CHAPTER focused towards either constant wall temperature or constant

CHAPTER
II

LITERATURE
REVIEW

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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.