Incident Heat Flux Research Articles

Effect of surface ablation on aerodynamic heating over a blunt cone in hypersonic airflow

The hypersonic airflow around a blunt cone coupled with carbonaceous surface ablation is numerically simulated to analyze the impacts of ablation on aerodynamic heating. Ablative reactions, including oxidation, nitridation, and sublimation, are found to affect aerodynamic heating mainly through the surface thermal effect (STE) and the gas-phase chemistry thermal effect (GCTE). Specifically, the sublimation reactions produce significant STE and GCTE. The STE of the sublimation reactions is not always endothermic. Downstream along the cone surface, the condensation of gas-phase C atoms on the wall leads to heat release. The GCTE is exothermic and increases the boundary layer temperature. The oxidation reaction generates weak GCTE but powerful STE, which is exothermic overall. On the contrary, nitridation reaction only leads to appreciable GCTE, characterized by releasing heat. The STE of nitridation is quite small and can be neglected. Oxidation and nitridation reactions often dominate the downstream region of the cone. In addition, in the head region where the ablation reactions are intense, ablation processes could generate considerable diffusive heat flux, increasing the total incident heat flux. Simultaneously, mass ejection can carry away much heat from the surface.

Numerical and experimental analysis of thermal and flow operating conditions of waterwall tubes connected by fins

This paper presents a method for determining the temperature distribution in the cross-section of waterwall tubes connected by fins using an in-house numerical algorithm prepared in the MATLAB environment, based on differential equations with separable variables. In order to verify the correctness of the algorithm operation, the temperature values obtained from it, determined for the frontal area of the tubes, are compared with the temperatures found in the Ansys Fluent environment and those measured on the test stand. A system corresponding to a fragment of the combustion chamber wall of a supercritical steam boiler was selected to perform the analysis. The system consists of three tubes connected by fins. The temperature distributions in the cross-sections of the tubes were compared for the case when each of the tubes was heated on one side with the same heat flux and when the heat flux falling on the central tube was by 50% higher than the heat flux incident on the neighbouring tubes. Experimental verification was carried out on a stand equipped with three vertical tubes connected by fins, heated on one side by infrared radiators.

High temperature and fire properties of sustainable syntactic foam reinforced by end‐of‐life tyre‐derived rubber particles

AbstractThe management of end‐of‐life tyres faces challenges due to insufficient recycling infrastructure and technologies, as well as limited markets for the materials recovered from them. To mitigate this, waste rubber can be upcycled and used as filler material for polymer matrix composites. Before rubber‐reinforced composites can be certified for fire‐prone applications, their thermal and flammability properties must be understood. This research investigates the effect of rubber fillers on the thermal stability, flammability and flame spread characteristics of epoxy matrix syntactic foam. Thermogravimetric analysis, Fourier Transform Infrared spectrometry (FTIR), scanning electron microscopy and attenuated total reflection FTIR spectrometry were employed to elucidate changes in thermal degradation behaviours. The influence of rubber fillers on the flammability of syntactic foam was assessed using the cone calorimeter. The fire reaction properties of rubber‐reinforced foam were affected by the intensity of the incident heat flux. Regardless of the incident heat flux, an increase in rubber content led to higher total heat release. At the lower heat flux of 35 kW/m2, the fire growth rate increased with rubber content, but at the higher heat flux of 50 kW/m2, the fire growth rate decreased as the rubber content increased. Importantly, all rubber‐reinforced syntactic foams achieved a UL94 HB ranking and exhibited reduced flame spread rates compared to the unmodified foam. This study demonstrated the potential for upcycling waste rubber into sustainable engineering products and expanded the knowledge base on fire reaction properties and flame spread characteristics of such hybrid composite materials.

A comparative investigation of charging conditions on thermal runaway of lithium-ion batteries induced by different incident heat fluxes

Thermal runaway poses a great challenge for lithium-ion battery applications, an in-depth delve into the thermal characteristics and fire behaviors is paramount to battery thermal management system design and fire prevention. This work intends to investigated the thermal characteristics and fire behaviors of batteries when subjected to different charging rates and without charging treatment under different incident heat fluxes by using of a cone calorimeter. Experimental results indicate that the battery thermal risk increased with increasing charging rate, manifesting lower internal short circuit and onset temperature, as well as higher maximum temperature and heat release rate. It was observed that the flame extension length rose from 32.85 mm at 0.5 C to 68.85 mm at 3 C during the venting stage. In addition, the charging time terminated prematurely which led to the final SOC declined by 5.2%−22%. Thermal runaway onset time reduced by between 746 s and 962 s as the incident heat flux increased from 20 kW/m2 to 50 kW/m2. Furthermore, the increasing thermal runaway duration without charging treatment, suggesting the accelerating effect of charging current. Subsequently, a semiempirical ignition model was developed. This work aims to give a comprehensive understanding of thermal runaway characteristics in the presence of dual charging and heat flux conditions and hopes to provide meritorious references for the safety of lithium-ion battery applications.

Thermo-hydraulic performance of tungsten flat tile type divertor target with two types of novel engineered cooling channel wall

The divertor is one of the key components in magnetic confinement fusion reactors. Improving the heat transfer performance of the divertor has great importance for promoting its service capacity. This work proposed a new kind of cooling surface configuration for a flat-type divertor design, which is based on the concept of synergetic heat transfer enhancement via combination of a sinusoidal undulation structure with a micro-nano structured surface (copper-mesh brazed coating). The combination surface configuration is preliminarily obtained by numerical optimization based on the calibrated model of subcooled flow boiling heat transfer. Then, experimental mockups were fabricated and tests with high heat flux (HHF) conditions were conducted to examine the effects of the new design. Both the numerical and experimental results confirm that the incident critical heat flux is increased from 15 MW/m2 of the micro-nano structured flat surface case to over 22 MW/m2 of the combination case under the conditions of relatively low flow velocity of 3 m/s and low flow resistance increase of 7.3 %.

Numerical investigation to optimize the modified cavity receiver for enhancement of thermal performance of solar parabolic dish collector system

The concentrated parabolic dish collector (PDC) systems are widely utilized for the purpose of generating high-temperature heat by receiving and converting solar energy efficiently into thermal energy. Cavity receivers including spiral coils have a common use in PDC system for optimal thermal performance. Therefore, to enhance the thermal performance of a PDC system, the present study investigates different cavity receiver configurations, such as cylinder and conical shaped receivers with flat and spherical absorber surface as well as the effect of spiral coil arrangement. To identify the most optimal spirally coiled cavity receiver, the comparison is conducted based on outlet temperature, heat exchange rate, thermal efficiency, effectiveness, and thermal performance capability (TPC). The investigations were conducted using the ANSYS Fluent CFD simulation software package. The SolTrace software package was utilized to compute the maximum heat flux incident on the absorber surface of the receiver, and peak heat flux data was used as input parameter at the absorber surface of the receiver. Furthermore, to select the suitable working fluid in PDC system, different heat transfer fluids, such as Syltherm 800, Xceltherm 600, ParathermHR™ and water, respectively were studied. From the thermal analysis of various proposed receiver geometries, the receiver with conical shape and spherical absorber surface is more efficient over other configurations. It provides 50 % higher heat transfer rate thancylindrical receiver with flat absorber surface. Moreover, receiver's thermal performance could modestly be improved by modifying the spiral coil pitch (variable pitch). In terms of pumping power consumption, readily availability, low viscosity, and high specific heat, water is suggested as working fluid for receiver of PDC system.

Development of Rotary Solar Receiver and Solar Simulator Facility for Concentrated Solar Power Applications

A prototype rotary solar receiver and a solar simulator facility have been designed, built and commissioned by Odqa Renewable Energy Technologies in conjunction with The Oxford Thermofluids Institute, University of Oxford for the validation of concentrated solar power technologies. The key features of the rotary solar thermal receiver include the rotating absorber surface which incorporates a high temperature heat transfer design. The innovative design also includes a compact solar power inlet aperture for very high incident heat flux to reduce the radiant heat loss from the high temperature surface. Air is used as the heat transfer fluid as this has system integration advantages over other types of heat transfer media, such as CO2, steam, molten salt, or solid particles. A 30kWe Xenon-arc lamp high heat flux incident light has also been developed to enable experimental validation of the solar receiver technologies. The Xenon-arc lamp array consists of three lamps combined with a custom designed, 5-degree-of-freedom focusing mechanism to achieve precise focus and maximise the heat flux. The receiver has successfully produced working fluid temperatures reaching 1200°C, demonstrating the concept viability for high temperature processes.

Varying Heat Release Rates per Unit Area – The Impact in Underground Mines

Abstract The fire behaviour and smoke behaviour of a fire in an underground mine will be largely dictated by the heat release rate of the fire. The heat release rate per unit area is generally the output from small-scale fire experiments and can be applied for larger objects with varying surface area. This paper studies the impact of varying heat release rate per unit area for fires in materials and substances typically found underground. Data from earlier fire experiments were analysed and discussed, and several different heat release rate curves and flame spread velocities for various fuel components were developed and discussed. It was found that the hydraulic hose presented the highest peak heat release rate per unit area of the solid fuel items, followed by the low-voltage cable, cab interior, and tyre in descending order. The increase in the peak heat release rate was highest for the highest incident heat flux levels, representative for scenarios underneath a mining vehicle. The heat release rate per unit area of pool fires underneath a vehicle is significantly higher than a corresponding free burning case - attributed to the higher incident heat flux – but a gravel surface underneath a pool fire will reduce the heat release rate considerably. Line fires in larger bundles of electrical cables were modelled and found to attain the peak flame spread velocity more rapidly compared with a fire in hydraulic hoses. This was caused by the higher heat release rate of the initial fire and ignited segments in the electrical cable case.

Understanding the effect of char oxidation on wood temperature profiles for varying heating and oxygen conditions

The use of mass timber framing as a sustainable material, particularly in high-rise buildings, requires detailed structural fire performance calculations. Thermal models describing only the solid phase are cost-effective alternatives to provide information to structural behavior models. Their accuracy depends on an adequate description of drying, pyrolysis, charring and eventually flaming phenomena. While in recent years there have been considerable contributions to the development of such models, there are still open questions. This work proposes a thermal model which incorporates char oxidation, describing both the kinetic- and diffusion-controlled regimes. The model was used to replicate two sets of experimental results which used standard fire calorimeters to study the ignition of thick wood specimens within a range of incident heat fluxes and oxygen concentrations, respectively. The model yields adequate temperature predictions in the early heating stages, but fails to replicate the behavior at later stages, when the effect of the surface combustion is noticeable. In terms of mass loss rates, a poorer performance is observed. To change from one oxidation regime to another, a Damköhler number is proposed, based on char oxidation reaction rates. It is found that for compartment fire conditions, char oxidation will mostly occur develop under diffusion-controlled conditions.

High temperature vacuum brazing of tungsten to tungsten alloy with structural material

The present paper offers an overview of experimental research on the vacuum brazing procedure used to join tungsten-based refractory materials and tungsten alloy with SS316L (a structural material) at high temperatures. The objective of joining tungsten (W) to tungsten alloy material (WL10) and SS316L materials is to develop a joining technique for fabricating the helium-cooled divertor target relevant to a DEMO-like fusion reactor. These divertor fingers are expected to withstand an incident heat flux of 10 MW/m2 and to be cooled by helium jets in an environment of high pressure (10 MPa) and high-temperature (600 °C) helium gas. High-temperature vacuum brazing has been performed at temperatures exceeding 1000 °C using selective brazing fillers in the Gleeble Machine-3800 thermo-mechanical simulator to join W (tungsten) to WL10 alloy (tungsten + 1 % Lanthanum oxide). Furthermore, vacuum brazing of multilayer materials, such as W-WL10-SS316L, has been conducted at 1050 °C for a 3 min hold time using AMS 4777 filler material in a single run on the Gleeble machine. The brazed specimens have undergone testing for 500 thermal cycles from 950 °C to 815 °C on the Gleeble-3800 machine to pre-qualify the brazed joints. Non-destructive testing (NDT), including ultrasonic testing (UT), microstructural analysis, and mechanical characterization, is employed to assess the quality of the brazed joints. The study presents the experimental techniques and the results of the characterization.

A Full-scale Fire Test on Electric Vehiclein an Underground Car Parks

A full-scale test was conducted to evaluate the risk of a modern battery electric vehicle (BEV) fire in an underground car park. A test rig 7,800 × 7,800 × 2,300 mm in size was built to simulate the corner segment of an underground car park. The BEV was positioned in a parking bay at a corner of the structure . The lithium-ion battery pack was heated using an electric heating sheet to activate thermal runaway inside the pack. The temperature distribution in the internal space of the test rig was measured using a thermocouple. The incident heat fluxes on neighboring cars were also estimated using heat flux gauges positioned around the test vehicle. Deflagration venting was observed instantaneously after the initial ignition of the flammable gas accumulated under the ceiling of the test rig. This phenomenon accelerated the growth of the BEV fire, resulting in an average ceiling jet temperature of 1,000 ℃ and a peak heat flux of 225 kW/m<sup>2</sup>.

Heat transfer in wind-driven fires stabilized under a mixed-convective turbulent flow environment

This study experimentally investigates the heat flux distribution over a horizontally placed condensed fuel surface under mixed-convective turbulent flow conditions. The study explores different freestream conditions, including variations in crossflow velocity (ranging from 1 m/s to 1.8 m/s) and turbulent intensity (ranging from 3% to 9%) on local heat fluxes at the condensed fuel surface. Further, to decipher the collective influence of momentum, buoyancy, and turbulence on local heat fluxes, a mixed-convection variable ξx was defined in the form of ξx=Grx1/ψx2n. The defined variable also encapsulates the property of fuel as a function of mass transfer number B, thereby establishing a fuel-dependency factor in the formulation of the mixed-convective parameter (ξx). A methodology was adopted to quantify heat flux components (convective and radiative) with the knowledge of local mass burning rates and local temperature gradients at the condensed fuel surface in the pyrolysis zone. An inverse relationship was observed for incident heat flux with flame standoff distance and ξx. The present study considered the experimental data for two fuels, n-heptane, and ethanol, which showed almost similar trends. However, the radiative heat flux was dominant for the n-heptane case due to the higher sooting propensity of heptane.

Upward flame spread behaviour of cladding materials on a medium-scale ventilated façade experimental setup with a single combustible wall

A parametric experimental study was performed to characterise the fire spread dynamics in a simplified ventilated façade using a medium-scale testing rig comprised of a non-combustible and a combustible cladding wall (1800 × 600 mm). Three different cavity widths and four different cladding materials were tested. Measurements of the flame height, the incident heat flux on the non-combustible cavity wall and oxygen consumption calorimetry were performed. A strong relationship between flame height and heat release rate was found for the growth phase of the fire. It has been shown that the time for encapsulation failure and subsequent cladding material core ignition decreased as the cavity width was reduced since the heat transfer to the walls was enhanced. The increase in the heat transfer to the opposite wall with all the materials could lead to external heat fluxes above the critical heat flux for ignition of a number of combustible cladding materials. This highlights the importance of considering the interaction of the products used in the façade and its geometry for the design of façade assemblies when accounting for the fire performance of the system. The results also show the need to understand the impact of the interaction between the design variables and the system performance, since the material performance observed at bench-scale may fail to capture the performance in heat transfer and flame spread scenarios observed at a system scale.

The influence of feedback and convection on imposed heating conditions when using gas-fired radiant panels in fire testing

Gas-fired radiant panel arrays (RPAs) are a common experimental tool used in fire science and material testing. Unlike devices such as Cone Calorimeter or the Fire Propagation Apparatus (FPA), RPAs typically consume gaseous fuel within a porous medium through which fuel is burnt. When RPAs are used, thermal feedback from the surface of heated samples, as well as the effects of hot gases within the zone of convective influence of the RPA will cause an increase in the surface temperature of the RPA. To investigate this, experiments were conducted using a gas-fired RPA. Target samples made from vermiculite board, concrete, and a water-cooled aluminium plate were exposed to various severities of pre-calibrated incident radiant heat fluxes (HF). It was confirmed that the presence of a target sample led to an increased surface temperature for the RPA of nearly 80 °C (for a calibrated incident HF of 144 kW/m2). This increased surface temperature results in an incident HF nearly 78% higher than the pre-calibrated value at the sample's surface. Based on the results in this paper, a correction method has been proposed which can be used by gas-fired RPA users to account for the increase in incident heat fluxes.

Measurements of heat flux components due to charged and non-charged particles in DIII-D divertor near and under detachment

Surfacing Eroding Thermocouples (SETC) have been used to provide high spatial and temporal resolution heat flux measurement in DIII-D. SETCs were first tested in the lower divertor of DIII-D using the Divertor Material Evaluation System (DiMES), then an array of SETCS was permanently installed in the upper SAS-VW divertor for studying the heat flux mitigation in closed divertor geometry. SETCs proved that inducing divertor detachment with various methods is an effective way to reduce the peak heat flux at the divertor targets. In the investigated discharges the heat flux is reduced to less than 30% under detachment compared to attached conditions. A new method of using the combination of recessed SETC and flush SETC was demonstrated to be able to measure the heat flux from volumetric radiation and the charge exchange neutrals. It was observed that the magnitude of the heat flux from radiation and charge exchange neutrals increased during the process of detachment. While 30% of the total incident heat flux is attributable to the volumetric radiation and the charge exchange neutrals in the fully detached divertor condition with B×∇B drift into the divertor, it could reach more than 60% with B×∇B drift away from the divertor.

Solar power tower plants with Bimetallic receiver tubes: A thermomechanical study of two- and three-layer composite tubes configurations

The present study numerically investigated the use of bimetallic tubes for concentrating solar energy applications. Specifically, a billboard receiver employing supercritical carbon dioxide (sCO2) as the heat transfer fluid is considered, with tubes made of stainless steel 316 and GRCop-84. Two- and three-layer tube configurations are compared, exploring the impact of more thermally conductive layer thickness and placement on temperature and stress fields. The findings demonstrate that the use of bimetallic tubes can effectively reduce temperature and stress in the receiver tubes. In light of the results, it can be concluded that the higher thermal conductivity of GRCop-84 leads to a more uniform temperature distribution, resulting in lower temperature peaks on the outer tube surface, and reduced maximum stresses. Furthermore, it has been found that the incident heat flux necessary to achieve the same temperature increment of sCO2 inside the panel is 1.7% lower in a two-layer tube configuration where GRCop-84 is placed in the outer layer. Besides, the stress in the 316 layer can be reduced up to 53.2% with the cited configuration. Nevertheless, it is observed that it is more beneficial to tube performance to place the more conductive layer inside, since it reduces stress in the GRCop-84 layer, and its compressive stress and corrosion-resistant properties help to avoid the risk of stress-corrosion-cracking. In a three-layer composite tube configuration, placing the more thermally conductive GRCop-84 layer close to the outer tube wall decreases the maximum temperature while increasing stress. The opposite effect is achieved by placing the more conductive layer closer to the inner tube wall. Overall, the results demonstrate the potential benefits of using bimetallic tubes for solar energy applications, when the layers have similar thicknesses, since their use can enhance the thermomechanical performance of conventional tubes made with one layer of 316 stainless steel. This fact has important implications for the design of efficient and reliable solar thermal systems.

An empirical firebrand pile heat flux model

Firebrand exposure has been identified as a leading cause of structure losses during wildfires. However, only a limited number of studies have attempted to quantify the incident firebrand heat flux to a target substrate surface. In this study, a bench-scale wind tunnel retrofitted with an IR thermal imaging system was used to generate high-resolution temperature data for the back surface of a thin, non-combustible insulation board exposed to a glowing firebrand pile at 0.9–2.7 m s−1 air flow velocities and 0.06 and 0.16 g cm−2 firebrand coverage densities. An inverse heat flux modelling technique utilizing a solid-phase pyrolysis solver, ThermaKin, was employed to determine transient firebrand pile heat flux profiles underneath and directly in front of a glowing firebrand pile. The inverse modelling technique used both the experimental back surface temperature data and the insulation's thermophysical properties. Average incident firebrand pile heat flux values obtained for all testing conditions ranged between 28 and 80 kW m−2. For the first time, an empirical model capturing the firebrand pile incident heat flux dependence on time, air flow velocity and firebrand coverage density was developed.

Uncertainty in Modeling Ablation Heat Transfer in Rocket Nozzles

The ablation of carbon cloth phenolic insulators used in solid rocket motor (SRM) nozzles involves highly complex phenomena that are difficult to accurately predict. Historical and even more modern ablation predictions rely heavily on anchoring to SRM test data to improve predictability and SRM reliability. Accelerated schedules, reductions in static SRM testing prior to flight, and a highly competitive global market are placing a substantial onus on computational capability and predictive uncertainty. Quantifying uncertainty in SRM nozzle ablation predictions is essential for motor reliability. This paper provides the details of a modern uncertainty quantification methodology applied to ablation predictions in carbon cloth phenolic insulators exposed to SRM nozzle environments. A particular historical test motor is used as a demonstration case. The system response quantities of interest are the erosion depth and char depth. A representative model and input uncertainty are provided, and a sensitivity analysis is performed to identify influential parameters. Uncertainties in the numerical models and inputs are propagated through a two-dimensional uncertainty quantification methodology using a Latin Hypercube Sampling approach. The results show that the primary sources of uncertainty in SRM nozzle thermal modeling are the heat transfer coefficient, incident radiation heat flux, char material thermal conductivity, virgin material density, char material density, char material specific heat, and pyrolysis gas enthalpy. Uncertainties in the predictions of nozzle insulation erosion and char for the test case are provided relative to the nozzle location at the 99 th percentile and 95th confidence interval. Uncertainty in the char depth is roughly ±28% along the entire axial length of the nozzle. Uncertainty in the erosion depth ranges from about ±45% for the entrance and throat regions to ±80% at the nozzle exit.

Ignition of Wildland Fuels Exposed to a Time-Decreasing Incident Heat Flux

ABSTRACT The effect of decreasing incident heat fluxes on the ignition delay time of dry pine needles is addressed in the present study. A customized modular instrument (I-FIT) that uses radiant heaters to simulate idealized firebrands ensures experimental repeatability for combustion experiments. Linear incident heat flux ramps are obtained by controlling the power of the heating element, thus simulating idealized firebrands. An analytical model based on the thermal ignition theory was developed and solved analytically using an integral approach. This model includes convective losses and in-depth penetration of radiation. Radiation was modeled using the P1 approximation. The theoretical model is complemented and validated by experimental data, showing increments of the ignition delay times when the negative slope steepness over time increases for the same heat flux. For given values of the initial incident heat flux on the sample, a critical slope is observed. For slopes steeper than this critical value, ignition is not attained.

Reflector Design for Simulating Varying Heat Flux on Nose Cap of a Re-entry Module

For thermo-structural tests of a nose cap for a re-entry module accurate simulation of time and space varying heat flux is essential for generating the actual temperature gradients as expected in flight. Temperature gradient simulations along with simulated structural loads are essential for testing the structural integrity of the C-C nose cap.
 Contoured reflectors are essential for obtaining spatial distribution of incident heat flux. For realizing the reflector for thermo-structural testing, it is important to design radiation reflectors that will generate required heat flux envelope along the nose cap geometry. Reflector design for simulating varying heat flux on re-entry nose cap has been carried out by estimating view factor between nose cap and for different geometries of reflector.
 Design could be accomplished by making use of view factor algebra. Elemental heat flux values were validated with literature data and methodology for reflector heat flux design was validated against test data. With single flat reflectors and multiple flat reflectors, the required spatial variation of incident radiative heat flux could be realized efficiently. View factor variation is compared with heat flux variation along the nose cap. It is recommended practice to have multiple reflectors for simulating required varying heat flux on the nose cap due to better flexibility.