UPPSC Lec'21 ME: Daily Practices Quiz. 16-Nov-2021

Quiz: Mechanical Engineering
Exam: UPPSC AE/Lecturer
Topic: Power Plant

Each question carries 1 mark
Negative marking: 1/4 mark
Time: 8 Minutes

Q1. In simple gas turbine the effect of reheating is
(a) increases thermal efficiency
(b) increases compressor work
(c) increases turbine work
(d) decreases thermal efficiency

Q2. The formula of work ratio in a simple gas turbine plant is
Where pressure ratio is r_p, T_1 is minimum temperature and T_3 is maximum temperature.
(a) 1-T_1/T_3 (r_p )^((γ-1)/γ)
(b) 1-T_3/T_1 (r_p )^((γ-1)/γ)
(c) 1-T_1/T_3 (r_p )^(γ/(γ-1))
(d) 1-T_1/T_3 (r_p )^((γ-1)/2γ)

Q3. An ideal closed-cycle gas turbine plant is working between the temperatures 927°C and 27°C using air as working fluid. The pressure ratio for maximum output is
(a) 11.3
(b) 13.3
(c) 15.3
(d) 17.3

Q4. The cycle which consists of three processes is?
(a) Stirling cycle
(b) Ericsson cycle
(c) Atkinson cycle
(d) Lenoir cycle

Q5. Which of the following is correct to increase work ratio in a simple gas turbine power plant is?
1. Reheating
2. Regeneration
3. Intercooling
(a) 1, 2 and 3
(b) 2 and 3
(c) 1 and 3
(d) 1 and 2

Q6. What is the propulsion efficiency of a jet engine (where u is flight velocity and v is jet velocity relative to aircraft)
(a) 2u/(v-u)
(b) (v+u)/v
(c) 2u/(v+u)
(d) (v+u)/2v

Solutions

S1. Ans. (c)
Sol. Generally in gas turbine, reheat means increasing the mean temperature of heat addition in lower pressure turbine after expanding of the gas in higher pressure turbine.
So, the work output in the turbine is increases but no change in the compressor work.
Thermal efficiency decreases because the mean temperature of heat rejection is also increases.
By reheating, the fuel consumption rate decreases for the given power output of the gas turbine
Work ratio also increases

S2. Ans. (a)
Work ratio=W_net/W_turbune =1-T_1/T_3 (r_p )^((γ-1)/γ)

S3. Ans. (a)
As, we know that for maximum output, the pressure ratio r_p=(T_3/T_1 )^((γ-1)/2γ)
Where T_1=27°C=300K and T_2=927°C=1200K
For air γ=1.4
r_p=(1200/300)^((1.4-1)/(2×1.4))=11.3
S4. Ans. (d)
Sol. The Lenoir cycle consists of three processes i.e., constant pressure, constant volume and isentropic process.
It is an idealized thermodynamic cycle often used to model a pulse jet engine

S5. Ans. (b)
Sol. Work ratio=W_net/W_turbune =(W_turbune-W_compressor)/W_turbune
In case of regeneration, there is no effect on work done. So, there is no effect on the work ratio.
In case of intercooling, the compressor work reduces, hence the work ratio increases.
In case of reheating, the turbine work increases, hence the work ratio increases.
S6. Ans. (c)
Sol. Propulsive efficiency of jet engine=2u/(v+u)