Design of MMIC oscillators using GaAs 0.2 μm PHEMT technology

We propose a feedback type oscillator and two negative resistance oscillators.These microwave oscillators have been designed in the S band frequency.A relatively symmetric resonator is used in the feedback type oscillator.The first negative resistance oscillator uses a simple lumped element resonator which is substituted by a microstrip resonator in the second oscillator to improve results.The negative resistance oscillator produces 4.207 dBm and 7.124 dBm output power with the lumped element resonator and microstrip resonator respectively,and the feedback type oscillator produces ?10.707 dBm output power.The feedback type oscillator operates at 3 GHz with phase noise levels at-83.30 dBc/Hz and-103.3 dBc/Hz at 100 kHz and 1 MHz offset frequencies respectively.The phase noise levels of the negative resistance oscillator with the lumped element resonator are-94.64 dBc/Hz and-116 dBc/Hz at 100 kHz and 1 MHz offset frequencies respectively,at an oscillation frequency of 3.053 GHz.With the microstrip resonator the phase noise levels are-99.49 dBc/Hz and-119.641 dBc/Hz at 100 kHz and 1 MHz offset frequencies respectively,at an oscillation frequency of 3.072 GHz.The results showed that both the output power and the phase noise of the negative resistance oscillators were better than those of the feedback type oscillator.     

摩托车发动机气门机构的分析与综合

本文以摩托车发动机气门机构为研究对象 ,在已知平底从动件升程的条件下 ,通过对升程曲线进行“预光顺”和“精光顺” ,然后进行运动学和动力学计算 ,为摩托车气门机构的设计提供依据。     

中线蝶阀的定常与突变流场分析

运用FLUENT软件,对大口径中线蝶阀的流场作定常分析。在此基础上考虑速度突变的情况,对相应的流场作非定常分析。结果表明入口速度突然增加将引起流场压力值及流速的瞬间提升,继而逐渐降低达到基本稳定,其峰值比高速稳态情况下的压力、流速值更大。同样当入口速度突然由高速恢复至低速,将引起流场压力值及流速的瞬间降低,继而逐渐升高达到基本稳定,其谷值比低速稳态下的压力、流速值还低。此外,开度大小对突变下的蝶阀的流场参数及其恢复平衡状态的快慢有很大影响。     

进气歧管长度与气门晚关角对动态效应影响的对比研究

进气歧管长度和进气门晚关角是两个影响充气动态效应的重要因素。从调节机理上分析,前者较简单而后者较复杂,但后者的应用范围更加广泛,对于此原因的探讨在现有资料中较少。采用AVL-BOOST软件,建立了某款发动机的热力循环模型,对比分析了两者在提高充气效率上的差异及产生差异的根源。模拟结果及原因分析表明:匹配适当长度的歧管,增强了进气道压力波动效应,从而提高了正向进气量,但无法控制进气倒流量;而合理的进气晚关角不仅充分的利用了惯性效应,增加了正向进气量,而且很好地控制了进气倒流量。因此,在利用充气动态效应,提高充气效率,提升发动机性能方面,调整进气晚关角所产生的效果优于改变进气歧管长度所产生的效果,从而说明了调整进气门晚关角应用更为广泛的根本原因。     

环形阀片局部加载变形研究

提出了减振装置环形节流阀片受局部均载作用时挠曲变形量的一种研究方法,根据节流阀片物理模型以及薄板力学的相关理论,推导了阀片受局部均载作用时挠曲变形量的解析式,并整理得出了便于理论分析和工程应用的表达形式.与有限元计算结果对比验证了公式以及推导过程的正确性,研究了不同载荷、载荷作用半径以及节流阀片相关结构参数和材料特性对其任意半径处变形量的影响规律,得出的解析式和结论为减振装置阻尼阀的精确设计提供了依据.     

发动机气门颈部失效原因分析与对策

本文从发动机气门颈部失效断口分析出发,运用理论与实际相结合的方法,指出了失效的类型、原因,提出了具体的改进措施。     

Design of a rotary valve orifice for a continuous wave mud pulse generator

High-speed downhole transmission technology plays an important role in measurement while drilling (MWD) and logging while drilling (LWD) systems, where the continuous wave mud pulse transmission method is currently the most advantageous method for wireless downhole transmission. To increase the production rate, transmission distance and testing intensity and to decrease the ground detected difficulty of continuous wave mud signals, the valve orifice must be optimized to satisfy the requirements for the continuous sinusoidal pressure output. In this study, an improved arc-fillet-line triangular valve orifice is designed based on a general line triangular valve orifice according to the relationships between the fluid differential pressure of a thin-walled cutting edge and the fluid flow area and between the correlation coefficient of the theoretical pressure difference and standard sinusoidal signal. The improved orifice is designed by calculating the variation between the flow area and relative rotation angle of the rotor to the stator through the established polar coordinate equations. The optimized valve structure is simple and easy to machine, and highly similar sinusoidal pressure wave signals can be achieved during practical operation to meet the requirements of the instrument design.     

Numerical investigation and Pareto optimization on performance,emission and in-cylinder reforming characteristics for two cylinder reciprocating engine with COx free fuel

The main objective of this study is to introduce the applicability of ammonia to the downsized compression ignition diesel engine for power generation or range extender. For this research objective, the two cylinder engine, which was the result of the previous study, fueled with diesel-ammonia blends was considered and the performance and NOx emission tendency were identified using the numerical method. Ammonia was mixed with diesel via injection at a specific fuel energy fraction (0%, 5%, 10%, or 15%) to evaluate the engine performance and emission characteristics. In addition, concept of “in-cylinder reforming” was introduced adopting negative valve overlap (NVO) by advancing the exhaust valve closing time to investigate the effect of adding ammonia as a hydrogen carrier. Subsequently, the primary variables affecting the brake-specific fuel consumption and NO_X are determined via multi-objective Pareto analysis. The optimal Pareto front confirms that exhaust valve timing exerts a greater effect on the performance and emissions than injection timing. Moreover, in-cylinder reformed hydrogen was increased under negative valve overlap strategy.     

Enhanced negative thermal expansion of (KMg)3+-substituted Fe2Mo3O12 ceramics

Herein, we report the solid-state synthesis of (KMg)_xFe_2-xMo₃O₁₂ (0 = x ≤ 1.5) ceramics. Phase composition, crystal structure, morphology, phase transition and thermal expansion behavior of the (KMg)_xFe_2-xMo₃O₁₂ ceramics were investigated by XRD, Raman, XPS, HRTEM, EDX, SEM, TMA and high-temperature XRD. Results indicate that (KMg)³⁺ dual-cations have successfully replaced Fe³⁺ in Fe₂Mo₃O₁₂ ceramics and single-phase monoclinic (KMg)_xFe_2-xMo₃O₁₂ ceramics were prepared for 0.25 = x ≤ 1. (KMg)³⁺ introduction can increase the density of (KMg)_xFe_2-xMo₃O₁₂ ceramics and effectively improve their negative thermal expansion (NTE) performance. In addition, the phase transition temperature (Tc) of Fe₂Mo₃O₁₂ was reduced from 508.1 °C to room temperature with the increase of (KMg)³⁺-substitution. Monoclinic KMgFeMo₃O₁₂ ceramics was observed to show stronger NTE in a wider temperature range of 30–700 °C for the first time. Its corresponding coefficient of thermal expansion (CTE) is as high as ?17.21 × 10^?6 °C^?1. The distortion of [FeO₆/MgO₆] polyhedra in (KMg)_xFe_2-xMo₃O₁₂ caused by (KMg)³⁺-substitution contributed to the stronger NTE.