The experimental and numerical investigation of flow are carried out in a circular conical diffuser with the generator slope of 0.3° and a length equal to 70 entry diameters within the framework of the analysis of the problem of existence and the value of the lower critical Reynolds number for flow in a circular tube. The measurements are carried out in an air flow for Reynolds numbers Re2 in the diffuser exit section ranging from 600 to 3000. It is shown that, in contrast to a tube, where only laminar flow can be realized as steady for Re < 2000, a developed turbulent flow is formed for Re2 > 1000 at the exit section of the diffuser. This flow is steady for Re2 > 1300, that is, almost independent of the turbulence level at the entry, and is determined by Reynolds number Re2 at the exit section. For Re2 ~1000 the turbulent flow continuously goes over into a laminar flow. The flow parameters measured at the diffuser exit correspond to the calculations using the three-equation turbulence model.
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