Flashcards in Q's Deck (159)
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1
In flow boiling “slug” flow is possible
In saturated flow boiling
2
In the PDB region the heat flux is mainly due to
c. single phase LIQUID convection
3
F/T. As the excess temperature increases, the sustainable heat flux will continuously increase in boiling.
False
4
Saturated steam condenses over a vertical flat surface and the condensate film flows down the surface. The local heat transfer coefficient for condensation
DECREASES with increasing distance from the top of the surface
5
A vertical, 3.5m long EVAPORATOR TUBE of 12mm internal diameter has water flowing through it from bottom to top. The mass flux is 1000 kg/m2s. The water enters the tube, subcooled with an enthalpy of 890 kJ/kg. The tube is heated with a constant heat flux q” = 7.58 x 105W/m2. Calculate the TUBE LENGTH in m (two decimal digits) at which the water reaches the saturation temperature, 270 °C. Use the data: enthalpy of saturated liquid hf = 1185 kJ/kg, enthalpy of
vaporization hfg= 1604 kJ/kg.
575 m
6
T/F. In a CONDENSER the CONTROLLING RESISTANCE will be on the HOT fluid side.
False
7
in the INERTIA controlled growth of a bubble, the growth rate is proportional to
t
8
T/F. In pool boiling, the ONB wall superheating for water is around 250°C
False
9
T/F. The surface should be non wetting for dropwise condensation.
True
10
T/F. For a condenser/evaporator the effectiveness is (1 – e–2NTU).
False
11
T/F. In condensation on a horizontal tube, the liquid film is turbulent at Re'c = 2000
False
12
T/F. In condensation film, linear temperature profile is generally assumed.
True
13
T/F. When inlet flow rates and temperatures for a heat exchanger are specified the easier method of analysis NTU method.
True
14
In laminar film condensation the convection coefficient varies as (where L is the height of plate)
L^(–0.25)
15
In a CONDENSER. WATER enters at 30°C and flows at the rate 1500 kg/hr. The condensing steam is at a temperature of 120°C and cooling water leaves the condenser at 80°C. Specific heat of water is 4.187 kJ/kg K. If the overall hea transfer coefficient is 2000 W/m2 K. the heat transfer AREA is
0.707 m^2
16
Consider the following statements: 1-MASS TRANSFER refers to mass in transit due to a species concentration gradient in a mixture. 2-Must have a mixture of two or more species for mass transfer to occur. 3-The species concentration gradient is the driving potential for mass transfer. 4-Mass transfer by diffusion is analogous to heat transfer by conduction. Which of these statements are correct?
1,2,3 and 4
17
The heat flux in nucleate pool boiling will be higher for
Indipendent of location
18
Homogeneous nucleation for water can occur with a minimum liquid superheating
250 °C
19
The inlet and outlet temperatures of the hot and cold fluids are 200, 40 and 180 and 100°C. The value of the LMTD for COUNTER flow is
118.88 °C
20
How can the rates of heat transfer from drop-wise condensation and film condensation be compared?
the rate of heat transfer from drop-wise condensation is HIGHER than that of film condensation
21
F/T. For the performance evaluation over the whole operating range of a heat exchanger LMTD method is more suitable.
False
22
In nucleate boiling, the nuclei of bubbles form when there is presence
dissolved gases
23
In the boiling curve, the point at which start the nucleation of the first bubble is termed
ONB
24
T/F. In film boiling excess temperature will be limited.
False
25
In the calculation of two phase pressure drop, the VAPOUR compressibility
Is neglected if a particular requirement is satisfied
26
Surface coating is suitable to maintain
drop-wise condensation
27
A vertical, 3.5m long evaporator tube of 12mm internal diameter has water flowing through it from bottom to top. The mass flux is 1000 kg/m2s. The water enters the tube, subcooled with an enthalpy of 890 kJ/kg. The tube is heated with a constant heat flux q” = 7.58 x 105W/m2. Calculate the vapour content at the outlet in % (one decimal digit). Use the data: enthalpy of saturated liquid hf = 1185 kJ/kg, enthalpy of vaporization hfg= 1604 kJ/kg.
241 %
28
Cross flow exchangers are popularly used for heat transfer
gas and gas or liquid and gas
29
T/F. The value Uo will be lower compared to value Ui in heat exchangers.
True
30
In film boiling the properties that contribute to the value of convection coefficient are
hfg, μg, kg and ρf
31
For a given channel and low heat flux, the highest heat transfer coefficients in flow boiling can be obtained
In the liquid deficient region
32
The thermal resistance for heat transfer is low in
drop-wise condensation
33
Water (Cp = 4.18 kJ/kgK) at 80°C enters a COUNTER FLOW heat exchanger with a mass flow rate of 0.5 kg/s. Air (Cp = 1 kJ/kgK) enter at 30°C with a mass flow rate 2.09 kg/s. If the effectiveness of the heat exchanger is 0.8, the LMTD (in °C) is
10 °C
34
A counter flow heat exchanger is used to heat water from 30°C to 80°C by using hot EXHAUST GAS entering at
140°C and leaving at 90°C. The log mean temperature difference for the heat exchanger is
60 °C
35
In a heat exchanger, the hot gases enter with a temperature of 150°C and leave at 75°C. The cold fluid enters at 25°C and leaves at 125°C. The heat capacity ratio of the exchanger is
0.75
36
T/F. For the same NTU, as the capacity ratio increases the effectiveness will decrease.
True
37
Typycal thermal boundary layer thickness for water in pool boiling is
0.1 mm
38
The CRITICAL heat flux in NUCLEATE pool boiling is proportional to (where σ is the surface tension)
σ^1/4
39
The wall superheating for bubble nucleation is strongly reduced if the contact angle is
a. higher than 90°
40
T/F. For water, the excess temperature range for maximum flux in nucleate pool boiling is about 200°C.
False
41
Generally, the convection heat transfer coefficient in drop-wise condensation is
higher than the convection heat transfer coefficient in film condensation
42
The logarithmic mean temperature difference (LMTD) of a counter flow heat exchanger is 20"C. The cold fluid
enters at 20°C and the hot fluid enters at 100°C. Mass flow rate of the cold fluid is twice that of the hot fluid.
Specific heat at constant pressure of the hot fluid is twice that of the cold fluid. The exit temperature of the cold fluid is
80 °C
43
A heat exchanger with heat transfer surface area A and overall heat transfer coefficient U handles two fluids of heat capacities C1 and C2 such that C1 > C2. The NTU of the heat exchanger is given by
UA/C2
44
T/F. For a given exchanger as the capacity ratio increases the final temperatures will increase.
False
45
With increase in excess temperature the heat flux in boiling
increases then decreases and again increases
46
"Taylor bubbles" are characteristic in
Slug flow
47
T/F. Increasing the system pressure, the equilibrium bubble radius will increase
True
48
The modified latent heat introduced by Rohsenow to the original Nusselt’s theory take into account
The subcooling of the condensate
49
A light oil flows through a copper tube of 2.6 cm ID and 3.2 cm OD. Air is flowing over the exterior of the tube. The convection heat transfer coefficient for the oil is 120 W/(m2 K) and for the air is 35 W/(m2 K). Calculate the overall heat transfer coefficient based on the outside area of the tube neglecting the resistance of the tube, but considering the following fouling resistances: 0.0009 (m2K)/W on the inside and 0.0004 (m2K)/W on the outside, respectively.
(one degimal digit)
24.8
50
The flow direction is not important in the case of heat exchange from
saturated steam to liquid water
51
Typical size of the water bubble at depart is
in the range of 1-2 mm
52
The presence of dissolved air in the liquid
REDUCES the needed superheating for the bubble formation
53
T/F. In nucleate boiling at higher heat flux levels convection coefficient is proportional to the third power of excess
temperature.
True
54
In a two phase mixture the velocity of sound
is generally lower than in both the phases
55
Consider the following statements regarding condensation heat transfer: 1. If a condensing liquid does not wet a surface then dropwise condensation will take place on it. 2. Dropwise condensation gives a higher heat transfer rate than filmwise condensation. 3. Reynolds number of condensing liquid is based on its mass flow rate. 4. Suitable coating or vapour additive is used to promote FILMWISE condensation. Which of these statements is/are correct?
1, 2 and 3
56
A cross-flow type air-heater has an area of 50 m2. The overall heat transfer coefficient is 100 W/m2 K and heat
capacity of both hot and cold stream is 1000 W/K. The value of NTU is
5
57
Consider the following statements: The flow configuration in a heat exchanger, whether counter flow or otherwise, will not matter if: 1. a liquid is evaporating. 2. a vapour is condensing. 3. mass flow rate of one of the fluids is far greater than the other. Which of these statements are correct?
1,2 and 3
58
T/F. Shell and tube arrangement is generally adopted because single tube type will be too long
True
59
Consider the following statements regarding NUCLEATE boiling: 1. The temperature of the surface is greater than the saturation temperature of the liquid. 2. Bubbles are created by the expansion of entrapped gas or vapour at small cavities in the surface. 3. The temperature is greater than the LEIDENFROST temperature. 4. The heat transfer
coefficient from the surface to the liquid is greater than that in film boiling. Which of these statements are correct?
1, 2 and 4
60
In nucleate pool boiling the heat flux depends on
fluid and material and surface roughness
61
The convective heat transfer coefficient in laminar film condensation varies as (ρ-density of liquid)
ρ^0.5
62
In pool boiling, as soon as the temperature of heating surface reaches the boiling point of the liquid, heat transfer takes place
by natural convection
63
T/F. At a fixed temperature, a liquid metastable state can occur at PRESSURES HIGHER than the saturation pressure
False
64
Critical film Reynolds number in condensation on a flat plate is
1800
65
Assertion (A): It is not possible to determine LMTD in a counter flow heat exchanger with equal heat capacity rates of hot and cold fluids. Reason (R): Because the temperature difference is invariant along the length of the heat exchanger.
A is false but R is true
66
In subcooled flow boiling zone, the wall temperature
After a first small increase, it remains practically constant
67
In the subcooled fully developed boiling region
Single phase FORCED convection contribution is negligible
68
In the saturated flow boiling, the microscopic contribution refers to
Nucleate boiling
69
T/F. DNB could occur in the liquid deficient region
False
70
A chocked flow occurs if
The RATIO between the Downstream and the Upstream pressure is LOWER than a critical value depending on the fluid
71
Which theory is widely used to determine the heat transfer coefficient for film condensation on surfaces?
Nusselt's theory
72
In a heat exchanger the LMTD was 26.2°C under certain flow conditions. The heat transfer rate was 11000 W. Due to changes in surrounding conditions the inlet temperature alone had changed and the value of LMTD was found as 22.3°C. The HEATT RATE at this conditions will be
67 W
73
T/F. When heat capacities of both fluids are equal, the temperature difference will be constant for parallel flow
arrangement.
False
74
In flow boiling, the critical heat flux value decreases if
G decreases
75
T/F. In flow boiling h is maximum at annular flow.
True
76
The acceleration term in two-phase pressure drop is
Often HIGHER than the other terms
77
T/F. In film wise condensation the value of h will be lower compared to dropwise condensation.
True
78
T/F. In Nusselt’s theory, temperature variation is assumed parabolic in condensate film.
False
79
A clean heat exchanger under test gave a U value of 2000 W/m2K. After one year of operation the value of U was determined as 1600 W/m2K. The fouling resistance is (6 decimal digits)
0.000125 m^2K/W
80
The overall heat transfer coefficient is the
sum of all conductances
81
The CRITICAL heat flux in nucleate pool boiling is proportional to (where hfg is the enthalphy of evaporation)
hfg
82
T/F: During bubble growing, the inertia-controlled regime is typical for small bubbles and at the early stage of their growth
True
83
In laminar film condensation the convection coefficient varies as (where L is the height of plate)
L^(–0.25)
84
Consider the following statements regarding condensation heat transfer: 1. For a SINGLE TUBE, horizontal position is preferred over vertical position for better heat transfer. 2. Heat transfer coefficient decreases if the vapour stream moves at high velocity. 3. Condensation of steam on an OILY SURFACE is dropwise. 4. Condensation of pure steam is always filmwise. Which of these statements are correct?
1 and 3
85
In a wall temperature-controlled system in pool boiling, the region after the thermal crisis is
transition boiling
86
T/F. Consider condensation on a vertical plate. The heat transfer coefficient is higher at the bottom of the plate
False
87
In a heat exchanger, the HOT GASES enter with a temperature of 150°C and leave at 75°C. The cold fluid enters at 25°C and leaves at 125°C. The HEAT CAPACITY RATIO of the exchanger is
0.75
88
T/F: Average value of convection coefficient in condensation up to distance L is (4/5)hL
False
89
When one of the fluid is condensing the best flow arrangement is
all are equal
90
T/F: The heat capacity of the fluid stream is the product of mass flow rate and enthalpy.
False
91
if bubble departs in the inertia-controlled regime, the frequency is related to the size of depart as
f2Dd = constant (comment: squared!)
92
Assertion (A): If the heat flux in pool boiling over a horizontal surface is increased above the cCRITICAL HEAT FLUX, the temperature difference between the surface and liquid decreases sharply. Reason (R): With increasing heat flux beyond the value corresponding to the CRITICAL HEAT FLUX, a stage is reached when the rate of formation of bubbles is so high that they start to coalesce and blanket the surface with a vapour film.
A is false but R is true
93
T/F: In the subcooled zone of flow boiling, the heat transfer coefficient increases approaching the mixture saturation point
True
94
The “flooding line” identifies:
The MAXIMUM countercurrent liquid flow rate for each gas flow rate
95
In nucleate boiling the wall superheating is proportional to the heat flux powered to
between 0.25 and 0.5
96
Considering an isolated bubble in a liquid pool, the early phase of its growth is
Inertia-controlled
97
In FORCED convection boiling process, a liquid flows through a tube with
SUBcooled or SATURATED boiling
98
T/F: After dryout, heat transfer is with superheated steam only
False
99
Which of the following is NOT a type of condensation heat transfer process?
BULK-wise condensation
100
In a heat exchanger the hot fluid inlet and outlet temperatures are 200 and 180°C. For the cold fluid the terminal temperatures are 40 and 100°C. The capacity ratio is
0.333
101
Effectiveness of a heat exchanger is
actual heat transfer/heat transfer when MINIMUM heat capacity fluid GOES through the MAXIMUM temperature difference in the exchanger
102
In a condenser of a power plant, the steam condenses at a temperature of 60°C. The cooling water enters at 30°C and leaves at 45°C. The Logarithmic Mean Temperature Difference (LMTD) of the condenser is
21.6 °C
103
The equilibrium radius of a spherical bubble:
DECREASES if the liquid superheating increases
104
In NUCLEATE pool boiling the convective heat transfer coefficient varies as (where ΔT is excess temperature)
ΔT^2
105
In NUCLEATE FILM boiling of WATERthe heat flux varies as (where k is the vapour thermal conductivity)
k^0.75
106
T/F. In film boiling an important mode of heat transfer is radiation.
True
107
“Flow reversal” occurs when
Liquid changes its flow direction with respect to the gas from CO-current to COUNTERcurrent
108
According to the Nusselt’s theory, if other parameters are kept constant, the LIQUID FILM thickness
Increases as the thermal conductivity of the LIQUID and its viscosity increase
109
COLD WATER flowing at 0.1 kg/s is heated from 20°C to 70°C in a counter flow type heat exchanger by a hot water stream flowing at 0.1 kg/s and entering at 90°C. The specific heat of water is 4200 J/(kgK) and density is 1000 kg/m3. If the overall heat transfer coefficient U for the heat exchanger is 2000 W/(m2K), the required heat exchange AREA (in m2) is
0.525 m^2
110
T/F: The flow direction affects the performance in the case of condensers or evaporators.
False
111
T/F: Homogeneous nucleation is not rare for water
False
112
Steam flows inside an externally cooled channel; as condensation starts, the mixture flow regime is
Annular flow
113
T/F: Maximum heat flux without damage can be sustained only in film boiling.
False
114
In condensing under same conditions, the convection coefficient will be lowest for
row of vertical pipes
115
In an ECONOMISER of a large steam generator pressurised water flows at a rate of 20 kg/s. The water temperature increases from 40°C to 160°C. The flue gas temperature drops from 360°C to 190°C. The heat capacity of the gas flow is
58984 W/K
116
T/F. When heat capacities of both fluids are equal, the temperature difference will be constant for PARALLEL flow
arrangement.
False
117
The HAET FLUX in nucleate pool boiling is proportional to (where hfg is enthalphy of evaporation)
1/hfg^2
118
In the Chen correlation for saturated flow boiling, increasing the quality
the factor F INCREASES and the suppression factor S decreases
119
During a blowdown in chocked flow the velocity of sound is reached
In the throat of the nozzle (minimum flow area)
120
T/F. In condensation, convection coefficient over a single tube is LOWER compared to condensation over a row of tubes.
False
121
A water mixture saturated at a temperature of 275.58 ◦C, enters into a VERTICAL STEEL TUBE of internal diameter 20 mm with an inlet quality of 25%. The mass flux is G = 1000 kg/m2s. The mixture is cooled by a uniform heat flux of q” = 8 x105W/m2. HOW LONG does the tube HAVE TO BE (in m, two decimal digits), if the enthalpy at the outlet is ho = 1085.7 kJ/kg? (specific enthalpy of the saturated liquid hf = 1213.9 kJ/kg, of the saturated vapour hg = 2785 kJ/kg).
8.06 m
122
The flow regime depends mainly on
direction of the flow, fluid properties, liquid and gas SUPERFICIAL VELOCITIES
123
Considering an isolated bubble in a liquid pool, the early phase of its growth is
inertia-controlled
124
T/F: At a fixed pressure, vapour metastable state can occur at TEMPERATURES LOWER than the saturation value
True
125
T/F: The average heat transfer coefficient on a vertical plate, always decreases as Reynolds number of the film increases
False
126
In an economiser water gets heated by 120°C while FLUE GASES are cooled by 170°C. The capacity ratio is
0.706
127
In nucleate pool boiling the heat flux for boiling of water is proportional to (μf is the viscosity of liquid)
μf^(–2)
128
T/F: In pool boiling, the ONB wall superheating for water is around 250°C
False
129
The Martinelli-Nelson method is used to evaluate:
total pressure drop
130
T/F: NTU is defined as UA/C(hot.)
False
131
The CRITICAL heat flux in NUCLEATE pool boiling is proportional to (where σ is the surface tension)
σ^(1/4)
132
The main assumption to adopt the HEM in two-phase pressure drop calculation is:
THERMAL AND MECHANICAL equilibrium between the phases
133
According to the Nusselt’s theory, if other parameters are kept constant, the LIQUID FILM thickness
INCREASES as the thermal conductivity of the liquid AND its viscosity increase
134
T/F: Condensation Reynolds number is defined IN TERMS OF mass flow G per unit width as 4GD/μ.
False
135
T/F: In a gas to liquid heat exchangers the CONTROLLING resistance will be on the GAS side.
True
136
in the HEAT TRANSFER CONTROLLED growth of a bubble, the growth rate is proportional to
t^(1/2)
137
A COUNTER FLOW heat exchanger has a surface area of 20 m2 and overall heat transfer coefficient of 20 W/m2-K. Air (Cp = 1000 J/kg-K) entering at 0.4 kg/s and 280 K is to be preheated by 0.4 kg/s of air leaving the system at 300 K. The exit temperature (in K) of the (pre)heated air
300 K
138
T/F: In a counter flow heat exchanger in which superheated fluid is COOLED, CONDENSED and UNDERCOOLED, using a cold
fluid, the PINCH point is at the end of the condensation section.
False
139
T/F: At a FIXED temperature, a liquid metastable state can occur at ALL pressures
False
140
Compared to the CHF, the physical burnout heat flux is
close to CHF in the SUBcooled thermal crisis
141
A counter flow SHELL and TUBE exchanger is used to heat water with hot exhaust gases. The water (Cp = 4180 J/kg°C) flows at the rate of 2 kg/s while the exhaust gas (Cp=1030 J/kg°C) flows at the rate of 5.25 kg/s. If the heat transfer surface area is 32.2 m2 and the overall heat transfer coefficient is 200 W/m2°C the NTU for the heat exchanger is
1.2
142
T/F: Capacity ratio is taken as zero for condensers and evaporators.
True
143
In the original Nusselt’s theory, the temperature profile through the film is LINEAR; indeed it is
With an UPward concavity (temperature gradient higher near the interface)
144
The condensate film on a HORIZONTAL tube is partially in turbulent conditions if the Reynolds number of the film for the total condensation rate is
Higher than 3600
145
T/F: The shape of the heating surface affects pool boiling heat transfer.
False
146
A heat exchanger with 10 m2 heat transfer area has an overall heat transfer coefficient of 600 W/m2K. The minimum heat capacity of the flow is 2100 W/K. The value of NTU is
2.86
147
For a PLANAR interface, during evaporation, the pressures of the liquid and gas phases are equal
False
148
During condensation at the LIQUID-VAPOUR interface
The liquid phase is SUBcooled at the VAPOUR phase pressure
149
In a post-CHF region of a heated channel, the heat transfer coefficient is higher in
In the MFB point. NO!!!! no single superheated!!
150
The inlet and outlet temperatures of the hot and cold fluids are 200, 40 and 180 and 100°C. The value of the LMTD for parallel flow is
25.28?
151
T/F: In flow boiling, the DROP FLOW REGIME exists in a saturated mixture only
False
152
In a heat exchanger the terminal temperatures of the hot fluid are 150 and 80°C. For the cold fluid the terminal temperatures are 30 and 100°C. The flow arrangement is counter flow and the value of LMTD is
50°C
153
In film boiling, the heat transfer coefficient is
h = h(conv)+ 0.75 h(rad)
154
According to the original Nusselt's theory, to determine the heat transfer coefficient for film condensation on
surfaces
all of the answers
155
An INDUSTRIAL GAS (Cp = 1 kJ/kgK) enters a parallel flow heat exchanger at 250°C with a flow rate of 2 kg/s to heat a water stream. The water stream (Cp = 4 kJ/kgK) enters the heat exchanger at 50°C with a flow rate of 1 kg/s. The heat exchanger has an effectiveness of 0.75. The gas steam EXIT temperature will be
100°C
156
T/F: In flow boiling drop flow will SUSTAIN higher heat flux.
False
157
The Net Transfer Unit is (NTU)
UA/Cmin
158
Two phase friction pressure drops are
HIGHER than in single phase at the same total mass flux
159
