共查询到20条相似文献，搜索用时 203 毫秒
1.
陈浩 《初中生世界(初三物理版)》2006,(4)
一、考点解读考点1声音及其特征一切正在发声的物体都在振动.固体、液体、气体都可以因振动而发出声音.相同的声音在不同的介质中传播速度不同,而不同的声音在同一种介质中传播速度却是相同的.声音在空气(15℃)中的速度是345m/s.一般情况下,声音在液体中传播较快,在固体中更快. 相似文献
2.
何琰 《中学物理教学参考》1998,(9)
一、本月知识学习指要：1．振动是物体发声的必要条件,声音的品质由发声体的振动来决定;介质是声音传播的物质基础,声音不能在真空中传播;在不同介质中声音的传播速度不同,在15℃的空气中声音的传播速度为340米／秒．2．固体、液体、气体是自然界中一般物质存在的三种状态．当满足一定条件时,一切物质都会发生状态变化．应注意：一些物质发生状态变化时,会出现吸热或放热而温度不改变的现象．二、练习题精选1．一切发声的物体都在,声音要依靠物质来传播,这种传播声音的物质叫,它可以是气体和．声音在中不能传播．2．下列说法中错… 相似文献
3.
应用Chemkin化学动力学软件包中的Senkin模块模拟了正庚烷在多孔介质发动机中的燃烧过程．通过修改Senkin程序,结合了Woschni传热模型和多孔介质换热模型,并在正庚烷详细氧化机理中加入氮氧化物的生成机理,将此程序纳入发动机燃烧的零维单区模型．对多种工况参数进行计算,讨论了运行参数对发动机性能的影响．当进气温度、压缩比增大,或过量空气系数降低时,多孔介质发动机着火时刻会明显提前．结果表明：多孔介质对混合气具有预热作用可强化发动机的点火燃烧,多孔介质的初始温度是决定压燃点火的决定性因素． 相似文献
4.
为了研究两亲聚合物泡沫在地层中封堵大孔道的能力及驱油性能,利用填砂管模型模拟地层环境,通过改变两亲聚合泡沫的气液比、注入速度等条件,探讨了两亲聚合泡沫的渗流特性。结果表明:两亲聚合物泡沫在多孔介质中的阻力因子随着气液比的增加先增大后减小,最佳气液比在1∶1到2∶1,此时两亲聚合物泡沫的封堵能力最强。两亲聚合物泡沫在多孔介质中的阻力因子随渗透率和注入速度的增加而变大;原油存在和温度升高时两亲聚合物泡沫在多孔介质中的渗流阻力变小;两亲聚合物泡沫可以有效封堵储层中的高渗透层,减小高、低渗透层的渗透率差异,从而使注入流体在不同渗透率层内比较均匀地推进,且对不同渗透率级差的岩心具有良好的适应性。 相似文献
5.
6.
一、单选题(每小题3分,共30分)
1.下列说法正确的是( ).
(A)声音的传播速度是340 m/s
(B)声音在固体和液体中传播的速度比在空气中传播的慢
(C)只要有振动,就一定能听到声音
(D)声音在介质中以声波的形式传播 相似文献
7.
《初中生世界(初三物理版)》2009,(Z1)
重点解读一、基本知识概述1.光的传播跟声音的传播不一样.声音只能在介质中传播,真空不能传声,而光既可以在介质中传播,也可以在真空中传播.并且,光在真空中的传播速度最大,是3.0×108m/s,在其他介质中的速度都比真空中小.例如,光在空气中的速度略比真空中小,可近似等于真空中的速度;在水中的 相似文献
8.
目的:低温风洞运行时大流量低温氮气被排放到大气环境中,对周围环境造成潜在的低温、缺氧危险。本文旨在研究羽流扩散过程中各变量(环境风速、环境风温度、相对湿度和排气出口流速)对羽流沉降的影响。创新点:采用考虑相变的低温羽流扩散模型,通过数值模拟对影响羽流扩散的各参数进行敏感性分析。方法:1.基于Hertz-Knudsen关系修正,考虑空气中水的相变,构建低温羽流扩散的数值模型;2.对照美国National TransonicFacility的羽流扩散数据和NASA的二阶分析模型的计算结果,验证本文所采用的数值模型的准确性;3.利用数值模拟,比较不同排放条件下近地面的最低氧含量和最低温度,并对各变量进行敏感性分析。结论:1.考虑相变的羽流扩散数值模型,相比NASA的二阶分析模型拥有更好的准确性。2.对于0.3m低温风洞的羽流扩散,高环境风速有利于羽流消散;高环境温度和高相对湿度能提升近地面的最低温度,但对近地面的最低氧含量影响甚微。3.当排气速度小于2 kg/s时,排气流速增大不利于羽流消散;当羽流速度大于2 kg/s时,排气流速增大有利于羽流消散。 相似文献
9.
与自由空间的燃烧相比,预混气体在多孔介质中燃烧具有功率密度大、较大的动力调节范围和较低的污染物排放的优点,已经成为近年来的研究热点.具有广阔的应用前景的多孔介质燃烧器已经在家用热水器上得到了应用.本文综述了预混气体多孔介质中燃烧的理论、实验和数值模拟的研究.最后,指出了以往研究中的不足,并对该领域今后应当重点开展的研究工作进行了探讨. 相似文献
10.
基于双相各向同性介质模型,推导了双相各向同性介质中波动方程的有限差分格式,并对其进行了数值模拟.结果表明,弹性波在双相各向同性介质中传播时除了存在常规的快纵波(qP1)和横波以外,还存在慢纵波(qP2),并且慢纵波的速度明显小于快纵波. 相似文献
11.
Based on a three-step kinetic mechanism, a one-dimensional, time dependent, numerical model is presented forthe smoldering propagation in a horizontally packed bed of cellulosic material. The kinetic processes include pyrolysis andoxidation degradation of fuel and oxidation of char. Heat transfer between solid and gas is taken into account, and the diffusioncoefficient varies with the temperature. Radiative heat transfer is included by using the diffusion approximation. The effectsof airflow velocity and oxygen concentration are simulated on the smoldering velocity and the averaged maximum temperatureof smoldering fuel. The results indicate that the spread rate varies linearly with increasing airflow velocity, and the inlet airvelocity has little effect on the maximum temperature. The evolutions of gas species and solid compositions are predicted.The effects of frequency factors (A1, A2 and A3) are analyzed. Simulations show that the smoldering spread rate increaseswith increasing A2 (fuel oxidation), but decreases with A1 (fuel pyrolysis) and A3 (char oxidation). 相似文献
12.
Jun Li Nan Gao Guang-yi Cao Heng-yong Tu Ming-ruo Hu Xin-jian Zhu Jian Li 《浙江大学学报(A卷英文版)》2010,11(1):61-70
In this paper, an application of a nonlinear predictive controller based on a self recurrent wavelet network （SRWN） model for a direct internal reforming solid oxide fuel cell （DIR-SOFC） is presented. As operating temperature and fuel utilization are two important parameters, the SOFC is identified using an SRWN with inlet fuel flow rate, inlet air flow rate and current as inputs, and temperature and fuel utilization as outputs. To improve the operating performance of the DIR-SOFC and guarantee proper operating conditions, the nonlinear predictive control is implemented using the off-line trained and on-line modified SRWN model, to manipulate the inlet flow rates to keep the temperature and the fuel utilization at desired levels. Simulation results show satisfactory predictive accuracy of the SRWN model, and demonstrate the excellence of the SRWN-based predictive controller for the DIR-SOFC. 相似文献
13.
By coupling the heat transfer equation with semi-global chemical reaction kinetic equations, a onedimensional, unsteady mathematical model is developed to describe the pyrolysis of single biomass pellet in the pyrolysis zone of downdraft gasifier. The simulation results in inert atmosphere and pyrolysis zone agree well with the published experimental results. The pyrolysis of biomass pellets in pyrolysis zone is investigated, and the results show that the estimated convective heat transfer coefficient and emissivity coefficient are suitable. The mean pyrolysis time is 15.22%, shorter than that in inert atmosphere, and the pellet pyrolysis process in pyrolysis zone belongs to fast pyrolysis. Among the pyrolysis products, tar yield is the most, gas the second, and char the least. During pyrolysis, the temperature change near the center is contrary to that near the surface. Pyrolysis gradually moves inwards layer by layer. With the increase of pyrolysis temperature and pellet diameter, the total pyrolysis time, tar yield, char yield and gas yield change in different ways. The height of pyrolysis zone is calculated to be 1.51—3.51 times of the characteristic pellet diameter. 相似文献
14.
针对气淬粒化装置中高炉渣颗粒的凝固行为,采用凝固熔化模型、流体体积函数模型和离散坐标辐射模型进行了三维瞬态模拟。考虑炉渣凝固过程中的物性参数变化,主要研究了颗粒的凝固过程、相界面移动速度、颗粒温度分布及周围空气速度分布。结果表明:颗粒凝固过程中固相分布不均匀,迎风面固相厚度大于背风面,这主要由迎风面空气速度比背风面快所致;固相-模糊区界面和模糊区-液相界面移动速度先增加后降低,这是由于在颗粒凝固前期导热系数的影响占主导地位、在凝固后期导热热阻的影响占主导地位;在凝固过程中,颗粒温度由外向内逐渐升高,内部温度降低缓慢。 相似文献
15.
Two numerical simulations were performed to investigate the protective effect of the foam cladding. One simulation is based on a previous experimental study, which is a ballistic pendulum with and without a foam cladding subjected to close-range blast loading. The other model is a steel beam with and without a foam cladding under blast loading. The overpressure due to the blast event can be calculated by the empirical function ConWep or by an arbitrary Lagrangian-Eulerian (ALE) coupling model. The first approach is relatively simple and widely used. The second approach can model the propagation of the blast wave in the air and the interaction between the air and the solid. It is found that the pendulum with the foam cladding always swings to a larger rotation angel compared to a bare pendulum. However, the steel beam with an appropriate foam cladding has a smaller deflection compared to the bare beam without a foam cladding. It is concluded that the protective effect of the foam cladding depends on the properties of the foam and the protected structure. 相似文献
16.
In the current studies a miniature silicon wafer fuel cell （FC） using L-ascorbic acid as fuel was developed. The cell employs L-ascorbic acid and air as reactants and a thin polymer electrolyte as a separator. Inductively coupled plasma （ICP） silicon etching was employed to fabricate high aspect-ratio columns on the silicon substrate to increase the surface area. A thin platinum layer deposited directly on the silicon surface by the sputtering was used as the catalyst layer for L-ascorbic acid electro-oxidation. Cyclic voltammetry shows that the oxidation of L-ascorbic acid on the sputtered platinum layer is irreversible and that the onset potentials for the oxidation of L-ascorbic acid are from 0.27 V to 0.35 V versus an Ag/AgCl reference electrode. It is found that at the room temperature, with 1 mol/L L-ascorbic acid/PBS （phosphate buffered solution） solution pumped to the anode at 1 ml/min flow rate and air spontaneously diffusing to the cathode as the oxidant, the maximum output power density of the cell was 1.95 mW/cm^2 at a current density of 10 mA/cm^2. 相似文献
17.
介绍了一种自行研制的空调表冷器热工性能测试系统,该系统是通过对实验室现有空气调节处理过程实验台进行改造制得。在压缩机和节流阀之间装设套管式蒸发器,制得冷冻水,经水泵送入表冷器中,与空气进行换热后重新返回套管式蒸发器中。通过温控器实现表冷器入口冷冻水温度的控制,表冷器进出口冷冻水温通过布置热电偶进行测量,冷冻水流量通过转子流量计进行测量,利用原有干湿球温度计测量空气进出口干湿球温度,进而在不同工况下实现表冷器析湿系数、热交换效率、接触系数和换热量的测试。依据测试结果分析风量、风速、冷冻水流量对表冷器热工性能的影响。 相似文献
18.
不同种类树脂基质对涂料炭化层内部泡孔结构与涂料防火性能有显著的影响,通过理论分析与流变性能测试得到以下两点主要结论：首先,高氯化聚乙烯树脂做为树脂基质可形成体积小、数量多且分布均匀的炭层泡孔可提升整体热阻,增强防火隔热效果;其次,高氯化聚乙烯树脂能在较高温度下保持较好的粘度和粘弹特性的树脂,有利于形成细密均匀泡孔结构,改善涂料防火性能. 相似文献
19.
An experimental and model-based study of the effect of rich air/fuel ratios （AFRs） and temperature on the NOx slip of a lean NOx trap （LNT） was conducted in a lean-burn gasoline engine with an LNT after-treatment system. The emissions of the engine test bench and the inlet temperature of the LNT were used as the major inlet boundary conditions of the LNT. The engine periodically operated between a constant lean AFR of 23 with alterable rich AFRs of 10, 11, 12, 13, and 14. A decrease in the rich AFR of the engine strengthened the desorption atmosphere in the LNT, an effect closely related to the number of reductants, and further heightened the NOx desorption of the LNT, but with a penalty in fuel consumption. To eliminate that penalty, the inlet boundary conditions of the LNT were varied by adjusting the inlet temperature within a range between 200 ℃ and 400℃. An increase in inlet temperature heightened the NOx desorption of the LNT, and a NOx breakthrough occurred after the inlet tem- perature exceeded 390 ℃. To control NOx breakthrough, the inlet temperature can be adjusted to offset the strong desorption atmosphere in the LNT commonly created by a rich AFR. 相似文献
20.
The effects of particle size of activated carbon （AC） on its wettability, electrode coating technology and electric chemical performance were studied to assemble nonaqueous electric double layer capacitors （EDLCs） for high power requirements. The results show that the specific surface area and total pore volume of AC decrease from 2 137 m^2/g to 1 683 m^2/g, and 0.95 cm^3/g to 0.78 cm^3/g, respectively, if it is ball-milled for 8 h. The pore size distributions are similar in the range of 0.7 nm to 3.5 nm for different ball-milling time. There exists oxidation on the surface of AC during the ball-milling process and the ratios of O-C=O oxygen compositions increase whereas those of C-O, C=O decrease. The peeling strength of AC coated on current collector is almost inverse proportion with the particle size of AC as well as the resistance of EDLCs, and its capacitance decreases about 6%. 相似文献