Numerical studies on the off-design performance of a cryogenic two-phase turbo-expander

The two-phase turbo-expanders are in more popular as their advantage of more provided cooling capacity in cryogenic applications. However, it makes the preliminary or alternative design and off-design prediction for this type turbine to remain an unworked area, due to a lack of better understanding for machine performance under two-phase conditions. This paper numerically studies the machine off-design performance based on a cryogenic turbo-expander, which is a key equipment to supply main cooling capacity for an air separation plant. The investigative off-design conditions including 420 numerical cases cover both superheated region and two-phase region. The simulations of gas expansion involving two-phase flow in nozzle and impeller are performed on non-equilibrium condensation model of ANSYS CFX. According to the state of gas expansion end point, the numerical results could be categorized as three types, i.e. superheated, supercooled and spontaneous condensation expansions. The effect of non-equilibrium thermodynamic process on the outlet parameters also is revealed via comparison with results from ideal equilibrium condensation model. Moreover, the turbo-expander performance maps under superheated and two-phase conditions, including mass flow rate, degree of reaction and total-to-static efficiency, are compared and analyzed. Finally, taking liquid mass fraction as a fundamental parameter as well as velocity ratio, new 3-D performance maps of reaction and efficiency are developed. The research achievements could be applicable to turbine design and performance prediction at two-phase conditions.