Hot Layer Research Articles

The impact of fire source location on hot gas layer temperature in multi-compartment pre-flashover fires: Analysis and semi-empirical model development

A numerical study of a two-room structure (multi-compartment) subjected to fire in the inner room was carried out, employing the CFD (Computational Fluid Dynamics) software called FDS (Fire Dynamics Simulator), to (i) analyze the influence of the fire source position on the hot gas layer temperature, Tu, and its behavior in a multi-compartment fire, and (ii) develop a semi-empirical engineering calculation model to predict Tu in the adjacent room to a pre-flashover well-ventilated fire room, taking into account the fire source position. The results showed that when the fire origin occurs close to walls or elevated above the floor level, a reduced air entrainment is observed in the fire plume and flame, causing an increase in the Tu and a reduction of the smoke production in the fire room. This reduction in the smoke production led to a lower increase in the Tu of the adjacent room in relation to the case when the fire source was at the center of the fire room. It was also observed that the Tu in both rooms was inversely proportional to the ventilation factor (A.H0.5), with a significant increase observed as the ventilation factor decreases, depending on the fire source position. Furthermore, a semi-empirical engineering calculation model was developed to predict the Tu in the adjacent room, considering different fire source locations at ground level and elevated. The model was validated against experimental data from the literature, showing a good agreement (deviation of ≈20 %), thus demonstrating its applicability to other cases.

Multi-layer topology optimization of dual-fluid convective heat transfer in printed circuit heat exchangers

Optimizing channel configurations in printed circuit heat exchangers is essential for enhancing heat transfer and minimizing pumping power. The present study aims to optimize channel configurations by developing a multi-layer topology optimization (TO) model that simultaneously optimizes the fin structures in both the cold and hot fluid layers. The TO model directly integrates the heat transfer rate as the target function while using the pressure drop from an airfoil structure as the constraint. Counterintuitive channel configurations are generated by the TO model under various Reynolds numbers (Re). The evaluation under both laminar and turbulent conditions demonstrates that the TO-designed channel configuration outperforms conventional channel configurations including empty, airfoil, heatric, louver, modified louver, sine curve, and S-shaped designs. Compared with the airfoil structure, at Re = 300, the optimized structure can reduce pumping power by 17.0 % while increasing heat transfer by 10.8 %, and at Re = 20000 it reduces pumping power by 52.0 % and enhances heat transfer by 1.2 %. The present study introduces a promising method for designing novel printed circuit heat exchangers.

Quantum coupling and self-heating impacts on threshold voltages of GaN metal–insulator-semiconductor high-electron-mobility transistors

A transient heat conduction equation has been proposed to describe the cooling process of a hot two-dimensional electron gas layer in a GaN-based transistor. This equation serves as the basis for a physical analytical model that explains the impacts of hot electrons in the hot two-dimensional electron gas layer via quantum coupling on the threshold voltage of GaN-based transistors. The temperature of the two-dimensional electron gas layer decreases to ambient temperature as time increases, and the threshold voltage shifts caused by such a temperature will recover with time. The proposed model accurately reflects the observed recovery of threshold voltage over time in experiments. At the same time, it offers insight into how the gate voltage, the source-drain voltage, and the ambient temperature can shift the threshold voltage in these transistors. The simplicity and analytic nature of this proposed model not only provide a physical origin of the threshold voltage instability in GaN-based devices but also offer the possibility for improving device reliability by selecting appropriate device physical parameters.

Experimentally Validated Potential Theory Approach for Investigation of the Flow Field Structure of an Open Pool of a Nuclear Reactor

Detailed information on the flow field structure is often important in numerous industrial applications. Although commercial computational fluid dynamics packages are often capable of providing the required data, they are costly and not universally available. This study was motivated by the operation of an open-pool nuclear research reactor where low radiation levels can be maintained by the installation of a stable purified hot water layer in the upper part of the pool. Maintaining a stable stratification requires a detailed description of the structure of the velocity field. Due to the inherent complications and restrictions of performing accurate measurements in a pool of a real-size operating reactor, either smaller-scale models or oversimplified fluid dynamics computational schemes are routinely used. These methods cannot be validated, and therefore do not necessarily capture the large-scale behavior correctly. We present an alternative approach to evaluate the velocity components in the pool that is based on the potential flow theory. The model results are validated by measurements using particle image velocimetry. The presented potential theory allows for the quick and easy assessment of the global properties of the fluid velocity distribution within the pool, and in particular, close to its surface. The suggested computational models are flexible and allow for easily varying the spatial dimensions of the flow field. The technique thus can be upscaled, and enables the validation of numerical computations in various fluid mechanical installations where the flow field cannot be resolved.

Decorative 3D printing on textiles using elastomer TPU filament under different printing conditions

PurposeNew sustainable approaches to fashion products are needed due to the demand for customization, better quality and cost reduction. Therefore, the decoration of fashion products using 3D printing technology can create a new direction for manufacturing science.Design/methodology/approachThis study aims to optimize the 3D printing of soft TPU material on textiles. In the past decade, trials of using 3D printing in tailored fashion products have been done due to the 3D printing simplicity, low cost of materials and time reduction. Therefore, soft polymers can be multi-layer stepped-deposited smoothly with the fused filament fabrication process.FindingsEven though there have been many attempts in the literature to 3D print multilayer polymer filaments directly onto textile fabrics by special-purpose 3D printers, only a few reports of decorative or personalized artefact 3D printing using open-platform filament material extrusion 3D printers. Printing speed, nozzle Z distance, textile fabric thickness and deposited strand height significantly affect 3D printing on textile fabric.Originality/valueThis study investigates the potential of 3D printing on textiles by changing the printing speed, nozzle hot end, Z distance and layer thickness. It presents two critical case studies of 3D printing soft thermoplastic polyurethane material on a cotton T-shirt and on a tulle textile to reveal the 3D printing on textile fabrics manufacturing challenges.

Guldborgsund Arson House Fire Experiment and Numerical Investigation

AbstractThis paper describes the Guldborgsund arson house fire experiment performed in Denmark and the subsequent numerical investigation. Gas temperatures were measured with four thermocouple trees, and smoke detector activation times were recorded in all rooms. A two-step approach was used to perform the numerical modelling for reproduction of the fire scene. The measured temperatures in the room of fire origin were used as an input for back calculating the Heat Release Rate (HRR) with the two-zone model Argos. As a next step, this HRR was used in the Fire Dynamic Simulator (FDS) to predict the temperatures and the smoke detectors’ activation times in other rooms. The FDS model was partly build using output files from the laser scanning. A sensitivity analysis is presented, where the effect of nine input parameters was investigated, including HRR, the material properties, the height of the fuel bed, the fire area, level of geometrical detail of the first item ignited etc. This study showed that the measured soot deposition heights on the walls differed from the heights of measured sharp temperature gradients used to indicate the hot smoke layer. The numerical simulations resulted in less than 50% error for most of the temperature measurement points during the fuel-controlled stage of the fire and results were the most sensitive to the input HRR. Material properties in FDS had a significant influence on the computed upper-layer gas temperatures at late stages of the fire.

Corrosion mechanism of postmortem converters slag-blocking ZrO2 sliding gate

The post-service ZrO2 sliding gate for converter slag-retaining was examined by XRD, SEM, EDS and X-CT to investigate its corrosion mechanism. The results show that m-ZrO2 in the hot working layer transform into stable t-ZrO2, and cracks and flaws are generated due to the accompanied volume contraction, leading to severe slag penetration. The CaO in the slag preferentially reacts with ZrO2, forming a CaZrO3 dense layer with a high melting point to prevent slag penetration. While SiO2-Al2O3-FeO component in the slag removes the MgO stabilizer out of Mg-PSZ crystals, causing destabilization and decomposition of the Mg-PSZ grains. The microstructure loses its integrity and is easily spalled. A corrosion model was established.

Effect of viscosity variation with temperature on convective instability in a hot dusty ferrofluid layer with permeable boundaries

As the subject under consideration has numerous industrial, biological, and environmental applications, so the prime objective of this article is to investigate the onset of thermal convection in a dusty ferrofluid layer under the effect of change in viscosity due to temperature in the presence of permeable boundaries. The linear stability analysis is employed to acquire the eigenvalue problem and then the eigenvalue problem is solved for the stationary convection mode using a single-term Galerkin method. The numerical values of critical wave number and critical Rayleigh number are calculated using MATLAB software, and the results are graphically displayed. The investigation shows that the permeability of the boundaries affects the size of the convection cells that form at the beginning of thermal convection. Further, it is found that the parameters related to temperature-dependent viscosity and permeable boundaries have a stabilizing effect on the system, while magnetic and dust particle parameters destabilize the system.

GOTHIC and MELCOR benchmarking of the CNL strong condensation containment apparatus experiments

Understanding severe accident behaviour in water-cooled small modular reactors is crucial for their deployment, as well as having qualified software tools for safety analysis. A model benchmarking exercise was conducted where predictions from the software tools GOTHIC and MELCOR were compared against past experiments from the Strong Condensation Containment Apparatus (SCCA). Key phenomena that can be expected in small modular reactor (SMR) containments, given their small size and external passive water cooling, are explored, including how well the codes replicate stratification between a hot upper layer and a cooler under layer. This overall thermalhydraulics behaviour impacts other important phenomena, like heat transfer and aerosol deposition. MELCOR, an integrated severe accident code, and GOTHIC, a thermalhydraulic and containment code, were used in benchmarking activities, wherein the experimental results were compared to their simulation results. The findings demonstrated MELCOR’s capabilities to simulate the SCCA experiments and GOTHIC’s strength in predicting multiphase flow and thermalhydraulics in containment and cooling systems. In addition, the benchmark comparisons provided new insights toward modelling experiments with these codes as well as the behaviour observed in the SCCA experiment. These findings will guide accident management and emergency preparedness requirements for SMRs.

Optical Temperature Field Measurements With Two-Color Laser Induced Fluorescence In A Stratified Thermal Energy Storage

Thermal energy storage systems (TES) are an important part for storing surplus electric energy from renewable, volatile energy sources. TES using liquid storage materials operate most efficiently by stratifying the storage fluid based on its thermal density gradient into a hot and cold layer, separated by a thermocline region. Previous studies of such stratified TES have shown that heat conduction from the hot, upper layer to the cold, lower layer through the storage tank wall induces counter-directed convective flows (wall jets) along the inner walls. These wall jets carry hot fluid from the upper layer and cold fluid from the lower layer towards the thermocline region, which leads to mixing of the layers and therefore to a decreased thermal efficiency. To quantify the influence of the wall jets on the mixing process, optical temperature field measurements in the thermocline region of a 105 L stratified TES model experiment are performed with two-color planar laser induced fluorescence (2C-PLIF). The results show that the method is feasible for evaluating the thermocline temperature distribution over time in such a system with an accuracy of 1.5 °C. Further measurements in the hot wall jet region show that the temperature resolution of the measurement system limits the evaluation of the thermal boundary layers. However, the 2C-PLIF system provides sufficient temperature field information to determine the local heat fluxes in future studies in combination with simultaneous PIV measurements.

CFD simulation of thermal stratification and mixing in a Nordic BWR pressure suppression pool

Boiling Water Reactor (BWR) employs the Pressure Suppression Pool (PSP) as a heat sink to prevent overpressure of the reactor vessel and containment. Steam can be injected into the PSP through spargers in normal and accident conditions and through blowdown pipes in case of a loss of coolant accident (LOCA). There is a safety limit on the maximum PSP temperature at which such steam injection might cause dynamic loads on the containment structures. The performance of the pool can be affected if thermal stratification is developed when temperature of the hot layer grows rapidly while cold layer remains inactive. Simulation of pool behavior during realistic accident scenarios requires validated models that can sufficiently address the interaction between phenomena, safety systems and operational procedures. Direct modeling of steam injection into a water pool in long-term transients is computationally expensive due to the need to resolve simultaneously the smallest space and time scales of individual steam bubbles and the scales of the whole PSP. To enable PSP analysis for practical purposes, Effective Heat source and Effective Momentum source (EHS/EMS) models have been proposed that avoid the need to resolve steam-water interface. This paper aims to implement mechanistic approaches previously developed by authors for the simulation of transient thermal stratification and mixing phenomena induced by steam injection through spargers in a Nordic BWR PSP. The latest version of the EHS/EMS models using the ‘Unit cell’ approach has been validated against integral effect pool tests and applied to plant simulations. Several scenarios with boundary conditions corresponding to postulated accident sequences were simulated to investigate the possibility of stratification development and the effects of activation of different systems (e.g., blowdown pipes, high momentum nozzle) on the pool behavior.

Machine Learning Based IoT Intrusion Detection System

Today, the use of IoT devices has increased rapidly. Every home network has at least one IoT device. These devices communicate over the Internet through various additional Networking protocols, one of the main protocols is The MQTT Protocol. This protocol is one of the hot layers for attacks on IoT devices. Various unauthorized devices or authorized devices with malicious intent try to connect to the network and compromise the devices. Hence, it's very important to detect the Intruder in our network. For the detection of Intruders, we use the IDS (Intrusion Detection system). This system should be trained to identify the MQTT attacks and get them to the notice of the Network Administrator. We'll be using ML techniques to train the IDS in identifying the attacks. We'll be using datasets that include unidirectional data flow, bidirectional data flow, and packet-based data flow information. Thus, through this project, an IDS with enough Accuracy in detecting the attacks will be developed. To train our IDS, we will be using MQTT-IoT-IDS2020 Dataset. We will be training our IDS with this dataset and will be analyzing the above dataset with our networking knowledge and will be making respective alterations in the dataset according to the attacks to improve the accuracy.

The Influence of Asphalt Concrete Underlayment on Slope Stability of the Ballasted High-speed Railways

In recent years, a hot mix asphalt layer, known as an underlayment, has been replacing granular sub-ballast in high-speed and heavy haul tracks, aiming to overcome the limitations of traditional ballasted railways. However, there is a lack of literature regarding the impact of this layer on the slope stability of railway embankments. Ensuring the safety of trains and track availability requires a thorough understanding of embankment slope stability. Hence, this study examined the influence of the underlayment. layer on slope stability in ballasted high-speed railway embankments. Using Slope/W software, twelve embankment models with 2H:1V and 1.5H:1V slopes were analyzed. Half the number of the models were traditional ballasted, while the others incorporated the underlayment layer. Different heights (3, 6, and 8m) were simulated to evaluate the effect on slope stability for both slope levels and track cross-sections. The analysis focused on gravitational force and neglected the increase in pore water pressure. Safety factors for slope slipping surface were used to quantify stability. The findings revealed that the underlayment layer enhances slope stability by 8% to 31% in high-speed railway embankments. However, the degree of improvement decreases with increasing embankment height, and steeper slopes benefit more from the underlayment layer compared to gentler slopes. While these results are promising, further research is needed to investigate the impact of varying pore water pressure on asphaltic tracks.

Heat Transfer in Pyroclastic Density Current‐Ice Interactions: Insights From Experimental and Numerical Simulations

AbstractStratovolcanoes are common globally, with high‐altitude summit regions that are often glacier‐clad and intersect the seasonal and perennial snow line. During an eruption, interaction between snow/ice and hot, pyroclastic deposits will potentially lead to extensive melt and steam production. This is particularly pertinent when pyroclastic density currents (PDCs) are emplaced onto and propagate over glacierised substrates. Generated melt and steam are incorporated into the flow, which can cause a transformation from a hot, dry granular flow, to a water‐saturated, sediment‐laden flow, termed a lahar. Both PDCs and ice‐melt lahars are highly hazardous due to their high energy during flow and long runout distances. Knowledge of the physics that underpin these interactions and the transformation to ice‐melt lahar is extremely limited, preventing accurate descriptions within hazard models. To physically constrain the thermal interactions we conduct static melting experiments, where a hot granular layer was emplaced onto an ice substrate. The rate of heat transfer through the particle layer, melt and steam generation were quantified. Experiments revealed systematic increases in melt and steam with increasing particle layer thicknesses and temperatures. We also present a one‐dimensional numerical model for heat transfer, calibrated against experimental data, capable of accurately predicting temperature and associated melting. Furthermore, similarity solutions are presented for early‐time melting which are used to benchmark our numerical scheme, and to provide rapid estimates for meltwater flux hydrographs. These data are vital for predicting melt volume and incorporation into PDCs required to facilitate the transformation to and evolution of ice‐melt lahars.

Impact of deposition time per layer in large format additive manufacturing with glass fiber reinforced ABS

This paper presents a parametric study of layer adhesion in Large Format Additive Manufacturing (LFAM) applications using glass fiber (GF) reinforced acrylonitrile butadiene styrene (ABS). The study focuses on the deposition time per layer (tL) and includes an innovative approach to monitor temperature variation throughout the printing process using an infrared camera. The paper provides a detailed account of the sample fabrication process and experimental tensile tests. Specifically, this study shows how lower times per layer yield better mechanical properties as material is deposited over hotter layers, resulting in smaller thermal gradients, hence better layer adhesion and reduced warpage.

Design, Simulation and Optimization of a Novel Transpired Tubular Solar Air Heater

In this paper, a novel tubular solar air heater is introduced. In this air heater, the hot boundary layer is drawn into the absorber tube and can provide thermal energy at moderate temperatures. Several different cases were simulated and a correlation was proposed to predict the collector’s effectiveness as a function Rayleigh number and Reynolds number. An equation was derived to find the effectiveness of this collector. Finally, a real case was studied with non-uniform solar flux distribution, as well as radiation heat loss. Good agreement was found between the results and those derived by the proposed analytical method. For different suction values, the first-law and the second-law efficiencies were calculated. Based on the exergy analysis, exergy destruction in absorption is the dominant factor that is unavoidable in low-temperature collectors. It was shown that there is an optimum suction value at which the second-law efficiency is maximized. At the optimum point, temperature rise can reach 54 K, which is hardly possible with a flat plate collector. Based on the exergy analysis, the relation between tube wall temperature and air outlet temperature in their dimensionless forms at the optimum working condition was derived, and it was shown that effectiveness at the optimum working condition is around 0.5. This means that the air temperature rise shall be half of the temperature difference between collector wall and the ambient temperatures. A high outlet temperature besides the low cost of construction and maintenance are the main advantages of this air heater. With such a high temperature rise, this type of collector can increase the use of solar energy in domestic applications.

Identification of primary input parameters affecting evacuation in ventilated main control room through CFAST simulations and application of a machine learning algorithm to replace CFAST model

Accurately predicting evacuation time in a ventilated main control room (MCR) during fire emergencies is crucial for ensuring the safety of personnel at nuclear power plants. This study proposes to use neural networks alongside consolidated fire and smoke transport (CFAST) simulations to serve as a surrogate model for physics-based simulation tools. Our neural networks can promptly predict the evacuation time in MCRs, proving to be a valuable asset in fire emergencies and eliminating the need for time-consuming rollouts of the CFAST simulations. The CFAST model simulates fire and evacuation scenarios in a ventilated MCR with variations in input parameters such as door conditions, ventilation flow rate, leakage area, and fire propagation time. Target output parameters, such as hot gas layer temperature (HGLT), heat flux (HF), and optical density (OD), are used alongside standardized evacuation variables to train a machine learning model for predicting evacuation time. The findings suggest that high ventilation flow rates help to dilute smoke and discharge hot gas, leading to lower target output parameters and quicker evacuation. Standardized evacuation variables exceed the required abandonment criteria for all door conditions, indicating the importance of proper evacuation procedures. The results show that neural networks can generate evacuation times close to those obtained from CFAST simulations.

High Efficiency Triplet‐Triplet Fusion Blue Fluorescence OLEDs by introducing a “Hot Exciton” Efficiency Enhancement Layer

AbstractTriplet‐triplet fusion (TTF) can effectively transfer triplet excitons to singlet energy level and exhibit good stability, making it widely used in blue fluorescence organic light emitting diodes (OLEDs) for practical application. However, the efficiency of the resulting blue fluorescence OLEDs is still fundamentally limited. Herein, this work introduces a hot exciton efficiency enhancement layer (EEL) between TTF emissive layer and electron‐transporting layer for the first time. It can be found that the triplet excitons are greatly utilized by the efficient hot exciton EEL. As a result, a high efficiency blue fluorescence OLED is successfully developed, exhibiting a maximum external quantum efficiency of 15.2% at the luminance of 4068.0 cd m−2, achieving a breakthrough in the efficiency of blue TTF‐OLEDs. Transient electroluminescence (TrEL) measurements and detailed theoretical calculations confirm the key role of the effective hot exciton process from the high‐lying triplet energy level (Tn, n ≥ 2) to S1 via a reverse intersystem crossing (hRISC). Furthermore, the device lifetime is also greatly improved owing to the fast and efficient hRISC in the introduced hot exciton EEL. This work has important guiding significance for constructing high efficiency blue fluorescence OLEDs by introducing hot exciton molecules.

Pressure Behaviour of Hot Gases and Smoke in Fires of Large Enclosures with Different Ventilation Systems

This study investigates numerically the pressure behavior of hot gases and smoke in fires of large enclosures. It mainly covers the effectiveness of using natural and mechanical ventilation in changing the pressure behaviour of hot gases and smoke inside different structures. Fire Dynamics Simulator (FDS), provided by NIST, has been used to run the simulations for all proposed cases. For the assumed scenarios, the values of hot gas pressure and temperature are extracted at different points, especially in the hot layer close to the ceiling, for further analysis. The effect of changing the location of the fire source has been investigated, too. Final results show noticeable changes in the behavior of pressure, and temperature, in the presence of different ventilation systems, both natural and mechanical. Thus, appropriate ventilation systems can improve the pressure behaviour inside the place, which offers a better environment and higher safety level, in case of fire, in such large enclosures, open plan offices,

Fretting fatigue damage mechanism of Ti6Al4V dovetail joint specimens at high temperature treated with salt layer

Salt layer preparation on the contact surface of Ti6Al4V dovetail joint specimens using solid NaCl and Na2SO4 deposition, the effect of hot salt layer on fretting fatigue properties of Ti6Al4V alloy at high temperature was studied. The maximum stress load and temperature are 10 kN and 500 °C, respectively. The results show that the average fatigue life of the base metal dovetail joint sample (BM sample) is 9.93×104, and the average fatigue life of the hot salt dovetail joint sample (HS sample) is increased to 69.9×104. The increase in fatigue life stems from the fact that the hot salt layer promotes the generation of thicker loose oxides on the surface of the Ti6Al4V alloy, besides, the lubricating effect of the oxide with higher hardness and the wear debris alleviates the fretting damage between the dovetail joint sample and the contact surface of the fretting pad. Fracture analysis shows that the hot salt layer does not change the crack initiation mechanism, and all of them show single crack source propagation and ductile fracture mechanism. Analysis of crack formation zone by electron back-scattered diffraction (EBSD), it is found that the presence of less dislocation distribution in the HS sample relative to the BM sample. The dislocations are more concentrated on the crack propagation paths and the fretting fatigue contact surfaces. This indicates that the BM sample produced more significant plastic deformation in the crack formation zone, accelerating crack initiation. The contact surface plastic deformation of the HS sample is small and the fretting fatigue life is enhanced.