3. Measurement uncertainty
For all LDA measurements, many uncertainties are involved. Unfortunately, a few important factors are sometimes beyond the researchers’ control [J. F. Meyers, personal communication, March 2014], such as the particle tacking fidelity, seeding bias, influence of the scattered light on the signal quality, and photo resolved signal bursts owing to insufficient scattered light. The particle size distribution is another source of uncertainty which is difficult to quantify. However, a study from Durst and Ruck  suggested that the maximum diameter of the particle should be less than 2–2.5 times the average diameter to achieve the optimum size distribution for LDA measurements. The particles used in this RX-3117 work have a nominal mean diameter of 8–12 μm. 90% of the particles have a diameter of 14–17 μm. Therefore, the ratio between the maximum diameter and the average diameter meets the requirement.
In this study, the LDA measurements cannot be finished within a short period of time due to the low data rates in some regions. So, a single measurement task may last a few days and require the experimental facility to be turned on and off multiple times. The flow rate is controlled by two plastic ball valves and sometimes flow rates shift. The stability of the pumps and the velocity shift are monitored by recording the readings of the flow meters. The maximum velocity shift within each measurement period was about 0.66%. Moreover, readings from the flow rate fluctuate continuously due to the nature of turbulence. Thus, considering all these effects, a certain degree of errors must have been introduced to results reported in this work.