Utilizing preferential vaporization to enhance the stability of spray combustion for high water content fuels

In a previous study on the stability of spray combustion for mixtures of alcohols (ethanol or 1-propanol) and water, the feasibility of burning fuels heavily diluted with water was demonstrated. In that study it was found that the preferential vaporization of alcohols in water can significantly enhance flame stability. Due to their high volatility and high activity coefficient in aqueous solution, the alcohols quickly evaporate from the droplets and generate a concentrated fuel vapor at the base of the jet. Therefore, a flame can be ignited and stabilize even though the water content of the fuel is quite high (up to 90 wt%) (Yi and Axelbaum, 2013). In this study, we develop a procedure for selecting chemical fuels showing strong preferential vaporization in water. t-Butanol was identified as an excellent candidate based on its physical and chemical properties, including activity coefficient, vapor pressure, heat of vaporization and heat of combustion. Flame stability was evaluated for aqueous solutions of both ethanol and t-butanol using a spray burner where the extent of swirl was adjustable. Under both high and low swirl intensity, the flame stability of t-butanol aqueous solutions was better than that of ethanol. The characteristic time for fuel release from a droplet was modeled for both ethanol and t-butanol. The time to release 99% of the fuel from the droplet for t-butanol is over 70% shorter compared to that for ethanol, which supports the improved flame stability observed for t-butanol in the experiments.     

The mechanism of flame propagation affected by flow/shock wave in a confined combustion chamber equipped with a perforated plate

The whole evolution of flame propagation in a confined combustion chamber was firstly experimentally observed in a newly designed experimental apparatus equipped with a perforated plate. The effect of the flame-flow/acoustic/shock wave interaction on the flame propagation was studied. The experiment was conducted with a stoichiometric hydrogen-air mixture. According to the flame morphology and the flame tip velocity, the whole evolution of flame propagation in the experimental apparatus was classified into the following three stages: laminar flame, jet flame and turbulent flame. In the present work, different flame propagation modes were obtained in different conditions. Depending on the initial pressure, three different flame propagation modes were observed. At an initial pressure of 1 bar, the flame propagation after perforated plate was mainly controlled by the interactions of the flame and combustion-generated flow ahead of the flame front. As initial pressures went up to 3 bar and 5 bar, shock waves were clearly observed ahead of the flame, which played a significant role on the flame propagation. The flame decelerated sharply and even propagated backwards, induced by the flame-shock wave interactions. Depending on the intensity of the shock wave, the backward-propagation velocity was higher at 5 bar with a stronger shock wave. In addition, the pressure oscillation at different initial pressures was discussed.     

Burning rates and temperatures of flames in excess-enthalpy burners: A numerical study of flame propagation in small heat-recirculating tubes

This study investigates flame propagation in small thermally-participating tubes where the wall acts as a heat-recirculating medium. This fundamental configuration allows heat in the combustion products to be recirculated into the reactants, resulting in excess enthalpy and enhanced burning rates. Preheating of the reactants by heat recirculation has traditionally been considered to be the dominant mechanism leading to large burning rates observed in such systems. This is mainly supported by results from physical models based on a one-dimensional (1-D) representation of the system, where the radial diffusion of heat from wall surface to channel centerline is not accurately captured. In this study, a 2-D formulation with conjugate heat transfer, which accurately resolves the transport of heat inside the gas-wall system, is used to model the excess-enthalpy phenomenon. Steadily-propagating stoichiometric methane–air flames are simulated inside an adiabatic tube of finite wall-thickness, over a wide range of inlet flow velocities and small tube diameters. Burning-rate enhancement is found to be caused not only by preheating, associated with heat recirculation, but also by an increase in flame-front area. Flame elongation is more pronounced with increasing tube diameter and inlet velocity, up to a point where the change in flame-front area becomes dominant in enhancing burning rate. In that case, heat recirculation is a necessary condition for flames to couple to the thermal wave in the wall and elongate, but does not provide a significant increase in enthalpy or temperature that would otherwise be needed for high burning rates to be observed. As the diameter is reduced, the effect of preheating becomes increasingly important for burning-rate enhancement compared to flame area increase. At very small diameters, smaller than the flame thickness, the increase in burning rate is seen to be predominantly attributable to preheating. However, preheating is seen to become limited as inflow velocity is increased, due to 2-D effects inside the fluid that interfere with heat recirculation. These findings demonstrate that 2-D effects inside the fluid can have a prohibitive influence on the burning-rate enhancement attributed to preheating, but that they also give rise to an additional mechanism, associated with the change in flame surface area, responsible for burning-rate enhancement in heat-recirculating burners.     

A phenomenological model for the prediction of soot formation in diesel spray combustion

A phenomenological soot model coupled with complex chemistry mechanism for the prediction of soot formation in diesel spray combustion is presented. The prototype of the model is one proposed by Leung and Lindstedt in which soot formation is treated by four global stages: particle nucleation, surface growth, surface oxidation, and particle coagulation, each of which is represented by only a few reaction steps. In the present study, the model is modified according to recent literature data. The formation of soot particles is linked with gas-phase chemistry via diacetylene and naphthalene, which are presumed to be indicative species of particle inception/nucleation. The soot surface growth is described using Frenklach et al.'s active site model, and the oxidation mechanism includes both Nagle and Strickland-Constable's O₂ oxidation and Neoh et al.'s OH oxidation models. The soot model integrated with the gas-phase kinetics is then applied in multidimensional spray simulations. The KIVA3 code that is widely used in diesel combustion studies is modified and employed for the simulations. The turbulent flow is predicted using the compressible k-ε model, and the turbulence-chemistry interaction handled by a partially stirred reactor model. The IDEA experimental data for n-heptane sprays in diesel-like conditions (800 K and 50 bar) are used for evaluation of the model. Some reaction rate constants are adjusted to achieve better agreement with the measurements. Further, sensitivity studies have been carried out and the effects of some parameters that affect the predictions are discussed. The results indicate that the model, if applied together with other models that properly describe sprays and turbulent flow, can be used for qualitative and even quantitative prediction of soot formation in diesel combustion.     

The enigmatic mechanism of the flame ionization detector: Its overlooked implications for fossil fuel combustion modeling

The flame ionization detector (FID) has been a commercial analyzer now for about 50 years. It still finds significant use as a sensitive quantitative monitor of organic compounds in gas chromatography and for monitoring mixtures of hydrocarbons. Its carbon counting ability to integrate, for example, total unburned hydrocarbon emissions from a source, now is accepted without question. This is especially noteworthy as the fundamental chemistry on which the instrument is based has always been uncertain. Although now largely overlooked, its mechanism has significant implications and suggests that there is an underlying simplicity to hydrocarbon combustion. As a result, in the light of recent discoveries concerning the very rapid formation of a pool of hydrocarbon radicals in hydrocarbon combustion, a re-examination of the chemi-ionization mechanisms in hydrocarbon flames has been undertaken. Many of the previous speculations have been scrutinized and it is confirmed that the primary chemi-ionizing reaction of CH(X²Π) with O atoms is most likely the sole source in combustion including the FID. The oft suggested roles of electronically excited states of CH now are ruled out but with some slight uncertainty remaining on the still unknown importance of the metastable CH(a⁴Σ⁻) state in flames. The reason for the “equal per carbon” response of the FID with any hydrocarbon finally has been resolved. From isotopically labeled studies and measurements of the concentrations of CH and C₂ it is seen, under the same conditions, that different hydrocarbons do produce approximately the same levels of CH on a unit carbon basis. This results from the very rapid destruction and reformulation kinetics in the reaction zone of flames, and formation of a hydrocarbon radical pool that constitutes the unburned carbon. These radicals then are gradually eroded by the continuing oxidation or by soot precursor growth. As a result, the nature of the carbon in a hydrocarbon fuel is mainly irrelevant, only its quantity. The one well-documented exception has always been C₂H₂ but the data now show this so-called anomalous behavior to be no more than a reflection of its uniquely slower combustion nature in the reaction zone. It is not apparent in substituted acetylene fuels. Close to the reaction zone its kinetics produce a larger profile of unburned carbon that is evidenced by enhanced levels observed for CH and C₂. The nature of the specific responses of the FID to other organic structural categories also is a reflection of their primary combustion breakdown and a measure of the initial pool of unburned carbon. Exactly similar responses are seen in both the FID and in soot formation tendencies. The connection though is indirect in that both processes relate to and result from the same pool of non-oxidized carbon, rather than any implied inceptive role. As a result, the observed sensitivities previously recorded with the FID now can be a useful aid in validating the primary dominant steps in combustion mechanisms and the example of dimethyl ether combustion is used as an illustration. At present, this rich analytical database could be particularly useful in modeling the more complex partially oxygenated fuels that now are being extensively studied.     

A study of thin‐flame quasisteady sphericosymmetric combustion of multicomponent fuel droplets. Part I: modelling for droplet surface regression and non‐unity gas‐phase Lewis number

A model for sphericosymmetric thin‐flame combustion of multicomponent fuel droplets has been developed in the first part of this two‐part work. The model incorporates effects of droplet surface regression and gas‐phase Lewis number. It is observed that both these effects affect the results substantially. The study also reveals the transient nature of the combustion process. Copyright © 1999 John Wiley and Sons, Ltd.     

Willingness to pay for hydrogen fuel cell electric vehicles in China: A choice experiment analysis

The hydrogen energy is considered to be main power source of transport sector in the future, and a huge amount of funds have been invested into developing hydrogen fuel cell electric vehicles (FCEVs). Since FCEVs are in initial development stage and there're few FCEVs on the road, before their expansion this paper intends to conduct an economic analysis for FCEVs by using the choice experiment method. In the choice experiment, 1072 participants were required to select among two FCEVs and one conventional fuel vehicle. Logit models were estimated and then the results were used to calculate the willingness to pay for FCEVs. Results showed that purchase price, driving range, refueling time, fuel cost, emissions reduction, refueling accessibility are significant influences, and the marginal values for every 200 km improvement in driving range, 5 min reduction in refueling time, RMB 0.5/kilometer reduction in fuel cost, 20% reduction in emissions, and 20% improvement of refueling accessibility were estimated to be RMB 49,091, 12,727, 3818, 47,818, and 12,909, respectively. A range of FCEV configurations were calculated, and compared to a gasoline-powered counterpart the extra value that customers were likely to pay for a FCEV ranged from RMB 20,810 to 95,310. These results have significant implications for promoting FCEVs and contribute to better sustainability in transport sector.