• <tr id="yyy80"></tr>
  • <sup id="yyy80"></sup>
  • <tfoot id="yyy80"><noscript id="yyy80"></noscript></tfoot>
  • 99热精品在线国产_美女午夜性视频免费_国产精品国产高清国产av_av欧美777_自拍偷自拍亚洲精品老妇_亚洲熟女精品中文字幕_www日本黄色视频网_国产精品野战在线观看 ?

    Effect of dilution gas composition on the evolution of graphite electrode characteristics in the spark gap switch

    2021-06-21 02:00:24HongyuDAI戴宏宇LeeLI李黎ShuaiREN任帥JingrunGUO郭景潤(rùn)XinGONG宮鑫andAnthonyBruceMURPHY
    Plasma Science and Technology 2021年6期
    關(guān)鍵詞:李黎

    Hongyu DAI(戴宏宇),Lee LI(李黎),Shuai REN(任帥),Jingrun GUO(郭景潤(rùn)),Xin GONG(宮鑫) and Anthony Bruce MURPHY

    1 State Key Laboratory of Advanced Electromagnetic Engineering and Technology,Key Laboratory of Pulsed Power Technology(Ministry of Education),School of Electrical and Electronic Engineering,Huazhong University of Science &Technology,Wuhan 430074,People’s Republic of China

    2 CSIRO Manufacturing,Lindfield NSW 2070,Australia

    Abstract As the widely implemented electrode material,graphite has the characteristic of sublimation by the thermal shock of the switching arc,and the produced carbon vapor is easy to condense into carbon powders and deposit in the switch.The impact of the type of dilution gas in a mixture of 20% oxygen and 80% dilution gas on the sublimation and oxidation characteristics of the graphite electrode is investigated.It is found that when nitrogen dilution gas was replaced by argon,the heat flux to the electrodes decreased,which led to a 63% reduction of graphite sublimation.At the same time,the cooling rate of the arc was slower in argon,which promotes oxidation of the carbon vapor.The residual solid carbon can be reduced by 70%–85%by using argon as the dilution gas.Consequently,it is demonstrated that the stability and working life of the switch could be increased by appropriate selection of the dilution gas.

    Keywords:pulsed discharge,graphite electrode,thermal arc,dilution gas,electrode erosion

    1.Introduction

    The graphite-electrode spark gap switch is a type of gap switch that uses a non-metallic conductor as the electrode material.Its peak power is up to several gigawatts and its current-carrying capacity is several hundreds of kiloamperes.The spark gap switch is the optimal choice for use in the laser power supplies because of its extremely high current-carry capacity[1,2].At the same time,it has been broadly applied in the underwater pulsed discharge[3],controlled nuclear fusion[4],and other devices that require high pulsed current as the input condition.The arc,which transfers high energy fluxes across the gas gap,is a kind of thermal plasma,which has a temperature in the order of 104K.The root of the arc is in direct contact with the electrode,and the heat flux is extremely high at the contact area.As a result,electrode erosion is an ongoing problem.

    The heat flux from the arc leads to sublimation of the graphite electrodes[5].The erosion processes of metal and non-metal materials are quite different.After heating by the arc,the metal material first melts into liquid state.Then,the liquid metal film on the electrode surface is splashed under the action of the Lorentz force,the shock wave force[6],and the thermal explosion force of the arc[7],and continues to vaporize in the high-temperature arc column[8],resulting in a severe mass loss from the electrode.However,the phase transition temperature of graphite is different from that of metal electrodes[9],and its melting point and boiling point are approximately the same.When the temperature exceeds about 4000 K,the graphite begins to sublimate[10].Because graphite has no liquid phase at atmospheric pressure,the surface of the graphite electrode is relatively flat after a discharge[11].In addition,the thermal conductivity of graphite is low at high temperatures.So the ablation range is effectively controlled because there is an approximately adiabatic process[12],which reduces the mass loss of the electrode.Donaldson and Kristiansen have confirmed that the graphite electrode has a lower mass-loss rate than a tungsten–copper electrode when the amount of transferred charge is greater than 25 C[13].Although the selection of graphite electrodes has significantly decreased the rate of electrode loss,the maintenance cycle of gap switches is still determined by the lifetime of the electrodes.The mass loss of electrodes still restricts the development of long-lifetime spark gap switches.

    The mechanism of thermal erosion of graphite has been studied for a long time.Graphite has a layered structure,and van der Waals bonds connect each layer.The carbon atoms in each layer are bonded covalently to form a hexagonal network[7].On the surface of graphite electrode,the bond structure is weak at the locations with geometry defects,and carbon atoms can be easily removed under the effect of the arc.Then,a local ablation pit is formed[13].Early studies show the amount of mass loss of graphite electrodes is proportional to the transferred charges[14].The insulating gas composition is also an important factor in determining the mass loss of the electrode.

    Affinito et al[15]studied the phenomena associated with the gas composition in the switch,demonstrating that pure N2is more likely than air to lead to failure of the switch.The same result was obtained by Donaldson et al[16],who found a large amount of amorphous carbon ejected from the graphite surface and accumulated on surrounding insulator for repetitive pulse discharges in pure N2.Vinaricky et al[17]noted that Ar will not react with graphite in the way that air and N2do.Therefore,it has a less destructive effect on the surface morphology of the electrode.For other gases,the damaging effects of SF6are stronger than N2[18].The ablation intensity depends on the ionization state and collision frequency of the discharge gas molecule.

    The high-current arc is a form of local thermodynamic equilibrium(LTE)plasma[19].The numerical analysis of thermal plasmas,taking into gas properties is well established[20].Nowadays,the ‘a(chǎn)tmosphere control method’ is being used as an efficiency optimization strategy in the fields of plasma gas decomposition[21],material modification[22],disinfection and sterilization[23].It has also been widely used in arc welding[24],nano material preparation[25,26]and other fields.In this way,the intensity of the heat source of the arc can be changed.Previous research has shown that the temperature of Ar arcs is lower than those of many other gases,the inert nature of Ar also prevents chemical reactions[27,28].

    Unoxidized graphite vapor will condense as solid carbon powder,which can lead to damage to the insulation of the switch.A volume fraction of 20% O2has been shown to reduce the production of solid carbon by promoting gas-phase oxidation reactions[29].In this work,the erosion characteristics for the traditional dilution gas of 80%N2and the novel dilution gas of 80% Ar,both combined 20% O2,are compared in pulsed discharges.The morphological changes to the graphite electrode are examined on microscopic level by surface morphological analysis and on macroscopic level by mass analysis,revealing the differences in the erosion reactions in the different dilution gases.

    2.Experimental methods

    2.1.Experimental setup

    A high-current discharge platform was built to study electrode ablation.As shown in figure 1,there was a trigger circuit and a main discharge circuit.The main discharge circuit was mainly composed of energy storage capacitors Cs,and damping inductors LD(in which the resistance is RD).The spark gap switch included an insulating shell with epoxy material,inlet and outlet nozzles and a pair of graphite electrodes.A magnetic switch was used to maintain the cathode potential of the spark gap switch when the trigger voltage is generated[30].The working principle of the discharge platform was explained in detail in a previous paper[31].

    Figure 1.Schematic diagram of the experimental apparatus.(a)Electrical circuit,including trigger circuit and main discharge circuit;(b)five different current waveforms under discharge voltages of 1 kV,3 kV,5 kV,7 kV,9 kV;(c)schematic diagram of the shape change of the arc with the pulsed current.

    The energy storage capacitor Csand damping inductor LDconstitute an energy storage module.There were 48 groups of energy storage modules.The capacitance of each module was 110 μF,and the inductance and resistance were 73 μH and 112 mΩ respectively.An inductor LL=73 μH with resistance RL=112 mΩ was chosen as the load.The amplitude of the pulsed current and the amount of transferred charges can be adjusted by changing the voltage across Cs.

    The electrodes of the spark gap switch were made from EDM-AF5 graphite supplied by the POCO company.The surface was polished by ultrafine sandpaper.The properties and geometric parameters of the electrodes are shown in table 1.

    Table 1.Parameters of graphite electrodes.

    Two gas nozzles were connected to the pneumatic circuit,which was a control apparatus of the outer gas cylinders and can be used to control the gas pressure of the switch chamber.The working pressure was set to 230 kPa.

    2.2.Experimental procedure

    The experimental steps used in this paper were as follows:

    (1)The pneumatic circuit was connected to the switch to ensure the chamber is completely sealed.A Pearson Rogowski coil(0.02 mV A?1)was connected between the switch and the load to monitor the discharge current.A Tektronix P6015A was connected to the anode of the switch to monitor the voltage.

    (2)The pre-mixed gas with 80% N2+20% O2was introduced into the pneumatic circuit and circulated for 1 min at a rate of 2 l s?1.Then,the pneumatic circuit was set to ‘internal recycle’,and the pressure was increased to 230 kPa.

    (3)The storage capacitor was charged to the predetermined voltage.The voltage range was 1–9 kV with a step size of 2 kV.The current waveform,shown in figure 1(b),was recorded by a Tektronix DPO4104B oscilloscope.

    (4)After the discharge,the gas in the switch cavity was collected,and measured by a Temet GASMET DX4000 Fourier infrared flue gas analyzer.

    (5)The switch was then disassembled and the electrodes removed.A Carl Zeiss GeminiSEM300 scanning electron microscope(SEM)was used to view the surface morphology.A Keyence VK-X200K laser confocal scanning microscope(LCSM)was used to detect surface roughness.

    (6)The platform was set to ‘a(chǎn)uto run’ mode.Under the rigid condition,repeated discharge was carried out 100 times,and the electrode mass before and after discharge was recorded by an Ohaus AR224CN electronic balance with the accuracy of 0.1 mg.

    (7)The gas was replaced with the 80% Ar+20% O2mixture.The above steps were repeated.

    3.Measurements and analysis of graphite electrode erosion

    The effects of thermal erosion and oxidation on graphite electrodes are analyzed for the two dilution gases.The mass loss of the electrodes is analyzed with the influence of gas composition studied on microscopic and macroscopic levels.

    3.1.Effect of dilution gas on graphite geometric properties

    The micro-morphological characteristics of the ablated area on the graphite surface are measured for the two dilution gases.A charging voltage of 9 kV was used.As shown in figure 1(b),the peak current was 30 kA.The ablated regions after three discharges are shown in figure 2.The shape and location of these regions show that the center of the contact area between the anode and the arc is located at the edge of the electrode.The discharge channel is formed in the area where the electric field distortion is strongest.The center of the ablated region on the cathode is directly above the center of the ablated region on the anode.The ablated spots on the surface can be divided into two types.The region in the center in which erosion is uniform can be called the erosion center(EC).There is also a region with a sputtered texture around the ablation center,which can be attributed to heat flow damage(HFD)around the arc’s periphery.

    Figure 2.Surface morphology of graphite electrode after three discharges at a peak current of 30 kA.

    The morphology changes detected using the SEM are shown in figure 3.The clearly delineated grains visible on the original graphite surface disappear in the HFD and EC area.

    Figure 3.Surface morphology of anode measured by SEM,(a)initial state in the scale of 10 μm,(b)initial state in the scale of 2 μm,(c)HFD area in the scale of 2 μm,(d)EC area in the scale of 2 μm.

    Table 2.Surface roughness of the center and edge of the ablated region in N2 and Ar dilution gases.

    The EC area on the anode is smaller and more uniform than that on the cathode.This may attribute to the movement of the arc attachment location on the cathode.The arc tends to concentrate in the high electrical conductivity region,so the EC area carries more current.When using N2as the dilution gas,the radius of the EC is between 12.2 and 14.1 mm.For Ar dilution gas,the radius of the EC is smaller.However,the region with sputtered texture is larger,and the radius of the HFD area is larger,up to 25.3 mm.The radius of the HFD region when using N2as the dilution gas is between 13.8 and 16.3 mm,which is slightly larger than the EC radius.

    Both the area and roughness of the damaged region can reflect the intensity of the ablation.The difference in the roughness between the anode and cathode is obvious from the LCSM measurements,as shown in table 2.In the table,roughness is expressed as Ra,Rz,and Rkuaccording to the the ISO 4287:1997 standard.Rais the arithmetical mean deviation of the assessed profile,Rzis the maximum peak to valley height of the profile,and Rkuis the kurtosis of the assessed profile.Under the thermal shock of the arc,the surface roughness of both electrodes increases significantly;the changes to the morphology of the cathode are more significant.This demonstrates that the heat flux is distributed unevenly in the sputtering area.The roughness changes on the surface of the anode and cathode are lower in the Ar dilution gas than in N2,which indicates that the heat flux at the contact area is smaller in the Ar case.

    3.2.Effect of dilution gas on the chemical reaction of graphite

    The solid graphite forms carbon vapor when it is heated to the sublimation point.The vapor enters the arc column,and reacts with the ionized gas to form gaseous carbon oxides.The concentration of the products CO and CO2is shown in figure 4.The trends of CO and CO2concentration are similar for low transferred charge in different dilution gases,but the CO2concentration increases rapidly in N2dilution gas when the transferred charge exceeds 30 C.

    Figure 4.Variations in CO and CO2 concentrations with the transferred charge.

    After one single discharge at a charging voltage of 9 kV,the composition and concentration of the gaseous products are measured by the Fourier infrared flue gas analyzer.The results are shown in table 3.

    Table 3.Composition and concentration of insulating gas after one discharge with the peak current of 30 kA.

    When Ar is used as the dilution gas,there are no nitrogen oxides formed,and the consumption of O2is lower.HCN is discovered in a high-current graphite-electrode spark gap switch.When the arc temperature rises to about 2773 K,free nitrogen atoms still exist[32],which easily combine with carbon atoms to generate cyanide radicals,including CN and C2N2.CN is not stable at room temperature[33],and as the temperature decreases,it reacts with O2,as shown in formulas(1)and(2).However,HCN is stable,and can be formed by reactions with water vapors,as shown in formula(3).The detection of HCN and CH4indicates that water vapor is present in the chamber.

    In general,the oxidation of carbon vapor is incomplete,and it quickly reaches a ‘hypoxic’ state.Therefore,unoxidized carbon vapor condenses as solid carbon,which remains in the switch chamber in the form of powder or aerosol[29].

    3.3.Effect of dilution gas on mass loss of graphite electrodes

    The carbon in the gaseous products is mainly in the form of CO2and CO.The mass of carbon in these gases can be expressed as

    where MCis the relative molecular mass of carbon,V is the volume of the switch chamber,T is the temperature,and P is the pressure in Pa.

    The thermal erosion effect of the arc on the graphite electrode is manifested by the removal of graphite particles.In the contact area between arc and electrode,graphite particles escape from the close-packed structure,leaving pits of micro-scale diameter.When the discharge time is increased,the steep edges of the pit become vulnerable to be shocked,which increases the area of the pit.The surface of the electrode subsides when the switch is discharged tens of thousands of times.The gap distance between the anode and cathode gradually increases until it is too long to be triggered.

    The dependence of electrode mass change on the transferred charge was studied[34,35].The switch discharge was repeated 100 times for the transferred charges of 4.6 C,14.6 C,24.5 C,35.1 C and 45.3 C to measure the mass loss of the electrode.The total mass of the cathode and anode before and after discharges was measured by an electronic balance,and the mass of carbon in the gas phase was calculated by equation(4).

    As shown in figure 5,in the same discharge atmosphere,the mass loss of the electrode is approximately linearly related to the amount of transferred charge.By linear fitting,it is found that the mass-loss rate of the electrode in the dilution gas of Ar is 0.0557 mg C?1,64% lower than that for N2.According to the conservation of mass,the mass of residual solid carbon is the difference between the sublimated mass of graphite and the mass of carbon in the gas phase.The residual solid carbon can be reduced by 70%–85%by replacing N2by Ar dilution gas.

    Figure 5.The dependence of carbon mass lost from the electrodes and in the gas phase on the amount of transferred charge for N2 and Ar dilution gases.

    4.Discussion

    4.1.Sublimation of the graphite electrode

    The pulsed discharge is a strongly transient process.As shown in figure 1,the overvoltage of the trigger system breaks down the insulating gas to form the initial arc channel.The arc current rises to peak value in 0.6 ms.As the gas surrounding the arc column was ionized,the radius of the discharge channel increases.New spots on the cathode emitted more electrons during the current rising phase.The new discharge channels that are formed converge to the main ionization channel by the influence of Lorentz force in the arc.The arc channel approximately conforms to the LTE state[36].

    The erosion rate of the electrode depends mainly on the intensity of the heat flux to the electrode[37].For the graphite electrode,there is no liquid phase at the working pressure.The evaporation caused by the heat flux is the main factor affecting the mass loss of the electrode[18].

    The plasma composition data are required in the calculation of the thermodynamic properties of the switching arc.The composition was calculated using the method of minimization of Gibbs free energy,which assumes the existence of local chemical equilibrium,and follows from the assumption of LTE.The required thermodynamic data were obtained from the tabulated by Moore[38]and JANAF Tables[39].

    The oxygen species considered in the calculation were

    Figure 6.Thermodynamic properties and transport coefficients for the N2 and Ar dilution gases at 230 kPa.

    A magneto-hydrodynamic model was established to obtain the temperature distribution of arc plasma.The arc in the model was assumed to be in the LTE state.The mass conservation,momentum conservation,and energy conservation equations were coupled with current continuity to describe the fluid mechanics and current density of the arc.

    The equation of mass conservation is

    The equation of momentum conservation is

    The equation of energy conservation is

    wheretis the time,ρis the mass density,is the velocity,pis the pressure,is the viscous stress tensor,is the current density,is the Lorentz force produced by the selfinduced magnetic field,his the enthalpy,kis the thermal conductivity,cpis the specific heat,Uis the NEC,andkBis the Boltzmann constant.

    The equation of current continuity is

    The magnetic fieldis calculated from the vector potential.

    whereσis the electrical conductivity,φis the electric potential,andμ0is the magnetic permeability in vacuum.

    The maximum current waveform in figure 2 is selected as the input condition of the calculation model.The temperature distribution is shown in figure 7.At the time 0.1 ms,when the current is rising,the temperature of the N2arc is obviously higher than the that in the Ar arc.When the current reaches its peak value at 0.6 ms,the N2arc is more constricted.The hightemperature regions near the electrodes are concentrated in small spots.In contrast,the temperature is more uniform in Ar.At the time of 1.8 ms,the temperature in the Ar dilution gas is higher because of the lower thermal conductivity of the Ar.

    Figure 7.Temperature distribution in the arc column.(a)Temperature in the dilution gas of 80% N2,(b)temperature in the dilution gas of 80% Ar.

    The thermal conduction from the arc and the impact of electrons plays a major role in anode heating[44].The heat flux to the anodeSaand on the cathodeSccan be expressed as

    wherejeis the electron current density,andjiis the ion current density.The current in the arc channel is mainly provided by the electrons emitted from the cathode.Here,is the work function of graphite,which is 4.7 eV[45],andis the energy required for the electron to escape from the electrode surface.Viis the cathode sheath voltage.The termdescribes the radiation heat loss of the electrode,whereTis the temperature of the arc surface,εais the surface radiation coefficient,andαis the Stefan–Boltzmann constant.

    The total thermal energy density is the integral of the heat flux over time,and is shown in figure 8.The absorption rate of heat in the graphite sublimation area is higher than the cooling rate,so heat accumulates rapidly on the surface.When the absorbed energy exceeds the latent heat of the graphite,the graphite begins to sublimate from the surface.Figure 8 shows that density of the heat energy in Ar is lower than that in N2,so the damage to the electrodes is expected to be less.

    Figure 8.Thermal energy heat density on the anode.

    4.2.Oxidation of the graphite vapor

    The diffused graphite vapor is oxidized to form a C–O chemical bond and exists as gaseous CO and CO2.The specific reactions include:

    Carbon oxides are the main gaseous product in the spark gap switch,and their amount increases as the arc cools more slowly.In addition,when the dilution gas is N2,N2and O2are dissociated in the arc,and the atoms react during the cooling phase.Nitrogen oxides are found in the dilution gas of N2[46].The nitrogen oxidation reactions consume oxygen and energy.

    However,changing the insulating gas affects the breakdown voltage of the switch.According to the experimental tests,at a pressure of 230 kPa,the breakdown voltage of the traditional dilution gas of N2is around 43 kV.In the dilution gas of Ar,the breakdown voltage is about 22 kV.If the switch should apply to the same insulation voltage of 43 kV using the dilution gas of Ar,the working pressure should be increased to 570 kPa.Figure 9 shows the mass loss of the electrodes and the amount of carbon in the gas under the different pressures of Ar dilution gas.When the pressure rises,the mass loss of the electrodes increases,but more carbon vapor is oxidized due to the higher oxygen atoms density.The residual solid carbon decreases about 42%compared with the traditional N2dilution gas.It appears that the Ar dilution gas still has advantages at high working pressure.

    Figure 9.Comparison of mass loss of the electrodes and mass of carbon element in the discharge gas under different dilution gases and different working pressures.

    In the different application scenarios,the dielectric strength of the switch is different.So it is hope that the different gas selection schemes can match different parameter conditions.The combined influence of working pressure and gas composition on the mass loss of graphite electrodes needs to be studied further.

    5.Conclusions

    The effect of insulating gas composition on the changes of the surface and the mass loss of the graphite electrodes is studied in mixture of the dilution gas of N2and Ar with 20%O2.The result shows that the gas composition is an important factor in the design of long-life,high-coulomb spark gap switches.The conclusions can be summarized as follows.

    (1)Under the thermal shock of the pulsed arc,the sublimation of the graphite leads to the mass loss from the electrode.The composition of arcing gas has a significant effect on the extent and intensity of the erosion.The roughness of the erosion surface is smaller in the Ar dilution gas than that in the N2dilution gas.

    (2)The mass-loss rate of graphite electrode in traditional N2–O2insulating gas is 0.1531 mg C?1.After replacing N2with Ar,under the same discharge conditions,the mass loss rate of the graphite electrode is reduced by 64%,which prolonged the working life of the electrode.

    (3)The temperature and heat transfer characteristics of the arc are determined by the thermodynamic parameters and transport coefficients of the discharge gas.Argon has low thermal conductivity and low specific heat,which will help to weaken the thermal flux to the electrode.

    There is no guarantee that the mixture of Ar with 20%O2is optimal,but it can prove that Ar has better performance than N2at the breakdown voltage below 9 kV.Further,the optimal insulating gas composition and the effect of pressure need to be further researched.

    Acknowledgments

    The study was supported by National Natural Science Foundation of China(Nos.51777082 and 52077091),and Chinese Scholarship Council(No.201906160101).The authors thank Huazhong University of Science and Technology Analytical and Testing Center for providing testing equipment.

    猜你喜歡
    李黎
    A comparative study of the self-propelled jumping capabilities of coalesced droplets on RTV surfaces and superhydrophobic surfaces
    依戀祖國(guó)
    新學(xué)期新氣象(上)
    In April, I Was Murmuring Your Name
    我是這么拿的第二名
    愛你(2017年21期)2017-11-14 18:28:39
    我是這么拿的第二名
    好倒霉呀!
    美人如玉王力可
    BOSS臻品(2016年8期)2016-08-01 22:46:21
    BEAUTIFUL美麗的秘密
    BOSS臻品(2015年9期)2015-09-16 01:53:36
    人生若只如初見 王韻壹
    BOSS臻品(2015年6期)2015-06-17 08:07:24
    精品久久蜜臀av无| 久久亚洲精品不卡| 久久中文字幕一级| 极品人妻少妇av视频| 一级毛片女人18水好多| 一级黄色大片毛片| 国产主播在线观看一区二区| 最近最新免费中文字幕在线| 亚洲国产中文字幕在线视频| 日韩国内少妇激情av| 婷婷精品国产亚洲av在线| 在线免费观看的www视频| av超薄肉色丝袜交足视频| 人妻久久中文字幕网| 午夜91福利影院| 亚洲精品成人av观看孕妇| 亚洲自拍偷在线| 日韩欧美一区二区三区在线观看| 久久香蕉国产精品| 久久精品91无色码中文字幕| 视频区图区小说| xxx96com| 国产一区二区三区视频了| 国产欧美日韩一区二区三| 一个人观看的视频www高清免费观看 | 在线十欧美十亚洲十日本专区| 91大片在线观看| 免费av中文字幕在线| 亚洲精品av麻豆狂野| 老司机午夜十八禁免费视频| 精品久久蜜臀av无| 国产1区2区3区精品| 久久 成人 亚洲| 久热爱精品视频在线9| 伊人久久大香线蕉亚洲五| 国产xxxxx性猛交| 一区二区日韩欧美中文字幕| 亚洲性夜色夜夜综合| 一级,二级,三级黄色视频| 日韩人妻精品一区2区三区| 久久久久久大精品| 欧美日韩亚洲高清精品| 国产真人三级小视频在线观看| 亚洲专区中文字幕在线| 国产欧美日韩一区二区三| 日本a在线网址| 国产麻豆69| 黑人欧美特级aaaaaa片| 国产精品国产高清国产av| 天天躁夜夜躁狠狠躁躁| 巨乳人妻的诱惑在线观看| 他把我摸到了高潮在线观看| 长腿黑丝高跟| 露出奶头的视频| av在线天堂中文字幕 | 欧美乱妇无乱码| 久久午夜综合久久蜜桃| av免费在线观看网站| 婷婷六月久久综合丁香| 一区二区三区精品91| 精品第一国产精品| 午夜精品国产一区二区电影| 日韩视频一区二区在线观看| 亚洲男人的天堂狠狠| 亚洲色图av天堂| 一区福利在线观看| 久久亚洲真实| 久久热在线av| 亚洲自偷自拍图片 自拍| 一边摸一边抽搐一进一出视频| 亚洲精品一卡2卡三卡4卡5卡| 亚洲欧美激情在线| 亚洲精品在线美女| 村上凉子中文字幕在线| 在线免费观看的www视频| 国产精品av久久久久免费| 日韩大码丰满熟妇| 97超级碰碰碰精品色视频在线观看| 搡老乐熟女国产| 好看av亚洲va欧美ⅴa在| 在线观看66精品国产| 日韩欧美国产一区二区入口| 在线永久观看黄色视频| 精品国产乱码久久久久久男人| 性欧美人与动物交配| 中文字幕精品免费在线观看视频| 日韩精品免费视频一区二区三区| tocl精华| 美国免费a级毛片| 成人三级黄色视频| 黄片大片在线免费观看| 亚洲欧美日韩高清在线视频| 国产男靠女视频免费网站| 少妇裸体淫交视频免费看高清 | 亚洲精品国产区一区二| 国产一区二区三区在线臀色熟女 | 日韩欧美一区二区三区在线观看| 日日爽夜夜爽网站| 国产精品 欧美亚洲| 免费观看精品视频网站| 99国产精品一区二区三区| 亚洲欧美日韩无卡精品| 桃红色精品国产亚洲av| 欧美乱码精品一区二区三区| 亚洲片人在线观看| 99热国产这里只有精品6| 中文字幕av电影在线播放| 好男人电影高清在线观看| 国产精品自产拍在线观看55亚洲| 亚洲成a人片在线一区二区| 看黄色毛片网站| 嫩草影视91久久| 叶爱在线成人免费视频播放| 久久久久精品国产欧美久久久| 曰老女人黄片| 99在线人妻在线中文字幕| 免费在线观看亚洲国产| 91九色精品人成在线观看| 亚洲自拍偷在线| 天堂中文最新版在线下载| 久久久国产一区二区| 少妇 在线观看| 中文字幕人妻丝袜一区二区| 在线观看日韩欧美| 不卡av一区二区三区| 欧美黑人精品巨大| 国产熟女午夜一区二区三区| 91在线观看av| 久久精品国产清高在天天线| 亚洲人成电影免费在线| 国产伦一二天堂av在线观看| 精品一品国产午夜福利视频| 男人操女人黄网站| 高清欧美精品videossex| 午夜福利欧美成人| 日韩高清综合在线| 丰满迷人的少妇在线观看| 悠悠久久av| 亚洲人成77777在线视频| 色婷婷av一区二区三区视频| 97人妻天天添夜夜摸| 亚洲视频免费观看视频| 精品国产一区二区久久| 国产精品乱码一区二三区的特点 | 交换朋友夫妻互换小说| 亚洲色图综合在线观看| 欧美在线黄色| 18禁裸乳无遮挡免费网站照片 | 久久久国产成人免费| 国产亚洲精品综合一区在线观看 | 黄色怎么调成土黄色| 久久久久久人人人人人| 交换朋友夫妻互换小说| 最近最新中文字幕大全电影3 | 91精品国产国语对白视频| 亚洲成人免费av在线播放| 久久久久久久久久久久大奶| 国产99久久九九免费精品| 99久久人妻综合| 日本a在线网址| 成人18禁在线播放| 妹子高潮喷水视频| 免费观看人在逋| 一级片'在线观看视频| 国产亚洲av高清不卡| 天堂√8在线中文| 极品人妻少妇av视频| 欧美日韩瑟瑟在线播放| 国产精品国产av在线观看| 成人手机av| 一级毛片高清免费大全| 男女做爰动态图高潮gif福利片 | 精品午夜福利视频在线观看一区| 精品国产亚洲在线| av在线天堂中文字幕 | 亚洲成人免费av在线播放| 免费人成视频x8x8入口观看| 国产高清国产精品国产三级| 午夜影院日韩av| 母亲3免费完整高清在线观看| 黄片播放在线免费| 成人三级黄色视频| 国产av又大| 午夜91福利影院| 他把我摸到了高潮在线观看| 夜夜夜夜夜久久久久| 欧美精品亚洲一区二区| 成人免费观看视频高清| 欧美中文综合在线视频| 国产深夜福利视频在线观看| 99riav亚洲国产免费| 一级毛片高清免费大全| 80岁老熟妇乱子伦牲交| 免费在线观看黄色视频的| 日韩免费高清中文字幕av| 欧美黑人精品巨大| av福利片在线| 亚洲精品一卡2卡三卡4卡5卡| 国产精品九九99| 国产成人欧美在线观看| 不卡一级毛片| 51午夜福利影视在线观看| 国产人伦9x9x在线观看| 精品久久久久久,| 视频在线观看一区二区三区| 黑人操中国人逼视频| 国产不卡一卡二| 久久精品亚洲av国产电影网| 国产精品乱码一区二三区的特点 | 亚洲一卡2卡3卡4卡5卡精品中文| a级毛片黄视频| 国产黄a三级三级三级人| 老司机午夜福利在线观看视频| 亚洲欧美激情在线| 亚洲av第一区精品v没综合| 9色porny在线观看| 国产不卡一卡二| 制服诱惑二区| 亚洲一区中文字幕在线| 国产成人精品在线电影| 一级毛片女人18水好多| 国产免费av片在线观看野外av| 精品一区二区三区四区五区乱码| 麻豆久久精品国产亚洲av | 超碰97精品在线观看| 亚洲欧美一区二区三区黑人| 一区二区三区国产精品乱码| 日韩欧美三级三区| 中出人妻视频一区二区| 日韩免费av在线播放| 男女下面插进去视频免费观看| 黄色丝袜av网址大全| 久久欧美精品欧美久久欧美| 真人一进一出gif抽搐免费| 757午夜福利合集在线观看| 在线天堂中文资源库| 国产激情久久老熟女| 久久人人97超碰香蕉20202| 午夜福利在线观看吧| 欧美成人免费av一区二区三区| 老汉色∧v一级毛片| 亚洲aⅴ乱码一区二区在线播放 | а√天堂www在线а√下载| 久久人妻av系列| 欧美日本亚洲视频在线播放| 嫁个100分男人电影在线观看| 91九色精品人成在线观看| 亚洲精品一二三| av片东京热男人的天堂| 国产成人欧美| av超薄肉色丝袜交足视频| 丝袜美足系列| 天堂动漫精品| av天堂久久9| 国产亚洲精品第一综合不卡| 男人操女人黄网站| 午夜福利影视在线免费观看| 在线天堂中文资源库| www.精华液| 韩国精品一区二区三区| 久久久久九九精品影院| 欧美日韩av久久| 大陆偷拍与自拍| 久久热在线av| 日韩欧美一区二区三区在线观看| 成人三级做爰电影| 国产三级在线视频| 欧美日韩一级在线毛片| 久久香蕉精品热| 中文字幕人妻丝袜制服| 亚洲中文日韩欧美视频| 欧美日本中文国产一区发布| 黄色a级毛片大全视频| 美女午夜性视频免费| 欧美日韩一级在线毛片| 激情视频va一区二区三区| 欧美激情高清一区二区三区| 久久 成人 亚洲| 亚洲激情在线av| 久久久国产一区二区| 自线自在国产av| 国产精华一区二区三区| 欧美精品啪啪一区二区三区| 极品教师在线免费播放| 国产1区2区3区精品| 午夜免费观看网址| 精品久久久久久成人av| 啦啦啦免费观看视频1| 黄色片一级片一级黄色片| 亚洲五月色婷婷综合| 中文字幕人妻丝袜制服| netflix在线观看网站| 欧美老熟妇乱子伦牲交| √禁漫天堂资源中文www| 欧美+亚洲+日韩+国产| a在线观看视频网站| 又黄又爽又免费观看的视频| 叶爱在线成人免费视频播放| 美女福利国产在线| 国产午夜精品久久久久久| 搡老乐熟女国产| 免费少妇av软件| 人人妻人人澡人人看| 亚洲五月色婷婷综合| 大型av网站在线播放| 精品卡一卡二卡四卡免费| 91精品三级在线观看| 高清在线国产一区| 久久精品aⅴ一区二区三区四区| 欧美丝袜亚洲另类 | 久久久水蜜桃国产精品网| 亚洲精品国产色婷婷电影| 国产成人系列免费观看| 色婷婷av一区二区三区视频| 乱人伦中国视频| 99在线视频只有这里精品首页| 亚洲色图av天堂| 欧美激情高清一区二区三区| 久久精品亚洲精品国产色婷小说| 嫩草影院精品99| 欧美最黄视频在线播放免费 | 国产精品久久电影中文字幕| 一级a爱片免费观看的视频| 精品久久久久久电影网| 国产男靠女视频免费网站| 精品熟女少妇八av免费久了| 亚洲成人久久性| 大香蕉久久成人网| 久久香蕉精品热| 一个人观看的视频www高清免费观看 | 国产成人免费无遮挡视频| 宅男免费午夜| 91大片在线观看| 精品国产美女av久久久久小说| 精品久久蜜臀av无| 搡老岳熟女国产| 亚洲一区高清亚洲精品| 亚洲精品一区av在线观看| 久久久久久久久久久久大奶| 国产日韩一区二区三区精品不卡| 丰满饥渴人妻一区二区三| 国产91精品成人一区二区三区| 国产黄色免费在线视频| 久久热在线av| 亚洲国产精品999在线| 亚洲精品国产精品久久久不卡| 久久香蕉激情| 久久精品国产综合久久久| 久久亚洲精品不卡| 桃红色精品国产亚洲av| 久久青草综合色| 日韩 欧美 亚洲 中文字幕| a在线观看视频网站| 高清欧美精品videossex| 美女高潮喷水抽搐中文字幕| 久久久久国产一级毛片高清牌| 成人特级黄色片久久久久久久| 精品国产乱码久久久久久男人| 精品久久久久久久久久免费视频 | 女生性感内裤真人,穿戴方法视频| 国产熟女午夜一区二区三区| 一二三四在线观看免费中文在| 91成年电影在线观看| 校园春色视频在线观看| 久久精品91蜜桃| 深夜精品福利| 成年人黄色毛片网站| 精品国内亚洲2022精品成人| 欧美日韩亚洲高清精品| 老汉色av国产亚洲站长工具| 校园春色视频在线观看| 免费在线观看影片大全网站| a级毛片在线看网站| 午夜免费鲁丝| 日韩欧美国产一区二区入口| 亚洲国产欧美一区二区综合| 成人精品一区二区免费| 久久精品国产清高在天天线| bbb黄色大片| 亚洲欧美日韩另类电影网站| 日韩三级视频一区二区三区| 欧美一级毛片孕妇| 久久婷婷成人综合色麻豆| 精品久久久久久久毛片微露脸| 国产激情久久老熟女| 精品电影一区二区在线| 91av网站免费观看| 制服人妻中文乱码| 成人av一区二区三区在线看| 90打野战视频偷拍视频| 少妇的丰满在线观看| 777久久人妻少妇嫩草av网站| 亚洲av美国av| 老司机午夜十八禁免费视频| 亚洲av成人av| 久久性视频一级片| 高清欧美精品videossex| 欧美激情 高清一区二区三区| 国产麻豆69| 国产精品1区2区在线观看.| videosex国产| 老司机福利观看| 不卡一级毛片| 亚洲七黄色美女视频| 欧美久久黑人一区二区| 亚洲国产精品999在线| 欧美色视频一区免费| 麻豆一二三区av精品| 国产一区在线观看成人免费| 国产精华一区二区三区| 黑人操中国人逼视频| 国产精品久久久人人做人人爽| 欧美色视频一区免费| 日韩国内少妇激情av| 男女床上黄色一级片免费看| 久久久国产成人精品二区 | 欧美激情极品国产一区二区三区| 99久久综合精品五月天人人| 天堂俺去俺来也www色官网| 久久久精品欧美日韩精品| 成人18禁高潮啪啪吃奶动态图| 亚洲精品一区av在线观看| 电影成人av| 国产亚洲精品综合一区在线观看 | 欧美日韩福利视频一区二区| 久久久久久人人人人人| 黄色片一级片一级黄色片| 97碰自拍视频| 亚洲免费av在线视频| 午夜福利欧美成人| 欧美大码av| 亚洲av美国av| 中文字幕av电影在线播放| 欧美日韩中文字幕国产精品一区二区三区 | 在线观看免费午夜福利视频| 天天躁夜夜躁狠狠躁躁| 中文字幕色久视频| 国产高清激情床上av| 正在播放国产对白刺激| 88av欧美| 亚洲欧美一区二区三区久久| 少妇裸体淫交视频免费看高清 | 一个人免费在线观看的高清视频| 在线观看舔阴道视频| 久久人妻福利社区极品人妻图片| 老熟妇乱子伦视频在线观看| 青草久久国产| 日韩精品青青久久久久久| 色婷婷av一区二区三区视频| 在线观看一区二区三区激情| 一级片'在线观看视频| 少妇被粗大的猛进出69影院| 黄片播放在线免费| 成人亚洲精品一区在线观看| 久久久久国产精品人妻aⅴ院| 亚洲国产欧美一区二区综合| 动漫黄色视频在线观看| av网站免费在线观看视频| 99国产精品一区二区三区| cao死你这个sao货| 一边摸一边做爽爽视频免费| 在线观看免费午夜福利视频| 丁香欧美五月| 丰满人妻熟妇乱又伦精品不卡| 欧美激情 高清一区二区三区| xxxhd国产人妻xxx| 国产精品日韩av在线免费观看 | 久久久国产欧美日韩av| √禁漫天堂资源中文www| 18禁国产床啪视频网站| 窝窝影院91人妻| 国产一区二区三区在线臀色熟女 | 国产精品偷伦视频观看了| 日本黄色视频三级网站网址| 色精品久久人妻99蜜桃| 国产精品乱码一区二三区的特点 | 在线av久久热| 久久亚洲精品不卡| 亚洲在线自拍视频| 露出奶头的视频| 两性午夜刺激爽爽歪歪视频在线观看 | 黄色毛片三级朝国网站| 亚洲九九香蕉| 亚洲精品一卡2卡三卡4卡5卡| 国产精品偷伦视频观看了| 天天躁夜夜躁狠狠躁躁| 精品午夜福利视频在线观看一区| 国产成年人精品一区二区 | 免费人成视频x8x8入口观看| 亚洲 欧美 日韩 在线 免费| 视频区图区小说| 一级a爱片免费观看的视频| 亚洲欧洲精品一区二区精品久久久| 欧美激情极品国产一区二区三区| 免费在线观看视频国产中文字幕亚洲| 麻豆av在线久日| 免费日韩欧美在线观看| 久久中文字幕一级| 精品高清国产在线一区| 免费高清视频大片| 在线播放国产精品三级| 精品一区二区三卡| 亚洲免费av在线视频| 日本免费一区二区三区高清不卡 | 欧美成狂野欧美在线观看| 国产精品99久久99久久久不卡| 51午夜福利影视在线观看| 国产熟女xx| 琪琪午夜伦伦电影理论片6080| 久久久久久久久免费视频了| 在线永久观看黄色视频| 国产激情久久老熟女| 国产又爽黄色视频| 99久久精品国产亚洲精品| 亚洲全国av大片| 制服诱惑二区| 不卡一级毛片| 成人手机av| 日本黄色视频三级网站网址| 国产精品秋霞免费鲁丝片| 国产真人三级小视频在线观看| 精品第一国产精品| 黄色视频,在线免费观看| 国产精品亚洲一级av第二区| 国产成人av教育| 在线观看一区二区三区| 国产精品电影一区二区三区| 亚洲欧美一区二区三区黑人| 亚洲 欧美 日韩 在线 免费| 免费不卡黄色视频| 亚洲精品一二三| 男男h啪啪无遮挡| 高清黄色对白视频在线免费看| 夜夜看夜夜爽夜夜摸 | 激情视频va一区二区三区| x7x7x7水蜜桃| 午夜a级毛片| 人妻丰满熟妇av一区二区三区| 丝袜美腿诱惑在线| 国产视频一区二区在线看| 黄色片一级片一级黄色片| 午夜a级毛片| 成人精品一区二区免费| 国产精品 国内视频| 午夜视频精品福利| 婷婷丁香在线五月| 欧美黑人欧美精品刺激| 久久久久久久久中文| 一级片免费观看大全| 天天躁夜夜躁狠狠躁躁| 欧美中文综合在线视频| 琪琪午夜伦伦电影理论片6080| 岛国在线观看网站| 麻豆国产av国片精品| 无遮挡黄片免费观看| 最新美女视频免费是黄的| 黄色视频,在线免费观看| 国产精品亚洲一级av第二区| 十分钟在线观看高清视频www| 国产精品免费一区二区三区在线| 在线观看舔阴道视频| 午夜福利在线观看吧| 嫩草影视91久久| √禁漫天堂资源中文www| 女生性感内裤真人,穿戴方法视频| 激情在线观看视频在线高清| 国产极品粉嫩免费观看在线| 国产蜜桃级精品一区二区三区| 黄色成人免费大全| 视频区图区小说| 国产不卡一卡二| 黄色怎么调成土黄色| 最近最新中文字幕大全免费视频| 精品电影一区二区在线| svipshipincom国产片| 琪琪午夜伦伦电影理论片6080| 亚洲午夜精品一区,二区,三区| 在线观看免费午夜福利视频| 精品一品国产午夜福利视频| 色综合婷婷激情| 免费看十八禁软件| 国产野战对白在线观看| 国产av一区在线观看免费| 久久久久久久午夜电影 | 免费在线观看黄色视频的| 久热这里只有精品99| 欧美乱码精品一区二区三区| 高清毛片免费观看视频网站 | 日韩有码中文字幕| 男女下面进入的视频免费午夜 | 18禁黄网站禁片午夜丰满| 亚洲第一av免费看| 欧洲精品卡2卡3卡4卡5卡区| 久久天堂一区二区三区四区| 精品国产乱子伦一区二区三区| 叶爱在线成人免费视频播放| 日本黄色视频三级网站网址| 久久久国产一区二区| 亚洲中文av在线| 国产精品免费视频内射| 亚洲欧美激情综合另类| 免费在线观看完整版高清| a级毛片黄视频| 精品国产一区二区久久| 亚洲第一青青草原| a级毛片黄视频| 久久这里只有精品19| 精品乱码久久久久久99久播| 老司机亚洲免费影院| 18禁黄网站禁片午夜丰满| 国产欧美日韩一区二区三| 美国免费a级毛片| tocl精华| 久久精品aⅴ一区二区三区四区| 一本综合久久免费|