CHAPTER focused towards either constant wall temperature or constant

CHAPTERIILITERATUREREVIEWShelland helical coiled tube heat exchangers are known as the most common sort ofheat exchangers due to their compactness, ease of manufacture and heat transferefficiency. Shell and helical coil heat exchanger is preferred over otherconventional heat exchanger due to its ability to transfer more heat in givenspace limitation. The coil curvature provide centrifugal forces to act on thefluid flowing inside heat exchanger, which result in secondary flow patternwhich is perpendicular to the axial flow pattern.

This secondary patternconsists of two vortices. Heat transfer rates are increased by secondary flowas it moves fluid across the temperature gradient. Hence, there is anadditional convective heat transfer mechanism that is perpendicular to theaxial flow, which does not exist in straight tube heat exchangers.Severalresearchers have analyzed the helical coil heat exchangers, which involvesvarious dimensionless numbers and geometric parameter variation in order toimprove the heat transfer rate and effectiveness of heat exchanger and somestudies are focused towards either constant wall temperature or constant wallheat flux boundary conditions of heat exchanger. However some studied arefocused on the effect of air bubble injection into helical coil heat exchanger,the variations of NTU and effectiveness due to the air bubbles injection withdifferent air flow rates.Inlast few years more computational methods have been developed as technologicaladvancement. These computational methods helped researchers in analyzingcombustion, fluid dynamics and behaviour of different models of heat transferetc.

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the heat exchangers have wide range of applications in various industries,which have attracted more researchers to work in this field.Followingarticles presents a review of important published literature:- 2.1.        Previous workAustenet al. (1988) Studied the influence of pitch on thepressure drop and heat transfer characteristics of helical coils is exploredfor the condition of uniform input heat flux. Two pairs of coils were tested;each pair corresponds to the same diametric ratio but substantially differentpitch ratio.

Water (3 < Pr < 6) was used as the test fluid. The resultsinclude the isothermal and diabatic friction factors, wall temperature, andlocal and fully developed Nusselt numbers. Significant pitch effects were notedin the friction factor and Nusselt number results at low Reynolds numbers.These effects are attributed to free convection, and they diminish as Reynoldsnumber increases.

Yildizet al. (1997) Studied  a heat exchanger which is constructed byplacing spring-shaped wires with varying pitch within a helical pipe was considered. The pressure drop and theoverall heat transfer rates were measured for the case of air flow at variousReynolds numbers inside and constant water flow outside. The results show thatthe Nusselt number increases with decreasing pitch/wire diameter ratio, as muchas five times with respect to an empty pipe for the same Dean number, and forthis relationship, a tentative empirical formula is suggested. Although a riseup to 10 times in the inlet/outlet pressure drop values with respect to theconventional empty helical case is observed, the increase in Nusselt number,naturally, reflects an increase of about 30% in the effectiveness of thehelical 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 thedynamical effects of the nucleation of bubbles. Their findings showed that themean velocity is nearly the same as in single phase flow, except near the wallwhere the shear stress is greater than in single phase flow.Prabhanjanet al. (2002) studied the comparison of heattransfer rates between a straight tube heat exchanger and a helically coiledheat exchanger. The studies focus on constant wall temperature and constantheat flux with fluid-to-fluid heat exchanger.

The results showed that the heattransfer coefficient was affected by the geometry of the heat exchanger.Koand Ting (2005) produced analyses the optimalReynolds number for the steady, laminar, fully developed forced convection in ahelical coiled tube with constant wall heat flux based on minimal entropygeneration principle. It is found that the entropy generation distributions arerelatively insensitive to coil pitch. An experimental investigation regardingthe laminar to turbulent flow transition in helically coiled pipes was studied.Timothyet al.

(2005) have studied experimental of adouble-pipe helical heat exchanger. Two heat exchanger sizes and both parallelflow and counter-flow configuration were tested. The result showed that, theheat transfer rates were much higher in the counter-flow configuration due tothe larger average temperature difference between the two fluids. Akpinaret al. (2005) Experimental investigations wereperformed by for analysis of the heat transfer and exergy loss in a concentricdouble pipe heat exchanger equipped with swirl generators.

Their results showedup to 130% increase in heat transfer. Akpinar(2006)investigated the exergy loss and heattransfer in a concentric double pipe heat exchanger equipped with helicalwires. Their experiments showed an augmentation of up to 1.16 times in thedimensionless exergy loss compared to the empty pipe.Funfschilling et. al.

(2006)studied the influence of the injectionperiod on the bubble rise velocity. They found that the rise velocity decreasessignificantly with the injection period.Ide et al. (2007)measured the void fraction and bubble size distributions in a microchannel.Naphon (2007) studiedthe thermal performance and pressure drop of the helical-coil heat exchangerwith and without helical crimped fins are studied. The heat exchanger consistsof 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 intoa helical-coil tube of thirteen turns. Cold and hot water are used as workingfluids in shell side and tube side, respectively. The experiments are done atthe cold and hot water mass flow rates ranging between 0.10 and 0.22 kg/s, andbetween 0.02 and 0.

12 kg/s, respectively. The inlet temperatures of cold andhot water are between 15 and 25 °C, and between 35 and 45 °C, respectively. Thecold water entering the heat exchanger at the outer channel flows across thehelical tube and flows out at the inner channel. The hot water enters the heatexchanger at the inner helical-coil tube and flows along the helical tube. Theeffects of the inlet conditions of both working fluids flowing through the testsection on the heat transfer characteristics are discussed.Salimpour(2008) studied, the heat transfer coefficients ofshell and helically coiled tube heat exchangers were investigatedexperimentally.

Three heat exchangers with different coil pitches were selectedas test section for both parallel-flow and counter-flow configurations. All therequired parameters like inlet and outlet temperatures of tube-side and shell-sidefluids, flow rate of fluids, etc. were measured using appropriate instruments.Totally, 75 test runs were performed from which the tube-side and shell-sideheat transfer coefficients were calculated. Empirical correlations wereproposed for shell-side and tube-side.

The calculated heat transfercoefficients of tube-side were also compared to the existing correlations forother boundary conditions and a reasonable agreement were observed.Salimpour(2008) presented an experimental investigationto study the heat transfer characteristics of temperature dependent- propertyengine-oil inside shell and coiled tube heat exchangers. Three heat exchangerswith different coil pitches were selected as the test section for counter-flowconfiguration. All the required parameters like inlet and outlet temperaturesof tube-side and shell-side fluids, flow rate of fluids, etc. were measuredusing appropriate instruments.

An empirical correlation existed in the previousliterature for evaluating the shell-side Nusselt number was invoked tocalculate the heat transfer coefficients of the temperature-dependent-propertyfluid flowing in the tube-side of the heat exchangers. Kitagawa et al. (2010) presentedan experimental investigation of laminar mixed-convection flows of water withsub-millimeter bubbles in a vertical channel. particle tracking velocimetrytechnique for the temperature and velocity measurements. The working fluid usedis tap water, and hydrogen bubbles generated by electrolysis of the water areused as the submillimeter bubbles. The Reynolds number of the main flow rangesfrom 100 to 200.

The ratio of the heat transfer coefficient withsub-millimeter-bubble injection to that without injection (the heat transfercoefficient ratio) ranges from 1.24 to 1.38. The heat transfer coefficientratio decreases with the increase in the Reynolds number.

Moawed(2011) studied experimentally the forcedconvection heat transfer from helical coiled tubes under constant heat fluxcondition. He developed a general correlation to describe the average Nusselt(Nu) number.Behabadiet al. (2012) studied, heat transfer enhancement ofa nanofluid flow inside vertical helically coiled tubes has been investigatedexperimentally in the thermal entrance region. The temperature of the tube wallwas kept constant at around 95 °C to have isothermal boundary condition.Experiments were conducted for fluid flow inside straight and helical tubes.

Inthese experiments, the effects of a wide range of different parameters such asReynolds and Dean numbers, geometrical parameters and nanofluid weightfractions have been studied. In order to investigate the effect of the fluidtype on the heat transfer, pure heat transfer oil and nanofluids with weightconcentrations of 0.1, 0.2 and 0.4% were utilized as the working fluid. Thethermo-physical properties of the working fluids were extremely temperaturedependent; therefore, rough correlations were proposed to predict theirproperties. Based on the experimental data, utilizing helical coiled tubes insteadof straight ones enhances the heat transfer rate remarkably.

Besides, nanofluidflows showed much higher Nusselt numbers compared to the base fluid flow.Finally, it was observed that combination of the two enhancing methods has anoticeably high capability to the heat transfer rate.Huminicet al. (2012) studied the purpose of thisreview summarizes the important published articles on the enhancement of theconvection heat transfer in heat exchangers using nanofluids on two topics. Thefirst section focuses on presenting the theoretical and experimental resultsfor the effective thermal conductivity, viscosity and the Nusselt numberreported by several authors. The second section concentrates on application ofnanofluids in various types of heat exchangers: plate heat exchangers, shelland tube heat exchangers, compact heat exchangers and double pipe heatexchanger.Akbaridous et al. (2013)studied numerically and experimentally laminar, steady state flow in helicallycoiled tubes at a constant wall temperature.

Pressure drop and the convectiveheat transfer behaviour of nanofluid were investigated. In the experimentalsection, a heat exchanger was designed, capable of providing constant walltemperature for coils with different curvature and torsion ratio for the easeof assembly. Pressure drop measurement and average convective heat transfercoefficient calculation were carried out. In the numerical study, the three-dimensionalgoverning equations were solved by finite difference method with projectionalgorithm using FORTRAN programming language.

Homogeneous model with constanteffective properties was used. The difference between numerical andexperimental results was significant. Dispersion model was employed to make theobserved difference between numerical and experimental results negligible.Dispersion model was modified to be applicable for helical tubes. Thismodification resulted in negligible difference between the numerical and theexperimental results.

More enhanced heat transfer was observed for tubes withgreater curvature ratio. Moreover, the performance evaluation of these enhancedheat transfer methods presented. Utilization of base fluid in helical tube withgreater curvature compared to the use of nanofluid in straight tubes enhanced heattransfer more effectively.Jamshidiet al.

(2013) attempts are made toenhance the heat transfer rate in shell and coiled tube heat exchangersexperimentally. Hot water flows in helical tube and cold water flows in theshell side. Tube and shell side heat transfer coefficients are determined usingWilson plots. Experimental apparatus and Taguchi method are used to investigatethe effect of fluid flow and geometrical parameters on heat transfer rate.After experiments, Taguchi method is used for finding the optimum condition forthe desired parameters in the range of 0.0813 < Dc < 0.

116,13 < Pc < 18, tube and shell flowrates from 1 to 4 LPM. Then the optimum condition according to the overall heattransfer coefficient for the whole heat exchanger is found. Results indicatethat the higher coil diameter, coil pitch and mass flow rate in shell and tubecan 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 designparameters in coiled heat exchangers.Aly(2013) studied A computational fluid dynamics(CFD) study has been carried out to study the heat transfer and pressure dropcharacteristics of water-based Al2O3 nanofluid flowing inside coiledtube-in-tube heat exchangers. The 3D realizable k–e turbulent model withenhanced wall treatment was used.

Temperature dependent thermos physicalproperties of nanofluid and water were used and heat exchangers were analyzedconsidering conjugate heat transfer from hot fluid in the inner-coiled tube tocold fluid in the annulus region. The overall performance of the tested heatexchangers was assessed based on the thermo-hydrodynamic performance index.Design parameters were in the range of; nanoparticles volume concentrations0.5%, 1.

0% and 2.0%, coil diameters 0.18, 0.

24 and 0.30 m, inner tube andannulus 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 obtainedshowed a different behavior depending on the parameter selected for thecomparison with the base fluid. Moreover, when compared at the same Re or Dn,the heat transfer coefficient increases by increasing the coil diameter andnanoparticles volume concentration. Also, the friction factor increases withthe increase in curvature ratio and pressure drop penalty is negligible withincreasing the nanoparticles volume concentration.

Conventional correlationsfor predicting average heat transfer and friction factor in turbulent flowregime such as Gnielinski correlation and Mishra and Gupta correlation,respectively, for helical tubes are also valid for the tested nanofluids whichsuggests that nanofluids behave like a homogeneous fluid.Ankannaet al. (2014) Proposed in the present days Heatexchangers are the important engineering systems with wide variety ofapplications including power plants, nuclear reactors, refrigeration andair-conditioning systems, heat recovery systems, chemical processing and foodindustries. Helical coil configuration is very effective for heat exchangersand chemical reactors because they can accommodate a large heat transfer areain a small space, with high heat transfer coefficients. This paper focus on anincrease in the effectiveness of a heat exchanger and analysis of variousparameters that affect the effectiveness of a heat exchanger and also dealswith the performance analysis of heat exchanger by varying various parameterslike number of coils, flow rate and temperature. The results of the helicaltube heat exchanger are compared with the straight tube heat exchanger in bothparallel and counter flow by varying parameters like temperature, flow rate ofcold water and number of turns of helical coil.

Kitagawaet al. (2014) Study is based on the experimentalfinding that microbubble swarms dramatically promote heat transfer from avertical heated wall, despite their potentially adiabatic nature, tests ofmicrobubble fluid mechanics in the isothermal state are performed to clarifythe unique motion characteristics of microbubble swarms. At constant bubbleflow rate, the microbubble swarm shows a significant pulsatory rise along avertical flat wall, particularly for small bubbles. Particle trackingvelocimetry applied to the microbubbles shows that a two-way interactionbetween the microbubbles and the liquid flow self-excites the pulsation duringtheir – 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 theliquid velocity gradient driven by the microbubbles themselves; ii) wavegeneration inside the microbubble swarm to induce the pulsatory rise of theswarm; and iii) amplification of the waves, which results in void-burstingmotion in the final stage.

Dizajiet al. (2015) attempts were made to increase thenumber of thermal units (NTU) and performance in a vertical shell and coiledtube heat exchanger via air bubble injection into the shell side of heatexchanger. 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 andincreasing the turbulence level of the fluid flow.

Air bubbles were injectedinside the heat exchanger via a special method and at new different conditionsin this paper. It was demonstrated that the amount of NTU and effectiveness canbe significantly improved due to air bubbles injection.Dizajiet al.

(2015) studied experimentally the effect ofair bubble injection on the heat transfer rate and effectiveness through ahorizontal double pipe heat exchanger.Dizajiet al. (2015) performed experimental investigationson the effects of flow, thermodynamic and geometrical characteristics on exergyloss in shell and coiled tubes heat exchangers.

Pressure drop and heat transfercharacteristics in shell and coiled tube heat exchangers have been widelystudied in the resent years. However, the effects of flow, thermodynamic andgeometrical parameters on energetic characteristics have not been explicitlyand experimentally studied. Hence, the main scope of the present work is toclarify the effect of shell and coil side flow rates, inlet temperatures, coilpitch and coil diameter on exergy loss in shell and coiled tube heatexchangers. Both of the total exergy loss and dimensionless exergy loss arestudied.Andrewet 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 andapplications.

In fact, helical tubes are the most popular from the family ofcoiled tube heat exchangers. This review summarises and critically reviews thestudies reported in the pertinent literature on the pressure dropcharacteristics of two-phase flow in helically coiled tubes. The main findingsand correlations for the frictional two-phase pressure drops due to:steam-water flow boiling, R-134a evaporation and condensation, air-watertwo-phase flow and nanofluid flows are reviewed. Therefore, the purpose of thisstudy is to provide researchers in academia and industry with a practicalsummary of the relevant correlations and supporting theory for the calculationof the two-phase pressure drop in helically coiled tubes.

A significant scopefor further research was also identified in the fields of: air-water bubblyflow and nanofluid two phase and three-phase flows in helically coiled tubes.Khorasaniet al. (2017) studied experimentally the effects ofair bubble injection on the performance of a horizontal helical shell andcoiled tube heat exchanger. The variations of number of thermal units (NTU),exergy loss and effectiveness due to the air bubbles injection with differentair flow rates are evaluated. A new procedure for injecting the air bubblesinto the shell side flow of the heat exchanger is proposed. The resultsexhibited a significant increase in the effectiveness and NTU of the heatexchanger as the air bubbles were injected.

It is suggested that thedisturbance and perhaps the turbulence intensity of the shell side flow areincreased due to the motion of air bubbles resulting in an increment in thevalue of NTU and exergy loss. In addition, the mixing effect of the bubbles andthe interaction with the thermal boundary layer can increase the velocity(hence the Reynolds number) of the shell side flow.