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Electricity and gas-based heat treatment of metal is an energy-intensive process. To mitigate the use of such high-grade energy, the concept of an open volumetric air receiver-based solar convective furnace (SCF) system is developed for the heat treatment of metal. This system includes an in situ waste heat recovery mechanism. This paper presents a Joule heating-based, controlled, experimental assessment of a laboratory-scale, retrofitted, SCF system for generating benchmark data. The reported measurements illustrate the heat transfer for (a) the charging and discharging process of thermal energy storage and (b) the two-stage heat treatment of metal with an in situ heat recovery process. The overall system efficiency, including heat recovery, heat storage, and heat transfer, is found to be 24%. Thus, the SCF system can serve as a viable alternative to an electrical energy-based heat treatment furnace. [ABSTRACT FROM AUTHOR]

• Laminar fluid flow based transport and deposition of dust on a flat plate is investigated using the one-way Euler-Lagrange coupling. • The effect of Stokes number, the particle shape and size, the particle release height and heating is described. • The analysis reveals (a) a way of mitigating the deposition and (b) the size of a solar PV panel or a solar water heater should be based on the local conditions. Hydraulically and thermally developing laminar flow induced dust deposition on a flat plate is of practical and fundamental interest with isothermal/non-isothermal and heat flux boundary conditions. The paper deals with the analysis of a steady, incompressible, two-dimensional, laminar, Newtonian fluid flow over an inclined flat plate. The reduced forms of momentum and continuity equations using boundary layer approximation are solved using the fourth order Runge-Kutta numerical scheme integrated with the shooting method. Subsequently, the energy equation with boundary layer approximations is solved for the considered flow over a flat plate using a constant heat flux or constant/variable wall temperature boundary conditions along the length. The calculated velocity and temperature profiles are used for analyzing the particle trajectory over the flat plate. Finally, the one-way coupling of Eulerian and Lagrangian approaches is presented and the effects of Stokes number, particle shape/size, and heating condition are analyzed on the transport and deposition of an inelastic isolated particle on the flat plate. The methodology will be helpful in addressing for instance, the dust deposition over a photovoltaic panel or a solar water heater, and a way of mitigating its effect on the same. [ABSTRACT FROM AUTHOR]

This paper aims at (a) improving the vertical velocity distribution in the wake of a rising isolated bubble for isothermal water layers and (b) evaluating the proposed distribution for thermally stratified therminol layers before and after the initiation of vortex shedding. To address these objectives, numerical investigations are performed, for the rise of an isolated bubble in isothermal and thermally stratified liquid layers, with a combination of the monotonic upwind scheme for conservation laws and pressure implicit with splitting of operators numerical scheme. The analysis revealed that the vertical velocity in the wake of a rising isolated bubble, for isothermal and thermally stratified liquid layers, differs remarkably from the Gaussian distribution. Based on the detailed investigations, region-wise wake velocity distribution comprising a linear superposition of Gaussian approximation with Burr distribution is proposed. Furthermore, this distribution is utilized to predict the rise velocity for a chain of bubbles having different frequencies of departure. Thus, the findings will be useful for the design of heat exchangers or cooling devices, which rely on the heat transfer augmentation with rising air bubbles from a heated surface for isothermal (buoyancy suppressed) and thermally stratified (buoyancy assisted) liquid layers. [ABSTRACT FROM AUTHOR]

The turbulent flow of binary hybrid nano-oils is investigated and Nusselt number correlation is developed for futuristic concentrated solar thermal application. Available Nusselt number correlations for water-based hybrid nanofluids are nanoparticle-specific property correlations dependent and substantially over-predict the values for hybrid nano-oils. Therefore, a generalized Nusselt number correlation for turbulent flow of water and oil-based binary hybrid nanofluids is deduced using the separation approach. Dissimilar to the available correlations, the developed correlation needs only the thermophysical properties of base fluid and nanoparticles. It is valid for the Reynolds number range 10,000-30,000 and the Prandtl number range 0.5-2000. It is found that the proposed correlation predicts the published experimental values for different hybrid nanofluids mostly within ±10-20%. Computational fluid dynamics simulation is also performed for turbulent flow of different hybrid nano-oils to assess the developed correlation. The comparative assessment also confirms that the developed correlation predicts the numerical values for hybrid nano-oils within ±10-20%. The deduced Nusselt number correlation will be useful for a realistic heat transfer analysis with different water or oil-based hybrid nanofluids. The need for experiments with different hybrid nano-oils is realized for further improvement. [ABSTRACT FROM AUTHOR]

Thermal/vegetable oil-based mono/hybrid nanofluids (Nano-oils) may be potential working fluids in single-phase natural circulation loops for medium temperature applications due to their favorable thermophysical properties (high thermal expansion coefficient and low viscosity) and high Prandtl number; however, not studied yet. Hence, the present study investigates the different nano-oils in a natural circulation loop. Therminol-VP1 and soybean-based mono/hybrid nano-oils with different nanoparticles (Al 2 O 3 , CuO, SiC, and MWCNT) are used at a volume fraction of 0.1%. The influence of power input and loop inclination on the energy-exergy performance parameters (Effectiveness, Entropy generation rate and Induced mass flow rate) are presented. The use of nano-oil shows enhanced heat exchanger effectiveness and lower entropy generation rate, whereas the induced mass flow rate may increase or decrease depending on nanoparticle shape and power input. Al 2 O 3 +MWCNT nano-oil exhibits superior performance, with a maximum mass flow rate reduction of 8%, effectiveness enhancement of 36%, and entropy generation reduction of 20% as compared to base oil. Therminol-VP1 shows quick flow establishment than soyabean oil. The usage of nano-oil is more beneficial at the higher power input. Loop inclination reduces the mass flow rate while increasing the effectiveness and entropy generation; however, mass flow reduction is much lower for counter-clockwise inclination. • Hybrid nano-oils having dissimilar shaped particles are used in natural circulation loop. • Mass flow rate, heat exchanger effectiveness and entropy generation rate are evaluated. • Effect of power input and loop inclination are investigated for a medium temperature range. • Therminol-VP1 shows quick flow establishment and better performance than soyabean oil. • Counter-clockwise inclination shows higher mass flow rate than the clockwise inclination. [ABSTRACT FROM AUTHOR]

• Transport of an isolated, dust particle in a circular, straight, absorber pore is analyzed. • The effect of dust accumulation, in an absorber pore, on the heat transfer is analyzed. • Analysis show that dust deposition, in a straight pore, may be mitigated by applying heat-flux. • Dust accumulation is found to be detrimental, for heat transfer in an absorber pore. The deposition of dust in the absorber pores of an open volumetric air receiver (OVAR) offers a significant challenge in arid deserts. In this paper, to address this problem, two aspects are investigated, (i) the transport of an isolated dust particle in a straight, circular absorber pore, for a steady, thermally-hydraulically developing, laminar flow, and (ii) the consequences on heat transfer due to dust particles accumulation, in a straight, circular, absorber pore. The first problem is investigated with a semi-analytical approach and the second problem makes use of computational fluid dynamics simulations. The finding shows (a) a heat flux may be beneficial, for mitigating the dust deposition in a, straight, circular, clean absorber pore, (b) dust deposition in a straight, circular, absorber pore is detrimental to heat transfer, which may lead to its failure, (c) the thermal development length will be reduced for the dust deposited pore in comparison to a clean pore. The findings reveal a way of mitigating dust deposition in a clean pore, and the need for a cleaning strategy of dust deposited absorber pore, for sustainable operation of an open volumetric air receiver. [ABSTRACT FROM AUTHOR]

Nanoparticle shape strongly affects the thermal-hydraulic behavior of nanofluid-enhanced single-phase natural circulation loop (SPNCL) but is yet to be explored experimentally. Hence, various mono/hybrid nanofluids having dissimilar shaped nanoparticles (Al 2 O 3 +Water, Al 2 O 3 +CuO + Water, Al 2 O 3 +SiC + Water, Al 2 O 3 +MWCNT + Water) with a 0.1% volume concentration, are used in SPNCL to investigate mass flow rate (MFR), heat exchanger effectiveness (HEE) and total entropy generation rate (TEGR). The effect of power input, coolant inlet temperature and loop inclination (both counter-clockwise and clockwise) on transient and steady-state performances are presented. The result reveals that mono/hybrid nanofluid shows enhanced HEE and lower TEGR as compared to base fluids, whereas the MFR may increase or decrease since the performance of SPNCL is significantly impacted by the nanoparticle's shape. Spherical-shaped nanoparticle-dispersed nanofluids show early flow initiation and higher MFR than cylindrical-shaped nanoparticle-dispersed nanofluids. MFR and TEGR rise with higher power input, whereas HEE declines for all working fluids. MFR and TEGR increase as the coolant inlet temperature rises, whereas HEE decreases for all working fluids. The loop inclination decreases the MFR, while it increases the HEE and TEGR. The Counter-clockwise inclination of the loop shows a higher MFR than the clockwise inclination at the same angle of inclination. • Hybrid nanofluids having dissimilar shaped particles are used in natural circulation loop. • Mass flow rate, heat exchanger effectiveness and entropy generation rate are evaluated. • Effect of power input, coolant inlet temperature and loop inclination are investigated. • Spherical-shaped shows early flow initiation and higher flow rate than cylindrical-shaped. • Counter-clockwise inclination shows higher mass flow rate than the clockwise inclination. [ABSTRACT FROM AUTHOR]

Several heliostats are arranged on the field for concentrating beam solar radiation onto a receiver. The flow-induced dust deposition and vibration of heliostats will lead to failure of the receiver and the entire system in eventuality. Therefore, fluid flow analysis behind a heliostat model in a narrow channel of limited length is presented depicting a field-type condition. In this paper, experiment and Large Eddy Simulation (LES) are performed. Laser Doppler Velocimetry (LDV) technique is used for horizontal velocity measurement around a heliostat with 25° angle of inclination at a Reynolds number of 60,000 basing on the length of an inclined plate. The wall-resolved coarse, intermediate and fine grids are utilized for LES with dynamic- Smagorinsky–Lilly and subgrid-scale kinetic energy transport equation based models. Comparison between local instantaneous and statistical quantities, like horizontal velocity and its root-mean-square values are presented. The flow development in the wake region of heliostat model is described using the time-dependent and the statistical horizontal velocity. Based on the adopted LES approach the coherent structures are identified with the λ ci -criterion. The analysis of power spectral density and swirling strength isosurface reveal the responsible coherent structures in the inertial subrange for transport of energy to smaller structures. Interestingly, the presence of hairpin-type vortices is observed extending from the two sides of model heliostat using the λ ci -criterion. Further, instantaneous span-wise vorticity contour reveals their alignment with flow represented by streamlines. The provided insight to these flow features and their organization is expected to frame strategies for mitigating the unsteady flow-induced vibrations and dust depositions on heliostats. [ABSTRACT FROM AUTHOR]

In India, the desert regions of Rajasthan and Gujarat are preferred for installation of solar photovoltaic (PV) and concentrated solar thermal (CST) systems because of a high level of both DNI and GHI components of solar radiation. However, such regions pose substantial challenges in terms of dust, a high ambient temperature and scarcity of water. This paper is an attempt to address issues related to the deposition of dust (a) on a mirror with stand depicting PV panel or heliostat in the wake of a similar geometry and (b) in the porous absorber of an open volumetric air receiver (OVAR). A detailed insight to the wake-region behind an inclined flat-plate or heliostat is provided based on analysis and experiment. These indicate possible measures such as modification of the geometry of collectors to mitigate the problem of dust deposition. Also, the use of a cyclone separator with defined parameters based on a decision variable has been suggested for reliable operation of an OVAR. These problems being generic in nature, similar strategies are expected to be applicable in other desert regions subject to their specific conditions. [ABSTRACT FROM AUTHOR]

In India, the desert regions of Rajasthan and Gujarat are preferred for installation of solar photovoltaic (PV) and concentrated solar thermal (CST) systems because of a high level of both DNI and GHI components of solar radiation. However, such regions pose substantial challenges in terms of dust, a high ambient temperature and scarcity of water. This paper is an attempt to address issues related to the deposition of dust (a) on a mirror with stand depicting PV panel or heliostat in the wake of a similar geometry and (b) in the porous absorber of an open volumetric air receiver (OVAR). A detailed insight to the wake-region behind an inclined flat-plate or heliostat is provided based on analysis and experiment. These indicate possible measures such as modification of the geometry of collectors to mitigate the problem of dust deposition. Also, the use of a cyclone separator with defined parameters based on a decision variable has been suggested for reliable operation of an OVAR. These problems being generic in nature, similar strategies are expected to be applicable in other desert regions subject to their specific conditions. [ABSTRACT FROM AUTHOR]

• A multi-zone model for unsteady heat transfer is proposed for an open volumetric air receiver. • The model includes all the modes of heat transfer in receiver internals. • Detailed investigations are performed to identify the important parameters. • Recommendations are made for the scale-up receiver design. A comprehensive, multi-zone, unsteady heat transfer model is developed for a straight absorber pore-based open volumetric air receiver. This includes heat exchange between the porous absorbers, absorbers and receiver casing, and absorbers and return air. The validation revealed its predictive capability within an uncertainty of ±7%. The model is used for the scale-up of receiver design with several intermediate absorber layers. The major recommendations for scale-up are a) multiple absorber layers is beneficial for mitigating thermal stress, b) higher flux concentration is required for non-volumetric heating, to achieve the desired air temperature, compared to volumetric heating, c) air return ratio should be 0.6, d) absorber porosity should be 0.6 for volumetric heating and higher for non-volumetric heating, e) absorber gap to length ratio should be 0.15 -0.25, f) the radiative heat loss is substantial for non-volumetric heating; therefore, the exposed surface area to ambient should be reduced, and g) absorber diameter to length ratio should be 1-2. The developed approach is generic and adaptable for the different open volumetric air receiver designs. [ABSTRACT FROM AUTHOR]

India receives abundant radiant energy from the sun on account of being located at the equatorial solar belt. Especially, an annual global solar radiation of about ⩾2400 kW h/m 2 is received in Rajasthan and in northern Gujarat. As a part of research initiative at IIT Jodhpur, heliostat based concentrated solar tower using an open volumetric air receiver is being developed for generation of high-temperature process heat, which can be utilized for metal processing operations. This technology includes sub-systems such as thermal energy storage and heat exchangers. In particular, the presented paper describes the design evaluation of an open volumetric air receiver. Experiments and computational fluid dynamics tool, ANSYS-FLUENT are used for this purpose. The obtained hot air (heat transfer fluid) can be employed, for instance, in heat treatment of metal such as Aluminum. The considered design aspects of the open volumetric air receiver are as follows: a. Influence of material thermal conductivity on the heat transfer modelling and thermally induced flow instability; b. Thermal–hydraulic analysis of recirculating air injection system; c. Influence of absorber porosity on the heat transfer using the performed experiments; d. Design and evaluation of the designed mixer-assembly for uniform and non-uniform heating of porous absorber. In this paper, the first section presents order-of-magnitude analysis to quantify the effect of thermal conductivity of the solid on heat transfer process with porous absorbers. The second section deals with analysis of thermally induced flow instability in porous absorbers with the validated and numerically adopted computational fluid dynamics tool, namely, FLUENT using the reported experiment by Fend et al. (2004a). In addition, the effect of thermal conductivity on thermally induced flow instability is presented. The next section presents three-dimensional analysis of the designed mixer-assembly for receiver using the validated CFD tool FLUENT. For this purpose, analyses are performed with different designs of mixer-plate and for different receiver geometries. Further, based on the detailed analysis, the injection mechanisms of recirculating air for circular and square absorber based receivers are proposed. Finally, a 4kWth experimental facility, which is being commissioned at IIT Jodhpur to test solar thermal sub-systems such as receiver, heat exchanger, thermal energy storage and experimental evaluation of receiver components are described. These experiments demonstrate the effect of absorber-porosity, uniform and non-uniform concentrated solar irradiance level on heat transfer with porous absorbers and effectiveness of the selected mixer-assembly design in such a condition. [ABSTRACT FROM AUTHOR]

It is necessary to investigate the relative effect of various phenomena before neglecting the same for simulating the single-phase natural circulation loop to obtain an acceptable prediction. Hence, the present study focuses on the validation and effect of different assumptions related to Boussinesq approximation, property variation, bend effect, heat loss, axial fluid and wall conduction on the transient and steady-state characteristics. Effect of assumptions for different working fluids (water, brines and hybrid nanofluids), and assess the effect of different heat flux distributions, like uniform, linear, non-linear, sinusoidal and Gaussian, applied to heater are also explored. Mass flow rate (predicts heat transport capability), effectiveness (predicts heat transfer ability) and entropy generation rate (predicts irreversibility) are evaluated. The results disclose that the Boussinesq approximation with constant other properties under-predicts the steady-state mass flow rate and effectiveness and over-predicts the entropy generation rate. This assumption cannot be applied for all working fluids. The fluid and solid conduction is the crucial parameter for simulating transient characteristics. Gaussian heat flux distribution shows the highest mass flow rate and non-linear increasing heat flux shows the highest effectiveness and entropy generation rate. This study will help to consider reasonable assumptions for the numerical simulation. [ABSTRACT FROM AUTHOR]

This paper aims at (a) comparative assessment of the different volume of fluid (VoF) based numerical schemes for a rising, single air bubble in isothermal liquid (water) layers and (b) investigation of bubble rise in thermally stratified liquid (therminol) layers. For numerical investigation of bubble rise in isothermal liquid layers, three available numerical schemes, Upwind, Quadratic Upstream Interpolation for Convective Kinematics (QUICK) and Monotonic Upwind Scheme for Conservation Laws (MUSCL) schemes, combined with the pressure-velocity coupling(p-v) approaches, such as, Pressure Implicit with Splitting of Operators (PISO) and Semi Implicit Method for Pressure-Linked Equations (SIMPLE), are considered. The wake analysis revealed that the region of influence grows beyond 10 times the diameter of the bubble. Moreover, based on the comparative assessment, MUSCL scheme with PISO is selected, for investigating the bubble rise in thermally stratified therminol layers. Based on these investigations, a 3D diagram, describing bubble shape as f (Ra, Eo, Ga), is proposed, and a new insight to the micro-convection, inside the rising bubbles is provided. Furthermore, a time-scale analysis is performed to describe the heat transfer mechanisms, (a) inside air bubble, and (b) between air bubble and the external surrounding liquid. Thus, the findings will be useful for the design of heat exchangers or cooling devices, which rely on the heat transfer augmentation with rising air bubble. [ABSTRACT FROM AUTHOR]

Transient analysis of vertical heating and horizontal cooling rectangular single-phase natural circulation loop with various water-based hybrid nanofluids (Al 2 O 3 + Ag, Al 2 O 3 + Cu, Al 2 O 3 + TiO 2 , Al 2 O 3 + CNT, Al 2 O 3 + Graphene) with 1% volumetric concentration is studied numerically. Temporal fluctuation and time required to attain the steady-state, transient mass flow rate and energy-exergy performance parameters (effectiveness and total entropy generation) using hybrid nanofluids are compared with water. The effect of power input and geometry parameter (diameter and height) of the loop on transient performances is studied as well. The result reveals that the fluctuation of mass flow rate and time required to attain the steady-state are less for hybrid nanofluids as compared to water. However, platelet and cylindrical shaped nanoparticles yield a lower stability as compared to spherical shaped. The mass flow rate is enhanced with hybrid nanofluids, except Al 2 O 3 + CNT and Al 2 O 3 + Graphene, as compared to water. The energy-exergy performance of hybrid nanofluids is higher than water. The maximum increment in mass flow rate is shown by Al 2 O 3 + Ag hybrid nanofluid (3%), whereas Al 2 O 3 + Graphene shows the highest increment in effectiveness (25.4%) and highest decrement in total entropy generation (14.3%) as compared to water. Smaller diameter and lower height of loop are found preferable for the flow stability. [ABSTRACT FROM AUTHOR]

Radiation calorimeter is a device for assessment of the incident local concentrated solar irradiance (CSI) onto a receiver surface. This is required to evaluate the performance of a concentrated solar thermal system using, for instance a volumetric receiver, for applications. In this paper a low-temperature water cooled radiation calorimeter (RADCAL) is presented. This is designed as a cavity based on the concept of blackbody to maximize the absorption of the incident CSI on its absorber surfaces. Solar selective coatings are deposited on its reflector and absorber surface to achieve the same and are characterized using standard methodologies. This depicts an absorptivity of 0.95 and a reflectivity of 0.87 in the desired range of the solar spectrum and is theoretically verified. The absorption of energy results in temperature rise of RADCAL, which is controlled by an external water jacket to limit its value at 100 °C for mitigating the use of pressurized water. At any instant and eventually at the steady-state the CSI is estimated following the conservation of energy principle. The developed RADCAL is experimentally evaluated up to a CSI of 800 Suns (1 Sun = 1 kW/m 2 ) using Joule heating. The entire heat transfer process is analysed with the developed unsteady state one-dimensional mathematical model. A comparative assessment of the measured and calculated RADCAL body temperature provides the underlying uncertainty and confirms its design basis. Furthermore, the design and given consideration allows its use in arid desert condition with dust and wind. Thus, RADCAL is likely to serve in future for evaluating concentrated solar thermal system in arid deserts. [ABSTRACT FROM AUTHOR]