Laltu Chandra

We investigate several critical aspects of a solar tower simulator for thermo‐catalytic ammonia cracking at an ammonia flow rate of 1 kg h ⁻¹ . These include (a) assessing the relative impact of optical, conversion, and receiver efficiencies and waste heat recovery (WHR) fraction on the net solar‐to‐hydrogen generation efficiency, (b) designing an optimal WHR and reuse subsystem for the simulator, (c) mitigating the overheating of a 350 We Xe‐short‐arc lamp, and (d) optimizing the catalyst for efficient ammonia decomposition. We conclude that the conversion efficiency has the highest impact on the net solar‐to‐hydrogen generation efficiency. Therminol 55 mass flow rate of 1.75 kg h ⁻¹ offers the highest WHR fraction of ~68% and a net solar‐to‐hydrogen efficiency of ~75.8%. A correlation predicts the surface temperature of the Xe‐short‐arc lamp, for airspeed in the range 1.4–5.2 m s ⁻¹ , within an uncertainty of about ±5–7%. Effective thermo‐catalytic ammonia decomposition was achieved by precisely tuning the catalyst composition through the selection of an appropriate support and optimized active metal loading. All of these findings will benefit the development and scale‐up of an integrated system.

The concentrated solar thermal systems, operating in the medium temperature range of 373–573 K, will be extremely useful for several industrial processes. However, the need for an in-depth understanding of the turbulent heat transfer in parabolic trough absorbers with pure and hybrid nano-oils, including the effect of buoyancy or gravity, is realized. This paper presents the Reynolds-averaged Navier–Stokes (RANS)-based turbulent heat transfer analyses in a 3D, long, straight for Reynolds number from 5000 to 20,000 and discrete heating conditions with different heat flux ratios such as 1, 5, 10, 20, 40, and 50 for pure oil and hybrid nano-oils having 1, 4, and 6% volume concentration of the nanoparticles. The major findings are, (a) gravity-induced anisotropy leads to high and low-speed fluid flows near the lower and upper walls, and temperature redistribution at a plane, which is beneficial, (b) the statistical axial-velocity deviates from the standard logarithmic law at a Reynolds number of 5000, and (c) the ratio of surface-area-averaged Nusselt number between the lower half and upper half of the tube is 4–12. Some important recommendations are (a) the effect of gravity must be included, (b) the local Richardson number may be used for improving the standard logarithmic law for the axial velocity, and (c) Nusselt number correlations are deduced for the upper half surface and lower half surfaces. The findings, albeit for limited parameters, will be useful for improving the heat transfer aspects in the parabolic trough absorber.

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.

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.

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.

In the present study, different thermal oil-based hybrid nanofluids (Al₂O₃+Ag+therminol-VP1, Al₂O₃+Ag+Paratherm-CR, Al₂O₃+Ag+Dowtherm-Q) has been used as a primary fluid in the passive indirect solar water heating (PISWH) system. The performance of the passive indirect solar water heating system has been evaluated by using three performance parameters i.e mass flow rate, effectiveness of the heat exchange and total entropy generation rate of the system for the laminar flow regime. The effect of geometric parameter such as tube diameter (25, 30, and 35 mm), solar collector inclination (θ=300,450,600) and different type of heat exchangers (Tube in Tube, Shell and Tube and Plate type) on the performance parameters has been studied. The study reveals that the mass flow rate increases with increasing diameter of tube and collector inclination angle. Whereas, the effectiveness decreases with increasing tube diameter and collector inclination angle. Moreover, the total entropy generation rate increases for increasing tube diameter and collector inclination. Therminol-VP1 hybrid nanofluid and plate heat exchanger show the best performance among oil-based hybrid nanofluids and heat exchangers.

Solar thermal energy systems are future sustainable solutions for both domestic as well as industrial use. Solar thermal systems, operating in medium temperature range (373–673 K), require concentrated solar thermal heating (CSH). In this work, a comprehensive numerical tool is developed to design and study multipurpose on-sun CSH system. The model uses a combined Monte-Carlo ray tracing, finite difference method, and all heat transfer modes. The model is validated with in-house experiment, which demonstrates its predictive capability. Next, the tool is used to optimize the cavity receiver geometry and predict the performance of the optimized CSH system under different direct normal irradiance (DNI) conditions. A CSH system using Therminol D12 as heat transfer fluid (HTF) is presented. Therminol D12 HTF-based system is predicted to take longer time than the system using water as HTF, for heating water to a specified temperature because of the heat exchanger effectiveness. However, the designed CSH system using Therminol D12 can attain higher temperatures than water without pressurization and through the heat exchanger can be used as a multipurpose system suitable for cooking, laundry, sterilization, process industry, etc.