Thermocouple Measurements Research Articles

The Different Welding Layers and Heat Source Energy on Residual Stresses in Swing Arc Narrow Gap MAG Welding.

In this paper, in order to reduce the time cost of prediction experiments in industry, a new narrow gap oscillation calculation method is developed in ABAQUS thermomechanical coupling analysis to study the distribution trend of residual weld stresses in comparison with conventional multi-layer welding processes. The blind hole detection technique and thermocouple measurement method verify the reliability of the prediction experiment. The results show that the experimental and simulation results have a high degree of agreement. In the prediction experiments, the calculation time of the high-energy single-layer welding experiments is 1/4 of the traditional multi-layer welding. Two welding processes of longitudinal residual stress and transverse residual stress distribution trends are the same. The high-energy single-layer welding experiment stress distribution range and transverse residual stress peak are smaller, but the longitudinal residual stress peak is slightly higher, which can be effectively reduced by increasing the preheating temperature of the welded parts. This implies that in the specific case of increasing the initial temperature of the workpiece, the use of high-energy single-layer welding instead of multi-layer welding to study the residual stress distribution trend not only optimizes the weld quality but also reduces the time cost to a large extent.

A mathematical examination of the impact of mould transparency to infrared radiation on solidification during the investment casting process

Investment casting is a highly dynamic process during which multiple competing physical phenomena are at work. Those seeking to understand and simulate such processes computationally are confronted with a considerable task, balancing accuracy with efficiency. Approximations and models based on well-understood and documented fundamental physics are powerful tools in a modeller’s arsenal. Driven by observed discrepancies between experimental thermocouple measurements and simulation predictions of casting temperatures, this work explores the additional alloy cooling mechanism of mould transparency to infrared radiation, targeting a new mathematical approximation applicable in such situations. Direct attenuation, scattering from coarse sand, sand distribution in the mould and material temperatures play a role in the extent of radiation transparency that must be considered. From this model, estimation of the additional cooling rate resulting from expected mould transparency can be determined and applied as a corrective measure to computation fluid dynamics (CFD) simulation results that do not capture this phenomenon.

Experimental and Numerical Study on Regulation of Cutting Temperature during the Circular Sawing of 45 Steel

Studying the cutting temperature is critical for unlocking the secrets of sawblade wear, lifespan, and the metallurgical alterations beneath the surface. This paper describes an investigation into the temperature of 45 steel during dry sawing, using a cemented carbide circular saw blade under various cutting conditions. A temperature acquisition system was developed, enabling the determination of the average temperature of the arc zone in the workpiece and the temperature of the sawtooth tip via a semi-automated thermocouple measurement and an embedded dynamic artificial thermocouple method, respectively. Results obtained from these two methods indicate a positive correlation between the sawing temperature and the saw blade speed and feed rate, with an optimal combination of cutting process parameters identified for maintaining stability within reasonable ranges. Finite element simulations reveal a cyclical fluctuation in temperature along the workpiece surface and sawtooth, with a gradual decrease after an increase in the intermittent step, and confirm the relationship between the sawing temperature and the saw blade and feed rates observed experimentally. Overall, this study presents valuable insights into the temperature changes occurring during the sawing process, with important implications for improving productivity and maintaining stability in industrial applications.

Multi-point temperature measurements in packed beds using phosphor thermometry and ray tracing simulations

Packed bed reactors are commonly found in the process industry, for example in flame-assisted calcination for cement production. Understanding the heat transfer inside the bed is essential for process control, product quality and energy efficiency. Here we propose a technique to determine the internal temperature distribution of packed beds based on a combination of lifetime-based phosphor thermometry, ray tracing simulations, and assimilation of temperature data using finite element heat transfer simulations. To establish and validate the technique, we considered a reproducible regular packing of 6 mm diameter aluminum spheres, with one of the spheres in the top layer being electrically heated. If a sphere inside the packing is coated with thermographic phosphors and excitation light is directed towards the packing, luminescence from the coated sphere exits the packed bed after multiple reflection and the sphere's temperature can be determined. Isothermal measurements showed that the temperature obtained by phosphor thermometry is independent of the luminescent sphere location. When imaging the luminescence on a camera, the luminescence distribution in recorded image depended, however, on the position of the sphere. Therefore, in setups with multiple phosphor-coated spheres, their signals can be separated using a least squares fit. We demonstrate the approach using a setup with three luminescent spheres and validated the temperature readings against thermocouple measurements. To obtain the spatial signatures for individual sphere positions required for the least squares fit, ray tracing simulations were used. These provide an efficient alternative to single sphere measurements that are only practical for regular spherical packed beds. Multi-point measurements were used as input to a finite element heat transfer simulations to determine parameters such as particle-to-particle air gap distance. With these, the full temperature distribution inside the bed could be assimilated from the measured values.

Acoustic reconstruction of the vortex field in the nonuniform temperature field of a simulated furnace

The high-temperature thermal vortex flows generated by the tangential combustion of large-scale coal-fired power plant boilers represent a complex medium that causes acoustic ray bending. In this paper, nonlinear acoustic tomography is combined with particle swarm optimization as a means of reconstructing the two-dimensional thermal vortex field. The nonuniform and large-gradient temperature distribution is the main cause of acoustic ray bending. We establish curved paths of acoustic propagation that are closely related to the temperature distribution. Based on these curved paths, a set of radial basis functions combined with Tikhonov regularization is derived to achieve more accurate reconstruction of the temperature field. The nonuniform temperature field affects the reconstruction of the flow field information. Our model considers a variation in the angle between the direction vector of the acoustic curved paths and the horizontal and vertical components of the flow velocity, and incorporates a priori information and a six-parameter hypersurface into the flow field reconstruction scheme. Particle swarm optimization is applied to minimize the objective function and obtain the vortex velocity field. Numerical experiments show that the proposed mathematical model of acoustic ray bending improves the reconstruction accuracy of the temperature distribution and vortex flow field. The acoustic time-of-flight is experimentally measured through the generalized cross-correlation with different window functions. The temperature and vortex fields reconstructed based on the actual time-of-flight are compared with thermocouple and anemometer measurements to demonstrate the validity and robustness of the proposed reconstruction model.

Equivalent value of non-uniform temperature field for thermocouple measurement

The incoming flow is assumed to have a uniform temperature profile when the energy balance of a thermocouple bead is analyzed for the purpose of measurement corrections, which is hardly true in real applications. The incoming temperature is normally non-uniform and the temperature of the flow closer to the bead has a stronger influence on the bead temperature. Considering this, for the first time, a Gaussian probability density function is established to generate an equivalent temperature of the non-uniform temperature field, which can be used in the corrections. With the one-dimensional thermocouple heat transfer program, the bead temperatures are calculated with five non-uniform temperature profiles and compared with the bead temperatures with the uniform temperatures to find the variance correlation of the Gaussian function. The variance is correlated to the Nusselt number and the wire diameter and the bead diameter of the thermocouple. Many cases with different thermocouple sizes, velocities and temperature ranges and widths of the power function profile are simulated. For all the tested cases, the bead temperature differences between the real profiles and the equivalent profiles are less than 27K, demonstrating the good accuracy of the current method.

Emissivity Influence on Thermocouple Correction

The reading of a thermocouple is not equal to the gas temperature because of the complicated heat transfer process among the thermocouple, the gas, and the environment. So the corrections for thermocouple measurements are necessary to obtain the real gas temperature. The current correction methods using multiple thermocouples assume that the thermocouples have the same surface emissivity. However, the emissivity of a thermocouple depends on its surface condition, and the emissivity of the thermocouples normally are different. CFD simulations are carried out to study the influence of emissivity difference on the correction accuracy. For the extrapolation method, the correction accuracy could be improved or weakened depending on the emissivity variation of the thermocouples. For the equation methods by De, the correction accuracy is poor for the low speed flow and good for the high speed flow. However, the correction accuracy for the high speed flow is weakened if the emissivity of the thermocouples have large difference. For the equation method by Brohez, the overall correction accuracy is good but the accuracy degrades greatly with the large emissivity difference of the thermocouples.

Development of an US, MRI, and CT imaging compatible realistic mouse phantom for thermal ablation and focused ultrasound evaluation

Tissue mimicking phantoms (TMPs) play an essential role in modern biomedical research as cost-effective quality assurance and training tools, simultaneously contributing to the reduction of animal use. Herein, we present the development and evaluation of an anatomically accurate mouse phantom intended for image-guided thermal ablation and Focused Ultrasound (FUS) applications. The proposed mouse model consists of skeletal and soft tissue mimics, whose design was based on the Computed tomography (CT) scans data of a live mouse. Advantageously, it is compatible with US, CT, and Magnetic Resonance Imaging (MRI). The compatibility assessment was focused on the radiological behavior of the phantom due to the lack of relevant literature. The X-ray linear attenuation coefficient of candidate materials was estimated to assess the one that matches best the radiological behavior of living tissues. The bone part was manufactured by Fused Deposition Modeling (FDM) printing using Acrylonitrile styrene acrylate (ASA) material. For the soft-tissue mimic, a special mold was 3D printed having a cavity with the unique shape of the mouse body and filled with an agar-based silica-doped gel. The mouse phantom accurately matched the size and reproduced the body surface of the imaged mouse. Tissue-equivalency in terms of X-ray attenuation was demonstrated for the agar-based soft-tissue mimic. The phantom demonstrated excellent MRI visibility of the skeletal and soft-tissue mimics. Good radiological contrast between the skeletal and soft-tissue models was also observed in the CT scans. The model was also able to reproduce realistic behavior during trans-skull sonication as proved by thermocouple measurements. Overall, the proposed phantom is inexpensive, ergonomic, and realistic. It could constitute a powerful tool for image-guided thermal ablation and FUS studies in terms of testing and optimizing the performance of relevant equipment and protocols. It also possess great potential for use in transcranial FUS applications, including the emerging topic of FUS-mediated blood brain barrier (BBB) disruption.

1,1-Diamino-2,2-dinitroethylene: The riddles of thermal decomposition and combustion

The thermal decomposition of deuterium derivatives α-FOX-7 and γ-FOX-7 have been studied using non-isothermal and isothermal kinetic methods. It was found that deuteration only affects the kinetics of the low-temperature decomposition step of α-FOX-7. A unique feature of the low-temperature stage of the decomposition of α-FOX-7 is that topochemical processes on crystal defects lead to intermolecular hydrogen transfer with the formation of its highly reactive aci-form. The decomposition of the aci-form of FOX-7 proceeds in an extremely unusual way via the molecular elimination of nitric acid and formation of nitrile N-oxide. The high-temperature decomposition stage involves the olefinic form of FOX-7 in a defect-free crystal lattice, and the decomposition is initiated by breaking the C–NO2 bond. In the gas phase, the FOX-7 molecule is also in its olefin form and exhibits high thermal stability.The combustion behavior and flame structure of α–FOX-7 and heat-treated substance (γ–FOX-7t) have been studied using a constant-pressure bomb and micro-thermocouple technique. Analysis of the thermocouple measurement data showed that the burning rate of both FOX-7 isomers is controlled by the rate of heat release in the condensed phase. This reaction is the decomposition of the substance in the melt. The higher burning rates of α-FOX-7 were due to the fact that the highly reactive aci-form of FOX-7 formed on the crystal defects remains in the melt and decomposes at the surface temperature. The break in the dependence of the burning rate on pressure observed for the α- and γ-polymorphs is associated with their complete isomerization into the internal aci-form of FOX-7 due to intramolecular hydrogen transfer. In this case, the surface temperature is determined by the vaporization of the internal aci-form of FOX-7, whose enthalpy of vaporization is higher than that of its olefin form.

Microstructure estimation and validation of ER110S-G steel structures produced by wire and arc additive manufacturing

Wire and Arc Additive Manufacturing (WAAM) emerged as a manufacturing process for large scale structures with extensive form and design freedom. WAAM can be fully exploited once the relation between the transient thermal history and its relation to microstructure development and resultant mechanical properties is established. This relation can be further used for computational design tools such as Topology Optimization. This paper presents a model to predict the relation between the thermal history and solid-state phase transformations in a widely applicable High Strength Low Alloy steel ER110S-G. The transient thermal history of parts manufactured by WAAM is modelled using finite element analysis. The modelled thermal history is validated with thermocouple measurements. Our results show that a critical cooling cycle is responsible for the solid-state phase transformation in an AM part. The cooling rate of this particular cooling cycle is superimposed onto an experimentally constructed Continuous Cooling Transformation (CCT) diagram to determine the local solid-state phase fractions. The predicted phase fractions in three wall samples with different design and processing conditions of AM parts are used to predict the hardness. The predicted hardness is 10% higher than the measured hardness of AM samples. The effect of tempering is also considered in the model through JMAK equation. The results show that the tempering is caused in regions with high martensite content and it lowers the hardness by 8 − 10%. The micrographs of the AM parts show that the microstructural features are same for the AM parts with similar critical cooling rates.

Considerations for the temperature stratification in a pre-burn constant-volume combustion chamber

In recent years, the Engine Combustion Network (ECN) has developed as a worldwide reference for understanding and describing engine combustion processes, successfully bringing together experimental and numerical efforts. Since experiments and numerical simulations both target the same boundary conditions, an accurate characterization of the stratified environment that is inevitably present in experimental facilities is required. The difference between the core-, and pressure-derived bulk-temperature of pre-burn combustion vessels has been addressed in various previous publications. Additionally, thermocouple measurements have provided initial data on the boundary layer close to the injector nozzle, showing a transition to reduced ambient temperatures. The conditions at the start of fuel injection influence physicochemical properties of a fuel spray, including near nozzle mixing, heat release computations, and combustion parameters. To address the temperature stratification in more detail, thermocouple measurements at larger distances from the spray axis have been conducted. Both the temperature field prior to the pre-combustion event that preconditions the high-temperature, high-pressure ambient, as well as the stratification at the moment of fuel injection were studied. To reveal the cold boundary layer near the injector with a better spatial resolution, Rayleigh scattering experiments and thermocouple measurements at various distances close to the nozzle have been carried out. The impact of the boundary layers and temperature stratification are illustrated and quantified using numerical simulations at Spray A conditions. Next to a reference simulation with a uniform temperature field, six different stratified temperature distributions have been generated. These distributions were based on the mean experimental temperature superimposed by a randomized variance, again derived from the experiments. The results showed that an asymmetric flame structure arises in the computed results when the temperature stratification input is used. In these predictions, first-stage ignition is advanced by 24μs, while second-stage ignition is delayed by 11μs. At the same time a lift-off length difference between the top and the bottom of up to 1.1 mm is observed. Furthermore, the lift-off length is less stable over time. Given the shown dependency, the temperature data is made available along with the vessel geometry data as a recommended basis for future numerical simulations.

Method of measuring the temperature of wood exposed to fire with type K thermocouples

The study of wood behaviour in the event of fire is of increasing interest in the civil engineering field. Wood is one of the most prevalent combustible materials used in buildings. However, this material is protected by the char layer during fire exposure. It can slow down the charring rate of timber in fire and protect the internal wood structure. In fire tests, it is very important to use the most effective method of measuring the temperature of wood. In this paper, experimental tests are performed in furnaces and the charring rate is determined by measuring the depth of the charred part of timber at the end of the time. This approach is compared to direct measurements given by thermocouples positioned at different depths in the wood sample in different directions (radial, longitudinal, and tangential). All test samples were tested in a furnace where three surfaces of the sample were exposed to ISO-834 fire standards. The comparison between the two methods of measuring the charring rate allowed us to evaluate the accuracy of the thermocouple measurements according to the implementation adopted in the sample. Besides, the factors affecting the temperature measurement results are also mentioned in this study such as the position of the thermocouple in the wood, the size of the measuring sample and the size of the thermocouple.

Studying the thermal characteristics and effectiveness of structural fire proofing made of PROSASK Firepanel cement boards by means of reproducing the high-temperature effect

Introduction.The application of PROZASK fire-retardant panels demonstrates an option for and the expediency of a comprehensive study of fire proofing characteristics.Goals and objectives.The mission of this research project is to (1) obtain the results of experimental studies of fireproofing panels, containing the cement binder, (2) determine their fire proofing efficiency, using the radiant heating test bench that reproduces pre-set modes of high-temperature exposure, and (2) analyze the testing results using the method of mathematical modelling of temperature fields inside fireproof structures.Methods.Standardized laboratory techniques were used to clarify the thermal-physical characteristics of boards at relatively low temperatures. Their fire proofing efficiency was evaluated in the course of additional testing of specimens using the radiant heating test bench. A reliable and relatively uncomplicated method and programme for calculating unsteady temperature fields in fireproof structures were used to conduct the thermal analysis and generalization of experimental results. The authors summarized the results of standardized tests, conducted in a fire furnace, to determine the flame-retardant efficiency of PROZASK Firepanel boards and the fire-resistance of the full-size structures they protect.Results.Values of specific heat capacity and the thermal conductivity coefficient of boards, tested using laboratory benches at relatively low temperatures, were verified. The results of thermocouple measurements, taken during the testing of specimens with the help of the radiant heating bench in standard and hydrocarbon temperature modes, were obtained. The processing of these results, using thermal engineering analysis, allowed determining the values of the thermal conductivity coefficient in the range of operating temperatures. The influence of moisture, contained in the boards, on their fire protection efficiency was evaluated. Comparison between the results of calculations and tests, conducted in the fire furnaces, showed the practical usability of the obtained characteristics of boards and the thermo-engineering analysis used to (1) clarify the fire-proofing efficiency and the design developed using PROZASK Firepanels and (2) evaluate the fire-resistance of the constructions they protect.Conclusions.The presented integrated studies generated a considerable amount of important information, required to prognosticate the fire proofing properties and the fire-resistance of constructions that contain PROZASK Firepanels. The role of additional testing of specimens using a radiant heating test bench and the effectiveness of thermal-engineering calculations as a tool for assessing the parameters of fire proofing and the fire resistance of structures are demonstrated.

Online tomography algorithm based on laser absorption spectroscopy

Conventional calibration-free wavelength modulation spectroscopy generally requires complex absorption spectrum simulations in combination with spectral databases and laser modulation parameters, placing high demands on the accuracy of a priori spectral parameters and hardware parameters. Meanwhile, inappropriate initial values can increase the computation time and even lead to local optimal solutions. In order to improve the computational efficiency, a rapid calibration-free wavelength modulation spectroscopy to obtain the integrated absorbance is presented in this work. First, this method is computationally efficient, requiring only algebraic calculations by using the 2nd, 4th, and 6th harmonic center peak height parameters to obtain the integrated absorbance, eliminating the need for computationally intensive harmonic fitting calculations. Secondly, this method has low dependence on the spectral database, requiring only line intensity and low-state energy level spectral parameters. Finally, this method is highly adaptable and does not require scanning the complete absorption spectral line shape, which solves the problem of incomplete harmonic signals caused by the conventional method at high temperature and high pressure due to the broadening of the absorption spectral line. This method has previously been used only for line-of-sight measurements at low-frequency experimental signals in stable environments, and for calculating the integrated absorbance at average temperature, concentration and pressure states. In this work, the method is applied to non-uniform complex combustion field tomography and combined with the proposed tomographic system to achieve online reconstructing temperature and concentration distributions. The accuracy and computational efficiency of the method in obtaining the integrated absorbance are verified by numerical simulations and experiments on the butane burner flame. The results show that the presented method is consistent with the reconstructed distribution compared with the conventional wavelength modulation method, with a maximum relative deviation of only 0.94% from the measurement and 3.5% from the thermocouple measurement, verifying the accuracy of the method. The computational efficiencies of the two methods for obtaining the integrated absorbance are analyzed. The average calculation time per path is 0.15 s for the present method and 21.10 s for the conventional method. The calculation efficiency of the present method is at least two orders of magnitude higher than that of the conventional method, which provides a fast and reliable research method and technical means to realize the industrial-grade online reconstruction of temperature and concentration distribution of combustion fields.

Experiments with reduced single pass absorption at ASDEX Upgrade – instrumentation and applications

Reflecting gratings have been installed in the vacuum vessel of ASDEX Upgrade for all beamlines of the electron cyclotron resonance heating system. Potentially unabsorbed millimetre wave power after the first pass through the plasma is redirected towards the plasma centre. This increases the efficiency of heating schemes with reduced single pass absorption like O-2 or X-3. In order to monitor beam position and power, thermocouples were installed into the gratings. A numerical model was developed to evaluate the beam intensity during short pulses from the thermocouple measurement in a non-stationary environment. An experiment was carried out, where only the X-3 resonance is present in the plasma, and the millimetre wave beam shine-through was measured successfully as a function of the central plasma electron temperature. This allows to deduce the X-3 absorption experimentally. Scanning the launching angles, it seems possible to measure the 2D beam cross section after the first pass through the plasma.

Comparisons of Structured Surface Floors for Pool Boiling Enhancement at Low Heat Fluxes: Hands-On Learning Setup for Heat Transfer Classroom

Various enhanced surfaces have been proposed over the years to improve boiling heat transfer. This paper introduces an experimental setup designed for boiling demonstration in the graduate-level Heat Transfer course. The pool boiling performance of water under atmospheric pressure of 1.025 bar is investigated by using several structured surfaces at heat fluxes of 28 and 35 kW/m2. Surfaces with holes, rectangular grooves, and mushroom fins are manufactured by an NC-controlled vertical milling machine. The heat flux versus excess temperature graph is plotted by using thermocouple measurements of water and base temperatures of the boiling vessel. The separation, rise, and growth of individual vapor bubbles from the surface during boiling were recorded with a digital camera. The results for the plain surface are compared to the Rohsenow correlation. The enhancement of heat transfer coefficient (h) ranged between 15% - 44.5% for all structured surfaces. The highest heat transfer coefficient enhancement is observed between 41% - 56.5% for holed surface-3 (405 holes) compared to the plain surface. The excess temperature dropped around 29% - 34% for holed surface-3 (405 holes) compared to the plain surface. The heat transfer coefficient increases as the spacing between channels or holes decreases. While the bubbles on holed and mushroomed surfaces were spherical, the bubbles on the flat and grooved surfaces were observed as formless. The suggested economical test design could be appropriate to keep students focused and participating in the classroom.

ANALYSIS OF THE FORM AND EVOLUTION OF THE SOLID PHASE DURING DIRECTIONAL CRYSTALLIZATION OF NON-FERROUS METALS WITH ELECTROMAGNETIC INFLUENCE BY ULTRASONIC AND TEMPERATURE METHODS

The paper is devoted to the experimental study of new mechanisms of controlling the process of directional crystallization of non-ferrous metals. The focus of the study is on the development and testing of measuring techniques applicable both to laboratory modeling and to processes under real operating conditions. The mechanism of controlling the rate and homogeneity of crystallization of a metal melt by changing the phase angles between the supply currents of a traveling magnetic field induction stirrer is proposed. This makes it possible to generate vortex flows of various topologies in the liquid metal, in particular, to change the number of large-scale vortices or to suppress the large-scale flow. It is shown that the hydrodynamic flows have an impact on the crystallization front shape, which allows one to control the homogeneity of metal solidification by changing the characteristics of the power supply of the inductor. It is important to note that a change in the phase angles of the currents while maintaining the supply amplitude does not significantly affect the crystallization rate, which opens up wide possibilities for controlling processes by changing both the current strength and the phase angles. The temperature method for determining the position of the crystallization front was successfully applied and verified by ultrasonic velocimetry measurements. It has been found that, in the presence of developed flows in a liquid medium, thermocouple measurements provide good agreement (up to a few percent) of the measured position and geometric shape of the crystallization front with the ultrasonic measurement data. In the absence of liquid phase stirring, the difference between the thermocouple and ultrasonic measurements increases slightly. Nevertheless, even in this case, the thermocouple method can be used to correctly determine the position and velocity of the crystallization front.

Simultaneous reconstruction of flame temperature and soot volume fraction through weighted non-negative least squares and light field imaging techniques

The flame temperature and soot volume fraction are important parameters for combustion diagnosis, which reflect the combustion reaction process and play a significant role in controlling pollutants emissions. This research introduces a direct method based on Weighted Non-Negative Least Squares (WNNLS) and light field imaging techniques to recover the flame temperature and soot volume fraction simultaneously. This method considers the flame self-absorption effect. Numerical studies were conducted on both Non-Negative Least Squares and proposed algorithms to compare the reconstruction performance. The anti-noise ability of the proposed reconstruction method was also investigated by adding various noises to radiative intensity. The effectiveness of the proposed method was also further verified by experimental studies on bimodal ethylene-diffusion flames. Thermocouple measurements were conducted and compared with the reconstructed temperatures. Both the numerical and experimental studies manifest that the proposed direct reconstruction method combing with WNNLS and light field imaging techniques is effective and accurate.

Laboratory model of electrovortex flow with thermal gradients for liquid metal batteries

We present a novel laboratory setup for studying the fluid dynamics in liquid metal batteries (LMBs). LMBs are a promising technology suited for grid-scale energy storage, but flows remain a confounding factor in determining their viability. Two important drivers of flow are thermal gradients, caused by internal heating during operation, and electrovortex flow (EVF), induced by diverging current densities. Our setup explores thermal gradients and electrovortex flow separately and in combination in a cylindrical layer of liquid gallium, simulating the behavior in a single layer of an LMB. In this work, we discuss the design principles underlying our choices of materials, thermal control, and current control. We also detail our diagnostic tools - thermocouple measurements for temperature and Ultrasonic Doppler Velocimetry (UDV) probes for velocities - and the design principles which go into choosing their placement on the setup. We also include a discussion of our post-processing tools for quantifying and visualizing the flow. Finally, we validate convection and EVF in our setup: we show that scaling relationships between the nondimensional parameters produced by our data agree well with theory and previous studies.

Analysis of thermocouple-based finger contact temperature measurements

The understanding of heat conduction during finger contact with cooler or hotter objects is important for designing many electronic devices and for setting safety standards in a variety of occupational settings. In the most common experimental approach to study this process, a micro-thermocouple is placed at the finger-object interface. However, the interpretation of what this measurement corresponds to is not clear. To this end, we develop a three-dimensional thermal simulation of the finger-thermocouple-substrate configuration. The model predictions match finger cooling measurements in eight distinct cases available in prior literature (finger pressed with 1 N or 9.8 N against a steel or an aluminum substrate held at −2 °C or −10 °C). We demonstrate that the thermocouple can be represented accurately as a truncated sphere with emerging cylindrical wires while a multilayer block model of the finger provides similar results to an anatomically representative model. Our simulations show that in the eight previously studied cooling cases, the average surface temperature of skin that is in contact with the substrate follows nearly the same but offset cooling trend as the thermocouple tip temperature. The value of the offset is predominantly determined by the substrate material, with the thermocouple tip temperature being lower than the average skin surface temperature by 1–5 °C and 3–10 °C for steel and aluminum substrate cases, respectively. This temperature difference results in a moderate to an extreme thermocouple underprediction of the time necessary for the skin surface to reach the experimental safety threshold of 1 °C. Consequently, from the perspective of the safety related applications the thermocouple measurement provides a conservative limit on the contact duration and thereby is suitable for such purposes, but for applications requiring accurate skin temperature measurements alternative experimental approaches should be used.