## Heat Transfer Miscellaneous

#### Heat-Transfer

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

1. 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

1. 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 (δt) is true

1.  δ = ( Pr )n δt

Here, Pr = 6 > 1
 or δ > 1 δt

∴ δ > δt

##### Correct Option: B

 δ = ( Pr )n δt

Here, Pr = 6 > 1
 or δ > 1 δt

∴ δ > δt

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

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

##### Correct Option: C

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

1. 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.  Pr = μCp K

 = 25 × 10-6 × 2 × 103 = 1 0.05

 ⇒ δ = 1 δt

Now , δ = 0.5 mm
∴ δ = δt = 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 = δt = 0.5 mm

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

1.  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