N-heptane Droplets Research Articles

Investigation of thermodynamic state evolution, phase transition and mixing of the hydrocarbon droplet in convective supercritical environments

Liquid alkane droplets transit to supercritical fluids under supercritical conditions. However, the subsequent thermodynamic state evolution, phase transition, and mixing in this case, which directly influences combustion in actual diesel engines, need further investigation. We studied the thermodynamic state evolution, phase transition, and mixing of n-heptane droplets in supercritical laminar nitrogen streams through a set of conservation equations. Non-ideal thermodynamic and transport properties were calculated to incorporate the dramatic variation of physical properties in the supercritical regime. We focus on the effects of Reynolds number, ambient pressure, and droplet size on the thermodynamic state evolution, phase transition and mixing of droplets. Results showed that increasing the Reynolds number can accelerate the evolution of thermodynamic states, which leads to the faster transition of droplets from the liquid phase and supercritical fluids to gas-phase mixtures. With the increased Reynolds number, the phase transition rate is increased and the mixing homogeneity is improved due to the enhanced mass transfer. Caused by the negative correlation between mass transfer and ambient pressure, in higher pressure, droplets after the supercritical transition require more time to transition from supercritical fluids to vapor-phase mixtures. Increasing ambient pressure inhibited the droplet phase transition process and deteriorated the mixing homogeneity, but no significant influence was shown on the droplet dynamic evolution process. Smaller droplets transition faster from liquid phase and supercritical fluids to vapor-phase mixtures. The faster thermodynamic state evolution and faster dynamic evolution process of the smaller droplet leads to its more rapid homogeneous mixing with ambient gasses, but the mode of dynamics is independent of the droplet size.

Experiment and modeling of stochastic ignition and combustion of fuel droplets impacting a hot surface

The ignition dynamic of liquid fuel droplets impacting hot surfaces is critical for fire-safety analysis in engineering systems, as well as for controlling wall-filming effects in IC engines. The scenario of slow fuel-leakage rates poses challenges associated with the stochastic processes of droplet splashing, break-up, turbulent mixing, and combustion. To address this, we conduct controlled experiments of liquid n-heptane droplets impacting a heated surface in the Leidenfrost regime, targeting the individual-droplet-deposition conditions. The experiment encompasses a range of surface temperatures and droplet-deposition rates. The experiments are complemented by theoretical analysis, where we developed a stochastic low-order numerical model, demonstrating good accuracy for predicting ignition probability and overall combustion dynamics. Notably, we observe a broad region of intermittent combustion behavior, with ignition probability varying based on surface temperature and droplet deposition rate. Additionally, we find that the transition to consistent ignition relies heavily on both surface temperature and deposition rate. Experimental and numerical model results shed light on the roles of the complex interplay between droplet breakup, chemical kinetics, and evaporation and mixing time scales, as well as the interaction among subsequent droplet combustion events, in governing the ignition and combustion of impacting droplet trains. The revealed dynamic of droplet/hot-surface ignition and the proposed stochastic model hold promise for advancing predictive capabilities of hot-surface-induced ignition and combustion arising from accidental leaks in flammable-liquid piping and wall-filming, particularly in the stochasticity-dominated individual-droplet-deposition regime.

Detailed simulations of one-dimensional detonation propagation in dilute n-heptane spray and air mixtures

One-dimensional numerical simulations based on the hybrid Eulerian–Lagrangian approach are performed to investigate detonation dynamics in two-phase gas-droplet n-heptane/air mixtures with and without liquid fuel pre-vaporization. The reactive Navier–Stokes equations considering the two-way coupling for interphase exchanges of mass, momentum, energy, and species are solved with a skeletal mechanism consisting of 44 species and 112 reactions. The effects of n-heptane droplet diameter and equivalence ratio (ER) on average detonation speed and mode are studied. For pre-vaporization cases, the average detonation speed first decreases and then increases with droplet diameter ranging from 2.5 to 40 μm, which is minimum at 7.5–10 μm due to the competition between fuel vapor addition and droplet evaporative heat absorption. However, the average speed increases monotonically as the droplet ER increases from 0.2 to 1.2. A further increase in the droplet ER (e.g., 2.4) would lead to detonation suppression in the presence of large droplets (e.g., above 30 μm). The detonation is fully quenched when the droplet ER is 3.2. Similar observations are also made for the pure sprayed cases without n-heptane pre-vaporization, where the average speed increases rapidly for droplet ER of 0.2–0.8 and slowly for ER of 0.8–1.6. Various detonation modes are observed with respect to droplet diameter and equivalence ratio, either with or without fuel pre-vaporization. Generally, the pure sprayed cases show more irregular behaviors in detonation propagation. The laden droplets provide a new approach to control the intrinsically unstable or highly irregular behaviors of pure gas or pure sprayed detonations. The finite, small disturbances from the spatially non-uniform droplets, and enrichment from the droplet evaporative mass addition, are two essential mechanisms for the mitigation of the pulsating detonation.

The interaction between evaporation and chemistry on two-stage auto-ignition of an n-heptane droplet

This paper investigates the interaction between droplet evaporation and chemical reactions on two-stage auto-ignition. A two-phase model is established by embedding evaporation source terms into a well-stirred reactor. Evaporation Damkholer number (Daeva) is first proposed to investigate auto-ignition characteristics because it controls the evaporation-induced decrease of gas temperature and the increase of available fuel vapor. Three auto-ignition modes are observed with increasing Daeva. Both mode 1# and 2# are two-stage auto-ignition and their difference is whether the evaporation is completed before (mode 1#, Daeva ≤ 1.0) or after the low-temperature auto-ignition (mode 2#, Daeva > 1.0). The feature of mode 1# is that the second auto-ignition delay is unchanged with increasing Daeva. However, the second delay sharply increases for mode 2#. The mode 3# has a slow evaporation and it is a single-stage auto-ignition due to the disappearance of low-temperature auto-ignition. Ambient pressure does not influence Daeva at the 1#-2# boundary (Daeva≈1.0), while an increased equivalence ratio slightly decreases the turning Daeva. Furthermore, a decreased total auto-ignition delay is observed in two special areas with increasing evaporation timescale due to the shortening of the first or second auto-ignition delay. In the end, a new estimation method for auto-ignition delays is proposed for three auto-ignition modes and its error is controlled in the range of ±10%.

Quantitative investigation of sooting dynamics in droplet combustion using an automated image analysis algorithm

This paper reports an image analysis approach using a newly-developed open-source program to extract quantitative measurements of soot volume fraction (SVF) from digital video images of burning n-heptane droplets. The automated program developed in this work can analyze images of fixed and untethered droplets to quantify sooting dynamics. The images analyzed in the program were taken from experiments carried out in the Multi-user Droplet Combustion Apparatus (MDCA) onboard the International Space Station (ISS). In these experiments, video imaging of burning droplets was obtained using backlighting by a laser diode with a wavelength of 653 nm. The light was collimated before it passed through the droplet and soot-containing region, after which the light was then attenuated and projected onto the camera’s sensors. This technique facilitates the measurements of SVF based on the principles of the full field light extinction method (FFLEM). The measurements provide quantitative data that reveal the sooting dynamics of liquid fuels during droplet combustion processes.The analyses of a soot-attenuating image (ISS n-heptane, untethered droplet) at an instant during the burning show that the SVF distribution has a peak at the soot shell location. It then decreases due to soot oxidation when the location is further away from the burning droplet. Regarding temporal effects on the maximum SVF (SVFmax), results show that SVFmax first increases after the burning is initiated until a peak value is reached, after which SVFmax decreases. The SVFmax values identified in this study for n-heptane are quantitatively consistent with previously reported values for fiber-supported droplets and are reached relatively early in the burning history. n-Heptane images are also analyzed to show the effects of initial droplet sizes on the maximum soot volume fraction. Results show that SVFmax decreases with increasing initial droplet size, which is consistent with visual observations of less soot formed and dimmer flame brightness as Do increases.

Supersonic combustion of n-Heptane droplets with cavity-based fuel injection

PurposeThe purpose of this study is to investigate the combustion of the n-Heptane droplets in the supersonic combustor with a cavity-based fuel injection configuration. The focus is on the impacts of the droplet size on combustion efficiency.Design/methodology/approachThe finite volume solver is developed to simulate the two-phase reacting turbulent compressible flow using a single step reaction mechanism as finite rate chemistry. Three different fuel injection settings are studied for the considered physical geometry and flow conditions: the gas fuel injection, small droplet liquid fuel injection and big droplet fuel. The fuel is injected as a slot wall jet from the bottom of the cavity.FindingsThe results show that using the small droplet size, the complete fuel consumption and combustion efficiency can be achieved but using the big droplet sizes, most fuel exit the combustor in the liquid phase and gasified unburned fuel. It is also demonstrated that the cavity's temperature distribution of the liquid fuel case is different from the gas fuel, and two flame branches are observed there due to the droplet evaporation and combustion in the cavity.Originality/valueTo the best of the authors’ knowledge, this study is performed for the first time on the combustion of the n-Heptane fuel droplets in scramjet configuration, which is promising propulsion system for the future economic flights.

Effects of pulse power on evaporation characteristics of an n-heptane droplet in a microtube

When combustion instability arises in liquid rocket engines, heat release fluctuations cause local temperature oscillations and affect the evaporation of fuel droplets. In this study, to investigate the responses of droplet evaporation characteristics to temperature oscillations, an n-heptane droplet is formed by continuous fuel injection, and a pulse power is applied to generate temperature oscillations. The pulse power outputs a square wave with a frequency of 0.1–10 Hz and a power of 16.6–30.0 W. A transient heat transfer and droplet evaporation model is established, and measured droplet diameters serve to validate the model. The effects of heating frequency, heating power, and waveforms on droplet vaporization are studied. The results show that pulse power induces oscillations in air temperature, droplet diameter, and evaporation rate, which correspond to the applied heating frequency. Due to heat transfer within the tube, a phase lag is observed in the air temperature relative to the applied power. The dynamic equilibrium between the evaporation rate and fuel flow rate causes the evaporation rate phase to lag behind the droplet diameter phase by a constant of 0.5π, and to lead slightly ahead of the air temperature phase. Increases in heating frequency reduce the amplitudes of air temperature, droplet diameter, and evaporation rate while maintaining their averages constant. Decreases in heating power lower the air temperature, consequently increasing the evaporation rate amplitude and average droplet diameter. At 16.6 W, the droplet descends due to exceeding the critical diameter of 2.02 mm. Replacing the square wave of pulse power with a sinusoidal or triangular wave only results in a decline in air temperature amplitude, consequently reducing the evaporation rate amplitude by an equivalent degree. The amplitude percentage ratio between them remains unchanged at 4.77 but is subject to interference from their phase difference. As frequency increases from 0.1 to 1 Hz, the phase difference rises from −0.08π and converges to 0, while the amplitude percentage ratio increases from 4.77, converging to 4.87.

Analysis of Water Droplet Interaction with Turbulent Premixed and Spray Flames Using Carrier Phase Direct Numerical Simulations

ABSTRACT Carrier-phase direct numerical simulations (DNS) of n-heptane/air combustion with water droplet injection are reported in this paper. The influence of water droplet injection has been analyzed for statistically planar gaseous premixed flames and spray flames where the fuel is supplied in the form of monodisperse fuel droplets. The effects of water injection in both types of flames are compared, including their sensitivities on the initial diameter of water droplets. The simulations are based on an Eulerian-Lagrangian-Lagrangian approach to consider the multi-phase interaction and the simultaneous effects of n-heptane and water droplets. It is shown that the two types of flames are different in terms of several aspects. In particular, the spray flames are characterized by lower temperatures, and their propagation speed is lower than for premixed flames. The main reason for these differences lies in the gaseous equivalence ratio experienced by the reaction zone. For the cases investigated here, where the overall equivalence ratio is unity, the spray combustion occurs under predominantly fuel-lean conditions. Furthermore, the flame temperature is influenced by the initial diameter of water droplets, which affects the strength of thermal expansion within the flame. This in turn influences the evaporation characteristics of both fuel and water droplets due to different residence times within the flame. Although the effects of water droplets on the combustion mode are dependent on the relative position within the spray flame, the net effect is a partial shift from non-premixed to premixed mode. At the same time, the spray combustion occurs under relatively fuel-richer conditions with water injection and these trends appear consistently in laminar and turbulent flows.

Simulation of Low-Temperature Oxidation and Combustion of N-Dodecane Droplets under Microgravity Conditions

Fires are considered among the most dangerous accidents on manned spacecraft. That is why several programs of combustion experiments were implemented at the International Space Station (ISS) since 2008. In the experiments with n-heptane and n-dodecane droplet combustion, a new phenomenon was discovered, namely, the phenomenon of the radiative extinction of a burning droplet with subsequent multiple flashes of flame. In this paper, n-dodecane droplet ignition, combustion, radiative extinction, and subsequent low-temperature oxidation with multiple flashes of cool, blue, and hot flames under microgravity conditions are studied computationally. The mathematical model takes into account multiple elementary chemical reactions in the vicinity of a droplet in combination with heat and mass transfer in liquid and gas, heat release, convection, soot formation, and heat removal by radiation. The model is based on the non-stationary one-dimensional differential equations of the conservation of mass and energy in liquid and gas phases with variable thermophysical properties within the multicomponent diffusion concept in the gas phase. Calculations confirm the important role of the soot shell formed around the droplet and low-temperature reactions in the phenomenon of droplet radiative extinction with multiple flame flashes in the space experiment at the ISS. Calculations reveal the decisive role of the blue flame, arising due to the decomposition of hydrogen peroxide, in the multiple flame flashes. Calculations with forced ignition of the droplet reveal the effect of the ignition procedure on droplet evolution in terms of the timing and the number of cool, blue, and hot flame flashes, as well as in terms of the combustion rate constant of the droplet. Calculations with droplet self-ignition reveal the possible existence of new modes of low-temperature oxidation of droplets with the main reaction zone located very close to the droplet surface and with only partial conversion of fuel vapor in it.

Acoustic excitation of an n-heptane droplet: Evaporation, ignition and combustion characteristics

In aerospace propulsion systems, the coupling between droplet combustion and sound waves can be a key mechanism for driving thermoacoustic instability. In this experimental study, we examine the evaporation, ignition and combustion characteristics of an n-heptane droplet subjected to time-periodic acoustic excitation of different amplitudes (12.5 to 37.5 Pa) and frequencies (100 to 500 Hz). On applying acoustic excitation, we find that the ignition delay time of the droplet decreases, while both the evaporation rate and the combustion rate increase. All three effects strengthen as the excitation amplitude increases, while the first and third effects strengthen as the excitation frequency decreases. These trends are corroborated by analysis of time-resolved CH* chemiluminescence images, which reveal that the heat-release-rate oscillations of the burning droplet grow in amplitude either as the excitation amplitude increases or as the excitation frequency decreases. The pronounced effects on droplet ignition and combustion observed during low-frequency acoustic excitation can be attributed to the time scale of chemical reaction being significantly shorter than that of the acoustic excitation. This study contributes to a better understanding of droplet combustion in the presence of time-periodic acoustic oscillations, facilitating the development of aerospace propulsion systems that can potentially be more resistant to thermoacoustic instability.

IMPROVING THE PERFORMANCE OF ZNO NANOPARTICLES FOR EOR IN HIGH-TEMPERATURE AND HIGH SALINITY CARBONATE RESERVOIRS

The main challenge of the application of nanofluids for enhanced oil recovery (EOR) in carbonate reservoirs is to maintain colloidal stability under reservoir conditions with high salinity and high temperature. In this study, we address this issue by increasing the stability and hydrophilicity of ZnO nanoparticles by adding TTIP. Adding TTIP on the surface of ZnO nanoparticles results in an increase in the hydrophilic heads in the final product. Then, these nanoparticles are used to coat carbonate rock surfaces to change their wettability. The coated rock plates are obtained by aging them in nanofluids. The modified ZnO-based coatings show to be more effective for wettability alteration purposes compared to the conventional ZnO coating. The un-coated rock plate is strongly oil-wet, where the water and n-heptane droplets contact angles on this surface are 168° and 0°, respectively. After aging the rock samples in nanofluids, superhydrophilic coatings form on the superhydrophobic surfaces. The rock surface before and after treatment, as well as the synthesized nanoparticles, are characterized by scanning electron microscope (SEM), x-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR) analyses. Results of this study indicate the possibility of using the materials for wettability alteration of oil-wet carbonate rock in the EOR process. The results of core flooding tests demonstrate that the oil recovery enhances significantly through the nanofluid flooding.

Investigation of supercritical transition and evaporation process of a hydrocarbon droplet under diesel engine-relevant conditions

Fuel spray supercritical transition in the diesel engine has been concerned widely, especially supercharged ones, however, whether the transition can significantly affect the fuel–air mixing under diesel engine-relevant conditions is an issue that still needs evaluating carefully. Thus, the transition and evaporation of individual n-heptane droplets in a nitrogen environment under various ambient conditions were investigated through a high-pressure evaporation model. The influence of ambient pressures on droplet lifetime (τlife) and evaporation rate constant (K) varies with ambient temperature. When Tr < 0.98, with increasing pressure, τlife first increases and then decreases, and K decreases. When 0.98 < Tr < 1.28, as pressure increases, τlife would increase even if K increases, because the increment of initial heat-up periods exceeds the decrease in quasi-steady evaporation periods. When Tr > 1.28, with increasing pressure, τlife decreases, and K increases. As the droplet heats up, the droplet interface is where the binary mixture would reach the critical mixing state if ambient conditions are high enough. Increasing ambient density decreases the minimum ambient temperature (Tm) required for the transition to occur. Above the Tm, transitions occur earlier with the increasing energy density of ambient gas. Increasing ambient density decreases the minimum ambient temperature required for diffusive fuel-gas mixing modes to dominate the mixing process. For droplets large enough to ignore gas–liquid interfacial thickness, the Tm required for the attainment of critical mixing states is almost independent of droplet diameter.

Numerical study of drop spread and rebound on heated surfaces with consideration of high pressure

The impact of a drop on a solid surface has been studied for many years. However, most of the previous numerical simulations were focused on the drop impact on a surface at room temperature and standard atmospheric pressure. This paper presents a numerical study of n-heptane and n-decane drops impacting solid surfaces with the consideration of high temperature and high pressure using smoothed particle hydrodynamics (SPH). The SPH method is validated against experiments from our work and literature. This work is focused on two typical drop-impact regimes, namely, spread and rebound. Different drop impact sequences were simulated at the wall temperature in the range of 27–400 °C and the ambient pressure between 1–20 bars. The difference between the inception of film boiling and liquid saturation temperature was found to decrease with elevating ambient pressure. The spread factor and apex height are investigated for the regime of spread. The results indicate that the lower viscosity fluid has a smaller spread factor as compared to the fluid with higher viscosity. The variation of Leidenfrost temperature with ambient pressure for both n-heptane and n-decane droplets is established numerically and compared with the trend observed in the experiment. The simulation outcomes of drop rebound for high boiling point liquid (n-decane) in the film boiling regime at atmospheric pressure show that with the increasing wall temperature, the drop rebound height and vapor layer height increase. Finally, the effect of ambient pressure on drop rebound height and velocity is investigated. The numerical results indicate that the increase in ambient pressure reduces the droplet rebound velocity and rebound height.

A Most Reactive Mixture Analysis of Ozone- and Temperature-Enhanced n-Heptane Droplet Autoignition and Cool Flame Burning in Microgravity

ABSTRACT Numerical simulations are used to study effects of ozone (O3) and thermal enhancement on autoignition and subsequent burning characteristics of isolated n-heptane droplets in microgravity conditions. Particular focus is given to effects of O3 on low temperature combustion, or cool flames. Levels of ambient O3 mole fraction and ambient temperature are varied from 0 to 0.03 and 400 K to 750 K, respectively. A comparison between two low temperature chemistry enhancement strategies – O3 addition and temperature increase – are investigated on an equivalent energy addition basis to evaluate relative effects on combustion characteristics. Results of this study are analyzed using physical and mixture fraction approaches because of unique insights offered by these frameworks. Ignition and ensuing flame propagation are investigated via a most reactive mixture analysis, evaluated using homogeneous reactor simulations of mixture compositions and temperatures surrounding the droplet. Lastly, a Damköhler number analysis is performed to explore oscillatory ignition dynamics.

Extinction characteristics of isolated n-alkane fuel droplets during low temperature cool flame burning in air

Large carbon number n-alkanes are a notable component in all real transportation fuels, and their chemical structure fosters substantial low temperature kinetic reactivity. Normal alkanes have been studied in various canonical configurations but rarely in systems with strong coupling between low temperature chemistry and transport for pure as well as for multi-component n-alkane mixtures. The Flame Extinguishment (FLEX) experiments onboard the International Space Station provided a unique platform for investigating low temperature multi-phase n-alkane and iso-alkane combustion. Among the many interesting phenomena experimentally observed, cool flame extinction can occur, accompanied by the concurrent formation of a surrounding cloud of condensed vapor. In this work we conduct numerical simulations of high and low temperature combustion of large, initially single-component n-heptane, n-decane and n-dodecane droplets. The role of initial droplet diameter, operating pressure, and n-alkyl carbon number on the extinction of hot and low temperature flames is investigated and compared against the available experimental data. While all three fuels exhibit similar hot flame behavior, cool flame activity increases with the carbon number, resulting in an increased cool flame temperature and decreased extinction diameter. Multi-cyclic “hot/cool flame transitions” are found in air as pressure is slightly increased above one atmosphere. The cyclic behaviors correspond to continuously varying hot and cool flame transitions across the high, low, and negative temperature coefficient (NTC) kinetic regimes. Further increase in pressure results in a second stage steady “Warm flame” transition. The extinction of hot and cool flame has a strong non-linear dependence on ambient pressure but as the hot flame extinction diameter increases with pressure the extinction diameter of the cool flame decreases. The computational results are compared with a recent asymptotic analysis of FLEX n-alkane cool flames.

Surface temperature measurement of cooling and heating droplets by rainbow refractometry.

Rainbow refractometry was used to measure the temperature and size of transparent spherical particles. In practice, however, there are limitations to the application of heating and cooling droplets, as the temperature measured is neither the average nor the surface or core temperature of the droplet. Reported here is an exploitation of this technique for droplet surface temperature determination. Droplet surface tension was measured by detecting the evolution of interference fringes of oscillating droplets. The dependence of surface tension on temperature facilitated the study of surface temperature of an evaporating droplet with time. Moving ethanol, n-heptane, and n-decane droplets were investigated under heating and cooling conditions. The capabilities and limitations of rainbow refractometry were verified by comparing the droplet temperature values measured directly by rainbow refractometry with the surface temperature.

Autoignition of isolated n-heptane droplets in air and hot combustion products at microturbine conditions

Spontaneous ignition of isolated n-heptane droplets with initial diameters of 20–100 is simulated using air at 4 atm and 700–1200 K, which includes the typical operating conditions of recuperated microturbines. Because some fuel droplets in a combustor may be sprayed or carried to near the recirculation zone, the simulations use a mixture of pure air and hot combustion products as the oxidiser. The flame structures, evaporation times, and autoignition times in both physical and mixture fraction spaces for the different conditions are presented and compared. The variables examined include the air preheat temperature, amount of dilution with hot products, initial fuel droplet diameter, oxidiser temperature, and oxygen concentration. The results show that droplets in pure air at microturbine conditions fully evaporate before ignition, suggesting that a prevaporised concept is suitable for microturbines. The dilution with hot combustion products decreases the ignition delay time mainly by raising the oxidiser temperature. Low-temperature chemistry does not have a significant effect on droplet ignition because adding even a small amount of hot combustion products can increase the oxidiser temperature to higher than the temperatures favourable for low-temperature kinetics. The cool flame is only observed for 100 droplets at low temperatures, but two-stage ignition is not observed.

Simulations of rotating detonation combustion with in-situ evaporating bi-disperse n-heptane sprays

Eulerian-Lagrangian simulations are conducted for two-dimensional Rotating Detonative Combustion fueled by bi-disperse n-heptane sprays without any fuel pre-vaporization. Parametric studies are performed to study the influences of droplet diameter and droplet distribution on the rotating detonation wave. The extinction process of the detonation wave is also been analyzed. It is found that small n-heptane droplets (e.g., 2 µm) are completely vaporized in the fuel refilling area. Increasing the droplet diameter causes the droplet to fail to evaporate completely within the fuel refilling area and exist after the detonation wave. A reflected shock can be observed after the detonation wave. When the droplet diameter is larger than 10 μm, the higher pressure after the detonation wave leads to the reactants cannot be sprayed into the combustor, eventually leading to extinction of the detonation wave. In bi-disperse n-heptane sprays, presence of droplets with small diameter stabilizes the detonation wave. The average equivalence ratio (up to 0.66 only) in the fuel refilling area is lower than total equivalence ratio (1.0 in this work), and the average equivalence ratio decreases with increased droplet diameter in the bi-disperse n-heptane sprays. The increase in droplet diameter decreases the detonated fuel fraction and detonation wave speed. The detonation speeds in bi-disperse n-heptane sprays are 3–9% lower than the respective gaseous cases. Moreover, the results also show that propulsion performance of rotating detonation combustor, such as thrust and specific impulse, decreases with the droplet diameter.

Effects of Pulse Power on Evaporation Characteristics of N-Heptane Droplet in a Micro-Tube

Effects of Pulse Power on Evaporation Characteristics of N-Heptane Droplet in a Micro-Tube

Numerical Simulation of Monodisperse Lube Oil Multiple Droplet Evaporation and Autoignition under Nonconstant Cylinder Conditions of Low-Speed Two-Stroke Gas Engines.

The possibility of lube oil droplets’ existence in cylinders for two-stroke low-speed gas engines is higher because of the much higher lube oil consumption rate. Some droplets are directly injected into cylinders by lube oil injectors, and some are blown into cylinders through the scavenging ports. Autoignition of cylinder oil droplets is the main cause of preignition. This research study indicates that under in-cylinder conditions, overlarge single droplets cannot autoignite due to the long evaporation time, and overly small single droplets cannot autoignite because of the low vapor concentration. To find out what kinds of oil droplet groups could autoignite and cause preignition, 3-D computational fluid dynamics simulation in OpenFOAM was carried out. The model predictions were validated against the experimental results, including the evaporation rate of n-heptane droplets and the ignition delay of lube oil droplets. Also, the simulation was used to investigate the characteristics of multiple droplets under different ambient temperatures and pressures. The evaporation lifetime and the ignition delay of a large single droplet are dozens of times longer than that of multiple droplets, which confirms that the droplet group is more dangerous than a large single droplet. The evaporation rate and ignition delay are affected by the distance and number of droplets. A larger number causes a lower average evaporation rate. A smaller distance causes a shorter ignition delay. The local vapor concentration and temperature could be greatly reduced due to the existence of multiple evaporation and ignition cores. Additionally, these findings of multiple droplets were confirmed under nonconstant cylinder conditions of a natural gas engine. This research provides a guide to design the lube oil injectors and scavenging ports so as to reduce the preignition caused by lube oil autoignition.