A large diameter Pulsating Heat Pipe (PHP) is a device operating as a thermosyphon on ground and as a PHP under microgravity conditions. Such a prototype with a transparent (sapphire) tube section in the adiabatic region has been tested during the 67th ESA parabolic flight campaign. Infrared visualizations of the fluid in the sapphire section along with measurements of all the relevant quantities, which characterize the device state (pressures, temperatures), are acquired during the tests and exploited for the validation of CASCO code of PHP simulation. After accurate implementation of the PHP geometry and material properties, transient simulations have been carried out. A comparison with the experiment is possible only for the cases where the PHP can be initially assumed at equilibrium. The transients for tube wall temperatures, liquid plug velocities, lengths and temperatures show a good agreement with the experiments during the start-up phase in microgravity conditions reducing the gap towards the development of a fully validated PHP design tool.