β Scribed by G.A. Turner
No coin nor oath required. For personal study only.
l?zermal properties are discussed: the mathematics applies equally well to mass transfer.)
IT is often necessary to know the value of the diffusivity of solid particles that are small enough to make uncertain the classical methods of finding diffusivities by measuring temperatures in a body of defined geometry. Furthermore the material may not be available in large s&s.
It is possible to deduce the value of their diiusivity by measuring the change in a temperature transient in a fluid flowing through a bed of such particles. The mathematics describing this effect is simplified if the flow is onedimensional and if there are no radial variations of temperature, velocity or of longitudinal dispersion coefficient. Even so, the change in the transient is governed not only by the values of the diffusivity of the solid, k, but also by the (unknown) values of the o&Bcients of (Slm) heat transfer, H, between solid and fluid and of longitudinal dispersion, D, in the fluid. It follows that the effects of these three factors must be disentangled. A method (believed to be novel) of doing this requires a knowledge of the frequency response of the system and this can be most easily and accurately found by generating steady sinusoidal temperature waves in the fluid. The method makes use of some of the following mathematical results: it is assumed, in the first place, that only k H and D are unknown, and that the particles are uniform spheres.
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