Current Issue : July - September Volume : 2017 Issue Number : 3 Articles : 6 Articles
This study numerically investigates the effects of changing vaned diffuser angles and vaneless diffuser widths of a twostage\ncentrifugal refrigerant compressor on the reduction of surge flow rate and the improvement of part-load performance.\nThe first stage has a low-solidity vaned diffuser, while the second stage has a vaneless diffuser. The analysis results\nshow that adjusting the angle of low-solidity vaned diffuser in the first stage can lower the surge flow rate and keeps high\nefficiencies at low flow rates. The inlet guide vane of the first stage can also lower the surge flow rate; however, their\nsimultaneous actuation cannot lower the surge flow rate further. Reducing the vaneless diffuser width of the second\nstage can lower the surge flow rate. Meanwhile, simultaneously adjusting the individual inlet guide vanes of the second\nstage can also effectively lower the surge flow rate. If the diffuser width of the first stage is reduced while the low-solidity\nvaned diffuser blade angle is fixed, the surge flow rate will be lowered obviously as the inlet guide vane is adjusted.\nHowever, reduction in diffuser width decreases the efficiency at low flow rate. Controlling the inlet guide vane and diffuser\nwidth well can lower the surge flow rate to 30% of the design flow....
The reliability of high-power light-emitting-diode (LED) devices strongly depends on the die-attach quality because voids may\nincrease junction temperature and total thermal resistance of LED devices. Die-attach material has a key role in the thermal\nmanagement of high-power LED package by providing low-contact thermal resistance. Thermal and mechanical analyses were\ncarried out by experiments and thermal simulation.The quantitative analysis results show that thermal resistance of die-attach layer\n(thermal resistance caused by die-attach material and voids in die-attach layer) plays an important role in total thermal resistance\nof high-power LED packaging according to the differential structure function of thermal transient characteristics. The increase of\nvoid fraction in die-attach layer causes the increases of thermal resistance of die-attach layer; the thermal resistance increased by\n1.95 K/Wwhen the void fraction increased to 62.45%.Thevoids also make an obvious influence on thermal stress and thermal strain\nof chip; the biggest thermal stress of chip was as high as 847.1MPa compared to the 565.2MPa when the void fraction increases\nfrom being void-free to 30% in the die-attach layer....
The problem about identification of elastic bending deformation of a drill\nstring in a curve wells based on the theory of flexible curved rods and the direct\ninverse problems of drill string bending in the channels of curvilinear\nbore-holes is stated. The problem is solved which determines the resistance\nforces and moments during performing ascending-descending operations in\ncurvilinear bore-holes with trajectories of the second order curve shapes. The\nsensitivity of the resistance forces relative to geometric parameters of the\nbore-hole axial line trajectories is analyzed....
Failure of cutterhead driving system (CDS) of tunnel boring machine (TBM) often occurs under shock and vibration conditions.\nTo investigate the dynamic characteristics and reduce system vibration further, an electromechanical coupling model of CDS is\nestablished which includes the model of direct torque control (DTC) system for three-phase asynchronous motor and purely\ntorsional dynamic model of multistage gear transmission system. The proposed DTC model can provide driving torque just as the\npractical inverter motor operates so that the influence of motor operating behavior will not be erroneously estimated. Moreover,\nnonlinear gear meshing factors, such as time-variant mesh stiffness and transmission error, are involved in the dynamic model.\nBased on the established nonlinear model of CDS, vibration modes can be classified into three types, that is, rigid motion mode,\nrotational vibration mode, and planet vibration mode. Moreover, dynamic responses under actual driving torque and idealized\nequivalent torque are compared,which reveals that the ripple of actual driving torquewould aggravate vibration of gear transmission\nsystem. Influence index of torque ripple is proposed to show that vibration of system increases with torque ripple. This study\nprovides useful guideline for antivibration design and motor control of CDS in TBM....
The paper presents the evaluation of the performance of various\naeroderivative gas turbine models (AGTM) derived from a high bypass turbofan\nengine (HBTE). The objective of the study is to determine the performance and\nsuitability of the developed AGTM for power generation. Furthermore, the models\nwere specified as Model I (two shafts with FPT), Model II (two shafts with\ndirect-drive LP turbine), and Model III (one shaft with FPT). The useful performance\ndata for the AGTM was generated using the turbomatch simulation code.\nAdditionally, the preliminary design point (DP) performance results estimated the\npower output and thermal efficiency of Models I, II, and III which ranged between\n3.46 � POT � 10.67 MW and 29.8 � ...
This paper proposes a sensor-less robust force control method for improving the control\nperformance of an electro-mechanical brake (EMB) which is applicable to commercial city buses.\nThe EMB generates the accurate clamping force commanded by a driver through an independent\nmotor control at each wheel instead of using existing mechanical components. In general, an EMB\nundergoes parameter variation and a backdrivability problem. For this reason, the cascade control\nstrategy (e.g., force-position cascade control structure) is proposed and the disturbance observer is\nemployed to enhance control robustness against model variations. Additionally, this paper proposed\nthe clamping force estimation method for a sensor-less control, i.e., the clamping force observer (CFO).\nFinally, in order to confirm the performance and effectiveness of a proposed robust control method,\nseveral experiments are performed and analyzed....
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