Radiative Heat Flux Research Articles

Evapotranspiration of an abandoned grassland in the Italian Alps: Modeling the impact of shrub encroachment

This study analyzes the effect of shrub encroachment on actual evapotranspiration (ETa), a still poorly studied phenomenon in the Alps. The effect of shrub encroachment is investigated on an Alpine grassland in Western Italy using both data and a soil hydrological model (Hydrus 1D), which is used to model three different land covers: grassland, shrubland, and a mixture of the two land covers with a novel double vegetation approach recently introduced. Four growing seasons of eddy covariance measurements are used as an approximate reference for the interpretation and consistency of the model outputs. Also, the impact of meteorological inter-annual variability and of different environmental conditions on both modeled and measured evapotranspiration is analyzed. The modeling results show that the model is able to capture the inter-annual variability of ETa. The double vegetation approach suggests that the percentage of total transpiration flux assigned to the shrubland is between 20 and 60 %. Single-vegetation simulations show that shrubs lead to an enhancement of ETa equal to + 27.1 %, +26.0 %, +26.8 %, and + 23.9 % (range 2014–2017) compared to grassland, which could lead to an alteration of the hydrological cycle. Moreover, chambers measurements of shrubs transpiration show a good agreement with the eddy covariance measurement, suggesting that the ecosystem’s behavior is already close to a shrubland, which yields an increased ETa if compared to grassland. The evaporative index from the modeled shrubland is higher (range +24–27 %) than the case of a modeled grassland.Finally, ETa and the evaporative fraction (EF) are in the energy-limited regime in most cases. This result was obtained from the analysis of the relationship between ETa (and EF) and either meteorological variables or soil water content including the simulated one in the 0–100 cm horizon. The following analysis, more focused on micrometeorological variables, namely vapor pressure deficit, net radiation, wind speed, air temperature, and ground heat flux, indicates that ETa is mostly affected by the vapor pressure deficit.

Experimental study on flame interaction, gas temperature distribution, and radiative heat flux from two rectangular burning propane sources

This work experimentally investigates the flame interaction (flame merging and flame height), gas temperature distribution, and radiative heat flux from two rectangular propane burners by varying total heat release rate (80–360 kW) and burner spacing (0–1.28 m). The results show that the dominant mechanism of air entrainment rate changes from spacing to heat release rate with increasing spacing, making two flames from fully merging to non-merging. A method for predicting the flame height is proposed in terms of air entrainment mechanism, which is proportional to the 2/3 power of the group dimensionless parameter (Q˙total*,Q˙total*=Q˙totalρ0CpT0g1/2LP3/2). With the decrease of spacing, gas temperature in the additional region is distributed in an inverted U-shaped manner in the horizontal direction, reaching its extreme value at the center of the two burners. The vertical gas temperature can be employed to estimate the critical flame merging, which depends on the dimensionless parameters of Q˙total* and S/W. When S/W < 51 Q˙total*, two flames are merged; otherwise, the two flames are separated. For two rectangular fires with different burner spacings, the radiative heat flux received by the external targets is accurately predicted using the Richardson Number (Rix, Rix=gβ(Tf−T0)xU2).

Compressive Strength parallel to grains of Charred Paulownia Wood

Wood is a common flammable in the building fire and the dominant fuel in the wildland fire. In this study, the surface of wood specimens has been pre-heated by various radiative heat fluxes and heating times to achieve varying char layer thicknesses. The compressive strength parallel to grains versus various charring thicknesses has been obtained by an electronic universal testing machine (WDW-20). Experimental results demonstrate that a higher heat flux and longer heating times promote the formation of the char layer, but the compressive strength versus the charring thickness first decreases to its minimum value and subsequently remains stable. For the 7 kW/m2 case, there is a rapid decrease in compressive strength for the 5-7 heating period. For example, the compressive strength is 31.7 MPa after the initial 5-minute period, and 3.69 MPa after the initial 7-minute period. For the radiative heat fluxes from 7 kW/m2 to 10 kW/m2, the reduction rate of compressive strength will increase from 1.32 MPa/min to 3.75 MPa/min for the initial 5-minute period, and increase from 4.95 MPa/min to 5.34 MPa/min for the initial 7-minute heating period. This study helps us understand the influence of charring processes on the mechanical properties of wood and the failure of timber structures.

Thermal Characteristics of Surface Fire Spreading Front Under Ambient Wind

Surface fire is the most common form of forest fire, primarily spreading along the forest land's surface and causing damage to nearby low-lying seedlings, shrubs, and the root systems of vegetation at its base. In actual forest fire scenarios, the occurrence of fires is often accompanied by environmental winds due to the typically complex terrain in forested areas. Forest vegetation is densely distributed, and when surface fires encounter obstacles such as tree trunks, the advancing fire front may interact with the tree trunks under the influence of environmental winds. This study focuses on understanding the thermal characteristics of the interaction between the advancing fire front and tree trunks in the presence of environmental winds. To investigate this interaction, temperature measurements are conducted on both sides of the obstacles and in the downstream region of the flame. Additionally, measurements of radiant heat flux are taken in the upper space above the experimental setup. By analyzing flame morphology parameters and experimental measurements, trends in thermal characteristic parameters with spatial position are identified. Regression formulas are developed to describe the temperature difference between the windward and leeward sides of the obstacles and the height above the experimental platform. Furthermore, the influence of flame climbing on radiant heat transfer in the upper space is discussed.

Non-similar analysis of radially magnetized flow and heat transfer of Reiner–Philippoff based nanofluid over a curved stretching surface with viscous dissipation

The aim of this study is to conduct a non-similar analysis of flow and heat transfer rheology of non-Newtonian fluid over a radially magnetized curved stretching surface in the presence of multiple type of nanoparticles. Reiner–Philippoff viscosity and homogenous models are incorporated in the governing equation to study the effects of non-Newtonian fluid and nanoparticles respectively. Reiner–Philippoff viscosity model is a complicated non-Newtonian fluid model presenting an implicit stress-strain relationship. This fluid model has ability to present viscosity behavior in response to varying viscoelastic conditions of shear-thinning, shear-thickening as well as Newtonian fluid. The nanoparticles of nickel, molybdenum disulfide, and tantalum are used to study the enhancement of the thermal conductivity of the fluid with influences of radiative heat flux and viscous dissipation. To attain a more precise and generalized analysis of the heat transfer and flow, the governing boundary layer equations are converted to non-similar form. The local non-similarity method up to second level of truncation is employed to numerically solve the resulting non-similar PDEs via bvp4c in MATLAB tools. To examine the impact of physical factors on flow and heat transfer, numerical data are visually and tabulated displayed. It is worth mentioning that the thermal radiation, curvature parameter, and magnetic number play important role in controlling wall drag and heat flux.

A refined cylinder fire model for thermal radiation calculation from localized fire to exposed horizontal surface

Fire is the most frequent hazard to buildings. The current design methods for predicting the temperatures of structures in fire are originally developed for the standard fire that assumes a uniform gas temperature field and are invalid for natural fires with high non-uniform gas temperature fields. Localized fire is a typical natural fire with high non-uniform gas temperature field, which is often considered in fire safety design. There are currently no validated simple general models for calculating the heat transfer from localized fire to exposed structures. To address the need of design method, this paper develops a refined cylinder fire model for calculating the incident radiant heat fluxes on horizontal structural members inside and outside the flame region of a localized fire. The model uses a cylinder, centered at the fire plume axis, to represent the localized fire. The height of the cylinder is not fixed, different to any other fire models. The temperatures in the cylinder are highly non-uniform in three dimensions and are determined based on the classic fire plume theory. For radiation calculation, the cylinder is evenly divided into several elements along the radius and height. Each element is assumed to be a homogenous, isotropic, and absorbing-emitting (participating) graybody. By careful consideration of the spatial gas temperature distribution, the radiation properties (emissivity and transmissivity) of the elementary graybody, and the configuration factor between the elementary graybody and the horizontal target surface, the model can get relatively accurate and conservative incident radiant heat flux. The relative deviations against both the experiment data and numerical results are within ± 30 %. The model introduces a new concept of calculating the heat fluxes from fire to structures, is a relatively simple general model (compared with the sophisticated computational fluid dynamics models), and is more precise and accurate than other simple ones. The model is recommended for use in performance-based fire safety design.

The Influence of the Heat Transfer Mode on the Stability of Foam Extinguishing Agents

The mass loss mechanisms of an aqueous film-forming foam (AF foam), an AR/AFFF water-soluble film-forming foam extinguishing agent (AR foam), and a Class A foam extinguishing agent (A foam) at different levels of thermal radiation, thermal convection, and heat conduction intensity were studied. At a relatively low thermal radiation intensity, the liquid separation rate of the AF, AR, and A foams is related to the properties of the foam itself, such as viscosity and surface/interface tension, which are relatively independent of the external radiation heat flux of the foam. At low radiation intensity (15 kW/m2 and 25 kW/m2), the liquid separation rate of the AF and A foams is relatively stable. When the heat flux intensity is 35 kW/m2, the liquid separation rate of the AF and A foams increases notably, which may be mainly due to the rapid decrease in foam viscosity. And the mass loss behavior is dominated by liquid separation in the AF, AR, and A foams under the influence of thermal radiation and thermal convection. Under the same experimental conditions, the liquid separation rate of AF is the fastest. There is no significant difference in the evaporation rates of the three kinds of foam in the same heat conduction condition. In addition, the AR and A foams usually have a 25% longer liquid separation time (t) under thermal radiation and thermal convection, and the thermal stability is better than AF foam. The temperature reached by the AF foam layer under thermal convection was lower than that of the AR and A foams, and the time for the foam layer to reach the highest temperature under heat conduction was longer than that of the AR and A foams.

Pyrolysis-based modelling of 18650-type lithium-ion battery fires in thermal runaway with LCO, LFP and NMC cathodes

This study presents a modelling approach for simulating fires in thermal runaway and here, it is applied to 18650-type batteries with LCO, NMC, and LFP cathodes at 100% SOC. The model assumes a single-step pyrolysis, where a solid lumped substance, representing the battery components, degrades into volatiles which are vent gases here. Coupling an in-house MATLAB code and FDS solver, a parameter estimation using Bayesian optimization was performed to fit the simulated mass loss and heat release rate to the experimental measurements. One-step kinetics are used for vent gas combustion, with CO yield obtained from literature and soot yield determined via 1-D diffusion flame simulations with CRECK kinetic mechanism in Cantera solver. 3-D simulations using the obtained models yielded maximum values of flame temperature, CO2 and H2O mole fractions, velocity, radiative heat flux, and flame height as 2520 K, 0.47, 0.15, 10.08 m/s, 13.29 kW/m2, and 0.62 m, respectively. The novelties associated with this approach are that, unlike the past studies, there is no need to model the thermal runaway process prior to flaming and, the resulting heat release rate is experimentally verified which makes further analyses, such as flame temperature, height and heat emissions possible.

Research on the influence and optimization of sunshade effect on radiative cooling performance

Intense solar radiation is the main reason why radiative cooling is difficult to achieve during the daytime, as the radiative heat flux from the sun will offset the heat emitted by the cooler. Most previous studies have focused on providing coolers with high solar reflectance and high mid-infrared spectral emissivity by designing photonic structures. In this study, a macroscopic sunshade device based on the solar visual motion trajectory is proposed to reduces the direct solar radiation received by the cooler surface through setting a blocking on the solar path, while hardly affecting the radiative capacity of the cooler. Here, we built a three-dimensional sunshade device simulation model based on the surface-to-surface radiative heat transfer module in COMSOL Multiphysics® software, which can effectively improve the cooling performance of the cooler. Then, using polished aluminum plates, black paint-coated cooler, and reflective radi-cool film to test the impact of macroscopic sunshade device based on the solar apparent motion trajectory on improving the cooling performance of difference coolers. The results verified the effectiveness of the proposed sunshade devices in improving radiative cooling performance, and the results showed that the maximum temperature reduction could reach 3.35 °C, 26.96 °C, and 3.42 °C, respectively.

A Comprehensive Seismic Monitoring of the Pillar Threatening the World Cultural Heritage Site Chauvet‐Pont d'Arc Cave, Toward Rock Damage Assessment

AbstractFragile geological features must undergo frequent structural health assessments to prevent catastrophic failure events. The mechanical behavior of natural sites is largely guided by vibrations of the earth and environmental exposure, but damage is rarely assessed, except empirically. The Chauvet‐Pont d’Arc cave, a UNESCO World Heritage Site, represents a shining example of fragility that would benefit from monitoring. It is overhung by a rock column known as Abraham's pillar that extends out from the cliff like a natural tuning fork. For this study, we monitored dancing movements of this pillar for over 2 years to analyze its elastic response to weather conditions. Using ambient‐seismic‐noise‐based methods, we identified the pillar's first two natural resonance modes. Through extensive monitoring of the site, we observed the striking temporal evolution of these two resonance frequencies on hourly, daily, seasonal, and pluriannual scales in response to changes in air temperature and insolation. Based on thermo‐acousto‐elastic modeling with a simplified 3D geometric structure, we determined how thermally‐induced stress stiffening affects the rock material, by applying convective and radiative heat fluxes to the model. From the results obtained, we suggest a novel quantitative method based on daily observations that can estimate the level of damage within the rock material. Our work provides a foundation for distinguishing between reversible processes and damage for hazard studies in the frame of climate change. Such knowledge is crucial not only for the preservation of heritage sites but also for enhancing risk assessment protocols and informing conservation efforts worldwide.

A comprehensive heat transfer prediction model for tubular moving bed heat exchangers using CFD-DEM: Validation and sensitivity analysis

This study established a complete heat transfer prediction model that integrates four heat transfer sub-models, including contact heat conduction, gas film heat conduction, heat radiation, and heat convection, along with an artificial softening correction based on combined approach of computational fluid dynamics and discrete element method (CFD-DEM). The model’s precision was confirmed by experimental results with relative errors below 5 %. The discussion on the heat flux contributions of the above four heat transfer mechanisms shows that with an initial particle temperature of 200℃, tube wall temperature of 30℃, and particle descent velocity of 1.3 mm/s, gas film conduction heat flux is the primary contributor to the total tube wall heat flux at 60.5 %, followed by radiation heat flux (19.5 %), contact conduction heat flux (12.1 %), and convection heat flux (7.9 %). Besides, the presentation of physical fields related to particles and the fluid showcases the model’s capability in elucidating the heat and mass transfer characteristics of the particle–fluid-wall system within tubular MBHEs. Furthermore, a local nominal range sensitivity analysis, along with a study of factors’ influence rules, was performed to quantify the effects of the 8 key model parameters on heat transfer results. The analysis indicates that fluid thermal conductivity is the predominant factor with its nondimensionalized first-order sensitivity coefficient of 54.9 %, succeeded by particle-to-wall angle factor (15.4 %), wall emissivity (14.4 %), gas film thickness (9.6 %), wall thermal conductivity (2.3 %), wall roughness (-1.6 %), particle Young’s modulus (-1.4 %), and particle specific heat capacity (0.4 %). Finally, their impacts on the heat prediction results were presented in detail.

Experimental Study on the Fire Smoke Characteristics of an Insulated Wire Jacket for Indoor Wiring Using a Smoke Density Chamber

A wire jacket that prevents leakage of electric current flowing through wires is made of polymer composites. The flame spreads widely with a large amount of smoke passing through the wire jacket when a fire occurs. As smoke hinders safe evacuation, it is important to determine the smoke characteristics of the wire jacket for safe evacuation. Therefore, in this study, the specific optical density was measured for 1800 s in the presence and absence of ignition flame under a radiant heat flux of 25 kW/m2 according to the ISO 5659-2 combustion chamber method. From the experimental results, the extinction coefficient and visibility indicating the degree of transmission of light and VOF4 indicating the smoke density characteristics at the beginning of combustion were derived. The experiment was conducted with a wire jacket comprising PVC and a wire jacket comprising XLPE of low-toxicity flame-retardant cross-linked polyolefin. The maximum specific optical density was higher for PVC than for XLPE with and without the ignition flame. Except for the case of XLPE with an ignition flame, the extinction coefficient of the wire jacket was &gt; 3.5 m-1, with a visibility of 0.2-2 m. PVC exhibited a higher maximum specific optical density in the absence of the ignition flame than in the presence of the ignition flame; however, VOF4 was higher in the presence of the ignition flame.

Modelling flame-to-fuel heat transfer by deep learning and fire images

In numerical fire simulations, the calculation of thermal feedback from the flame to the solid and liquid fuel surface plays a critical role as it connects the fundamental gas-phase flame burning and condensed-phase fuel gasification. However, it is a computationally intensive task in CFD fire modelling methods because of the requirement of a high-resolution grid for calculating the interface heat transfer. This paper proposed a real-time prediction of the flame-to-fuel heat transfer by using simulated flame images and a computer-vision deep learning method. Different methanol pool fires were selected to produce the image database for training the model. As the pool diameters increase from 20 to 40 cm, the dominant flame-to-fuel heat transfer shifts from convection to radiation. Results show that the proposed AI algorithm trained by flame images can predict both the convective and radiative heat flux distributions on the condensed fuel surface with a relative error below 20%, based on the input of real-time flame morphology that can be captured by a larger grid size. Regardless of growing or decaying fires or puffing flames induced by buoyancy, this method can further predict the non-uniform distribution of heat transfer coefficient on the interface rather than using empirical correlations. This work demonstrates the use of AI and computer vision in accelerating numerical fire simulation, which helps simulate complex fire behaviours with simpler models and smaller computational costs. Highlights A total framework between AI model and fire simulation software is designed to further enhance the reliability of AI-based fire simulations. A standard pool fire simulation database is built using numerical model recommend by the IAFSS Computation Group. A deep learning model is developed to predict both the convective and radiative heat flux distributions on the condensed fuel surface using numerical images database. The demonstration showcases the application of AI and computer vision to accelerate numerical fire simulation.

Experimental Study on Effects of Moisture Content and Tree Height on Crown Fire Behaviors of Live Cypress Trees

ABSTRACT Once a fire occurs in urban green belts, the live foliage of some tree and shrub species with larger moisture content than dead fuel can support fire spread, which poses a great threat to the wildland-urban interface (WUI). Cypress tree is widely used as an ornamental vegetation worldwide. This work aims to study the fire and spread behaviors through cypress trees. Experiments were conducted using three tree heights between 0.8 m and 1.6 m and five values of fuel moisture content (FMC) between 30% and 160%. The fuel mass loss, flame shape, vertical temperature distribution and outward vertical heat fluxes were measured. The results showed that for a given tree height, a small enough FMC promoted the burning process and decreased both the maximum heat release rate (HRR) and the time to reach peak HRR compared to a larger FMC. The tree height correlated strongly with the flame height, while the influence of FMC was much smaller. Then, a correlation between the normalized maximum flame height and the normalized maximum HRR was established by taking the crown height as the characteristic length. Reducing the FMC did not necessarily promote the fire spread and trees with large FMCs can support relatively large flame front spread rates. The effect of the tree height on the radiant heat flux was greater than the effect of FMC.

The mechanism of the influence of radiation heat flux on the combustion behavior of raw rubbers

Understanding the combustion behavior of rubbers is crucial for predicting their fire hazards. The cone calorimeter experimental results in this study confirm that the combustion behavior of raw rubbers depends on the rubber type and the heat flux. Natural rubber and ethylene-propylene-diene monomer rubber exhibit heat release rate (HRR) curves with only one peak at the end of the burning stage under all tested heat fluxes. At higher heat fluxes, the HRR curves of polybutadiene rubber (BR) and styrene butadiene rubber (SBR) exhibit a single peak; at heat fluxes lower than 20 kWm−2, two HRR peaks were observed. The proposed mechanism model with the crosslink reaction as the key factor can explain the emergence of two-peak HRR curve and has been proved through experiments and simulations. For BR and SBR at 15 kWm−2, the specimens exhibit two upper layers at ignition time. The surface pyrolysis layer seems to be covered by black film or char, with the crosslink degree as high as 84.23–100 %. The second crosslink reaction layer exhibits a higher decomposition temperature Td,5 % than the original rubber. Based on reaction kinetics and thermodynamics data derived from TG/DSC experiments, the novel pyrolysis model taking crosslink reaction and thermal decomposition of crosslinked intermediate into account well predicts the HRR curves obtained at varied heat fluxes. The conversion of raw rubber to the crosslinked intermediate plays an important role in the reduction of HRR immediately after ignition, resulting in the first HRR peak.

Transient thermal behavior and efficiency in fully wet porous longitudinal fin: The influence of shape-dependent hybrid nanofluid and internal heat generation

The main focus of this study is to analyze the transient heat transfer characteristics of a convective–radiative longitudinal porous fin fully wetted in hybrid nanofluid along with internal heat generation. The Darcy formulation is considered for analyzing the velocity of the fluid passing through the fin and the Rosseland approximation determines the radiation heat flux. The hybrid nanofluid is obtained by immersing GO and Mo S 2 nanoparticles in the hybrid base fluid C 2 H 6 O 2 ‐ H 2 O . Furthermore, three different shapes of nanoparticles namely spherical, platelet, and blade have been selected for the investigation. The derived nonlinear partial differential equation is nondimensionalized and solved numerically by employing the finite difference method. The effect of shape factor, nanoparticle volume fraction, dimensionless time, wet porous parameter, radiation–conduction parameter, heat generation parameters, and other relevant parameters on the transient thermal analysis of the fin structure has been graphically analyzed and discussed. Furthermore, the transient thermal fin efficiency has been modeled and its variation with the significant parameters has been examined. One of the major outcomes is that efficiency increases with nanoparticle volume fraction. The examination has resulted in a novel outcome that the presence of hybrid nanofluid enhances the fin efficiency and also the high fin efficiency is achieved in the presence of blade-shaped nanoparticles. The findings of the investigation play a prominent role in the heat transfer enhancement of industrial processes.

Influence of solar insolation on energy and exergy efficiency of a rectangular duct solar air heater with various types of V rib roughness: An analytical approach

In the current work, the energy and exergy evaluations for several V ribs in a rectangular duct SAH are evaluated and compared with smooth SAH for a heat flux range of 800 to 1200 W/m2. The analysis uses correlations for the friction factor and Nusselt number for different V ribs that have been developed by various researchers. The investigators' recommended optimum design parameters are employed to analyse the specific rib. The thermohydraulic and exergy efficiency of various V ribs are evaluated using the MATLAB code and comparisons of the findings are provided. The staggered discrete multi-V-shape rib configurations achieves the maximum thermal efficiency of 76.63% for Reynolds number of 21,000 and solar radiation heat flux of 800 W/m2. Corresponding to a Reynolds number of 21,000 and solar radiation heat flux of 1200 W/m2, the perforated multi-V-shaped rib gives a maximum thermohydraulic efficiency of 74.82%. At 800 W/m2 of heat flux, a maximum exergy efficiency of 5.17% is obtained for the staggered discrete multi-V-shape rib configuration at lower Reynolds number. Whereas, corresponding to higher Reynolds number, the maximum exergy efficiency is 4.18% for perforated multi-V rib.

Snow thermal conductivity and conductive flux in the Central Arctic: Estimates from observations and implications for models

During the Arctic winter, the conductive heat flux through the sea ice and snow balances the radiative and turbulent heat fluxes at the surface. Snow on sea ice is a thermal insulator that reduces the magnitude of the conductive flux. The thermal conductivity of snow, that is, how readily energy is conducted, is known to vary significantly in time and space from observations, but most forecast and climate models use a constant value. This work begins with a demonstration of the importance of snow thermal conductivity in a regional coupled forecast model. Varying snow thermal conductivity impacts the magnitudes of all surface fluxes, not just conduction, and their responses to atmospheric forcing. Given the importance of snow thermal conductivity in models, we use observations from sea ice mass balance buoys installed during the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition to derive the profiles of thermal conductivity, density, and conductive flux. From 13 sites, median snow thermal conductivity ranges from 0.33 W m−1 K−1 to 0.47 W m−1 K−1 with a median from all data of 0.39 W m−1 K−1 from October to February. In terms of surface energy budget closure, estimated conductive fluxes are generally smaller than the net atmospheric flux by as much as 20 W m−2, but the average residual during winter is −6 W m−2, which is within the uncertainties. The spatial variability of conductive heat flux is highest during clear and cold time periods. Higher surface temperature, which often occurs during cloudy conditions, and thicker snowpacks reduce temporal and spatial variability. These relationships are compared between observations and the coupled forecast model, emphasizing both the importance and challenge of describing thermodynamic parameters of snow cover for modeling the Arctic as a coupled system.

Adjoint-based shape optimization for radiative transfer in porous structure for volumetric solar receiver

Due to their large surface areas, porous structures are often used for volumetric solar receivers for efficient utilization of the solar energy. However, the precise simulation and optimization of radiative heat transfer inside porous structures are still challenging due to their complex geometries. In the present study, an adjoint-based shape optimization framework for radiative transfer in porous structures is first proposed. Specifically, an arbitrary complex geometry of a porous structure is represented by a level-set function embedded in a Cartesian grid system. The volume penalization method combined with the discrete ordinate method is employed to simulate a forward radiative transfer problem in a porous structure. Then, a cost functional for an adjoint analysis is defined by the volume average of the divergence of the radiative heat flux, which corresponds to the total heat absorbed by the porous structure with a negative sign. The adjoint equations for minimizing the cost functional, and the corresponding shape update formula are derived and implemented in an open-source software, OpenFOAM. The developed adjoint-based shape optimization framework is applied to three types of initial porous structures, i.e., SC Kelvin, BCC Kelvin, and square honeycomb models. For all the initial porous models, the front surface commonly extends towards the direction of the incident radiation, and consequently notable improvements of the absorption efficiency can be achieved through the optimization. In addition, a systematic parametric survey for the initial shape of the SC Kelvin model is conducted to reveal the impacts of the number of unit cells, the solid emissivity, and the solid temperature on the optimal shapes and the resulting absorption efficiencies.

Experimental and Numerical Studies on the Efficacy of Water Mist to Suppress Hydrocarbon Fires in Enclosures

Fire is one of the most undesirable events onboard a ship. The engine room is one of the most critical spaces in the ship in terms of fire protection, as it includes machinery, hydrocarbon fuel systems, and different electrical equipment. With the phasing out of Halon 1301 as a fire suppressant over recent decades, there has been an intensive effort to explore the efficacy of water-mist spray in mitigating fires within machinery spaces. This exploration entails a comprehensive investigation through experimental and simulation studies aimed at identifying suppression mechanisms and evaluating their effectiveness. While experimental setups typically encompass measurements of gas temperature, thermal radiation heat flux, oxygen concentration, and fire extinction time, limited attention has been paid to quantifying the heat release rate (HRR), a crucial indicator of fire magnitude. Furthermore, research into shielded fire scenarios remains sparse, despite their significance in maritime fire dynamics. Addressing shielded fires with water mist proves particularly challenging due to the potential obstruction impeding the direct interaction between the fire source and the water droplets. In the existing literature, most of the computational fluid dynamics (CFD) modelling of fires and suppression was performed using a Fire Dynamics Simulator (FDS). Alternate studies were performed using FireFOAM. and very few employed FLUENT and other analogous software codes. In the majority of reported computational studies, the determination of HRR was typically relied upon for its calculation derived from the measured data of fuel mass loss rate. Moreover, certain studies were undertaken for numerical simulations without conducting thorough model validation, either by omitting validation altogether or solely validating against dry fire experiments (i.e., without water-mist suppression). This critical review of the literature has identified several notable research gaps in the context of extinguishing hydrocarbon fires utilising water-mist spray, warranting further investigations. Additionally, this review paper highlights recent advancements in both experimental and numerical investigations pertaining to the efficacy of water-mist fire-suppression systems in enclosed spaces regarding hydrocarbon fires.