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Lewis number: ratio of thermal diffusivity (a) to the diffusion coefficient (S)
Application: phenomena with simultaneous heat and mass transfer.
Nusselt number: ratio of the total heat transferred (a • /) to the heat by conduction (X)
Application: convective heat transfer.
Pedet number: ratio of heat transfer by convection (pc -w-l) to the heat by conduction
a)
Application: heat transfer by forced convection.
Nahme or Griffith number: ratio of viscous dissipation (j3T W 2 J]) to the heat by
conduction (X) perpendicular to the direction of flow
Application: heat transfer in melt flow
Prandtl number: ratio of the kinematic viscosity (v) to thermal diffusivity (a)
Application: convective heat transfer
Reynolds number: ratio of the inertial force (p w I) to viscous force (T])
Application: The Reynolds number serves as a criterium to judge the type of flow. In
pipe flow, when Re is less than 2300 the flow is laminar. The flow is turbulent at Re
greater than about 4000. Between 2100 and 4000 the flow may be laminar or turbulent
depending on conditions at the entrance of the tube and on the distance from the entrance
[2]
Application: fluid flow and heat transfer.
Sherwood number: ratio of the resistance to diffusion (/ / S) to the resistance to mass
transfer (l/j8s)
Application: mass transfer problems
Schmidt number: ratio of kinematic viscosity (v) to the diffusion coefficient (S)
Application: heat and mass transfer problems
Stokes number: ratio of pressure forces (p • Z) to viscous forces (rj • w)
Application: pressure flow of viscous media like polymer melts.
The use of dimensionless numbers in calculating non Newtonian flow problems is
illustrated in Section 4.3.3 with an example.
3.5 Heat Transfer by Convection
Heat transfer by convection, particularly by forced convection, plays an important role
in many polymer processing operations such as in cooling a blown film or a part in an
injection mold, to mention only two examples. A number of expressions can be found in
the literature on heat transfer [3] for calculating the heat transfer coefficient a (see
Section 3.1.6). The general relationship for forced convection has the form
(3.53)