Numerical simulation of a parabolic trough solar collector with nonuniform solar flux conditions by coupling FVM and MCRT method

In this paper, a more detailed three-dimensional computational model of the whole parabolic trough solar collector (PTC) system and corresponding numerical simulations by combining the Finite Volume Method (FVM) and the Monte Carlo Ray-Trace (MCRT) method were presented. Corresponding codes and solving methods were also developed and applied to simulate and analyze the total involuted photo-thermal conversion process of an experimental LS2 PTC system. The numerical results were compared with experimental data and good agreement was obtained, proving that the model and method used in the present study is feasible and reliable. More details of the characteristics of solar concentrating, solar collecting, fluid dynamics, coupled heat transfer and the whole flow and temperature fields in the receiver were also revealed and discussed. Then some typical heat transfer fluid (HTF) types and residual gas conditions were further studied. It was revealed that the properties of these HTFs/conditions and their varying relations of the fluid temperature affected the characteristics of fluid dynamics, coupled heat transfer and the whole temperature distributions in the receiver, thus affected the thermal loss and the collector efficiency synthetically.     

Hydrogen effect on the intergranular failure in polycrystal ɑ-iron with different crystal sizes

We studied the fracture strain of polycrystal ɑ-iron at three different hydrogen concentrations and for three crystal sizes with molecular dynamics simulations (MDs). As the hydrogen concentration increases, a fine crystal model's fracture resistance is prone to below a comparatively coarse grain model. This finding elucidates that the most vulnerable area can alter from coarse grain zone to fine-grain zone in the welding heat-affected zone (HAZ) with the effect of hydrogen. A simplified model is thus built to predict the strain energy increment in different crystal size systems caused by the introduction of hydrogen atoms. This strain energy increment is not equal to the fracture energy reduction induced by the same amount of hydrogen insertion, demonstrating that elastic volume expansion of grain boundary (GB) caused by hydrogen is not the determining mechanism of intergranular failure. The density of triple or multi-junctions of GBs, which is typically dependent on the volume fraction of GBs, is the crucial factor for intergranular failure caused by hydrogen embrittlement.     

20#钢在油田采出液中的腐蚀行为研究

采用腐蚀失重法、丝束电极技术和电化学测量技术研究了20#钢在油田采出液中的腐蚀行为。结果表明,在实验周期内可将腐蚀发展过程分为腐蚀初期、腐蚀发展期和腐蚀稳定期,腐蚀初期对应腐蚀情况最为严重;金属同一位置随实验时间延长发生阴阳极极性反转现象,并且随着温度升高,反转周期变短;通过对腐蚀金属整体和局部进行电化学测量研究发现,不同温度、不同位置腐蚀发生类型有所差别。     

Thermodynamic and economic assessment of a PEMFC-based micro-CCHP system integrated with geothermal-assisted methanol reforming

A micro-combined cooling heating and power (CCHP) system integrated with geothermal-assisted methanol reforming and incorporating a proton exchange membrane fuel cell (PEMFC) stack is presented. The novel CCHP system consists of a geothermal-based methanol steam reforming subsystem, PEMFC, micro gas turbine and lithium bromide (LiBr) absorption chiller. Geothermal energy is used as a heat source to drive methanol steam reforming to produce hydrogen. The unreacted methanol and hydrogen are efficiently utilized via the gas turbine and PEMFC to generate electricity, respectively. For thermodynamic and economic analysis, the effects of the thermodynamic parameters (geothermal temperature and molar ratio of water to methanol) and economic factors (such as methanol price, hydrogen price and service life) on the proposed system performance are investigated. The results indicate that the ExUF (exergy utilization factor the exergy utilization factor), TPES (trigeneration primary energy saving) and energy efficiency of the novel system can be reached at 8.8%, 47.24% and 66.3%, respectively; the levelized cost of energy is 0.0422 $/kWh, and the annual total cost saving ratio can be reached at 20.9%, compared with the conventional system. The novel system achieves thermodynamic and economic potential, and provides an alternative and promising way for efficiently utilizing abundant geothermal energy and methanol resources.     

Modelling of fractured horizontal wells with complex fracture network in natural gas hydrate reservoirs

Shale gas resources (SGR), as a representative of natural gas hydrate reservoirs, have been the main energy supply for the energy consumption currently. The multi-scale pore structure of shale, complicated seepage mechanisms, including Knudsen diffusion, matrix deformation, stress sensitivity, non-Darcy flow and spatial fracture network stimulated by hydraulic fracturing technology have posed huge challenges to an accurate prediction and assessment of shale gas recovery. A full understanding of gas seepage mechanism of shale gas is the critical and scientific issue to develop carbon hydrogen energy resources effectively. It is very urgent to establish a comprehensive mathematical model to analyze the productivity capacity through simultaneously considering various flow mechanisms and fractures network system. To fill this gap, this paper presents a comprehensive numerical model of hydraulic fracturing horizontal well with discrete fracture network where embedded discrete fracture model (EDFM) is employed to characterize the coupled phenomenon between discrete fracture network and fractured SGR. And then two numerical discretization methods, e.g., finite difference and finite-volume, are used to numerically discretize the equations, subsequently, the Newton-Raphson iterative method is adopted to obtain the final solutions. Finally, the sensitivity analysis experiments are employed to investigate the effects of the key parameters. The results can provide some certain guidance for the optimization of stimulated treatment in natural gas hydrate reservoirs.     

On the application of natural algorithms to structural design optimization

Powerful computational methods based on metaphors of ‘urvival of the fittest’ (the genetic algorithm) and human brain activity (the neural network) have made significant progress in engineering where there are needs for search and learning mechanisms. The principal subject of the paper is the ‘genetic algorithm’, a ‘population-based’ method of searching large combinatorial (design) spaces to find the optimum combination of design variables. Attention needs to be given to the form and organization of the algorithm if it is to be applied to large-scale problems. Consideration is given to the development of a space condensation heuristic which progressively reduces the size of the multidimensional space being searched thus leading to a more economical application of the algorithm. The approach to adaptivity of controls and the type of penalty function used for the design constraints are explained. Some results from a study of optimum design of a multistorey frame are included by way of illustration.     

Ni–SiC composite plated under a modulated current

Ni–SiC composite coatings were plated under a modulated current in order to control coating properties including ceramic content, hardness, ductility, internal stresses and hot-oxidation resistance. Composites with SiC gradient distribution were deposited from a Watts-type bath by reducing the current density from 8 to 3 A dm⁻². When a pulse regime with a frequency of 0.05 Hz was used, the composite possessed a combination of enhanced hardness and higher ductility together with lower internal stresses. The best resistance to hot-corrosion at 800 and 1000 °C was obtained for the composite deposited in pulse-reverse regime and containing 17 vol% of SiC.     

A heuristic method for simultaneous tower and pattern-free field optimization on solar power systems

A heuristic method for optimizing a solar power tower system is proposed, in which both heliostat field (heliostat locations and number) and the tower (tower height and receiver size) are simultaneously considered.Maximizing the thermal energy collected per unit cost leads to a difficult optimization problem due to its characteristics: it has a nonconvex black-box objective function with computationally expensive evaluation and nonconvex constraints.The proposed method sequentially optimizes the field layout for a given tower configuration and then, the tower design is optimized for the previously obtained field layout. A greedy-based heuristic algorithm is presented to address the heliostat location problem. This algorithm follows a pattern-free method. The only constraints to be considered are the field region and the nonconvex constraints (which allow heliostats to not collide).The absence of a geometrical pattern to design the field and the simultaneous optimization of the field and the tower designs make this approach different from the existing ones. Our method is compared against other proposals in the literature of heliostat field optimization.     

A study on ceiling jet characteristics in an inclined tunnel

The characteristics of a ceiling jet of an inclined tunnel in a fire will be studied and reported in this paper. Scale modeling experiments on a ceiling jet in a model tunnel of length 3.0 m, width 0.8 m and height 1.0 m inclined at different angles of 0°, 10°, 20° and 30° were carried out. Numerical studies by large eddy simulation were then performed. Both experimental observation and numerical simulation indicated that the characteristics of the temperature and velocity fields near the upper tunnel are different from those obtained using the empirical equations reported in the literature. Another set of empirical equations for gas temperature and flow velocity along the tunnel were fitted by experimental data. These derived empirical equations are useful for estimating the temperature and flow velocity patterns for the ceiling jet in an inclined tunnel with an angle within the range 0–30°.