ISRO RAC 2018 Question paper with Answers

ANSWER KEYS OF THE WRITTEN TEST FOR RECRUITMENT TO THE POST OF SCIENTIST/ENGINEER 'SC' (REFRIGERATION & AIRCONDITIONING) [BE006 & BE010] HELD ON 22.04.2018

1. Carnot Cycle consists of
(a) Two constant pressure and two isentropic processes
(b) One constant isenthalpic, one constant volume and two constant pressure processes
(c) Two isothermal and two isentropic processes
(d) One constant pressure, one constant volume and two isothermal processes

Answer1 (c) Two isothermal and two isentropic processes

2. One reversible heat engine operates between 1600°K and T2°K and another reversible heat engine operates between T2°K and 400°K. If both the engines have the same heat input and output, then temperature T2 is equal to
(a) 800 K                                                                         (b) 1600 K
(c) 1200 K                                                                       (d) 6400 K

Answer: (a)
since same input and output, thus
\[\frac{1600}{T_2} = \frac{T_2}{400}\] \[T_2 = 800K\]

3. Isentropic flow is

(a) Reversible adiabatic flow
(b) Irreversible adiabatic flow
(c) Frictionless fluid flow
(d) Reversible isothermal flow

Answer: (a) Reversible adiabatic flow

4. Reversed Joule cycle is known as

(a) Rankine cycle                                                                       (b) Carnot cycle
(c) Bell-Coleman cycle                                                              (d) Otto cycle

Answer: (c) Bell-Coleman cycle 
      The Reverse Brayton Cycle is known as Joule or Bell - Coleman cycle

5. The thermal efficiency of Gas turbine plant is
(a) $r^{γ-1}$                                                        (b) $1- r^{γ-1}$
(c) $1 - (1/r)^{γ/(γ-1)}$                                        (d)  $1 - (1/r)^{(γ-1)/γ}$
Where r is pressure ratio and γ is relation between specific heats.

Answer: (d)  The Brayton cycle is a theoretical cycle for simple gas turbine.

6. A 100 W electric bulb was switched on in a 2.5 m × 3m × 3 m size thermally insulated room
having temperature of 20°C. The room temperature at the end of 24 hrs. will be
(a) 100°C                                                                        (b) 470°C
(c) 370°C                                                                        (d) 600°C
Consider density and specific heat of air as 1.2 kg/m3 and 0.718 kW/°C/kg

No correct Answer : Excluded for evaluation 


7. A solar energy based heat engine which receives 80 kJ of heat at 100°C and rejects 70 kJ of
heat to the ambient at 30°C is to be designed. The thermal efficiency of heat engine is
(a) 70%                                                                      (b) 18.8%
(c) 12.5%                                                                   (d) cannot be calculated

Answer: (c)
Thermal Efficiency  = Work/heat input = (heat input - heat rejected)/ heat input
                                = (80-70)/80 = 0.125 ot 12.5%

8. A mixure of gases expand from 0.03 m3 to 0.06 m3 at constant pressure of 1 MPA and absorb
84 kJ of heat during the process. The change in internal energy of the mixture is
(a) 54 kJ                                                                     (b) 30 kJ
(c) 84 kJ                                                                     (d) 110 kJ

Answer: (a)
Isobaric process: Work, W = P(V2-V1) = $10^6(0.06-0.03) = 30 kJ$
change in internal energy, ΔE = Q - W = 84 -30 = 54 kJ

9. If the concentrated load applied at the free end of a cantilever beam is doubled along with its
length and moment of inertia also, then the deflection at free end will increase by
(a) 2 times                                                                      (b) 4 times
(c) 8 times                                                                      (d) 12 times

Answer: (c)

\[δ'_B = \frac{2F*(2L)^3}{3E(2I)} = 8\frac{FL^3}{3EI} = 8*δ_B \]

10. In the case of a beam simply supported at both ends, if the same load instead of being
concentrated at centre is distributed uniformly throughout the length, then deflection at
centre will get reduced by
(a) 1/2 times                                                                   (b) 1/4 times
(c) 5/8 times                                                                   (d) 3/8 times

Answer: (d)
Concentrated load at the centre of simply supported beam, $δ_1 = \frac{PL^3}{48EI} $

Uniformly distributed load of simply supported beam $δ_2 = \frac{5qL^4}{384EI} $ and P = qL

deflection is reduced = $δ_2 - δ_1 = \frac{PL^3}{48EI}(1-\frac{5}{8}) = \frac{3}{8} \frac{PL^3}{48EI}  =\frac{3}{8}δ_1 $
Related Notes

11. If the Bending moment is increased three times, then to keep the stress in the beam same,
sectional modulus shall be
(a) decreased 3 times                                                        (b) increased 3 times
(c) unchanged                                                                   (d) increased 6 times

Answer: (b)
\(σ = \frac{M}{Z}\)   Related Notes

12. A beam with rectangular section of 120 mm × 40 mm is placed horizontally by mistake, (with
width as 120 mm and depth as 40 mm) whereas it was designed to be placed vertically. (with
width as 40 mm and depth as 120 mm). The ratio of section modulus will be
(a) 1/3                                                                   (b) 1/2
(c) 1/6                                                                   (d) 1/8

Answer: (a)
\(Z = \frac{bh^2}{12}\)  (Related Notes)  thus,
\[\frac{Z1}{Z2} = \frac{120*(40^2)}{40*(120^2)} = 1/3\]

13. If a shaft is required to transmit twice the power at twice the speed for which it is designed,
its diameter must
(a) increase two times                                                          (b) reduce two times
(c) remain same                                                                   (d) increase three times

Answer: (c)
Since power P = T*w so if torque(T) will remains same that means induced stresses will also be same. Therefore no need to change diameter.

14. The strength of a hollow shaft in comparison to solid shaft, both having outer diameter D
and inner diameter of hollow shaft as D/2 is
(a) Half                                                                              (b) 7/8th
(c) 15/16 times                                                                   (d) Remain same

Answer: (c)
Polar section modulus Zp=J/r     (Related Notes)
\[Z_{p,h} = \frac{\pi(D_o^4 - D_i^4)}{32r} = \frac{\pi(D^4 - (D/2)^4)}{32r} =  \frac{15}{16}\frac{\pi(D^4)}{32r} =  \frac{15}{16}Z_{p,s}\]

15. Steel bar of 40 mm × 40 mm square section is subjected to an axial compressive load of
200 kN. If the length of the bar is 2 m and E (Youngs modulus) = 200 GPa. The elongation of
bar will be
(a) 5.4 mm                                                                   (b) 1.25 mm
(c) 2.7 mm                                                                   (d) 3.4 mm

Answer: (b)
Axial elongation, Δ = PL/AE    (Related Notes)

16. The shear force diagram for a simply supported beam carrying uniformly distributed load
of w per unit length, consists of
(a) One right angled triangle
(b) One equilateral triangle
(c) Two right angled triangle
(d) Two rectangles

Answer: (c) Two right angled triangle

17. The power transmitted by a shaft 60 mm diameter at 180 RPM, if the permissible stress is
85 N/mm2
(a) 68 kW                                                                  (b) 650 KW
(c) 1200 kW                                                                  (d) 7 KW

Answer: (a)
Power, $P = \frac{2\pi NT}{60}$ and $T = \frac{τ J}{r} = \frac{\pi d^3τ}{16}$
\[ P = \frac{2(\pi)^2 N d^3 τ}{8} = 67.95 kW\]

18. A material has a young’s modulus of 1.25 × 105 N/mm2 and a poisson’s ratio of 0.25. The bulk
modulus of the material will be
(a) 83 × 105 N/mm2                                                                  (b) 0.83 × 105 N/mm2
(c) 8.3 × 103 N/mm2                                                                  (d) 8.3 × 105 N/mm2

Answer: (b)

19. Neutral solution is one which has pH value
(a) Greater than 7                                                                  (b) Equal to 7
(c) Less than 7                                                                       (d) None of the above

Answer: (b) PH =7

20. Forced feed lubrication system means that oil is delivered to the engine by
(a) Gravity
(b) Pressure created by oil pump
(c) Splashing of the crank shaft
(d) None of the above

Answer: (b)

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