Fundamentals of cell opening in polymer foaming

Polymeric foams are ubiquitous in foods and industrial manufacturing. Since, they are used in a number of applications as thermal and acoustic insulators, in some cases it is desirable to create foams with cells not interconnected (i.e. closed cells), while in others cases an efficient interconnections between cells (i.e. opened cells) is required, as instance for culture substrates for living cells. In both cases, a fundamental understanding of the physics governing the cell opening process is needed to improve the final product and reduce the polymeric manufacturing cost. In this dissertation, the physical mechanisms leading to cell opening in foams is investigated from a fundamental point of view. As such, the complex foaming process (i.e. involving different physical mechanisms) was studied with a bottom-up process, dividing it in four elementary steps namely: 1) cells growth, 2) cells interaction, 3) rupture and 4) retraction of the cells walls. Different experimental techniques are employed in this thesis; most of them were designed during the Ph.D. to reproduce particular experimental conditions, which are difficult to be obtained with typical foaming equipment. In fact, different new experimental apparatus were developed (i.e. Mini-batch, Interfacial bubble, Breaking bubble) and specifically designed to make unique measurements. The new apparata are particularly useful for testing theoretical predictions on some types of simplified systems useful for the study of the foaming process. The main and novel result of this thesis is the fundamentals understanding of the entire foaming process that leads to a fundamental comprehension of how to produce a particular foam morphology, called fully opened cell. In the literature, there was not fundamentals understanding of the mechanisms behind the cell opening in thermoplastic foaming, since the reported foaming models stop the modeling at the rupture event in the cell walls, without considering the retraction event of the produced hole. The introduction of the retraction as the fundamental step to produce a fully open cell morphology is the novelty of this thesis. Moreover, the comprehension of the retraction step, leads to us to identify the importance of the role of the viscoelasticity for making a fully opened cell foam, that is a new concept that is unique and it was not covered earlier in the previous literature. Moreover, a model of the entire foaming process was developed and it was identified a criterion that employs the computed stresses, the elongational rate and the film thickness among the bubbles to predict the rupture of the polymeric layer between the bubbles and its retraction. As a result, the foaming process model is able to make predictions on the final foam morphology, starting from any polymer/gas solution. Independent experiments to assess the validity of each step of the proposed approach were performed. In conclusion, the developed methodology allows to design the materials and processing conditions to control foam morphology. In the first part of this thesis, a general overview of the foaming process is supplied, focusing the attention on the crucial points of each foaming steps pointed out by the existent literature. The main part will be occupied by the contributions published during the years of this Ph.D. because they represent the steps ahead achieved with respect to the literature

Stratospheric constituent measurements using UV solar occultation technique

The photochemistry of the stratospheric ozone layer was studied as the result of predictions that trace amounts of pollutants can significantly affect the layer. One of the key species in the determination of the effects of these pollutants is the OH radical. A balloon flight was made to determine whether data on atmospheric OH could be obtained from lower resolution solar spectra obtained from high altitude during sunset

Pressure Gain Combustion: Fuel Spray and Shockwave Interaction

Pressure gain combustion can attain higher thermodynamic cycle efficiency in gas turbine power systems, resulting in the reduction of specific fuel consumption/fuel burn and Carbon dioxide emissions.There are many ways to achieve pressure gain and the present research investigates pressure gain through shock bubble (gas and liquid bubble) interaction (SBI) using computational fluid dynamics (CFD) simulations. The numerical simulations have been performed in 2D and 3D representations of the shock tube to depict the interaction of a planar shock wave with distinct gas and liquid inhomogeneities. The three scenarios considered cover the interaction of a planar shock wave in air with: spherical helium bubble (Mach number, Ma = 1.25); cylindrical helium bubble (Ma = 1.22) and cylindrical water bubble (Ma = 1.47). To perform these simulations, the Unsteady Reynolds-Averaged Navier-Stokes (URANS) mathematical model and the coupled level set and VOF method within the commercial CFD code, ANSYS FLUENT, have been applied. A finite volume method (FVM) is also employed to solve the governing equations. For the spherical and cylindrical gas bubble cases, various quantitative analyses are presented and compared to the experimental work of Haas and Sturtevant (1987). These include: refracted wave, transmitted wave, upstream interface, downstream interface, jet, vortex filament, non-dimensional bubble, and vortex velocities. The predicted non-dimensional bubble and vortex velocities have also been compared with experimental data, a simple model of shock- induced Rayleigh Taylor (RT) instability and other theoretical models. Comparisons are also shown between the predicted bubble length/width and the experimentally measured results to elucidate changes in the shape and size of the 2D and 3D bubbles. Additional quantitative analyses are also presented for the spherical bubble involving the size estimation of the vortex pair as well as their spacing. For the shock cylindrical water bubble interaction case, the quantitative predictions include: displacement/drift, acceleration, distortion in the lateral direction, distortion in flow direction, area variation from bubble distortion, as well as drag coefficient and are compared to the experimental measurements of Igra et al. (2002). It has been demonstrated that 3D simulations compare very well with the experimental data, suggesting that 3D simulations are necessary to capture SBI process accurately. Finally, comprehensive flow visualization has been used to elucidate the shock-bubble interaction (SBI) process from bubble compression to the formation of the vortex filaments (cylindrical helium bubble), vortex rings (spherical helium bubble), vortices (cylindrical water bubble) as well as the production and distribution of vorticity. It is demonstrated for the first time that turbulence is generated at the early phase of the SBI process, with the maximum turbulence intensity reaching about 20% around the vortex filaments/vortex rings regions for the cylindrical/spherical helium bubble cases respectively and about 22% for the cylindrical water bubble case at the later phase of the interaction process

Foam management in distillation plants

Foam formation occurs for various substrates during distillation processes. A concrete prediction of the foam formation can only be approximated due to the physical, chemical, and biochemical complexity of the influencing factors. Foam formations affect both the design and the operation of distillation plants, due to various undesirable negative effects of foams on the process and the product. In this work, foam formations under boiling conditions in distillation plants of the spirit industry were investigated and different foam control methods for a holistic foam management system were developed. It was intended to make the distillation process more foam-resilient, and less subjected to foam-induced process disruption. To investigate foam formation in distillation processes lab-scale experiments and experiments on a column still were carried out. In the first step for a foam management system, the inhibition of foams by modification of substrate properties was investigated. In experiments various physical and rheological parameters of mashes as well as other foam-relevant parameters were determined. The aim was to derive a possible link with foam formation. It was shown that the viscosity and viscosity-determining compounds of the substrate have a significant influence on the foaming behavior of mashes. Rye mash was used as the demonstration medium in these experiments. In rye mash the compound pentosan was, in particular, influencing the viscosity. The experiments demonstrated, that the degradation of pentosans prior to distillation resulted in a decrease in viscosity and reduced foam accumulation. Next to foam-promoting substrate properties, foam-promoting operating conditions were investigated. The aim was to link passive process parameters to foam formations. On a laboratory scale, the foam formation in rye mashes was investigated as a function of passive process parameters and operating conditions, respectively, during distillation. It was demonstrated, that foam formations only occurred in a narrow temperature range of 89.5 98.2 °C. Additionally, foam formations were significantly lower with reduced energy input. The findings of the lab scale experiments were applied to develop foam-resilient heating profiles for distillations in the column still. In addition, it was focused on the separation effectiveness and economic efficiency of the new heating profiles, particularly with regard to process duration and the quality of the distillates obtained. Promising foam-resilient heating profiles were transferred to different substrates and their effectiveness was tested. Based on the findings, recommendations for distilleries for a foam-resilient distillation process could be derived, as well as predictions regarding effects on the product quality and process effectiveness. As the last step, active measures for foam destruction utilizing ultrasound were investigated. Ultrasound was introduced into the column at the level of the foam retention device of the distillation unit. The introduction of ultrasound into the column at the level of the foam retention device resulted in a reduction of foams. The observed decrease in foams was attributed to ultrasound-induced drainage of the liquid phase and subsequent destruction of the foam. However, also limitations of the method were found, e.g. limited area of effect. Further research is needed to validate the results and overcome these limitations. Overall, it was shown that foam management, which is not based on chemical defoamers, is possible in foam formation under boiling conditions in distillation processes. Several proposed measures, including inhibition, reduction, and destruction of foams were proposed. By combining them a holistic foam management is feasible.Bei Destillationsprozessen kommt es bei verschiedenen Substraten zur Schaumbildung. Eine konkrete Vorhersage des Schaumbildung kann aufgrund der physikalischen, chemischen und biochemischen Komplexität der Einflussfaktoren nur näherungsweise erfolgen. Schaumbildungen beeinflussen sowohl die Auslegung als auch den Betrieb von Destillationsanlagen, da Schäume verschiedene unerwünschte Auswirkungen auf den Prozess und das Produkt haben können. In dieser Arbeit wurde die Schaumbildung unter Siedebedingungen in Destillationsanlagen der Spirituosenindustrie untersucht und verschiedene Methoden zur Schaumkontrolle für ein ganzheitliches Schaummanagementsystem entwickelt. Ziel war es, den Destillationsprozess resistenter gegen schaumbedingte Prozessunterbrechungen zu machen. Zur Untersuchung der Schaumbildung in Destillationsprozessen wurden Experimente im Labormaßstab und an einem Brenngerät durchgeführt. Als erstes Element eines Schaummanagementsystem wurde die Inhibition von Schaumbildung durch Veränderung der Substrateigenschaften untersucht. In Versuchen wurden verschiedene physikalische und rheologische Parameter von Maischen sowie weitere schaumrelevante Parameter bestimmt. Ziel war es, einen möglichen Zusammenhang mit dem Schaumbildungsvermögen der Substrate herzustellen. Es zeigte sich, dass die Viskosität und viskositätsbestimmende Verbindungen im Substrat einen wesentlichen Einfluss auf das Schaumverhalten haben. Als Demonstrationsmedium für diese Versuche wurde Roggenmaische verwendet. In Roggenmaische war insbesondere die Verbindung Pentosan viskositätsbestimmend. In Versuchen konnte gezeigt werden, dass der Abbau von Pentosanen vor der Destillation zu einer Verringerung der Viskosität und einer geringeren Schaumbildung führte. Neben den schaumfördernden Substrateigenschaften wurden auch schaumfördernde Betriebsbedingungen untersucht. Ziel war es, passive Prozessparameter bzw. daraus resultierende Betriebsbedingungen mit Schaumbildung zu korrelieren. Im Labormaßstab wurde die Schaumbildung in Roggenmaischen in Abhängigkeit von passiven Prozessparametern bzw. Betriebsbedingungen während der Destillation untersucht. Es zeigte sich, dass Schaumbildung nur in einem engen Temperaturbereich von 89,5 - 98,2 °C auftrat. Außerdem war die Schaumbildung bei reduziertem Energieeintrag signifikant reduziert. Die Erkenntnisse aus den Laborversuchen wurden genutzt, um schaumresistente Heizprofile für Destillationen mit dem Brenngerät zu entwickeln. Darüber hinaus wurde die Trenneffektivität und die Wirtschaftlichkeit der neuen Heizprofile, insbesondere im Hinblick auf die Prozessdauer und die Qualität der gewonnenen Destillate, untersucht. Vielversprechende schaumresistente Heizprofile wurden auf unterschiedlichen Substraten angewendet um ihre allgemeine Wirksamkeit zu prüfen. Aus den Ergebnissen konnten Empfehlungen für Brennereien für einen schaumresistenten Destillationsprozess abgeleitet, sowie Vorhersagen über Auswirkungen auf die Produktqualität und Prozesseffektivität durch Änderung der passiven Prozessparameter gemacht werden. In einem letzten Schritt wurden aktive Maßnahmen zur Schaumzerstörung mit Hilfe von Ultraschall untersucht. Der Ultraschall wurde auf Höhe der Schaumrückhaltevorrichtung des brenngeräts in die Kolonne eingebracht. Die Einführung von Ultraschall in die Kolonne auf der Höhe der Schaumrückhaltevorrichtung führte zu einer Zerstörung von Schaumbildungen. Der beobachtete Rückgang der Schaumbildung wurde auf die durch den Ultraschall induzierte Entwässerung der flüssigen Phase und damit einhergehende Zerstörung des Schaums zurückgeführt. Es wurden jedoch auch Limitierungen der Methode festgestellt, z.B. ein begrenzter Wirkbereich. Weitere Forschungsarbeiten sind erforderlich, um die Ergebnisse zu validieren und diese Limitierungen zu überwinden. Insgesamt wurde durch diese Arbeit gezeigt, dass ein Schaummanagement, das nicht auf chemischen Entschäumern basiert, bei der Schaumbildung unter Siedebedingungen in Destillationsprozessen möglich ist. Es wurden mehrere Maßnahmen vorgeschlagen, darunter die Inhibierung, Reduzierung und Zerstörung von Schaum. Durch deren Kombination ist ein ganzheitliches Schaummanagement möglich

Non-Immersion Ultrasonic Cleaning: An Efficient Green Process for Large Surfaces with Low Water Consumption

Ultrasonic cleaning is a developed and widespread technology used in the cleaning industry. The key to its success over other cleaning methods lies in its capacity to penetrate seemingly inaccessible, hard-to-reach corners, cleaning them successfully. However, its major drawback is the need to immerse the product into a tank, making it impossible to work with large or anchored elements. With the aim of revealing the scope of the technology, this paper will attempt to describe a more innovative approach to cleaning large area surfaces (walls, floors, façades, etc.) which involves applying ultrasonic cavitation onto a thin film of water, which is then deposited onto a dirty surface. Ultrasonic cleaning is an example of the proliferation of green technology, requiring 15 times less water and 115 times less power than conventional high-pressurized waterjet cleaning mechanisms. This paper will account for the physical phenomena that govern this new cleaning mechanism and the competition it poses towards more conventional pressurized waterjet technology. Being easy to use as a measure of success, specular surface cleaning has been selected to measure the degree of cleanliness (reflectance) as a function of the process’s parameters. A design of experiments has been developed in line with the main process parameters: amplitude, gap, and sweeping speed. Regression models have also been used to interpret the results for different degrees of soiling. The work concludes with the finding that the proposed new cleaning technology and process can reach up to 98% total cleanliness, without the use of any chemical product and with very low water and power consumption.This research was funded by European Union’s Horizon 2020 research and innovation programme under grant agreement Nº 654479 WASCOP and Nº 792103 SOLWAR

Micro Channel Cooler Performance Improvement by Insonation

The motivation for this work is the need to remove waste heat from laser diodes and high speed transistors in processes which are exponentially increasing past 1 kW/cm2 as anticipated by Moore\u27s Law. The hypothesis guiding the work is that ultrasonic insonation of micro coolers employed to dissipate these heat loads can improve heat removal. It is thought that the mechanism promoting the benefit is enhancement of the ability of the coolant to remove latent heat in two-phase operation by managing entrained bubble size near the cooler\u27s exit so as to forestall flow reduction or blockage caused by large bubbles, wedges and slugs accumulating there. Insonation experiments to prove the hypothesis have been done on several micro channel coolers in the range 4-80 kHz to quantify improvement in heat flux removal. In order to understand how insonation would produce benefit in heat removal, a research effort was undertaken to study the affect of 5-30 Pa acoustic fields on air bubbles rising in small aquariums. This involved developing a Faraday cage shielded acoustic probe, along with a force-beam calibration tool, for measuring field levels near a strongly electromagnetic-radiating ultrasonic source. Experiments were conducted on columns of pseudo monodisperse, sub-millimeter diameter air bubbles in water, and other fluids using bubble generators optimized for this purpose. A numerical analysis model based on energy balance of the acoustic work done on a bubble resulted in predicting mass transfer flux, and in quantifying bubble shrinkage and growth when irradiated on either side of its resonance. The model, and experiments show that bubble populations can be predictably altered by ultrasound. The research was concluded by identifying and quantifying micro channel cooler performance change when insonated in the range 4-80 kHz. It was discovered that 28 and 58 kHz radiation of exchangers having hydraulic diameters spanning 0.02 to 0.6 mm could produce heat flux removal improvements of 5 W/cm2 in devices normally removing less than 30 W/cm2, a factor of 17%. Peak thermal resistance improvement approaching 60 % has been observed