Page 209 - 48Fundamentals of Compressible Fluid Mechanics

Rayleigh flow is (frictionless) flow with heat transfer through a pipe of constant
cross sectional area. In practice Rayleigh flow is difficult to achieve. Yet Rayleigh
flow is practical and useful concept in a obtaining trends and limits. The density
and pressure change due to external cooling or heating. As opposed to the two
previous models, the heat transfer can be in two directions not like the friction (there
is no negative friction). This fact create situation different compare to the previous
two models. This model applied to case where the heat transfer is significant.

10.1 Introduction

The third simple model for an one dimensional flow is for constant heat transfer for
frictionless flow. This flow referred in the literature as Rayleigh Flow (see historical
notes). This flow is another extreme case in which the friction effect are neglected
because their relative effect is much smaller the heat transfer effect. While the
isothermal flow model has heat transfer and friction the main assumption was that
relative length is so the heat transfer occurs between the surrounding and tube.
In contract the heat transfer in Rayleigh flow occurs either between unknown tem-
perature to tube but the heat flux maintained constant. As before, a simple model
is build around assumption of constant properties (poorer prediction to case were
chemical reaction take palace).
This model usage is to a rough predict the situation involve mostly chemical
reaction. In analysis the flow one has to be aware that properties do change signif-
icantly for a large range of temperature. Yet, for smaller range of temperature and
length the calculations are more accurate. Nevertheless, the main characteristic of
the flow such as chocking condition etc. are encapsulated in this model.
The basic physics of the flow revolves around the fact that the gas is highly

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