Porous Bed Research Articles

Evaluation of the dryout region of heat-generating conical porous beds

This study evaluates how multi-phase fluids flow and how a large volume becomes a dryout region in a volumetrically heat-generating conical porous bed formed in the pre-flooded reactor cavity of a hypothetical 1,000 MWe-class NPP whose reactor cavity is circular and 9 m in diameter. At repose angles of 25°, 35°, and 45°, effective particle diameters of 0.65, 0.9 and 1.65 mm with porosity of 0.37, the system pressures from 1 bar to 5 bar and the power densities of 400, 600, and 800 kW/m3, the coolability of debris beds is assessed. It is observed that dryout occurs in the debris beds for all the analyzed cases and that the volumetric ratio of dryout region is from 1.6% to 90.4%. The evaluation finds that the dryout region decreases as the effective particle diameter increases, the system pressure increases, the repose angle increases and the power density decreases.

NUMERICAL SIMULATION STUDY ON THE REACTION PERFORMANCE OF A METHANOL STEAM REFORMING TO HYDROGEN MICROREACTOR

Hydrogen has received widespread attention as a new clean energy in order to reduce the carbon emissions of fuel vehicles. This paper studies a tubular microreactor based on methanol steam reforming. Methanol and steam are mixed in proportion and the chemical reaction takes place in a porous catalytic bed. For heating purposes, hot gas from the burner penetrates the reactor bed through heating tubes. Energy is supplied through the heating tubes to drive the endothermic reaction system. The microreactor is enclosed in an insulated jacket. In this paper, parameters such as methanol conversion and hydrogen concentration are evaluated by considering microreactor materials, heating gas temperature and flow direction, heating tube distribution, pressure drop and reaction channel length. First of all, choosing a microreactor material with a smaller thermal conductivity can avoid excessive heat loss, and improve heat transfer performance. Increasing the heating gas temperature leads to an increase in the temperature of the reaction zone, thereby increasing the CH3OH conversion rate and H2 mass fraction. Changing the flow direction of the heating gas affects the reaction rate, but has little effect on the reaction result. Through the research on the distribution of the heating tubes, the results show that the hydrogen production rate is higher when the contact area between the heating tubes and the reaction zone is larger. Secondly, through the comparison of the data under different pressure drops, the best parameter [Formula: see text][Formula: see text]pa is obtained, and the CH3OH conversion rate is 80.6% at this time. Finally, increasing the length of the reaction channel can make the reaction more complete. For example, when the reaction channel length [Formula: see text][Formula: see text]m, the CH3OH conversion rate is as high as 83.7%.

Experimental study on burning characteristic of liquid fuel immersed in porous media bed: Effect of particle gradation

A series of burning test was conducted to investigate the effect mechanism of particle gradation on burning characteristic of liquid fuel immersed in porous media. High-purity quartz equipped with two-component discontinuous gradation and ethanol liquid were employed as porous bed and liquid fuel. The characteristic parameters, e.g., burning rate, temperature distribution, flame feature and burning duration, etc., were recorded and identified. Results show that the burning characteristic of liquid fuel immersed in porous media with gradation configuration is conjunctively influenced by capillary effect and thermal conductivity of porous media. For the porous media bed with fine particles, capillary effect plays a dominant role in the variation trend of burning rate. While for the porous media bed with good gradation, under the effect of better contact condition and more coordination numbers, thermal conduction enhanced by particle gradation dominates the burning characteristic. Meanwhile, the burning rate of porous bed immersed by liquid fuel demonstrates stage-variation feature: a rapid growth stage dominated by direct flame thermal feedback and an attenuation-stable stage dominated by thermal conduction. Due to the “back effect”, the burning rate has a quasi-stable phase in middle-and-late stage, which is positively correlated with the one-half power of media porosity. Moreover, due to the enhancement of thermal conductivity, the downward heat transfer rate of porous media with good gradation is relatively faster. In addition, there is still a positive tendency between flame height and burning rate. This work is expected to provide some references for understanding the burning characteristics of liquid fuel immersed in porous media and burning-decontamination operation.

Darcy–Brinkman Double Diffusive Convection in an Anisotropic Porous Layer with Gravity Fluctuation and Throughflow

The influence of the throughflow and gravity fluctuation on thermosolutal convection in an anisotropic porous bed with the Darcy–Brinkman effect is considered numerically. The critical Rayleigh numbers for the onset of stationary and oscillatory modes have been found via linear instability analysis. The impact of various gravitational functions in the presence of throughflow on stability is studied. The analysis has been carried out for decreasing and increasing gravity fluctuations. The convective problem has been numerically analyzed using a single-term Galerkin approach. The results show that the mechanical anisotropy parameter and Lewis number have a destabilizing effect, while the thermal anisotropy parameter, Darcy number, solutal Rayleigh number, throughflow parameter, and gravity parameter have a stabilizing effect on stationary and oscillatory convection. It is clear that the system changes in a way that makes it more stable for case (iii) gravity fluctuation and more unstable for case (iv) gravity fluctuation.

Experimental study on continuous spill fire of liquid fuel on porous bed

ABSTRACT As a traditional fossil fuel, petroleum fuel is prone to spills and fire accidents during storage, transportation, and use, which poses a great threat to the safe utilization of energy. This work aims to study the effect of porous media on the combustion characteristics of continuous spill fires. The transformer oil spill fire experiment is carried out on a porous bed with a length of 120 cm, a width of 15 cm, and a depth of 5 cm. The effects of sand diameter (0.75 mm, 1.5 mm, 3 mm, and 6 mm) and discharge rates (25 ml/min, 50 ml/min, and 75 ml/min) on the development of spill fires are analyzed. The results show that the spill fire fuel transport mechanism mainly includes gas phase diffusion caused by fuel consumption on the surface of a porous bed and liquid phase convection caused by capillary flow. Thus, three zones are formed from top to bottom: (i) the top vapor zone; (ii) the middle gas–liquid two-phase zone; and (iii) the bottom liquid phase zone. The presence of sand reduces the efficiency of the heat and mass. And it makes the spill fire stable combustion area increase compared with the smooth substrate. The stable combustion area has a positive correlation with the mass discharge rate and a negative correlation with the sand size. The stable combustion area of 1.5 mm, 3 mm, and 6 mm sand is 87.1%, 81.9%, and 77.5% of that of the 0.75 mm sand. In contrast to transformer oil pool fires and spill fires on smooth substrates, the burning rate is negatively related to the equivalent burning diameter. Meanwhile, the burning rate of spill fire on porous beds is significantly reduced compared with the first two. Based on the analysis of heat and mass transfer, a model for the burning rate of spill fire in porous media is formed and its correctness is verified. The experimental results are of great significance to the safe utilization of liquid energy.

Second stage porous media burner for syngas enrichment

The syngas production from hydrocarbons by porous media combustion (or conventional gasification) processes has been intensively and extensively studied due its calorific value and its applications in the energy sector. However, the syngas produced in a first stage gasifier can still have concentrations of light hydrocarbons (e.g., methane) which can be post-processing for further enrichment of hydrogen and carbon monoxide. The present work numerically investigates the performance of a second stage porous media burner to enrich the syngas content, mainly hydrogen and carbon monoxide. A one-dimensional model based on a two-temperature approximation is implemented, based on the PREMIX code, and supported by CHEMKIN and GRI-MECH 3.0 database and routines. From hybrid porous bed reactor experiments, five types of mixtures in the equivalence ratio range between 0.4 and 2.4 are tested: pure methane as baseline, pure Eucalyptus nitens syngas, pure Pinus radiata syngas, and two mixtures of methane with each of the biomass syngas in equal volume quantities, as transition points. The results obtained show that in the enriched syngas, in comparison to the first content after the first stage reactor, the concentrations of hydrogen and carbon monoxide increase, due to the partial oxidation of methane, however part of the hydrogen is consumed in the process. The intermediate species present in methane processing for pure syngas mixtures have broader reaction zones and in lower concentrations compared to the use of pure methane. For equivalence ratios greater than 1.9, pure syngas mixtures show higher conversion efficiencies compared to the baseline. At the equivalence ratio of 2.4, the pure syngas from Eucalyptus nitens and Pinus radiata has an energy return over energy invested (EROI) of 58.27% and 53.95%, respectively, and a maximum hydrogen and carbon monoxide yields of 31.66% and 48.40%, respectively. In the case of the Pinus radiata, the outlet syngas concentrations of the hydrogen and carbon monoxide on dry basis expose an increment about two times in comparison to the initial concentrations.

Linear Model for Two-Layer Porous Bed Suspended with Nano Sized Particles

Two immiscible fluids flows are materialized in science and technology; the combined convection of the two immiscible fluids in a square conduit is reviewed in this study. The nanofluid and pure viscous fluid which do not mix are discussed, and both layers saturated with a porous matrix have different permeabilities. The Dupuit–Forchheimer and Tiwari–Das models are applied to outline the permeability of the layer and nanofluids, respectively. The finite difference method is utilized to find the solutions of conservation equations along with suitable boundary and interface conditions. The boundary condition for the velocity is no slip at all the boundaries, while continuity of velocity and shear stress are used at the interface. The left and right walls are kept at constant but different temperatures, the top and bottom walls are isolated, and the continuity of temperature and heat flux is assumed at the interface. Grashof number, Brinkman number, Darcy number, inertia parameter, permeability ratio, solid volume fraction, thermal conductivity and viscosity ratios, different nanoparticles, and various base liquids of the two-layered fluids are engaged. The velocity is depleted by the inertia and viscosity ratio while it is accelerated with the Darcy and Grashof numbers. The energy distribution was not modulated significantly with any of the dimensionless numbers. Using copper nanoparticles doped in mineral oil and ethylene glycol produced the peak momentum. Diamond nanoparticles dropped in water catalysis showed the best heat transfer rate.

Long-term trajectory of the macroinvertebrate community in the downstream channel of a hydropower dam after the operation of a sediment bypass tunnel

A sediment bypass tunnel (SBT) is a dam facility used to mitigate reservoir sedimentation and is expected to contribute to river ecosystem restoration by delivering sediment to the degraded downstream channel. We analyzed past monitoring data of benthic invertebrates sampled downstream of the Asahi Dam in Nara Prefecture Japan to understand the long-term changes in the invertebrate community after the installation of the SBT. Before the SBT installation, the reaches upstream of the dam (one Up site) and downstream of the dam (two Down sites) showed different invertebrate communities, dominated by invertebrates that preferred clean cobble beds, sand and gravel layers, or run habitats in the former and invertebrates that preferred stable or porous beds or pool habitats in the latter. A significant increase in the Up–Down site community similarity within a few years after the SBT installation indicated a rapid recovery of the invertebrate community at the Down sites. During the 16 years of the after-SBT period, the total density and taxon richness at the Down sites increased in the early half, while they decreased in the latter half due to bed changes associated with the continuous accumulation of sediments. The change in dominant invertebrates from taxa of the large sizes to those of the small sizes and the increase in the interhabitat community similarity suggest that the bed of the Down sites became finer and flatter by sand and gravel that filled interstitial spaces of cobbles in the late stage. While the invertebrate community at the Down sites recovered dramatically by the sediment supply through the SBT, additional actions (e.g., cobble augmentation, installation of flow guiding structures) to maintain a balanced sediment size distribution and developed pool-riffle structures would be necessary to maintain the potential habitat heterogeneity and biodiversity in the long term. Our study also demonstrated that invertebrates responded to the sediment supply according to their habitat preferences, which suggests that the classifications of invertebrates by habitat-related traits (e.g., bed-residence types and pool-riffle types in this study) would help to properly understand the sediment and bed status of river reaches under channel degradation or aggradation.

DAVINCI-SP tests for debris bed formation and spreading in a pool with center conical structure under two-phase condition

In case of severe accidents in light water reactors (LWRs), the coolability of relocated corium from the reactor vessel is a significant safety issue. The failure in cooling and stabilizing the molten core in the containment vessel can threaten the integrity of the containment boundary. In a flooded cavity, a porous corium debris bed is expected to develop on the bottom of the cavity pool due to the melt jet breakup and fragmentation. The geometric configuration of the porous debris bed is important for accurate coolability assessment under accident conditions. Earlier, Kim et al. (2016a, 2016b) presented an analytical model to estimate the spreading of the debris bed in terms of two-phase flow and the debris injection parameters. Here we are testing the effect of conical bottom surface in passively dispersing the debris particle to obtain a levelled bed formation. For this, the previous model for debris bed development is modified to simulate the development of debris bed over conical bottom surface. The existing ‘The Debris Bed Research Apparatus for Validation of the Bubble-Induced Natural Convection Effect Issue (DAVINCI) - Spreading (SP) experimental facility was modified with a center conical structure. Five kilograms of stainless steel simulant debris were injected stepwise from the top of the water level, while air bubbles simulating the two-phase vapor flow were injected from the bottom of the particle catcher plate. The test results from the current study to describe the spreading of the debris bed in terms of two-phase flow, debris injection parameters, and conical bottom surface geometrical parameters are discussed. Subsequently, the model for characteristics length and side slope angle of debris bed were modified using the empirical parameters from the tests. Comparison between the tests results and modified model prediction are presented. Lastly, the effectiveness of the conical bottom surface on particle dispersion is discussed.

Development of capacitance sensor for void fraction measurement in a packed bed of spheres

A capacitance void fraction sensor (CVS) is applied to measure the volumetric averaged void fraction in a packed bed of spheres. The void fraction in the packed bed is one of the most important parameters to evaluate cooling characteristics in a porous debris bed during a severe accident of nuclear reactors, and the quantitative void fraction measuring technique for such porous flow channels should be developed. The CVS is a very simple method, and the void fraction is estimated from the electrical capacitance measured between the electrodes installed on the pipe. Generally, the linear relationship or Maxwell equation could be applied to estimate the void fraction from the capacitance measured by the CVS. However, the electrical field in the packed bed becomes complex due to the existence of spheres. Therefore, they may not be applied to the void fraction estimation in the packed bed. In this study, the CVS with a ring-type electrode configuration is used for the sphere-packed beds, and the applicability of the CVS is investigated. At first, the particle size and the pipe diameter are varied in the packed test section, and X-ray transmission imaging is used to clarify the relation between the void fraction and the capacitance in the packed bed. Then, it is found that the void fraction can be obtained by the coefficient in Maxwell's equation, depending on the packed bed properties. Finally, the measurement accuracy of the CVS for the sphere-packed bed is estimated by comparing it with a volumetric method, and the availability of the proposed method is shown.

Numerical investigation of cooling ability in heat-generating porous debris bed after severe accident in PWR

A porous debris bed formed after a severe accident is a structure composed of different sized particles. Due to the fission reaction in the debris bed, decay heat is continuously released. Therefore, the cooling ability should be investigated to determine the safety of the debris bed. In this paper, two-phase conservation equations with closure correlations are proposed for the boiling phenomenon inside the pool. For the flow resistance, the drag force between the gas and liquid in the continuous fluid is considered as well as the flow resistance of the solid to fluid in the porous medium area. The heat transfer model takes into account the heat transfer between solid phase and fluid phases as well as the heat and mass transfer between gas and liquid. All calculations are conducted based on the CFD method, and the related models are written into the CFD calculation program in the form of a User Defined Function (UDF). After the necessary validation of the proposed correlations, the analysis and discussion are based on the effect of the heating type, the non-uniform distribution of structural parameters, and the shape of the geometry. The results show the key effect of natural convection between the different boundary settings of the heating type. The time series of strong natural convection formation and the decay power of heat are the factors that are determined for the position of the boiling crisis. In addition, the limited power density is determined by the top half of the debris bed. The increase in structural parameters and operating pressure leads to a better cooling ability. For the shape of the debris bed, a regular cylinder is a better structure for heat removal, while the conical shape significantly reduces the limited power density, which is dangerous for the long-term cooling of the debris bed. The cooling ability would be improved if a downcomer existed in the porous debris bed. These three research parts are conducive to deepening the understanding of the process of a serious accident.

Coherent structures, turbulence intermittency, and anisotropy of gravity currents propagating on a rough and porous bed

This study experimentally investigated the impacts of rough and porous (RP) bed and sedimentation processes on the coherent structures, turbulence intermittency, and anisotropy of saline and turbidity currents. The results reveal that the local current concentration responds immediately (saline current) or languidly (turbidity current) to turbulence bursting events. Inside the dense current, the turbulent momentum fluxes in the streamwise (u′u′¯) and vertical (w′w′¯) directions transfer downstream and downward, which favor the sweep events. Inside the ambient water, u′u′¯ and w′w′¯ transfer upstream and upward, contributing to the formation of ejection events. At the current–ambient water interface, u′u′¯ and w′w′¯ do not tend to transfer in particular directions resulting in almost equal quantities of sweep and ejection events. The Gram–Charlier series expansion is strictly applicable to probability density functions (PDFs) of the sweep and ejection events but not suitable ideally for PDFs of the outward and inward interaction events. The primary anisotropy invariant map (AIM) of gravity currents starts from the two-component plain strain limit (near the bed). It is followed by the three-dimensional isotropy (inside the dense current and ambient water) and the axisymmetric contraction limit (current–ambient water interface). Finally, it ends in two-dimensional isotropy (near the free surface). This AIM is sensitive to the RP boundary and the sedimentation processes. Along the streamwise direction, the RP boundary causes alternations between the anisotropic and isotropic turbulence, but the arranged pattern of the rough units determines the period of this alternation.

Experimental study and numerical simulation of resistance to airflow in a storage bin of rough rice with three inlet duct configurations

The measurement and estimation of pressure drop through porous beds are important in the design and development of grain drying and aeration systems. Three inlet interconnected channels were designed and developed as inlet duct floors of an experimental grain silo. The channels were formed into shapes resembling the characters E, F and H. The pressure drop and air distribution were evaluated at different airflow rates for use as a small scale on-farm grain silo. It was observed that the pressure drop increased with the airflow rate and the height of the rough rice in the storage bin. Duct configuration of F and H represents the maximum and minimum values for pressure drop, respectively. The pressure at different heights inside the bed was uniform for all three air inlet duct configurations. Hukill & Ives and Ergun equations were used to fit with experimental data. The modified constant coefficients of both equations were determined for each of the three configurations. The Ergun equation fitted best the experimental data for the entire airflows range. Using the modified Ergun equation, a numerical simulation of the flow inside the rough rice bed was performed in the COMSOL Multiphysics software v6.1 and the velocity distribution inside the bed was presented. The simulated results revealed that the bin with H inlet duct configuration has better performance in continuous aeration process compared to E and F configurations, whereas an F duct configuration has better air penetration ability at the bottom and near the centre of the bin.

Numerical investigation of working fluid properties impacting performance of magnetocaloric cooling device

This paper presents a virtual experiment test stand of a magnetocaloric cooling device that utilizes the magnetocaloric effect (MCE). The virtual test stand is based on a comprehensive and novel numerical model that integrates pumps, check valves, tanks, and a gadolinium porous bed in a time-dependent system. The MCE is achieved by cyclic stages of magnetization and demagnetization of the porous bed of an active magnetic regenerator (AMR) and is synchronized with flow of a working fluid. The virtual test stand is utilized to investigate the influence of the properties of the working fluid on the performance of the cooling device. The isolated and combined effects of the density, specific heat, viscosity, and thermal conductivity of the working fluid are analyzed using the Design of Experiments (DOE) approach. The response variables considered are the temperature span between the cold and hot tanks and the available cooling load of the cold tank. The conducted study has revealed that the temperature span is majorly influenced by the isolated effects of thermal conductivity, specific heat, and density, whereas the cooling power is influenced by isolated and combined effects of most of the properties of the working fluid. The virtual test stand developed and the results achieved can be used to define an optimal working fluid and operational setting, such as the cycle frequency, mass flow, and bed structure of a real device. • Virtual test stand of a magnetocaloric cooling device was developed. • The influence of thermophysical properties of the working fluid was investigated. • DOE was used to study their isolated and coupled effects. • Temperature span was mostly influenced by the isolated effects of thermal properties. • Cooling performance was also influenced by the coupled effects.

Packed bed thermal energy storage system using form-stable high-density polyethylene

Energy storage is a pressing need throughout a range of applications, and storage of thermal energy is an increasingly important element in energy management. This study describes the implementation and performance characterization of a new latent heat thermal energy storage system applicable to medium temperature processes requiring heat below 125 °C. The system utilizes a packed bed of form-stable polymer latent heat storage media. The media consists of a high-density polyethylene composite reinforced with glass fiber and a thin surface coating of epoxy resin. Stabilization with traditional composite synthesis approaches provides potential for low cost and high performance. The media is stable over repeated melting/solidification cycles and shows excellent thermal capacity, with more than 160 kJ/kg attributable to latent heat. We characterize the performance of direct contact heat exchange between the storage media and heat transfer fluids. The study includes effects of the mass flux of the heat transfer fluid, fluid inlet temperature during charging and discharging, and initial temperature of the porous bed. The system can be charged and discharged at relatively high rates, e.g. > 100 W/kg, and provides excellent energy density with high exergetic efficiency averaging approximately 79% (i.e. low temperature differences between charge and discharge). The approach presented offers opportunities to enhance the use of thermal storage in medium temperature applications (e.g. a charge/discharge operational range between 120 °C – 140 °C). • Form-stable, fiber reinforced, polymer thermal storage media was synthesized. • Packed bed thermal energy storage performance was characterized. • Media shows excellent latent heat capacity exceeding 160 kJ/kg. • System can be charged and discharged at high rates, e.g. > 100 W/kg. • System can operate with low temperature differences between charge and discharge.

Modeling of Coupled Heat and Mass Transfer in a Trapezoidal Porous Bed on a Grid

We investigate heat and mass transfer in an isosceles trapezoidal cavity, filled with charcoal considered as a granular porous medium. The Darcy-Brinkman-Forchheimer flow model is coupled to the energy and mass equations with the assumption of non-thermal equilibrium. These equations are discretized by the finite volume method with an offset mesh and then solved by the line-by-line method of Thomas. The coupling between pressure and velocity is obtained by Semi-Implicit Method for Pressure Linked Equations. (SIMPLE) algorithm. The results show that the temperature in the cavity increases when the inclination angle of the sides walls decreases. The 15° inclination is selected as being able to offer better thermal performance in the cookstove combustion chamber.

Experimental investigation of quenching phenomena in high temperature conical debris bed – Part 1: MONET tests

In case of severe accidents in light water reactors (LWRs), the coolability of relocated corium from the reactor vessel is a significant safety issue. In a flooded cavity, a porous corium debris bed is expected to develop on the bottom of the cavity pool due to the melt jet breakup and fragmentation. The debris bed needs to be quenched by water either flooding from the top or flooding from the bottom until continuous cooling is established. If it is not quenched, this mass will begin to re-melt due to decay heat and leads to Molten Corium Concreate Interaction (MCCI). Most of the studies reported have conservative assumptions of one-dimensional homogeneous particulate beds with top or bottom flooding condition of coolant. Understanding of debris beds quenching with multi-dimensional characteristics are limited. To investigate the quenching behavior of the conical debris bed by means of experiments, a test facility MCCI-mitigatiON through passive cooling Effect Test (MONET) apparatus is established. Specifically, the objective is to understand the mechanism of heat transfer and quench front movement inside the conical heated debris bed during quenching by cooling water supplied from the bottom of the bed. Two different particle bed bottom wall boundary conditions are studied; closed bottom wall and open bottom wall to understand the effect of water ingression from the bottom wall. The particulate debris bed is formed with two different types of particles; Ø 3 mm SS 304 and Ø 3 mm alumina. In addition, the effect on the quench front movement inside the particle bed has been reported by using temperature measurements together with direct visualizations.

Role of curved walls on efficient thermal convection in porous beds confined within enclosures: heatline and entropy production maps

PurposeThis study aims to elucidate the role of curved walls in the presence of identical mass of porous bed with identical heating at a wall for two heating objectives: enhancement of heat transfer to fluid saturated porous beds and reduction of entropy production for thermal and flow irreversibilities.Design/methodology/approachTwo heating configurations have been proposed: Case 1: isothermal heating at bottom straight wall with cold side curved walls and Case 2: isothermal heating at left straight wall with cold horizontal curved walls. Galerkin finite element method is used to obtain the streamfunctions and heatfunctions associated with local entropy generation terms.FindingsThe flow and thermal maps show significant variation from Case 1 to Case 2 arrangements. Case 1 configuration may be the optimal strategy as it offers larger heat transfer rates at larger values of Darcy number, Dam. However, Case 2 may be the optimal strategy as it provides moderate heat transfer rates involving savings on entropy production at larger values of Dam. On the other hand, at lower values of Dam (Dam ≤ 10−3), Case 1 or 2 exhibits almost similar heat transfer rates, while Case 1 is preferred for savings of entropy production.Originality/valueThe concave wall is found to be effective to enhance heat transfer rates to promote convection, while convex wall exhibits reduction of entropy production rate. Comparison between Case 1 and Case 2 heating strategies enlightens efficient heating strategies involving concave or convex walls for various values of Dam.

Predicting convection configurations in coupled fluid–porous systems

A ubiquitous arrangement in nature is a free-flowing fluid coupled to a porous medium, for example a river or lake lying above a porous bed. Depending on the environmental conditions, thermal convection can occur and may be confined to the clear fluid region, forming shallow convection cells, or it can penetrate into the porous medium, forming deep cells. Here, we combine three complementary approaches – linear stability analysis, fully nonlinear numerical simulations and a coarse-grained model – to determine the circumstances that lead to each configuration. The coarse-grained model yields an explicit formula for the transition between deep and shallow convection in the physically relevant limit of small Darcy number. Near the onset of convection, all three of the approaches agree, validating the predictive capability of the explicit formula. The numerical simulations extend these results into the strongly nonlinear regime, revealing novel hybrid configurations in which the flow exhibits a dynamic shift from shallow to deep convection. This hybrid shallow-to-deep convection begins with small, random initial data, progresses through a metastable shallow state and arrives at the preferred steady state of deep convection. We construct a phase diagram that incorporates information from all three approaches and depicts the regions in parameter space that give rise to each convective state.

Parametric analysis of a high-temperature solar-driven propane steam reformer for hydrogen production in a porous bed catalytic reactor

In this study, a solar-driven porous bed catalytic reactor (PBCR) is investigated with the aim of propane reforming and hydrogen production using a clean energy source. The process consists of two major parts: a PBCR equipped with internal heating tubes and a parabolic dish collector (PDC) with its cylindrical cavity receiver (CCR) to supply the thermal energy required for the reforming process. A proper heat transfer fluid (HTF) is thermally charged in the CCR and discharged in the PBCR. In addition to the reforming reaction, the reversible water–gas shift (WGS) reaction takes place in the PBCR. In the reverse WGS, hydrogen produced in the previous stage is consumed and undesired carbon monoxide is generated. This unfavorable condition should be controlled to restrict the production of undesirable byproducts. In the present study, two CCR and PBCR parts are modeled/simulated using the CFD approach, and then validated using available experimental data. After model validation, the impacts of critical parameters such as inlet gas temperature, pressure and composition, bed porosity, PDC aperture diameter, and HTF circulation rate within the system on the performance of the entire system are assessed. Propane conversion, hydrogen yield, and carbon monoxide selectivity (CMS) are chosen as the system's performance indicators. At optimal conditions, for an inlet pressure of 100 Pa and temperature of 300 K, inlet propane mass fraction of 0.05, bed porosity of 0.3, PDC aperture diameter of 3 m, and HTF circulation rate of 0.05 kg/s, the time average propane conversion, hydrogen yield, and CMS during a typical day and a part of nighttime are obtained at about 62.93 %, 10.326, and 1.0122 %, respectively.