Abstract
Angle resolved 2-D PIV measurements were performed to characterise the flow and turbulence as well as indicate potential droplet break up mechanisms in an in-line Silverson 150/250 high shear mixer, using water as the working fluid in the turbulent regime (120,000 < Re < 420,000). Distributions of Reynolds stresses, turbulent kinetic energy (TKE), and energy dissipation rates (ε) were examined. The regions of interest (ROI) were: A – jet emanating from a stator hole and B – the rotor swept volume. The complex flow pattern can cause droplet break up under either laminar or turbulent conditions depending on the characteristic length and velocity in the ROI; break up due to turbulence in the inertial regime was identified as the dominant mechanism in this study. Evaluated energy dissipation rates obtained assuming either a fully resolved velocity field (DE) or using the Smagorinsky closure model (SGS) were found to depend on rotor speed e.g. ε∝Nb with b exponents of 1.59–1.90 (DE) and 2.42–2.84 (SGS), which are comparable to existing literature values. The influence on ε of the rotor speed, external pump flow rate and induced backpressure on the mixer outlet, were also investigated. Analysis revealed that the intensity and propensity of ε is dictated by the dominant flow in the mixing head e.g. radial flow at high pump flow rates, prominent in ROI A or tangential flow at high rotor speeds and when backpressure is induced, prominent in ROI B.
Highlights
Angle resolved 2-D Particle Image Velocimetry (PIV) measurements were performed to characterise the flow and turbulence as well as indicate potential droplet break up mechanisms in an in-line Silverson 150/250 high shear mixer, using water as the working fluid in the turbulent regime (120,000 < Re < 420,000)
This paper presents a novel study of the hydrodynamics of a custom-built pilot-scale, Silverson 150/250 mixer equipped with an external pump for flow control
Air entrainment occurred for rotor speeds higher than 60.0 rps as the external pump acts as a flow restriction in the suction line
Summary
Angle resolved 2-D PIV measurements were performed to characterise the flow and turbulence as well as indicate potential droplet break up mechanisms in an in-line Silverson 150/250 high shear mixer, using water as the working fluid in the turbulent regime (120,000 < Re < 420,000). ‘Rotor-stator’ Mixers (RSM), or high shear mixers (HSM), are extensively used in industrial processes for liquid-liquid dispersion, emulsification and homogenisation due to their capability to produce high levels of shear and energy dissipation rates (three orders of magnitude greater than a mechanically stirred vessel) on a relatively small volume of fluid. They are used in the food, cosmetics, home and personal care as well as pharmaceutical industries for manufacturing products such as mayonnaise, detergents, shampoos and conditioners (Håkansson, 2018; Zhang et al, 2012) and can be operated in batch, semi-continuous or continuous (in-line) modes. Symbols B CS D Dh Dinlet Dps, inner Dps, outer Dr,i Dr,o Dss, inner Dss, outer Dvolute k k1 L LIA Lp MP MS MT N P POUT PoZ Q R11 R12 R22 Re ReT S SÀr UU h Up U Uf Uhole Uinlet Ujet Umax Urms
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