Turbulent mixing and residence time distribution in novel multifunctional heat exchangers-reactors

(1)

Context & Objectives Results

Methods

Numerical simulations:

• CFD code Fluent®

• double precision, segregated and second-order accuracy with SIMPLE algorithm

• standard k-ε turbulence model with a two-layer model for computing the near-wall region

• fully developed turbulent flow velocity profile at the computational domain inlet

• 1/8 of the mixers volumes is computed due to the symmetry

• unstructured 3D mesh with hexahedral volumes refined at the solid boundaries

LDV (Laser Doppler Velocimetry) measurements:

• Dantec LDV system equipped with a 10 W argon-ion laser source and two BSA- enhanced signal-processing units (57N20 BSA and 57N35 BSA enhanced models)

• measurements are taken along different radial profiles in each array, at 3 mm downstream from the tab

• the global mean standard deviation is 6% for the mean velocity and 5% for the fluctuating velocity

• the test section is preceded by a preconditioner (1500 mm straight Plexiglas pipe) to produce a fully developed turbulent flow at the test section inlet

Conclusions & Perspectives

The reversed-array arrangement provides the best performance in micro- (50%) and meso-mixing (25%), but exhibits an approximately 40% increase in power consumption over the classical HEV geometry, i.e. aligned arrays here

The RTD exhibits bimodal behavior in alternating and reversed arrays, and a plug flow with axial dispersion in the aligned arrays

Future work will aim to study the thermal performances of these three configurations

Turbulent mixing and residence time distribution in novel multifunctional heat exchangers-reactors

Charbel Habchi

^{1, 2}

, Thierry Lemenand

¹

, Dominique Della Valle

¹

and Hassan Peerhossaini

¹

1 Thermofluids, Complex Flows and Energy Research Group, Laboratoire de Thermocinétique de Nantes, CNRS UMR 6607, École Polytechnique de l’Université de Nantes, rue Christian Pauc, B.P. 50609, 44306 Nantes, France

2 Agence de l’Environnement et de la Maîtrise de l’Énergie (ADEME), 20 avenue du Grésillé, B.P. 90406, 49004 Angers, France

Multifunctional heat exchangers-reactors

• Heat removal of exothermal reactions

• Efficient mixing for low strain rates

• Plug flow reactors

• Compactedness

• No power required except pumping

• Self-cleaning

• Water and waste water treatment

• Petrochemicals and refining

• Pharmaceuticals

• Food processing

• Minerals processing

• Agricultural chemicals Advantages Applications

Alternating arrays (Novel geometry) 45°

4.7 mm

6.2 mm

z y

β β = 30°

3.9 mm x

y

Trapezoidal vortex generator

Tube wall

• The internal diameter of the test sections is of 20 mm, with a total length of 140 mm.

• Working fluid is water at 298 K

• Turbulent flow with Reynolds numbers ranging between 7500 and 15000.

Flow

Water tank

Test section

Safety valve

Digital balance

Small container

Personal computer Gear rotary

pump

Flexible connections

Preconditioner Postconditioner Water tank

Test section

Safety valve

Digital balance

Small container

Personal computer Gear rotary

pump

Flexible connections

Preconditioner Postconditioner

Experimental rig The grid system on a longitudinal

section of a tab array

Experimental validation 3 mm downstream from the 7^thtabs array, Re= 15000

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5

Aligned arrays:

Numerical Experiments Alternating arrays:

Numerical Experiments Reversed arrays:

Numerical Experiments

Radial coordinate, r/R Streamwise mean velocity, W(m s-1)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0 1 2 3 4 5 6 7 8 9 10 11 12

εl, m (m2s-3)

Aligned arrays Alternating arrays Reversed arrays

z/L Flow direction

0.72 0.74 0.76 0.78 0.80 0.82 0.84 0.86

0 1 2 3 4 5 6 7 8 9 10 11 12

z/L εl, m (m2s-3)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

Flow direction

kl, m (m2s-2)

z/L

Aligned arrays Alternating arrays Reversed arrays

0.72 0.74 0.76 0.78 0.80 0.82 0.84 0.86

0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

z/L kl, m (m2s-2)

8000 10000 12000 14000 16000

10 100

Aligned arrays, f = 0.209 Re^-0.061 Alternating arrays, f = 0.230 Re^-0.048 Reversed arrays, f = 0.183 Re^-0.006

Z= f/ fempty pipe

Reynolds number, Re Helical KenicsTM

ISGTM SulzerTM

SMX SulzerTM

SMV

Flow pattern

Distribution of the mean streamwise velocity (Re= 15000) for (a) aligned (b) alternating (c) reversed arrays

Turbulence kinetic energy

& Meso-mixing

Turbulence energy dissipation rate

& Micro-mixing

Residence time distribution

& Macro-mixing

Power consumption

0.50 0.75 1.00 1.25 1.50 1.75 2.00

0.00 0.05 0.10 0.15 0.20 0.25 0.00 0.05 0.10 0.15 0.20 0.25 0.00 0.05 0.10 0.15 0.20 0.25

Reversed arrays

θ = t/tm

E(t)

Alternating arrays Aligned arrays

Plug flow with axial dispersion, Pe= 112

Geometry Aligned arrays

Alternating arrays

Reversed arrays Abs(tm,num - tm,th)/tm,th

9.77 %

☺

17.50 % 3.25 %

☺

Re= 15000 Aligned arrays Alternating arrays Reversed arrays

Macro-mixingtm(s) 0.162 0.148 0.174

σ/ tm 0.230 0.286 0.202

Meso-mixing max(kl,m) (m².s^-2) 0.060 0.065 0.076 max(kl,m/εl,m) (1/s^-1) 9.9x10^-3 8x10^-3 8.4x10^-3 Micro-mixing max(εl,m) (m².s^-3) 6.088 8.088 9.056

∆P (Pa) 916.15 1143.75 1365

kl,m= TKE averaged on reactor cross sections εl,m= Turbulence energy dissipation rate averaged on reactor cross sections