1. Ph.D Theses

Permanent URI for this collectionhttps://idr.nitk.ac.in/handle/1/11

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Subject: search.filters.subject.Nanofluids

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    Nanofluid Mediated Gas- Liquid Mass Transfer Enhancement In Pulsed Plate Column
    (National Institute of Technology Karnataka, Surathkal, 2023) Shet, Amruta S; K, Vidya Shetty
    Oxygen transfer limitations result in poor performance in bioreactors and reduced efficiency in catalytic and photocatalytic reaction systems where oxygen transfer is involved. Adequate oxygen transfer can be achieved by increasing the volumetric oxygen transfer coefficient (kLa). Several investigations have shown that the enhancement of kLa can be achieved in the presence of nanofluid which is a colloidal suspension of nanoparticles in any base fluid. Nanoparticles may be intentionally added to the reactor fluid or may be inherently present in the reactor to form a nanofluid. Pulsed plate column (PPC) is widely used as an aerobic bioreactor and gas-liquid contactor for various applications. In the present study, the influence of TiO2, SiO2, and α-Fe2O3 nanofluids with water as the base fluid on kLa was studied in PPC. The effect of nanofluid parameters such as nanofluid type in terms of nanoparticles used, their size and loading along with the column parameters such as frequency (f) and amplitude (A) of pulsation, pulsing velocity (A×f) and gas velocity (Ug) was studied. The use of nanofluids led to kLa enhancement. It was found that kLa increased as the nanoparticle loading increased, attained a maximum at the critical loading, and then reduced as the loading was further increased. The critical loading depended on the nanofluid. kLa was found to increase with the increase in A, f, and Ug. The nanoparticle loading and A×f showed an interacting effect on kLa resulting in one or more hydrodynamic regimes depending upon the type of nanofluids, size, and loading of the nanoparticles. Nanofluids with lower-size nanoparticles showed higher kLa compared to those with larger sizes. TiO2 nanofluid provided a better kLa enhancement than SiO2 and α-Fe2O3 nanofluid. The maximum enhancement factors were obtained with TiO2, SiO2, and α-Fe2O3 nanofluids at the critical loading conditions. The order of magnitude analysis implied that the convective currents caused by the Brownian movement of the nanoparticles in the fluid can be the possible reason for mass transfer enhancement in PPC. Pseudo-homogeneous model was tested and it was found to accurately predict the enhancement only till the critical loading conditions. The developed dimensionless correlations and artificial neural network models could accurately predict kLa and thus may find potential applications in the design of pulsed plate column when used as gas-liquid mass transfer contactors, bioreactors, or photocatalytic reactors. The results of this study indicate that the pulsing conditions required to achieve the desired mass transfer characteristics can be reduced by using a nanofluid instead of the base fluid, thus potentially leading to tremendous saving of energy.
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    Characterisation of Select Nanofluid and Vegetable Oil Quenchants and Assessment of Heat Transfer during Quench Hardening of Steels
    (National Institute of Technology Karnataka, Surathkal, 2018) Ullal, Vignesh Nayak; Narayan Prabhu, K.
    The present investigation involved the characterization of nanofluids and study of heat transfer characteristics of vegetable oil quench media for heat treatment of steels. Nanoquenchants were formulated by the two-step method. The wetting kinetics and kinematics of quench media were studied by measuring the contact angle and online video imaging during quenching. CuO, MWCNT and graphene based nanofluids showed better wetting and spreading ability compared to distilled water. In nanofluids, the stabilization of the vapour phase stage resulted in low severity of quenching. Spatiotemporal heat flux transients were estimated by using a 2-D IHCP model during of quenching of ISO 9950 inconel probe. The study showed increased heat extraction capabilities of graphene and MWCNT nanofluids compared to distilled water under agitated quenching conditions. Heat extraction rates were found to be lower for CuO nanofluids. The use of edible and non-edible vegetable oils for quench hardening was investigated by comparing their heat transfer characteristics with a mineral oil. The study showed the excellent potential of non-edible vegetable oils for quench hardening of steels. Karanja oil was found to be superior compared to neem and sunflower oils. To simulate the industrial quench heat treatment, reference probes made of medium and high carbon steels were quenched and heat flux transients were estimated by taking into account the phase transformation. The cooling curves obtained with reference probes made from G 10450 and G 10900 steels showed kinks indicating enthalpy change accompanied with phase transformations during continuous cooling. This was reflected in the estimated heat flux curves. The effect of viscosity, density and surface tension of quench media on the mean peak heat flux was quantified using a power fit model. The section thickness effect on heat flux transients was examined by using probes of diameters 25 mm and 50 mm. The cooling rates measured at various locations along the cross section of reference probes of both thicknesses were related to the hardness using the Quench Factor technique. The heat transfer characteristics of the quench media, the evolved microstructure and the resulting hardness were in complete agreement.