This work presents visualization of the evaporation/boiling process and thermal measurements of operating horizontal transparent heat pipes. The heat pipes consisted of a two-layered copper mesh wick consisting of 100 and/or 200 mesh screens, a glass tube and water as the working fluid. Experimental results indicated that nucleate boiling was prompted for a wick having a fine 200-mesh bottom layer. When the fluid charge approximately equaled the pore Volume in the wick, the water-vapor interface receded into more curved menisci with increasing heat load Q. Thus, larger capillary forces and evaporation areas were attained to meet the increasing need of liquid supply and evaporation rate at the water-vapor interface. At Q = 40 and 45 W, the water film became less than 100 mu m and the nucleate boiling observed at lower heat loads disappeared. Optimal thermal characteristics with smallest thermal resistances across the evaporator and lowest overall temperature distributions were found for Such a wick/charge combination. Under a smaller charge, partial dry-out was observed in the evaporator. Under a larger charge, liquid recession with increasing heat load was limited and bubbles grew and burst violently at high heat loads. The effects of different wicks and fluid charges on the evaporation/boiling characteristics were discussed. (c) 2008 Elsevier Ltd. All rights reserved.