Low Gas Volume Research Articles

Gas–Liquid Mixability Study in a Jet-Stirred Tank for Mineral Flotation

Micro- and nano-bubble jet stirring, as an emerging technology, shows great potential in complex mineral sorting. Flow field characteristics and structural parameters of the gas–liquid two-phase system can lead to uneven bubble distribution inside the reaction vessel. Gas–liquid mixing uniformity is crucial for evaluating stirring effects, as increasing the contact area enhances reaction efficiency. To improve flotation process efficiency and resource recovery, further investigation into flow field characteristics and structural optimization is necessary. The internal flow field of the jet-stirred tank was analyzed using computational fluid dynamics (CFDs) with the Eulerian multiphase flow model and the Renormalization Group (RNG) k − ε turbulence model. Various operating (feeding and aerating volumes) and structural parameters (nozzle direction, height, inner diameter, and radius ratio) were simulated. Dimensionless variance is a statistical metric used to assess gas–liquid mixing uniformity. The results indicated bubbles accumulated in the middle of the vessel, leading to uneven mixing. Lower velocities resulted in low gas volume fractions, while excessively high velocities increased differences between the center and near-wall regions. Optimal mixing uniformity was achieved with a circumferential nozzle direction, 80 mm height, 5.0 mm inner diameter, and 0.55 radius ratio.

Diagnostic Ability of Ultrasonography Compared with Computed Tomography for Assessing Rectal Feces.

Chronic constipation is a common gastrointestinal disorder, and management is crucial. Computed tomography (CT) is useful for evaluating rectal fecal mass but limited owing to radiation exposure, cost, and inaccessibility at certain facilities. Ultrasonography (US) avoids these pitfalls, but it is unknown whether it accurately assesses rectal feces. In this study, we evaluated the diagnostic performance of US compared with CT as the gold standard for assessing rectal feces. We prospectively evaluated rectal fecal mass retention using US performed within 1 h of CT to assess the degree of agreement between methodologies. Rectal stool findings were evaluated on three levels: no stool (R1), presence of stool (R2), and hard stool filling (R3). The sample included 100 patients (55 men, 45 women), of whom 47 were constipated. The kappa coefficients for rectal content detection were excellent between US and CT (p <0.001). Eighty-two cases (R1: 46 cases; R2: 28 cases; R3: 8 cases) were matched with CT and US findings, and 18 were not. Cases that did not match had low urine or high gas volumes. CT and US findings showed high agreement in constipation (kappa coefficient 0.674, p <0.001) and non-constipation groups (kappa coefficient 0.677, p <0.001). All cases with R3 on CT were found in the constipation group, while more than half of the cases with R1 on CT were in the non-constipation group. CT and US showed high agreement in evaluating rectal fecal mass retention, indicating that US can substitute CT.

Synthesis of (5-Iminio-4,5-dihydro-1,3,4-oxadiazol-2-yl)tetrazol-1-ide: A Zwitterionic Compound Containing a Tetrazole Anion as a Promising Blunt Energetic Material

Abstract(5-Iminio-4,5-dihydro-1,3,4-oxadiazol-2-yl)tetrazol-1-ide is the first zwitterionic energetic material containing tetrazole as an anion and 1,3,4 oxadiazole as a cation. Its low mechanical sensitivity (IS &gt;40 J, FS &gt;360 N) and high gas volume after detonation at standard temperature and pressure (767.0 dm3/kg) make it a prime example of a novel type of blunt energetic material with promising applications.

Critical consideration of gas bubbles to replace fat globules in acidified milk matrices

The influence of low gas volume fraction and bubble deformability on rheological properties and microstructure of acidified milk matrices was investigated; functionality was compared with that of fat globules. With increasing fat volume fraction, stiffness and yield stress of the acidified milk models increased, but decreased with increasing gas volume fraction. This was caused by the large size of the bubbles compared with the fat globules and different processing of the samples containing gas and fat, respectively. A mixture of 1% fat together with gas bubbles resulted in a smaller mean bubble size and thus stiffer bubbles. Simultaneously, an increase in stiffness and yield stress of the bubble-containing matrices was observed, although not to the same extent as for unfoamed matrices with increasing fat content. Beyond a critical gas volume fraction of 0.27, gas bubbles were no longer separated and interacted with other matrix components and themselves, weakening the structure.

A numerical model for simulating separated gas-liquid two-phase flow with low GVF in a V-cone flowmeter

One of the relatively new types of differential pressure flowmeters is the V-Cone (conical or cone) meter. In addition to having many advantages over other types, this flowmeter can also be used in multiphase flows. In the last few decades, many numerical works have been presented for single-phase flows. But in the discussion of two-phase flow, most of the available works are related to experimental research. Therefore, in this paper, the separated two-phase flow with a low gas volume fraction (GVF) has been numerically investigated. For this purpose, an unstructured grid and finite volume numerical method were used. In order to model the two-phase flow and turbulence, the existing approaches were compared. According to the results obtained, the volume of fluid (VOF) method for simulating two-phase flow and the Realizable k-ε model for turbulence modeling lead to better results than other investigated models. Also, by stimulating the flow with the aforementioned methods, it was found that the accuracy of the pressure calculation decreases with the reduction of the superficial velocity and volume fraction of the gas. Furthermore, for a more detailed analysis, the superiority of the Realizable k-ε turbulence model compared to other investigated models was proved quantitatively.

Optical and gas sensing properties of TiO2/RGO for methanol, ethanol and acetone vapors

• Preparation of TiO 2 /RGO nanocomposite by annealing method at 180 °C. • Employing TiO 2 /RGO nanocomposite toward sensing methanol, ethanol and acetone vapors with different ppm. • Observation an excellent response for ethanol detection even in low gas volume. Reduced graphene oxide and metal oxides derivatives are introduced as relatively benign materials for improving gas sensors' performance as exemplified by the synthesis of TiO 2 /GO nanocomposite via ultrasonication, and then the assembly of RGO adorning TiO 2 nanoparticles (NPs) (TiO 2 /RGO) is accomplished using the thermal annealing method. The characterization of the as-fabricated sample indicated that irregular shaped TiO 2 NPs with size of 26 nm covered the RGO nanosheets. The optical band gap measurement indicated the values of 3.19, 2.1 and 2.5 eV for TiO 2 , GO and TiO 2 /RGO samples, respectively. The ensued nanocomposites are then deployed for the gas sensing response of three common organic vapors of ethanol, methanol, and acetone with concentrations of 20, 40, 60, 80, 100 ppm, at ambient temperature. The TiO 2 /RGO sensor relatively exhibited an excellent response (3–4 times) for ethanol detection. These results indicated that the application of TiO 2 /RGO can help assist in the evolvement of portable gas sensing devices endowed with the capability to detect common place organic vapors.

Modeling of deteriorated heat transfer in supercritical fluids using a two-phase method

In heat transfer processes of supercritical fluids, heat transfer deterioration (HTD) is normally manifested by unusual rise of wall temperature with one or multiple peaks. Despite being dangerous for the heat exchanger equipment, there is a lack of accurate predictions and reasonable explanations of such HTD phenomena. In this work, a two-phase numerical method is validated and optimized in terms of the two-phase model and interval temperature ΔT. The results showed that the Mixture model outperformed the volume of fluid model, where the modeling results were more consistent with the experimental values when ΔT was set as 1 K. In addition, considering the pseudo-boiling theory and the two-phase numerical method, three cases with different forms of HTD were selected for further investigation. The selection of the saturation temperature Tsat and condensation frequency fe was found to also have a significant impact on calculation results. The best predicted results were obtained at a saturation temperature equal to the termination of pseudo-boiling temperature with a fe of around 200. Overall, the two-phase numerical method was found to yield more accurate predictions compared to the single-phase numerical method in this work. Through the analysis of the average gas volume fraction along the flow direction, a link between this fraction and HTD was developed. Moreover, HTD in the case of supercritical fluids could also be divided into two types of pseudo-film boiling, similar to that under subcritical pressure. The first type of HTD is similar to stable pseudo-film boiling where the gas volume fraction is very high at the deterioration point. By dividing the supercritical fluid into three regimes, a good agreement was reported between the wall temperature and the vapor-like regime. Also, the wall temperature and the vapor-like regime were found to peak at the same position. On the other hand, the second type of HTD is similar to the departure from nuclear boiling, which can occur at the inlet or in the low gas volume fraction zone.

Comparative Osteogenesis and Degradation Behavior of Magnesium Implant in Epiphysis and Diaphysis of the Long Bone in the Rat Model.

Magnesium (Mg), as a biodegradable material, is a promising candidate for orthopedic surgery. Long-bone fractures usually occur in cancellous-bone-rich epiphysis at each end or the cortical-rich diaphysis in the center, with different bone healing processes. Little is known about the differences in results between the two regions when applying Mg implants. Therefore, this study aimed to compare the biodegradation and osteogenesis of Mg implants in a rat model’s epiphysis and diaphysis of the long bone. Twelve male Sprague Dawley rats underwent Mg rod implantation in the distal femoral epiphyses and tibial diaphyses. Every three weeks for up to twelve weeks, degradation behavior, gas evolution, and new bone formation were measured by micro CT. Histomorphology was analyzed by Hematoxylin and Eosin, Villanueva bone staining, and TRAP staining for osteoclastogenesis evaluations. Micro-CT analysis showed statistically significant higher new bone formation in the epiphysis group than in the diaphysis group, which correlated with a lower gas volume. Histological analysis showed higher osseointegration of Mg implants in the epiphyseal region than in the diaphyseal region. The magnesium implant’s osteoclastogenesis-inhibiting properties were shown in the surrounding areas in both the cortical bone of the diaphysis and the cancellous bone of the epiphysis. Our findings show the differences in the magnesium implant’s osteogenesis and biodegradation in the epiphysis and the diaphysis. These dissimilarities indicate a better response of the epiphyseal region to the Mg implants, a promising biomaterial for orthopedic surgery applications.

The rise of bubbles in shear thinning viscoelastic fluids

Bubbles in a liquid rise under gravity and separate to the top. Bubbly liquids exist commonly in nature and play a significant role in energy-conversion, oil and chemical industries. Therefore, understanding how bubbles rise is of great importance. Rheological properties of the fluid have a strong impact on single bubble rise and have been shown to change collective bubble rise at low gas volume fractions significantly. We expect that a viscoelastic fluid can strongly modify the rise of bubbles in more concentrated suspensions.We generate bubbly liquids up to gas fractions of 30 %. We measure the bubble size and the rise velocity in micellar solutions made of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal), a common system to create shear-thinning solutions.We show that when the NaSal concentration is small and the solutions are Newtonian, the bubble rise velocity decreases with increasing volume fraction of bubbles and the relationship between the two follows the Richardson-Zaki prediction. For the shear thinning viscoelastic solutions, the Richardson-Zaki relation no longer applies. Bubble clustering leads to faster rise velocities and a weaker dependence on the bubble volume fraction. At the largest concentration two rise regimes are observed. A fast one similar to that in the other shear thinning samples, followed by a very slow bubble rise. The slow rise velocity is attributed to the smallest bubbles rising so slowly, that at the shear rates around them, the fluid behaves as a Newtonian fluid. Therefore, bubble rise becomes again comparable to Stokes expectations. We also show that the peculiar dependence of the rise velocity with volume fraction of bubbles in the shear thinning viscoelastic solutions can have important implications in flotation as the area flux changes strongly with bubble volume fraction.

Chemical and nutritional characteristics of Cannabis sativa L. co-products.

Cannabis sativa L. is an annual herbaceous plant. It was used for centuries to obtain different products. In the last century, hemp cultivation was forbidden due to the psychoactive effects of tetrahydrocannabinol acid (THCA). In the last years, new strains, characterized by high cannabidiolic acid (CBDA) and low THCA level, were developed renewing the interest in hemp cultivation to obtain food or to extract essential oils from flowers. All these processes produce many residues with different chemical–physical characteristics. In order to evaluate their potential use also in animal nutrition, some hemp co‐products were evaluated. Two different co‐products of seed processes (flour and oil) and two co‐products obtained trimming the flowers, differing in granulometry were used. The samples were analysed for chemical composition and evaluated in vitro using the gas production technique with buffaloes' ruminal inoculum. All hemp co‐products showed interesting nutritional characteristics, such as crude protein content always higher than 20% on a dry matter basis, and high neutral detergent fibre concentration partially lignified. The in vitro gas production parameters at 120 h of incubation showed quite low fermentability testified by the low organic matter degradability and cumulative gas volume (OMD from 28.09 to 45.64% and OMCV from 110 to 164 ml/g, respectively). Also, the methane produced after 24 h of incubation was particularly low (from 1.78 to 11.73 ml/g dOM). These results could be due to the high lipid and ash amounts or to the CBDA content that probably affected the CH4 formation processes. According to preliminary results obtained by this study, it is possible to hypothesize that these co‐products could be useful to mitigate the methane production into the rumen. Further studies are necessary in order to evaluate the correct inclusion into the diet for ruminants.

Feasibility of Detonation in Porous Silicon Nanoenergetics

AbstractPorous silicon with sodium perchlorate oxidizer is hypothesized to be a detonable explosive, which is atypical for heterogenous fuel/oxidizer composites. The existence of detonation in energetic porous silicon remains contested and would theoretically feature several unique behaviors illustrated by this study. Calculations to predict detonation performance are performed using CHEETAH thermochemical code for silicon porosities of 45–85 % surrounding 69 %, which represents an experimentally typical case. Uniquely, e‐PS detonation is predicted to produce low gas volume. At 69 % porosity, condensed detonation products comprise 78 % of the mass (varying from 86 % to 46 % for 45–85 % porosity), and therefore a low TNT equivalent mechanical energy of 0.429 (but 1.32 when comparing total detonation energy). The calculated pressure of detonation for 69 % porosity is 1.065 GPa, only about 4 % of that of typical military explosives but over 50 times greater than compression wave amplitudes estimated for fast‐burning nanothermites, which are comparable heterogenous fuel/oxidizer composites. At porosities above 67 %, computed detonation velocities are shown to exceed estimates for the unreacted speed of sound and therefore a detonation structure consistent with classical CJ theory is proposed. Indeed, published maximum experimental propagation speeds for energetic porous silicon in this upper porosity range when pore size is optimal agree well with CHEETAH computations of detonation velocity, thereby supporting that detonation is possible. Below 66 % porosity, sound speed in unreacted material overtakes the calculated detonation velocities. Traditionally this precludes formation of a detonation wave since the shockwave would decay as energy propagates ahead acoustically into unreacted material. However, experimental agreement is still observed between 60–67 % porosity. By comparing computed detonation pressure with estimates for the strength of unreacted porous silicon, this range is proposed to be a transition zone in which a detonation‐like structure could be maintained, despite the inverted sound speed comparison, by rapid catastrophic fracture of porous silicon at the detonation front. This hypothesis implies that a sonic precompression wave must precede the detonation wave since at least a minor fraction of shock energy would propagate ahead in unreacted material. Finally, comparisons are made with baseline primary explosives and nanothermites showing that among metal‐based composite energetics on the basis of reaction rate and detonation pressure, energetic porous silicon currently exhibits the most promise for replacing primary explosives at reasonable densities in igniters and augmenting initiator formulations.

Evaluation of Different Flare Gas Recovery Alternatives with Exergy and Exergoeconomic Analyses

Natural gas flaring is considered one of the significant sources of greenhouse gas emissions in the upstream oil and gas industry. It plays an essential role in reducing the energy efficiency of oil and gas production. Depending on flare gas properties, it can be used for different destinations. Herein, we have evaluated the main flare gas recovery alternatives, including sweet gas production, Gas to Liquid (GTL), and power generation, by using exergy and exergoeconomic analysis tools. For this purpose, flare gas condition in two phases of the Pars Special Economic Energy Zone in south of Iran was chosen as a case study. The mass sensitivity analysis is conducted to examine the processes in different flare gas volumes. Mass flow fluctuation analysis is carried out to analyze the effect of mass flow fluctuation as an intrinsic characteristic of flare systems. Results indicate that the production of electrical power from flaring gas is an economical method regarding the exergoeconomic criteria. While the power generation process has disadvantages such as high exergy destruction and high capital cost, advantages such as higher revenue, local potentials for designing, equipment, and construction should be regarded. On the other hand, the sweetening plant has the highest exergetic efficiency rather than other substitutes. This method also has a lower exergoeconomic factor, especially in lower flaring volume. The mass flow sensitivity analysis in these processes shows the GTL production unit's appropriate performance in low flare gas volume. The results also offer the superior performance of sweetening plants in high flare gas volume. Combined Cycle Power Plant (CCPP) process shows an acceptable performance in a broad range of flare gas volumes. Additionally, mass flow fluctuation analysis has been conducted to determine the effect of mass flow fluctuation in these alternatives. The investigation suggested that the CCPP process has lower fluctuation in terms of cycle product (power generation).

Numerical study of the equivalent roughness effect on the low-flow centrifugal compressor impeller characteristics

The article presents the results of a numerical study of the effect of equivalent sandgrain roughness on the characteristics of a low-flow centrifugal compressor stage. Low-flow stages are widely used in high-pressure compressors as the last stages. These compressors are used for natural gas booster compressor stations, as well as in technological processes for the production of methanol, ammonia, high - pressure polyethylene, etc. The efficiency of the low-flow stages is lower than that of medium-and high-flow centrifugal compressors stages. Stages operate at high pressure and have low gas volume flow rates. For this reason they have narrow channels of the flow part, significant friction and leakage losses. The study was conducted in the Ansys CFX 19.2 software package. In the first part of the study, the low-flow stage numerical model is validated with the test results. Full-scale tests of the stage were carried out at the LPI-SPbPU earlie. In the second part of the study, impeller equivalent sand-grain roughness numerical study carried out. As a result, the characteristics of the stage were obtained at four different values of equivalent sand-grain roughness ks. The impeller losses were estimated. It is established that when the ks values increase, the polytropic head and efficiency values do not decrease equally over the entire characteristic. Characteristics are significantly reduced in areas of high expenditure and change slightly with minimal expenditure. The internal head coefficient varies slightly depending on ks.

Distribution and motion characteristics of bubbles in a multiphase rotodynamic pump based on modified non-uniform bubble model

Multiphase rotodynamic pumps are important equipment that transport gas-liquid mixtures in offshore oilfields, suggesting it is necessary to investigate the flow characteristics in such pumps. In this study, the Eulerian two-fluid model is solved to obtain the bubble distribution in the pump, while the non-uniform bubble model is used to predict the break-up and coalescence of bubbles. The gas cavity equation is introduced to capture the rapid coalescence of bubbles near the suction side of the impeller blades, and the interphase forces in the cavities are modified. Compared with the experimental measurements for the gas volume fraction and bubble size, it is found that the simulations with the modification have better accuracies than without it. The effects of the inlet gas volume fraction (IGVF) on the bubble distribution are analyzed. Due to the formation of vortices, a large bubble number density is observed near the shroud of the impeller inlet, which increases with greater IGVF. The gas-liquid separation in the diffuser becomes more significant as the IGVF increases. Bubbles from the impeller inlet are observed to continuously gather near the suction side of the impeller blades. This forms a gas cavity that moves towards the impeller outlet with a low gas volume fraction. The rotor-stator interactions cause the air pockets to discontinuously move along the pressure side of the diffuser blade.

Challenges of fast sampling of volatiles for thermal desorption gas chromatography - mass spectrometry

Fast active sampling of volatile organic compounds (VOCs) under field conditions still is a great challenge especially when the exposure time to the source of emissions is a restricting factor. Hence, to identify ideal conditions for such applications, we systematically compared fast active sampling of VOCs collected on two common adsorbents under two regimes: first, very low gas volumes (from 300 mL) sampled at nominal flow rate and, second, sampling at the maximal applicable flow rate (0.5 L/min) before loss of sorbent material was experienced. For XAD-2 and Tenax TA, efficient sorbents for on-site VOC-sampling followed by thermal desorption GC–MS, significant differences in the signal response of volatile compounds were related not only to the varied experimental factors alone, but also to their interactions and to compound volatility. In the first regime, volatiles (∼0.004–3.13 mM) from Tenax TA gave the highest signal response only above 800 mL sampled gas volume while at low concentrations (∼0.004–0.12 mM), satisfactory recovery from XAD-2 required longer analyte-sorbent interaction. For the second regime, the relative recovery was severely impaired down to 73 ± 23%, n = 56 for Tenax TA and 72 ± 17%, n = 56 for XAD-2 at intermediate concentration, and 79 ± 11%, n = 84 for Tenax TA at high concentration compared to the relative recovery at standard flow rate. Neither Tenax TA nor XAD-2 provided a 100% total recovery (calculated using breakthrough values) for any of the evaluated compounds. Finally, two-way and three-way interactions identified in a multi-variable model, explained not only the dependence of the signal response on different experimental variables, but also their complex interplay affecting the recovery of the VOCs. In conclusion, we show for the first time that XAD-2, a material only recently introduced for the adsorption of volatiles from the gas phase, competes well with the standard material Tenax TA under conditions of fast sampling. Due to the similar absolute recovery with Tenax TA even at low concentration and with regard to the better detection limits, we consider XAD-2 the better choice for fast sampling of VOCs, particularly with low sample volumes at regular flow. For fast sampling with high flow rate, however, both sorbents might be selected only if the corresponding recovery loss can be accepted for the study.

The effects of sodium reduction on the gas phase of bread doughs using synchrotron X-ray microtomography.

Globally, the bakery industry has a target of reducing sodium content in bread products. However, removing salt results in changes in the quality of bread through effects on dough's gas phase during the breadmaking process. Using synchrotron X-ray microtomography, the objective of this study was to investigate how sodium reduction induced changes in the gas phase parameters (i.e., gas volume fraction, bubble size distribution (BSD) and its time evolution) of non-yeasted doughs made from a wide range of formulations (i.e., wheat cultivar and water content) prepared with different mixing times. As salt content was reduced, a lower gas volume was retained in the dough by the end of mixing. Less gas bubbles were also retained if doughs were prepared from a stronger wheat cultivar, higher water content, and/or mixed for a shorter time. Rates of change in the median (R0) and the width (ε) of the fitted lognormal radius dependence of bubble volume fraction [BVF(R)] indicated that reduced sodium content permitted disproportionation to proceed more rapidly. Higher water content or longer mixing time also resulted in faster disproportionation, indicating that water content and mixing time can be manipulated as a means of increasing bubble stability against disproportionation during low-sodium breadmaking. An examination of relative changes in dough's gas phase parameters arising from sodium reduction demonstrated that wheat cultivar, water content and mixing time all affected dough's tolerance to sodium reduction. Therefore, attainment of good bread crumb cell structure in low-sodium bread formulas is a function of salt's effects on dough rheology in addition to its effect on yeast activity, so that dough formulation and mixing conditions also need to be considered.

Analysis on the performance of a 3D positive ion detector as propone and argon sensor

Nanodosimetry is a technique that measures the energy deposited by ionizing radiation under the principle of ion induced impact ionization in low pressure gas volume which correlates the number of ionization produced in nanometric scale of the DNA. In the modern field of nanodosimetry, the multilayer Printed Circuit Board (PCB) technology based 3D positive ion detector plays a more prominent role in the field of radiation biology, radiation dosimetry, oncology, and also as gamma spectrometer and gas sensor. The present study is targeted to analyze the performance of the 3D positive ion detector as gas sensor under propane, methane, argon, nitrogen, and air medium at various pressures ranging from 0 to 10 Torr using Co-60 source. From this study, it is observed that the detector shows varying magnitude in its efficiency and amplitude under different media with a maximum efficiency of 12.286% and amplitude of 168 V under propone medium at 0.9 Torr. The detector also shows variation in its cluster size distribution and ion drift time under different media. Hence, it is concluded that the 3D positive ion detector can be considered as a gas sensor.

Analyze the positive ion detector in terms of thickness

Nanodosimetry is a technique that measures the energy deposited by ionizing radiation in low pressure gas volume which correlates the number of ionization produced in nanometric scale of the DNA. In the modern field of nanodosimetry, the structure of the PCB technology based positive ion detector play a more prominent role to improve its efficiency. Hence, the present study analyzes the variation in efficiency of the PCB technology based detector in terms of its thickness. In our study, the copper strips, cathode material and insulating material of the PCB technology based detector proposed by Bashkirov et al., were modified and it is used for further analysis. The detector 1 was constructed to have 3.220 mm thickness which includes 3.0 mm dielectric materials and 0.220 mm gold. The detector 2 was constructed to have 3.483 mm thickness which includes 3.261 mm dielectric materials and 0.222 mm gold. These two detectors were characterized by Nanodosimeter under Propane and argon medium at 0 to 10 Torr pressure using Am-241 and Co-60 sources. It is concluded that as the thickness of the PCB technology based positive ion detector increases, the ion collection efficiency is also increased. This is due to the increase in dielectric materials thickness which enhances the ion impact ionization.

Mixing mechanism in a two-dimensional bubble column

The present contribution investigates the mixing of a passive scalar by a homogeneous bidimensional bubble swarm rising at high Reynolds number in a liquid initially at rest. Mixing experiments are performed in a Hele-Shaw cell for gas volume fractions α ranging from 3.0% to 14.0%. A weakly diffusive passive dye is injected within the swarm, and the temporal evolution of the spatial distribution of concentration is measured. The vertical distribution of concentration shows an upward propagation and a spreading due to the mixing induced by the unsteady open wakes of the bubbles. A one-dimensional large-scale model involving an intermittent and convective mechanism has been developed to describe the global evolution of the concentration distribution in the vertical direction. Based on experimental observations, it assumes that each bubble catches a given volume of fluid V t in its wake and transports it over a certain length L before releasing it. A good agreement is found between the experimental concentration profiles measured in the vertical direction and the model prediction. The comparison between the model and the experiments allows the determination of the transported volume V t and the transport length L as a function of the gas volume fraction. It appears that the transported volume is related to the characteristic length of the velocity deficit at the rear of the bubble. The transport length, which is related to the correlation length of the dye patches, shows two regimes. At low gas volume fraction, it is controlled by the viscous length related to the flow damping at the walls, whereas, at higher gas volume fraction, it is limited by the distance between two successive bubbles. The mixing properties are finally characterized from the first three-order moments of the dye distribution, which are determined by means of the model. The upward propagation of the dye is shown to scale as αV , where V is the bubble rise velocity. The spreading of the concentration distribution is characterized by an effective diffusivity, which presents strong similarities with the diffusion coefficient measured in a three-dimensional bubble column [Almeras et al. J. Fluid Mech. 776, 458 (2015)]. At low gas volume fraction, it √ increases as α, whereas it saturates at high gas volume fraction. The dye distribution also is asymmetric with a significant skewness coefficient which slowly decreases in time. Therefore, the dye transport cannot be described as a purely diffusive process over the time required for the dye to spread over the cell.

Location of the inlets and outlets of Chinese dome digesters to mitigate biogas emission

A model based on three equations was developed and validated for the design of the inlet and outlet of the Chinese dome digester (CDD) to prevent biogas emission or leakage. The model was implemented in MATLAB software and validated with results from a pilot study. Biogas and temperature data from the pilot experiment were used to validate the model and the results fitted well with the experimental data at low gas volume ( 20 mols). The model can be used to predict the optimal size of the CDD for daily biogas storage based on different reactor and expansion chamber sizes, in order to mitigate the emission of surplus biogas from the inlet and outlet pipes.