To solve the blockage and fouling issues of sewage heat exchangers in sewage source heat pump, a novel de-foulant hydrocyclone with ejection device was proposed and studied with FLUENT software. The pressure and velocity fields were coupled by the SIMPLEC algorithm, whereas the Reynolds Stress Model was used to predict the 3-D strong swirling turbulent flow due to its anisotropic nature. The Discrete Phase Model was employed to study the particle motion. Results showed that, compared with the variable underflow-pipe diameter, adjusting the ejection velocity could significantly enhance the separation performance by achieving the low split ratio without the risk of blocking the underflow pipe. Unlike the effects of conventional ejectors, the increasing ejection velocity increased the back pressure of the underflow and hence decreased the split ratio. Specifically, if the ejection velocity was higher than a certain value (e.g., 2.0 m/s in this paper), all the 3-D strong swirling turbulent flows in hydrocyclones would be flushed out through the vortex finder by the ejected fluid. In addition, the range of the optimum ejection velocity did exist (e.g., 1.5-2.0 m/s in this paper). In general, the increasing ejection velocity increased the pressure and velocity in hydrocyclones. (C) 2018 Elsevier Ltd. All rights reserved.