Thermal performance of nanofluids in heat transfer loops

Theoretical investigations have been carried out to find the advantages of using different nanofluids in Forced Circulation Loop (FCL) are presented in this paper. FCL finds its application in different heat transfer systems. Compared to the Natural Circulation Loop (NCL), FCL offers better heat transfer due to the higher heat transfer coefficient and more diffusion. Comparison among different chosen nanofluids has been made. Five different water based nanofluids (Al2O3, CuO, Cu, SiO2 and TiO2) have been chosen for the study. In order to calculate performance parameters, thermal conductivity and viscosity of nanofluids are measured and remaining properties are calculated based on the available correlations. From the present investigation it is found that using of nanofluids is beneficial in the way that it brings compactness to the heat transfer loops. Of course, there is trade-off between pressure drop and heat transfer characteristics and these aspects are also discussed.


Introduction
Miniaturization and quick heat transfer are becoming essential in every engineering field. It is always a challenge to an engineer to design an effective heat transfer system by compromising between pressure drop and heat transfer rate. Review papers on nanofluids in heat transfer applications are presented by several researchers [1][2]. Now-a-days, heat generation rate from the electronic equipment is increasing rapidly. There is a big challenge for researchers to identify better heat transport fluid for cooling of electronic equipments. Nakayama et al [3] presented different types of cooling methods for electronic equipments. Thermo physical properties of working fluids plays vital role for better heat transfer enhancement. Several researchers proposed correlations for the measurement of nanofluid properties [4][5][6]. Saidur et al [9] presented overall review on applications and challenges of nanofluids in different engineering disciplines. Iskander Tlili [10] reported the advantage of nanofluid as working fluid in NCL. Praveena [11] conducted numerical and experimental studies on nanofluid when the nanofluid flows through a circular tube. N. P. Devi [12]  loops gives better heat transfer rate compared to natural circulation loops. Extensive literature is available for natural circulation loops. In this paper five types of nanofluids (Al2O3, CuO, Cu, SiO2 and TiO2) are chosen for study of its use in FCL in order to increase the heat transfer in Forced circulation loops (FCL).

Mathematical formulation
In order to investigate the effect of nanofluids in FCL, a simple mathematical model has been developed. Figure 1 represents the FCL considered for study.
By applying suitable correlation for friction factor, eqn.1 can be written as: is termed as figure of merit.
Pumper power can be calculated by: Rate of heat transfer is given by: The ratio of heat transfer rate to volume flow rate is called as volumetric heat capacity. From equations 2, 3 and 4 an expression for ratio of diameters is derived. By considering heat input, temperature drop across the heat source and total loop length same for the same pumping power, the ratio of the essential diameter of the loop when nanofluid (NF) as working fluid to that of water (dNF/dF) is given by: Density of nanofluid is calculated Using Eq. (6) [7]. Vajjha et al. [2] presented that the experimental values of density are well matched with the prediction via Eq. (6) for different nanofluids. So, for this present study Eq. (6) is chosen for the estimation of density of various nanofluids.

Results and discussion
Theoretical investigations are carried out to find the advantages of using different nanofluids in Forced Circulation Loop (FCL). Effect of nanoparticle concentration on density of nanofluids is represented in Fig.4. Figure 4 depicts that there is linear increment of density with nanoparticle concentration for nanofluids. Density of nanofluid is directly influenced by density of nanoparticle which is dispersed in the base fluid. From Table 1, it can be noticed that among all nanoparticles, Cu nanoparticle has higher density which is the reason for higher density of Cu/Water nanofluid compared to other nanofluids.  In order to check the suitability of nanofluids for forced circulation loops (FCL) equation 5 has been derived. From the equation it is clear that diameter ratio depends upon thermo physical properties and also the variation of each property is not similar. Figure 5 represents the variation of diameter ratio for different water based nanofluids with various particle concentrations. Observation reveals that, diameter of the loop decreases with increase in particle concentration. However, there is magnitude difference from fluid to fluid. The diameter ratio is lower for Cu/water nanofluid than for other nanofluids due to its higher density, low specific heat for a specific concentration. So, Cu/water nanofluid exhibits more compactness compared to other fluids considered for the study. Irrespective of type of nanofluid, one can conclude that the use of nanofluid in place of water offers compactness to FCL.    types nanofluids compared with water. Volumetric heat capacity of Cu/Water nanofluid is higher than the other nanofluids for the concentration of 1 to 5%. At higher concentrations the deviation is more.

Conclusion
Advantage of using different nanofluids in Forced Circulation Loop is studied. FCL with different nanofluids is compared with water based FCL in terms of diameter ratio to check the suitability of nanofluids in FCL. Analytical expression for FCL is derived to calculate the size reduction of heat transfer loops.
Following are the conclusions drawn: • Usage of nanofluid in place of water offers compactness to FCL. Cu/water nanofluid exhibits more compactness compared to other fluids considered for the study. • Among all nanoparticles, Cu nanoparticle has higher density which is the reason for higher density of Cu/Water nanofluid compared to other nanofluids. • Volumetric heat capability increases with percentage of particle concentration.
• Among all nanoparticles, copper is having higher thermal conductivity. So, Cu/Water nanofluid is more favourable as far as heat transfer rate is concerned.