Reverse Gas Flow Research Articles

CO2 conversion in a microwave plasma torch: 2D vs 1D approaches

In the present study, we computationally investigate the splitting of CO2 to carbon monoxide and oxygen in an atmospheric pressure microwave (MW) plasma torch. We demonstrate different stages of CO2 conversion while using 2D and 1D models. For both models, we use identical sets of chemical reactions, cross sections, power profiles and dimensions of the plasma region. Based on the real MW plasma torch device, we first constructed two-dimensional geometry and obtained results using the 2D model. Then, the 1D plug-flow model was employed. With 1D model we expected to obtain the results close to those we already had from the 2D approach. However, we revealed that the gas temperature and plasma species behaviour in 1D model was quite different from those obtained with the 2D code. We revisited the 2D results and found that the reverse (upstream) gas flow near the central electrode was responsible for the observed discrepancies. In 2D model, the residence time of a certain portion of gas was much longer. When the flow rate in 1D model was adjusted, the reasonable agreement between both models was achieved.

Gas/particle flow and pressure variation characteristics of Ende pulverized coal gasifier equipped with ejector

AbstractEnde pulverized coal gasifier (EPCG), as a typical representative of circulating fluidized bed gasifier, has been widely used in gasification field. The return conduit of EPCG is generally not equipped with a feedback control device (FCD), which tends to result in a low solid circulation rate (Gs) and high unburned combustibles in fly ash. To address these issues, a new technique using the ejector as the FCD was designed to provide a stable pressure barrier and improve solid circulation. Preliminary thermal experiments demonstrated the feasibility of improving the EPCG with injector system. Currently, the specific mechanisms that contribute to further optimize this technology have not been systematically studied. Considering the large impact of ejector operation on solid circulation rate and stability as well as syngas production, this work adopts a gas/particle 1:15 cold EPCG bench to investigate the gas/particle flow and pressure variation characteristics before and after ejector operation and under different working medium pressures (Pp). When the ejector operates at the design pressure Ps = 0.187 MPa, the outlet pressure of the return conduit is slightly larger than the furnace bottom pressure, thus preventing the reverse gas flow at the furnace bottom. The particle velocity and concentration distribution in the furnace are basically consistent with that without the ejector, showing a similar central spout region and near‐wall annulus flow region. As Pp increases from 0.04 to 0.24 MPa, the average particle velocity in the return conduit increases from 0.95 to 2.4 m/s, the particle concentration decreases from 0.088 to 0.051, and the solid circulation rate increases from 0.37 to 0.53 kg/h. The ejector outlet pressure gradually increases, the inlet pressure decreases, and the pressure barrier provided by the ejector rises from 1052 to 3640 Pa. When Pp is 0.12 MPa, the ratio of GN2/Gair is 0.91%, which can achieve the purpose of continuous and stable pressurization with a small amount of nitrogen.

Hybrid digital twin for conditional lubricant oil transport simulation and oil consumption prediction in internal combustion engines

Reducing lubricant oil consumption (LOC) has long been an interest to engine manufacturers in an effort to reduce overall emissions from internal combustion (IC) engines. However, directly correlating engine operating conditions with LOC rates is a challenging task given its complex and multi-physics nature. Building on previous experimental observations and modeling efforts, a baseline hybrid model, which combines traditional physics-based numerical simulation with modern machine learning techniques, was developed. As the first step, the model is to compute the oil exiting the top ring gap with given oil supply rate and locations to the second land. Studies performed using the model reveal a strong dependency of LOC on engine speed, load as well as the relative locations of piston ring gaps. In particular, the close proximity of the oil source on second land to the top ring gap and the reverse gas flow at low load conditions can both induce high LOC. Piston ring gap rotations and the different oil supply mechanisms to the piston ring pack are also found to heavily influence the oil consumption behavior and oil film thickness distribution pattern. The present work is the first step to a more comprehensive model for predicting LOC for healthy systems of piston ring packs. The efficiency of the model made it possible to study the oil redistribution that takes many engine cycles to reach equilibrium and rotation of the ring gaps that could happen in reality at a time scale of minutes and even longer.

Effect of drainage channel on separation performance of swirl-vane separator

Swirl-vane separator in nuclear reactors removes liquid from gas by centrifugal force. Since liquid film is accumulated on the wall, drainage channels are designed to remove liquid film in time and reduce secondary droplet entrainment. Euler-Euler multiphase numerical model and Realizable k-ε turbulence model are developed to investigate the effect of drainage channels. The results indicate separation efficiency is improved by removing water through drainage channels at low inlet gas velocity, while reverse gas flow from down-comer to riser through drainage channels pushes water near wall to the axis of riser, deteriorating separation at high inlet gas velocity. Moreover, the effect of channel area, size, shape and arrangement are analyzed. Separation performance is most sensitive to drainage channel arrangement compared to size and shape. Evenly distributing drainage channels over a wide range of wall benefits separation of water and reduces reverse gas flow.

An improved method for controllable growth of monolayer WSe2 and its optical characterization

Due to the van der Waals interaction between the transition metal sulfide layers, it can be stripped into a single atomic layer, and tungsten diselenides(WSe2) as one of the typical p-type semiconductors has become a research hotspot. WSe2 thin flakes have been successfully prepared by chemical vapor deposition(CVD) methods, but the synthesis conditions of WSe2 are usually highly sensitive and difficult to control, which makes it difficult to grow monolayer WSe2 with good lattice quality in a large area. Here, we use a new CVD growth method to improve the product quality, through the combination of reverse gas flow and rapid heating method to improve the nucleation and lattice quality of WSe2.This work will systematically study the effects of key growth conditions, focus on elucidating the growth mechanism, and optimize the growth parameters. In order to verify the preparation of monolayer and multi-layer WSe2 thin flakes with good lattice quality. WSe2 thin flakes were characterized by optical microscope, atomic force microscope, Raman spectrum and photoluminescence (PL) spectrum. Moreover, monolayer WSe2 and multi-layer WSe2 can be distinguished by Raman test and fluorescence spectrum test, and they have different characteristic peaks. We can judge whether the material is monolayer by observing the peak position.

Interlayer interference during coalbed methane coproduction in multilayer superimposed gas-bearing system by 3D monitoring of reservoir pressure: An experimental study

Coproduction is an important measure for improving coalbed methane flow and production efficiency in multilayer superimposed gas-bearing systems. By monitoring the reservoir pressure in 3D spaces, we analyzed the mechanism of fluid migration and interlayer interference during coproduction through a four-layer coproduction experiment with a reservoir pressure gradient of 0.4 MPa. The results revealed the coupling between reservoir pressure gradient, matrix shrinkage, and effective stress, along with their joint effect on the reservoir pressure evolution in stages. The increase in reservoir pressure or reverse gas flow in low-pressure coal seams is the direct manifestation of interlayer interference. This interference increases gradually as the difference between the pressure gradients of the reservoirs increases. The gas elastic energy was modeled considering the free and adsorption states. We found that the difference in gas elastic energies between reservoirs is a major cause of interlayer interference. The wellbore connects coal seams with different reservoir pressures, that is, it connects different energy bodies. This destroys the state of dynamic energy balance between the systems, causing fluid flow from high-energy gas-bearing systems to those with low energy to achieve a new dynamic balance. When the reservoir energy difference is significant, the fluid in higher energy systems shields or inhibits the fluid flow to the wellbore in lower energy systems and may even cause “backflow” to lower energy systems.

Effect of ignition position on premixed hydrogen-air flame quenching behaviors under action of metal wire mesh

For obtaining effective inhibitory strategies for hydrogen combustion in terms of safety, the effect of ignition position on premixed hydrogen-air flame quenching behaviors under action of metal wire mesh is experimentally analyzed. Combustible hydrogen/air gases with volume ratio of 2:3 is adopted. Metal wire mesh of 60 meshes-60 layers is selected as the flame inhibitory structure. Three different ignition positions are arranged for obtaining different stages of flame entering suppression section. The results indicate that ignition position affects flame quenching results significantly. The flame quenching difficulty increases as the distance between ignition position and wire mesh location increases. Besides, the ignition position influences flame behaviors greatly. Tulip flame is actually led by the vortices in IP1 case, and the internal mechanism is attributed to the interactions of flame front and pressure waves. However, for IP3, the special tulip flame with an extremely long inversion cusp is actually led by the enhanced reverse gas flow in burnt field, and the root cause is metal wire mesh enlarges the discrepancies of combustion intensity between flame propagating toward positive and negative directions. Moreover, the ignition position changes flame tip speed and pressure greatly. As ignition position changes from IP1 to IP2 and IP3, the maximum flame tip speed toward positive direction decreases from 120.5 m/s to 105.2 m/s and 56.9 m/s, separately. Also, the maximum pressure reduces from 0.13 MPa to 0.08 MPa and 0.07 MPa, respectively, which further proves a more pronounced inhibitory effect when setting ignition position closer to wire mesh.

Physical Simulations of Gas Production Mechanism in Constant-Rate Co-production from Multiple Coal Reservoirs

Coalbed methane co-production is an efficient technique for producing gas from multiple coal reservoirs. In this study, four large-scale specimens were connected in parallel to model a system of coalbed methane production. The results revealed that a low-pressure coal reservoir is disrupted owing to the pressure equilibrium, causing the cross-formational flow of gas through the wellbore. The production and productivity contribution rates were positively correlated with the initial reservoir pressure over a stable production period, forming a “dynamic supply” state among the coal reservoirs. The intensity of the interlayer interference was negatively correlated with reservoir pressure such that with significant interlayer interference, reverse gas flow or a halt in production could occur in the low-pressure coal reservoir. The constant-rate production value and the effect of interlayer interference had an inverse relationship in terms of production; the smaller the former, the greater was the latter. Increasing the constant-rate value can restrict large-scale production from the high-pressure coal reservoir and improve the productivity recovery rate and production distribution of the low-pressure coal reservoir.

Controllable Epitaxial Growth of MoSe2 Bilayers with Different Stacking Orders by Reverse-Flow Chemical Vapor Deposition.

The stacking order plays a critical role in the electronic and optical properties of two-dimensional materials. It is however of great challenge to achieve large-size and homogeneous bilayer crystals with precisely controlled stacking orders. Here, we demonstrate an optimized chemical vapor deposition strategy to grow MoSe2 bilayers with controlled AA or AB stacking sequences. Reverse gas flow effectively suppresses the random nucleation centers, leading to uniform growth of the second layer of MoSe2 on the first monolayer. A customized temperature profile selectively activates the growth of the MoSe2 bilayer with different stacking orders: the AA stacking MoSe2 bilayer tends to form at 810 °C, and the AB stacking MoSe2 bilayer prefers to grow at a higher temperature of 860 °C. A series of characterization methods confirm that MoSe2 bilayers with different stacking orders exhibit distinct crystal structures and physical properties. Our results demonstrate a robust and effective route for the controllable synthesis of transition metal dichalcogenide bilayers, which will benefit the development of two-dimensional materials and van der Waals heterostructures.

The use of the gas flow model to improve the design of the piston-rings-cylinder system of a diesel engine

The article presents the results of simulation research aimed at assessing the impact of selected dimensions of the rings and piston on the ring-pack operation and developing proposals of changes that will improve its performance. The object of the research was a diesel engine for delivery vehicles. A mathematical model of the ring-pack developed by the author of this work was used for the research. The results have shown that with restrictions imposed by the engine manufacturer regarding the scope of modernization, it is possible to significantly reduce or even eliminate the reverse gas flow from the inter-ring space to the combustion chamber, but this must be done at the expense of increased blow-by. The effect of proposed changes on the axial displacement of the rings in the grooves has also been discussed.

Numerical simulation of thermal edge flow in ratchet-like periodically patterned micro-channels

This paper presents a numerical analysis of a novel configuration of the Knudsen pump for rarefied gas flow. A basic unit of the ratchet-like periodically patterned micro-channel consists of two facing parallel cold walls with a triangular hot plate in the center. The gas flow is primarily triggered by the thermal edge flow formed by the dramatic variation in the temperature field near the tips of the triangular plate. The direct simulation Monte Carlo (DSMC) method is applied to study the influence of the operation conditions and geometric parameters on the gas flow, such as the temperature difference, Knudsen number, channel dimension, periodic length, geometric parameters of the triangular hot plate, and surface accommodation coefficients. The results demonstrate that the temperature difference of the walls is the basis for the gas flow. The mass flow rate increases nonlinearly with the temperature difference, and the maximum mass flow rate occurs when Kn is approximately 0.15. The pumping speed level can be improved by increasing the channel dimension appropriately; however, reverse gas flow occurs for exceedingly large channel dimensions. By simply adjusting the dimensions of the two triangular hot plates, the mass flow rate can be increased by 3.7 times. The mass flow rate reaches its maximum when the length-width ratio of the triangular hot plate is 2.0. In particular, by changing the surface accommodation coefficients of the inclined and vertical walls of the triangular plate, the pumping speed level can be maximally increased by approximately 35 times.

Transition metal dichalcogenides bilayer single crystals by reverse-flow chemical vapor epitaxy

Epitaxial growth of atomically thin two-dimensional crystals such as transition metal dichalcogenides remains challenging, especially for producing large-size transition metal dichalcogenides bilayer crystals featuring high density of states, carrier mobility and stability at room temperature. Here we achieve in epitaxial growth of the second monolayer from the first monolayer by reverse-flow chemical vapor epitaxy and produce high-quality, large-size transition metal dichalcogenides bilayer crystals with high yield, control, and reliability. Customized temperature profiles and reverse gas flow help activate the first layer without introducing new nucleation centers leading to near-defect-free epitaxial growth of the second layer from the existing nucleation centers. A series of bilayer crystals including MoS2 and WS2, ternary Mo1−xWxS2 and quaternary Mo1−xWxS2(1−y)Se2y are synthesized with variable structural configurations and tunable electronic and optical properties. The robust, potentially universal approach for the synthesis of large-size transition metal dichalcogenides bilayer single crystals is highly-promising for fundamental studies and technological applications.

Controllable one-step growth of bilayer MoS2–WS2/WS2 heterostructures by chemical vapor deposition

Heterostructures of two-dimensional (2D) transition metal dichalcogenides (TMDs) offer attractive prospects for practical applications by combining unique physical properties that are distinct from those of traditional structures. In this paper, we demonstrate a three-stage chemical vapor deposition method for the growth of bilayer MoS2–WS2/WS2 heterostructures with the bottom layers being the lateral MoS2-center/WS2-edge monolayer heterostructures and the top layers being the WS2 monolayers. The alternative growth of lateral and vertical heterostructures can be realized by adjusting both the temperature and the carrier gas flow direction. The combined effect of both reverse gas flow and higher growing temperature can promote the epitaxial growth of second layer on the activated nucleation centers of the first monolayer heterostructures. By using customized temperature profiles, single heterostructures including monolayer lateral MoS2–WS2 heterostructures and bilayer lateral WS2(2L)–MoS2(2L) heterostructures could also be obtained. Atomic force microscopy, photoluminescence and Raman mapping studies clearly reveal that these different heterostructure samples are highly uniform. These results thus provide a promising and efficient method for the synthesis of complex heterostructures based on different TMDs materials, which would greatly expand the heterostructure family and broaden their applications.

Реверс газа на Украину: суть проблемы и её влияние на международные отношения в сфере энергетики

The article examines the most important aspects of the problem of the reverse of gas to Ukraine, its significance and consequences for the further development of contemporary international relations in the energy sector between the Russian Federation (gas exporter), Ukraine (transit country) and the European Union (importer). Considering all the elements that are important for the analyzed problem, the answer to the question “how the reverse gas flow influenced the energy security of Russia, Ukraine and the European Union, and the relationship between them” is given, using the explicative method of content analysis and the analysis of several press conferences. To understand all the nuances of relationships in this triangle, a detailed description of the term “reverse gas flow” is offered, as well as an explanation of the differences between “physical” (or real) and “virtual” reverse, which play a key role in the study of this problem. The essence of the conflict lies in the fact that the parties have different points of view on the legitimacy of the so-called “virtual” backhaul. In contrast to the physical reverse, i.e. to the real gas flow through the pipeline, in case of a virtual or a paper reverse, a one-way gas flow takes place, part of the total volume of which the transit state buys and leaves on its territory. The EU and Ukraine believe that a virtual backhaul between them is possible according to the provisions of the Third Energy Package and its relevant norms, based on the fact that the EU countries and Ukraine are part of the so-called. Energy Community. Another point of view regarding virtual backhaul is Russian, which initially opposed virtual reverse on the grounds that the raw material is the property of the state-owned company OAO Gazprom until it crosses the western border of Ukraine. The article details the legal documents on which the above-mentioned countries rely, as well as the official statements of the parties. Based on the analysis conducted, it is concluded that reverse gas supplies to Ukraine represent a temporary and far from perfect scheme, which does not fundamentally solve the problem of disagreements between the EU, Ukraine and Russia in the energy sector, namely, the gas transit. Nevertheless, the observed increase in the volume of Russian natural gas supplies to the EU in 2016 and early 2017 and the forecast for a further demand increase for gas from Russia in the EU in the future indicates that the provision of gas transit through the territory of the Visegrad Group countries, Members of the EU from Southeastern Europe and the search for new ways of supply, as well as the preservation of some of the current ones, occupy the leading positions on the political agenda of the EU countries, Russia and Ukraine, as they relate to their energy security

Analysis of design parameters of pistons and piston rings of a combustion engine

This paper analyzes the effect of gaps in the lock of the first and second sealing rings and selected dimensions of the ring belt of the piston on the piston-ring-cylinder seal. The research was performed using a mathematical model of the seal system. Simulation results have shown that a significant reduction in the exhaust gas flow to the crankcase – without compromising other parameters characterizing the performance of the seal – would require increased production rigor. Reducing the blow-by without the need to narrow the tolerance intervals of the engine components is possible, but this would be accompanied by increased reverse gas flow from the inter-ring space towards the combustion chamber. The research has also shown that the ring seal of the automotive compression ignition engine, which was the starting point for testing, was optimized for minimizing gas flow towards the combustion chamber at the expense of increased flow of exhaust gases to the crankcase.

Ejector type solid circulation system analysis for circulating fluidized beds

Proper solid circulation is of crucial importance for the stable operation of circulating fluidized beds. There should be a pressure barrier between the riser and the downcomer to avoid reverse gas flow through the downcomer and to maintain a stable solid circulation. In conventional systems, loop seals, L, J, V, valves are mostly used as solid circulation valves. In this study, ejector (eductor) type solid circulation was investigated in an experimental CFB setup (94mm in diameter, 7950mm in height). The solid flux at the downcomer (34mm in diameter) and the motive gas flowrate of the ejector were investigated under different cold flow conditions. There are two different operation modes of the ejector type solid circulation depending on the solid flux through the downcomer. For low solid fluxes, there is no solid accumulation at the downcomer. As the solid flux increases, a solid column formation is observed above the ejector system. The desired pressure difference obtained with the ejector is higher for the latter mode of operation and requires less motive gas flow compared to the former for proper operation. The experimental results show that the ejector is capable to provide the required pressure barrier for the CFB loop in both cases. In addition to the experimental study, ejector system was analyzed with CFD model in order to investigate the effect of basic geometric and operational parameters on the pressure difference generated by the ejector. Furthermore a novel semi-empirical method was proposed as a tool for predicting the ejector performance comparable with CFD results.

Gradient-structure titanium carbide filter for liquid and gas filtration

Gradient-pore-structure filters have been prepared via consolidation of titanium carbide powders, followed by heat treatment. The gradient structure allows one to improve the efficiency of high-temperature purification of aggressive media with the aim of removing submicron-sized dispersed contaminants, in comparison with an isotropic highly porous material. The advantages of such filters are not only the possibility of regeneration by heat treatment and rinsing with acids or other solvents but also the possibility of regeneration using a reverse liquid or gas flow.

The study of thermal decomposition of RDX by corona discharge-ion mobility spectrometry-mass spectrometry

In this study we have examined the formation of ions from hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) sample evaporated at 473 K in reactions with reactant ions (RI) produced in negative corona discharge (CD). The ions have been analysed and detected by ion mobility spectrometry coupled to orthogonal acceleration time of flight spectrometer (IMS-oaTOF). The chemical ionisation was carried out by O2−, O2− (H2O), N2O2−, N2O3− and N2O3− (H2O) RI formed in CD operated in zero air in the reverse gas flow mode at elevated temperatures 438 K and by Cl−(H2O)n (n = 0,1,2) at 332 K. In case of 332 K, the RI were prepared by admixture of carbon tetrachloride (CCl4 – dopant gas) in order to enhance sensitivity of RDX.

Experimental simulation of negative ion chemistry in Martian atmosphere using ion mobility spectrometry-mass spectrometry

We have studied the formation of negative ions in a negative Corona Discharge (CD) fed by CO2/N2 mixtures (with 0, 2, 4, 6, 8, 10% N2) using the technique of ion mobility spectrometry-orthogonal acceleration time of flight mass spectrometry (IMS-oaTOF). The composition of the negative ions was found to be dependent on the initial gas composition, the gas flow regime, the concentrations of neutral reactive species formed in the discharge and the trace amounts on water in the gases were found to play an important role in the negative ions formation. In a pure CO2 discharge operating under standard gas flow conditions of IMS (associated with strong interaction of ions with neutral reactive species formed in discharge) the ions CO3− (H2O) and CO4−(H2O) dominated the measured negative ion spectrum while in CO2/N2 mixtures NO3−(H2O)n, NO3−(HNO3) (n = 0, 1) ions prevailed. In the case of reverse gas flow regime (low interaction of ions with neutral reactive species formed in discharge), the negative ions detected were O2−(H2O)n, and O2−.CO2(H2O)n both in pure CO2 and N2/CO2 mixtures. The spectra of negative ions recorded for a gas mixture containing 4% N2 in CO2 were compared with theoretical predictions of negative ion composition in the lower atmosphere of Mars.

Ion mobility spectrometry–mass spectrometry studies of ion processes in air at atmospheric pressure and their application to thermal desorption of 2,4,6-trinitrotoluene

In this study we have investigated the negative reactant ion formation in a negative corona discharge (CD) using the corona discharge ion mobility spectrometry orthogonal acceleration time-of-flight (CD-IMS-oaTOF) technique. The reactant ions were formed in the CD operating in the reverse gas flow mode at an elevated temperature of 363.5 K in synthetic and ambient air. Under these conditions mainly and their clusters were formed. We have also studied the influence of CCl4 admixture to air (dopant gas) on the composition of the reactant ions, which resulted in the formation of Cl− and its clusters with a reduced ion mobility of 3.05 cm2 V−1 s−1 as a major reactant ion peak. Additional IMS peaks with reduced ion mobilities of 2.49, 2.25 and 2.03 cm2 V−1 s−1 were detected, and Cl− · (NO2) and Cl− · (NO)n(n = 2, 3) anions were identified. The negative reactant ions were used to detect 2,4,6 trinitrotoluene (TNT) using the thermal desorption (TD) technique using a CD-IMS instrument. Using TD sampling and a negative CD ion source doped by CCl4 we have achieved a limit of detection of 350 pg for direct surface analysis of TNT.