Sampling characteristics of inlets operated at low U/U0 ratios: new insights from computational fluid dynamics (CFX) modeling

Belyaev and Levin (Journal of Aerosol Science, 5 (1974) 325) (B& L) empirically derived a function for aspiration efficiencies of thin-walled inlets operated at low velocity ratios U/U0 (U flow velocity inside the inlet, U0 velocity of the free flow). This function, which is especially applicable for particle sampling on fast, high-altitude aircraft, has not been experimentally or theoretically verified. By means of the computational fluid dynamics model (CFX), we evaluate this B&L function by explicitly determining inlet aspiration efficiencies over a wide range of high-altitude airborne applications. We find good agreement in the range where B&L performed their measurements (U/U0=0.18–0.2, particle Stokes numbers Stk>0.1), but for lower U/U0 and smaller Stk the B&L function deviates from the CFX results. Therefore, we provide here a modified B&L function covering the ranges U/U0=0.007–0.2 and Stk>0.001: In addition, we used CFX to analyze the performance of a short inlet and demonstrate the strong sensitivity of the sampling characteristics to the inlet design, i.e. the particle transmission efficiency changes from zero for a standard inlet to 30% for a short inlet and the aspiration efficiencies of smaller particles are lower by approximately a factor of two when sampled with the short in comparison to the standard inlet.