Passive, gravity-driven thermosyphons represent a step-change in technology towards the goal of greatly reducing PUE (Power Usage Effectiveness) of datacenters by replacing energy hungry fans of air-cooling approach with a highly-reliable solution able to dissipate the rising heat loads demanded in a cost-effective manner. The European Union has launched a zero carbon-footprint target for datacenters by the timeline of 2030, which would include new standards for implementing green solutions. In the present study, a newly updated version of the general thermosyphon simulation code previously presented at InterPACK 2019 and InterPACK 2020 is considered. To facilitate the industrial transition to thermosyphon cooling technology, with its intrinsic complex flow phenomena, the availability of a general-use, widely validated design tool that handles both air-cooled and liquid-cooled types of thermosyphons is of paramount importance. The solver must be able to analyze and design thermosyphon-based cooling systems with high accuracy and handle the numerous geometric singularities in the working fluid’s flow path, besides that of the secondary coolant. Therefore, a new extensive validation of the thermosyphon simulation solver is performed and presented here versus experimental data gathered for a compact liquid-cooled thermosyphon design, which is being considered for the cooling of high-performance servers. The new experimental database has been gathered to be able to characterize the effect of filling ratio, heat load, secondary coolant temperature and mass flow rate on the cooling performance, using R1234ze(E) as a low GWP (Global Warming Potential) working fluid. This compact design has experimentally demonstrated high performance, maintaining the pseudo chip’s temperature lower than 45°C for evaporator footprint heat fluxes up to 18W/cm2. The comparison shows that the solver is able to accurately predict thermosyphon thermal-hydraulic performance, and based on this prediction, characterize the internal flow rate generated by the thermosyphon, which is key to correctly estimate the maximum heat removal capability.