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储油罐内气体空间传热传质规律及风洞实验相似准则数的研究

作者:完美论文网  来源:www.wmlunwen.com  发布时间:2019/10/8 9:14:04  

摘要:目前,集成油气回收系统和密闭排气系统的常压拱顶罐作为储存汽油等高挥发性油品的设施虽然得到了较为广泛的应用,但罐内气体空间的传热传质机理尚不清楚。因此,本文通过小型拱顶罐的小呼吸损耗实验、自编程序模拟和计算流体力学(CFD)技术来研究太阳辐射、大气温度和环境风速对拱顶罐内气体空间传热传质规律的影响。对于外浮顶罐而言,油气通过浮盘上的孔隙直接进入外界环境中,对于此类有别于拱顶罐内的传质问题,风洞实验是一个很好的研究手段。为此,本文以外浮顶罐为例,结合数值模拟技术和风洞实验平台,研究油气扩散风洞实验准则数的适用性问题。取得研究成果如下:

(1) 搭建小型拱顶罐的小呼吸损耗实验平台,研究罐内的传热传质规律。液体空间罐壁温度的变化幅度远小于气体空间罐壁温度的变化幅度,气体空间罐壁温度在垂直方向上的温度梯度很小;罐顶中心位置温度约等于罐顶平均温度。白天时沿罐内气体空间中心线自下而上,温度逐渐升高,温度梯度逐渐增大;夜晚时罐内中心线上的温度几乎相等,温度梯度几乎为0。一天之内,罐内液面温差为12 K左右。日出之后,罐内油气浓度逐渐升高,在13:00~14:00左右达到最高值,而后渐渐降低,在下个日出时达到最低值,在后半夜,罐内油气浓度趋于稳定,变化幅度不大。一天之内,储罐小呼吸损耗率为0.20‰左右。

(2) 利用集中参数法,建立拱顶罐的0维非稳态传热传质理论模型,并利用Matlab软件编写程序,在春分日6:00至18:00的时间段内,对储罐温度、储罐传热系数和液相蒸发量随时间的变化规律进行了数值模拟。罐内自然对流换热系数、储罐边界(罐壁和罐顶)的辐射换热系数和总传热系数的变化幅度很小。罐顶、气体空间罐壁及液体空间罐壁温度的变化趋势跟大气温度变化趋势一致,最大值均出现于14:00时分;气体空间罐壁温度最高,罐顶温度次之,气相温度又次之,液体空间罐壁温度最低;气相温度居于大气温度和储罐边界(气体空间罐壁和罐顶)温度之间;液位为2875mm的1000 m3拱顶罐内液相蒸发量为421.13 kg,在不考虑小呼吸损耗的情况下,罐内油气浓度达到495.45 g•m-3。

(3) 以正己烷作为油气的替代物,综合拱顶罐传热因素,利用ANSYS Fluent软件分别研究1000 m3拱顶罐内2维气体空间在春分日、夏至日和冬至日,以及不同液位时的传热传质规律。上午和下午时,靠近阳面的温度等值线上凸,靠近阴面的温度等值线下凹,中午时,温度等值线较为平稳;罐内气体空间平均温度随时间呈抛物线趋势变化,且随静储液位的升高而降低,下午2点左右达到最高值。在罐壁和罐顶处有很明显的速度边界层,且罐内有明显的逆时针漩涡;同一时刻,静止储存液位越高,气流最大速度越大;同一静止储存液位,中午时分气流最大速度最小。气体空间正己烷的浓度自下至上逐渐减小,越往上浓度梯度越小;靠近液面附近存在一个浓度较高的大浓度层;气体空间纵向浓度差随静止储存液位的升高而变小,气体空间的平均浓度随着时间的推移和静止储存液位的升高而增大。

(4) 结合风洞实验和数值模拟技术,研究外浮顶罐油气扩散的风洞实验所遵循的准则数。在3种不同液位下,模型的临界雷诺数(Recrit)相等,均为22436。当Re≥Recrit时,油罐内的流动结构不再发生变化,此时满足原型与模型的相似与Re数无关。在满足Re无关的基础上,分别基于Frρ数相等原则和Nemoto准则设计风洞实验和原型模拟,得到风洞实验和原型模拟的浓度分布对比,前者具有较高的一致性,而后者相差较大。研究成果可为油气扩散实验提供一定的指导。

The breathing loss experiment of the smalldome roof tank (DRT) and the numerical simulation with self-compiled programand ANSYS Fluent software were conducted to investigate the mechanism of heatand mass transfer in the gas space of the DRT. The applicability of thecriteria in the wind tunnel experiment for oil vapor diffusion was investigatedby combining numerical simulation technology and wind tunnel experimentalplatform. The research results are as follows:

(1) The results of breathing lossexperiment of the small DRT are showed that the temperature variation range ofthe wall contacting with liquid (WCWL) is smaller than that of the wallcontacting with gas (WCWG) and the temperature gradient of the WCWG is verysmall in the vertical direction; The tank roof temperature can be treated asthe average temperature of the tank roof. During the day, the temperaturegradually increases along the central line of the tank from liquid surface toroof, while at night, the temperature in the gas space of the tank is almostequal. The variation trends of the average gas temperature and liquid surfacetemperature in the tank are consistent with that of ambient temperature. Withinone day, the temperature difference of liquid surface is about 12K. Aftersunrise, the concentration of oil vapor in the tank rises gradually, reachesthe highest value at 1~2 p.m., then decreases gradually, reaches the lowestvalue at the next sunrise. In the latter half of the night, the concentrationof oil vapor tends to be stable, with little change. Within one day, thebreathing loss is about 0.20‰.

(2) A theoretical model of 0-dimensionalunsteady heat and mass transfer for the DRT was established by using the lumpedparameter method and considering the heat transfer factors of storage tankssynthetically. A program was compiled with the MATLAB software to simulate thevariation of the tank temperature, heat transfer coefficient and liquid phaseevaporation with time in the spring equinox period from 6:00 to 18:00.

The variation of the heat transfercoefficients, including the natural convection in the tank, radiation heattransfer coefficients of the tank boundary and total heat transfer coefficientsof tank are stable. The variation trends of temperature of gas phase, tankroof, the WCWG and the WCWL are consistent with that of air temperature. TheWCWG has the highest temperature, the tank roof temperature takes the secondplace, the gas phase takes the third place, and the WCWL takes the least place.The gas phase  temperature is betweenatmospheric temperature and the tank boundary (WCWG and roof)temperature. Theliquid evaporation in the 1000 m3 DRT with a liquid level of 2875 mm is 421.13kg under the initial condition of zero oil vapor concentration in the tank.

(3) Considering the storage conditions andtaking n-hexane as the representative of the light oil products, the unsteadyheat and mass transfer in 2-dimensional gas space of the 1000 m3 DRT with differentliquid levels was simulated by ANSYS Fluent software on the spring equinox,summer solstice and winter solstice. In the morning and afternoon, thetemperature isoline near the sunny side is convex, while the one near the nightside is concave; at noon, the temperature isoline is relatively stable; theaverage temperature of the gas space in the tank decreases with the increase ofthe static storage level, changing with time in a parabolic trend, reaching thehighest at about 2 p.m. under the solar radiation. There is an obvious velocityboundary layer near the tank wall and roof, and there is an obviouscounter-clockwise eddy in the tank. The maximum velocity of gas in the tank atdiferent time is in the range of 0.08~0.49 m•s-1. The gas velocity which is thelowest at noon increases with the rising of the stationary level.The averageconcentration of gas space increases with the rising of the stationary storagelevel and time.

关键词:小呼吸损耗;传热系数;ANSYSFluent;风洞;准则数

breathing loss;heattransfer coefficient;ANSYS Fluent;wind tunnel;criterion number

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