Hemispherical Cavity Research Articles

Experimental investigations for enhancing the performance of hemispherical cavity receivers using additively manufactured heat exchangers

ABSTRACT This research explores a pioneering approach in cavity-type solar parabolic dish collector technology (SPDCT) by introducing a novel, additive-manufactured thermally conductive polymer composite heat exchanger over a modified hemispherical cavity receiver for heat recovery applications. This manuscript investigates the performance of innovative configuration of SPDC for Wardha (442001), Maharashtra State (027), INDIA, 20.75° N, 78.65° E, modifying the traditional setup by employing copper coil with different arrangements. The unique aspect of this research lies in the implementation of a hemispherical cavity receiver with preheating water arrangement by installing a thermally conductive polymer composite (TCPC) heat exchanger developed with compositions TPU/MWCNT/GNP of 93/3.5/3.5 by wt.% using an additive manufacturing technique, a strategy not previously explored in the existing literature. A computational study (steady state thermal analysis) conducted using ANSYS Student Version R2023 revealed substantial heat availability over the receiver’s top exterior, forming the basis for the experimental tests. Under different test conditions, the highest thermal efficiency improvement reached to 10.91%. This paper contributes novel ideas to the field, emphasizing the potential of additive-manufactured thermally conductive polymer heat exchangers to increase the performance of solar parabolic dish collector systems.

Evaluation of thermo-fluidic performance of micro pulsating heat pipe with and without evaporator side surface structures

Interfacial behaviour and thermal performance of a 4-loop micro pulsating heat pipe are examined through experiments performed at different filling ratio (30–60 %), working fluid (acetone, ethanol, and methanol), and heat input (0–40 W). During the heat pipe operation, nucleation, growth and merging of bubbles, elongation, and movement of vapor plug, and dry out conditions are noticed upon ramp heating (0–40 W) inside smooth surface evaporator pulsating heat pipe. Experiments show that preparation and boiling inception lead to steady operation of heat pipe. Pulsating heat pipe showed minimum thermal resistance when operated with 40 % filling ratio and acetone as working fluid, which has low viscosity, surface tension and boiling point. Additionally, effect of surface structures (rectangular pillar protrusion and hemispherical dimple cavity) on the evaporator section is tested against base smooth surface micro pulsating heat pipe to understand augmentation of heat transfer. Hemispherical dimple cavities show better nucleation, bubble growth, and vapor plug elongation and thereby reduction in thermal resistance. On the other hand, rectangular pillar protrusions showed clogged bubble, least bubble detachment and vapor plug length in the adiabatic zone which results in higher thermal resistance than even smooth surface.

An Experiment-Based Variable Compensation Method to Improve the Geometric Accuracy of Sub-mm Features Fabricated by Stereolithography (SLA)

In this paper, we present an experimental procedure to enhance the dimensional accuracy of fabrication via stereolithography (SLA) of features at the sub-mm scale. Deviations in sub-mm hemispherical cavity diameters were detected and measured on customized samples by confocal microscopy. The characterization and experimental observations of samples allowed the identification of inaccuracy sources, mainly due to the laser beam scanning strategy and the incomplete removal of uncured liquid resin in post-processing (i.e., IPA washing). As a technology baseline, the measured dimensional errors on cavity diameters were up to −46%. A compensation method was defined and implemented, resulting in relevant improvements in dimensional accuracy. However, measurements on sub-mm cavities having different sizes revealed that a constant compensation parameter (i.e., C = 85, 96, 120 μm) is not fully effective at the sub-mm scale, where average errors remain at −24%, −18.8%, and −16% for compensations equal to 85, 96 and 120 μm, respectively. A further experimental campaign allowed the identification of an effective nonlinear compensation law where the compensation parameter depends on the sub-mm feature size C = f(D). Results show a sharp improvement in dimensional accuracy on sub-mm cavity fabrication, with errors consistently below +8.2%. The proposed method can be extended for the fabrication of any sub-mm features without restrictions on the specific technology implementation.

Cavity effects on spontaneous ignition of pressurized hydrogen jets

The release of pressurized hydrogen jets can lead to spontaneous ignition, which poses a major challenge to the safe utilization of hydrogen energy. Previous studies have primarily focused on the spontaneous ignition of pressurized hydrogen released into tubes. However, in real industrial scenarios, pressurized hydrogen is usually released into the atmosphere or a complex equipment environment. Therefore, the effect of obstacles on spontaneous ignition needs to be considered. In this study, the spontaneous ignition of sudden releases of pressurized hydrogen entering various cavity shapes was modeled using the standard k-ω turbulence model, the eddy dissipation concept (EDC) model and a detailed 21-step chemistry model. The effects of the cavities on the jet flow fields and the spontaneous ignition were analyzed for various cavity shapes and release pressures. For the pressures considered in this study, the cylindrical cavity did not lead to ignition. The highest hydrogen jet temperature occurred in the contact region due to the shock wave reflection, after which spontaneous ignition could not occur due to the flow divergence. The pressurized hydrogen entering hemispherical and conical cavities enhanced the hydrogen-air mixing which led to combustible regions and the formation of multidimensional shock waves, which significantly increased the spontaneous ignition probability. The cavities created a high-temperature region inside the cavity that experienced ignition but the flames did not spread out of the cavity. The conical cavity produced lower flame temperatures than the hemispherical cavity but prolonged the flame lifetime. Higher initial hydrogen release pressures resulted in more violent multidimensional shock wave interactions and higher contact surface temperatures, which lead to earlier ignition times but with little impact on the initial location of the spontaneous ignition. The results of this study provide a scientific foundation for hydrogen safety codes, safe equipment designs and the development of industry codes and standards.

Morphometrical parameters of acetabulum of hip bone - An osteological study

Background: Acetabulum is an approximately hemispherical cavity on the lateral aspect of the hip bone. Acetabulum is not always of the same shape, size, or depth. Any deviations of the parameters from normal range have strong correlation with the development of various kinds of pathologies. Aims and Objectives: Acetabulum is an approximately hemispherical cavity on the lateral aspect of the hip bone. Acetabulum is not always of the same shape, size or depth. Any deviations of the parameters from normal range have strong correlation with the development of various kinds of pathologies. Materials and Methods: This study was conducted in department of Anatomy, Yenepoya Medical College, Mangalore. Two hundred hip bones were included in the study. Various parameters of the acetabulum were measured. The morphometrical parameters measured were: (a) Transverse diameter of acetabulum, (b) acetabular height, (c) acetabular depth, (d) acetabular volume, (e) distance from pubic tubercle to anterior rim of acetabulum, (f) ratio between distance from pubic tubercle to anterior rim of acetabulum and acetabular height. Range, mean, standard deviation, and standard error of mean were determined for each parameter. P value was also calculated to compare the parameters side wise. Results: The independent T test was done and the P value was calculated for each parameter. Acetabular volume showed a significant difference between left- and right-sided bones. Most of the parameters showed highly significant difference between male and female hip bones in both right side and left side. Conclusion: As total and partial hip replacements are common surgeries performed these days, knowledge of the dimensions of acetabulum will assist prosthetists to construct suitable prosthesis for Indian population. It would be also useful in the fields of anatomy, osteology, and anthropology.

Research on the Performance of Biodiesel using Diesel Engines with Different Combustion Chamber Shapes

The performance dynamics of diesel engines powered by biodiesel are examined in this study, with a focus on the impact of various combustion chamber configurations. Understanding how diverse combustion chamber designs impact engine performance, emissions profiles, and combustion efficiency under varied operating situations is the main goal. The study used a 25% blend of Corn Oil Methyl Ester (COME25) biodiesel to test two types of engines: a Toroidal Cavity Piston (TCP) engine and a regular Hemispherical Cavity Piston (HCP) engine. When compared to the COME25 HCP engine, the TCP engine exhibits a notable 2.14% gain in Brake Thermal Efficiency (BTE) at maximum power. This improvement is attributable to the TCP engine's capacity to produce more cylinder swirl and turbulence, which leads to more effective combustion. This efficiency improvement comes with trade-offs, too, since full-power operation results in a simultaneous 28% reduction in Carbon Monoxide (CO) and a 25% reduction in Hydrocarbon (HC) emissions, but a 14% rise in nitrous oxide (NOx) emissions. These results demonstrate the intricate relationship between emissions control and combustion chamber shape. Along with the TCP engine and COME25 blend, the study also examines changes in Compression Ratio (CR). It finds that optimising both CR and combustion chamber geometry can result in a 9.6% decrease in Brake-Specific Fuel Consumption (BSFC) and a 2.31% rise in BTE. This illustrates the possible advantages of optimising engine settings to increase performance. The study offers insightful information for optimising diesel engines using biodiesel mixes, presenting a viable path for raising combustion efficiency and lowering emissions.

Influence of High-Intensity Pumping on Gain Medium Temperature Increase and Laser Mode Tunability in a Hemispherical Short Cavity

To increase the efficiency of laser oscillators by overcoming losses, the unsaturated gain must be increased. For this purpose, high-intensity pumping, typically higher than 100 kW/cm2, is effective. However, the temperature increase and strong thermo-optic effects of the gain medium have been pointed out as obstacles to high efficiency in solid-state lasers. Therefore, the effect of high pump intensity on the laser mode tunability required for high efficiency is investigated by studying the dependence of the laser threshold on the cavity length using a continuous-wave hemispherical short-cavity laser. The results show that the laser mode can be tuned with sufficient range and precision for high efficiency under various loss conditions regardless of the high pump intensity and are in qualitative agreement with a simple theory. Furthermore, according to the heat transport theory, microchip Yb:YAG, the gain medium of this study, does not have a high cooling efficiency, but the maximum temperature increase is estimated to be only about 12 K despite the high pump intensity of about 110 kW/cm2. This is because it is the pump power, not the pump intensity, that is proportional to the temperature increase, as the maximum pump power is only 900 mW. These results indicate that high-intensity pumping is a promising approach to achieve efficient lasing at low cost.

Role of Micro-Architectures on Insects' Elytra Affects the Nanomechanical and Optical Properties: Inspired for Designing the Lightweight Materials

The structural coloration of insects has received significant attention in the field of nanophotonic, which has related properties with biophotonic crystals. The main focus in this study is on structural and mechanical features of wasp and beetle elytra such as Chrysidinae (the specimen of parasitoid or kleptoparasitic wasp), Rhomborhina gigantea (RG is the specimen of bettle in the family of Cetoniidae), and Chrysolina coerulans (CC is a species of beetle in the family Chrysomelidae). The nanostructures present on the surface of elytra show the optical and deformable material properties. Surface morphology has been measured by scanning electron microscopy and atomic force microscopy (AFM). The depths of the hemispherical cavities present on the elytra have been analyzed using AFM. The optical and nanomechanical properties of the beetle elytra have also been studied. To measure the reflectivity in transverse electric and TM mode, UV-visible spectroscopy has been used. The value of the average index of refraction nav is observed as 1.8 in all three species and the half-pitch of elytra's varies from 0.139-0.136 μm in CC, 0.123-0.114 μm in Chrysidinae, and 0.154-0.161 μm in RG. Nanoindentation was used to measure the modulus and hardness of the different beetle's elytra. Higher value of elastic modulus (1.72 GPa) and hardness (0.89 GPa) in RG of elytra plays a significant role in resisting force. The photonic structure exhibits optical intrinsic color mixing properties, which are used in the anticounterfeiting fields (currency and passport identification codes). The structural properties of beetle elytra would help to design the lightweight, high strength, anti-coating sensor, color-changing micro air vehicles, and novel optical materials.

Integration of a high finesse cryogenic build-up cavity with an ion trap

We report on the realization of a hemispherical optical cavity with a finesse of F = 13 000 and sustaining inter-cavity powers of 10 kW, which we operate in a closed-cycle cryostat vacuum system close to 4 K. This was designed and built with an integrated radio-frequency Paul trap in order to combine optical and radio-frequency trapping. The cavity provides a power build-up factor of 2300. We describe a number of aspects of the system's design and operation, including low-vibration mounting and locking and thermal effects at high powers. Thermal self-locking in the high intracavity power regime was observed to enhance passive stability below 1 kHz. Observations made over repeated cool-downs over the course of a year show a repeatable shift between the ion trap center and the cavity mode.

Experimental study of vortex ring impingement on concave hemispherical cavities

Discrete vortex rings impinging on concave hemispherical cavities were explored experimentally. Planar laser-induced fluorescence, two-dimensional particle image velocimetry and flow visualization techniques were employed. Five different ratios of vortex ring to hemisphere cavity radius ( $\gamma$ ) were investigated, namely, $\gamma = 1/4,1/3,2/5,$ $1/2, 2/3$ . For $\gamma = 1/4,1/3, 2/5$ , the geometric confinement of the primary ring due to the hemispherical cavity induced loop-like instabilities in the secondary ring, which led to head-on collision and ejection of the looped ends as they orbited the primary ring. As the hemispherical cavity decreased in diameter (increasing $\gamma$ ), the dynamics were altered significantly due to the increased generation of vorticity along the edge of the hemisphere. For $\gamma = 1/2$ , vorticity produced at the edge/lip of the hemisphere ultimately disrupted the classical formation of a secondary vortex ring from the wall-bounded vorticity. For $\gamma = 2/3$ , the primary ring and hemisphere radius were close enough in size that the interaction was dominated by direct impact of the primary ring with the lip of the cavity. The primary vortex ring produced a vortex ring at the lip of the hemisphere that ultimately separated from the cavity, orbited around the primary ring, and then self-advected in the direction opposite to the primary vortex ring trajectory. A detailed investigation of the dynamics provided.

Hemispherical Cavity Cobalt Oxyhydroxides as Oxygen Evolution Electrocatalysts Electrodeposited in Borate-buffered Solution Using a Polystyrene Colloidal Template Method

Cobalt oxyhydroxide (CoOOH) electrocatalysts with a hemispherical cavity structure were electrodeposited in borate-buffered solution using a polystyrene colloidal template method. The hemispherical-cavity catalysts exhibited higher activity for the oxygen evolution reaction than the corresponding flat thin-film catalysts. Analysis by operando Co K-edge X-ray absorption spectroscopy revealed that, under the electrode working conditions, the local structure of the Co species was composed of nanosized CoOOH clusters with high-oxidation-number Co species. In addition, a B K-edge X-ray absorption spectroscopic analysis showed that borate anions were adsorbed onto the catalysts, stabilizing the catalyst structure. Thus, the highly oxidized CoOOH clusters with adsorbed borate anions are likely to function as active sites, possibly because the clusters are generated around anisotropic Co sites in the catalysts with hemispherical cavities. Cobalt oxyhydroxide (CoOOH) electrocatalysts with a hemispherical cavity structure were electrodeposited in borate-buffered solution using a polystyrene colloidal template method. The hemispherical-cavity catalysts exhibited higher activity for the oxygen evolution reaction than the corresponding flat thin-film catalysts.

A numerical investigation on thermal analysis of RPC based solar thermochemical reactor for two-step H2O splitting cycle for hydrogen production

Ceria based solar thermochemical cycle is a high-temperature based redox chemical reactions to split H2O or CO2to produce hydrogen and/or syngas. The redox reactions are carried out in a reactor cavity thus the analysis and optimization of design as well as thermal analysis is a crucial factor to improve the solar-to-fuel conversion efficiency. This paper proposes the hybrid design of cylindrical and hemispherical cavity and its effects of geometrical parame-ters such as reticulated porous ceria (RPC) thickness (15 mm, 20 mm, and 25 mm) and gas flow gap (5 mm&10 mm) on temperature and flux distribution and solar-to-fuel efficiency for both steady-state and transient condition. A numerical computational fluid dynamics (CFD) analysis is carried out to study heat and mass transfer as well as geometrical design consid-eration of the STCR cavity under SolTrace generated Gaussian distributed concentrated solar flux. Two-step water-splitting reaction in the Solar Thermochemical cavity reactor (STCR) using ceria (CeO2) has been modeled to explore the oxygen evolution/reaction rate and to estimate solar-to-fuel efficiency and its relationship with geometrical factors. The RPC of 25 mm thickness yields the highest oxygen evolution rate of 0.34 mL/min/gCeO2 and solar-to-fuel efficiencies are 7.82%, 12.07% and 16.18% for 15 mm, 20 mm and 25 mm of RPC thickness, respectively without heat recovery. The operating conditions and optimized geometric factors, based on result analysis and comparison, are discussed in detail.

Experimental and numerical investigations on the explosions nearby a free surface from both sides

The explosion near a free surface is a complex multiphase flow problem involving large deformation and even fragmentation of the fluid interface. Experiments and numerical simulations are presented to investigate the transient fluid dynamics of explosions approaching the free surface from the air and water sides. The numerical model is established based on the Eulerian finite element method and the Volume of Fluid method. The explosion experiments are conducted in a water tank. We classify the problem into cavities and bubbles according to whether the explosive is exposed to the air. Explosions above the free surface generate hemispherical cavities that evolve into W-shaped ones after an upward jet develops from the bottom. The hemispherical cavity will close if the explosion is close enough to the free surface. The simulations reveal that the distance parameter dominates the bubble’s bursting and that the explosive equivalent and distance parameter compete to determine the cavity’s closure. In addition, the impact pressure characteristics generated by the explosion near the free surface are also analyzed.

Numerical analysis of combined free convection and radiation heat transfer from an open hemispherical cavity

The present work aims to numerical study the combined effect of free convection and surface radiation from an open hemispherical cavity and the estimation of cooling time using ANSYS-Fluent 15.0. This study warrants a significant novelty. Various parameters, such as Rayleigh numbers (in the range of to ), surface temperatures, and the radius of curvatures varied to elucidate the effect of these parameters on the heat transfer characteristics considering surface radiation. The temperature contours and velocity vectors are obtained pictorially to understand the thermo-fluid characteristics around the hemispherical surface better. For all cases, the radiative heat transfer rate is found to be higher than the convective heat transfer rate for a given condition. The estimation of cooling time is done using lumped system analysis, taking both convection and radiation into account. It is observed that the cooling time estimated with radiation is much lower than that without radiation (only convection). This signifies that the radiation effect should be considered for similar problems. Finally, for the open hemispherical cavity, a correlation for the Nusselt number is developed by incorporating the regression analysis of the available data.

Рельефные формы на поверхности космогенных сферул как признак их происхождения

When studying the material composition of sedimentary formations, the problem of identifying extraterrestrial matter arises, in particular, distinguishing cosmic spherules from technogenic and volcanogenic ones. Based on the results of study of a collection of 482 volumetric spherules separated from ferromanganese crusts from the Pacific Ocean floor, an additional feature of such identification based on relief forms on the surface of spherules is proposed. The article gives a detailed description of such relief forms arising during the passage by cosmic particles through the upper layers of the Earth’s atmosphere, isolation and separation of the Fe-Ni cores from the spherules. The presence of a round hemispherical cavity on the flattened side of the spherule, a depression with concentric circles, or a «button» structure, as well as the presence of a Fe-Ni core is a sufficient but not necessary sign to consider a spherule cosmogenic.

Experimental Investigation on the Vector Characteristics of Concentrated Solar Radiation Flux Map

It is difficult to measure the concentrated solar radiation flux (CSRF) inside a cavity receiver directly due to the complex geometry of cavity receivers. At present, most reports measure the CSRF at the aperture plane (generally coinciding with the focal plane of solar concentrators) of the cavity receiver instead of at the absorber wall, which would result in serious aberration because of the obvious difference in configuration between the aperture plane and the absorber wall. To obtain the CSRF at the absorber wall with complex geometry, the vector characteristics (consisting of both directional distributions and spatial distributions) of the CSRF at the focal plane were measured using developed double water-cooled Lambertian targets together with a CCD (charge-coupled device) camera. Then the CSRF images at the absorber wall of a hemisphere cavity receiver were carried out by applying MCRTM (Monte Carlo ray-tracing method) in combination with the measured vector results of the CSRF at the aperture plane. Results show that the directional distributions of the CSRF at the aperture plane are rather nonhomogeneous along the zenith angle and the circumferential angle. The directional distribution performance of the CSRF at the focal plane plays an equally important role in the CSRF images of the cavity receivers. In addition, the relative error of the peak CSRF value of the cavity receiver between the uniform and the measured directional distribution cases is up to 16%. The conclusions provide an important reference for the development of the CSRF measurement.

Numerical modeling of the magneto convection of a Newtonian fluid confined between two vertically eccentric hemispheres

This work is a contribution to the numerical study of the phenomenon of heat transfer by laminar natural convection of an electrically conductive Newtonian fluid subjected to a uniform horizontal magnetic field. The study focused on a hemispherical cavity delimited by two vertically eccentric hemispheres. A constant flux density is imposed on the inner hemisphere while the outer hemisphere is maintained at a constant temperature. The combination of thermal and electrical boundary conditions is exploited to obtain the critical values of the parameters marking the onset of instability. The Boussinesq approximation is used to study the equations governing this fluid instability. The projection of these equations in the bispheric coordinate system as well as the discretization by the finite difference method facilitated the development of a computer code in Fortran. The exploitation of this code made it possible to determine the growth rates for Hartmann values equal to 1; 10 and 100, from Rayleigh equal to 103; 104; 105 and 106, with eccentricity equal to ± 0.2; ± 0.5 and 0 and a radius ratio equal to 2. The aim is to highlight the effect of the magnetic field on the heat transfer. At the end of the study, the results obtained are consistent and revealing: they are in good agreement with those of references drawn from the literature.

Theoretical and experimental investigations for geometrical error during hemispherical cavity machining on CO2 laser

CO2 laser material processing has become popular due to its unique advantages such as high productivity, ease of automation, non-contact processing, greater material utilization, and minimum heat-affected zone. Polymethylmethacrylate (PMMA) is found to be one of the most suitable thermoplastics for CO2 laser machining due to its higher melting point and higher absorptivity at the CO2 laser wavelength. The scope of the present work deals with geometrical error analysis during machining of the three-dimensional spherical cavity with the help of a 2D setup of the laser. The influence of process parameters like power, cutting speed, and the number of passes of laser probe on cusp height is examined. The cusp height is measured after machining equal and unequal step size on PMMA. The experiments were performed for profile controlled approach for equal step size and accuracy approach for unequal step size. An algorithm is proposed for predicting unequal step diameter with a constant value of cusp height. It was observed that higher power, lower speed, and more number of passes of laser probe leads to minimum cusp height. Hence, the proposed algorithm for predicting unequal step diameter for a constant cusp height leads to a reduction in staircase effect, thereby, producing a smoother curve as compared to the profile controlled (equal step diameter) approach.

Thermal Performance Improvement of Modified Hemispherical Cavity Receiver with Air Curtain

Heat losses from solar cavity receivers degrade the efficiency of solar parabolic dish collector system. Specifically, increased convection losses emulate a significant contribution in decreasing the performance of the modified solar cavity receiver and hence, reducing these convection losses has become a matter of great concern. The Air curtain plays a crucial role in minimizing the convection losses. This work proposes an air curtain-based modified cavity receiver wherein a plane nozzle is placed on the face of receiver to produce air curtain effect. Air curtain will prevent hot air from escaping the modified cavity receiver and increase the stagnation zone inside it to reduce natural convection on the surfaces. Numerical analyses are performed considering different air velocities and receiver orientations to evaluate the effect of air curtains on the convection losses. The impact of plane air nozzle parameters like outlet temperature, inclination, and outlet velocity of the nozzle on convection losses are shown and these parameters’ optimal value is evaluated. Our results state that the air curtain attached to the modified solar cavity receiver significantly improves performance by reducing the convection losses.

Nusselt Number Prediction for Oil and Water in Solar Tubular Cavity Receivers

In this study, a numerical model was developed for prediction of Nusselt number in solar cavity receivers. Thermal oil and water were used as the working fluid. A dish concentrator with different shapes of the cavity receiver, including hemispherical, cylindrical, and cubical, was investigated. The different shapes of cavity receiver were studied under the same operating conditions for prediction of the internal heat transfer coefficient correlation for each cavity receiver. The system is investigated under the variation of solar radiation, flow rate, and inlet temperature of solar working fluids. The developed thermal model is validated based on the experimental data for the cylindrical cavity receiver using thermal oil. The results reveal that the hemispherical cavity receiver had the highest cavity heat gain, heat transfer coefficient, and Nusselt number values compared to two other cavity receivers. It could be concluded that the cavity heat gain, and heat transfer coefficient, and Nusselt number amounts had improved with increasing solar radiation, increasing flow rate, and decreasing inlet temperature of the working fluid. Some equations were suggested for prediction of Nusselt number with the variation of solar radiation, flow rate of the working fluid, and inlet temperature of working. It was concluded that application of thermal oil had resulted in higher Nusselt numbers than the use of water as the solar working fluid. Consequently, the application of oil is suggested for high-temperature systems.