Thermal Engineering

TSadhu Singh, Sukumar Pati

Chapter 16

Gas Turbines - all with Video Answers

Educators

Chapter Questions

Problem 1

Consider the following statements:
I. Intercooling is effective only at lower pressure ratios and high turbine inlet temperatures.
II. There is very little gain in thermal efficiency when intercooling is used without the benefit of regeneration.
III. With higher value of $\gamma$ and $c_{p}$ of the working fluid, the net power output the Brayton cycle will increase.
Of these statements,
(a) I, II, and III are correct
(b) I and II are correct
(c) I and III are correct
(d) II and III are correct

Hunza Gilgit

Hunza Gilgit

Numerade Educator

Problem 2

In a gas turbine cycle with two stages of reheating, working between maximum pressure $p_{1}$ and minimum pressure $p_{4}$ the optimum reheat reassure would be
(a) $\left(p_{1} p_{4}\right)^{1 / 2}$ and $\left(p_{1} p_{2}\right)^{2 / 3}$
(b) $\left(p_{1}^{2} p_{4}\right)^{2 / 3}$ and $\left(p_{1} p_{4}^{2}\right)^{1 / 2}$
(c) $\sqrt{p_{1} p_{4}}$ and $p_{1} \sqrt{p_{4}}$
(c) $\left(p_{1} p_{4}\right)^{1 / 2}$ and $\left(p_{1} p_{4}\right)^{2 / 3}$

Hunza Gilgit

Numerade Educator

Problem 3

Intercooling in gas turbines
(a) decreases net output but increases thermal efficiency
(b) increases net output but decreases thermal efficiency
(c) decreases both net output and thermal efficiency
(d) increases both net output and thermal efficiency

Hunza Gilgit

Numerade Educator

Problem 4

Figure $16.46$ shows four plots $\mathrm{A}, \mathrm{B}, \mathrm{C}$, and $\mathrm{D}$ of thermal efficiency against pressure ratio.
The curve which represents that of a gas turbine plant using Brayton cycle (without regeneration) is the one labelled
(a) $\mathrm{A}$
(b) $\mathrm{B}$
(c) C
(d) $\mathrm{D}$

Hunza Gilgit

Numerade Educator

Problem 5

The optimum intermediate pressure $p_{i}$ for a gas turbine plant operating between pressure limits $p_{1}$ and $p_{2}$ with perfect intercooling between the two stages of compression (with identical isentropic
$\begin{array}{l}\text { efficiency) is given by } \\ \text { (a) } p_{1}=p_{2}-p_{1} & \text { (b) } p_{1}=\frac{1}{2}\left(p_{1}+p_{2}\right)\end{array}$
(c) $p_{1}=\sqrt{p_{1} p_{2}}$
(d) $p_{1}=\sqrt{p_{2}^{2} p_{1}^{2}}$

Hunza Gilgit

Numerade Educator

Problem 6

In a gas turbine cycle, the turbine output is $600 \mathrm{~kJ} /$ $\mathrm{kg}$, the compressor work is $400 \mathrm{~kJ} / \mathrm{kg}$ and the heat supplied is $1000 \mathrm{~kJ} / \mathrm{kg} .$ The thermal efficiency of this cycle is:
(a) $80 \%$
(b) $60 \%$
(c) $40 \%$
(d) $20 \%$

Hunza Gilgit

Numerade Educator

Problem 7

In a single-stage open-cycle gas turbine, the mass flow through the turbine is higher than the mass flows through compressor, because
(a) the specific volume of air increases by use of intercooler
(b) the temperature of air increases in the reheater
(c) the combustion of fuel takes place in the combustion chamber
(d) the specific heats at constant pressure for incoming air and exhaust gases are different

Hunza Gilgit

Numerade Educator

Problem 8

A gas turbine develops $120 \mathrm{~kJ}$ of work while the compressor absorbed $60 \mathrm{~kJ}$ of work and the heat supplied is $200 \mathrm{~kJ}$. If a regenerator which would recover $40 \%$ of the heat in the exhaust were used, then the increase in the overall thermal efficiency would be:
(a) $10.2 \%$
(b) $8.6 \%$
(c) $6.9 \%$
(d) $5.7 \%$

Hunza Gilgit

Numerade Educator

Problem 9

A gas turbine works on which one of the following cycles?
(a) Brayton
(b) Rankine
(c) Stirling
(d) Otto

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Numerade Educator

Problem 10

Reheating in a gas turbine
(a) increases the compressor work
(b) decreases the compressor work
(c) increase the turbine work
(d) decreases the turbine work

Hunza Gilgit

Numerade Educator

Problem 11

Consider the following statements relating to a closed gas turbine cycle:
I. The cycle can employ monatomic gas like helium instead of air to increase the cycle efficiency if other conditions are the same.
II. The efficiency of heat exchanger increases with the use of helium.
III. The turbine blades suffer higher corrosion damages.
IV. Higher output can be obtained for the same size. Which of these statements are correct?
(a) I, II, and III
(b) I, II, and IV
(c) II, III, and IV
(d) I, III, and IV

Hunza Gilgit

Numerade Educator

Problem 12

The efficiency of a simple gas turbine can be improved by using a regenerator, because the
(a) work of compression is reduced
(b) heat required to be supplied is reduced
(c) work output of the turbine is increased
(d) heat rejected is increased

Hunza Gilgit

Numerade Educator

Problem 13

In a regenerative feed heating cycle, the economic number of the stages of regeneration
(a) increases as the initial pressure and temperature increase
(b) decreases as the initial pressure and temperature increase
(c) is independent of the initial pressure and temperature
(d) depends only on the condenser pressure

Hunza Gilgit

Numerade Educator

Problem 14

Match List I (Blades) with List II (Features) and select the correct answer using the codes given below the Lists:
$$
\begin{array}{l}
\text { Codes: }\\
\begin{array}{lllll}
& \mathrm{A} & \mathrm{B} & \mathrm{C} & \mathrm{D} \\
\text { (a) } & 2 & 1 & 5 & 4 \\
\text { (b) } & 3 & 4 & 5 & 1 \\
\text { (c) } & 2 & 4 & 3 & 5 \\
\text { (d) } & 3 & 2 & 1 & 5
\end{array}
\end{array}
$$

Hunza Gilgit

Numerade Educator

Problem 15

Consider the following statements about modification in a gas turbine power plant working on a simple Brayton cycle:
I. Incorporation of the regeneration process increases specific work output as well as thermal efficiency.
II. Incorporation of regeneration process increases thermal efficiency but specific work output remains unchanged.
III. Incorporation of intercooling process in a multi-stage compression increases specific work output but the heat input also increases.
IV. Incorporation of intercooling process in a multi-stage compression system increases specific work output, the heat addition remains unchanged. Which of the above statements are correct?
(a) 1 and 3
(b) 1 and 4
(c) 2 and 3
(d) 2 and 4

Hunza Gilgit

Numerade Educator

Problem 16

A power plant, which used a gas turbine followed by steam turbine for power generation, is called
(a) Topping cycle
(b) Bottoming cycle
(c) Brayton cycle
(d) Combined cycle

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Numerade Educator

Problem 17

Consider the following statements:
I. The speed of rotation of the moving elements of gas turbines is much higher than those of steam turbines.
II. Gas turbine plants are heavier and larger in size than steam turbine plants.
III. Gas turbines require cooling water for its operations.
IV. Almost any kind of fuel can be used with gas turbines. Which of the statements given above are correct?
(a) I and II
(b) I and III
(c) I and IV
(d) III and IV

Hunza Gilgit

Numerade Educator

Problem 18

The thermal efficiency of a gas turbine cycle with regeneration in terms of $\mathrm{T}_{3}$ (maximum temperature), $\mathrm{T}_{1}$ (minimum temperature), $r_{p}$ (pressure ratio) and $\gamma\left(=\frac{c_{p}}{c_{v}}\right)$ is given by
(a) $1-\frac{\mathrm{T}_{1}}{\mathrm{~T}_{3}} r_{p}^{\left(\frac{\gamma}{\gamma-1}\right)}$
(b) $1-\frac{\mathrm{T}_{3}}{\mathrm{~T}_{1}} r_{p}^{\left(\frac{\gamma}{\gamma-1}\right)}$
(c) $1-\frac{\mathrm{T}_{3}}{\mathrm{~T}_{1}} r_{p}^{\left(\frac{\gamma-1}{\gamma}\right)}$
(d) $1-\frac{\mathrm{T}_{1}}{\mathrm{~T}_{3}} r_{p}^{\left(\frac{\gamma-1}{\gamma}\right)}$

Hunza Gilgit

Numerade Educator

Problem 19

Consider the following features for a gas turbine plant:
I. Intercooling
II. Regeneration
III. Reheat
Which of the above features in a simple gas turbine cycle increase the work ratio?
(a) I, II, and III
(b) Only I and II
(c) Only II and III
(d) Only I and IV

Hunza Gilgit

Numerade Educator

Problem 20

Consider the following statements:
For a large aviation gas turbine, an axial flow compressor is usually preferred over centrifugal compressor because
I. the maximum efficiency is higher
II. the frontal area is lower
III. the pressure rise per stage is more IV. the cost is lower Which of the statements given above are correct?
(a) I and IV
(b) Only I and II
(c) I, II, and III
(d) II, III, and IV

Hunza Gilgit

Numerade Educator

Problem 21

In a reaction turbine, the heat drop in fixed blade is $8 \mathrm{~kJ} / \mathrm{kg}$ and the total heat drop per stage is $20 \mathrm{~kJ} /$ $\mathrm{kg}$. The degree of reaction is
(a) $40 \%$
(b) $66.7 \%$
(c) $60 \%$
(d) $25 \%$

Hunza Gilgit

Numerade Educator

Problem 22

Consider the following statements:
Which of the following increase the work ratio in a simple gas turbine plant?
I. Heat exchanger
II. Inter cooling III. Reheating
Select the correct answer using the code given below:
(a) 1 and 2
(b) 2 and 3
(c) 1 and 3
(d) 1,2 , and 3

Hunza Gilgit

Numerade Educator

Problem 23

Optimum pressure ratio for maximum specific output for ideal gas turbine plant operating at initial temperature of $300 \mathrm{~K}$ and maximum temperature of $1000 \mathrm{~K}$ is closer to
(a) 4
(b) $\underline{8}$
(c) 12
(d) 16

Hunza Gilgit

Numerade Educator

Problem 24

The cycle shown in Fig. $16.47$ represents a gas turbine cycle with intercooling and reheating.
Match List-X (Units) with List-Y (Processes) and select the correct answer using the codes given
below the Lists:
$$
\begin{array}{l}
\text { Codes: }\\
\begin{array}{lllll}
& \text { A } & \text { B } & \text { C } & \text { D } \\
\text { (a) } & \text { I } & \text { II } & \text { IV } & \text { III } \\
\text { (b) } & \text { II } & \text { IV } & \text { V } & \text { III } \\
\text { (c) } & \text { III } & \text { IV } & \text { V } & \text { II } \\
\text { (d) } & \text { II } & \text { V } & \text { IV } & \text { I }
\end{array}
\end{array}
$$

Hunza Gilgit

Numerade Educator

Problem 25

Which one of the following is the correct statement? In a two-stage gas turbine plant with intercooling and reheating,
(a) both work ratio and thermal efficiency improve
(b) work ratio improves but thermal efficiency decreases
(c) thermal efficiency improves but work ratio decreases
(d) both work ratio and thermal efficiency decrease

Hunza Gilgit

Numerade Educator