Phase change materials used in thermal energy storage systems are critical for energy utilization. Organic phase change materials have received considerable attention both for applications and ...research due to their favorable properties, such as large latent heat, low cost, stability, nontoxicity, and corrosion resistance. Because of the inherently low thermal conductivity, there has been a lot of research on various combinations of organic phase change materials with high conductivity materials. Carbon nanotubes, graphene and porous carbon have excellent properties, such as ultrahigh surface area and high thermal conductivity, which have become the preferred materials for thermal properties enhancement. Benefit from the varied dimensional spaces for retaining materials, these carbon-based materials can act as excellent confinement for phase change materials to form shape stabilized, phase change tunable, and thermally enhanced phase change composites. In this paper, a review about recent advancement and challenges for organic phase change materials confined in different types of carbon-based materials is presented, associated with some relevant theories trying to understand the mechanism of heat transfer and phase change in micro/nano confinement. Some promising applications in thermal management and energy conversion that have great potential for more explorations are also introduced.
•Carbon-based materials are suitable for infiltrating organic phase change materials.•The thermal properties of organic phase change materials can be improved effectively.•The challenges and perspectives of organic phase change materials are proposed.
Large amounts of iron ore sinter are produced throughout the world. The development of sustainable and energy-efficient technologies has become increasingly important due to the huge pressure ...associated with energy consumption and greenhouse gases emissions by iron ore sinter plants. This review describes recent progress in the substitution of renewable alternative fuels, modifying the properties of solid fuels and reactant gases, and process control implementation, as well as addressing the limitations of these techniques and identifying opportunities for future research. Alternative fuels are examined including biomass, hydrogen-based gaseous fuel, and recycled carbon sources for coke breeze in iron ore sintering to address environmental and energy-saving considerations. The negative impacts of alternative fuel substitution on the heat pattern and sinter quality, especially at a high substitution rate, are analyzed in terms of the fuel surface area, fuel reactivity, and matching the performance of the flame front speed and heat front speed. Pre-granulation and surface modifications of solid fuels are discussed, followed by flue gas recirculation and oxygen enrichment techniques. Finally, processes comprising raw material granulation control, fuel segregation control, and in situ process control are described, which aim to satisfy the ambitious goals of maximizing the sintering performance and minimizing energy consumption and environmental impacts.
•Progress of renewable fuel substitution in iron ore sintering process is presented.•Fuel/reactant gas modifications and process control techniques are summarized.•Future research needs in sustainable and energy-efficient sintering are identified.
3D computational analysis was performed to investigate heat transfer and pressure drop characteristics of flow in SWFET (Smooth Wavy Fin-and-Elliptical Tube) heat exchanger with four new VGs (vortex ...generators), RTW (rectangular trapezoidal winglet), ARW (angle rectangular winglet), CARW (curved angle rectangular winglet) and WW (Wheeler wishbone). The numerical model was well validated with the available experimental results. Numerical results illustrate that vortex generators can bring about further heat transfer enhancement through careful adjustment of the position with respect to the elliptical tube, type and attack angle of vortex generators. The influences of the geometrical factors including attack angles of the winglets (αVG = 15∘,30∘,45∘,60∘ and 75°) and width/length aspect ratio (w/l = 0.5,1.0) of the Wheeler wishbones on enhancing the heat transfer performance of a smooth wavy fin heat exchanger with a three-row staggered elliptical tube bundle are investigated. A parametric study on the winglet vortex generators indicated that for the small attack angle, CARW vortex generators gives better thermohydraulic performance under the present conditions. The best thermal performance with winglet VGs in larger attack angle, was obtained at RTW VGs arrangement. For the SWFET heat exchangers, the WW VGs with w/l = 0.5 provide the best heat transfer performance.
•A SWFET heat exchanger is numerically studied with 3D SST k−ω turbulence model.•Four new types of vortex generators, RTW, ARW, CARW and WW, were considered.•Vortex generators can bring about further heat transfer enhancement through careful adjustment.•For small attack angle, CARW vortex generators give better thermohydraulic performance.•WW vortex generators with w/l = 0.5 provide the best heat transfer performance.
Improving the heat transfer performance of phase change material (PCM) plays a crucial role in designing efficient latent heat thermal energy storage (LHTES) systems. The purpose of this study is to ...address and elucidate the effects of the metal foam-fin hybrid structure and the inclination angle on the phase change process by using the numerical simulation method. An experimental system for the validation of the numerical models is established. The solid–liquid phase interfaces, streamlines, liquid fraction (f), the dimensionless time (Fo×Ste), and average Nusselt number (Nu¯) of PCM in the container enclosure at inclination angles of 0°, 30°, 60°, and 90° with six kinds of enhanced heat transfer structures, including fin, metal foam, and metal foam-fin hybrid structures, are compared. Besides, the liquid fraction and Nu¯ during the phase change process are predicted by the artificial neural network (ANN). Results demonstrate that the optimized heat transfer performance of the metal foam-fin hybrid structure could reduce the melting time. In addition, the increase in the number of fins can improve the heat transfer performance and reduce heat accumulation in the top area with the inclination angle increasing. Compared to pure PCM at the inclination angle of 90°, the values of Fo×Ste of metal foam-1 fin and metal foam-5 fins hybrid structures are reduced by 52.69% and 60.02%, respectively. However, the energy storage density per unit volume decreases as a function of the increasing inclination angles and the number of fins within a case. Furthermore, the excellent predictions of f and Nu¯ are obtained by ANN with MSE and R2 of 9.6480 × 10−5, 0.9990 and 0.0150, 0.9937, respectively.
•Heat transfer rate of PCM was improved by metal foam-fin hybrid structure.•Inserting more fins suppressed heat accumulation with inclination angle increasing.•Metal foam-fin hybrid structure reduced energy storage density per unit volume.•MLP-GA model showed good predictions of liquid fraction and Nusselt number.
•Detail thermal profiles and gas concentrations in sintering process were obtained.•Equivalent fixed carbon replacement was proposed to ensure sufficient heat supply.•Effects of charcoal size and ...distribution on combustion rate were presented.•Sintering performance was improved with coarse and coated charcoal combustion.
Using biomass for partial or complete replacement of coke breeze in iron ore sintering process is an attractive technique for reducing emissions of greenhouse gas and gaseous pollutants. But one drawback of this technique is that low or medium grade charcoal may lead to the failure in achieving proper sintering performance. In this paper, the behaviors of coke combustion versus charcoal combustion in sintering bed were compared. The results of thermal profile and exhaust gas composition indicated that the sinters quality was deteriorated at high charcoal proportion. Lacking heat release in melting zone and the excessively high combustion rate were the reasons to weak sinters. In order to ensure the sintering performance when using medium grade charcoal in sintering bed, the effects of three improving measures (proposing the equivalent fixed carbon substitution approach, increasing charcoal particle size and adopting coated charcoal combustion) were experimentally tested. The results showed that equivalent fixed carbon substitution approach was more effective to produce sufficient heat in melting zone at medium grade charcoal combustion. Additionally, it was also found that increasing charcoal particles size and applying coated charcoal combustion method could reduce combustion rate to achieve a proper matching condition between flame front speed and heat transfer front speed. Consequently, with the help of equivalent fixed carbon substitution approach, coarse charcoal and coated charcoal particles, the peak temperature, holding time above 1100°C, melting quantity index and combustion efficiency were increased in the charcoal sintering process.
Randomly packed bed reactors are widely used in chemical process industries, because of their low cost and ease of use compared to other packing methods. However, the pressure drops in such packed ...beds are usually much higher than those in other packed beds, and the overall heat transfer performances may be greatly lowered. In order to reduce the pressure drops and improve the overall heat transfer performances of packed beds, structured packed beds are considered to be promising choices. In this paper, the flow and heat transfer inside small pores of some novel structured packed beds are numerically studied, where the packed beds with ellipsoidal or non-uniform spherical particles are investigated for the first time and some new transport phenomena are obtained. Three-dimensional Navier–Stokes equations and RNG
k–ε turbulence model with scalable wall function are adopted for present computations. The effects of packing form and particle shape are studied in detail and the flow and heat transfer performances in uniform and non-uniform packed beds are also compared with each other. Firstly, it is found that, with proper selection of packing form and particle shape, the pressure drops in structured packed beds can be greatly reduced and the overall heat transfer performances will be improved. The traditional correlations of flow and heat transfer extracted from randomly packings are found to overpredict the pressure drops and Nusselt number for all these structured packings, and new correlations of flow and heat transfer are obtained. Secondly, it is also revealed that, both the effects of packing form and particle shape are significant on the flow and heat transfer in structured packed beds. With the same particle shape (sphere), the overall heat transfer efficiency of simple cubic (SC) packing is the highest. With the same packing form, such as face center cubic (FCC) packing, the overall heat transfer performance of long ellipsoidal particle model is the best. Furthermore, with the same particle shape and packing form, such as body center cubic (BCC) packing with spheres, the overall heat transfer performance of uniform packing model is higher than that of non-uniform packing model. The models and results presented in this paper would be useful for the optimum design of packed bed reactors.
•The heat pattern can be controlled by adjusting the gaseous fuel concentration.•Gaseous fuel segregation was firstly proposed for energy efficiency optimization.•The new method has great potential ...for green and efficient production of sinters.
It has been widely reported that the sinter strength and heat pattern would be weakened when adopting the low grade solid fuels, such as biomass, semi-coke and anthracite. Moreover, the imbalance of heat distribution in the sintering bed is considered to be problematic on the energy efficiency. To solve the above problems simultaneously, the gaseous fuel segregation method was firstly proposed in this paper. The gaseous fuel was injected to the melting zone from the top and auto-ignited near the solid fuel combustion zone. Firstly, methane concentrations of 0.0% and 0.5% vol. were tested, keeping the total calorific heat input unchanged. The heat pattern in melting zone was recorded by both contact thermocouples and non-contact thermal infrared imager. The results indicated that the methane injection could significantly extend the melting zone from the upstream and raise the sinter strength higher than that of coke sintering, without increasing the energy consumption. Then, the energy saving potential of the novel method was evaluated by reducing the calorific heat input 4, 6 and 8%. Furthermore, in the segregation case, the gaseous fuel injecting concentration was increased in the upper bed to enhance the weak heat pattern, and decreased in the lower bed to avoid the energy waste. It was observed that the melting zone became much more uniform in the infrared images. Finally, the optimum segregation degree of 1.0%/mm was recommended, where the sinter strength grew 2.31%. The present study provides an effective way for optimizing the energy efficiency in the sintering process.
•Recent research of three kinds of surface heat exchangers are reviewed.•The STHXs with novel combined helical baffles have been reviewed.•The primary surface heat exchangers with compact surfaces ...have been reviewed.•The direct and indirect air-cooled heat exchangers have been reviewed.
In the present study, the recent research of three kinds of surface heat exchangers, i.e., shell-and-tube heat exchangers with helical baffles, air-cooled heat exchangers used in large air-cooled systems, and primary surface heat exchangers are reviewed. They are used in the energy conversion and utilization for liquid to liquid, gas to gas and liquid to gas heat exchange, respectively. It can be concluded that the helical baffled shell-and-tube heat exchangers (STHXs) should be used to replace the conventional segmental baffled STHXs in industries, despite there are a lot of research work have to be done, especially on the novel combined helical baffles. The primary surface gas to gas heat exchangers are developing towards to the more complex 3D CC primary surfaces, such as the double-wave CC primary surface, offset-bubble primary surface and 3D anti-phase secondary corrugation. The whole performance for the air-cooled heat exchangers in the air cooling system and the multi-objectives optimization for air-cooled heat exchangers should be paid more attention, considering the heat transfer, pumper power, space usage and other economic influence factors.
Followed by the numerical study of Yang et al. (2010), the macroscopic hydrodynamic and heat transfer characteristics in some novel structured packed beds are experimentally studied in this paper, ...where the packings of ellipsoidal or non-uniform spherical particles are investigated for the first time with experiments and some important results are obtained. For present experiments, the interstitial heat transfer coefficient in the packed bed is determined using an inverse method of transient single-blow technique. The effects of packing form and particle shape are carefully investigated and the experimental and numerical results (Yang et al., 2010) are also compared in detail. Firstly, it is discovered that, the computational method reported by Yang et al. (2010) might be appropriate for heat transfer predictions in structured packings, while it might underestimate the friction factors, especially when the porosity is relatively low. Secondly, it is found that, the traditional Ergun's and Wakao's equations might overpredict the friction factors and Nusselt numbers for the structured packings, respectively, and some experimental modified correlations are obtained. Furthermore, it is also revealed that, both the effects of packing form and particle shape are significant to the macroscopic hydrodynamic and heat transfer characteristics in structured packed beds. With proper selection of packing form, such as simple cubic packing (SC) and particle shape, such as ellipsoidal particle, the pressure drops in the structured packed beds can be greatly reduced and the overall heat transfer performances will be improved. These experimental results would be reliable and useful for the optimum design in industry applications.
► The forced convections in some novel structured packed beds are experimentally investigated. ► The computational method of Yang et al. (2010) underestimates the friction factors. ► Ergun's and Wakao's equations overpredict the friction factors and Nusselt numbers. ► The modified correlations are obtained from the experiments. ► Both the effects of packing form and particle shape are quite significant.
