Elsevier

Powder Technology

Volume 286, December 2015, Pages 55-63
Powder Technology

Experimental investigation on particle entrainment behaviors near a nozzle in gas–particle coaxial jets

https://doi.org/10.1016/j.powtec.2015.07.041Get rights and content

Highlights

  • We observe the particle entrainment behavior near the nozzle.

  • The particle entrainment behavior occurs due to the drag force of gas backflow.

  • The influencing factors of the entrainment behavior are investigated.

  • Three empirical correlations are derived to research quantitatively.

Abstract

The particle entrainment behaviors at the outlet of a nozzle in the gas–particle coaxial jets are experimentally investigated by morphology. The particles in the surface of the granular stream discharging from an inner channel are entrained into the recirculation region due to the drag force of the gas backflow. Hence, the gas backflow is the most essential and crucial factor in the entrainment phenomena. Moreover, the experimental results demonstrate that the particle mass flow rate, the inner duct thickness and the particle size have influences on the formation as well as the characteristics of the entrainment phenomena. The particle entrainment critical annular gas velocity increases linearly with the initial particle velocity and is responsible for the inner duct thickness and the particle size. The entrainment length of the particles and the reverse dispersion angle are employed to analyze qualitatively and quantitatively the motions of the particles. Finally, two empirical correlations on the entrainment length of the particles and the reverse dispersion angle are achieved.

Graphical abstract

(a) Snapshot of the entrainment performance near the nozzle. (b) Partial view of the red rectangle.

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Introduction

Gas–particle two-phase flows are crucial for both scientific researches and engineering processes, such as pulverized coal gasifiers [1], [2], [3] and pulverized coal combustors [4]. As a matter of fact, the behaviors of gas–particle two-phase flows are used widely in diverse industrial equipment. In spite of the relatively simple geometric configuration, the particle dispersion of a coaxial nozzle is a rather complex issue. It is characterized by complicated dispersion phenomena including the flow behaviors of particles and the interaction between gas and particles, which makes it attractive for basic investigations of the two-phase flow physics.

Dating back to the 1980s, Crowe et al. [5] investigated the particle dispersion for various size particles and explained that the larger particle dispersion rates were greater than the smaller particle dispersion rates through the coherent structures. Chein and Chung [6] researched numerically the effects of the vortex pairing on the particle dispersion and proposed that the extent of the particle dispersion depends strongly on the Stokes number. Thereafter, studies [7], [8], [9] about air turbulent modulation in the particle-laden jets became the crucial research focus. In addition, an increasing number of researchers [10], [11], [12], [13], [14], [15] paid great attention to the interactions between gas and particles, especially the two-phase velocity measurements. Recently, a kinetic theory model and granular temperature model [16], [17], [18], [19] are developed to evaluate the particle dynamics in the particle-laden turbulent flows. As far as two-phase coaxial jets were concerned, Fan et al. [20], [21] conducted experimental studies and highlighted significant influences of the solid particles on the coaxial jet behaviors. Lately, a set of experiments have been carried out by Liu et al. [22], [23] and three typical dispersion modes, which emerged in a granular jet surrounded by an annular air jet, have been identified. Thereafter, Lu et al. [24], [25], [26] observed that the particle bubbles in a periodic manner were formed thought the high-speed digital photography. Next, they got insights into the factors (e.g., particle mass flow rate and annular channel thickness) influencing the characterization of the particle bubbles. Moreover, they studied experimentally an annular granular jet with acoustic excitation and suggested that excitation frequency and SPL contributed to encourage the growth of particle bubbles in radial direction. On the other hand, Gui et al. [27] and Liu et al. [28] focused on the numerical simulation of swirling particle-laden coaxial jets and studied the particle motions affected by the gas vortices. Michael Breuer and Michael Alletto improved the simulation of particle-laden jets with high mass loadings and discussed the importance of the inter-particle collisions in comparison to the particle–fluid interaction.

Even though the single-phase and two-phase coaxial jets have been studied theoretically, experimentally and numerically, as shown in Table 1, the researches on the characteristics of the particle dispersion in the near-field region of coaxial jets are relatively limited so far. In previous studies, the liquid entrainment behaviors at the nozzle exit in the coaxial jets were observed by Tian et al. [29], [30]. Consequently the motivation of the present paper is to explore, experimentally, the interaction between particles and gas behind the thick wall of an inner channel in the near field.

Section snippets

Experimental setup

The experiments were conducted with a dense gas–particle coaxial jet in the gaseous environment, as shown schematically in Fig. 1. The gas from the blower flowed into the annular channel through the storage tank, which contributed to the gas turbulence elimination from the blower. On the other hand, the particle mass flow rates in the inner channel were controlled by the air pressure in the hopper connected with the steel cylinder. To measure the particle mass flow rates, the granules flowing

Entrainment phenomenon of granular coaxial jets

In brief, the annular gas discharges from the nozzle at a high speed and ejects into the surrounding air, as a result of a negative pressure zone emerging behind the wall of the inner nozzle. Moreover, the annular gas creates a stagnation point on the surface of the granular stream, namely the maximum pressure point, owing to the gas divergence from out of the nozzle. Meanwhile Chigier et al. [31] proposed that there was a vortex center, namely the minimum pressure point, between the annular

Conclusions

In this study, the interactions between gas and particles in the scenario of the granular coaxial jets are experimentally investigated by morphology. We research the entrainment phenomenon in the near field region using the high-speed camera. The experimental results reseal that the particles in the surface of the granular stream are entrained into the recirculation region due to the drag force of the backflow gas. Hence, the backflow gas is the most essential and crucial factor in the

Nomenclature

    X

    downstream distance from the nozzle exit (m)

    M

    momentum flux ratio

    uo

    fluid velocity in the outer channel (m/s)

    ui

    fluid velocity in the inner channel (m/s)

    D0

    inner diameter of the inner nozzle (m)

    D1

    outer diameter of the inner nozzle (m)

    D2

    inner diameter of the outer nozzle (m)

    h

    wall thickness of the inner nozzle (m)

    Reg

    Reynolds number of the gas

    ug

    superficial gas velocity (m/s)

    dp

    particle diameter (m)

    mp

    particle mass flow rate (kg/s)

    St

    Stoke number

    Rep

    Reynolds number of the particle

    ugc

    critical gas velocity

Acknowledgments

This research was supported by the National High Technology Research and Development Program of China (2013AA051103), the National Natural Science Foundation of China (U1402272), Fundamental Research Funds for the Central Universities (WB1314046), and Shanghai Natural Science Foundation (15ZR1409500).

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