Transient Heating Conditions Research Articles

Numerical study on temperature response of microheater and evolution characteristics of superheated zone under fast transient heating

The nucleate boiling process that occurs on the surface of a microheater immersed in a liquid pool under fast transient heating conditions can be regarded as homogeneous or near-homogeneous nucleation, exhibiting a significant difference when compared to boiling at conventional time scales on large and even micro spatial scales. This paper numerically investigates the boiling process at both small spatial and temporal scale by using a validated model. Results indicate a significant difference between the average microheater temperature and the maximum temperature at the boiling surface, which can reach up to 80 °C within the scope of present study. Consequently, the nucleation temperature, typically determined by the average temperature of the microheater in most references, may not precisely represent the homogeneous (or nearly homogeneous) nucleation temperature. At the initiation of nucleation, the profiles at the boundary of superheated zone are approximately circular in the z-plane and arched in the x-plane, respectively. Meanwhile, the profile of the superheated zone boundary at the onset of nucleation under direct heating mode (DHM) appears approximately trapezoidal, exhibiting a significant difference from that observed under indirect heating mode (IHM). It is also noted that the local temperature distribution in the fluid region can be considerably affected by natural convection, potentially exerting a substantial influence on the bubble dynamic behaviors during the subsequent bubble growing phase. These findings may provide valuable references for future experimental study on the mechanisms involving in microscale boiling heat transfer process under rapid heating conditions.

Relationships between Electroencephalogram and Thermal Perception of Passenger in Winter Vehicle Compartments

The development of electric vehicles (EVs) has prompted a critical examination of the trade-off between range and human thermal comfort. Therefore, an accurate, real-time assessment of human thermal perception inside vehicles is important. This study investigates an electroencephalogram- (EEG-) based method for evaluating human thermal comfort in the vehicle passenger compartment. Under transient winter heating conditions, the study experimentally investigates the correlation between objective physiological parameters (skin temperature and electroencephalogram) and subjective human thermal perception. The results reveal distinct patterns in EEG signals corresponding to changes in thermal perception. Specifically, the δ rhythm exhibits a U-shape variation with increasing thermal perception, while the θ, α, β, and γ rhythms display an inverted U-shape variation. Differences in each frequency band across thermal comfort states in humans are greater than differences in the frequency band across thermal sensation states. Furthermore, the relative power of the θ rhythm emerges as the most effective in discerning the thermal perception state of the human body. The EEG signal characteristics of the T7 and T8 channels align more closely with human thermal sensation, whereas the AF4 channel excels at discriminating the state of human thermal comfort. The insights gained from this study serve as a foundation for evaluating human thermal perception in vehicles, enhancing human-vehicle interaction, and addressing challenges related to human thermal comfort and vehicle range.

Model development for oxidation and degradation behavior of accident tolerant Cr coating on Zr alloy cladding under high temperature steam atmosphere

The oxidation and degradation mechanisms of Cr coating on Zr alloy cladding under high temperature steam atmosphere are summarized. Six basic phenomena are considered for model development, including: oxidation of Cr, growth of ZrCr2, formation of Zr(Cr) solid solution, outward diffusion of Zr, reduction of Cr2O3, and volatilization of Cr2O3. The mathematical analysis model is established and validated, demonstrating its good capacity to predict the increase and subsequent decrease in the thickness of Cr2O3 scale, as well as the thickness changes in other individual layers. The developed model is applied in the analysis of the structure evolution of Cr coating under steady state, transient heating conditions and postulated accident scenarios.

Computational modelling and investigation of nucleate boiling with periodic exponential heat flux-based power-transients

The present work proposes a numerical methodology to perform single bubble nucleate boiling simulation subjected to power transients with exponential heating. Cyclic variation in transient heat flux is utilized which enables—proper characterization of the power transient parameters, parametric investigation of power transients on single bubble dynamics, and comparison between steady and transient heating condition. For constant time averaged heat flux in power cycle, it was observed that heating parameters are not having any influence on bubble departure diameter and bubble growth period whereas the bubble departure frequency is enhanced until vertical coalescence is observed. Parametric investigations based on time period of periodic heat flux and time averaged heat flux reveal the existence of a range in excursion time period which cause increase in bubble departure frequency, with applicability for controlling bubble dynamics with power transients and improving efficiency of thermal management devices using nucleate boiling.

Spalling mitigation techniques for high-performance concrete at elevated temperature

High-performance concrete (HPC) is highly susceptible to temperature-induced spalling, which unveils severe damage of HPC structural members. Therefore, this research work is aimed at developing spalling mitigation techniques for HPC and experimentally assessing spalling behaviour at transient heating conditions up to 1000 °C for a 4-hour fire resistance rating. A total of four HPC mixes were comprised for spalling behaviour evaluation, in which Mix H1 was designed only using main binder ordinary Portland cement (OPC), whereas Mix-H2, Mix-H3, and Mix-H4 were designed by partial replacement of OPC with three different supplementary cementitious materials (SCMs), i.e., fly ash (FA), silica fume (SF), ground granulated blast furnace slag (GGBS) one at a time as a binary composite system. Polypropylene (PP) fibres and steel (ST) fibres one at a time from a range of 0.00, 0.62, 1.24, 1.86, and 2.48 kg/m3 and ST fibres range of 0.00,12.43, 24.87, 37.30, and 49.74 kg/m3 were incorporated into HPC mixes for evaluating spalling mitigation behaviour at elevated temperature. For each HPC mix, a total of three standard cylindrical specimens (100 mm diameter × 200 mm height) were exposed to transit temperature 1000 °C for up to 4 h to assess spalling behaviour. It is observed that Mix-H1 content only OPC does not reveal any severe damage and spalling behaviour, whereas Mix-H2, Mix-H3, and Mix-H4 comprising SCMs revealed very severe damage and spalling behaviour. This result indicates that the highly dense microstructure with a highly durable HPC matrix using SCMs does not allow any moisture movement/migration from the inherent concrete binding matrix to the outer exposed surface of cylinder specimens and resulting in explosive-type spalling of HPC. However, using 1.86 kg/m3 of PP fibres and 37.30 kg/m3 of ST fibres in HPC binary systems has a positive impact on mitigating spalling of HPC at elevated temperatures.

Thermal creep of austenitic stainless steel plate material under transient heating conditions

Owing to its exceptional features, ASTM Grade 304 (EN 1.4301) austenitic stainless steel is the material of choice for numerous industrial, structural, and architectural applications, particularly where high resistance to corrosion and elevated temperatures is required. Like other metals, stainless steel undergoes time-dependent inelastic deformations at fire temperatures, called ”thermal creep”. The thermal creep of Grade 304 stainless steel has been previously studied under steady-state (isothermal) conditions. However, due to the complexities involved in testing and data analysis, the ”transient creep”, i.e. thermal creep under transient heating regimes, of this material has not yet been studied. In response to this knowledge gap, the present study investigates the thermal creep of Grade 304 stainless steel plate material when subjected to continuously rising temperatures. First, the thermal expansion and the transient stress–strain curves of the stainless steel are obtained at elevated temperatures up to 900 °C. Then, by comparing the material’s response under two different heating rates of 5 and 20 °C/min, an analytical creep model is developed that is not only capable of simulating all three stages of creep under transient conditions, but also computationally efficient. Finally, to demonstrate the practical applicability of the developed model for component-level analyses, it is implemented as a user-defined subroutine in ABAQUS finite element software to simulate transient creep in a typical plate structural component. Results show that by reducing the heating rate from 20 to 5 °C/min, due to the transient creep effects, the plate’s buckling and failure temperatures can drop by up to around 12% and 6%, respectively, suggesting the significance of transient creep effects in the stainless steel plates.

Transient Flow Boiling and Maldistribution Characteristics in Heated Parallel Channels Induced by Flow Regime Oscillations

Flow boiling provides an effective means of heat removal but can suffer from thermal and hydrodynamic transients that compromise heat transfer performance and trigger device failure. In this study, the transient flow boiling characteristics in two thermally isolated, hydrodynamically coupled parallel microchannels are investigated experimentally. High-speed flow visualization is synchronized to high-frequency heat flux, wall temperature, pressure drop, and mass flux measurements to provide time-resolved characterization. Two constant and two transient heating conditions are presented. For a constant heat flux of 63 kW/m² into each channel, boiling occurs continuously in both channels and the parallel channel instability is observed to occur at 15 Hz. Time-periodic oscillations in the pressure drop and average mass flux are observed, but corresponding oscillations in the wall temperatures are virtually nonexistent at this condition. At a slightly lower constant heat flux of 60 kW/m², boiling remains continuous in one of the channels, but the other channel experiences time-periodic flow regime oscillations between single-phase and two-phase flow. At this condition, extreme time-periodic wall temperature oscillations are observed in both channels with a long period (~7 s) due to oscillations in the severity of the flow maldistribution. For the transient heating conditions, square-wave heating profiles oscillating between different heat flux levels are applied to the channels. Because of their relatively high frequency, the heating transients are attenuated by the microchannel walls, resulting in effectively constant heating conditions and flow boiling characteristics like that of the aforementioned constant heating conditions. This study illustrates the susceptibility of parallel two-phase heat sinks to flow maldistribution, particularly when undergoing transient flow regime oscillations.

The effect of adhesive type and ply number on the compressive strength retention of CLT at elevated temperatures

This paper describes novel experiments to identify the reduction in compressive strength of cross-laminated timber (CLT) at elevated temperatures. The adhesive type and number of timber plies was varied between CLT samples to allow an assessment of the CLT ‘system’, rather than timber only. Samples were subjected to steady state and transient heating conditions and their ultimate load bearing capacities were measured. At ambient reference temperatures, a statistically significant difference in ultimate compressive strength was observed between ply numbers but not between adhesive types. For transiently heated samples, capacity was reduced at 100 °C for CLT bonded with polyurethane adhesive type as compared to samples bonded with melamine formaldehyde. The presented results indicate that the composition of CLT can influence the whole system structural response, both at ambient temperature and under fire conditions.

In-situ deformation measurement of Zircaloy-4 cladding tube under various transient heating conditions using optical image analysis

In a loss of coolant accident (LOCA), fuel cladding deformation is an important factor in terms of maintaining coolable geometry. In order to simulate the cladding deformation behavior, experimental research and the development of a fuel performance code have been conducted. In addition, relevant experimental data concerning the major phenomena for validating the developed code are required. Therefore, in this study, an experimental apparatus named ‘DIMAT’ (Deformation In-situ Measurement Apparatus by image-analysis Technique) is developed to produce high temperature cladding tube deformation data. In order to simulate various accident conditions, an infrared (IR) furnace and pressure injection equipment were assembled. Additionally, the cladding specimen in the IR furnace was clearly recorded by application of high temperature resistant black spray paint and modification of a quartz tube. Some uncertainty factors that might arise due to the paint application were considered and evaluated. After the deformation behavior in a scenario was recorded, the in-situ hoop strain of the cladding was calculated through the developed image analysis method. To ensure accuracy of the established image analysis system, a validation experiment and uncertainty evaluation were performed. Finally, the burst experiments were carried out at low and high heating rates of 1, 14, and 28 °C/s, and consequently the in-situ high temperature strain rate was successfully measured in each experimental case.

Measurement of Zircaloy-4 cladding tube deformation using a three-dimensional digital image correlation system with internal transient heating and pressurization

In this study, multi-dimensional (Multi-D) deformation of a Zircaloy-4 cladding tube under internal transient heating and pressurization conditions was measured by experimental setup named ‘Experimental setup to investigate high temperature deformation of cladding tube under internal pressurizing and heating (EIGEN)’ and three-dimensional digital image correlation (3D DIC) system. The ‘EIGEN’ experimental setup was designed to investigate high temperature deformation of the cladding tube under inert gas atmosphere. The 3D DIC system based on stereovision was further developed to be applied to high temperature by using high temperature resistant speckle pattern and bandpass filter. To evaluate measurement accuracy of the 3D DIC system, validation works of 3D DIC calibration were performed with proposed methodology. Additionally, a 3D finite element model (FEM) of the ‘EIGEN’ experiment was developed to evaluate the results of the 3D DIC. Finally, ‘EIGEN’ experiment was conducted under several fill pressure conditions such as 30, 50, 60, 70 bars. As a result, the established 3D DIC system could successfully measure in-situ and real-time large deformation such as ballooning and rupture as well as full-field multi-D deformation of the cladding tube at the high temperature under the inert atmosphere. Multi-D strains from the 3D DIC showed good agreement with calculated ones of the 3D FEM, and they were also compared with 3D laser scanning results for validation.

Time-resolved characterization of microchannel flow boiling during transient heating: Part 2 – Dynamic response to time-periodic heat flux pulses

Flow boiling in microchannels is an effective method for dissipating high heat fluxes. However, two-phase heat sink operation during transient heating conditions remains relatively unexplored. In Part 1 of this two-part study, the dynamic response of flow boiling to a single heat flux pulse was experimentally studied. In this Part 2, the effect of heating pulse frequency on microchannel flow boiling is explored when a time-periodic series of pulses is applied to the channel. HFE-7100 is driven through a single 500 μm-diameter glass microchannel using a constant pressure reservoir. A thin indium tin oxide layer on the outside surface of the microchannel enables simultaneous transient heating and flow visualization. High-frequency measurements of heat flux, wall temperature, pressure drop, and mass flux are synchronized to the flow visualizations to characterize the boiling process. A square-wave heating profile is used with pulse frequencies ranging from 0.1 to 100 Hz and three different heat fluxes levels (15, 75, and 150 kW/m2). Three different time-periodic flow boiling fluctuations were observed for the heat flux levels and pulse frequencies investigated in this study: flow regime transitions, pressure drop oscillations, and heating pulse propagation. For heat flux pulses between 15 and 75 kW/m2 and heating pulse frequencies above 1 Hz, time-periodic flow regime transitions between single-phase and two-phase flow are reported. For heating profiles involving 150 kW/m2 heat flux pulses, fluid in the microchannel is always boiling and thus the flow regime transitions are eliminated. For heating pulse frequencies between approximately 1 and 10 Hz, the thermal and flow fluctuations are heavily coupled to the heating characteristics, forcing the pressure drop instability frequency to match the heating frequency. Outside this heating pulse frequency range, the pressure drop instability occurs at the intrinsic frequency of the system. For heating pulse frequencies above 25 Hz, the microchannel wall attenuates the transient heating profile and the fluid essentially experiences a constant heat flux.

Validation of pyrolysis model in transient heating scenarios and diverse spectral boundary conditions

This study aims to provide a robust validation to the condensed-phase pyrolysis model approach used in the FireFOAM CFD code. Firstly, bench-scale pyrolysis tests were conducted for two engineering wood products (EWPs), namely plywood and medium density fiberboard (MDF), at three constant heat flux levels. These tests were used to derive model-effective material properties via inverse modeling and optimization approaches. For MDF, a better model fit was obtained when the virgin material density was allowed to vary along its thickness, which is consistent with its known anisotropic nature. Next, three test cases were developed for the purpose of pyrolysis model validation by subjecting the EWPs to fire-relevant dynamic heating scenarios. In two tests, the materials were exposed to transient heating conditions where the heat flux to the materials increased over the duration of pyrolysis. In the remaining test, the spectral emissivity boundary condition of the sample material was altered significantly. Very good agreement was observed between the validation experimental data and the model predictions for all three scenarios. The validation studies provide confidence that the model-effective material properties, determined using inverse modeling and optimization for constant heating conditions, can be confidently applied to fire-relevant dynamic heating scenarios and boundary conditions.

Heat pipe dryout and temperature hysteresis in response to transient heat pulses exceeding the capillary limit

The balance between the capillary pressure provided by the wick in a heat pipe or vapor chamber and the flow resistance to liquid resupply at the evaporator determines the maximum heat load that can be sustained at steady state. This maximum heat load is termed as the capillary limit; operation at steady heat loads above the capillary limit will result in dryout at the evaporator wick. However, different user needs and device workloads can lead to highly transient heat loads in applications ranging from consumer electronic devices to server processors. In these instances, the operation of heat pipes must be assessed in response to brief transient heat loads which could be higher than the notional capillary limit that governs dryout at steady state. In the current study, experiments are performed to characterize the transient thermal response of a heat pipe subjected to heat input pulses of varying duration that exceed the capillary limit. Transient dryout events due to a wick pressure drop exceeding the maximum available capillary pressure can be detected from an analysis of the measured temperature signatures. It is demonstrated that under such transient heating conditions, a heat pipe can sustain heat loads higher than the steady-state capillary limit for brief periods of time without experiencing dryout. If the heating pulse is sufficiently long as to induce transient dryout, the heat pipe may experience an elevated steady-state temperature even after the heat load is reduced back to a level lower than the capillary limit. The steady-state heat load must then be reduced to a level much below the capillary limit to fully recover the original thermal resistance of the heat pipe. This characteristic temperature hysteresis following transient dryout has significant implications for the use of heat pipes for pulsed power dissipation. Further experiments are performed to bound the range of heat loads over which the temperature hysteresis is present following a transient dryout event.

Optimized high performance thermoelectric generator with combined segmented and asymmetrical legs under pulsed heat input power

In this study, a segmented asymmetrical thermoelectric generator (SASTEG) is numerically investigated to optimize its electrical performance and mechanical reliability under transient and steady state conditions. The thermal and electrical performance of the SASTEG and TEG under transient and steady state heating conditions are studied and compared. A three-dimensional numerical model is developed and solved using finite element method in COMSOL 5.3 Multiphysics software. Temperature dependent thermoelectric material properties are considered, and the elastoplastic behaviour of copper and solder is accounted for in the thermal stress analysis. The initial SASTEG geometry used in this study is subsequently optimized to reduce the maximum von Mises stress developed in its legs while maintaining its enhanced power output. Results obtained show that the optimized SASTEG provided a power output enhancement of 117.11% compared to that of the conventional TEG under rectangular pulsed heat condition. Also, the asymmetrical leg geometry used in the SASTEG n-type leg provided a reduced thermal stress of 39.21% in the lower segment (cold side) compared to the symmetrical leg geometry used in the p-type leg lower segment. This study will provide vital guidance in the design of high-performance TEG with improved mechanical reliability.

Study of clad ballooning and rupture behaviour of Indian PHWR fuel pins under transient heating condition in steam environment

This paper presents the results of the investigation on the deformation and rupture characteristics of Indian pressurized heavy water reactor (IPHWR) fuel pins under simulated loss of coolant accident (LOCA) condition in steam environment. Transient heating experiments were carried out on single fuel pin internally pressurized with argon gas in the range 3–70 bar. Effect of internal pressure on burst temperature, influence of burst temperature on the circumferential strain and rupture opening area were also studied. Two circumferential strain maxima at the burst temperatures of 740 & ∼979 °C and a minimum at the burst temperature of ∼868 °C were observed. It was found that oxidation had considerable effect on the burst behavior. Test data were used to derive a direct empirical correlation for burst stress exclusively as a function of temperature. The ballooning and rupture behaviours in steam and argon environments have been compared. Experimental data were examined against various correlations using Erbacher equation and author's previous correlation in argon.A second burst correlation has also been developed combining the equation in argon from the previous work of the authors and an exponential factor with oxygen content as a parameter assuming the burst stress to be a function of both temperature and oxygen concentration. The burst temperatures predicted by this empirical correlation are in good agreement with the test data.

A Comparison of the Plastic-Flow Response of a Powder-Metallurgy Nickel-Base Superalloy Under Nominally-Isothermal and Transient-Heating Hot-Working Conditions

The flow-stress behavior at hot-working temperatures and strain rates of the powder-metallurgy superalloy LSHR was determined under nominally-isothermal and transient-heating conditions. Two conventional methods, compression of right-circular cylinders and torsion of thin-walled tubes, were used for isothermal tests. A direct-resistance-heating technique utilizing torsion of round-bar specimens in a Gleeble® machine was applied for both isothermal and transient-heating conditions. When expressed in terms of effective stress and strain, baseline data determined by the two conventional methods showed good agreement. With the aid of a flow-localization analysis to assess the confounding influence of axial (and radial) temperature gradients on deformation uniformity, the flow stresses determined from nominally-isothermal Gleeble® torsion tests were shown to be broadly similar to those from the conventional tests. With regard to transient phenomena, Gleeble® tests were also useful in quantifying the effect of rapid heating and short soak time on the observed higher flow stress associated with a metastable microstructure. The present work also introduces two new test techniques using direct-resistance-heated torsion specimens. One involves continuous heating under constant-torque conditions, and the other comprises testing an individual specimen at a series of temperatures and strain rates. Using a single specimen, the former method enabled the determination of the apparent activation energy for plastic flow, which was similar to that determined from the series of isothermal tests; the latter provided a low-cost, high-throughput approach to quantify the flow behavior.

A unified rheological model for modelling steel behaviour in fire conditions

This paper presents a newly developed rheological model capable of modelling the behaviour of carbon steel at high temperature under stress- and strain-rate controlled tests. By combining two serial Kelvin elements with the appropriate spring-and-damper constitutive behaviour models it is possible to model creep strain development under stationary and transient heating conditions. Furthermore, the model is able to take into account the inherent increase of the yield strength if the strain rate is raised to moderately high levels usually expected in a fire-induced collapse of the structure. Constitutive behaviour models for each of the rheological elements are based on the test data from which the Eurocode 3 stress-strain law originated. The model was verified by using the test results of constant stress- and strain-rate tests from various sources. Overall comparison of results indicates the applicability of the proposed rheological model to structural fire engineering analysis for steel grades S275 and S355.

Response of Phase-Change-Material-Filled Porous Foams Under Transient Heating Conditions

Thermal energy storage using phase-change materials is a common method to cool systems passively. These phase-change materials often have low thermal conductivity, compromising their effectiveness in cooling. One solution commonly employed is to fill thermally conductive foams with phase-change materials to increase overall thermal conductivity. These systems are often designed to use solid/liquid phase change from the phase-change material to control a system’s temperature for as long as possible. However, the transient response of the thermal storage system has not been fully explored, particularly for aerospace applications in which the conditions of the surrounding environment are rarely steady. Consequently, an experimental study focusing on changing boundary conditions was developed, the results of which are reported here, and recommendations on the use of different foams for certain conditions are provided. The experimental results are compared to an existing numerical model to determine its predictive value.

Combination of heat storage and thermal spreading for high power portable electronics cooling

This study proposes a novel cooling method by combining a liquid–metal phase-change material module (LMPCM) and a flat heat pipe thermal spreader (FHP) for the application of cooling electronic devices. In this mechanism, the liquid–metal phase-change material module and the flat heat pipe function as a heat storage and thermal spreader, respectively. Thermal performance of this novel design was experimentally evaluated at both steady and transient heating conditions with different heating power inputs. Results indicate that the LMPCM+FHP modules could effectively restrain the temperature rise of electronic chips and prolong the usage time of electronic devices. Furthermore, the FHP–LMPCM module outperformed other scenarios that included the LMPCM–FHP module, copper–LMPCM module, LMPCM module, FHP module and copper sheet. Moreover, results indicate that the FHP–LMPCM module could maintain the temperature of electronic devices within an allowable temperature range even under the conditions of the PCM completely melting at transient-state heating conditions, proving this cooling method to be a very promising solution for the thermal management of high power portable electronics.

The effect of azimuthal temperature distribution on the ballooning and rupture behavior of Zircaloy-4 cladding tube under transient-heating conditions

ABSTRACTFor the purpose of investigating the effect of azimuthal temperature distribution on the ballooning and rupture behavior of Zircaloy-4 (Zry-4) cladding tube, laboratory-scale experiments on non-irradiated Zry-4 cladding tube specimens were performed under transient-heating conditions which simulate loss-of-coolant accident (LOCA) conditions by using an external heating method, and the data obtained were compared to those from a previous study, where an internal heating method was used. The maximum circumferential strains of the cladding tube specimens were firstly divided by the engineering hoop stress. The divided maximum circumferential strains, ks, of the previous study, which used the internal heating method, were then corrected based on the azimuthal temperature difference (ATD) in the cladding tube specimen. The ks for the external heating method which was used in this study agreed fairly well with the corrected ks obtained in the previous study which employed the internal heating method in the burst temperature range below ∼1200 K. Also, the area of rupture opening tended to increase with increasing of the value which is defined as the engineering hoop stress multiplied by the maximum circumferential strain. From the results obtained in this study, it was suggested that the maximum circumferential strain and the size of rupture opening of a cladding tube under LOCA-simulated conditions can be estimated mainly by using the engineering hoop stress, the maximum circumferential strain, and ATD in the cladding tube specimen, irrespective of heating methods.