title: Experimental and Numerical Investigations of Skim Milk Powder Stickiness and Deposition Mechanisms

authors: Zhao, SongXin

subjects: Deposition

abstract: The particle gun method and Computational Fluid Dynamics (CFD) modelling was used to study stickiness and deposition mechanisms of skim milk powder in an impingement jet hitting a stainless steel plate. The particular focus was on the effect of jet velocity and particle size distribution on deposition. Low jet velocities of 10.3, 14.8 and 19.4 m/s were studied at fine particle size levels of 30, 51, and 61 mm, using a jet-plate height to jet diameter ratio of 4. For skim milk powder with a bulk particle size (d(0.5) = 61 μm), lowering the air velocity from 19.4 m/s to 10.3 m/s increased the level of deposition and decreased the point at which deposition first occur as measured by the temperature difference between the glass transition temperature (Tg) of amorphous lactose and the air jet temperature of the particle gun. This point is called (T-Tg)critical. The critical point decreased from 39.0 °C to 18.6 °C as the velocity decreased from 19.4 to 10.3 m/s and the (T-Tg)critical obtained at the lower velocity is in closer agreement with previously reported fluid bed rig results. The (T-Tg)critical point and level of deposition was also found to be highly dependent on particle size. Increasing the average particle size from 30 μm to 61μm increased the (T-Tg)critical from 8.2 °C to 18.6 °C and 14.8 °C to 39.0 °C for jet velocities of 10.3 m/s and 19.4 m/s respectively. Levels of particle deposition also dramatically decreased for both velocity ranges. Ring shaped deposit morphologies were observed with increasing particle stickiness. Beyond (T-Tg)critical powder deposits formed at the periphery of the plate creating a large round clear zone which decreased until a striped deposit ring formed and finally deposits formed only at the centre. Particles were observed to bounce radially from the centre of the plate before sticking. Milk powder deposits are therefore governed by the kinetic energy of the impinging particle in addition to particle surface stickiness. The particle gun method was modelled using Fluent CFD software as an adhesion phenomenon arising from the particle-surface contact dynamics of a particle laden impingement jet contacting a vertical collection plate. The development of a wall boundary condition for specifying the particle-surface interaction has been the focus. A particle is captured by the wall if the impinging kinetic energy is below the prescribed critical normal kinetic energy; otherwise the particle rebounds with reduced kinetic energy. The model was developed through the User Define Function option of Fluent. The CFD model confirmed that particles rebound radially from the collection plate several times before sticking. Circular deposit morphology results from such modelled contact dynamics which are similar to the observed experimental deposits. The level of deposition predicted by CFD increased with increasing levels of critical normal kinetic energy, in the same way experimental deposits increased with increasing particle stickiness. The current model did not considered the contribution from the tangential velocity component to particle stick/rebound behaviour, but it is expected the tangential velocity may also play a significant role and should be included in future CFD models. It is recommended that the particle-surface interaction needs to be studied in more detail, preferably with imaging systems such as Particle Image Velocimetry (PIV), so that individual particle trajectory and deposition behaviour can be followed and analysed.

text: Experimental and Numerical Investigations of Skim Milk Powder Stickiness and Deposition Mechanisms The particle gun method and Computational Fluid Dynamics (CFD) modelling was used to study stickiness and deposition mechanisms of skim milk powder in an impingement jet hitting a stainless steel plate. The particular focus was on the effect of jet velocity and particle size distribution on deposition. Low jet velocities of 10.3, 14.8 and 19.4 m/s were studied at fine particle size levels of 30, 51, and 61 mm, using a jet-plate height to jet diameter ratio of 4. For skim milk powder with a bulk particle size (d(0.5) = 61 μm), lowering the air velocity from 19.4 m/s to 10.3 m/s increased the level of deposition and decreased the point at which deposition first occur as measured by the temperature difference between the glass transition temperature (Tg) of amorphous lactose and the air jet temperature of the particle gun. This point is called (T-Tg)critical. The critical point decreased from 39.0 °C to 18.6 °C as the velocity decreased from 19.4 to 10.3 m/s and the (T-Tg)critical obtained at the lower velocity is in closer agreement with previously reported fluid bed rig results. The (T-Tg)critical point and level of deposition was also found to be highly dependent on particle size. Increasing the average particle size from 30 μm to 61μm increased the (T-Tg)critical from 8.2 °C to 18.6 °C and 14.8 °C to 39.0 °C for jet velocities of 10.3 m/s and 19.4 m/s respectively. Levels of particle deposition also dramatically decreased for both velocity ranges. Ring shaped deposit morphologies were observed with increasing particle stickiness. Beyond (T-Tg)critical powder deposits formed at the periphery of the plate creating a large round clear zone which decreased until a striped deposit ring formed and finally deposits formed only at the centre. Particles were observed to bounce radially from the centre of the plate before sticking. Milk powder deposits are therefore governed by the kinetic energy of the impinging particle in addition to particle surface stickiness. The particle gun method was modelled using Fluent CFD software as an adhesion phenomenon arising from the particle-surface contact dynamics of a particle laden impingement jet contacting a vertical collection plate. The development of a wall boundary condition for specifying the particle-surface interaction has been the focus. A particle is captured by the wall if the impinging kinetic energy is below the prescribed critical normal kinetic energy; otherwise the particle rebounds with reduced kinetic energy. The model was developed through the User Define Function option of Fluent. The CFD model confirmed that particles rebound radially from the collection plate several times before sticking. Circular deposit morphology results from such modelled contact dynamics which are similar to the observed experimental deposits. The level of deposition predicted by CFD increased with increasing levels of critical normal kinetic energy, in the same way experimental deposits increased with increasing particle stickiness. The current model did not considered the contribution from the tangential velocity component to particle stick/rebound behaviour, but it is expected the tangential velocity may also play a significant role and should be included in future CFD models. It is recommended that the particle-surface interaction needs to be studied in more detail, preferably with imaging systems such as Particle Image Velocimetry (PIV), so that individual particle trajectory and deposition behaviour can be followed and analysed.

year: 2009

title: Formation of equiaxed crystal structures in directionally solidified Al-Si alloys using Nb-based heterogeneous nuclei

authors: Bolzoni, Leandro

abstract: The design of chemical compositions containing potent nuclei for the enhancement of heterogeneous nucleation in aluminium, especially cast alloys such as Al-Si alloys, is a matter of importance in order to achieve homogeneous properties in castings with complex geometries. We identified that Al3Nb/NbB2 compounds are effective heterogeneous nuclei and are successfully produced in the form of Al-2Nb-xB (x = 0.5, 1 and 2) master alloys. Our study shows that the inoculation of Al-10Si braze alloy with these compounds effectively promotes the heterogeneous nucleation of primary α-Al crystals and reduces the undercooling needed for solidification to take place. Moreover, we present evidences that these Nb-based compounds prevent the growth of columnar crystals and permit to obtain, for the first time, fine and equiaxed crystals in directionally solidified Al-10Si braze alloy. As a consequence of the potent heterogeneous particles, the size of the α-Al crystals was found to be less dependent on the processing conditions, especially the thermal gradient. Finally, we also demonstrate that the enhanced nucleation leads to the refinement of secondary phases such as eutectic silicon and primary silicon particles.

text: Formation of equiaxed crystal structures in directionally solidified Al-Si alloys using Nb-based heterogeneous nuclei The design of chemical compositions containing potent nuclei for the enhancement of heterogeneous nucleation in aluminium, especially cast alloys such as Al-Si alloys, is a matter of importance in order to achieve homogeneous properties in castings with complex geometries. We identified that Al3Nb/NbB2 compounds are effective heterogeneous nuclei and are successfully produced in the form of Al-2Nb-xB (x = 0.5, 1 and 2) master alloys. Our study shows that the inoculation of Al-10Si braze alloy with these compounds effectively promotes the heterogeneous nucleation of primary α-Al crystals and reduces the undercooling needed for solidification to take place. Moreover, we present evidences that these Nb-based compounds prevent the growth of columnar crystals and permit to obtain, for the first time, fine and equiaxed crystals in directionally solidified Al-10Si braze alloy. As a consequence of the potent heterogeneous particles, the size of the α-Al crystals was found to be less dependent on the processing conditions, especially the thermal gradient. Finally, we also demonstrate that the enhanced nucleation leads to the refinement of secondary phases such as eutectic silicon and primary silicon particles.

year: 2016

title: The importance of whakapapa for understanding fertility

authors: Rarere, Moana

abstract: The Māori fertility transition—which saw a shift from high to low birth rates between 1966 to 1976—was one of the most rapid fertility declines observed anywhere in the world. Since then, Māori fertility has hovered around replacement level (2.1 births per woman), somewhat above that of Pākehā (European) New Zealanders. More striking are differences in timing with Māori women bearing their children younger and over a longer duration. This paper sits within a broader research project that asks: What are the important influences that have sustained contemporary Māori fertility patterns? Drawing on Mana Wahine (Māori women’s discourses) and whakawhiti kōrero (interviews) with wāhine Māori (Māori women) this paper highlights whakapapa (genealogy) as an important concept in broadening and deepening our understandings of fertility, and situating individual fertility and reproduction within a broader set of relations.

text: The importance of whakapapa for understanding fertility The Māori fertility transition—which saw a shift from high to low birth rates between 1966 to 1976—was one of the most rapid fertility declines observed anywhere in the world. Since then, Māori fertility has hovered around replacement level (2.1 births per woman), somewhat above that of Pākehā (European) New Zealanders. More striking are differences in timing with Māori women bearing their children younger and over a longer duration. This paper sits within a broader research project that asks: What are the important influences that have sustained contemporary Māori fertility patterns? Drawing on Mana Wahine (Māori women’s discourses) and whakawhiti kōrero (interviews) with wāhine Māori (Māori women) this paper highlights whakapapa (genealogy) as an important concept in broadening and deepening our understandings of fertility, and situating individual fertility and reproduction within a broader set of relations.

year: 2022