Vapor Penetration Research Articles

Experimental study on the non-reacting spray characterization of gasoline compression ignition fuel

Gasoline compression ignition (GCI) engines have recently become a topic of interest due to its potential for very high thermal efficiencies with significant pollution reduction. High reactivity gasoline-like fuels with shorter ignition delay and similar volatility compared to market gasoline have been identified as a viable option for the GCI engines. To fully realize the potential of gasoline-like fuels in GCI engines, spray characterization of the fuel is needed for developing a better understanding of these fuels’ behavior in perspective of GCI engine applications. In this study, the spray characteristics of high reactivity gasoline-like fuel specially designed for GCI engines were investigated and compared with commercial gasoline fuel under non-reacting conditions. The new GCI fuel of RON 77 and E10 market gasoline of RON 91 were tested using a high-pressure custom-made multi-hole injector. The rate of injection, liquid and vapor penetration lengths were measured for both the fuels. It was found that the spray characteristics of GCI fuel are similar to that of the commercial E10 gasoline under non-reacting conditions.

Robust cellulose nanocomposite films based on covalently cross-linked network with effective resistance to water permeability

Cellulose films are of poor water-vapor barrier performance. Herein, we put forward an effective way to suppress adsorption by crosslinking of hydroxyl groups via epichlorohydrin (ECH), meanwhile graphene oxide (GO) nanosheets are utilized to prolong the pathway of vapor penetration. The strategy confers a significant enhancement of vapor barrier performance as well as mechanical properties to cellulose-based films. Specifically, an outstanding reduction of 67.4% in water-vapor permeability coefficient is achieved in nanocomposite films compared to the uncrosslinked cellulose films. Furthermore, for the first time, two-dimensional correlation analysis reveals that crosslinking of ECH do not alter penetration direction, while GO can eminently act as shielding for the formation of bound water which change the sequential order of firstly-interacted vapor area from crystalline to amorphous area. Free volume is the penetration destination. The retarding effect introduced by the GO in amorphous area gives rise to the improvement of the vapor-barrier.

Encapsulation of Organic and Perovskite Solar Cells: A Review

Photovoltaic is one of the promising renewable sources of power to meet the future challenge of energy need. Organic and perovskite thin film solar cells are an emerging cost-effective photovoltaic technology because of low-cost manufacturing processing and their light weight. The main barrier of commercial use of organic and perovskite solar cells is the poor stability of devices. Encapsulation of these photovoltaic devices is one of the best ways to address this stability issue and enhance the device lifetime by employing materials and structures that possess high barrier performance for oxygen and moisture. The aim of this review paper is to find different encapsulation materials and techniques for perovskite and organic solar cells according to the present understanding of reliability issues. It discusses the available encapsulate materials and their utility in limiting chemicals, such as water vapour and oxygen penetration. It also covers the mechanisms of mechanical degradation within the individual layers and solar cell as a whole, and possible obstacles to their application in both organic and perovskite solar cells. The contemporary understanding of these degradation mechanisms, their interplay, and their initiating factors (both internal and external) are also discussed.

Spray performance of air-assisted kerosene injection in a constant volume chamber under various in-cylinder GDI engine conditions

The performance of kerosene spray from an air-assisted system was investigated in a constant volume chamber by backlit imaging and shadowgraph technologies over various chamber pressures, temperatures, and fuel amount. The results show that the penetration decreases with the increase in the chamber pressures from 0.5 bar to 3.5 bar due to the reduction in the differential pressure between the air/fuel interface and ambience and the rise in the resistance to penetration as well. Liquid-phase penetration increases with an increase in temperature from 400 K to 500 K both at 1.0 bar and 3.0 bar for decreasing density of ambient gas. When ambient temperature was increased at 1.0 bar, there was no significant change in the vapor penetration. Therefore, increasing ambient temperature by internal EGR cannot reduce wall impingement when GDI engines fueled with kerosene. Furthermore, liquid-phase penetration decreases with the increase in fuel amount, while at the front end of spray the cross-sectional area increases since the spray is increasingly disturbed by the ambient gas.

Screen Compliance Experiments for Application of Liquid Acquisition Device in Space

The purpose of a liquid acquisition device (LAD) is to separate liquid and vapor phases inside a spacecraft propellant storage tank in the reduced gravity and microgravity conditions of space so that vapor-free liquid can be extracted to the transfer line. A popular type of LAD called a screen channel LAD or gallery arm, uses a fine porous screen and surface tension forces of the liquid to allow pure liquid to flow through the screen while blocking vapor penetration. To analyze, size, and optimize the design of LADs for future in-space propellant transfer systems, models and data are required for the four fundamental influential factors for LAD systems, including bubble point, flow-through-screen pressure drop, wicking rate, and screen compliance for a wide variety of screen meshes. While there is sporadic data available for three of these parameters, there is no published quantitative data for screen compliance. During the transient startup of propellant transfer, the liquid must be accelerated from rest to the steady flow demand velocity, which causes the screen to deform or comply, so compliance data is required for accurate transient LAD analyses; most design codes only consider steady state analysis. This paper presents screen compliance experiments on 14 different screens, examining the effects of fineness of mesh, open area, and screen metal type on compliance. A basic equation of state is also developed and validated against the data which can be easily integrated into any transient LAD flow code to model propellant transfer.

Large eddy simulations of diesel-fuel injection and auto-ignition at transcritical conditions

Large eddy simulations of transcritical injection and auto-ignition of n-dodecane in a combustion chamber are performed. To this end, a diffuse-interface method is employed that solves the compressible multi-species conservation equations, and a cubic state equation together with real-fluid transport properties is employed to describe the transcritical fluid state. The reaction chemistry is represented by a finite-rate chemistry model involving a 33-species reduced mechanism for n-dodecane. Compared to commonly employed two-phase approaches, the method presented in this work does not introduce tunable parameters for spray-breakup. Large eddy simulation calculations are performed by considering the Spray A single-hole injector at non-reacting and reacting conditions at a pressure of 60 bar and temperatures between 800 and 1200 K. Quantitative comparisons with measurements for liquid and vapor penetration lengths are performed for non-reacting conditions, and sensitivity to threshold values on mixture fraction are examined. The analysis of reacting flow simulations focuses on comparisons of the instantaneous temperature and species fields for OH and CH2O at 800 and 900 K, respectively. Quantitative comparisons with measurements for ignition delay and lift-off heights as a function of ambient temperature are performed. To examine the transient ignition phase, comparisons of radially integrated OH profiles obtained from the simulations with reported measurements for OH* are performed, showing good agreement. These results show that the large eddy simulation modeling framework adequately reproduces the corresponding ignition processes, which are relevant to realistic diesel-fuel injection systems.

Large-eddy simulation of dual-fuel ignition: Diesel spray injection into a lean methane-air mixture

In the present study, large-eddy simulation (LES) together with a finite-rate chemistry model is utilized for the investigation of a dual-fuel (DF) ignition process where a diesel surrogate (n-dodecane) spray ignites a lean methane-air mixture in engine relevant conditions. The spray setup corresponds to the Engine Combustion Network (ECN) Spray A configuration enabling an extensive validation of the present numerical models in terms of liquid and vapor penetration, mixture distribution, ignition delay time (IDT) and spatial formaldehyde concentration. The suitability of two n-dodecane mechanisms (54 and 96 species) to cover dual-fuel chemical kinetics is investigated by comparing the predicted homogeneous IDTs and laminar flame speeds to reference values in single-fuel methane-air mixtures. LES of an n-dodecane spray in DF conditions is carried out and compared against the baseline ECN Spray A results. The main results of the study are: (1) ambient methane impacts the ignition chemistry throughout the oxidation process. In particular, the activation of the low-temperature chemistry is delayed by a factor of 2.6 with both mechanisms, whereas the high-temperature chemistry is delayed by a factor of 1.6–2.4, depending on the mechanism. (2) The ignition process starts from the spray tip. (3) There exists a characteristic induction time in the order of 0.1 ms between the start of the first high-temperature reactions and the time when maximum methane consumption rate is achieved. (4) The high-temperature ignition process begins near the most reactive mixture fraction conditions. (5) The role of low-temperature reactions is of particular importance for initiation of the production of intermediate species and heat, required in methane oxidation and (6) both applied mechanisms yield qualitatively the same features (1)–(5) in the DF configuration.

Understanding spray and atomization characteristics of butanol isomers and isooctane under engine like hot injector body conditions

Spray characterization of GDI injectors is of significant importance as it influences the overall engine performance and its emissions. Spray morphology under various engine like conditions influences engine design. Vapor bubbles are formed inside liquid droplets when hot fuel is injected into the sub-saturation pressure conditions leading to flash boiling of GDI sprays. This dramatically alters the spray morphology and therefore needs to be understood thoroughly. A spray undergoing flash boiling experiences catastrophic breakup of the liquid jet resulting in fine atomization which in turn enhances droplet evaporation. However, spray structure is significantly altered under such conditions. In the present work, effect of hot injector body was studied to understand flash boiling behaviour of alcohol fuels and which were then compared to isooctane spray characteristics under similar conditions. Experiments were performed at five different injector temperatures: 298 K, 373 K, 423 K, 473 K and 523 K. Significant difference in spray morphology was observed when the fuel temperature was increased. Reduction in overall spray cone angle was observed with increase in temperature for all the fuels considered in this study. Vapour penetration lengths increased with increase in temperature, whereas liquid penetration initially increased and then decreased significantly due to higher evaporation rate. PDPA data showed significant reduction in SMD and AMD when the temperature was raised from 298 K to 423 K. SMD for n-butanol, isobutanol and isooctane reduced by 58.45%, 54.51% and 64.87%, respectively. Similarly, AMD reduced by 65.58%, 57.12% and 74.24%, respectively.

Irradiation-induced mixing of Na and K in graphite in molten salt reactor: An estimation based on ion beam irradiation

The massive molten salt-graphite interface produced by liquid infiltration or vapor penetration in a molten salt reactor suggests the importance of irradiation-induced mixing and diffusion. We employed He+ ion beam irradiation to simulate neutron irradiation-induced mixing of Na and K in graphite. The depth distributions of both Na and K were measured by RBS (Rutherford backscattering spectrometry) and XPS (X-ray photoelectron spectroscopy) depth profiling. Both the mixing rate and the diffusion rates of the Na and K mark layers in the graphite induced by the ion beam irradiation were calculated. The results indicate that the diffusion rates of Na and K in the graphite reflector and/or moderator were several hundred nanometers per dpa irradiation dose. This could induce a change in the properties and cohesion of graphite.

Experimental investigation on GDI spray behavior of isooctane and alcohols at elevated pressure and temperature conditions

In this study macroscopic spray characteristics and droplet size distribution were measured for spray emanating from a six-hole solenoid GDI fuel injector. Three test fuels namely Isooctane, ethanol and n-butanol were used in this study. Macroscopic spray characterization was performed using Mie scattering and Schlieren Shadowgraph technique along with high speed videography. Liquid and vapor penetration length and along with the liquid/vapor spray cone angle was determined using these techniques. Phase Doppler Particle Analyzer (PDPA) was used to determine droplet size distribution. Results showed that thermo-physical properties such as saturation temperature, latent heat of vaporization, surface tension, density and viscosity of a fuel play a very important role in the spray plume penetration. Isooctane showed the least penetration lengths for all pressure and temperature conditions that were evaluated in this study, whereas butanol showed the highest penetration length. Due to the higher values surface tension, viscosity and latent heat of evaporation, alcohol sprays consisted of droplets of larger diameter.

Large Eddy Simulation on the Flame Structure for Split Injections of n-dodecane at Different Temperatures and Densities

ABSTRACTLarge eddy simulation coupled with linear eddy model is applied for the simulation of n-dodecane spray flames with multiple injections (0.5 ms injection/0.5 ms dwell/0.5 ms injection). The numerical model is validated by comparing the vapour penetration length at inner condition and the predicted ignition delay times at reacting conditions with the experimental results. Good agreement between the predicted and measured data is observed. The effects of initial conditions, including the initial temperature and ambient gas density on the development of double injections are analyzed. By decreasing the ambient gas density or initial temperature, the appearance of high-temperature kernels is delayed. The tip penetration travels faster both for the first and second injections when the initial gas density is reduced from 22.8 to 15.2 kg/m3 at an initial temperature of 900 K. By decreasing the initial gas temperature from 900 to 800 K, the tip penetration travels much slower. This is because that the high-temperature combustion appears very early at 900 K. The combustion of the first injection leads to the increase in local velocity and this effect becomes more obvious as the ignition delay (ID) is shortened. At latter times, the penetration velocity is almost the same for the second injection. Moreover, as the second spray is injected into hotter environment containing different key intermediate species, ID is reduced for the second injection. The reduction in ID is also related to the time when the second spray catches up with the first one. The inhibited mixing between fuel and air leads to richer ignition in the second injection compared with the first injection. For fuel-rich cells with the mixture fraction greater than 0.1, formaldehyde (CH2O) is quickly formed. Moreover, CH2O mass fractions in all of these cells have higher values compared with the first injection.

Characteristics of trans-critical propane spray discharged from multi-hole GDI injector

This paper is to investigate the characteristics of trans-critical propane spray compared with the flash boiling spray ejected from a multi-hole injector in a constant volume chamber by using the Schlieren and backlit imaging methods. The fuel temperature (Tf) is set from 30 °C to 120 °C, injection pressure (pf) from 60 bar to 120 bar and ambient pressure (pa) from 0.2 bar to 10 bar.The results show that the trans-critical spray has longer vapor penetration and shorter liquid penetration than the flash boiling spray, but those two sprays have similar curve liquid boundaries near the nozzle. Several discernable collapsed shock structures near the nozzle are observed at Tf = 120 °C, pf=120 bar, pa = 1 bar, but they disappear when pf drops to 60 bar for the transition path may not enter the two-phase region, and the liquid phase hardly occurs. The Mach disk distance of trans-critical spray in this study is larger than that of the under-expanded ideal gas jet because of the collapse phenomenon. At Tf = 120 °C, pf = 120 bar, the liquid length decreases when paincreases from 0.2 bar to 2.5 bar and increases when pa continues to rise.

Scaling analysis of heat and mass transfer processes in an adsorption packed bed

In this article, a scaling analysis of heat and mass transfer processes in an adsorption packed bed is presented. New scaling parameters that characterize the performance of the adsorption cooling bed are derived and their importance are discussed. In addition, a numerical study is performed to illustrate the roles played by the newly derived scaling parameters. It is found that the presence of heat of adsorption makes the apparent heat capacity of an adsorption bed much larger than the heat capacity of the adsorbent material itself. The present results indicate that the heat diffusion and vapor penetration depths can be used to specify the desired working pair properties and the adsorbent layer thickness for producing the maximum cooling power. From the results of scaling analysis, it can be concluded that the inter-particle permeation resistance has a considerable effect on the performance of an adsorption silica gel bed when the particle diameter to adsorbent layer thickness ratio is less than 0.1. It can also be concluded that performance of a silica gel packed bed is controlled by the conductive thermal resistance when the Fourier number of the adsorbent-adsorbate layer (Fo) is less than 1.0. The convective resistance (i.e., external resistance) dominates the bed performance when the dimensionless temperature ratio (Θ) of adsorption silica gel packed bed is more than 0.2.

Low-Temperature Atomic Layer Deposition of Metal Oxide Layers for Perovskite Solar Cells with High Efficiency and Stability under Harsh Environmental Conditions

Rapid progress achieved on perovskite solar cells raises the expectation for their further development toward practical applications. Moisture sensitivity of perovskite materials is one of the major obstacles which limits the long-term durability of the perovskite solar cells, especially in outdoor operation where rainfall and water accumulation on the solar panels often occur. Micro/nanopinholes within the functional layers of the devices usually lead to water vapor penetration, thus subsequent decomposition of perovskites, and finally poor device performance and shortened operational lifetime. In this work, low-temperature atomic layer deposition (ALD) technique was utilized to incorporate pinhole-free metal oxide layers (TiO2 and Al2O3) into an inverted perovskite solar cell consisting of indium tin oxide/NiO/perovskite/PC61BM/TiO2/Ag. The interface properties between the inserted TiO2 layer and the perovskite layer were investigated by X-ray photoelectron spectroscopy. The results showed that TiO2 ALD fabrication process had made negligible degradation to the perovskite layer. The TiO2 layer can significantly reduce interfacial charge recombination loss, improve interfacial contact, and enhance water resistance. A maximum power conversion efficiency (PCE) of 18.3% was achieved for devices with TiO2 interface layers. A stacked Al2O3 encapsulation layer was designed and deposited on top of the devices to further improve device stability under harsh environmental conditions. The encapsulated devices with the best performance retained 97% of the initial PCE after being stored in ambient condition for a thousand hours. They also showed great water resistance, and no significant degradation in terms of PCE and photocurrent of the devices was observed after they were immersed in deionized water for as long as 2 h. Our approach offers a promising way of developing highly efficient and stable perovskite solar cells under real-world operational conditions.

Modeling of frost layer growth considering frost porosity

A numerical model for predicting frost layer growth based on computational fluid dynamics is developed. This model can predict the growth behavior of a highly porous frost layer formed by desublimation. A new volumetric mass transfer rate equation is proposed, which can consider water vapor penetration into a frost layer. The model is validated through experimental results under various operating conditions and used for analyzing the frost layer growth process. The density distribution inside the frost layer is almost linearly changed in the direction perpendicular to the cooling surface under the operating conditions favorable for desublimation, showing different characteristics from the case in the operating conditions favorable for freezing after condensation. In addition, the average mass transfer rate is analyzed as a function of time. As time passes, the porosity of the frost layer decreases and the mass transfer rate due to water vapor penetration decreases gradually.

An optimization method for formulating model-based jet fuel surrogate by emulating physical, gas phase chemical properties and threshold sooting index (TSI) of real jet fuel under engine relevant conditions

Two jet fuel surrogates (S1 and S2) were proposed in this work, aimed at improving the quantitative accuracy of fuel properties that affect both premixed and spray-guided combustion modes under engine relevant conditions by emulating real jet fuel properties including physical, gas phase chemical properties and threshold sooting index (TSI). An intelligent optimization approach was employed to calculate the composition that inherently satisfies both the physical and gas phase chemical characteristics as well as sooting tendency. The jet fuel surrogate S1 was composed of five components including decalin, n-dodecane, iso-cetane, iso-octane and toluene (0.005/0.4011/0.1249/0.098/0.371 by mole fraction), while surrogate S2 is a mixture of the same components but different in proportions (0.1449/0.3706/0.2059/0.0195/0.2591 by mole fraction). Based on the newly proposed surrogates, a skeletal jet fuel surrogate chemical reaction mechanism was developed by describing the chemistries for the oxidation of large molecules C4Cn and smaller H2/CO/C1 molecules respectively. The skeletal jet fuel surrogate mechanism was significantly compacted into 74 species and 189 reactions, making it practical to be used in 3-dimensional (3-D) engine combustion simulations. This newly developed mechanism was verified against the experimental results of ignition delay times, species concentrations and laminar flame speed under a wide range of conditions, while 3-D validations were conducted for spray liquid and vapor penetrations in a constant volume chamber. As a result, the proposed fuel surrogate is capable of predicting the spray and combustion characteristics and main species profiles under engine relevant circumstance. Due to the stringent target properties used in this work, the surrogate S1 performed best in assessing ignition characteristics attributed to its elaborate chemical properties, making it the most suitable candidate for chemical dominated combustion like premixed engine combustion. Alternatively, S2 displayed outstanding spray-guided combustion behavior because of the stringent chemical and physical target properties assigned. It is worthwhile to note that this new method of formulating surrogates for different applications was efficient and time-saving. Finally, we aim to perform experimental validation tests for CN, density, viscosity, surface tension, TSI, sooting tendency and particle size distribution to further support the validity of the current proposed jet fuel surrogates (S1 and S2) in the future.

Experimental study of spray characteristics of diesel/hydrogenated catalytic biodiesel blended fuels under inert and reacting conditions

Biodiesel has the potential to replace the conventional diesel fuel, thus the increasing interest and research in the use of biodiesel fuels for transport applications to improve the emisions. In this work, the spray characteristics of diesel with hydrogenated catalytic biodiesel (HCB) fuel blends were investigated in a constant volume combustion chamber to provide an accessible tool to predict spray behavior based on cheap and off-engine condition measurements for applying HCB in diesel engines. As two important indexes of spray characteristics, the liquid penetration and vapor penetration were researched using Mie-scattering and Schlieren methods under non-reacting conditions to avoid the influence of combustion on the mixing and vaporization processes. Besides, the liquid length of blended fuel under reacting conditions is measured by a laser system to figure out the effect of combustion on atomization process. The results show that the liquid length decreases with increasing HCB ratio in the blends and the fuel density has greater influence on the liquid length than the fuel viscosity. By comparing the liquid length result, a slight difference between the different blends on vapor penetration was observed under the same boundary condition. The spray characteristics of blends indicate that HCB is a good blending component for blended fuel which can be applied in diesel engine directly in large-scale. Moreover, the liquid length of laser Mie-scattering method is higher than that of LED Mie-scattering and the liquid length under reacting conditions is shorter than that of inert conditions.

Experimental and analytical study on vapor phase and liquid penetration for a high pressure diesel injector

In this study, a macroscopic characterization has been performed on a solenoid diesel injector (2200 bar-8 hole nozzle) under various non-reacting but evaporative conditions. For vapor penetration a two pass Schlieren visualization set up was selected. A high speed camera was used to record high speed images of the injection event to analyze the transient evolution of the vapor phase of the spray. The transient liquid penetration of the spray has been measured via MIE-Scattering imaging technique using a high speed camera as well. Unsteady RANS based CFD Simulations have been performed to simulate the experimental conditions and correlation results are presented. Built-in models from commercial code StarCD have been used to model spray formation which includes submodels for turbulence, nozzle flow, break-up and fuel properties. A novel CAE process using an automation and optimization tool has been used to achieve robust model settings, and the final model prediction are compared with the experimental observation for the injector characterization with respect to the non-reacting spray penetration with change in ambient and injection conditions. The model correlates well with the sensitivities for temperature and injection pressures qualitatively however improvements required to capture the density effects mainly related to the mesh orientation, fixed time step size where further analysis required.

Effect of hybrid breakup modelling on flame lift-off length and soot predictions

An improved droplet breakup model coupled with the effect of turbulence flow within the nozzle was implemented into the general transport equation analysis code to describe the flame lift-off length and predict the soot distribution. This model was first validated by the non-evaporating and evaporating spray experimental data. The computational results demonstrate that the breakup model is capable of predicted spray penetration and liquid length with reasonable accuracy. The inclusion of turbulence enhanced the breakup model, increased the droplet breakup rate, decreased spray penetration for about 6–12% compared to the results of Kelvin-Helmholtz Rayleigh-Taylor (KH-RT) breakup model. Then, the model was applied to investigate the influence of ambient density, temperature, oxygen concentration and injection pressure on the flame lift-off length under typical diesel combustion conditions. The predictions showed good agreement with the experimental data. The result also indicated that the turbulence inside the nozzle strengthen the rate of breakup, resulting in more smaller droplets, leading to high evaporation rate and smaller vapour penetration lengths, thus decreases the lift-off length about 8%. Finally, the model was used to explore the soot distribution. The overall trend of soot with the variations in injection pressure was well reproduced by the breakup model. It was found that the droplet with faster velocity under high injection pressure, this could lead to larger lift-off length, which will play a significant role for the fuel–air mixing process and thus cause a decrease in soot in the fuel jet. Results further indicated that the turbulence term can decrease the soot mass about 5–9% by improved the droplet breakup process.

Multi-environment PDF modeling for n-dodecane spray combustion processes using tabulated chemistry

The multi-environment probability density function approach together with a tabulated chemistry model has been developed to numerically simulate spray combustion processes under Diesel-like combustion conditions. In this study, the joint composition PDF is approximated using the multi-environment PDF, consisting of a combination of weights and weighted scalars in both chemical composition space and physical space. To account for the auto-ignition process and subsequent flame propagation, a flamelet generated manifold is generated by utilizing steady and unsteady flamelet solutions. The spray dynamics are represented by the state-of-art physical models in context with the Eulerian–Lagrangian approach. The RANS-based computations are made for the n-dodecane non-reacting and reacting spray jets. In terms of spray and vapor penetration, the mean and rms mixture fraction, ignition delay, and lift-off height, the numerical results are in reasonably good agreement with the available experimental data of the Engine Combustion Network. Based on the numerical results, detailed discussions are made for the effects of the ambient conditions on the structures and characteristics of high-pressure Diesel-fueled spray jet flames.