Countercurrent Flow Of Gas Research Articles

Experimental study on flow characteristics of falling film over horizontal tubes under countercurrent gas flow

Open-absorption heat pump serves as an important means of industrial waste heat recovery, and the overall performance of the heat pump system depends on the absorber’s performance. Horizontal-tube–falling-film absorption refers to a complex process involving the coupling of fluid flow, heat transfer, and mass transfer. For the complete understanding of the intertube flow characteristics of horizontal-tube falling film under countercurrent gas flow (We), Visual experimental studies on falling-film flow were conducted using high-speed photography. The critical Reynolds number (Re) was quantitatively characterized to represent the transition of different flow patterns between tubes. The experiment was conducted to investigate the variation in the critical Re under the coupling effects of various factors and establish an experimental prediction correlation formula for the critical Re and solution properties, We, s, and d with an error margin of less than 10%. The influence of single factors was characterized qualitatively and then subjected to sensitivity analysis. The factors affecting the critical Re in descending order of magnitude were as follows: solution mass fraction > s > d > countercurrent gas flow (We). A reduction of approximately 50% in the critical Re for every 10% increase in the solution mass fraction.

An Integrated Modeling Approach for Vertical Gas Migration Along Leaking Wells Using a Compressible Two-Fluid Flow Model

Gas migration behind casings can occur in wells where the annular cement barrier fails to provide adequate zonal isolation. A direct consequence of gas migration is annular pressure build-up at wellhead, referred to as sustained casing pressure (SCP). Current mathematical models for analyzing SCP normally assume gas migration along the cemented interval to be single-phase steady-state Darcy flow in the absence of gravity and use a drift-flux model for two-phase transport through the mud column above the cement. By design, such models do not account for the possible simultaneous flow of gas and liquid along the annulus cement or the impact of liquid saturation within the cemented intervals on the surface pressure build-up. We introduce a general compressible two-fluid model which is solved over the entire well using a newly developed numerical scheme. The model is first validated against field observations and used for a parametric study. Next, detailed studies are performed, and the results demonstrate that the surface pressure build-up depends on the location of cement intervals with low permeability, and the significance of two-phase co-current or counter-current flow of liquid and gas occurs along cement barriers that have an initial liquid saturation. As the magnitude of the frictional pressure gradient associated with counter-current of liquid and gas can be comparable to the relevant hydrostatic pressure gradient, two-phase flow effects can significantly impact the interpretation of the wellhead pressure build-up.

How do the fluid dynamics change for gravity-destabilized film flow on structured surfaces? An experimental investigation using light-induced fluorescence

Liquid film flow is the dominant flow regime in distillation and absorption processes within structured packings. Extensive research has been carried out to improve the understanding of the involved fluid dynamics. Up to now, these investigations mainly focused on gravity-stabilized film flow, i.e., the liquid phase flows over a packing in counter-current flow with a gas phase. In contrast, gravity-destabilized film flow is studied much less frequently although this type of flow applies to about half of the cases in a column with structured packings. Here, the liquid runs along the underside of the packing, also in counter-current flow with the gas phase. To close the gap in the experimental data, this contribution investigates the fundamental fluid dynamics of gravity-destabilized liquid film flows on a smooth plate, a 2D wave texture, and a 3D pyramidal texture. Results on critical convective inclination angle, drained liquid mass flow, and liquid film thickness without counter-current gas flow are reported and compared to numerical results from literature. In the experiments, the Reynolds number is varied from 28.4 to 113.5 using a surfactant-modified aqueous system. The 2D structure showed decreased liquid flow stability compared to the smooth surface. However, the 3D structure seems to have a stabilizing effect on the flow.

A reduced-order modeling of a tubular solar reactor for long duration thermochemical energy storage

The storage of solar energy in a solid form, referred to as a “solar fuel”, can be achieved through a process known as endothermic solar thermochemistry. This process transforms the absorbed solar energy into a stable and retrievable form that can be stored for extended periods of time. This paper presents a low–order heat transfer model of a counter–current tubular falling bed reactor designed to produce thermally reduced magnesium manganese oxide pellets for long duration thermochemical energy storage. The energy required for the endothermic reduction was supplied by concentrated solar energy or renewable electricity via indirect heating of the gas and solid reactants flowing in a ceramic tube. The counter-current gas flow enhances the mixing of the solid particles with the heat recuperation zone, allowing the gas and particles to enter and exit the tubular reactor close to room temperature. The reactor was vertically oriented and was heated circumferentially by an adjustable level heat flux along a finite segment of its length. The temperature distribution of the reactor in response to transient changes along the tube was modeled by considering conduction, convection, and radiation heat transfer. Governing equations for the heat transfer model were solved by discretizing the reactor tube into a finite number of control volumes and using an energy balance for the heat exchange between the reactor wall, gas, and particles within the control volume. The energy absorbed during this endothermic reaction was modeled numerically by fitting the data of the chemical conversion rate with the corresponding temperature of particles in the heating zone. The numerical model has been experimentally validated using a reactor prototype made of a 121.92 cm alumina tube heated by a 7 kW electric tube–furnace. The alumina tube receives magnesium manganese oxide pellets of 3.66 ± 0.516 mm in diameter from the top, and a counter–current gas flow from the bottom. The reactor wall temperature was monitored by six thermocouples installed along the reactor tube length. The experimental procedure was numerically simulated, and the temperature variation along the reactor tube was compared with a matrix of experimental runs for a range of particles mass flowrates (0.75–1.25 g/s) and corresponding gas flowrates (36–65 SLPM). The reactor system was heated gradually from room temperature to a steady state temperature of 1673 K, and then cooled down to room temperature. The heating and cooling processes were simulated, and the numerical and experimental results were compared throughout processes. The numerical model showed similar trends to the experimental results, with an error of 0.69 to 7.9 % for the particle inlet and 0.7 to 7.9 % for the gas inlet during steady-state operation. The proposed numerical model can be implemented as a simplified physical model to design a feedback control system to regulate reactor temperature.

Modeling and comprehensive analysis of hydrogen production in a newly designed steam methane reformer with membrane system

The use of a hydrogen perm-selective membrane in the conventional steam methane reformer (SMR) is a promising technology for producing ultra-pure hydrogen under moderate operating conditions. A detailed computational fluid dynamics (CFD) analysis is performed on a membrane SMR unit for an on-site hydrogen refueling station (HRS) to get the optimum operating conditions. The developed model was validated with experimental results. The obtained results predicted that the high temperature of the process gas inlet, the high operating pressure, and the low gas hourly space velocity (GHSV) were advantageous. However, these factors must be carefully adjusted to achieve optimal efficiency. Furthermore, the simulation with a counter-current sweep gas flow configuration gave better results than a co-current operation. The estimated optimal conditions for improved CH4 conversion and H2 recovery were 4500 h−1 GHSV, 500 K inlet temperature, and the 8.5 bar operating pressure, representing CH4 conversion and H2 recovery of approximately 94.7% and 95.8%, respectively.

Well-scale experimental and numerical modeling studies of gas bullheading using fiber-optic DAS and DTS

Bullheading is an effective well control technique to pump produced fluids back into the formation during drilling and completion operations to regain well control. However, existing bullheading procedures are often based on a trial-and-error approach due to the complexity of modeling counter-current gas flow mechanisms in well control simulators and the lack of empirical bullheading datasets in full-scale conditions. To address this gap, a 5163-ft-deep experimental wellbore was utilized to conduct bullheading tests using nitrogen and water to simulate a “gas below BOP” scenario. We present a novel application of fiber-optic distributed acoustic sensor (DAS) and distributed temperature sensor (DTS) in facilitating the measurement and monitoring of gas slugs in the wellbore for an improved understanding and modeling of the gas bullheading processes.Full scale tests were performed by injecting a nitrogen slug into the wellbore annulus which was bullheaded with water while taking returns via tubing. A detailed analysis of the bullheading data for two different gas slug volumes and bullheading rates is presented. DTS and DAS data were analyzed using a variety of time- and frequency-domain signal processing techniques to estimate displacement velocities and changing gas slug lengths during injection, subsequent expansion, and displacement in the annulus. Numerical simulations were also performed for an improved understanding of the bullheading processes using a drift flux model (DFM)-based transient multiphase flow simulator modified for counter-current flow situations. Different gas slip models, used in the DFM, were benchmarked for the best match with the DAS and DTS measurements obtained from the bullheading experiments. The data from downhole pressure gauges were also compared to the DAS and DTS measurements as well as the results from numerical simulations and satisfactory agreements was obtained.This study demonstrates a novel application of DAS and DTS in obtaining high-resolution spatial-temporal measurements for the facilitation of improved understanding and modeling of well-scale bullheading hydrodynamic behaviors. The evaluation, selection, and calibration of transient multiphase flow sub-models used in this study improved the understanding of the counter-current flow in a well and provided references for applying the DFM in simulating bullheading operations.

Spatially Resolved Tomographic Assessment of Axial Mixing in Structured and Random Packings

AbstractThe liquid axial dispersion inside modern structured packings (Sulzer Mellapak™ 500.Y, Raschig Super‐Pak 350Y) and one random packing of the 4th generation (Raschig Super‐Ring 0.3) was measured by means of computed tomography. As the gas velocity is known to have little effect on the liquid axial dispersion at low hydraulic load, all experiments were conducted without countercurrent gas flow at liquid loads of B = 10, 15, and 20 m3m−2h−1 in a column with an inner diameter of 100 mm. Injection of the X‐ray‐active tracer potassium iodide allows the identification of active and passive liquid accumulations inside the structured packing. Good agreement was found between the active liquid holdup from tomographic images and the total holdup calculated from the mean residence time for the structured packings.

Nonlinear wave attenuation in strongly confined falling liquid films sheared by a laminar counter-current gas flow

Experiments are conducted in water films falling along the bottom wall of a weakly inclined rectangular channel of height$5$mm in the presence of a laminar counter-current air flow. Boundary conditions have been specifically designed to avoid flooding at the liquid outlet, thus allowing us to focus on the wave dynamics in the core of the channel. Surface waves are excited via coherent inlet forcing before they come into contact with the air flow. The effect of the air flow on the height, shape and speed of two-dimensional travelling nonlinear waves is investigated and contrasted with experiments of Kofman, Mergui & Ruyer-Quil (Intl J. Multiphase Flow, vol. 95, 2017, pp. 22–34), which were performed in a weakly confined channel. We observe a striking difference between these two cases. In our strongly confined configuration, a monotonic stabilizing effect or a non-monotonic trend (i.e. the wave height first increases and then diminishes upon increasing the gas flow rate) is observed, in contrast to the weakly confined configuration where the gas flow is always destabilizing. This stabilizing effect implies the possibility of attenuating waves via the gas flow and it confirms recent numerical results obtained by Lavalleet al.(J. Fluid Mech., vol. 919, 2021, R2) in a superconfined channel.

Numerical simulation of evaporating wavy falling liquid films in laminar gas streams

Numerical simulations are performed to investigate the interfacial heat and mass transfer from evaporating wavy falling liquid films in interaction with laminar gas streams. The OpenFOAM solver has been used to conduct the simulations where the liquid-gas interface is resolved using the Volume of Fluid method of solving for three phases, i.e., liquid, vapor, and air. The configuration considered is a falling liquid (water) film on a heated vertical plate with a confined laminar moist air (gas) flow that is either (a) co-current or (b) counter-current to the downward liquid flow. The evaporation at the liquid-gas interface is driven by the interfacial gradient of the vapor mass fraction. Interfacial waves are triggered using a monochromatic forcing disturbance that leads to sinusoidal or solitary waves forming at the liquid-gas interface under respective forcing frequencies. The numerical model is validated with the available experimental data. The results show nearly a 15% enhancement in time-averaged Sherwood number (Sh) due to film waviness (sinusoidal or solitary) at the lower volumetric gas flow rate, Qg = +50 (co-current) and Qg = −50 (counter-current). This enhancement in the Sh for both the waves further increases by 11% with Qg = +800 and 196% with Qg = −800. A closer examination of the mass transfer process over a wave demonstrates that with Qg = +50, the concentration of the gas side streamlines at the trough locations of the wave leads to higher values of Sh at these locations. However, with Qg = +800, although the overall Sh increases, vortices appear at the wave trough locations, leading to a corresponding decrease in the local Sh values. Correlations are proposed for predicting Sh under co-current and counter-current gas flow effects.

Development of a Moving Bed Reactor for Thermochemical Heat Storage Based on Granulated Ca(OH)2

Calcium hydroxide is promising for thermal energy storage due to its low cost and high energy density. Nevertheless, the powdered material is cohesive and has low thermal conductivity which is a major challenge for the operation of moving bed reactors. One approach to facilitate the movement of the reaction bed is the stabilisation of the particles through the coating of Ca(OH)2 granules with Al2O3 particles. In this work, a newly designed reactor concept was specifically developed for testing coated Ca(OH)2 granules. The design allows for the movement of the reaction bed by gravity assistance and direct heating of the particles by a counter current gas flow. The operation was successfully demonstrated and proved to achieve high heat transfer between gas and granules. Furthermore, the movement of the reaction bed was achieved after the discharging phase. Two batches of uncoated and coated Ca(OH)2 granules were subject of 10 thermochemical cycles in this reactor. The cycling stability, structural integrity, mechanical stability, morphology and phase composition of the granules were analysed. Full conversion of both samples was demonstrated for the entire experimental series. It was found that the alumina coating enhances the mechanical stability of the granules under reaction conditions.

MODELING OF A DUSTED GAS FLOW CLEANING PROCESS USING A KINEMATIC MODEL OF THE INTERACTION OF DISPERSED PARTICLES WITH DROPLETS IN A WET SCRUBBER

A mathematical model is proposed for the scavenging process of the dispersed particles by droplets in a wet scrubber under excess spray density in Venturi tube within kinematic approach of the interaction of particles in countercurrent gas-dispersed flows, which refines the existing engineering model, taking into account the spatial size variation of the droplets, due to their coagulation with wet slurry droplets and uncaptured particles entering a wet scrubber from the Venturi tube. The results of calculations with the adopted mathematical model showed that in case of possibility to organize the spraying of a gas flow in a scrubber with 300–500 micron droplets aerosol at a specific spray density of about 1 liter/m3, a 1–2 meters wide layer of droplets ensures effective absorption of both uncaptured PM2.5 solid particles, and the slurry droplets from the Venturi tube. The ejection of the slurry droplets into a wet scrubber from the Venturi tube, and the associated increase in the size of the scrubber droplets due to coagulation with slurry droplets, does not noticeably affect the efficiency of the dusted gas stream cleaning. An adopted mathematical model was applied to calculate the capture of particles by droplets in cylindrical and conical scrubbers. Due to the increase in a residence time of the droplets upon increased velocity of the countercurrent gas flow, the efficiency of gas cleaning from dispersed particles in a conical scrubber appears to be higher than in a cylindrical scrubber. However, with an increase in the spray density above 2 liter/m3 and with droplet diameters greater than 1000 microns, the efficiency of the conical scrubber decreases, which is associated to an increase in the escape of a significant proportion of massive drops to the walls with a reduction in the scrubber reactor cross-section. Bibl. 21, Fig. 4.

Model based random packing optimisation for absorption processes using the hydrodynamic analogy concept

A novel approach is presented that supports the development of new packing types for absorption processes by “virtual experiments”. Based on a hydrodynamic analogy model for the absorption in random packed beds, a simulation study highlighting the influence of packing geometry on separation efficiency is performed. In the model, the liquid flow is reproduced by film, jet and droplet flow patterns with countercurrent gas flow. To incorporate the actual redirection of liquid and gas flow inside the packing, these flows are mixed after specific lengths of undisturbed flow. The lengths are derived from the packing geometry. These lengths as well as the hold-up ratios for the liquid flow patterns and the hydraulic diameter of the packed bed are varied in the simulation study. It is found that a small hydraulic diameter, an increased frequency of liquid film mixing and an increased film hold-up lead to higher absorption rates. In contrast, the frequency of gas or jet/droplet mixing has negligible influence.

Synthesis of Zero Carbon Solid-State Fuel for Long Duration Energy Storage

This work presents a unique thermochemical process for synthesizing magnesium manganese oxide-based zero carbon solid-state fuel, which can be used for long duration energy storage. High temperature (1450°C) heating of the processing furnace can be driven by either renewable electricity or concentrated solar power. The very simple fuel synthesis concept is based on a tubular falling bed reactor with countercurrent oxygen-depleted gas flow for complete heat recuperation. A sophisticated control strategy using pulsed width modulated gas flow and an L-valve maintains consistent magnesium manganese oxide (Mg-Mn-O) particle flow up to 1450°C. The measured extent of the thermal reduction reaction after cooling the Mg-Mn-O particles is more than 90% of the fully reduced state at equilibrium. The thermal to chemical efficiency (ratio of chemical energy stored to thermal energy input) and overall system efficiency are above 96% and 28%, respectively. Both, the solid particle reactor flow at the extreme temperature of 1450°C and the system efficiency of 28% are the highest reported for thermochemical fuels in the literature to date.

A new hydrodynamic analogy model for the determination of transport phenomena in random packings

A new model for absorption processes in columns equipped with random packings based on the hydrodynamic analogy approach is presented. The packed bed is represented by a bundle of cylindrical channels with the same diameter equal to the hydraulic diameter of the bed. The liquid flow in the real packing is captured by a combination of film, jet and drop flow which are assumed to be the dominating flow patterns in the bed. The film flow is considered as attached to the inner surface of the channels, while jets and drops are falling through the channel center. Each of the flow patterns is modelled independently of one another, in a separate channel type with counter-current gas flow. After certain packing specific lengths of undisturbed flow, each phase is mixed to govern the redirection of the flow and the transition from film to jet and drop flow and vice versa as well as the mixing of gas flows from neighboring packing elements. The fluid dynamics as well as heat and mass transfer are described by the respective rigorous conservation equations. Both phases are related by conjugate boundary conditions at the phase interface. The reactive CO2 absorption into aqueous sodium hydroxide solution is investigated experimentally for two random packings of type 50 mm Pall-Ring and ENVIPAC 2 in an industrial-scale column. The model is validated against the experimentally obtained data.

Stability of gravity-driven thin-film flow in the presence of an adjacent gas phase

The stability of a falling liquid film flowing down an inclined channel is revisited. The classical Kapitza criterion for the onset of long wave instability ignored the presence of the adjacent gas phase and provided the critical liquid Reynolds number, Recr=5/6cot(β) for the flow destabilization. In the current study, the impact of the adjacent gas on Recr is studied via solution of the Orr-Sommerfeld equations in both the liquid and gas phases. The particular case of zero net (recirculating) gas flow is investigated, though this case is shown to be relevant to a wider range of concurrent and countercurrent gas flows. The results obtained confirm the recent finding that Kapitza instability may be fully suppressed in sufficiently small channels. However, in large channels, where the critical perturbation for the flow destabilization is long wave, the Kapitza criterion largely over predicts the critical Reynolds number. This is shown to be related to the increasing impact of the dynamic interactions between the gas and liquid at the interface, whereas the role of the liquid inertia diminishes. A revised analytical expression for Recr is derived, which accounts for those interactions in gravity driven thin film flows.

Assessment of mass transfer intensification potential for a CO2 capture process using three-phase fluidized bed

In order to assess the post-combustion CO2 capture intensification potential, a detailed mathematical model was developed for a three-phase fluidized bed. As solvents, various aqueous solutions were used e.g. NaOH and MEA with and without glycerol addition. An innovative mass transfer equipment is presented, a three-phase gas-solid-liquid fluidized bed absorber, in which the bed of low-density inert solid particles is fluidized by the counter current flow of gas as a continuous phase and liquid as a dispersed phase. Experimental data from a pilot plant was used to validate the developed model. The hydrodynamic parameters determined were the followings: fluidized bed expansion ratio, liquid holdup and bed pressure. The most important efficiency parameter, the effective mass transfer area, was assessed. The results show that by using a three-phase fluidization column, the mass transfer parameters exhibit significant increases (8–10 times) in comparison to conventional packed beds. In addition, introducing glycerol to NaOH and MEA solutions was investigated as a potential method to further improve the overall CO2 absorption performance. As the results show, the glycerol addition intensifies the CO2 absorption process by about 10 %.

Falling liquid films in narrow tubes: occlusion scenarios

We study a gravity-driven wavy liquid film falling down the inner surface of a narrow cylindrical tube in the presence of an active core gas flow. We employ the model of Dietze and Ruyer-Quil (J. Fluid Mech., vol. 762, 2015, pp. 68-109) to investigate the role of surface waves in the occlusion of the tube. We consider four real working liquids and reproduce several experiments from the literature, focusing on conditions where the Bond number is greater or equal to unity. We prove that occlusion is triggered by spatially growing surface waves beyond the limit of saturated travelling-wave solutions, and delimit three possible regimes for a naturally evolving wavy film: (i) certain occlusion, when the liquid Reynolds number is greater than the limit of the spatially most-amplified travelling waves. Occlusion is caused by surface waves emerging from linear wave selection (scenario I); (ii) conditional occlusion, when the most-amplified waves possess travelling states but longer waves don't. Occlusion is triggered by secondary instability, generating long waves through nonlinear coarsening dynamics (scenario II); and (iii) impossible occlusion, when travelling waves always exist, no matter how great their wavelength. We show that certain occlusion is delayed by gravity and precipitated by a counter-current gas flow, axial viscous diffusion (high-viscosity liquids), and inertia (low-viscosity liquids). The latter two effects are also found to determine whether the occlusion mechanism is dictated by loss of travelling-wave solutions or absolute instability. Finally, we show that occlusion can be prevented through coherent inlet forcing. As a side benefit, we introduce an augmented version of our model based on a localized additional force term that allows representing stable travelling liquid pseudo-plugs.

Three-dimensional multi-phase simulation of cooling patterns for proton exchange membrane fuel cell based on a modified Bruggeman equation

A three-dimensional multi-phase non-isothermal model of proton exchange membrane fuel cell (PEMFC) is developed based on a modified Bruggeman equation. The developed model is validated with experimental results and it shows the modified model can more accurately simulate actual working conditions of PEMFC at medium and high current density. The influences of different cooling patterns on the performances of PEMFC are explored. Results reveal that different gas flow patterns have roughly the same effect on the temperature distribution of the membrane in the case of the cooling water counter-current flowing. Under the counter-current gas flow pattern, pattern 3 is the best cooling pattern, that is to keep the cooling water flow direction consistent with the cathode gas flow direction. Under the co-current gas flow pattern, pattern 6 is the best cooling pattern, that is to keep the cooling water flow direction consistent with the gas flow direction. Meanwhile, the temperature distribution of the cathode and the anode is basically symmetrical at the interface of the cathode gas diffusion layer and the cathode catalyst layer. Additionally, the optimal cooling patterns need to ensure the cooling water co-current flowing, and it has obvious advantages of system integration and cost reduction.

Solitary waves on superconfined falling liquid films

Solitary waves on the surface of a vertically falling liquid film in contact with an extremely confined counter-current gas flow are studied numerically. As the gas velocity is increased, traveling waves display a secondary oscillatory instability and then a catastrophic instability. The periodic amplitude modulations produced by the oscillatory instability intensify mixing and can be tuned through the gas velocity. The catastrophic instability leads to wave reversal and liquid arrest.

Gasification of powdered coal in filtration regime with a super adiabatic heating

A new method is proposed for gasification of a powdered coal in filtration regime with superadiabatic heating. The fuel is supplied suspended with the flow of oxidant gas, whereas the gas flow filters through a porous bed of large particles making a heat recovery medium. Owing to the heat exchange between countercurrent solid and gas flows part of the heat is recuperated and transferred from the products to the reaction zone. The heat accumulation in a relatively narrow combustion zone can result in the temperature within the combustion front substantially higher than the adiabatic temperature calculated for the initial temperature of the reactants being that ambient and the heat of the reaction is evenly distributed over the products.