CFD analysis of Al2O3-syltherm oil nanofluid on parabolic trough solar collector with a new flange-shaped turbulator model

Behzd Shker,Mosyeb Gholini, Mohsen Pourfllh , D.D. Gnji

a Department of Mechanical Engineering, Mazandaran University of science and Technology, Babol, Iran

b Department of Mechanical Engineering, Babol Noushirvani University of Technology, Babol, Iran

Keywords:Al 2O 3-syltherm oil nanofluid Flange-shaped turbulator Pressure drop Friction factor

ABSTRACT In this paper, in order to improve the performance of a linear parabolic collector, the thermal effects of using Al 2O 3-syltherm oil nanofluid with different concentrations and new flange-shaped turbulators are investigated.The simulation was performed by ANSYS-FLUENT-18.2 commercial software using Realizable k-εtwo-equation turbulence model.In accordance with the results, it was realized that increasing the volume fraction of nanoparticles (to 5%) and number of turbulators causes the heat transfer coefficient(h) of the fluid to elevate and ultimately the uniform temperature is created in the absorber.For instance,at a flow rate of 4.5kg/s and an inlet temperature of 350 K, the value ofhincreases by about 8.5 % by changing the number of turbulators from 10 to 15 sets.On the other hand, the results indicate that by changing the arrangement of the turbulators, the heat transfer efficiency of the collector can be increased by 5 % for 350 K, 3.5 % for 450 K and 1% for 550 K inlet temperature.

There have been always efforts to find an unlimited source of energy that is pollution-free and causes the least harm to the environment [1].The reason is that most of the energy resources used by humans are fuels with limited resources and abundant fossil pollution, which cause great damage to the environment, the ozone layer and problems such as global warming [2].One of the energy resources that have the necessary properties to replace this type of fuels is solar energy [3].In recent years, many technologies have been developed and studied in order to use solar energy.One of these technologies is the use of parabolic solar collectors that can be used with unique features in various fields.These devices are composed of several components, the two main components of which are parabolic mirror and tubular absorber [4].The operation of this device is such that the fluid in the tube is warmed up by the energy of the rays focused on the tube by the mirror and its temperature rises to at least 400 °C [5].The steam generated in this process can be used in many systems, the most important of which are power generation systems and heating systems [6].This type of collector has more uses in the industry due to its high efficiency compared to other solar collectors.Thappa et al.[7] published an article to investigate the effect of the size of the absorber pipe diameter on the performance of the parabolic collector.Their results showed that heat loss decreases to 5 W/m2by decreasing the pipe diameter and the overall performance of the system increases by 3%.The effect of deformation of the absorbent tube floor on the thermal performance of the parabolic collector with working fluid of oil has been published by Bellos et al.[8].Their results indicated that the deformation of the tube floor in the form of convex waves can increase the thermal performance of the absorbent by 4.55%.Valizade et al.[9] in an article analyzed the use of metal foam in parabolic collectors.The results of their work showed that the use of this foam increases the friction factor, reduces the inlet temperature and increases the thermal performance of the system.Also, an article has been published by Bucadom et al [10] with the aim of investigating the addition of two types of gears to the floor of a solar absorber, which indicates that the presence of these gears due to perturbation in the thermal boundary layer, improves system temperature and performance.

According to studies, many factors affect heat transfer, including: material, temperature difference between two surfaces, the rate of inlet and outlet of fluid, the area of heat transfer surfaces, etc.[11-13].Increasing the surface of heat transfer because of being the more diverse and suitable effect is one of the bestmethods.For this purpose, turbulators and fins can be used to in- crease heat transfer surfaces.As seen in most heat transfer systems such as heat sinks, radiators and heat exchangers, the addition of a fin helps a lot to improve heat transfer to the system, which will increase the efficiency of the whole system [14-17] .On the other hand, choosing a suitable fluid is very important since it has the greatest effect on heat transfer.Using fluids made by nanotechnol- ogy, much progress has been made in the field of heat transfer.A fluid with a high heat transfer coefficient can be made by combin- ing a base fluid and a type of metal in nano-dimensions [18-21] .In these types of fluids, the high heat transfer coefficient is due to the presence of metal in them, which can be used in parabolic collectors to make more exploitation of solar energy [22] .Improv- ing the performance of parabolic collectors using nanofluids was studied by Cázares et al.[23] .Their results showed that nanofluids with higher volume fraction will have a better effect on improving the thermal performance of the system and also it will increase the thermal performance by up to 80%.Subramani et al.[24] in another work investigated the use of TiO2/ DI-H 2 O nanofluid in a linear parabolic collector.Their results indicate that: This new nanofluid with the appropriate ratio of nanoparticles can improve system performance up to 8.66%.By analyzing the exergy of a so- lar collector with Al2O3/oil working fluid in which the nanoparti- cles were considered to vary from 0 to 5%, Khakrah et al.[25] con- cluded that adding a maximum of 5% of the nanoparticles to the base fluid increases the efficiency up to 19%.Bellos et al.[26] by simulating several fins with different sizes in a parabolic collector, concluded that the use of a 10 mm height and 2 mm thick fin in- creases the Nusselt number up to 65.8% and the thermal efficiency up to 0.82%.

The main purpose of the simulation performed in this study is to investigate the factors affecting heat transfer and design a new absorber tube for a parabolic trough collector (PTC), that has the highest heat transfer rate.For this purpose, two basic innovations have been used in this paper, one of them is the use of special flange-shaped turbulators in the absorber tube and the other is the use of Al 2 O 3 - syltherm oil nanofluid as heat transfer fluid.For this aim, the simulation is performed in 3 different cases of flange- shaped turbulators using ANSYS-FLUENT-18.2 commercial software and using Realizablek-εturbulence model.The heat flux in the en- vironment is calculated by using the Monte Carlo radiation track- ing method [27] .

The PTC in this study is a type of concentrating collector that reflects sunlight on a line.This collector consists of a parabolic re- flective sheet that is glossy and made of glass or metal.A steel absorber tube with a glass cover around it, is located along the focal length of this parabolic reflector.Heat transfer fluid flows in the tube and the heat absorbed by the absorbent is transferred to this fluid, and the heated fluid can be used for various purposes.The studied parabolic collector has special geometric and optical properties that are presented in Table 1 [27] .Also, this PTC has a concentration factor of 22.74, which has an optical efficiency of approximately 75.50%.In this study, flange-shaped turbulators areused in the absorber tube; these turbulators are used in differ- ent numbers 10-15-20 sets in the form of 3 loops and in a row.Another innovation discussed in this paper is the use of Al 2 O3nanoparticles in Syltherm oil base fluid [28] .Design details of the absorbent tubes are presented in Fig.1 and Table 2 .

Table 1 The specifications related to the collector size and its optical prop- erties.

Fig. 1. The linear parabolic collector geometry.

Table 2 Geometric specifications of turbulators used in ab- sorber tube.

Table 3 Correlation for the Syltherm-800 properties according to Dow Chemical Company [28] .

Table 4 Nanoparticle properties at different temperatures.

In studies on absorbents, the fluid flow inside the tube is fully developed and the system is considered stable and the fluid flow is considered turbulence.In this paper, we have tried to investi- gate the effect of new flange-shaped turbulators and the use of Al 2 O 3 nanofluid in improving the heat transfer of the absorbent compared to the smooth absorbent tube (SAT).First, we study the SAT as a reference so that we can make an accurate comparison between the designed absorbers and the made turbulators, and ex- amine the impact of the innovation on improving heat transfer.

In this paper, Al2O3-syltherm oil heat transfer fluid has been used to investigate the designed absorbents.This fluid is a nanofluid made from the dispersion of Al 2 O 3 nanoparticles with different concentrations in the Syltherm-800 base fluid, therefore it has good heat transfer properties and is used as a working fluid in parabolic trough collector (PTC) (See Table 3).This fluid is not in the ANSYS -FLUENT software library by default, so we have to write its properties in UDF code and insert it in the software.The equations to calculate the specifications of the base fluid and nanofluid are given below [28] .

In addition, Al2O3nanoparticle properties are given in differ- ent temperatures (see Table 4) [28] .The equations to calculate the thermophysical properties of nanofluid are given below [ 27 , 28 ].

Table 5 The specifications of stainless steel [27-28] .

Stainless steel is used to make the absorber tube and flange- shaped turbulators, the properties of which are shown in Table 5 [ 27 , 28 ].For all the studied cases, the amount of solar radiation based on the Fig.2 (Monte Carlo radiation tracking method) is written with UDF code and inserted in fluent software [27] .

Fig. 2. A graph of heat flux distribution.

The mathematical equations and definitions of used parameters in this study are as follows [26-30] .

To calculate the Nusselt number [29] :

The heat transfer coefficient (HTC) between the absorber tube and the heat transfer fluid can be obtained as follows:

where theQpro is the amount of useful generated thermal energy;Tris the average internal surface temperature of the absorbent andTmis the average temperature of the heat transfer fluid used in the calculations.

The Reynolds number is obtained by the following [30] :

whereρ,υinandμare dynamic density, inlet velocity and viscos- ity, respectively.The Gnielinski equation [31] for the Nusselt num- ber is also given as follows:

In Eq.(10) ,fwill be as follows [32] :

We use the Eq.(12) to calculate the friction factor [30] :

The Filonenko equation for the friction factor is also given as follows [32] :

Equation (14) is exploited to calculate the efficiency of the parabolic trough collector (PTC) [ 27 , 28 ]:

The expressionΔTin the above equation is obtained from the Eq.(15) :

The system is considered steady for this paper, and the flow is turbulent, incompressible and with forced convection heat transfer, and its equations will be as follows [ 15 , 27 ].(See Eqs.(16)~(18))

Momentum equation:

The discretization of the governing equations is performed by the finite element method (FEM) [ 33 , 34 ] and by ANSYS-Fluent 18.2 software.For fluid flow, Realizablek-εturbulence model, convec- tion term discretization by QUICK method, pressure by PRESTO method and energy equation by second-order upwind scheme method have been used.The SIMPLEC method is used to relate the pressure and velocity.The plate between the fluid and the solid body is coupled before initialization.The residual value for the convergence of the numerical solutions obtained with the equa- tionsk,ε,continuity and motion is considered to be less than 10−6, while for the energy equation, the residual value is less than 10−9.In this study, convergence is obtained in the abovementioned residual values.Also, we considered a constant number for the av- erage temperature and pressure outside the system for all cases.

Figure 1 shows the location of the turbulators as well as the fluid inlet and outlet.To study the absorbents, we must use the appropriate and uniform boundary conditions for all the designed cases so that we can make a correct comparison between them.Boundary conditions are as follows:

The simulations performed in this paper have been done with 3 different mass flow rates so that we can determine the effect of the amount of inlet fluid on heat transfer.The ambient temperature in all cases was 350K [ 27 , 28 ] and also we used 3 inlet temperatures of 350 K, 450 K and 550 K.

Figure 2 shows a diagram of the heat flux distribution at dif- ferent angles of the pipe environment.This diagram is drawn with different simulations and by Monte Carlo radiation tracking method (MCRT) [29] .The non-slip condition is also applied to the pipe walls, both in the upper and lower halves.

The only condition for the absorber outlet is that we set the pressure gradient to zero.

The authors of the present study examined the effects of meshes for greater confidence in the results.The number of meshes in each case is different, so that for a collector with 10 sets of turbulators, the number of meshes is 5,753,0 0 0 and with increasing the number of wide surfaces to 15 and 20 sets of turbu- lators, the number of meshes increases to 7,330,0 0 0 and 9,80 0,0 0 0 respectively.The calculations required for the simulation were per- formed with a computer system with 32 GB-RAM and core-i9 pro- cessors.For instance, Fig.3 shows the mesh independence test for the absorber tube with 10 sets of turbulators (case 1).Based on Fig.3 , more than 5,753,0 0 0 meshes don’t have effect on the re- sults.

We should have a proper validation of the new model in order to make a correct comparison and examine the impact of the pro- posed innovations on improving heat transfer.First, for a smooth absorber, we check the accuracy of the numerical model.The re- sults of simulations and calculations performed on the smooth tube indicated that the results correspond well with the classical values of Gnielinski and Filonenko [ 31 , 32 ].As shown in Fig.4 , the graphs of Nusselt changes and friction factor have a small error compared to the classical value, which is equal to 7.5% for Nusselt and for the friction factor it is equal to 5.5 %.Since the obtained er- rors are in the appropriate range, the simulation and analysis per- formed is valid and this simulation process can be a good criterion for other models.

Fig. 3. Heat transfer coefficient in different number of meshes for 10 turbulator case.

Fig. 4. Validation graph for Nusselt number and friction factor.

The main focus of this paper is to investigate the results of ve- locity, temperature changes, heat transfer coefficient, and turbu- lence kinetic energy on a linear parabolic collector with the pres- ence of turbulators in the path of the fluid and the absorbent wall.Also, the effect of important parameters on the collector efficiency is evaluated in this paper.The working fluid in this set is Al 2 O 3 - syltherm-800, which is considered to be single-phase.

Temperature distribution on the absorbent surface is one of the most important parameters that should be considered in PTC.Since the heat flux applied to the bottom of the absorber pipe is high, there is a possibility of damage to the pipe and deformation over- time for the pipe.The designed absorbent pipe has eliminated this problem to some extent compared to SAT.The turbulators in the absorber tube are designed to absorb heat from the absorber floor and reduce its temperature significantly due to its high heat trans- fer coefficient.In addition, the turbulators will be able to transfer this received heat into the fluid and provide a more uniform temp.In the designed examples of this study, by increasing the number of turbulators, the average temperature of the inner surface of the absorber pipe decreases.The outlet temperature of the fluid in all the designed cases increases by about 4.5 K compared to the inlet temperature.Meanwhile, the average temperature at a distance of 2.5 mm from the bottom of the absorber wall for simulations per- formed at the temperature of 450 K and with a mass flow rate of 4.5 kg/s for 10, 15 and 20 sets of turbulators are equal to 513.498 K, 509.139 K and 503.924 K, respectively.This decrease in mean temperature indicates that the temperature will be more uniformly distributed as the number of turbulators increases.The presence of a turbulators can turbulence the internal flow of the pipe and in- crease the heat transfer between the absorber wall and the work- ing fluid, which reduces the average temperature of the inside of the absorber pipe.Figure 5 shows the temperature contour for the different number of turbulators and smooth tube.Also, Fig.6 dis- plays the temperature contour in close-up, showing the effect of the turbulators on making the temperature more uniform.As can be seen, the presence of the turbulator, in addition to reducing the temperature of the absorbent tube, can spread the tempera- ture more uniformly in the fluid.

Fig. 5. The temperature contours inTin 450 K and mass flow rate of 4.5 kg/s for different turbulators.

Fig. 6. The temperature contour in close-up, showing the effect of the turbulators.

Heat transfer coefficient is one of the indicators by which we can better understand the rate of heat transfer between a fluid and a solid.We will always seek to increase this coefficient to create better heat transfer in the designed adsorbents.As shown in the results, the presence of turbulators in the absorber pipe will increase the heat transfer coefficient.This increase in heat transfer is firstly due to the increase in heat transfer surfaces between the fluid and the turbulators, and secondly, the pres- ence of these turbulators destroys the thermal boundary layer and also causes better mixing of the flow, which in turn increases the heat transfer coefficient.On the other hand, by increasing the number of turbulators, there is a higher heat transfer coeffi- cient.In addition to the turbulator, increasing the mass flow rate causes the flow to be more turbulent and improves heat trans- fer to a large extent.As shown in the diagrams, for the sample designed with 15 sets of turbulators at a temperature of 450K with a mass flow rate of 4.5 kg/s , thehamount is 695.659, but by increasing the mass flow rate to 8.5 kg/s , thehwill be 1276.959.Moreover, by increasing temperature and increasing nanofluid concentration, heat transfer coefficient will increase due to changes in the thermophysical properties of the nanofluid.Heat transfer coefficient in different inlet temperatures is shown in the Fig.7 .

Fig. 7. Heat transfer coefficient in different inlet temperatures.

As can be seen from the flow lines in the velocity contours, the presence of a turbulator in the absorber tube causes a loss of or- der in the flow.In the absence of order in the flow lines near the absorber wall, no boundary layer is created, so heat transfer near the absorber wall can be done properly.In addition, the fluid ve- locity and the presence of a turbulator cause vortices in the ab- sorber tube.The higher the fluid velocity, the greater the number and intensity of these vortices.The turbulators used in this pa- per are placed in a circle along the fluid.These turbulators, which are placed in the tube in triplicate, can affect the velocity and order of the flow.As shown in temperature contours and veloc- ity contours (Fig.8), in areas where the fluid hits the turbulators and changes the direction of the velocity vectors, heat transfer is better.

Nusselt number diagrams for 1% nanofluid concentration are shown in Fig.9 .These graphs are taken at different temperatures.One of the factors increasing the Nusselt number is increasing the mass flow rate.For the absorber designed with 10 sets of turbu- lators at the input temperature of 350 K with increasing the mass flow rate from 4.5 to 6.5 Nusselt number will increase from 220 to 280.Also, with increasing the mass flow rate to 8.5 the Nusselt number will reaches 346.In the other hand increasing the number of turbulators inside the tube will also increase the Nusselt num- ber.Increasing in the Nusselt number shows better convection heat transfer than conduction heat transfer.

Fig. 8. Velocity contours for different mass flow inlets.

Examining the results between the simple pipe and the ab- sorber pipe designed with 20 sets of turbulators, it is observed that the turbulence energy of the current will increase significantly.As shown in TKE contours Fig.10 , the amount of TKE does not in- crease in a simple pipe due to the uniformity of the flow, but in the designed absorber pipe, this amount will improve due to the presence of tandem turbulators, which will increase the heat trans- fer.In addition to the presence of a turbulators, the amount of in- put mass flow rate will also have a large impact on increasing the TKE.By comparing the designed absorber tube with 20 sets of tur- bulators in different mass flow rate s and observing their contours, it can be understood that the turbulence energy is directly related to the fluid inlet mass flow rate.As the mass flow rate increases from 4.5 kg/s to 6.5 kg/s , the TKE value increases by 83% and by increasing the mass flow rate from 6.5 kg/s to 8.5 kg/s , this value increases by 62%.These percentages were calculated in the case with 10 sets of turbulators and inlet temperature of 350 K.As shown in Fig.10 , TKE will increase in area where the fluid collision to turbulators.

Because excessive pressure drop can affect the overall perfor- mance of the parabolic collector, it is necessary to determine the importance of each factor affecting the pressure drop.Many pa- rameters affect the pressure drop, such as inlet mass flow rate, number of turbulators, pipe diameter and nanofluid concentration.Since the main cause of friction pressure drop is due to the col- lision of the fluid with the solid body, the amount of mass flow rate and the number of turbulators will have a great impact on the pressure drop.As shown in the Fig.11 contours, increasing the mass flow rate will increase the pressure drop.Considering the ad- sorber tube designed with 15 sets of turbulators, if the inlet mass flow rate is 4.5 kg/s , the amount of pressure drop will be equal to 106,259 Pa, with the inlet mass flow rate of 6.5 kg/s , this value will be equal to 220,370 Pa, and in the simulation with the in- let mass flow rate of 8.5 kg/s , the pressure drop will be equal to 375,0 0 0 Pa.Also, by increasing the number of turbulators, the amount of friction caused by nanofluid collisions with the wall in- creases and this in turn increases the amount of pressure drop.In this simulation, by changing 10 sets of turbulators to 20 sets, the pressure drop increases by 96%, temperature inlet in this samples is equal to 350 K.

To calculate the dimensionless friction coefficient, we use Eq.(12) .As shown in the equation, changes in fluid pressure and velocity will have the greatest effect on the coefficient of friction.It can be seen from the diagrams of Fig.12 that as the fluid ve- locity increases, the amount of friction coefficient decreases.Also, if the fluid inlet pressure is greater than the fluid outlet pressure, the amount of friction coefficient will be higher.Moreover, increas- ing the number of turbulators increases the coefficient of friction because it increases the collisions inside the pipe and causes more pressure drop.In the inlet mass flow rate of 4.5 kg/s and inlet temperature of 350 K for a tube designed with 10 sets of turbula- tors, the friction coefficient is 0.788, which under the same condi- tions for a absorber designed with a 15 sets of turbulators is 1.02, in which we will have an increase of 29.45%.

Fig. 9. Nusselt number graph in different inlet temperatures.

Fig. 10. TKE contours for different mass flow inlets.

Fig. 11. Pressure contour for various mass flow and turbulators.

Fig. 12. Friction factor in different inlet temperatures.

Also, in another comparison for turbulators with different sizes in a tube designed with 15 sets of turbulators, it was found that if we increase the height of the turbulators to 5 mm, the coefficient of friction will increase by 15%.This result is due to the increase in the amount of fluid collisions with the turbulators.

Equation (14) is used to calculate the efficiency of the parabolic collector.According to this equation, the efficiency depends onΔTand the only variable in this formula isΔT, which we can get from Eq.(15) .WhereTinis the temperature of the inlet fluid andToutis the temperature of the outlet fluid.Tamb is also the ambient tem- perature and is considered to be a constant for all cases.Therefore, according to the given equations for efficiency andΔT, the only effective factor on efficiency is fluid inlet and outlet temperature.As shown in Tables 6 , 7 , and 8 , increasing the fluid inlet temper- ature reduces the efficiency, but increasing mass flow rate slightly improves the efficiency.In general, if the working temperature of the absorbent pipe is closer to the ambient temperature, the value ofΔTis lower and the efficiency is higher.The maximum effi- ciency is for the absorber pipe designed with 20 sets turbulators and with an inlet mass flow rate of 8.5 kg/s .

Table 6 Efficiency of absorber with 10 sets turbulators.

Table 7 Efficiency of absorber with 15 sets of turbulators.

Table 8 Efficiency of absorber with 20 sets of turbulators.

Heat transfer plays an important role in industry and power plants, so in the present study, the heat transfer in the linear parabolic solar collector has been numerically simulated by replac- ing the nanofluid with pure oil fluid.The heat flux in the periph- eral direction is done using Monte Carlo radiation tracking method and Navier-Stokes three-dimensional equations of mass, momen- tum and energy using ANSYS-FLUENT-18.2 commercial software and the SIMPLEC method is considered for speed-pressure cou- pling.The main innovation in this study is the use of special flange-shaped turbulators in the wall of the absorber tube, which are placed in three rows and 10, 15 and 20 sets of turbulators.Re- sults indicate that:

•If the fluid inlet pressure is higher than the fluid outlet pressure (Pin＞Pout), the amount of friction coefficient will be higher.

•If the working temperature of the absorbent pipe is closer to the ambient temperature, the value ofΔTis lower and the ef- ficiency is higher.

•If the number of turbulators in the absorber wall increases from 10 to 20, the pressure drop increases about 96%.

•If the mass flow rate increases from 4.5 kg/s to 6.5 kg/s, the TKE value increases to 83% and by increasing the mass flow rate from 6.5 kg/s to 8.5 kg/s, this value increases to 62%.

•Increasing the number of turbulators and the mass flow rate will increase the Nusselt number and shows the better convec- tion heat transfer rate.

•If the number of turbulators (10 to 20) and the mass flow rate (4.5 to 8.5 kg/s) change, the Nusselt number increases and shows the better convection heat transfer rate.

In general, it can be concluded that the model presented in this study is a suitable model for utilizing clean solar energy.Since this model can reduce the wall temperature of the absorber pipe and distribute the heat more uniformly on the absorber pipe, this can significantly reduce the damage to the absorber pipe.

Ethics approval

This article does not contain any studies with human partici- pants or animals performed by any of the authors.

Declaration of Competing Interest

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

Acknowledgements

The authors would like to acknowledge the Department of Me- chanical Engineering, Babol Noushirvani University of Technology, Iran.