Recent progress and remaining challenges in post-combustion CO2 capture using metal-organic frameworks (MOFs)

The emissions of carbon dioxide (CO₂) and other greenhouse gases from rapidly growing industries and households are of great concern. These emissions cause problems like global warming and climate change. Although various technologies can be used to decline the alarming levels of greenhouse gases, CO₂ capturing is deemed more cost-effective. The metal-organic frameworks (MOFs) are proving to be effective adsorbent material for CO₂ capture due to their microporous structure. MOFs exhibit varying extents of chemical and thermal stabilities; hence, distinct MOFs can be selected for applications based on the working environment. In this article, thermal, chemical, mechanical, and hydrothermal stabilities of MOFs and adsorption mechanisms of CO₂ capture in MOFs were overviewed. Also, the approaches for enhancing the adsorption capacity and efficacy of MOFs were discussed. Moreover, the utilization of MOFs to improve the separation efficacy of mixed matrix membranes (MMMs) is also discussed. Furthermore, as the conversion of CO₂ to fuels and other useful products is a viable next step to CO₂ capture, therefore, recent progress in the utilization of MOFs as catalysts for CO₂ conversion was also briefly discussed. Present work attempts to link the chemistry of MOFs to process economics for post-combustion CO₂ capture.     

Electrochemical hydrogen compressor: Recent progress and challenges

Hydrogen has higher specific energy than conventional fuels but compared per unit volume under normal conditions, its energy density is lower. This difference is compensated with compression. Theoretically, compression is possible with a proton exchange membrane electrolyzer (PEME), in the process of hydrogen production, but the hydrogen permeation to the oxygen side forms a potentially explosive mixture. An electrochemical hydrogen compressor (EHC) with an analogous working principle presents the most promising solution due to its noiseless and vibration-free operation, modularity, absence of moving parts, and higher efficiency compared to mechanical compressors. Hydrogen purification and its extraction from gaseous mixtures are additional benefits that give electrochemical compression further advantage. This paper discusses the working principle of electrochemical hydrogen compression technology and its design development. The focus is on research trends, recent advances, and transpired challenges. In addition, reviewed literature aspects not studied sufficiently are highlighted, and future research directions are proposed.     

Effect of crossflow regulation by varying jet diameters in streamwise direction on jet impingement heat transfer under maximum crossflow condition

Jet impingement heat transfer has been studied numerically for a maximum crossflow condition using a 3?×?9 array of jets. Five-hole configurations have been studied for jet average Reynolds numbers ranging from 10,000 to 20,000. Crossflow has been mitigated by varying the jet diameters in the streamwise direction to reduce the impact of crossflow on downstream jet impingement. The design criteria for all five configurations were to keep the average of the jet diameters equal to the constant jet diameter configuration (baseline). It has been found that the configuration with increasing and then decreasing jet diameters provided higher levels of heat transfer with more uniform cooling when compared to the traditional constant diameter configuration and other configurations.     

Control of jet impingement heat transfer in crossflow by using a rib

Experimental studies are carried out to investigate the jet impingement heat transfer in crossflow by liquid crystal thermography (LCT). The aim is to assess the possibility of controlling heat transfer by using a rib. The crossflow Reynolds number spans from 80,000 to 160,000 and the velocity ratio ranges from 1.0 to 2.8. The results show that the presence of rib can significantly modify the heat transfer pattern of impinging jet. For all the tested cases, the presence of rib makes the Nusselt number profiles across the stagnation point change from a classical bell-shaped profile to a plateau-like pattern, indicating the enhanced heat transfer region expands more as the rib is present. In particular, the presence of rib has a more pronounced effect on the enhancement of heat transfer at lower velocity ratios (R = 1.0 and R = 1.4). However, in such cases, the local heat transfer in the rib corner region deteriorates. At higher velocity ratios (R = 2.0 and R = 2.8), the presence of rib makes the heat transfer rate more uniform, but meanwhile, it is found that the impinging jet effect tends to be weaker.     

Experimental study on flow characteristics of a sleeved jet into a main crossflow

Experiments were carried out on the hydraulic mechanism of the thermal shock caused by cold jet injection at a T‐junction with thermal sleeve in the reactor cooling system using digital particle imaging velocimetry (DPIV) technique to measure the flow in the main duct and in the annular space of the sleeve tube. The flow and vorticity characteristics were investigated at jet‐to‐crossflow velocity ratios of 0.5 to 4. There was a stream of discharge from the annular space at the rear part of the sleeve near the jet exit, which resulted in decreasing the influence of the jet on the downstream wall. The intensive vorticity in the near wake mainly originated from the shear layer vorticity of the jet and the annular discharge stream. The intensive vorticity soon broke down and dissipated, and further developed into the counterrotating vortex pair in the far wake. The flow in the annulus was closely dependent on R, and thermal protection of the sleeve would become evident at higher R. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(1): 24–31, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10131     

Characterization of confined hydrogen-air jet flame in a crossflow configuration using design of experiments

Hydrogen combustion has many industrial applications and development of new hydrogen burners is required to fulfil new demands. A novel configuration of hydrogen burner utilizing crossflow injection of fuel jets into swirling combustion air is characterized empirically in this work. It is intended as a first step in the development of new burner technologies having reduced emission levels and improved efficiency. Experiments were designed using the full factorial design method. Operating parameters were varied simultaneously and the NO_X emissions from the flame stabilized on the burner were measured. Statistical analysis of the experimental data showed that overall equivalence ratio is the dominant factor and lower NO_X emissions are observed at low equivalence ratios, irrespective of the burner power level. The analysis yielded an empirical relationship among NO_X emission, overall equivalence ratio, and power level that is useful in the design activity for a future combustion system based on the proposed configuration.     

Vortices and heat flux around a wall-mounted cube cooled simultaneously by a jet and a crossflow

Vortex morphology and heat transfer over a wall-mounted heated cube in an in-line array, cooled simultaneously by a crossflow and a normally impinging round jet, have been studied by conjugate large-eddy simulations. The interaction of the two streams and the cubes leads to the formation of complex vortical structures that govern heat removal from the cube surface. The strongest and the most evenly distributed cooling were found on the cube top and the front face. The heat flux on the side faces is lower in the zones where the flow separates, while it increases downstream where a fresh fluid from the crossflow flushes the faces. The separation on the back face of the cube creates an arch vortex, which dictates the heat transfer from that face. Despite its persistence and relative steadiness, significant nonuniformity of the temperature field has been detected on the rear face, characterised by the time meandering of hot spots. Vortex rings, created in the jet shear layer before its impact on the cube, break up upon impingement, leading to the re-establishing of the thermal boundary layer, and the consequent enhancement of heat transfer. The turbulent heat flux and its budget correlate well with the corresponding turbulent stress components.     

Numerical investigation on mixing process of a sonic fuel jet into a supersonic crossflow

The mixing process of a fuel jet in a supersonic crossflow is one of the significant issues for the design of the scramjet combustor. In this paper, the orthogonal analysis was employed to investigate the influences of the parameters of the supersonic mainstream and the fuel jet on the mixing process. Eight variables were considered and 27 cases were performed by the three-dimensional Reynolds-averaged Navier-Stokes (RANS) coupled with the shear stress transport (SST) turbulence model. The results show that the jet patterns can be divided into three categories by calculating the velocity ratio, named attachment pattern, transition pattern, and separation pattern, respectively. The extreme difference analysis indicates that the total pressure and Mach number of the mainstream, the total pressure of the fuel jet, and the diameter of the jet hole have a remarkable impact on the penetration depth and total pressure recovery. Additionally, a new dimensionless number named BS was proposed. And the penetration depth and total pressure recovery can be fitted to different functions of the BS. The fitted curves show that the larger penetration depth and smaller total pressure loss are generated as the BS increases. Finally, another new dimensionless number named LJ was proposed. And a positive correlation between the LJ and mixing efficiency has been elaborated based on analyzing the influence mechanism of the streamwise vortexes and the shockwaves on the mixing process. These correlations can provide help for primary optimization of supersonic combustor.     

URANS study of pulsed hydrogen jet characteristics and mixing enhancement in supersonic crossflow

In this paper, three-dimensional pulsed hydrogen jet in supersonic crossflow (PJISC) is investigated by the unsteady Reynolds Averaged Navier-Stokes (URANS) simulations with the k-ω shear stress transport (SST) turbulence model. The numerical validation and mesh resolution have been carried out against experiment firstly. The effects of the pulsed frequency and amplitude on the jet flow field and mixing performance in supersonic cross-flow are all addressed. It significantly changes the distribution of the hydrogen jet flow by comparing with the steady jet in supersonic crossflow. The fuel jet penetration, mixing efficiency, decay rate of the maximum hydrogen mass fraction and total pressure losses are used to quantitatively analyze the mixing performance. The mixing of fuel and incoming air flow is enhanced by the pulsed jet, especially for the case of 50 kHz, which is the optimal pulsed frequency while considering the effects of jet excitation frequency in the present simulations. The decay rate of the maximum mass fraction of hydrogen in the far field downstream is related to the frequency of the pulse jet. Moreover, the pulsed frequency and amplitude have little effects on the total pressure recovery coefficient for the cases studied in the present simulations.     

Numerical simulation of transient 3-D turbulent heated jet into crossflow in a thick-wall T-junction pipe

The present work is to investigate the transient three-dimensional heated turbulent jet into crossflow in a thickwall T-junction pipe using CFD package.Two cases with the jet-to-crossflow velocity ratio of 0.05 and 0.5 are computed,with a finite-volume method utilizing k-ε turbulent model.Comparison of the steady-state computations with measured data shows good qualitative agreement.Transient process of injection is simulated to examine the thermal shock on the T-junction component.Temporal temperature of the component is acquired by thermal coupling with the fluid.Via analysis of the flow and thermal characteristics,factors causing the thermal shock are studied.Optimal flow rates are discussed to reduce the thermal shock.     

Three-dimensional numerical study of flow and species transport in an elevated jet in crossflow

The jet fluid coming out of the elevated source behaves as the efflux of exhaust gases from a stack and the crossflow stream representing the natural wind flow. Direct numerical simulations (DNS) in an elevated jet in crossflow have been performed at a Reynolds number of 250 and a Schmidt number of 7.25. The flow field and concentration distribution of pollutants have been explored by solving three-dimensional unsteady Navier–Stokes and species transport equations using second order spatial and temporal discretization. Three different blowing ratios (0.5, 1.0 and 1.5) have been used to quantify the effect of blowing ratio on the flow field and pollutant dispersion. The blowing ratio is found have significant effect on both the flow and dispersion of pollutant. The evolution of various flow structures associated with the elevated jet in crossflow such as wakes, loop vortices is discussed. The instantaneous behavior of jet has been observed to be dependent on the unsteadiness of flow structures that varies with the blowing ratio. The downwash of jet has been observed at the lowest blowing ratio.     

Numerical model of gaseous fuel jet injection into a fluidized furnace

The paper presents a fluid-porous medium model, developed for stationary 2D predictions of fluidized bed. Dense phase is considered a fixed porous medium, while gas–particle interactions and bubbling phase are modeled regarding balance of friction forces between gas and particles. Like referent measurements, predictions of lateral jet injection into the bed suggest the jet penetration length is strongly affected by fluid velocity at the nozzle outlet, while influences of the nozzle vertical position and inclination angle are not significant. Also, the fluid velocity and the nozzle vertical position exert pronounced effects on mixing rate of components (fuel and oxidizer).     

Recent progress in electric-field assisted combustion: a brief review

The control of combustion is a hot and classical topic. Among the combustion technologies, electric-field assisted combustion is an advanced techno-logy that enjoys major advantages such as fast response and low power consumption compared with thermal power. However, its fundamental principle and impacts on the flames are complicated due to the coupling between physics, chemistry, and electromagnetics. In the last two decades, tremendous efforts have been made to understand electric-field assisted combustion. New observations have been reported based on different combustion systems and improved diagnostics. The main impacts, including flame stabilization, emission reduction, and flame propagation, have been revealed by both simulative and experimental studies. These findings significantly facilitate the application of electric-field assisted combustion. This brief review is intended to provide a comprehensive overview of the recent progress of this combustion technology and further point out research opportunities worth investigation.     

Financing energy efficiency in developing countries—lessons learned and remaining challenges

Although energy efficiency implementation is increasingly being recognized by policymakers worldwide as one of the most effective means to mitigating rising energy prices, tackling potential environmental risks, and enhancing energy security, mainstreaming its financing in developing country markets continues to be a challenge. Experience shows that converting cost-effective energy savings potential, particularly the demand-side improvement opportunities across sectors, into investments face many barriers and unforeseen transaction costs. This paper draws upon selected experiences with financing energy efficiency in developing countries to explore the key factors of various programmatic approaches and financing instruments that have been applied successfully for delivering energy efficiency solutions. Through case studies, a diverse range of institutional issues are examined related to the identification, packaging, designing, and monitoring approaches that have been used to catalyze traditional and innovative financing of energy efficiency projects. With adequate liquidity in major developing country markets and availability of modern energy savings technologies, it is often the institutional issues that become a key challenge to address in order to finance and implement robust programs. As further operational experience is gained, increased knowledge sharing can lead to scaling-up of such energy efficiency investments. The paper concludes with some ideas for accelerating implementation.     

Mixing augmentation induced by the combination of the oblique shock wave and secondary recirculation jet in a supersonic crossflow

The mixing process of fuel-air in the supersonic crossflow is a pivotal technology for the scramjet engine. In this paper, numerical simulation of the transverse sonic hydrogen jet into a supersonic Mach 3 crossflow with the mixing augmentation strategy induced by the combination of the oblique shock wave and secondary recirculation jet has been carried out. Detailed flow field structures, hydrogen mass fraction distributions, vortex structures, heat flux and some parameters have been explored in order to investigate its mixing enhancement mechanism. Results of the three-dimensional Reynolds-average Navier-Stokes (RANS) equations coupled with the two-equation shear stress transport (SST) κ-ω turbulence model show that the combined strategy of the oblique shock wave and secondary recirculation jet device can effectively improve the mixing speed and mixing efficiency with little total pressure loss. Also, the secondary recirculation jet device can reduce the peak of the heat flux effectively. In this study, the case with the single bleed hole owns the best effect with improving the mixing efficiency by 82.75% locally and reducing the maximum heat flux by 15.24% respectively。     

Design exploration of three-dimensional transverse jet in a supersonic crossflow based on data mining and multi-objective design optimization approaches

The information in the three-dimensional transverse injection flow field is very important for the design of a scramjet combustor, and it should be explored by using the data mining and multi-objective design optimization methods. In the current study, the three-dimensional Reynolds-averaged Navier–Stokes (RANS) equations coupled with the two equation SST k-ω turbulence model has been utilized to simulation the transverse injection flow field with a freestream Mach number 3.75, and the influence of the turbulence model on the flow field properties has been evaluated as well. At the same time, three grid scales have been employed to perform the grid independency analysis, and the predicted results have been compared with the available experimental data in the open literature in order to carry out the code validation process. Further, the effect of the injector geometric configuration on the mixing efficiency of the transverse injection flow field has been investigated, and four different configurations have been considered in the current study, namely the square port, the diamond port, the equilateral triangular port and the circular port. The obtained results show that the case with the square injection port can obtain the largest mixing efficiency, and it can offer the rapidest near-field mixing between the injectant and the air. At last, the transverse injection flow field with the square injection port has been optimized by the surrogate-based evolutionary algorithm, and the relationships between the design variables and the objective functions have been explored by the variance analysis method. It is shown that the jet-to-crossflow pressure ratio has a high remarkable impact on the total pressure recovery efficiency, as well as the number of the injection ports on the drag force performance. The drag force increases with the increase of the number of the injection ports due to the deeper penetration of the rear jets.     

Modeling of n-Hexane and n-Octane liquid fuel jets in gaseous crossflow for evaporation,combustion and breakup evaluation

This paper investigates the phenomena of liquid fuel jets in gaseous crossflow for two types of fuels, namely n-Hexane and n-Octane. In this regard, a numerical model is developed to predict a droplet behavior including trajectory, velocity, evaporation and combustion, size degradation, breakup time and radius of produced child droplets. Therefore, the mass, concentration, energy and momentum conversation equations are derived to evaluate the droplet acceleration from initial conditions. The velocity distribution is then obtained through a numerical integration of the acceleration over time. A further integration is made to determine the droplet position. In addition, evaporation and combustion and Taylor Analogy Breakup (TAB) models are integrated to assess droplet evaporation, combustion rate, and breakup behavior during the injection process. The professional version of the Engineering Equation Solver (EES) software is used to solve the model which has the advantage of providing the thermodynamic properties of the different fluids involved through predefined functions. The behaviors of droplets are investigated for two injection cases: evaporation only and evaporation and combustion. The results obtained are presented in the variations in trajectories, velocities, droplet size and surface temperature corresponding to each case and type of fuel.     

Recent progress and challenges in photocatalytic water splitting using layered double hydroxides (LDH) based nanocomposites

The worldwide energy demand is steadily increasing and estimated to be doubled by the year 2050 due to a continuous hike in economies and population. A large part of the global energy requirement procures using traditional energy sources such as fossil fuels, which are non-renewable. Also, their excessive consumption imparts negative impacts on the environment by CO₂, and CO emissions, which constantly increase the average global temperature. Therefore, the need for a more reliable, sustainable, inexpensive, renewable and environmentally-friendly form of energy is imperative. From these perspectives, hydrogen energy is emerging as one of the most promising alternatives to overcome rising energy demand with a zero-carbon footprint.Herein, various layered double hydroxides (LDH) nanocomposite owing to their attractive physicochemical properties and synergistic effect with other materials in the field of hydrogen production are reviewed. Why the class of LDHs materials is critical and their ideographic properties which make them promising materials in the field of water splitting via photocatalysis and electrocatalysis are also discussed. The synthetic methods of LDHs based nanocomposites fabrication are summarized. Various challenges and strategies from the viewpoint of a different method of hydrogen production through LDHs are reported. Additionally, multiple techniques like surface plasmon resonance (SPR), heterojunction formation, and doping with co-catalyst to increase the efficiency for photocatalytic hydrogen production are also presented. Hopefully, this review will help the readers explore highly efficient, inexpensive and stable LDH catalysts toward photocatalytic water splitting.     

The blowout limit of a jet diffusion flame in a coflowing stream of lean gaseous fuel-air mixtures

The blowout limit of a circular jet diffusion flame in a low velocity coflowing stream of air is extended significantly by the introduction of a small amount of some fuel vapor in a surrounding flow. The jet fuels employed were methane or hydrogen, while the coflowing stream contained, in turn, methane, hydrogen, propane, or ethylene. The widening of the flame blowout limit could be correlated to the concentration of the fuel in the surrounding stream relative to the corresponding concentration that caused a flame flashback within the surrounding stream in the presence of the jet flame.Moreover, the blowout limit of the flame of a jet of methane containing significant proportions of a diluent such as nitrogen was also extended markedly by the presence of fuel in the surrounding stream. As expected, when carbon dioxide was the diluent in the central jet instead of nitrogen, relatively higher fuel concentrations were needed in the coflowing stream to provide the same jet blowout velocities.