An osmotic heat pipe is a top heat mode heat pipe driven by an osmotic force. Therefore, a concentration and a solution flow rate in a membrane module of the osmotic heat pipe are especially of great importance for the heat transport. Thus, the flow in the membrane module is investigated in detail. As a result, with a ratio of a concentration on the membrane wall to a mixed mean concentration derived semi-theoretically, correlations for the mixed mean concentration, the concentration on the wall and the solution flow rate along the channel in the membrane module were proposed, which can correlate the experimental data from reference (2) within±20% errors. In addition, a method of increasing in the osmotic pumping rate and heat transport rate was proposed.