Thesis presented August 31, 2023

Abstract:
The pulse tube cryocooler is one of the key technologies in cryogenic satellite cooling systems. Although compact, there is a real interest to reduce its size to optimize the payload of space missions. The regenerator is at the heart of the pulse tubes technology. A promising miniaturization method lies in the use of micro fabrication techniques to design reticular-type exchangers with controlled porosity. In this context, this thesis work focuses on the study of micro-fabricated diamond-shape arrays in oscillating flows at frequencies between 20 and 150 Hz. In the steady state, the hydraulic performance of these devices is a function of the friction factor that can be expressed as a function of the pressure drop across the device and the flow rate. In the oscillating regime (where the pressure and flow conditions vary sinusoidally), a similar relationship can be derived. A careful study performed during this work shows that the phase shift between the pressure drop and the flow rate, that increases with frequency, also plays a role. The microstructure also plays a role on the friction factor. As an analytical study would be too complex, we set up an experimental bench dedicated to the measurement of the Poiseuille number associated to the friction factor through the measurement of the flow rate and of the pressure drop between the device's ends. We manufactured samples with a network of diamonds-shape pillars of different characteristics (sizes and angular opening) and different porosities. These samples were then measured in the stationary and oscillating regimes. Similar to the stationary regime, the results in the oscillating regime show an increase in the Poiseuille number with the angle of the diamond and the porosity of the device. The measurements also seem to corroborate the effect of the phase shift on the Poiseuille number. However, this remains to be confirmed, as the phase shift range swept by our samples is not large enough.

Keywords:
Energy, Engineering science, Thermal energy, Combustion​

On-line thesis.