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    Energy Engineering, Mechanics & Materials
    Ventilation System Heating Demand Forecasting Based on Long Short-Term Memory Network
    ZHANG Zhanluo (张战罗), ZHANG Zhinan (张执南), EIKEVIK Trygve Magne, SMITT Silje Marie
    2021, 26 (2):  129-137.  doi: 10.1007/s12204-021-2277-5
    Abstract ( 318 )   PDF (1198KB) ( 140 )  
    Load forecasting can increase the efficiency of modern energy systems with built-in measuring systems by providing a more accurate peak power shaving performance and thus more reliable control. An analysis of an integrated CO2 heat pump and chiller system with a hot water storage system is presented in this paper. Drastic power fluctuations, which can be reduced with load forecasting, are found in historical operation records. A model that aims to forecast the ventilation system heating demand is thus established on the basis of a long short-term memory (LSTM) network. The model can successfully forecast the one-hour-ahead power using records of the past 48 h of the system operation data and the ambient temperature. The mean absolute percentage error (MAPE) of the forecast results of the LSTM-based model is 10.70%, which is respectively 2.2% and 7.25% better than the MAPEs of the forecast results of the support vector regression based and persistence method based models.

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    Exergy Analysis of Direct-Expansion Solar-Assisted Heat Pump Based on Experimental Data
    KONG Xiangqiang (孔祥强), CUI Fulin (崔福林), LI Jianbo (李见波), ZHANG Maoyuan (张茂远)
    2021, 26 (2):  138-145.  doi: 10.1007/s12204-020-2228-6
    Abstract ( 200 )   PDF (948KB) ( 55 )  
    The purpose of this research is to reveal the exergy variation of each component in a direct-expansion solar-assisted heat pump (DX-SAHP) system. Exergy analysis of the DX-SAHP system with R134a is conducted,and the performance evaluation is done. The proposed system is mainly composed of a bare plate solar collector/evaporator, a hermetic variable-frequency rotary-type compressor, a micro-channel condenser and an electronic expansion valve. The experimental data include the exergy loss rates, irreversibility rates and exergy loss ratios of all components and the influences of ambient temperature on these parameters. The analysis results show that the average irreversibility rate of the compressor is 204.8 W at an ambient temperature of 16 °C, and 149.9 W at an ambient temperature of 27 °C. The highest irreversibility rate occurs in the compression process, followed by the throttling process, the evaporation process and the condensation process.

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    Numerical Study of Sodium Bentonite Extrusion into a Planar Fracture
    LIU Miaomiao (刘苗苗), LI Xiaoyue (李晓月), XU Yongfu (徐永福)
    2021, 26 (2):  146-154.  doi: 10.1007/s12204-021-2268-6
    Abstract ( 195 )   PDF (940KB) ( 61 )  
     As a candidate buffer/backfill material for high-level radioactive waste geological repositories, bentonite has numerous favorable properties, such as low permeability, high expansibility, and a high sorption capacity for radionuclides. The radionuclide-isolating performance of a buffer is strongly influenced by its extrusion. In this study, the bentonite extrusion process is explored: its basic mechanism can be considered free swelling of the bentonite. A 2D extrusion model of bentonite that is based on the 1D free swelling model of bentonite is presented. A numerical method is proposed to investigate the extrusion process of Na-bentonite into fractures over time under no-seepage conditions based on the free swelling model. The influences of the electrolyte concentration and dry density on the extrusion depth and mass of the bentonite are discussed, and the distribution of montmorillonite inside the bentonite is analysed. The rationale of the proposed bentonite extrusion model is then illustrated in comparison with the results of the bentonite extrusion test.
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    Comparative Study on Two-Stage Absorption Refrigeration Systems with Different Working Pairs
    KONG Xiangqiang (孔祥强), MENG Xiangxi (孟祥熙), LI Jianbo (李见波), SHANG Yanping (尚燕平), CUI Fulin (崔福林)
    2021, 26 (2):  155-162.  doi: 10.1007/s12204-020-2209-9
    Abstract ( 209 )   PDF (854KB) ( 36 )  
    The objective of this paper is to present a simulation study on the two-stage absorption refrigeration systems of 2.5 kW capacity using LiBr-H2O, NH3-H2O and R124-DMAC as working pairs. Under the design condition that the generating, absorbing, evaporating and condensing temperatures are 75 °C, 45 °C, 5 °C and 40 °C, respectively, the high and low pressure side solution circulation ratios and the coefficient of performance (COP) for the systems are calculated. Then the influences of medium, generating, absorbing, evaporating and condensing temperatures on system performances are analyzed. The results show that under the design condition, the COP of the LiBr-H2O system can reach 0.49, superior to those of the NH3-H2O and R124-DMAC systems, which are 0.32 and 0.31, respectively. Furthermore, the medium temperature for higher COP lies in an interval of 64—67 °C for the LiBr-H2O, NH3-H2O and R124-DMAC systems. High generating temperature and low absorbing temperature can decrease the high and low pressure side solution circulation ratios, and can also increase the COP. High evaporating temperature can decrease the low pressure side solution circulation ratio and increase the COP. Low condensing temperature can decrease the high pressure side solution circulation ratio and increase the COP.

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    Effect of Deflectors on the Flow Characteristics of a Square Pipe with a 90° Bend
    JIANG Chenqi (江晨琦), GONG Zhaoxin (宫兆新)
    2021, 26 (2):  163-169.  doi: 10.1007/s12204-021-2278-4
    Abstract ( 270 )   PDF (2608KB) ( 51 )  
    Pipe flow is a classic hydrodynamic issue. Most pipelines contain bends, and bends cause energy loss and distort the flow because of secondary flow. Deflectors are often used to adjust the flow quality in a bend. In this study, a numerical simulation using ANSYS Fluent 19.0 is used to analyze the effects of the deflector number, location and angle on the flow characteristics of a square pipe with a 90° bend. The velocity non-uniformity and the head loss are analyzed quantitatively. The secondary flow is presented visually, and its evolution characteristics are assessed. An optimized installation scheme for the deflectors is discussed, and a reference is provided for controlling the flow quality in bends via deflectors.

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    Scaling Relation of the Scalar Diffusion in a Rotating Mixer
    SUN Na (孙娜), WANG Lipo (王利坡), LI Yuanbo (李渊博), LI Lin (李琳), QI Shuaipeng (齐帅鹏), SHEN Yongxing (沈泳星)
    2021, 26 (2):  170-175.  doi: 10.1007/s12204-021-2275-7
    Abstract ( 284 )   PDF (1495KB) ( 26 )  
    Scalar mixing is under the joint control of convection and diffusion. The ratio of the dissipative scale of velocity field to that of the scalar field depends on the Schmidt number. In the high Schmidt number limit, the scalar scale is much smaller than that of the momentum, which then requires either special treatment or ad hoc models for the scalar quantity in numerical simulations. In order to avoid model uncertainty or unnecessary numerical complexity, the direct numerical simulation is performed for studying the scalar mixing process in a confined rotating mixer tank. It has been found that in the range of negligible numerical diffusivity,the characteristic scalar mixing time is inversely proportional to the scalar diffusivity. Analysis based on the dimensional argument justifies such scaling relation as well, from which the unaccepted computational time of the mixing process in the high Schmidt number limit can be efficiently determined, without the use of ad hoc models. This scaling idea is also of practical meaningfulness for other similar problems.
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    Simulation of Bimodal Fiber Distribution Effect on Transient Accumulation of Particles During Filtration
    AKAMPUMUZA Obed, WU Jiajun (吴佳骏), QUAN Zhenzhen (权震震), QIN Xiaohong (覃小红)
    2021, 26 (2):  176-185.  doi: 10.1007/s12204-020-2251-7
    Abstract ( 188 )   PDF (1849KB) ( 24 )  
    Modeling has become phenomenal in developing new products. In the case of filters, one of the most applied procedures is via the construction of idealized physical computational models bearing close semblance to real filter media. It is upon these that multi-physics tools were applied to analyze the flow of fluid and the resulting typical performance parameters. In this work, two 3D filter membranes were constructed with MATLAB; one had a random distribution of unimodal nanofibers, and the other, a novel modification, formed a bimodal distribution; both of them had similar dimensions and solid volume fractions. A comparison of their performance in a dust-loading environment was made by using computational fluid dynamic-discrete element method (CFD-DEM) coupling technique in STAR-CCM+. It was found that the bimodal nanofiber membrane greatly improved the particle capture efficiency. Whereas this increased the pressure drop, the gain was not too significant. Thus, overall, the results of the figure of merit proved that adopting a bimodal formation improved the filter's quality.
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    Auto-Tuning Parameters of Fractional PID Controller Design for Air-Conditioning Fan Coil Unit
    LI Shaoyong (李绍勇), WANG Duo (王铎), HAN Xilian (韩喜莲), CHENG Kang (程康), ZHAO Chunrun (赵春润)
    2021, 26 (2):  186-192.  doi: 10.1007/s12204-020-2245-5
    Abstract ( 227 )   PDF (642KB) ( 23 )  
     The traditional integer order PID controller manipulates the air-conditioning fan coil unit (FCU) that offers cooling and heating loads to each air-conditioning room in summer and winter, respectively. In order to maintain a steady indoor temperature in summer and winter, the control quality cannot meet the related requirements of air-conditioning automation, such as large overshoot, large steady state error, long regulating time, etc. In view of these factors, this paper develops a fractional order PID controller to deal with such problem associated with FCU. Then, by varying mutation factor and crossover rate of basic differential evolution algorithm adaptively, a modified differential evolution algorithm (MDEA) is designed to tune the satisfactory values of five parameters of indoor temperature fractional order PID controller. This fractional order PID control system is configured and the corresponding numerical simulation is conducted by means of MATLAB software. The results indicate that the proposed fractional order PID control system and MDEA are reliable and the related control performance indexes meet with the related requirements of comfortable air-conditioning design and control criteria.

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    Gas Expansion Process in the Dynamic Vacuum Calibration
    CHENG Yongjun (成永军), CHEN Shuping (陈叔平), WANG Chenghong (汪乘红), XI Zhenhua (习振华), SUN Wenjun (孙雯君), LIU Dan (刘丹)
    2021, 26 (2):  193-200.  doi: 10.1007/s12204-021-2279-3
    Abstract ( 230 )   PDF (1228KB) ( 22 )  
     By analyzing the gas expansion process from the upstream chamber via an orifice and a very rapidopening ultra-high vacuum (UHV) gate valve to downstream one, the standard pressure analytical model based on the dynamic vacuum calibration apparatus in millisecond range developed by Lanzhou Institute of Physics (LIP) is deduced theoretically and corrected by real gas characteristics and temperature changes. According to the Knudsen criterion, there is no free molecular flow regime in the area in front of the orifice during the gas expansion, so the chocked flow approximation is adopted to reduce the difficulty of numerical computation. Under this approximation and the full opening of the rapid valve, the standard pressure expression is calculated theoretically, and the upstream chamber pressure and temperature changes are obtained by numerical simulation during the gas expansion from 100 kPa to 10 kPa. Also, experiments are performed using capacitance diaphragm gauges (model CDG045Dhs). The uncertainties between the measured pressure and the simulated and theoretical ones are 10% and 4.65%, respectively, which indicates that the apparatus can generate predictable pressure changes in the millisecond range and the conceptual model can better approximate the calibration results. Finally, the orifice conductance, and the correction factors of real gas characteristics and temperature change to the standard pressure are calculated according to the simulation results; the corrected standard pressure expression is obtained.

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    Evaporation Characteristics of Ethanol Diesel Droplets Containing Nanoparticles
    WANG Xiaorong (王筱蓉), WEI Ning (韦宁), GAO Ji (高吉), YAN Jun (严俊), JIANG Genzhu (姜根柱)
    2021, 26 (2):  201-209.  doi: 10.1007/s12204-021-2280-x
    Abstract ( 206 )   PDF (1876KB) ( 29 )  
    Evaporation plays an important role in the cylinder combustion process, so the study of fuel evaporation characteristics is very important to improve the combustion performance of internal combustion engine. In this study, ethanol diesel was used as the base fuel. The evaporation characteristics of droplets at different ambient temperatures (623K, 923K) were investigated by adding alumina nanoparticles with different mass fractions (1%, 2.5%) to ethanol diesel. The results show that the evaporation states of three kinds of droplets (ethanol diesel, base fuel with 1% alumina nanoparticles, base fuel with 2.5% alumina nanoparticles) are similar in the low temperature environment, and nanoparticles inhibit the evaporation of the droplets. At low temperature, the higher the concentration of nanoparticles is, the slower the evaporation rate is. However, in the high temperature environment, nanoparticles significantly promote droplet evaporation, and the phenomenon of micro explosion is obvious. By analyzing the evaporation processes of two different temperatures, we know that nanoparticles first gather on the surface of the droplet, then form a protective shell, and finally reduce the evaporation rate of the droplet at low temperature. In the high temperature environment, nucleation sites and bubbles are formed in the interior of the droplet; as the droplet is heated further, nucleation sites and bubbles first gather and then fuse; the bubbles gradually expand and burst rapidly, causing micro explosion of the droplets, and most of them are evaporated in the micro explosion.

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    Ethanol Evaporation Characteristics of the Blends of Fatty Acid Methyl Ester and Ethanol
    WANG Xiaorong (王筱蓉), WANG Gang (王刚), WANG Lihui (王黎辉), ZHANG Jie (张杰), YAN Jun (严俊)
    2021, 26 (2):  210-217.  doi: 10.1007/s12204-021-2281-9
    Abstract ( 207 )   PDF (1023KB) ( 23 )  
     The evaporation characteristics of fatty acid methyl ester (FAME) mixed with four concentrations of ethanol at 873K and normal atmospheric pressure are studied herein. FAME is used as base oils, and ethanol mass fractions vary from 10%, 20%, 30% to 40%. The experimental results show that the evaporation process of the binary component droplets of FAME-ethanol can be divided into two stages: a fluctuation evaporation stage, and an equilibrium evaporation stage. In these four concentration gradients, micro-explosions occur in the droplet evaporation process. The fluctuation evaporation stage is divided into two stages: a strong fluctuation stage and a weak fluctuation stage. After the micro-explosion, there is still a small amount of ethanol in the droplet. Due to the surface tension of the droplet, a small amount of ethanol cannot make the droplet violently fluctuate. The results show that the earlier the droplet micro-explosion occurs, the more intense it is, and the shorter the lifetime of the droplet is. Different concentrations of ethanol have different improvements in droplet evaporation characteristics. Generally, the higher the ethanol concentration is, the shorter the lifetime of the droplet is. However, increasing the ethanol concentration from 20% to 30% has the most obvious effect on the lifetime of the droplet.

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    Experiment and Simulation of Impact Response of Woven CFRP Laminates with Different Stacking Angles
    ZHUANG Weimin (庄蔚敏), WANG Pengyue (王鹏跃), AO Wenhong (熬文宏), CHEN Gang (陈刚)
    2021, 26 (2):  218-230.  doi: 10.1007/s12204-021-2271-y
    Abstract ( 166 )   PDF (3045KB) ( 24 )  
    The effects of different laying angles on impact resistance and damage types of woven carbon fibrereinforced polymer (CFRP) laminates were studied. The drop hammer impact test and simulation analysis of woven CFRP laminates at four different velocities were carried out by means of the combination of experiment and explicit finite element analysis. The effect of impact velocity on damage pattern and fracture of woven CFRP laminates with eight layers and four laying angles (0°, 15°, 30°, and 45°) was studied by drop weight impact test, and the damage area of woven CFRP laminates with different laying angles was measured by ultrasonic nondestructive testing method. A user-defined material model was developed based on LS-DYNA to simulate the impact behavior of woven CFRP laminates. A finite element simulation model was established to effectively predict the complex damage evolution of woven CFRP laminates under low-speed impact loading. Its effectiveness was verified by impact response and damage crack pattern. The energy absorption and impact force of the laminated plate increase gradually from 0° to 45° in the laying angle of woven fabric. With the increase of laying angle from 0° to 45°, the length of fracture crack and the damage area increase. The developed model can accurately predict the impact properties of the composites.

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    Global Fabric Defect Detection Based on Unsupervised Characterization
    WU Ying (吴莹), LOU Lin (娄琳), WANG Jun (汪军)
    2021, 26 (2):  231-238.  doi: 10.1007/s12204-020-2246-4
    Abstract ( 238 )   PDF (2085KB) ( 29 )  
    Fabric texture intelligent analysis comprises the following characteristics: objective detection results, high detection efficiency, and accuracy. It is significantly vital to replace manual inspection for smart green manufacturing in the textile industry, such as quality control and rating, and online testing. For detecting the global image, an unsupervised method is proposed to characterize the woven fabric texture image, which is the combination of principal component analysis (PCA) and dictionary learning. First of all, the PCA approach is used to reduce the dimension of fabric samples, the obtained eigenvector is used as the initial dictionary, and then the dictionary learning method is operated on the defect-free region to get the standard templates. Secondly, the standard templates are optimized by choosing the appropriate dictionary size to construct a fabric texture representation model that can effectively characterize the defect-free texture region, while ineffectively representing the defective sector. That is to say, through the mechanism of identifying normal texture from imperfect texture, a learned dictionary with robustness and discrimination is obtained to adapt the fabric texture. Thirdly, after matching the detected image with the standard templates, the average filter is used to remove the noise and suppress the background texture, while retaining and enhancing the defect region. In the final part, the image segmentation is operated to identify the defect. The experimental results show that the proposed algorithm can adequately inspect fabrics with defects such as holes, oil stains, skipping, other defective types, and non-defective materials, while the detection results are good and the algorithm can be operated flexibly.

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    Welding Automation & Computer Technology
    Weld Geometry Monitoring for Metal Inert Gas Welding Process with Galvanized Steel Plates Using Bayesian Network
    MA Guohong (马国红), LI Jian (李健), HE Yinshui (何银水), XIAO Wenbo (肖文波)
    2021, 26 (2):  239-244.  doi: 10.1007/s12204-020-2234-8
    Abstract ( 223 )   PDF (1319KB) ( 34 )  
    We present a novel method to monitor the weld geometry for metal inert gas (MIG) welding process with galvanized steel plates using Bayesian network (BN), and propose an effective method of extracting the weld reinforcement and width online. The laser vision sensor is mounted after the welding torch and used to profile the weld. With the extracted weld geometry and the adopted process parameters, a back propagation neural network (BPNN) is constructed offline and used to predict the weld reinforcement and width corresponding to the current parameter settings. A BN from welding experience and tests is presented to implement the decision making of welding current/voltage when the error between the predictive geometry and the actual one occurs. This study can deal with the negative welding tendency to adapt to welding randomness and indicates a valuable application prospect in the welding field.

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    Bypass-Enabled Thread Compaction for Divergent Control Flow in Graphics Processing Units
    LI Bingchao (李炳超), WEI Jizeng (魏继增), GUO Wei (郭炜), SUN Jizhou (孙济州)
    2021, 26 (2):  245-256.  doi: 10.1007/s12204-020-2240-x
    Abstract ( 215 )   PDF (1101KB) ( 36 )  
    Graphics processing units (GPUs) employ the single instruction multiple data (SIMD) hardware to run threads in parallel and allow each thread to maintain an arbitrary control flow. Threads running concurrently within a warp may jump to different paths after conditional branches. Such divergent control flow makes some lanes idle and hence reduces the SIMD utilization of GPUs. To alleviate the waste of SIMD lanes, threads from multiple warps can be collected together to improve the SIMD lane utilization by compacting threads into idle lanes. However, this mechanism induces extra barrier synchronizations since warps have to be stalled to wait for other warps for compactions, resulting in that no warps are scheduled in some cases. In this paper, we propose an approach to reduce the overhead of barrier synchronizations induced by compactions. In our approach, a compaction is bypassed by warps whose threads all jump to the same path after branches. Moreover, warps waiting for a compaction can also bypass this compaction when no warps are ready for issuing. In addition, a compaction is canceled if idle lanes can not be reduced via this compaction. The experimental results demonstrate that our approach provides an average improvement of 21% over the baseline GPU for applications with massive divergent branches, while recovering the performance loss induced by compactions by 13% on average for applications with many non-divergent control flows.

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