Spot Heating Research Articles

Rapid curing prospects of geopolymer cementitious composite using frontal polymerization of methyl methacrylate monomer

This paper investigates the prospects of thermal frontal polymerization (FP) technique for rapid curing of fly ash-based geopolymer composite (GPC). In this research, FP is used as an on-demand heat source alternative to conventional external heat application method. This method involves the mixing of an organic monomer (methyl methacrylate - MMA), crosslinker (trimethylolpropane triacrylate – TMPTA), and organic soluble initiator (Aliquat® persulfate - AQPS) with the GPC mortar (fly ash, sodium silicate, sodium hydroxide, and sand) for geopolymerization. An instantaneous application of spot heat initiates the exothermic polymerization of the organic monomer (MMA), forming a reaction front that provides heat needed for the geopolymerization activity, which ultimately produces a solid finished product by propagating through the GPC matrix. This solid finished product is termed in this article as frontally polymerized GPC (FPGPC). The polymerization of the above-mentioned organic monomer (MMA) in the FPGPC matrix was established by FTIR and Raman Spectroscopy. In addition, SEM analysis revealed a hybrid composite consisting of an interpenetrating network (IPN) of inorganic and organic components. This initial study showed that FPGPC achieved impressive compressive strength at early age (within an hour) as compared to both GPC and Ordinary Portland Cement (OPC) companion specimens. However, the strength at latter days and porosity of FPGPC did not match with that of GPC and OPC, which could be improved by optimizing the FPGPC mix and controlling the temperature rise during MMA polymerization. Hence, significant potential of FPGPC as construction material is apparent with limitations to be resolved through further research.

The ICECool Fundamentals Effort on Evaporative Cooling of Microelectronics

The Intrachip Enhanced Cooling Fundamentals (ICECool Fun) effort was launched by the Defense Advanced Research Projects Agency (DARPA) under the leadership of Dr. Avram Bar-Cohen during 2012–2015 to target an order of magnitude improvement in chip level and hot spot heat fluxes, compared to the then state-of-the-art (SOA). Evaporative cooling technologies to achieve potential targets of 1 kW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at the chip level and 5 kW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at the hot spot level were targeted. A key goal was to improve fundamental understanding of the evaporative cooling physics at the relevant scales, and a numerical modeling capability to enable the co-design of such solutions in emerging computing and communications systems. A summary of the five projects pursued under this effort is provided, including the key accomplishments and developed capabilities.

Catalytic Activity of Various Carbons during the Microwave-Initiated Deep Dehydrogenation of Hexadecane.

Carbon materials have been widely used as microwave susceptors in many chemical processes because they are highly effective at transforming incoming electromagnetic energy for local (hot spot) heating. This property raises the intriguing possibility of using the all-pervasive carbonaceous deposits in operating heterogeneous catalytic processes to augment the catalytic performance of microwave-initiated reactions. Here, the catalytic activities of a range of carbon materials, together with carbon residues produced from a “test” reaction—the dehydrogenation of hexadecane under microwave-initiated heterogeneous catalytic processes, have been investigated. Despite the excellent microwave absorption properties observed among these various carbons, only activated carbons and graphene nanoplatelets were found to be highly effective for the microwave-initiated dehydrogenation of hexadecane. During the dehydrogenation of hexadecane on a Fe/SiC catalyst, active carbon species were formed at the early stage of the reactions but were subsequently transformed into filamentous but catalytically inert carbons that ultimately deactivated the operating catalyst.

Interaction of flow pattern and heat transfer in oscillating heat pipes for hot spot applications

Oscillating heat pipes (OHP) are increasingly used for the thermal management of hot spots. Thereby, a local evaporation zone occurs that is still not fully understood due to complex interactions of flow patterns and heat transfer. This study quantifies the thermal performance and the flow behavior of oscillating heat pipes with a centrally located hot spot heater. The copper OHPs were filled with acetone and tested with different orientations. The filling ratios varied from 0% to 90%, the heat inputs increased from 25 W to 200 W. Our results illustrate that the flow velocity values and the resulting thermal resistance are highly interrelated. Within one channel, there is a wide range of velocity values from 30 mm/s to 1200 mm/s; the thermal resistance decreases with higher average flow velocity. By characterizing the flow regime, we show that a reliable start-up depends on the initial vapor liquid pattern. Depending on the initial pattern, an individual temperature difference is required for the start-up, even for similar operating conditions. The analysis of the stopping behavior shows that a uniform temperature difference of 14 K is required for a stable fluid oscillation. Our study substantiates a strong interrelation between flow pattern and heat transfer in OHPs, which also shows a strong potential for future optimizations of OHP applications.

Photothermal Agarose Microfabrication Technology for Collective Cell Migration Analysis.

Agarose photothermal microfabrication technology is one of the micropatterning techniques that has the advantage of simple and flexible real-time fabrication even during the cultivation of cells. To examine the ability and limitation of the agarose microstructures, we investigated the collective epithelial cell migration behavior in two-dimensional agarose confined structures. Agarose microchannels from 10 to 211 micrometer width were fabricated with a spot heating of a focused 1480 nm wavelength infrared laser to the thin agarose layer coated on the cultivation dish after the cells occupied the reservoir. The collective cell migration velocity maintained constant regardless of their extension distance, whereas the width dependency of those velocities was maximized around 30 micrometer width and decreased both in the narrower and wider microchannels. The single-cell tracking revealed that the decrease of velocity in the narrower width was caused by the apparent increase of aspect ratio of cell shape (up to 8.9). In contrast, the decrease in the wider channels was mainly caused by the increase of the random walk-like behavior of component cells. The results confirmed the advantages of this method: (1) flexible fabrication without any pre-designing, (2) modification even during cultivation, and (3) the cells were confined in the agarose geometry.

A highly-sensitive genetically encoded temperature indicator exploiting a temperature-responsive elastin-like polypeptide

Genetically encoded temperature indicators (GETIs) allow for real-time measurement of subcellular temperature dynamics in live cells. However, GETIs have suffered from poor temperature sensitivity, which may not be sufficient to resolve small heat production from a biological process. Here, we develop a highly-sensitive GETI, denoted as ELP-TEMP, comprised of a temperature-responsive elastin-like polypeptide (ELP) fused with a cyan fluorescent protein (FP), mTurquoise2 (mT), and a yellow FP, mVenus (mV), as the donor and acceptor, respectively, of Förster resonance energy transfer (FRET). At elevated temperatures, the ELP moiety in ELP-TEMP undergoes a phase transition leading to an increase in the FRET efficiency. In HeLa cells, ELP-TEMP responded to the temperature from 33 to 40 °C with a maximum temperature sensitivity of 45.1 ± 8.1%/°C, which was the highest ever temperature sensitivity among hitherto-developed fluorescent nanothermometers. Although ELP-TEMP showed sensitivity not only to temperature but also to macromolecular crowding and self-concentration, we were able to correct the output of ELP-TEMP to achieve accurate temperature measurements at a subcellular resolution. We successfully applied ELP-TEMP to accurately measure temperature changes in cells induced by a local heat spot, even if the temperature difference was as small as < 1 °C, and to visualize heat production from stimulated Ca2+ influx in live HeLa cells induced by a chemical stimulation. Furthermore, we investigated temperatures in the nucleus and cytoplasm of live HeLa cells and found that their temperatures were almost the same within the temperature resolution of our measurement. Our study would contribute to better understanding of cellular temperature dynamics, and ELP-TEMP would be a useful GETI for the investigation of cell thermobiology.

HEAT SOURCE FOR TIG WELDING MODELLING

A new model of heat source applicable to TIG welding is proposed. The model uses three calibration parameters - efficiency, effective heating spot radius and heat source concentration factor. Based on the experimental results, the model was calibrated and the results obtained for the form of penetration were compared with the experimental ones.

Discrimination of skyrmion chirality via spin\u2013orbit and \u2013transfer torques for logic operation

Recently many works on magnetic memories and logic circuits, which use a magnetic skyrmion have been reported. Previously we micromagnetically simulated a method to switch a chirality of a magnetic skyrmion formed in a magnetic thin film by introducing a pulsed heat spot. In this paper, we propose a method to discriminate the chirality of a skyrmion in a branched nanowire by using spin–orbit torque (SOT) and spin-transfer torque (STT), and confirm the validity of the method by using simulation. The simulated results show that the motion changes depending on the chirality when additional SOT is applied on a skyrmion moving in a branch by STT. This method can be used as a fundamental building block for electrical detection in memory and logic devices using the chirality of skyrmions as a data bit in addition to the presence (and polarity) of the skyrmions as conventionally used, which can be lead to multiple-valued operation.

Влияние структуры столба дугового разряда в вакууме на энергетическую эффективность процесса сварки

An arc discharge with a hollow cathode in vacuum has two forms of steady state: diffuse ("blurred" external column) and constricted (a visible cylindrical column providing a high directivity of the discharge energy transfer with minimal scattering in the radial direction). The article considers the dependence of the energy concentration in the heating spot and, consequently, the energy efficiency of the welding process on the structure of the arc discharge column with a hollow cathode when changing its mode parameters. The distributions of local parameters of the arc discharge with hollow cathode plasma are shown in a wide range of its modes based on probe studies. Considering the distributions allows obtaining a fairly complete picture of the physical processes in the plasma of the external discharge column and the effect of its structure on the energy efficiency of the welding process. It is shown that an arc discharge with a hollow cathode in a vacuum, the outer column of which has the structure of a plasma beam, is a relatively highly concentrated energy source and in terms of specific energy indicators can reach an energy concentration second only to beam energy sources.

The role of geometry in the generation of a shock wave by a femtosecond laser pulse

Laser exposure at a sufficient intensity creates a shock wave (SW), propagating in the irradiated target. The process is used in many technological applications. As a result of femtosecond exposure, a warmed up layer with a thickness of d T ∼ 0.1 μm occurs. The radius of the heating spot R L varies from values of the order of a micron (focusing on the diffraction limit) up to tens or hundreds of microns depending on the experiment. As you can see, R L ≫ d T, therefore one-dimensional motion with a plane surface is generated. The quasi-plane SW stage ends when the SW moves away from the target surface to a depth of about RL. Then the stage of quasi-hemispherical propagation begins. The paper analyzes the transition from plane to hemispherical SW. The motion of the “wings” of a hemispherical wave on the target surface bordering on a gas or vacuum is investigated. Theoretical estimates and numerical simulation results are presented. Analysis of the movement of the “wings” on the surface is important for experimental diagnostics of phenomena inside the target.

Analysis of delamination and heat conductivity of epoxy impregnated pancake coils using a cohesive zone model

Epoxy-impregnated pancake coil is observed to delaminate locally when its temperature is cooled from room temperature to 77 K. The delaminations cause the degradation of the critical current of the coil. Besides, the delamination may degrade the thermal conductivity and affect the thermal conductive path. Thus, in this study, the delamination behavior of epoxy-impregnated pancake coil during the cooling process is analyzed through a coupled thermal–mechanical cohesive zone model. The measured transverse tensile strength of coated conductors has shown considerable scatter and Weibull distribution has been adopted to fit the transverse tensile strength. The simulations are able to capture the main characterizations of the observed delaminations with the cohesive strength of the cohesive element following a Weibull distribution. After the cooling process, a heat spot is applied to the degraded positions to analyze the temperature field and the thermal conductive path in the damaged coil. Compared to the original undamaged coil, the delaminations indeed degrade the thermal conductivity and increase the thermal resistance. What’s more, the effects of the variation of the thickness of the epoxy and different interfacial cohesive strength distributions of the coated conductor are considered. The results show that reduction of the thickness of the epoxy can lower the radial stress and release the damage, and the random distribution of the cohesive strength inside the coated conductor dominantly determines the delamination pattern of the coil.

The relation between petroleum product prices and crude oil prices

We present an empirical examination of the relation between real spot Brent oil prices and real spot petroleum product prices, specifically gasoline and heating oil prices. Based upon weekly price data spanning a 30-year period ending in April 2019, we provide consistent evidence of Granger causality running from oil prices to product prices accounting for both non-stationarity of the series, and separately conditional heteroskedasticity. No evidence of Granger causality from product prices to oil prices is found for the full sample period nor the period up to the end of 2005, but evidence that gasoline prices Granger-caused oil prices is found for post-2005. Similar results are found for an extended model that also includes potentially endogenous real market variables related to supply and demand in the oil, heating oil and gasoline markets. Given the reduced form model results that oil prices Granger-cause gasoline and heating oil prices, we specify and estimate recursively identified structural vector autoregressive models that specifically account for structural supply and demand shocks in the oil market as well as the petroleum product markets. The oil market block follows Kilian (2009). We find that real spot gasoline prices and real heating oil prices do not respond to structural oil supply shocks, respond transiently to global commodity demand shocks, but do respond to oil-specific demand shocks. We conclude the result that spot oil prices Granger-cause spot gasoline and heating oil prices largely occurs through the channel of oil-specific demand shocks.

Forming the Convective Flows and a Cluster of Particles under Spot Heating

ABSTRACT The behavior of self-organization of convective flows in a thin layer of liquid under point (local) heating is investigated experimentally. The interaction of thermocapillary and thermogravitational-free convection can lead both to self-organization of a cluster of micro-vortices in the form of hexagonal structures and to its partial disintegration. Correlation analysis of the velocity field shows that the characteristic convection scales change continuously over time. The largest size of the vortex flow corresponds to the layer diameter (20 mm); the integral convection scale (2.5 mm) characterizes the established interaction of vortex structures in a wide range of sizes; and the dimensions of hexagonal convective cells (80–100 µm) show the lower limit of the characteristic scale of vortex structures. The observed flow macrostructure is determined by the complex nonlinear interaction of vortices of the specified scales. The resulting value of the average integral convection scale can be effectively used to predict the convection velocity.

Analytical Evaluation of the Dendritic Structure Parameters and Crystallization Rate of Laser-Deposited Cu-Fe Functionally Graded Materials.

The paper is devoted to the direct energy deposition (DED) of functionally graded materials (FGMs) created from stainless steel and aluminum bronze with 10% content of Al and 1% of Fe. The results of the microstructure analysis using scanning electronic microscopy (SEM) demonstrate the existence of a dendritic structure in the specimens. The crystallization rate of the gradient binary Cu-Fe system structures was investigated and calculated using the model of a fast-moving concentrated source with an ellipsoid crystallization front. The width of the secondary elements of the dendrites in the crystallized slab was numerically estimated as 0.2 nm at the center point of the circle heat spot, and the two types of dendrites were predicted in the specimen: the dendrites from 0.2 to approximately 50 nm and from approximately 0.1 to 0.3 μm in width of the secondary elements. The results were found to be in good accordance with the measured experimental values of the dendritic structure geometry parameters.

Investigation of Heat Transfer Characteristics in a Rotating Convection System With Bi-Directional Thermal Gradients

Abstract A series of rotating convection experiments have been conducted in a novel configuration, which comprises a cylindrical annulus with spot heating on the bottom outer edge and uniform cooling on the inner surface. Such a system provides bi-directional thermal gradients in both radial and vertical directions, thereby reenacting the thermal gradient patterns encountered in the atmosphere. Bulk heat transfer characteristics are studied by quantifying the overall Nusselt number, Nu, for a range of Taylor number, Ta, heating rate, Q, and Rayleigh number, Ra. Temperature measurements are carried out at different locations with the help of thermocouples. The Nusselt number is found to be quite sensitive to the buoyancy and relatively insensitive to the rotation rate. The correlation for Nu as a function of Ra revealed different power law exponents for low and high Ta values. The varying exponent is attributed to the presence of baroclinic eddies at high Ta, which in turn is verified with the help of flow visualization. The heat transfer characteristics in this new configuration are significantly different compared to other conventional rotational convection systems, where thermal gradients are present in only one direction.

Эксплуатационная проверка эффективности применения газового лучистого отопления в локомотивном депо

Gas-powered radiant heaters are a highly effective energy efficient solution that provides spot heating for spacious industrial facilities. The authors analyze efficiency of gas-powered radiant heating in a railroad motive power depot as the result of an experimental radiant heating implementation. An algorithm of efficiency estimation has been developed taking into account a lack of power consumption metering. Natural gas consumption has been compared between two heat supply options before and after modernization.

Measuring the radiation coefficient distribution and surface temperature distribution of a tungsten body heated by a powerful laser

In this article, we present what we believe to be the first successful demonstration of the emissivity distribution together with the temperature distribution of a tungsten plate at the laser heating spot. The measurements were made in the 740–800 nm wavelength range by using the multispectral imaging method on a laser-heating apparatus with a tandem acousto-optic filter (TAOTF). The TAOTF consists of two conjugated acousto-optic crystals connected to a high-definition video camera. The emissivity measurement error was less 7% for a maximum point temperature (2540 K) at the laser heating spot.

Photothermal testing of composite materials: Virtual wave concept with prior information for parameter estimation and image reconstruction

In this paper, we propose a new parameter estimation and image reconstruction approach for the photothermal testing of composite materials. Therefore, the full multidimensional evaluation method, virtual wave concept, is extended to estimate the orthotropic thermal diffusivity tensor and to reconstruct the initial temperature distribution after a laser spot heating in an orthotropic material. We establish a formal relationship between the virtual speed of sound tensor and the thermal diffusivity tensor. Furthermore, we show how prior information in the form of positivity and sparsity can be incorporated in the regularization process to improve the solution of the inverse imaging problem. In a second step, the initial temperature distribution is reconstructed by applying ultrasonic imaging methods on the calculated 3D bimodal virtual wave field. This new approach is validated on simulation and experimental data of a unidirectional carbon fiber reinforced polymer. The information loss that results from entropy production during heat diffusion can be partly compensated by including prior information. This allows an accurate parameter estimation and a high-resolution image reconstruction.

Development of Stellite 1 hardfacing procedure for crack-free SS 304 stainless steel orifice plate

This paper studies establishing an effective deposition technique of cobalt-based Stellite1 on austenitic stainless steel (SS 304) orifice plate component, used in offshore application. Stellite1 hardfacing was done using gas tungsten arc welding (GTAW) and flux-cored arc welding (FCAW) techniques, which resulted in intergranular and longitudinal cracks. In finite element analysis (FEA), a three-dimensional solid model was generated and simulated for spot heat welding using ANSYS software. After simulation for heat distribution with and without weld bordering, crack-free hardfacing of Stellite1 was carried out and validated experimentally using FCAW technique. A control on substrate preheating, thermal insulation, job clamping and cooling time reduced the intersecting cracks. Metallurgical analysis using Optical microscopy, SEM, EDS and XRD along with hardness measurements was done to investigate the mechanisms causing failure. Crack area measurements were done in gradient edge detection technique using MATLAB software and quantified.

Evaluation of electrical characteristics of locally degraded areas on exfoliated multilayered ceramic capacitors by thermal distribution analysis

The temperature distribution around a degraded area of an exfoliated multilayer ceramic capacitor (MLCC), just before electrical breakdown (pre-breakdown) was measured. The temperature increase of the heating spot was in good agreement with the temperature evaluated from Joule heating by the applied voltages and the current. The current–voltage characteristics and the temperature dependence on the insulation resistance of the exfoliated sample showed the same characteristics as unexfoliated samples. We have confirmed that the locally degraded areas dominate the electrical properties of pre-breakdown MLCCs.