This work experimentally compares the evaporation characteristics of three working fluids: water, methanol and acetone, in groove-wicked flat-plate heat pipes. In addition, the performance enhancement by filling copper powders in the evaporation section is investigated. The parallel, U-shaped grooves have a width of 0.25 mm and a depth of 0.16 mm. With stepwise increase of heat load, the behavior of the working fluid in the grooves is visualized and the evaporator resistances are measured. For all the three fluids, longitudinal liquid recession with a steep-sloped liquid front can be visualized above a certain heat load. Behind the short liquid front is the accommodation region where the meniscus appears to anchor on the top corners of the groove walls. Under a thermally stable situation, independent longitudinal oscillations of the liquid front exist in different grooves, forming a constantly varying zigzag front line. In response to increasing heat load, the liquid fronts gradually leave the heated zone, accompanied by increasing plate temperatures. Water exhibits the best performance with highest maximum heat loads and lowest evaporator resistances. However, the differences in the maximum heat loads for different fluids are much smaller than the differences in their figures of merit. No boiling is observed in the groove wick for all the three fluids. With the groove-powder evaporator, the evaporation performance is greatly improved for water. However, the improvements for methanol and acetone are not as significant as for water. Condensation modes for the three fluids are also compared. Dropwise condensation is observed for water, and film condensation for methanol and acetone. (C) 2013 Elsevier Ltd. All rights reserved.