  A multi-parametric transient numerical simulation of the start-up of a large
diameter Pulsating Heat Pipe (PHP) specially designed for future experiments on
the International Space Station (ISS) are compared to the results obtained
during a parabolic flight campaign supported by the European Space Agency.
Since the channel diameter is larger than the capillary limit in normal
gravity, such a device behaves as a loop thermosyphon on ground and as a PHP in
weightless conditions; therefore, the microgravity environment is mandatory for
pulsating mode. Because of a short duration of microgravity during a parabolic
flight, the data concerns only the transient start-up behavior of the device.
One of the most comprehensive models in the literature, namely the in-house 1-D
transient code CASCO (French acronym for Code Avanc{\'e} de Simulation du
Caloduc Oscillant: Advanced PHP Simulation Code in English), has been
configured in terms of geometry, topology, material properties and thermal
boundary conditions to model the experimental device.The comparison between
numerical and experimental results is performed simultaneously on the temporal
evolution of multiple parameters: tube wall temperature, pressure and, wherever
possible, velocity of liquid plugs, their length and temperature distribution
within them. The simulation results agree with the experiment for different
input powers. Temperatures are predicted with a maximum deviation of 7%.
Pressure variation trend is qualitatively captured as well as the liquid plug
velocity, length and temperature distribution. The model also shows the ability
of capturing the instant when the fluid pressure begins to oscillate after the
heat load is supplied, which is a fundamental information for the correct
design of the engineering model that will be tested on the ISS. We also reveal
the existence of strong liquid temperature gradients near the ends of liquid
plugs both experimentally and by simulation. Finally, a theoretical prediction
of the stable functioning of a large diameter PHP in microgravity is given.
Results show that the system provided with an input power of 185W should be
able to reach the steady state after 1min and maintain a stable operation from
then on.
