Academic Journal of Science and Technology 
ISSN: 2771-3032 | Vol. 15, No. 1, 2025 

 

238 

Research	on	the	Flow	Characteristics	of	Low	Liquid	
holdup	Gas	Liquid	Two	Phase	
Shunan He1, Denghai Wang3, Yao Li1, Mao Li2, Gao Feng4, Yongfei Gao1 

1Second Gas Production Plant, Changqing Oilfield Company, CNPC, China 
2Machine Manufacture Plant, Changqing Oilfield Company, CNPC, China 
3Changqing Oilfield Company, CNPC, China 
4Changqing Engineering Design Co.LTD, China 

 

Abstract: With the continuous growth of natural gas demand, the low liquid content gas-liquid two-phase flow characteristics 
of wet natural gas during pipeline transportation have become a research hotspot. Accurate prediction of pressure drop gradient 
is crucial for pipeline design, operational safety, and economic benefits. This article summarizes the flow characteristics of gas-
liquid two-phase flow with low liquid content in wet natural gas pipelines, and focuses on analyzing the applicability and 
predictive performance of three typical theoretical models (FLAT model, ARS model, and double circle model). The FLAT model 
is based on the assumption of a horizontal interface, which is computationally simple but ignores interface fluctuations; The ARS 
model considers the gas-liquid shear effect and is suitable for low liquid content conditions; The double circle model assumes a 
more realistic flow through a double arc interface, but the calculation is complex. Research has shown that existing models still 
have limitations in predicting complex conditions such as inclined pipelines, high flow rates, and droplet entrainment. Future 
research needs to combine multi-scale simulation, intelligent algorithms, and high-precision experiments to further enhance the 
universality and engineering applicability of the model. This article provides a systematic reference for the theoretical research 
and engineering practice of the flow characteristics of wet natural gas pipelines. 

Keywords: Wet natural gas; Gas-liquid two-phase flow; Flow characteristics. 
 

1. Introduction 
With the rapid development of our country's economy, the 

demand for energy in our country is also increasing. At the 
same time, due to the increasingly tight and scarce non 
renewable resources such as oil and natural gas, the extraction 
and transportation of natural gas have received more and 
more attention.  After natural gas is extracted from gas wells, 
it is often accompanied by components such as free water, 
water vapor, and liquid hydrocarbons. This type of natural gas 
is called wet natural gas. 

At present, the exploration and development of oil and gas 
resources in China are gradually shifting towards unmanned 
areas such as deserts and oceans, and many large-scale oil and 
gas reservoirs have been detected.  In order to transport the 
extracted oil and gas resources to downstream users, the 
following two transportation methods are generally adopted: 
the first is to establish a gas processing plant around the well 
site to separate the wet natural gas (also known as condensate 
gas) extracted from the gas well into condensate oil and dry 
natural gas, and then transport it in a single-phase manner.  
The second method is to not separate the wet natural gas 
extracted from the gas well first, and use the pressure at the 
wellhead to transport the wet natural gas through a long-
distance pipeline to a gas treatment plant near downstream 
users for processing. 

The first wet natural gas processing method requires the 
construction of corresponding natural gas processing 
equipment and related operators near the oil and gas field.  
As natural gas extraction gradually shifts to areas with harsh 
natural environments such as deserts and oceans, adopting the 
first treatment method will undoubtedly increase the cost of 
natural gas extraction and reduce its economic benefits. 
Therefore, in order to reduce the extraction cost of enterprises, 

the second treatment method is generally adopted, which first 
transports wet natural gas to downstream users for processing. 

Long distance pipeline transportation of wet natural gas 
reduces extraction costs, but during the natural gas 
transportation process, with changes in temperature, pressure, 
and other parameters along the pipeline, liquids such as 
condensate may precipitate inside the pipeline. [1-3]  The 
precipitated liquid deposits in the low-lying areas of the 
pipeline, forming a liquid accumulation that can affect the 
efficiency of natural gas pipeline transportation, corrode the 
pipeline, and even cause ice blockage and accidents in severe 
cases. Even single-phase natural gas will precipitate liquid in 
the pipeline during transportation when the temperature 
inside the pipeline is lower than the saturation temperature of 
water vapor or the dew point of heavy hydrocarbons. 

During the transportation of wet natural gas, the released 
liquid can affect the safe and stable operation of the entire 
pipeline, and even cause corresponding economic losses.  
Natural gas transmission pipelines often pass through areas 
with undulating terrain, causing downward and upward tilting 
of the pipeline. When the gas velocity is lower than the critical 
liquid carrying velocity of the pipeline, it will accumulate in 
low-lying areas of the pipeline, forming liquid accumulation 
and causing a series of problems [4,5]. 

When the transportation speed of natural gas is higher than 
the critical liquid carrying velocity of natural gas, the liquid 
deposited in the low-lying area of the pipeline will be carried 
by the natural gas and transported along the pipeline with the 
gas, forming a gas-liquid two-phase flow.  Therefore, natural 
gas pipeline flow belongs to the category of multiphase flow, 
and its flow state is relatively complex, which is an urgent 
issue in contemporary oil and gas industry production. 
Therefore, countries with relatively developed petroleum 
industries have invested a large amount of material and 



 

239 

human resources in recent decades to study the characteristics 
of gas-liquid two-phase flow in natural gas transmission 
pipelines. 

The hydraulic characteristics such as pressure drop and 
liquid holdup play a very important role in the design and safe 
operation of condensate natural gas pipelines. Studies have 
shown that the presence of a small amount of liquid can have 
a significant impact on the flow of gas phase in pipelines.  
Hart et al. [6] pointed out that the pressure drop of natural gas 
containing 0.5% condensate is 30% higher than that of single-
phase natural gas. Therefore, the low content of liquid in 
pipelines has a significant impact on the transport capacity 
and efficiency of natural gas transportation systems. Accurate 
prediction of liquid holdup and pressure drop in gas-liquid 
two-phase flow under operating conditions is of great 
significance for pipeline size design and selection of 
compression equipment.  At the same time, the liquid holdup 
rate is also an important parameter that determines the 
frequency of natural gas pipeline cleaning and the design of 
downstream equipment. It has significant reference value for 
analyzing pipeline corrosion, wear, paraffin deposition, and 
hydrate formation. 

Therefore, it is of great significance to conduct in-depth 
research on the flow characteristics of liquid accumulation in 
low-lying areas of wet natural gas pipelines during 
transportation, accurately predict the critical liquid carrying 
velocity, pipeline pressure drop gradient, and liquid holdup 
characteristics of pipeline liquid accumulation [7-9] .It can not 
only reduce costs for enterprises, but also reduce resource 
waste for the country, which has significant economic and 
social significance. 

2. Low Liquid Content Gas Liquid 
Two Phase Flow Model 

The shape of the gas-liquid interface is an important factor 
in studying the flow characteristics of low liquid content gas-
liquid two-phase flow in wet natural gas pipelines, and is 
crucial for predicting critical liquid carrying velocity, pressure 
drop, and holdup.  When the gas phase flow rate is high, the 
gas-liquid interface will bend, and the interface shape is 
therefore very different from the flat interface assumption of 
the basic two fluid model. Researchers have not yet reached a 
consensus on how to determine the interface shape. [10]  Due 
to the shear effect of high-speed airflow, a large number of 
small droplets appear in the wavy stratified flow. The 
mechanism of droplet carrying and deposition makes the 
interface rougher. Therefore, how to better predict the basic 
flow parameters after determining the interface shape requires 
comprehensive consideration of various factors such as 
interface friction and droplet entrainment in theoretical 
calculations.  And different phase interfaces correspond to 
different wet wall fractions, interface perimeters, and friction 
coefficients.  And the closed form relationships of these 
parameters are essential for predicting pressure drop and 
holdup.  Domestic and foreign scholars have successively 
established different interface shape models (Figure 2-1) to 
study the flow characteristics of gas-liquid two-phase 
stratification. 

 
Figure 2-1. Schematic diagram of different gas-liquid 

interface shapes 

2.1. FLAT Model 
In 1976, Taitel and Dukler first proposed the FLAT model, 

an interface shape model for gas-liquid two-phase laminar 
flow, which assumes that the gas-liquid interface shape during 
the flow process is horizontal, as shown in the above figure. 

Liquid phase channel area: 

 

   (1) 

 
Gas phase flow channel area: 

 

    (2) 

 
Pipeline cross-sectional area: 

 

                   (3) 

 
Liquid phase wetting circumference: 

 

            (4) 

 
Gas phase wetting circumference: 

 

          (5) 

 
Pipeline cross-sectional circumference: 
 

                         (6) 

 
Wetting circumference at gas-liquid interface: 

 

                  (7) 

 

2

2 1 L L L
L 0 25 π cos 2 1 2 1 1 2 1- h h h

A . d
d d d

    
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2

2 1 L L L
G 0 25 π cos 2 1 2 1 1 2 1- h h h

A =A . d +
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



 

240 

In the formula, represents the height of the bottom liquid 
film, m. 

2.2. ARS Model 
In 1989, Hart  proposed a new interface shape model 

called ARS (Apparel Rough Surface) model for low liquid 
holdup (HL<0.06), and established a correlation between 
frictional pressure drop and liquid holdup in horizontal gas-
liquid pipe flow based on this model. 

Liquid phase wet wall fraction: Hart pointed out that when 
the gas phase flow rate is low, the liquid phase wet wall 
fraction can be calculated by the following formula 

 

             (8) 

 
In the formula, Θ represents the wet wall fraction of the 

liquid phase, dimensionless, and FrL represents the Froude 
number of the liquid phase, dimensionless. 

Liquid phase Froude number: 

 

              (9) 

 
In the formula, d represents the inner diameter of the 

pipeline, m; g represents the acceleration due to gravity, m2/s。 
Liquid phase channel area: 

 

                (10) 

 
Gas phase flow channel area: 

 

             (11) 

 
Liquid phase wetting circumference: 

 

                  (12) 

 
Wetting circumference at gas-liquid interface: Hart pointed 

out that in the ARS model, the thickness of the liquid film at 
the pipe wall is much smaller than the inner diameter of the 
pipeline, so the wetting circumference at the gas-liquid 
interface in the ARS model can be approximately equal to the 
wetting circumference of the liquid phase; 

 

                  (13) 

 
Liquid phase wetting circumference: 

 

             (14) 

 
Friction factor at the gas wall: 

 

           (15) 

 
In the equation,epresents the gas phase Reynolds number, 

which is dimensionless. 
Gas phase Reynolds number: 

 

             (16) 

 
In the formula, μ G represents the gas-phase dynamic 

viscosity, Pa•s 
Friction factor at gas-liquid interface: 

 

         (17) 

 
In the formula, k/d represents the relative roughness of the 

pipeline, which is dimensionless. 
Pipeline roughness: 

 

           (18) 

 
δ represents the average thickness of the liquid film at the 

pipe wall, m. 
Average thickness of liquid film: 

 

                  (19) 

 
Friction factor at the liquid wall: 

 

             (20) 

 
In the equation, represents the apparent Reynolds number 

in the liquid phase, 

 

             (21) 

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241 

2.3. Double circle model 
In 1997, Chen et al. proposed the Double Circle model and 

theoretically studied the liquid holdup and pressure drop of 
gas-liquid two-phase stratified wavy flow in horizontal pipes 
under low liquid holdup. The model suggests that the 
interface shape of gas-liquid two-phase flow is a double circle. 

According to the empirical relationship proposed by Hart 
et al. [27] (1989) based on experimental data, the wet wall 
fraction of the pipeline can be calculated based on the liquid 
holdup HL: 

 

           (22) 

 
Liquid phase Froude number: 

 

               (23) 

 
Liquid holding capacity; 

 

                    (24) 

 
According to the geometric relationship in the double ring, 

the liquid phase flow channel area can be obtained: 

 

     (25) 

 
According to the definition of liquid holding capacity: 

 

                  (26) 

 
Based on the geometric relationship of double circular 

rings, the relationship between pipeline diameter, circular ring 
diameter, and central angle can be obtained: 

 

              (27) 

 
From the above equation, we can obtain: 

 

 (28) 

 
Gas phase flow channel area: 

             (29) 

 
Liquid phase wetting circumference: 

 

                   (30) 

 
Gas phase wetting circumference: 

 

              (31) 

 
Diameter of the circular ring: 

 

                (32) 

 
Wetting circumference at gas-liquid interface: 

 

                   (33) 

 
Thickness of liquid film at the bottom of the pipeline: 

 

     (34) 

 
Friction factor at the liquid wall: 

 

           (35) 

 

         (36) 

 
In the equation, ReL represents the liquid phase Reynolds 

number, which is dimensionless. 

 

                    (37) 

 
In the formula, dL represents the diameter of the liquid 

phase water, dimensionless. 

 

                   (38) 

0 374 0 580 52 0 26. .
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 
2

L L
L

L G

u
Fr

gd


 


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A
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A
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1 2 1 2

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D D
A D D

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1

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D
A H

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sin H

sin tan

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D
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1
2 1

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sin
D D

sin


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

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   1 2
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2 2L

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h cos cos    

L L
L

16
2000f             Re

Re
 

0 2
L L L0 046 2000.f . Re        Re 

L SL L
L

L L

V d
Re

H




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4 L
L

L

A
d

S




 

242 

Friction factor at the gas wall: 
In the equation, ReG represents the gas phase apparent 

Reynolds number 
 

               (39) 

 
In the formula, dG represents the gas phase hydraulic 

diameter, dimensionless. 
Gas phase hydraulic diameter: 

 

                (40) 

 
Friction factor at gas-liquid interface: 

 

    (41) 

 

3. Results and Discussion 
This article studies the flow characteristics of gas-liquid 

two-phase flow with low liquid content in wet natural gas 
pipelines, with a focus on analyzing the performance of FLAT 
model, ARS model, and Double Circle model in predicting 
pressure gradients. This article uses experimental data from 
Mantilla et al. for analysis[11]. 

 

 
Figure 2-1. Comparison of Model Prediction Results 

 
The FLAT model assumes that the gas-liquid interface is a 

horizontal plane, simplifying the calculation of geometric 
parameters such as wetted perimeter and flow area, and is 
suitable for gas-liquid two-phase flow under low velocity and 

laminar conditions. It has high computational efficiency and 
a clear theoretical foundation. However, it ignores interface 
fluctuations and has significant prediction errors under 
turbulent or high gas velocity conditions. 

The ARS model more accurately describes the effect of 
gas-liquid shear on pressure gradient. However, its 
application is limited and only suitable for low liquid content 
conditions, without considering droplet entrainment. 

The double circle model assumes that the gas-liquid 
interface is a double circular arc shape, which is closer to the 
geometric characteristics of actual flow. It can calculate the 
liquid holdup and wetted perimeter more accurately. The 
interface shape parameters can be dynamically adjusted. 
However, its computational complexity is high and lacks 
universality. 

As shown in Figure 2-1, the prediction of pressure gradient 
in horizontal pipelines must take into account the accurate 
prediction of shear stress, which has a significant impact on 
model prediction.  

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and Analysis of Wet Natural Gas [J]. Natural Gas Technology, 
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[2] Meng, W. Low-Liquid Loading Gas-Liquid Two-Phase Flow 
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[3] Li Yansong  Research progress on water thermal models of 
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[4] Gu Qiong  Research on the Accumulation Pattern of Wet Gas 
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[5] Li Yuxing, Feng Shuchu. Study on Transient Simulation and 
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[11] Mantilla, I.: Mechanistic Modeling of Liquid Entrainment in 
Gas in Horizontal Pipes. Ph.D., University of Tulsa, OK,2008. 

 

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