NK型凝汽式汽轮机调节系统的设计【说明书+CAD】
收藏
资源目录
压缩包内文档预览：(预览前20页/共33页)
编号：120935925
类型：共享资源
大小：2.27MB
格式：ZIP
上传时间：2021-04-11
上传人：221589****qq.com
认证信息
个人认证
李**（实名认证）
湖南
IP属地：湖南
40
积分
- 关 键 词：
-
NK
型凝汽式
汽轮机
调节
系统
设计
说明书
CAD
- 资源描述：
-
NK型凝汽式汽轮机调节系统的设计【说明书+CAD】,NK,型凝汽式,汽轮机,调节,系统,设计,说明书,CAD
- 内容简介：
-
内蒙古科技大学毕业设计外文翻译Decay of Rotational Airflow with Flow Conditioner in Larger DiameterDucts for Dust Concentration Measurement using Isokinetic SamplingZ. C. Tan, Y. Zhang, S. E. FordDepartment of Agricultural and Biological Engineering, University of Illinois at Urbana-Champaign, 1304 West Pennsylvania Avenue, Room 332, Urbana, Illinois 61801.Phone: (217) 244-6316, Fax: (217) 244-0323. Email: AbstractWithin this study the decay speed of centrifugal device induced rotational airflow in round ducts and its conditioning for dust concentration measurement was investigated. Tests were conducted under room conditions at average Reynolds numbers within the range of 0.7-1.14x105 . Duct diameters were 152 mm and 203 mm. Airflow in the ducts was found nonuniform and unsteady within the axial location to diameter ratio (x/D) of 10.5. The nonuniformity of axial airflow was nearly independent of the average Reynolds number. The study proved that it is impractical to decay the rotational airflow simply by using straight extension duct. A flow conditioner with minimal particle loss is needed to make the air flow uniform and steady. As a case study, a simple flow conditioner was tested. It decayed the rotational airflow to an acceptable level within x/D=2. The measured dust concentrations at 5 radial locations at x/D=2 were uniform: 1% for total mass concentration, and 10% for all the particles smaller than 10 m.Keywords. Rotational airflow, airflow conditioning, dust samplingIntroductionCyclones have been widely used in many agricultural engineering applications. They were primarily used for larger particle separation (Carpenter 1986) or dust sampling at low airflow rates (Crook 1995, Lacey and Venette 1995). Although cyclones have been well industrialized and commercialized, uncertainties remain and they encourage researchers to further study the particle separation mechanism. Recent advances in this area indicate that it is very likely to be further developed for separating respirable particles at low cost and higher airflow rates (Zhang et al. 2001, Wang et al. 2002). During the investigation process, dust separation efficiency of a cyclone needs to be measured. With the increasing concern of respirable particles in environments, air-cleaning devices are often tested based on particle size (ANSI/ASHRAE 1999). Isokinetic sampling systems are often used for dust concentration measurements in ventilation ducts. In order to minimize the anisokinetic effects (Hinds 1999), the sampling nozzles should be correctly aligned to match the air velocity profile. Consequently rotational airflow exiting a cyclone has to be conditioned to an acceptable steadiness before air velocity profile can be determined and dust concentration can be measured. In addition, uniform airflow is also preferred to reduce other sampling errors. It is a challenge to pick a representative sampling point in the rotational airflow. A multipoint sampling system could be employed. One apparent disadvantage is that many sampling heads will inference with the airflow pattern and introduce a significant error factor. Another disadvantage is that more sampling points require more sampling tubes, and consequently increase particle loss in sampling tubes. Therefore, a single point sampling system is preferred. The upstream airflow is not rotational. A single sample point is acceptable. However, before a single downstream sample point can be taken, we must condition the rotational airflow to an acceptable uniformity. ANSI/ASHRAE (1999) standard requires that qualification tests shall be performed for uniformities of air velocity and aerosol in the test duct. The ANSI/ASHRAE standard also recommends bending the test duct 180 to bring the downstream sample locations relatively close to the upstream location, which allows short sample lines to the particle counter. It reduces the overall length of the test duct, facilitating its placement within the test room. However, it should be noted that this introduces another error factor. The bending could create rotational airflow. This swirl poses challenges in accurate measurements of airflow rate (McManus et al. 1985, Laribi et al. 2001), and maybe consequently, the dust concentration. Two methods are often considered for airflow conditioning. One is the use of long straight extension ducts and another is the use of flow conditioners. The rotational airflow in a duct will decay to an acceptable level provided the straight duct is sufficiently long. The question is how long it should be. If the duct is too long, excessive space is needed to setup the measurement system, and more importantly, there might be too much particle loss on the inner surface of the duct. Studies on rotational flow have been reported since the 1950s. The majority of these studies dealt with rotational flows in burners and air mixing in combustion areas. Only few of them are about decay of rotational flows. One of the early studies on rotational flow in ducts is an analysis of rotational flow of a constant property fluid in a straight duct of circular cross-section by Talbot (1954). The subject was steady and laminar flow in ducts. Kreith and Sonju (1965) later presented the average decay of tape-induced fully developed turbulent swirl in water flow through a duct. The duct was 25.4 mm in diameter and 2,540 mm long. The twisted tape inducers consisted of 0.85 mm thick galvanized steel strips. The experiments were done with Reynolds numbers from 104 to 105 . It indicated that the rate of decay depended on the axial Reynolds number: the decay rate increased as the Reynolds number decreased, but the decay was found to be independent of the initial swirl intensity. It was observed that turbulent swirl decayed to about 10-20% of its initial intensity at an axial location to diameter ratio (x/D) of 50. Recently Laribi et al. (2001) experimentally studied the decay of swirling turbulent gas (petroleum) flow in a long duct. The inner diameter of the duct was 100 mm. The swirling flow was generated by double 90-degree elbow. Tests were conducted at Reynolds number ranging from 3x104 to 1.1x105 . It was observed that the flow was fully developed at x/D=90. The decay of a swirling flow of nitrogen gas in long ducts was reported by McManus et al. (1985). Tests were conducted under ambient temperature and pressure of 40 kPa at Re=8x105 to 1.4x106 . The decay of 60 degrees of swirling across a 100 mm smooth-walled duct measured at x/D=0, 10, 33, and 70. It was found that at x/D=70, the decay was 59%. Based on 6 sets of experimental data, they gave a fit decay equation, Percent Decay = where x is the axial location in the duct, D the inner diameter of the duct, and x/D the axial location to diameter ratio. From this equation, we can predict that the decay can be reduced to 10% at x/D=200. Although the exact x/D numbers are different for the rotational flow to become fully developed, the above representative studies have shown that the duct has to be very long to reduce swirl to acceptable levels. In a laboratory, it might not be a practical solution for eliminating possible measurement errors attributed to swirl simply by using long ducts. ObjectiveAmong the limited amount of literature on decay of rotational fluid flow, few of them are about decay of rotational airflow in long ducts, and most of them were conducted in ducts less than 100 mm in diameter. The objective of this research is to study the decay of a rotational airflow created by a rotation device (i.e. centrifugal fan or cyclone) in ducts larger than 100 mm in diameter. It will be proved that a long duct is impractical, and a flow conditioner has to be used to condition the rotational airflow. As a case study, a flow conditioner with a large open ratio and with smooth surfaces was made and its effect on flow conditioning was tested. It is also expected that researchers who measure dust emission rate downstream of exhausting fans can find useful information from the results herein. Experimental MethodsDucts used in this research were commercial galvanized steel ducts with inner diameters of D= 152 mm and 203 mm. Maximum length of the ducts was 2,100 mm. The maximum length to diameter ratio, L/D, was about 10.8 due to space limitations in the laboratory. Axial velocity profile was measured using an anemometer from TSI, Inc. The anemometers measurement range was 0 to 20.3 m/s, with an accuracy of 5.0% of reading or 0.025 m/s whichever is greater. In this paper, airflow direction is our major concern for isokinetic sampling purpose. We wanted to condition the rotational airflow into a one-dimensional airflow. Therefore, only the axial velocity profile was measured. If axial velocity is not steady or uniform, the total velocity is not steady or uniform. While rotating the anemometer during test, other velocity components were obvious if the axial flow was not steady or uniform. On the other hand, other two velocity components were not apparent when the axial flow was steady and uniform. Samples were taken at different axial locations (x/D). At each location, velocities were measured at different radial locations. The rotational airflow was generated with an in-line centrifugal fan. The airflow rate was adjusted with a variable transformer. The transformers maximum output settings were 120 V and 140 V. It could be adjusted from 0% to 100% of each maximum output. In this project, seven voltage levels of fan power were used: 90% and 100% of 140V, and 60%, 70%, 80%, 90% and 100% of 120V. For each power level and each x/D in duct, axial air velocities at up to 13 evenly distributed points were measured, with increase of 12.7 mm or 25.4 mm (see Figure 5). To verify the conditioning effect of the simple airflow conditioner, dust spatial distribution downstream was measured using an isokinetic sampling system. Particle concentrations were measured using APS 3321 (TSI, Inc.). The sampling points were 2D downstream of the flow conditioner, and their relative radial locations are shown in Figure 1. The corresponding radial locations of #1, #2, #3, #4 and #5 are 1/3 R, 1/6 R, 1/6 R, 7/12 R, and 5/6 R, respectively. 10 samples were taken at each of the 5 radial locations. The tests were conducted under Reynolds number of Re=2.2x104 .Figure 1. Particle sampling locations Standard Arizona test dust was dispersed using a turntable dust generator. The size distribution of particles entering the air stream is shown in Figure 2. As seen in the graph, 95% by volume are less than 12 m with a mass median of 4.8 m.Figure 2. Particle size distribution Results and DiscussionIn this paper, a coordinate system as illustrated in Figure 3 is used. Axial direction is x, and radial direction is r. The length, radius and diameter of the duct are L, R, and D, respectively. At any axial location x, the velocity at different radial location r is Vr. The velocity on the centerline is Vc. The average velocity at x is V. Figure 3. Illustration of the parameters referred in this paper In this paper, dimensionless parameters including Reynolds numbers, nonuniformity parameter, and dimensionless velocity are used for data analysis. For airflow inside a uct, Reynolds number is defined as (ANSI/ASME 1979),Re= where: V is the average fluid velocity (m/s)., l a characteristic dimension of the system in which the flow occurs (m), and v the kinematic viscosity of the fluid (m2 /s). The characteristic dimension is the inner diameter of the duct, D. The kinematic viscosity is 1.5x10-5 m2 /s for room air at 20 oC and 1 atm.Average Reynolds numbers are calculated using the averaged axial velocities. The average Reynolds numbers in ducts in our tests were within the range of 0.7-1.14x105. 1. Effect of centrifugal device location The airflow pattern inside a duct created by pushing air into a duct is different from pulling air out. Curves in Figure 4 are dimensionless velocities, the velocity at r over the average velocity (Vr/V), against dimensionless radial location, radial location r over the radius of the duct (r/R). For the example cases in Figure 4, the average Reynolds number was 1.14x105 . The effects under other operation conditions were similar. When the fan was pulling air out of the duct, Vr/Vs at all the measured locations, even at x/D=0.46, were very close 1. It indicates that airflow was uniform. However, when the duct was downstream the device, Vr/Vs at most of the measured points are way off unity, even at x/D=8.8. It indicates that downstream centrifugal device does not create strong rotational flow in duct, but upstream centrifugal device does. Figure 4. Effect of device location (pulling air out of and pushing air in duct) (r is the radial location, R is the Radius of the duct, Vr is the velocity at r, V is the average velocity, x is the axial location, and D is the diameter of the duct). Since the rotational effect was negligible when the duct was upstream of our fan, only those cases where the duct was downstream of the fan are presented below. An example velocity profile is first given to visualize the airflow inside the ducts with and without flow conditioner. Then the uniformity and steadiness of the airflow are compared. At the end, as a case study, the dust concentration distribution after the flow conditioner is presented.2. Axial velocity profile Axial velocity patterns were similar for all 7 average Reynolds numbers (fan levels). Figure 5 is an example that illustrates the axial velocity profile in ducts with and without flow conditioner at different x/D. The corresponding average Reynolds number was 1.14x105 (140V x 100% fan power). The flow conditioner was made of galvanized steel sheet metal. It had 4 vanes with L/D=2. When a flow conditioner is used, we have to take into consideration the particle loss on the flow conditioner. Particle loss on the flow conditioner should be minimized as much as possible. Therefore, the open area of the conditioner should to be as large as possible, and the surfaces should be as smooth as possible. Flow conditioners that are made up of multiple small tubes (i.e. 1 mm diameter) were not considered. The sheet metal is about 1 mm thick with smooth surface. The open ratio of a conditioner in a 152 mm duct is about 98.3%. Considering the large open ratio and smooth surface of the vanes, and small downstream particles, it is expected that the pressure drop and particle loss were minimal. This should be verified.Figure 5. Decay of rotational airflow in a long duct without and with a flow conditioner 3. Nonuniformity As mentioned above, at each axial location (x/D), velocities at about 10 radial locations (r/Rs) were measured. Then we have an average velocity (V) and a standard deviation () for each x/D. A nonuniformity parameter was defined to quantitatively compare the rotational effect at different x/Ds under different operational conditions. The nonuniformity parameter is defined as the standard deviation over the average value of the velocities. The higher the standard deviation over the average, the higher the level of nonuniformity is. Figure 6 shows some examples of the variation of /V with average Reynolds number. In most cases of rotational airflow, /V dropped slightly with average Reynolds number. For conditioned airflow, the variation is negligible. For any fixed point in the duct, the value of /V did not change significantly with fan power. This also explained that the velocity profile mentioned above was similar at different average Reynolds numbers (velocities). Therefore, only the average /V was used for each Reynolds number to quantitatively illustrate the nonuniformity at each x/D. Figure 7 is based on this idea.Figure 6. Nonuniformity of velocity under test conditions (V is the average velocity, is standard deviation of velocity. x/D is the axial location over diameter of the duct) Figure 7 compares the nonuniformity of airflow in duct. Each point in Figure 7 is the average of 7 values of /Vs as shown in Figure 6. Within the accuracy in the tests, /V of air velocities at any axial location in round ducts was higher than 0.1 (or 10%) (for x/D2/3) is always less than 0.1. The experimental results here show that airflow approached uniformity level of 0.05 (5%) after x/D=1.5. On the other hand, even with a flow conditioner, it is not recommended to take samples right after the conditioner. Taking samples at x/D1.5 after the flow conditioner is a safe choice, unless it is impractical like in the situations reported by Marsh et al. (2003). The fitting curve gives the relationship between /V and x/D as, Figure 7. Nonuniformity of velocity over x/D for flow with and without flow conditioner (V is the average velocity, is standard deviation of velocity. x/D is the axial location over diameter of the duct) 4. Steadiness Another parameter to evaluate the effect of the flow conditioner is flow steadiness. In this paper steadiness of the flow is characterized using centerline dimensionless velocity, Vc /V. Where, Vc is air velocity at the centerline, and is the average velocity at x/D. It shows the velocity change along the duct. For tested conditions without flow conditioner, the centerline axial velocity keeps increasing along the duct length (see Figure 5). Figure 8 gives an example when average Reynolds number is 1.14x105 . It shows that the centerline velocity approached the average velocity as x/D increased. For airflow without flow conditioner, the axial velocity kept increasing and approaching steady state, but it did not reach steady state within our test conditions (x/D10.5). For flow after conditioner, the centerline velocity number reached the average velocity quickly after x/D=1.5. It indicates that the flow conditioner made the rotational air flow quickly approach steady state. Figure 8. Steadiness over x/D for flow with and without flow conditioner (Vc/V is the velocity at the centerline over average velocity at x, and x/D the axial location over diameter of the duct) Overall, the tests confirmed that it is impractical to decay the rotational airflow in ducts to an acceptable level simply by using long ducts. 5. Particle Concentration Steadiness and Uniformity Steadiness at each location as illustrated in Figure 1 was characterized by the standard deviation over the average concentration(/) of the 10 samples. Steadiness of particle concentration at each of the 5 locations is shown in Figure 9. For each location, as seen in the graph, the standard deviation of the average particle concentration is less than 15% with majority less than 10%. The steadiness of the turntable might also have contributed to the variation.Figure 9. Steadiness downstream the flow conditioner (Re=2.3x105 ) (is standard deviation and is the average particle concentration; dp is the particle diameter) At each radial location, there is an average particle concentration calculated from the 10 samples. Then the average particle concentration at the axial location (x/D=2) is calculated based on the 5 concentrations at the 5 radial locations. The average particle size distribution at x/D=2 is shown in Figure 10. Each error bar marks the corresponding standard deviation (). The corresponding standard deviation over average (/) is also shown in the same graph on the right y-axis. As seen in the graph, / is less than 10% for any particle that is smaller than 10 m. A higher / is associated with lower particle concentrations. Note that at lower particle concentration, the sampling efficiency of APS 3321 is low, and it introduces errors. This is out of the scope of this paper and will be addressed separately. On the basis of mass concentration, the average concentration of the 5 locations was =8.42 mg/m3 with a standard deviation of =0.075 mg/m3 , which indicates that / is less than 1%.Figure 10. Uniformity downstream the flow conditioner (5 locations, Re=2.3x105 ) 6. Comparison with previous studies Results in this paper also expanded the database of study of rotational flow in long ducts. A comparison of previous tests with ours is shown in Table 1. Table 1 Comparison of test conditions with previous studiesTable 1 shows that rotational flow exists in many general engineering applications. Rotational flow could be created by, but not limited to, 1) bending vanes, such as vanes used in cyclones, 2) sudden changes in flow direction, including 90 bends (McManus et al. 1985) and double 90 elbows (Laribi et al., 2001), and 3) air flow downstream of a centrifugal device such as a centrifugal fan. The rotational airflow will affect the accuracy in airflow rate measurement using an orifice meter (Laribi et al., 2001) as well as dust concentration measurements using isokinetic sampling techniques, as addressed in this paper.Summary and ConclusionsThis research investigated decay of centrifugal device created rotational airflow in round ducts under room condition. An example velocity profile was first given to visualize the airflow inside the ducts with and without flow conditioner. Then the uniformity and steadiness of the airflow were compared. As a case study, the dust concentration distribution after the flow conditioner was presented. The following conclusions can be drawn from this study: 1. Centrifugal device has negligible effect in creating rotational flow on upstream airflow in round ducts. The same device can create very strong nonuniform and unsteady rotational airflow in downstream ducts. 2. Once a rotational airflow is created in a duct, it is impractical to lessen it to an acceptable level simply by using long ducts. 3. The nonuniformity parameter of the airflow at certain point in a duct is nearly independent of the average Reynolds number. When , the airflow is considered uniform. 4. A simple flow conditioner can significantly reduce the airflow swirl. The flow conditioner with length to diameter ration (L/D) of 2 lessens the rotational airflow to an acceptable level quickly within x/D=2. Dust concentrations at 5 radial locations at x/D=2 downstream the conditioner was observed uniform (/0.01). On the other hand, it is not recommended to take samples right after the flow conditioner. ReferencesANSI/ASME MFC-1M-1979. Glossary of terms used in the measurement of fluid flow in pipes. ANSI/ASHRAE Standard 52.2-1999. Method of testing general ventilation air-cleaning devices for removal efficiency by particle size. Carpenter, G. A. 1986. Dust in Animal indoor environments-Review of Some Aspects, Journal of Agricultural Engineering Research. 33: 227-241. Crook, B. 1995. Inertial samplers: biological perspectives, in Bioaerosols Handbook (edited by Cox and Wathes), chapter 9, Florida: CRC Press, Inc. Hinds, W. C. 1999. Aerosol Technology: Properties, Behavior and Measurement of Airborne Particles. New York: Wiley. Kreith, F. and O. K. Sonju. 1965. The decay of a turbulent swirl in a pipe. Journal of Fluid Mechanics. 22(2): 257-271. Lacey, L. and J. Venette. 1995. Outdoor air sampling techniques, in Bioaerosols Handbook (edited by Cox and Wathes), chapter 16, Florida: CRC Press, Inc.Laribi, B., Wauters, P., and M. Aichouni. 2001. Experimental study of the decay of swirling turbulent pipe flow and its effect on orifice meter performance. Proceedings of the FEDSM2001, 2001 ASME Fluid Engineering Conference. May 29-June 1, 2001. New Orleans, USA. FEDSM2001-18039: 93-96. Marsh, L. S., S. W. Gay, G. L. Van Wicken, and T. Crouse. 2003. Performance evaluation of the SanScentair scrubber for removal of dust, ammonia, and hydrogen sulfide from the exhaust air of a swine nursery. ASAE paper #034052. ASAE 2003 annual meeting. July 27-30, 2003. Las Vegas, NV. McManus, S. E., B. R. Bateman, J. A. Brenham, I. V. Pantoja, and D. Mann. 1985. The decay of swirling gas flow in long pipes. American Gas Association Operating Section Distribution/Transmission Conference, Boston, MA. May 1985. Talbot, L. 1954. Laminar swirling pipe flow. Journal of Applied Mechanics. Transactions of ASME. 21(76): 1-7. Wang, L., C.B. Parnell and B.W. Shaw. 2002. Performance Characteristics of Cyclones in Cotton-Gin Dust Removal. Agricultural Engineering International: the CIGR Journal of Scientific Research and Development. Volume IV. Manuscript # BC 02 001. Zhang, Y., X. Wang, G. L. Riskowski, S. E. Ford, and L. L. Christianson. 2001. Particle separation efficiency of a uniflow deduster with different types of dusts. ASHRAE Transactions. 107 (2): 93-98. Nomenclatured p Particle diameter (m)D Duct diameters (m) l Characteristic dimension (m) L Length of the duct, length of the flow conditioner (m) r Radial location (m) R Radius of the duct (m) r/R Dimensionless radial location Re Reynolds numbers V Average fluid velocity (m/s) Vc Velocity along the center line (m/s) Vr Velocity at radial location at r (m/s) x Axial location of the duct (m) x/D Axial location to diameter ratio Standard deviation of velocity (m/s), particle concentration (number/cm3 ) Average value particle concentration (number/cm3 ) Non-uniformity of velocity or particle concentration v Kinematic viscosity of the fluid (m2 /s). _Z. Tan, Y. Zhang, and S. Ford. !Decay of Rotational Airflow with Flow Conditioner in Larger Diameter Ducts for Dust Concentration Measurement using Isokinetic Sampling!. Agricultural Engineering International: the CIGR Journal of Scientific Research and Development. Manuscript BC 03 007. Vol. V. December 2003.Z.C. 谭，Y.张，S.E.福特!为测量粉尘浓度，在具有旋转气流衰变和流量调节器的大直径管道中使用等速采样!国际农业工程：国际农业工程学会杂志科学的研究和发展。公元前2003年12月手稿第5卷， 编号03 007 。在具有旋转气流衰变和流量调节器的大直径管道中利用等速采样测量粉尘浓度Z.C. 谭，Y.张，S.E.福特伊利诺斯州厄巴纳市宾夕法尼亚西大街第1304号伊利诺斯大学乌尔班纳原野分校农业及生物工程系332室 61801电话：(217) 244-6316，传真：(217) 244-0323，Email: 摘要在这项研究中离心设备的速度衰变引起了圆形输送管道中气流的旋转，并调节被研究测量的粉尘浓度。在平均雷诺数为0.7-1.14x105范围内的房间条件下进行测试。管道直径为152 mm和203 mm。发现输送管道在直径比率(x/D)为10.5的轴向位置内气流的不均匀性和不平稳性。轴向气流的不一致几乎不依赖于平均雷诺数。研究证明通过使用延长直管道，衰变的旋转气流是不存在的。带有最小微粒损失的液体调节器是需要使气流均匀和稳定。作为一个研究实例，测试一个简单的流量调节器，它在x/D=2之内把旋转气流衰变成一个可以接受的标准。在x/D=2的5个辐射状位置的被测量粉尘浓度是一致的：大多数微粒的1%和所有较10 m更小的微粒的10%。关键词：旋转气流，气流调节，粉尘取样前言旋风分离器曾经广泛地用于许多农业的工程应用。他们主要以低的气流率(弯曲 1995，花边和Venette 1995 )用于较大的微粒分离(木匠1986 )或者粉尘取样。虽然旋风分离器已充分被工业化和商业化，但是它们的不确定性仍然激励研究者进一步研究微粒的分离机制。最新进展在这方面表明，它是十分有可能以低的费用和较高的气流率（张等人， 2001年，王等人2002年）在分离可吸入微粒中得到更进一步的发展。在调查过程中，需要测量一台旋风分离器的粉尘分离效率。有了在环境中可吸入微粒增加的关注，空气清洁设备经常基于微粒大小进行测试（美国国家标准学会/美国采暖、制冷与空调工程师协会标准 1999年）。等速取样系统粉尘浓度测量方法常用于通风导管中，为了最大限度地减少非同流态影响（汉斯 1999年），取样喷管应该正确地对齐匹配空气速度概况。因此，旋转气流旋风分离器具有一个可以接受的稳定性之前，要求空气流速概况可以判定及粉尘浓度可测。此外，一致的气流也倾向于减少其它取样误差。在旋转气流中选择一个典型的取样点是个难题，使用一个多点取样系统，其中一个明显的缺点是，许多采样头必须与气流样式的结论，并采用一个有效的误差因素。另一个不利条件是更多取样点要求更多的管子，并因此增加取样管子中的微粒损失。因此，优先选择单点取样系统。在进入端的气流不是旋转的，一个单点取样系统是可以接受的。然而，在一个单一出口端取样点被采用之前，我们必须以旋转气流可接受的一致性为条件。美国国家标准学会/美国采暖、制冷与空调工程师协会（1999年）标准要求，为了空气流速的一致性和测试导管的气溶胶必须做资格测试。美国国家标准学会/美国采暖、制冷与空调工程师协会标准也建议把测试导管折弯180，使下游取样位置较为接近上游的位置，允许短的采样线对微粒计数。它减少测试导管的整体长度，有利于其在测试房间之内的安置。然而，值得注意的是这采用另一个误差因素，折弯能够造成旋转气流。在精确测量的气流率（麦克马纳斯等人， 1985年，拉里比等人， 2001年）中，也许因粉尘浓度使这个涡流状态带来各种挑战。经常被考虑的两种气流调节方法。一种是使用长的直延长导管和另一种是使用流量调节器。提供足够长的直导管，旋转气流在导管将衰减到可以接受的标准。问题是管道应该多长，如果导管过长，需要过多的空间安装测量系统，以及更重要的是，在导管的内表面可能会有太多的微粒损失。关于旋流的研究从20世纪50年代就有报告了，这些研究的大多数是在燃烧器的旋流处理和空气混合燃烧领域。这些研究中只有极少数是关于旋流的衰变，其中一项关于旋流的早期研究，是由塔博（Talbot）(1954)在圆形横截面直导管内定值流体属性的旋流的分析。主题是在管道的平稳和层流，克莱特(kreith)和sonju （ 1965 ）后来提出磁带引起的平均衰变在水穿过导管的混乱旋动中得到充分发展。导管直径是25.4mm和长2，540mm，扭转的磁带电感器包括了0.85mm厚度的镀锌钢小条，实验完成的雷诺数从104 到105 ，它表明衰变的比率依赖于轴的雷诺数：衰变率随雷诺数有所减少，但衰变被发现不依赖于初始涡流强度，据指出，混乱旋流衰变约10-20%，其初始强度在轴向位置以直径比（x/D）为50。最近拉里比等人（2001）在一长管道的实验研究混乱旋流燃气（油）的流量，管道的内径是100mm，由双90度弯头产生旋流，测试进行的雷诺数从3x104 到 1.1x105 。据观察所得，这股巨流，在x/D=90得到充分开发。据麦克马纳斯等人（1985）发表在长管道中氮气体的一种旋流衰变，在Re=8x105 to 1.4x106下，根据环境温度和压力为40kPa进行测试 ，旋流横截面100mm光滑管壁的60度的衰变在x/D=0, 10, 33,和70处测量，人们发现x/D=70时，衰变是59%，基于6套实验的数据，他们给出适宜的衰变方程，Percent Decay = 其中X导管轴向位置，D是管道的内径，和x/D是轴向位置的直径比率。从这个方程，我们可以预测在x/D=200处衰变可降低至10% 。虽然精确的x/D数据是对不同的旋流得到充分发展，上述的典型研究显示，该导管要很长，以减少涡流到可以接受的标准，在实验室里，通过使用长的导管简单消除可能的测量误差，它可能不是一个实用的解决办法。宗旨在有关液体旋流的衰变的数量有限的文献之中，他们中很少几个是关于在长导管中旋转气流的衰变的，而他们中大多数是在直径少于100mm的导管中实施的。本调查的目的是研究通过一个旋转装置（即离心风机或旋风分离器）在直径大于100mm的管道中形成一个衰变旋转气流，它将证明在一长导管中使用流量调节器来决定旋转气流是不切实际的。作为一个案例研究，流量调节器制成一个大开放比率和光滑表面及其对流量调节的影响进行了测试。人们还预计，研究人员测量排气风扇下游的粉尘排放率，从如下研究结果中可以找到有用的资料。实验方法在这项研究中采用的管道是内径为D=152mm和203mm商业镀锌钢导管。管道的最大长度是2100mm，最大长径比由于受实验室中空间的限制，L/D约为10.8。轴向流速剖面测量采用TSI公司的风速计，该风速计的测量范围为0至20.3m/s，读数精度为5.0%或0.025m/s以较高者为准。在本文中，我们主要关注的目的是在气流方向上等速取样。我们希望把旋转气流状态变成一维气流，因此，只测量轴向剖面流速。如果轴向速度并不稳定或不一致，其总速度也就不稳定或不一致。当在实验期间旋转风速计，如果轴向流量不稳定或不一致，则其他分量是显而易见的。另一方面，当轴向流量稳定并一致，则其他两个分量并不明显。样本送往不同的轴向位置（x/D），在每一个位置测量不同径向位置的流速。一台同轴离心风机产生旋转气流后，用一个可变的变压器调整气流率,该变压器的最大输出设置为120V和140V，它的最大输出功率可以从0%至100%进行调整。在这个项目中，7个电压等级的风机功率分别是：140V的有90%和100%，120V的有60%、70%、80%、90%和100%。每个功率等级和在管道中每个x/D，在多达13个均匀分布点用12.7mm或25.4mm的增量进行轴向风速测量（参见图5）。检验一个简单的气流调节器的调节效果，采用等速取样系统测量分布在下游空间的粉尘。采用APS 3321测量微粒浓度（TSI公司），采样点是流量调节器的二维下游，以及它们的相对径向位置如图1所示。#1、#2、#3、#4和#5的对应的径向位置分别为1/3 R， 1/6 R， 1/6 R， 7/12 R和5/6 R，10个样本，分别采用了在5个径向位置的每一个。测试是在雷诺数Re=2.2x104 下进行的。图1.微粒取样位置亚利桑那州标准使用转盘粉尘发电器分散粉尘，进入空气系统的微粒的大小分配如图2所示。在曲线图中看出，95%的体积小于12m，大部分体积为4.8m。图2.微粒分布图结果与讨论在本文中，使用坐标系统如图3所示。轴向方向是X，和径向方向是R，导管的长度、半径和直径分别是L、R和D。任何位置X在不同径向位置R的速度是Vr。这个速度在中线是Vc。平均速率在X处是V。图3.例证在本文中提到的参量在本文中，无量纲参数包括雷诺数、非均匀性参数和用于数据分析的无因次速率。在开普敦大学里面，气流的雷诺数被定义为（美国国家标准学会/机场地面活动目标显示设备 1979）：Re= 其中：V是平均的流体速度（m/s），L是流量重现（m）系统的一个典型尺寸和V是流体（m2 /s）的运动学黏度，D是管道内径的特征尺寸。在20 oC 和 1 atm的室内空气速度为m2 /s的运动学黏度是1.5x10-5 m2 /s。用平均轴向速度计算平均雷诺数，在我们的实验导管内平均雷诺数的范围是0.7-1.14x105。1. 离心设备位置的效果在造成气流模式的管道里面，空气输入管道与排出空气的不同。在图4中曲线的无因次速度，流速在r以上的平均速度（Vr/V），对无量纲径向位置，导管（r/R）半径超过r的径向位置。例如，例图4中，平均雷诺数是1.14x105 ，在其它操作条件下面的结果是相似的。当风扇从管道里排除空气，在x/D=0.46时所有被测位置Vr/Vs是非常接近1的，它表明气流是一致的。然而，当管道下游设备，在x/D=8.8时Vr/Vs在大多数测量点是一致的，它表明下游的离心设备没有造成强有力的旋转流管，但上游的离心设备不会造成。图4.设备位置的作用（空气从导管里排除和推入）（r是径向位置，R是管道的半径，Vr是在r处的速度，V是平均速度，X是轴向位置，D是管道的直径）当管道是在上游时，我们的风扇旋转效应是可以忽略的。只有那些情况下，风扇下游的管道下面呈现出来，一个例子速度剖面，首先给导管内部和流量调节器外可视化气流，然后比较气流的均匀性和稳定性。在去年底，作为一个案例研究，流量调节器呈现粉尘浓度分布。2. 轴向流速剖面图轴的速度模式对所有7个平均的雷诺数（风机标准）是相似的。图5是一个例子，说明了在管道中轴向速度剖面和无流量调节器，在不同的x/D相应的平均雷诺数是1.14x105 (140V x 100% 风扇电源)。流量调节器是镀锌合金钢板制成，它有4个L/D=2的叶片。当我们使用流量调节器时，我们必须考虑到流量调节器的微粒损失。应该尽量减小流量调节器的微粒损失。所以，调节器的开放区域应该可能的大，并且表面应该尽可能的光滑。流量调节器是弥补了多重小管（即1mm直径）进行考虑。表面光滑的1mm厚合金钢板，在一152mm口径的管道中调节器的开口比率为98.3%。考虑到大的开口比率及表面光滑的叶片，以及下游的小微粒，但预计到压力降和微粒损失甚微，这应加以核实。图5.有没有流量调节器在长管道中旋转气流的衰变3. 不均匀性如上所述，在每个轴向位置（x/D），测量大约10个径向位置（r/R）的速度。于是，对于每x/D我们有一个平均速度（V）和标准差（）。不均匀性参数的定义是在不同的x/Ds根据不同的运行条件以定量比较旋转效应。不均匀参数定义为标准偏差超过平均值的速度。较高的标准差以上的平均值来说，等级越高的不均匀度越高。图6.的一些例子显示/V的变动与平均雷诺数，在大多数情况下的旋转气流，其/V与雷诺数略为下降。对于经过调整的气流，其变化是微不足道的。对于任何固定点导管，其/V的值与风机电源没有明显改变。这也说明以上提到的速度剖面在不同的平均雷诺数（速度）相似。因此，只有/V的平均值用于每个雷诺数在x/D的数量说明了非均匀性。图7就是基于这种想法的。
- 温馨提示:
1: 本站所有资源如无特殊说明，都需要本地电脑安装OFFICE2007和PDF阅读器。图纸软件为CAD,CAXA,PROE,UG,SolidWorks等.压缩文件请下载最新的WinRAR软件解压。
2: 本站的文档不包含任何第三方提供的附件图纸等，如果需要附件，请联系上传者。文件的所有权益归上传用户所有。
3.本站RAR压缩包中若带图纸，网页内容里面会有图纸预览，若没有图纸预览就没有图纸。
4. 未经权益所有人同意不得将文件中的内容挪作商业或盈利用途。
5. 人人文库网仅提供信息存储空间，仅对用户上传内容的表现方式做保护处理，对用户上传分享的文档内容本身不做任何修改或编辑，并不能对任何下载内容负责。
6. 下载文件中如有侵权或不适当内容，请与我们联系，我们立即纠正。
7. 本站不保证下载资源的准确性、安全性和完整性, 同时也不承担用户因使用这些下载资源对自己和他人造成任何形式的伤害或损失。
人人文库网所有资源均是用户自行上传分享，仅供网友学习交流，未经上传用户书面授权，请勿作他用。
川公网安备: 51019002004831号