slip flow and free molecule flow

Eckert, E. R., and Drake, R. M., 1987, Analysis of Heat and Mass Transfer, Hemisphere Publishing Co., New York, NY.

Page 481

Another interesting, and at the same time surprising conclusion, which has almost always been obtained in slip-flow analyses is that the results are found to correspond closely with free-molecule-flow results if the Knudsen numbers are taken as very large. Since slip flow is a phenomenon supposedly related to small Knudsen numbers, the exact significance of this correspondence to free-molecule-flow results at high Knudsen numbers is unknown; however, successful use of the correspondence has been made in semiempirical analysis to correlate experimental data over the entire flow regime form gas dynamics to free-molecule flow