[en] Least squares support vector machine (LS-SVM) for nonlinear regression is sensitive to outliers in the field of machine learning. Weighted LS-SVM (WLS-SVM) overcomes this drawback by adding weight to each training sample. However, as the number of outliers increases, the accuracy of WLS-SVM may decrease. In order to improve the robustness of WLS-SVM, a new robust regression method based on WLS-SVM and penalized trimmed squares (WLSSVM–PTS) has been proposed. The algorithm comprises three main stages. The initial parameters are obtained by least trimmed squares at first. Then, the significant outliers are identified and eliminated by the Fast-PTS algorithm. The remaining samples with little outliers are estimated by WLS-SVM at last. The statistical tests of experimental results carried out on numerical datasets and real-world datasets show that the proposed WLSSVM–PTS is significantly robust than LS-SVM, WLS-SVM and LSSVM–LTS.

[en] Thermal error is a major factor influencing the accuracy of large precision electrical discharge machining (EDM) machine tools, especially when processing continuously for a long time. In this paper, a novel thermal analysis model was set up to identify the static and dynamic thermal behaviour of the large EDM machine tool. The thermal effect of multiple spark discharges is considered. An equivalent heat flux method was proposed to model the intermittent heat flux for the first time. Both the steady and transient analyses were applied to investigate the thermal equilibrium time of critical points. It is found that when the study point is far away from the heat source, the longer thermal equilibrium time is needed. And the thermal equilibrium time of the machine tool was also estimated. Verification experiment has been performed, indicating the simulation accuracy of 87 % on the temperature rise of the electrode. Moreover, on the displacement of the spindle, the simulated result matched with the experimental result in Z direction error of 7 %. Finally, suggestions for reducing the thermal deformation were proposed to further improve the machining accuracy of large EDM machine tools.

[en] In this paper we discuss MDP with distribution function criterion of first-passage time. Some properties of several kinds of optimal policies are given. Existence results and algorithms for these optimal policies are given in this paper

[en] Thermal error is a major factor influencing the accuracy of large precision electrical discharge machining (EDM) machine tools, especially when processing continuously for a long time. In this paper, a novel thermal analysis model was set up to identify the static and dynamic thermal behaviour of the large EDM machine tool. The thermal effect of multiple spark discharges is considered. An equivalent heat flux method was proposed to model the intermittent heat flux for the first time. Both the steady and transient analyses were applied to investigate the thermal equilibrium time of critical points. It is found that when the study point is far away from the heat source, the longer thermal equilibrium time is needed. And the thermal equilibrium time of the machine tool was also estimated. Verification experiment has been performed, indicating the simulation accuracy of 87 % on the temperature rise of the electrode. Moreover, on the displacement of the spindle, the simulated result matched with the experimental result in Z direction error of 7 %. Finally, suggestions for reducing the thermal deformation were proposed to further improve the machining accuracy of large EDM machine tools.

[en] Thermal error which has been widely studied in cutting machine tools, was ignored in the EDM machines in most cases, since there is usually no high-speed rotation for spindles. However, for large die-sinking EDM machines, due to heavy load of drive system and long processing cycle of large aeronautical parts, thermal error induced by jump motion has seriously impaired the machining accuracy and gradually been recognized. In this paper, the dynamic thermal behavior of spindle induced by periodic jump motions in large precision die-sinking EDM machine was studied for the first time. Noted that the Z-axis base and column show obvious temperature rise and the thermal error in Y direction is the largest, which is about 6.5 and 5 times compared with that in X and Z directions. Based on this, an efficient thermal error prediction model was presented. Thermal sensitive points were picked out through fuzzy clustering and correlation theory, taken as inputs of radial basis function (RBF) neural network to guarantee the accuracy. As a result, the prediction accuracy in X, Y and Z directions are 95.2 %, 92.5 % and 94.4 %, respectively. Finally, the effect of jump period on spindle thermal behavior was investigated, and suggestions for optimizing jump motion parameters were proposed to further improve the machining accuracy of large EDM machines.

[en] Using the Weather Research and Forecasting (WRF) model, this paper analyzes the spatiotemporal features of heat waves in 20-year regional climate simulations over East Asia, and investigates the capability of WRF to reproduce observational heat waves in China. Within the framework of the Coordinated Regional Climate Downscaling Experiment (CORDEX), the WRF model is driven by the ERA-Interim (ERAIN) reanalysis, and five continuous simulations are conducted from 1989 to 2008. Of these, four runs apply the interior spectral nudging (SN) technique with different wavenumbers, nudging variables and nudging coefficients. Model validations show that WRF can reasonably reproduce the spatiotemporal features of heat waves in China. Compared with the experiment without SN, the application of SN is effectie on improving the skill of the model in simulating both the spatial distributions and temporal variations of heat waves of different intensities. The WRF model shows advantages in reproducing the synoptic circulations with SN and therefore yields better representations for heat wave events. Besides, the SN method is able to preserve the variability of large-scale circulations quite well, which in turn adjusts the extreme temperature variability towards the observation. Among the four SN experiments, those with stronger nudging coefficients perform better in modulating both the spatial and temporal features of heat waves. In contrast, smaller nudging coefficients weaken the effects of SN on improving WRF’s performances.

[en] Thermal error which has been widely studied in cutting machine tools, was ignored in the EDM machines in most cases, since there is usually no high-speed rotation for spindles. However, for large die-sinking EDM machines, due to heavy load of drive system and long processing cycle of large aeronautical parts, thermal error induced by jump motion has seriously impaired the machining accuracy and gradually been recognized. In this paper, the dynamic thermal behavior of spindle induced by periodic jump motions in large precision die-sinking EDM machine was studied for the first time. Noted that the Z-axis base and column show obvious temperature rise and the thermal error in Y direction is the largest, which is about 6.5 and 5 times compared with that in X and Z directions. Based on this, an efficient thermal error prediction model was presented. Thermal sensitive points were picked out through fuzzy clustering and correlation theory, taken as inputs of radial basis function (RBF) neural network to guarantee the accuracy. As a result, the prediction accuracy in X, Y and Z directions are 95.2 %, 92.5 % and 94.4 %, respectively. Finally, the effect of jump period on spindle thermal behavior was investigated, and suggestions for optimizing jump motion parameters were proposed to further improve the machining accuracy of large EDM machines.

[en] Highlights: • Supercritical water gasification applied to convert food waste to H₂-rich syngas • Hydrothermal pretreatment shortened the residence time of supercritical water gasification. • Highest H₂ yield and efficiencies were obtained with 200 °C, 20 min hydrothermal pretreatment. • Electricity input was reduced with hydrothermal pretreatment. The huge amount of food waste (FW), containing high organic matter content and moisture, is difficult to be well treated. Supercritical water gasification (SCWG) can efficiently convert FW to H₂-rich syngas. However, it requires high energy input due to the high temperature and high pressure. This study provided an innovative “two-steps heating process” for the SCWG of FW, which firstly utilized hydrothermal (HT) pretreatment to shorter time of SCWG. The effects of different HT temperature (200 °C, 250 °C, 300 °C, 30 min) to SCWG temperature (480 °C, 30 min) and the different residence time (20 min HT - 40 min SCWG, 30 min HT - 30 min SCWG, and 40 min HT - 20 min SCWG) on total syngas yield, carbon conversion efficiency (CE), cold gas efficiency (CGE), and hydrogen conversion efficiency (HE) were studied. Moreover, the energy input by means of electricity consumption in each experiment was measured to determine the energy saving rate. The optimal condition (200 °C, 20 min HT - 40 min SCWG), obtaining the gas yield (17.22 mol/kg), CE (20.10%), CGE (22.13%), and HE (41.54%), was higher than the gas yield (16.53 mol/kg), CE (19.98%), CGE (20%), and HE (38.08%) of directly SCWG (60 min, 0 °C–480 °C). Moreover, the TOC of derived liquid and the pyrolysis characteristics of solid residues were analyzed. Additionally, it was also observed the HT pretreatment helped to reduce the electricity consumption. The highest energy saving rate was 15.58%.

[en] Highlights: • The different synthetic routes of 1-Methyl-2,4,5-trinitroimidazole (MTNI) was studied in detail by using DFT method. • The nitration reaction path is affected by the solvent molecule. • The yield is affected by the different nitration mechanisms. During the synthesis of 1-methyl-2,4,5-trinitroimidazole (MTNI), the product yield was affected by group introduction order. This phenomenon has not been explained in the reaction mechanism. In this paper, the DFT method was used to study the synthesis mechanism. For the nitration reaction, an N-H$\cdots$O hydrogen bond was formed between the midazole ring and water molecule in the solution to promote the nitration reaction, so the nitration mechanism was affected by the introduction of methyl group at the N site. For the methylation reaction, the electrophilic attack of the methyl group was affected by the introduction of more electron-withdrawing nitro group.

[en] Highlights: • The two formation mechanisms of 3,4-dinitrofuroxan (DNFO) via nitration of furoxan with two typical nitration reagents BF₄NO₂ and HNO₃ are clarified. • The formation mechanisms are mainly the electrophilic substitutions of nitronium ions from the nitration reagents and the abstractions of protons in the complex intermediates. • BF₄⁻ is shown to be a better proton abstracter than HNO₃ and H₂O due to its no barrier combination with H⁺. • Chloroform is a feasible solvent and heating properly is necessary for the two reactions. A systematic investigations on the nitration of furoxan by two typical nitration reagents nitronium tetrafluoroborate (BF₄NO₂) and dilute nitric acid (HNO₃) in chloroform (CHCl₃) solvent using density functional theory (DFT) method to reveal the formation mechanism of 3,4-dinitrofuroxan (DNFO) and explore new synthesis routes. The geometry optimizations of the minima and transition states involved in the two nitration reactions are performed at the B3LYP/6-311++G** basis set level. The CCSD single-point energy corrections at the same level are carried out on top of the optimized geometries to obtain the accurate energy. Calculated results demonstrate that the electrophilic substitutions of nitronium ions from the nitration reagents and the abstractions of protons in the complex intermediates are the main formation mechanism of DNFO. BF₄⁻ is shown to be a better proton abstracter than HNO₃ and H₂O due to its no barrier combination with H⁺. The abstraction of proton by HNO₃ is predicted to be more feasible than H₂O because it can supply the nitration attacker (NO₂⁺) and release more heat. Chloroform is a feasible solvent and heating properly is necessary for the two reactions due to the relatively high barrier of 37 kcal/mol. These conclusions provide some significant indications on the new experimental synthesis of DNFO.