Cavity Heat Research Articles

Performance Assessment of a Solar Dryer System Using Small Parabolic Dish and Alumina/Oil Nanofluid: Simulation and Experimental Study

In this study, a small dish concentrator with a cylindrical cavity receiver was experimentally investigated as the heat source of a dryer. The system was examined for operation with pure thermal oil and Al2O3/oil nanofluid as the working fluids in the solar system. Moreover, the design, the development, and the evaluation of the dried mint plant are presented in this work. Also, the solar dryer system was simulated by the SolidWorks and ANSYS CFX software. On the other side, the color histogram of the wet and dried mint samples based on the RGB method was considered. The results revealed that the different temperatures of the solar working fluids at the inlet and outlet of the cavity receiver showed similar trend data compared to the variation of the solar radiation during the experimental test. Moreover, it is found that the cavity heat gain and thermal efficiency of the solar system was improved by using the nanofluid as the solar working fluid. Furthermore, the required time for mint drying had decreased by increasing the drying temperature and increasing air speed. The highest drying time was measured equal to 320 min for the condition of the air speed equal to 0.5 m/s and the drying temperature of 30 °C. A good agreement was observed between the calculated numerical results and measured experimental data. Finally, based on the color histogram of the wet and dried mint samples, it was concluded that intensity amount of the red color of the mint increased with the drying process compared to intensity amount of the red color of the wet mint sample.

Evaluation of local heating in Doppler-broadening thermometry based on cavity ring-down spectroscopy

We report on an optical and thermal model that allows one to quantify the local heating effect in a cavity ring-down spectroscopy (CRDS) experiment. The effect is due to the intracavity absorption of the laser radiation propagating inside the optical cavity, when this latter is filled with an absorbing gas. The local heating generates a temperature profile into the volume probed by the laser radiation, thus leading to a systematical deviation in temperature measurements by means of Doppler-broadening gas thermometry. Exploiting the representation of the cavity as a linear system for the electric field in the frequency domain, we were able to determine the spatial and temporal behaviors of the heating source and solve the inhomogeneous Fourier heat transfer equation using the method of the Green's functions. The local heating effect is strongly dependent on several parameters characterizing the CRDS experiment, including the geometrical size of the cavity, the two mirrors' reflectivity and loss, the absorption coefficient of the intracavity gaseous medium, the incident power, and the intracavity beam waist. The dynamical behavior of the gas temperature was calculated in three experimental regimes, depending on the laser switch-on time interval as compared to the cavity decay time and heat diffusion characteristic time. In the worst case, the local heating provides a systematical shift in Doppler broadening thermometry of about one part per million.

THERMAL EVALUATION OF CAVITY RECEIVER USING WATER/PG AS THE SOLAR WORKING FLUID

In this study, a parabolic dish concentrator with a cavity receiver was investigated. Water/ Propylene Glycol (PG) was used as the solar heat transfer fluid. Thermal numerical modelling was developed for prediction of the cavity receiver performance. The water/PG in different volume fractions (VF) of the PG was examined consist of 0%, 25%, 50%, and 55%. The working fluid inlet temperature is investigated in ranging 0oC to 100oC. The results revealed that the thermal efficiency and the cavity heat gain decreased by increasing the GP volume fraction. The pressure drop and pumping work demand decreased by increasing the working fluid inlet temperature as well as decreasing the PG volume fraction in the pure water. Consequently, the pure water had the lowest amount of the pressure drop among the investigated working fluids. The cavity surface temperature increased by increasing the working fluid inlet temperature as well as increasing the PG volume fraction in the pure water. Consequently, the application of the higher amount of PG is recommended for the Bryton cycle.

Investigation of Exergy of Double-Pipe Heat Exchanger Using Synthesized Hybrid Nanofluid Developed by Modeling

Synthesized copper oxide nanoparticles were incorporated on the surfaces of carbon nanotubes using ultrasonication condition and incipient wetness impregnation technique (IWI). Morphological surfaces and structures of the prepared nanocomposite were investigated by the Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD). Results showed a modification of thermophysical properties of prepared hybrid nanofluids compared to untreated carbon nanotube-water. Four parameters (Reynolds number, nanofluid volume fraction, twisted (pitch) ratio, and cavity diameter ratio) were investigated, and their impacts on the exergy efficiency enhancement of double-pipe heat exchanger were determined. Modifying response surface methodology-central composite design (RSM-CCD) to realize the optimal exergy efficiency of the system (98.4 %) showed the possibility to improve all control variables contemporarily and with significant accuracy. There was a strong correlation between the experimental and predicted values by Artificial Neural Network-Genetic Algorithm (ANN-GA) (R2 = 0.92). Minimum mean square error (MSE) was found using a three-layer ANN with 10 neurons in the hidden layer. The results show that the increased Reynolds number and nanofluid volume fraction lead to increases in the heat transfer and exergy efficiency of the system. However, while adding pitches and cavities of the spiral strip to heat exchanger increases heat transfer due to the increasing turbulence flow, at larger pitches and cavity numbers, heat transfer and exergy efficiency were reduced. This was confirmed using two computer-modeling approaches which allow optimization of the exergy of the system.

Modelling and performance evaluation of an integrated receiver-storage for concentrating solar power beam-down system under heterogeneous radiative conditions

An integrated receiver-storage (IRS) system for a concentrating solar power (CSP) beam-down system is analyzed for optimal performance and to enable efficient round-the-clock operation. For this purpose, a new in-house programme based on a transient 2-D simulation model coupling a cavity receiver and heat storage together was developed. The programme employs the Matlab® software and it was successfully validated against previous simulation results. The IRS-system was analyzed under realistic radiative boundary conditions. Charging and discharging processes considering radial and axial heat transfer were simulated. It was found that the impact of differences in the heterogeneous radiative boundary conditions on the system efficiencies were limited. The differences between the 2-D coupled model and a decoupled 1-D thermal models were at largest in the start-up of the IRS with a relative error of 5.6%, but the differences smoothened out over time. Analyzing the overall efficiency of the IRS system indicated that the performance of IRS is very good and the IRS could well be applicable for beam-down CSP. Charging and discharging efficiencies of 99% and 93% and a solar-to-exergy conversion ratio of 0.53 could be reached.

Natural Convection of Non-Newtonian Power-Law Fluid in a Square Cavity with a Heat-Generating Element

Development of modern technology in microelectronics and power engineering necessitates the creation of effective cooling systems. This is made possible by the use of the special fins technology within the cavity or special heat transfer liquids in order to intensify the heat removal from the heat-generating elements. The present work is devoted to the mathematical modeling of thermogravitational convection of a non-Newtonian fluid in a closed square cavity with a local source of internal volumetric heat generation. The behavior of the fluid is described by the Ostwald-de Waele power law model. The defining Navier–Stokes equations written using the dimensionless stream function, vorticity and temperature are solved using the finite difference method. The effects of the Rayleigh number, power-law index, and thermal conductivity ratio on heat transfer and the flow structure are studied. The obtained results are presented in the form of isolines of the stream function and temperature, as well as the dependences of the average Nusselt number and average temperature on the governing parameters.

Prediction and parametric analysis of cavity growth for the underground coal gasification project Thar

Underground coal gasification (UCG) is a promising clean coal technology to convert unmineable and deep coal reserves into syngas, which can be used in many industrial applications. In UCG field, real time monitoring of hydrological and geological conditions such as water influx rate, cavity growth and its interaction with overburden is a formidable task. UCG project Thar (UPT) lacks real time data acquisition system to monitor these parameters. In this work, a 3D axisymmetric cavity simulation model (CAVSIM) is parameterized with operating conditions of UPT and properties of Lignite B coal of Thar coal fields. For model validation, a comparison has been made between simulated and the UPT field data for the composition and heating value of syngas. The results of CAVSIM are also compared with our previous ID packed bed model, which show the superiority of CAVSIM model. Moreover, a comprehensive simulation study has been carried out to predict the cavity growth and its interaction with overburden. The effect of operating parameters of UPT on volumetric cavity growth and heating value of syngas are also investigated.

Design and analysis of thermo-structure for cryogenic radiometer

Absolute cryogenic radiometer is built based on a new theory of electrical-substitution measurement, which is for measuring the radiant power by using the equivalent electrical power and has recently served as a primary standard for radiant power measurements. This study aims to design and implement a cryogenic radiometers to measure the optical power in a range from <inline-formula><tex-math id="M1">\begin{document}$0.1\;{\text{μ}}{\rm{W}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181880_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="6-20181880_M1.png"/></alternatives></inline-formula> to 2 mW, which can substitute for the imported products. Intensive experiments are performed to study the thermal circuit of cryogenic radiometer, and systematically analyze the influences of cavity assembly and heat link materials on the responsivity and thermal time constant of cryogenic radiometer. On this basis, the thermo-structure mechanical parts are developed, which are comprised of a blackbody cavity, heat link and heat sink. Both the heat sink and the blackbody cavity are made of OFHC copper that is plated with gold. All surfaces are highly polished and reflective to reduce any radiative effects. The absorptance of the cavity can reach up to 0.999995 at 633 nm. And then, a characteristic parameters’ test system of cryogenic radiometer is built. Through optimizing the temperature control system and improving the design of the heat sink, the standard deviation of the heat sink can be kept under 0.2 mK for 30 min. By using that test system, the responsivity and thermal time constant of cryogenic radiometer with four different kinds of heat link materials (OHFC copper, 6061 Al, SS304 stainless steel, and polyimide) are tested experimentally. The experimental results show that the responsivity and thermal time constant are 35.5 K/W and 23 s for OHFC copper, 318.9 K/W and 106 s for 6061 Al, 434.8 K/W and 297 s for SS304 stainless steel, 714.8 K/W and 506 s for polyimide. As the thermal conductivity of heat link material changes, the two parameters of responsivity and thermal time constant will simultaneously change significantly. The responsivity and thermal time constant are a pair of mutually constrained parameters, and temperature stability is an important parameter for designing the thermo-structure. After increasing the responsivity, it will not only significantly increase the measurement time and resource consumption, but also affect the temperature control stability, and hence limiting the measurement accuracy. All the test data indicate that the characteristic parameter of cryogenic radiometer can be adjusted by changing the material and structure of heat link. The obtained results will have a certain reference value for the index distribution of cryogenic radiometer characteristic parameters and designing the next generation of absolute cryogenic radiometers.

Numerical modeling of heat transfer through the air interlayer considering the surface radiation

Current numerical models of façade’s cladding do not account for ventilated air cavity heat resistance. The major problem in assigning its properties is the complexity of the air thermal conductivity coefficient calculation inside a ventilated air cavity. National standards and building codes provide design methods for calculating thermal resistance values for non-ventilated air layers depending on their thickness and location in the structure. The procedure of ventilated air cavity thermal design is not correct when its thermal conductivity coefficient is used as for a non-ventilated interlayer. In order to overcome the lack in the adequate design method, an approach utilizing both the finite element and analytical solutions of the heat exchange equation has been proposed in the present paper. In doing so, the formula derived for calculating the equivalent value of the air thermal conductivity in a ventilated air layer takes into account the heat exchange by radiation as a design value.

Novel wavelet-homotopy Galerkin technique for analysis of lid-driven cavity flow and heat transfer with non-uniform boundary conditions

In this paper, a brand-new wavelet-homotopy Galerkin technique is developed to solve nonlinear ordinary or partial differential equations. Before this investigation, few studies have been done for handling nonlinear problems with non-uniform boundary conditions by means of the wavelet Galerkin technique, especially in the field of fluid mechanics and heat transfer. The lid-driven cavity flow and heat transfer are illustrated as a typical example to verify the validity and correctness of this proposed technique. The cavity is subject to the upper and lower walls’ motions in the same or opposite directions. The inclined angle of the square cavity is from 0 to π/2. Four different modes including uniform, linear, exponential, and sinusoidal heating are considered on the top and bottom walls, respectively, while the left and right walls are thermally isolated and stationary. A parametric analysis of heating distribution between upper and lower walls including the amplitude ratio from 0 to 1 and the phase deviation from 0 to 2π is conducted. The governing equations are non-dimensionalized in terms of the stream function-vorticity formulation and the temperature distribution function and then solved analytically subject to various boundary conditions. Comparisons with previous publications are given, showing high efficiency and great feasibility of the proposed technique.

Simulation of nanofluid natural convection in a U-shaped cavity equipped by a heating obstacle: Effect of cavity's aspect ratio

Abstract Research on nanofluid for heat transfer enhancement of thermal systems has received great attention owing to the lack of energy sources. In this study, fluid flow and natural convection heat transfer of Al2O3–Water or TiO2–water nanofluid inside a U-shaped cavity consist of a hot obstacle has been investigated numerically by lattice Boltzmann method (LBM). In this paper, different parameters are investigated such as Rayleigh number, the solid volume fraction of the nanoparticles, the U-shaped cavity's aspect ratio and heating obstacle's height on the flow field and heat transfer in the enclosure. The results showed that the Rayleigh number (Ra), cavity aspect ratio (AR) and obstacle's height can be affected on isotherms, streamlines and local and average Nusselt number. The average Nusselt number of the obstacle sides increased by increasing the Ra number and solid volume fraction of nanoparticles (ϕ) regardless the AR. In addition, by increasing the AR, the average Nusselt number increased. At low Ra, the effect of nanoparticles on increment of heat transfer for narrow cavities was more than wide ones.

Numerical simulation for the investment casting process of a large-size titanium alloy thin-wall casing

To optimize the investment casting process when producing high quality large-size titanium alloy thin-wall components is a time-consuming job due to the complicated metallurgical process. Numerical simulation is a high-efficiency method compared with trial and error, and therefore is introduced to the investment casting process optimization to shorten the new product development cycle and reduce the production cost. In this study, weakly compressible model (WCM) and ununiformed finite difference mesh (UFDM) was developed to reduce the memory consumption and ensure the simulation efficiency. The precision of the WCM and UFDM were verified by numerical simulation of cavity heat convection in a square cavity and hydraulics simulation of centrifugal filling in a transparent cavity. The numerical simulation of the investment casting process of a titanium alloy thin-wall casing under different process conditions was accomplished using a self-developed software, and the distribution characteristics of potential shrinkage defects were predicted. It was found that the predicted defects in the titanium alloy casing matched well with the actual X-ray experimental results. For the components investigated in this paper, more numerical simulation results show that the centrifugal casting process with respect to gravity casting had no obvious improvements in the concentrated shrinkage defects, and the gravity casting process can be more reasonable from the engineering point of view.

GMDH modeling and experimental investigation of thermal performance enhancement of hemispherical cavity receiver using MWCNT/oil nanofluid

Nowadays, nanofluids are introduced as an effective technique for enhancing the thermal efficiency in solar collectors. In this work, the MWCNT/oil nanofluid with 0.8% nanoparticle mass fraction is investigated experimentally in a solar dish concentrator coupled to a hemispherical cavity receiver. The main objective of this research is to examine the effect of the nanofluid application for improving the solar thermal performance of the hemispherical cavity receiver. The results revealed that thermal efficiency of the hemispherical cavity receiver has averagely increased equal to 12.93%, using application of the MWCNT/oil nanofluid compared to the application of the basefluid. In the steady state periods, the average thermal efficiency of 76.23% and 67.50% were calculated for the hemispherical cavity receiver using the MWCNT/oil nanofluid and pure oil, respectively. It was conducted that the receiver heats gain and the thermal efficiency of the cavity receiver has a similar trend with the working fluid difference temperature. Two experimental models were suggested based on the overall performance versus Tin-TambIbeam for the MWCNT/oil nanofluid and the pure oil. Finally, it is found that the use of the examined nanofluid is able to enhance the thermal performance of the examined collector about 13%. Also, in this study, group method of data handling (GMDH) artificial neural networks is applied in order to model the dependency of thermal efficiency and the cavity heat gain of the Hemispherical Cavity Receiver on the mentioned factors.

Heat dissipation characteristics from photovoltaic cells within the partitioned or non-partitioned glazed cavity to the windy environment

Abstract A numerical model, whose computation domain contains the glazed shallow cavity and surrounding wind field, has been proposed to investigate the characteristics of heat dissipation from photovoltaic cells within the glazed cavity to the windy environment. To reflect the actual working characteristics, both the photoelectric conversion characteristic and surface radiation were considered. The temperature distribution of PV cells particularly the hot spot temperature has been displayed to reveal the effect of tilt angle (α) and wind velocity (v). Then a thorough analysis was presented in virtue of visualized results, convection Nusselt number inside the cavity (Nucin) and total heat loss coefficient (Ut). Meanwhile, efforts were also performed to compare the partitioned and non-partitioned cavities. Results show that hot spot position at α = 15° has the obvious diversity due to the variation of wind velocity. Unlike the increased Nucin, total heat loss coefficient of PV cells at v = 5 m/s decreases when the tilt angle shifts from 15° to 30°. Further, it is found that hot spot temperature difference between the three cells shrinks in the partitioned cavity, and the partition indeed contributes to the improvement of PV cells temperature uniformity, which plays a positive role in the protection of PV cell property.

Receding horizon H∞ guaranteed cost tracking control for microwave heating medium with temperature-dependent permittivity

This paper considers the temperature spectrum tracking control of microwave heating model, in the presence of asymmetrical input saturation, nonhomogeneous Neumann boundary condition and temperature-dependent permittivity. The sufficient condition for the existence of receding horizon H∞ guaranteed cost control is proposed based on the derived finite-dimensional ordinary differential equation (ODE) error model. Furthermore, by on-line updating and solving linear matrix inequalities (LMIs) optimization problem, the constrained tracking controller can be obtained in the sense of minimizing H∞ norm and satisfying the quadratic cost performance. The proposed control strategy is implemented on a one-dimensional cavity heating model and its performance is evaluated through the simulation.

Cooling concentrator photovoltaic systems using various configurations of phase-change material heat sinks

A new hybrid concentrator photovoltaic-phase change material system is developed to attain rapid thermal dissipation by enhancing the typically low thermal conductivity of phase change materials. The developed system includes four different configurations of phase change material heat sinks: single cavity, three-parallel cavity, five-parallel cavity, and three-series cavity configuration. Furthermore, nine different pattern arrangements of phase change materials are studied. A comprehensive two-dimension model of photovoltaic layers integrated with phase change material heat sink is developed to predict the transient temperature variation at different concentration ratios of 10 and 20. The model is numerically simulated and validated with the available experimental data; the heat sink configurations with three and five parallel cavities are found to significantly reduce the solar cell temperature compared to the single cavity and three-series cavity heat sink configurations. Furthermore, the use of a five-parallel cavity heat sink is found to greatly enhance temperature uniformity of the solar cell. Substantial enhancement in temperature uniformity is also observed with different pattern arrangements of the phase change materials using the three-parallel cavity configuration. These findings can help identify the optimal configuration of heat sinks and pattern arrangements of phase change materials in order to achieve higher performance with concentrator photovoltaic systems.

LATTICE BOLTZMANN SIMULATION OF FREE CONVECTION IN AN INCLINED OPEN-ENDED CAVITY PARTIALLY FILLED WITH FIBROUS POROUS MEDIA

The present study deals with natural convection flow in an open-ended cavity partially filled with fibrous porous media. The numerical investigations are performed using the lattice Boltzmann method. The considered porous media are formed of random arrangements of square obstacles and simulated in pore scale. The effects of some dominant variables, such as the Rayleigh number, angle of inclination, porosity, and eccentricity of the porous media on the flow field and the cavity heat transfer, are investigated. Results are reported in terms of isotherms, streamlines, and average Nusselt numbers. The numerical results obtained indicate that appropriate use of porous media can enhance the rate of heat transfer. Also, increasing the inclination angle at high Rayleigh number or decreasing the porosity at low Rayleigh number increases the average Nusselt number. Comprehensive discussions on the effectiveness of the porous cavity compared to a nonporous one, in different situations, are also provided.

Study on stabilities, thermophysical properties and natural convective heat transfer characteristics of TiO2-water nanofluids

TiO2-water nanofluids with different mass fractions (ω = 0.1 wt%, ω = 0.3 wt% and ω = 0.5 wt%) are prepared respectively, and the stabilities are studied by scanning electron microscope, transmission electron microscope, dynamic analysis method and settlement observation method. Additionally, thermophysical properties of nanofluids are discussed, and models of thermophysical properties are deduced. Then, an experimental installation and a two-phase lattice Boltzmann model for natural convection heat transfer are established in this paper and the effects of cavity ratio, heating power and nanoparticle mass fraction on heat transfer are discussed respectively. It can be obtained that the thermal conductivities of TiO2-water nanofluids can be improved by 5.23% to the utmost extent. However, the heat transfer can be enhanced by 34.2% in the maximum with the increase of nanoparticle mass fraction at the lowest heating power and the largest cavity ratio.

A Hot-Polymer Fiber Fabry-Perot Interferometer Anemometer for Sensing Airflow.

This work proposes the first hot-polymer fiber Fabry–Perot interferometer (HPFFPI) anemometer for sensing airflow. The proposed HPFFPI is based on a single-mode fiber (SMF) endface that is attached to a UV-cured polymer to form an ultracompact fiber Fabry–Perot microcavity. The proposed polymer microcavity was heated using a low-cost chip resistor with a controllable dc driving power to achieve a desired polymer’s steady-state temperature (T) that exceeds the T of the surrounding environment. The polymer is highly sensitive to variations of T with high repeatability. When the hot polymer was cooled by the measured flowing air, the wavelength fringes of its optical spectra shifted. The HPFFPI anemometers have been experimentally evaluated for different cavity lengths and heating power values. Experimental results demonstrate that the proposed HPFFPI responses well in terms of airflow measurement. A high sensitivity of 1.139 nm/(m/s) and a good resolution of 0.0088 m/s over the 0~2.54 m/s range of airflow were achieved with a cavity length of 10 μm and a heating power of 0.402 W.

Numerical and experimental investigation of wind effect on a hemispherical cavity receiver

In this study, a parabolic dish concentrator with a hemispherical cavity receiver was investigated under windy weather conditions. In the first section, a numerical analysis was carried out to determine an optimum structural parameter of the hemispherical cavity receiver. Two equations were numerically determined for prediction of the optimum aperture area of the cavity receiver and the maximum receiver thermal efficiency versus the optical errors in the range of 5mrad to 35mrad. In the second section, the optimized hemispherical cavity receiver was experimentally constructed, and the experimental tests were carried out in a real weather condition. The effect of the wind speed on the cavity heat losses was experimentally investigated. A correlation between the heat losses, due to the wind, and the wind speed was experimentally estimated in form of the Q̇loss,wind=59.545×Vwind+76.724. The correlation was obtained for the investigated hemispherical cavity receiver with R2=0.9477 for the side wall direction of the wind. The cavity wall temperatures show an inverse behavior with the wind speed. Also, the cavity top wall temperatures indicate higher amount than the cavity side wall temperatures. Finally, the effect of the wind speed was considered on the energy performance of the hemispherical cavity receiver. The results indicate that the wind is an effective parameter in the energy performance of the hemispherical cavity receiver. Therefore, the application of the glass cover of the hemispherical cavity receiver will be recommended.