M2dcR2 Advisory Board, Ghent, 19/06/2014

CFD simulations of droplet-wall interaction upon impingement of heavy

hydrocarbon droplets

Amit V. Mahulkar, Pieter Verhees, Kevin M. Van Geem, Geraldine J. Heynderickx* and Guy B. Marin

http://www.lct.UGent.be E-mail: Geraldine.Heynderickx@UGent.be

*Laboratory for Chemical Technology Technologiepark 914, 9052 Ghent, Belgium

European Research Institute of Catalysis

ANSYS FLUENT 13.0 Multiphase model: Volume Of Fluid (VOF)

Interface tracking: Geo-reconstruct Actual interface

Piece-wise

linear interface

Accurate when mesh size is an order

of magnitude smaller than radius of

curvature

Phase change model

Evaporation (T > Tsat ) Condensation (T < Tsat )

rlv=rvl= 900 s-1

𝑚𝑣𝑙 = 𝑟𝑣𝑙𝛼𝑙𝜌𝑙𝑇 − 𝑇𝑠𝑎𝑡𝑇𝑠𝑎𝑡

𝑚𝑙𝑣 = 𝑟𝑙𝑣𝛼𝑙𝜌𝑙𝑇 − 𝑇𝑠𝑎𝑡𝑇𝑠𝑎𝑡

Geometry & Mesh Gravity

Inlet

Outlet

¤ 880,000 cells (~ 2 m)

¤ Diameter of domain ~ 6 droplet diameter

¤ Height of domain ~ 2 droplet diameter

¤ Time step size was such that 50 time steps

were needed for the droplet to reach wall

Mesh on horizontal bottom wall

Droplet in vertical plane of

applied geometry completed

with mesh

CFD Model Composition

Singlecomponent droplet

Multicomponent droplet

Parameter Vapor phase Liquid phase

Density (kg/m3) 9.4 830

Viscosity (kg/m.s) 710-6 0.0032

Surface tension (N/m) 0.05

Boiling point (K) 511

→Pseudocomponent: Gasoil properties

→Gas condensate

¤ Represented by 11 species

¤ 86wt% already evaporated

=86wt%

Multicomponent Singlecomponent Splash with ring formation

Stick

Splash with ligament formation

Breakup

Rebound

Mechanism:

The film keeps

growing, becomes

unstable and

ligaments are

formed. They

grow until they

break up.

Mechanism:

The entire droplet

mass gets

deposited on the

wall and

instantaneously the

film starts boiling

and disintigrates

¤ Mechanism: The droplet impinges on the wall, undergoes an elastic deformation

and spreads until a maximum stretching diameter is reached. At that moment the

surface energy of the film becomes dominant. The liquid contracts and the

droplet is formed again.

¤ Outwards velocity is determined by making an energy balance.

Mechanism:

The droplet spreads as

a film over wall. No

droplet mass

disengages from the

wall.

Mechanism:

The film keeps growing.

Liquid is accumulated at

the periphery of the film.

This liquid, detaches

from the film and wall,

keeps growing until it

disintegrates.

l = Liquid density; = Surface tension;

l = liquid viscosity; D = droplet diameter

𝑊𝑒𝑁𝑜𝑟𝑚 =𝐷𝑉𝑁𝑜𝑟𝑚

2 𝜌𝑙𝜎

=inertial force

surface tension

Normal Weber number

VNorm

Stick + Breakup

For singlecomponent droplets there is no

Stick + Breakup regime due to a single

boiling point

Multicomponent droplets have a range of

boiling point temperatures.

Mechanism:

Due to instantaneous boiling the

deposited mass fragments into smaller

entities that remain deposited on the wall.

Wall

Surface

tension

Inertia

Viscosity

Adhesion/

Vapor film

Droplet

Wall roughness

Wall temperature

A customized regime map for

impingement behavior in the

convection section of a steam cracker

is needed

Fouling in the convection section is a major

problem for crackers operating with heavy

petroleum fractions.

Droplet impingement on the heat exchanger

walls and subsequent fouling due to thermal

degradation of the liquid resulting in coke

formation are observed.

The amount of coke formed on the superheater

walls is proportional to the amount of liquid

deposited on those walls. This in turn depends

on the droplet-wall interaction. Thus the ability to

correctly predict the droplet behavior upon its

impact on a wall is of prime importance.

Superheated Steam

Feed

Nozzle

Heavy Liquid

Feed

Evaporator

Impinging droplets

→ Coke formation

Feed-steam mixture

overheater-1

Justification Convection section Droplet impingement Introduction

Abstract

Future work Acknowledgements

This work presents the construction of

regime maps based on CFD simulations

determining Stick, Splash, Rebound and

Breakup behavior of heavy hydrocarbon

droplets upon impingement on a hot wall.

First regime maps for droplets consisting of a

single pseudo-component with gasoil

properties, are constructed. Several CFD

simulations for different combinations of wall

temperature and incoming normal Weber

number are performed.

In a second step the CFD model is used to

construct regime maps for multicomponent

hydrocarbon droplets.

¤ This work was supported by Fund for Scientific Research Flanders (FWO-N: G.0022.09N)

and the Long Term Structural Methusalem Funding by the Flemish Government.

¤ This work was carried out using the STEVIN Supercomputer Infrastructure at Ghent

University, funded by Ghent University, the Flemish Supercomputer Center (VSC), the

Hercules Foundation and the Flemish Government – department EWI.

¤ Perform simulations of the superheater with implementation of the developed regime maps

and a coking mechanism to quantify coking.

¤ Test the applicability of the developed regime maps for different hydrocarbon mixtures and

validate the regime maps with data available in literature.

Splash-R

Splash-(L+R)

Splash-L

Splash-R+Breakup

Splash-(L+R)+Breakup

Splash-L+Breakup

Stick Breakup

Rebound

Splash-R

Splash-

(L+R)

Splash-L

Splash-R+Breakup

Splash-(L+R)+Breakup

Splash-L+Breakup

Stick Breakup

Rebound

Stick+Breakup