Thermo & Fluid

Question 1

A ship floating on still water has its metacentre located below its centre of gravity. The ship’s state is:

a. Neutral

b. Stable

c. Unstable

Answer 1

c. Unstable

Question 2

Two flat-bottomed flasks, A and B, with equal areas at their bases are filled with a liquid of constant density up to the same height h. Flask A is perfectly cylindrical while flask B has a tapering profile up to its neck, so that the volume of the liquid in B is less than in A. Assuming the liquid is static, which flask has a greater pressure at its bottom face?

a. B

b. A

c. Neither

Answer 2

c. Neither

Question 3

Which of these statements best describes a fluid?

a.
It resists an applied shear stress

b.
It continuously deforms under an applied shear stress

c.
It circulates continuously once a shear stress has been applied

d.
There are no shear stresses present

Answer 3

b.
It continuously deforms under an applied shear stress.

Question 4

Which of the following does not contribute to the convection coefficient of the interface between a fluid and a surface?

a.
The properties of the fluid

b.
The surface geometry

c.
The nature of fluid motion

d.
The emissivity of the surface

e.
The bulk fluid velocity

Answer 4

d.
The emissivity of the surface.

Question 5

At an elevation of 2000 m, it is found that the air pressure is 79.5 kPa and the ambient temperature is 12 °C. What is the density of the air? (Assume air is an ideal gas).

a.
0.32 kg/m3

b.
0.97 kg/m3

c.
1.03 kg/m3

d.
1.20 kg/m3

Answer 5

b.
0.97 kg/m3.

Question 6

A ventilating fan in a bathroom has a volume flow rate of 20 litres per second. If the air has a density of 1.2 kg/m3, what mass of air is vented out in 2 hours?

a.
288.0 kg

b.
48.0 kg

c.
345.6 kg

d.
172.8 kg

Answer 6

d.
172.8 kg

Question 7

A frictionless piston-cylinder device containing a fluid in equilibrium at 120 °c is placed on a heater at 150 °c until the temperature of the fluid rises by 12 °c and the piston moves through 4 cm. This process is:

a.
Isobaric

b.
Isentropic

c.
Isothermal

d.
Isochoric

Answer 7

a.
Isobaric

Question 8

A football floats on still water. The diameter of the football is 22 cm and its mass is 0.42 kg. If the density of the water is 1000 kg/m³, what percentage of the football’s volume is submerged in the water?

a.
9.42%

b.
0%

c.
7.53%

d.
0.19%

Answer 8

c.
7.53%

Question 9

A 3 m by 4 m wall is to be made of wood with a thermal conductivity k = 0.15 W/m·K. If the temperature inside is to be maintained at 20 °c and the outside temperature is -5 °c, what is the minimum thickness the wall needs to be to maintain a heat loss of at most 100 W?

a.
11 cm

b.
15 cm

c.
222 cm

d.
45 cm

Answer 9

d.
45 cm

Question 10

For internal flow within a pipe, the hydrodynamic entry length is shorter for which type of flow?

a.
Turbulent

b.
Laminar

Answer 10

a.
Turbulent

Question 11

Using the steam tables, determine the enthalpy per unit mass of water at a pressure of 300 kPa and a temperature of 325 °c:

a.
3123.3 kJ/kg

b.
3225.8 kJ/kg

c.
3277.0 kJ/kg

d.
3174.6 kJ/kg

Answer 11

a.
3123.3 kJ/kg

Question 12

A system consisting of a fluid in a piston-cylinder device at 80 °c is brought into contact with a heat reservoir at 10 °c, so that heat transfers from the system to the reservoir. The entropy of the system:

a.
Remains constant

b.
Decrease

c.
Increases

Answer 12

b.
Decrease

Question 13

A fluid in a horizontal cylinder is heated at a constant pressure of 95 kPa so that the piston, of area 4 cm2, is moved by 2 cm. How much work has been done by the fluid?

a.
76 J

b.
0.076 J

c.
0.76 J

d.
760 J

Answer 13

c.
0.76 J

Question 14

An iceberg floats in seawater such that 90% of its volume is submerged below the surface. If the density of the seawater is 1030 kg/m3, what is the density of the iceberg? (Assume the iceberg is homogeneous).

a.
912.25 kg/m3

b.
927 kg/m3

c.
1000 kg/m3

d.
105 kg/m3

Answer 14

b.
927 kg/m3

Question 15

In a Newtonian fluid, when the shear stress is increased the rate of deformation:

a.
Decreases

b.
Increases

c.
Is zero

d.
Remains the same

Answer 15

b.
Increases

Question 16

In a static, incompressible fluid, the pressure depends on:

a.
Direction of application but not distance from the free surface

b.
Distance from the free surface and direction of application

c.
Neither distance from the free surface nor direction of application

d.
Distance from the free surface but, not direction of application

Answer 16

d.
Distance from the free surface but, not direction of application

Question 17

If the emissivity of a surface of a body increases, will the rate at which the body cools down increase or decrease?

a.
Increase

b.
Decrease

c.
No effect

Answer 17

a.
Increase

Question 18

A piston-cylinder device containing 0.25 kg of water is heated until it begins to boil at 100 °c. Heat is continuously applied until the volume is measured to be 0.209 m3. At this point, what proportion of the water has been converted to saturated vapour?

a.
100%

b.
0%

c.
50%

d.
25%

Answer 18

c.
50%

Question 19

Water is flowing through a horizontal venturi metre at a rate of 0.0004 m3/s. The cross sectional area at the inlet is 0.0007 m2, and at its narrowest point is 0.00018 m2. Assuming the fluid flow to be incompressible, frictionless and with no heat transfer or pumps/turbines present, what is the difference in pressure head between the inlet and the narrowest part of the venturi?

a.
165 cm

b.
23.5 cm

c.
14.7 cm

d.
18.9 cm

Answer 19

b.
23.5 cm

Question 20

On a particular morning, the atmospheric pressure is 98 kPa and the temperature is -12 °c. What is the density of the air? (Assume air is an ideal gas).

a.
1.31 kg/m3

b.
0.76 kg/m3

c.
1.55 kg/m3

d.
1.19 kg/m3

Answer 20

a.
1.31 kg/m3

Question 21

Which of the following is not a valid expression of the second law of thermodynamics?

a.
Heat will not spontaneously flow from a low temperature body to a high temperature body

b.
A device that operates on a cycle cannot transfer heat from a low temperature body to a high temperature body

c.
During a thermodynamic process the entropy of the system and surroundings will stay the same or increase

d.
It is impossible to build a device that operates on a cycle whose net effect is to extract heat at a single temperature and convert it to work

Answer 21

b.
A device that operates on a cycle cannot transfer heat from a low temperature body to a high temperature body

Question 22

A fixed container of water is heated until the temperature of the fluid rises by 10 °c. This process is:

a.
Isochoric

b.
Isothermal

c.
Isentropic

d.
Isobaric

Answer 22

a.
Isochoric

Question 23

A steady flow mixing chamber has two inlets and one outlet, and is used to mix water of different temperatures. The cross sectional area of the inlets are 0.0002 m2 each and the velocity of the water at the inlets is 0.5 m/s. Taking the density of the water to be 1000 kg/m3, what mass of water exits the outlet in 10 seconds?

a.
1 kg

b.
20 kg

c.
2 kg

d.
0.5 kg

e.
10 kg

Answer 23

c.
2 kg

Question 24

A container full of refrigerant R-134a is found to be at a temperature of -10 °c and pressure of 60 kPa. What state is the fluid in?

a.
Saturated liquid

b.
Superheated vapour

c.
Saturated vapour

d.
Compressed liquid

Answer 24

b.
Superheated vapour

Question 25

A piston-cylinder device contains water at 250 kPa and 120 °c. What state is the water in? a. Compressed liquid b. Saturated liquid c. Saturated vapour d. Superheated vapour

Answer 25

a. Compressed liquid

Question 26

A piston-cylinder device contains refrigerant R-134a at 90 kPa, -28.65 °c, and specific volume 0.15 m3/kg. What state is the refrigerant in? a. Saturated vapour b. Saturated liquid c. Saturated liquid-vapour mixture d. Superheated vapour e. Compressed liquid f. Not enough information to tell

Answer 26

c. Saturated liquid-vapour mixture

Question 27

Water with a density of 1000 kg/m3 and dynamic viscosity of 0.001 kg/m·s is flowing in a cylindrical pipe of diameter 20 cm at a velocity of 80 cm/s. The flow is: a. Turbulent b. Transitional c. Laminar

Answer 27

a. Turbulent

Question 28

On a sunny day, with the ambient temperature at 25 °c, air blows over the roof of a car with a convection heat transfer coefficient of 12 W/m2.K. The surface of the roof is at 35 °c. If heat flows from the roof at a rate of 180 W, what is the area of the roof? a. 1.5 m2 b. 1.67 m2 c. 1.33 m2 d. 0.75 m2

Answer 28

a. 1.5 m2

Question 29

A fully reversible heat engine extracts heat at 600 °c and rejects heat at 10 °c every cycle, producing 250 kw of power. What is the efficiency of this engine? a. 58.3% b. 98.3% c. 41.7% d. 67.6%

Answer 29

d. 67.6%

Question 30

A fully reversible heat pump extracts heat energy from outdoors, where the ambient temperature is 15 °c, to heat a room at 20 °c. What is its coefficient of performance (COP)? a. 4 b. 58.6 c. 3 d. 1 e. 29

Answer 30

b. 58.6

Question 31

Water is flowing through a circular pipe of diameter 15 cm at a velocity of 2 m/s with a dynamic viscosity of 0.0015 kg/m.s. Taking the density of the water to be 1000 kg/m3, the relative roughness of the pipe walls to be 0.001, from the Moody chart, determine the friction factor: a. 0.021 b. 0.020 c. 0.031 d. 0.029

Answer 31

b. 0.020

Question 32

A wall made of breeze blocks measures 2 m by 2 m. If its height were increased to 3 m, but the thickness, temperature gradient and heat transfer rate remain constant, by what factor does the thermal conductivity change (i.e. if knew = Xkold, what is X)? a. 0.75 b. 1.5 c. 3 d. 0.667

Answer 32

d. 0.667

Question 33

A fluid contained within a frictionless, thermally insulated piston-cylinder device is compressed slowly. What type of process is this? a. Isothermal b. Isochoric c. Isobaric d. Isentropic

Answer 33

d. Isentropic

Question 34

A tank containing 500 litres of water has a tap at the bottom. When opened, the water flows through the tap at a rate of 0.25 kg/s. Taking the density to be constant at 1000 kg/m3, how long does it take to empty the tank? a. 2000 s b. 250 s c. 1000 s d. 500 s

Answer 34

a. 2000 s

Question 35

Water is pumped through a cylindrical frictionless pipe at constant velocity from ground level to a height of 100 m. What is the change in pressure head of the water? a. 10.2 m b. 0 c. 98.1 m d. 100 m

Answer 35

d. 100 m

Question 36

For which of the following conditions does Bernoulli’s equation not apply? a. Flow through a heat exchanger b. Incompressible flow c. Frictionless flow d. Steady flow

Answer 36

a. Flow through a heat exchanger

Question 37

Air in a container is maintained at a pressure of 3 bar with a density of 3.40 kg/m3. Assuming the air to be an ideal gas, what is its temperature in the container? a. 307.4 K b. 3554 K c. 355.4 K d. 30.7 oC

Answer 37

a. 307.4 K

Question 38

Saturated water is heated at a constant temperature of 190 °c until all the liquid water converts to saturated water vapour. What is the change in entropy during this process? a. 5.5968 kJ/kg.K b. 6.0470 kJ/kg.K c. 6.5059 kJ/kg.K d. 4.2705 kJ/kg.K

Answer 38

d. 4.2705 kJ/kg.K

Question 39

Air in a reciprocating engine is heated from a temperature of 600 K to 1000 K. Treating the air as an ideal gas, what is the change in specific enthalpy? a. 402 kJ/kg b. 287 kJ/kg c. 400 kJ/kg d. 0

Answer 39

a. 402 kJ/kg

Question 40

A heat engine operating on the ideal Carnot cycle extracts heat at 327 °c and rejects it at a temperature of 27 °c, producing 60 kJ of work in each cycle. How much heat energy is extracted during a cycle? a. 0 J b. 120 J c. 180 J d. 60 J