Abstract: With requirements of designing an adiabatic capillary tube in engineering, an approximate analytic solution to the tube length derived from the approximate integral model is developed. In order to take the choked flow into account and avoid the consequent iterative calculation in the approximate integral model, a direct method for judging and evaluating the choked flow is presented on the basis of the relationship between length and pressure. Case study shows that the present approximate analytic solution could meet engineering precision. Based on the approximate analytic solution, the choked characteristics of flow in adiabatic capillary tubes are theoretically analyzed.

Key words: Approximate analytic solution, Capillary tube, Choked flow