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Browsing Theses by Supervisor "Chandra, Laltu"

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    On the Evaluation and Cleaning of an Open Volumetric Air Receiver: A Step towards Realizing Solar Convective Furnace System in Arid Deserts
    (Indian Institute of Technology Jodhpur, 2018-12) Chandra, Laltu
    Arid desert regions of Rajasthan and Gujarat are exposed to a high level of solar irradiance (> 5 kWh/m2/day), which can be harnessed with the concentrated solar thermal technologies. For example, the central receiver based solar air tower is a point focusing system, which uses heliostat for concentrating the beam radiation onto an open volumetric air receiver. This comprises of porous absorbers that are exposed to a high heat flux beyond 800 kW/m2 and are open to atmosphere. In this thesis, the addressed pertinent challenges for operating such a receiver in arid deserts are as follows: (a) The concentrated solar irradiance onto an open volumetric air receiver follows quasi-Gaussian distribution along the radial direction, which can promote the thermally induced flow instability, more probably in the centrally located absorbers. The underpinning theoretical and numerical investigations with the circular straight pore based cylindrical absorber are presented. The findings are necessary for operating a system using such a receiver. Based on assumptions, it is found that the thermal induced flow instability is unforeseen up to the desired temperature of 750 K for solar convective furnace. However, a need of more detailed analysis is realized. (b) The absorber pores are prone to dust deposition leading to a partial or the complete blockage. As a consequence, the formation of a local or a wide area hotspot and an eventual failure of the receiver are envisaged. To investigate the same, both the theoretical and the computational analyses are performed with volumetric and non-volumetric heating conditions including the radiation based heat losses. The investigations revealed that even a 100 µm thick layer of the deposited dust in an absorber pore of diameter 2 mm can lead to an elevation of temperature up to 150 K at a given operating condition. To mitigate the effect of dust deposition a cyclone separator based cleaning strategy is proposed. (c) Finally, a seven equations based one-dimensional zonal model is developed and is validated with the reported in-house experimental data. In this model the absorbers are divided into categories viz. central and peripheral. The performed analysis aims at identifying the important parameters viz. porosity, geometric dimensions, air return ratio that are relevant to the performance assessment of such a receiver in terms of the thermal efficiency.Therefore, the undertaken research may be considered as a step towards realizing the open volumetric air receiver based furnace system in desert regions.
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    Open Volumetric Air Receiver based Solar Convective Furnace System
    (Indian Institute of Tehcnology, Jodhpur, 30-06-2023) Chandra, Laltu; Mukhopadhyay, Sudipto
    Electrical energy from fossil fuels or gas-fired systems is commonly used as a heat source for industrial process heating, such as the heat treatment of metals, which leads to harmful emissions. Freely available solar energy is a viable option for transitioning to a net zero carbon economy and reducing emissions. For instance, harnessed solar energy with a concentrated solar thermal (CST) system may be utilized, for example, in the melting, coating, and joining of metals. Recently, a novel, retrofitted solar convective furnace (SCF) system was developed for the heat treatment of Aluminum using hot air from a heliostat-based CST system. The developed SCF system comprises an open volumetric air receiver (OVAR), two pebble-bed sensible thermal energy storage (TES) systems, viz. primary and secondary, and the furnace itself. The OVAR produces hot air using the concentrated solar irradiance onto its aperture. The generated hot air is transported to the SCF directly or indirectly via the primary TES. The secondary TES is utilized for waste heat recovery from hot air at the furnace outlet. The feasibility assessment of the SCF system is performed using a two-step approach: (1) experiments are performed for each of the sub-systems and the integrated system, and (2) mathematical models are developed for scaling of OVAR, each of the sub-systems, and the integrated system analysis. The details are described as follows: Parametric experimental investigations are performed with primary TES for charging and discharging processes to evaluate its thermal evaluation. The experimental investigations for primary TES showed that the time-averaged charging and discharging efficiencies are 65-70% and 72-75%, respectively. Further analyses revealed the exergy efficiencies for charging and discharging are 45-50% and 58-60%, respectively. Experiments are performed to investigate heat transfer in the retrofitted solar convective furnace. These experiments include a provision for external heating for SCF. Experimental results of SCF, such as temperature profile and heating process, show the capability of heat treatment of metal-ingot via forced convection. For the numerical design, OVAR size (a key component of SCF) is selected based on a preliminary calculation for 0.58 MW capacity Aluminum furnace, with direct normal irradiance (DNI) of 220 W/m2 and a concentration ratio of 600. The modelling of unsteady heat transfer in OVAR is done by considering multiple zones of the receiver, viz. central, intermediate, and peripheral zones. The model was validated with experimental data and a two-zone model, which demonstrated the model's prediction capability within ±7%. A parametric investigation is carried out for the planned scale up OVAR design. An unsteady heat transfer model is developed for the TES. Analysis revealed that the deviation between the computed and experimental temperature is within ±15%. Also, a one-dimensional mathematical model is developed to analyze the unsteady heat transfer process for the retrofitted SCF. The calculations show a deviation of about ±15% from the experimental data. Finally, a mathematical model is developed for the installed lab-scale SCF system, including OVAR, connecting pipes with insulation, and TES. Findings demonstrate the potential of using the developed CST-based SCF system for the heat treatment of metal. However, the integrated model needs to be refined for better results and may be addressed in future.