Aerosol sampling at small forward-facing angles: differentiation of yaw from pitch

Abstract

Abstract When sampling aerosols in ambient or industrial air environments with an inlet, small changes in wind direction or physical constraints in positioning the inlet in the system necessitate the assessment of sampling efficiency in both the vertical and horizontal planes at small angles. In this study we have experimentally investigated the overall sampling efficiency of tubular sharp-edged inlets in yaw (horizontal plane) and pitch (vertival plane) orientations at 0–20° from horizontal aerosol flows. We have developed a model for the overall sampling efficiency at yaw and pitch by extending our previously developed model for pitch to yaw based on our new wind tunnel data. In our model, the difference between yaw and pitch is expressed by the effect of gravity on the wall impaction process inside the inlet described by a gravity effect angle. At yaw, the gravity effect on the wall impaction process does not change with sampling angle. At pitch, the gravity effect on the impaction process results in particle loss increase for upward and decrease for downward sampling. A new relationship has been developed between the gravity effect and physical sampling angles. Using our model, we have developed graphical representations for aerosol sampling at small angles. These can be used in the field to determine the overall sampling efficiency of inlets at several operating conditions and the operating conditions that results in an acceptable sampling error. Pitch and diameter factors have been introduced for relating the efficiency values over a wide range of operating conditions to those of a reference condition. The pitch factor determines the overall sampling efficiency at pitch from yaw values, and the diameter factor determines the overall sampling efficiency at different inlet diameters.