Laltu Chandra

Fluid flow analysis behind heliostat using LES and RANS: A step towards optimized field design in desert regions

Heliostats are used for concentrating beam radiation onto a receiver. The flow induced dust deposition on these reflectors will lead to failure of the receiver. For this purpose, the wake behind a heliostat is analyzed at 25° of inclination and at a Reynolds number of 60000. In this paper the Reynolds Averaged Navier-Stokes (RANS) and the Large Eddy Simulation (LES) approaches are used for analyzing the air-flow behind a heliostat. LES and RANS are performed with a wall-resolved grid. For the purpose of validation, the horizontal velocity is measured in a wind-tunnel with a model heliostat using laser Doppler velocimetry technique. RANS and LES approaches are found to qualitatively predict the statistical quantities, like the mean horizontal-velocity in comparison to experiment. RANS under-predicts root-mean-square of the horizontal-velocity and even failed to capture the flow features behind heliostat. Thus, it is concluded that RANS will suffice with well-resolved grid for analyzing mean flow features. For analyzing wake and to understand the induced dust deposition LES is required. Further, the analysis reveals that the wake-affected region is up to three times the length of the heliostat’s mirror. This can be recommended as the minimum distance between any two aligned heliostats in Jodhpur.Heliostats are used for concentrating beam radiation onto a receiver. The flow induced dust deposition on these reflectors will lead to failure of the receiver. For this purpose, the wake behind a heliostat is analyzed at 25° of inclination and at a Reynolds number of 60000. In this paper the Reynolds Averaged Navier-Stokes (RANS) and the Large Eddy Simulation (LES) approaches are used for analyzing the air-flow behind a heliostat. LES and RANS are performed with a wall-resolved grid. For the purpose of validation, the horizontal velocity is measured in a wind-tunnel with a model heliostat using laser Doppler velocimetry technique. RANS and LES approaches are found to qualitatively predict the statistical quantities, like the mean horizontal-velocity in comparison to experiment. RANS under-predicts root-mean-square of the horizontal-velocity and even failed to capture the flow features behind heliostat. Thus, it is concluded that RANS will suffice with well-resolved grid for analyzing mean flow features. ...

Design of a Cyclone Separator for Cleaning of Dust from Volumetric Air Receiver

Arid regions, like Rajasthan, receive abundant solar energy and are prone to dust/sand storms. Line and point focusing technologies are used for harnessing this energy. At IIT Jodhpur, open porous volumetric air receiver based point focusing technology is considered for process heat application. This system uses air as heat transfer fluid and is expected to achieve a temperature as high as 800 °C. This receiver on account of volumetric heating, is exposed to a high heat flux, even, up-to 1000 suns (1sun = 1 kW/m2). As this receiver is open to atmosphere, the dust storms in these regions can block the absorber pores and enter the system. Due to lower thermal conductivity of sand in comparison to absorber material, high temperature gradients and thermal stresses are expected on the absorber. It can result in failure of the system. In view of this the current activity aims at cleaning and collection of the removed dust from pores of receiver. A 2D2D geometry of cyclone separator is proposed for deposited dust collection. The experiments on collection efficiency and pressure drop are performed and compared with empirical model. Pressure drop is estimated using CFD and experimentally validated for an improved relation for pressure drop coefficient.

Transient Heat Transfer Analysis in Insulated Pipe with Constant and Time-Dependent Heat Flux for Solar Convective Furnace

The paper deals with heat transfer during the transportation of the hot air from receiver via an insulated pipe to solar convective furnace for metal processing. In the developed concept of solar convective furnace (SCF) hot air at a temperature of 300–600 °C is required for a duration of about 3–6 h. However, the availability of solar irradiance is 8–10 h per day, whereas, the typical required time for achieving the steady condition is in the order of hours. Thus, prediction of time-dependent air temperature development at the outlet of insulated pipe is required for a given flow condition. Considering these aspects, the developed and validated transient heat transfer analysis tool is used to analyze effect of (a) pipe length and thickness of insulation, (b) mass-flow-rate of air, (c) constant and variable inlet air temperature and (d) preheating of pipe. Following are the broad observations based on the performed analysis: (i) time to reach steady state for the considered insulated pipe and the ratio of the heat loss to input reduces with increasing mass-flow-rate; (ii) increasing insulation thickness beyond critical thickness reduces the heat loss that results in a higher temperature at outlet. However, it increases the required time to reach the steady state condition; (iii) the achieved maximum temperature corresponding to a solar irradiance is forward shifted in time.

Transition Metal-Based Spectrally Selective Coatings Using In-House Developed Spray System

The development of transition metal (Fe, Co, Ni, Mn) oxides based thin coatings is reported on stainless steel substrates for solar absorber applications. Absorber layers are synthesized using in-house developed spray system for metal–metal oxide composite coating structures. The optimized combinations of transition metal precursors are sprayed for thin film deposition. The post-spray heat treatment has been carried out to convert the transition metal precursor layers into spectrally selective absorber coatings showing metal–metal oxide composite structures. Three combinations of bi-transition metals Co–Mn, Co–Fe, and Co–Ni precursors are used to synthesize thin film structures. The thickness of these thin films structures is nearly 1 μm. These spectrally selective coatings exhibit high absorptivity (α ~ 0.9) in 300–900 nm wavelength range and emissivity (e ~ 0.18–0.44) in 2.5–25 μm wavelength range. The extension of this work is focused on the development of a large-scale system, capable of fabricating spectrally selective coating on desired structures and optimization of annealing conditions, leading to the optimum solar thermal performance with high absorptivity (α > 0.95) and low emissivity (e < 0.1) in the desired wavelength ranges.

Thermal and Materials Perspective on the Design of Open Volumetric Air Receiver for Process Heat Applications

The concentrated solar thermal technologies (CST) are versatile in view of their multi-faceted applications, such as, process heat, cooling, and electricity generation. These are of line and point-focusing types with the later having much higher flux concentration (in Suns). This allows achieving a temperature in excess of 1200 K using, for instance, the open volumetric air receiver (OVAR). Such a high temperature is useful for applications, like the one which is developed at IIT Jodhpur, namely the solar convective furnace for heat treatment of aluminum. This requires a temperature of up to 750 K in the first phase of development. Thus, a suitable solar selective coating withstanding such a high temperature and having a thermal conductivity close to the base material for operating in an open atmosphere is desirable. Because of its atmospheric exposure, air and dust-induced degradation is inevitable, which may lead to its failure. These challenges are to be addressed for adapting such high-temperature CST technologies in arid deserts of India, the Middle-East, and Africa. In view of such challenges, the following details and foreseen developments are discussed in the paper: (a) design of OVAR including various sub-components; (b) flow-stability and the effect of heat-flux distribution on an absorber pore; (c) the developed coating and its characterization for OVAR.

Experimental and Computational Investigation of Heat Transfer in an Open Volumetric Air Receiver for Process Heat Application

India receives abundant solar irradiance with an annual average of ~19.97 MJ/m2 per day in Jodhpur only. This solar energy can be harnessed for electricity generation, melting or heat treatment of metals. Use of air as heat transfer fluid offers significant advantages of being nontoxic, freely available and operating temperature beyond 800 °C. Considering these aspects, as a research initiative, open volumetric air receiver (OVAR) is being developed with a peak-power capacity of 4 kWth. The installed testing facility at IIT Jodhpur includes sub systems, which are thermal energy storage (TES), air–water heat exchanger. In the absence of solar simulator electrical heating is being employed for circumferential (external) heating of the absorbers. In particular, the presented paper presents: (a) Effect of pore diameters (2 and 3 mm) on the average outlet temperature of absorber with porosity (Ɛ) ~52% at \( {\text{POA/MFR}} = 100\;{\text{kJ/kgK}} \), where POA is the equivalent Power-On-Aperture and MFR is mass-flow rate of air; (b) Efficiency performance curve for absorbers with Ɛ ~ 52%; (c) Modeling of heat transfer in absorber with adopted commercial CFD tool FLUENT including returned air circulation; (d) Comparison between CFD analyzed and experimentally obtained temperature for absorbers with Ɛ ~ 42, 52, and 62%; (e) Predictions with incident radiation onto the front surface of porous absorber.

Selective Design of an Experiment for Evaluating Air–Water Hybrid Steam Condenser for Concentrated Solar Power

In thermal power plants water-cooled steam condenser is used to reject heat from steam at the turbine outlet. In this system, makeup water of about 1500–3000 L per MWh of electricity generation is required. Therefore, in the arid places, air-cooled condensers are recommended, especially, for concentrated solar thermal power (CSP) plants. It is reported that 10 °C rise in air temperature reduces electricity output by about 4.2% Bustamante et al. (Appl Therm Eng xxx:1–10, 2015) [1]. For steam condensation with air at a temperature of 37 °C, an exergetic efficiency of the condenser is 26% in comparison to 63% for water-cooled steam condenser Blanco-Marigorta et al. (Energy, 36:1966–1972, 2011) [2], Bustamante et al. (Appl Therm Eng xxx:1–10, 2015) [1]. Therefore, air-cooled condenser needs higher initial temperature difference (ITD) between condensing steam and air to achieve a high power output. In dry and hot places, like Rajasthan in India, during summer, air temperature reaches up to 45–50 °C. Whereas, condensing steam at the exit of turbine is available at a pressure of ~0.1 atm and at a temperature of ~45 °C. In view of this, the paper presents design of an experiment for evaluation of an air–water hybrid steam condenser. The proposed concept is based on: (a) the temperature difference between air and condensing steam in dry and arid regions and (b) achieving a high value of Nusselt/Colburn-factor to friction-factor ratio. A plate fin-and-circular tube with staggered arrangement air-cooled condenser designs is selected, as the starting point. Two possible modifications are suggested: (a) Earth (underground) for pre-cooling of ambient air or water and (b) use of water spray for additional cooling of the pre-cooled air. The obtained cold air is employed for heat transfer in condenser. In this paper, the selection and evaluation of characteristic design parameters are presented. Finally, the designed experimental setup using all these aspects is described.

Solar Convective Furnace for Heat Treatment of Aluminium

Heat treatment of metal is an energy intensive process. Currently direct electrical energy is employed for this purpose. The harnessed heat using concentrated solar thermal technology can be employed for processing of metal, like, annealing of Aluminium. This paper presents a technology that uses solar energy as fuel and converts the harnessed heat for this purpose. An existing heat treatment furnace is scaled down and retrofitted to demonstrate solar convective heating concept. Air-flow velocity measurements in furnace side-duct are performed using laser Doppler velocimeter (LDV) along the centerline of injection circular nozzle. Comparison with the reported impinging jet profiles shows that air-flow in the side-duct is asymmetric. Computational fluid dynamics (CFD) approach based on different turbulent models is used for air-flow analysis. The selected CFD approach is validated by comparing measured velocities. Validated CFD tool is used for details flow analysis in the furnace. Detailed analysis revealed the formation of a vortex structure under the heart region, which is expected to influence the metal processing quality.

Development Of Nickel Modified Fe 3 O 4 Solar Selective Coatings For Solar Absorber Applications

We report the development of Fe3O4/Cu, and Ni-Fe3O4/Cu based spectrally selective coatings for solar absorber applications using two different electrochemical baths. The deposition processes were optimized for both electrochemical baths and it was observed that the controlled introduction of nickel in Fe3O4 matrix is important to achieve enhanced solar thermal response. The fabricated coatings were characterized to understand the structural, micro-structural and optical properties, to investigate their phase, chemical composition, surface morphology, thickness and solar thermal properties. Ni-Fe3O4 composite coatings exhibited improved adhesion to Cu substrate and allowed better thickness control when compared to Fe3O4 structures without Ni. Improved substrate adhesion and optimized thickness resulted in better optical properties for these coatings. The optimized Ni-Fe3O4 coatings exhibit maximum spectrally averaged absorptivity (α = 0.87) in the 300 – 900 nm wavelength range, and minimum spectrally averaged emissivity (ε = 0.18) in the 2.5 – 25 μm infrared range at room temperature. For Fe3O4 coatings developed on Cu substrates without Ni, maximum absorptance and minimum emittance values obtained were 0.76 and 0.08 respectively. Ni-Fe3O4 coating thickness was measured ~32 μm. Copyright

Development of a Technique for Measurement of High Heat Flux

High heat flux measurement is necessary for various experiments and industrial applications. For example, to estimate a very high irradiance onto a surface as in solar thermal and in testing of plasma facing component in ITER. In order to estimate high heat flux of the order of MW/m2, an experiment with plasma jet of non-transferred 9MBM type is employed. This paper describes the design of a non-intercepted calorimeter. This is for estimation of average incident heat flux of the plasma jet along the axial direction. Temperature at different axial locations on the surface of calorimeter and at the outlet is measured. The heat flux associated with the incident plasma is estimated from these measured values of temperature. Rate of heat transfer from plasma jet to the employed target surface at different axial position is measured with the help of intercepted calorimetric method. Based on the estimated heat flux and heat transfer rate, the electrothermal efficiency of 9MBM plasma torch is estimated. Torch heat efficiency (THE), plasma heat efficiency (PHE) and heat transfer effectiveness (HTE) of the plasma jet are also estimated for copper materials. PHE and HTE are required for mathematical modelling of plasma surface interaction.