Critical components in coal-fired power plants such as final superheater heat exchangers experience severe conditions associated with high metal temperatures and high temperature gradients during base load and transient operations. Such adverse conditions could significantly reduce the life span of the components, especially due to the requirement of greater plant flexibility that is an essential part of the global power system transformation. Integrated thermofluid process models can be employed to obtain a better understanding of the relationship between the operational conditions and the metal temperatures. Thus, a methodology was developed to model radiant superheater heat exchangers in steady state and transient operations. The methodology is based on a network approach which entails solving the transient one-dimensional forms of the conservation equations for mass, energy and momentum. The model building blocks account for the convective thermal resistance on the steam side, the conductive thermal resistances of the tube wall and scaling or fouling on the tube walls, as well as the convective and radiative thermal resistances and direct radiation on the flue gas side. The model captures the physical layout of the tube passes in a tubesheet via the arrangement of the network building blocks. It is also possible to connect tubesheets together across the width of the boiler as per the arrangement in a real plant. The modelling methodology was first used to develop a process model of a convective cross-flow primary superheater heat exchanger with complex flow arrangement. The dual-tube 12-pass superheater was discretized along the flue gas flow path as well as along the steam flow path. The model was qualitatively validated using real plant data from literature and for reference purposes also systematically compared to conventional lumped parameter models. The ability of the model to analyse the effect of ramp rate during load changes on the tube metal temperature was demonstrated, as well as the ability to determine the maldistribution of flow and temperature on the steam and flue gas sides. The methodology was also applied to model a U-shaped radiant superheater heat exchanger. Due to the challenges associated with obtaining comprehensive real plant data in an industrial setting, a validation methodology was proposed that is based on a combination of plant design C-schedules and a boiler mass and energy balance, as well as limited plant measurements. The consistent comparisons with C-schedule data provide evidence of the validity of the model, which was further demonstrated via the comparisons with real plant data. The model allows prediction of the steam mass flow and temperature distribution going into the outlet stub headers as well as the main outlet headers for different inlet flow and temperature distributions on the steam and flue gas sides. These results were compared to detail real-plant measurements of the outlet header temperatures. The model also allows prediction of the metal temperatures along the length of the tubes which cannot readily be measured in the plant. The model was applied to demonstrate the impact of different operational conditions on the tube metal temperatures. Such integrated process models can be employed to study complex thermofluid process phenomena that may occur during intermittent, transient and low load operation of power plants. In addition, such models could be useful for predictive and preventative maintenance as well as online condition monitoring.

Accurate modeling of radiative heat transfer through combustion gases has received considerable attention in recent years. The spectral line weighted-sum-of-gray-gases (SLW) model was developed based on detailed line-by-line spectral data of gases. A critical element of the SLW model is the absorption line blackbody distribution function (ALBDF). This function was designed to utilize the spectral properties of gases in an efficient and compact manner. However, there are several limitations of the ALBDF in its original form. First, the valid ranges of temperature and pressure are not large enough to include important applications, such as oxy-combustion, where temperatures can exceed 2500 K, and pressurized combustion, where non-atmospheric pressures are expected. In addition, since the original ALBDF correlation was developed, new spectral data have become available which extend the accuracy of the previous work. Finally, it is desirable to be able to represent the ALBDF of CO in addition to H2O and CO2. Improving the SLW model in this manner will make it more generally applicable and ensure greater confidence in its accuracy. Line-by-line absorption cross-section data were generated carefully using a recently released spectroscopic database, HITEMP 2010. The Voigt line profile was implemented, and line wings were included in regions where they maintain a significant contribution. Line-by-line calculation of the ALBDF, total emissivity, and radiative transfer were also performed in order to provide benchmark data and to explore the influence of variable total pressure. It was found that increasing total pressure causes the ALBDF to shift to lower values at a given absorption cross-section, although this change is weaker at increasing temperature. Total emissivity is strongly affected by total pressure changes, although the change is modest if the product of partial pressure and path length is held constant. Increasing total pressure in a layer of gas increases the radiative flux exiting the gas layer; this was also found to be true for both the case of constant layer length and constant mass of radiating material. Efficient representations of the ALBDF were generated. The hyperbolic tangent correlation of Denison and Webb was updated to reflect improved spectroscopic data and to cover a wider range of temperature (400 K = T = 3000 K) and pressure (0.1 atm = p = 50 atm). The correlation was also extended to CO, which had not been correlated previously. Using tabulated line-by-line data directly was also explored, and these data have been made available for H2O, CO2, and CO. Finally, these efficient representations of the ALBDF were successfully validated by comparison with line-by-line calculations and experimental data for both total emissivity and radiative transfer. The latter included comparisons with intensity measurements and a comprehensive combustion simulation implementing the SLW model.

Les effets des transferts radiatifs sur les écoulements de convection naturelle sont étudiés en régimes transitionnel et turbulent. On considère des mélanges air/H2O/CO2 confinés dans des cavités cubiques différentiellement chauffées. Des simulations numériques de référence sont entreprises jusqu'à Ra=3x108 en couplant une méthode spectrale de collocation pour l'écoulement et une méthode de lancer de rayons, associée à un modèle ADF, pour le rayonnement. Pour l'étude du régime turbulent, une modélisation des transferts radiatifs basée sur un filtrage spatial est proposée : les contributions filtrées sont résolues par la méthode de lancer de rayons sur un maillage lâche et les contributions de sous-maille sont résolues de manière analytique dans l'espace de Fourier. Ce modèle est combiné à la simulation numérique directe de l'écoulement à Ra=3x109. Les transferts radiatifs ont pour effet de diminuer la stratification thermique verticale et d’augmenter la circulation générale. Lorsque les six parois de la cavité sont noires et le gaz transparent, deux zones de stratification thermique instable apparaissent en amont des couches limites verticales. Dès Ra=5x106, une instabilité de type Rayleigh-Bénard se développe dans ces zones, induisant des écoulements instationnaires. Lorsque les parois adiabatiques sont parfaitement réfléchissantes, les parois isothermes noires et le gaz rayonnant, des écoulements instationnaires chaotiques sont obtenus à partir de Ra=3x107. Des rouleaux contra-rotatifs à la sortie des couches limites verticales sont observés, ce qui suggère qu'une instabilité de force centrifuge soit responsable de la transition. / Radiative transfer effects on natural convection flows are investigated in transitional and turbulent regimes. Air/H2O/CO2 mixtures contained in cubical differentially heated cavities are considered. Benchmark numerical simulations are carried out up to Ra=3x108 by coupling a spectral collocation method for the flow and a ray tracing method, associated with an ADF model, for radiation. In order to study the turbulent regime, a radiative transfer model based on spatial filtering is proposed: filtered contributions are solved with the ray tracing method on a coarse grid and sub-grid contributions are obtained analytically in Fourier space. This model is combined with the direct numerical simulation of the flow at Ra=3x109. The effects of radiative transfer are a decrease of the vertical thermal stratification and an increase of the flow driven in the cavity. When the six cavity walls are black and the gas is transparent, two unstably stratified zones appear upstream the vertical boundary layers. From Ra=5x106, a Rayleigh-Bénard type instability in these zones triggers the unsteadiness. When the adiabatic walls are perfectly reflecting, the isothermal walls are black and the gas is participating, unsteady chaotic flows are obtained in this case from Ra=3x107. Counter rotating rolls at the exit of the vertical boundary layers are observed, which suggests that transition to unsteadiness is due to centrifugal forces.

No description available.

Methane and oxygen are a promising propellant combination in future rocket propulsion engines mainly due to its advantages like reusability and cost reduction. In order to have a comprehensive understanding of this propellant combination extensive research work is being done. Especially, for reusable rocket engines the thermal calculations become vital as an effective and efficient cooling system is crucial for extending the engine life. The design of cooling channels may significantly be influenced by radiation. Within the framework of this thesis, the gas radiation heat transfer is modelled for CFD simulation of rocket thrust chambers and analysed for the 𝐶𝐻4/𝑂2 fuel combination. The radiation is modelled within ArianeGroup’s in-house spray combustion CFD tool - Rocflam3, which is used to carry out the simulations. Radiation properties can have strong influence for certain chemical compositions, especially 𝐶𝑂2 and 𝐻2𝑂 which are the products of the 𝐶𝐻4 and 𝑂2 combustion. A simplified gas radiation transport equation is implemented along with various spectral models which compute the gas emissivity for higher temperature. Also, Rocflam-II code which has an existing gas radiation model is used to compare and validate the simplified model. Finally the combination of the convective and radiative heat transfer values are compared to the experimental test data. In contrast to the previously existing emissivity models with a certain temperature limit, the model used here enables the inclusion for the total emissivity of 𝐶𝑂2 and 𝐻2𝑂 for temperatures up to 3400 K and hence more appropriate for hydrocarbon combustion in space propulsion systems. It turns out that the gas radiation is responsible for 2-4% of the total heat flux for a 𝐶𝐻4/𝑂2 combustion chamber with maximum integrated temperature of 2700 K. The influence of gas radiation would be greater than 4% respective of the integrated temperature. Gas radiation heat flux effects are higher in stream-tube combustion zone compared to the other sections of the thrust chamber. The individual contribution of radiative heat flux by 𝐶𝑂2 was noted to be 1.5-2 times higher than that to 𝐻2𝑂. It was shown that the analytically derived simplified expression for gas radiation along with the various spectral models had reasonable approximation of the measured radiation. The estimated radiation was correct to the measured radiation from the Rocflam-II model for a temperature range of 400-3400 K. / Metan och syre är en lovande kombination av drivmedel i framtida raketframdrivningsmotorer främst på grund av dess fördelar som återanvändbarhet och kostnadsminskning. För att få en omfattande förståelse av denna drivmedelkombination görs ett omfattande forskningsarbete. Speciellt för återanvändbara raketmotorer blir värmeberäkningarna viktiga eftersom ett effektivt och effektivt kylsystem är avgörande för att förlänga livslängden på motorn. Utformningen av kylkanaler kan betydligt påverkas av strålning. Inom ramen för denna avhandling modelleras gasstrålningsvärmeöverföringen för CFD-simulering av rakettryckkamrar och analyseras för 𝐶𝐻4/𝑂2 -bränslekombinationen. Strålningen är modellerad i ArianeGroup’s egen förbränning CFD-verktyg - Rocflam3, som används för att utföra simuleringarna. Strålningsegenskaper kan ha starkt inflytande för vissa kemiska kompositioner, särskilt 𝐶𝑂2 och 𝐻2𝑂 som är produkterna från förbränningen 𝐶𝐻4 och 𝑂2. En förenklad gasstrålningstransportekvation implementeras tillsammans med olika spektralmodeller som beräknar gasemissiviteten för högre temperatur. Dessutom används Rocflam-II-kod som har en befintlig gasstrålningsmodell för att jämföra och validera den förenklade modellen. Slutligen jämförs kombinationen av konvektiva och strålningsvärmeöverföringsvärden med de experimentella testdata. Till skillnad från de tidigare existerande utsläppsmodellerna med en viss temperaturgräns möjliggör modellen som används här att inkludera den totala emissiviteten för 𝐶𝑂2 och 𝐻2𝑂 för temperaturer upp till 3400 K och därmed mer lämplig för kolväteförbränning i rymdframdrivningssystem. Det visar sig att gasstrålningen svarar för 2-4% av det totala värmeflödet för en 𝐶𝐻4/𝑂2 förbränningskammare med maximal integrerad temperatur på 2700 K. Påverkan av gasstrålning skulle vara större än 4% av den integrerade temperaturen. Effekter på värmeströmning av gasstrålning är högre i strömrörs förbränningszon jämfört med de andra sektionerna av tryckkammaren. Det individuella bidraget från strålningsvärmeflöde med 𝐶𝑂2 noterades vara 1.5-2 gånger högre än det 𝐻2𝑂. Det visades att det analytiskt härledda förenklade uttrycket för gasstrålning tillsammans med de olika spektralmodellerna hade en rimlig tillnärmning av det uppmätta strålning. Den uppskattade strålningen var korrekt den uppmätta strålningen från Rocflam-II-modellen för ett temperaturintervall på 400-3400 K.

Methane and oxygen are a promising propellant combination in future rocket propulsion engines mainly due to its advantages like reusability and cost reduction. In order to have a comprehensive understanding of this propellant combination extensive research work is being done. Especially, for reusable rocket engines the thermal calculations become vital as an effective and efficient cooling system is crucial for extending the engine life. The design of cooling channels may significantly be influenced by radiation. Within the framework of this thesis, the gas radiation heat transfer is modelled for CFD simulation of rocket thrust chambers and analysed for the 𝐶𝐻4/𝑂2 fuel combination. The radiation is modelled within ArianeGroup’s in-house spray combustion CFD tool - Rocflam3, which is used to carry out the simulations. Radiation properties can have strong influence for certain chemical compositions, especially 𝐶𝑂2 and 𝐻2𝑂 which are the products of the 𝐶𝐻4 and 𝑂2 combustion. A simplified gas radiation transport equation is implemented along with various spectral models which compute the gas emissivity for higher temperature. Also, Rocflam-II code which has an existing gas radiation model is used to compare and validate the simplified model. Finally the combination of the convective and radiative heat transfer values are compared to the experimental test data. In contrast to the previously existing emissivity models with a certain temperature limit, the model used here enables the inclusion for the total emissivity of 𝐶𝑂2 and 𝐻2𝑂 for temperatures up to 3400 K and hence more appropriate for hydrocarbon combustion in space propulsion systems. It turns out that the gas radiation is responsible for 2-4% of the total heat flux for a 𝐶𝐻4/𝑂2 combustion chamber with maximum integrated temperature of 2700 K. The influence of gas radiation would be greater than 4% respective of the integrated temperature. Gas radiation heat flux effects are higher in stream-tube combustion zone compared to the other sections of the thrust chamber. The individual contribution of radiative heat flux by 𝐶𝑂2 was noted to be 1.5-2 times higher than that to 𝐻2𝑂. It was shown that the analytically derived simplified expression for gas radiation along with the various spectral models had reasonable approximation of the measured radiation. The estimated radiation was correct to the measured radiation from the Rocflam-II model for a temperature range of 400-3400 K. / Metan och syre är en lovande kombination av drivmedel i framtida raketframdrivningsmotorer främst på grund av dess fördelar som återanvändbarhet och kostnadsminskning. För att få en omfattande förståelse av denna drivmedelkombination görs ett omfattande forskningsarbete. Speciellt för återanvändbara raketmotorer blir värmeberäkningarna viktiga eftersom ett effektivt och effektivt kylsystem är avgörande för att förlänga livslängden på motorn. Utformningen av kylkanaler kan betydligt påverkas av strålning. Inom ramen för denna avhandling modelleras gasstrålningsvärmeöverföringen för CFD-simulering av rakettryckkamrar och analyseras för 𝐶𝐻4/𝑂2 -bränslekombinationen. Strålningen är modellerad i ArianeGroup’s egen förbränning CFD-verktyg - Rocflam3, som används för att utföra simuleringarna. Strålningsegenskaper kan ha starkt inflytande för vissa kemiska kompositioner, särskilt 𝐶𝑂2 och 𝐻2𝑂 som är produkterna från förbränningen 𝐶𝐻4 och 𝑂2. En förenklad gasstrålningstransportekvation implementeras tillsammans med olika spektralmodeller som beräknar gasemissiviteten för högre temperatur. Dessutom används Rocflam-II-kod som har en befintlig gasstrålningsmodell för att jämföra och validera den förenklade modellen. Slutligen jämförs kombinationen av konvektiva och strålningsvärmeöverföringsvärden med de experimentella testdata. Till skillnad från de tidigare existerande utsläppsmodellerna med en viss temperaturgräns möjliggör modellen som används här att inkludera den totala emissiviteten för 𝐶𝑂2 och 𝐻2𝑂 för temperaturer upp till 3400 K och därmed mer lämplig för kolväteförbränning i rymdframdrivningssystem. Det visar sig att gasstrålningen svarar för 2-4% av det totala värmeflödet för en 𝐶𝐻4/𝑂2 förbränningskammare med maximal integrerad temperatur på 2700 K. Påverkan av gasstrålning skulle vara större än 4% av den integrerade temperaturen. Effekter på värmeströmning av gasstrålning är högre i strömrörs förbränningszon jämfört med de andra sektionerna av tryckkammaren. Det individuella bidraget från strålningsvärmeflöde med 𝐶𝑂2 noterades vara 1.5-2 gånger högre än det 𝐻2𝑂. Det visades att det analytiskt härledda förenklade uttrycket för gasstrålning tillsammans med de olika spektralmodellerna hade en rimlig tillnärmning av det uppmätta strålning. Den uppskattade strålningen var korrekt den uppmätta strålningen från Rocflam-II-modellen för ett temperaturintervall på 400-3400 K.

This work thoroughly analyzes a previously created computational model for predicting characteristic properties of the combustion process in an experimental combustion chamber. Any found shortcomings of the original model are removed and the model is further improved prior to its application on 11 real cases of combustion tests performed at various conditions and with various fuels. Data provided by the model are confronted with the data obtained during the combustion tests and the model accuracy is evaluated, based on local heat flux along the length of the combustion chamber. Finally, the overall usefulness of the model is determined by the means of evaluating the acquired accuracy values, and further possibilities of model improvement and use are presented.