Comparative Study of Transient Conditions for Continuous Operation and Intermittent Operation of EATHE System Operated in Winter Season: A CFD Approach

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 04 Issue: 07 | July -2017 www.irjet.net p-ISSN: 2395-0072
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Comparative Study of Transient Conditions for Continuous
Operation and Intermittent Operation of EATHE System Operated in
Winter Season: A CFD Approach
Abhishek Agarwal1*, Mohd. Yunus Sheikh2, Rohit Misra3
-----------------------------------------------------------------------------------------***----------------------------------------------------------------------------------------------
Abstract - The objective of the present work is to
investigate the transient conditions of Earth Air Tunnel
Heat Exchanger under intermittent operation using
Computational Fluid Dynamics software FLUENT 6.3.
Simulation runs have been carried out for winter
heating case using three different soil thermal
conductivities. Results obtained for intermittent
operation of EATHE system have been compared with
those obtained for continuous operation in terms of
temperature rise of air and COP of the system.
Simulations reveal that the rise in air temperature
obtained during intermittent operation is better than
that obtained during continuous operation of EATHE.
These results shown significant improvement in
heating potential during intermittent operation is
observed for EATHE buried into soil having low
thermal conductivity.
1. Introduction
To reduce peak load passive cooling system are
recommended such as EATHE system. There are many
reported experimental and analytical studies on EAHE.
The ground temperature at depths of about 2 m to 3 m is
practically independent of seasonal variation. During
winter the ground temperature is higher than the ambient
temperature and during summer it is lower. This offers
great opportunities for coupling a building to the ground
to provide favourable protection from adverse outdoor
conditions, throughout the year. The indoor air is
circulated through small diameter cylindrical ducts that
are buried horizontally, at a depth of at least 2 m. Given the
large thermal inertia of the ground, it can considered as a
considerable heat source or heat sink, depending on the
season.
As a space cooling technology utilizing natural energy,
earth–air–pipe systems have attracted increasing interest
for energy conservation [1–5]. Mihalakakou et al. [6–8]
and Jacovides et al. [9,10] used earth–air–pipe heat
exchangers in cooling agricultural greenhouses. Kumar et
al. [11] evaluated the conservation potential of an earth–
air–pipe system coupled with a building with no air
conditioning. The cooling power for the earth pipe with
length of 60 m, diameter of 0.10 m and air flow velocity of
5 m/s was 19 kW, which was adequate to maintain an
average temperature of 27.65 °C for a single room in India.
To improve the feasibility and comfort for space cooling,
coupled a desiccant cooling set-up on the basis of the
earth–air–pipe. A specific study on the thermal saturation
and recovery of the soil under intermittent and continuous
EAHX operation is performed by Mathur et al. [22].
2. Description of CFD model
Computational fluid dynamics (CFD) is used to solve
the fluid flow, heat & mass transfer problems by dividing
the objects into grid form and applying governing
equations on each grid. CFD based model solved these
governing equations in the form of partial differential
equation. Numerical solution of these equation gives
temperature and pressure distribution, flow parameters.
CFD helps to reduce long tedious experimental work and
enhance the accuracy of work.
To examine the air temperature rises in winter
season of EATHE system, CFD software, FLUENT 6.3, was
used. CFD software, GAMBIT 2.4.6 has been used to design
and meshing 60 m long, 0.1 m diameter pipe. The model
incorporates the effect of turbulent air flow on the thermal
performance. The element type and the grid density were
selected to be variable according to the sensitivity of
temperature quantity, so that the calculation can adapt to
the actual situation and reach a high level of accuracy.
Because the temperature changes more sharply around
the pipe wall, the grid is designed to be denser in that area,
while it is sparser farther away from the pipe wall.
In the present study it has been assumed that air is
incompressible and the soil is homogeneous and its
physical properties are constant. It was also assumed that
the property of the pipes and ground materials do not
change with temperature and engineering materials used
in the CFD model are isotropic and homogeneous.This was
validated by experimental results.
Figure 1. Four different views of CFD model for EATHE

Table 1. Boundary Conditions and geometric parameters
used in simulation
Boundaries Unit Value
Air inlet temperature K 280.6
Air Velocity ms-1 5
Soil temperature K 300.2
Pipe Length m 60
Pipe Diameter m 0.1
Pipe inlet velocity ms-1 5
Thermo-physical parameters of materials used in
simulation listed in table 2. These properties were fetched
from depth of 3.7 m by physical testing other parameters
were taken from Cucumo et. al.[12]. CFD model of 60 m
length and 0.1 m diameter of pipe is validated with
experimental result. The validated CFD model has been
used for 24 h of intermittent operation. Ajmi et al.[13]
reported that no significant effect of soil surrounding the
pipe has been occurred therefore, for study purpose
diameter of soil cylinder has been taken two times of pipe
diameter.
3. Experimental set-up:
The mentioned diagram Figure 2 shown
experimental set-up of EATHE, having 60 m long
horizontal PVC pipe of inner diameter 0.10 m buried
underground at depth of 3.7 m. for movement of air in
EATHE pipe single phase blower is connected. The depth
of 0 m, 0.62 m, 1.24 m, 1.86 m, 2.48 m, 3.10 m and 3.7 m
respectively from ground surface.
Table 2. Thermo-Physical parameter used in simulation
Parameters
Density
kgm-3
Thermal
conductivity
Wm-1K-1
Specific
heat
capacity
Jkg-1K-1
Air 1.225 0.0242 1006
Soil
(SL1)
2050 0.52 1840
Soil
(SL2)
2050 2.0 1840
Soil
(SL3)
2050 4 1840
PVC 1380 1.16 900
Figure 2. Schematic of room integrated EATHE system
Additional RTD has been installed along the pipe length,
T7, T8, T9, T10, T11, T12, T13, T14 and T15 at 0.2 m, 1.7 m, 4.7 m,
9.3 m, 15.1 m, 24.2 m, 34.0 m, 44.4 m and 60.0 m
respectively from inlet point of pipe to measure air
temperature. A group of four RTD (Pt-100) temperature
sensors at axial distance of 6.4 m, 27.4 m and 48.8 m from
the inlet of EATHE were also provided to measure the
temperature of pipe–soil interface, temperature of soil at a
distance of 0.2 m, 0.4 m and 0.6 m from pipe surface,
respectively.
4. Validation of simulation model:
In this exercise, this grid independency test mainly
used for checking the effects of mesh size on accuracy of
solution. So, this graph revealed that inlet point of pipe
maintains temperature 312.6 K, 312.6 K, 312.6 K by
coarse, medium and fine mesh respectively. Similarly, at
30 m away from pipe surface these temperatures reduce
till 302.7 K, 301.7K and 301.6K by coarse, medium and fine
mesh respectively. Similarly, at outlet of pipe temperature
reduce till 301.3 K, 300.3 K and 300.2 K by coarse, medium
and fine mesh respectively.
Figure 3. Validation of EATHE system
298
300
302
304
306
308
310
312
314
0 10 20 30 40 50 60
StaticTemperatureof
Air(K)
Length of pipe (m)
Coarse Mesh
Medium Mesh
Fine Mesh

Study of transient behavior of EATHE system
in continuous operation:
Soil layers surrounding the pipe have also been
simulated to study the transient effects of thermal
conductivity of soil on the thermal performance of EATHE
system by using three different types of soil SL1, SL2 and
SL3. Comparison of simulated and experimental values of
temperature of air in the pipe at various points along the
length is summarized for air velocity of 5 m/s.
5. Following modes of operation of EATHE
system under transient conditions have
been investigated
Thermal performance of EATHE system operating under
various intermittent mode (Mode 2) has been compared
with the continuous mode of operation (Mode 1) in terms
of rise in air temperature along the length of pipe.
5.1. Continuous mode (Mode 1)
In this operation a long pvc pipe has been used. It
has analyzed by using three types of soils, having different
thermal conductivities which are mentioned in thermo-
physical condition. Air temperature of winter season has
been assumed about 280.6 K and surrounded soil
possessed near 300.7 K. when air pass through the pipe
due to temperature difference heat transfer occurs at pipe-
soil interface. Pipe gets heated up by the soil. Two mode of
heat transfer has been noticed in EATHE system,
conduction mode occurs in between soil and pipe,
convection mode occurs in between pipe and air. During
running position heat will accumulating between soil
layers which creates thermal influence zone and degrade
thermal performance by time.
5.2. Intermittent mode (Mode 2)
In this mode EATHE system has been used
intermittently. It is used continuously for 12 hours and
then kept OFF for next 12 hours making ON-OFF cycle of
24 hours duration. For the use of EATHE in such a manner
one cycle of intermittent operation shall be obtained.
Hourly variation in air temperature at outlet section of
EATHE pipe buried in soil (SL) with lowest thermal
conductivity (SL1= 0.52 Wm-1K-1), medium thermal
conductivity (SL2= 2.0 Wm-1K-1) and highest thermal
conductivity (SL3= 4.0 Wm-1K-1) during intermittent
operation (Mode 2) have been compared with that
obtained during continuous operation (Mode 1) in Fig 4-6.
Figure 4 shown that at outlet section of the length of pipe
buried under soil (SL1) is obtained 297.8 K after 25th hour
during continuous operation (Mode 1). However
corresponding value obtained during intermittent mode of
operation (Mode 2) is 299.4 K after 25th hour.
6. COP ANALYSIS
Coefficient of performance of EATHE system under
continuous and intermittent modes of operation has been
determined with the help of formula given as
COP = (1)
Where, ṁ =ρa A Vi
Figure 4. Hourly temperature of air for soil SL1 at
outlet section of pipe (0.1 m diameter) at 5 m/s flow
velocity for Mode 1 and Mode 2
297
298
299
300
301
302
1 5 9 13 17 21 25
AirTemperature(K)
Hours
Temperature variation of air for
soil SL1 at outlet section of pipe
during Mode 1 and Mode 2
operation
Mode
1
Mode
2

Figure 5. Hourly temperature of air for soil SL2 at
outlet section of pipe (0.1 m diameter) at 5 m/s flow
ṁ = mass flow rate of air (kg/s), Vi = Flow velocity of air
(m/s), ρa = Density of air (kg/m3), Cp = Specific heat of
soil and qin = Energy input to the blower (W).
Typical values of above parameters are ṁ = 0.048 kg/s,
Cp = 1005 Jkg-1K-1
, ∆T = Tout - Tin, Cd (Coefficient of
discharge) = 0.6, qin = 120 W, Tout = Hourly temperature at
60 m; Tin = Hourly temperature at inlet point.
It is observed from Table 3 that for continuous mode
(Mode 1) the average COP is 4.4, 4.7 and 4.7 for SL1, SL2
and SL3 respectively. However, corresponding values of
COP for intermittent mode (Mode 2) are 4.5, 4.7 and 4.7
for SL1, SL2 and SL3 respectively
Figure 6. Temperature of air for soil SL3 along the
length of pipe (0.1 m diameter) at 5 m/s flow
Table 3. Average COP
Mode
SL1 (
0.52W/mK)
SL2 (2
W/mK)
SL3
(4 W/mK)
Mode-1 4.4 4.7 4.7
Mode-2 4.5 4.7 4.7
CONCLUSION
The main concluding remarks of the study are as
follows:
1. Thermal performance of EATHE system in
intermittent operation (Mode 2) increases the outlet air
temperature of EATHE system by 8.13%, 1.04% and
0.01% for SL1, SL2 and SL3, respectively. These
results show that during non-working time period of
EATHE system soils are regenerated by dissipating the
accumulated heat to subsequent soil layers which helps
to improve thermal performance of EATHE system.
2. Coefficient of thermal performance of EATHE
system in intermittent operation (Mode 2) is increase by
0.1 for lowest thermal conductivity soil (SL1) where as
high thermal conductivity doesn’t affect the coefficient of
thermal performance of EATHE system.
3. Operating the EATHE in intermittent mode found
to be very useful. Heat accumulation in the nearby soil
during continuous operation of EATHE system can be
minimized by running the system in intermittent
mode. This study also revealed that EATHE system
with higher thermal conductivity soil (SL3) can be
operated continuously (Mode 1) while EATHE system
with lower thermal conductivity (SL1) must be used in
intermittently ( Mode 2).
Table 4. Temperature rise of air at outlet section of
pipe after 24 hours of continuous operation (Mode 1)
and intermittent operation (Mode 2)
Soil Mode 1 Mode 2
Improvement
%
SL1 17.2 18.8 9.30
SL2 19.2 19.4 1.04
SL3 19.5 19.5 0.00
297
298
299
300
301
302
1 5 9 13 17 21 25
AirTemperature(K)
Hours
Temperature variation of air for soil
SL2 at outlet section of pipe during
Mode 1 and Mode 2 operation
Mode1
Mode2
297
298
299
300
301
302
1 5 9 13 17 21 25
AirTemperature(K)
Hours
Temperature variation of air for soil
SL3 at outlet section of pipe during
Mode 1 and Mode 2 operation
Mode1
Mode2

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Comparative Study of Transient Conditions for Continuous Operation and Intermittent Operation of EATHE System Operated in Winter Season: A CFD Approach

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Comparative Study of Transient Conditions for Continuous Operation and Intermittent Operation of EATHE System Operated in Winter Season: A CFD Approach