76. Which one of the following is NOT a rotating machine?

1. 1. Centrifugal pump

   
   
   

   2. Gear pump

   
   
   

   3. Jet pump

   
   
   

   4. Vane pump

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: C

   NA

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77. A pump handling a liquid raises its pressure from 1 bar to 30 bar. Take the density of the liquid as 990 kg/m³. The isentropic specific work done by the pump in kJ/kg is

1. 1. 0.10

   
   
   

   2. 0.30

   
   
   

   3. 2.50

   
   
   

   4. 2.93

   
   
   

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   Work done by pump = υ(p₂ – p₁)
   

   =	(30 - 1) × 100
   	990

   
   = 2.93 kJ/kg

   Correct Option: D

   Work done by pump = υ(p₂ – p₁)
   

   =	(30 - 1) × 100
   	990

   
   = 2.93 kJ/kg

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78. Water flows through two different pipes A and B of the same circular cross-section but at different flow rates. The length of pipe A is 1.0 m and that of pipe B is 2.0 m. The flow in both the pipes is laminar and fully developed. If the frictional head loss across the length of the pipes is same, the ratio of volume flow rates Q_B /Q_A is _____ (round off to two decimal places)

1. 1. 0.5

   
   
   

   2. 0.4

   
   
   

   3. 0.6

   
   
   

   4. 0.3

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: A

   NA

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79. Water (density = 1000 kg/m³) at ambient temperature flows through a horizontal pipe of uniform cross-section at the rate of 1 kg/s. If the pressure drop across the pipe is 100 kPa, the minimum power required to pump the water across the pipe, in Watt, is _______.

1. 1. 100 Watt

   
   
   

   2. 150 Watt

   
   
   

   3. 50 Watt

   
   
   

   4. 200 Watt

   
   
   

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1. View Hint View Answer Discuss in Forum

   W = m v(P₁ - P₂)
   

   W = 1 ×	1	100 × 103 = 10 Watt
   	1000

   
   

   Correct Option: A

   W = m v(P₁ - P₂)
   

   W = 1 ×	1	100 × 103 = 10 Watt
   	1000

   
   

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80. For steady, fully developed flow Inside straight pipe of diameter D, neglecting gravity effects, the pressure drop ∆p over a length L and the wall shear stress τW are related by

1. 1. τW =	∆pD
      	4L

   
   
   

   2. τW =	∆pD²
      	4L²

   
   
   

   3. τW =	∆pD
      	2L

   
   
   

   4. τW =	4∆pD
      	D

   
   
   

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   Shear stress on wall,
   

   τW =		-δP	.	R
   		δx		2

   
   

   where		-δP	.	∆
   		δx		L

   
   

   & R =	D
   	2

   
   

   ∴ τW =		∆P	×	D	=	∆PD
   		L		2 x 2		4L

   
   

   Correct Option: A

   Shear stress on wall,
   

   τW =		-δP	.	R
   		δx		2

   
   

   where		-δP	.	∆
   		δx		L

   
   

   & R =	D
   	2

   
   

   ∴ τW =		∆P	×	D	=	∆PD
   		L		2 x 2		4L

   
   

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