Double pipe heat exchangers, owing to their simplicity in construction and ease of maintenance, are being widely used in refineries, food processing units and pharma industries. However, they have a limitation that they can only operate at low heat loads. In order to make them suitable
for higher loads, it is essentially required to enhance the rate of heat transfer. In this work, an attempt is made to this end using a combination of threaded rods and water-based aluminum oxide nanofluids. Experimental investigations were carried on a counter-flow double pipe return bend
heat exchanger (DPHE) using water-aluminum oxide (Al2O3) nanofluids with threaded rod inserts. The volume concentration of Al2O3 nanofluid added in water was 0.03% and 0.05%. The entire analysis was carried out under turbulent flow regime by varying
the mass flow rates of cold water from 3 to 15 LPM in steps of 2 LPM, with the hot water flow rate being fixed at 6 LPM. The Reynolds number range considered was from 3000 to 30000. From the obtained results, it was found that the maximum enhancement in the Nusselt number and friction factor
for 0.05% concentration of the nanofluid and threaded rod inserts respectively is 89.95% and 32.46% more than the plain tube. The maximum thermal performance factor was found to be 1.75 for the combination of 0.05% nanofluid and inserts.
Nanofluids are well known for their enhanced thermal properties. In spite of their excellent properties, there are certain hindrances to their applications on large scale. The issue of nanoparticle agglomeration in the base fluid with the consequent stability related issues is one of the main obstacles to the usage of nanofluids. Stability is crucial because the longer the nanofluids remain stable, the better their capacity to retain their thermal properties. Hence there is a need to evolve long-term stable nanofluids. Since there are a lot of factors, which are affecting the stability of the nanofluids, there is a need to optimize the process parameters. In this regard, central composite rotatable design (CCRD) was applied in this study to optimize the independent parameters of stability of ferric oxide nanofluids. For this, the performance of nanofluids was assessed by measuring the nephelometric turbidity units (NTU), based on the independent variables such as percentage of intensity of the nanoparticle, pH of the base fluid, and percentage volume concentration of the surfactant. All the parameters that are affecting individually and mutually were validated statistically using analysis of variance (ANOVA). A regression equation to evaluate the NTU was developed. The obtained results showed that the values predicted by the model and that obtained from the experiments were in good agreement with each other. It is observed that more than 99.65% of the variation could be predicted by the model developed for NTU. The response surface methodology (RSM) has revealed that the ideal process parameters for greater stability of nanofluids are 0.01% particle volume intensity, pH 3.2, and 0.6% surfactant intensity.
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