Journal of Power and Energy Engineering, 2014, 2, 15-22
Published Online September 2014 in SciRes. http://www.scirp.org/journal/jpee
http://dx.doi.org/10.4236/jpee.2014.29003
How to cite this paper: Mehravar, S. and Sabbaghi, S. (2014) Thermal Performance of MEMS-Based Heat Exchanger with
Micro-Encapsulated PCM Slurry. Journal of Power and Energy Engineering, 2, 15-22.
http://dx.doi.org/10.4236/jpee.2014.29003
Thermal Performance of MEMS-Based Heat
Exchanger with Micro-Encapsulated PCM
Slurry
Samira Mehravar, Samad Sabbaghi
Department of Nano Chemical Engin eering, Faculty of Advanced Technologies, Shiraz University, Shiraz, Iran
Received May 2014
Abstract
Latent heat thermal energy storage technique has demonstrate to be a better engineering option
mainly due to its benefit of supplying higher energy storage density in a smaller temperature dif-
ference between retrieval and storage. For this purpose, a micro electro-mechanical system,
MEMS-based heat exchanger with microencapsulated PCM (MEPCM) slurry as cold fluid, has been
simulated three dimensionally. This work investigates the influence of using MEPCM-slurry on the
temperature of the cold and hot fluids. The MEPCM and water properties have been considered
temperature dependent. MEPCM-slurry is used with different volume fractions. The result shows
that using MEPCM with 25% volume fraction leads to the improvement in fluids temperatures,
that is, for hot fluid the rate of temperature reduction increases up to 23.5% and for cold fluid the
rate of temperature rise decreases to 9%, compared to using only water in the MEMS.
Keywords
MEMS, Microchannel, MEPCM-Slurry, Latent Heat, Simulate
1. Introduction
Phase change materials (PCMs) are materials with the ability to change phase in a certain temperature range.
They absorb or release high energy during melting or solidifying. Their ability to storage high energy indicates
their high latent heat. Due to PCMs have inherent low thermal conductivity, their application in the latent heat
thermal storage (LHTS) systems has lower heat transfer rates. Different procedures have been offered for the
enhancement of PCMs thermal conductivity like dispersing nanoparticles with high thermal conductivity in
PCMs [1] and microencapsulating them. A large number of researchers have investigated the performance of
different types of heat exchangers used as latent heat thermal storage units, among which MEMS-heat ex-
changer has higher efficient for micro scales [2]. Fluids used in heat exchangers have significant influence on
cooling efficiency rate. Moreover, the fluids individual thermophysical properties are the major key for their
cooling ability.
Using encapsulated PCMs improves the performance of the heat exchanger heat storage due to their high la-

S. Mehravar, S. Sabbaghi
16
tent heat. At first Tuckerman and Pease [3] offered a microchannel heat exchanger for electronic cooling by us-
ing water. They demonstrated that the heat flux rate of 790 W/m2 could be dissipated in that microchannel heat
exchanger. Goel et al. [4] investigated the convective heat transfer performance of an MEPCM-slurry in volume
fraction from 0% - 20% in a circular tube. They showed that MEPCM-slurry affected by Stefan number and the
volumetric concentration considerably decreases the wall temperature rise up to 50% compared to water. Sabbah
et al. [5] studied the influence of MEPCM slurry on the microchannel heat sink. They observed that enhance-
ment of the heat transfer coefficient is significantly affected by MEPCM melting temperature, channel inlet and
outlet temperature. Yu et al. [6] examined the convective heat transfer of MEPCM with an average of 4.97 µm
at volume fraction range of 0% - 20% in rectangular minichannel. The result showed that the 5% suspension has
a better effect on cooling performance than water in lower wall temperature and improved heat transfer coeffi-
cients. In the presented work, a 3D-dimensional MEMS-heat exchanger has been designed. In this work effect of
MEPCM-slurry with octadecane core and polymethylmethacrylat shell on the thermal performance of micro-
channel has been investigated.
2. Methodology
2.1. MEMS—Heat Exchanger
Figure 1(a) shows a schematic of MEMS-heat exchanger made of Aluminum, modeled in this study. It consists
of 14 circular microchannels with 100 µm diameter. Due to the symmetry of the channel, we modeled only half
of the doubled channels as shown in Figure 1(b). The simulations are developed with water and then are re-
peated with MEPCM-slurry with volume fractions of 5%, 10%, 15%, 20% and 25%.
In order to solve this problem following assumptions are adopted: 1) fluid flow is laminar, steady state and
incompressible; 2) The viscous dissipation is ignored; 3) The thermophysical properties of water, PCM and
MEPCM-slurry are dependent on temperature; 4) External body force and gravitational body force are ignored.
The Governing Equations Are:
• Continuity equation
.0) .( =∇ U ρ
(1)
.wkvjuiU++=
(2)
(a)
(b)
Figure 1. (a) Geometry of the MEMS; (b) MEMS modeled.

S. Mehravar, S. Sabbaghi
17
• Momentum equation
()UUP gF ρ ρτ ∇⋅=−∇++∇⋅ +
(3)
where:
F
: External body forces (
0F=
).
g ρ
: The gravitational body force (
0g ρ =
).
• Energy equation
()()UHK TS ρ ∇⋅=∇⋅ ∇ +
(4)
where:
H: The carrier fluid enthalpy.
S: volumetric heat source (S = 0).
The total enthalpy (
H
) is described by Equation (5).
.Hh H= +∆
(5)
where:
H: sensible enthalpy.
ΔH: MEPCM latent heat.
The sensible heat is calculated by Equation (6).
ref p
ref d
T
T
h hcT
=+ ∫
(6 )
where
href: reference enthalpy.
Tref: reference temperature.
The latent heat of slurry content is calculated by Equation (7).
.HL βφ ∆=
(7)
where:
β
: liquid fraction.
φ
: NEPCM mass fraction.
L: PCM latent heat.
The mass ratio of melted PCM to the total mass of PCM is defined as the liquid fraction. The PCM melting
temperature ranges from
solidus
T
to
liquidus
T
. Therefore
β
is described by Equation (8).
solidus
0TT
if <
= β
liquidus
1TT
if >= β
.
liquidussolidus
solidusliquidus
solidus TTTif
T T
TT <<
−
−
= β
(8)
2.2. Numerical Method
The governing equations in the three dimensional system are solved by using CFD software Fluent. The distri-
bution of temperature and velocity are calculated by using finite volume technique with SIMPLE algorithm.
2.3. Fluids Properties
In this study, MEPCM-slurry with the core (PCM) and shell made of octadecane and polymethylmethacrylat
(PMMA) respectively is mathematically analyzed. The average diameter of ME PCM is 4.9 µm [4]. The core
material includes 70% of the MEPCM volume. The PCM latent heat is equal to 244 J/g [7]. The thermophysical
properties of octadecane and PMMA are shown in Table 1 [1] [8] [9].
The density, specific heat and thermal conductivity of microcapsules [4] are defined as Equations (9)-(11)

S. Mehravar, S. Sabbaghi
18
Table 1. The thermophysical properties of MEPCM.
Octadecane PMMA
Density
(kg/m)
( )
750
0.001 T319.151−+
1190
Specific heat
(J/kg∙K)
2000 1470
Thermal conductivity
(W/m∙K)
0.358 if
solidus
TT <
0.148 if
liquidus
TT >
0.21
liquidus
T
302 -
solidus
T
299 -
3
PCM c
PCM
10 c
7
d
d
ρρ

=

(9)
( )
()
pcpshc sh
pPCM csh PCM
73
37
cc
c
ρρ
ρ ρρ
+
=+
(10)
PCM c
PCM PCMc cshPCM c
11
dd
kdkdk dd
−
= +
(11)
where
d: diameter.
c: PCM particles (core).
sh: PMMA (shell).
PCM: MEPCM particle (core + shell).
Equations (12)-(15) are used to calculate the density, specific heat [6], viscosity [4], and thermal conductivity
[10], of MEPCM-slurry.
f npw
(1 ) cc ρρ ρ = +−
(12)
pf pnppw
(1) .cc c ϕϕ = +−
(13)
( )
2.5
2
fw
1 1.16.cc µµ
−
= −−
(14)
( )
w npnpw
fnp np
ww
22
21
k kckk
kkk
c
kk
++ −
=
+− −


(15)
where
W: water.
c
: volume fraction.
φ
: mass fraction of NEPCM.
The following relation is used to measure
φ
Equation (16).
pnp
wnp w
.
()
c
c
φρ ρρ =+−
(16)
3. Results and Discussion
Simulations were carried out first with pure water, and then were repeated with MEPCM-slurry with volume
fractions of 5%, 10%, 15%, 20% and 25%. Considering that, the inlet velocity (Vin) is equals to 1 m/s and inlet
temperatures of cold and inlet fluid are 298 and 300 K respectively.

S. Mehravar, S. Sabbaghi
19
3.1. The Mean Temperatures of Hot and Cold Fluids
Figure 2 shows the mean temperature of cold and hot water versus channel length. Figure 3 shows the mean
temperature of hot water and MEPCM-slutty at the volume fraction of 5% versus channel length. The results
show that using MEPCM-slurry reduces the rate of temperature rise of cold flow, moreover increases the rate of
temperature reduction of hot flow compared to using only water. It can be seen that hot water temperature de-
creases without significant increasing in MEPCM-slurry temperature. In this way, it can be found that PCMs
have capability of energy storage due to its high latent heat. In addition, Sabbah et al. [5] indicated the signifi-
cant influence of the channel inlet and outlet temperature on the heat transfer coefficient in the microchannels.
The mean flow temperature is described by Equation (17).
c
c
pc
A
m
pc
A
d
d
vc TA
Tvc A
ρ
ρ
=∫
∫
(17)
where
c
A
: channel cross-sectional area (m2).
Figure 4 shows the mean temperature of hot water when MEPCM-slurry in 5%, 10%, 15%, 20% and 25% of
volume fractions is used as cold fluid. As a result the increase of the rate of the hot water temperature reduction
is observed for all MEPCM volume fractions. Figure 5 shows the mean temperature of MEPCM-slurry as cold
fluid in various volume fractions. From Results, It can be seen that MEPCM-slurry in all of its volume fractions
Figure 2. Mean temperature of cold and hot water.
Figure 3. Mean temperature of MEPCM-slurry and hot water.

S. Mehravar, S. Sabbaghi
20
Figure 4. Mean temperature of hot water by using MEPCM-slurry as
cold fluid in various volume fractions.
Figure 5. Mean temperature of MEPCM-slurry in various volume
fractions.
reduces the rate of temperature rise of cold fluid. To illustrate, consider the list of data in Table 2. The results
show that increasing volume fraction of MEPCM reduces the temperature of both hot and cold fluids. The best
performance of thermal storage in MEMS-heat exchanger is observed at the volume fraction of 25%. It should
be noted that MEPCM particles must be less than 25% to be considered Newtonian [11].
3.2. Inlet Velocity
Figure 6 shows the mean temperature of hot water in various inlet velocities, Vin = 0.5, 1, 1.5 and 2 m/s versus
channel length. MEPCM-slurry in 25% of volume fraction is used as cold fluid. The results show that in all inlet
velocities the rate of temperature reduction of hot water increases. Besides, it can be seen that higher inlet veloc-
ity leads to lower water temperature. Figure 7 shows the mean temperature of MEPCM-slurry at the volume
fraction of 25% and in various inlet velocities. The results show that the rate of temperature rise of MEPCM-
slurry is decrease for all inlet velocities. In addition, it can be seen that higher inlet velocity leads to lower
MEPCM-slurry temperature. As a result, inlet velocity is one of the important factors for enhancement of ther-
mal performance of MEMS-heat exchanger.

S. Mehravar, S. Sabbaghi
21
Table 2. List of fluids temperature.
MEPCM
volume fraction
T
out
(K)
hot water
T
out
(K)
cold fluid
0% 305.57 315.52
5% 304.52 313.15
10% 303.82 311.38
15% 303.32 310
20% 302.96 308.92
25% 302.68 308
Figure 6. Mean temperature of hot water in various inlet velocities by
using MEPCM-slurry as cold at the 25% of volume fraction.
Figure 7. Mean temperature of MEPCM-slurry at the 25% of volume
fraction in water in various inlet velocities.
4. Conclusions
• Using MEPCM-slurry instead of water reduces The MEMS temperature due to high-energy storage capabil-
ity of PCM.

S. Mehravar, S. Sabbaghi
22
• Increasing volume fraction of MEPCM-slutty and inlet velocity are the most important factors for the ther-
mal performance enhancement of the MEMS-heat exchanger.
• An improvement of 23.5% can be achieved by using MEPCM-slurry as cold fluid at the volume fraction of
25%, compared to using only water in MEMS-heat exchanger.
Considering the temperature dependent of fluids properties, the achieved results are accurate and more similar
to the real situation.
References
[1] Marin, J.M., Zalba , B., Cabeza, L.F. and Me hli ng, H. (2005) Improvement of a Thermal Energystorage Using Plates
with Paraffin-Graphite Composite. Heat and Mass Transfer, 48, 2561-2570.
[2] Valan Arasu, A., Sasmito, A.P. and Mujumdar, A.S. (2013) Numerical Performance Study of Paraffin Wax Dispersed
with Alumina in a Concentric pipe Latent Heat Storage System. Thermal Science, 17, 419-430.
http://dx.doi.org/10.2298/TSCI110417004A
[3] Tuckerman, D.B. and Pease, R.F. (1981) High Performance Heat Sinking For VLSI. IEEE Electron. Dev. Lett. EDL-2,
126-129. http://dx.doi.org/10.1109/EDL.1981.25367
[4] Goel, M., Roy , S.K. and Sengupta, S. (1994) Laminar Forced Convection Heat Transfer In Microcapsulated Phase
Change Material Suspensions. Heat and Mass Transfer, 37, 593-604.
[5] Sabbah, R., Farid , M.M. and Al-Hallaj, S. (2008) Micr o-channel Heat Sink with Slurry of Water with Micro-Encapsu-
lated Phase Change Material: 3D-numerical Study. Applied Thermal Engineering, 29, 445-454.
http://dx.doi.org/10.1016/j.applthermaleng.2008.03.027
[6] Yu, R., Frank, D. and Peter, S. (2007) Convective Heat Transfer Characteristics of Microencapsulated Phase Change
Material Suspensions in Minichannels. Heat Mass Transfer, 44, 175-186. http://dx.doi.org/10.1007/s00231-007-0232-0
[7] Zalba, B., Marin, J.M., Cabeza, L.F. and Meh ling , H. (2003) Review on Thermal Energy Storage with Phase Change:
Materials, Heat Transfer Analysis and Applications. Applied Thermal Engineering, 23, 251-283.
http://dx.doi.org/10.1016/S1359-4311(02)00192-8
[8] Abhat, A. (1983) Low Temperature Latent Heat Thermal Energy Storage: Heat Storage Materials. Solar Energy, 30,
313-332. http://dx.doi.org/10.1016/0038-092X(83)90186-X
[9] Sasaguchi, K. and Viskanta, R. (1989) Phase Change Heat Transfer during Melting and Resolidification of Melt
Aroung Cylindrical Heat Source(s)/Sink(s). Energy Rsources Technology, 111, 43-49.
http://dx.doi.org/10.1115/1.3231400
[10] Maxwell, J.C. (1881) A Treatise on Electricity and Magnetism. Oxford University Press.
[11] Charunyakorn, P., Sengupta, S. and Roy, S.K. (1991) Forced Convection Heat Transfer in Microencapsulated Phase
Change Material Slurries: Flow in Circular Ducts. Heat and Mass Transfer, 34, 819-833.