Heat pipes, which are heat-transfer devices of high thermal conductance, have been studied intensively because of their applicability and efficiency. However, only a small number of studies have been conducted on relatively long heat pipes. Here, the thermal hydraulic characteristics of a long heat pipe are studied. In particular, the thermal performance of the heat pipe and the entrainment limit, one of the important design parameters, are examined, so as to determine their sensitivities to the length-to inner-diameter (L/D) ratio. Experiments are conducted to measure the heat removal performance and limitations of the heat pipe for various length scales. It is found that the heat-pipe performance exhibits a similar trend for various L/D, but with differing values. It is also found that the L/D effect on the entrainment limit for a large-L/D heat pipe differs significantly from the predictions of conventional correlations. Therefore, in order to develop a practical passive heat-pipe-based heat-removal device, an entrainment limit expression incorporating L/D correction is required. (C) 2016 Elsevier Ltd. All rights reserved.