Low-velocity slug-flow

If for a given material, Vmf > 50 mm s1 (i.e., above the boundary shown in Fig. 5) andX< 0.001 m3 s kg-1 (i.e., below the boundary shown in Fig. 6), then dense-phase low-velocity slug-flow (Wypych, 1995a) is possible (e.g., mustard seed, polyethylene powder, 1000 pm sand, polyethylene pellets and granulated sugar). Note that dilute-phase also is possible. 

Solids deposition in low-velocity slug flow pneumatic conveying... 

Dense-phase (low-velocity slug-flow) solids mass flow = 6th-1, air mass flow =0.065kgs-1, solids loading = 26, pipehne pressure drop = 120 kPa, air velocity = 2.9 to 6.3 ms-1. 

This paper presents results from a theoretical and experimental investigation into the unstable zone between dilute-phase and low-velocity slug-flow. The new boundary model predictions compare well with the experimental data. 

Fig. 1. Typical conveying characteristics (state diagram) for low-velocity slug-flow products.

For products suitable for low-velocity slug-flow, if the air flow selected is higher than that for boundary C, the two-phase flow will be in the form of suspended particles and/or strands. If the air flow is lower than Boundary B, the resulting flow will be in the form of low-velocity slugs. Operation between Boundary B and C can result in different flow modes, depending on the test rig. 

Boundary B delineates the condition that prevents the possibility of strand flow over the stationary layer. The model established in this work to predict the boundary of low-velocity slug-flow is based on this understanding and the theory of mass, force and momentum balance. 

It should be noted that the analyses and model developed and presented in this paper are based on the minimum conveying velocity model by Wirth [2-3]. For example, the method of analysis and some of the key dimensionless parameters used by Wirth [2-3] were found applicable to the current work on operating limits for low-velocity slug-flow. However, there are some importance differences between the two models ... 

An investigation into the boundary of low-velocity slug-flow pneumatic conveying has been conducted experimentally and theoretically. The mechanism for the formation of the imstable zone has been developed. Based on mass, force and momentum balances and the unstable zone mechanism, a theoretical model for the prediction of the unstable zone boundary has... 

Pressure drop prediction of low-velocity slug-flow materials in the unstable zone... 

The theory developed for low-velocity slug-flow [5] still can be used to predict pressure drop caused by long slugs in the unstable zone as long as kw can be determined accurately. 

R Pan, PW Wypych and J Yi, Design of low-velocity slug-flow pneumatic conveying, 6th Int Conf on Bulk Materials Storage, Handling and Transportation, Wollongong, NSW, Australia, 28-30 September, 1998, pp 211-217. 

Fig. 6 Example of pneumatic conveying at the unstable/low-velocity slug-flow boundary.

To design reliably a low-velocity slug-flow pneumatic conveying system, the total pipeline pressure drop as well as the lower and upper boundaries must be predicted accurately for a given product. 

The boundaries in low-velocity slug-flow can be located easily by using full-scale test data. However, such testing is labour intensive, time consuming and costly. Fundamental research for locating the boundaries via a simple and specific simulation rig has been conducted. The results show that the method is very promising. If it is possible to replace the need for extensive experimental work, the time and cost taken to determine the boundaries will be reduced dramatically. 

R. Pan and P.W. Wypych, Pressure Drop and Slug Velocity in Low-Velocity Slug-Flow Pneumatic Conveying of Bulk Solids, Powder Technology, 94, ppl23-132,1997. 

In addition to the obvious advantages of reduced air flow, velocity and power, dense-phase (low-velocity slug-flow) offers the following additional benefits ... 

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