Multiphase Flow Modelling of Fuel Tank Sloshing

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25 June
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Having already gone through the earlier blogs particularly, Multiphase Flow Modeling using CFD  wherein we discussed about multiphase flow physics i.e. What is a multiphase flow? What are its classifications ? and Mathematical Treatments in CFD Modeling of Multiphase Flows where we explained about the different mathematical modeling approaches and differences between them. 

Now talking about an industrial application of multiphase flow we shall discuss about fuel tank sloshing as a test case. 

Tank sloshing occurs when a tank filled with either fuel or any fluid is subjected to external excitation forces. Typical examples are ships with large ballast tanks and liquid cargo carriers (e.g. oil tankers) that are often at risk of exposure to sloshing loads during their operational life. Although inserting baffles and similar structural objects damps the sloshing movement intensity of liquid fuel, however cannot be used for fuels like liquefied natural gas (LNG) carriers and thus sloshing has evolved as a main design constraint wherein the accurate calculation of the sloshing loads becomes an essential element of tank design process for safe transport of fuels.

Test Case Name : Fuel Tank Sloshing

Domain Application : Oil & Gas, Automobile, Aerospace
Physics : Transient Multi-Phase Flow with Free Surface

Software Tools Used :
Pre-Processing : ANSYS ICEM-CFD 13.0
Solution : ANSYS FLUENT 13.0
Post-Processing : ANSYS FLUENT 13.0

Objective : Two objectives were set for the present study :

1. To investigate the free surface movement of liquid fuel in a tank under varying acceleration scenarios
2. To compare the effects of baffles (anti-wave baffles) on intensity of sloshing.

Problem Physics : Wherever we have tanks containing bulk volumes of liquid fuel and there is a change in acceleration of the vehicle carrying the tanks there is sloshing. The occurrence of sloshing is basically a two-phase flow between the air and the liquid fuel present in the volume. Depending upon the applications and where the tank is located, sloshing can pose challenges for the structural design or safe transportation of the fuel.

In real life scenarios such phenomenon is applicable for :

1. In automobile fuel tank, to investigate and ensure the continuous supply through fuel pick-up line and
2. In oil & gas companies, to investigate the effect of ship motion of sloshing of oil in production separator/ oil surge vessel/ hydrocarbon storage tank.

We take a case of a tank half filled with kerosene fuel on a vehicle. The vehicle undergoes acceleration in the positive X-axis direction of 9.81 m/s2 for 1.5 seconds and then the acceleration stops for the next 1 sec. In the state the tank is under the influence of gravitational force for all the time as shown in image below.

We shall solve the demonstration for three cases :

Case 1. Tank without baffles
Case 2. Tank with zero thickness baffles. The baffles have semi-circular holes, two on each baffle.
Case 3. Tank with porous baffles, having an array of holes.

Physically as it is very complex and computationally very expensive to model them as they are, hence we approximate these perforated baffles into porous media with some thickness with the same overall effect, as shown in figure below. The purpose of baffles is to damp the oscillation/ sloshing to provide structural stability.Below images shows the distinguishing factors for the geometries we have used for each case.

Three cases under study

Details of case set-up :

  • Primary phase: Air
  • Secondary phase: Kerosene
  • Distinct free surface between two phases
  • Flow Physics : Transient, porous media

Transient Behavior :

  • Total cycle time: 2.5 sec
  • Acceleration of 9.81 m/sin positive x-direction for 1.5 sec
  • Acceleration of 0 m/s for 1.5 to 2.5 sec
  • Gravitational force acting all the time

Case-2: Baffles with zero thickness

Case-3: Baffles with thickness & porosity

Simulation results for the three cases

Tanks sloshing animation for three cases :

Case 1 : Maximum splashing of kerosene occurs at 0.45 second, i.e. quite early. The volume of fluid (VOF) model was used to solve the case. 

Case 2 : Splashing occurred at 0.9 second. As can be seen the splashing intensity is reduced because of the presence of baffles and settlement of the fluid (kerosene) is more quicker as compared to in Case 1, as the splashing is damped with the baffles. 

Case 3 : Splashing occurred at 0.87 second. Here we have baffles with thickness and porosity and hence the damping is more as compared to the earlier cases. Thus kerosene settles down pretty faster also as can be seen the animation below. 

This demonstrates application of VOF multiphase flow model to carry out sloshing simulation studies. As CFD clearly shows the intensity of sloshing as a function of time, CFD simulations can serve as an important tool to tackle such industrial problems of tank sloshing providing detail insights for design optimization.


 

The Author

vijay-mali

Vijay is COO and Co-Founder of CCTech & LearnCAx. Vijay's major contribution in professional career is growing CCTech from team of two people to group of 30 technologists and now CCTech is a preferred partner to many engineering industries. At CCTech, Vijay look after business development for CFD division and a member of technical review committee.

Since beginning of Vijay's professional career, he has passion for education. At CCTech, he conceptualized a unique training program on CFD which was then taken by more than 500 students and most of them are now working as CFD analyst in industry. This program is now considered as benchmark for classroom training. The same passion caused birth of LearnCAx, an online education brand of CCTech. LearnCAx is first MOOC platform dedicated for CFD education.

Prior to CCTech, Vijay worked with ANSYS India (formerly Fluent India) in FloWizard development team. Vijay hold M.Tech. in Aerospace Engineering from IIT Bombay.


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+3 Vijay Mali 2014-02-26 03:01
This is the blog with application of one of the multiphase model in ANSYS FLUENT name VOF for industrial case. Preliminary overview is given. If you have specific question or query, write it in the comment and I will get back to you !!!
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0 Jade 2016-01-26 20:15
Hello, I am a french student and I am very interested in your article about the sloshing, but I don't really understand what are baffles and thickness baffles, what is it made of ? Can I build it myself if I want to reproduce the experience ?
Thank you.
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0 Jade 2016-01-26 20:17
Hello, I am a French student and I am very interested in your article about sloshing. But I don't really understand what are "baffles" and "thickness baffles", what it is made of ? Can I build it myself if I want to reproduce the experience ?
Thanks.
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0 Praveen Kumar 2016-01-28 16:16
Commonly, Baffle is referred as a flow-directing vane in ducts/vessels. Here it is just used to reduce/suppress the sloshing in the tank/vessel. The material can be same as that of the vessel or different. In general baffles has some thickness and when we say 'zero-thickness', it is just an assumption that is valid if the effect of thickness on the fluid(sloshing) is negligible.
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