Laminar heat transfer to non-Newtonian fluids in pipes with abrupt changes in diameter

In this thesis the results of experimental work performed to determine local heat transfer coefficients for non-Newtonian fluids in laminar flow through pipes with abrupt discontinuities are reported. The fluids investigated were water-based polymeric solutiorrs of time-indpendent, pseudoplastic materials, with flow indices "n" ranging from 0.39 to 0.9.The tube configurations were a 3.3 :1 sudden convergence, and a 1: 3.3 sudden divergence.The condition of a prescribed uniform wall heat flux was considered, with both upstream and downstream tube sections heated. Radial temperature traverses were also under­ taken primarily to justify the procedures used in estimating the tube wall and bulk fluid temperatures and secondly to give further insight into the mechanism of heat transfer beyond a sudden tube expansion. A theoretical assessment of the influence of viscous dissipation on a non-Newtonian pseudoplastic fluid of' arbitrary index "n" was carried out. The effects of other secondary factors such as free convection and temperature-dependent consistency were evaluated empirically. In the present investigations, the test conditions were chosen to minimise the effects of natural convection and the estimates of viscous heat generation showed the effect to be insignificant with the polymeric concentrations tested here. The final results have been presented as the relationships between local heat transfer coef'ficient and axial distance downstream of the discontinuities and relationships between dimensionless wall temperature and reduced radius. The influence of Reynolds number, Prandtl number, non-Newtonian index and heat flux have been indicated.

The modelling of buoyancy driven flow in bubble columns

Using the analogy between lateral convection of heat and the two-phase flow in bubble columns, alternative turbulence modelling methods were analysed. The k-ε turbulence and Reynolds stress models were used to predict the buoyant motion of fluids where a density difference arises due to the introduction of heat or a discrete phase. A large height to width aspect ratio cavity was employed in the transport of heat and it was shown that the Reynolds stress model with the use of velocity profiles including the laminar flow solution resulted in turbulent vortices developing. The turbulence models were then applied to the simulation of gas-liquid flow for a 5:1 height to width aspect ratio bubble column. In the case of a gas superficial velocity of 0.02 m s-1 it was determined that employing the Reynolds stress model yielded the most realistic simulation results. © 2003 Elsevier B.V. All rights reserved.

Conditional moment closure for chemical reactions in laminar chaotic flows

We implement conditional moment closure (CMC) for simulation of chemical reactions in laminar chaotic flows. The CMC approach predicts the expected concentration of reactive species, conditional upon the concentration of a corresponding nonreactive scalar. Closure is obtained by neglecting the difference between the local concentration of the reactive scalar and its conditional average. We first use a Monte Carlo method to calculate the evolution of the moments of a conserved scalar; we then reconstruct the corresponding probability density function and dissipation rate. Finally, the concentrations of the reactive scalars are determined. The results are compared (and show excellent agreement) with full numerical simulations of the reaction processes in a chaotic laminar flow. This is a preprint of an article published in AlChE Journal copyright (2007) American Institute of Chemical Engineers: http://www3.interscience.wiley.com/

Stenosis triggers spread of helical Pseudomonas biofilms in cylindrical flow systems

Biofilms are multicellular bacterial structures that adhere to surfaces and often endow the bacterial population with tolerance to antibiotics and other environmental insults. Biofilms frequently colonize the tubing of medical devices through mechanisms that are poorly understood. Here we studied the helicoidal spread of Pseudomonas putida biofilms through cylindrical conduits of varied diameters in slow laminar flow regimes. Numerical simulations of such flows reveal vortical motion at stenoses and junctions, which enhances bacterial adhesion and fosters formation of filamentous structures. Formation of long, downstream-flowing bacterial threads that stem from narrowings and connections was detected experimentally, as predicted by our model. Accumulation of bacterial biomass makes the resulting filaments undergo a helical instability. These incipient helices then coarsened until constrained by the tubing walls, and spread along the whole tube length without obstructing the flow. A three-dimensional discrete filament model supports this coarsening mechanism and yields simulations of helix dynamics in accordance with our experimental observations. These findings describe an unanticipated mechanism for bacterial spreading in tubing networks which might be involved in some hospital-acquired infections and bacterial contamination of catheters.

Influence of the length of remaining root canal filling and post space preparation on the coronal leakage of Enterococcus faecalis

This study evaluated the sealing ability of different lengths of remaining root canal filling and post space preparation against coronal leakage of Enterococcus faecalis. Forty-one roots of maxillary incisors were biomechanically prepared, maintaining standardized canal diameter at the middle and coronal thirds. The roots were autoclaved and all subsequent steps were undertaken in a laminar flow chamber. The canals of 33 roots were obturated with AH Plus sealer and gutta-percha. The root canal fillings were reduced to 3 predetermined lengths (n=11): G1=6 mm, G2=4 mm and G3=2 mm. The remaining roots served as positive and negative controls. Bacterial leakage test apparatuses were fabricated with the roots attached to Eppendorf tubes keeping 2 mm of apex submerged in BHI in glass flasks. The specimens received an E. faecalis inoculum of 1 x 107 cfu/mL every 3 days and were observed for bacterial leakage daily during 60 days. Data were submitted to ANOVA, Tukey's test and Fisher's test. At 60 days, G1 (6 mm) and G2 (4 mm) presented statistically similar results (p>0.05) (54.4% of specimens with bacterial leakage) and both groups differed significantly (p<0.01) from G3 (2 mm), which presented 100% of specimens with E. faecalis leakage. It may be concluded that the shortest endodontic obturation remnant leaked considerably more than the other lengths, although none of the tested conditions avoids coronal leakage of E. faecalis.

Catalytic oxidation of ethanol on gold electrode in alkaline media

This work presents a study of the catalytic oxidation of ethanol on polycrystalline gold electrode in alkaline media. The investigation was carried out by means of chronoamperometry, cyclic voltammetry, and in situ FTIR spectroscopy. The main goal was to investigate the early stages of ethanol electrooxidation, namely at fairly low potentials (E = 600 mV vs. RHE) and for moderate reaction times (t < 300 s). Chronoamperometric experiments show a current increase accompanying the increasing in the ethanol concentration up to about 2 M and then a slight decrease at 3 M. Adsorbed CO has been observed as early as about 200 mV vs. RHE and indicates that the cleavage of the C-C bond might occur, probably to a small extent, at very low overpotentials during ethanol adsorption on gold surface. The amount of dissolved acetate ions produced during the chronoamperomentry was followed by the asymmetric stretching band at 1558 cm(-1) as a function of time, and found to increase linearly with time up to 300 s. This allowed estimating the reaction order of acetate formation with respect to ethanol concentration.

Exploiting Pulsed Flows for Heating Improvement: Application to Determination of Total Reducing Sugars in Molasses and Sugar-Cane Juices

Laminar and pulsed flows typical of multi-commuted and multi-pumping flow systems, were evaluated in relation to analytical procedures carried out at high temperatures. As application, the spectrophotometric determination of total reducing sugars (TRS, hydrolyzed sucrose plus reducing sugars) in sugar-cane juice and molasses was selected. The method involves in-line hydrolysis of sucrose and alkaline degradation of the reducing sugars at about 98 degrees C. Better results were obtained with pulsed flows, due to the efficient radial mass transport inherent to the multi-pumping flow system. The proposed system presents favorable characteristics of ruggedness, analytical precision (r.s.d. < 0.013 for typical samples), stability (no measurable baseline drift during 4-h working periods), linearity of the analytical curve (r > 0.992, n = 5, 0.05-0.50% w/v TRS) and sampling rate (65 h(-1)). Results are in agreement with ion chromatography.

Residence time distribution in holding tubes using generalized convection model and numerical convolution for non-ideal tracer detection

A procedure is proposed for the determination of the residence time distribution (RTD) of curved tubes taking into account the non-ideal detection of the tracer. The procedure was applied to two holding tubes used for milk pasteurization in laboratory scale. Experimental data was obtained using an ionic tracer. The signal distortion caused by the detection system was considerable because of the short residence time. Four RTD models, namely axial dispersion, extended tanks in series, generalized convection and PER + CSTR association, were adjusted after convolution with the E-curve of the detection system. The generalized convection model provided the best fit because it could better represent the tail on the tracer concentration curve that is Caused by the laminar velocity profile and the recirculation regions. Adjusted model parameters were well cot-related with the now rate. (C) 2010 Elsevier Ltd. All rights reserved.

Investigation of the residence time distribution in a plate heat exchanger with series and parallel arrangements using a non-ideal tracer detection technique

The objective was to study the flow pattern in a plate heat exchanger (PHE) through residence time distribution (RTD) experiments. The tested PHE had flat plates and it was part of a laboratory scale pasteurization unit. Series flow and parallel flow configurations were tested with a variable number of passes and channels per pass. Owing to the small scale of the equipment and the short residence times, it was necessary to take into account the influence of the tracer detection unit on the RID data. Four theoretical RID models were adjusted: combined, series combined, generalized convection and axial dispersion. The combined model provided the best fit and it was useful to quantify the active and dead space volumes of the PHE and their dependence on its configuration. Results suggest that the axial dispersion model would present good results for a larger number of passes because of the turbulence associated with the changes of pass. This type of study can be useful to compare the hydraulic performance of different plates or to provide data for the evaluation of heat-induced changes that occur in the processing of heat-sensitive products. (C) 2011 Elsevier Ltd. All rights reserved.

Heat transfer and entropy generation optimization of forced convection in a porous-saturated duct of rectangular cross-section

We investigate analytically the first and the second law characteristics of fully developed forced convection inside a porous-saturated duct of rectangular cross-section. The Darcy-Brinkman flow model is employed. Three different types of thermal boundary conditions are examined. Expressions for the Nusselt number, the Bejan number, and the dimensionless entropy generation rate are presented in terms of the system parameters. The conclusions of this analytical study will make it possible to compare, evaluate, and optimize alternative rectangular duct design options in terms of heat transfer, pressure drop, and entropy generation. (c) 2006 Elsevier Ltd. All rights reserved.

Chemical sterilization in in vitro propagation of Arundina bambusifolia Lindl. and Epidendrum ibaguense Kunth

There is a great demand for simpler and less costly laboratory techniques and for more accessible procedures for orchid breeders who do not have the necessary theoretical basis to use the traditional seed and clone production methods of orchids in vitro. The aim of this study was to assess the use of sodium hypochlorite (NaClO) as a decontaminant in the process of inoculating adult orchid explants of Arundina bambusifolia and Epidendrum ibaguenses. Solutions of NaClO (1.200, 2.400, 3.600, 4.800 and 6.000 mg L-1 - equivalent to 50, 100, 150, 200 and 250 mL L-1 of commercial bleach - CB) were sprayed on the explants (1.0 mL) and the culture medium (GB5), in the presence or absence of activated charcoal (2 g L-1). The explants used were nodal segments of field-grown adult plants. The procedures for inoculating the explants were conducted outside the laminar flow chamber (LFC), except for the control treatment (autoclaved medium and explant inoculation inside the LFC). The best results for fresh weight yield, height and number of shoots were obtained using NaClO in solution at 1.200 mg L-1 (equivalent to 50 mL L-1 commercial bleach) with activated charcoal in the culture medium. Fresh weight figures were 1.10 g/jar for Arundina bambusifolia and 0.16 g/jar for Epidendrum ibaguenses. Spraying the NaClO solutions controls the contamination of the culture medium already inoculated with the explants.

Efeitos tridimensionais no escoamento em torno de cilindros confinados

Mestrado em Engenharia Química

Efeito da razão de submersão sobre os parâmetros hidrodinâmicos de uma coluna de recirculação de líquido através de um tubo exterior

No trabalho realizado nesta tese, procedeu-se ao estudo hidrodinâmico de uma coluna de borbulhamento de recirculação externa do líquido (CREL), que permitiu ampliar o conhecimento já existente sobre este tipo de colunas. Para realizar este estudo utilizaram-se líquidos viscosos Newtonianos, nomeadamente soluções aquosas de glicerina com viscosidades entre 0,007 e 0,522 Pa.s. Para a gama de caudais de ar injectados, 1,5x10-5 até 1,35x10-4 m3/s, o ar ascendia ao longo da coluna de borbulhamento sob a forma de bolhas tubulares. Após a realização dos ensaios, verificou-se que o regime de escoamento do líquido entre bolhas tubulares variou desde o tipo laminar até transição (1,9 laminar até ao regime turbulento (12,1 < ReV <3297). Nos ensaios experimentais foram utilizadas duas colunas de borbulhamento com diferentes diâmetros internos de 22 e 32 mm. A razão de submersão, , seleccionada foi de 0,85. Este valor foi escolhido por ser significativamente diferente dos usados em estudos anteriores (0,935 por Santos (2004), Santos (2005) e 0,91 por Rangel (2008)) de forma a poder analisar a influência da razão de submersão sobre os parâmetros hidrodinâmicos. Nos ensaios realizados foram determinadas a retenção do gás na coluna e a velocidade superficial do líquido em escoamento no interior da coluna de borbulhamento. Os valores da retenção do gás foram comparados com valores teóricos previstos por equações empíricas de Nicklin et al (1962). Estes valores também foram comparados com valores obtidos experimentalmente em trabalhos anteriores, onde se utilizaram razões de submersão de 0,935 e 0,91. Após análise dos valores da retenção verificou-se que estes eram mais elevados para as soluções de maior viscosidade e menor diâmetro da coluna. Para as experiências realizadas com as soluções com viscosidades entre 0,52 Pa.s e 0,053 Pa.s, os valores experimentais eram inferiores aos previstos por Nicklin et al (1962) para regime laminar tanto na coluna de 22 mm como na coluna de 32 mm. Para a solução com 0,007 Pa.s de viscosidade os valores experimentais situavam-se na zona de transição entre o regime laminar e o regime turbulento em ambas as colunas. Verificou-se que a diminuição da razão de submersão de 0,935 para 0,85 aumentou significativamente a retenção em todas as experiências realizadas. A velocidade superficial média de circulação do líquido variava na razão inversa do diâmetro e da viscosidade. A diminuição da retenção provocou uma diminuição significativa na velocidade do líquido.

POLYCLONAL OUTBREAK OF BLOODSTREAM INFECTIONS CAUSED BY Burkholderia cepacia COMPLEX IN HEMATOLOGY AND BONE MARROW TRANSPLANT OUTPATIENT UNITS

Aim: The objective was to describe an outbreak of bloodstream infections by Burkholderia cepacia complex (Bcc) in bone marrow transplant and hematology outpatients.Methods: On February 15, 2008 a Bcc outbreak was suspected. 24 cases were identified. Demographic and clinical data were evaluated. Environment and healthcare workers' (HCW) hands were cultured. Species were determined and typed. Reinforcement of hand hygiene, central venous catheter (CVC) care, infusion therapy, and maintenance of laminar flow cabinet were undertaken. 16 different HCWs had cared for the CVCs. Multi-dose heparin and saline were prepared on counter common to both units.Findings: 14 patients had B. multivorans(one patient had also B. cenopacia), six non-multivorans Bcc and one did not belong to Bcc. Clone A B. multivorans occurred in 12 patients (from Hematology); in 10 their CVC had been used on February 11/12. Environmental and HCW cultures were negative. All patients were treated with meropenem, and ceftazidime lock-therapy. Eight patients (30%) were hospitalized. No deaths occurred. After control measures (multidose vial for single patient; CVC lock with ceftazidime; cleaning of laminar flow cabinet; hand hygiene improvement; use of cabinet to store prepared medication), no new cases occurred.Conclusions: This polyclonal outbreak may be explained by a common source containing multiple species of Bcc, maybe the laminar flow cabinet common to both units. There may have been contamination by B. multivorans (clone A) of multi-dose vials.

Rapid fluidic exchange microsystem for recording of fast ion channel kinetics in Xenopus oocytes.

We present a new lab-on-a-chip system for electrophysiological measurements on Xenopus oocytes. Xenopus oocytes are widely used host cells in the field of pharmacological studies and drug development. We developed a novel non-invasive technique using immobilized non-devitellinized cells that replaces the traditional "two-electrode voltage-clamp" (TEVC) method. In particular, rapid fluidic exchange was implemented on-chip to allow recording of fast kinetic events of exogenous ion channels expressed in the cell membrane. Reducing fluidic exchange times of extracellular reagent solutions is a great challenge with these large millimetre-sized cells. Fluidic switching is obtained by shifting the laminar flow interface in a perfusion channel under the cell by means of integrated poly-dimethylsiloxane (PDMS) microvalves. Reagent solution exchange times down to 20 ms have been achieved. An on-chip purging system allows to perform complex pharmacological protocols, making the system suitable for screening of ion channel ligand libraries. The performance of the integrated rapid fluidic exchange system was demonstrated by investigating the self-inhibition of human epithelial sodium channels (ENaC). Our results show that the response time of this ion channel to a specific reactant is about an order of magnitude faster than could be estimated with the traditional TEVC technique.