Transient Temperature Rise Research Articles

Temperature Predictions Using Hybrid Heat Flux Gage Measurements

The hybrid heat flux gage is a high temperature device that does not require water cooling and can be placed directly onto a surface to quantify the heat exposure boundary conditions. In this research, the hybrid gage was mounted onto three different samples and used to determine exposure conditions for predicting the transient temperature rise of the samples. Tests were performed in an ASTM E1354 cone calorimeter at cold surface heat flux levels of 10 kW/m2, 20 kW/m2 and 35 kW/m2. Boundary condition details such as the radiant heat flux and the convective heat transfer coefficient were determined with the hybrid gage. The measured boundary conditions were input into the finite element package Abaqus to predict the transient through thickness temperature rise of aluminum alloy 5083, Marinite P, and UNIFRAX Duraboard LD ceramic board. Transient temperature predictions were generally within 6% of the experimental data, demonstrating the use of the hybrid gage to determine the mixed mode boundary condition details for fire applications.

A computational model for transient temperature rise in a dye laser gain medium pumped by a copper vapor laser

Spectrally stable dye lasers play an important role in techniques based on high resolution spectroscopy and atomic spectroscopy. The spectral purity of a dye laser is affected when the pump power to it is increased beyond the threshold. When the pump power is increased beyond the threshold, two mode oscillations occur which decrease the spectral purity of the dye laser. The effect of higher pump pulse energies on transient thermal effects has been studied using a computational fluid dynamics (CFD) model and the disturbances to the laser cavity have been studied using commercially available ray tracing software. The change in the cavity length was determined from the CFD model for several dye concentrations and pump powers. The results of the CFD model have been verified by published results and experimental results from our system. Our study shows that in the longitudinally pumped single mode laser change in the cavity length is a more dominant disturbance than thermal blooming. Our model is useful for the design of the dye cell.

Effect of Thermal Interface on Heat Flow in Carbon Nanofiber Composites

The thermal transport process in carbon nanofiber (CNF)/epoxy composites is addressed through combined micromechanics and finite element modeling, guided by experiments. The heat exchange between CNF constituents and matrix is studied by explicitly accounting for interface thermal resistance between the CNFs and the epoxy matrix. The effects of nanofiber orientation and discontinuity on heat flow and thermal conductivity of nanocomposites are investigated through simulation of the laser flash experiment technique and Fourier's model of heat conduction. Our results indicate that when continuous CNFs are misoriented with respect to the average temperature gradient, the presence of interfacial resistance does not affect the thermal conductivity of the nanocomposites, as most of the heat flow will be through CNFs; however, interface thermal resistance can significantly alter the patterns of heat flow within the nanocomposite. It was found that very high interface resistance leads to heat entrapment at the interface near to the heat source, which can promote interface thermal degradation. The magnitude of heat entrapment, quantified via the peak transient temperature rise at the interface, in the case of high thermal resistance interfaces becomes an order of magnitude more intense as compared to the case of low thermal resistance interfaces. Moreover, high interface thermal resistance in the case of discontinuous fibers leads to a nearly complete thermal isolation of the fibers from the matrix, which will marginalize the contribution of the CNF thermal conductivity to the heat transfer in the composite.

Thermal Model for Power Converters Based on Thermal Impedance

In this paper, the superposition principle of a heat sink temperature rise is verified based on the mathematical model of a plate-fin heat sink with two mounted heat sources. According to this, the distributed coupling thermal impedance matrix for a heat sink with multiple devices is present, and the equations for calculating the device transient junction temperatures are given. Then methods to extract the heat sink thermal impedance matrix and to measure the Epoxy Molding Compound (EMC) surface temperature of the power Metal Oxide Semiconductor Field Effect Transistor (MOSFET) instead of the junction temperature or device case temperature are proposed. The new thermal impedance model for the power converters in Switched Reluctance Motor (SRM) drivers is implemented in MATLAB/Simulink. The obtained simulation results are validated with experimental results. Compared with the Finite Element Method (FEM) thermal model and the traditional thermal impedance model, the proposed thermal model can provide a high simulation speed with a high accuracy. Finally, the temperature rise distributions of a power converter with two control strategies, the maximum junction temperature rise, the transient temperature rise characteristics, and the thermal coupling effect are discussed.

Contact measurement of thermal conductivity and thermal diffusivity of solid materials: Experimental validation of feasibility with a prototype sensor

A contact method has been proposed for measuring thermal transport properties of solids including soft materials. The method has an advantage of potential utilization for in-situ measurement without preparing a sample specimen. A unique feature of the method is to prepare a shallow cavity around a film sensor for a layer of a gel that is used to eliminate the thermal contact resistance between the sensor and the sample. A prototype sensor, 3mm in diameter, was fabricated on the surface of 0.16-mm thick glass substrate, and used with a 50-μm thick silicon rubber sheet as a spacer for the gel. The transient temperature rise of the sensor was determined from the electrical resistance after heating the sensor at a constant current. The thermal conductivity and the thermal diffusivity of a sample as well as the thickness of the gel layer were then determined from an iteratively obtained theoretical temperature rise that agreed with the measured temperature rise. The results obtained by the experiments with four different materials indicated that the thermal conductivity could be determined within 10% errors. The present study therefore demonstrated feasibility of the method, while improvement is still needed to reduce the error particularly in the thermal diffusivity.

Assessment of pulse conditions effects on reliability in GaN-based high electron mobility transistors by transient temperature measurements

The forward Schottky characteristic method, utilizing the temperature dependence of forward gate-source Schottky junction voltage, has been used to measure the transient temperature rise under DC and cycle pulse for multi-finger AlGaN/GaN high electron mobility transistor. The effect of electrical pulse on channel temperature has been studied. The transient temperature rise of channel under pulses with different duty cycles and frequencies are determined, respectively. The measurement results show that operation under high frequency and/or low duty cycle can improve the lifetime and performance reliability of AlGaN/GaN HEMT devices effectively. Furthermore, the measurement results are found to be consistent with the electro-thermal simulation results performed for the device.

Optical frequency domain reflectometry measurements of spatio-temporal temperature inside catalytic reactors: Applied to study wrong-way behavior

An Optical Backscatter Reflectometer (OBR) with optical frequency domain reflectometry (OFDR) enables measurement of the spatial temporal temperature inside catalytic reactors with unparalleled combined spatial and temporal resolution. It was used to measure the transient temperature inside a single channel of a washcoated monolith reactor in which the Pt-catalyzed oxidation of ammonia was carried out. Measurements of the monolith temperature response to either a rapid step decrease in the feed temperature or an increase of the flow rate were used to test the capability of the OFDR technique and to determine their impact on the transient temperature rise of the monolithic catalyst. The rapid step increase in the flow rate can generate a transient peak temperature of the solid phase higher than that at the steady state. This transient temperature rise was smaller than that caused by a sudden decrease of the feed temperature.

Numerical Calculation of Transient Thermal Characteristics in Gas-Insulated Transmission Lines

For further knowledge of the thermal characteristics in g as- i nsulated t ransmission l ines (GILs) installed above ground , a finite-element model of coupling fluid field and thermal field i s established , in which t he corresponding assumptions and boundary conditions ar e given. Transient temperature rise processes of the GIL under the conditions of v ariable ambient temperature, wind velocity and solar radiation are respectively investigated . E quivalent of surface convecti ve heat transfer coefficient and heat flux boundary conditions a re updated i n the analysis process. Unlike the traditional finite element method (FEM), the variability of the thermal properties with temperature is considered. The calculation results ar e validated by the tests results reported in the literature. The conclusion provides method and theory basis for the knowledge of transient temperature rise characteristics of GILs in open environment .

Characterization of vertical buried defects using lock-in vibrothermography: I. Direct problem

The ability of lock-in vibrothermography to distinguish between different sizes, shapes and locations of vertical kissing defects, such as cracks or corrosion, is analyzed in this work both theoretical and experimentally. We have computed the oscillating, AC, surface temperature of samples containing inner modulated heat sources, representing the defect, and analyzed the sensitivity of this oscillating temperature to the defect geometric parameters. Moreover, we have prepared samples with calibrated vertical defects. Vibrothermographic data are usually recorded under non-steady conditions (i.e. while the surface temperature is still rising). As this transient temperature rise has a non-negligible component at the excitation frequency, the measured AC temperature is strongly distorted. By subtracting the Fourier component of the transient temperature rise from vibrothermographic data, we obtain the pure AC temperature (amplitude and phase) in very good agreement with the theoretical predictions.

Deformation Index–Based Modeling of Transient, Thermo-mechanical Rolling Resistance for a Nonpneumatic Tire

ABSTRACT When competing in performance with their pneumatic counterparts, nonpneumatic tires should have several critical features, such as low energy loss when rolling over obstacles, low mass, low stiffness, and low contact pressure. In recent years, a nonpneumatic tire design was proposed to address each of these critical issues [1]. In this study, the steady state and transient energy losses due to rolling resistance for the proposed nonpneumatic tire are considered. Typically, such an analysis is complex because of the coupling of temperature on the structural deformation and the viscoelastic energy dissipation, which requires an iterative procedure. However, researchers have proposed a simplified analysis by using the sensitivity of the tire's elastic response to changes in material stiffness through a deformation index [2–4]. In the current study, the method is exploited to its full potential for the nonpneumatic tire due to the relatively simple nature of deformation in the tire's flexible ring and the lack of several complicating features present in pneumatic tires, namely, a heated air cavity and the complex stress state due to its composite structure. In this article, two models were developed to predict the transient and steady-state temperature rise. The first is a finite element model based on the deformation index approach, which can account for thermo-mechanical details in the tire. Motivated by the simplicity of the thermo-behavior predicted by this finite element model, a simple lumped parameter model for temperature prediction at the center of the shear band was developed, which in many cases compares very well with the more detailed finite element approach due to the nature of the nonpneumatic tire. The finite element model can be used to, for example, explore the design space of the nonpneumatic tire to reach target temperatures by modifying heat transfer coefficients and/or material properties.

Joule heating and current-induced domain wall motion

We investigate numerically and experimentally the Joule heating produced by current pulses and its contribution to current-induced domain wall (DW) motion in a (Ga,Mn)As ferromagnetic semiconductor. Different thermal coupling between tracks and substrates are explored. A direct contact leads to a logarithmic transient temperature rise and a stationary state determined by the substrate thickness. The introduction of a low thermal conducting (Ga,In)As interlayer produces an additional temperature rise whose time variation and magnitude are analyzed. Experimentally, the measured temperature rises present a good agreement with predictions over more than four orders of magnitude in time for values of the heat conductivity and of the heat capacity close to those reported in the literature. The Joule heating is shown to produce non-linearities in the domain wall velocity versus current density characteristics. A correction of Joule heating is proposed and permits the identification of the flow regimes from a comparison of domain-wall dynamics in tracks presenting different pinning characteristics.

Inlet Cone Effect on Diesel Particulate Filter Regeneration upon a Rapid Shift to Idle

The particulate matter (PM) emitted by a diesel engine is collected and then burned in a diesel particulate filter (DPF). A major technological challenge in the operation of the ceramic, often cordierite, filter is that a rapid shift to idle may create a local hot region with a temperature much higher than under stationary feed conditions. This excessive transient temperature rise may cause local melting or cracking of the ceramic filter. Almost all previous studies of temperature excursions during the DPF regeneration (combustion of the deposited PM) were of cases in which equal exhaust flow rate was fed to all the parallel inlet channels. The diesel engine exhaust pipe is sometimes connected to the DPF by a wide-angled cone (diffuser). This leads to a mal-distribution of the flow rate to the inlet channels and of the deposited PM. Simulations revealed that following a rapid shift to idle the highest regeneration temperature in a DPF fed by a cone exceeded that in one not fed by a cone and it may exceed the cordierite DPF melting temperature (1200 °C). Moreover, it may generate transient radial and axial temperature gradients several times higher than under stationary regeneration that may crack the cordierite DPF. The increase in the temperature gradient is especially large in the axial direction. One of the surprising findings is that the highest temperature attained following a step change to idle is not a monotonic function of the initial PM loading.

Experimental study on temperature rise of acoustic radiation force elastography.

Acoustic radiation force (ARF) elastography is potentially useful for imaging the elasticity of human tissue. Because a "push wave" that is used to generate ARF is a long burst wave comparable to that used in regular clinical imaging, detailed investigation of its safety is required. We focus on the transient temperature rise in the far field, where the beam paths are overlapped. Soft tissue mimicking a phantom and bone samples were exposed to a 2-MHz plane wave for 20s. The temperature rises in the far field were measured using a thermocouple. The temperature rises at 1ms, the time required for the displacement measurement, were estimated by fitting the experimental results. The results showed that the thermosensitivity of the bone was 36 times higher than that of the phantom, and the use of a repeated push wave may have exceeded the allowable maximum temperature rise, 1°C, on the bone surface. In conclusion, the imaging area, including the path of the push wave, should be carefully checked and the time interval for consecutive use should be adjusted to prevent thermal risk on the surface of the bone.

Nanoscale Heat Transport in Self-Organized Ge Clusters on Si(001)

ABSTRACTUltrafast time resolved transmission electron diffraction (TED) in a reflection geometry was used to study the cooling behavior of self-organized, well defined nanoscale germanium hut and dome clusters on Si(001). The clusters were heated in a pump-probe scheme by fs-laser pulses. The resulting transient temperature rise was then determined from the drop in diffraction intensity caused by the Debye-Waller effect. From a cooling time of τ =177 ps we estimated a strongly reduced heat transfer compared with homogeneous films of equivalent thickness.

Theoretical estimation of adiabatic temperature rise from the heat flow data obtained from a reaction calorimeter

A novel method for estimating the transient profile of adiabatic rise in temperature has been developed from the heat flow data for exothermic chemical reactions that are conducted in reaction calorimeter (RC). It has also been mathematically demonstrated by the present design that there exists a direct qualitative equivalence between the temporal evolution of the adiabatic temperature rise and the concentration of the limiting reactant for an exothermic chemical reaction, carried out in semi batch mode. The proposed procedure shows that the adiabatic temperature rise will always be less than that of the reaction executed at batch mode thereby affording a thermally safe corridor. Moreover, a unique reaction scheme has been designed to establish the independent heat effect of dissolution and reaction quantitatively. It is hoped that the testimony of the transient adiabatic temperature rise that can be prepared by the proposed method, may provide ample scope for further research.

Optoacoustic Imaging: An Emerging Modality for the Gastrointestinal Tract

Optoacoustic Imaging: An Emerging Modality for the Gastrointestinal Tract

Measurements of Hydrogen Thermal Conductivity at High Pressure and High Temperature

The thermal conductivity for normal hydrogen gas was measured in the range of temperatures from 323 K to 773 K at pressures up to 99 MPa using the transient short hot-wire method. The single-wire platinum probes had wire lengths of 10 mm to 15 mm with a nominal diameter of 10 μm. The volume-averaged transient temperature rise of the wire was calculated using a two-dimensional numerical solution to the unsteady heat conduction equation. A non-linear least-squares fitting procedure was employed to obtain the values of the thermal conductivity required for agreement between the measured temperature rise and the calculation. The experimental uncertainty in the thermal-conductivity measurements was estimated to be 2.2 % (k = 2). An existing thermal-conductivity equation of state was modified to include the expanded range of conditions covered in the present study. The new correlation is applicable from 78 K to 773 K with pressures to 100 MPa and is in agreement with the majority of the present thermal-conductivity measurements within ±2 %.

Experimental Test and Numerical Analysis to Estimate Permissible Transport Current Considering Protection of High-Tc Superconducting Tapes in Adiabatic Condition

High-Tc superconducting (HTS) wires have showed good capabilities as an alternative conductor for superconducting magnet. The HTS wires have more current carrying capability within high external magnetic field and its stability against external disturbance is much higher than the low temperature superconducting (LTS) wires. But the HTS wire has slower normal zone propagation (NZP) velocity which makes HTS magnets hard to be protected from unexpected hot spot. This paper presents experimental test and numerical analysis about the relation between transport current and rate of temperature rise in a hot spot. A sample which contained a HTS wire, a heater, and temperature sensor was fabricated and an experimental test on measuring normal zone propagation and temperature of the HTS wire was conducted. A numerical model was also built to estimate transient temperature rise of a winding pack model which was representing practical winding of HTS magnets. The numerical model and its numerical estimation results are expected to be a guideline about designing proper protection technique and operating current for HTS magnets.

Transient temperature rise during regeneration of diesel particulate filters

A major technological challenge in the regeneration of diesel particulate filters (DPFs) is that sometimes the exothermic regeneration generates excessively hot local zones that melt the cordierite ceramic filter. The cause of this melting is still an open question. The temperature rise under stationary (constant feed conditions) regeneration is not sufficiently high to cause this melting. We investigated a conjecture that the high temperature excursions in a DPF may be the dynamic response to a shift from normal driving mode to idle. This shift affects three inputs: It increases the oxygen concentration and simultaneously decreases the feed temperature and filtration velocity. Infrared measurements of the spatio-temporal temperature during soot combustion on a single layer DPF confirmed that the response to such a simultaneous step change leads to a transient temperature that exceeds the highest attained for stationary operation under either the initial or final operation conditions. The transient temperature rise caused by a simultaneous step change of the three inputs was rather close to the sum of the responses the three individual changes. The amplitude of the temperature excursion depended in a complex way on several factors, such as the direction of the moving temperature front and whether the feed step-change was done before or after the formation of a moving front, and the period after the initiation of the temperature front.

Dynamic Calibration and Performance of Reliable and Fast-Response Coaxial Temperature Probes in a Shock Tube Facility

An experimental dynamic calibration technique of reliable, rugged, low-cost, and fast-response coaxial temperature probes is presented. These probes were successfully designed and fabricated in-house, in conjunction with its signal processing circuit, which can be used for transient heat transfer measurements in a hypersonic testing facility. These probes have a response time less than 50 μs with a rise time less than 0.3 μs. Two types of scratches were used, mainly abrasive papers with different grit sizes and scalpel blades with different thicknesses to form the probe junction. The effect of the scratch technique on the probe's thermal product is investigated. The probes were tested and calibrated in the test section and end wall of the UNITEN shock tube facility at different axial and radial locations. The effects of placing the coaxial temperature probe at different axial and radial distances, different working fluids, and different Mach numbers on the transient surface temperature rise were examined. It was observed from the dynamic calibration results that the thermal product of a particular coaxial temperature probe depends on Mach number, junction scratch technique, and junction location, as well as on the enthalpy conditions. It was also noticed that the calibrated coaxial temperature probe using the scalpel blade technique with a particular blade size gives consistent thermal product values. Thus, it does not require an individual calibration. However, for a coaxial temperature probe whose junction was created using the abrasive paper technique with different grit sizes, a calibration for each coaxial temperature probe is likely to be needed.