DIMENSIONLESS NUMBERS IN HEAT TRANSFER

https://www.mesubjects.net/wp-admin/post.php?post=627&action=edit      Free & Forced Convection-2

https://www.mesubjects.net/wp-admin/post.php?post=3781&action=edit      Q.A.  Pool boiling

https://www.mesubjects.net/wp-admin/post.php?post=2717&action=edit         Q. A. Boiling Cond

https://www.mesubjects.net/wp-admin/post.php?post=2690&action=edit          Q.A. Fins

https://www.mesubjects.net/wp-admin/post.php?post=2318&action=edit        Q.A.Conduction-1

https://www.mesubjects.net/wp-admin/post.php?post=2742&action=edit    Q.ANS. CONVECTION

https://www.mesubjects.net/wp-admin/post.php?post=630&action=edit    CONVECTION HEAT TRANSFER-1

https://www.mesubjects.net/wp-admin/post.php?post=627&action=edit    CONVECTION HEAT TRANSFER-2

https://www.mesubjects.net/wp-admin/post.php?post=4132&action=edit    Mcq Overall HTCoeff

https://www.mesubjects.net/wp-admin/post.php?post=4107&action=edit           Mcq Basic HT

https://www.mesubjects.net/wp-admin/post.php?post=1339&action=edit    HEX Overall HT Coeff

https://www.mesubjects.net/wp-admin/post.php?post=675&action=edit              Q.A HT

https://www.mesubjects.net/wp-admin/post.php?post=11259&action=edit           Q ANS COND-3

DIMENSIONLESS NUMBERS IN HEAT TRANSFER

Prandtl number (Pr) is a dimensionless number.

Pr = momentum diffusivity/Thermal diffusivity =μ Cp/kf

PHYSICAL SIGNIFICANCE OF PRANDTL NUMBER

Value of Prandtl number signifies the thickness of thermal boundary layer as compared to the thickness of hydro boundary layer.

firstly if  Pr =1, δ =δth

Secondly if Pr <1 then δ < δth

Thirdly if Pr >1 then δ > δth

 SIGNIFICANCE OF ‘GRASHOF’ NUMBER

Grashof number (Gr) is the ratio of the buoyancy to viscous force acting on a fluid layer. It is used in free convection. Its value indicates whether the Buoyancy force is more predominant or  the viscous force. It is used where Buoyancy force is more predominant. It is similar to to the Reynolds number in forced convection.It is used both in fluid dynamics and heat transfer.

 

Gr = βg Δt l3/ ϑ2

Buoyancy force is due to density difference. The temperature difference causes the density difference. Greater is the temperature difference, greater will be density difference. Greater will be the Grashoff’s number.

 NUSSELT NUMBER

Nusselt number is the ratio of  heat transfer due to convection to heat transfer due to  conduction. Higher is the Nusselt number, more will be the rate of  heat transfer by convection than by conduction. It helps to find ‘h’ due to free or forced convection.

Nu = hL/kf

Where L is the characteristic length

kf is the thermal conductivity of the fluid

 REYNOLDS NUMBER “Re’

It is a ratio of the inertia force to the viscous force. More inertia force means a higher Reynolds number. Higher viscous force means a small[er Reynolds number. It is applicable in forced convection heat transfer. It also determines the laminar flow or the turbulent flow.

CASE 1   FORCED CONVECTION FLOW OVER A FLAT PLATE

Laminar flow, Re≤ 5 lac

Turbulent flow Re > 7 lac

CASE 2   FORCED CONVECTION IN A PIPE OR A TUBE

(a) For laminar flow Re ≤ 2100

  • For turbulent flow Re > 4000