Incident Heat Flux Research Articles

Thermal Hydraulics of Helium-Cooled Finger-Type Divertors at Higher Incident Heat Fluxes

Over the last decade, a number of studies at the Georgia Institute of Technology (GT) have evaluated the thermal hydraulics of the design of the helium-cooled modular divertor with multiple jets (HEMJ) originally developed at the Karlsruhe Institute of Technology. Using the GT helium loop, a test section of a single HEMJ finger heated by a radio-frequency (rf) induction heater was studied at near prototypical condition at pressures of ~10 MPa, maximum mass flow rates of 8 g/s, and maximum helium inlet temperatures Ti of 425°C. The area-averaged cooled surface temperature was estimated from embedded thermocouple measurements. This, together with the average incident heat flux , was used to determine the average heat transfer coefficient and the corresponding Nusselt number over the cooled surface. The normalized pressure loss coefficient KL was determined from the pressure drop measured across the test section.The helium loop was modified last year by enclosing the test section and heater within an argon-filled stainless steel chamber to minimize oxidation of the tungsten-alloy test section. Initial results, when extrapolated to prototypical conditions, suggested that was about 20% higher than our previous results. However, the maximum heat flux for these results was less than 3 MW/m2 due to rf coupling with the steel chamber walls. The chamber was then recently upgraded to a glass–stainless steel enclosure with modified feedthroughs for the induction heater connections to minimize this coupling. With this upgrade, a maximum incident heat flux = 8.1 MW/m2 was achieved. This work presents experimental estimates and correlations for and KL at higher heat fluxes. These results provide greater confidence when estimating the maximum heat flux that can be accommodated by the HEMJ at fully prototypical conditions.Finally, preliminary metrology results for the test section used to experimentally study the simplified flat design variant of the HEMJ are presented as part of an effort to resolve recently reported discrepancies between experimentally estimated and numerically simulated

Investigation of particle radiation and its effect on NO prediction in a pilot-scale facility for both air and oxy-coal combustion

Radiation heat transfer plays an important role in pulverised coal combustion, influencing the overall combustion efficiency, pollutant formation and flame ignition and propagation. In this paper, the radiation properties of the particles as well as gas property models on the overall influence of the prediction of the formation of NOx pollutants in a pulverised coal combustion have been investigated. The non-grey weighted sum of grey gases (WSGG) model has been employed to calculate the radiation of the gas phase coupled with the radiation interaction from the particulate phase. The Mie theory, as well as constant or linear models, have been employed to describe the particle radiative properties. The prediction results, calculated from the data from a 250 kW pilot scale combustion test facility (CTF), are compared against experimental measurements under air-fired condition and a range of oxyfuel conditions. The results show that the choice of radiation solution can have a considerable impact on the radiative heat transfer results, in which the Mie theory shows a significant improvement in the incident wall heat flux compared to the constant or linear models. Also, the more accurate solution employed for radiation of gases and particles considerably improves the NOx prediction in the flame region.

Formulized average surface binding energy elevation and lithium vaporization and redeposition investigations in capillary pore systems

An investigation into lithium evaporation and redeposition with multi-mesh capillary pore systems (CPS) has been performed on a one-cathode linear plasma device. A diagnostic system has been used to monitor the lithium evaporation process in the vicinity of the specimen surface, with the adjustable plasma parameters of electron temperature ranging from 0.4–0.9 eV and electron density from 0.8–3.2 × 1019 m−3. The experimental results show that the lithium evaporation rate follows the reduction scaling ratio of 1.94:1.69:1.00 as the number of mesh layers is increased from two to four, which is closely related to the time-dependent fraction weighted average surface binding energy (i.e. evaporation heat) elevation because a fraction of lithium atoms is adhered to the mesh surface. From the viewpoint of nanoscale physics, a new formula is derived for the first time to qualitatively explain the physical mechanism of evaporation heat, or of time-dependent fraction weighted dual average surface binding energy with minor elevation in this special concomitant configuration CPS, which consists of liquid lithium and multi-meshes. The reason for this phenomenon is that the mesh surface binding force acts on a fraction of the adhered lithium atoms, leading to a minor elevation in the evaporation heat. The lithium vapor plays a modest shielding role from the incident heat flux dumping by stopping power plus lithium impurity radiation enhancement, as in the impurity radiation divertor, otherwise the thermal equilibrium of the system could not be attained. In addition, an experimental comparison with a nonactive target and a semi-empirical temperature-dependent model calculation have been applied to further verify the shielding effect. It has been observed that the redeposition rate increases with the applied discharge current.

Assessing the onset of swelling for thin intumescent coatings under a range of heating conditions

In the past decades, the steel industry has experienced a rapid growth in the use of intumescent coatings for protecting load-bearing steel structures in the event of a fire. The onset of swelling is key for assuring the effectiveness of thin intumescent coatings for providing thermal insulation to the steel substrate during fire. The study presented herein investigates the onset of swelling for a commercially available thin intumescent coating applied on steel plates and exposed to a wide range of heating conditions. Experiments were performed using an array of radiant panels for controlling incident radiant heat flux at the exposed surface of coated steel samples. Within the scope of this research study, the onset of swelling was defined based on two conditions: (1) visual observation of swelling during heating or (2) time-history of the steel temperature. Research outcomes derived from this work defined a threshold for the onset of swelling in terms of steel and coating temperatures and it concluded that the onset of swelling is directly influenced by the heating conditions at the exposed surface and the original applied dry film thickness.

Development of tungsten armored high heat flux plasma facing components for ITER like divertor application

The dome and reflector plate are the parts of divertor plasma facing components (PFCs) of ITER tokamak which are mainly aimed for the removal of heat load of maximum 5 MW/m2 in steady state condition. The dome is a curved tungsten armoured component and the reflector plate is a straight component. These components have multi-layered joints made of various materials such as tungsten (W), OFHC copper (Cu), copper alloy (CuCrZr) and stainless steel (SS316LN). Joining of such multi-layered joints is known to be problematic due to joining of several dissimilar materials. In this paper, we report the indigenous development of medium size dome and reflector plate via vacuum brazing route for ITER like tokamak application. In order to evaluate the performance of the dome against ITER-like scenarios (maximum heat flux removal of 5MW/m2), the dome has been successfully tested for 1000 number of steady-state thermal cycles at incident heat fluxes of 3.87 MW/m2 in the High Heat Flux Test Facility (HHFTF) at IPR. Subsequent testing of additional 200 thermal cycles was also done at incident heat flux of 6 MW/m2. During the High heat flux (HHF) tests, surface temperature of W tiles reached 640oC and the beam power was restricted at 6MW/m2 to limit the temperature below 450oC at the CuCrZr heat sink. Total 1200 steady-state thermal cycles have been completed. At 6 MW/m2, the absorbed heat flux was 4 MW/m2. Engineering analysis on the HHFT of the dome has been performed using Finite element method (FEM) and Computational Fluid Dynamics (CFD) to simulate and to correlate with the experimental data. Ultrasonic immersion technique – Non destructive testing (NDT) was used to inspect the brazed joint quality of the dome before and after the HHFT. The results of the experimental details, engineering analysis and methodology adopted to fabricate the medium size dome and reflector plate are presented here.

Three-dimensional numerical study of direct steam generation in vertical tubes receiving concentrated solar radiation

Concentrated solar thermal energy is a good alternative to mitigate the environmental impacts generated by the growing use of energy. There are several commercial solar power tower plants in the world operating with direct steam generation. These plants operate with cavity-type and external tubular receivers. The conventional tubular receivers consist of a set of vertical circular tubes through which the working fluid circulates. This study is focused to analyze in detail the direct steam generation in a vertical tube receiving concentrated solar radiation. The mathematical model is based on the equations of continuity, momentum and energy for each phase. The modified model of Rensselaer Polytechnic Institute was used for the conditions of critical heat flux coupled to a Eulerian two fluid model. In the modified model of Rensselaer Polytechnic Institute, the total heat flux is divided into four components from the wall to the liquid: (a) liquid-phase convective heat flux, (b) heat transfer due to quenching, (c) heat flux due to evaporation and (d) convective heat flux of the vapor phase. The mathematical model was solved with a computational fluid dynamics software. The results were validated with experimental data reported in the literature and a parametric study was carried out to determinate the effect of the mass flux and heat flux incident on the wall, on: the steam quality, the volumetric fraction, the enthalpies and temperatures of liquid and steam. The reduction of mass flux by 44% increased the output mass flow of steam 4.28 times, whereas the increase of concentrated solar flux by 40% increased the output mass flow of steam 4 times.

Downstream radiative and convective heating from methane and propane fires with cross wind

Experiments were conducted to elucidate the radiative and convective heating occurring downstream of wind-driven fires produced by a gaseous burner. These flames model, at reduced scale, some of the dynamics observed in wind-driven fire spread through wildlands, buildings, mines or tunnels. Methane and propane were used to create fires ranging from 5 to 25 kW with ambient velocities ranging from 0.6 to 2.2 m/s. The total and incident radiative heat flux to a nearly-adiabatic downstream surface were measured by a water-cooled total heat flux gauge and a radiometer, respectively. The interaction between the buoyancy induced by the flame and momentum from the free stream was represented by a mixed-convection parameter, ξ=Grx2/Rex1n, where n = 3/2, 2 or 5/2. ξ was evaluated with two length scales in order to capture effects of both the boundary layer development length (x1) and heated distance downstream of the burner (x2). Results showed that the propane flame (high luminosity) exhibited slightly higher radiative heat fluxes than methane flames (low luminosity) under the same external conditions, while the convective heat flux followed an opposite trend. The downstream local radiative heat flux was quantified using a dimensionless flame thickness δx*, which showed a good relationship with ξ for n = 5/2 but not 3/2 or 2. The local convective heat transfer coefficient was expressed in the form of a local Nusselt number, Nux2Rex1−1/2, and correlated well as a piecewise function with ξ for n = 5/2. It was found that both δx* and Nux2Rex1−1/2 have a turning point at ξ ≈ 0.005, which was visually shown to denote the location where transition between an attachment and plume-like flame occurs. By separately describing both radiative and convective downstream heating, the mechanisms controlling heating which drives flame spread in wind-driven fires can be further understood.

Gaussian windowed frequency modulated thermal wave imaging for non-destructive testing and evaluation of carbon fibre reinforced polymers

Infrared thermographic techniques show their potential usage for non-destructive testing and evaluation of various materials due to their inherent capabilities such as safe, full field, remote, qualitative and quantitative defect detection capabilities. In this paper, a Gaussian Weighted Frequency Modulated Thermal Wave Imaging approach is reported for detection of sub-surface defects in Carbon Fiber Reinforced Polymer (CFRP) sample for a given frequency modulated incident heat flux. Artificial flat bottom holes and metallic inclusions as subsurface defects are prepared for the experimental investigation. Matched filter algorithm is applied for detection of sub surface defects by correlation coefficient images and compared the detection capabilities with conventional frequency domain phase images. The effect of spectral reshaping on frequency modulated thermal wave imaging is investigated. The results of the experiments show spectral reshaping is the most suitable selection for enhancing inspection capability and obtaining the highest Signal to Noise Ratio (SNR) for a given CFRP material.

Powder-epoxy resin/glass fabric composites with reduced flammability

This study investigates the possible enhancement of flame resistance in powder-epoxy resin/glass fabric composites. For this purpose, the halogen-free flame retardants containing phosphorous, nitrogen and aluminium were used. The total content of the fillers did not exceed 25 wt%. The laminates assessed for flame retardancy were designed specifically to be used as components of seats in public transport. Thermal resistance of the laminates and the surfaces of partially burned composites were also examined using thermogravimetric and scanning electron microscopy/energy-dispersive x-ray spectroscopy analyses, respectively. On the basis of the obtained results, it was found that the highest flame resistance (V-0 class, minimum oxygen concentration = 35.5% and maximum average rate of heat emission = 38.5 kW/m2 at an incident heat flux of 50 kW/m2) was identified in the laminates with matrix comprising 15 wt% aluminium diethyl phosphinate and 10 wt% melamine polyphosphate. In turn, the laminates with the matrix containing ammonium polyphosphate as the main component achieved only the V-1 flammability class.

MEASUREMENT OF THERMAL EFFICIENCY OF HEATING BOILER WITH FURNACE WALL WATERFLOW

This article deals with the improvement of thermal efficiency of heating boilers with furnace wall waterflow. During one cycle in a PK-38 boiler the average level of the heat flow decreases by 25–30 %. The incident heat flux is measured with a thermal probe which, however, gives a large error in the measurement results. Experiments show that the error depends on the penetration of the thermal probe into the outer surface of thermal zone as well as on cavities in sealing the thermal probe, and different thermophysical properties of the latter and metal material of the heating surface. The accuracy of the measured parameters is affected by the thermal probe sealing. It is found that the distortion of temperature fields is more significant at the lower boundary of the thermal probe junction at frequently used sealing. Studies show that the waterflow leads to the restoration of local coefficients of thermal efficiency to the previous values. The obtained results can be used in boiler design and allow improving the measurement methods for thermal efficiency of heating boilers with furnace wall waterflow.

Position of cones within cone clusters determines seed survival in black spruce during wildfire

It has recently become clear that the regeneration density of serotinous species within a burned area declines with local fire intensity. It is assumed that this occurs because variation in local fire intensity leads to variation in incident heat fluxes and, ultimately, seed necrosis. We argue here that this same relationship between incident heat flux and seed necrosis is important at the scale of individual plant crowns. Using Picea mariana (Mill.) B.S.P. (black spruce), we show that postfire seed viability increases with crown height, depth into the crown, and angle from wind direction (with the windward side enjoying greater viability). All three effects are what one would expect given the physics of buoyant plumes, interactions of moving fire lines with wake flow around cylinders, and heat transfer in porous bodies such as a tree crown. We conclude by discussing the broader consequences of cone cluster size and global change on regeneration in serotinous species.

Scientific and Practical Bases of a Method of Reception of Thin-Layer Heat-Insulating Coverings

The paper presents a brief overview of the thin-layer thermal insulation paints used now and their characteristics. A new composition of thin-layer heat-insulation coating is proposed. The introduction of solid phases of non-autoclaved foam concrete with the average density D150 with high values of the standard entropy of formation in it is scientifically substantiated from the point of view of increasing the thermal protection properties. It is shown that such phases have an advantage in comparison with the solid phases of the glass and ceramic microspheres used now. It is also proved that the presence of thin-layer thermal insulation coating of nanoscale particles in the form of silica in the composition favours the reflection of the incident heat flux due to the Tindal effect and provides an increase in the polydispersity of the composition. The calculation of the resulting composition by the Van Vleсk formula used in the classical science is given.

Pressure and particle property impacts on radiation in oxy-coal combustion

A numerical study evaluating the significance of scattering phase function, particle size, particle distribution and pressure in an oxy-coal combustor was conducted. Particle absorption and scattering coefficients were calculated from particle properties and Mie theory. Mie theory was also used to identify appropriate scattering phase functions as a function of particle size. A strongly forward scattering function represented by a Henyey-Greenstein function was found to match scattering behavior for particle diameters greater than approximately 10 μm. A comparison of highly forward scattering based on two mono-sized particle distributions (15 µm and 119 µm) and a polydisperse size distribution (27.9 µm SMD) showed small particles produced peak incident wall heat fluxes 30–40% greater than large particles due to the increased number density with the smaller particles. The polydisperse distribution produced flux profiles similar but slightly higher than the 15 µm mono-sized particles. Although strongly forward scattering was found to best represent particle scattering for this system, data did suggest that phase function/scattering coefficient influence becomes more important with increasingly small particle size. Two particle distribution profiles (narrow and wide) were used to represent different coal flame profiles. A wide particle distribution across the combustor diameter produced higher incident wall heat fluxes than a narrow distribution limited to the center region of the combustor. Increased particle size decreased the effect of distribution. Incident wall heat flux was found to increase with increasing combustor pressure up to approximately 5 atm and 10 atm, respectively, for wide and narrow particle distributions. This was primarily due to increased particle absorption and scattering coefficients. Additional pressure increases beyond this up to 20 atm had minimal impact on wall fluxes due to saturated values of the particle absorption coefficient.

Computer Tomography Based Structure Characterization of Expanded Intumescent Coatings for Fire Protection

Intumescent coatings are used to protect structural steel from fires. In this study, morphological characteristics of expanded and charred intumescent coatings, which is a key factor of its thermal insulating mechanisms, have been studied using a computer tomography (CT)-based analysis method. Specimens of the charred materials are prepared by a laboratory-scale mass-loss cone heating method for measuring thermal insulation performance. Three types of coatings are exposed to three constant incident heat fluxes (25 kW/m2, 50 kW/m2, and 75 kW/m2) for 2 h. The micro-CT scanning technology is applied on the charred materials to generate 3D reconstructed images with 20 µm resolution by ImageJ software. By using an image processing technique, the central region of each scanned 3D model is divided into three horizontal layers at different depths: top, middle and bottom. The morphological structure varies significantly with the type of coatings, the incident heat flux, and the depth in the char, but the median value of pore size (i.e. equivalent diameter) for all tested samples varies within a narrow range (0.352 mm to 0.424 mm). The qualitative morphological observations as well as the quantitative pore properties (i.e., 3D porosity, size distributions, and sphericity) are obtained. The present non-destructive structure analyses method is useful to assist in new formulation developments and to advance numerical modeling.

MEMOS 3D modelling of ELM-induced transient melt damage on an inclined tungsten surface in the ASDEX Upgrade outer divertor

The first MEMOS 3D simulations of liquid metal motion on an inclined bulk tungsten sample transiently molten by edge-localized modes (ELMs) are reported. The exposures took place at the outer ASDEX-Upgrade divertor with the tungsten surface tangent intersecting the magnetic field at ∼ 18°. Simulations confirm that the observed poloidal melt motion is caused by the volumetric J × B force with J the bulk replacement current triggered by thermionic emission. The final erosion profile and total melt build up are reproduced by employing the escaping thermionic current dependence on the incident heat flux derived from dedicated particle-in-cell simulations. Modelling reveals that melt dynamics is governed by the volumetric Lorentz force, capillary flows due to thermal surface tension gradients and viscous deceleration. The effect of the evolving surface deformation, that locally alters the field-line inclination modifying the absorbed power flux and the escaping thermionic current, in the final surface morphology is demonstrated to be significant.

Fire behavior of lithium-ion battery with different states of charge induced by high incident heat fluxes

Numerous accidents including fires and explosions occurring in transportation and storage of batteries enhance the need of studying the fire performances on batteries. In the current work, series bench-scale tests are conducted using a cone calorimeter to explore the fire behavior of lithium-ion battery. The influence of two key factors, namely state of charge (SOC) and incident external heat flux, on the battery fire characteristics is especially investigated. Combustion behavior, time to ignition (TTI), heat release rate (HRR) and fire risk assessment are obtained. The battery with higher SOC under high incident heat flux presents a fierce combustion process and higher surface temperature than the others. It is noteworthy that the time to ignition and time to peak HRR (TTP) decrease with the SOC, whereas the peak HRR (pHRR) and total heat release (THR) increase overall. On the whole, the increment of incident heat flux causes the decrease in TTI and TTP, while the pHRR and THR increase at higher incident heat flux. And the values of TTI agree well with the classical ignition model. Finally, the fire risk of battery with different SOCs under various incident heat fluxes is also highlighted.

Experimental study on the onset of swelling for thin intumescent coatings

Intumescent coatings (also known as reactive coatings) are widely used to protect load-bearing steel structural elements during fire. Upon heating, these initially thin coatings swell to form a low density, highly insulating foamed char that insulates steel substrate. Within the context of modern fire safety engineering design of steel structural systems, thermal insulation during fire may be key for assuring that steel does not reach ‘critical’ temperatures that may yield local structural instability and/or progressive failure (i.e. global instability).Traditionally, the fire performance of intumescent coatings is typically based on compliance (and to some extent their performance) to the standard fire resistance test in furnace, where single coated elements are exposed to the standard temperature-time curve. Numerous research studies have emphasised the influence of heating conditions on the onset of swelling and the overall effectiveness of intumescence. Moreover, some studies have shown that slow growing fires or low heating regimes may cause an incomplete swelling of the intumescent coating, or even melting or delamination. Consequently, the onset of swelling of is key for assuring its effectiveness in providing thermal insulation to the steel substrate during fire. Past researchers have shown that the onset of swelling occurs for temperatures in the range of 200-300°C. There are limited research studies that have look at the effects of varied, non-standard heating regimes on the onset or effectiveness of swelling of thin intumescent coatings.The study presented herein investigates the onset of swelling for a commercially available thin intumescent coating exposed to a wide range of heating regimes. Experiments are performed by using radiant panels and controlling incident radiant heat flux at the exposed surface of coated steel samples. This allows for the direct and precise control of the thermal boundary conditions at the exposed surface of tested samples. The onset of swelling criteria were defined in two ways: (1) visual observation of swelling during heating or (2) time-history of the protected steel temperature.The outcomes of this work aim at yielding a sound understanding of the true effectiveness of thin intumescent coatings. Experimental results mainly show that onset of swelling is directly influenced by the heating regime and it occurs for test samples heated at constant incident radiant heat flux above 20 kW/m2. The onset of swelling for the tested thin intumescent coating occurs for steel temperatures between 150 and 300°C and ‘mean coating temperature’ between 350 and 450°C. However, the steel temperature at onset of swelling is inversely proportional to the amount of constant incident heat flux at the exposed surface of tested samples, while the ‘mean coating temperature’ is directly proportional. As a conclusion, the time-history of net heat flux at the exposed surface of tested samples is not the only parameter that governs the onset of swelling for thin intumescent coatings.

Heat release and flame spread assessment of insulation in External Thermal Insulation Composite System (ETICS) façades

External Thermal Insulation Composite Systems (ETICS) represent a popular modern façade for reducing energy consumption, particularly in retrofit applications. Insulation – typically in the form of either EPS (expanded polystyrene) or mineral wool – is applied to the exterior of buildings using mortar adhesive, and then coated with a final layer of sand-cement render and glass fibre reinforcement. The materials are relatively cheap, and the labour can be easy and inexpensive compared to other solutions.The system does however mean that highly flammable EPS insulation is protected by only a thin layer of render, normally 3–8mm. The addition of this combustible material to the façade of the building represents a change in the fire risk, as the typical fire safety strategy involving compartmentation does not anticipate vertical fire spread on the exterior of a building. It is therefore necessary to characterise the fire risk, and ensure that an optimal fire barrier – namely, the render layer – is defined adequately.In this work, the insulation component of an EPS ETICS façade is tested using micro- to small-scale methodologies. The objective of this is to characterise some of the fundamental thermal material properties, ignitability, heat release, and flame spread of the specific components. The aim is to then extend this to a larger-scale methodology for testing of complete systems, which will be complemented by numerical modelling to enable scaling. Previous attempts at testing ETICS in a small-scale have been largely unsuccessful, and efforts have pointed to large-scale apparatuses – generally 4–8m high – as the only viable solution at present. These are however expensive and time consuming, and deliver little information on how the façade performs. Testing is especially problematic for EPS, which melts and shrinks at low temperatures, leading to difficulties in attempting to extract global flammability properties.MCC (Microscale Combustion Calorimeter) testing has been performed to benchmark the different EPS insulation that can be used in buildings. It is found that black EPS containing expanded graphite used in façades has substantially improved performance compared to ordinary white EPS used as building insulation. The peak heat release was reduced from 1160 W g−1 down to 490–740 W g−1, occurring at a temperature of 430–440 °C. An extra additive to improve the moisture performance of the system also enhances the fire performance slightly by reducing the heat release further.Previous attempts of testing ETICS and EPS using LIFT (Lateral Ignition and Flame spread Test) have in many cases been unproductive. For this experimental series, a sheet of paper has been applied to the surface of the first 100 mm of EPS in order to force ignition. Results for the flame spread velocity near to ignition have then been discarded to minimise the influence of this method. Flame spread results for the white EPS were not satisfactory due to the fact that the flame spread velocity had a weak dependence on the incident heat flux, and that that minimum critical heat flux for flame spread was 0.0 kW m−2, that is, no external energy was required to sustain burning. For black EPS specimens, the addition of the expanded graphite was effective in improving the flame spread performance. The minimum heat flux for flame spread was increased to 0.75–1.09 kW m−2, and the velocity was sufficiently moderate that a reasonable value for the flame spread modulus could be obtained.In future, attempts will be made to correlate the flame spread results of these individual EPS products to the flame spread as part of a complete ETICS façade. This would then provide a more meaningful method to compare different ETICS solutions as a first step without the need for expensive large-scale testing during the main development phase of systems.

Pyrolysis and Combustion of Polyvinyl Chloride (PVC) Sheath for New and Aged Cables via Thermogravimetric Analysis-Fourier Transform Infrared (TG-FTIR) and Calorimeter.

To fill the shortages in the knowledge of the pyrolysis and combustion properties of new and aged polyvinyl chloride (PVC) sheaths, several experiments were performed by thermogravimetric analysis (TG), Fourier transform infrared (FTIR), microscale combustion calorimetry (MCC), and cone calorimetry. The results show that the onset temperature of pyrolysis for an aged sheath shifts to higher temperatures. The value of the main derivative thermogravimetric analysis (DTG) peak of an aged sheath is greater than that of a new one. The mass of the final remaining residue for an aged sheath is also greater than that of a new one. The gas that is released by an aged sheath is later but faster than that of a new one. The results also show that, when compared with a new sheath, the heat release rate (HRR) is lower for an aged one. The total heat release (THR) of aged sheath is reduced by 16.9–18.5% compared to a new one. In addition, the cone calorimetry experiments illustrate that the ignition occurrence of an aged sheath is later than that of a new one under different incident heat fluxes. This work indicates that an aged sheath generally pyrolyzes and it combusts more weakly and incompletely.

Heat transfer characteristics of impinging methane diffusion and partially premixed flames

Flame jet impingement heat transfer is a well-established technique for obtaining high heat transfer rates in many domestic and industrial applications. The flame structure and the heat transfer characteristics of the impinging methane diffusion flame and partially premixed flame are experimentally investigated. The heat transfer characteristics of the impinging methane flame are determined using the minimization technique wherein the target surface is impinged by a methane flame from the bottom and is simultaneously cooled from the top by air jets of different Reynolds number. At steady state, one-dimensional energy balance across the impingement surface provides an over-determined system of equations which when solved using the minimization technique give the heat transfer coefficient of the flame jet, the reference temperature and the emissivity of the gas/flame beneath the impingement surface.The stagnation point heat transfer coefficient and reference temperature are found to be maximum for the case of flame just touching the impingement surface. The radial variation of the non-dimensional heat transfer coefficient and reference temperature is found to have a Gaussian profile. For low H/d’s where the flame is impinging the target surface, the temperature at the stagnation zone is lower than in the radial position as the hot jet turns in the radial direction and impinges at a distance slightly away from the stagnation region.The separation of heat transfer components indicates that the heat transfer is convective dominant with radiation accounting to 10% of the total incident heat flux in the case of methane diffusion flame and 3.5% in the case of partially premixed flame. Re-radiation becomes a significant component of heat transfer, accounting to 25–30% of the incident heat flux, due to higher wall temperatures. The present study also investigates the effect of tube burner to plate spacing.