Experimental Temperature Profiles Research Articles

On the behaviour of the temperature field around pool fires in controlled experiment and numerical modelling

This paper attempts to evaluate how the Fire Dynamics Simulator (FDS) correlates with experimental flame heights and temperature profiles around a small scale pool fire. Experimental studies were conducted in a 6 cm diameter burner inside an exhaustion hood with three liquid fuels largely used in industry: commercial hexane, hydrous ethanol and type C gasoline. The simulated fuels were defined as mixture of molecules from FDS fuel library. The burning rates were prescribed in the calculation routine and compared with the results when the burning rate is predicted by the simulator. FDS predicts flame heights with errors below 16% when the burning rate is prescribed as boundary condition. Good agreement with experimental data is observed for a given number of probe locations. However, FDS struggled to describe the temperature profile around the flame area in the region close to the pool rim, between X/D=0.25 and X/D=0.75, and with heights below 5.0 diameters.

Comparison between Theoretical and Experimental Radial Electron Temperature Profiles in a Low Density Weakly Ionized Plasma

Experimental and theoretical studies of the radial distribution function of the electron temperature (RDFT) in a low-density plasma and weakly ionized gas for the abnormal glow region are presented. Experimentally, the electron temperatures and densities are measured by a Langmuir probe moved radially from the center to the edge of the cathode electrode for helium gas at different pressures in the low-pressure glow discharge. The comparison of the final experimental data for the radial distribution of electron temperatures and densities for different low pressures ranging from 0.2 to 1.2 torr, with the final proved equation of RDFT confirms that the electron temperatures decrease with increasing product of radial distance and gas pressures, showing a radial decrement dependence of the electron temperature from the center to the edge of the electrode. This is attributed to the increase of the number of electron-atom collisions at higher gas pressures and consequently of the rate of ionization. For the axial distance (L) from the tip of the probe to cathode electrode and the cathode electrode radius (R), a theoretical and experimental comparison for the two conditions L R, for both cases the produced plasma temperatures decrease and densities increase. It is concluded that the RDFT accurately shows a dramatic decrease for L R similar as for conditions of magnetized and unmagnetized effect for DC plasma. This means that the rate of plasma loss by diffusion decreased for L < R, agrees well with the applied of magnetic field behavior

Multi-Physics Modeling and Process Simulation for a Frequency-Shifted Solid-State Source Microwave Oven

The recent rapid growth of the semiconductor industry in development of high power microwave source has open a new promising avenue by full electronic control over the electromagnetic field towards improve the microwave heating performance. In this paper, a coupled electromagnetic and heat transfer model was built to simulate microwave heating of multiple chicken blocks with temperature-dependent dielectric properties dependent on a solid-state microwave oven. Owing to the ability of solid-state source to precisely control the frequency, the temperature distribution simulated for the fixed frequency was compared with different frequency-shifted rates increasing process and decreacsing process between 2.4 GHz and 2.5 GHz. The simulated temperature distribution was validated by comparing with experimental temperature profiles obtained using a thermal imaging camera at the end of microwave heating. The results demonstrated the heating uniformity and efficiency is highly influenced by the head and tail frequency but also the frequency shifted rate. And a specific frequency-shifted rate decreasing process between 2.4 GHz and 2.5 GHz was proposed in improving the heating performance.

Quantitative study of local heat sources by Ultrasonic Infrared Thermography: An approach for estimating total energy released by low energy impact damage in C/C composite

An approach intended for the evaluation of total energy released in multi-component defects subjected to ultrasonic stimulation is described. Experimental research was conducted on carbon/carbon composite containing a 25 J impact damage defect. The test sample was ultrasonically stimulated at the frequency of 22 kHz by using a magnetostrictive device. The impact damage was modelled as a pyramid-like set of six single delaminations, and each of them represented an independent heat source. The calculated temperature response was matched to the experimental temperature profile by shape and amplitude thus allowing the evaluation of energy release within the defect.

Modeling Microwave Heating of Frozen Chinese Fast Foods Based on Dielectric Properties

Abstract Chinese steamed bread with chicken, which is a typical Chinese fast food, is popular among consumers. The model simulated a frozen combination consisting of six chicken nuggets and two Chinese steamed breads with the dynamic change of dielectric properties on the turntable. The temperature-dependent dielectric and thermo-physical properties of chicken meat were measured as a function of temperature from −5°C to 95°C. A mathematical model of the dielectric properties, specific heat and thermal conductivity with temperature dependence was inputted in the model as the material parameters. 7.5 rpm was observed to be the optimal rotation speed. Simulated temperature profiles were compared with experimental temperature profiles after 90 s of microwave (MW) heating at 700 W. The simulated results were consistent with the corresponding experimental results. The developed model can be applied in food products to identify the cold and hot spots to ensure quality in terms of temperature uniformity.

Transient Self-Heating at Nanowire Junctions in Silver Nanowire Network Conductors

Nanostructured transparent conducting electrodes (TCEs) may be suitable replacements for transparent conductive oxides due to their optical, electrical, and mechanical properties. Because nanowire (NW) nanowire or tube–tube junctions are the transport bottlenecks in network-based conductors, understanding the properties of these junctions and their connectivity within the network is crucial to understanding and controlling electrical conduction through these networks. Quantifying local self-heating within the network can provide information on the coupled electrothermal response, local conduction pathways, and potential reliability. In this study, self-heating thermal transients within a silver NW network are characterized using high-resolution transient thermoreflectance imaging that provides high temporal (∼200 ns) and spatial (∼200 nm) resolution. The self-heating induced by an applied voltage pulse results in distinct temperature changes at microscopic hotspots formed at individual NW–NW junctions. For both heating and cooling cycles, thermal time constants less than $\text{1 }\mu {\text{s}}$ are observed at various hotspots. For a representative hotspot, line scans along two crossing NWs, taken at different time instants ranging from 0 to 2 μ s, show the temporal and spatial evolution of the temperature profile. We estimate the van der Waals force (∼−4.0244 N), contact width (∼5 nm), and interface thermal resistance (∼1.6 × 105 K/W) between NWs and the underlying substrate. A heat transfer model that considers local power generated at a hotspot, local coupling between the NWs and substrate, heat conduction along the NWs and heat transfer into the substrate, is developed and used to interpret the experimental data. The heat transfer model and experimental temperature profile help to quantify the local power generated at the hotspot and the fraction of this power propagating along each wire. The ability to resolve the local self-heating with such temporal and spatial resolution uniquely enables understanding of electrothermal response and current pathways in the distributed conductors.

Thermal analysis of a finned receiver for a central tower solar system

Abstract In this study, a thermal analysis of a finned receiver prototype for a thermosolar tower system is presented. The experimental system consists of parallelepiped aluminum enclosure of 1.2 m high, 1.23 m wide and 0.1 m depth. At the interior, 1232 cylindrical fins with a diameter of 0.0095 m (3/8″) and 0.09 m length increases the heat transfer area up to 225%. The vertical wall receives the incoming solar concentrated radiation from a group of heliostats whilst at the interior a constant flow of water removes the absorbed energy. Experimental temperature profiles were obtained at different heights and depths and a comparison was made with numerical results obtained with the use of commercial CFD software. It was found that the maximum thermal efficiency of the receiver was 94.4%, decreasing as the radiative flux increases.

Effect of pressure on the combustion of an aqueous urea and ammonium nitrate monofuel

Temperature profiles and effluent concentrations were measured during combustion of aqueous urea and ammonium nitrate at pressures of 1–15 MPa. Pollutant levels decreased, while the combustion temperature showed a non-monotonic change with increasing pressure. Experimental temperature profiles were applied in kinetic gas-phase simulations, and resulting species concentrations were in good agreement with experimental values. Sensitivity analyses indicated the kinetic parameters of isocyanic acid hydrolysis are the main source of uncertainty, possibly leading to the lower agreement observed for experimental and simulation carbon species. Rate of production analyses indicated that isocyanic acid is mainly consumed by hydrolysis to carbon dioxide, while nitric acid reacts with nitrous acid to produce water. Ammonia exhibited two channels of decomposition, reacting with either hydroxyl or nitrogen dioxide to form amidogen and either water or nitrous acid, respectively. Nitrogen was mainly formed by the three-body reaction of diazenyl. As pressure increased, the aforementioned pathways became increasingly dominant.

Simulation study of parameters influencing microwave heating of biomass

Experimental investigation of microwave-assisted pyrolysis was successfully done using a specially modified domestic microwave oven. At conditions where no parameterised experimental study was conducted (except temperature measurement), changing other parameters that influence the microwave heating can be investigated qualitatively in detail by means of simulation work. Therefore, a computer based simulation using Comsol Multiphysics software was applied not only to predict how the electromagnetic field distributes within the cavity but also to study different parameters that influence heating distribution inside the microwave oven. The simulation work was initially performed with verification between experimental and simulation temperature profiles at temperature settings of 500 °C and 800 °C and an agreement was achieved in terms of the temperature profile and heating behaviour of the biomass. The simulation work has proved that the inhomogeneity of temperature of the biomass is reflected by the local occurrence of hot spots and cold spots. The effect of different positions of the waveguide is remarkable where the bottom-fed microwave energy oven was shown to have a poor electric field distribution. However, when simulation was done on combining the effect of having the microwave energy fed from the bottom and the presence of the mode stirrer, the electric field was greatly improved with the heating distribution of the biomass resembling that obtained from the side-fed microwaves energy oven (usually refers to a common home microwave oven). The effect of having a mode stirrer rotating inside the microwave oven is also pronounced where the mode stirrer acts to stir the electric field strength within the cavity so that a more uniform heating within the biomass can be achieved. Interestingly from the simulation, for a specified microwave cavity, an optimum bed size of biomass was found at 50 mm height where maximum microwaves energy absorption takes place. In this sense, more microwaves energy can be converted into heat thereby ultimately helping the biomass to reach the desired pyrolysis temperature in shorter time. The COMSOL modelling on microwave heating therefore has shown to be simple and practical for use as a framework in predicting temperature profile of the biomass and intensity of the electric field.

Finite-rate chemistry modelling of non-conventional combustion regimes using a Partially-Stirred Reactor closure: Combustion model formulation and implementation details

The present work focuses on the numerical simulation of Moderate or Intense Low oxygen Dilution combustion condition, using the Partially-Stirred Reactor model for turbulence-chemistry interactions. The Partially-Stirred Reactor model assumes that reactions are confined in a specific region of the computational cell, whose mass fraction depends both on the mixing and the chemical time scales. Therefore, the appropriate choice of mixing and chemical time scales becomes crucial to ensure the accuracy of the numerical simulation prediction. Results show that the most appropriate choice for mixing time scale in Moderate or Intense Low oxygen Dilution combustion regime is to use a dynamic evaluation, in which the ratio between the variance of mixture fraction and its dissipation rate is adopted, rather than global estimations based on Kolmogorov or integral mixing scales. This is supported by the validation of the numerical results against experimental profiles of temperature and species mass fractions, available from measurements on the Adelaide Jet in Hot Co-flow burner. Different approaches for chemical time scale evaluation are also compared, using the species formation rates, the reaction rates and the eigenvalues of the formation rate Jacobian matrix. Different co-flow oxygen dilution levels and Reynolds numbers are considered in the validation work, to evaluate the applicability of Partially-Stirred Reactor approach over a wide range of operating conditions. Moreover, the influence of specifying uniform and non-uniform boundary conditions for the chemical scalars is assessed. The present work sheds light on the key mechanisms of turbulence-chemistry interactions in advanced combustion regimes. At the same time, it provides essential information to advance the predictive nature of computational tools used by scientists and engineers, to support the development of new technologies.

Formation of NO and NH in NH3-doped CH4 + N2 + O2 flame: Experiments and modelling

Co-combustion of 5200 ppm NH3 with a stoichiometric, atmospheric pressure, CH4 + N2 + O2 flame has been investigated with experiments and kinetic modelling. Profiles of the amidogen (NH) radical and nitric oxide (NO) have been measured using laser-induced fluorescence, the latter being quantitatively determined. Temperature profiles were measured using Rayleigh scattering and thermocouple, the nonintrusive measurements were considered more reliable and were used for evaluation of LIF data as well as input for flame modelling. Experimental results are compared with predictions of a chemical mechanism developed by Mendiara and Glarborg (2009), with simulations based on solution of energy equation as well as on experimental temperature profiles as input. Compared with a neat flame, the NH3-doped flame shows a shift in position ∼0.7 mm downstream, as established from the measurements of the NH profile. Modelling prediction of post-flame NO concentrations in the NH3-doped flame, around 1160 ppm, was within the evaluated uncertainty with experimental data (1460 ppm). Reaction path analysis indicated NH2 as a key species in the formation of NO and N2 from the nitrogen added to the flame by NH3. Altogether, the mechanism predicts concentration levels rather well but fails to predict the shift in flame position obtained with addition of NH3 to the rather slowly burning hydrocarbon flame.

In-situ distortions in LMD additive manufacturing walls can be measured with digital image correlation and predicted using numerical simulations

Distortions in Additive Manufacturing (AM) Laser Metal Deposition (LMD) occur in the newly-built component due to rapid heating and solidification and can lead to shape deviations and cracking. This paper presents a novel approach to quantify the distortions experimentally and to use the results in numerical simulation validation. Digital Image Correlation (DIC) is applied together with optical filters to measure in-situ distortions directly on a wall geometry produced with LMD. The wall shows cyclic expansion and shrinking with the edges bending inward and the top of the sample exhibiting a slight u‐shape as residual distortions. Subsequently, a structural Finite Element Analysis (FEA) of the experiment is established, calibrated against experimental temperature profiles and used to predict the in-situ distortions of the sample. A comparison of the experimental and numerical results reveals a good agreement in length direction of the sample and quantitative deviations in height direction, which are attributed to the material model used. The suitability of the novel experimental approach for measurements on an AM sample is shown and the potential for the validated numerical model as a predictive tool to reduce trial-and-error and improve part quality is evaluated.

Linear friction welding process simulation of Ti-6Al-4V alloy: a heat transfer analysis of the conditioning phase

Linear Friction Welding is an emerging solid-state joining process used for complex geometries. The process is composed of four distinct phases: conditioning (also called initial), transition, friction, and forging. This paper proposes a new numerical approach to simulate the initial phase (phase 1) while reducing the computational time. An implicit fully coupled thermomechanical 2D analysis scheme in ABAQUS was employed to compare the proposed thermal model and the conventional oscillation model. Physical and mechanical temperature-dependent properties of Ti-6Al-4V were implemented in the two models. An excellent agreement was obtained with the experimental temperature profiles published in the literature. The new thermal model reduces the computational time significantly, up to 99% with respect to an oscillating model.

Experimental and numerical study of turbulent mixed convection in a cavity with an internal heat source

In this study, experimental and numerical results of heat transfer in a ventilated cavity with an internal heat source are presented. The cavity represents a ventilated room with a person inside in a 1:3 scale. It has a vertical wall receiving a constant and uniform heat flux, while the opposite wall is kept at a constant temperature. The rest of the walls is adiabatic. The cavity has multiple inlets and outlets of air, considering ventilation by ducts of an air-conditioning system. Experimental temperature profiles were obtained at six different depths and heights consisting of 14 thermocouples each. Six turbulence models were evaluated against experimental data. The minimum average percentage differences between numerical and experimental average Nusselt numbers of the hot wall and heat source were 12.9% and 4.1% with the renormalized k–ε turbulence model.

Finite-rate chemistry modelling of non-conventional combustion regimes

The present work deals with the numerical simulation of Adelaide Jet in Hot Co-flow burner under Moderate or Intense Low oxygen Dilution (MILD) conditions, using the Partially Stirred Reactor (PaSR) model for turbulence-chemistry interactions. The PaSR model assumes that reactions are confined in specific regions of the computational cells, whose mass fractions depend both on the mixing and the chemical time scales. Therefore, the appropriate choice of mixing and chemical time scales becomes crucial to ensure the predictivity of the numerical simulations. Results show that the most appropriate choice for the evaluation of the mixing time scale in MILD regime consists in using the geometric mean of the Kolmogorov and the integral mixing time, rather than Kolmogorov or integral scales. This is supported by the validation of the numerical results against experimental profiles of temperature, major and minor species mass fractions. Moreover, the identification of the key species in the oxidation process allows to determine a chemical time scale that improves the prediction of centerline and upstream temperature predictions, compared to the results obtained by including all species in the evaluation of the chemical scale.

Development and prototype testing of MgCl2/graphite foam latent heat thermal energy storage system

Composites of graphite foam infiltrated with a magnesium chloride phase-change material have been developed as high-temperature thermal energy storage media for concentrated solar power applications. This storage medium provides a high thermal energy storage density, a narrow operating temperature range, and excellent heat transfer characteristics. In this study, experimental investigations were conducted on laboratory-scale prototypes with magnesium chloride/graphite foam composite as the latent heat thermal energy storage system. Prototypes were designed and built to monitor the melt front movement during the charging/discharging tests. A test loop was built to ensure the charging/discharging of the prototypes at temperatures >700 °C. Repeated thermal cycling experiments were carried out on the fabricated prototypes, and the experimental temperature profiles were compared to the predicted results from numerical simulations using COMSOL Multiphysics software. Experimental results were found to be in good agreement with the simulations to validate the thermal models.

Towards self-consistent plasma modelisation in presence of neoclassical tearing mode and sawteeth: effects on transport coefficients

The neoclassical tearing modes (NTM) increase the effective heat and particle radial transport inside the plasma, leading to a flattening of the electron and ion temperature and density profiles at a given location depending on the safety factor q rational surface (Hegna and Callen 1997 Phys. Plasmas 4 2940). In burning plasma such as in ITER, this NTM-induced increased transport could reduce significantly the fusion performance and even lead to a disruption. Validating models describing the NTM-induced transport in present experiment is thus important to help quantifying this effect on future devices. In this work, we apply an NTM model to an integrated simulation of current, heat and particle transport on JET discharges using the European transport simulator. In this model, the heat and particle radial transport coefficients are modified by a Gaussian function locally centered at the NTM position and characterized by a full width proportional to the island size through a constant parameter adapted to obtain the best simulations of experimental profiles. In the simulation, the NTM model is turned on at the same time as the mode is triggered in the experiment. The island evolution is itself determined by the modified Rutherford equation, using self-consistent plasma parameters determined by the transport evolution. The achieved simulation reproduces the experimental measurements within the error bars, before and during the NTM. A small discrepancy is observed on the radial location of the island due to a shift of the position of the computed q = 3/2 surface compared to the experimental one. To explain such small shift (up to about 12% with respect to the position observed from the experimental electron temperature profiles), sensitivity studies of the NTM location as a function of the initialization parameters are presented. First results validate both the transport model and the transport modification calculated by the NTM model.

Multi-scale gyrokinetic simulations of an Alcator C-Mod, ELM-y H-mode plasma

High fidelity, multi-scale gyrokinetic simulations capable of capturing both ion () and electron-scale () turbulence were performed in the core of an Alcator C-Mod ELM-y H-mode discharge which exhibits reactor-relevant characteristics. These simulations, performed with all experimental inputs and realistic ion to electron mass ratio () provide insight into the physics fidelity that may be needed for accurate simulation of the core of fusion reactor discharges. Three multi-scale simulations and series of separate ion and electron-scale simulations performed using the GYRO code (Candy and Waltz 2003 J. Comput. Phys. 186 545) are presented. As with earlier multi-scale results in L-mode conditions (Howard et al 2016 Nucl. Fusion 56 014004), both ion and multi-scale simulations results are compared with experimentally inferred ion and electron heat fluxes, as well as the measured values of electron incremental thermal diffusivities—indicative of the experimental electron temperature profile stiffness. Consistent with the L-mode results, cross-scale coupling is found to play an important role in the simulation of these H-mode conditions. Extremely stiff ion-scale transport is observed in these high-performance conditions which is shown to likely play and important role in the reproduction of measurements of perturbative transport. These results provide important insight into the role of multi-scale plasma turbulence in the core of reactor-relevant plasmas and establish important constraints on the the fidelity of models needed for predictive simulations.

Radiation-driven m = 2 island formation and dynamics near density limit in experimental advanced superconducting tokamak ohmic plasma

A radiation-driven m = 2 island was observed in the experimental advanced superconducting tokamak (EAST) ohmic plasma, near the density limit. The mode onset occurs when the the ohmic heating input is less than the radiative cooling loss, which agrees with the mode onset behavior of the thermo-resistive model. The evolution of the equilibrium during the mode process was obtained using the ONETWO transport code, with inputs comprising the experimental electron temperature and density profiles. A large m = 2 island can drive an m = 1 sideband mode, which leads to an internal crash that appears as a large change in temperature that occurs not only in the q = 2 region but also in the core.

Sensible Heat Transfer during Droplet Cooling: Experimental and Numerical Analysis

This study presents the numerical reproduction of the entire surface temperature field resulting from a water droplet spreading on a heated surface, which is compared with experimental data. High-speed infrared thermography of the back side of the surface and high-speed images of the side view of the impinging droplet were used to infer on the solid surface temperature field and on droplet dynamics. Numerical reproduction of the phenomena was performed using OpenFOAM CFD toolbox. An enhanced volume of fluid (VOF) model was further modified for this purpose. The proposed modifications include the coupling of temperature fields between the fluid and the solid regions, to account for transient heat conduction within the solid. The results evidence an extremely good agreement between the temporal evolution of the measured and simulated spreading factors of the considered droplet impacts. The numerical and experimental dimensionless surface temperature profiles within the solid surface and along the droplet radius, were also in good agreement. Most of the differences were within the experimental measurements uncertainty. The numerical results allowed relating the solid surface temperature profiles with the fluid flow. During spreading, liquid recirculation within the rim, leads to the appearance of different regions of heat transfer that can be correlated with the vorticity field within the droplet.