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Abstract Directed relation graph with error propagation (DRGEP) method combined with extensive validation for 0D, 1D and 2D CFD modeling supported by sensitivity and Rate-Of-Production (ROP) analyses are implemented for comparative study of detailed and reduced kinetic mechanisms for CH4 + H2 combustion. To this end, two detailed kinetic mechanisms, namely AramcoMech 2.0 and recently updated Konnov mechanism, were validated using available measurements of ignition delay times and laminar burning velocities for hydrogen, methane and hydrogen + methane fuel mixtures. For all experimental conditions visited, both detailed mechanisms demonstrated good and close to each other performance. Two-stage DRGEP method and reaction reduction based on computational singular perturbation (CSP) were then implemented to achieve two skeletal models: 25 species and 105 reactions for AramcoMech 2.0 and 27 species and 107 reactions for the Konnov model. The conditions for skeletal models generation cover ɸ = 0.5–2.0, temperature 900–2000 K, and pressure 1–50 bar. Turbulent non-premixed flames of CH4 + H2 in the Jet in Hot Co-flow (JHC) burner for two different oxygen concentrations in a co-flow were modeled using both skeletal models. 2-D RANS simulations with OpenFOAM code of the flame structure using the two skeletal kinetic mechanisms are similar except for the mass fraction of OH and CO. To elucidate the differences between two skeletal mechanisms generated using the same reduction method, extensive validation for 0D, 1D and 2D CFD modeling were supported by sensitivity analysis for detailed and skeletal reaction models. Good agreement between the skeletal and detailed mechanisms was found in top reactions as well as their sensitivity coefficients, which affect auto-ignition process and laminar flame propagation. Further chemical and sensitivity analysis of the structure of laminar flames demonstrate that three important reactions, i.e. CO + OH = CO2 + H, H2 + OH = H + H2O, and CH4 + OH = CH3 + H2O have different rate constants in the Aramco and Konnov models that may contribute to the differences in the prediction of CO concentration profiles. The simulation predictions for CO concentrations are improved for laminar flames and JHC flame by using a 25-species modified version in which these rate constants were taken from the Konnov mechanism. It was noted that DRGEP method applied to different detailed kinetic schemes generate skeletal models with different, non-overlapping lists of retained species. The presence of CH2CHO in the Aramco 25-species skeletal mechanism and its absence in the Konnov 27-species mechanism, and the presence of CH, CH2, CH2CO in the latter and their absence in the former mechanism were analysed and explained using Rate-Of-Production analysis for conditions found in the CFD simulations.

Abstract The thermoelastic response due to a time-dependent rectangular heat source in a semi-infinite medium is analyzed. The problem originates from studies of nuclear waste repositories in rock. Canisters containing heat-emitting nuclear waste are deposited over a large rectangular area deep below the ground surface. The solution for a time-dependent heat source is obtained from the corresponding instantaneous heat source by superposition. The thermoelastic problem for the instantaneous rectangular heat source in a infinite surrounding is solved exactly. An important step is the introduction of so-called quadrantal heat sources. The solution for the rectangle is obtained from four quadrantal solutions. The solution for the quadrantal heat source depends on the three dimelasionless coordinates only. Time occurs in the scale factors only. The condition of zero normal and shear stresses at the ground surface is fulfilled by using a mirror heat source and a boundary solution. The boundary solution accounts for the residual normal stress at the ground surface. Using a Hertzian potential, a surprisingly simple solution is obtained. The final analytical solution is quite tractable considering the complexity of the initial problem. The solution may be used to test numerical models for coupled thermoelastic processes. It may also be used in more detailed numerical simulations of the process near the heat sources as boundary conditions to account for the three-dimensional global process.

The current and future emissions regulations and demands on internal combustion engines have increased the need for improvement of the thermal design and engine ef ciency. This paper reports an initial investigation concerning in-cylinder heat transfer. In addition, a literature review in this eld is presented. It was found that no extensive application studies have been performed on full cylinder heat transfer by taking the effect of combustion and gas exchange into the account. The available literature is mainly focused on individual component studies and sub-model improvements. Furthermore, a simple parameter study is performed in order to estimate the contribution of each parameter on the in-cylinder engine heat transfer. This is done by setting up an engine segment, using the commercial CFD tool AVL FIRE and the automatic mesh generator AVL Ese Diesel. Engine segment simulations are made for the period between IVC (intake valve closing) and EVO (exhaust valve opening). The parameter study revealed that most of the parameters selected do in-fact signi cantly affect the in-cylinder heat transfer. However, the effects on the indicated mean effective pressure, or indicated power output, are different. (Less)

This paper describes the development of a novel modelling tool for evaluation of solid oxide fuel cell (SOFC) performance. An artificial neural network (ANN) is trained with a reduced amount of data generated by a validated cell model, and it is then capable of learning the generic functional relationship between inputs and outputs of the system. Once the network is trained, the ANN-driven simulator can predict different operational parameters of the SOFC (i.e. gas flows, operational voltages, current density, etc.) avoiding the detailed description of the fuel cell processes. The highly parallel connectivity within the ANN further reduces the computational time. In a real case, the necessary data for training the ANN simulator would be extracted from experiments. This simulator could be suitable for different applications in the fuel cell field, such as, the construction of performance maps and operating point optimisation and analysis. All this is performed with minimum time demand and good accuracy. This intelligent model together with the operational conditions may provide useful insight into SOFC operating characteristics and improved means of selecting operating conditions, reducing costs and the need for extensive experiments.

Abstract In most studies on technology change, the analysis of cost reductions of new energy technologies has been narrow and has often neglected essential processes related to the deployment of new technologies, such as photovoltaics (PV). However, in the case of distributed PV systems, other costs than for the PV modules – aka the deployment or balance-of-system costs – are significant. This review study identifies the long-term dynamics of “hard” and “soft” costs associated with the deployment of building-sited PV systems in Germany since the early 1990s. The results show that the costs for central hardware components such as inverters and mounting systems have decreased by 70–87% since the 1990s. Results also show that "soft deployment costs" such as planning and installation decreased by 65–85%, and the corresponding experience curve has a progress ratio of 88–90%. The results imply that both hard and soft deployment costs have decreased with cumulative experience. Generally speaking, deployment processes, and support for such processes, are essential for the assessment of the overall cost dynamics related to the implementation of new energy technologies such as PV.

This discussion highlights and defines key factors and organisational structures for the transformation of local agro-biomass based bioenery systems in Sweden and Denmark. Cross case study analysis is applied to 13 examples where straw is used for energy at various scales within a conceptual framework developed by the authors. This also delivers a cross-country comparison of systems and of the stakeholder entrepreneurs involved in straw-to-energy activities. The paper then delineates four different types of agro-biomass based frameworks for organisation and action. These include ‘small scale local heat production’, ‘medium scale local heat provision with excess for sale’, ‘medium scale conversion and district heating’, and ‘large scale power or combined heat and power generation’. Three categories of farm-based entrepreneurs (i.e. pluriactive, resource exploiting and portfolio farmer) emerge from case studies and are described in the paper. (Less)

Abstract In this paper, a multi-scale model is used to assess the multiple mineral precipitation potential in a full-scale anaerobic granular sludge system. Reactor behaviour is analysed under different operational conditions (addition/no addition of reject water from dewatering of lime-stabilized biomass) and periods of time (short/long term). Model predictions suggest that a higher contribution of reject water promotes the risk of intra-granule CaCO3 formation as a result of the increased quantity of calcium arriving with that stream combined with strong pH gradients within the biofilm. The distribution of these precipitates depends on: (i) reactor height; and (ii) granule size. The study also exposes the potential undesirable effects of the long-term addition of reject water (a decrease in energy recovery of 20% over a 100-day period), caused by loss in biomass activity (due to microbial displacement), and the reduced buffer capacity. This demonstrates how both short-term and long-term operational conditions may affect the formation of precipitates within anaerobic granules, and how it may influence methane production and consequently energy recovery.

In order to contribute to knowledge on how-to expand bioenergy this paper examines an example of success from Sweden. The idea is to identify factors that can explain the difference between success and failure of bioenergy systems. In the aftermath of the oil crises in the 1970s, the local government in the town of Enkoping in Sweden was cautioned by the local Swedish military regiment to shift towards domestic energy supplies rather than imported fossil fuels. During the decades that followed the local energy companies developed a pioneering bioenergy system. There are 3 important conditions relevant, to explain the success: (1) the introduction of the carbon tax in Sweden provided market conditions making bioenergy sufficiently competitive with fossil fuels; (2) the know-how developed by the local energy companies through experimentation and collaboration with research institutions encouraged investments in local opportunities; and (3), the formation of partnerships in a regional network of actors, including private companies providing biofuels and technology, research institutions, and local government, helped to co-ordinate the development of the bioenergy system. (Less)

Abstract Fuel injection is one of the most important processes in compression-ignition internal combustion engines owing to its significant impact on the exhaust emissions and thermal efficiency. In this study, experiments were carried out to investigate the influence of injection pressure and injection timing on the temporal evolution of the injection rate and injection duration in a specially designed experiment rig equipped with a common rail injection system. It is well known that the injection signal from the electronic control unit (ECU) of the injection system, which is often the only injection information available in engine operation and experiments, gives little information about the actual injection rate profile. It is shown in the present experiments that the actual injection duration is usually longer than the energizing time (ET). The time delay between the actual injection of the fuel and the ECU signal is about 0.3–0.4 ms, and the time delay appears to be insensitive to the injector geometry and injection pressure condition. The injection process can be characterized as five stages, a fast injector valve opening stage, a slow valve opening stage, a valve fully open stage, followed by a slow valve closing stage, and finally a rapid valve closing stage. It is found that the first stage, the fast valve opening stage, is insensitive to the injection pressure and injector nozzle diameter; however, the peak injection rate is a strong function of these parameters. The second and the third stage may not appear with a short injection duration. A new injection model was developed for the common rail injection system, which was capable of simulating the instantaneous fuel injection rate and injection duration for a range of injection pressure and injection duration. The model was shown to be able to replicate the experimental injection rate profile of the present experiments and experiments found in the literature for common rail injection system. The new injection model was applied to predict the effect of injection pressure and injection duration on the performance of a diesel engine under various engine speed and load conditions. The new injection model was shown to be able to describe the injection mass flow rate, which eventually leads to a reasonably good prediction of the variations of the spray development, in-cylinder pressure, heat release rate, and emissions.