Gas-phase saturation and evaporative cooling effects during wet compression of a fuel aerosol under RCM conditions

Wet compression of a fuel aerosol has been proposed as a means of creating gas-phase mixtures of involatile diesel-representative fuels and oxidizer + diluent gases for rapid compression machine (RCM) experiments. The use of high concentration aerosols (e.g., ∼0.1 mL_fuel/L_gas, ∼1 × 10⁹ droplets/L_gas for stoichiometric fuel loading at ambient conditions) can result in droplet–droplet interactions which lead to significant gas-phase fuel saturation and evaporative cooling during the volumetric compression process. In addition, localized stratification (i.e., on the droplet scale) of the fuel vapor and of temperature can lead to non-homogeneous reaction and heat release processes – features which could prevent adequate segregation of the underlying chemical kinetic rates from rates of physical transport. These characteristics are dependent on many factors including physical parameters such as overall fuel loading and initial droplet size relative to the compression rate, as well as fuel and diluent properties such as the boiling curve, vaporization enthalpy, heat capacity, and mass and thermal diffusivities. This study investigates the physical issues, especially fuel saturation and evaporative cooling effects, using a spherically-symmetric, single-droplet wet compression model. n-Dodecane is used as the fuel with the gas containing 21% O₂ and 79% N₂. An overall compression time and compression ratio of 15.3 ms and 13.4 are used, respectively. It is found that smaller droplets (d₀ ∼ 2–3 μm) are more affected by ‘far-field’ saturation and cooling effects, while larger droplets (d₀ ∼ 14 μm) result in greater localized stratification of the gas-phase due to the larger diffusion distances for heat and mass transport. Vaporization of larger droplets is more affected by the volumetric compression process since evaporation requires more time to be completed even at the same overall fuel loading. All of the cases explored here yield greater compositional stratification than thermal stratification due to the high Lewis numbers of the fuel–air mixtures (Le_g ∼ 3.8).     

Combustion of gel fuels based on organic gellants

The phenomena involved in the combustion of non-metallized organic-gellant-based gelled fuels were investigated. The droplet combustion was photographed using a high-speed digital video camera. Experiments were conducted also with a number of control fuels that were prepared especially for simulating different phenomena occurring during combustion. The organic-gellant-based gel fuel droplet is characterized by a gellant layer formed on the droplet surface because of phase separation, resulting in bubbling and jetting. Phase separation is most likely the phenomenon that dominates the combustion process. The use of control fuels confirms that phase separation is not unique to gel fuels.     

A numerical assessment of the novel concept of crevice containment in a rapid compression machine

Rapid compression machines (RCMs) typically incorporate creviced pistons to suppress the formation of the roll-up vortex. The use of a creviced piston, however, can enhance other multi-dimensional effects inside the RCM due to the crevice zone being at lower temperature than the main reaction chamber. In this work, such undesirable effects of a creviced piston are highlighted through computational fluid dynamics simulations of n-heptane ignition in RCM. Specifically, the results show that in an RCM with a creviced piston, additional flow of mass takes place from the main combustion chamber to the crevice zone during the first-stage of the two-stage ignition. This phenomenon is not captured by the zero-dimensional modeling approaches that are currently adopted. Consequently, a novel approach of ‘crevice containment’ is introduced and computationally evaluated in this paper. In order to avoid the undesirable effects of creviced piston, the crevice zone is separated from the main reaction chamber at the end of compression. The results with ‘crevice containment’ show significant improvement in the fidelity of zero-dimensional modeling in terms of predicting the overall ignition delay and pressure rise in the first-stage of ignition. Although the implementation of ‘crevice containment’ requires a modification in RCM design, in practice there are significant advantages to be gained through a reduction in the rate of pressure drop in the RCM combustion chamber and a quantitative improvement in the data obtained from the species sampling experiments.     

CFD modeling of two-stage ignition in a rapid compression machine: Assessment of zero-dimensional approach

In modeling rapid compression machine (RCM) experiments, zero-dimensional approach is commonly used along with an associated heat loss model. The adequacy of such approach has not been validated for hydrocarbon fuels. The existence of multi-dimensional effects inside an RCM due to the boundary layer, roll-up vortex, non-uniform heat release, and piston crevice could result in deviation from the zero-dimensional assumption, particularly for hydrocarbons exhibiting two-stage ignition and strong thermokinetic interactions. The objective of this investigation is to assess the adequacy of zero-dimensional approach in modeling RCM experiments under conditions of two-stage ignition and negative temperature coefficient (NTC) response. Computational fluid dynamics simulations are conducted for n-heptane ignition in an RCM and the validity of zero-dimensional approach is assessed through comparisons over the entire NTC region. Results show that the zero-dimensional model based on the approach of ‘adiabatic volume expansion’ performs very well in adequately predicting the first-stage ignition delays, although quantitative discrepancy for the prediction of the total ignition delays and pressure rise in the first-stage ignition is noted even when the roll-up vortex is suppressed and a well-defined homogeneous core is retained within an RCM. Furthermore, the discrepancy is pressure dependent and decreases as compressed pressure is increased. Also, as ignition response becomes single-stage at higher compressed temperatures, discrepancy from the zero-dimensional simulations reduces. Despite of some quantitative discrepancy, the zero-dimensional modeling approach is deemed satisfactory from the viewpoint of the ignition delay simulation.     

Transient dynamic of hydrogen permeation through a palladium membrane

Transient dynamic of hydrogen permeation through a palladium membrane is studied in the present study. Three different pressure differences between the two sides of the membrane are considered; they are 3, 5 and 8 atm. The experimental results indicate that the variation in the hydrogen permeation process is notable at the selected pressure differences. When the pressure difference is relatively low (i.e. 3 atm), the hydrogen permeation process proceeds from a time-lag period, then to a concave up period and eventually to a concave down period. Therefore, the transient hydrogen permeation is characterized by a three-stage mass transfer process. When the pressure difference is increased to 5 atm, the time-lag period disappears, thereby evolving the three-stage mass transfer process into a two-stage one. However, the concave up period withers significantly. Once the pressure difference is as high as 8 atm, the transient hydrogen permeation is completely characterized by a concave down curve, yielding a single-stage mass transfer process. A quasi-steady state of hydrogen permeation is defined to evaluate the period of the transient mass transfer process. It suggests that, within the investigated conditions of operation, the time required for hydrogen permeation to reach the steady value is around or over 1 h. For the low pressure difference cases, the transient period is especially long, resulting from the time-lag characteristic. Once the hydrogen permeation is in the steady state, over 80% of hydrogen can be recovered from the membrane.     

An experimental study of the autoignition characteristics of conventional jet fuel/oxidizer mixtures: Jet-A and JP-8

Ignition delay times of Jet-A/oxidizer and JP-8/oxidizer mixtures are measured using a heated rapid compression machine at compressed charge pressures corresponding to 7, 15, and 30 bar, compressed temperatures ranging from 650 to 1100 K, and equivalence ratios varying from 0.42 to 2.26. When using air as the oxidant, two oxidizer-to-fuel mass ratios of 13 and 19 are investigated. To achieve higher compressed temperatures for fuel lean mixtures (equivalence ratio of ∼0.42), argon dilution is also used and the corresponding oxidizer-to-fuel mass ratio is 84.9. For the conditions studied, experimental results show two-stage ignition characteristics for both Jet-A and JP-8. Variations of both the first-stage and overall ignition delays with compressed temperature, compressed pressure, and equivalence ratio are reported and correlated. It is noted that the negative temperature coefficient phenomenon becomes more prominent at relatively lower pressures. Furthermore, the first-stage-ignition delay is found to be less sensitive to changes in equivalence ratio and primarily dependent on temperature.     

OH production by transient plasma and mechanism of flame ignition and propagation in quiescent methane-air mixtures

Transient plasma induced production of OH is followed in a quiescent, stoichiometric CH₄-air mixture using the planar laser induced fluorescence technique. Ignition and subsequent flame propagation, for both the transient plasma and traditional spark ignition, are observed with a high speed camera (2000 fps). The transient plasma is generated using a 70 ns FWHM, 60 kV, 800 mJ pulse. OH production was confirmed throughout the chamber volume; however, the mean number density was found to decay below 1.3×¹⁰¹⁴ cm⁻³ near 100 μs. Nonetheless, ignition induced by transient plasma was decidedly faster than by spark ignition. Using the high speed camera, ignition initiated by transient plasma was found to occur along the length of the anode at approximately 1 ms, leading to the formation of a wrinkled, cylindrically-shaped flame. Analysis of the flame front propagation rates shows that flames ignited by transient plasma propagate essentially at the speed consistent with well accepted literature values for the stoichiometric methane-air mixture. This supports the notion that residue plasma, if any, has little effect on flame propagation.     

Transient convective burning of interactive fuel droplets in double-layer arrays

The transient convective burning of n-octane droplets interacting within double-layer arrays in a hot gas flow perpendicular to the layers is studied numerically, with considerations of droplet surface regression, deceleration and relative movement due to the drag of the droplets, internal liquid motion, variable properties, non-uniform liquid temperature and surface tension. Each layer in the double-layer array is a periodic droplet array aligned orthogonal to the free stream direction. The droplets in different layers are arranged either in tandem or staggered. Several different flame structures are found for the double-layer arrays. The transient behaviors of the droplets in both upstream and downstream layers are studied and compared, for various initial relative stream velocity and initial transverse droplet spacing. The average surface temperature and vaporization rate for the front (or upstream) droplets and back (or downstream) droplets are influenced by the flame structure. The front droplets in a double-layer array behave similarly to the droplets in a single-layer array for the streamwise droplet spacing considered in this study. The back droplets approach the front droplets because they generally have lower drag.     

Spectroscopic and chemical-kinetic analysis of the phases of HCCI autoignition and combustion for single- and two-stage ignition fuels

The temporal phases of autoignition and combustion in an HCCI engine have been investigated in both an all-metal engine and a matching optical engine. Gasoline, a primary reference fuel mixture (PRF80), and several representative real-fuel constituents were examined. Only PRF80, which is a two-stage ignition fuel, exhibited a “cool-flame” low-temperature heat-release (LTHR) phase. For all fuels, slow exothermic reactions occurring at intermediate temperatures raised the charge temperature to the hot-ignition point. In addition to the amount of LTHR, differences in this intermediate-temperature heat-release (ITHR) phase affect the fuel ignition quality. Chemiluminescence images of iso-octane show a weak and uniform light emission during this phase. This is followed by the main high-temperature heat-release (HTHR) phase. Finally, a “burnout” phase was observed, with very weak uniform emission and near-zero heat-release rate (HRR). To better understand these combustion phases, chemiluminescence spectroscopy and chemical-kinetic analysis were applied for the single-stage ignition fuel, iso-octane, and the two-stage fuel, PRF80. For both fuels, the spectrum obtained during the ITHR phase was dominated by formaldehyde chemiluminescence. This was similar to the LTHR spectrum of PRF80, but the emission intensity and the temperature were much higher, indicating differences between the ITHR and LTHR phases. Chemical-kinetic modeling clarified the differences and similarities between the LTHR and ITHR phases and the cause of the enhanced ITHR with PRF80. The HTHR spectra for both fuels were dominated by a broad CO continuum with some contribution from bands of HCO, CH, and OH. The modeling showed that the CO+O→CO₂+hν reaction responsible for the CO continuum emission tracks the HTHR well, explaining the strong correlation observed experimentally between the total chemiluminescence and HRR during the HTHR phase. It also showed that the CO continuum does not contribute to the ITHR and LTHR chemiluminescence. Bands of H₂O and O₂ in the red and IR regions were also detected during the HTHR, which the data indicated were most likely due to thermal excitation. The very weak light emission in the “burnout” phase also appeared to be thermal emission from H₂O and O₂.     

The role of low temperature fuel chemistry on turbulent flame propagation

A new high temperature, high Reynolds number, Reactor Assisted Turbulent Slot (RATS) burner has been developed to investigate turbulent flame regimes and burning rates for large hydrocarbon transportation fuels, which exhibit strong low temperature chemistry behavior. The turbulent flow characteristics are quantified using hot wire anemometry. The turbulent flame structures and burning velocities of n-heptane/air mixtures are measured by using planar laser induced fluorescence of OH and CH₂O with reactant temperatures spanning from 400 K to 700 K. It is found for the first time that for n-heptane/air mixtures there are four unique turbulent flame regimes, a conventional chemically-frozen-flow regime, a low-temperature-ignition regime, a transitional regime between the low- to high-temperature-ignition regimes, and a high-temperature-ignition regime, depending on the initial reactant temperature and heated flow residence time prior to the flame. The turbulent burning velocities have been measured for the first two regimes, chemically-frozen-flow and low-temperature-ignition regimes, in order to quantitatively address the role of low temperature ignition on the turbulent burning velocity. In the latter case, large amount of CH₂O formation has been observed in the pre-flame zone, signaling a significant change in the reactant composition and chemistry. At a given reactant temperature and turbulent intensity, the normalized turbulent burning velocities can be varied depending on the extent of low temperature fuel oxidation by varying the heated flow residence time and reactant temperature. The present results suggest that contrary to the previous studies, the turbulent flame regimes and burning velocities for fuels with low temperature chemistry may not be uniquely defined at elevated temperatures.     

Speedy solution of quasi-steady-state species by combination of fixed-point iteration and matrix inversion

A speedy algorithm that utilizes both fixed-point iteration (FPI) and matrix inversion is developed for solving a set of nonlinear, coupled algebraic equations for the concentrations of the quasi-steady state (QSS) species in reduced mechanisms. It is shown that the slow convergence of FPI experienced in some reduced mechanisms is caused by QSS species that are strongly coupled. A method is developed for identifying groups of QSS species that are coupled and closely connected. During the iteration, the concentrations of those species without coupling to other species are obtained by FPI. The concentrations of species in coupled groups are solved by matrix inversion as FPI is inefficient. Using three reduced mechanisms with total number of QSS species ranging from 17 to 251, the performances of the proposed algorithm are extensively assessed. It is concluded that the proposed algorithm offers significant CPU savings when the coupling among species is strong.     

Numerical simulation of hydrogen combustion process in rotary engine with laser ignition system

In this paper the combustion and ignition process in the hydrogen-fueled peripheral-ported rotary engine with single and dual laser ignition systems was studied numerically. The computational method was established for the process simulation including interaction between turbulence and chemical reactions. The detailed chemical kinetic model of hydrogen combustion was used. It was shown that the ignition and combustion process in the H₂-fueled rotary engine is highly transient with specific distortion and stretching of the combustion front in the combustion chamber due to complex motion of the rotor relative to the engine housing. The single and dual laser ignition systems were simulated to compare the ignition efficiency and the rate of hydrogen burning out. The evaluation of pressure in the combustion chamber was performed and compared with the experimental data obtained for the rotary engine fueled by natural gas. It was shown that the H₂-fueled rotary engine with the dual laser ignition system has potential application in alternative automotive industry due to high efficiency and near-zero carbon-based emission.     

An experimental and theoretical study of the effects of heat conduction through the support fiber on the evaporation of a droplet in a weakly convective flow

This study investigates the effect of heat conduction through the support fiber on a droplet's evaporation in a weakly convective flow. Experimentally, a droplet of n-heptane or n-hexadecane with an initial diameter of 700 or 1000 μm was suspended at the tip of a horizontal or vertical quartz fiber (diameter 50, 150, or 300 μm) to evaporate in an upward hot gas flow (at 490 or 750 K). A simple one-dimensional model of transient conduction is formulated in combination with evaporation of the droplet. The calculations agree well with the experiments. In general, heat conduction through the fiber enhances evaporation, with a stronger effect for a lower gas temperature and a thicker fiber. However, the total heat inputs are attenuated when the fiber's diameter is 300 μm. Orientation of the fiber is unimportant. Also, the evaporation rate is enhanced in an oxygen-containing gas flow, due to the additional heating from oxidation around the droplet.     

On the suppression of negative temperature coefficient (NTC) in autoignition of n-heptane droplets

Autoignition of n-heptane droplets under microgravity is investigated numerically. The comprehensive model, considering the transience in both the gas and liquid phases and non-ideal thermophysical properties, includes the 116-step reaction mechanism of Griffiths. Two-stage ignition manifests for ambient temperature less than 900 K at elevated pressures of 0.5 and 1.0 MPa. The predicted first delays and total delays agree well with the experimental data in the literature. The second delay decreases greatly with increasing pressure because a stronger Stefan flow supplies more fuel vapor for reaction as the cool flame shifts closer to the droplet to enhance evaporation. The Stefan flow effect, in combination with the inhomogeneous temperature and fuel vapor distributions, explains why the NTC (negative temperature coefficient) present in homogeneous mixtures is not observed in droplet ignition experiments. Near the minimum ignition diameter, the ignition delay increases for smaller droplets at T_∞ = 700 K, P_∞ = 1.0 MPa. For a droplet smaller than the minimum ignition diameter, only first ignition with cool flame is reached. The absence of ZTC (zero temperature coefficient) in our simulations may be attributed to the weaker inverse temperature dependence of the reaction mechanism adopted.     

Transient simulation of household refrigerators: A semi-empirical quasi-steady approach

This paper outlines a simulation model for the cycling behavior of household refrigerators and, therefore, for predicting their energy consumption. The modeling methodology follows a quasi-steady approach, where the refrigeration system and also the refrigerated compartments are modeled following steady-state and transient approaches, respectively. The mass, energy and momentum conservation principles were used to put forward the equation set, whereas experimental data were collected and used to reduce the model closing parameters such as the thermal conductances and capacitances. The experiments were carried out using a controlled temperature and humidity environmental chamber. The model predictions were compared to experimental data, when it was found that the energy consumption is well predicted by the model with a maximum deviation of ±2%. A sensitivity analysis was also carried out to identify opportunities for energy savings.     

Autoignition of n-decane under elevated pressure and low-to-intermediate temperature conditions

An experimental investigation of the autoignition for various n-decane/oxidizer mixtures is conducted using a rapid compression machine, in the range of equivalence ratios of ?=0.5-2.2, dilution molar ratios of N₂/(O₂ + N₂) = 0.79-0.95, compressed gas pressures of PC=7-30 bar, and compressed gas temperatures of TC=635-770 K. The current experiments span a temperature range not fully investigated in previous autoignition studies on n-decane. Two-stage ignition, characteristic of large hydrocarbons, is observed over the entire range of conditions investigated, as demonstrated in the plots of raw experimental pressure traces. In addition, experimental results reveal the sensitivity of the first-stage and total ignition delays to variations in fuel and oxygen mole fractions, pressure, and temperature. Predictability of two kinetic mechanisms is compared against the present data. Discrepancies are noted and discussed, which are of direct relevance for further improvement of kinetic models of n-decane at conditions of elevated pressures and low-to-intermediate temperatures.