Experimental study on the particle velocity development profile and acceleration length in horizontal dilute phase pneumatic conveying systems Nir Santo b, ⁎, Dmitry Portnikov b , Naveen Mani Tripathi b , Haim Kalman a,b a Aaron Fish Chair in Mechanical Engineering – Fracture Mechanics, Israel b Laboratory for Conveying and Handling of Particulate Solids, Department of Mechanical Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer Sheva 84105, Israel abstract article info Article history: Received 10 December 2017 Received in revised form 3 June 2018 Accepted 19 July 2018 Available online 21 July 2018 The significance of the acceleration zone is expressed when designing a straight pipeline that follows either the feeding point or a bend in a pneumatic conveying system. It is an effective and reliable system that takes into ac- count the acceleration zone in the design process. The current paper presents a thorough experimental investi- gation of particle velocity profile at the acceleration region obtained from 3-in, 2-in and 1-in horizontal dilute phase pneumatic conveying systems with various operating conditions and conveyed materials. The velocity was obtained by using a high speed camera combined with image processing. Investigation of the statistical ve- locity distribution resulted in a new correlation for the particle velocity profile throughout the acceleration zone and the acceleration length in the range of the tested operating conditions. © 2018 Elsevier B.V. All rights reserved. Keywords: Pneumatic conveying Acceleration zone Acceleration length Particle velocity Entry length 1. Introduction A proper design process for a pneumatic conveying system is one that takes into account the pressure drop per unit length evaluation. In order to evaluate the pressure drop, one needs to know the accelera- tion length among other parameters. For this reason, the velocity profile that develops within the mentioned section of the pipe is needed. Pre- evaluation of the particle velocity profile is crucial for a reliable and ef- ficient system design. There are several methods of measurement of ac- celeration zone length. Monazam et al. [1] measured the acceleration length by monitoring the void fraction along a pipeline. The end of the acceleration zone is determined when a fully developed flow is achieved, meaning that the void fraction does not change along the pipeline. An additional common way of measuring an acceleration length uses pressure transducers [1–4]. The static air pressure drop per unit length is not constant in the acceleration zone due to the de- crease in the energy required to accelerate the particles as they advance further. Assuming that after the acceleration zone the static pressure of the conveying air should decrease linearly; identifying a constant pres- sure drop per unit length will indicate a steady state flow and the end of the acceleration zone. In a paper by Agarwal et al. [5], a method of eval- uating the acceleration length by bend erosion analysis was introduced. Although achieving a correlation for the acceleration length was not a part of their study, they have shown both the possibility of using the method, and the scarcity of calculation results based on various correla- tions given in the previous literature when trying to evaluate the accel- eration length for fine sand which was their primary tested material. Another measuring method suggested in past studies is individual par- ticle velocity tracking. Dividing the travelled distance of the particles by the time travelled, will implicitly result in the particles speed and acceleration. When measuring the acceleration zone through the particle velocity one can find several conventional ways for doing so [6–9]. Some of the methods will give an average cross sectional velocity values and some will result in specific values for each and every particle in the tested sec- tion of the pipe. Pipe intrusive ways can be applied by using a dia- phragm, impact plates, etc., although these are less common while trying to analyze a pneumatic conveying system for design purposes. Among the non intrusive means the most common relies on the cross correlation method, where a signal (electrostatic, electrical capacity, optic etc) is traced at two points along a pipe line. Knowing the time and distance of travel between the two points will allow calculation of the average velocity of the particles in the tested location. Another method is based on the Doppler shift, where the particle's velocity is proportional to the change of a pulse energy frequency that was applied to the pipe. The last method is of individual particle velocity measure- ment using a high speed camera that records a transparent section of the pipe. In the latter method, data can be acquired for each and every traced particle in any direction according to the angle between the Powder Technology 339 (2018) 368–376 ⁎ Corresponding author. E-mail address: santoni@bgu.ac.il (N. Santo). https://doi.org/10.1016/j.powtec.2018.07.074 0032-5910/© 2018 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec