Cristobal Cortes - Academia.edu (original) (raw)
Papers by Cristobal Cortes
International Journal of Thermal Sciences, 2001
As a result of the quick and vast development of instrumentation and software capabilities, the o... more As a result of the quick and vast development of instrumentation and software capabilities, the optimization and control of complex energy systems can presently take advantage of highly sophisticated engineering techniques, such as CFD calculations and correlation algorithms based on artificial intelligence concepts. However, the most advanced numerical prediction still relies on strong simplifications of the exact transport equations. Likewise, the output of a neural network, or any other refined data-processing device, is actually based in a long record of observed past responses. Therefore, the implementation of modern diagnosis tools generally requires a great amount of experimental data, in order to achieve an adequate validation of the method. Consequently, a sort of paradox results, since the validation data cannot be less accurate or complete than the predictions sought. To remedy this situation, there are several alternatives. In opposition to laboratory work or well-instrumented pilot plants, the information obtained in the full scale installation offers the advantages of realism and low cost. This paper presents the case-study of a large, pulverized-coal fired utility boiler, discussing both the evaluation of customary measurements and the adoption of supplementary instruments. The generic outcome is that it is possible to significantly improve the knowledge on combustion and heat transfer performance within a reasonable cost. Based on the experience and results, a general methodology is outlined to cope with this kind of analysis. 2001
RESUMEN La implementación de modelos de monitorización del funcionamiento de calderas de potencia... more RESUMEN La implementación de modelos de monitorización del funcionamiento de calderas de potencia requiere de gran cantidad de datos para su validación, que además deben cumplir unos mínimos requi- sitos de precisión y completitud. La importancia de estos modelos es cada día mayor, pues se aplican en el control de procesos complejos que suceden en este tipo de instalaciones. Frente
Applied Thermal Engineering, 2005
Thermal modelling of large pulverized fuel utility boilers has reached a very remarkable developm... more Thermal modelling of large pulverized fuel utility boilers has reached a very remarkable development, through the application of CFD techniques and other advanced mathematical methods. However, due to the computational requirements, on-line monitoring and simulation tools still rely on lumped models and semiempirical approaches, which are often strongly simplified and not well connected with sound theoretical basis. This paper reviews on-line modelling techniques, aiming at the improvement of their capabilities, by means of the revision and modification of conventional lumped models and the integration of off-line CFD predictions. The paper illustrates the coherence of monitoring calculations as well as the validation of the on-line thermal simulator, starting from real operation data from a case-study unit. The outcome is that it is possible to significantly improve the accuracy of on-line calculations provided by conventional models, taking into account the singularities of large combustion systems and coupling offline CFD predictions for selected scenarios.
Fuel, 2008
A CFD investigation has been carried out about the performance of a 600 MW e tangentially coal-fi... more A CFD investigation has been carried out about the performance of a 600 MW e tangentially coal-fired boiler, focusing on the reduction of NO x attainable by using overfire air. To this purpose, a comprehensive combination of NO x chemistry models has been used, coupled with the numerical simulation of fluid and particle flow, solid fuel combustion and heat and mass transfer. Predicted values of gas temperature and species concentration have been adopted to validate the model against actual measurements from the full-scale boiler, under conventional and overfire air arrangements. A reasonable agreement has been attained in most cases. Additionally, modelling sensitivity has been evaluated against variations in some fuel-dependent parameters hard to measure or estimate (devolatilisation rates, nitrogen content in volatiles and char, reburning rates).
Applied Thermal Engineering, 2005
This technical paper addresses an elementary analytic procedure for the approximate solution of t... more This technical paper addresses an elementary analytic procedure for the approximate solution of the quasi-one-dimensional heat conduction equation (a generalized Bessel equation) that governs the temperature variation in annular fins of hyperbolic profile. This fin shape is of remarkable importance because its heat transfer performance is close to that of the annular fin of convex parabolic profile, the so-called optimal annular fin that is capable of delivering maximum heat transfer for a given volume of material [Zeitschrift des Vereines Deutscher Ingenieure 70 (1926) 885]. The salient feature of the analytic procedure developed here is that for realistic combinations of the two parameters: the enlarged Biot number and the normalized radii ratio, the truncated power series solutions embracing a moderate number of terms yields unprecedented results of excellent quality. The analytic results are conveniently presented in terms of the two primary quantities of interest in thermal design applications, namely the heat transfer rates and the tip temperature.
Applied Energy, 2010
In order to gain an extra increment of efficiency to compensate for capital costs, one of the mai... more In order to gain an extra increment of efficiency to compensate for capital costs, one of the main issues in the design of advanced supercritical power plants is the reduction of boiler exit gas temperature below typical values of conventional, subcritical units.
International Journal of Thermal Sciences, 2010
Thermal performance of micro-pin fins of variable diameter and rough surface is calculated by mea... more Thermal performance of micro-pin fins of variable diameter and rough surface is calculated by means of an approximate procedure based on truncated power series. The geometric effect of roughness is modeled after Bahrami et al. [9], which greatly increases the intricacy of the fin equation, preventing its analytical solution even by symbolic computer codes. The approximate series solution is developed firstly
Convective heat dissipation from standard metallic fins can be significantly enlarged by coating ... more Convective heat dissipation from standard metallic fins can be significantly enlarged by coating the fin with a thin layer made of a high thermal conductivity material. Calculation of the heat release from this kind of two-material fins involves a complex conjugate system of two conduction equations in two space variables. For a coated fin of tapered profile, aiming at its volume optimization, there is not analytical solution and only numerical methods are feasible. This article describes the application of a largely simplified approximate method for the estimation of the efficiency of coated fins of tapered profile. To assess the correctness of the approximate method, the computed efficiencies are compared against those supplied by the Finite Element Method.
International Journal of Thermal Sciences, 2003
This paper re-examines the venerable lumped model of unsteady heat conduction by means of a detai... more This paper re-examines the venerable lumped model of unsteady heat conduction by means of a detailed study of the exact temperature distributions in bodies of elementary geometry (i.e., large slab, long cylinder and sphere). The space-mean temperature is used as a vehicle for demonstrating that the lumped calculation directly follows as a particular case from the infinite series solution of the general distributed model. In this manner, several methods to find a limit Biot number can be established as simpler alternatives to the traditional procedure. Additionally, the discussion offers a different perspective of this classical subject of heat conduction theory, gaining more insight on the limiting behavior of unsteady temperature distributions.
International Journal of Heat and Mass Transfer, 2008
This paper discusses the thermal calculation of composite, metallic fins of variable thickness. I... more This paper discusses the thermal calculation of composite, metallic fins of variable thickness. In the simpler case of a constant-thickness (rectangular profile), the complete procedure involves first the analytical solution of the two-dimensional, two-material conduction problem, under the form of an infinite series of orthogonal eigenfunctions. Then the limit as Bi→0 is sought, also analytically, which simplifies the series to
Applied Thermal Engineering, 2009
A heat sink normally consists of a substrate on which a bundle of pin fins made of a highly condu... more A heat sink normally consists of a substrate on which a bundle of pin fins made of a highly conducting metal like aluminum or copper is attached. Its performance can be qualified by the space they occupy in relation to the thermal resistance they provide, which requires the solution of the quasi one-dimensional heat conduction equation in the geometry of interest. This technical brief reports exact and approximate analytical procedures for the truncated pin fin of hyperbolic profile. The exact solution involves rather intricate Bessel functions, whereas the approximation is based on easily computable power series. Compared to the exact figures, simple, truncated power series having between five and eight terms yield results of excellent quality for practical combinations of the two controlling parameters: the thermal length mL and the coordinate ratio W. Results are presented for the two variables of interest in thermal design: the fin tip temperature (a local quantity) and the fin thermal efficiency (a global quality ratio).
International Journal of Thermal Sciences, 2010
In this paper we investigate the heat transfer from pin-fins of infinite length made of a high th... more In this paper we investigate the heat transfer from pin-fins of infinite length made of a high thermal conductivity core with a low thermal conductivity coating. Considering a two-dimensional thermal field in the coating and a one-dimensional field in the core, an exact, analytical solution is presented for this geometry, previously not available in the literature. This solution is obtained
Chemical Engineering Science, 2005
This paper describes a systematic approach to predict ash deposits in coal-fired boilers by means... more This paper describes a systematic approach to predict ash deposits in coal-fired boilers by means of artificial neural networks. The approach is of a “grey box” nature, decomposing the problem into logical parts, and avoiding the use of sophisticated data. Although it is relative to the specific fuel and equipment, the prediction is very detailed and can be used on-line; it accounts not only for the deposition rate, but also for short-term cleaning occurrences, thus simulating a complex and chaotic time-evolution. The model is developed with the aid of a case-study, that of the fouling of a furnace, as detected by heat flux meters. Provided that an adequate amount of heat transfer measurements can be gathered, the procedure can be used to simulate the evolution of boiler heat absorption under realistic conditions of deposition. Applications include obviously new possibilities for automatic control of the equipment, as well as the optimization of operating set points to maximize thermal efficiency, such as the sequence and operation of on-load cleaning devices. It is thought that the method developed would be applicable to other instances of fouling or equipment degradation exhibiting similar behavior, specially with respect to on-line corrective measures.
International Journal of Thermal Sciences, 2001
As a result of the quick and vast development of instrumentation and software capabilities, the o... more As a result of the quick and vast development of instrumentation and software capabilities, the optimization and control of complex energy systems can presently take advantage of highly sophisticated engineering techniques, such as CFD calculations and correlation algorithms based on artificial intelligence concepts. However, the most advanced numerical prediction still relies on strong simplifications of the exact transport equations. Likewise, the output of a neural network, or any other refined data-processing device, is actually based in a long record of observed past responses. Therefore, the implementation of modern diagnosis tools generally requires a great amount of experimental data, in order to achieve an adequate validation of the method. Consequently, a sort of paradox results, since the validation data cannot be less accurate or complete than the predictions sought. To remedy this situation, there are several alternatives. In opposition to laboratory work or well-instrumented pilot plants, the information obtained in the full scale installation offers the advantages of realism and low cost. This paper presents the case-study of a large, pulverized-coal fired utility boiler, discussing both the evaluation of customary measurements and the adoption of supplementary instruments. The generic outcome is that it is possible to significantly improve the knowledge on combustion and heat transfer performance within a reasonable cost. Based on the experience and results, a general methodology is outlined to cope with this kind of analysis. 2001
RESUMEN La implementación de modelos de monitorización del funcionamiento de calderas de potencia... more RESUMEN La implementación de modelos de monitorización del funcionamiento de calderas de potencia requiere de gran cantidad de datos para su validación, que además deben cumplir unos mínimos requi- sitos de precisión y completitud. La importancia de estos modelos es cada día mayor, pues se aplican en el control de procesos complejos que suceden en este tipo de instalaciones. Frente
Applied Thermal Engineering, 2005
Thermal modelling of large pulverized fuel utility boilers has reached a very remarkable developm... more Thermal modelling of large pulverized fuel utility boilers has reached a very remarkable development, through the application of CFD techniques and other advanced mathematical methods. However, due to the computational requirements, on-line monitoring and simulation tools still rely on lumped models and semiempirical approaches, which are often strongly simplified and not well connected with sound theoretical basis. This paper reviews on-line modelling techniques, aiming at the improvement of their capabilities, by means of the revision and modification of conventional lumped models and the integration of off-line CFD predictions. The paper illustrates the coherence of monitoring calculations as well as the validation of the on-line thermal simulator, starting from real operation data from a case-study unit. The outcome is that it is possible to significantly improve the accuracy of on-line calculations provided by conventional models, taking into account the singularities of large combustion systems and coupling offline CFD predictions for selected scenarios.
Fuel, 2008
A CFD investigation has been carried out about the performance of a 600 MW e tangentially coal-fi... more A CFD investigation has been carried out about the performance of a 600 MW e tangentially coal-fired boiler, focusing on the reduction of NO x attainable by using overfire air. To this purpose, a comprehensive combination of NO x chemistry models has been used, coupled with the numerical simulation of fluid and particle flow, solid fuel combustion and heat and mass transfer. Predicted values of gas temperature and species concentration have been adopted to validate the model against actual measurements from the full-scale boiler, under conventional and overfire air arrangements. A reasonable agreement has been attained in most cases. Additionally, modelling sensitivity has been evaluated against variations in some fuel-dependent parameters hard to measure or estimate (devolatilisation rates, nitrogen content in volatiles and char, reburning rates).
Applied Thermal Engineering, 2005
This technical paper addresses an elementary analytic procedure for the approximate solution of t... more This technical paper addresses an elementary analytic procedure for the approximate solution of the quasi-one-dimensional heat conduction equation (a generalized Bessel equation) that governs the temperature variation in annular fins of hyperbolic profile. This fin shape is of remarkable importance because its heat transfer performance is close to that of the annular fin of convex parabolic profile, the so-called optimal annular fin that is capable of delivering maximum heat transfer for a given volume of material [Zeitschrift des Vereines Deutscher Ingenieure 70 (1926) 885]. The salient feature of the analytic procedure developed here is that for realistic combinations of the two parameters: the enlarged Biot number and the normalized radii ratio, the truncated power series solutions embracing a moderate number of terms yields unprecedented results of excellent quality. The analytic results are conveniently presented in terms of the two primary quantities of interest in thermal design applications, namely the heat transfer rates and the tip temperature.
Applied Energy, 2010
In order to gain an extra increment of efficiency to compensate for capital costs, one of the mai... more In order to gain an extra increment of efficiency to compensate for capital costs, one of the main issues in the design of advanced supercritical power plants is the reduction of boiler exit gas temperature below typical values of conventional, subcritical units.
International Journal of Thermal Sciences, 2010
Thermal performance of micro-pin fins of variable diameter and rough surface is calculated by mea... more Thermal performance of micro-pin fins of variable diameter and rough surface is calculated by means of an approximate procedure based on truncated power series. The geometric effect of roughness is modeled after Bahrami et al. [9], which greatly increases the intricacy of the fin equation, preventing its analytical solution even by symbolic computer codes. The approximate series solution is developed firstly
Convective heat dissipation from standard metallic fins can be significantly enlarged by coating ... more Convective heat dissipation from standard metallic fins can be significantly enlarged by coating the fin with a thin layer made of a high thermal conductivity material. Calculation of the heat release from this kind of two-material fins involves a complex conjugate system of two conduction equations in two space variables. For a coated fin of tapered profile, aiming at its volume optimization, there is not analytical solution and only numerical methods are feasible. This article describes the application of a largely simplified approximate method for the estimation of the efficiency of coated fins of tapered profile. To assess the correctness of the approximate method, the computed efficiencies are compared against those supplied by the Finite Element Method.
International Journal of Thermal Sciences, 2003
This paper re-examines the venerable lumped model of unsteady heat conduction by means of a detai... more This paper re-examines the venerable lumped model of unsteady heat conduction by means of a detailed study of the exact temperature distributions in bodies of elementary geometry (i.e., large slab, long cylinder and sphere). The space-mean temperature is used as a vehicle for demonstrating that the lumped calculation directly follows as a particular case from the infinite series solution of the general distributed model. In this manner, several methods to find a limit Biot number can be established as simpler alternatives to the traditional procedure. Additionally, the discussion offers a different perspective of this classical subject of heat conduction theory, gaining more insight on the limiting behavior of unsteady temperature distributions.
International Journal of Heat and Mass Transfer, 2008
This paper discusses the thermal calculation of composite, metallic fins of variable thickness. I... more This paper discusses the thermal calculation of composite, metallic fins of variable thickness. In the simpler case of a constant-thickness (rectangular profile), the complete procedure involves first the analytical solution of the two-dimensional, two-material conduction problem, under the form of an infinite series of orthogonal eigenfunctions. Then the limit as Bi→0 is sought, also analytically, which simplifies the series to
Applied Thermal Engineering, 2009
A heat sink normally consists of a substrate on which a bundle of pin fins made of a highly condu... more A heat sink normally consists of a substrate on which a bundle of pin fins made of a highly conducting metal like aluminum or copper is attached. Its performance can be qualified by the space they occupy in relation to the thermal resistance they provide, which requires the solution of the quasi one-dimensional heat conduction equation in the geometry of interest. This technical brief reports exact and approximate analytical procedures for the truncated pin fin of hyperbolic profile. The exact solution involves rather intricate Bessel functions, whereas the approximation is based on easily computable power series. Compared to the exact figures, simple, truncated power series having between five and eight terms yield results of excellent quality for practical combinations of the two controlling parameters: the thermal length mL and the coordinate ratio W. Results are presented for the two variables of interest in thermal design: the fin tip temperature (a local quantity) and the fin thermal efficiency (a global quality ratio).
International Journal of Thermal Sciences, 2010
In this paper we investigate the heat transfer from pin-fins of infinite length made of a high th... more In this paper we investigate the heat transfer from pin-fins of infinite length made of a high thermal conductivity core with a low thermal conductivity coating. Considering a two-dimensional thermal field in the coating and a one-dimensional field in the core, an exact, analytical solution is presented for this geometry, previously not available in the literature. This solution is obtained
Chemical Engineering Science, 2005
This paper describes a systematic approach to predict ash deposits in coal-fired boilers by means... more This paper describes a systematic approach to predict ash deposits in coal-fired boilers by means of artificial neural networks. The approach is of a “grey box” nature, decomposing the problem into logical parts, and avoiding the use of sophisticated data. Although it is relative to the specific fuel and equipment, the prediction is very detailed and can be used on-line; it accounts not only for the deposition rate, but also for short-term cleaning occurrences, thus simulating a complex and chaotic time-evolution. The model is developed with the aid of a case-study, that of the fouling of a furnace, as detected by heat flux meters. Provided that an adequate amount of heat transfer measurements can be gathered, the procedure can be used to simulate the evolution of boiler heat absorption under realistic conditions of deposition. Applications include obviously new possibilities for automatic control of the equipment, as well as the optimization of operating set points to maximize thermal efficiency, such as the sequence and operation of on-load cleaning devices. It is thought that the method developed would be applicable to other instances of fouling or equipment degradation exhibiting similar behavior, specially with respect to on-line corrective measures.