volume of the fluid flowing through the device. The larger the specific volume, the larger the reversible work produced or consumed by the steady-flow device

True

The thermal contact resistance at the two 1-cm-thick aluminum plates can be greater than the sum of the thermal resistances of both plates.

True

The thermal efficiency of a Carnot cycle is independent of the type of working fluid used (ideal gas, steam, etc.) or whether the cycle is executed in a closed or steady-flow system.

True

The thermal efficiency of a gas-turbine engine depends on the allowable maximum gas temperature at the turbine inlet.

True

The thermal resistance concept is widely used in practice because it is intuitively easy to understand and it has proven to be a useful tool in the solution of a wide range of heat transfer problems. But its use is limited to systems through which the rate of heat transfer remains constant.

True

There are different types of heat transfer processes. We refer to them as modes: conduction, convection, and radiation.

True

The heat released from a household refrigerator to the room air comes most from the cold stuff inside the refrigerator, since the refrigerator is able to transfer the heat from a low-temperature body to a high-temperature body.

True & False

T/F - For surface that emits and reflects uniformly, called a diffuse surface, the spectral emissivity and absorptivity are equal, e_lambda(T) =a_lambda(T), for a given temperature.

True - Kirchhoff's Law says these are the same

T/F - The reflection from surfaces can be idealized as diffuse (uniformly in all directions, rough surfaces) or specular (mirror-like, polished surfaces).

True - refer to laser and mirror demonstration from class.

In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction.

True/False, True according to the book but false because radiation can also occur