The Properties of Combined Horizontal Flows of Air and Farm Livestock Slurries in a Tubular Loop Aerator

Pig slurries and dairy cattle slurries exhibit shear-thinning characteristics and therefore two-phase horizontal pipeline flows of water and slurries with air were studied in a novel Tubular Loop Aerator (TLA) to determine whether air injection could achieve shear-thinning effects, leading to reduced energy requirements. The TLA incorporated a pipeline of 40·2 mm internal diameter which was 65 m in length. It was found that the pressure gradients resulting from these two-phase flows were described satisfactorily by a correlation based on ratios of single- and two-phase pressure gradients. This technique enabled convenient comparison of the flow characteristics of slurries with different total solids (TS) concentrations at different air injection rates.

Pig slurries containing up to 3·5% TS showed no reductions in pressure loss due to air injection, whereas cattle slurries of 2–3% TS produced some instances of pressure reduction. These reductions occurred at low superficial liquid flow velocity (0·5 m/s) and <2% (vol/vol) air injected, but the results did not provide statistically significant evidence of an energy-saving effect.

Introduction

To design efficient pipeline transport systems for farm livestock slurries, the rheological properties, (i.e. relationships between shear stress and shear rate) must be known. Systems for handling and transporting such slurries have been widely used for many years. Nevertheless, designers of these systems are still without a good procedure for deciding these essential property parameters.

Many farm slurries are known to exhibit non-Newtonian flow characteristics (Hashimoto & Chen, 1975). Non-Newtonian properties lead to non-linear relationships between shear stress and shear rate. Previously, it has been found that relationships between shear stress and shear rate for farm livestock slurries can be expressed by a power law (Hashimoto & Chen, 1975):

$${\tau }_w=K{\gamma }^n$$

where: ${\tau }_w$ is the shear stress at the pipe wall in Pa; $\gamma$ is the shear rate in s⁻¹; and K and n are characteristic constants for the fluid. Consequently, it may be more energy-efficient, under certain circumstances, to pump at high velocities and thereby to take advantage of the lower viscosity that ensues. Similar effects may be created by injecting air into the flow. To investigate the effects that air injection has on the rheological properties of farm livestock slurries, experiments with pig slurries and dairy cattle slurries were undertaken in a Tubular Loop Aerator apparatus (TLA), of the type previously described by Cumby and Slater (1984). This type of apparatus had already exhibited good aeration performance in other applications (Russell et al., 1974; Ziegler et al., 1980) and offered the advantage of a plug flow reactor, a device that has been used for the treatment of industrial and domestic sewage in pumped mains (Boon et al., 1977; Newcomb et al., 1979; Carne et al., 1982). More importantly, this apparatus permitted the establishment of defined hydrodynamic conditions, and generation of a variety of shear rates. Using the TLA, the objectives of this study were to develop and assess a simple technique for the presentation and interpretation of multi-phase flow data and to complete a series of experiments to investigate the specific horizontal flow properties of mixtures of air and farm livestock slurries with a view to possible energy saving. As a precursor, experiments were completed with water and air to provide data representing Newtonian fluids with reproducible physical characteristics.