In this paper, the boiling limit of bi-porous sintered metal wicks is considered. The effects of the particle size (45 <= d <= 200 mu m) and the cluster size (250 <= D <= 675 mu m) on the boiling limit are investigated experimentally and analytically. In order to gain understanding of the boiling phenomena in bi-porous wicks, the bubble movements in the bi-porous wicks are visualized using glass particle wicks and a high speed camera. Based on the visualization results, an analytic model for predicting the boiling limit of the bi-porous wicks is developed. The model predicts the experimental data well within the error of 11%, and indicates that the permeabilities of liquid and vapor are key parameters for determining the boiling limits of wicks. The bi-porous wicks have higher boiling limits than the mono-porous wicks by up to 67%, since they have much higher liquid/vapor permeabilities, by having separate flow paths for each phase. From the model, an optimal configuration of bi-porous wick is found to be 4 < D/d < 6. (C) 2012 Elsevier Ltd. All rights reserved.