When a boiling liquid flows inside a very small and non-circular channel, the capillary forces become important in determining aspects of the flow phenomena such as two-phase flow pattern. The fundamental features of two-phase flow under such regimes determine flow boiling heat transfer. And yet understanding the underlying phenomena can be beneficial from the perspectives of design and optimization of compact evaporators and condensers. The thrust of this study is the nature of flow boiling heat transfer in a channel with a small gap and in a channel, filled with a packed bed, when the capillary forces govern the flow pattern. The current study involves flow boiling of refrigerant R318C in a transparent annulus with the 0.95-mm gap. The flow boiling of refrigerant R318C in a channel, filled with 1.25-mm glass spheres is also discussed. Local heat transfer was measured by thermocouples inside the inner heated tube for both cases. The heat transfer coefficient and flow pattern are presented both for subcooled and saturated flow boiling up to crisis conditions. The regimes of nucleate boiling and forced convection were defined as a function of film thickness.