Vol.:(0123456789) 1 3
Arabian Journal for Science and Engineering
https://doi.org/10.1007/s13369-019-04089-x
RESEARCH ARTICLE - MECHANICAL ENGINEERING
The Efect of Spacer Orientations on Temperature Polarization
in a Direct Contact Membrane Distillation Process Using 3‑d CFD
Modeling
S. M. F. Hasani
1
· A. S. Sowayan
1
· M. Shakaib
2
Received: 14 February 2019 / Accepted: 1 August 2019
© King Fahd University of Petroleum & Minerals 2019
Abstract
Membrane distillation is an emerging technology that uses hydrophobic membranes to separate nonvolatile solids from liq-
uids. The vapor pressure gradient between the feed and the permeate sides drives the process. Low-grade thermal energy is
used to heat feed water and create a pressure gradient. A large vapor pressure gradient across the membrane surfaces results
in high permeation rates. The feed spacer is an important element of the membrane module that forms channels for feed
and permeate fow. A good feed spacer design helps improve permeation. In this paper, 3-d CFD simulations are carried
out for spacer-flled channels, and the efect of inlet velocity, flament orientation and spacing on heat transfer is studied.
Temperature polarization is used as the parameter for heat transfer performance evaluation. Shear stress and temperature
polarization index have been calculated for diferent spacer orientations in a direct contact membrane distillation process.
The results show a major infuence of the studied parameters on temperature polarization and shear stress. A comparison of
2-d and 3-d analyses reveals that the average shear stress in the two approaches is nearly the same, but the standard deviation
of shear stress is lower for the 2-d case. Similarly, the average value and the standard deviation of temperature polarization
index are lower than those obtained in the 3-d analysis. The fndings also show that for staggered axial flaments, the tem-
perature polarization index distribution is more uniform suggesting that such orientations are more suitable for enhancing
heat transfer in a membrane distillation process.
Keywords CFD simulation · Feed spacer · Membrane distillation · Shear stress · Temperature polarization
List of Symbols
C
p
Specifc heat (J/kg K)
d
f
Filament diameter (m)
d
h
Hydraulic diameter (m)
h
(
=
q
w
T
h
-T
hm
=
q
w
T
cm
-T
c
)
≡ heat transfer coefcient (W/
m
2
K)
h
ch
Channel height (m)
k Thermal conductivity (W/m K)
L Channel length (m)
l
m
Mesh length/flament spacing (m)
Nu Nusselt number
(
=
hd
h
k
)
Pr Prandtl number
(
=
C
p
k
)
q
w
Heat fux (W/m
2
)
Re Reynolds number
(
=
vd
h
)
u
av
Average velocity (m/s)
T
c
Inlet temperature of cold fuid (K)
T
cm
Temperature at membrane surface in cold channel
(K)
T
h
Inlet temperature of hot fuid (K)
T
hm
Temperature at membrane surface in hot channel
(K)
ρ Density (kg/m
3
)
τ
av
Average value of shear stress
τ
sd
Standard deviation of shear stress
Φ Temperature polarization index
ϕ
av
Average value of temperature polarization index
ϕ
sd
Standard deviation of temperature polarization index
μ Viscosity (kg/m s)
ε Voidage of feed or permeate channel
θ Flow attack angle
* S. M. F. Hasani
smhasani@imamu.edu.sa
1
Department of Mechanical Engineering, Al Imam
Mohammad Ibn Saud Islamic University, Riyadh,
Saudi Arabia
2
Department of Mechanical Engineering, NED University
of Engineering and Technology, Karachi, Pakistan