COMPARISON BETWEEN TCOMPARISON BETWEEN TCOMPARISON BETWEEN TBUBBLY FLOW FOR MODEBUBBLY-FLOW FOR MODEBUBBLY FLOW FOR MODE

(*)E Amores Vera(*) J Rodríguez RuizE. Amores Vera( ), J. Rodríguez Ruiz, gCentro Nacional del Hidrógeno Prolongación FernandoCentro Nacional del Hidrógeno. Prolongación Fernando13500 Puertollano (Ciudad Real) SPAIN www cnh2 es13500 Puertollano (Ciudad Real). SPAIN. www.cnh2.es( )

(*) @ h2 T l 34 926 420 682(*) ernesto amores@cnh2 es Tel: +34 926 420 682( ) ernesto.amores@cnh2.es, Tel: +34 926 420 682

INTRODUCTIONINTRODUCTION

Hydrogen production by water electrolysis combined with renewableHydrogen production by water electrolysis combined with renewablei i f h i ll f i dl h d denergies is one of the most environmentally ‐ friendly methods, comparedg y y , p

to traditional technologies based on fossil fuels since no CO emissions areto traditional technologies based on fossil fuels since no CO2 emissions aregeneratedgenerated.

O tl t H O tl t H2Outlet H2 Outlet H2

One of the most critical aspects onOne of the most critical aspects ont l t l i i th li idwater electrolysis is the gas‐liquid

H2

H2y g q

separation especially in systems + H

+ H

separation, especially in systems

et

ter

et

ter

with an electrolyte being nle

wat nle

watwith an electrolyte being

i l t d b (f d

I w I w

recirculated by a pump (forcedy p p (convection) [1] The main problem r +

H

2convection) [1]. The main problem

ter

ater d H

of this kind of circulation is that a wat wa

rsedof this kind of circulation is that a

f ti ld t t th

et w

let

per

gas fraction could return to the

Out

l

Out

Dis

pgelectrolysis circuit which may have

O O D

electrolysis circuit, which may haveundesiderable consequences, suchundesiderable consequences, such

th f ti f l ias the formation of explosive (a) (b)pmixtures and pump damages [2]

(a) Pump OFF

(b) Pump ONmixtures and pump damages [2] Pump OFF Pump ON

(Figure 1). Fi 1 G Li id t(Figure 1). Fig 1. Gas-Liquid separator

A suitable design of separator devices could be a solution in order to avoidA suitable design of separator devices could be a solution in order to avoidt t th l t l i i it I thi th f ta gas return to the electrolysis circuit. In this sense, the use of gas traps org y g p

deflectors might reduce hydrogen suction by pump action Howeverdeflectors might reduce hydrogen suction by pump action. However,introduction of traps or new bodies inside the separator could stronglyintroduction of traps or new bodies inside the separator could stronglyi fl th fl id d i f d li id d t b l hinfluence the fluid dynamic of gas and liquids, and turbulence phenomenay g q pcould be generated (Figure 2)could be generated (Figure 2).

The present work reports a comparison between laminar and turbulentThe present work reports a comparison between laminar and turbulentb bbl fl d l h l h d ff b b hbubbly‐flow modules. The aim is to evaluate the differences between bothymodules and whether the simplifications adopted for laminar regimemodules, and whether the simplifications adopted for laminar regimecorrectly describe the behavior of two‐phase flow within the separatorcorrectly describe the behavior of two phase flow within the separator.

GEOMETRY & SET UPGEOMETRY & SET‐UPG O & S U

The geometry of theliquid-gas interface The geometry of the(out for hydrogen, model (Figure 3)water level)

ode ( gu e 3)as designed in 2Dwas designed in 2D,

including the gasincluding the gasInlet traps, and consi‐t aps, a d co s

dering onl thedering only theGas domain occupied bytraps domain occupied by

S

the two‐phase flownt- t e t o p ase o

( ater h drogen)Poi

(water‐hydrogen).P

Outlet h i lifiAnother simplifica‐

(a) Standard (b) Separatorp

tions were made in(a) Standard separator

(b) Separator used in the model Wall

tions were made inseparator used in the model Wall

order to reduce theFi 3 G t i l t d

order to reduce thed l l iFig 2. Types of separators Fig 3. Geometry implemented model complexity.p y

COMPUTATIONAL METHODSCOMPUTATIONAL METHODSCOMPUTATIONAL METHODS

O (k d l)LAMINAR BUBBLY FLOW TURBULENT BUBBLY FLOW (k‐emodel)

Both laminar bubbly flow equations and the following ones:

( ) ( ) FIu Tlrrrrrrrr

r⎤⎡⎟⎞

⎜⎛ ∇∇∇∇∇∇

∂ φφφφ 2Both laminar bubbly flow equations and the following ones:

⎤⎡ ⎞⎛( ) ( ) FgIuuupuut lll

TllTllllll

lll

rrrrrr+⋅⋅+⎥

⎦

⎤⎢⎣

⎡⎟⎠⎞

⎜⎝⎛ ⋅∇⋅−∇+∇⋅+⋅⋅∇+−∇=∇⋅⋅⋅+

∂∂⋅⋅ ρφηηφρφρφ

3 ( ) klkT

lll SPkkuk+−+⎥

⎤⎢⎡

∇⎟⎟⎞

⎜⎜⎛

+∇=∇+∂ ερμμρρ ··r⎦⎣ ⎠⎝

( ) ( )∂

( ) klkk

lll SPkkut

++⎥⎦

⎢⎣

∇⎟⎟⎠

⎜⎜⎝

+∇∇+∂

ερσ

μρρ

( ) ( ) 0=⋅⋅+⋅⋅⋅∇+⋅+⋅∂∂

ggglllggll uut

rr ρφρφρφρφ( ) T ⎥

⎤⎢⎡

⎟⎞

⎜⎛∂ εεεμε 2r∂t

( )⋅∂ ρφ( ) ep

kSC

kCP

kCu

t klkT

lll =+−+⎥⎦

⎢⎣

∇⎟⎟⎠

⎞⎜⎜⎝

⎛+∇=∇+

∂∂ εεερεε

σμμερερ εεεε

,·· 21r

( ) glggggg mu

t−=⋅⋅∇+

∂⋅∂ rρφρφ ⎦⎣ ⎠⎝ ε

( )( )[ ]Tk 2ggggt∂ ( )( )[ ] slipgkkT

lllkl upCSuuuPkCep rrrr · , : , , TT ∇−=∇+∇∇=== φμε

ρμε μ ε

TURBULENT AND LAMINARTURBULENT AND LAMINARTURBULENT AND LAMINAR LING H2 / H2O SEPARATIONLING H2 / H2O SEPARATIONLING H2 / H2O SEPARATION

o El Santo s/no El Santo s/n. ss

RESULTSRESULTS

Figure 4 and Figure 5 show the obtained results for laminar and turbulentgu e a d gu e 5 s o t e obta ed esu ts o a a a d tu bu e tb bbl flo fl id d namics sim lations of a gas liq id separator in thebubbly flow fluid dynamics simulations of a gas‐liquid separator in thesame operation conditions Evolution of gas distribution and speedsame operation conditions. Evolution of gas distribution and speedprofiles are shown. As can be seen, in both cases with increasing time gasp o es a e s o s ca be see , bot cases t c eas g t e gasgoes do n into the separator d e to effect of the p mp Ho e er stronggoes down into the separator, due to effect of the pump. However, strongfluctuations are observed in the case of laminar bubbly flow resultsfluctuations are observed in the case of laminar bubbly flow results.

0 040.04

0.03

0 020.02

0.01

0.00300s200s120s60s40s20s0s

Fig 4. Gas fraction (surface) and velocity profiles (arrows) obtained by laminar bubbly flowg ( ) y ( ) y y

0.016

0.0120.012

0 0080.008

0 0040.004

3002001206040200 0.00300s200s120s60s40s20s0s

Fig 5 Gas fraction (surface) and velocity profiles (arrows) obtained by turbulent bubbly flowFig 5. Gas fraction (surface) and velocity profiles (arrows) obtained by turbulent bubbly flow

I Fi 6 th0 003

In Figure 6 the evo‐0.003

lution of the gas

TO

(1)lution of the gas

0 0024RN

TIT

(fraction measured on 0.0024

UR

CUfraction measured on

i t S (Fi 3)Point-S

ET CIR

Cpoint‐S (Figure 3)0 0018N

R S C

with time is 0.0018

ON

YSISwith time is

CTI

OLYpresented. Laminar

0 0012RA

CTR

Opresented. Laminari l ti h 0.0012FR C

Tsimulations show

ME LEstrong fluctuations

0 0006LUM

E E TURBULENTstrong fluctuations,

0.0006

VOL

THE

LAMINARwhile a sweet curve V T LAMINARwhile a sweet curve

bt i d f 0.0was obtained for 0.040 80 120 160 200 240 280 320 360turbulent bubbly

TIME (s)turbulent bubbly

TIME (s)flow simulation (k-e

Fig 6 Volume gas fraction measured in point So s u at o (d l) Fig 6. Volume gas fraction measured in point-Smodel).

CONCLUSIONSCONCLUSIONS

• Introduction of deflectors or gas‐traps increases turbulence phenomenaIntroduction of deflectors or gas traps increases turbulence phenomena

• Turbulence bubbly flow model allows a suitable analysis of turbulenceTurbulence bubbly flow model allows a suitable analysis of turbulenced i i lti h fldynamics in multiphase flowy p

F t d l t ill b i t d t th i f th t• Future development will be oriented to the comparison of the presentresults with other simulation modules which take into account the gas‐results with other simulation modules which take into account the gas‐liquid interface.liquid interface.

REFERENCESREFERENCESC S

[1] Takeuchi M., Furtua T., Efficiency and two‐phase flow of alkaline water electrolysis under[ ] , , ff y p f f yforced convection of electrolyte Annals of Assembly for Int Heat Transfer Conference 13 2006forced convection of electrolyte, Annals of Assembly for Int. Heat Transfer Conference 13, 2006

[2] Hug W Divisek J Mergel J Seeger W Steeb H Highly efficient advanced alkaline[2] Hug W., Divisek J., Mergel J., Seeger W., Steeb H., Highly efficient advanced alkalinel l f l i I J H d E 9 (1992) 699electrolyzer for solar operation, Int. J. Hydrogen Energy 9 (1992) 699

Project EXSIVA was financed by: