O encruamento e a recristalização do aço inoxidável ferrítico AISI 430 com adição de nióbio e microestrutura formada por grãos grosseiros foram investigados. Os aços inoxidáveis ferríticos podem ser utilizados na indústria automotiva, por exemplo, nas partes mais quentes do sistema de exaustão de gases. Neste trabalho, três composições distintas foram investigadas, variando-se as quantidades de nióbio e de intersticiais (carbono e nitrogênio). Com o objetivo de se obter uma microestrutura formada por grãos grosseiros útil para o estudo dos efeitos de orientação, amostras dos aços laminado a quente foram recozidas a 1250?C por 2 h. Após recozimento, o tamanho de grão foi medido usando o método padrão dos interceptos lineares. Para o estudo do encruamento, as placas foram laminadas a frio entre reduções de 20% e 80%. As amostras com 80% de redução foram recozidas em temperaturas variando entre 400 e 1000?C por 15 min para o estudo da recristalização. As curvas de encruamento e de amolecimento isócrono foram realizadas mediante a determinação da variação da microdureza Vickers. A caracterização microestrutural das amostras foi realizada com o auxílio das microscopias ótica (MO) e eletrônica de varredura (MEV) no modo de elétrons retroespalhados (BSE). A macrotextura foi determinada com o auxílio da difração de raios X (DRX). A microtextura foi determinada mediante o mapeamento de amostras representativas via difração de elétrons retroespalhados (EBSD). Os precipitados presentes na matriz ferrítica dos aços laminados a quente (FSS-B) foram extraídos utilizando duas rotas distintas: eletrolítica e química. A natureza cristalográfica dos precipitados foi determinada via DRX e a morfologia foi observada com o auxílio da MEV. As microestruturas recuperadas na condição inicial consistem de grãos alongados na DL e por partículas de Nb(C,N). A textura é caracterizada por componentes típicas de laminação no centro e de cisalhamento na superfície devido às grandes reduções por passe. Após recozimento, as macrografias revelaram que os aços FSS-A e FSS-B apresentam tamanho de grão similar, enquanto que o aço FSS-C apresentou tamanho de grãos menor. Os resultados de textura apresentaram as componentes CH (centro), Goss e Brass (superfície). A laminação a frio ocorreu de forma homogênea para as reduções inferiores a 50%. A partir desta redução, regiões bandeadas surgem na microestrutura. Notou-se que existe uma relação de orientação entre os grãos originais e as regiões bandeadas de modo que volumes regulares do grão giram na DT. Esta relação também foi observada nas regiões ao redor das partículas de Nb(C,N). A textura de laminação a frio é constituída pelas fibras ? e ?. A recristalização dos aços investigados ocorre em temperaturas entre 650 e 850ºC. A partir da microtextura, não foram observadas componentes de textura associadas ao mecanismo PSN. O aço FSS-R apresentou componentes pertencentes à fibra ?, enquanto que os aços FSS-A, FSS-B e FSS-C apresentaram além da fibra ?, componentes CH e fibra η. O recozimento em temperaturas elevadas promove o crescimento de grão e a conseqüente formação das componentes CH e fibra η. / Work hardening and recrystallization behaviors of coarse-grained Nb-containing AISI 430 ferritic stainless steels were investigated. Ferritic stainless steels can be used in automotive industry in hot parts of the gas exhaust system. In this work, three different compositions were investigated varying niobium and interstitial contents (carbon and nitrogen). Aiming to obtain a useful coarse-grained microstructure for the study of orientation effects, hot-rolled samples were annealed at 1250°C for 2 h. After annealing, grain size was determined using a standard linear intercept method. Samples were cold rolled to reductions varying from 20% up to 80%. Samples after 80% cold rolling were annealed at temperatures ranging from 400°C up to 1000°C for 15 min to investigate their recrystallization behavior. Vickers microhardness testing was performed to follow hardening and softening behaviors in the samples. Microstructural characterization of the samples was performed using both light optical and scanning electron (SEM) microscopies in the backscattered electron mode (BSE). The macrotexture was determined by X-ray diffraction. Electron backscatter diffraction (EBSD) measurements were carried out in representative samples to determine microtexture. Precipitates in hot-rolled samples (FSS-B) were both electrolytically and chemically extracted. The crystallography of precipitates was determined by X-ray diffraction and their morphology was observed using SEM. The microstructures in hot-rolled condition consist of elongated recovered grains and dispersed Nb(C,N) particles. Texture is characterized by typical rolling components in the center layer, whereas shear components appear in the surface layers. After annealing, FSS-A and FSS-B steels displayed similar grain size, whereas FSS-C displayed a finer one. The results of macrotexture show CH in the center layer and both Goss and Brass components at the surface layer. The microstructure of samples cold rolled up to reductions below 50% do not display banding. Above 50% reduction, deformation heterogeneities (bands) appear in the microstructure. The banded regions and those around coarse particles tend to rotate in TD. Cold-rolling texture displays both ? and ? fiber components. Recrystallization takes place in temperature between 650 and 850ºC. Texture due to PSN mechanism was not observed. The FSS-C steel displayed only components belonging to ? fiber, whereas FSS-A, FSS-B e FSS-C steels displayed besides ? fiber, components CH and those belonging to η fiber. Grain-growth annealing at high temperature contributes to increase CH and η fiber components.

O encruamento e a recristalização do aço inoxidável ferrítico AISI 430 com adição de nióbio e microestrutura formada por grãos grosseiros foram investigados. Os aços inoxidáveis ferríticos podem ser utilizados na indústria automotiva, por exemplo, nas partes mais quentes do sistema de exaustão de gases. Neste trabalho, três composições distintas foram investigadas, variando-se as quantidades de nióbio e de intersticiais (carbono e nitrogênio). Com o objetivo de se obter uma microestrutura formada por grãos grosseiros útil para o estudo dos efeitos de orientação, amostras dos aços laminado a quente foram recozidas a 1250?C por 2 h. Após recozimento, o tamanho de grão foi medido usando o método padrão dos interceptos lineares. Para o estudo do encruamento, as placas foram laminadas a frio entre reduções de 20% e 80%. As amostras com 80% de redução foram recozidas em temperaturas variando entre 400 e 1000?C por 15 min para o estudo da recristalização. As curvas de encruamento e de amolecimento isócrono foram realizadas mediante a determinação da variação da microdureza Vickers. A caracterização microestrutural das amostras foi realizada com o auxílio das microscopias ótica (MO) e eletrônica de varredura (MEV) no modo de elétrons retroespalhados (BSE). A macrotextura foi determinada com o auxílio da difração de raios X (DRX). A microtextura foi determinada mediante o mapeamento de amostras representativas via difração de elétrons retroespalhados (EBSD). Os precipitados presentes na matriz ferrítica dos aços laminados a quente (FSS-B) foram extraídos utilizando duas rotas distintas: eletrolítica e química. A natureza cristalográfica dos precipitados foi determinada via DRX e a morfologia foi observada com o auxílio da MEV. As microestruturas recuperadas na condição inicial consistem de grãos alongados na DL e por partículas de Nb(C,N). A textura é caracterizada por componentes típicas de laminação no centro e de cisalhamento na superfície devido às grandes reduções por passe. Após recozimento, as macrografias revelaram que os aços FSS-A e FSS-B apresentam tamanho de grão similar, enquanto que o aço FSS-C apresentou tamanho de grãos menor. Os resultados de textura apresentaram as componentes CH (centro), Goss e Brass (superfície). A laminação a frio ocorreu de forma homogênea para as reduções inferiores a 50%. A partir desta redução, regiões bandeadas surgem na microestrutura. Notou-se que existe uma relação de orientação entre os grãos originais e as regiões bandeadas de modo que volumes regulares do grão giram na DT. Esta relação também foi observada nas regiões ao redor das partículas de Nb(C,N). A textura de laminação a frio é constituída pelas fibras ? e ?. A recristalização dos aços investigados ocorre em temperaturas entre 650 e 850ºC. A partir da microtextura, não foram observadas componentes de textura associadas ao mecanismo PSN. O aço FSS-R apresentou componentes pertencentes à fibra ?, enquanto que os aços FSS-A, FSS-B e FSS-C apresentaram além da fibra ?, componentes CH e fibra η. O recozimento em temperaturas elevadas promove o crescimento de grão e a conseqüente formação das componentes CH e fibra η. / Work hardening and recrystallization behaviors of coarse-grained Nb-containing AISI 430 ferritic stainless steels were investigated. Ferritic stainless steels can be used in automotive industry in hot parts of the gas exhaust system. In this work, three different compositions were investigated varying niobium and interstitial contents (carbon and nitrogen). Aiming to obtain a useful coarse-grained microstructure for the study of orientation effects, hot-rolled samples were annealed at 1250°C for 2 h. After annealing, grain size was determined using a standard linear intercept method. Samples were cold rolled to reductions varying from 20% up to 80%. Samples after 80% cold rolling were annealed at temperatures ranging from 400°C up to 1000°C for 15 min to investigate their recrystallization behavior. Vickers microhardness testing was performed to follow hardening and softening behaviors in the samples. Microstructural characterization of the samples was performed using both light optical and scanning electron (SEM) microscopies in the backscattered electron mode (BSE). The macrotexture was determined by X-ray diffraction. Electron backscatter diffraction (EBSD) measurements were carried out in representative samples to determine microtexture. Precipitates in hot-rolled samples (FSS-B) were both electrolytically and chemically extracted. The crystallography of precipitates was determined by X-ray diffraction and their morphology was observed using SEM. The microstructures in hot-rolled condition consist of elongated recovered grains and dispersed Nb(C,N) particles. Texture is characterized by typical rolling components in the center layer, whereas shear components appear in the surface layers. After annealing, FSS-A and FSS-B steels displayed similar grain size, whereas FSS-C displayed a finer one. The results of macrotexture show CH in the center layer and both Goss and Brass components at the surface layer. The microstructure of samples cold rolled up to reductions below 50% do not display banding. Above 50% reduction, deformation heterogeneities (bands) appear in the microstructure. The banded regions and those around coarse particles tend to rotate in TD. Cold-rolling texture displays both ? and ? fiber components. Recrystallization takes place in temperature between 650 and 850ºC. Texture due to PSN mechanism was not observed. The FSS-C steel displayed only components belonging to ? fiber, whereas FSS-A, FSS-B e FSS-C steels displayed besides ? fiber, components CH and those belonging to η fiber. Grain-growth annealing at high temperature contributes to increase CH and η fiber components.

<div>In order to better understand and quantify the effect of instabilities in systems utilizing flow boiling heat transfer, the present study explores dynamic results for pressure drop, mass velocity, thermodynamic equilibrium quality, and heated wall temperature to ascertain and analyze the dominant modes in which they oscillate. Flow boiling experiments are conducted for a range of mass velocities with both subcooled and saturated inlet conditions in vertical upflow, vertical downflow, and horizontal flow orientations. High frequency pressure measurements are used to investigate the influence of individual flow loop components (flow boiling module, pump, pre-heater, condenser, etc.) on dynamic behavior of the fluid, with fast Fourier transforms of the same used to provide critical frequency domain information. Conclusions from this analysis are used to isolate instabilities present within the system due to physical interplay between thermodynamic and hydrodynamic effects. Parametric analysis is undertaken to better understand the conditions under which these instabilities form and their impact on system performance. Several prior stability maps are presented, with new stability maps provided to better address contextual trends discovered in the present study.</div><div>Further, this study utilizes experimental results for vertical upflow boiling of FC-72 in a rectangular channel with finite inlet quality to investigate Density Wave Oscillations (DWOs) and assess their potential impact on design of two-phase systems for future space missions. High-speed flow visualization image sequences are presented and used to directly relate the cyclical passage of High and Low Density Fronts (HDFs and LDFs) to dominant low-frequency oscillations present in transient pressure signals commonly attributed to DWOs. A methodology is presented to determine frequency and amplitude of DWO induced pressure oscillations, which are then plotted for a wide range of relevant operating conditions. Mass velocity (flow inertia) is seen to be the dominant parameter influencing frequency and amplitude of DWOs. Amplitude of pressure oscillations is at most 7% of the time-averaged pressure level for current operating conditions, meaning there is little risk to space missions. Reconstruction of experimental pressure signals using a waveform defined by frequency and amplitude of DWO induced pressure fluctuations is seen to have only moderate agreement with the original signal due to the oversimplifications of treating DWO induced fluctuations as perfectly sinusoidal in nature, assuming they occur at a constant frequency value, and neglecting other transient flow features. This approach is nonetheless determined to have potential value for use as a boundary condition to introduce DWOs in two-phase flow simulations should a model be capable of accurately predicting frequency and amplitude of oscillation.</div><div>Additionally, this study presents a new mechanistic model for Density Wave Oscillations (DWOs) in vertical upflow boiling using conclusions drawn from analysis of flow visualization images and transient experimental results as a basis from which to begin modeling. Counter to many prior studies attributing DWOs to feedback effects between flow rate, pressure drop, and flow enthalpy causing oscillations in position of the bulk boiling boundary, the present instability mode stems primarily from body force acting on liquid and vapor phases in a separated flow regime leading to liquid accumulation in the near-inlet region of the test section, which eventually departs and moves along the channel, acting to re-wet liquid film along the channel walls and re-establish annular, co-current flow. This process was modeled by dividing the test section into three distinct control volumes and solving transient conservation equations for each, yielding predictions of frequencies at which this process occurs as well as amplitude of associated pressure oscillations. Values for these parameters were validated against an experimental database of 236 FC-72 points and show the model provides good predictive accuracy and capably captures the influence of parametric changes to operating conditions.</div><div>Also, this study shows analysis of pressure signals in condensing systems reveal the presence of relevant oscillatory phenomena during flow condensation as well, which may impact performance in applications concerned with precise system control. Towards this end, the present study presents results for oscillatory behavior observed in pressure measurements during flow condensation of FC-72 in a smooth circular tube in vertical upflow, vertical downflow, and horizontal flow orientations. Dynamic behavior observed within the test section is determined to be independent of other components within the flow loop, allowing it to be isolated and interpreted as resulting from physical aspects of two-phase flow with condensation. The presence of a peak oscillatory mode (one of significantly larger amplitude than any others present) is seen for 72% of</div><div>vertical upflow test cases, 61% of vertical downflow, and 54% of horizontal flow. Relative intensities of this peak oscillatory mode are evaluated through calculation of Q Factor for the corresponding frequency response peak. Frequency and amplitude of peak oscillatory modes are also evaluated. Overall, vertical upflow is seen to exhibit the most significant oscillatory behavior, although in its maximum case amplitude is only seen to be 7.9% of time-averaged module inlet pressure, indicating there is little safety risk posed by oscillations under current operating conditions. Flow visualization image sequences for each orientation are also presented and used to draw parallels between physical characteristics of condensate film behavior under different operating conditions and trends in oscillatory behavior detected in pressure signals</div><div>Further, the present work outlines a new methodology utilizing temperature and pressure measurements to identify condensation flow regimes. For vertical upflow condensation, amplitude of dynamic temperature and pressure oscillations are shown to clearly indicate transition from counter-current flow regimes (i.e., falling film, oscillating film, flooding) to annular, co-current flow (climbing film flow regime). In horizontal flow condensation, standard deviation between multiple thermocouple measurements distributed around the tube circumference was calculated at all axial (stream-wise) measurement locations. High values of standard deviation are present for stratified flow (stratified flow, wavy-stratified, plug flow), while axisymmetric flow regimes (i.e., slug flow, annular flow) yield significantly lower values. Successful development of this technique represents a valuable contribution to literature as it allows condensation flow regime to be identified without the often-costly restriction of designing a test section to allow optical access. Identified flow regimes in both vertical upflow and horizontal flow orientations are compared to regime maps commonly found in the literature in pursuit of optimum performing maps.</div><div>Finally, the present study aims to better analyze the influence of body force on flow condensation heat transfer by conducting tests at multiple orientations in Earth’s gravity. Dielectric FC-72 is condensed in a smooth stainless-steel tube with 7.12 mm diameter and 574.55 mm condensing length by counterflow of cooling water across the outer surface of the tube. Test conditions span FC-72 mass velocities of 50.3 – 360.3 kg/m2s, test section inlet pressures of 127.0 – 132.1 kPa, and test section inlet thermodynamic equilibrium qualities of 0.13 – 1.15. A subset of data gathered corresponding to axisymmetric, annular condensation heat transfer is identified and a detailed methodology for data reduction to calculate heat transfer coefficient presented. Uncertainty analysis is also presented and indicates channel average heat transfer coefficients are calculated within ±3.6% to ±26.7% (depending on operating conditions). Analysis of parametric trends for condensation heat transfer reveals the dominant influence of mass velocity (flow inertia), secondary influence of vapor mass fraction (thermodynamic equilibrium quality), and strong dependence on orientation (body force) at low mass velocities. At higher mass velocities results for all orientations investigated begin to converge, indicating body force independent annular condensation heat transfer is achieved. Separated Flow Model predictions of vertical downflow condensation heat transfer provide reasonable agreement with experimental results, evidence by a Mean Absolute Error (MAE) of 31.2%. Evaluation of condensation heat transfer correlations for horizontal flow reveal most correlations struggle for cases with high liquid content. Specific correlations are identified for superior accuracy in predicting the measured data.</div>

No / An extensive study of the fibre orientation structures developed in a transverse ribbed plate during injection moulding, and the use of these structures to investigate the effect of local fibre orientation state on crack initiation resistance, is reported. The fibre orientation results for the ribbed plate, measured using large area image analysis system developed at Leeds University, showed that after an initial settling down period, the central core region, where the fibres are aligned perpendicular to the flow direction, decreased in size monotonically, with an associated monotonic increase in the outer shell regions, where the fibres are aligned preferentially along the injection direction. Interestingly, the level of orientation in the two regions remained almost constant: only the proportions of the two regions were found to change with flow length. Across the plate, close to the gate, the central core region was found to have a lens-like shape, while at the other end of the plate the core was thinner and also consistent in thickness across the sample width. The transverse rib was found to cause little disturbance to the fibre orientation of the base plate. The different proportions of the shell and core regions at different locations over the ribbed plate provided an ideal case to test the proposition of Friedrich that the crack resistance of a short fibre reinforced material depends on the number of fibres that are perpendicular to the crack tip. The impact test results gathered in this way confirmed this hypothesis of Friedrich.

<p>This study comprises experimental investigations and theoretical/empirical analyses of three forced-convective (pumped) boiling schemes: (i) confined round single jet and jet array impingement boiling, and flow boiling through conventional-sized (ii) concentric circular annuli and (iii) rectangular channels. These schemes could be utilized in the thermal management of various applications including high-heat-flux electronic devices, power devices, electric vehicle charging cables, avionics, future space vehicles, etc.</p>