Theoretical and experimental study on the autoignition phenomena of homogeneous reactive mixtures

The main objective of this Thesis is the study of the autoignition phenomenon of reactive mixtures from a theoretical and experimental point of view. A wide parametric study has been carried out in a Rapid Compression-Expansion Machine (RCEM) for different initial temperatures, compression ratios, equivalence ratios and molar fractions of oxygen (by using synthetic EGR) for different fuels. The ignition delay referred to cool flames (if it can be identified), as well as the ignition delay referred to the high-temperature stage of the ignition, have been experimentally obtained and their trends have been explained regarding the chemical kinetics of each fuel. The different effects of the species that compose the synthetic EGR on the ignition delay have been studied, decoupling the thermodynamic effects from the chemical ones. Different compositions have been taken into account to generate the synthetic EGR, and validation limits have been obtained for each mixture. The thermodynamic and the chemical effects have shown to be opposed, while the dominant one is different depending on the working temperature. Several chemical kinetic mechanisms have been validated by comparison to the experimental results. A detailed mechanism for iso-octane and n-heptane blends and a reduced mechanisms for n-dodecane have been analyzed. Moreover, a sub-model for the generation and decay of excited OH* has been validated by comparison to chemiluminescence and spectroscopy results. The different radiation sources have been studied for iso-octane and n-heptane by means of spectroscopy techniques. Besides, chemiluminescence measurements filtered at 310nm (OH* emission wavelength) have been performed in order to analyze the generalization and propagation velocity of the autoignition front. The ignition propagation has shown to depend on the thermodynamic conditions reached in the combustion chamber when the first ignition spot occurs and not on the global reactivity of the mixture. Furthermore, two different radiation sources have been found at 310nm in the spectroscopic analysis depending on the ignition intensity: the decay of the OH* radical from excited to ground state and the oxidation of CO to CO2 (CO continuum). However, these optical techniques have been applied only in the experiments carried out with iso-octane and n-heptane due to technical limitations. Finally, a new predictive model has been theoretically developed starting from the Glassman's model for autoignition. This method is based on modeling the accumulation rate of chain carriers up to reach their critical concentration (obtaining the ignition delay referred to cool flames) and, afterwards, modeling the disappearance rate of such chain carriers up to their consumption (when the maximum heat release rate is reached, obtaining the ignition delay referred to the high-temperature stage of the process). The predictive capability of the model has been compared to the ability of other methods that can be found in the literature, such as the Livengood & Wu integral method. The validity of each method has been tested, defining a working methodology to obtain reasonable predictions for the ignition delay.El objetivo de esta Tesis Doctoral es el estudio del fenómeno de autoencendido de mezclas reactivas desde un punto de vista teórico y experimental. Se ha realizado un amplio estudio paramétrico en una Máquina de Compresión-Expansión Rápida (RCEM por sus siglas en inglés) barriendo diversas temperaturas iniciales, relaciones de compresión, dosados relativos y fracciones molares de oxígeno (mediante el uso de EGR sintético) para distintos combustibles. El tiempo de retraso del fenómeno de llamas frías (en el caso de existir), así como el tiempo de retraso de la etapa de alta temperatura, han sido obtenidos experimentalmente y sus tendencias explicadas mediante cinética química. Se han estudiado los diferentes efectos de las distintas especies involucradas en el EGR sintético sobre el tiempo de retraso, desligando aquellos de carácter termodinámico de los efectos puramente químicos. Se han tenido en cuenta distintas composiciones para definir dicho EGR, estableciendo límites de validez para cada una de las mezclas propuestas. Los efectos termodinámicos y químicos resultaron ser opuestos, siendo dominante uno u otro a distintos rangos de temperatura de trabajo. Varios mecanismos de cinética química han sido validados gracias a los resultados experimentales obtenidos. Además de un mecanismo detallado para mezclas PRF de iso-octano y n-heptano, se ha llevado a cabo la validación de otro mecanismo simplificado para el n-dodecano. Por otro lado, un submodelo de formación y decaimiento de OH* excitado ha sido validado contra resultados de quimioluminiscencia y espectroscopía. Se han estudiado las diferentes fuentes de radiación del proceso de autoencendido para el iso-octano y el n-heptano mediante técnicas de espectroscopía. Además, se han realizado medidas de quimioluminiscencia filtrada a 310nm (longitud de onda de emisión del radical OH*) para el análisis de la generalización y velocidad de propagación del frente de autoencendido. La propagación del encendido ha mostrado ser dependiente de las condiciones termodinámicas alcanzadas en la cámara de combustión en el instante de ignición más que de la reactividad de la mezcla. Se han encontrado dos fuentes de radiación distintas a 310nm mediante espectroscopía, dependiendo de la intensidad del encendido: el decaimiento del radical OH* de estado excitado a estado natural y la oxidación del CO a CO2 (continuo del CO). No obstante, estas técnicas han sido utilizadas solamente para los dos combustibles de referencia de la escala de octanaje debido a limitaciones técnicas. Finalmente, se ha desarrollado un nuevo modelo predictivo de manera teórica partiendo del modelo de Glassman para el autoencendido. Este método se basa en modelar primero la tasa de acumulación de portadores de cadena hasta su concentración crítica (obteniendo así el tiempo de retraso referido a la etapa de llamas frías) y, tras dicho instante, modelar la tasa de consumo de dichos portadores de cadena hasta su completa desaparición (instante en el cual se produce la máxima exotermia del proceso, prediciendo el tiempo de retraso referido a la etapa de alta temperatura del encendido). La capacidad predictiva del modelo ha sido comprobada para cada uno de los seis combustibles ensayados. Además, dicha capacidad predictiva ha sido comparada con la de otros métodos existentes en la literatura, como la integral de Livengood & Wu. La validez de cada uno de los métodos ha sido analizada, definiendo una metodología de uso para obtener predicciones razonables del tiempo de retraso.L'objectiu d'aquesta Tesi Doctoral és l'estudi del fenomen d'autoencesa de mescles reactives des d'un punt de vista teòric i experimental. S'ha realitzat un ampli estudi paramètric en una Màquina de Compressió-Expansió Ràpida (RCEM per les seues sigles en anglès) cobrint diverses temperatures inicials, relacions de compressió, dosatges relatius i fraccions molars d'oxigen (mitjançant l'ús de EGR sintètic) per a diferents combustibles. El temps de retard del fenomen de flames fredes (en el cas d'existir), així com el temps de retard de l'etapa d'alta temperatura, han sigut obtinguts experimentalment i les seues tendències explicades mitjançant cinètica química. S'han estudiat els diferents efectes de les diferents espècies involucrades en l'EGR sintètic sobre el temps de retard, deslligant aquells de caràcter termodinàmic dels efectes purament químics. S'han tingut en compte diferents composicions per a definir aquest EGR, establint límits de validesa per a cadascuna de les mescles proposades. Els efectes termodinàmics i químics van resultar ser oposats, sent dominant un o un altre a diferents rangs de temperatura de treball. Diversos mecanismes de cinètica química han sigut validats gràcies als resultats experimentals obtinguts. A més d'un mecanisme detallat per a mescles PRF d'iso-octà i n-heptà, s'ha dut a terme la validació d'un altre mecanisme simplificat per al n-dodecà. D'altra banda, un submodel de formació i decaïment d'OH* excitat ha sigut validat contra resultats de quimioluminescència i espectroscopía. S'han estudiat les diferents fonts de radiació del procés d'autoencesa per a l'iso-octà i l'n-heptà mitjançant tècniques d'espectroscopía. A més, s'han realitzat mesures de quimioluminescència filtrada a 310nm (longitud d'ona d'emissió del radical OH*) per a l'anàlisi de la generalització i velocitat de propagació del front d'autoencesa. La propagació de l'encesa ha mostrat ser depenent de les condicions termodinàmiques aconseguides en la cambra de combustió en l'instant d'ignició més que de la reactivitat de la mescla. S'han trobat dues fonts de radiació diferents a 310nm mitjançant espectroscopía, depenent de la intensitat de l'encesa: el decaïment del radical OH* d'estat excitat a estat natural i l'oxidació del CO a CO2 (continu del CO). No obstant açò, aquestes tècniques han sigut utilitzades solament per als dos combustibles de referència de l'escala de octanaje a causa de limitacions tècniques. Finalment, s'ha desenvolupat un nou model predictiu de manera teòrica partint del model de Glassman per a l'autoencesa. Aquest mètode es basa a modelar primer la taxa d'acumulació de portadors de cadena fins a la seua concentració crítica (obtenint així el temps de retard referit a l'etapa de flames fredes) i, després d'aquest instant, modelar la taxa de consum d'aquests portadors de cadena fins a la seua completa desaparició (instant en el qual es produeix la màxima exotermia del procés, predient el temps de retard referit a l'etapa d'alta temperatura de l'encesa). La capacitat predictiva del model ha sigut comprovada per a cadascun dels sis combustibles assajats. A més, aquesta capacitat predictiva ha sigut comparada amb la d'altres mètodes existents en la literatura, com la integral de Livengood & Wu. La validesa de cadascun dels mètodes ha sigut analitzada, definint una metodologia d'ús per a obtenir prediccions raonables del temps de retard.López Pintor, D. (2017). Theoretical and experimental study on the autoignition phenomena of homogeneous reactive mixtures [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/90642TESI

Correlations for the ignition characteristics of six different fuels and their application to predict ignition delays under transient thermodynami conditions

[EN] The ignition characteristics of six different fuels have been correlated as a function of the temperature, pressure, equivalence ratio and oxygen molar fraction in this investigation. More specifically, the ignition delay referred to cool flames, the high-temperature ignition delay and the critical concentrations and ignition times of HO2 and CH2O have been parameterized for n-dodecane, PRFO, PRF25, PRF50, PRF75 and PRF100. To do so, a wide database of ignition data of the aforementioned fuels has been generated by means of chemical simulations in CHEMKIN, solving a detailed mechanism for PRF mixtures and a reduced mechanism for n-dodecane. In fact, in cylinder engine-like conditions reached in a Rapid Compression Expansion Machine (RCEM) have been replicated. The mathematical correlations have shown a relative deviation around 20% with the database in the low-temperature, low-pressure zone, which is the typical accuracy of usual correlations for the ignition delay. Finally, the ignition delay under transient conditions measured in the RCEM has been predicted by means of different integral methods coupled to both the proposed correlations and the generated database. It has been found that deviations between the predictions obtained with the correlations or with the database are lower than 1%. This means that the correlations are accurate enough to predict the ignition time in spite of showing high deviation with the database, since the low-temperature, low-pressure zone has a minor contribution to the ignition delay.The authors would like to thank different members of the CMT-Motores Termicos team of the Universitat Politecnica de Valencia for their contribution to this work. The authors would also like to thank the Spanish Ministry of Education for financing the PhD. Studies of Dario Lopez-Pintor (grant FPU13/02329). This research has been partially funded by FEDER and the Spanish Government through project TRA2015-67136-R.Desantes, J.; Bermúdez, V.; López, JJ.; López-Pintor, D. (2017). Correlations for the ignition characteristics of six different fuels and their application to predict ignition delays under transient thermodynami conditions. Energy Conversion and Management. 152:124-135. https://doi.org/10.1016/j.enconman.2017.09.030S12413515

Sensitivity analysis and validation of a predictive procedure for high and low-temperature ignition delays under engine conditions for n-dodecane using a Rapid Compression-Expansion Machine

[EN] A predictive procedure for cool flames and high-temperature ignition delays based on the accumulation and consumption of chain carriers has been validated for n-dodecane under engine conditions. To do so, an experimental parametric study has been carried out in a Rapid Compression-Expansion Machine, measuring the ignition times for different compression ratios (14 and 19), initial temperatures (from 403 K to 463 K), O-2 molar fractions (from 0.21 to 0.16) and equivalence ratios (from 0.4 to 0.7). The measured ignition delays have been compared to results from chemical kinetic simulations performed in CHEMKIN using a 0-D reactor that replicates the experimental conditions by solving five different chemical kinetic mechanisms, as a way to evaluate the mechanisms accuracy and variability. In general, all chemical kinetic mechanisms are able to accurately replicate the experimental ignition delays, being the mean relative deviation lower than 1.9% and 1.6% for both ignition stages, cool flames and the high-temperature ignition respectively. Furthermore, small differences have been appreciated between mechanisms in terms of ignition delay. Then, the predictive method has been applied using different databases obtained from each mechanism and a sensitivity analysis has been performed in order to evaluate the effects of the selected database on the predicted ignition delay. It has been found that while cool flames seems to be independent on the selected mechanism, the predicted high-temperature ignition delay is very sensitive to the species selected as chain carrier. Thus, if formaldehyde is assumed as ignition tracer, the predicted ignition time can vary up to 3%, while this percent decreases up to 1.3% when hydrogen peroxide takes the role of chain carrier. (C) 2017 Elsevier Ltd. All rights reserved.The authors are grateful to the Generalitat Valenciana for the financial support to acquire the RCEM (references PPC/2013/011 and FEDER Operativo 2007/2013 F07010203PCI00CIMETUPV001). Finally, the authors would like to thank the Spanish Ministry of Education for financing the PhD. Studies of Darío López-Pintor (grant FPU13/02329).Desantes, J.; Bermúdez, V.; López, JJ.; López-Pintor, D. (2017). Sensitivity analysis and validation of a predictive procedure for high and low-temperature ignition delays under engine conditions for n-dodecane using a Rapid Compression-Expansion Machine. Energy Conversion and Management. 145:64-81. https://doi.org/10.1016/j.enconman.2017.04.092S648114

A phenomenological explanation of the autoignition propagation under HCCI conditions

[EN] A phenomenological explanation about the autoignition propagation under HCCI conditions is developed in this paper. To do so, diffusive effects from the burned zones to the fresh mixture, pressure waves based effects and expansion effects caused by combustion are taken into account. Additionally, different Damkohler numbers have been defined and evaluated in order to characterize the phenomenon and quantify the relevance of each effect. The theoretical explanation has been evaluated by means of chemiluminescence measurements performed in a Rapid Compression Expansion Machine (RCEM), which allow to estimate the velocity of propagation of the autoignition front. The results showed that under HCCI conditions the autoignition propagation is controlled, in general, by the pressure waves established in the combustion chamber, since the characteristic time of the autoignition propagation is too short to assume the absence of pressure gradients in the chamber. Thus, the thermodynamic conditions reached behind the pressure wave promote the autoignition and explain the high propagation velocities associated to the reaction front. Besides, the results also showed that the contribution of diffusive phenomena on the propagation is negligible, since the characteristic time of diffusion is too long compared to the characteristic time of the autoignition propagation. Finally, the experimental measurements showed that the autoignition propagation is affected by a really relevant cycle-to-cycle variation. The turbulence generated by the combustion has, by definition, an aleatory behavior, leading to random heterogeneity distribution and, therefore, to somewhat random autoignition propagation.The authors would like to thank different members of the CMT-Motores TTrmicos team of the Universitat Politecnica de Valencia for their contribution to this work. The authors would also like to thank the Spanish Ministry of Education for financing the PhD. Studies of Dario Lopez-Pintor (grant FPU13/02329). This research has been partially funded by FEDER and the Spanish Government through project TRA2015-67136-R.Desantes, J.; López, JJ.; García-Oliver, JM.; López-Pintor, D. (2017). A phenomenological explanation of the autoignition propagation under HCCI conditions. Fuel. 206:43-57. https://doi.org/10.1016/j.fuel.2017.05.075S435720

Experimental validation of an alternative method to predict high and low-temperature ignition delays under transient thermodynamic conditions for PRF mixtures using a Rapid Compression-Expansion Machine

An alternative procedure to predict both high-temperature stage and cool flames ignition delays under transient thermodynamic conditions is intended to be validated in this paper. An experimental study has been carried out in a Rapid Compression-Expansion Machine (RCEM), using different iso-octane/nheptane blends in order to cover a wide range of octane numbers (from 25 to 75) under a wide range of initial temperatures (from 363 K to 423 K), compression ratios (14 and 19), O2 molar rates (from 21% to 16%) and equivalence ratios (from 0.4 to 0.8). The results obtained have been used to validate direct chemical kinetic simulations, as well as to evaluate the alternative predictive method and the Livengood & Wu integral method. Simulations have been performed solving a detailed chemical kinetic mechanism in CHEMKIN. The experimental results show good agreement with the chemical kinetic simulations and with the alternative predictive method. In fact, the mean relative deviation between experiments and simulations is equal to 1.7%, 2.2% and 3.1% for PRF25, PRF50 and PRF75, respectively. Besides, the alternative method has shown good predictive capability not only for the high-temperature stage of the process, but also for cool flames, being the mean relative deviation versus the experimental data lower than 3.3% for all fuels. Better predictions of the ignition delay have been obtained with the alternative procedure than the ones obtained with the classic Livengood & Wu expression, especially in those cases showing a two-stage ignition pattern, in which the Livengood & Wu integral method is not able to predict the high-temperature stage of the process.The authors would like to thank different members of the CMT-Motores Termicos team of the Universitat Politecnica de Valencia for their contribution to this work. The authors would also like to thank the Spanish Ministry of Education for financing the PhD. Studies of Dario Lopez-Pintor (Grant FPU13/02329). This work was partly funded by the Spanish Ministry of Economy and Competitiveness, project TRA2015-67136-R.Desantes Fernández, JM.; Bermúdez, V.; López, JJ.; López Pintor, D. (2016). Experimental validation of an alternative method to predict high and low-temperature ignition delays under transient thermodynamic conditions for PRF mixtures using a Rapid Compression-Expansion Machine. Energy Conversion and Management. 129:23-33. https://doi.org/10.1016/j.enconman.2016.09.089S233312

Experimental validation and analysis of seven different chemical kinetic mechanisms for n-dodecane using a Rapid Compression-Expansion Machine

[EN] Seven different chemical kinetic mechanisms for n-dodecane, two detailed and five reduced, have been evaluated under Engine Combustion Network (ECN) thermodynamic conditions by comparison to experimental measurements in a Rapid Compression-Expansion Machine (RCEM). The target ECN conditions are imposed at Top Dead Center (TDC), which cover a wide range of temperatures (from 850 K to 1000 K), oxygen molar fractions (0.21 and 0.15) and equivalence ratios (0.8, 0.9 and 1), while the pressure is fixed to keep a constant density at TDC equal to 22.8 kg/m(3). The results obtained have been used to validate the chemical kinetic simulations, which have been performed with CHEMKIN, by comparing both cool flames and high temperature ignition delays, as well as the heat released in each stage of the combustion process in case of having a two-stage ignition pattern. The experimental results show good agreement with the chemical kinetic simulations. In fact, the mean relative deviation in ignition delay between experiments and simulations among all the chemical mechanisms is equal to 18.0% (3 CAD) for both cool flames and high temperature ignition. In general, closer correspondence has been obtained for the ignition delay referred to the high-temperature stage of the process, being the cool flames phenomenon more difficult to reproduce. Moreover, the differences between the reduced mechanisms and the most detailed one have been analyzed, concluding that the enhanced specific reaction rates of the most reduced mechanisms cause differences not only on the ignition delays, but also on the Negative Temperature Coefficient (NTC) behavior and on the heat released during cool flames. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.The authors would also like to thank the Spanish Ministry of Education for financing the PhD. Studies of Dario Lopez-Pintor (grant FPU13/02329). This study was partially funded by the Spanish Ministry of Economy and Competitiveness in the frame of the COMEFF (TRA2014-59483-R) project.Desantes, J.; López, JJ.; García-Oliver, JM.; López-Pintor, D. (2017). Experimental validation and analysis of seven different chemical kinetic mechanisms for n-dodecane using a Rapid Compression-Expansion Machine. Combustion and Flame. 182:76-89. https://doi.org/10.1016/j.combustflame.2017.04.004S768918

A new method to predict high and low-temperature ignition delays under transient thermodynamic conditions and its experimental validation using a Rapid Compression-Expansion Machine

A new procedure to predict both high-temperature stage and cool flames ignition delays under transient thermodynamic conditions has been developed in this paper. The results obtained have been compared with those obtained from the Livengood & Wu integral method, as well as with other predictive methods and with direct chemical kinetic simulations and experimental data. All simulations have been performed with CHEMKIN, employing a detailed chemical kinetic mechanism. The simulations and predictions have been validated in the working range versus experimental results obtained from a Rapid CompressionExpansion Machine (RCEM). The study has been carried out with n-heptane and iso-octane, as diesel and gasoline fuel surrogates, under a wide range of initial temperatures (from 358 K to 458 K), initial pressures (0.14 MPa and 0.17 MPa), compression ratios (15 and 17), EGR rates (from 0% to 50%) and equivalence ratios (from 0.3 to 0.8). The experimental results show good agreement with the direct chemical kinetic simulations and with the new predictive method proposed. In fact, the mean relative deviation between experiments and simulations is equal to 1.719% for n-heptane and equal to 1.504% for iso-octane. Besides, the new method has shown good predictive capability not only for the hightemperature stage of the process but also for cool flames, being the mean relative deviation versus the experimental data lower than 2.900%. Better predictions of the ignition delay have been obtained with the new procedure than the ones obtained with the classic Livengood & Wu expression, especially in those cases showing a two-stage ignition pattern.The authors would like to thank different members of the CMT-Motores Termicos team of the Universitat Politecnica de Valencia for their contribution to this work. The authors would also like to thank the director of LAV-ETH, Konstantinos Boulouchos, for the Dario Lopez-Pintor's internship at LAV. Finally, the authors would like to thank the Spanish Ministry of Education for financing the PhD. Studies of Dario Lopez-Pintor (grant FPU13/02329).Desantes Fernández, JM.; Bermúdez, V.; López, JJ.; López Pintor, D. (2016). A new method to predict high and low-temperature ignition delays under transient thermodynamic conditions and its experimental validation using a Rapid Compression-Expansion Machine. Energy Conversion and Management. 123:512-522. https://doi.org/10.1016/j.enconman.2016.06.051S51252212

Characterization and prediction of the discharge coefficient of non-cavitating diesel injection nozzles

An experimental and theoretical study about the characterization of the discharge coefficient of diesel injection nozzles under non-cavitating conditions is presented in this paper. A theoretical development based on the boundary layer equations has been performed to define the discharge coefficient of a convergent nozzle. The discharge coefficient has been experimentally obtained for a standard diesel fuel under a wide range of Reynolds numbers by two different techniques: mass flow rate measurements and permeability measurements. Five different nozzles have been used: two multi-hole nozzles that have been tested in the frame of this work, and three other single-hole nozzles, the data of which have been taken from previous studies. The experimental results show good agreement with the theoretical expressions, proving that it is possible to predict the discharge coefficient of a non-cavitating nozzle with the equations shown in this paper.The authors would like to thank different members of the CMT-Motores Termicos team of the Universitat Politecnica de Valencia for their contribution to this work, specially to R. Payri, F.J. Salvador, J. Gimeno and G. Bracho. This work was partly sponsored by "Ministerio de Economia y Competitividad" in the frame of the project "Comprension de la influencia de combustibles no convencionales en el proceso de inyeccion y combustion tipo diesel", reference TRA2012-36932. The equipment used in this work has been partially supported by FEDER project funds "Dotacion de infraestructuras cientifico tecnicas para el Centro Integral de Mejora Energetica y Medioambiental de Sistemas de Transporte (CiMeT), (FEDER-ICTS-2012-06)", in the frame of the operation program of unique scientific and technical infrastructure of the Ministry of Science and Innovation of Spain. This support is gratefully acknowledged by the authors. Finally, the authors would like to thank the Spanish Ministry of Education for financing the PhD. studies of Dario Lopez-Pintor (grant FPU13/02329).Desantes Fernández, JM.; López, JJ.; Carreres Talens, M.; López-Pintor, D. (2016). Characterization and prediction of the discharge coefficient of non-cavitating diesel injection nozzles. Fuel. 184:371-381. https://doi.org/10.1016/j.fuel.2016.07.02637138118

Study of ignition delay time and generalization of auto-ignition for PRFs in a RCEM by means of natural chemiluminescence

An investigation of the effects of contour conditions and fuel properties on ignition delay time under Homogeneous Charge Compression Ignition (HCCI) conditions is presented in this study. A parametric variation of initial temperature, intake pressure, compression ratio, oxygen concentration and equivalence ratio has been carried out for Primary Reference Fuels (PRFs) in a Rapid Compression Expansion Machine (RCEM) while applying the optical technique of natural chemiluminescence along with a photo-multiplier. Additionally, the ignition delay time has been calculated from the pressure rise rate and also corresponding numerical simulations with CHEMKIN have been done. The results show that the ignition delay times from the chemical kinetic mechanisms agree with the trends obtained from the experiments. Moreover, the same mechanism proved to yield consistent results for both fuels at a wide range of conditions. On the other hand, the results from natural chemiluminescence also showed agreement with the ignition delay from the pressure signals. A 310 nm interference filter was used in order to detect the chemiluminescence of the OH* radical. In fact, the maximum area and peak intensity of the chemiluminescence measured during the combustion showed that the process of auto-ignition is generalized in the whole chamber. Moreover, the correlation of peak intensity, maximum area and ignition delay time demonstrated that natural chemiluminescence can also be used to calculate ignition delay times under different operating conditions. Finally, the area of chemiluminescence was proved to be more dependant on the fuel and ignition delay time than on the operating conditions. (C) 2015 Elsevier Ltd. All rights reserved.The authors would like to thank different members of the LAV team of the ETH-Zurich for their contribution to this work. The authors are grateful to the Universitat Politecnica de Valencia for financing the Ph.D. studies of Vera-Tudela (FPI SP1 Grant 30/05/2012) and his stay at ETH-Zurich (grant 30/12/2014). Finally, the authors would like to thank the Spanish Ministry of Education for financing the Ph.D. studies of Lopez-Pintor (Grant FPU13/02329) and his stay at ETH-Zurich (Grant EST14/00626).Desantes Fernández, JM.; García Oliver, JM.; Vera-Tudela-Fajardo, WM.; López Pintor, D.; Schneider, B.; Boulouchos, K. (2016). Study of ignition delay time and generalization of auto-ignition for PRFs in a RCEM by means of natural chemiluminescence. Energy Conversion and Management. 111:217-228. https://doi.org/10.1016/j.enconman.2015.12.052S21722811

Validity of the Livengood & Wu correlation and theoretical development of an alternative procedure to predict ignition delays under variable thermodynamic conditions

A theoretical study about the autoignition phenomenon has been performed in this article. The hypotheses of the Livengood & Wu integral have been revised, concluding that the critical concentration of chain carriers is not constant. However, its validity under engine conditions has been justified. Expressions to characterize the temporal evolution of the concentration of chain carriers, as well as the critical concentration of active radicals and the ignition delay, have been obtained starting from the Glassman s model. A new expression to predict ignition delays under variable conditions has been developed and the results obtained with this expression have been compared with those obtained from the Livengood & Wu integral. Two different fuels have been studied: isooctane (as a gasoline surrogate) and n-heptane (as a diesel fuel surrogate). The new method to predict ignition delays under variable conditions has shown, in general, better results than the classic Livengood & Wu integral, but the inability of the Glassman s model to reproduce the negative temperature coefficient regime should be improved in future works.The authors would like to thank different members of the CMT-Motores Termicos team of the Universitat Politecnica de Valencia for their contribution to this work. The authors would also like to thank the Spanish Ministry of Education for financing the PhD. Studies of Dario Lopez-Pintor (Grant FPU13/02329). This work was partly founded by the Generalitat Valenciana, Project PROMETEOII/2014/043.k.Desantes Fernández, JM.; López Sánchez, JJ.; Molina Alcaide, SA.; López Pintor, D. (2015). Validity of the Livengood & Wu correlation and theoretical development of an alternative procedure to predict ignition delays under variable thermodynamic conditions. Energy Conversion and Management. 105:836-847. https://doi.org/10.1016/j.enconman.2015.08.013S83684710