• <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 ?

    THREE-DIMENSIONAL NUMERICAL SIMULATION OF THERMALHYDRAULIC PERFORMANCE OF A CIRCULAR TUBE WITH EDGEFOLD-TWISTED-TAPE INSERTS*

    2010-05-06 08:22:15CUIYongzhang

    CUI Yong-zhang

    School of Energy and Power Engineering, Shandong University, Jinan 250061, China

    School of Thermal Engineering, Shandong Jianzhu University, Jinan 250101, China, E-mail: cyz@sdjzu.edu.cn

    TIAN Mao-cheng

    School of Energy and Power Engineering, Shandong University, Jinan 250061, China

    THREE-DIMENSIONAL NUMERICAL SIMULATION OF THERMALHYDRAULIC PERFORMANCE OF A CIRCULAR TUBE WITH EDGEFOLD-TWISTED-TAPE INSERTS*

    CUI Yong-zhang

    School of Energy and Power Engineering, Shandong University, Jinan 250061, China

    School of Thermal Engineering, Shandong Jianzhu University, Jinan 250101, China, E-mail: cyz@sdjzu.edu.cn

    TIAN Mao-cheng

    School of Energy and Power Engineering, Shandong University, Jinan 250061, China

    (Received November 30, 2009, Revised July 7, 2010)

    Three-dimensional numerical simulations and experiments were carried out to study the heat transfer characteristics and the pressure drop of air flow in a circular tube with Edgefold-Twisted Tape (ETT) inserts and with classic Spiral-Twisted-Tape (STT) inserts of the same twist ratio. The RNG turbulence model for mildly swirling flows, the enhanced wall treatment for low Reynolds numbers, and the SIMPLE pressure-velocity method were adopted to simulate the flow and heat transfer characteristics. Within the range of Reynolds number from 2 500 to 9 500 and the twist ratio y from 5.4 to 11.4, the Nusselt number of the tube with ETT inserts is found to be 3.9% - 9.2% higher than that with STT inserts, and the friction factor of the tube with ETT inserts is 8.7% -74% higher than that of STT inserts. The heat enhancement is due to higher tangential velocity and asymmetrical velocity profile with the increase and decrease of the periodic velocity within an edgefold length. It is found that main factors affecting the heat transfer of ETT inserts are the twist angle and the gap width between the tube and inserts. A larger twist angle leads to a higher tangential velocity, and larger Nusselt number and friction factor. The thermal-hydraulic performance slowly decreases as the twist angle increases. The gap width between tube and inserts has a significant influence on the heat transfer, while little influence on pressure drops. The thermal-hydraulic performance increases in average by 124% and 140% when the gap width reduces from 1.5 mm to 1.0 mm and 0.5 mm. The larger the gap width, the higher velocity through the gap will be, which would reduce the main flow velocity and tangential velocity. So a small gap is desirable. Comparing experimental and numerical results at variable air flow and tube wall temperature, the numerical results are found to be in a reasonable agreement with the experiment results, with difference of the Nusselt number in a range of 1.6% - 3.6%, and that of the friction factor in a range of 8.2% - 13.6%.

    heat transfer enhancement, Edgefold-Twisted-Tape (ETT), Spiral-Twisted-Tape (STT), thermal-hydraulic performance

    1. Introduction

    The heat transfer enhancement technology has been developed rapidly and employed in a wide variety of engineering problems, such as condensing gas boiler and water heater. Tape inserts are frequently used to reduce exhaust flue temperature and to make heat exchangers compact. There are mainly five effects of twisted tape inserts in the heat transfer enhancement: (1) increase in flow velocity, (2) decrease in hydraulic diameter, (3) increase in flow path, (4) secondary motion, (5) fin contribution, if tape inserts are in good thermal contact with the tube wall.

    The swirl flow in a tube was suggested by Kreit and Margolis (1959), and most of the swirl flows were created by long and short classic Spiral Twisted-Tape (STT) inserts[1-22]with and without holes[1-5], regularly spaced tape inserts[6-7], louvered strip[8]and wire coil inserts[9]. Studies were carried out[1,4,10-12]the heat transfer characteristics and pressure drops in circular tubes with twisted-tape inserts under fully turbulent flow conditions. Fahed[10]studied the effect of the tube-tape clearance on the heat transfer in fully developed turbulent flow in a horizontal isothermal tube, and it is shown that the heat transfer enhancement increases as the tube-tape clearance decreases. Recently, three-dimensional numerical analyses were carried out to study the thermal-hydraulic characteristics of the flow inside a circular tube with different twisted-tape inserts[12-21,23]. The RNG k?ε turbulent model[24,25]was used to simulate self-rotating STT inserts by Zhang[21], and to model STT inserts and perforated and jagged twisted tapes by Rahimi et al.[2]. Results show that the higher turbulence intensity of the fluid close to the wall and the tangential velocities were mainly attributed for the heat transfer enhancement. Eiamsa-Ard[12]adopted the SIMPLE technique, together with four turbulence models to simulate the flow in a circular tube induced by means of loose-fit twisted tapes, and the numerical results show that the shear stress transport k?ω turbulence models give the most consistent results with those of Manglik and Bergles.

    Table 1 Geometrical parameters and the twist ratio of inserts

    The transition flow regimes in a tube with twisted-tape inserts, specially Edgefold-Twisted-Tape (ETT) inserts, were not well studied. The arrangement enhances the structural stability and makes it possible to adopt thinner stainless inserts. This article presents three-dimensional numerical analyses and experiments on heat transfer characteristics and pressure drops of the air flow in a circular tube fitted with ETT inserts and STT inserts under constant wall temperature.

    2. Physical model and mathematical analysis

    2.1 Circular tube with inserts

    A circular tube with ETT inserts is shown in Fig.1. The tube’s inner diameter is D. The main geometrical parameters of ETT inserts include edgefold length ( L ), twist angle (A), tape width ( B), and tape thickness (δ). The twist angle is a rotation within an edgefold length, with H beingothe twist pitch and n the edgefold number within 360, the gap between tubes and inserts (b ) and the twist ratio ( y) can be expressed as:

    Fig.1 Circular tube with edgefold-twisted-tape inserts

    The STT inserts have the same twist ratio and the twist width as the ETT inserts in order to compare the thermal and hydraulic performance. Geometrical parameters and the twist ratio of the investigated inserts are listed in Table 1.

    2.2 Mathematical analysis

    The studied area includes the air between twist tape inserts and inside the tube. The following assumptions are adopted to simplify the physical model: (1) the radiation and natural convection heat transfer can be ignored, (2) the viscosity heating can be ignored, (3) the change of the air composition can be ignored, (4) the twisted-tape surface can be considered as adiabatic, and the conduction along tape inserts can be ignored, (5) no slip motion on tube walland inserts surface, (6) constant wall temperature. For transition turbulent flows, the three-dimensional equations of continuity, momentum, energy, turbulent kinetic energy (k), and the dissipation rate (ε) in the fluid region are as follows:

    Continuity equation:

    The numerical simulation is carried out using Fluent, with RNG k?ε, SIMPLE pressure-velocity coupling algorithm, and the second upwind discretization scheme for momentum, energy, turbulent kinetic energy and dissipation energy. The convergence criterion is satisfied when the residuals of variables are less than 1×10-4except for the energy where a value of 1×10-7is used. Under-relaxation factors of turbulent kinetic energy, turbulent dissipation rate and turbulent viscosity are changed within the range between 0.2 and 0.4, others take the default values. For accounting for the low Reynolds number and the near wall flow, an enhanced wall treatment is also adopted.

    The air inlet is specified with the mass flow rate inlet boundary condition, the air outlet with the pressure outlet boundary condition, the tube wall is a wall with constant temperature and the surface of the inserts is an adiabatic wall.

    2.3 Grid-independence

    The region near the inner tube wall is meshed with refined hexahedron cells, the other regions are meshed with tetrahedron cells, and the grid number is varied with the tube’s inner diameter. The accuracy and the validity of the numerical results are ensured by a careful check of the grid-independence. Table 2 shows the grid numbers and numerical results for inner diameters of 21 mm and 1 m of No.2 ETT inserts, so the internal count of 80 is used to grid all inserts. By using the boundary adaptation on the tube wall and the insert wall and the gradient adaptation on the whole region, the convergence rate is high. The computations are performed on the workstation with Intel Xeon E4505 CPU and the time is in the range of 2 h to 4 h for each case.

    Table2 grid number and numerical results

    3. Numerical results and discussions

    The steps involved in calculating the tubesideheat transfer coefficient and the friction factor from the simulation temperature, flow rate, and pressure drop are outlined below. All intube flow parameters are based on the inner diameter of the empty tube, all fluid properties are evaluated at the length average bulk temperature, unless otherwise indicated.

    The log-mean temperature difference, ΔTm, is defined as

    where Tinis the inlet temperature of air, Toutis the outlet temperature of air, Twis the inner wall temperature.

    The Nusselt number, Nu, is defined as

    where α is the convective heat transfer coefficient, λ is the thermal conductivity, Q is the heat transfer rate and F is the heat transfer area.

    The friction factor, f, is calculated from the following equation

    where ΔP is the pressure drop in the entire length ( Ltotal), u is the bulk averaged velocity.

    The thermal-hydraulic performance with different inserts, φ, is defined as

    3.1 STT and ETT insert performance

    Figures 2 and 3 show the Nusselt number and the friction factor of a tube with No.5 and No.7 STT inserts and No.2 and No.4 ETT inserts at the same inlet and tube wall temperature, with gap width b of 0.5 mm. It can be seen that the Nusselt number and the friction factor of the tube with ETT inserts are larger than those with STT inserts with the same twist ratio. The Nusselt number of No.2 ETT inserts is 3.9% greater than that of No.5 STT inserts, the Nusselt number of No.4 ETT inserts is 9.2% greater than that of No.7 STT inserts in average. The friction factor of No.2 ETT inserts is 8.7% larger than that of No.5 STT inserts, the friction factor of No.4 ETT inserts is 74% larger than that of No.7 STT inserts in average. The thermal-hydraulic performance φ of No.2 ETT inserts is 1.01 on the base of No.5 STT inserts, that of No.4 ETT inserts is 0.91 on the base of No.7 STT inserts.

    Fig.2 Nusselt number of tube with S TT and ETT inserts

    Fig.3 Friction factor of tube with STT and ETT inserts

    The enhancement is mainly due to a higher tangential velocity and the main flow velocity profile. The tube with STT inserts has a symmetrical profile with the same main velocity and tangential velocity, but the tube with ETT inserts has asymmetrical velocity magnitude and tangential velocity profiles, as shown in Figs.4 and 5. It can be seen that the velocity of ETT assumes a periodic variation within an edgefold length, the velocity first increases and later decreases on one side, but it first decreases and later increases on the other side The tangential velocity has the same variation trend. Such velocity variations help gas mixing.than that for twist angle 20o, and the Nusselt number for twist angle 20ois 1.9% larger than that for twist angle 15oin average. The friction factor for twist angle 30ois 45% greater than that for twist angle 20o, and the friction factor for twist angle 20ois 0.4% greater than that for twist angle 15o. Therefore, under the transition flow, the flow disturbance increases with the increase of the twisted ratio, and the low twisted ratio tape has a strong effect. On the base of 15otape, the thermal-hydraulic performance φ of 20oand 30otapes is 0.996 and 0.988, respectively, and it decreases with the increase of the twist angle.

    Fig.4 Velocity magnitude profile of No.3 inserts

    Fig.5 Tangential velocity profile of No.3 inserts

    Fig.6 Effect of twist angle (A) on Nusselt number

    Fig.7 Effect of twist angle (A) on friction factor

    Fig.8 Tangential velocity profile at z =0.180 m for twist angles of 20oand 30o

    The enhancement by the twist angle is due to different tangential velocities. Figure 8 is the

    Fig.9 Effect of gap width (b) on Nusselt number

    Fig.10 Effect of gap width (b) on friction factor

    3.2.2 Gap between tube and inserts b

    Figures 9 and 10 show the Nusselt number and the friction factor for different gap widths at the same inlet temperature and wall temperature. In general, the gap width has a significant influence on the Nusselt number, but little influence on the pressure drop. The Nusselt number and the friction factor decrease as the gap width increases. The Nusselt number for b=1mm and b=0.5 mm is 7.1% and 23.7% larger than that for b=1.5 mm. The friction factor for b=1mm and b=0.5 mm is 3.2% and 12.1% greater than that for b=1.5 mm. On the base of the case b=1.5 mm, the thermal-hydraulic performances for the cases b=1mm and b=0.5 mm are shown in Fig.11, which increase in average by 124% and 140%, respectively. So a small gap width is desirable. The gap width effect comes from the different velocity through the gap. Figures 12 and 13 show the velocity magnitude and the tangential velocity for the cases b=1mm and b=1.5 mm at section z =0.100 m. The traveling velocity through the gap increases with the gap width, which leads to significantly lower main velocity and tangential velocity.

    Fig.11 Effect of gap width ( b) on thermal-hydraulic performance

    Fig.12 Velocity profile of different gap widths at the same inlet velocity of 3 m/s

    3.2.3 Edgefold length L

    Figures 14 and 15 show that the edgefold length has little effect on the Nusselt number and the friction factor, which in the case, L=20 mmtakes value 2.4% and 1.4% smaller those in the case L=15 mm. A large edgefold length can be used for easy fabrication.

    Fig.13 Tangential velocity profile of different gap widths at the same inlet velocity of 3 m/s

    Fig.14 Effect of edgefold length on Nusselt number

    Fig.15 Effect of edgefold length on friction factor

    Fig.16 Schematic diagram of the test facility

    4. Validity test

    4.1 Test facility

    The experiment setup is shown in Fig.16, where a copper tube with inner diameter d=21mm and wall thickness 2 mm is used as the test section. Length of the test section is 1 000 mm. The tube with ETT inserts is placed in a box and cooled by water, and the working medium inside the tube is air. Cooling water is provided by a thermostatic gas water heater. Air temperature is adjusted by adjustable electric heater, and is measured by eight RTDs. The volume flow rate of air is measured with Swema Flow 125, with a measuring range from 2 l/s to 125 l/s and the accuracy of ±3%. The pressure drop of air is measured with Swema 3 000 , with pressure range from –150 Pa to 1 500 Pa. The average temperature of the tube wall is determined by means of 10 thermocouples located along the tube. All the data signals are collected by a data acquisition system and stored in computer for further analysis.

    4.2 Test results and discussions

    For a high cooling water rate and a low heat transfer rate, the water temperature rise is within 0.4 k - 0.8 k, so the tube wall temperature is represented by the average temperature at ten locations on the test tube wall

    where N is the thermocouple number, TNis the temperature measured at a location on the tube wall.

    The total heat transfer capacity

    where G is the air flow rate, Tinand Toutare the air inlet and outlet temperature.

    The test and numerical results are shown in Figs.17 and 18. The test air inlet temperature is 393 K, the tube wall temperature is 313 K, the air flow rate is adjusted by the fan speed. The Nusselt number in the test is 1.6% - 3.6% smaller than that obtained by the simulation, and the friction factor is 8.2% - 13.6% greater than that of the simulation. So experiment results are in a reasonable agreement with simulation results.

    Fig.17 Nusslet number vs. air inlet velocity

    Fig.18 Friction factor vs. air inlet velocity

    5. Conclusions

    Three-dimensional numerical simulations and experiments were carried out to study the heat transfer, friction factor and thermal-hydraulic performance of tubes with STT inserts and ETT inserts. Experiment results are in a reasonable agreement with numerical results. The following conclusions are reached.

    (1) The heat transfer of a tube with ETT inserts is enhanced as compared with a tube with STT inserts.Within the range of Reynolds number from 2 500 to 9 500 and the twist ratio y from 5.4 to 11.4, the Nusselt number and the friction factor of the tube with ETT inserts are 3.9% - 9.2% and 8.7% - 74% larger than those with STT inserts, and the thermal-hydraulic performance is within 0.91 to 1.01. The major enhancement of the heat transfer is found due to higher tangential velocity and asymmetrical velocity profile with the increase and decrease of the periodic velocity within an edgefold length

    (2) The twist angle is the most important structural factor. A larger twist angle leads to larger Nusselt number and friction factor. The larger the twist angle, the higher tangential velocity will be. As the twist angle increases, the thermal-hydraulic performance decreases slowly.

    (3) The gap width has a significant influence on the heat transfer, but little influence on the pressure drop. When the gap width is reduced from 1.5 mm to 1.0 mm and 0.5 mm, the Nusselt number increases by 7.1% and 23.7%, the friction factor increases by 3.2% and 12.1%. The thermal-hydraulic performance increases in average by 124% and 140%. The traveling velocity increases as the gap width increases, which leads to significantly lower main velocity and tangential velocity, therefore, a small gap width is desirable.

    [1] CHIU Yu-wei, JANG Jiin-yuh. 3D numerical and experimental analysis for thermal-hydraulic characteristics of air flow inside a circular tube with different tube inserts[J]. Applied Thermal Engineering, 2009, 29(2-3): 250-258.

    [2] RAHIMI M., SHABANIAN S. R. and ALASAIRAFI A. A. Experimental and CFD studies on heat transfer and friction factor characteristics of a tube equipped with modified twisted tape inserts[J]. Chemical Engineering and Processing, 2009, 48(3): 762-770.

    [3] ZHANG Hua, ZHOU Qiang-tai. Experimental investigation on heat transfer and flow resistance characteristics of smooth round tubes with twisted-tape inserts[J]. Physical Examination and Testing, 2005, 23(5): 15-18(in Chinese).

    [4] KLACZAK A. Heat transfer and pressure drop in tubes with short tabulators[J]. Heat and Mass Transfer, 1996, 31(6): 399-401.

    [5] EIAMASA-ARD S., THIANPONG C. and PETPICES E. et al. Convective heat transfer in a circular tube with short-length twisted tape insert[J]. International Communication in Heat and Mass Transfer, 2009, 36(4): 365-371.

    [6] EIAMSA-ARD S., THIANPONG C. and PROMVONGE P. Experimental investigation of heat transfer and flow friction in a circular tube fitted with regularly spaced twisted tape elements[J]. International Communications in Heat and Mass Transfer, 2006, 33(10): 1225-1233.

    [7] SAHA S. K., DUTTA A. and DHAL S. K. Friction and heat transfer characteristics of laminar swirl flow through a circular tube fitted with regularly spaced twisted tape insert[J]. International Journal of Heat and Mass Transfer, 2001, 44(22): 4211-4223.

    [8] EIAMSA-ARD S., PETHKOOL S. and THIANPONG S. Turbulent flow heat transfer and pressure loss in a double pipe heat exchanger with louvered strip inserts[J]. International Communications in Heat and Mass Transfer, 2008, 35(2): 120-129.

    [9] AHMED M., DEJU L. and SARKAR M. A. R. et al.Heat transfer in turbulent flow through a circular tube with twisted tape inserts[C]. Proceedings of the International Conference on Mechanical Engineering. Dhaka, Bangladesh, 2005, ICME05-TH-08.

    [10] FAHED S. A., CHAKROUN W. Effect of tube-tape clearance on heat transfer for fully developed turbulent flow in a horizontal isothermal tube[J]. International Journal of Heat and Fluid Flow, 1996,17(2): 173-178.

    [11] DATE A. W. Prediction of fully developed flow in a tube containing a twisted tape[J]. International Journal of Heat and Mass Transfer, 1974, 17(8): 845-859.

    [12] EIAMSA-ARD S., WONGCHAREE K. and SRIPATTANAPIPAT S. 3-D Numerical simulation of swirling flow and convective heat transfer in a circular tube induced by means of loose-fit twisted tapes[J]. International Communication in Heat and Mass Transfer, 2009, 36(9): 947-955.

    [13] SAMA P. K., SUBRAMANYAM T. and KISHORE P. S. et al. A new method to predict convective heat transfer in a tube with twisted tape inserts for turbulent flow[J]. International Journal of Thermal Science, 2002, 41(10): 955-960.

    [14] SIVASHANMUGAN P., SURESH S. Experimental studies on heat transfer and friction factor characteristics of turbulent flow through a circular tube fitted with helical screw-tape inserts[J]. Applied Thermal Engineering, 2007, 46(16): 1292-1298.

    [15] MAZEN M., KHADER A. Further understanding of twisted tape inserts effects as tube insert for heat transfer enhancement[J]. Heat and Mass Transfer, 2006, 43(2): 123-134.

    [16] SIVASHANMUGAM P., SURESH S. Experimental studies on heat transfer and friction factor characteristics of turbulent flow through a circular tube fitted with regularly spaced helical screw-tape inserts[J]. Applied Thermal Engineering, 2007, 27(8-9): 1311-1319.

    [17] JIN Zhi-hao, WANG Guan-qing and LIU Jie et al. Numerical simulation of fluid flow characteristics in wavy plates[J]. Journal of Hydrodynamics, Ser. A, 2004, 19(1): 26-30(in Chinese).

    [18] WU Mei-wei, ZHANG Zao-sun. Numerical research on the structure in turbulent pipe flow[J]. Journal of Hydrodynamics, Ser. A, 2002, 17(3): 324-333(in Chinese).

    [19] TANG Zhi-wei, YAN Gui-lan and GAO Li-li. Numerical simulation of heat transfer enhancement for twisted inserts in tubes[J]. Journal of Engineering Thermophysics, 2008, 29(7): 1211-1214(in Chinese).

    [20] SUN Dong-liang, WANG Liang-bi. Numerical simulation of fluid flow and heat transfer in tube inserting twisted-tape[J]. Journal Chemical Industry and Engineering, 2004, 55(9): 1422-1427(in Chinese).

    [21] ZHANG Lin, QIAN Wei-hong. 3D numerical simulation of flow and heat transfer in self-rotating twistedtape-inserted tube[J]. Journal Chemical Industry and Engineering, 2005, 56(9): 1633-1638(in Chinese).

    [22] SARMA P. K., KISHORE P. S. and RAO V. D. et al. A combined approach to predict friction coefficients and convective heat transfer characteristics in a tube with twisted tape inserts for a wide range of Re and Pr[J]. International Journal of Thermal Science, 2005, 44(4): 393-398.

    [23] ZENG Zhuo-xiong. A new turbulence modulation in second-order moment two-phase model and its application to horizontal channel[J]. Journal of Hydrodynamics, 2008 ,20(3): 331-338.

    [24] ZHANG Ming-liang, SHEN Yong-ming. Threedimensional simulation of meandering river based on 3-D RNG k-ε turbulence model[J]. Journal of Hydrodynamics, 2008, 20(4): 116-125.

    [25] LU Chang-gen, CAO Wei-dong and QIAN Jian-hua. A study on numerical method of Navier-Stokes equation and non-linear evolution of the coherent structures in a laminar boundary layer[J]. Journal of Hydrodynamics, Ser. B, 2006 ,18(3): 372-377.

    10.1016/S1001-6058(09)60101-3

    * Project supported by the National Basic Research Program of China (973 Program, Grant No. 2007CB206903).

    Biography: CUI Yong-zhang (1970- ), Male, Ph. D. Candidate, Associate Professor

    TIAN Mao-cheng, E-mail: tianmc65@sdu.edu.cn

    色精品久久人妻99蜜桃| 99热精品在线国产| 久久精品91蜜桃| 丁香欧美五月| 免费观看人在逋| 热99re8久久精品国产| 久久人妻av系列| 国内精品一区二区在线观看| 久久精品国产自在天天线| 国内毛片毛片毛片毛片毛片| 麻豆国产av国片精品| 亚洲天堂国产精品一区在线| 欧美区成人在线视频| 美女免费视频网站| 欧美zozozo另类| 最新在线观看一区二区三区| 亚洲国产日韩欧美精品在线观看 | 欧美zozozo另类| 欧美3d第一页| 国产精品乱码一区二三区的特点| 精品久久久久久,| a级毛片a级免费在线| 99riav亚洲国产免费| 校园春色视频在线观看| 成人高潮视频无遮挡免费网站| av中文乱码字幕在线| 丰满乱子伦码专区| 免费在线观看亚洲国产| 国产熟女xx| 波多野结衣高清无吗| 成年女人永久免费观看视频| 无人区码免费观看不卡| 国产亚洲精品综合一区在线观看| 久久精品人妻少妇| 女生性感内裤真人,穿戴方法视频| 午夜精品在线福利| 国产激情欧美一区二区| 午夜福利高清视频| 久久久久久久午夜电影| 国产综合懂色| 九九在线视频观看精品| 久久久成人免费电影| 精品免费久久久久久久清纯| 精品99又大又爽又粗少妇毛片 | 99久久精品一区二区三区| 国内精品美女久久久久久| 精品乱码久久久久久99久播| 亚洲国产色片| 窝窝影院91人妻| 三级国产精品欧美在线观看| 国产欧美日韩精品亚洲av| 青草久久国产| 小蜜桃在线观看免费完整版高清| 嫩草影视91久久| 国产亚洲欧美在线一区二区| 一级作爱视频免费观看| 免费av观看视频| 亚洲精品亚洲一区二区| 一个人免费在线观看的高清视频| 三级毛片av免费| 麻豆一二三区av精品| 日韩免费av在线播放| 十八禁网站免费在线| 国产精品嫩草影院av在线观看 | 久久午夜亚洲精品久久| 麻豆一二三区av精品| 97碰自拍视频| 久久久国产精品麻豆| 免费在线观看成人毛片| 亚洲最大成人中文| 日韩 欧美 亚洲 中文字幕| av视频在线观看入口| 欧美极品一区二区三区四区| 成人特级黄色片久久久久久久| 日韩欧美三级三区| 成人av在线播放网站| 精品国内亚洲2022精品成人| 午夜福利在线在线| 欧美激情在线99| 亚洲欧美精品综合久久99| 欧美绝顶高潮抽搐喷水| 91麻豆精品激情在线观看国产| 成人无遮挡网站| 日韩欧美国产在线观看| 精品一区二区三区人妻视频| 欧美性猛交黑人性爽| 免费在线观看亚洲国产| 老司机在亚洲福利影院| 久久久成人免费电影| 日韩欧美精品免费久久 | 一区二区三区激情视频| 国产成年人精品一区二区| 午夜福利视频1000在线观看| 窝窝影院91人妻| 亚洲中文字幕一区二区三区有码在线看| 伊人久久大香线蕉亚洲五| 亚洲熟妇熟女久久| 午夜福利在线观看吧| 国产精品久久电影中文字幕| 999久久久精品免费观看国产| АⅤ资源中文在线天堂| 最新在线观看一区二区三区| 变态另类成人亚洲欧美熟女| 精品国产美女av久久久久小说| 欧美xxxx黑人xx丫x性爽| 久久人妻av系列| 1000部很黄的大片| 久久久精品欧美日韩精品| 97超级碰碰碰精品色视频在线观看| 国产亚洲精品av在线| 国产三级在线视频| 国产成人影院久久av| 99热这里只有是精品50| 2021天堂中文幕一二区在线观| 一个人免费在线观看电影| 精品日产1卡2卡| 国产精品98久久久久久宅男小说| 有码 亚洲区| 国内精品久久久久久久电影| 一个人看的www免费观看视频| 欧美午夜高清在线| 国产成人av教育| 亚洲国产日韩欧美精品在线观看 | 日本三级黄在线观看| 香蕉久久夜色| 夜夜看夜夜爽夜夜摸| 亚洲精品亚洲一区二区| 久久久成人免费电影| 国产精品女同一区二区软件 | 国产真实伦视频高清在线观看 | 老司机在亚洲福利影院| 午夜免费成人在线视频| 变态另类丝袜制服| x7x7x7水蜜桃| 国产探花极品一区二区| 深夜精品福利| 午夜激情欧美在线| 久久精品国产亚洲av涩爱 | 中国美女看黄片| 国产色爽女视频免费观看| 国产99白浆流出| 国产一区二区亚洲精品在线观看| 女人被狂操c到高潮| 国产精品99久久99久久久不卡| 欧美黄色片欧美黄色片| 欧美一级毛片孕妇| 两个人的视频大全免费| 欧美成人性av电影在线观看| 国内精品久久久久精免费| 精品人妻1区二区| av视频在线观看入口| 老师上课跳d突然被开到最大视频 久久午夜综合久久蜜桃 | 亚洲精品在线观看二区| 免费看十八禁软件| 香蕉久久夜色| 国产蜜桃级精品一区二区三区| 久9热在线精品视频| 午夜免费男女啪啪视频观看 | 精品一区二区三区av网在线观看| 欧美av亚洲av综合av国产av| 一本一本综合久久| 国产伦在线观看视频一区| 国产主播在线观看一区二区| tocl精华| 国产精品国产高清国产av| 好看av亚洲va欧美ⅴa在| 欧美在线黄色| 国产成人av教育| 欧美绝顶高潮抽搐喷水| 少妇的逼好多水| 亚洲成a人片在线一区二区| 国产黄片美女视频| 黄色视频,在线免费观看| 欧美中文综合在线视频| 波多野结衣高清无吗| 一个人观看的视频www高清免费观看| 老师上课跳d突然被开到最大视频 久久午夜综合久久蜜桃 | 99国产极品粉嫩在线观看| 国产精品精品国产色婷婷| 亚洲人成网站在线播| 精品一区二区三区人妻视频| 免费看美女性在线毛片视频| 怎么达到女性高潮| 国产高清激情床上av| 欧美色视频一区免费| 一本综合久久免费| 亚洲熟妇熟女久久| 久久伊人香网站| 国产爱豆传媒在线观看| 久久天躁狠狠躁夜夜2o2o| 免费一级毛片在线播放高清视频| 久久久成人免费电影| 18禁黄网站禁片午夜丰满| 少妇熟女aⅴ在线视频| 亚洲精品色激情综合| 两人在一起打扑克的视频| 三级男女做爰猛烈吃奶摸视频| 亚洲内射少妇av| 99久国产av精品| 动漫黄色视频在线观看| 欧美性猛交╳xxx乱大交人| 国产精品久久电影中文字幕| 男女午夜视频在线观看| 男插女下体视频免费在线播放| 一级毛片高清免费大全| 国产私拍福利视频在线观看| 亚洲第一欧美日韩一区二区三区| 一进一出抽搐gif免费好疼| 国产高清有码在线观看视频| 久久久久久久久中文| 久久这里只有精品中国| 狂野欧美白嫩少妇大欣赏| 99久久九九国产精品国产免费| 变态另类丝袜制服| 亚洲人成网站在线播| 少妇熟女aⅴ在线视频| 色吧在线观看| 老司机午夜十八禁免费视频| 精品久久久久久久人妻蜜臀av| 一级a爱片免费观看的视频| 国产亚洲精品久久久com| 欧美绝顶高潮抽搐喷水| 长腿黑丝高跟| 亚洲第一电影网av| 18禁黄网站禁片免费观看直播| 久久久色成人| 欧美日韩综合久久久久久 | 色在线成人网| 女警被强在线播放| 全区人妻精品视频| 亚洲黑人精品在线| 日韩国内少妇激情av| 亚洲精品美女久久久久99蜜臀| www国产在线视频色| 麻豆久久精品国产亚洲av| 51午夜福利影视在线观看| 中文字幕久久专区| 亚洲激情在线av| 日本黄色片子视频| 亚洲精品亚洲一区二区| 久久久国产成人免费| 国产乱人伦免费视频| 久久久久久大精品| 欧美最黄视频在线播放免费| 色吧在线观看| 亚洲不卡免费看| 精品人妻1区二区| 国产亚洲精品久久久com| 在线十欧美十亚洲十日本专区| 19禁男女啪啪无遮挡网站| 亚洲不卡免费看| 最好的美女福利视频网| 可以在线观看的亚洲视频| 波野结衣二区三区在线 | 国产精品永久免费网站| av天堂在线播放| 国产亚洲av嫩草精品影院| 岛国在线观看网站| 观看美女的网站| 免费人成在线观看视频色| 99久久精品热视频| 亚洲男人的天堂狠狠| 久久欧美精品欧美久久欧美| 一级作爱视频免费观看| 国产视频一区二区在线看| 超碰av人人做人人爽久久 | 五月伊人婷婷丁香| 欧美成人性av电影在线观看| 亚洲人与动物交配视频| 国产aⅴ精品一区二区三区波| 国产视频一区二区在线看| 亚洲av免费在线观看| 国产欧美日韩一区二区精品| 丁香六月欧美| 日韩欧美国产一区二区入口| 少妇裸体淫交视频免费看高清| 99久久综合精品五月天人人| 国产毛片a区久久久久| 一a级毛片在线观看| 日韩欧美精品免费久久 | 欧美最新免费一区二区三区 | 国产精品久久视频播放| 少妇高潮的动态图| 性色av乱码一区二区三区2| 窝窝影院91人妻| 免费电影在线观看免费观看| 久久伊人香网站| 亚洲国产精品999在线| 在线观看免费午夜福利视频| 亚洲一区二区三区色噜噜| 久久精品91无色码中文字幕| 亚洲av五月六月丁香网| 岛国在线免费视频观看| 好看av亚洲va欧美ⅴa在| 久久香蕉国产精品| 日日夜夜操网爽| 宅男免费午夜| 制服人妻中文乱码| 国产精品久久久久久精品电影| 又爽又黄无遮挡网站| 国产乱人视频| 国产精品美女特级片免费视频播放器| 国产极品精品免费视频能看的| 久久亚洲真实| 免费无遮挡裸体视频| 我要搜黄色片| 亚洲成av人片在线播放无| 国产成人福利小说| 国产欧美日韩精品一区二区| 日本精品一区二区三区蜜桃| 国产高清三级在线| 岛国视频午夜一区免费看| svipshipincom国产片| 五月伊人婷婷丁香| 亚洲av不卡在线观看| 亚洲精品亚洲一区二区| 中文字幕精品亚洲无线码一区| 老汉色∧v一级毛片| 久久久久九九精品影院| 午夜两性在线视频| 免费观看的影片在线观看| 精品福利观看| 日本 欧美在线| 亚洲欧美日韩东京热| 国产av不卡久久| 亚洲天堂国产精品一区在线| 国产精品,欧美在线| 亚洲男人的天堂狠狠| 91av网一区二区| 国产97色在线日韩免费| 国产三级在线视频| 欧美中文日本在线观看视频| 亚洲国产精品999在线| 国产日本99.免费观看| 亚洲 国产 在线| 成人av在线播放网站| 老汉色∧v一级毛片| 国产精华一区二区三区| 夜夜看夜夜爽夜夜摸| 成人无遮挡网站| 婷婷精品国产亚洲av| 在线观看美女被高潮喷水网站 | 国产精品1区2区在线观看.| 搡老妇女老女人老熟妇| 香蕉久久夜色| 我要搜黄色片| 在线十欧美十亚洲十日本专区| 免费观看人在逋| 国产亚洲精品一区二区www| 精品久久久久久久毛片微露脸| 真人做人爱边吃奶动态| 亚洲狠狠婷婷综合久久图片| 在线播放无遮挡| 九九热线精品视视频播放| 丁香六月欧美| 一个人免费在线观看的高清视频| 中文资源天堂在线| 观看美女的网站| 无限看片的www在线观看| 国产一区二区三区在线臀色熟女| 在线播放无遮挡| 久久久精品欧美日韩精品| 日韩欧美在线乱码| 在线观看美女被高潮喷水网站 | 国产精品亚洲一级av第二区| 午夜福利视频1000在线观看| 我的老师免费观看完整版| 久久九九热精品免费| 精品一区二区三区视频在线观看免费| 日韩亚洲欧美综合| 亚洲 国产 在线| 热99re8久久精品国产| 久久精品人妻少妇| av片东京热男人的天堂| 美女大奶头视频| 99久久综合精品五月天人人| 亚洲美女视频黄频| 国产综合懂色| www.999成人在线观看| 天堂网av新在线| 国产欧美日韩精品亚洲av| 成人一区二区视频在线观看| a级毛片a级免费在线| 在线播放无遮挡| 欧美一区二区亚洲| 国产精品国产高清国产av| 色播亚洲综合网| 黄色成人免费大全| 十八禁人妻一区二区| 亚洲电影在线观看av| 久久性视频一级片| 国内少妇人妻偷人精品xxx网站| 99在线视频只有这里精品首页| 12—13女人毛片做爰片一| 最近在线观看免费完整版| 悠悠久久av| 熟女人妻精品中文字幕| 亚洲真实伦在线观看| e午夜精品久久久久久久| 国产真实伦视频高清在线观看 | 日本撒尿小便嘘嘘汇集6| 99国产极品粉嫩在线观看| 国产97色在线日韩免费| 在线观看一区二区三区| 制服人妻中文乱码| 有码 亚洲区| 18+在线观看网站| 叶爱在线成人免费视频播放| 国产精品一区二区三区四区免费观看 | 国产真实乱freesex| 91久久精品电影网| 99久久精品国产亚洲精品| 精品久久久久久,| 特大巨黑吊av在线直播| 久久99热这里只有精品18| 最近最新中文字幕大全电影3| 88av欧美| 人人妻人人看人人澡| 听说在线观看完整版免费高清| 特级一级黄色大片| 久久精品91无色码中文字幕| 色哟哟哟哟哟哟| 亚洲人与动物交配视频| 欧美av亚洲av综合av国产av| 日本三级黄在线观看| 精品乱码久久久久久99久播| 国产伦精品一区二区三区四那| 久久久久久大精品| 欧美精品啪啪一区二区三区| 日韩高清综合在线| 午夜福利高清视频| 国产成人欧美在线观看| 婷婷六月久久综合丁香| 久久久精品大字幕| 露出奶头的视频| 国产伦在线观看视频一区| 欧美zozozo另类| 小说图片视频综合网站| 欧美日韩一级在线毛片| 精品久久久久久,| 亚洲久久久久久中文字幕| 午夜免费观看网址| a级一级毛片免费在线观看| 俄罗斯特黄特色一大片| 欧美日韩精品网址| 给我免费播放毛片高清在线观看| 日韩欧美国产一区二区入口| 精品日产1卡2卡| 国产私拍福利视频在线观看| 国产av麻豆久久久久久久| 嫩草影视91久久| 性色avwww在线观看| 1000部很黄的大片| 国产69精品久久久久777片| 少妇丰满av| 最近最新中文字幕大全电影3| 九色成人免费人妻av| 91在线观看av| 国产精品 国内视频| 国产一区二区三区视频了| 在线免费观看不下载黄p国产 | 97碰自拍视频| 99热这里只有精品一区| 欧美黑人欧美精品刺激| 看黄色毛片网站| 国产黄色小视频在线观看| 变态另类成人亚洲欧美熟女| 精品熟女少妇八av免费久了| 美女大奶头视频| 天堂影院成人在线观看| 麻豆国产97在线/欧美| 欧美日韩国产亚洲二区| 国模一区二区三区四区视频| 国产高清视频在线播放一区| 又爽又黄无遮挡网站| www.色视频.com| 日韩大尺度精品在线看网址| 中文字幕熟女人妻在线| 亚洲av二区三区四区| 亚洲成av人片免费观看| 人人妻,人人澡人人爽秒播| 老司机福利观看| 美女高潮喷水抽搐中文字幕| 亚洲午夜理论影院| 天天躁日日操中文字幕| 1000部很黄的大片| 老司机午夜福利在线观看视频| 18禁国产床啪视频网站| 亚洲成av人片在线播放无| 啪啪无遮挡十八禁网站| 三级国产精品欧美在线观看| 国产单亲对白刺激| 亚洲av美国av| 9191精品国产免费久久| 亚洲精品影视一区二区三区av| 每晚都被弄得嗷嗷叫到高潮| 欧美黄色淫秽网站| 久久中文看片网| 18+在线观看网站| 啪啪无遮挡十八禁网站| 老熟妇仑乱视频hdxx| 91麻豆av在线| 一二三四社区在线视频社区8| 国产伦一二天堂av在线观看| 嫁个100分男人电影在线观看| 91字幕亚洲| 老司机福利观看| 国产精品一区二区三区四区久久| 好看av亚洲va欧美ⅴa在| 久久天躁狠狠躁夜夜2o2o| 1000部很黄的大片| 特大巨黑吊av在线直播| 免费高清视频大片| 成人高潮视频无遮挡免费网站| 毛片女人毛片| 无遮挡黄片免费观看| av黄色大香蕉| 亚洲性夜色夜夜综合| av福利片在线观看| 国产美女午夜福利| 热99在线观看视频| 一级毛片高清免费大全| 中亚洲国语对白在线视频| 好男人电影高清在线观看| 亚洲久久久久久中文字幕| 我要搜黄色片| 成年女人看的毛片在线观看| 又爽又黄无遮挡网站| 岛国视频午夜一区免费看| 免费人成在线观看视频色| 亚洲av日韩精品久久久久久密| 在线看三级毛片| 亚洲欧美日韩高清在线视频| 香蕉av资源在线| 黄色丝袜av网址大全| 五月玫瑰六月丁香| 午夜视频国产福利| 欧美日韩乱码在线| 又黄又爽又免费观看的视频| 男女视频在线观看网站免费| 在线a可以看的网站| 亚洲av免费高清在线观看| 国产精品影院久久| 99久久综合精品五月天人人| 在线视频色国产色| 欧美3d第一页| 午夜a级毛片| 天堂影院成人在线观看| 黄片小视频在线播放| 欧美一级a爱片免费观看看| 亚洲内射少妇av| 久久久久久久久中文| 观看美女的网站| 精品福利观看| 桃红色精品国产亚洲av| 精品一区二区三区人妻视频| 日韩高清综合在线| 动漫黄色视频在线观看| 亚洲最大成人手机在线| 悠悠久久av| 在线看三级毛片| 91av网一区二区| 久久伊人香网站| 深夜精品福利| 波多野结衣巨乳人妻| av在线天堂中文字幕| 国产熟女xx| 欧美激情在线99| 国产精品亚洲一级av第二区| 成年人黄色毛片网站| 91久久精品电影网| 日本撒尿小便嘘嘘汇集6| 啦啦啦免费观看视频1| 丰满人妻熟妇乱又伦精品不卡| 99热只有精品国产| 麻豆一二三区av精品| 欧美日韩乱码在线| 国产精品1区2区在线观看.| 欧美乱妇无乱码| 精品人妻偷拍中文字幕| av片东京热男人的天堂| 丰满的人妻完整版| 中文字幕高清在线视频| 啪啪无遮挡十八禁网站| 动漫黄色视频在线观看| 精品熟女少妇八av免费久了| 国产一区在线观看成人免费| 免费一级毛片在线播放高清视频| 欧美xxxx黑人xx丫x性爽| 色在线成人网| 日日夜夜操网爽| 国模一区二区三区四区视频| 国产真实伦视频高清在线观看 | 老司机午夜十八禁免费视频| 亚洲精品色激情综合| 激情在线观看视频在线高清| 老司机午夜十八禁免费视频| 国产高清视频在线观看网站| 精品福利观看| 日本黄色片子视频| 亚洲内射少妇av| 女同久久另类99精品国产91| 老司机午夜福利在线观看视频| 国产精品日韩av在线免费观看| 欧美区成人在线视频| 99久国产av精品| a级毛片a级免费在线| 国产极品精品免费视频能看的| 国产精品久久电影中文字幕| 极品教师在线免费播放| 12—13女人毛片做爰片一| 久久久久精品国产欧美久久久| 国产黄色小视频在线观看| 久久精品人妻少妇| 精品久久久久久久久久免费视频|