Ivar S. Ertesvåg

The Eddy Dissipation Turbulence Energy Cascade Model

Abstract The turbulence energy cascade model used in the Eddy Dissipation Concept for combusting flow is presented and discussed in relation to existing knowledge of relevant turbulent flows. The cascade consists of a stepwise model for energy transfer from larger to smaller scales and for energy dissipation from each scale level by viscous forces. The cascade model makes a connection between the viscous fine structures, where combustion takes place, and the larger transporting eddies which are simulated by turbulence models. Thus, fine-structure quantities are expressed in terms of turbulence energy and dissipation. The model is compared to turbulence-energy-spectrum data for the inertial subrange and the dissipative range for nonreacting and reacting flows. The model is also discussed in relation to isotropic decaying turbulence in the transition from initial to final periods of decay. It is concluded that the energy cascade model captures important features of the turbulence structural interaction and dissipation.

Society exergy analysis: a comparison of different societies

Data from exergy analyses for a number of different countries found in the literature are compared and the differences are discussed. In studies of Sweden, Ghana, Japan, Italy and Norway, the exergy in material flows had been considered, in addition to the flows of energy carriers. In other studies, the use of energy carriers was analyzed for the USA, Finland, Canada, Brazil, Turkey, the Organization for Economic Co-operation and Development (OECD) countries and the World. The exergy of material flows in these societies was estimated. The total annual exergy input per capita to the societies ranged over one order of magnitude. The total exergy efficiency varied from approximately 0.1 to 0.3, whereas the end-use exergy efficiency in general was slightly higher. It was found that different investigators had made somewhat different assumptions on exergy efficiencies in specific sectors, and these assumptions are discussed. However, the structure of the energy system appeared to be more important for the total exergy efficiency than the assumptions on the sectors. In particular, the residential–commercial sector represents major irreversibilities in many societies. In countries where electricity from thermal power plants has a significant contribution to the end use, this also caused large irreversibilities. Finally, the method of society exergy analysis is discussed. It is pointed out that, because of structural dissimilarities, different countries should be compared with care. However, the development within each society can be evaluated using exergy analyses. Furthermore, such analyses can be used as a means to increase the awareness of the notion of energy quality and degradation.

EXERGY ANALYSIS OF THE NORWEGIAN SOCIETY

The use and conversion of energy carriers and materials in the Norwegian society in 1995 were investigated by an exergy analysis. All resources that enter the society, such as waterfall energy, fossile fuels, ores and minerals, harvested crops, fishing and hunting, and wood, were included. However, exported oil and gas were excluded from the analysis. The end use was broken down into nine end-use sectors: forest industry, food sector, aluminum industry, steel and other metal industry, chemical industry, transportation, households, public service, and other industry. Each of these sectors was treated in further detail. The last three sectors were also regrouped into mechanical work, space heating, and lighting, equipment, etc. The total exergy input was 1184 PJ, which was 278 GJ per capita. The output of products and services contained 280 PJ exergy or 68 GJ per capita. This was 24% of the input. The household and public-service sectors had exergy efficiencies of approx. 10%, whereas the aluminium and forestry sectors had efficiencies of approx. 40% and 50%, respectively.

Exergy analysis of solid-oxide fuel-cell (SOFC) systems

The exergy concept has been used to analyze two methane-fueled SOFC systems. The systems include preheating of fuel and air, reforming of methane to hydrogen, and combustion of the remaining fuel in an afterburner. An iterative computer program using a sequential-modular approach was developed and used for the analyses. Simulation of an SOFC system with external reforming yielded first-law and second-law efficiencies of 58 and 56%, respectively, with 600% theoretical air. Heat released from the afterburner was used to reform methane, vaporize water, and preheat air and fuel. When these heat requirements were satisfied, the exhaust-gas temperature was so low that it could only be used for heating rooms or water. Because of heat requirements in the system, fuel utilization (FU) in the FC was limited to 75%. The remaining fuel was used for preheating and reforming. Reduced excess air led to reduced heat requirements and the possibility of a higher FU in the FC. Irreversibilities were also reduced and efficiencies increased. Recycling fuel and water vapor from the FC resulted in first-law and second-law efficiencies of 75.5 and 73%, respectively, with 600% theoretical air, vaporization of water was avoided and the FU was greater.

A NUMERICAL INVESTIGATION OF A LIFTED H2/N2 TURBULENT JET FLAME IN A VITIATED COFLOW

Numerical calculations of a lifted H2/N2 turbulent jet flame in a vitiated coflow of hot gases are presented. The calculations are performed using Magnussens Eddy Dissipation Concept (EDC) for turbulent combustion, and are an extension to previously reported EDC modeling results presented by Cabra et al. (2002). Four different turbulence models are employed to investigate in more detail the turbulence modeling effect on the EDC combustion model with detailed chemistry. A series of simulations are presented that indicate the extent to which turbulence models influence the predicted lift-off height with the EDC combustion model. Several flow conditions were tested. For all calculations, EDC predicts more lift-off by using the standard k-ϵ model than by using Reynolds-stress-equation (RSE) models, whereas a modified k-ϵ model predicts less lift-off than the RSE models. The reason for the lower predicted lift-off with the modified k-ϵ model is because a modified turbulence Prandtl or Schmidt number in the scalar equations in the modified k-ϵ model allows an earlier mixing of the hot coflow with the fuel jet. All models overpredict the lift-off height for the standard flow conditions. Recent experiments and numerical calculations by others have shown that the vitiated coflow flame is extremely sensitive to variations in the coflow temperature. The present calculations show that this sensitivity is captured by the EDC combustion model, however to a smaller degree than that previously reported. Calculations with variations in coflow temperature and jet flow velocity indicate that for each flow condition, the various turbulence models predict the same percentage increase or decrease in lift-off height. These EDC calculations show that the turbulence model effect on the EDC predicted lift-off height is important and that a better flame structure is predicted with the RSE model by Jones and Musonge than with the other turbulence models.

Influence of Turbulence Modeling on Predictions of Turbulent Combustion

Computations of an axisymmetric bluff-body stabilized turbulent diffusion flame are presented. The effects of turbulence modeling on turbulent combustion predictions are studied. The test case is simulated using κ-e and Reynolds-stress-equation turbulence models with and without extensions for low Reynolds numbers. Turbulent combustion is modeled by two different combustion models with fast chemistry. Effects of chemical kinetics are studied by including detailed chemistry in one combustion model. The combustion predictions are considerably affected by the choice of turbulence model. The nonpremixed flame is stabilized by a recirculation zone behind the bluff body. In isothermal, nonreacting flow, the predictions of the recirculation zone are quite similar for the four models. With combustion, a Reynolds-stress-equation closure predicts a significantly weaker recirculation compared with the κ-e results. This allows a larger spreading of the fuel and better mixing in the bluff-body wake. When finite-rate chemistry is introduced, the κ-e model predicts blow out, whereas the Reynolds-stress-equation model does not. This is due to the larger spreading and mixing by the latter model. The low-Reynolds-number extensions gave a much too strong recirculation, which reduced the spreading of the fuel jet.

Exergy Analysis of Gas-Turbine Combined Cycle With CO2 Capture Using Pre-Combustion Decarbonization of Natural Gas

A concept for natural-gas fired power plants with CO2 capture has been investigated using exergy analysis. The present approach involves decarbonization of the natural gas by authothermal reforming prior to combustion, producing a hydrogen-rich fuel. An important aspect of this type of process is the integration between the combined cycle and the reforming process. The net electric power production was 47.7% of the Lower Heating Value (LHV) or 45.8% of the chemical exergy of the supplied natural-gas. In addition, the chemical exergy of the captured CO2 and the compression of this CO2 to 80 bar represented 2.1% and 2.7%, respectively, of the natural-gas chemical exergy. For a corresponding conventional combined cycle without CO2 capture, the net electric power production was 58.4% of the LHV or 56.1% of the fuel chemical exergy. A detailed breakdown of irreversibility is presented. In the decarbonized natural-gas power plant, the effect of varying supplementary firing (SF) for reformer-feed preheating was investigated. This showed that SF increased the total irreversibility and decreased the net output of the plant. Next, the effects of increased gas-turbine inlet temperature and of gas-turbine pressure ratio were studied. For the conventional plant, higher pressure led to increased efficiency for some cases. In the decarbonized natural-gas process, however, higher pressure ratio led to higher irreversibility and reduced thermal-plant efficiency.Copyright

Large Eddy Simulation of Methane Diffusion Flame: Comparison of Chemical Kinetics Mechanisms

Large Eddy Simulations (LES) of the Sandia Flame D methane diffusion flame are carried out. An extended LES version of the Eddy Dissipation Concept (EDC) model is used as a turbulence combustion closure model. In LES the computational cost is high due to the fine grid resolution and unsteady nature of the governing equations. Using a detailed chemical mechanism in LES with EDC is too expensive, and hence, to reduce computational cost reduced mechanisms are preferred. In this study a number of PSR calculations for methane‐air combustion using two reduced methane mechanisms and one detailed methane mechanism are carried out. The reduced mechanisms tend to strongly overpredict the temperature and major species mass fraction in either the lean or the rich mixture zone. Both mechanisms overpredict temperature for lean mixtures. The two reduced methane mechanisms are also studied with LES for the Sandia Flame D. The results of the two reduced mechanisms are compared with the fast chemistry approach and it is ob...

Modeling Instabilities in Lean Premixed Turbulent Combustors Using Detailed Chemical Kinetics

The development of non-conventional combustion technology with ultra-low emissions and the safe operation of combustion systems require a thorough understanding of the mechanisms of combustion instabilities. The objective of the present work is to investigate the role of unmixedness and chemical kinetics in driving combustion instabilities. The reaction-rate responses of different species to inlet flow variations have been studied using a perfectly stirred reactor model. Transient simulations of combustion of methane and propane with air, using both global single-step and detailed chemical kinetic mechanisms, have been conducted with imposed oscillations on inflow mass flow rate, temperature, and mixture equivalence ratio. The detailed mechanisms predicted fuel reaction-rate oscillations with amplitudes proportional to the imposed oscillations. However, increased amplitudes of the reaction rates of CO2 and OH were observed when the combustion became leaner, while the reaction-rate amplitudes of CO and H2 decreased. The single-step mechanisms predicted to some degree a similar reaction-rate behavior as the detailed mechanisms. However, near stoichiometric conditions, the fuel reaction rate of propane showed little influence by the imposed oscillations. When the mean equivalence ratio was lowered below a certain value, the fuel reaction-rate oscillations grew stronger and became larger than those seen with the detailed mechanism. This shows that simple mechanisms can by themselves introduce instabilities not seen with detailed mechanisms.

Thermodynamic Performance Indicators for Offshore Oil and Gas Processing: Application to Four North Sea Facilities

Oil and gas extraction have been responsible for 25—28% of the total greenhouse gas emissions in Norway the last 10 years. The part from offshore oil and gas processing, including power production, flaring, and cold ventilation on production platforms, accounted for 20—22%. Exergy analysis is a method for systematic assessment of potential to perform work. It gives the possibility to identify where in a process inefficiencies occur: both losses to the surroundings and internal irreversibilities, and can be used as a tool for pinpointing improvement potential and for evaluation of industrial processes. When used in the petroleum sector, this can motivate more efficient oil and gas extraction, leading to a better utilisation of the resources and less greenhouse gas emissions.The objectives of this thesis were to: (i) establish exergy analyses of the oil and gas processing plants on different types of North Sea platforms; (ii) identify and discuss improvement potentials for each case, compare them and draw general conclusions if possible; and (iii) define meaningful thermodynamic performance parameters for evaluation of the platforms.Four real platforms (Platforms A—D) and one generic platform of the North Sea type were simulated with the process simulators Aspen HYSYS and Aspen Plus. The real platforms were simulated using process data provided by the oil companies. The generic platform was simulated based on literature data, with six different feed compositions (Cases 1—6). These five platforms presented different process conditions; they differed for instance by their exported products, gas-to-oil ratios, reservoir characteristics and recovery strategies.Exergy analyses were carried out, and it was shown that for the cases studied in this work, the power consumption was in the range of 5.5—30 MW, or 20—660 MJ/Sm3 o.e. exported. The heat demand was very small and covered by electric heating for two of the platforms, and higher, but low enough to be covered by waste heat recovery from the power turbines and by heat integration between process streams, for the other three platforms. The main part of the power was consumed by compressors in the gas treatment section for all cases, except Platform B and Case 4 of the generic model. Platform B had lower pressures in the products than in the feeds, resulting in a low compression demand. Case 4 of the generic model had a high content of heavy hydrocarbons in the feed, resulting in large power demand in the oil export pumping section. The recompression and oil pumping sections appeared to be the other major power consumers, together with the seawater injection system, if installed.The total exergy destruction was in the range of 12—32 MW, or 43—517 MJ/Sm3 o.e. exported. Most exergy destruction was related to pressure increase or decrease. Exergy destruction in the gas treatment section made up 8—57% of the total amount, destruction in the recompression section accounted for 11—29%, while 10—28% took place in the production manifolds. Exergy losses due to flaring varied in the range of 0—13 MW.Platforms with high gas-to-oil ratios and high pressures required in the gas product presented the highest power consumption and exergy destruction.Several measures were proposed for reduction of exergy destruction and losses. Two alternatives included use of mature technologies with potential to increase efficiency significantly: (i) limit flaring by installation of gas recovery systems, and (ii) improve gas compression performance by updating/exchanging the compressors.Several thermodynamic performance indicators were discussed, with Platforms A—D as case studies. None of the indicators could at the same time evaluate (i) utilisation of technical achievable potential, (ii) utilisation of theoretical achievable potential and (iii) total use of energy resources. It was concluded that a set of indicators had to be used to evaluate the thermodynamic performance. The following indicators were suggested: BAT efficiency on exergy basis, exergy efficiency, and specific exergy destruction.The formulation of exergy efficiency for offshore processing plants is difficult because of (i) the high throughput of chemical exergy, (ii) the large variety of chemical components in the process streams and (iii) the differences in operating conditions. Approaches found in the literature for similar processes were applied to Platforms A—D. These approaches had several drawbacks when applied to offshore processing plants; they showed low sensitivity to performance improvements, gave inconsistent results, or favoured platforms operating under certain conditions. A new exergy efficiency, called the component-by-component efficiency, was proposed. This efficiency could successfully evaluate the theoretical improvement potential.Eksergianalyse av offshore olje- og gassprosesseringOlje- og gassutvinning har vaert kilde til 25—28% av de totale klimagassutslippene i Norge de siste 10 arene. Den delen som stammer fra offshore olje- og gassprosessering (kraftproduksjon, fakling og kaldventilering pa produksjonsplattformer) stod for 20—22%. Eksergianalyse er en metode for systematisk bestemmelse av potensiale til a utfore arbeid. Det gir mulighet til a identifisere hvor i en prosess ineffektiviteter oppstar: bade i form av tap til omgivelsene og i form av interne irreversibiliteter. Det kan brukes som et verktoy for a finne forbedringsmuligheter og for evaluering av industrielle prosesser. Ved bruk innen petroleumssektoren kan dette motivere for mer effektiv olje- og gassutvinning, noe som gir bedre utnyttelse av ressursene og mindre utslipp av klimagasser.Formalet med denne avhandlingen er a: (i) etablere eksergianalyser av olje- og gassprosessering pa ulike typer Nordsjo-plattformer; (ii) identifisere og diskutere forbedringspotensialer for hvert tilfelle, sammenligne dem og trekke generelle konklusjoner om mulig; og (iii) definere meningsfulle termodynamiske ytelsesindikatorer for evaluering av plattformene.Fire virkelige plattformer (Plattform A—D) og en generisk Nordsjo-type plattform er simulert med prosessimulatorene Aspen HYSYS og Aspen Plus. De virkelige plattformene er simulert ved a bruke prosessdata stilt til radighet av operatorene av plattformene. Den generiske plattformen er simulert basert pa litteraturdata, med seks ulike fodesammensetninger (Case 1—6). Disse fem plattformene har ulike prosessbetingelser; de har for eksempel ulike eksporterte produkter, gass/olje-forhold, reservoaregenskaper og utvinningsstrategier.Eksergianalyser viser at for tilfellene studert i dette arbeidet er kraftforbruket i storrelsesorden 5,5—30 MW, eller 20—660 MJ/Sm3 o.e. eksportert. Varmebehovet er svaert lite og blir dekket med elektrisitet for to av plattformene, og noe hoyere men lavt nok til a bli dekket med varmegjenvinning fra kraftturbinene og ved varmeveksling mellom prosesstrommer for de tre andre plattformene. Hoveddelen av kraften blir konsumert av kompressorene i gassbehandlingsseksjonen for alle tilfellene bortsett fra Plattform B og Case 4 i den generiske modellen. Plattform B har lavere trykk i produktstrommene enn i fodestrommene, noe som resulterer i lavt behov for kompresjon. Case 4 i den generiske modellen har et hoyt innhold av tunge hydrokarboner i foden, noe som resulterer i hoyt kraftbehov i seksjonen for eksportpumping. Seksjonene for rekompresjon og eksportpumping viser seg a vaere de andre viktigste kraftforbrukerene, sammen med systemet for sjovannsinjeksjon hvis dette er installert.Den totale ekserginedbrytingen er 12—32 MW, eller 43—517 MJ/Sm3 o.e. eksportert. Mest ekserginedbryting er relatert til trykkoking eller trykkreduksjon. Ekserginedbryting i gassbehandlingsdelen utgjor 8—57% av den totale mengden, nedbryting i rekompresjonsseksjonen utgjor 11-29%, mens nedbryting i produksjonsmanifoldene utgjor 10—28%. Eksergitap pa grunn av fakling varierer mellom 0—13 MW.Plattformene med hoye gass/olje-forhold og behov for hoyt trykk i gassproduktene har hoyest kraftforbruk og ekserginedbryting.Ulike tiltak for reduksjon av ekserginedbryting og eksergitap er foreslatt. To alternativer inkluderer bruk av modne teknologier og har potensiale til a oke effektiviteten betydelig: (i) begrensning av fakling av gass ved installasjon av gassgjenvinningssystemer, og (ii) forbedring av gasskompresjonen ved a oppdatere/bytte ut kompressorer.Flere termodynamiske ytelsesindikatorer er diskutert med utgangspunkt i Plattform A—D. Ingen av indikatorene kan pa samme tid evaluere (i) utnyttelse av teknisk oppnaelig potensiale, (ii) utnyttelse av teoretisk potensiale og (iii) total bruk av energiressurser. Det konkluderes med at et sett med indikatorer ma brukes for a evaluere termodynamisk ytelse. De folgende indikatorene foreslas: BAT (best tilgjengelig teknologi) effektivitet pa eksergibasis, eksergieffektivitet og spesifikk ekserginedbryting.Formuleringen av eksergieffektivitet for offshore olje- og gassprosessering er utfordrende pa grunn av (i) den hoye gjennomgangen av kjemisk eksergi, (ii) den store variasjonen av kjemiske komponenter i prosesstrommene og (iii) de store forskjellene i driftsbetingelser. En ny type eksergieffektivitet foreslas. Denne effektiviteten kan evaluere utnyttelsen av det teoretiske potensialet pa tross av punktene nevnt ovenfor.