Shell

and helical coiled tube heat exchangers are known as the most common sort of

heat exchangers due to their compactness, ease of manufacture and heat transfer

efficiency. Shell and helical coil heat exchanger is preferred over other

conventional heat exchanger due to its ability to transfer more heat in given

space limitation. The coil curvature provide centrifugal forces to act on the

fluid flowing inside heat exchanger, which result in secondary flow pattern

which is perpendicular to the axial flow pattern. This secondary pattern

consists of two vortices. Heat transfer rates are increased by secondary flow

as it moves fluid across the temperature gradient. Hence, there is an

additional convective heat transfer mechanism that is perpendicular to the

axial flow, which does not exist in straight tube heat exchangers.

Several

researchers have analyzed the helical coil heat exchangers, which involves

various dimensionless numbers and geometric parameter variation in order to

improve the heat transfer rate and effectiveness of heat exchanger and some

studies are focused towards either constant wall temperature or constant wall

heat flux boundary conditions of heat exchanger. However some studied are

focused on the effect of air bubble injection into helical coil heat exchanger,

the variations of NTU and effectiveness due to the air bubbles injection with

different air flow rates.

In

last few years more computational methods have been developed as technological

advancement. These computational methods helped researchers in analyzing

combustion, fluid dynamics and behaviour of different models of heat transfer

etc. the heat exchangers have wide range of applications in various industries,

which have attracted more researchers to work in this field.

Following

articles presents a review of important published literature:-

2.1.

Previous work

Austen

et al. (1988) Studied the influence of pitch on the

pressure drop and heat transfer characteristics of helical coils is explored

for the condition of uniform input heat flux. Two pairs of coils were tested;

each pair corresponds to the same diametric ratio but substantially different

pitch ratio. Water (3 < Pr < 6) was used as the test fluid. The results
include the isothermal and diabatic friction factors, wall temperature, and
local and fully developed Nusselt numbers. Significant pitch effects were noted
in the friction factor and Nusselt number results at low Reynolds numbers.
These effects are attributed to free convection, and they diminish as Reynolds
number increases.
Yildiz
et al. (1997) Studied a heat exchanger which is constructed by
placing spring-shaped wires with varying
pitch within a helical pipe was considered. The pressure drop and the
overall heat transfer rates were measured for the case of air flow at various
Reynolds numbers inside and constant water flow outside. The results show that
the Nusselt number increases with decreasing pitch/wire diameter ratio, as much
as five times with respect to an empty pipe for the same Dean number, and for
this relationship, a tentative empirical formula is suggested. Although a rise
up to 10 times in the inlet/outlet pressure drop values with respect to the
conventional empty helical case is observed, the increase in Nusselt number,
naturally, reflects an increase of about 30% in the effectiveness of the
helical heat exchanger.
Gabillet et al. (2002)
experimentally studied on the bubble injection in a turbulent boundary layer.
Experiments were performed in a horizontal channel in order to simulate the
dynamical effects of the nucleation of bubbles. Their findings showed that the
mean velocity is nearly the same as in single phase flow, except near the wall
where the shear stress is greater than in single phase flow.
Prabhanjan
et al. (2002) studied the comparison of heat
transfer rates between a straight tube heat exchanger and a helically coiled
heat exchanger. The studies focus on constant wall temperature and constant
heat flux with fluid-to-fluid heat exchanger. The results showed that the heat
transfer coefficient was affected by the geometry of the heat exchanger.
Ko
and Ting (2005) produced analyses the optimal
Reynolds number for the steady, laminar, fully developed forced convection in a
helical coiled tube with constant wall heat flux based on minimal entropy
generation principle. It is found that the entropy generation distributions are
relatively insensitive to coil pitch. An experimental investigation regarding
the laminar to turbulent flow transition in helically coiled pipes was studied.
Timothy
et al. (2005) have studied experimental of a
double-pipe helical heat exchanger. Two heat exchanger sizes and both parallel
flow and counter-flow configuration were tested. The result showed that, the
heat transfer rates were much higher in the counter-flow configuration due to
the larger average temperature difference between the two fluids.
Akpinar
et al. (2005) Experimental investigations were
performed by for analysis of the heat transfer and exergy loss in a concentric
double pipe heat exchanger equipped with swirl generators. Their results showed
up to 130% increase in heat transfer.
Akpinar(2006)
investigated the exergy loss and heat
transfer in a concentric double pipe heat exchanger equipped with helical
wires. Their experiments showed an augmentation of up to 1.16 times in the
dimensionless exergy loss compared to the empty pipe.
Funfschilling et. al. (2006)
studied the influence of the injection
period on the bubble rise velocity. They found that the rise velocity decreases
significantly with the injection period.
Ide et al. (2007)
measured the void fraction and bubble size distributions in a microchannel.
Naphon (2007) studied
the thermal performance and pressure drop of the helical-coil heat exchanger
with and without helical crimped fins are studied. The heat exchanger consists
of a shell and helically coiled tube unit with two different coil diameters.
Each coil is fabricated by bending a 9.50 mm diameter straight copper tube into
a helical-coil tube of thirteen turns. Cold and hot water are used as working
fluids in shell side and tube side, respectively. The experiments are done at
the cold and hot water mass flow rates ranging between 0.10 and 0.22 kg/s, and
between 0.02 and 0.12 kg/s, respectively. The inlet temperatures of cold and
hot water are between 15 and 25 °C, and between 35 and 45 °C, respectively. The
cold water entering the heat exchanger at the outer channel flows across the
helical tube and flows out at the inner channel. The hot water enters the heat
exchanger at the inner helical-coil tube and flows along the helical tube. The
effects of the inlet conditions of both working fluids flowing through the test
section on the heat transfer characteristics are discussed.
Salimpour
(2008) studied, the heat transfer coefficients of
shell and helically coiled tube heat exchangers were investigated
experimentally. Three heat exchangers with different coil pitches were selected
as test section for both parallel-flow and counter-flow configurations. All the
required parameters like inlet and outlet temperatures of tube-side and shell-side
fluids, flow rate of fluids, etc. were measured using appropriate instruments.
Totally, 75 test runs were performed from which the tube-side and shell-side
heat transfer coefficients were calculated. Empirical correlations were
proposed for shell-side and tube-side. The calculated heat transfer
coefficients of tube-side were also compared to the existing correlations for
other boundary conditions and a reasonable agreement were observed.
Salimpour
(2008) presented an experimental investigation
to study the heat transfer characteristics of temperature dependent- property
engine-oil inside shell and coiled tube heat exchangers. Three heat exchangers
with different coil pitches were selected as the test section for counter-flow
configuration. All the required parameters like inlet and outlet temperatures
of tube-side and shell-side fluids, flow rate of fluids, etc. were measured
using appropriate instruments. An empirical correlation existed in the previous
literature for evaluating the shell-side Nusselt number was invoked to
calculate the heat transfer coefficients of the temperature-dependent-property
fluid flowing in the tube-side of the heat exchangers.
Kitagawa et al. (2010) presented
an experimental investigation of laminar mixed-convection flows of water with
sub-millimeter bubbles in a vertical channel. particle tracking velocimetry
technique for the temperature and velocity measurements. The working fluid used
is tap water, and hydrogen bubbles generated by electrolysis of the water are
used as the submillimeter bubbles. The Reynolds number of the main flow ranges
from 100 to 200. The ratio of the heat transfer coefficient with
sub-millimeter-bubble injection to that without injection (the heat transfer
coefficient ratio) ranges from 1.24 to 1.38. The heat transfer coefficient
ratio decreases with the increase in the Reynolds number.
Moawed
(2011) studied experimentally the forced
convection heat transfer from helical coiled tubes under constant heat flux
condition. He developed a general correlation to describe the average Nusselt
(Nu) number.
Behabadi
et al. (2012) studied, heat transfer enhancement of
a nanofluid flow inside vertical helically coiled tubes has been investigated
experimentally in the thermal entrance region. The temperature of the tube wall
was kept constant at around 95 °C to have isothermal boundary condition.
Experiments were conducted for fluid flow inside straight and helical tubes. In
these experiments, the effects of a wide range of different parameters such as
Reynolds and Dean numbers, geometrical parameters and nanofluid weight
fractions have been studied. In order to investigate the effect of the fluid
type on the heat transfer, pure heat transfer oil and nanofluids with weight
concentrations of 0.1, 0.2 and 0.4% were utilized as the working fluid. The
thermo-physical properties of the working fluids were extremely temperature
dependent; therefore, rough correlations were proposed to predict their
properties. Based on the experimental data, utilizing helical coiled tubes instead
of straight ones enhances the heat transfer rate remarkably. Besides, nanofluid
flows showed much higher Nusselt numbers compared to the base fluid flow.
Finally, it was observed that combination of the two enhancing methods has a
noticeably high capability to the heat transfer rate.
Huminic
et al. (2012) studied the purpose of this
review summarizes the important published articles on the enhancement of the
convection heat transfer in heat exchangers using nanofluids on two topics. The
first section focuses on presenting the theoretical and experimental results
for the effective thermal conductivity, viscosity and the Nusselt number
reported by several authors. The second section concentrates on application of
nanofluids in various types of heat exchangers: plate heat exchangers, shell
and tube heat exchangers, compact heat exchangers and double pipe heat
exchanger.
Akbaridous et al. (2013)
studied numerically and experimentally laminar, steady state flow in helically
coiled tubes at a constant wall temperature. Pressure drop and the convective
heat transfer behaviour of nanofluid were investigated. In the experimental
section, a heat exchanger was designed, capable of providing constant wall
temperature for coils with different curvature and torsion ratio for the ease
of assembly. Pressure drop measurement and average convective heat transfer
coefficient calculation were carried out. In the numerical study, the three-dimensional
governing equations were solved by finite difference method with projection
algorithm using FORTRAN programming language. Homogeneous model with constant
effective properties was used. The difference between numerical and
experimental results was significant. Dispersion model was employed to make the
observed difference between numerical and experimental results negligible.
Dispersion model was modified to be applicable for helical tubes. This
modification resulted in negligible difference between the numerical and the
experimental results. More enhanced heat transfer was observed for tubes with
greater curvature ratio. Moreover, the performance evaluation of these enhanced
heat transfer methods presented. Utilization of base fluid in helical tube with
greater curvature compared to the use of nanofluid in straight tubes enhanced heat
transfer more effectively.
Jamshidi
et al. (2013) attempts are made to
enhance the heat transfer rate in shell and coiled tube heat exchangers
experimentally. Hot water flows in helical tube and cold water flows in the
shell side. Tube and shell side heat transfer coefficients are determined using
Wilson plots. Experimental apparatus and Taguchi method are used to investigate
the effect of fluid flow and geometrical parameters on heat transfer rate.
After experiments, Taguchi method is used for finding the optimum condition for
the desired parameters in the range of 0.0813 < Dc < 0.116,
13 < Pc < 18, tube and shell flow
rates from 1 to 4 LPM. Then the optimum condition according to the overall heat
transfer coefficient for the whole heat exchanger is found. Results indicate
that the higher coil diameter, coil pitch and mass flow rate in shell and tube
can enhance the heat transfer rate in these types of heat exchangers.
Contribution ratio obtained by Taguchi method shows that shell side flow rate,
coil diameter, tube side flow rate and coil pitch are the most important design
parameters in coiled heat exchangers.
Aly
(2013) studied A computational fluid dynamics
(CFD) study has been carried out to study the heat transfer and pressure drop
characteristics of water-based Al2O3 nanofluid flowing inside coiled
tube-in-tube heat exchangers. The 3D realizable k–e turbulent model with
enhanced wall treatment was used. Temperature dependent thermos physical
properties of nanofluid and water were used and heat exchangers were analyzed
considering conjugate heat transfer from hot fluid in the inner-coiled tube to
cold fluid in the annulus region. The overall performance of the tested heat
exchangers was assessed based on the thermo-hydrodynamic performance index.
Design parameters were in the range of; nanoparticles volume concentrations
0.5%, 1.0% and 2.0%, coil diameters 0.18, 0.24 and 0.30 m, inner tube and
annulus sides flow rates from 2 to 5 LPM and 10 to 25 LPM, respectively.
Nanofluid flows inside inner tube side or annular side. The results obtained
showed a different behavior depending on the parameter selected for the
comparison with the base fluid. Moreover, when compared at the same Re or Dn,
the heat transfer coefficient increases by increasing the coil diameter and
nanoparticles volume concentration. Also, the friction factor increases with
the increase in curvature ratio and pressure drop penalty is negligible with
increasing the nanoparticles volume concentration. Conventional correlations
for predicting average heat transfer and friction factor in turbulent flow
regime such as Gnielinski correlation and Mishra and Gupta correlation,
respectively, for helical tubes are also valid for the tested nanofluids which
suggests that nanofluids behave like a homogeneous fluid.
Ankanna
et al. (2014) Proposed in the present days Heat
exchangers are the important engineering systems with wide variety of
applications including power plants, nuclear reactors, refrigeration and
air-conditioning systems, heat recovery systems, chemical processing and food
industries. Helical coil configuration is very effective for heat exchangers
and chemical reactors because they can accommodate a large heat transfer area
in a small space, with high heat transfer coefficients. This paper focus on an
increase in the effectiveness of a heat exchanger and analysis of various
parameters that affect the effectiveness of a heat exchanger and also deals
with the performance analysis of heat exchanger by varying various parameters
like number of coils, flow rate and temperature. The results of the helical
tube heat exchanger are compared with the straight tube heat exchanger in both
parallel and counter flow by varying parameters like temperature, flow rate of
cold water and number of turns of helical coil.
Kitagawa
et al. (2014) Study is based on the experimental
finding that microbubble swarms dramatically promote heat transfer from a
vertical heated wall, despite their potentially adiabatic nature, tests of
microbubble fluid mechanics in the isothermal state are performed to clarify
the unique motion characteristics of microbubble swarms. At constant bubble
flow rate, the microbubble swarm shows a significant pulsatory rise along a
vertical flat wall, particularly for small bubbles. Particle tracking
velocimetry applied to the microbubbles shows that a two-way interaction
between the microbubbles and the liquid flow self-excites the pulsation during
their - 2 - co-current rise. The sequence consists of the following processes:
i) increase in the bubble number density close to the wall as a result of the
liquid velocity gradient driven by the microbubbles themselves; ii) wave
generation inside the microbubble swarm to induce the pulsatory rise of the
swarm; and iii) amplification of the waves, which results in void-bursting
motion in the final stage.
Dizaji
et al. (2015) attempts were made to increase the
number of thermal units (NTU) and performance in a vertical shell and coiled
tube heat exchanger via air bubble injection into the shell side of heat
exchanger. Besides, exergy loss due to air bubble injection is investigated.
Indeed, air bubble injection and bubbles mobility (because of buoyancy force)
can intensify the NTU and exergy loss by mixing the thermal boundary layer and
increasing the turbulence level of the fluid flow. Air bubbles were injected
inside the heat exchanger via a special method and at new different conditions
in this paper. It was demonstrated that the amount of NTU and effectiveness can
be significantly improved due to air bubbles injection.
Dizaji
et al. (2015) studied experimentally the effect of
air bubble injection on the heat transfer rate and effectiveness through a
horizontal double pipe heat exchanger.
Dizaji
et al. (2015) performed experimental investigations
on the effects of flow, thermodynamic and geometrical characteristics on exergy
loss in shell and coiled tubes heat exchangers. Pressure drop and heat transfer
characteristics in shell and coiled tube heat exchangers have been widely
studied in the resent years. However, the effects of flow, thermodynamic and
geometrical parameters on energetic characteristics have not been explicitly
and experimentally studied. Hence, the main scope of the present work is to
clarify the effect of shell and coil side flow rates, inlet temperatures, coil
pitch and coil diameter on exergy loss in shell and coiled tube heat
exchangers. Both of the total exergy loss and dimensionless exergy loss are
studied.
Andrew
et al. (2016) studied due to their compact design,
ease of manufacture and enhanced heat transfer and fluid mixing properties,
helically coiled tubes are widely used in a variety of industries and
applications. In fact, helical tubes are the most popular from the family of
coiled tube heat exchangers. This review summarises and critically reviews the
studies reported in the pertinent literature on the pressure drop
characteristics of two-phase flow in helically coiled tubes. The main findings
and correlations for the frictional two-phase pressure drops due to:
steam-water flow boiling, R-134a evaporation and condensation, air-water
two-phase flow and nanofluid flows are reviewed. Therefore, the purpose of this
study is to provide researchers in academia and industry with a practical
summary of the relevant correlations and supporting theory for the calculation
of the two-phase pressure drop in helically coiled tubes. A significant scope
for further research was also identified in the fields of: air-water bubbly
flow and nanofluid two phase and three-phase flows in helically coiled tubes.
Khorasani
et al. (2017) studied experimentally the effects of
air bubble injection on the performance of a horizontal helical shell and
coiled tube heat exchanger. The variations of number of thermal units (NTU),
exergy loss and effectiveness due to the air bubbles injection with different
air flow rates are evaluated. A new procedure for injecting the air bubbles
into the shell side flow of the heat exchanger is proposed. The results
exhibited a significant increase in the effectiveness and NTU of the heat
exchanger as the air bubbles were injected. It is suggested that the
disturbance and perhaps the turbulence intensity of the shell side flow are
increased due to the motion of air bubbles resulting in an increment in the
value of NTU and exergy loss. In addition, the mixing effect of the bubbles and
the interaction with the thermal boundary layer can increase the velocity
(hence the Reynolds number) of the shell side flow.