Multiphase Flow Modelling of Fuel Tank Sloshinglearncax.com /blog/2013/06/25/multiphase-f low-modelling-of - f uel- tank-sloshing/

Ganesh Visavale

Having already gone through the earlier blogs particularly, Multiphase Flow Modeling using CFD wherein wediscussed 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 differentmathematical modeling approaches and differences between them. Now talking about an industrial application ofmultiphase 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. Typicalexamples are ships with large ballast tanks and liquid cargo carriers (e.g. oil tankers) that are often at risk ofexposure to sloshing loads during their operational life.

Typical examples are ships with large ballast tanks and liquid cargo carriers (e.g. oil tankers) that are often at risk ofexposure 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 maindesign constraint wherein the accurate calculation of the sloshing loads becomes an essential element of tankdesign process for safe transport of fuels.

Test Case Name : Fuel Tank Sloshing

Domain Applicat ion: Oil & Gas, Automobile, AerospacePhysics: Transient Multi-Phase Flow with Free Surface

Sof tware Tools Used:Pre-Processing: ANSYS ICEM-CFD 13.0Solut ion: ANSYS FLUENT 13.0Post -Processing : ANSYS FLUENT 13.0

Object ive: Basically the two objectives were set for the present study :1. To investigate the free surface movement of liquid fuel in a tank under varying acceleration scenarios2. 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 inacceleration of the vehicle carrying the tanks there is sloshing. The occurrence of sloshing is basically a two-phaseflow between the air and the liquid fuel present in the volume. Depending upon the applications and where the tankis located, sloshing can pose challenges for the structural design or safe transportation of the fuel.In real life scenarios such phenomenon is applicable:1. In automobile fuel tank, to investigate and ensure the continuous supply through fuel pick-up line and2. In oil & gas companies, to investigate the effect of ship motion of sloshing of oil in production separator/ oil surgevessel/ hydrocarbon storage tank.

Case study:We take a case of a tank half filled withkerosene fuel on a vehicle. The vehicleundergoes acceleration in the positive X-axisdirection of 9.81 for seconds and then theacceleration stops for the next sec. In the statethe tank is under the influence of gravitationalforce for all the time as shown in image below.

We shall solve the demonstration for threecases i.e.:Case 1. Tank without bafflesCase 2. Tank with zero thickness baffles. Thebaffles have semi-circular holes, two on eachbaffle.

Im age-02: Three cas es under s tudy

Cas e-2: Baffles wi th z ero th icknes s

Cas e-3: Baffles wi th th icknes s & poros i ty

Case 3. Tank with porous baffles, having anarray of holes. Physically as it is very complexand computationally very expensive to modelthem as they are, hence we approximate these perforated baffles into porous media with some thickness with thesame overall effect, as shown in figure (Image-02) below. The purpose of baffles is to damp the oscillation/ sloshingto provide structural stability.Below images shows the distinguishing factors for the geometries we have used foreach case.

Details ofcase set -up:Number ofphases: 2- Primary phase:Air- Secondaryphase:Kerosene- Distinct freesurface betweentwo phases

Flow Physics:- Transient, porous media

Transient Behavior:- Total cycle time: sec- Acceleration of in positive x-direction for 1.5 sec- Acceleration of for to sec- Gravitational force acting all the time

From imagesbelow we cansee the freesurface wheresloshing occursfor the threecases withrespect to time.

Also with theanimations (incases below)we can seethat:Case 1:Maximumsplashing ofkeroseneoccurs at 0.45second, i.e.quite early. Thevolume of fluid(VOF) modelwas used tosolve the case.

Sim ula tion res u l ts fo r the three cas es

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

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

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

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