Showing papers in "International Journal of Thermodynamics in 2005"

Exhaust Gas Recirculation in Gas Turbines for Reduction of CO2 Emissions; Combustion Testing with Focus on Stability and Emissions

Abstract: Exhaust gas recirculation can be applied with the intention of reducing CO2 emissions. When a fraction of the exhaust gas is injected in the entry of a gas turbine, the amount of CO2 in the exhaust gas not being recirculated will be higher and less complicated to capture. However, with this change in combustion air composition, especially the reduced concentration of oxygen, the combustion process will be affected. The lower oxygen concentration decreases the stability and the increased amount of CO2, H2O and N2 will decrease the combustion temperature and thus, the NOx emissions. Testing has been performed on a 65 kW gas turbine combustor, to investigate the effect of adding N2, CO2 and O2 in the combustion process, with focus on stability and emissions of NOx. Results show that adding N2 and CO2 decreases the NOx emissions, whereas O2 addition increases the NOx emissions. The tests have been performed both in a diffusion flame (pilot burner) and a premixed flame (main burner), and for additives being injected with the fuel or with the air stream. Addition into the fuel stream is proven to affect the NOx emissions the most. The stability limits of the flames are indicated with respect to mass-based additive-to-fuel ratios.

Exergetic Optimization of Solar Air Heaters and Comparison with Energy Analysis

Abstract: In this paper, an exergetic optimization of the solar air heater is developed. For this means, an integrated mathematical model of thermal and optical performance of the solar heater is derived. The most geometric parameters and operation conditions are considered as variables in this analysis. Some correlations for exergy efficiency of heater components are obtained. Then, exergy efficiency of the heater is derived by using these correlations. In the process of deriving an equation for the exergy efficiency, while the overall thermal loss coefficient and other heat transfer coefficients of the heater are assumed to be variable, the common error of using the Petela efficiency is corrected to reach the improved equation of solar radiation exergy. Finally, through the MATLAB toolbox the exergy efficiency equation is maximized. Then exergy efficiency is compared with the thermal efficiency of the heater, resulting in an extraordinary increase of the exergy efficiency according to the optimized parameters and benefit of this approach for such systems.

Kinetics of Evaporation: Statistical Rate Theory Approach

Abstract: Currently there are three theoretical approaches to study evaporation: Continuum Mechanics, Classical Kinetic Theory, and recently Statistical Rate Theory (SRT). The assumptions being used and the predictions resulting from the first two methods have not been supported by experimental results which are in agreement with the SRT predictions. It seems that SRT can predict the conditions existing at the interface during evaporation better than other methods. This paper reviews some of the published evaporation studies, particularly evaporation rate, thermocapillary convection, and temperature discontinuity at the interface during evaporation and compares the results of different approaches.

3-D Numerical Calculation of the Local Entropy Generation Rates in a Radial Compressor Stage

Abstract: The flow field in a low-ns radial compressor stage is computed by means of a LES simulation performed with a commercial CFD package. Radial and tangential secondary flows are identified both in the rotor and in the diffuser. In the latter, a regime of transient stall with a blockage region oscillating between the two sidewalls is observed. Rotor secondary flows influence the diffuser regime, and some of the diffuser instabilities seem to be phase-locked to some of the unsteady large flow structures detectable in the rotor. The entropy generation rate is computed locally, to assess if and where the design could be improved. Our results confirm that entropy generation maps (both in space and time) provide designers with detailed and unequivocal information about the causes of the intrinsic flow irreversibilities.

Heat, Mass and Charge Transport, and Chemical Reactions at Surfaces

Abstract: In this work we derive the excess entropy production rate for heat, mass and charge transport into, out of and across a surface, using as basic variables the excess densities proposed by Gibbs With the help of these variables we define the surface as an autonomous system (ie a surface in local equilibrium) and find its excess entropy production rate This then determines the conjugate fluxes and forces Equivalent forms of the entropy production rate are given The forms contain finite differences of intensive variables into and across the surface as driving forces The general form of the force-flux relations is given The expressions for the fluxes serve as boundary conditions for integration across heterogeneous systems Two examples are discussed in more detail The first example is the practically important coupled transport of heat and mass into and through a liquid-vapor surface The second example concerns phenomena at electrode surfaces: the coupled transport of heat, mass and charge and a chemical reaction By assuming that the two sides of the surface can be described as resistances in series, we are able to reduce the number of unknown transport coefficients considerably For both examples it is shown that the coupling coefficients for heat and mass flow are large at the surface, when the homogeneous phases have a large enthalpy difference As a consequence it is not sufficient to use, for instance, Fourier’s law for transport of heat across surfaces

Application of the Maximum Entropy Principle in the Analysis of a Non-Equilibrium Chemically Reacting Mixture

Abstract: The Maximum Entropy Principle has been used to model complex chemical reaction processes. The maximum entropy principle has been employed by the Rate-Controlled Constrained-Equilibrium (RCCE) method to determine concentration of different species during non-equilibrium combustion process. In this model, it is assumed that the system evolves through constrained equilibrium states where entropy of the mixture is maximized subject to constraints. Mixture composition is determined by integrating set of differential equations of constraints rather than integration of differential equations for species as is done with detailed kinetics techniques. Since the number of constraints is much smaller than the number of species present, the number of rate equations required to describe the time evolution of the system is considerably reduced. This method has been used to model the stoichiometric mixture of the formaldehyde-oxygen combustion process. In this study 29 species and 139 reactions has been used, while keeping the energy and volume of the system constant. Calculations have been done at different sets of pressures and temperatures, ranging from 1 atm to 100 atm, and from 900 K to 1500 K respectively. Three fixed elemental constraints: conservation of elemental carbon, elemental oxygen and elemental hydrogen and from one to six variable constraints were used. The four to nine rate equations for the constraint potentials (Lagrange multipliers conjugate to the constraints) were integrated and as expected, RCCE calculations gave correct equilibrium values in all cases. Only 8 constraints were required to give very good agreement with detailed calculations. Ignition delay times and major species concentrations were within 0.5% to 5% of the values predicted by detailed chemistry calculations. Adding more constraints improved the accuracy of the mole fractions of minor species at early times, but had only a little effect on the ignition delay times. Rate-Controlled Constrained-Equilibrium calculations reduced the computation time by 50% when using eight constraints.

Procedures for the Search of the Optimal Configuration of District Heating Networks

Abstract: This paper deals with the choice of the optimal configuration of the district heating network to be built in an urban area. The users to be connected with the network are determined so that an economic objective function is optimized. In this approach, the average unit cost of heat is considered as the function to be minimized. An alternative heating system is considered for the users not connected with the network. In this work, three different iterative procedures are presented. All these procedures start with an initial superstructure connecting the possible users. The initial structure is progressively simplified by disconnecting one user at each iteration. The three procedures differ in the algorithm for the network simplification: the first procedure is deterministic, while the others use probabilistic approaches derived from the simulated annealing technique. The procedures are applied to a small portion of the urban tissue of Turin and their effectiveness is compared.

Exergy and Structural Analysis of an Absorption Cooling Cycle and the Effect of Efficiency Parameters

Abstract: Absorption cycles are an alternative to compression cycles in cooling and refrigeration applications. Our analysis of an absorption cycle is based on the exergy and the structural analysis. Once the exergy analysis has been achieved, the coefficients of structural bonds (CSBs) of the main heat and mass exchangers can be determined by a structural analysis. The CSBs show how the modification of the irreversibility of one component, by means of a variation of its efficiency, affects the whole cycle. It will be wise to put much of the design effort in improving the efficiency of a component, knowing that a slight decrease of the irreversibility of that component, thanks to a higher efficiency, results in an important improvement in the total irreversibility of the cycle. This methodology is applied to a single effect ammonia-water absorption cooling cycle. We also study how the selection of efficiency parameters affects the results comparing CSBs of heat exchangers obtained from the minimum temperature differences or the UA-values. Results show that the UA is a more suitable parameter than the minimum temperature difference. Concerning the CSB values, we obtain very high values for the refrigerant heat exchanger. Values above one are also observed for the absorber, condenser and generator. Lower values are found for the generator and the solution heat exchanger. A more detailed analysis should investigate the dependence of the CSB values on the range of efficiencies. As a further step, these results could be used in the thermoeconomic analysis and economical optimization.

Influence of Regenerative Feed Water Heaters on the Operational Costs of Steam Power Plants and HP Plants

Abstract: The influence of particular regenerative feed-water heaters on the operational costs of a steam power plant and HP plant has been determined by means of the incremental energy efficiency expressing the ratio of the increase of electricity production to the increase of the consumption of chemical energy of fuel, assuming a constant flow rate of steam at the outlet of the turbine (power plant) or a given production rate of useful heat (HP plant). Exemplary calculations are included.

Effects of Immersed Surfaces on the Combustor Efficiency of Small-Scale Fluidized Beds

Abstract: In this study, effects of the different types of heat exchanger surfaces on the second law efficiency of a small-scale circulating fluidized bed (CFB) combustor are analyzed and the results are compared with the bubbling fluidized bed coal combustor effectiveness values. Using a previously developed simulation program, combustor efficiency and entropy generation values are obtained at different operation velocities at different height and volume ratios of the immersed surfaces, both for circulating and bubbling fluidized bed combustors. Besides that, the influence of the immersed surface types on the combustor efficiency was compared for different fluidized bed combustors. Through this analysis, the dimensions, arrangement and type of the immersed surfaces which achieve maximum efficiency are obtained.

Constructal Optimization of Top Contact Metallization of a Photovoltaic Solar Cell

Abstract: A top contact metallization of a photovoltaic solar cell collects the current generated by incident solar radiation. Several power-loss mechanisms are associated with the current flow through the front contact grid. The design of the top metal contact grid is one of the most important areas of efficient photovoltaic solar cell design. In this paper, an approach based on the constructal theory is proposed to design the grid pattern in a photovoltaic solar cell, minimizing total resistive losses. Constructal theory explains the geometric form (shape and structure) of most volume-to-point systems in nature. In this paper, the applicability of the constructal theory to design top contact metallization for a photovoltaic solar cell has been extended.

Investigation of the Start-up Strategy for a Solid Oxide Fuel Cell Based Auxiliary Power Unit under Transient Conditions

Abstract: A typical approach to the synthesis/design optimization of energy systems is to only use steady state operation and high efficiency (or low total life cycle cost) at full load as the basis for the synthesis/design. Transient operation as reflected by changes in power demand, shut-down, and start-up are left as secondary tasks to be solved by system and control engineers once the synthesis/design is fixed. However, start-up and shut-down may be events that happen quite often and, thus, may be quite important in the creative process of developing the system. This is especially true for small power units used in transportation applications or for domestic energy supplies, where the load demand changes frequently and peaks in load of short duration are common. The duration of start-up is, of course, a major factor which must be considered since rapid system response is an important factor in determining the feasibility of solid oxide fuel cell (SOFC) based auxiliary power units (APUs). Start-up and shut-down may also significantly affect the life span of the system due to thermal stresses on all system components. Therefore, a proper balance must be struck between a fast response and the costs of owning and operating the system so that start-up or any other transient process can be accomplished in as short a time as possible yet with a minimum in fuel consumption. In this research work we have been studying the effects of control laws and strategies and transients on system performance. The results presented in this paper are based on a set of transient models developed and implemented for the components of a 5 kWe net power SOFC based APU and for the high-fidelity system which results from their integration. The simulation results given below are for two different start-up approaches: one with steam recirculation and component pre-heating and the second without either. These start-up simulations were performed for fixed values of a number of system-level parameters (e.g., fuel utilization, steam-to-methane ratio, etc.) and were used to generate sufficient information to permit the development of appropriate control strategies for this critical operating point. These strategies are based on a balance between fuel consumption and response time. In addition, energy buffering hardware was added to the system configuration in order to minimize the effects of transients on fuel cell stack performance and lifetime.

An Innovative Solution for Suburban Railroad Transportation: The Gas Turbine-Hybrid Train

Abstract: The paper reports the latest results of a study conducted on a hybrid train in which a gas turbine, operating in several alternative control modes (fixed point, on-off or load-following), generates the electrical energy for recharging a battery package and for driving the electric motors of a suburban train. The model, originally developed for automotive applications, has been validated by experimental tests performed on an ELLIOTT TA-45 GT group in the ENEA-Casaccia Research Center. This paper describes the preliminary design of the traction system and the choice of the energy flow control strategy. Numerical simulations have been carried out, based on an actual train mission (the Norwegian railroad track between Asker and Lillehammer) and on industrial data for the single components. Following two different approaches, separate optimizations of the gas turbine set and battery package are performed, in which the objective function is the monetary cost per mission or, which is equivalent, the kWh/(t·km) for a given mission profile.

Stability of Transport and Rate Processes

Abstract: About fifty years ago, the Turing instability demonstrated that even simple reactiondiffusion systems might lead to spatial order and differentiation, while the Rayleigh-Benard instability showed that the maintenance of nonequilibrium might be the source of order in fluids subjected to a thermodynamic force above a critical value. Therefore, distance from global equilibrium in the form of magnitude of a thermodynamic force emerges as another constraint of stability; some systems may enhance perturbations, and evolve to highly organized states called the dissipative structures after a critical distance on the thermodynamic branch. Although the kinetics and transport coefficients represent shortrange interactions, chemical instabilities may lead to long-range order and coherent time behavior, such as a chemical clock, known as Hopf bifurcation. Stability analyses of linear and nonlinear modes for stationary homogeneous systems are useful in understanding the formation of organized structures. This review presents the stability of equilibrium and nonequilibrium systems of transport and rate processes with some case studies. It underlines the relationships between complex behavior and stability of systems using the classical and nonequilibrium thermodynamics approaches.

Technical Note: Zero Emissions Power Generation with CO2 Reduction by Fayalite

Abstract: In this paper a new ‘fuel’, fayalite Fe2SiO4, is proposed tentatively and a new concept involving the convergence of a power plant and its fuel source is described. The CO2 from the power plant would be injected underground where it would be reduced to methane. The methane would then serve as the ‘derived fuel’ of the power plant having been produced by the reaction of fayalite with the CO2. One of the possible chemical reactions of CO2 with rock is indicated, along with properties of the mineral fayalite. Calculations of the Gibbs free energy and enthalpy show that the reaction of carbon dioxide with fayalite is exothermic and might be self-sustaining.

Effect of Gas-Properties Evaluation Method on the Optimum Point of Gas Turbine Cycles

Abstract: Recent work has revealed that the assumption regarding the behavior of gases (perfect, ideal, real) and, consequently, the way their properties are evaluated may alter critically the picture obtained about the performance of gas turbine systems This fact prompted an investigation of how the aforementioned assumption may affect the optimal design point of gas turbine systems The present study is restricted to a comparison between the ideal and perfect gas assumption Three systems have been selected for study and three optimization problems have been formulated and solved for each system: two thermodynamic and one thermoeconomic The results demonstrate that the method used for the evaluation of properties of gases has a very significant effect on the optimal point of each system The article is published with the permission of the organizers of ECOS 2005, where it was first presented

An Unifying Thermodynamic Framework for Nonlinear Macrokinetics in Reaction-Diffusion Systems

Abstract: Two competing directions in elementary chemical or transport steps are analyzed from the viewpoint of their contribution to the overall rates. Systems with nonlinear transport phenomena and chemical reactions are described by the equations of nonlinear kinetics of the Marcelin-Kohnstamm–de Donder type that contain terms exponential with respect to the Planck potentials and temperature reciprocal. Simultaneously these equations are analytical expressions characterizing the transport of the substance or energy through the energy barrier. They constitute potential representations of a generalized law of mass action that includes the effect of transfer phenomena and external fields. Important are the physical consequences of these kinetics near and far from equilibrium. In these developments nonlinear symmetries and generalized affinity are important. The affinity picture - new for transport phenomena - and the traditional Onsagerian picture are shown to constitute two equivalent representations for kinetics of chemical reactions and transfer processes. Correspondence with the Onsager’s theory is shown closely to the thermodynamic equilibrium. Yet, it can be shown that rates of transport processes and chemical reactions far-from-equilibrium cannot be determined uniquely in terms of their affinities since these rates depend on all state coordinates of the system.

Generalized Gibbs Entropy, Irreversibility and Stationary States

Abstract: A generalization of the Gibbs entropy postulate is proposed, based on the BBGKY hierarchy as the non-equilibrium entropy for a system of N interacting particles. This entropy satisfies the basic principles of thermodynamics in the sense that it reaches its maximum at equilibrium and is coherent with the second law. By using this entropy and the methods of non-equilibrium thermodynamics in the phase space, a generalization of the Liouville equation describing the evolution of the distribution vector in the form of a master equation is obtained. After neglecting correlations in this master equation, the Boltzmann equation was obtained. Moreover, this entropy remains constant in nonequilibrium stationary states and leads to macroscopic hydrodynamics. Non-equilibrium Green-Kubo type relations and the probability for the non-equilibrium fluctuations are also derived.

Study of Interfacial Mass Transfer on Vapor Bubbles in Microgravity

Abstract: The knowledge of interfacial heat and mass transfer is important for environmental and technical applications, especially nowadays for numerical simulations of two phase problems. However, the data available up to now are inconsistent, because most experiments performed on earth suffer under buoyancy and convection, and thus the boundary conditions at the evaluation could not clearly be defined. Therefore, we seized the opportunity to investigate interfacial heat and mass transfer in microgravity environment. In these experiments the growth and collapse in the overall superheated and subcooled bubles, respectively, liquid or free vapor bubbles were observed at various liquid temperature and pressure states and over periods of from a few seconds up to 300 seconds. It was for the first time that such very long periods of bubble growth could be observed. The experimental set-up allowed the control of the liquid supersaturation before the bubbles were initiated by a short heat pulse at a miniaturized heater. Therefore it was possible to perform a systematic parametric study. The measured curves for vapor bubble growth are in good agreement with our numerical simulation. Based on this model the kinetic coefficients for the evaporation and condensation according to Hertz-Knudsen have been derived from the experimental data.

Review - Stability of Transport and Rate Processes

Abstract: About fifty years ago, the Turing instability demonstrated that even simple reaction-diffusion systems might lead to spatial order and differentiation, while the Rayleigh-Benard instability showed that the maintenance of nonequilibrium might be the source of order in fluids subjected to a thermodynamic force above a critical value. Therefore, distance from global equilibrium in the form of magnitude of a thermodynamic force emerges as another constraint of stability; some systems may enhance perturbations, and evolve to highly organized states called the dissipative structures after a critical distance on the thermodynamic branch. Although the kinetics and transport coefficients represent short-range interactions, chemical instabilities may lead to long-range order and coherent time behavior, such as a chemical clock, known as Hopf bifurcation. Stability analyses of linear and nonlinear modes for stationary homogeneous systems are useful in understanding the formation of organized structures. This review presents the stability of equilibrium and nonequilibrium systems of transport and rate processes with some case studies. It underlines the relationships between complex behavior and stability of systems using the classical and nonequilibrium thermodynamics approaches.