The pressure drop in the entrance region of an abrupt inlet circular tube was determined experimentally. Local values of K, the dimensionless factor for laminar flow which expresses the excess of pressure drop over that of fully developed flow, were determined for the Reynolds range from 200 to 600. Variations in absolute pressure were used at each Reynolds number in order to produce different bulk velocities. The results for this Reynolds number range agreed with the analyses for smooth entrance tubes, and no dependency on bulk velocity was observed. The absolute pressure of the test section was also reduced sufficiently to obtain a large mean free path for air, thus producing a slip effect at the tube wall. Pressure-drop measurements were made for a Knudsen number range of 0.001 to 0.07, and the slip-flow correction was determined in this range. The data agreed well with Kennard’s prediction for slip flow in a tube if a value of 0.9 was used for the diffuse factor.

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