arXiv subjects

Vikrant Khullar

Publications and source records attributed to Vikrant Khullar.

7 recordsLinked to original sources

arXiv2026

Understanding Heat Transport Mechanisms in Optically Transparent Thermal Loss Mitigators

Optically transparent thermal loss mitigators have recently seen renewed research interests owing to their increasing relevance in the realms ranging from smart windows, efficient greenhouse designs and high-performance-low-cost solar thermal systems. In depth understanding of the heat transport mechanisms and their quantification is crucial for building efficient opto-thermal management strategies for optimization of the aforementioned systems. The present work serves to identify and quantify the key heat transfer mechanisms operative in a host of optically transparent thermal loss mitigators. In particular, comprehensive experimental modelling frameworks have been developed to investigate the efficacy of carbon dioxide gas (CO2), air, vacuum (0.07mbar), transparent heat mirrors (Indium tin oxide coated glass) and aerogels (silica-based) in mitigating thermal losses. Detailed and careful experimental modelling reveals that it is imperative to employ more than one thermal loss mitigator and choose correct absorber surface orientation (relative to the irradiation direction) to maximize thermal loss mitigation. Magnitude of absorber surface stagnation temperature has been employed as the figure of merit to quantitatively compare various optically transparent thermal loss mitigators. Under un-evacuated conditions, CO2 has emerged as potent alternative to more sophisticated optically transparent thermal loss mitigators like aerogels and transparent heat mirrors. Enhancements (relative to air) on the order of 2%-7%, 46%-84%, 57%-84% and 66%-86% are observed in case of CO2, vacuum, transparent heat mirrors (vacuum) and aerogel (vacuum) respectively.

physics.app-ph↗

arXiv2023

Understanding Photo-thermal and Melting Mechanisms in Optical Charging of Nano and Micro Particles Laden Organic PCMs

The realm of latent heat storage has witnessed emergence of optical charging as a promising route of solar thermal latent heat storage. However, it is still in its initial stages of development and warrants further investigations to take it to the next level i.e., realization of optical charging based real-world systems. Engineering efficient optical charging process in turn necessitates efficient photo-thermal energy conversion, transfer as well as storage of the incident solar radiant energy. The present work is a determining step in deciphering, quantifying, and understanding the aforementioned steps involved in the optical charging process. In particular, experiments have been designed carefully to investigate optical charging of composite-PCMs (particles laden organic PCMs) with and without thermochromism assistance. Spatial-temporal temperature distribution curves reveal that temperature spread (in the liquid phase) in case of optical charging of non-thermochromic particles (carbon soot nanoparticles) laden PCMs is significantly high (as high as approximately 24 {\deg}C) relative to that observed in case of thermochromic particles (microcapsules) laden PCMs (approximately, 4 {\deg}C). The magnitude of the temperature spread (being representative of the deviation from thermostatic optical charging) clearly points out that opposed to non-thermochromic laden PCMs, nearly thermostatic optical charging can be achieved in case of thermochromic particles laden PCMs. Furthermore, in case of optical charging without thermochromic assistance, the temperature spread, peak temperatures and the melting rates increase with increase in particles concentration. Whereas, in the latter case, although the temperature spread and peak temperatures are nearly independent; the melting rates do depend on the particles concentration.

physics.app-ph↗

arXiv2023

Predicted Performance Bounds of Thermochromism Assisted Photon Transport for Efficient Solar Thermal Energy Storage

Efficient storage of solar thermal energy is still one of the major bottlenecks in realizing dispatchable solar thermal systems. Present work is a significant step in this direction, wherein, we propose, thermochromism assisted photon transport based optical charging for efficient latent heat storage. Seeding thermochromic nanoparticles into the phase change material (PCM) allows for dynamic control of PCM's optical properties - aiding deeper penetration of photons and hence significantly enhancing the photon-nanoparticle interactions. Moreover, carefully tailoring of transition temperature near the melting temperature allows for efficient non-radiative decay of the absorbed photon energy and that too under nearly thermostatic conditions. In particular, the present work serves to develop a mechanistic opto-thermal theoretical modelling framework to compute melting front progression, latent heat storage and sensible heat discharging capacities pertinent to thermochromism assisted photon transport. Moreover, to truly assess and quantify the benefits of the aforementioned charging route, a host of other possible charging routes (viz., thermal and non-thermochromic optical charging) have also been dealt with. Detailed analysis reveals that relative to the thermal charging route, thermochromism assisted optical charging offers significant enhancements in terms of melting front progression (approximately 152%) and latent heat storage capacity (approximately 167%). Overall, thermochromism assisted photon transport is a synergistic approach which allows for simultaneous collection and storage of solar energy at accelerated rates without requiring the PCM to be heated to high temperatures.

physics.app-ph↗

arXiv2022

Nanofluid Filled Enclosures: Potential Photo-thermal Energy Conversion and Sensible Heat Storage Devices

In the present work we propose "nanofluid filled enclosures" as potential photo-thermal energy conversion and sensible heat storage devices. Herein, the optical charging of the nanofluid has been modeled as "solar radiant energy - nanoparticles" interaction. The subsequent energy redistribution has been modeled as coupled transport phenomena involving mass, momentum and energy transport. In particular, nanofluid filled enclosure with adiabatic, and isothermal (through convective) boundaries have been analyzed to decipher the fundamental limits of sensible heat storage and thermal discharging capacities respectively. Furthermore, the effect of nanoparticles volume fraction on the photo-thermal energy conversion mechanisms and its redistribution thereof has been critically investigated. Detailed analysis reveals that under similar operating conditions, in volumetric absorption mode (i.e., at low nanoparticles volume fraction) nanofluid filled enclosure has higher sensible heat storage (7% - 27% higher) and thermal discharging (16% - 47% higher) capacities than in the corresponding surface absorption mode (i.e., at high nanoparticles volume fraction). Overall, "nanofluid filled enclosures", particularly in volumetric absorption mode, could be deployed for efficient solar thermal conversion and storage.

physics.app-ph↗

arXiv2022

Modeling and Analysis of Heat Transfer and Fluid Flow Mechanisms in Nanofluid Filled Enclosures Irradiated from Below

Radiation driven transport mechanisms are ubiquitous in many natural flows and industrial processes. To mimic and to better understand these processes, recently, radiatively heated nanofluid filled enclosures have been extensively researched. The present work is essentially a determining step in quantifying and understanding the transport mechanisms involved in such enclosures. In particular, a two dimensional square nanofluid filled enclosure irradiated from the bottom has been investigated in laminar flow situation. Effects of nanofluid optical depth, inclination angle of the enclosure, incident flux, and boundary conditions (adiabatic and isothermal) have been investigated. Moreover, the temperature and flow fields have been carefully analyzed in the situation ranging from volumetric to mixed to surface absorption modes. Under adiabatic boundary conditions, steady state is unconditionally achieved irrespective of the incident flux magnitude (varied between 5Wm-2 to 50Wm-2), enclosure inclination angle (varied between 0 to 60 degrees) and mode of absorption (surface, mixed or volumetric). However, in case of isothermal boundaries; onset of natural convection and its transition into transient regime is significantly impacted by the mode of absorption and the enclosure inclination angle.

physics.flu-dyn↗

arXiv2019

Performance Bounds of Nanoparticles Laden Volumetric Absorption Solar Thermal Platforms in Laminar Flow Regime

Recent success in synthesizing thermally stable nanofluids at low costs is a significant breakthrough in the evolution of volumetric absorption based solar thermal systems. However, we have yet not been able to clearly identify the range of operating and design parameters in which volumetric absorption could prove to be beneficial. One of the key reasons being that we have not been able to fully understand the heat transfer mechanisms involved in these novel systems. The present work takes a few steps further in this direction wherein we have developed a comprehensive and mechanistic theoretical framework which is robust enough to account for coupled transport phenomena and orders of magnitudes of operating parameters for host of receiver design configurations. Moreover, we have also modeled equivalent surface absorption based systems to provide a comparison between volumetric and surface absorption processes under similar operating conditions. Performance characteristics reveal that particularly at high solar concentration ratios, volumetric absorption-based receivers could have 35% - 49% higher thermal efficiencies compared to their surface absorption-based counterparts. Finally, the present work serves to define optimal performance domains of these solar thermal systems, particularly in the laminar flow regime (200 < Re < 1600) and over a wide range of solar concentration ratios (5-100) and inlet fluid temperatures (293-593K).

physics.app-ph↗

arXiv2018

Efficient Volumetric Absorption Solar Thermal Platforms Employing Thermally Stable - Solar Selective Nanofluids Engineered from Used Engine Oil

We report a low cost and scalable method to synthesize solar selective nanofluids from 'used engine oil'. The as-prepared nanofluids exhibit excellent long-term stability and photo-thermal conversion efficiency. Moreover, these were found to retain their stability and functional characteristics even after extended periods of high temperature (300°C) heating, ultra violet light exposure and thermal cyclic loading. Building upon it, we have been able to successfully engineer an efficient volumetric absorption solar thermal platform that employs the as-prepared nanofluids and achieves higher steady state temperatures (approximately 5% higher) relative to the conventional surface absorption based solar thermal system under the sun. The developed volumetric absorption solar thermal platform could prove to be significant step in the evolution of efficient solar thermal systems which could potentially be deployed for host of applications ranging from solar driven heating, air-conditioning, and desalination units to solar energy electricity generation systems.

physics.app-ph↗