Browsing by Author "K., Narayan Prabhu"

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Development of Novel Thermal Analysis Techniques For Characterization Of Salt Based Phase Change Materials For Thermal Energy Storage Applications
(National Institute of Technology Karnataka, Surathkal, 2022) Agarwala, Swati; K., Narayan Prabhu
Solar energy storage technologies have proved to be promising in terms of providing an uninterrupted power supply. Latent heat thermal energy storage systems in the form of salt phase change materials have been successfully used in concentrated solar plants for energy storage applications due to their superior energy storage parameters and functionalities. In the present work, to characterize the salt phase change materials, the development, suitability, and reliability of a characterization method devoid of the limitations of the conventional thermal characterization techniques have been researched and proposed. A new approach based on the solution to inverse heat conduction problem for assessing solidification parameters of phase change materials (PCMs) salts in steel mold with furnace cooling has been proposed. The method estimates the steel mold -salt interfacial heat flux, and it is used to calculate the latent heat and phase change parameters of salt PCMs using calorimetry-based energy balance equations. This method is more accurate than the conventional Computer-Aided Cooling Curve Analysis (CACCA) techniques. It eliminates the drawbacks of baseline fitting calculations and errors introduced due to the improper selection of solidification points. Pure Salt PCMs such as potassium nitrate (KNO3) and solar salt mixture of 60 wt%NaNO3 and 40 wt%KNO3 were used for the validation of this technique in the work. The solidification parameters such as rate of cooling, time taken for solidification and latent heat of the PCMs were determined and were found to be in close accordance with the reported literature data. A quantitative method for this determination of phase change parameters of salt-based PCMs has also been proposed. This technique involves estimating mold-salt interfacial heat flux by solving Fourier’s law of heat conduction within the salt and using it to calculate the phase change enthalpy of salt PCMs. Pure salt PCMs such as potassium nitrate (KNO3), sodium nitrate (NaNO3)), and solar salt mixture (60wt.%NaNO3+40wt.%KNO3) were used for validation of this technique Further, the effects of the addition of MWCNT carbon-based nanostructures on thermal energy storage (TES) parameters of lithium-based eutectic binary, ternary and quaternary salts were investigated. The addition of nanoparticles showed no effect on the solidification time and temperatures of the PCMs. In binary and ternary salts, lower concentrations of nanoparticles showed a positive impact on the TES parameters. LiK and LiT with 0.1% MWCNT showed an enhancement in the latent heat values of 27.6% and 19.28% respectively. However, the effects were limited at higher MWCNT weight concentrations. It was observed that the addition of nanoparticles had no significant effect on the TES parameters of the quaternary salts. In addition, the micrographic studies to analyze the agglomeration at higher concentrations of nanoparticles and wettability studies were also performed.
Heat Transfer Studies During Quench Hardening of Steels
(National Institute of Technology Karnataka, Surathkal., 2024) Alberts, Augustine Samuel; K., Narayan Prabhu
In the present work, heat transfer during quench hardening of steels was investigated incorporating the effect of phase transformation, section thickness, and the quench medium. The work is divided into three sections. In the first part of the work, the effect of section thickness on the steel/quenchant interfacial heat flux is studied. The second part defines a critical heat transfer coefficient as the minimum heat transfer coefficient required to form 50% martensite at the core of the steel cylinder of a particular grade and section thickness. A simulation study was carried out for this purpose. In the last section of the work, an attempt was made to repurpose and reuse the used cooking oil as a blend for oil quench media. The first part of the work investigates the effect of phase transformation on interfacial heat flux during quench hardening treatment of steel. Experimental and modeling approaches comprising the inverse heat conduction problem (IHCP) phase transformation coupling were employed to analyze the thermal behaviour of different steel grades with varying section thicknesses. The study revealed a distinct heat flux pattern with phase transformation, showing a dip followed by a rise. Increasing section thickness increased the surface heat flux for stainless-steel probes (no phase transformation), the heat flux was reduced for plain carbon steel probes due to phase transformation. A was introduced for quantifying the enthalpy change during quenching of steel probes. was found to be consistent for a steel grade and independent of section thickness but varied with cooling rate and quench medium. Incorporating IHCP models with phase transformation simplifies the process of quenching simulation and minimizes data inputs. A database on Q as a function of temperature and cooling rate would greatly facilitate heat transfer modeling during quench hardening of steels. In the second part of the work, a simulation study is performed to obtain the critical heat transfer coefficient for the selection of quenchants for a particular grade of steel and section thickness. The simulation study is conducted by solving phase transformation coupled transient heat conduction equation using the finite element method. The finite element model adopted in this work uses the one-dimensional radially symmetric model with the constant heat transfer coefficient boundary condition at the surface. The variables in the simulation study are the carbon content, the diameter of steel, and the heat transfer coefficient. The effect of the variables on the martensite formation at the core of the steel is studied. The critical heat transfer coefficient significantly varied with the section thickness and the carbon content of the steel. The usefulness of the study in selecting a quenchant for quench hardening plain carbon steels with varying carbon content is illustrated. In the last part of the work, the fatty acid methyl ester (FAME) produced from the used sunflower cooking oil through transesterification was blended with sunflower and mineral oils at various proportions. The cooling characteristics of the FAME/oil blends were assessed using the cooling curve analysis according to ASTM D6200 and ISO9950 standards. The inverse heat conduction method estimated the spatiotemporal metal/quenchant interfacial heat flux. The uniformity of heat flux was analyzed. The results indicated that blending of used cooking oil-derived FAME with sunflower oil up to 60vol% and mineral oil up to 50vol% provided comparable cooling characteristics to pure oils. The estimated heat flux transients showed a marginal decrease in peak heat flux for FAME blends in sunflower oil, whereas an increased peak heat flux with mineral oil. In the subsequent part, the thermal-oxidative stability of the FAME blend is improved through the epoxidation reaction. The stability of epoxidized FAME/mineral blended oil is assessed by thermogravimetric analysis (TGA) and thermal quench cycling. The quench cycles were performed using an ISO 9950 Inconel 600 standard probe. The viscosity and cooling performance of the oil was assessed periodically at the 1 st , 10 th , 50 th, and 100 th quench cycle. The results indicated that the thermal stability of the blend quenchant was improved with the epoxidation of FAME. The study showed that blending the epoxidized form of FAME (EFAME) in oil provided better thermal-oxidative stability than the FAME/oil blend.
Thermal Conformance Between Metal-Metal Contacts Under Transient Conditions – A Study (The Effect of Tims, Metal Thermophysical Properties, and the Interfacial Pressure)
(National Institute of Technology Karnataka, Surathkal., 2024) Ramakrishna, Devanand P; K., Narayan Prabhu
A study of interfacial heat transfer between metal-metal contacts is essential in various engineering applications for designing thermal systems with increased efficiency. Interfacial heat transfer between metal-metal contacts under transient conditions is discussed in the present investigation. A novel technique based on solution to inverse heat conduction problem was adopted to estimate the interfacial heat flux transients between metal contacts. Thermal conformance parameters and a thermal conformance assessment parameter (TCAP) were proposed to assess conformance of metal-metal contacts. The metal-metal contacts with different combinations, such as Cu-Cu, Al-Al, Brass-Brass, Cu Al, and Al-Cu, were subjected to varying degrees of interfacial pressure, and the heat flux transients were estimated using lumped heat capacitance and inverse heat conduction approaches. The effects of thermal interface materials (TIMs), thermophysical properties and surface roughness on the thermal conformance were also investigated. The study used polymer-based TIMs with thermal conductivity ranging from 0.193 W/m K to 7 W/m K. The interfacial pressure had a significant impact on heat transfer across the interface under bare conditions (without TIMs). However, the effect of interfacial pressure on contact heat transfer is reduced with the application of TIMs of high thermal conductivity. The study also revealed that the effect of pressure was negligible on heat transfer beyond a limiting value of interfacial pressure. A smooth surface texture of the contacting materials resulted in enhanced heat transfer across the interface. The addition of multi-walled carbon nanotubes (MWCNT) in low conductive polymer-based TIMs on heat transfer was investigated. At low weight percentages, the heat flux transients increased marginally by about 6%. However, increasing the weight percentages of MWCNTs resulted in deterioration in heat flux owing to the agglomeration of MWCNTs. The study also found that the thermophysical properties of metals in contact affected the interfacial heat transfer with thermal conformance parameters increasing linearly with TCAP for all the combinations of metal-metal contacts except for the Cu-Cu combination, where interfacial pressure becomes dominant. Further, 1-Methyl-2-pyrrolidone (NMP), a cleaning solvent generally used in the semiconductor industry, was applied on the source and sink surfaces to assess its effect on heat transfer at the interface. The use of NMP resulted in a significant increase in the thermal conformance at the interface. The increase in the thermal conformance on the application of NMP was attributed to the (i) removal of oxide films on the source and sink surfaces and (ii) surface smoothening effect increasing the microscopic contact points and effectively reducing the mean gap width. The surface smoothening effect was validated by carrying out surface roughness measurements.