Computational modeling of n-heptane droplet combustion in air–diluent environments under reduced-gravity

The combustion and extinguishment of n-heptane droplets in air–diluent environments under reduced-gravity are studied, where carbon dioxide, helium and xenon are used as diluents. A gasification model employing the volume of fluid (VOF) method was developed and implemented into the CFD package FLUENT. The droplet evaporation, combustion and parasitic current were validated by comparing with the previous study. The numerical results agree well with experimental data. The Soret effect is predicted to sometimes be important during combustion, changing the flame temperature by several hundred degrees K in some cases. Transport of oxygen to the flame is weakened by thermal diffusion if helium is used as a diluent gas, while it is strengthened by thermal diffusion if xenon is introduced.     

On the extinction characteristics of alcohol droplet combustion under microgravity conditions – A numerical study

Quasi-steady burning and extinction of droplets are of interest from both fundamental and application viewpoints. The latter is related to combustor performance and fire safety issues in reduced gravity environments. Influences of diluent in the atmosphere on isolated droplet combustion characteristics including extinction provide insights to fire extinguishment phenomena and the effectiveness of various diluents as fire suppressants. Extinction of pure methanol and methanol–water droplets ranging from 1.5 to 7 mm size, for varying levels of ambient carbon-dioxide, helium and oxygen concentration – burning in a quiescent microgravity environment were studied numerically to compare the effectiveness of fire suppressant diluent selection and determining the limiting oxygen index. The results show distinct regimes of diffusive and radiative extinction. The transition from diffusive to radiative extinction is strongly influenced by the ambient diluent selection, especially by carbon dioxide concentration. Results for helium as the diluent showed increased burning rate and extinction due to diffusive heat loss. An “extinction characteristic” correlation is proposed that depends on burning rate, ambient diffusivity and flame standoff ratio. Recent methanol droplet experiments conducted over a wide range of operating conditions onboard the International Space Station were found to yield results that agree well with the proposed “extinction characteristic” correlation.     

Experimental study of the influence of λ and intake temperature on combustion characteristics in an HCCI engine fueled with n-heptane

Combustion characteristics fueled with n-heptane were investigated experimentally in a retrofitted engine to realize homogeneous charge compression ignition (HCCI) combustion mode. The influence of intake temperature and λ on chemical reactions was analyzed. In-cylinder pressure, temperature, heat release rate and ignition timing profiles were obtained with variations of the intake temperature and λ. The oxidation process was divided into three phases and the main reactions were analyzed. The results show that the combustion of n-heptane is more sensitive to intake temperature than to λ. The reason is that the intake temperature has a more profound effect on the oxidation process than the effect of λ. Moreover, the increase of intake temperature can extend the range of λ, and an optimal intake temperature exists corresponding to the value of λ.     

Mixtures of hydrogen and methane in the internal combustion engine – Synergies,potential and regulations

Taking advantage of the multi-fuel ability of internal combustion engines, mixtures of hydrogen and methane as fuel allow a substantial reduction of CO₂-emissions at affordable costs. By means of a prototype vehicle that was adapted for the operation with mixtures of methane and hydrogen and approved for road traffic, necessary technical adjustments and safety measures for gas-powered vehicles are explained.     

Influence of injector technology on injection and combustion development – Part 1: Hydraulic characterization

An experimental study of two real multi-hole Diesel injectors is performed under current DI Diesel engine operating conditions. The aim of the investigation is to study the influence of injector technology on the flow at the nozzle exit and to analyse its effect on the spray in evaporative conditions and combustion development. The injectors used are two of the most common technologies used nowadays: solenoid and piezoelectric. The nozzles for both injectors are very similar since the objective of the work is the understanding of the influence of the injector technology on spray characteristics for a given nozzle geometry. In the first part of the study, experimental measurements of hydraulic characterization have been analyzed for both systems. Analysis of spray behaviour in evaporative conditions and combustion development will be carried out in the second part of the work. Important differences between both injectors have been observed, especially in their transient opening and closing of the needle, leading to a more efficient air–fuel mixing and combustion processes for the piezoelectric actuated injector.     

Influence of injector technology on injection and combustion development – Part 2: Combustion analysis

The influence of injection technology on the fuel–air mixing process and the combustion development are analyzed by means of visualization techniques. For this purpose, two injectors (one solenoid and one piezoelectric) are characterized using an optical accessible two stroke engine. Visualization of liquid penetration has allowed the measurement of the stabilized liquid length, which is related with the efficiency of fuel–air mixing process. A theoretical derivation is used in order to relate this liquid length with chamber conditions, as well as to make a temporal analysis of these phenomena. After this, natural flame emission and chemiluminescence techniques are carried out. These results indicate that the piezoelectric system has a more efficient fuel–air mixing and combustion, reducing the characteristic times as well as soot formation. Finally, a correlation for the ignition delay of the two systems is obtained.     

Modelling of the gas-turbine colorless distributed combustion: An application to hydrogen enriched – kerosene fuel

This study examines hydrogen-enriched kerosene combustion under distributed regime in a gas turbine combustion chamber. With hydrogen enrichment, it is aimed at increasing combustion performance of those fuels. However, in this circumstance, it is obvious to increase the flame temperature with taking place hydrogen enrichment. Thus colorless distributed combustion (CDC), which is one of the advanced combustion techniques, can be suggested to control flame temperature with slowing down the reaction rate, resulting in ultra-low NO_X emissions and more uniform temperature distribution with a broadened flame. For this purpose, the hydrogen-enriched kerosene fuels were examined by modeling a CFD code using the eddy dissipation concept, the radiation model (P-1) and the turbulence model (standard k-ε). In this way, the thermal fields and the NO_X distributions have been obtained. The results showed that hydrogen enrichment increased the flame temperatures from about 2490 K to 2605 K at air-combustion conditions until 30% H₂, resulting in the NO_X levels predicted increased in the combustor. With reducing oxygen percentage the flame started to be the broadened one. The flame temperatures decreased, for instance, from about 2605 K to 2230 K at 15% O₂ for the 30% H₂ containing fuel. As a result of this, the NO_X levels reduced from about 30 ppm to the values lower than 1 ppm in the combustor. It is concluded that increments in temperature and NO_X levels with hydrogen can be suppressed with distributed regime, which enables that gas turbines can be operated at wider flammability limits with hydrogen enrichment.     

Cool or school?: the role of building attributes in explaining residential energy burdens in California

This paper quantifies the dimensions of an important energy efficiency conversation: the energy burden of low-income households. Due to budgetary constraints, low-income households face a stark consumption trade-offs described as “cool or school”. This study is the first to apply multivariate building energy regression modeling to assess the independent effects of various building characteristics and appliances on a household’s energy burden in the USA. We find that more attic insulation and newer air conditioners significantly predict lower energy burdens. Furthermore, homeowners enjoy ~?27% more attic insulation compared to renters. Our results offer empirical support for programs that offer deep retrofits to low-income households. We conclude by offering suggestions for leveraging weatherization funding to fund building energy retrofits.     

Turbulent Sprays Evaporating Under “Stabilized Cool Flame” Conditions: Assessment of two CFD Approaches

An “in-house” computational fluid dynamics code implementing a Euler-Lagrange approach is extended by incorporating the Euler-Euler (two-fluid model) approach, to improve prediction capabilities of flow and thermal characteristics of turbulent evaporating sprays. The performance of both approaches is assessed by comparing predictions with experimental data for a variety of evaporating-spray test cases. The applicability of the Euler-Lagrange and Euler-Euler approach is established in an isopropyl alcohol–air turbulent flow, in which characteristic droplet quantity predictions are in satisfactory overall agreement with measurements. The evaporating spray characteristics are then predicted under “stabilized cool flame” conditions and the computational results are compared to experimental data for a nonreactive case and a reactive case. In both cases, the velocity and thermal fields are successfully captured by both approaches. Overall, the article demonstrates an approach toward the development of performance-based computational tools.     

Continuous detonation combustion of ternary “hydrogen–liquid propane–air” mixture in annular combustor

Experiments are performed on continuous detonation combustion of ternary hydrogen–liquid propane–air mixture in a large-scale annular combustor 406 mm in outer diameter with an annular gap of 25 mm. Liquid propane is fed into the combustor at the time when sustained continuous-detonation combustion of hydrogen–air mixture is attained therein. Mass flow rates of hydrogen, propane and air in the experiments ranged from 0.1 to 0.5 kg/s (hydrogen), 0.1 to 0.5 kg/s (propane), and 5 to 12 kg/s (air). Continuous-detonation combustion of liquid propane in air is obtained for the first time due to addition of hydrogen rather than due to enrichment of air with oxygen. Combustor operation with a single continuously rotating detonation wave (DW) for about 0.1 s has been obtained when the flow rates of propane and air remained constant while the flow rate of hydrogen was rapidly decreasing.     

Techno-economic analysis of processes with integration of fluidized bed heat exchangers for H2 production – Part 2: Chemical-looping combustion

This work covers a techno-economic assessment for processes with inherent CO₂ separation, where a fluidized bed heat exchanger (FBHE) is used as heat source for steam reforming in a hydrogen production plant. This article builds upon the work presented in Part 1 of this study by Stenberg et al. [1], where a process excluding CO₂ capture was examined. Part 2 suggests two process configurations integrating steam reforming with a chemical-looping combustion (CLC) system, thus providing inherent CO₂ capture. The first system (case CM) uses natural gas as supplementary fuel whereas the second system (case CB) uses solid biomass, which enables net negative CO₂ emissions. In both systems, the reformer tubes are immersed in a bubbling fluidized bed where heat for steam reforming is efficiently transferred to the tubes. The processes include CO₂ compression for pipeline transportation, but excludes transport and storage. The CLC system is designed based on key parameters, such as the oxygen carrier circulation rate and oxygen transport capacity. The first system displays a process with net zero emissions and a hydrogen production efficiency which is estimated to 76.2%, which is almost 8% higher than the conventional process. The levelized production cost is 1.6% lower at below 2.6 €/kg H₂. The second system shows the possibility to reduce the emissions to ?34.1 g CO₂/MJ_H2 compared to the conventional plant which emits 80.7 g CO₂/MJ_H2. The hydrogen production efficiency is above 72% and around 2% higher than the conventional process. The capital investments are higher in this plant and the levelized hydrogen production cost is estimated to around 2.67 €/kg. The cost of CO₂ avoidance, based on a reference SMR plant with CO₂ capture, is low for both cases (?4.3 €/ton_CO2 for case CM and 2.7 €/ton_CO2 for case CB).     

Advances and challenges in explaining fuel spray impingement: How much of single droplet impact research is useful?

The impingement of fuel spray onto interposed surfaces in an IC engine, equipped either with a direct or an indirect injection system, is a fundamental issue affecting mixture preparation prior to combustion and, therefore, also affecting engine performance and pollutant emissions. In this context, the development of fuel injection systems relies on accurate knowledge of the fluid dynamic and thermal processes occurring during spray/wall interaction.     

Techno-economic analysis of H2 production processes using fluidized bed heat exchangers with steam reforming – Part 1: Oxygen carrier aided combustion

This work presents the techno-economic assessment for a new process where a fluidized bed heat exchanger (FBHE) is used as heat source for steam reforming in a hydrogen production plant. This suggested process configuration is compared with a reference case representing a conventional steam methane reforming (SMR) large-scale hydrogen production plant. The use of a FBHE as a heat source for the endothermic reforming is an advantage because of the high heat transfer coefficient to the reformer tubes. The suggested process configuration utilizes oxygen carrier particles as bed material and a bubbling fluidized bed reactor with immersed reformer tubes to ensure sufficient heat production for the reforming and improved heat transfer to the reformer tubes compared a conventional plant. The results include a comparison of hydrogen production efficiency and levelized production costs (LCOH) of the two plants where the production efficiency is more than 11% higher and the LCOH is more than 7% lower for the suggested process configuration.     

Experimental study of hydrogen explosion in repeated pipe congestion – Part 1: Effects of increase in congestion

Experimental studies were conducted with the objective of gaining a better understanding of the potential explosion hazard consequences that could be associated with a high-pressure leak from a hydrogen vehicle refuelling system. The first part of the study, described in this paper, was a series of experiments designed to establish hydrogen–air explosion overpressures in a well-defined and well understood 3 m × 3 m x 2 m (high) repeated pipe congestion. The results of the experiments are discussed in terms of the conditions leading to the greatest overpressures. It is concluded from the study that stoichiometric ratio in the range of 1.2–1.3 gives highest overpressure. Moreover, it was observed that increasing the congestion from 4-gate to 9-gate congestion leads to significant increase in the overpressure. In addition, it was concluded that, explosion in a hydrogen-air mixture is significantly more severe than the explosion in an ethane-air, methane-air or propane-air mixtures. This is attributed to higher laminar flame speed of hydrogen-air mixtures.     

Enhanced ionic conductivity in calcium doped ceria – Carbonate electrolyte: A composite effect

Recently, ceria-based nanocomposites, as a proton and oxygen ion conductor, has been developed as promising electrolyte candidates for low-temperature solid oxide fuel cells (LTSOFCs). Up to now, samarium doped ceria (SDC) was studied as a main oxide for nanocomposite electrolyte; while calcium doped ceria (CDC) is considered as a good alternative from both material performance and economical aspects. Yet the conduction behavior of CDC-based composite has not been reported. In the present study, calcium doped ceria was prepared by oxalate co-precipitation method, and used for the fabrication of CDC/Na₂CO₃ composite. The thermal decomposition process, structure and morphology of the samples were characterized by TGA, XRD, SEM, etc. The oxygen ion conductivity of single phase CDC sample was measured by electrochemical impedance spectroscopy (EIS), while the proton and oxygen ion conductivity of CDC/Na₂CO₃ nanocomposite sample were determined by four-probe d.c. measurements. The CDC/Na₂CO₃ samples show significantly enhanced overall ionic conductivity compared to that of single phase CDC samples, demonstrating pronounced composite effect. This confirms that the use of nanocomposite as electrolyte can effectively lower the operation temperature of SOFC due to improved ionic conductivity.     

Managing fuel variability in LPG-based PEM fuel cell systems – I: Thermodynamic simulations

Hydrocarbon-based polymer electrolyte membrane fuel cell systems are promising solutions for auxiliary power units and remote systems. The sequence of steam reforming, water–gas shift and preferential oxidation reactors is a common fuel processing methodology. The heat released by burning the hydrogen-depleted anode waste gas is utilized to drive the endothermic reforming reaction and to generate steam, while the other two sub-processes are exothermic. Balancing these heat fluxes, while maintaining the narrow temperature windows required for each of these reactions, is a key system control challenge. Strategies based on a priori knowledge of fuel composition result in instability, reduction in system efficiency or transgression of safe limits in critical parameters when fuel composition varies. To address this, we have developed and implemented a control strategy that uses more readily measurable quantities to perform control actions, and is independent of knowledge about exact fuel composition or flow rate. In this paper, we analyze the effectiveness of the new control strategy by quantifying its effects on liquefied petroleum gas based polymer electrolyte membrane fuel cell systems. Results indicate that even in extreme cases of fuel composition variation, this control strategy enables the determination of set points such that the system efficiency and other critical parameters are held in a narrow range around optimal values.     

Condensation in a vertical tube bundle passive condenser – Part 2: Complete condensation

An experimental study of the tube bundle effect on heat removal capabilities in complete condensation mode of a passive condenser was performed. A full scale test section, with four condenser tubes, was designed and constructed to simulate operating conditions of a passive containment cooling system. For complete condensation analysis, pure steam was supplied to the test section and heat transfer properties were measured for pressure from 100 to 280 kPa. The condensation heat transfer results were similar to the findings from single tubes, except for a slightly higher condensate mass flux. This was determined to be a result of turbulent mixing in the secondary boiling water caused by the tube bundle.     

Experimental study of hydrogen explosion in repeated pipe congestion – Part 2: Effects of increase in hydrogen concentration in hydrogen-methane-air mixture

Hydrogen is seen as an important energy carrier for the future which offers carbon free emissions. At present it is mainly used in refueling hydrogen fuel cell cars. However, it can also be used together with natural gas in existing gas fired equipment with the benefit of lower carbon emissions. This can be achieved by introducing hydrogen into existing natural gas pipelines. These pipelines are designed, constructed and operated to safely transport natural gas, which is mostly methane. Because hydrogen has significantly different physical and chemical properties than natural gas, any addition of hydrogen my adversely affect the integrity of the pipeline network, increasing the likelihood and consequences of an accidental leak. Since it increases the likelihood and consequences of an accidental leak, it increases the risk of explosion. In order to address various safety issues related to addition of hydrogen in to a natural gas pipeline a EU project NATURALHY was introduced. A major objective of the NATURALHY project was to identify how much hydrogen could be introduced into the natural gas pipeline network. Such that it does not adversely impact the safety of the pipeline network and significantly increase the risk to the public. This paper reports experimental work conducted to measure the explosion overpressure generated by ignition of hydrogen-methane-air mixture in a highly congested region consisting of interconnected pipes. The composition of the methane/hydrogen mixture used was varied from 0% hydrogen (100% methane) to 100% hydrogen (0% methane) to understand its effect on generated explosion overpressure. It was observed that the maximum overpressures generated by methane-hydrogen mixtures with 25% (by volume) or less hydrogen content are not likely to be significantly greater than those generated by methane alone. Therefore, it can be concluded that the addition of less than 25% by volume of hydrogen into pipeline networks would not significantly increase the risk of explosion.     

Condensation in a vertical tube bundle passive condenser – Part 1: Through flow condensation

An experimental study and a boundary layer analysis were performed for the steam condensation in a vertical tube bundle passive condenser operating in a through flow mode. Four condenser tubes were submerged in a water pool and the heat from the condenser tube was removed through boiling. Experimental data were obtained for various system pressures (100–170 kPa), inlet steam flow rates (15–47 g/s) and non-condensable gas concentration (0–15%). The experimental results showed substantial deterioration in condensation when non-condensable gas was present. With increase in steam flow rate and system pressure the condensate rate increased. The boundary layer thickness and non-condensable gas concentration increased along the condenser tube length.