Heat exchangers are used widely in industrial application such as chemical, food processing, power production, refrigeration and air-conditioning industries. Helical coiled heat exchangers are used in order to obtain a large heat transfer per unit volume and to enhance the heat transfer rate on the inside surface. In the present study, CFD simulations are carried out for a counter flow tube in tube helical heat exchanger where hot water flows through the inner tube and cold water flows through the outer tube. From the simulation results heat transfer coefficient, pressure drop and nusselt number are calculated. The heat transfer characteristics of the same are compared with that of a counter flow tube in tube straight tube heat exchanger of same length under same temperature and flow conditions. CFD simulation results showed that the helical tube in tube heat exchanger is more effective than the straight tube in tube heat exchanger.

1. International Journal of Applied Engineering Research
ISSN 0973-4562 Volume 9, Number 18 (2014) pp. 5209-5220
© Research India Publications
http://www.ripublication.com
Numerical Simulation of a Tube in Tube Helical Coiled Heat
Exchanger using CFD
Soby P. Sunny1
, Siddharth D. Mhaske2
, Yash B. Parikh3
1
(PG Student, Mechanical Engineering
Department, Symbiosis Institute of Technology, Pune)
2
(PG Student, Mechanical Engineering Department,,
Symbiosis Institute of Technology, Pune)
3
(Assistant Professor, Mechanical Engineering
Department, Symbiosis Institute of Technology, Pune)
1
sobzsunny@gmail.com 2
mhaskesiddharth@gmail.com
3
yash.parikh@sitpune.edu.in
Abstract
Heat exchangers are used widely in industrial application such as chemical,
food processing, power production, refrigeration and air-conditioning
industries. Helical coiled heat exchangers are used in order to obtain a large
heat transfer per unit volume and to enhance the heat transfer rate on the inside
surface. In the present study, CFD simulations are carried out for a counter
flow tube in tube helical heat exchanger where hot water flows through the
inner tube and cold water flows through the outer tube. From the simulation
results heat transfer coefficient, pressure drop and nusselt number are
calculated. The heat transfer characteristics of the same are compared with that
of a counter flow tube in tube straight tube heat exchanger of same length
under same temperature and flow conditions. CFD simulation results showed
that the helical tube in tube heat exchanger is more effective than the straight
tube in tube heat exchanger.
Keywords: Heat exchanger, helical coil, CFD, simulation
Introduction
Flow through a helical coiled tube has observed extensive applications including
power plants, chemical, refrigeration, air conditioning and food processing industries.
Fluid flow in a coiled tube in tube will experience a centrifugal force, which results in
a secondary flow. The secondary flow enhances the heat transfer rates as it reduces

2. 5210 Soby P. Sunny et al
the temperature gradient across the cross-section of the tube. Thus there is one more
convective heat transfer mechanism in helical coils, perpendicular to the main flow,
which does not exist in straight tube heat exchangers.
An experimental study on helical coiled heat exchanger were done and results
showed that heat transfer coefficient was affected by the geometry of the heat
exchanger. It also showed that heat transfer coefficient was more for helical coiled
tubes compared to the straight tube [1]. A CFD model was developed to be validated
with the experimental values, the results showed the outlet temperatures and nusselt
number were very close to the experimental values [2]. Numerical study of tube in
tube helical heat exchanger for both parallel and counter flow was done. The result
indicated that in the design of double pipe helical heat exchanger there must be more
attention given to the annulus side as the thermal resistance is dominating in the
annulus part [3]. Experimental and CFD study of helically coiled heat exchangers
were done, results showed that CFD simulation results match reasonably well with the
experiment [4]. A CFD analysis of helical tube in tube heat exchanger was done to
determine the nusselt number and heat transfer coefficient. The result showed along
the outer side of the pipes the velocity and pressure values were higher in comparison
to the inner values [7]. A study of pressure drop characteristics of nano fluid flow
inside a vertical helical coiled tube was conducted, the results indicated that using
helical tubes instead of straight tube increases the pressure drop exponentially [5]. A
comparative analysis was carried out for different correlations by different researchers
for helical coil heat exchanger. Results showed that the ratio of the tube diameter to
coil diameter should be large enough for large intensities of secondary flow inside the
tube [6].
Objective
The objective of the study is to determine the heat transfer characteristics of helical
tube in tube heat exchanger. The helical coil tube in tube heat exchanger was initially
modeled and then simulated using computational fluid domain with fixed wall
temperature boundary conditions. The parameters like heat transfer rate, heat transfer
coefficient, and Nusselt number were calculated. The result obtained from simulation
is then validated with the literature study done on helical tube in tube with same
dimensions and same flow rates. The straight tube in tube heat exchanger was also
modeled of equal length and operating conditions, so as to compare with the heat
transfer characteristics of helical tube in tube heat exchanger.
Numerical modelling and simulation
Geometry and parameters of tube in tube helical coil
The major geometric dimensions include the inner diameter of the tube (d1), outer
diameter of the tube (d2), curvature diameter (D) and the coil pitch (p). Table 1 below
shows dimensional and used in the present study.

3. Numerical Simulation of a Tube 5211
Table 1: Dimensional and operating parameters of the heat exchanger
S. No. Dimensional Parameters Values
1 Inner tube diameter 15.8mm
2 Outer tube diameter 22.5mm
3 Curvature radius 76.2mm
4 Inlet temperature 345
5 Outlet temperature 280
6 Working fluid Water
Figure 1. Schematic diagram of helical and straight tube in tube helical heat
exchanger

4. 5212 Soby P. Sunny et al
Calculation of heat transfer coefficient and nusselt number
The heat transfer Q, can be obtained from the simulated result of the heat exchanger.
Then heat transfer coefficient, h can be calculated from the equation,
TA
Q
h


.
(1)
Q = heat transfer (W),
h = heat transfer coefficient (W/m2
K),
A = area of heat transfer (m2
),
∆T = difference between temperature average fluid temperature and average helical
coil temperature (K)
Nusselt number, Nu can be calculated by using the relations,
K
hD
Nu
h)(
 (2)
Nu = Nusselt Number,
h = heat transfer coefficient,
Dh = hydraulic diameter,
k = thermal conductivity
The heat transfer coefficient, nusselt number are calculated for the tube in tube
helical heat exchanger and further compared with straight tube in tube heat exchanger.
3.2. Numerical Simulation
The CFD software ANSYS Fluent 14.0 was used to solve the governing equations of
mass, momentum and heat transfer. The tube in tube helical heat exchanger was
modeled in ANSYS fluent geometry module, then the meshing of the model was done
in ANSYS fluent meshing module. In order to get good results fine mesh was
employed, hexahedral and tetrahedral mesh was used for the model. Also at high
temperature region near boundaries we employed structured hexahedral mesh. The k-
Ɛ standard turbulence model was used as the viscous model suggested by Wang and
Chen. In the simulation of the turbulent flow, simple scheme algorithm for pressure
velocity coupling was used. The assumptions used for the simulation of the helical
heat exchanger are:
1. The flow is steady and incompressible.
2. Radiation and natural convection effects are ignored.
3. Constant wall temperature at the boundary.
The inputs are given at the two inlets of the inner tube and outer tube. The
conservation equations were solved for the control volume to yield the velocity and
temperature fields for the fluid flow in the model. The model was converged when all
the residuals fell below 10-6
in the computational domain.

5. Numerical Simulation of a Tube 5213
Results and discussions
The CFD simulation was done for the three different mass flow rates of water
respectively for both straight and helical tube ranging from 0.2kg/s to 0.7kg/s.
Reynolds number corresponding to these mass flow rates were varying from 4000 to
9000. The parameters that are adopted for comparison are temperature of water at the
outlet, heat transfer rate, nusselt number, pressure drop and heat transfer coefficient.
The flow of fluid in helical coil is shown in the figure 2, the fluid particles moves
towards the outer wall then returns to the inner portion of tube by flowing back along
the wall. The figure 3 shows the close view of the velocity vectors, the fluid in the
curve of the tube moves towards the outer wall and then returns to the inner portion of
tube by flowing back along the wall.
Figure 4 shows the temperature contour for the helical tube in tube. It can be seen
that temperature drop for helical tube in tube is higher than the straight tube, which is
due to the curvature effect of the helical shape of the coil. Fluid flow in the outer layer
of the tube in tube moves faster than the fluid flow in the inner layer due this a
secondary flow is set which enhances the heat transfer. Figure 5 shows the
temperature contour for the straight tube in tube, it shows the temperature drop in the
inner tube from inlet to outlet.
Figure 6 and 7 shows the variation of heat transfer coefficient for different mass
flow rates for the inner and outer tube of helical and straight tube in tube heat
exchanger. From the graph it is clear that as mass flow rate increases heat transfer
coefficient also increases as expected since heat transfer rate is proportional to the
mass flow rate. The heat transfer coefficient for helical tube in tube shows a
remarkable increase of heat transfer coefficient by 10.5% when compared to the
straight tube in tube heat exchanger.
Figure 8 and 9 shows the variation of inner tube and outer tube nusselt number for
helical and straight tube in tube heat exchanger. Nusselt numbers corresponding to the
helical is higher than the straight tube in tube for all mass flow rates. This is due to the
secondary flow in the helical tube in tube which aids the heat transfer. The nusselt
number increased by 10.9% for helical tube in tube when compared to straight tube in
tube for a particular mass flow rate. In the helical coils at higher mass flow rates the
Reynolds number increases and the fluid turbulence also increases. Thereby due to
higher turbulence the intensity of secondary flow increases and hence nusselt number.
Figure 10 shows the variation of pressure drop over the entire length of helical
tube in tube heat exchanger. From the figure 11 it is clear that the pressure drop in
helical tube in tube is higher than that in straight tube in tube, this is due to the
centrifugal force and secondary flow in helical tube in tube. Secondary flow dissipates
kinetic energy which increases the resistance to flow.
The present study was validated from the literature work done by Soumya Ranjan,
2013. Figure 12 shows the Total pressure at different points along the pipe length for
the outer wall were checked and an accuracy of 94.56% was achieved.

6. 5214 Soby P. Sunny et al
Figure 2. Velocity vectors colored by velocity magnitude (m/s)
Figure 3. Close view of velocity vectors of helical tube in tube

7. Numerical Simulation of a Tube 5215
Figure 4. Contours of static temperature in K of helical tube in tube
Figure 5. Contours of static temperature for inner tube of straight tube in tube

8. 5216 Soby P. Sunny et al
Figure 6: Variation of heat transfer coefficient with mass flow rate for inner tube
of helical and straight tube in tube
Figure 7: Variation of heat transfer coefficient with mass flow rate for outer tube
of helical and straight tube in tube

9. Numerical Simulation of a Tube 5217
Figure 8: Variation of Nusselt Number with mass flow rate for inner tube of
helical and straight tube in tube
Figure 9: Variation of Nusselt number with mass flow rate for outer tube of
helical and straight tube in tube

10. 5218 Soby P. Sunny et al
Figure 10: Pressure drop along the length of the helical tube in tube
Figure 11: Total Pressure Plot for outer wall of helical tube in tube and straight
tube in tube

11. Numerical Simulation of a Tube 5219
Figure 12: Total Pressure Plot for outer wall of helical tube in tube
Conclusion
In this present work CFD simulation for helical tube in tube heat exchanger was
carried out and the results of heat transfer parameters have been compared with the
straight tube in tube under same geometrical and operating conditions. The CFD
results are validated with the literature work, the results achieved were well within the
error limits. Simulation results indicated that heat transfer rate, nusselt number and
heat transfer coefficient are higher in case of helical tube in tube when compared with
the straight tube in tube. For different mass flow rates, the helical tube in tube heat
exchanger provides an increase in heat transfer coefficient by 10.5%. The pressure
drop for helical coil tube in tube is found to be more when compared with the straight
tube in tube for identical conditions and varies exponentially indicating the necessity
of higher pumping power for helical tube in tube heat exchanger.
Table 2: Nomenclature
Nomenclature
Re, Reynolds number Dh, Hydraulic Diameter (m)
Q, Heat transfer rate (W) Nu, Nusselt Number
A, Area of heat transfer (m2
) k, Thermal conductivity (W/mK)
∆T, Temperature difference (K) ∆P, Pressure drop (Pa)
6. References
[1] Prabhanjan D.G., Raghavan G.S.V., and Rennie T.J., “Comparison of heat
transfer rates between a straight tube heat exchanger and a helically coiled heat
exchanger”, Heat and mass transfer, Vol. 29, No. 2, (2002), pp.185-191.

12. 5220 Soby P. Sunny et al
[2] Prabhanjan D.G., Raghavan G.S.V., and Rennie T.J., “Natural convection heat
transfer from helical coiled tubes”, International Journal of Science, Vol. 43,
No. 4, (2004), pp. 359-365.
[3] Timothy J.R., Vijaya G.S., “Numerical studies of a double-pipe helical heat
exchanger”, Applied Thermal Engineering, Vol. 26, (2006), pp. 1266-1273.
[4] J.S. Jayakumar, S.M. Mahajani, J.C. Mandal, “Experimental and CFD
estimation of heat transfer in helically coiled heat exchangers”, Chemical
engineering research and design, Vol. 86, (2008), pp. 221-232.
[5] Pramod S. P., Mandar M. L., Rajkumar G., “Parametric analysis of helical coil
heat exchanger”, International Journal of Engineering Research & Technology,
Vol.1, Issue 8, (2012), pp. 1-5.
[6] M. P. Fakoor-Pakdaman, M.A. Akhavan-Behabadi, P. Razi, “An empirical
study on the pressure drop characteristics of Nano fluid flow inside helically
coiled tubes” International Journal of Thermal Sciences, Vol. 65, (2013), pp.
206-213.
[7] Soumya R.M., “CFD analysis of heat transfer in a helical coil heat exchanger
using fluent”, Maser Thesis, NIT Rourkela, India, (2013), pp. 1-33.