查看更多>>摘要：The thermo-fluidic transport characteristics of ferrofluids can be influenced by the application of a magnetic field. The magnetic manipulations of ferrofluids have been useful in augmenting heat transfer, as evident from recent investigations. In the present study, we examine a novel strategy for augmenting two-phase heat transfer and show that the magnetic manipulation of non-boiling Taylor bubble flow (TBF) of ferrofluids can provide on-demand augmentation. In an earlier investigation (https://doi.org/10.1016/j.colsurfa.2020.124589), we had shown that the characteristics of the TBF of ferrofluids could be altered through external magnetic manipulations. As transport characteristics of TBFs primarily depend on their flow morphology, it was anticipated that such alteration would affect their thermal transport characteristics, which are examined in the present work. The generation of smaller bubbles and unit-cells through magnetic manipulations decreased the void fraction of the resulting TBF. In addition, a greater number of units participated in the heat exchange process compared to larger bubble-slug systems at any given time instance. Such flow modifications cause considerable augmentation (which can go up to 100%) in two-phase heat transfer. The extent of augmentation depends on the applied magnetic field and induced magnetic force, homogeneous gas fraction, liquid film thickness/void fraction and flow morphology of the resulting TBF, which are examined in the present study. The application of ferrofluids in TBFs provides multiple benefits, such as suspension of nanoparticles with better thermal properties and additional functionality of the flow manipulations through external means. The proposed application with the suggested manipulation technique provides an effective alternative for an on-demand augmentation in two-phase heat transfer in low Reynolds number flows.
查看更多>>摘要：In this study, a detailed numerical analysis of the fluid flow and heat transfer of a three-dimensional microchannel is performed to evaluate the effect of using periodic vertical porous and solid ribs with various geometrical shapes, including rectangular, elliptical, isosceles triangular, backward triangular and forward triangular on the walls of this microchannel. Darcy-Brinkman-Forchheimer equations are used to model transport through the porous medium. The results for microchannels with solid ribs and with porous ribs are compared to each other. It is found that at the lowest studied inlet velocity (uin=0.25m/s), for the rib heights of Hr=0.025mm,0.05mmand0.07mm, the average Nusselt number of the porous-rib microchannel is 2.16, 2.1 and 1.3 times greater than the solid case, respectively; and, 2.505, 3.62 and 4.01 times greater than the no-ribs microchannel, respectively. It is found that for the same increase in the Nusselt number, the porous ribs induces much less pressure drop compared to the solid cases. Accounting the thermal and hydraulic performance simultaneously, Figure of Merit (FOM) is calculated. The results show that porous ribs lead in much better FOMs compared to the solid ribs case. Effects of the height, porosity and Darcy number of the ribs are also investigated.
查看更多>>摘要：The water injection (WI) technique represents a viable way to improve the performance of spark ignition (SI) engines. The main benefit of using this technique is to take advantage of the high heat of vaporization of water. If injected into the intake port, its vaporization cools the air–fuel mixture entering the engine. This could lead to greater spark advances, with optimal combustion phasing, and to a reduction of the combustion temperature with consequent lower heat losses. Moreover, due to the vaporization cooling effect, the heat capacity ratio of the in-cylinder gases increases. These three main factors allow improving the thermal efficiency of the engine. In this paper, the numerical simulation has been utilized in order to assess the influence of water injection on the performance of a “downsized” turbocharged spark-ignition engine. Calculations have been carried out using a 1-D model validated with respect to the experimental data of the analyzed engine. A knock model has been implemented in the 1-D model in order to identify the knock-limited parameters in engine optimization. Threshold values for knock index and exhaust turbine inlet temperature have been imposed as optimization constraints. The model has been subsequently modified by adding water injection into the intake ports. A simple thermodynamic approach has been utilized in order to obtain a first estimation of effects of water evaporation on the temperature of the fresh charge entering the cylinder and on that of the unburned mixture at the end of the compression stroke. The authors analyzed the effects of water injection at various operating points. An optimization procedure of the engine parameters in case of WI operation is presented and its effects on combustion characteristics and engine performance are described in detail. The engine control parameters have been optimized in order to obtain minimum specific fuel consumption. The most remarkable results show that, at high load operation, it is possible to increase both spark advance and air-to-fuel ratio with a consequent improvement in terms of both brake mean effective pressure (BMEP) and brake specific fuel consumption (BSFC). In particular, at full load, the BSFC registered a 12% mean improvement in the engine speed range, while BMEP mean increase was about 9%. The obtained results show water injection technique could be a useful tool in performance improving of turbocharged spark-ignition engines.
查看更多>>摘要：The thermal-lag Stirling engine is a type of external combustion engine that differs from traditional Stirling engines in that it has only one cylinder and one piston. In the cylinder, a porous medium with a temperature gradient is installed to separate high- and low-temperature regions. Although its construction is simpler than that of the traditional Stirling engine, the thermal-lag Stirling engine has numerous operating modes at different heating temperatures and initial speeds. In this study, a theoretical model for analyzing the performance and operating modes of a thermal-lag Stirling engine is proposed. The model was solved by the method of multiple scales. The results indicate that the engine is operated by the thermal-lag effect caused by imperfect heat transfer in working spaces. Three operating modes were predicted: the decay, swinging, and rotating modes. Seven operating regions for the different modes were illustrated in the temperature-ratio and frequency-ratio domain. The transitions between modes were also predicted using the proposed theory. The performance of the proposed engine under different loadings in a stable operating state was evaluated. The dependence of the indicated power on engine speed was also determined. The results reveal that an optimal loading exists for achieving maximum power. A minimum value of the operating engine speed was also predicted by the proposed model.
查看更多>>摘要：In this paper, the flue gas-assisted steam flooding process from a novel perspective of condensation heat transfer performance was investigated to reduce carbon emissions of heavy oil thermal recovery. More specifically, steam condensation heat transfer coefficients with and without flue gas were tested, and heat transfer and oil recovery for steam flooding assisted by flue gas in a sandpack were studied. The experimental results indicate that the existence of the gas film on the condensation block increased the heat transfer resistance and decreased the condensation heat transfer coefficient. The gas channeling of flue gas provided a flow path for the steam and promoted heat transfer deep into the sandpack. Flue gas-assisted steam flooding improved the oil recovery factor by 20.02% compared with steam flooding, and 46.4% of the injected flue gas could be stored in the sandpack. The storage of flue gas could be divided into two parts: the flue gas dissolved in the residual heavy oil and the flue gas trapped in the porous media. Flue gas treatment and injection processes for steam injection wells were designed. Site application of flue gas-assisted steam injection showed that the oil production increased by 28.3%, and the underground CO2 storage rate was 74.5%. The novel strategy of flue gas treatment and injection with steam greatly reduces the carbon emissions of heavy oil thermal recovery, which is very applicable for small-scale steam generator used in oilfields.
查看更多>>摘要：Membrane technology is one promising technology for CO2 capture from industrial gases. The application of membrane system in gasification-based power plants is particularly appealing considering the elevated pressure of syngas subject to decarbonization resulting in lower energy and economic penalties for CO2 capture. As key novelty element of this work, the membrane technology was evaluated in both alone and hybrid configuration with gas-liquid absorption in view of decarbonization of Integrated Gasification Combined Cycle power plants. Assessed decarbonized gasification-based power plant concepts produce about 450 MW net output with 90% CO2 capture rate. An integrated techno-economic and environmental evaluation methodology (based on modelling, simulation and process integration) was applied to quantify the most important plant performance indicators. For comparison, similar IGCC designs without decarbonization feature or with decarbonization by chemical and physical gas-liquid absorption were also analysed. The overall conclusion is that the membrane technology has important techno-economic benefits in comparison to chemical and physical absorption e.g. greater overall net energy efficiency (up to about one net percentage point), lower specific capital investment costs (down to 9%), lower operational & maintenance costs (down to 10%), lower electricity production costs (down to 7%), lower CO2 capture costs (down to 50%).
查看更多>>摘要：This study examined the cooling performance of a crescent-shaped crater (CSC) hole design along the pressure side of a transonic vane. The adiabatic effectiveness of the CSC holes and circular holes was measured via an endoscopic pressure-sensitive paint (PSP) technique in a transonic wind tunnel at Ma = 0.84. Coolant flow (CO2) was discharged into the mainstream flow through two rows of holes, with the blowing ratio (M) varying from 0.6 to 1.2. The measured cooling effectiveness of the CSC hole was significantly higher than that of the circular design, demonstrating ～ 57% and ～ 38% enhancements in area-averaged effectiveness at M = 0.8 and 1.0, respectively. The physics of the CSC hole design was further explored by numerical simulations, which reveal the generated vortex structures and aerodynamic losses under various conditions. Specifically, the simulations demonstrated the features of anti-counter-rotating vortex pairs for CSC holes, which partially counteracted the detrimental effect of counter-rotating vortex pairs, thereby greatly improving the cooling effectiveness along the vane's pressure side.
查看更多>>摘要：The paper studies the pressure drop and heat transfer coefficient of boiling flows in PCHEs. This study presents results obtained by measuring pressure drops and heat transfer coefficients in relevant flows. The effect of the cross-sectional shape of the flow path is evaluated. The results were compared to a previous study. The authors would like to correct the following aspect of this paper: ? EquationPage 8 wrong: [Formula presented] (22) correct: [Formula presented] (22)This mistake does not affect the results. However, the authors would like to avoid any confusion that the wrong equation may cause to the reader. The authors would like to apologise for any inconvenience caused.
查看更多>>摘要：The paper presents pressure drop and flow patterns of boiling flows in mini-channels with semi-circular cross-sections. This study presents results obtained by measuring pressure drops and visualization. The results were compared to a previous study and a new prediction model for pressure drop in boiling flows. The authors would like to correct the following aspect of this paper: ? EquationPage 4 wrong: [Formula presented] (8) correct: [Formula presented] (8)? Author name of referencePage 9 (2 places) wrong: Mori et al. [38] correct: Mori [38]? Missing unitsPage 7 wrong: 1.04 correct: 1.04 mm? Wrong reference numberPage 10 wrong: [23, 30–32] correct: [31–34]? Incorrect description of aspect ratioPage 10 wrong: 3.1 correct: 3:1? Uncertainty description errorTable 1 Page 3 wrong: ±3.4% correct: ±2.7% wrong: ±4.9% correct: ±3.8%Neither mistake has a significant effect on the content of the text and does not affect the results. However, the authors would like to avoid any confusion that the wrong equation, author name of the cited reference and reference number may cause to the reader. The authors would like to apologise for any inconvenience caused.
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Elsevier