王 栋
论文题目：分流分相式气液两相流体流量计
作者简介：王 栋，男，1960年02月出生，1995年02月师从于西安交通大学林宗虎教授，于2000年06月获博士学位。
摘 要
气液两相流是石油、化工、动力等工业领域常见的流动工况，但气液两相流体的流量测量方法却一直是国际上没有很好解决的一个难题。近年来随着现代工程规模的迅速发展，该问题显得更加突出，世界各国都在投入巨资进行开发研究。本学位论文在国家自然科学基金会重大项目（59995460）的资助下，研究探索测量气液两相流体流量的新方法，作者在本论文中发明的测量方法已获得发明专利。
论文首先综述了气液两相流体流量测量方法的现状，根据测量过程中是否对来流进行分离，可以将现有的技术分为分离法和非分离法。传统的分离法采用分离设备将气液混合物分离后再进行测量，具有测量精度较高、测量结果可靠、测量过程不受流型变化的影响等优点。但缺点是分离设备体积庞大价格昂贵，并需建立专门的计量站和测试管线。近年来出现的“粗分离法（partial flow
separation）”通过把来流粗分离为以气相为主和液相为主的两股两相流后再进行测量，虽然在一定程度上能够缩小分离器的体积，并降低两相流流量测量难度，但因未能将气液混合物完全分离，故实际上对提高测量精度的作用是很有限的，况且由于测量仪表的数量增加了一倍，反而有可能使测量精度降低，仪表成本增加。非分离法的主要缺点是，测量仪表和传感器都在两相流中工作，易受两相流的强烈波动性和流型变化等因素的影响，在可靠性、稳定性和测量精度等方面远低于在单相流中的情况，甚至根本就无法正常工作。目前除核磁共振和放射性示踪等技术外，其他的非分离法技术在测量原理上仍缺乏较坚实的理论基础。迄今还没有一种两相流量计能够达到公认的商用标准：测量误差小于5%，测量结果不受流型影响，并能长期在各种工程环境下工作。核磁共振和放射性示踪等技术虽很有希望，但由于这类测量设备过于昂贵（几十万到上百万美元），因而很难在工程中广泛应用。
本文提出的测量方法简称分流分相法。这种测量方法在测量原理上突破了现有的气液两相流体流量测量模式（即分离法与非分离法），其具体过程为，首先通过一种分配器从被测两相流体中成比例地分流出一部分（5%～20%）气液混合物，接着使用一小型分离器将这股两相流分离成单相气体和液体，然后分别用单相流量计测量出气相和液相流量并根据比例关系将测量值换算成被测两相流体的流量，最后再分别将这部分气体和液体返回被测两相流中。与传统的分离法相比，由于进行了分流，分离器的负荷仅为原来的5%～20%，因而体积可以比原来全部分离时缩小5～20倍，基本上接近于一个普通单相流量计的体积。与其他各种非分离法相比，测量仪表都在单相流中工作，测量结果不受两相流的波动性和流型变化的影响。因而在测量精度、可靠性以及测量范围等方面都远优于各种非分离式仪表。
如何实现“成比例”地分流，保证分流出的流体流量与被测两相流体流量之间具有稳定和确定的比例关系（即分流系数），是本文研究的主要内容。如何构造能“成比例”地分流的分配器是论文要解决的关键问题。作者共探索研究了以下4种基本类型的分流分相式两相流体流量计：
1. 三通型。首先从三通管入手进行探索，利用T-型三通的相分离特性从被测两相流中分流分离出一股单相气流，通过测量这部分单相气体的流量来确定被测两相流体的流量和干度。三通管既是分配器同时又起分离器的作用。在大量实验观测的基础上建立了分流体回路和主流体回路的基本方程式，并推导了分流系数的计算式，证明这种分流分相式两相流体流量计的分流系数与干度成线性关系而不是一个常数，根据得到的气体流量测量值可以确定流量和干度中的一个参数，另一个参数需用其他方法来确定，即这种流量计属于一种单参数两相流量计。
2. 取样管型。应用取样管作为分配器来进行分流。系统地测试了传统取样管的分流特性，在此基础上对其进行了改造，增加了混合器，改端口形状为“S”型，从而使分流系数能在一个较宽的范围内保持稳定。
3. 转鼓型。为从根本上克服流型变化对分流的影响，在分流原理上突破了从“空间”上进行分流的现有模式，首次探索通过“分时”的方法进行分流，即通过分配来流分别流向分流体回路和主流体回路的“时间”来实现精确分流。这样不论来流的流型如何变化，两回路的流量大小只取决于所分配的时间长短，而与流型无关。由于分流时间是很容易控制的，从而能比较容易地保证分流系数在各种情况下都保持稳定。作者首创的转鼓分配器就是这样一种基于分时原理的分配器。分配器由壳体和转鼓两部分组成，壳体除用来支撑转鼓外，本身不含任何可动部件。转鼓的外型为圆柱体，内部均匀对称地分隔为若干互不相通的通道。沿转鼓轴线方向，通道呈螺旋型走向。各个通道的出口大都通向转鼓下游的两相流体管道（主流体回路），只有少数几个通道作为分流通道通向了分离器（分流体回路）。当两相流体流过转鼓时，就会冲击转鼓高速旋转。随着转鼓的旋转，各通道的入口端面不断地掠过管道流通截面上的每一点，使每一点上的两相流体都能机会均等地流入每一个通道。在一个旋转周期内，流通截面上各点的两相流体流入每一通道的时间均相等，这等价于来流以相等的时间份额分别依次流向了各个通道。这样，来流流向分离器（分流体回路）的时间份额，就与转鼓中通向分离器的通道数（分流通道数）成正比，而与两相流的流型无关，分流系数保持为常数。论文中已给出了简明的理论证明和实验验证。
4． 旋流型。与转鼓型分配器相反，旋流型分配器内不含任何运动部件，而是通过几组形状不同的叶栅让两相流体本身做特定的流动来实现两相流体在一个多通道元件内的均匀分配。该多通道元件的通道出口大都通向分配器下游的两相流体管道，只有少数几个通道作为分流通道通向了分离器。这样，流向分离器的流量，仅仅取决于通向分离器的通道数（分流通道数），而与两相流体的流型和其他因素基本无关，也就是说分流系数为常数。这是一种非常适合于工程应用的分配器。
在空气—水实验台上，对上述4种形式的分流分相式气液两相流体流量计进行了实验研究。实验管道内径为30毫米，水平放置。实验压力温度接近于常温常压。空气折算流速范围为4～40m/s，水折算流速范围为0～0.28 m/s。实验中出现的流型包括分层流、波状分层流和环状流。实验结果表明，上述四种流量计都具有稳定的或变化规律确定的分流系数。流量计工作平稳，测量范围广，测量精度高。平均误差小于5%，最好的结果可以达到3%以内。
分流分相法是本论文首创的完全具有中国知识产权的气液两相流体流量测量方法，它为解决工业领域内的气液两相流体流量测量问题提供了一条新的途径。通过分流使分离器的体积成倍地缩小，通过分相把两相流体的流量测量转化成了单相流体的流量测量，通过采用合理的分配器，可以获得稳定的分流系数值，从而使气液两相流体的流量测量精度有可能接近甚至达到单相流的水平。其成本约为3万元人民币，而一台与其测量精度接近的核磁共振两相流量计的造价高达几十万美元。因而本方法使气液两相流体流量计有可能象单相流量计一样低成本地批量生产和广泛应用。
关键词：流量计 两相流 分流分相
Gas-liquid Two-phase Flowmeters
with Extracting and
Separating Method
ABSTRACT
Gas-liquid two-phase flows
are common in the petroleum、chemical and power industries，but the measurement of their
flowrate are still a difficult problem in engineering．In recent years，with the rapid
development of modern engineering project，this problem become more
urgent to solve，a great deal of investments throughout the world have been made to
develop this kind of flowrate instruments．The objective of this
thesis, financially supported by the National Natural Science foundation of
China key project（59995460），is to research and develop new measurement methods
of two-phase flow, and the methods invented in this paper have been granted to
be Chinese patents．
In the
first part of the thesis, a review of the prior arts of two phase flow
measurement was made, and these arts can be classified as separation method and
Non-separating method, depending on whether the two phase mixture is separated
in the process of measurement. In traditional separation method, a large
separator is always employed to separate the gas-liquid mixture first, and then
meter the flowrate of each phase with conventional single phase flow meters.
This solution offers definite advantages, such as metering reliability and flow
regime independent. Usually the separation equipment is very large and
expensive, and some special pipe lines and metering stations also have to be
built. The partial flow separation method, developed in recent years, partially
separate the two phase mixture into predominantly liquid and predominantly gas
streams before measuring with two-phase flowmeters. Although the separator can
be reduced in size and each flow stream only needs to be measured over a
limited range of component fraction, the improvement in measurement precision
is limited and the cost of flowmeter may increase, for the reason that the
gas-liquid mixture is not fully separated, and the number of sensors and
instruments needed is doubled. The major disadvantages of Non-separating method
lie in the facts that all the sensors and instruments are directly disposed to
the two-phase flow, although some of them are non-intrusive. The violent
fluctuations and flow pattern changes of two phase flow will make the sensors
and instruments be far less stable、reliable and precise than
that in single phase flow, and even can not work at all. Except for Nuclear
Magnetic Resonance (NMR) and Radio type techniques such as PNA (pulsed neutron
activation), other non-separating meters are lack of reasonable theoretical
foundations. So far, none of two-phase flow meters can meet the commercial
stand: measuring error less than ±5%, flow regime independent, and suitable for
use in industrial environment. NMR and PNA are hopeful, but they are too
expensive (cost～$105－106) to be used in engineering.
The
new method proposed in this thesis is called “Extracting and Separating Method
or abbreviated to ESM” which break through the prior models of gas liquid
two-phase flow measurement (separating or non-separating). The process is that：first，proportionally
extracting a stream of gas-liquid mixture（5%～20%）from the
two-phase flow to be measured with a distributor，then separating it into
single-phase gas and liquid with a small compact separator，after that，measuring each
phase with conventional single phase flowmeters and converting the measured
values to the two-phase flowrate to be measured according to the extraction
ratio，at the end,
returning the extracted stream into the two-phase flow pipe again．Because only a
small portion of the total flow is extracted, the separator can be reduced 5～20 times in size
compared with the traditional separation technique in which all the two-phase
mixture is separated, and the volume of the two-phase flowmeter may nearly
reach the bulk of a common single phase flowmeter．Furthermore, the ESM is also
much better than the most non-separating methods in reliability 、stability 、precision 、measuring range
and so on. Because the instruments utilized in ESM system work in the single
phase flow, the measurement results would not be influenced by the flow
patterns and the transit nature of the two-phase flow．
How
the “proportional extraction” can be approached to assure that the extracted
stream is the representative of total fluid stream or the extraction ratio is
stable, is the major task of the thesis．How to construct such
distributors is the key problem to be solved in the thesis. Four basic types of
distributor were proposed and studied in the thesis：
1. The T-junction type By utilizing the phase split phenomena in T-junctions, a
single-phase gas stream is extracted and separated from the total fluid stream,
and a conventional flow meter is provided to meter the gas flow rate which then
is used to calculate the mass flow rate or mass quality of the two-phase mixture
to be measured. The junctions not only act as a distributor but also a
separator. Based on a series of experiments, the basic equations on the
extraction loop and the main loop were proposed and the equation of extraction
ratio was also derived consequently which show that the extraction ratio is not
a constant, it varies with the mass quality according to a linear function.
Only one of the parameters (the flow rate or mass quality) can be determined by
the metered value of extracted gas flow, another parameter has to be metered
with other means i.e. this is a single parameter two-phase flow meter.
2.
The sample tube type Having tested
the extracting behavior of conventional sample tube systematically, a new
reformed one was constructed, which has a “S” front port, and a mixer is also
added in front of the sample tube. Quite stable extraction ratio were appeared
over a big span of flow rates．
3. The
rotational drum type. In order to
avoid the influence of flow regimes on extracting effectively, the thesis broke
through the conventional manner of flow distribution, which divide flow only
from “space”, and tried to extract flow by “time” sharing for the first time.
The extraction stream is proportionally obtained by accurately control the
extraction time fraction in which total flow is completely conducted into the
extraction loop. The extraction ratio is equal to the time fraction and
independent of flow patterns. Because the time fraction can be easily
controlled, so the task of assuring extraction ratio stable becomes much
simpler. The distributor comprises of a rotational drum and shell．The shell is to
support the drum and has not any moveable part. The outline of the drum is a
cylinder, the inside space is equally segregated into a series of small flow
channels which twisted around the drum axis．The outputs of the most
channels are directed to the down stream of the pipe, only a few of the
extraction channels are connected to the separator．As two-phase
mixture passes through the channels, the drum will be forced to run at a high
speed around its axis by the fluid．With the running of the
drum, the entrance of each channel will continuously scan over every point on
the cross section of the pipe, and so the fluid at any point on the cross
section of the pipe will have the same possibility to enter each channel. In a
rotation period of the drum, the time fraction of fluid at any point on the
cross section of pipe flowing into each channel is equal, so the drum seemingly
acts as a time controlled switch which equally directs the total flow into each
channel, the extraction time fraction and extraction ratio is proportional to
the number of extraction channels and equal to the ratio of extraction channel
number to the whole channels in the drum．An ingenious proof was made
in the thesis, and a further modified equation to consider the effect of liquid
leakage through the gap between drum and shell was established too.
4. Swirler
type. Contrasting to the
rotational drum distributor, there is no any running parts in a swirler
distributor that is very suitable for use in engineering．By using two or
three swirler assembles and conditioners, the two-phase mixture itself is
guided to pass through a special routing to complete the equal distribution in
a multi-channel assemble in which each channel has the same geometrical shape
but is not connected each other．The outputs of the most channels is directed to the down stream of the
pipe, only a few of extraction channels is connected to the separator．Therefore, the
flowrate of the stream (the extracted stream)entering the separator is only
dependent on the number of channels(the extracting channel) connected to the
separator and is independent of the flow pattern and other factors, so the
extraction ratio remain constant．
Experiments
for the four types of ESM two-phase flowmeters mentioned above were conducted
in an air-water two-phase flow loop．The inner diameter of the
loop was 30 mm and the test section was horizontally placed．The pressure and
temperature were near that at room conditions．The air superficial velocity
ranged from 4 to 40m/s, and the water superficial velocity varied between 0 and
0.28m/s．The flow patterns observed during the tests included stratified flow、wave flow and
annular flow．The experiment results showed that all the four types of ESM two-phase
flowmeters mentioned above have stable extracting ratio or the extraction ratio
change with definite manners．The flowmeters were able to work steadily and accurately over a wide range．The average error
was less than 5%, and the best results ever reached was within 3% .
The ESM
with Chinese intellectual property provide a new way to solve the difficult
problem of two-phase flow measurement in engineering．By extracting,
the size of separator can be reduced greatly；by separating, the
measurement of two-phase flow is translated into that of single phase flow and
by choosing an appropriate distributor very stable extraction ratio can be
obtained．All these would make it possible for the two-phase flowmeters to
approach or even reach the precision of single phase flowmeters．The cost of a ESM
type two-phase flowmeter is about 30000 RMB (a NMR two-phase flowmeter with the
precision of ESM, costs 105～106 US dollar).
So two-phase flowmeters would also be as largely manufactured and widely used
as single phase flowmeters．
keywords：Flowmeter Two-phase flow Extracting and Separating Method
