Question

0 In spite of being forced', a forced convection is not as efficient as free convection in removing heat from a vertical hot plate. True / False (ü) Computer components are designed such that the are cooled due to natural convection. This cooling will be even more effective if the computer is on the space station. True / False (ii) Birds take advantage of natural convection while soaring. True / False (iv) A fictitious planet similar to earth has an atmosphere, otherwise similar to earth's, has a B value twice as that of earth's. Hot air ballooning on that planet will be tougher there compared to that on earth. True / False (v) A hot plate will cool better when held horizontal than when held vertical as larger area of hot air will be exposed to gravity. True / False (vi) Boussinesg approximation transplants incompressibility in an otherwise compressible boundary layer equation. True / False (vii) Free convection involves a higher value of convection heat transfer coefficient h compared to a forced convection. True / False (viii) Assuming that moon has the same atmosphere as earth, a boiled egg will take same time to cool off as it will take on the earth. True / False (ix) If you observe the Grashof number, Grz force and viscous force. True / False 9B (Ts-T.) 2 carefully it is ratio of buoyancy (x) What Grashof number is to free convection flow, Reynolds number is to forced convection flow. True / False

Answer 1

Problems iii, vii, ix and x are solved below.

iii) Birds take advantage of natural convection while soaring.

• TRUE
• ​​​​​​​The given statement is true. The birds always uses convection currents as the additional advantage while soaring to save their energy for flying high.
• Instead of using their wings to flap by using their energy, this convection currents helps birds to fly for much more time without flap at such heights.
• Hence, this makes some advantage for birds.

vii) Free convection involves a higher value of convection heat transfer coefficient h compared to a forced convection:

• FALSE
• ​​​​​​Heat transfer coeffficient is mainly depends on flow velocity, properties of fluid, type of flow.
• Hence, in natural convection, there not be much flow velocity. But, in forced convection, there is large flow velocity.
• This helps in inducing more heat transfer coeffficient values.
• Hence, heat transfer coeffficient for forced convection is more than that of free convection.
• But the given statement says, h is more for free convection. Hence, the given statement is wrong.

ix) If you observe the Grashof number,
g8(T- TL Gri
carefully, it is ratio of buoyancy force and viscous force.

• TRUE

• We know that, Grashof number is a ratio of buoyancy force to viscous force which represents
  g8(T- TL Gri
  , where g= acceleration due to gravity, $\beta$ = coeffficient of thermal expansion, $T_{s}$ = surface temperature, $T_{\infty}$ = outside fluid temperature, L= characteristic length, = kinematic viscosity.
• Hence, the given statement is true.

x) What Grashof number is to free convection flow, Reynolds number is to forced convection flow.

• TRUE
• ​​​​​​​We know that, function of Grashof number and Reynolds number are same. Because both are used to find the type of flow.
• Only difference is that, Reynolds number is used in case of forced convection and Grashof number is used in case of free convection.
• Hence, the given statement is true.

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