Les presses hydrauliques à compression sont utilisées afin de mettre en oeuvre des thermoplastiques. La maîtrise des champs thermiques à la surface des plateaux de presse est essentielle pour garantir une transformation de qualité des matériaux. La mise en forme des composites dits « haute performance » met en jeu d’importants transferts thermiques au niveau du plateau : montée en température jusqu’à 450 °C par résistances électriques, puis refroidissement par écoulement d’eau au sein de canaux cylindriques. L’apparition de phénomènes d’ébullition convective liés aux niveaux de température mis en jeu rend délicate la maîtrise des vitesses de refroidissement lors du procédé. L’objectif des travaux présentés dans ce manuscrit est d’étudier la phase de refroidissement à l’eau de plateaux de presse, dans des conditions représentatives du procédé industriel. Deux configurations expérimentales sont retenues. Dans un premier temps, l’étude se concentre sur l’analyse de l’écoulement diphasique eau-vapeur dans un canal horizontal. Une caméra rapide permet d’observer les régimes d’écoulements apparaissant dans un canal en quartz. Puis, l’évolution des champs thermiques le long d’un canal en acier inoxydable est menée, à l’aide d’une instrumentation thermique fine, dont notamment un capteur intrusif mobile spécialement conçu pour mesurer la température au coeur de l’écoulement. Dans un deuxième temps, l’étude est menée sur un élément de plateau de presse, système plus représentatif de la réalité industrielle. Plusieurs essais expérimentaux sont menés afin de quantifier l’influence du débit d’eau et de la température initiale sur la vitesse de refroidissement. Un modèle numérique vient en appui de l’analyse des résultats expérimentaux pour aider à la compréhension des phénomènes. Enfin, le montage expérimental est modifié, afin d’optimiser le refroidissement du plateau et respecter une vitesse de descente en surface de 25 °C.min-1 et ouvrir des perspectives en terme de régulation du procédé. / Hydraulic press are used to develop thermoplastic materials. The control of the thermal fields at the surface of the press platens are essential to ensure a good-quality transformation. The forming of “high performance” composites is realized within high temperature levels : heating to 450°C, with electrical cartridge ; cooling, with a water flow inside cylindrical channels. The incipience of flow boiling, due to high temperature, makes the control of cooling rate difficult during the process. The goal of the works presented here is to study the cooling process of the platen, in industrial conditions. Two experimental setups are tested. On the one hand, the study focuses on the analysis of the two-phase flow boiling. A high-speed camera enables the different flow regimes to be observed in a quartz channel. Then, the evolution of the thermal fields along a stainless steel channel is led, thanks to a fine thermal instrumentation, including an intrusive mobile sensor which has been especially made to measure the flow temperature. On the other hand, the study focuses on a single part of a platen, which is more representative of the industrial conditions. Several tests are led in order to quantify the influence of the water flowrate and the initial temperature level on the cooling rate. In parallel, a numerical model is developed to comprehensively understand the phenomenon. Finally, the experimental test bench is modified to improve the cooling of the platen and to control a 25°C.min-1 cooling rate, in the purpose of finding some perspectives concerning the process regulation.

Degradation of permafrost in peatlands can convert forested peat plateaus to inundated collapse bogs. Due to increased unfrozen soil carbon stocks and more saturated conditions, collapse bogs can potentially be large emitters of methane. Using a network of bubble traps permanently installed in peat, I tested several hypotheses about controls on ebullition in collapse bogs with varying time since thaw in interior Alaska. Ebullition increased during the growing season, likely due to increased substrate availability and warmer soils. Bubbles were found primarily in shallow peat layers, and were dominated by modern carbon. Ebullition hot spots were associated with high sedge density throughout the collapse sites. Episodic ebullition occurred during atmospheric pressure changes. Overall, my study demonstrated that permafrost thaw in peatlands will result in methane emissions through ebullition that include both young and old carbon, contradictory to the generally accepted paradigm; that ebullition in peatlands is solely a surface process.

Thesis (M.S.)--Rochester Institute of Technology, 1993. / Typescript. Includes bibliographical references (leaves 89-90).

Thesis (M.S.)--University of Nevada, Reno, 2008. / "December, 2008." Includes bibliographical references (leaves 54-57). Online version available on the World Wide Web.

Thesis (M.S.)--Rochester Institute of Technology, 1995. / Typescript. Includes bibliographical references (leaves 81-83).

Thesis (M.S.)--Rochester Institute of Technology, 1995. / Typescript. Bibliography: leaves 86-89.

<p> Entrapped gas (V g) greatly affects peatland biogeochemistry and hydrology by altering volumetric water content, buoyancy, hydraulic conductivity and generating overpressure zones. These over pressure zones affect hydraulic gradients which influence water and nutrient flow direction and rate. The loss of this entrapped gas to the atmosphere via ebullition (bubbling) has been proposed as the dominant transport mechanisms for CH4 from peatlands, releasing significant amounts of CH4 to the atmosphere in a single event. Atmospheric pressure has been linked to ebullition events and is known to affect gas volumes; similarly, temperature affects gas production and volume. This thesis investigates the relationship between these environmental factors (atmospheric pressure and temperature) on both V g and ebullition processes. </p> <p> An incubation experiment using six peat cores at three incubation temperatures ( 4 °C, 11 °C, 20°C) was conducted in 2004 where each core was incubated in a sealed PVC cylinder and instrumented to measure Vg, pore-water C~ concentrations, and ebullition (volume and C~concentrations). Temperature data for each incubation group and atmospheric pressure were measured within the laboratory setting. </p> <p> Increasing bulk density was associated with decreased frequency of ebullition events and higher average ebullition volumes, indicating a relationship between bulk density ebullition characteristics. Future work will be needed to identify the direct relationship between V g, bulk density and ebullition. </p> <p> Evaluation of ebullition and atmospheric pressure data revealed a strong relationship between periods of falling pressure and ebullition events where 71% of measured events (n = 391) occurred during periods of decreasing pressure. Investigation of falling pressure characteristics revealed that drop duration (days) had a more significant effect on total ebullition volumes than did magnitude (kPa). As such, long periods of decreasing pressure trigger greater gas releases via ebullition than short decreases of large magnitude. This has implications for the prediction and modelling of ebullition events in natural systems, and for the estimation of CH4 fluxes and carbon budgets of peatlands. </p> The V g variability model accounted for changes in V g caused by gas transfer between aqueous and gaseous phases (Henry's law) and thermal and pressure induced volume changes (Ideal gas law) using measured temperature and atmospheric pressure data. Gas loss via ebullition and CH4 production were also accounted for. Good agreement was found between measured and modeled V g values where gas contents were greater than 10% (average r2 value of 0.78). Accuracy of the model indicates a general understanding of the processes, however it also suggests that further factors are influencing internal gas dynamics that require further investigation. </p? / Thesis / Master of Science (MSc)

Freshwater ecosystems cover about 4 % of the Earth’s surface. Yet they are important components in the global C cycle, as they in addition to their within-lake primary production, receive and process organic matter loads originating from terrestrial environments. The boreal forest biome has the highest density of lakes globally. In general, lakes in the boreal forest biome are shallow with high proportions of littoral sediments and are considered methane hotspots in the landscape. The major pathway of methane (CH4) from lakes (depth ≤ 10 m) to the atmosphere is via CH4 ebullition (i.e., gas bubbles). Moreover, CH4 ebullition is highly irregular in space and time. While enhanced CH4 ebullition rates have been reported to coincide with temporal forces (e.g., pressure drop) and showed a spatial variability with higher CH4 ebullition rates at lake inlets, none of the present models can currently represent the variability of CH4 ebullition over space and time. To improve the understanding behind spatial drivers of CH4 ebullition, sediment characteristics in relation to CH4 ebullition were investigated in Lake Alsta, a shallow and eutrophic lake in Sweden. In-situ CH4 ebullition rates were analysed along with sediment TN, TOC, C:N ratio and potential CH4 production rates. Sediment TN could explain CH4 production (R20.39, p-value&lt;0.001), while the degree of explanation of CH4 ebullition rate was low yet significant (R2=0.14, p-value=0.03). However, the combination of fine sediments, together with high loads of aquatic- and terrestrial organic matter and nitrogen are likely spatial factors driving the high CH4 ebullition rates at both inlets in Lake Alsta, Sweden. The CH4 ebullition at its deepest point cannot be distinguished from the ebullition rates at the inlets, and the outlet has a significantly lower CH4 ebullition rate than all other sites. This highlights that the distribution of sediment and the quality of organic matter (i.e., C:N ratio) within the lake affects CH4 ebullition. In addition, including littoral vegetation into CH4 ebullition models and analysing sediment redox potentials might further give explanation to spatial differences.

An experimental investigation of the flow boiling of water at atmospheric pressure was undertaken, including a high—speed cine photographic study of the flow structure near the Critical Heat Flux (CHF). Experimental tests from single-phase forced convection to burnout were conducted at different flow velocities and inlet subcoolings for water flowing upwards through a vertical channel of rectangular cross—section electrically heated on one wall with a glass window forming the opposite wall. The test surfaces were stainless steel strips of constant dimensions, except that wall thickness ranged from 0.08 mm to 0.20 mm. Quantitative measurements of the bubble parameters for the same heating surface under the same operating conditions with varying levels of heat flux (70&percnt; to 95&percnt; of CHF) were carried out. A nucleation site deactivation/reactivation process was observed with increasing heat flux. A proposed site deactivation mechanism explained this behaviour. A nucleate boiling heat transfer model was proposed for the fully— developed nucleate boiling region, with allowance made for the overlapping areas of bubble influence. It compared favourably with the experimental data. The effect of wall thickness on CHF was investigated: increases in CHF as between the 0.08 mm and the 0.20 mm wall thickness ranging from 38&percnt; to 57&percnt; were observed. An empirical expression for CHF, including wall thickness as a parameter was developed, correlating the experimental data to within 15&percnt; and indicating a limiting value for wall thickness affecting CHF. The flow regimes near burnout were identified as bubbly and slug, these being independent of wall thickness. Other models proposed for the CHF mechanism were tested against the detailed experimental observations at high subcoolings. They were found to be inconsistent with the experimental evidence. A possible alternative for the CHF mechanism points towards stabilisation/ growth of a vapour patch following bubble coalescence as a most likely cause for burnout.

Gas Emissions from Contaminated Fibrous Sediments in Sweden The discharge of untreated wastewater from pulp and paper mills have resulted in the accumulation offibrous sediments on the bottom of many nearby aquatic recipients. Some accumulations are multiplemeters thick and consist almost entirely of cellulose fibre or wood chips; these are called fiberbanks.The often hypoxic conditions and high organic content in fiberbanks makes them favourable for methaneproducing microorganisms, and gas release by ebullition has been observed. CH4 has high globalwarming potential and this study therefore aims to investigate GHG emissions from Swedish fiberbanks.Since methanogenesis is influenced by temperature and organic content, the gas ebullition is expectedto vary with season and between fiberbanks. As such it was necessary to examine differences inebullition rate, bubble volume and bubble quantity between different fiberbanks and to test the influenceof temperature on ebullition. To achieve this, the gas ebullition from two fiberbanks with very differentcomposition (Väja and Sandviken), were investigated using optical ebullition sensors measuring thequantity and volume of released gas bubbles. The ebullition measurements were performed in laboratoryat room temperature (20oC) and with sediments in incubation (4 – 15oC). The results indicate differencesin both ebullition rate and mean bubble volume between these two fiberbanks, with only minordifferences in the quantity of bubbles released. In a period of stable ebullition over five consecutivedays, sediment from Väja released 83 – 90% larger volumes of gas per day, and also produced bubblesthat were on average 67 – 89 % larger in volume when compared to Sandviken. The incubationexperiments show that ebullition from both fiberbanks increases exponentially with temperature, at ratessimilar to those found in natural sediments (Väja Q10 3.9, Sandviken Q10 4.9). The rate of accelerationin ebullition from both sediments is very strong &gt;10oC, which is also similar to what has been observedin natural sediments. If estimating the combined GHG emissions from Swedish fiberbanks based on theresults from this study, it shows that fiberbanks could emit as much as 550 000 – 900 000 tonnes of CO2equivalents annually. That would correspond to 1.1 – 1.7% of the combined annual Swedish GHGemissions in 2018, and with fiberbank ebullition showing such a strong temperature dependence, thatestimate would grow rapidly when water temperatures increase with a warming climate / Utsläpp av orenat processvatten från svensk pappersindustri har resulterat i ansamlingar av fiberhaltigasediment på botten av närliggande vattendrag. På vissa platser bildar de fiberhaltiga sedimenten flerameter tjocka fiberbankar som består nästan uteslutande av cellulosafibrer eller träflis. Det högaorganiska innehållet i fiberbankarna resulterar ofta i syrefria förhållanden vilket gör dem gynnsammaför metanproducerande mikroorganismer, och frisläppning av gasbubblor har observerats. Metangasbidrar starkt till växthuseffekten och det här projektet utformades därför med huvudmålet att uppskattaväxthusgasutsläppen via ebullition från svenska fiberbankar. Eftersom metangasproduktionenförväntades variera beroende på temperatur och fiberbankskomposition, undersöktes skillnader igasutsläpp från två olika fiberbankar, gällande koncentrationen på utsläppt gas, mängd utsläppt gas,volym på bubblor, antal bubblor, samt hur gasutsläppen från fiberbankarna påverkades av temperatur.Undersökningen inkluderade sediment från två fiberbankar med väldigt olika sammansättning (Väja ochSandviken) och gasutsläppen studerades med hjälp av optiska sensorer i rumstemperatur och underinkubation vid temperaturer från 4 – 15oC. Data från undersökningarna användas sedan till att uppskattade årliga växthusgasutsläppen från svenska fiberbankar. Resultaten indikerar att det är stora skillnader imängden utsläppt gas och volymen på frisläppta bubblor mellan dessa två fiberbanksediment, menendast små skillnader i antalet frisläppta bubblor. Fiberbankssediment från Väja släppte ut en 83 - 90%större gasvolym per dag och producerade också i genomsnitt 67 - 89% större bubblor jämfört medsediment från Sandviken. Inkubationsexperimenten visar att gasutsläppen från de bådafiberbanksedimenten ökar exponentiellt med temperatur, och tilltar i liknande hastighet som hosnaturliga sediment (Väja Q10 3.9, Sandviken Q10 4.9). Ökningen i gasutsläpp vid temperaturer över 10oCär mycket stark hos båda sedimenten, vilket också liknar observationer från naturliga sediment. Närresultaten används för att uppskatta växthusgasutsläppen från den totala mängden fiberbanksedimentsom kan finnas i Sverige, visar de att fiberbankar kan släppa ut så mycket som 550 000 - 900 000 tonCO2-ekvivalenter årligen. Det skulle innebära 1.1 – 1.7% av de sammanlagda årliga svenskaväxthusgasutsläppen.