A general differential equation in oblique curvilinear coordinates has been established according to the tensor analysis and has been used to simulate the 3-D flowfield in a particle separator.Computational grid are set up with the body-fitted coordinates and a k-ε turbulence model is adopted in computation.Special fluid phenomena in a helicoidal pipe have been simulated.The particle trajectory model has also been established and has been used to analyze the performance of the vortex tube.The results are in good agreement with the experiment.