CFD simulation on membrane distillation of NaCl solution

Abstract

A computational fluid dynamics (CFD) simulation that coupled an established heat and mass transfer model was carried out for the air-gap membrane distillation (AGMD) of NaCl solution to predict mass and heat behaviors of the process. The effects of temperature and flowrate on fluxes were first simulated and compared with available experimental data to verify the approach. The profiles of temperature, temperature polarization factor, and mass flux adjacent to the tubular carbon membrane surface were then examined under different feed Reynolds number in the computational domain. Results show that the temperature polarization phenomena can be reduced, and mass flux can be enhanced with increase in the feed Reynolds number.

Key words： membrane distillation; computational fluid dynamics (CFD) simulation; temperature polarization; carbon membrane

Acknowledgements

The authors are grateful to the Center of Membrane Science and Technology in Dalian University of Technology for providing the commercial CFD package FLUENT v6.

Nomenclature

D	inner diameter of the tubular carbon membrane
Hv	latent heat of evaporation
h1	the condensation heat transfer coefficient
hc	heat transfer coefficient at cooling plate
J	mass flux
k	thermal conductivity
L	length of the tubular carbon membrane
Mw	molecular weight of water
N	molar flux
Pavg	average totoal pressure
pfm	partial pressure of water on the feed side
pmg	partial pressure of water on the membrane/air-gap interface
pv	saturation pressure
q	heat flux
qcm	conductive heat flux across the membrane
qvm	vaporization heat flux across the membrane
R	universal gas constant
Re	Reynolds number (Re= ρuD/μ)
r0	the average pore radius of the membrane
Tavg,m	the average temperature of the membrane
Tf	the feed temperature at the inlet
Tfm	the feed temperature adjacent to membrane surface
Ufc	overall heat transfer coefficient
u	axial velocity
v	radial velocity
x	axial coordinate
xNaCl	molar fraction of the NaCl solution
y	radial coordinate

Greeks letters

ΔP	pressure difference beteween the membrane
αw	the activity coefficient of water
δ	thicknes
ϵ	porosity
μ	viscosity
Θ	temperature polarization factor
ρ	density
τ	tortuosity

Subscripts

a	air
c	coolant
fp	condensate film/cooling plate interface
g	air-gap
gf	air-gap/condensate film interface
m	membrane
mg	membrane/air-gap interface
p	cooling plate
pc	cooling plate/coolant interface

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