Increase In Temperature Gradient Research Articles

Modeling and Simulation of a High Temperature Proton Exchange Membrane Fuel Cell

High temperature (HT) (around 120-200°C) PEM FCs are predicted to be the next generation of PEMFCs particularly for hydrogen-powered automobiles and combined heat and power (CHP) systems because the water management can be simplified at such temperatures as only a single phase of water vapor needs to be considered. Additionally, the cooling system can be streamlined due to an increase in temperature gradient between the coolant and the FC stack. This will also allow easy recovery of waste heat that can be used as a practical heat source. Furthermore, the CO tolerance improves dramatically at high temperatures thereby allowing HT PEMFCs to utilize reformed and impure hydrogen.A single phase three-dimensional, steady-state, isothermal model for a single 5 HT PEM fuel cell with serpentine flow channels is implemented in COMSOL to investigate the effect of various operating conditions like temperature, back pressure and cathode flow rate and design parameters like catalyst layer loading, cathode GDL porosity, and flow field channels including parallel and interdigitated channels. Different performance indicators in terms of polarization curve, loss mechanisms, oxygen molar concentration, and anode and cathode pressure are represented for a complete overall analysis. In fact, the breakdown of different overpotential loss mechanism for HT PEMFC presented here is unique that not only quantifies these losses but also provides an accurate comparison with corresponding low temperature counterpart. The result from the computational model follows the experimental result very closely, thus, validating our model. The simulations stipulates that the performance of a HT PEMFC improves with increasing temperature, back pressure, and air flow rate. Increasing catalyst layer loading improves the performance up to a point after which it starts to drop at low voltages because of hindered gas diffusion. Similarly, a comparative study among different flow field channel is also presented indicating improved performance when going from parallel to interdigitated and serpentine channels. Furthermore, this study provides guidelines to optimize HT PEMFC performance through comprehensive parametric study.

Effect of hydrogen pressure on pore growth and microstructure of GASAR porous Mg–Ag eutectic alloys

In order to study the effect of hydrogen, as a third non-metallic component, on the macroscopic pore growth and eutectic solidification structure in the directional solidification process, GASAR porous Mg–Ag eutectic alloy ingots were prepared by mould casting technology at different hydrogen pressures. The results show that with an increasing hydrogen pressure, the increased temperature gradient promotes the nucleation and growth of pores; for the microstructure, the increased temperature gradient cause an increasing driving force for solidification structure growth, which leads to the instability of the eutectic cells, and the eutectic structure transitions from lamellar to rod-like.

Warping deformation of the track slab and their influence on the interlayer stiffness and vibration transmission characteristics of the unit ballastless track

The unit ballastless track partially segments the concrete track bed in the longitudinal direction, and relieves the concrete temperature stress by isolating the upper and lower layers. However, the layer isolation leads to the incompatible deformation of the upper and lower track structures under the effect of the temperature gradient. A large temperature gradient on the track slab will cause local contact between layers, the contact area between layers will then decrease, thus leading to “stiffness softening” between layers. When the vehicle passes, the contact area between layers will gradually increase, which, in turn, leads to “stiffness hardening”. The analysis results show that the greater the design stiffness between layers, the more significant the “stiffness softening” effect will be under the temperature gradient. When k0 = 400 × 106 (N·m−1)·m−2 and Tg = 90 °C·m−1, the interlayer stiffness decreases by 88%.The simulation analysis results of the wheel-drop impact test show that under a positive temperature gradient, the peak value of rail vibration acceleration increases, and the peak value of the track slab and base plate vibration acceleration decreases. The greater the stiffness designed between layers, the more obvious the influence will be. With the increase of the positive temperature gradient, the vibration acceleration of the rail and the track slab will be reduced in the 1–30 Hz and 40–100 Hz frequency domains. With the increase of the negative temperature gradient, the vibration acceleration of the rail in the 4–10 Hz, 20–50 Hz frequency domains, and the vibration acceleration of the track slab in the 70–200 Hz frequency domains will be increased. A large positive temperature gradient inhibits the transmission of high-frequency vibrations above 40 Hz to the substructure, while a negative temperature gradient inhibits the transmission of low-frequency vibrations within 30 Hz and 70–200 Hz to the substructure. A positive temperature gradient is beneficial to restrain the downward transmission of low-frequency vibration. When k0 = 200 × 106 (N·m−1)·m−2, the low-frequency vibration transmission loss of the track slab to the base plate is close to 0 dB without temperature gradient, but the positive temperature gradient increases the transmission loss by about 10 dB, the greater the temperature gradient, the higher the transmission loss of the track slab to the base plate under high stiffness.

Response of high-altitude clouds to the galactic cosmic ray cycles in tropical regions

Galactic cosmic rays are one of the possible mediators of the solar influence on climate. However, the impacts of GCR on clouds and climate systems are not fully understood. In this paper, we show that the high-altitude clouds associated with deep convective activities are responding to the decadal-scale cycles of GCRs and that the susceptible areas are seasonally variable. Most notable responses were found in August over tropical land areas, suggesting that the susceptivity of clouds to GCRs depends on the depth of convective activities and the abundance of aerosol precursor materials. Furthermore, following the activation of high-altitude cloud formation, an increase in sea surface temperature (SST) gradient was observed over the Pacific. Although the response of sea surface temperature to solar activity has mostly been discussed as mediated by solar radiations, we propose that another mechanism is possible: through the impact of GCRs on clouds and the resultant changes in atmospheric circulations.

Computational analysis and biomechanical study of Oldroyd-B fluid with homogeneous and heterogeneous reactions through a vertical non-uniform channel

Abstract Homogeneous and heterogeneous reactions play a decisive role in biological procedures such as burning, polymer creation, ceramic construction, distillation, and catalysis. The magnetic properties of hemoglobin molecules are organic. Magnetic resonance imaging (MRI) and electronic components with an electromagnetic field are now readily available, allowing for the explanation of fundamental biological processes. These ideas form the foundation of an ongoing study that attempts to look into the impact of both homogeneous and heterogeneous reactivity on the peristaltic transport of magnetohydrodynamics Oldroyd-B fluid. When convective and partial sliding conditions are present, the configuration changes to a non-uniform vertical channel. The fundamental partial differential equations are resolved utilizing the Homotopy Analysis Method. Entropy optimization has been carried out. The primary limits entering the problem are investigated, and then a graph is used to show the influences of temperature, velocity, skin fraction, Nusselt number, and pressure increase against mean circulation, trapping phenomena, homogeneous reactions, and heterogeneous way to respond. When magnetic parameter rises, the velocity of Oldroyd-B fluid and Bejan number decrease, while temperature, entropy generation, and pressure gradient increase. The tables show that the skin friction coefficient rises for accumulative values of the Grashof number and magnetic parameter, while the skin friction coefficient drops for rising values of the velocity slip parameter and Reynolds number. The Nusselt number increases for large values of Eckert, Grashof numbers, and magnetic parameters.

Digital assessment of phase changes and heat transfer for hydrates in microstructures using X-ray CT imaging

Natural gas-hydrates are plural clean energy resource with rich deposits, and hydrate behavior significantly affects the exploration, production and utilization of these energy resources. However, technical difficulties for effective production and utilization of these energy sources still exist. Hydrate formation using xenon and NaBr brine and X-ray computed tomography (CT) imaging experiments are conducted to investigate microstructural morphology phase behaviors of hydrates, and a novel multi-phase color difference separation method to segment the pore, hydrate, water and grain phases is proposed. Effects of underground temperature gradient on thermal properties during the phase changes of hydrates are analyzed, and their effects on flow properties are studied. Results indicate that the temperature fraction, hydrate phase fraction, latent-heat thermal energy (LTE) and total enthalpy decrease with increasing temperature gradient. Hydrates rapidly form approximately at time = 250s for temperature gradient of 50 K and time = 500s for temperature gradients of 100 K, 150 K, 200 K and 300 K, and the hydrate phase equals to each other approximately at time = 95s. The stable LTE and total enthalpy approaches to a maximum when the temperature gradient is 150 K. Moreover, the temperature fraction and total enthalpy decrease on average by 4.73%, 4.69% and 76.13%, 75.87%, and the hydrate phase fraction increases by 7.89% and 7.88% with increasing microporosity and micropore size from 34.66% to 41.93% and from 37.53 μm to 43.22 μm, respectively. The temperature fraction and total enthalpy increase on average by 5.29% and 76.67% with increasing pore-scale hydrate saturation from 0.73 to 1.00. The LTE remains constant with an increase in hydrate pore-scale variables of microporosity, micropore size and hydrate saturation from 34.66% to 41.93%, from 37.53 μm to 43.22 μm and from 0.73 to 1.00, respectively. In addition, numerical permeability computed by the proposed relation of microporosity and pore-scale hydrate saturation agrees well with experimental results, with the average errors all less than 5%. The temperature and total enthalpy in phase change heat transfer of hydrates improve the permeability. The LTE in phase change heat transfer of hydrates hardly changes the permeability, and hydrate phase decreases the permeability. This work provides the quantitative and qualitative evaluations of phase change and heat transfer behaviors of gas-hydrates, which are helpful and provide excellent tools to develop the techniques for the exploration, production and utilization of these energy resources to decrease the energy crisis.

Mesoscale FE analysis of thermal conductivity of steel fiber-reinforced cementitious materials considering fiber-matrix interface and pore effects

The thermal conductivity (λ-value) of cementitious materials with steel fibers is not always better than that of plain cementitious materials due to the interface between the steel fiber and cementitious materials and the increased porosity. A mesoscale finite element (FE) model considering both fiber-matrix interface and pore effects was developed to quantify the influences of porosity, pore water saturation degree, fiber-matrix interface property, fiber content, fiber length, fiber shape, and fiber orientation on the λ-value of cementitious materials, which can explain well the various relationships between the λ-value of cementitious materials and steel fiber content reported in the literature. In the FE model, cementitious materials containing steel fibers were divided into two scales where Scale I included solid phase and pore, and Scale II was made up of the homogenous composites from Scale I and steel fibers. To achieve a more accurate representation of the pore structures, the pores at Scale I were created randomly without specifying their geometry. The fiber-matrix interface in cementitious materials at Scale II was modelled as a zero-thickness interface with constant interfacial thermal conductance. It is found that the λ-value of cementitious materials decreases linearly with increasing porosity but increases linearly with increasing pore water saturation degree. The influence of steel fiber on the λ-value of cementitious materials depends on the fiber-matrix interface property. When the fiber content or fiber length increases, the λ-value will increase for good fiber-matrix interfaces with high interfacial thermal conductance; while poor interfaces with low interfacial thermal conductance lead to an opposite trend. Fiber geometry has a limited impact on the λ-value. The fiber orientation can affect the λ-value of cementitious materials. When the fiber rotational angle with respect to the direction of temperature gradient increases, the λ-value of cementitious materials with the same fiber-matrix interface property will decrease.

The coupled system response to 250 years of freshwater forcing: Last Interglacial CMIP6–PMIP4 HadGEM3 simulations

Abstract. The lig127k-H11 simulation of the Paleoclimate Modelling Intercomparison Project (PMIP4) is run using the HadGEM3-GC3.1 model. We focus on the coupled system response to the applied meltwater forcing. We show here that the coupling between the atmosphere and the ocean is altered in the hosing experiment compared to a Last Interglacial simulation with no meltwater forcing applied. Two aspects in particular of the atmosphere–ocean coupling are found to be affected: Northern Hemisphere (NH) gyre heat transport and Antarctic sea ice area. We apply 0.2 Sv of meltwater forcing across the North Atlantic during a 250-year-long simulation. We find that the strength of the Atlantic Meridional Overturning Circulation (AMOC) is reduced by 60 % after 150 years of meltwater forcing, with an associated decrease of 0.2 to 0.4 PW in meridional ocean heat transport at all latitudes. The changes in ocean heat transport affect surface temperatures. The largest increase in the meridional surface temperature gradient occurs between 40–50∘ N. This increase is associated with a strengthening of 20 % in 850 hPa winds. The jet stream intensification in the Northern Hemisphere in return alters the temperature structure of the ocean by increasing the gyre circulation at the mid-latitudes and the associated heat transport by +0.1–0.2 PW, and it decreases the gyre circulation at high latitudes with a decrease of ocean heat transport of −0.2 PW. The changes in meridional surface temperature and pressure gradients cause the Intertropical Convergence Zone (ITCZ) to move southward, leading to stronger westerlies and a more positive Southern Annual Mode (SAM) in the Southern Hemisphere (SH). The positive SAM influences sea ice formation, leading to an increase in Antarctic sea ice.

Flow Characteristics and Switching Mechanism of Bistable Slit Flow Actuated by Temperature.

The bistable flow is attractive as it can be analogous to a switch to realize flow control. Based on the previous studies on actuation technique, the present study first proposed temperature-driven switching of bistable slit flow. A two-dimensional numerical simulation was conducted to investigate the flow deflection characteristics and switching mechanism. It was concluded that the temperature gradient not only biases the slit flow but also locks it to the high-temperature side. The flow deflection angle became larger with the increase in temperature gradient. Being driven by the temperature, the flow can be switched from one side to the other. Furthermore, the fluid viscosity, which varies with temperature, determines the degree of flow deflection and the entire switching time. This research can enrich the active regulation of flow and has significant potential applications in thermal sensors, thermal detectors, microelectromechanical systems, biomedicine, and other equivalent fields.

Investigation of condensate droplet movement in marangoni condensation of ethanol-water mixtures by infrared thermography

Marangoni condensation is a phenomenon that occurs during the condensation of a positive system with binary vapor mixtures. Applying infrared thermography to the investigation of Marangoni condensation could not only help to obtain quantitative data on droplet movement but also establish the relationship between the droplet velocity and surface temperature of condensate droplets. In this study, the droplet movement in Marangoni condensation of ethanol-water mixtures was experimentally investigated by infrared thermography. The effects of vapor concentration, impinging cooling water temperature, and vapor velocity on droplet velocity were investigated, and the relationships between droplet velocity, temperature gradient, and surface tension gradient were studied. The results indicated that the droplet velocity first increased and then decreased with the increase in the droplet radius. The maximum droplet velocity decreased with the increase in vapor concentration and vapor velocity. As impinging cooling water temperature increased, the droplet velocity decreased, and the decline was more obvious at the high impinging cooling water temperature. In addition, the droplet velocity increased and then decreased with the increase in the surface temperature gradient and surface tension gradient.

Impact of Stratospheric Geoengineering on Sea Surface Temperature in the Northern Gulf of Guinea

Among techniques proposed to limit global warming, there is Stratospheric Aerosol Geoengineering (SAG) which is aiming to increase Earth-atmosphere albedo by injecting sulfur dioxide into the stratosphere in order to reduce the solar radiation that reaches the earth. This study aims to assess the potential impact of SAG on Sea Surface Temperature (SST) in the Northern Gulf of Guinea and its causes using GLENS (Geoengineering Large Ensemble) simulations performed under a high anthropogenic emission scenario (RCP8.5). Here, we focus on two dynamically different regions: Sassandra Upwelling in Côte d’Ivoire (SUC, located east of Cape Palmas) and Takoradi Upwelling in Ghana (TUG, located east of Cape Three Points). Results show that in the SUC region, under climate change, there is an increase in SST (referred to as the current climate) all year long (by 1.52 °C on average) mainly due to an increase in net heat flux (lead by the decrease in longwave radiation) and also in weak vertical mixing (caused by strong stratification which dominates the vertical shear). Under SAG, SST decreases all the seasonal cycle with its maximum in December (−0.4 °C) due to a reduction in the net heat flux (caused by a diminution of solar radiation) and an increase in vertical advection (due to an increase in vertical temperature gradient and vertical velocity). In the TUG region, under climate change, SST warming is a little more intense than in the SUC region and SST changes are driven by an increase in the net heat flux and strong stratification. The cooling of the SST in TUG is similar to the SUC region, but contrary to this region, the cooling under SAG is not only explained by a decrease in the net heat flux but also by the remote forcing of wind changes at the western equatorial Atlantic.

Membrane distillation for separation and recovery of valuable compounds from anaerobic digestates

Digestate - the wet by-product of the anaerobic digestion (AD) of organic waste - is composed of a mixture of residual organic matter, microbial biomass and inorganic compounds. Although digestate is rich in nutrients, the occurrence of high levels of salinity and soluble ammoniacal nitrogen (N-NH4+) limits its direct exploitation as organic fertilizer and imposes a complex disposal.This study assesses at laboratory-scale the potential of Direct Contact Membrane Distillation (DCMD) for the separation of nutrients and the recovery of water from the liquid phase of the digestate produced by a digester that treats cattle manure and agricultural residues. Experimental tests, carried out at different feed temperature (50–70 °C) and initial pH (6–8.2) of the liquid digestate, aimed at investigating the effect of the operating conditions on the transmembrane flux of water vapor, the concentration of N-NH4+, P-PO3-, and Chemical Oxygen Demand (COD) in both permeate (also referred to as distillate) and retentate streams.For raw digestate, the initial transmembrane flux enhanced from 1.83 L·m−2·h−1 to 5.55 L·m−2·h−1, in response to increasing temperature gradient. Overall, the highest achieved water recovery factor was 40% at a feed temperature of 70 °C. The pH had a limited impact on the transmembrane flux, whereas notably affected the DCMD performance in terms of separation efficiency. In particular, a pH reduction from 8.2 (raw digestate) to 6 was able to completely avoid the transfer of N-NH4+ into the distillate. Compared to the concentration in the raw digestate, the reduction of COD and P-PO3- in the distillate was 99.0% and 99.9%, respectively. Regardless feed temperature, the performance of the process was subjected to a significant decline of evaporation rate due to occurrence of membrane fouling; however, cycles of cleaning with citric acid allowed to partially regenerate the membrane by removing the reversible cake of foulants accumulated on the surface. The results demonstrate an adequate performance of MD process for recovering water and nutrients, leading to a dehydrated fraction of the digestate phase that can be easily stored and used in agronomic practices.

Numerical study on reaction-induced force of a coal particle: Effect of temperature gradient in entrained-flow gasifier

The ambient temperature in entrained-flow gasifier varies in the large range, and temperature gradient can reach 60,000 ∼ 80,000 K/m in the flame zone. This work numerically studies the effect of temperature gradient on the conversion characteristic and reaction-induced force of a single carbon particle during the gasification process. The single carbon particle is placed in the H2O/CO2 mixture atmosphere with a temperature gradient. The particle size and the average ambient temperature around the particle are considered. The temperature gradient leads to the difference of conversion rate on the particle surface. The net reaction-induced force is formed. The reaction-induced force (Fr) is proportional to d2 (d is the particle diameter). For small particles (d < 30 μm), the Fr can reach the same magnitude as the particle gravity. Additionally, Fr increases linearly with the increase of temperature gradient. With the increase of ambient temperature around the particle, the Fr rapidly increases.

Horizontal Distribution of Temperature Effect in Rubberized Concrete Pavement: A Case Study

Temperature distribution and the deformation behavior under temperature are important parameters in the design and evaluation of concrete pavements. In this paper, in order to study the horizontal distribution of the temperature effect on rubberized concrete pavement (RCP), the distribution differences of temperature, temperature gradient and strain at different horizontal locations were analyzed based on fiber Bragg grating test technology. The relationships between temperature and strain and between temperature gradient and strain were also investigated. The results show that within a cycle of temperature or temperature gradient change, the time of temperature increase or temperature gradient increase is only 1/4 of the whole cycle, significantly less than the time of the temperature or temperature gradient decrease. Comparing the center, edges and corner of the pavement, the horizontal distribution of temperature and temperature gradients in the RCP is uneven, and the greatest negative temperature gradient is experienced at the corner of the pavement, which is 25 °C·m−1 greater than the temperature gradient at the center. The negative temperature gradient at the corner of the concrete pavement exacerbates the bottom deformation at the center and edge of the pavement, especially in the X-axis direction at the center and in the Y-axis and Z-axis directions at the edge. The relationships between temperature and horizontal strain at the center and edge of the RCP have a significant hysteresis effect and are markedly stronger than those at the corner. Moreover, when the temperature gradient is less than −23.4 °C·m−1 or greater than 14.5 °C·m−1, the curling effect at the edge of the RCP is more obvious.

Drivers of Biases in the CMIP6 Extratropical Storm Tracks. Part II: Southern Hemisphere

Abstract The Southern Hemisphere storm tracks are commonly simulated too far equatorward in climate models for the historical period. In the latest generation of climate models from phase 6 of the Coupled Model Intercomparison Project (CMIP6), the equatorward bias that was present in CMIP5 models still persists, although it is reduced considerably. A further reduction of the equatorward bias is found in atmosphere-only simulations. Using diagnostic large-scale fields, we propose that an increase in the midlatitude temperature gradients contributes to the reduced equatorward bias in CMIP6 and AMIP6 models, reducing the biases relative to ERA5. These changes increase baroclinicity in the atmosphere and are associated with a storm track that is situated farther poleward. In CMIP6 models, the poleward shift of the storm tracks is associated with an amelioration of cold midlatitude SST biases in CMIP5 and not through a reduction of the long-standing warm Southern Ocean SST bias. We propose that increases in midlatitude temperature gradients in the atmosphere and ocean are connected to changes in the cloud radiative effect. Persistent track density biases to the south of Australia are shown to be connected to an apparent standing-wave pattern originating in the tropics, which modifies the split jet structure near Australia and subsequently the paths of cyclones.

The response of Nassella trichotoma (serrated tussock) seeds and seedlings to different levels of fire intensity

Context Fire is an important disturbance regime in grassland communities, since it is responsible for stimulating the regeneration of many species and for maintaining levels of biodiversity. When invasive plants, such as Nassella trichotoma, establish and become widespread in a grassland community, these important fire events can be altered in intensity and frequency, which means that they are able to facilitate the establishment of the exotic species. Therefore, before fire can be recommended as a suitable control technique for invasive species, or alternatively to be integrated into grassland restoration programs, understanding the response of the seeds of exotic species to high temperatures, such as those experienced during a fire, should be well understood. Aims Our aim was to identify their response to a gradient of temperatures associated with different levels of fire intensity. We examined how increased duration of exposure affects their response, and whether seed age or seed moisture content affect the germination response of this species. Methods To gain a fuller understanding of the fire response of N. trichotoma’s seedbank, seeds were collected in 2016, 2017, 2018 and 2019 and then stored until the commencement of the experiments in 2020. Selected seeds were first subjected to an increasing temperature gradient (80°C, 100°C, 120°C, 140°C, and a control), and an increasing duration of exposure (of 1, 3, 6, and 9 min). In the second experiment, one population was selected to test these same temperatures and duration of exposure after the seeds were hydrated to 15%, 50%, or 95%. Last, seedlings were grown for 3 months under glasshouse conditions and then exposed to increasing temperatures (20°C, 60°C, 80°C, 100°C, and 120°C), and an increasing duration of exposure (3, 6, and 9 min). The seedlings were assessed 2 weeks after the heat exposure for signs of damage. Key results It was found that increased temperatures and duration of exposure had a subtle negative effect on germination parameters, including reduced total germination and increased time to 50% germination. The 140°C treatment was seen to be a significant threshold because it killed all the seeds at any duration of exposure. A significant difference among the ages of each seed lot was observed to be a factor on the tested germination metrics, with the oldest tested population (2016) demonstrating the highest germination percentage, uniformity, and rate. Seed germination percentage was significantly reduced for seeds hydrated to 95% compared with the control treatment, whereas no significant difference was observed for the seeds hydrated to 15% and 50%. For the heat treatment of the seedlings, damage to the leaves was observed in the 80°C, 100°C, and 120°C treatments, with some plants in the 120°C treatment experiencing extensive damage prior to resprouting. No seedlings were killed at the tested temperatures. Conclusions Results of this study indicated that fire may be a useful tool for reducing seedbank density by killing a high proportion of the seeds on the soil surface, or located within the top 1 cm of the soil profile, but not for seeds buried more deeply. Efficacy of fire on surface and shallow-buried seeds is improved with high seed moisture content; however, these seeds buried below this depth are still protected by the soil from the lethal effects of temperature. Implications Fire implemented before seed set could be used to effectively kill a large proportion of N. trichotoma seeds. However, for more comprehensive control, it is recommended that chemical treatment is integrated with the fire treatment to improve the overall control efficiency.

Effect of heterogeneity on interphase heat transfer for gas–solid flow: A particle-resolved direct numerical simulation

Particle-resolved direct numerical simulation (PR-DNS) of flow past a particle cluster is conducted to analyze the influence of heterogeneous particle distribution on the gas–solid heat transfer calculation. Then, the heat transfer rates calculated using Gunn's correlation are systematically compared with the DNS results for virtual computational fluid dynamics-discrete element method (CFD-DEM) grids with different levels of heterogeneity. The results show that, for a grid located at the interface between the dense cluster region and dilute region, Gunn's correlation significantly overestimates the heat transfer rate, especially at small Reynolds numbers. This is caused by the large temperature difference between the dense and dilute regions in the heterogeneous CFD-DEM grid. The value calculated by Gunn's correlation can be up to ten times the DNS result. For a homogeneous grid inside a dense region, the conventional Nusselt correlation fails to capture the rapid increase in the fluid temperature gradient around the near-interface particles when the grid approaches the cluster–fluid interface. Furthermore, even if the size of the CFD-DEM grid is reduced to twice the particle diameter, the heterogeneous particle distribution still leads to a remarkable error in the heat transfer calculation. Finally, modifications to Gunn's correlation are proposed for three typical cross-interface cases, which can well reflect the influence of the heterogeneous distribution of particles and yield a heat transfer rate close to the PR-DNS results. The mean relative deviations of the three fitted correlations are 5.8%, 14.3%, and 22.4%, respectively.

Multi-regime reaction front and detonation initiation by temperature inhomogeneity

Auto-ignition, reaction front propagation, and detonation development are foundational events in combustion, and are relevant to the occurrence of engine knock. It is generally understood that different auto-ignition modes can be initiated by non-uniform initial temperatures, manifesting the transition from supersonic to subsonic combustion modes with increasing temperature gradients. In this work, we have investigated the auto-ignition and reaction front propagation of syngas/air mixtures initiated by wide-ranging temperature gradients, in both spherical and planar coordinates, and have identified a universal detonation response diagram with multiple, non-monotonic boundaries of auto-ignition modes under engine-relevant conditions. Specifically, it is shown that with increasing gradient steepness, in addition to the conventional three regimes of supersonic auto-ignition deflagration, detonation development, and subsonic auto-ignition deflagration, the reaction front propagation speed would first decrease dramatically and then increase, hence inducing additional detonation regimes. Consequently, two detonation peninsulas are identified, with the first corresponding to the well-established Bradley detonation peninsula and the second manifesting a broader detonation regime. Both detonation peninsulas depend on the hotspot size and they can connect together when the hotspot radius becomes sufficiently large. The transient auto-ignition processes and chemical-gas dynamic interactions agree with the typical characteristics of various auto-ignition modes. Finally, auto-ignition modes are summarized in the detonation diagram, in which the Bradley detonation peninsula is well reproduced and the new detonation peninsula is quantitatively determined. The present study demonstrates that auto-ignition modes are significantly affected by the non-monotonic behavior of reaction front propagation, and the use of actual propagation speed is necessary for steeper temperature gradients in order to determine more accurate dimensionless parameters.

Effect of radial flows in fuel channels on thermal performance of counterflow tubular solid oxide fuel cells

Increasing the volumetric power density of solid oxide fuel cell (SOFC) stacks could enable their wider adoption in powering vehicles. The radial flows generated by inserts in gas channels of the tubular SOFC could enhance electrochemical reactions in porous electrodes and improve the power density. In this study, numerical simulations are performed to examine the influence of the enhanced exothermic electrochemical reactions on the temperature distributions in a counterflow tubular SOFC with inserts. We show that the radial flow in the fuel channel could improve the SOFC power density by 30% but lead to an increase in the maximum cell temperature gradient from 12 ℃/cm to 23 ℃/cm. The effects of radial flows under different inlet mass flow rates of hydrogen and methane are also examined. Compared with the conventional tubular SOFC, the percentage of power density improvements and the maximum cell temperature gradients of the tubular SOFC with inserts both increase with increasing inlet hydrogen mass flow rates. The maximum cell temperature gradient is increased from 12.8 ℃/cm to 47.5 ℃/cm as the percentage of net power density improvement is increased from 20% to 45%. We also demonstrate that increasing the inlet methane flow rate could adversely affect the SOFC performance, since the strong radial flow can remove the steam in porous anode, which impedes the methane steam reforming reaction and consequently reduces the output power density. The present work helps improve our understanding of heat and mass transfer characteristics of the radial flows in the tubular SOFC and helps build a foundation for SOFC channel designs.

Solar pyrolysis of biomass - part II: The physicochemical structure evolution of char

Dynamic solar pyrolysis experiments of willow pellets with different thicknesses (3 mm, 1 mm and 0.5 mm) were conducted at heat flux densities of 0.5 and 1.1 MW/m2 to study the physicochemical structure evolution of char. Characterizations of the physical structure showed that the reflectivity of char surface firstly reached the minimum value of 7.3 % and then rebounded to 15.3 %. The specific surface area decreased with reaction time from 13.2 to 2.0 m2/g, indicating its poor microscale pore structure (pore size ≤500 nm). The 3D scanning conducted by computed tomography (micro-CT system) showed that the residue pellet has unique hierarchical layers with different density and porosity. The lamellar structure can be also found inside the char layer which may be due to the increasing temperature gradient and the internal pressure caused by the volatile release. Concerning the chemical structure evolution, the secondary coking of the heavy compounds in volatiles introduced more oxygen-containing groups and alkyl side chains into the char layer. The aromatic rings system in the inner part of char was developed through removal of side-chains and functional groups. Further structure ordering of char surface was contributed by dehydrogenation and condensation. These differences between two parts of char can be attributed to the poor heat and mass transfer properties of lamellar structure. This work reveals the unique structural characteristics of solar pyrolysis char, which complements the solar pyrolysis mechanism and provides a new insight into the structural evolutions.