Inlet and outlet temperature fluctuations of the fluid passing through the inner tube of a concentric-tube counterflow heat exchanger are related by a temperature transfer function. This function is affected by the velocity distribution of the stream, a distribution which is usually in the shape of a paraboloid. Such a distribution has a center-line velocity that is higher than the bulk velocity. The center-line velocity is responsible for longitudinal temperature diffusion effects which cannot be ignored. Slug flow at bulk velocity, a convenient assumption made by many authors, does not account for the shape of experimental fluid-temperature transients discussed in this paper. Rather, the center-line velocity and the bulk velocity, both convenient quantities, are recommended as effective aids in the successful prediction of temperature transients.

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