Developent of Helium Re-condenser for 1.5 T ZBO MRI Scanner

Abstract

The heart of any MRI scanner is the superconducting magnet which produces a homogeneous field at the field of view of the magnet. Niobium-Titanium (Nb -Ti), having a critical temperature of 9.2K, is still the workhorse for MRI magnet. The Nb-Ti magnet s dipped into the liquid Helium bath inside the MRI cryostat. Modern MRI Scanners are designed with zero boil off (ZBO) technology. Helium. A CCR is installed at Helium cryost at, a Helium re-condenser is incorporated at second stage of the CCR for continuous re-condensation of Helium vapour. In this project a Helium re-condenser is developed to maintain zero boil off operation in 1.5 T MRI scanner. The Helium cryostats deploy a two stage Sumitomo RDK 415-D GM Cryo-cooler with refrigeration capacity of 40 W at 52 K and 1.5 W at 4.2K. However optimum design of Helium cryostat will ensure lesser thermal load than specified capacity of CCR, affluent zero boil off operation depends on the proper functioning of the Helium re-condenser. This thesis presents an appropriate design of re-condenser with straight rectangular fins. Design of the fins are optimized on the basis of the performance parameters like surface area for heat transfer, convective heat transfer coefficient, fin efficiency and fin effectiveness. Present designed re-condenser has 17 integral fins each of 38 mm height, 2 mm thickness with a gap of 2 mm between two fins. Theoretical fin efficiency and fin effectiveness are 88.90% and 11.23 respectively. Numerical simulation of the re-condensation efficiency of Helium re-condenser is completed. Numerical simulation includes both steady and transient state simulation, considering mass flow rate inlet at 4.31 K and fin wall at 4.2K. Re-condensation rate is 9.57e-05 kg/sec according to CFD modelling through ANSYS Fluent. Designed Helium re-condenser is in the stage of fabrication and to examine actual rate of re-condensation a Helium re-condensation test rig is developed at IUAC, New Delhi.