Numerical investigation of grooves effects on the thermal performance of helically grooved shell and coil tube heat exchanger

Mehdi Miansari ,Mehdi Rajabtabar DarvishiDavood Toghraie,Pouya BarnoonMojtaba Shirzad,As’ad Alizadeh

1 Department of Mechanical Engineering,Qaemshahr Branch,Islamic Azad University,Qaemshahr,Iran

2 Department of Mechanical Engineering,Khomeinishahr Branch,Islamic Azad University,Khomeinishahr,Iran

3 Department of Mechanical Engineering,Department of Mechanical Engineering,Babol Noshirvani University of Technology,Babol,Iran

4 Department of Mechanical Engineering,Urmia University,Urmia,Iran

5 Department of Mechanical Engineering,College of Engineering,University of Zakho,Zakho,Iraq

Keywords:Numerical simulation Heat transfer Turbulent flow Shell and coil Helically grooved shell Heat exchanger

ABSTRACT Heat exchangers are integral parts of important industrial units such as petrochemicals,medicine and power plants.Due to the importance of systems energy consumption,different modifications have been applied on heat exchangers in terms of size and structure.In this study,a novel heat exchanger with helically grooved annulus shell and helically coiled tube was investigated by numerical simulation.Helically grooves with the same pitch of the helical coil tube and different depth are created on the inner and outer wall of annulus shell to improve the thermal performance of heat exchanger.In the first section,thermal performance of the shell and coil heat exchanger with the helical grooves on its outer shell wall was compared with same but without helical grooves.At the second section,helically grooves created on both outer and inner wall of the annulus shell with different groove depths.The results showed that the heat exchanger with grooves on both inner and outer shell wall has better thermal performance up to 20%compared to the heat exchanger with grooves on only outer shell wall.The highest thermal performance achieves at lower flow rates and higher groove depths whereas the pressure drop did not increase significantly.

1.Introduction

Heat exchangers are one of the most used equipment in various industries such as refineries,pharmaceuticals,power plant and so on.A heat exchanger allows heat transfer between at least two media with different phases [1–16].Many researches have been done for improvement of the heat transfer by the use of the inner helical corrugated tube [1,17–19].The coiling of a tube helps in designing a compact heat exchanger.Helical and serpentine coils are the conventional coil designs prevalent in heat transfer applications,such as radiators in automobiles,evaporators,and condenser coils in refrigerators and air conditioners,and shell and coil heat exchangers in distillers.Flowing fluid experiences,a centrifugal force in coiled tubes,causing the fluid from the inner wall to move toward the outer wall.The effect of helical corrugation and nanoparticles on the thermal performance was investigated by Rabienatajet al.[20].They showed that these changes are much more effective than the use of plain tube.Vicenteet al.[21] investigated the effect of corrugated tubes in different flow regimes.They showed that this change reduces the critical Reynolds number to 1300.The effect of using helical ribs on horizontal double pipes performance was considered by Nafonet al.[22].According to their results,helical ribs height is a more effective factor in the performance of this equipment.Hassanpouret al.[23] investigated the simultaneous use of corrugated tube and twisted tapes.The twisted tapes used in this study were in three types of perforated,U-cut and V-cut.Huaizhiet al.[24] studied the direction of flow on the hydrothermal performance of the asymmetrical corrugated tubes.The effect of using spirally corrugated tubes instead of plain one in the shell and tube heat exchanger studied by Liuet al.[25].According to their results,in all the investigated cases,Nusselt number has also improved with decreasing pressure drop.In another study,Hanet al.[26] performed an optimization of double-tube heat exchangers with inner corrugated tube.Sadeghi Dizajiet al.[27] studied the effect of flow arrangements on a double corrugated tube heat exchanger.Six different arrangement models with corrugated tube were tested and compared with the case of plain tube.Salimpouret al.[28] with studying the effect of three parameters such as corrugation depth,corrugation pitch and corrugation width on the helically corrugated tubes thermal performance showed that the use of a corrugation tube instead of plain tube can be effective especially in high Reynolds numbers.Kareemet al.[29] with use of new type of corrugation shape improved the heat transfer in spirally corrugated tubes showed that corrugated tubes enhance thermal performance 2.4–3.7 times of smooth tubes.Hanet al.[30] studied the turbulent flow in outward convex corrugated tube and the results of these different models were compared with each other.Panahi and Zamzamian [31] studied a shell and coiled tube heat exchanger.The simulations were performed for different coils side flow rates.Their results show that using helical wire as a tabulator,can improved the heat transfer rate on the coil side.Etghani and Baboli [32] with considering four effective parameters (cold and hot flow rate,tube diameter and coil pitch) and by use of Taguchi method optimized the heat transfer rate of the shell and helical tube heat exchanger.According to their results,cold flow rate and also tube diameter are the most influential parameters and maximum Nusselt number occurs in most hot and cold flow rates.The thermal performance of annular shell and helical tube heat exchanger,at various shell side flow rates,coil pitches and coil-to-tube diameter ratios were investigated numerically by Mirgolbabaei [33].Gouet al.[34] with use of experimental data evaluated the relationships provided for heat transfer coefficient of water flow in helical coiled tube.Alimoradiet al.[35] investigated the thermal performance of the shell and helically coiled tube heat exchangers.According to their results,the heat transfer rate can be increased up to 44%.Miansariet al.[36] compared the performance of different type of fins,such as cut (V-shaped) circular fins and circular fins with applying on the helical coil tube in shell and helical tube heat exchanger.Their results reviled that this change on the coil tube can be improved the performance of the heat exchanger especially for the case of circular fins.The effects of adding helical grooves on the outer shell surface of the annulus shell and helical coil tube heat exchanger investigated numerically by Miansariet al.[37].They showed that the rate of heat transfer improved up to 5% by this helical groove does not affect the pressure drop significantly.

According to the aforementioned literature review,the usual shell walls that used in shell and helical coil tube heat exchangers had a smooth surface.Therefore,in this study,to improve the hydrothermal performance of these heat exchangers,a new shape of shell has been designed and presented.The proposed scheme will use an annulus shell with a grooved surface at both its inner and outer surfaces.Numerical simulation are used to study the effects of using this novel shell and tube heat exchanger and the influence of the grooves depth of the inner shell wall on the hydrothermal performance of the heat exchanger.

2.Geometrical Model

Fig.1 illustrates the geometry of the base shell and helical tube heat exchanger.In fact,the best geometry that presented in the study of Mirgolbabaeiet al.[33] is considered as the basic geometry to continue this work.In this heat exchanger,hot fluid with an inlet temperature of 80 °C is on the coil side and a cold fluid with an inlet temperature of 20 °C is on the shell side.Water used as working fluid with constant physical properties.Geometrical characteristics of novel heat exchanger are presented in Table 1.Miansariet al.[37] investigated the thermal performance of this heat exchanger by applying a helical groove on the outer shell wall (as shown in Fig.2) with different groove depths of 5,10 and 15 mm.The aim of present study is investigating the effects of helical grooves on the inner shell wall in addition to the outer surface (Fig.3) and different groove depths on the thermal performance of heat exchanger.According to Table 2,four cases including different geometrical and fluid flow parameters are investigated.As can be seen,the outer shell wall groove depth is fixed to 5 mm,which is the best case that presented by Miansariet al.[37].Also the coil side inlet flow rate and inlet temperature are constant and only the shell side flow rate is variable.

Table 1 Sizes of base heat exchanger geometry

Table 2 Different cases for consideration in present study

2.1.Governing equations

The continuity,momentum and energy equation are respectively as follows [35]:

The transfer equations for realizablek-ε turbulence model are as follows [35]:

where σkand σεare the turbulence Prandtl numbers for thekand ε,respectively;C1andC2are the model coefficients;Gkrepresents the generation of turbulence kinetic energy,and is as follows [35]:

Sijis the strain rate tensor which is

and the eddy viscosity is presented as below

The coefficientCμis

Fig.1.Basic annulus shell and helical coil tube heat exchanger [33].

Fig.2.Annulus shell and helically coil tube heat exchanger with helicalgrooves on the outer shell surface [37].

Fig.3.Annulus shell and helically coil tube heat exchanger with helical grooves on the inner and outer shell walls in present study.

with constant values of:C2=1?9,σk=1?0,σε=1?2

The performance evaluation criteria can be used to quantify the efficiency of a heat exchanger.This metric consolidates the influence of heat-transfer enhancements for a specified pumping power and is usually defined as [38–43]:

3.Numerical Modeling

A three-dimensional numerical simulation for steady-state turbulent flow is carried out.Numerical simulation is performed in ANSYS Fluent 18.1.The governing equations are solved by the finite-volume method with double-precision and pressure-based segregated algorithm named SIMPLE.Presto algorithm is used pressure–velocity coupling and for the energy,momentum,kinetic turbulence and dissipation rate,second-order upwind scheme is selected.The realizablek-ε is selected for modeling and solving the turbulent flow.Also,it is assumed in that the fluid is Newtonian,incompressible,continuous and isotropic.

3.1.Boundary conditions

Hot fluid with a flow rate of 0.03 kg?s-1and a temperature of 80°C,and cold fluid in four different flow rates were investigated.The inlet cold fluid flow rates in four conditions are 0.015,0.03,0.045 and 0.06 kg?s-1with temperature is 20 °C.The outer and inner shell wall are considered as insulated wall with no heat flux.Coupled wall is applied as boundary condition for helically tube wall.The no-slip boundary conditions are adopted on all wall.

3.2.Meshing and grid independency

For the geometry of current study,an irregular tetrahedron mesh is used for all computational domains except for helical coil tube of heat exchanger that regular mesh is applied.For more accuracy and solving the equation near the wall,the boundary layermesh with five layers and growth rate 1.2 is applied for the areas near the shell and tube walls.The first layer thickness is 0.2 and 0.3 mm,for the surfaces near the tube and shell respectively.With adopting four different mesh size groups such as 1.7,3.2,4.9 and 6.4 million meshes for the geometry of under study,grid independency test is carried out.As shown in Fig.4,it was revealed the difference between the last two mesh systems is less than 1%.Thus,with respect to convergent time and also solution accuracy,the mesh system with 4.9 million cells is adopted (see Fig.5).

4.Results and Discussion

4.1.Validation

For model validation,results of numerical simulation heat exchanger without helical grooves are compared with Jamshidiet al.[44]as shown in Fig.6.It is revealed that the result of present numerical study is in good agreement with and experimental result.According to Fig.6 the differences between them is about 1.5%–5.2% for range of Reynolds number from 400 to 1200.Then it can be concluded that this numerical modeling is accurate enough.

4.2.The effect of inner shell diameter

The annulus shell of heat exchanger that was presented by Miansariet al.[27] had an inner and outer diameter of 10 cm and 14 cm,respectively,which due to the distance between the inner shell surface and the helical coil tube,it is not possible to create a grooves depth of 10 mm on the inner shell wall.Therefore,it is necessary to reduce the inner shell diameter to increase the distance between the inner shell surface and the helical coil tube of the heat exchanger.For this purpose,by keeping the other geometrical parameters constant,the inner shell diameter is only reducing to 8 cm.Before studying the desired parameters of this study,first examines the effect of this change on the heat exchanger performance compared to the as mentioned above by Miansariet al.[27].

Fig.4.Grid independency for the present study.

Fig.5.Generated mesh at the(a)longitudinal section and(b)helical coil tube of the heat exchanger.

Fig.6.Validation of present study with Jamshidi et al. [29].

Fig.7 shows the heat flux passing through the helical coil tube surface for two heat exchangers with different inner shell diameters.As shown,by decreasing the inner shell diameter,the heat flux increased slightly in most flow rate except in the flow rate of 0.015 kg?s-1.As it is presented in Fig.8,this increase is about 5%for the flow rate of 0.015 kg?s-1and in other flow rates;it is less than 1%.According to Fig.8,it can be concluded that when the inner shell diameter is changed from 10 to 8 cm has no significant heat transfer enhancement has been seen.

As it was observed,the change of inner shell diameter from 10 mm to 8 mm has no significant effect on thermal performance of heat exchanger.To considering the hydraulic performance,Fig.9 presents the pressure drop variations for the two cases at different flows rate.As shown,the pressure drop is almost constant and has not changed except atQ=0.06 kg?s-1that pressure drop increased by 3.5%.

4.3.The effect of grooves on the outer shell surface

After considering and comparing the heat exchanger hydrothermal performance with inner shell diameter of 8 mm relative to the one presented by Miansariet al.[27],and ensuring that there is no major change in its performance due to the change in inner shell diameter,in following,by focusing on new heat exchanger,the parameters considered in this study will be discussed.Fig.10 shows the effect of grooves only on the outer shell surface.As indicate in this figure,the addition of grooves on the outer shell surface of this heat exchanger does not have considerable effect on the increase of the heat flux.

Fig.11 presents the velocity contour on the longitudinal section plane forQ=0.015 kg?s-1.As shown below,adding a groove to the outer shell surface of the heat exchanger has not been able to improve the current distribution around the helical coil tube.In fact,the addition of a groove to the outer shell surface redirects the flow to the space between the helical coil tube and the inner shell surface,and the fluid flow through the space between the helical coil tube and the outer shell surface is significantly reduced.

Fig.12 shows the temperature contour at the longitudinal section plane forQ=0.015 kg?s-1.As shown,adding the grooves on the outer shell surface has no effect on the temperature contour and there is no improvement in the temperature distribution.This is also happening for pressure drop as shown in Fig.13.At the higher flowrates,the pressure drop of heat exchanger withh1=5 mm slightly decreases in compare toh1=0 mm.

Fig.7.Comparison of heat flux in two heat exchangers with different inner shell diameter.

Fig.9.Comparison of pressure drop in two heat exchangers with different inner shell diameter.

Fig.10.The effect of adding grooves on the outer shell surface on the heat exchanger thermal performance.

Fig.11.The effect of outer shell surface grooves on velocity distribution at the longitudinal section plane for Q=0.015 kg?s-1.

Fig.12.The effect of outer shell surface grooves on temperature distribution at the longitudinal section plane for Q=0.015 kg?s-1.

Fig.13.The effect of outer shell surface grooves on pressure drop at different flow rates.

Fig.14.Effect of outer shell wall grooves on the performance evaluation criteria of heat exchanger.

We compare the effect of outer shell wall grooves on the performance evaluation criteria (PEC) in Fig.14.As shown,the PEC decrease when the mass flow rate increases.

4.4.The effect of adding grooves on the inner and outer shell surfaces

As observed in the previous section,the addition of grooves on the outer shell surface of the heat exchanger did not significantly affect the heat exchanger performance.In this section,we consider the grooves on the inner and outer shell surfaces of heat exchanger at the same time.Therefore,grooves with depth of 5 mm are added on the both sides of shell and the results are compared with no grooves one.According to the Fig.15,by adding grooves,the heat flux is improved in all flow rates.

Fig.16 shows the velocity contour for this case forQ=0.015 k g?s-1.According to the proposed velocity contour,the grooves on the heat exchanger inner and outer shell walls results in a better distribution of velocity around the helical coil tube,and therefore,the local heat transfer coefficient can be improved as the local velocity increases.

Fig.15.Effect of both inner and outer shell surfaces grooves on heat flux.

Fig.16.Effect of both inner and outer shell walls grooves on velocity contour for Q=0.015 kg?s-1.

Fig.17.Effect of both inner and outer shell surfaces grooves on flow distribution.

For further consideration,the stream lines of the two models without grooves and with grooves on the inner and outer shell surfaces are presented in Fig.17.As shown in the figure,the grooves on the shell surfaces results in a better distribution and uniformity of the stream lines around the heat exchanger helical coil tubes,whereas for the no-grooved case,this uniform distribution is not observed and the concentration of the stream lines is more on one side of the shell.

Fig.18.The effect of groove on temperature distribution on the helical coil tube.

Fig.19.Effect of both inner and outer shell surfaces grooves on pressure drop.

Fig.20.Effect of inner and outer shell wall grooves on the performance evaluation criteria of heat exchanger.

This better flow distribution and increased local velocity around the heat exchanger helical coil tubes as indicated in Fig.18,resulted in a more uniform temperature distribution and lowered helical coil tube surface temperature (better heat transfer) compared to the no-groove case.But despite these positive effects of adding groove on shell surfaces,as indicated in Fig.19,the pressure drop for both two cases are the same that indicating the effectiveness of this method in increasing the heat transfer without excess pressure drops.

Fig.21.Effect of groove depths on the heat exchanger heat flux.

Fig.22.Velocity contours at middle plane of heat exchanger on different outer and inner shell wall grooves depth.

The effect of inner and outer shell wall grooves on the performance evaluation criteria (PEC) in Fig.20.As shown,the PEC of model in all mass flow rate is bigger than one and this means the inner shell wall groove have a goof effect on the performance of heat exchanger

4.5.The effect of the groove depths

As mentioned before,adding grooves on both annulus shell surfaces is more effective than on one shell surface with no significant increase in pressure drop.In following,the effect of two inner shell surface groove depth such as 5 and 10 mm with a constant outer shell surface groove depth equal to 5 mm on the heat exchanger performance will be considered.As shown in Fig.21,the heat flux increased with increasing groove depth in all the investigated flow rates.

Fig.23.Stream line for different groove depths.

Fig.24.Effect of groove depth on temperature distribution.

Fig.22 indicates the effect of groove depth on the velocity contour.As shown in the figure,as the depth of the inner shell surface grooves increases,the velocity distribution around the helical coil tube becomes more uniform and the local velocity around the helical coil tube increases(Fig.23),which helps to more uniform temperature distribution(Fig.24)and then improves the heat transfer thermal performance.

Fig.25.Comparison of temperature distribution on helical coil tube for different groove depths.

Fig.26.Effect of groove depth on the shell side pressure drop.

Fig.27.Percentage of increase in heat transfer at different shell groove depths compared to no-grooved shell.

The temperature distribution on the helical coil tube is shown in Fig.25 for two groove depths 5 mm and 10 mm.As shown,the temperature of the helical coil tube surface is significantly lower for the case of bigger groove depth,indicating better heat transfer.

According to Fig.26 and as shows before the effect of groove depth on the pressure drop of the shell side is insignificant,and there is no change in pressure drop as the groove depth increased.While the grooves depth increase,the fluid flow in a helically channel between grooves of shell and coil tube.This means that the fluid resistance become lower because fluid flow in direction of helical channel with low incidence.In other words,the changes of pressure drop is insignificant.

Fig.27 shows the increase present in heat transfer at different cases of the grooved heat exchanger shells compared to the nogrooved shell.As shown,adding grooves only to the outer shell surface does not improve heat transfer compared to the nogroove mode,whereas grooving on both shell surfaces improved heat transfer,especially at low flow rates.For example,in a model with an internal shell groove depth of 10 mm at a flow rate of 0.015 kg?s-1,the heat transfer increased by about 20%,while at a flow rate of 0.06 kg?s-1it decreased to about 5%.

Fig.28 indicates the effect of the groove on the shell side outlet temperature.As the flow rate increases,the outlet temperature is reduced,also due to the better performance of the groove depth 10 mm than the other models,the outlet temperature in this model is higher than the other models in the whole range of studied flow rates.

Fig.28.Shell outlet temperature in different shell groove depths.

Fig.29.Coil outlet temperature in different shell groove depths.

Fig.30.Effect of different shell wall grooves depth on the performance evaluation criteria of heat exchanger.

Fig.29 shows the coil outlet temperature in different models.In this section,due to the better performance of the model with the internal groove depth 10 mm,the coil outlet temperature (hot fluid) has decreased further (more heat transfer rate).

The effect of different shell wall grooves depth on the performance evaluation criteria (PEC) in Fig.30.As shown,the PEC of model with 10 mm grooves on the outer shell wall has the better performance compare to other one.Also,while the mass flow rate increases the PEC decrease.

5.Conclusions

In this research,in order to increase the thermal performance of annulus shell and helical coil tube heat exchanger,helical grooves have been created on the inner and outer shell surfaces.The 3D numerical modeling results showed that the use of grooves at both the inner and outer shell surfaces of the heat exchanger,due to better rotation of the fluid flow and increased local velocity around the heat exchanger helical coil tubes,presents better thermal performance than the groove on the single shell surface especially for low flow rates.For example,in a model with an inner shell depth of 10 mm atQ=0.015 kg?s-1,heat transfer increased by about 20%,while atQ=0.06 kg?s-1it decreased to about 5%.The results demonstrated that increasing in inner shell surface groove depth from 5 mm to 10 mm can also improve the flow distribution around the helical coil tube and heat exchanger thermal performance.Also,addition of the groove on both the inner and outer surfaces,can increasing its thermal performance.Although the increase of grooves can slightly increase the heat transfer efficiency,the increase in pressure drop is more obvious.It can be seen from the decrease of PEC,which means that the comprehensive heat transfer performance is reduced.The shell side structure with grooves is not conducive to improving the comprehensive heat transfer performance.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.