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Flow turbidity, when introduced between the transmitting and receiving optics and the measuring volume of a laser-Doppler anemometer, changes the pedestal amplitude and visibility of the signal. The purpose of this work is to assess the effect on the accuracy of particle sizing, based on measurements of these two quantities, for depths of field of 5 and 10 cm, interrupting particle diameters between 14 to 212 μm in three discrete ranges and void fractions up to 0.1 percent. The turbidity introduces random fluctuations in visibility which increase with void fraction and the resulting rms errors in particle diameter for turbidity introduced on the receiving side of the optics are smaller than 10 percent at void fractions below 0.1 percent. For particles larger than about one third of the beam diameter, the influence of turbidity is largely due to the interruption of the incident beams over the 5 cm nearest to the measuring volume.

Simultaneous measurements of the mean streamwise and radial velocities and the associated Reynolds stresses were made in an air-solid two-phase flow in a square sectioned (10×10 cm) 90° vertical to horizontal bend using laser Doppler velocimetry. The gas phase measurements were performed in the absence of solid particles. The radius ratio of the bend was 1.76. The results are presented for two different Reynolds numbers, 2.2×105 and 3.47×105, corresponding to mass ratios of 1.5×10−4 and 9.5×10−5, respectively. Glass spheres 50 and 100 μm in diameter were employed to represent the solid phase. The measurements of the gas and solid phase were performed separately. The streamwise velocity profiles for the gas and the solids crossed over near the outer wall with the solids having the higher speed near the wall. The solid velocity profiles were quite flat. Higher negative slip velocities are observed for the 100 μm particles than those for the 50 gm particles. At angular displacement θ=0°, the radial velocity is directed towards the inner wall for both the 50 and 100 μm particles. At θ=30° and 45°, particle wall collisions cause a clear change in the radial velocity of the solids in the region close to the outer wall. The 100 μm particle trajectories are very close to being straight lines. Most of the particle wall collisions occur between the θ=30° and 60° stations. The level of turbulence of the solids was higher than that of the air.

Measurements have been made of the effect of flow turbidity on the visibility and pedestal amplitude of an anemometer signal when incident laser beams are interrupted by particulate flow. The purpose is to assess the likely accuracy of particle sizing and the reliability of discrimination between continuous and particulate phase velocities. Optical depths of field were varied between 2.5 × 10−2 and 14 × 10−2 mm the diameter of the interrupting particles ranged between 14 and 800 μm in six discrete ranges and the corresponding void fractions lay between 0.003% and 0.378%. The incident beam diameter was approximately 400 μm. The measured size is subject to both systematic and random errors when inferred from measurements of pedestal amplitude: the random error increases as the ratio of the incident beam diameter to that of the particulate phase decreases. Systematic errors corresponding to a 10% underestimation of diameter occur at void fractions of 0.003%, 0.01% and 0.018% for particles below 40 μm 75 μm and 105 μm respectively over a 5 cm depth of field. The r.m.s. error is smaller than 7% for particles below 40 μm for all conditions studied but increases with increasing diameter and exceeds 10% at void fractions greater than 0.1% for particles above about 100 μm. The random error in measured diameter derived from measurements of visibility is influenced mostly by the flow turbidity over the 5 cm of the incident beams closest to the measuring volume. For interrupting particles smaller than about 100 μm the r.m.s. error is similar to that for measurements based on the pedestal amplitude. Discrimination of the velocity signal between the particulate and dispersed phase, based on the separation of pedestal amplitudes, is likely to be unreliable if the particle diameter is comparable to the diameter of the incident beams and if the probability of two particles simultaneously present in each beam is not negligible. A method for estimating the level of turbidity at which discrimination is no longer possible is described.

The mean streamwise and radial velocities and the associated Reynolds stresses hare been measured in a square-section duct with a 90' bend of 1.76 radius ratio using laser-Doppler velocimetry. The results are presented for two different Reynolds number5 2.2 X IO5 and 3.47 X radial velocities ws observed up to the 0 = 30° station. At 8 = 30' the structure of the pressure-driven secon. dary Row was well established. At u = 7S0, the stress- driven secondary flow was overwhelmed by the secondary motion of the first kind. There was qualitative and quan- titative agreement between the high and low-speed flows as far as the radial to bulk velocity ratio f (up to -0.42) were recorded for both Reynolds numbers. Between B = 90' and z' = 6.35, 0' had decayed considerably, but secondary motion persisted with the same sense of roto- tion as that in the bend. ur recovered from the bend effect at a slower rate than UB, in the downstream region. Highly anisotropic turbulence appeared throughout the bend. In the downstream region, the flow was round to be more isotropic than it was in the bend. The results are presented in a form suitable for the evaluation of numerical techniques. NOMENCLATURE IO 5 . A superposition of stress-driven and pressure driven