Gas Crossflow Research Articles

Numerical simulation of the atomization of liquid transverse jet in supersonic airflow

The present study provides a numerical method for liquid jet atomization in supersonic gas crossflow. Compressibility of the gas and incompressibility of the liquid are considered. High-order accurate weighted essentially non-oscillatory schemes and the Harten–Lax–van Leer contact approximate Riemann solver are used for gas flows. Liquid flow is simulated by the Chorin projection method. The motion of the sharp interface between the gas and liquid is simulated by the volume of fluid method. In order to verify the accuracy of the numerical method, numerical and experimental results for the droplet breakup in the supersonic gas flow are compared. The method is employed to simulate the liquid jet atomization in the supersonic gas crossflow. According to numerical results, the breakup process is analyzed for four different stages. The discussion for the effect of the Mach number for the gas crossflow on the liquid jet atomization is given.

Concentration of aqueous solutions of low-volatile liquids in a falling film with evaporation into a cross-gas flow

The article presents the results of the investigation of the process of concentrating solutions of low-volatile liquids in a flowing film under the conditions of evaporation in the cross-flow of neutral gas. The purpose of the study was to establish the features of solution composition change along the film length. The study was carried out using the developed mathematical model with experimental determination of model parameters: heat and mass transfer coefficients. It was found that at the beginning of the film, the change in the concentration of the solution has a character close to linear, and then the concentration sharply increases until the solvent evaporates completely. It was shown that this pattern of change in the composition of the solution is related to the distribution of the temperature of the solution along the length of the film. The intensity of the concentration process increases with decreasing initial flow rate of the solution and its initial concentration as well as with increasing initial temperature of the solution, initial temperature of the gas and temperature of the surface of the wall along which the film flows. The concentration intensity decreases with an increase in the velocity of the gas entering the space above the film. Comparison of the calculation results concerning aqueous glycerol solution with the experimental data showed their good agreement. The data obtained in the article allow calculating the height of the film concentrator nozzle at which a given concentration of the solution is provided.

Mathematical modeling of CO2 separation using different diameter hollow fiber membranes

This research presents a 2D mass-transfer simulation model using computational fluid dynamics (CFD) for separation of CO2 from a binary gas mixture of CO2/CH4 by means of polytetrafluoroethylene (PTFE) hollow fiber membrane contactor (HFMC). Governing equations with their corresponding boundary conditions are solved using COMSOL Multiphysics and the results are validated against reported experimental data. Convection and diffusion flux vectors and concentration gradient of CO2 species in the radial and axial directions of the HFMC are investigated. This study provides an opportunity to investigate the effects of gas and liquid cross flow velocities on the overall performance of membrane contacting system. The results demonstrate that increasing the liquid phase velocity improves the CO2 absorption performance of the membrane system, while increasing the gas mixture velocity deteriorates the CO2 separation of the system. The impact of hollow fiber geometry on the removal of CO2 is investigated and the results indicate that hollow fibers with smaller inner diameter provides higher effective mass-transfer area and therefore superior CO2 removal performance. Furthermore, the modeling predictions for the CO2 removal are in good agreement with the experimental data under various ratios of gas to liquid velocity. The PTFE hollow fiber membrane contacting system showed a great potential for separation of CO2 from gas mixtures.

Visualisation and numerical analysis of laser powder bed fusion under cross-flow

As well as preventing oxidation, a cross-flow of inert gas over the powder bed extracts process by-products, making it critical for process repeatability and build quality during the additive manufacturing of metallic parts. In this study, high-magnification schlieren imaging was employed to visualise the interactions between the cross-flow, laser plume and individual powder particles for the first time. Novel 3D multiphysics simulations of the cross-flow, including the vapour jet and laser plume, were used to calculate energy, momentum and species transport over the processed area in order to understand and quantify the observed interactions. During processing, metallic fumes as well as a significant number of airborne powder particles and agglomerates were observed to remain close to the powder bed, over the scanned area. These particles interact in a range of stochastic collisions that affect the build consistency. Refractive index gradients were formed in the atmosphere above the island scans, which extended many tens of mm beyond the processing area. The rate of extraction of these by-products, regulated by the velocity of the cross-flow, was related to the quality of the builds. A cross-flow perpendicular to the laser scan direction was observed to remove airborne particles from adjacent laser scans most effectively, limiting laser-particle and particle-particle interactions. The model demonstrated that the refractive index gradients away from the vapour jet are due to the metal concentration rather than the temperature of the heated gas.

Effect of part-length rods for different BWR subchannel geometries

Despite the beneficial effects of part-length rods (PLRs) in BWR fuel, they also introduce inhomogeneities to the coolant flow. These effects are hard to simulate and might affect cooling efficiency, dryout margin and CRUD deposition. So far, no fundamental experimental research was conducted to explore the details of their influence to the liquid film behavior in the dominating annular flow regime.To investigate the fluid dynamical effects of PLRs, we analyzed three different types of subchannel geometries. We varied the positions of PLRs in a pair of adjacent subchannels as well as the liquid and gas flow rates, staying within the annular flow regime. The adiabatic air-water test section was equipped with two types of sensors measuring the liquid film thickness distribution on half a fuel rod and on a fuel element wall. The measurement frequency was 10 kHz and the lateral resolution in the two-dimensional measuring domain was 2 mm. This corresponds to 64 × 16 and 16 × 32 measuring points, for rod and wall sensor respectively.Closely downstream of a PLR, we found a recirculation zone with less shear driving the liquid film. This is reflected by an increase of the average film thickness and a decrease of the wave velocity. A cross-flow of gas and liquid towards the open space downstream of the PLR was detected. The side of neighboring fuel rods facing this open space sees an increase of liquid flow. On the PLR-averted side, the coolant flow decreases, reducing the dryout margin. Further downstream, more gas tends to traverse into the open space left behind the PLR, which again increases the driving shear stress on the liquid film. The film becomes thinner and faster. The precise channel geometry featuring PLRs have a strong and complex influence on the local average liquid film thickness and the wave velocities. Their qualitative distribution at different flow conditions however, is similar.

Instability breakup model of power-law fuel annular jets in slight multiple airflows

In this paper, a temporal instability model has been derived to explore the influence of slight multiple airflow movements for the power-law fuel annular jet. Adopting the method of linear approximation and considering two different disturbance modes, the power-law fuel jet dispersion equation has been obtained based on the initial and boundary conditions. The influence of dimensionless characteristic parameters for the annular jet is investigated. By solving the dispersion equation, it can be found that the para-sinuous mode is more likely to play a leading role. For low-speed cases, the outer crossflow gas promotes the instability of fuel annular jets more effectively, while the inner coaxial airflow has an obvious promotion impact on high-speed jets. Reducing the thickness of the fuel annular film will weaken the stable inertia of the fuel, make the fuel annular jet easier to break up, and enhance the primary breakup scale. Furthermore, increasing the outer or inner gas density can accelerate the annular spray breakup process, and also amplify the gain effect of airflows. Besides, pseudoplastic fluid annular jets are more unstable and more suitable as fuel for future use. These discussions aim for a better understanding of the power-law fuel annular jet breakup process with multiple airflows and provide theoretical guidance for practical applications.

Ionic-Liquid-Based Contactors for Carbon Dioxide Removal from Simulated Spacecraft Cabin Atmospheres

The ionic liquid, 1-butyl-3-methylimidazolium acetate ([bmim][Ac]), was used to remove carbon dioxide () from a simulated spacecraft cabin atmosphere (2 mm Hg (267 Pa)) partial pressure of with balance of nitrogen (630 mm Hg total pressure). Three gas–liquid contactor configurations were experimentally characterized to measure the rates of removal from the simulated atmosphere with [bmim][Ac]. Two of the contactors, a parallel path flat plate and hollow fiber membrane, used hydrophobic porous membranes (silicone and polypropylene, respectively) to separate the gas and ionic liquid streams. The third, a three-dimensional printed interior-corner capillary-driven contactor, was configured for the crossflow of gas directly over the free liquid surface. At circa liquid flow and gas flow rate (0.24 slpm), inlet and outlet concentration differentials in the gas flow were 990, 2420, and for the flat plate, hollow fiber, and interior-corner contactors, respectively. For these same contactors, the removal rates were (), (), and , respectively. Overall mass transfer coefficients were (), (), and ( for each contactor, respectively. The coefficients generally decreased in direct proportion to increased gas flow. These performance metrics were nearly insensitive to variations in the flow of ionic liquid. The maximum uptake of by [bmim][Ac] was measured at 7.45 w% at and room temperature (23°C). The loading was 1.67 w% when exposed to the simulated spacecraft cabin atmosphere. In addition, desorption with thermal vacuum and thermal sparge processes were studied at temperatures from 20 to 80°C, the latter using dry inert gases (argon and nitrogen) to remove from the ionic liquid. After 4 h at 71°C under rough vacuum (0.5 mm Hg) without stirring, gravimetric measurements indicated a decrease in loading of from 1.75 w% to 1.29 w%. In comparison, for a gas sparge flow through -saturated [bmim][Ac] at 65°C, the loading decreased from 1.67 w% to 0.75 w% over the same period. The results suggest the importance of elevated temperature coupled with agitation to increase the rate of desorption from the ionic liquid.

Simulation of heat transfer in a flowing film of a evaporating liquid into a cross flow of a neutral gas

Simulation of heat transfer in a flowing film of a evaporating liquid into a cross flow of a neutral gas

Energy efficiency optimization of the waste heat recovery system with embedded phase change materials in greenhouses: A thermo-economic-environmental study

Abstract The continuous increase of greenhouse gas emission and the growing cost of fossil fuels are two motive forces to utilize energy sources more effectively. The agricultural greenhouses are developed to supply the food resources safely and with the higher quality, especially in off-season periods of year. In developing countries like Iran, the fuel usage of the greenhouse is very high, and there is a large margin for improving heating systems. The waste heat recovery system (HRS) in two forms of with and without embedded phase change material (PCM) added to the existing heating system, and their effects studied experimentally. The HRS is a cross-flow gas flow heat exchanger with dimensions of 193cm*75cm*85 cm, which is located between the exhaust gas and the circulating air streams. The study case is a greenhouse of 20×66 m2 in city of Isfahan located nearly in central region of Iran. The mean energy efficiency improved by 40% and 33% and the 2nd law efficiency enhanced by 263% and 127% for heating systems equipped with HRSs and PCM HRS, respectively. Using PCM HRS, the inside air and exhaust gas temperatures changed by +17% and -8.2%, respectively. The fuel savings gained by using HRS with and without embedded PCM are 19% and 48% with the investment return back periods of 3 and 4 months, respectively.

Numerical simulation on horizontal tube falling film column flow with influence of gas crossflow

Numerical simulation on horizontal tube falling film column flow with influence of gas crossflow

Unstable breakup of a power-law liquid fuel jet in the presence of a gas crossflow

A theoretical model is presented for the effect of a slight gas crossflow on the instability of the power-law fuel jet in this paper. Based on a linear approximation, the dispersion equation was deduced theoretically with axisymmetric disturbance. The influence of various dimensionless factors for jet instability is studied. Theoretical calculation results of temporal mode show that the transverse motion of gas phase could effectively accelerate the power-law fuel breakup. With the increase of transverse gaseous velocity, power-law fuel jets would become more susceptible to be broken up. Increasing gas-liquid density ratio could enhance the aerodynamic forces in the axial and vertical directions and promote fuel fragmentation. Simultaneously, a huge gas density could weaken the influence of transverse airflow. With the increase of the transvers airflow, Weber number has a weakening effect on the fuel breakup, but it has a significant positive effect on the primary breakup scale. However, with gas crossflow, the liquid viscous force also takes strong disadvantage of development of surface wave. Besides, the effects of crossflow and co-current airflow environments on the instability of power-law liquid jet are studied. For the generalized Reynolds number and power law exponent, the fuel jet under cross-flow conditions is more unstable, but the jet under the co-current flow condition is smaller on the fracture scale. These issues are beneficial to understand the disintegration of the power-law fuel jet with gas crossflow, and supply reference for the application of power-law liquids in real engineering technologies.

Characteristics of vertically falling liquid film affected by cross gas flow in a narrow flat channel

Gas crossflow on a falling liquid film occurs in many practical cases in industries, particularly, in the downcomer of the APR1400, when accidents such as damage to the cold legs occur, hot steam crossflows over the vertical falling water film formed by the water impingement on the wall through a direct-vessel injection nozzle. Such crossflow may degrade the heat transfer ability of the liquid film. To investigate the film characteristics of downward flowing water, across which air was blowing horizontally, an experimental setup was designed and assembled. Experiments were conducted under different water flow rates (2, 3, 4, and 5 L/min) and different air velocities (0, 3.4, 4.9, 6.9, 8.4, and 9.8 m/s) to determine their effect on the film structure. A confocal chromatic sensor was employed to detect film thickness, film width, and wave amplitude. It was found that the base film thickness and wave amplitude do not vary when superficial air velocity is not larger than 4.9 m/s. As air velocity increases, the base film thickness exhibits a decreasing trend and wave amplitude exhibits an increasing trend. In particular, wave amplitude shows a different tendency from that found in annular flow. Moreover, local base film thickness and wave amplitude decreased and increased along the film flowing direction, respectively. Furthermore, the average film thickness was found constant throughout the whole air velocity range. In addition, film width expanded nonlinearly as the water flow rate increased, and the slope of the film centerline under different superficial gas velocities was presented for the first time.

A Hierarchical Space-Time Spectral Element and Moment-of-Fluid Method for Improved Capturing of Vortical Structures in Incompressible Multi-phase/Multi-material Flows

A novel block structured adaptive space-time spectral element and moment-of-fluid method is described for computing solutions to incompressible multi-phase/multi-material flows. The new method implements a space-time spectrally accurate method in the bulk regions of a multi-phase/multi-material flow and implements the cell integrated semi-Lagrangian moment-of-fluid method in the vicinity of mixed material computational cells. In the new method, the space-time order can be prescribed to be $$2\le p_{\ell }^{(x)}\le 16$$ (space) and $$2\le p^{(t)}\le 16$$ (time) respectively. $$\ell $$ represents the adaptive mesh refinement level. Regardless of the space-time order, only one ghost layer of cells is communicated between neighboring grid patches that are on different compute nodes or different adaptive levels $$\ell $$. The new method is first tested on incompressible vortical flow benchmark tests, then the new method is tested on the following incompressible multi-phase/multi-material problems: (i) vortex shedding past a tilted cone and (ii) atomization and spray of a liquid jet in a gas cross-flow.

Gas crossflow between coal and sandstone with fused interface: Experiments and modeling

Multi-layered CBM (Coalbed Methane) reservoirs contain dozens (20–40) of thin coal seams (0.5 m–10 m) and sandstone seams and a commingled production strategy is preferred. Gas crossflow between coal seams and sandstone seams contributes much to the commingled gas production. Our study is focused on gas crossflow between coal and sandstone with fused interface. The experimental apparatus designed for this study consisted of a sandstone sample holder and a coal sample holder, where nitrogen was injected at a constant pressure difference between the sandstone and coal. The gas crossflow rate between coal and sandstone was monitored. Factors including confining stress, effective horizontal stress, axial stress and the permeability of sandstone influencing the gas crossflow were analyzed based on the experimental results. Based on the interface characteristics, a crossflow model for fused interface between coal and sandstone was proposed and gas crossflow resistant coefficients were defined. The experimental results show that gas crossflow rates decreased dramatically with the increment of confining stresses loading on the coal sample and increased linearly with the increment of average pore pressure on the condition of constant effective horizontal stresses. Compared with stresses loading on sandstone samples, the axial stresses and confining stresses loading on coal make larger influence on the gas crossflow rates. With the increment of the sandstone permeability, gas crossflow rates become less sensitive to the change of sandstone permeability. The comparison between model results and experimental results demonstrate that the proposed mathematical models can effectively predict gas crossflow between coal and sandstone with fused interface.

Microbubble generation with shear flow on large-area membrane for fine particle flotation

Fine particle flotation is carried out in a flotation column with a 19-channel ceramic membrane sparger. The microbubble generation is controlled by the shear flow in the membrane channels. Bubbles in the flotation column are photographed via a high speed video camera. Gas holdup and quartz recovery is investigated under different operating conditions. Gas holdup in the flotation column firstly increases and then decreases with the increase in superficial gas velocity. Similar trend in gas holdup is also found by increasing cross flow velocity. Larger particle concentration in the feed results in smaller gas holdup, while the gas holdup positively changes with collector concentration. The quartz recovery is strongly influenced by gas holdup and bubble size. Similarly, the recovery firstly increases and then decreases when increasing superficial gas velocity and cross flow velocity. The collector concentration plays a more obvious role in flotation recovery at high superficial gas velocity.

Comparison of simulation and experiments for multimode aerodynamic breakup of a liquid metal column in a shock-induced cross-flow

While the mechanisms that drive breakup and aerodynamic dispersion of traditional liquids such as water have been extensively studied, it is not yet clear if models for traditional liquids can be used to accurately describe the behavior of molten metals. In this paper, multiphase simulations with the interface-capturing combined level-set volume-of-fluid approach are used to provide time-resolved morphology and breakup data for a liquid column subject to a shock-induced cross-flow. For the first time, numerical simulation of the behavior of a liquid metal (Galinstan alloy composed of gallium, indium, and tin) is compared to the well-documented behavior of water. Simulations consider a gas cross-flow Weber number between 10 and 12, which produces a multimode breakup morphology consisting of multiple baglike structures. Up to bag breakup, we confirm that the deformation rate of Galinstan follows the same dependence on the gas cross-flow Weber number as ordinary liquids when time is nondimensionalized by including the liquid-gas density ratio. Moreover, we determine that the appearance of a central stem along the column upstream surface in multimode bag breakup is consistent with the occurrence of Rayleigh-Taylor instability. We also resolve bag stretching and fragmentation, to the full extent allowed by our computational resources, and carry out a direct comparison with the measurements of size and velocity of secondary droplets from high-speed digital inline holography. For Galinstan, we illustrate the differences between simulation and experiment that emerge because of the modification of the surface properties of the metal exposed to air.

The Dynamics of transient mode dispersed jet moving drops in gas crossflow

It is considered the problem of the dynamics of sole drop dispersed jet in gas crossflow in active zone of contact heat exchanger with ring-gap nozzle. The equations are offered to estimate the motion trajectory of drop and residence time in active apparatus zone for transient mode 2<Red<500.

Experimental study on working characteristics of direct current plasma jet igniter

The flameout of the combustor at high altitude poses a great threat to the flight safety of the aircraft. The low temperature and pressure as well as thin air at high altitude make it difficult to successfully and reliably reignite. The direct current plasma jet, one of the promising plasma assisted ignition technologies, has been applied in the different engines. In this paper, the working characteristics (mainly including discharge characteristics and jet characteristics) of direct current plasma jet igniter is investigated experimentally by using electric signal, synchronously acquired arc movement pictures, and high speed schlieren pictures of plasma jet in both static flow field and cross flow field. The influence of the gas flow rate and cross flow velocity on the working characteristics of direct current plasma jet igniter is also studied. Results show that there are three stages (jet formation stage, jet expansion stage, and jet stability stage) form the breakdown of the igniter electrodes to the stable distribution of the jet in the combustion section. The operating voltage and fluctuation frequency rises significantly (from 92.55 V to153.11 V, and from 9.2 kHz to17.3 kHz), the influence area and penetration depth of plasma jet increased in cross flow environment with the increase of the gas flow rate. As the cross flow velocity increases, the three-dimensional fluctuation of arc movement is more obvious, the influence area and penetration depth of plasma jet decreases quickly. Both the discharge characteristics and jet characteristics affect the ignition capability of plasma jet igniter, and this study has certain guiding significance for the design of direct current plasma jet igniter.

Effects of additives on atmospheric pressure gliding arc applied to the modification of polypropylene

The polypropylene (PP) strips were modified with the industrial gliding arc discharge working in air flow at 50 Hz. The experiments aimed at increased wettability and improved strength of the epoxy adhesive DP190 bond with PP. While working only in dry air, the deterioration of the plasma treatment uniformity was observed at higher treatment speeds than 100 mm/s. At the same time, the tensile strength of the adhesive bond and the atomic concentrations of O and N decreased whereas the water contact angle (WCA) measurement error increased. The fast camera measurements revealed that the PP surface was treated by the direct contact with active plasma filaments. The non-uniformity of the PP surface treatment for increased treatment speed can be explained by taking into account the stochastic character of the filament propagation direction and the characteristic time scales of the process.As an improvement of the gliding arc treatment technology a new geometry utilizing the gas cross flow was proposed with the help of gas dynamic simulations. The addition of argon cross flow led to an improvement of the treatment uniformity and, for the optimized conditions, the measured tensile strength of the adhesive bond to PP was 6.5 times higher than for the untreated PP sample. The treatments with the cross flow of argon mixed with water vapors and volatile organic compounds (VOCs), ethanol and isopropanol, did not increase the tensile strength of the adhesive joint even though the wettability was higher for the mixtures with VOCs. Since the concentration of functional groups measured by X-ray photoelectron spectroscopy was well correlated with the tensile strength of adhesive joints, WCA measurements cannot be used as the only parameter judging the success of the plasma treatment aimed at an improvement of adhesive joint strength.

Spatter transport by inert gas flow in selective laser melting: A simulation study

Spattering is an unavoidable phenomenon during Selective Laser Melting (SLM). The distinctively large and dark solidified molten particles have always been observed on the powder bed, leading to the potential deterioration of the final printed parts. In commercial SLM machines, inert gas flow is pumped into the chamber over the powder bed to remove spatter and other unwanted by-products such as metal vapour and plasma plumes. However, traces of spatter still remain on the powder bed and the regions close to the chamber outlet. In this work, the trajectories of spatter particles were tracked using the Discrete Phase Model (DPM). The continuous fluid domain was solved using three-dimensional Computational Fluid Dynamics (CFD) simulations. The entrainment effects on the hot ejections as well as the recoil pressure driven ejections were considered. In other words, it was assumed that the latter were also entrained by the metal vapour and laser plume. The final distribution characteristics downstream of the cross-flow (-x direction) were validated by an earlier experimental study. The results show that a gradually decreasing trend in terms of the spatter size and mass distributions were obtained from the simulations which showed good agreement with the earlier experiments. It was also shown that the trajectories of the spatter particles were governed mainly by their initial momentum when the inert gas flow velocity was increased. While complete removal of the spatter particles from the powder bed was not achieved, the presence of the inert gas flow aided in the transport of a majority of the particles further downstream giving a more even mass distribution. Also, in spite of not accounting for the complete multi-physics of the SLM process and the associated recoil pressure driven ejections, the proposed spatter transport simulations can be applied in future gas cross-flow optimisation designs for the effective removal of spatter over the powder bed in commercial SLM machines.