A hydrodynamic study on the soil removal mechanisms of liquid jets and sprays

This thesis presents a hydrodynamic study on the cleaning mechanisms of liquid jets and sprays used in batch cleaning processes in the pharmaceutical industry. By analysing an exemplar wash rack employed for cleaning-out-of-place (COP) on a pharmaceutical manufacturing facility, the flow rate distribution through the rack was predicted using the open source software EPANET. The flow rates obtained using this method were then applied to the design of experiments (DOE). Two jet cases were considered; horizontal impingement on to a vertical wall and vertical impingement through a pipe. For the horizontal case, flow rates used were in the range 1 – 4 l/min. Water temperatures were in the range 20 – 60 degrees centigrade and soil layer thicknesses 0.19 – 1.9 mm. The soil used for horizontal experiments was white soft paraffin (WSP), an excipient commonly used in the manufacture of liquids, creams and ointments. Results from these experiments showed that cleaning occurred via a roll-up mechanism of WSP. Using an energy framework it was observed that cleaning efficiency decreased with time as the distance of the cleaning frontier from the impingement point of the jet increased. Cleaning beyond the drop point of WSP showed a significant increase in cleaning performance as phase transition occurred to mobile. For the vertical jet, the material cleaned was a gel. The primary failure mode was misalignment of the jet relative to the pipe, causing flow to miss the entrance to the pipe. Also residues were observed when the gel was preheated at 85 degrees centigrade prior to cleaning. For the spray experiments, WSP was again used and the same flow conditions. Large residual films were observed at room temperature but again beyond the WSP drop point all material in contact with the flow was removed. Computational simulations of single droplet impingement on to a wetted wall with contact angles varying from 0 – 83 degrees and a wall with a liquid film on the surface were also conducted using the computational fluid dynamics software COMSOL. This was intended to represent a constituent droplet of a spray and gauge its cleaning mechanism on a wall. Shear exerted on the wall was observed to significantly reduce with the presence of a liquid film on the surface

Impact Dynamics of Surfactant-Laden Droplets on Non-Wettable Coatings

Owing to their excellent water repellency, non-wettable (superhydrophobic) coatings have gained tremendous attention in the past couple of decades. Alkyl ketene dimer (AKD), an inexpensive polymer frequently used in paper industry as a sizing agent, has shown potentials to become superhydrophobic. The formation of a porous structure after curing the solidified AKD for an extra-long time (4–6 days) results in superhydrophobicity, i.e., a static contact angle with water of \u3e150° and a roll-off angle of \u3c10°. In this work, a facile and low-cost method was used to turn the surface of AKD superhydrophobic in a very short period of time by briefly treating the coatings, obtained from isothermally heated molten AKD at 40 °C for 3 min, with ethanol. The resulting superhydrophobicity is due to the formation of porous, entangled irregular micro/nano textures that create air cushions on the surface leading to droplet state transition from Wenzel to Cassie. As a proof of concept, the same material was applied to the co-sputtered nickel-tungsten thin films, commonly used in micro/nano-electro-mechanical systems, to improve their hydrophobicity. According to the results, at least 20% increase was observed in the dynamic contact angles of the treated substrates. In addition, this work presents a detailed high-speed imaging analysis of the influence of the molecular weight, concentration and ionic nature of surfactants on droplet impact of such solutions on hydrophilic, hydrophobic and superhydrophobic substrates. Among all these surfaces, the concentration and ionic nature of the solutions were found to be more dominant parameters in determining the energy dissipation in the retraction phase of the droplet impact on the superhydrophobic (AKD) surfaces at room temperature. As the concentration decreases or when positive charges are present in the solution, it is more likely to observe a similar retraction dynamic to pure water when the droplet hits the superhydrophobic AKD having negatively charged surface sites. Finally, the impact dynamics and freezing behavior of these solutions were studied at very low temperatures of –10 to –30 °C. The results show that the dynamic behavior of the solutions is also a function of their temperature-dependent viscosity. The surfactant-laden droplets generally demonstrated an accelerated freezing compared to pure water. This might be due to the fact that the presence of surfactants can promote heterogeneous ice nucleation both from within the liquid as well as a larger solid-liquid interfacial area, resulted from filling the air pockets of the surface by surfactants, leading to enhanced heat transfer. The behavior of the cationic surfactant at certain concentrations was, however, an exception leading to a freezing delay, for which a mechanism will be proposed

Numerical investigation on the evaporation of droplets depositing on heated surfaces at low Weber numbers

The evaporation of water droplets, impinging with low Weber number and gently depositing on heated surfaces of stainless steel is studied numerically using a combination of fluid flow and heat transfer models. The coupled problem of heat transfer between the surrounding air, the droplet and the wall together with the liquid vaporisation from the droplet’s free surface is predicted using a modified VOF methodology accounting for phase-change and variable liquid properties. The surface cooling during droplet’s evaporation is predicted by solving simultaneously with the fluid flow and heat transfer equations, the heat conduction equation within the solid wall. The droplet’s evaporation rate is predicted using a model from the kinetic theory of gases coupled with the Spalding mass transfer model, for different initial contact angles and substrate’s temperatures, which have been varied between 20–90° and 60–100 °C, respectively. Additionally, results from a simplified and computationally less demanding simulation methodology, accounting only for the heat transfer and vaporisation processes using a time-dependent but pre-described droplet shape while neglecting fluid flow are compared with those from the full solution. The numerical results are compared against experiments for the droplet volume regression, life time and droplet shape change, showing a good agreement

Substrate Wettability Influences Internal Jet Formation and Mixing during Droplet Coalescence

The internal dynamics during the axisymmetric coalescence of an initially static free droplet and a sessile droplet of the same fluid are studied using both laboratory experiments and numerical simulations. A high-speed camera captured internal flows from the side, visualized by adding a dye to the free droplet. The numerical simulations employ the volume of fluid method, with the Kistler dynamic contact angle model to capture substrate wettability, quantitatively validated against the image-processed experiments. It is shown that an internal jet can be formed when capillary waves reflected from the contact line create a small tip with high curvature on top of the coalesced droplet that propels fluid toward the substrate. Jet formation is found to depend on the substrate wettability, which influences capillary wave reflection; the importance of the advancing contact angle subordinated to that of the receding contact angle. It is systematically shown via regime maps that jet formation is enhanced by increasing the receding contact angle and by decreasing the droplet viscosity. Jets are seen at volume ratios very different from those accepted for free droplets, showing that a substrate with appropriate wettability can improve the efficiency of fluid mixing

Modeling of Spray/Wall Interactions: Based on Droplet Morphology Dynamics

The present work has the objective of perfecting our knowledge related to spray impact, which is of paramount importance for the optimization of a wide variety of investigation areas, such as combustion systems, coating and cooling processes, and also pollutant emissions. This last referred area has been gaining more and more importance due to the obvious environmental concerns that we face in our age. For these reasons,a remarkable effort by the scientific community has been made in order to deepen the understanding of the mechanisms underlying the spray impingement process. In this dissertation, and through numerical analysis, our in-house code was adapted to reflect the impingement conditions and secondary atomization treatment proposed by Ma et al. [41]. The complex relations between incident spray and the corresponding impact surface are yet far from being duly elucidated, whereby this paper aims to bring us closer to that objective. Evidently, an extensive bibliographic review was performed about theoretical and computational concepts. There are numerous computational models in literature that intend to portray the relation between the impinging spray and the impact surface. Although, not all of these models display the complexity necessary to represent different types of conditions, such as the presence of liquid film or even the existence of a temperature so high that prevents the contact between spray and wall through the generation of a vapor layer. This phenomenon is commonly known as ”Leidenfrost effect” and is usually neglected. One of the first to emerge was proposed by Naber and Reitz, employing the KIVA code, and proposed a single threshold to determine if splash occurred or not. At first glimpse, this model was obviously flawed by way of not accounting for the conditions of occurrence of each impingement regime. Later on, Senda presented a model of their own that was able to predict not only secondary atomization and liquid film formation resulting from the impinging droplets, but also the heat transfer process present in such situation. Sendas’s model despite presenting moderate accuracy, lacked the adaptability to a wider spectrum of applications. Bai and Gosman, using the " model for the gas phase and a stochastic Lagrangian method for the spray, tried to solve this lack of adaptability by modelling the effect of wall conditions and introducing several new regimes. The results translated in improvements describing the secondary droplets, mainly through fitting secondary droplets in a chi-squared distribution and by including surface energy and film dissipation in the conservation equations. Despite these satisfactory results, this model also failed to attain general applicability. Taking into account recent literature alterations, parameters such as saturation temperature and liquid film thickness were utilized to establish more detailed boundary conditions with the intent to represent a more extended range of possible scenarios. In the application of this model a distinction was made between corona splash and prompt splash due to the fact that secondary droplets present different characteristics for each case. Questions such as expansion of the lamella, crown formation and propagation, as well as splashed film mass or transformed mass from crown to secondary droplets became of paramount importance during all the stages of the identified regime and were all detailed in this model. The size and velocity of secondary droplets depend strongly on the initial conditions of the spray at the injector exit, as well as the interaction between incident droplets, crossflow, liquid film, evaporation rate, and interposed hot wall. All these parameters are considered in this macroscopic model of the spray/wall interactions. This dissertation allows us to obtain a detailed analysis about the properties of secondary droplets. In what concerns this subject, a new regime was implemented to a specific gap of boundary conditions and denominated ”uncertain region”. This regime quantifies the probability of splash or rebound occurrence through a uniform distribution since the available information for these conditions is very scarce. Moreover, simulations are carried out for predicting the outcome of flows, including liquid film formation, droplet breakup, and spray evaporation. The numerical results are then compared against experimental data available in open literature to ascertain the predictions capabilities and validate the model.O presente trabalho tem como objetivo aperfeiçoar o conhecimento relativo ao impacto de sprays, que é de extrema importância para a otimização de uma variedade de áreas de investigação, tais como sistemas de combustão, processos de coating ou até de arrefecimento, e também emissões de diversos poluentes. Esta última área de investigação referida tem vindo a ganhar cada vez mais relevância devido a óbvias preocupações ambientais que enfrentamos em pleno século vinte e um. Ao longo deste trabalho, o nosso in-house code foi adaptado para aplicar as condições de fronteira e tratamento de atomização secondária propostas por Ma et al. [41]. As complexas relações entre o spray incidente e a correspondente superfície de impacto ainda estão longe de estarem devidamente elucidadas, pelo que este trabalho visa aproximarnos desse objetivo. Foi também, evidentemente, realizada uma extensa revisão bibliográfica relativa a todas as diferentes facetas deste trabalho, ou seja, tanto ao campo teórico como ao computacional. Existem na literatura diversos modelos computacionais que pretendem retratar a relação entre o spray projetado e a superfície de impacto. Contudo, existe também uma clara impossibilidade física de reproduzir todas as condições possíveis desse mesmo impacto, o que torna todo o tipo de adição a este contexto extremamente útil. Um dos primeiros modelos a emergir foi proposto por Naber e Reitz, pelo que utilizava um código KIVA, e propunha um único critério de transição para determinar se ocorria ou não splash. À primeira vista, este modelo apresentava limitações claras uma vez que não contabilizava as condições de ocorrência de cada regime de impacto. Anos mais tarde, Senda apresentou um modelo que permitia prever não só atomização secondária e a formação de liquid film resultante do impacto, mas também o processo de transferência de calor resultante deste processo. Este modelo proposto por Senda, apresentava resultados relativamente precisos, contudo carecia de adaptabilidade, algo que limitava o seu leque de aplicação gravemente. Bai e Gosman, utilizando o modelo " para a fase de gás e um método estocástico Lagrangiano para a fase de spray, tentaram preencher a falta de aplicabilidade do modelo de Senda, através da introdução de de novos regimes e da modelação das condições da parede. Os resultados traduziram-se em melhorias no que toca à descrição das gotas secondárias, principalmente através da utilização de uma distribuição qui-quadrado, e da inclusão de energia de superfície e dissipação de film nas equações de conservação. Apesar destes esforços, também este modelo não conseguiu vingar amplamente. Tendo em conta alguma alterações que emergiram de literatura recente, foram utilizados parâmetros como a temperatura de saturação e altura do filme de líquido para estabelecer condições de fronteira mais pormenorizadas e precisas que representassem fielmente um mais alargado leque de situações. Estas alterações de condições de fronteira, juntamente com a adição de novas equações representativas da conservação de massa e energia, permitiram que se tratassem as gotas secundárias como uma coroa em formação e posterior rim. Este trabalho permite obter uma análise pormenorizada aos resultados obtidos, pelo que há um claro foco de análise no que respeita às propriedades das gotas secundárias. No que toca a este assunto, um novo regime relativo às propriedades das gotas secundárias foi inserido e denominado ”região incerta”. Este quantifica a probabilidade de ocorrer splash ou ressalto mediante uma distribuição gaussiana, uma vez que a quantidade de informação disponível para estas condições de aplicação é escassa. Este trabalho permite, igualmente, estabelecer uma clara distinção entre corona e prompt splash, o que acrescenta uma quantidade de condições reprodutíveis considerável ao nosso código in-house

Determination Of Adhesive Strength And Freezing Rate Of Ice On Aircraft Structures At Subcooled Temperatures

Icing is widely recognized as one of the most dangerous, and potentially fatal, weather hazards in aircraft operations. Ice accretion on lifting surfaces is known to increase flow separation and drag, decrease lift, alter the moment and pitch of an aircraft, and cause undesired vibrations throughout the aircraft structure, all of which can lead to loss of control of an aircraft and accidents. It is for these reasons that developing methods to deter ice adhesion to aircraft structures is important to the aircraft operations. The average adhesive strength of ice on aluminum at -5, -10, -20, and -30C, was measured to be 0.2150.031, 0.1840.031, 0.2130.041, and 0.2020.035 MPa respectively, suggesting that temperature does not affect on the adhesive strength of ice. The adhesive strength of ice was then measured on bare and methoxymethylethoxypropanol, polymethylhydrosiloxane, and octylphenol ethoxylate treated aluminum, stainless steel, copper, and polycarbonate substrates at -10C. None of the surfactants used in the present study were found to be truly ice-phobic. Wettability was measured on the surfaces of all substrates used. The octylphenol ethoxylate, a surfactant that caused all of the materials observed in the present study to exhibit superhydrophilic surface properties, was revealed to be the only surfactant to reduce the adhesive strength of ice on all of the substrates. At -40C the volumetric freeze rate of a sessile droplet was measured to be 4.62 mm3/second, and the duration of the entire freezing process of a sessile droplet was 10.67 seconds. Icing is widely recognized as one of the most dangerous, and potentially fatal, weather hazards in aircraft operations. Ice accretion on lifting surfaces is known to increase flow separation and drag, decrease lift, alter the moment and pitch of an aircraft, and cause undesired vibrations throughout the aircraft structure, all of which can lead to loss of control of an aircraft and accidents. It is for these reasons that developing methods to deter ice adhesion to aircraft structures is important to the aircraft operations. The average adhesive strength of ice on aluminum at -5, -10, -20, and -30C, was measured to be 0.2150.031, 0.1840.031, 0.2130.041, and 0.2020.035 MPa respectively, suggesting that temperature does not affect on the adhesive strength of ice. The adhesive strength of ice was then measured on bare and methoxymethylethoxypropanol, polymethylhydrosiloxane, and octylphenol ethoxylate treated aluminum, stainless steel, copper, and polycarbonate substrates at -10C. None of the surfactants used in the present study were found to be truly ice-phobic. Wettability was measured on the surfaces of all substrates used. The octylphenol ethoxylate, a surfactant that caused all of the materials observed in the present study to exhibit superhydrophilic surface properties, was revealed to be the only surfactant to reduce the adhesive strength of ice on all of the substrates. At -40C the volumetric freeze rate of a sessile droplet was measured to be 4.62 mm3/second, and the duration of the entire freezing process of a sessile droplet was 10.67 seconds

Advances and challenges of ammonia delivery by urea-water sprays in SCR systems

Over the past decades, selective catalytic reduction (SCR) using aqueous urea sprays as ammonia precursor has become the prevalent technique for NO$_{X}$ emission control in mobile applications. Preparation of ammonia from urea water sprays still represents a challenge in aftertreatment engineering as complex interactions of multi-phase physics and chemical reactions have to be handled. Increasingly stringent emission legislations and the ongoing development of fuel-efficient engines and close-coupled aftertreatment systems raise high demands to SCR systems. Due to highly transient conditions and short mixing lengths, incomplete spray evaporation can result in liquid/wall contact and formation of solid urea deposits lowering ammonia selectivity and homogeneity. This article reviews the ongoing development of SCR systems with focus on the efficient evaporation and decomposition of the injected spray for a homogeneous ammonia distribution in front of the SCR catalyst. Critical aspects of spray evaporation and impingement, liquid film and deposit formation are pointed out and potentials for system optimization are discussed

Surfactants, Thermal And Surface Energy Effects On Emulsions’ Transport Properties: A Study Using Lattice Boltzmann Method

This work aims to provide an efficient Gunstensen LBM based CFD model, capable of solving complex problems related to droplets behavior in shear and parabolic flows. Thermal conditions determine the outcome of the physical and transport properties of emulsions during their various processing phases. A better understanding of the intricate relationship between thermal, surfactants and hydrodynamics can help in the optimization of these processes during the production of emulsions. To investigate the outcome of coupling thermal, surfactants and hydrodynamics on emulsions behavior, a robust quasi-steady thermal-surfactants numerical scheme is presented and used here. To validate the model, the rheological behavior of oil-in-water system was investigated. The numerical results matched well the experimental results of the similar oil-in-water system under steady-state thermal conditions. Furthermore, it is shown that the proposed numerical model can handle cases with transient thermal conditions while maintaining good accuracy. The model has been improved to study the combined effects of temperature, and contact angle on the movement of slugs and droplets of oil in water (O/W) system flowing between two parallel plates and in 3D confined flow study. This is found in the enhanced oil recovery technique which includes thermal, contact angle and surfactant effects for breaking up trapped crude oil. The model static contact angle due to the deposition of the O/W droplet on a flat surface with simulated hydrophilic characteristic at different fluid temperatures, matched very well the proposed theoretical calculation. Furthermore, the model was used to simulate the dynamic behavior of droplets and slugs deposited on the domain\u27s upper and lower surfaces, while subjected to parabolic flow conditions. The model accurately simulated the contact angle hysteresis for the dynamic droplets cases. It was also shown that at elevated temperatures the required power to transport the mixture diminished remarkably. The aim is to improve our understanding of the underlying physics associated with the secondary and tertiary extraction process of trapped crude oil in wells by injecting hot water. Finally, the model was utilized for the investigation of the flow behavior of O/W emulsions with the goal of delineating the best practices for transporting these emulsions in circular ducts. The effects of temperature, volume fraction, flow pressure gradient, and surfactants concentration are investigated in a Poiseuille flow setup. A dimensionless power number ratio was introduced and successfully used for guiding the selection of the most cost-efficient means for transporting O/W emulsion