106. For a glass plate transmissivity and reflectivity are specified as 0.86 and 0.08 respectively, the absorptivity of the glass plate is
     

1. 1. 0.86
      

   
   
   

   2. 0.08
      

   
   
   

   3. 1.00
      

   
   
   

   4. 0.06

   
   
   

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1. View Hint View Answer Discuss in Forum

   τ = 0.86
   ρ = 0.08
   α = ?
   α + τ + ρ = 1
   α + 0.86 + 0.08 = 1
   α = 0.06

   Correct Option: D

   τ = 0.86
   ρ = 0.08
   α = ?
   α + τ + ρ = 1
   α + 0.86 + 0.08 = 1
   α = 0.06

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107. Water (Prandtl number = 6) flows over a flat plate which is heated over the entire length. Which one of the following relationship between the hydrodynamic boundary layer thickness (δ) and thermal boundary layer thickness (δ_t) is true
     

1. 1. δ_t = δ
      

   
   
   

   2. δ_t < δ
      

   
   
   

   3. δ_t > δ
      

   
   
   

   4. Can not be predicted

   
   
   

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1. View Hint View Answer Discuss in Forum

   	δ	= ( Pr )n
   	δt

   
   Here, Pr = 6 > 1
   

   or	δ	> 1
   	δt

   
   ∴ δ > δ_t

   Correct Option: B

   	δ	= ( Pr )n
   	δt

   
   Here, Pr = 6 > 1
   

   or	δ	> 1
   	δt

   
   ∴ δ > δ_t

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108. For air near atmospheric condition flowing over a flat plate the laminar thermal boundary layer is thicker than hydrodynamic boundary layer

1. 1. True / False

   
   
   

   2. True

   
   
   

   3. False

   
   
   

   4. None of these

   
   
   

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1. View Hint View Answer Discuss in Forum

   The given statement is False.
   At near atmospheric or ambient condition,
   Pr = 1, i.e., δ = δ_t

   Correct Option: C

   The given statement is False.
   At near atmospheric or ambient condition,
   Pr = 1, i.e., δ = δ_t

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109. A fluid flowing over a flat plate has the following properties: dynamic viscosity = 25 × 10^–6 kg/ ms, specific heat = 2.0 kJ/kgK, thermal conductivity 0.05 W/mK. The hydrodynamic boundary layer thickness is measured to be 0.5 mm. The thickness of the thermal boundary layer would be
     

1. 1. 0.1 mm
      

   
   
   

   2. 0.5 mm
      

   
   
   

   3. 1.0 mm
      

   
   
   

   4. None of these

   
   
   

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1. View Hint View Answer Discuss in Forum

   Pr =	μCp
   	K

   
   

   =	25 × 10-6 × 2 × 103	= 1
   	0.05

   
   

   ⇒	δ	= 1
   	δt

   
   Now , δ = 0.5 mm
   ∴ δ = δ_t = 0.5 mm

   Correct Option: B

   Pr =	μCp
   	K

   
   

   =	25 × 10-6 × 2 × 103	= 1
   	0.05

   
   

   ⇒	δ	= 1
   	δt

   
   Now , δ = 0.5 mm
   ∴ δ = δ_t = 0.5 mm

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110. For the fluid flowing over a flat plate with Prandtl number greater than unity, the thermal boundary layer for laminar forced convection.
     

1. 1. is thinner than the hydrodynamic boundary layer
      

   
   
   

   2. has thickness equal to zero
      

   
   
   

   3. is of same thickness as hydrodynamic boundary layer
      

   
   
   

   4. is thicker than the hydrodynamic boundary layer
      

   
   
   

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1. View Hint View Answer Discuss in Forum

   Pr1 / 3 =	δ	, if Pr > 1 , it means δ < δt
   	δt

   Correct Option: A

   Pr1 / 3 =	δ	, if Pr > 1 , it means δ < δt
   	δt

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