heat

Question 1

If the fluid dynamics boundary layer is larger than the thermal one, the value of the Prandtl number is:

Answer 1

larger than 1

Question 2

In which of the following answers is the Wien
law correctly written:

Answer 2

λmax emission · T = const(2898)

Question 3

In steady-state conditions, the general heat
flux profile Q’‘(r) in a homogeneous, hollow
sphere in which a heat source U’’’ is present
reads as (C1 > 0):
a)Q’‘(r) = U’’‘/3r − C1λ/r^2
b)Q’‘(r) = -U’’’/(3λ)r + C1/r^2

Answer 3

a

Question 4

The most general form of the conduction
equation for homogeneous, isotropic media with conductivity varying with temperature is:
a)∇2T + U’’’/(ρc) = −λ ∂T /∂τ
b)∇ (λ∇T) + U’’’= ρc ∂T /∂τ
c)λ∇2T + U’’’ = ρc ∂T /∂τ

Answer 4

b

Question 5

A sequence of flow patterns encountered
during flow condensation in a pipe is:
A)single phase vapour - annular flow - slug/plug flow- bubbly flow - single-phase liquid
B)single phase vapour - bubbly flow - mist flow - annular flow - single-phase liquid
C)single phase liquid - bubbly flow - annular flow - mist-flow - single-phase vapour
D)single phase liquid - annular flow - bubbly flow -plug/slug flow - single-phase vapour

Answer 5

A

Question 6

Which of the following sentences is true
for mirrors?
A First-surface mirrors do not suffer from debris impact because the reflecting surface is protected by a glass or polymer layer.
B Second-surface mirrors are preferred for heat rejection because they can offer a more favourable emission/absorption ratio.
C First-surface mirrors are preferred for heat rejection because because they avoid the “ghost reflections” issue.
D Second-surface mirrors do not suffer from debris impact because the emitting layer is fully protected by a metal shield

Answer 6

B

Question 7

Which of the following is a necessary condition for the stability of an explicit finite difference scheme (FTCS -Forward Time Central Scheme)?
A Pr < 0.5
B Bi < 0.5
C Fo < 0.5
D Nu < 0.5

Answer 7

C

Question 8

The temperature profile T (τ ) of a small, very conductive sphere (with uniform and constant thermophysical properties), initially at T0 and convectively heated by a constant temperature ambient at T∞:
A has oscillations when a constant internal heat source is present
B has oscillations if no internal heat source is present
C tends to T∞ if no internal heat source is present
D tends to T∞ if an internal heat source is present

Answer 8

C

Question 9

Concerning capillary phenomena, the Young
equation reads

Answer 9

cos θ = (σSA − σSL)/σLA

Question 10

In which of the following answers are the
substances correctly ordered according to
decreasing values of the Prandtl number?

A viscous mineral oil - water - air - liquid metal
B air - water - viscous mineral oil - liquid metal
C liquid metal - air - water - viscous mineral oil
D air - water - liquid metal - viscous mineral oil

Answer 10

A

Question 11

For a medium with constant conductivity, the conduction equation can be written as;
A) λ∇^2T + U’’’= ρc ∂T /∂τ
B) ∇2T + U’’’/(ρc) = −λ ∂T /∂τ
C) ∇ (λ∇T) − λU’’’= ρc ∂T /∂τ
D ∇^2T = −λ

Answer 11

A

Question 12

The Richardson dimensionless group is defined
as:

Answer 12

Ri = Gr/Re^2

Question 13

During pool boiling, the maximum heat flux occurs:

A at the onset of nucleate boiling
B immediately before of the jets&columns phase
C at the onset of film boiling
D immediately before of the transition phase

Answer 13

D

Question 14

Concerning capillary phenomena the Laplace-Young eq;

A ∆P=σ/(1/R1+1/R2)
B ∆P=σ*(1/R1+1/R2)

Answer 14

b

Question 15

In which of the following answers are the
substances correctly ordered according to
increasing values of the Prandtl number?

A viscous mineral oil - water - air - liquid metal
B air - water - viscous mineral oil - liquid metal
C liquid metal - air - water - viscous mineral oil
D air - water - liquid metal - viscous mineral oil

Answer 15

c

Question 16

During pool boiling, the minimum heat flux
occurs:
A at the onset of nucleate boiling
B immediately before of the jets&columns phase
C at the onset of film boiling
D immediately before of the transition phase

Answer 16

C

Question 17

In which of the following answers are the
substances correctly ordered according to
increasing thermal conductivity?

A Air - water - iron - stainless steel - diamond - copper
B Air - water - stainless steel - iron - copper - diamond
C Water - air - copper - stainless steel - iron - diamond
D Diamond - iron - copper - stainless steel - air - water

Answer 17

B

Question 18

Concerning capillary phenomena, the Young equation reads:

A cos θ =(σSA − σSL)/σLA
B cos θ =σLA/(σSA − σSL)

Answer 18

A

Question 19

The radiosity for a diffuse, gray, opaque
surface can be calculated as:

A radiosity = α · irradiation + ε σ T^4
surface
B radiosity = ρ · irradiation + ε σ T^4
surface
C radiosity = ρ · irradiation + α σ Asurface T^4surface
D radiosity = ρ Asurface · irradiation + ε σ T^4surface

Answer 19

B

Question 20

Active sites for nucleation are of importance:

A During film condensation to avoid the destruction of the film that would lead to dropwise condensation.
B During nucleate boiling to favour the formation of the bubbles
C During dropwise condensation to avoid that it becomes film condensation.
D During film boiling, to avoid the film dryout

Answer 20

b

Question 21

Which of the following sentences is true
for mirrors?
A First-surface mirrors do not suffer from debris impact because the reflecting surface is protected by a glass or polymer layer.
B Second-surface mirrors are preferred for high-end optics because they avoid the “ghost reflections” issue.
C Second-surface mirrors are preferred for heat rejection because they can offer a more favourable emission/absorption ratio.
D First-surface mirrors are preferred for heat rejection because they can be thinner, minimizing conduction.

Answer 21

C

Question 22

The sequence of flow patterns encountered
during flow boiling is:
A single phase liquid - annular flow - plug/slug flow -mist flow - bubbly flow - single-phase vapour
B single phase liquid - mist flow - bubbly flow - annular flow - plug/slug flow - single-phase vapour
C single phase liquid - bubbly flow - plug/slug flow -annular flow - mist flow - single-phase vapour

Answer 22

c

Question 23

Which of the following sentences is true
for a capillary heat pipe?
A In presence of gravity, a capillary heat pipe can cover longer distances if the evaporator is at an higher height with respect to the condenser.
B In presence of gravity, a capillary heat pipe can cover longer distances if the condenser is at an higher height with respect to the evaporator.
C Also in presence of gravity, orientation is not important for capillary heat pipes because the fluid motion is due to capillary actions.
D Also in presence of gravity, orientation is not important for capillary heat pipes because the gravitational pressure drop for the vapour is very small.

Answer 23

b

Question 24

Which of the following sentences is correct for meshing in CFD?
A) more accurate solution can be obtained if the mesh is coarser in the regions of the domain where larger gradients are expected, to reduce the amount of numerical noise.
B) A more accurate solution can be obtained if the mesh is more refined in the regions of the domain where larger values of the quantities are expected, as given the total number of elements this reduces the noise-to-signal ratio.
C) A more accurate solution can be obtained if the mesh is coarser in the regions of the domain where larger values of the quantities are expected, as this improves both convergence and stability.
D) A more accurate solution can be obtained if the mesh is more refined in the regions of the domain where larger gradients are expected, to better capture the distributions of the quantities

Answer 24

d

Question 25

Comparing Rayleigh and Mie scattering:

Answer 25

Rayleigh scattering is due to smaller particles and is more isotropic

Question 26

During pool boiling, the maximum heat flux is:
A)at the onset of nucleate boiling
B )just before the onset of nucleate boiling
C )just before the dryout
D )just after the dryout

Answer 26

c

Question 27

The temperature profile T (τ ) of a small, very
conductive sphere convectively heated by a
constant temperature ambient:

A may have oscillations
B cannot have oscillations
C has a maximum for T = (hAT∞) / (ρcV )
D has a minimum for T = (hAT∞) / (ρcV )

Answer 27

b

Question 28

You have to simulate free convection over a vertical cylinder, hotter than the surrounding air. For the whole length of the cylinder, half of its side surface receives radiation from the sun. Which of the following domains would be a correct choice for the simulation?
A 2D axi-symmetric domain
B 3D domain with no symmetries
C 3D domain considering 1/4 of the cylinder
D 2D domain considering a proper horizontal slice of the cylinder

Answer 28

b

Question 29

In which of the following answers is the
Stefan-Boltzmann law correctly written?

A ρλ,T + αλ,T + τλ,T = 1
B λmax emission · T = const
C ελ,T, = αλ,T,
D ET’’ = εT σ T4

Answer 29

d

Question 30

Which of the following is usually true when
considering temperatures or temperature ranges for thermal control?
A ∆Toperating > ∆Tnon-operating > ∆Tsurvival
B ∆Toperating < ∆Tnon-operating < ∆Tsurvival
C Toperating > Tnon-operating > Tsurvival
D Toperating < Tnon-operating < Tsurvival

Answer 30

b

Question 31

The general form of the Laplace-Young
equation reads:
∆P = 2 σ / R
∆P = σ (1/RI + 1/RII )
∆P = σ / R cosθ
∆P = σ (1/RI − 1/RII )

Answer 31

b

Question 32

Considering nucleate boiling, which of the
following is true:
A superheating is needed for bubble birth to overcome the additional capillary pressure difference
B superheating is caused by the the pressure reduction due to capillary action
C superheating is desirable but also dangerous due to the risk of burnout
D superheating only occurs with self-rewetting fluids

Answer 32

a

Question 33

The one-term approximation for the solution of 1D, transient, conduction problems is valid when…

A the Biot number is much lower than 1
B the Fourier number is larger than 0.2
C the Richardson number is much larger than 1
D the Rayleigh number is larger than 0.2

Answer 33

b