Study on the performance of the boil-off gas twin-screw compressor based on the thermal-fluid-structure coupling method

The boil-off gas (BOG) twin-screw compressors are widely used in the transportation of liquefied natural gas. A thermal-fluid-structure (TFS) simulation model of the screw compressor is established based on the conjugated heat transfer and weak fluid-structure coupling method. The thermal deformation of the structure at a steady state is considered in the CFD simulation to improve the calculation accuracy. The accuracy of the calculation results was verified by the experiment on a high-speed oil-free twin-screw air compressor. The thermal performance and the compressor efficiencies at different operating conditions were obtained by simulations. It was found that the volumetric efficiency increased from 67 to 78 % while the isentropic efficiency increased from 64 to 75 % with increasing the suction temperature. The volumetric efficiency reduced from 76 to 71 % with increasing the discharge pressure while the isentropic efficiency reached the maximum value of 70 % when the discharge pressure was equal to the end-of-compression pressure. Besides, when the rotational speed was lower than 6000 rpm, increasing the rotational speed had a significantly positive effect on the compressor efficiencies. The volumetric efficiency increased slowly when the speed raised from 6000 to 10,000 rpm. The isentropic efficiency reached a maximum of 73 % at the speed of 8000 rpm, which decreased to 72 % at 10,000 rpm.