Experimental and numerical investigation on the effects of the seeding properties on LDA measurements

The characteristics of the seeding particles, which are necessary to implement the laser Doppler anemometry (LDA) technique, may significantly influence measurement accuracy. LDA data were taken on a steady-flow rig, at the entrance of the trumpet of the intake system of a high-performance engine head. Five sets of measurements were carried out using different seeding particles: samples of micro-balloons sieved to give three different size ranges (25-63 mu m, 90-200 mu m, and standard as received from the manufacturer 1-200 mu m), smoke from a "home-made" sawdust burner (particle size <= 1 mu m), and fog from a commercial device (particle size around 1 mu m). The LDA data were compared with the results of two-phase computational fluid dynamics simulations. The comparison showed a very good agreement between the experimental and numerical results and confirmed that LDA measurements with particle dimensions in the order of 1 mu m or less represent the actual gas velocity. On the contrary, quite large particles, which are often used because of their cost and cleanliness advantages, introduce nonnegligible errors.