Thermal Properties and Thermodynamics

Question 1

Multiple choice

A cyclic device operates between three thermal reservoirs, as shown in the figure. Heat is transferred to form the cyclic device. It is assumed that heat transfer between each thermal reservoir and the cyclic device takes place across negligible temperature difference. Intersections between the cyclic device, the respective thermal reservoirs that are shown in the figure are all in the form of heat transfer.

The cyclic device can be

a reversible heat pump or a reversible refrigerator
an irreversible heat engine
an irreversible heat pump or an irreversible refrigerator

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Answer

a reversible hear engine

Question 2

Multiple choice

In arc welding of a butt joint, the welding speed is to be selected such that highest cooling rate is achieved. Melting efficiency and heat transfer efficiency are 0.5 and 0.7, respectively. The area of weld cross section is 5 mm² and the unit energy required to melt the metal is 10 J/mm³. If the welding power is 2 kW, the welding speed in mm/s is closest to

4
14
24
34

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Answer

$\frac{\partial T}{\partial X}=0K/m,\frac{\partial T}{\partial Y}=20K/m$

Answer

Superheated vapour will be left in the system.

Answer

is valid for any reversible process

Answer

k₁ = 2k₂

Answer

Both [a] and [r] are true and [r] is the correct reason for [a].

Answer

P - S - 5, R - U - 3, P - S - 1, Q - T - 2

Answer

$\frac{\partial^2T}{\partial X^2}$

Question 3

Multiple choice

Steady two-dimensional heat conduction takes place in the body shown in the figure below. The normal temperature gradients over surfaces P and Q can be considered to be uniform. The temperature gradient $\frac{\partial T}{\partial X}$ at surface Q is equal to 10 K/m. Surfaces P and Q are maintained at constant temperatures as shown in the figure, while the remaining part of the boundary is insulated. The body has constant thermal conductivity of 0.1 W/m.K. The values of $\frac{\partial T}{\partial X}and\frac{\partial T}{\partial Y}$ at surface P are

$\frac{\partial T}{\partial X}=20k/m,\frac{\partial T}{\partial Y}=0 k/m$
$\frac{\partial T}{\partial X}=0K/m,\frac{\partial T}{\partial Y}=10K/m$
$\frac{\partial T}{\partial X}=10K/m,\frac{\partial T}{\partial Y}=10K/m$

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Question 4

Multiple choice

In the figure shown, the system is a pure substance kept in a piston cylinder arrangement. The system is initially a two phase mixture containing 1 kg of liquid and 0.03 kg of vapour at a pressure of 100 kPa. Initially, the piston rest on a set of stops, as shown in the figure. A pressure of 200 kPa is required to exactly balance the weight of the piston and the outside atmospheric pressure. Heat transfer takes place into the system until its volume increases by 50%. Heat transfer to the system occurs in such a manner that the piston, when allowed to move, does so in a very slow (quasi-static / quasi - equilibrium) process. The thermal reservoir from which heat is transferred to the system has a temperature of 400⁰C. Average temperature of the system boundary can be taken as 175⁰C. The heat transfer to the system is 1 kJ during which its entropy increased by 10 J/K.

Specific volume of liquid (v_f) and vapour (v_g) phase, as well as values of saturation temperatures, are given in the table below ||||| |---|---|---|---| | Pressure (kPa) | Saturation temperature, T_sat(⁰C) | V_f(m³/kg) | V_g(m₃/kg) | | 100 | 100 | 0.001 | 0.1 | | 200 | 200 | 0.0015 | 0.002 |

At the end of the process, which one of the following situations will be true?

No vapour will be left in the system.
A liquid + vapour mixture will be left in the system.
The mixture will exist at a dry saturate vapour state.

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Question 5

Multiple choice

A pressure of 100 kPa is applied. Initially, the piston rest on a set of stops, as shown in the figure. A pressure of 200 kPa is required to exactly balance the weight of the piston and the outside atmospheric pressure. Heat transfer takes place into the system until its volume increases by 50%. Heat transfer to the system occurs in such a manner that the piston, when allowed to move, does so in a very slow (quasi-static / quasi - equilibrium) process. The thermal reservoir from which heat is transferred to the system has a temperature of 400⁰C. Average temperature of the system boundary can be taken as 175⁰C. The heat transfer to the system is 1 kJ during which its entropy increased by 10 J/K.

Specific volume of liquid (v_f) and vapour (v_g) phase, as well as values of saturation temperatures, are given in the table below ||||| |---|---|---|---| | Pressure (kPa) | Saturation temperature, T_sat(⁰C) | V_f(m³/kg) | V_g(m³/kg) | | 100 | 100 | 0.001 | 0.1 | | 200 | 200 | 0.0015 | 0.002 |

The net entropy generation (considering the system and the thermal reservoir together) during the process is closest to

7.5 J/K
7.7 J/K
8.5 J/K
10 J/K

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Question 6

Multiple choice

The logarithmic mean temperature difference (LMTD) of a counter flow heat exchanger is 20⁰C. The cold fluid enters at 20⁰C and the hot fluid enters at 100⁰C. 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

40⁰C
60⁰C
80⁰C
can't be determined

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Question 7

Multiple choice

In the figure shown, the system is a pure substance kept in a piston cylinder arrangement. The system is initially a two phase mixture containing 1 kg of liquid and 0.03 kg of vapour at a pressure of 100 kPa. Initially, the piston rests on a set of stops, as shown in the figure. A pressure of 200 kPa is required to exactly balance the weight of the piston and the outside atmospheric pressure. Heat transfer takes place into the system until its volume increases by 50%. Heat transfer to the system occurs in such a manner that the piston, when allowed to move, does so in a very slow (quasi-static / quasi - equilibrium) process. The thermal reservoir from which heat is transferred to the system has a temperature of 400⁰C. Average temperature of the system boundary can be taken as 175⁰C. The heat transfer to the system is 1 kJ during which its entropy increased by 10 J/K.

Specific volume of liquid (v_f) and vapour (v_g) phase, as well as values of saturation temperatures, are given in the table below ||||| |---|---|---|---| | Pressure (kPa) | Saturation temperature, T_sat(⁰C) | V_f(m³/kg) | V_g(m₃/kg) | | 100 | 100 | 0.001 | 0.1 | | 200 | 200 | 0.0015 | 0.002 |

The work done by the system during the process is

0.1 kJ
0.2 kJ
0.3 kJ
0.4 kJ

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Question 8

Multiple choice

In a steady state steady flow process taking place in a device with a single inlet and a single outlet, the work done per unit mass flow rate is given by $w=\int\limits^{outlet}_{inlet}vdp$, where v is the specific volume and p is the pressure. The expression for w given above

is valid only if the process is both reversible and adiabatic
is valid only if the process is both reversible and isothermal
is incorrect; it must be $w=\int\limits^{outlet}_{inlet}vdp$

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Question 9

Multiple choice

In a composite slab, the temperature at the interface (T_inter) between two materials is equal to the average of the temperatures at the two ends. Assuming steady one-dimensional heat conduction, which of the following statements is true about the respective thermal conductivities?

2k₁ = k₂
k₁ = k₂
2k₁ = 3k₂

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Question 10

Multiple choice

A 100 W electric bulb was switched on in a 2.5 m × 3 m × 3 m size thermally insulated room having a temperature of 20°C. The room temperature at the end of 24 hours will be

321 °C
341 °C
450 °C
470 °C

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Question 11

Multiple choice

Determine the correctness or otherwise of the following Assertion [a] and the Reason [r]. Assertion [a]: In a power plant working on a Ranking cycle, the regenerative feed water heating improves the efficiency of the steam turbine. Reason [r]: The regenerative feed water heating raises the average temperature of heat addition in the Rankine cycle.

Both [a] and [r] are true but [r] is NOT the correct reason for [a].
Both [a] and [r] are false
[a] is false and [r] is true.

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Question 12

Multiple choice

Group I shows different heat addition processes in power cycles. Likewise, Group II shows different heat removal processes. Group III lists power cycles. Match items from Groups I, II and III.

P - S - 1, R - U - 3, P - S - 4, P - T - 2
R - T - 3, P - S - 1, P - T - 4, Q - S - 5
P - T - 4, R - S - 3, P - S - 1, P - S - 5

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Question 13

Multiple choice

In an arc welding process, the voltage and current are 25 V and 300 A respectively. The arc heat transfer efficiency is 0.85 and welding speed is 8 mm/sec. The net heat input (in J/mm) is

64
797
1103
79700

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Question 14

Multiple choice

In a case of one dimensional heat conduction in a medium with constant properties, T is the temperature at position x, at time t. then $\frac{\partial T}{\partial T}$ is proportional to

$\frac{T}{X}$
$\frac{\partial T}{\partial X}$
$\frac{\partial^2 T}{\partial x\partial t}$

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Question 15

Multiple choice

Heat flows through a composite slab, as shown below. The depth of the slab is 1 m. the k values are in W/m K. the overall thermal resistance in K/W is

17.2
21.9
28.6
39.2

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Thermal Properties and Thermodynamics

Thermal Properties and Thermodynamics Questions