Investigation on fluid flow heat transfer and frictional properties of Al 2 O 3 nanofluids used in shell and tube heat exchanger

Nanofluids are generally utilized in providing cooling, lubrication phenomenon, controlling the thermophysical properties of the working fluid. In this work, nanoparticles of Al 2 O 3 are added to the base fluid which flows through the counter flow arrangement in a turbulent flow condition. The hot and cold fluids used are ethylbenzene and water respectively and have different velocities on both shell and tube side. This study emphasizes the analysis of flow properties, friction loss, and energy transfer in terms of heat using nanofluid in the heat exchanger. The heat transfer rate of present investigation with nanoparticle addition is 4.63% higher in comparision to Dittus Boelter correlation. Apart from this, the obtained friction factor is 0.0376 very much closer to Gnielinski and Blasius correlations. This investigation proved that appropriate nanoparticle additions and baffle inclinations have fabulous impact upon the performance of heat exchanger and its effectiveness.


Introduction
The world of thermal engineering centred on the term Heat Exchanger (HX), which form the necessary for several industries where requisite for heat reduction economically. Hence the HX with low functional and management cost was designed with operating cooling fluids that extract the generated heat. The shell-tube style HX is recognized for its easiness in design, in general, it comprises up of following parts: Shell, Tube, baffles and the fluids. The shell forms the outermost portion by enclosing all other parts and liable for carrying the cooling fluids from entry to exit port over the tubes. The tube transmits the boiling fluid generated from the system by the cooling fluid where there is a transfer of heat among the fluids. The baffles are arrayed to alter the flow course of the cooling fluid in the HX. These varieties of HX have higher reliability in comparison with the other types as it can be operated at high pressure, possess higher surface area to volume ratio and effectiveness that can be easily enhanced by accumulating the tubes. In general, the heat transfer calculation by CFD is a complicated process as it requires a computer with more power and space. Hence the resolving of models is required.
There are numerous sorts of cooling fluids implemented in the HXs that involve varieties of water, oils and other organic compounds. Applications of heat exchangers are vast and require a thorough knowledge to cover each aspect. Among the applications, their main use is in the process industry, mechanical equipment, and home appliances. Heat exchangers are nowadays employed for heating district systems extensively Air conditioners, and refrigerators use heat exchangers to condense or evaporate the fluid. Moreover, it also has applications in milk processing to do pasteurization.
Nanofluids are solid-liquid compound materials comprising of solid nanoparticles or nanofibers with proportions usually from 1 to 100 nm dispersed in the fluid medium. This type of fluid is not just a plain liquid-solid combination while the utmost important condition of a nanofluid is an agglomerate-free stable suspension over an extended period without instigating any chemical alterations to its parent fluid. This can be well accomplished by reducing the density amid solids as well as liquids or by enhancing the viscosity of fluids, i.e., through the addition of nanometersized particles as well as by hindering particles from agglomeration, the settling of particles could be eluded (Sridhara & Sutapathy, 2011 [2010] showed the escalation in the factor of friction and dynamic viscosity with the order of Alumina nanoparticle dispersion in water.  illustrated the augmentation of heat transfer and turbulence when nanoparticles were added to the base fluids. Namburu et al.
[2009] had led the experiment with several nanofluids added to the ethylene glycol water and analyzed the heat transfer performance numerically concluded that nanofluid had enhanced features than base fluid. There was an increase in Re and Nu numbers when the concentration of nanoparticle increases observed by Rott [1990]. The study conducted by Wakeham, et al. [1991] identified that the transport property depends on size, shape and volume fraction of

Shell tube type heat exchanger (STHX)
Shell being the wall of STHX comprises of a tube arrangement which carries the hot fluid, and corresponding cooling fluid flows along the arrangement of the baffles in the shell side. The size and length of the shell depend largely on the number of tubes and its arrangement. Here, the geometry modeling was carried out using ANSYS Space Claim while the analysis was made using finite volume method as in Computational Fluid Dynamics (CFD) tool.
This study deals with the estimation of fluid flow and friction properties of a cold fluid added with nanoparticles of spherical dimensions in the heat exchanger. Here, ethyl-benzene is used as a hot fluid at a temperature of about 340 K whereas, the cold fluid is of two types, i.e., water and water-Al 2 O 3 nanofluid fluid (WANF) at a temperature of 300 K. The tube parameters such as diameter, pitch layout, and counts were determined for this HX.
It is noted from previous studies that pitch arrangement, number, orientation, and spacing of baffles along with their orientation can extraordinarily alter the overall efficacy of the heat exchanger. In this study, the triangular pitch has been selected for the tube bundles as it offers better results regarding enhanced surface area per unit length, i.e., maximum tube density. These tubes are generally built as bundles that can be easily dismounted from the tube arrangement (TFD-HE13 -STHX Design). The properties of Al 2 O 3 nanoparticles are given in Table 1.  Table 2. Table 2 Geometric dimension of STHX.

Specification of STHX Dimension
The inner diameter of the shell 90 mm The parameters of STHX were chosen according to Tubular Exchanger Manufacturers Association (TEMA) Standards (Bell, 2004) and were designed as in Figure 1 (a), and Figure 1  Hence, for this criteria, a fine mesh was made with maximum care at the wall regions and edges which are all the regions of high temperature and pressure gradients.
The contours of initially made coarse mesh were analyzed with that of the fine mesh and were observed that the latter mesh resolves in a better way over the regions of high pressure as well as  -Specific Heat capacity (J kg -1 K -1 )

Governing equation of motion
Momentum equation: Energy Equation: where is the density of the fluid, is the velocity of fluid, is the viscous stress tensor, is the pressure, is the body forces in the system, is the internal energy of the fluid, is the heat transfer, is the time, is the dissipation and ∇ . is the heat lost by conduction.

Data analysis
The flow and friction properties of nanofluids, are determined from the base values of particles which are used in the heat exchanger. These are determined using the below-mentioned formulae. The density of nanofluid resulting through the mixing of the base fluid; which means water and alumina nanoparticles is obtained through, In the same manner, the specific heat capacity of the nanofluid is given as, The where, the subscripts , and refers to the base fluid, nanoparticles and the nanofluids respectively. The next parameter that is to be determined is the overall heat transfer rate of the system which is given as, The overall heat transfer rate, = ̇( − ) Here, ̇ is the mass flow rate of the nanofluid system, and are the outlet and inlet temperatures of the nanofluids. In case of the friction factor of the system, it is obtained through, 198 . The nanofluid correlation for friction factor is given by where Re is the Reynolds number and Ø is the particle concentration. In the present work, turbulent flow is considered. It is added that Reynolds' number also determines the nature of the flow and is given as, = The obtained results regarding heat transfer rate and friction factor were then correlated with various models of heat transfer correlations by Dittus -Boelter (Eqn.11, 12), Gnielinski (Eqn. 13) and Blasius (Eqn.14) presented as in the below equations.

Grid independence test
Grid independence study is considered as an important procedure in all CFD analysis. The reason is that the solution which is delivered by the CFD software should be independent of the grid size. This study helps to find out an optimum point where a suitable accurate solution for the

Results and discussion
This section deals with the experimental calculation of flow and friction properties of the nanofluid within the shell tube type heat exchanger. The properties of base fluid and nanoparticle are tabulated in Table 3.  To determine the other parameters like friction factor and heat transfer rate, the outlet temperature of the nanofluid was needed. To determine these temperatures, the analysis was carried out in ANSYS -Fluent and the temperature contours achieved through the test were expressed below.
During CFD analysis, a realizable k-ε turbulence has been utilized. The selection of the turbulence model is generally very critical in any CFD simulation, and this sort of convergence was selected for this study as the obtained Mach number was below the selected range of 0.3.
The boundary walls are assigned individually with preferable boundary conditions. Similarly, the conditions of no-slip were assumed while all the boundary walls except tube walls are set to zero heat flux.
In addition to the above-mentioned assumptions, the convergence criterion was assumed to the 10 -3 range, and thus the boundary conditions assigned at the initial stage was tabulated in Table   4. The temperature contours obtained through the flow of base fluid alone and nanofluid with 1% Al 2 O 3 over STHX can be expressed in Figure 4 and Figure 5 respectively. These temperature

Validation and correlation
The obtained results for the assumed inlet condition of 300 K with 0° baffle inclination angle were validated with the data available through results from various literature studies. The results were tabulated in Table 5  between the heat transfer rates was found to be 6.9 % while that of outlet temperature was found to be 2.71%. Here, relative deviation for outlet temperature is suitable to use in the present work because the outlet temperature difference is less than 10 0 C. For general heat exchanger calculations, a temperature difference up to 10 0 C can be taken under the acceptable limit. This validates that the obtained results are almost in parallel to the ideal results that are to be obtained through a shell and tube type heat exchanger with 0° inclined baffles.
The correlations have been checked with Dittus -Boelter relation and are tabulated in Table 6. In case of friction factor through the implemented technique along with Gnielinski correlation and Blasius correlation, the values are tabulated in Table 7. The results obtained depicts that, there exists a substantial variation in the friction factor correlation in case of both Gnielinski as well as Blasius correlations.

Conclusion
Investigation and numerical analysis have been carried out to determine the flow and friction 356 properties of nanofluids in a shell and tube heat exchanger using Al 2 O 3 nanofluid. It was found from the experiments that, water along with Al 2 O 3 nanofluid of 1% volume concentration has better heat transfer rate compared with normal base fluid alone, i.e. only water. All the analysis have proved that there occurs a substantial increase in the heat transfer rates followed by an earlier convergence history. It is clear from the experiment that, the addition of nanoparticles produced a positive effect on the flow as well as friction properties of cold fluids. The quantitative value of friction factor for the present investigation was observed to be 0.0376 which is very much nearer to results obtained from Gnielinski correlation and Blasius correlation. It can be further noted that the heat transfer rate can be increased if baffle inclinations are provided with the shell.

Declaration
I do hereby declare that the informations furnished in the manuscript is unique and genuine in nature.