Numerical Study for Subcooling of Liquid Nitrogen in GM Cryocooler

Abstract

A three dimensional study of helical coil heat exchanger for subcooling of liquid nitrogen is carried out, where liquid nitrogen flows in helical coil heat exchanger and get subcooled. The heat exchanger is attached at the coldhead end of GM cryocooler. The numerical simulation is carried out by using CFD software package ANSYS Fluent®. The liquid nitrogen is flows though the helical coil heat exchanger, the coldhead of GM cryocooler extracts heat from heat exchanger. The low temperature is produced in cold head due to expansion of high pressure helium gas. The liquid nitrogen get subcooled up to 120C-150C. Subcooling avoids evaporation of liquid nitrogen due to improper insulation, loss of heat while flowing through coils and other means. The cylinder having length (l), outer diameter (Do) and thickness (t) having one end close created. The helical coil of inner diameter (di), outer diameter (do) and pitch (p) is wound over cylinder for first case. In second case, the helical coil is 0.35 mm inside the cylinder wall for increasing contact area. Now in third case the halfway cut helical coil is wound over a cylinder wall, which provides direct contact of liquid nitrogen with cylinder wall. The liquid nitrogen supplied at different velocities for all three cases. The results of wall temperature, fluid temperature, heat flux, Nusselt Number, temperature contours are obtained and compared for three different cases of helical coil heat exchanger assembly. The results reveals that there is more subcooling performance and heat transfer rate for third case. It also observed that, there is chance of freezing at lower velocity. It can be concluded that, the third case i.e. helical coil halfway wound over a cylinder is more optimum case for subcooling of liquid nitrogen.