Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-10

Performance investigation of finned tube type adsorber for different finspecifications
Khanam Marzia1,2, Skander Jribi3, Takahiko Miyazaki1,2, Bidyut Baran Saha2, Shigeru
Koyama1,2
1Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Japan
2
International Institute for Carbon-Neutral Energy Research, Kyushu University, Japan
3
Laboratory of Electro-Mechanical Systems, National Engineering School of Sfax,
University of Sfax, Tunisia

Abstract
In this study, 2D axisymmetric heat and mass transfer simulation of finned tube type adsorber
bed employing activated carbon-ethanol working pair has been performed. The model uses
mass, momentum, and energy conservation equations as well as adsorption equilibrium andadsorption rate equations. Moreover, almost identical experimental conditions have been
provided for the simulation to validate the simulation results. In the result section, temperatureand pressure profiles are presented and validated by comparing with the experimental results.
Specific cooling power and COP were evaluated and found 210.63 W/kg and 0.33
respectively. Finally, this CFD model is used for performance optimization of the system byvarying heat exchanger fin specifications.
analysis is necessary to improve the system
performance and optimize the heat exchanger
design parameter. Some researchers have alreadystarted to analyze heat and mass transfer ofadsorber bed heat exchanger through CFD
simulation[1][2][3].
The objective of this study is to validate the CFD
model of finned tube type adsorber withexperimental results and investigate theperformance of the system in terms of coefficientof performance (COP) and Specific cooling power(SCP). Moreover, this CFD model will be used for
performance optimization of the system by varying
heat exchanger fin specifications.

1. Introduction
Adsorption cooling system (ACS), a
feasible alternative to vapor compression
system, is gaining more attention nowadays
as it employs natural and environment
friendly refrigerants such as water, ethanol,ammonia, CO2 etc. Moreover, it can bedriven by waste heat or solar energy.
Althoughthissystemhasbeencommercialized, however, still it has someproblems such as relatively low efficiency
and bulkiness, which hinder this systemfrom mass production. Several researchers
are working continuously to improve theperformance of ACS system. The
performances of ACS already has been
examined through lump-sum modelingusing several adsorbent-adsorbate pairs suchassilicagel-water,water-zeolite,
ammonia-activatedcarbon,ethanolactivated carbon. Although performance
prediction by lump-sum modeling is morepractical rather than ideal cooling cycle
simulation, however, the assumption, same
temperature in the bed is not realistic.
Consequently, heat and mass transfer ofadsorber heat exchanger through CFD

2. CFD Model
The experiment of finned tube adsorber withactivated carbon-ethanol pair was conducted in ourlaboratory earlier [4]. For similar experimental
conditions, the CFD simulation is carried out with
Ansys fluent software v.18.1. Geometry andmeshing were performed by Ansys Design
Modeler and Ansys Meshing respectively.
2.1. Geometry
Figure 1 is showing the schematic geometry
where the studied domain is decreased the half
space between two fins; therefore, the simulation

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Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-10
was a 2D-axisymmetric simulation. Specificationsof finned tube adsorber are furnished in table 1.
2.2. Materials & porous zone properties
Materials used in the simulations are as
follows:
i. Activated carbon powder of type Maxsorb-III
packed between the fins,ii. Copper material for tube and fins,iii. Ethanol as gas phase refrigerant.

and (5) to Fluent as user-defined functions (UDFs).
Table 2. Properties of activated carbon powder
Parameter

SymbolValuePacking density (kg.m-3)

ρa275

Skeletal density (kg.m-3)

ρs

2200

Particle density (kg.m )ρp464.14

Average particle diameter (μm)

Dp70

Micropore volume (cm .g )vμ1.7

specific heat (kJ.kg-1.K-1)

Cp,s

1

Thermal conductivity(W.m-1.K-1)

ks

0.066

-3

3-1

𝑑𝑞

𝑆𝑚 = −(1 − 𝛾)𝜌𝑝 𝑑𝑡(4)𝑑𝑞

𝑆ℎ = −(1 − 𝛾)𝜌𝑝 𝑄𝑠𝑡 𝑑𝑡
Here,

𝑑𝑞𝑑𝑡

(5)

denotes adsorption rate which is

calculated by using linear driving force (LDF)
equation (6).
𝑑𝑞Figure 1. 2D axisymmetric geometry of finnedtube adsorber
Table 1. Specification of finned tube adsorberParameter
Tube inner diameter (mm)Tube outer diameter (mm)Fin thickness (mm)Fin pitch (mm)Fin height (mm)SymbolValuedidoδ
lh

26
29
0.53
3.710𝑑𝑡𝜌𝑠

𝛼=𝐷𝑃2𝛾3150 (1−𝛾)23.5 (1−𝛾)𝐶2 = 𝐷𝑃𝛾3(6)Where k and q* are diffusion time constant andequilibrium uptake and these are modeled by
Arrhenius and Dubinin-Astakhov (D-A) equations((7) and (8)) respectively.
𝐸𝑘 = 𝐴 exp (− 𝑎 )(7)
𝑅𝑇Here, A = 0.2415 s-1 and Ea = 225 kJ/kg which
denote pre-exponential factor and activationenergy respectively [5].

Table 2 presents the properties of activated carbon
powder. The porous zone properties of theactivated carbon powder are characterized byequations (1-3).
𝜀𝑏 = 1 − 𝜌𝑎 − 𝑣𝜇 𝜌𝑎= 𝑘 (𝑞∗ − 𝑞)

𝑅𝑇𝑃𝑛𝑞 ∗ = 𝑞𝑠 exp (− ( 𝐸 𝑙𝑛 ( 𝑃𝑠 )) )(8)

Where, qs = 1.2 kg.kg−1, E = 139.5 kJ.kg−1 and n= 1.8 [5].
2.4 Boundary conditions
Heating/Cooling surface: The supply watertemperature is 20 ˚C during pre-cooling andadsorption processes and 80 ˚C during preheating
and desorption processes. These conditions aregiven as convection boundary condition, andconvection heat transfer coefficient is calculated
for the water flow of 3 l/min by using Gnielinski
correlation.
Inlet/Outlet condition: Adsorber is connectedto the evaporator having the pressure of 3.85 kPa
which is the inlet condition. Besides, the(1)(2)(3)

Where𝜀𝑏 , 𝛼 and𝐶2 denote porosity,
permeability and inertial resistance coefficient ofporous zone respectively.
2.3 Adsorbent-adsorbate interactions
The adsorbent-adsorbate characteristics arethemass
adsorbed/desorbed
andheat
released/adsorbed which is given by equations (4)27Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-10
refrigerant outlet condition during desorption
process is at condenser pressure of 10.35 kPa.
2.5 Performance investigation
Performance indicating parameters of theadsorption cooling system, specific cooling powerand coefficient of performance (COP), arecalculated by using equations (9-10).

Therefore, ethanol adsorption onto activatedcarbon is only 66% of its capacity within the 400s
adsorption time. The calculated specific coolingpower of the system is found 210.63 W/kg andCOP is 0.33 which are calculated by using theequation (9) and (10) respectively.
80

𝐶𝑂𝑃 =

𝑡𝑐𝑦𝑐𝑙𝑒𝑄𝑐ℎ𝑖𝑙𝑙(10)

𝑄𝑑𝑒𝑠

Sim. Ads.1 mm70(9)Temperature [oC]

𝑄𝑐ℎ𝑖𝑙𝑙 =𝑎𝑑𝑠_𝑒𝑛𝑑ℎ𝑓𝑔 .∫𝑎𝑑𝑠_𝑠𝑡𝑎𝑟𝑡 𝑚𝑎𝑑𝑠 𝑑𝑡

3. Results and discussion
Figure 2 shows the pressure profile in theadsorber/desorber bed where adsorption takes
place at 3.85 kPa pressure and desorption pressureis 10.35 kPa. There is a sharp change of pressureduring the pre-cooling and preheating process, andconstant during adsorption and desorption.

Sim. Ads. 5 mm
60Exp. Ads.5mm

50Exp. Ads. 1mm

403020800

1000

1200

1400

1600

Time [s]Figure 3. Simulated vs. experimental temperatures
profile at 1 and 5mm adsorbent thickness.
10.9

12000

0.80.7Uptake [kg/kg]

Pressure [Pa]

10000
8000
6000

0.6
0.5
0.4
0.3

equilibrium uptake

0.2

4000

instantaneous uptake

0.1
2000

080001000

1200

1400

1600

1800

Time [s]05001000Time [s]

1500

2000

Figure 4. Simulated instantaneous and equilibrium
uptake profile.

Figure 2. Simulated pressure profile in theadsorber/desorber bed.

Experimental temperature change vs. simulated
temperature profiles at 1 and 5 mm adsorbent
thickness from the outer surface of the copper tube
is displayed in figure 3. Good agreement is foundbetween experimental and simulated temperature
results. In figure 5, temperature distributions inside
the adsorber bed at the end of adsorption,
pre-heating, desorption and pre-cooling processes
is shown. As convection and conduction heattransfer at the center of the bed is not good,
therefore, after pre-heating and desorption process,temperature distribution inside the bed is not same.
Simulated equilibrium uptake and instantaneous
uptake of the bed is shown in figure 4. The
difference between maximum and minimum valuesof equilibrium uptake is 0.494 kg/kg whereas thechange in case of instant uptake is 0.32 kg/kg.

Figure 5. Temperature change in the finned tube
adsorber at the end of adsorption, pre-heating,desorption and pre-cooling process.

28

Proceedings of International Forum for Green Asia 2017
Special issue of Evergreen - Joint Journal of Novel Carbon Resource Sciences & Green Asia Strategy

P-10transfer to the adsorbent will become worse,
therefore, the SCP will decrease.

4. Performance study for different fin
specifications
4.1 Effect of fin height on performance
The effect of fin height at 5,10, 15 mm withconstant fin pitch 3.7 mm have been checked toevaluate the system performance. From the figure
6, it is seen that with the increase of fin height
SCP decreases, however, COP increases and after
10 mm fin height it decreases. As the massdiffusion into the adsorbent layer will be loweredwith the larger fin height, therefore, it will result inthe lower SCP. Besides, the larger fin height causes
the decrease in the heat capacity ratio so that theCOP can increase, however, due to a large decreasein the SCP at the fin height of 15 mm, the COP isdeceased.

5. Conclusion

The summary of this study are as followsI. CFD modeling of finned tube adsorber
employing activated carbon-ethanol has been
done and validated with experimental results.
II. Good agreement is found between
experimental and simulation results.
III. Performance study for different finspecification has been done and results showthat fin height less than 10mm and fin pitch
around 4 mm is good regarding COP andSCP.
IV. This CFD model will allow performance
optimization of the system by optimizing theoperating conditions as well.

0.4

250

0.3
0.25

150

0.2

100

0.15

50

SCP

0.1

COP

0.05

0Acknowledgment
This work has been supported by Mayekawa
Houonkai Foundation, Japan.
References

COP [-]SCP [W/kg]

0.35
200[1]

0246

81012Fin height [mm]1416Figure 6. Effect of fin height variation on COPand SCP[2]

4.2 Effect of fin pitch on performance
The effect of fin pitch of 2, 3.7, 5.5 mm withconstant fin height 10 mm have been simulated.
From the figure 7, it is evident that with theincrease of fin height SCP decreases, however,
COP increases. The increase of the fin pitch resultsin the decrease in heat capacity ratio. Therefore,
the COP will be improved. Moreover, the heat
250

[3]

[4]

0.4

0.35
0.30.25150
100

SCP0.2

COP0.15

COP [-]SCP [W/kg]

200

[5]

0.1

500.05001234
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Email: marzia@phase.cm.kyushu-u.ac.jp
Figure 7. Effect of fin pitch variation on COPand SCP29