The effects of the number of turns and the inclination angle on the operating limit of micro pulsating heat pipes (MPHPs) are investigated through a series of experiments. Square channels with 5, 10, 15, and 20 turns are engraved onto a silicon wafer to form a serpentine closed loop. A glass wafer is anodically bonded to the silicon wafer for flow visualization, and FC-72 is used as the working fluid. Experimental results show that the dependence of the maximum allowable heat flux on the inclination angle becomes weaker as the number of turns increases, and it is negligible when the number of turns is 20. For this orientation-independent case with a large number of turns (20 turns or more), a new film formation model of the operating limit is proposed: it is postulated that the MPHPs reach the operating limit when the average rate of film formation caused by the motion of the liquid slugs is smaller than the evaporation rate in the evaporator section. This postulate is experimentally confirmed through comparison between the two rates. By combining the film-formation model with the existing falling-film model, a correlation for predicting the maximum allowable heat flux covering a complete range of the number of turns is developed. (C) 2018 Elsevier Ltd. All rights reserved.