Page 269 - Mechanical Engineers' Handbook (Volume 4)

258 Furnaces

Figure 36 Reynolds number (Re) for flow of air or flue gas through tubes or across tube banks. 1

the advantage of limited horsepower demand with reduced back pressure and increasing
volume, and can be used where system resistance is unpredictable. The operating point on
the pressure–volume curve is determined by the increase of duct resistance with flow,
matched against the reduced outlet pressure, as shown in the upper curve.

9.3 Laminar and Turbulent Flows

The laminar flow of a fluid over a boundary surface is a shearing process, with velocity
varying from zero at the wall to a maximum at the center of cross section or the center of
the top surface for liquids in an open channel. Above a critical Reynolds number, between
2000 and 3000 in most cases, flow becomes a rolling action with a uniform velocity ex-
tending almost to the walls of the duct, and is identified as turbulent flow.
With turbulent flow the pressure drop is proportional to D; the flow in a large duct can
be converted from turbulent to laminar by dividing the cross-sectional area into a number
of parallel channels. If flow extends beyond the termination of these channels, the conversion
from laminar to turbulent flow will occur over some distance in the direction of flow.
Radial mixing with laminar flow is by the process of diffusion, which is the mixing
effect that occurs in a chamber filled with two different gases separated by a partition after
the partition is removed. Delayed mixing and high luminosity in the combustion of hydro-
carbon gases can be accomplished by ‘‘diffusion combustion,’’ in which air and fuel enter
the combustion chamber in parallel streams at equal and low velocity.