Two-phase flow loop technologies capable of acquiring high heat fluxes (>1kW/cm 2 ) from large area heat sources (10cm 2 ) are being considered for the next generation naval thermal requirements. A loop thermosyphon device (∼1 meter tall) was fabricated and tested that included several copper porous wick structures in cylindrical evaporators. The first two were standard annular monoporous and biporous wick designs. The third wick consists of an annular evaporator wick and an integral secondary slab wick for improved liquid transport. In this configuration a circular array of cylindrical vapor vents are formed integral to the primary and secondary transport wick composite. Critical heat fluxes using these wick structures were measured between 240W/cm 2 and 465W/cm 2 over a 10cm 2 area with water as the working fluid at 70°C saturation temperature. A thermosyphon model capable of predicting flow rate at various operating conditions based on a separated flow model is presented.