Radiative Heat Loss Research Articles

A mechanistic analysis of H2O and CO2 diluent effect on hydrogen flammability limit considering flame extinction mechanism

The released hydrogen can be ignited even with weak ignition sources. This emphasizes the importance of the hydrogen flammability evaluation to prevent catastrophic failure in hydrogen related facilities including a nuclear power plant. Historically numerous attempts have been made to determine the flammability limit of hydrogen mixtures including several diluents. However, no analytical model has been developed to accurately predict the limit concentration for mixtures containing radiating gases. In this study, the effect of H2O and CO2 on flammability limit was investigated through a numerical simulation of lean limit hydrogen flames. The previous flammability limit model was improved based on the mechanistic investigation, with which the amount of indirect radiation heat loss could be estimated by the optically thin approximation. As a result, the sharp increase in limit concentration by H2O could be explained by high thermal diffusivity and radiation rate. Despite the high radiation rate, however, CO2 with the lower thermal diffusivity than the threshold cannot produce a noticeable increase in heat loss and ultimately limit concentration. We concluded that the proposed mechanistic analysis successfully explained the experimental results even including radiating gases. The accuracy of the improved model was verified through several flammability experiments for H2-air-diluent.

Experimental and numerical investigations on extinction strain rates in non-premixed counterflow methane and propane flames in an oxygen reduced environment

The aim of this work is to contribute to the better understanding of the oxidation process of light hydrocarbons, low calorific value gas and components of natural gas in the presence of inert gases and the influence of oxygen reduction in exhaust gas recirculation situations. The extinction behavior of laminar counterflow diffusion flames of methane (CH4), ethene (C2H4) and propane (C3H8) under nitrogen diluted condition has been investigated experimentally and numerically. In the experimental set-up, a counterflow burner was employed to investigate the extinction behavior under atmospheric conditions (1 bar and 298 K). The present study highlights the non-linear influence on the decrease in the extinction strain rate (ESR) that were found with decreasing fuel content and decreasing oxygen content. The oxygen content on the oxide side was stepwise diluted with nitrogen in order to distinctly show the possible oxygen-depleted influence in the recirculation areas. Additionally, it has been shown that the distance between the nozzles has an impact on the ESR but not on the general rising trend with the increasing fuel content in the non-premixed flames. Moreover, to clarify the influence, the flame zones were visualized using a system of high-speed OH* chemiluminescence camera. It was possible to investigate the width, intensity and position of the flame front from the radical detection. This provided a comprehensive data base for the subsequent numerical validation. In order to complement the experimental investigations, extensive uncertainty analyses were conducted and a new laser-based approach for the detection of flame extinction were demonstrated for the low-visible flames. Besides the experimental study, ESRs of CH4 and C3H8 combustion obtained from the experimental measurements were compared with those predicted by the numerical simulations. The San Diego-2014 reaction mechanism was used to represent the detailed chemical reaction, in which the predicted ESRs agree well with the experimental data was shown. Additionally, the effects of radiative heat loss, molecular transport model and chemical reactions on the ESRs were studied numerically. The present results suggest that the radiative heat loss plays a minor role on the ESRs, the molecular transport model is more significant and the ESRs are quite sensitive to several key reactions.

Identification of the extinction mechanism of lean limit hydrogen flames based on Lewis number effect

The conventional flame stretch analysis showed some technical limitations in describing the observed extinction mechanism without considering the heat transfer characteristics related to flame flow patterns. To improve the flame stretch analysis, this study investigated the extinction mechanism of lean limit hydrogen flames with the stabilized flame method. Multidimensional lean limit hydrogen flames stabilized in a tube of inner diameter of 25 mm were simulated numerically. Unlike the methane flames, the positive effect of a stretched hydrogen flame (Le≪1) on the flame tip dominated the negative stretch effect by increasing the preferential diffusion. Hence the primary extinction of hydrogen flames occurred at the trailing edge by heat loss mechanism. The simulation confirmed that the radiation heat loss rate did not attain 5% of the conduction heat loss rate at the trailing edge. When the flame skirt length was shortened enough by the primary extinction, an enhanced recirculation flow was generated. The combined effect of conduction and recirculation flow associated with the Lewis number effect was the key to understand the extinction mechanism of lean limit hydrogen flames. Finally, generation of recirculating flow causing the stretch extinction plays a dominant role in final flame extinction.

Flammability Limits of Flat Materials with Moderate Thickness in Microgravity

The flammability limits of flat materials with different moderate thickness in opposed flow were investigated to clarify the effect of sample thickness on flammability limit. Thin poly-methyl methacrylate (PMMA) sheets and polycarbonate (PC) sheets were used as samples. The thickness was varied as 0.2 mm, 0.5 mm and 1.0 mm for PMMA, and 0.1 mm, 0.2 mm, 0.5 mm and 1.0 mm for PC. The limiting oxygen concentration (LOC) of each sample was obtained by 20 s parabolic flight experiments in mild and low flow velocity condition, and by blow-off tests on ground in high flow velocity condition. The flight experiments showed that there existed significant difference in LOC according to the sample thickness especially in mild flow condition; the thicker the sample was, the larger the LOC became. We applied a hypothesis of heat penetration zone shape into the heat balance equation around the preheat zone to account the effect of sample thickness on quenching phenomena. Although the hypothesis was not confirmed by experimentally, the obtained empirical model based on the hypothesis well represented the trend of the change of LOC when the sample thickness varied not only for the PMMA and PC samples, but also for low-density polyethylene (LDPE) and high-density polyethylene (HDPE) samples with relatively thick thickness. The modified model states that increase in the sample thickness has equivalent effect to increase in the radiative heat loss and such thickness effect is amplified when the pyrolysis temperature of the material is high.

High thermoelectric energy conversion efficiency of a unicouple of n-type Mg3Bi2 and p-type Bi2Te3

Low-grade heat harvesting by thermoelectric technology can contribute to efficient energy utilization. However, the large-scale application of thermoelectric devices is partially hindered by their unsatisfactory performance and relatively high cost. Here we prepared a cost-effective n-type Mg3Bi2-based compound with a peak zT of 1.24 at 573 K. A unicouple comprised of n-type Mg3.2Bi1.29Sb0.7Te0.01 and p-type Bi0.2Sb1.8Te3 was constructed and its energy conversion efficiency was evaluated. The unicouple exhibited a record-high efficiency of (9.0 ± 0.5)% or (8.3 ± 0.4)% [considering the radiation heat loss] under a temperature difference of 265 K at the hot-side temperature of 573 K. Our results demonstrate the great potential of Mg3Bi2-based materials for low-grade heat recovery.

Effects of Acetylene Addition to the Fuel Stream on Soot Formation and Flame Properties in an Axisymmetric Laminar Coflow Ethylene/Air Diffusion Flame.

The effects of adding acetylene to the fuel stream on soot formation and flame properties were investigated numerically in a laminar axisymmetric coflow ethylene/air diffusion flame using the open-source flame code Co-Flame in conjunction with an elementary gas-phase chemistry scheme and detailed transport and thermodynamic database. Radiation heat transfer of the radiating gases (H2O, C2H2, CO, and CO2) and soot was calculated using a statistical narrow-band correlated-k-based wide band model coupled with the discrete-ordinates method. The soot formation was described by the consecutive steps of soot nucleation, surface growth of soot particles via polycyclic aromatic hydrocarbons (PAHs)-soot condensation or the hydrogen abstraction acetylene addition (HACA) mechanism, and soot oxidation. The added acetylene affected the flame structure and soot concentration through not only chemical reactions among different species but also radiation effects. The chemical effect due to the added acetylene had a significant impact on soot formation. Specifically, it was confirmed that the addition of 10% acetylene caused an increase in the peak soot volumetric fraction (SVF) by 14.9% and the peak particle number density by about 21.1% (z = 1.5 cm). Furthermore, increasing acetylene concentration led to higher concentrations of propargyl, benzene, and PAHs and consequently directly enhanced soot nucleation rates. In addition, the increased H mole fractions also accentuated the soot surface growth. In contrast, the radiation effect of the addition of 10% acetylene was much weaker, resulting in slightly lower flame temperature and SVF, which in turn reduced the radiant heat loss.

Heat dissipation performance of a heat pipe radiator with rectangular longitudinal fin under natural convection

To solve the cooling problem of electronic equipment with small heat dissipation area under high uneven power, this paper presents a natural convection radiator with L-shaped heat pipe and rectangular longitudinal fin module. The radiator has the advantages of compact structure, flexible layout, and didn’t occupy the side space due to the structural advantages. A numerical study was carried out, and the results are in good agreement with the experimental results. The thermal performance of the rectangular longitudinal fin module is also studied, the radiation heat loss of the fins accounts for about 25% of the total heat dissipation. The temperature and velocity vector distribution near the fin was analysed to research the influence of flow characteristics on the heat transfer of the fin. Additionally, the numerical results also show that the fin heat sink has an optimal fin height, which is 14mm.

Radiative heat loss estimation of building envelopes based on 3D thermographic models utilizing small unmanned aerial systems (sUAS)

The advent of sUAS (small unmanned aerial systems – or surveying drones) technology has made comprehensive thermographic analyses of the built environment feasible. Advantages over traditional thermal survey techniques include a significant increase in ease of three-dimensional (3D) navigation around large structures as well as the ability to perform data-rich measurements. 3D building envelope thermographic models represent the building surface temperatures and can be created using structure-from-motion photogrammetry techniques with the collected high-overlap sUAS thermographic images. This paper introduces an end-to-end workflow for measuring radiative heat loss from building envelopes. This includes the setup and process required for quickly creating accurate 3D thermographic models and their analysis with software developed for this purpose. Three analytical models for building envelope radiative heat loss are applied and tested for viability through controlled experiments. The workflow is then demonstrated by evaluating the thermal radiation losses of two buildings on the campuses of the University of North Georgia. Our findings suggest the workflow can quickly provide good radiative heat loss measurements and may be a valuable addition to more comprehensive thermal analysis techniques.

Combustion waves in narrow samples of solid energetic material: Chaotic versus spinning dynamics

In this work, we study the conditions determining the appearance of chaotic or spinning dynamic regimes for flames propagating in narrow cylindrical samples of energetic materials. Two models for heat losses from the sample surface are explored. The first model allows for a linear dependence of heat-loss intensity on the surface temperature. Another model assumes radiative heat losses modeled by the fourth degree of temperature. The global stability analysis of steady-state solutions is carried out for both models.It is shown that the main factor determining the wave dynamics is the sample thickness. In sufficiently thin samples, the axisymmetric instability mode prevails and the flame dynamics depends on the intensity of heat-losses. With an increase in their intensity, the auto-oscillatory dynamics first transforms into chaotic through the period doubling Feigenbaum’s cascade and, with a further increase in heat-losses, the dynamical extinction occurs. However if the sample is sufficiently thick, the most unstable mode is the mode with a nonzero azimuthal wave number. In this case, the spinning propagation regime is finally realized with a multi-headed flame structure formed. It is found that, qualitatively, this scenario is independent of the specifics of the heat-loss model.

Numerical modeling of non-premixed combustion of micron-sized Ti particles in counter-flow arrangement: The effects of heat losses

In this study, a numerical model was introduced for simulation of preparation of TiO2 by non-premixed combustion of Ti particles in counter-flow arrangement. Preheating, reaction, melting, and vaporization processes were modeled using the asymptotic concept. To reach a viable model, convective and radiative heat losses were included in the model derivation. The conservation equations for mass and energy were presented in each zone with the heat loss terms. Boundary and jump conditions were employed to solve the governing equations to preserve continuity throughout the system. The numerical modeling was first validated against literature data, where good agreement was achieved. Then, the focus of results and discussion were given to the influence of heat loss terms on the spatial distributions of temperature and species mass fractions. It was clearly shown in that heat losses can significantly shorten the distances between the stagnation plane and flame fronts. This is the also the case for melting and vaporization fronts. However, the heat losses can obviously suppress thermophoretic force. Finally, the effects of initial fuel temperature on positions of flame, melting, and vaporization fronts, where almost linear dependences were found that higher initial fuel temperature results in closer fronts to the stagnation plane.

Thermocapillary deformation induced by laser heating of thin liquid layers: Physical and numerical experiments

Characterizing the profile of the laser-induced thermocapillary deformation of a thin liquid layer on a laser absorbing solid is of great fundamental and applied importance. The thermocapillary effect is the basis of several non-contact methods for measuring the physicochemical and thermal properties of liquids and solids, and methods of non-destructive testing in material science and thermal physics. The study of the layer rupturing caused by laser beam heating could contribute to solving the problem of the dry spots formation in thin–film heat exchangers. In this regard, the development of physical and numerical tools to determine the thermocapillary profiles of thin liquid layers is of great interest to researchers. In the present work, we developed a home-made setup to scan a deformed surface of a liquid layer with a laser sheet. Its accuracy was verified by scanning the surface of a solid standard specimen with a given Gaussian profile. By scanning the thermocapillary deformed layers of silicone oil, we found that for the Gaussian distribution of the radiation intensity in the heating laser beam, the thermocapillary deformations are significantly not the Gaussian. A modified Agnesi function, which is characterized by high accuracy and allows defining the surface profile using a minimum of experimental data, is shown to be an optimal function for approximating the thermocapillary deformation. Using Agnesi approximation can significantly reduce the time of the experiment and the processing of results. To validate new experimental data, an axisymmetric numerical model of the thermocapillary convection in a thin liquid layer was developed using commercial software Comsol Multiphysics. It helped calculating the surface profile and the temperature field on the substrate for two boundary conditions of radiative heat exchange in the system corresponding to maximum and minimum radiation heat loss. For the case of the ebonite-silicone oil system, when the laser beam is absorbed by the ebonite surface, it was numerically shown that the difference between the types of boundary conditions becomes noticeable only on very thin layers close to the pseudo-rupture. In general, a comparison of the stationary profiles of the thermocapillary surface deformation and temperature distributions obtained experimentally and numerically for the entire range of the studied layer thicknesses shows satisfactory agreement between the physical and numerical results.

Observations of long duration microgravity spherical diffusion flames aboard the International Space Station

Spherical diffusion flames of gaseous fuels supported on a porous spherical burner are investigated experimentally in microgravity for long durations aboard the International Space Station (ISS). Experimental results are compared with a transient numerical model. The experiments involve normal spherical diffusion flames (i.e., fuel flowing into oxidizer) burning ethylene in an oxygen/nitrogen mixture at atmospheric pressure. The stoichiometric mixture fraction and adiabatic flame temperature are varied by diluting the fuel with nitrogen and varying the oxygen concentration in the ambient. The diagnostics consist of color video, thin-filament pyrometry for gas temperature, thermocouple measurements of burner temperature, and photomultiplier tube measurements of broad and narrow-band flame emission. Burn times range from 28 – 336 s. All flames grow monotonically after ignition. Peak gas temperature generally decreases with time. Radiative extinction is observed when, at constant reactant flow rate, the flame expands until radiation heat loss approaches the chemical heat release. Before a flame experiences complete extinction, it may enter an oscillatory mode in which part of the flame partially extinguishes and then reforms. The oscillations grow in magnitude until complete extinction. Sufficiently small flames with low radiation losses exhibit quasi-steady behavior, but burner heating is significant, and the experimental duration is dictated by the temperature limit of the porous sphere.

Semi-analytic model of a carbon fiber thermal-field emitter

Carbon fibers passing current are subject to resistive heating. When failure occurs, this is related to their local temperature. The failure temperature and its location are estimated. The temperature variation is calculated using analytical models for electrical and thermal conductivities based on the temperature dependent electron–phonon relaxation time. In the absence of radiative heat loss, an analytic expression of temperature along the fiber is given from which a maximum possible emission current is derived and is governed by a single introduced parameter ωo. A method of treating the radiative heat loss is developed and is governed by a second parameter γ, which allows a rapid numerical means to calculate the correction to the analytic form. Heat variation along a thick carbon fiber is contrasted to that along a multi-walled carbon nanotube (MWNT): it is shown that the relative magnitude of ωo compared to γ determines that the analytical formula is a good approximation for MWNTs but requires numerical correction for fibers. Furthermore, it is shown that the analytical form of ωo specified a maximum current beyond which the carbon emitter fails due to thermal runaway. The theoretical models are used to interpret observed behavior of field emission from carbon fibers and the resulting damage they endure when the extracted field-emission current is high. Results from implementing the developed temperature variation model into the MICHELLE beam optics simulation code are presented, with an example application predicting the conditions for stable equilibrium operation as well as for the onset of fiber failure.

Convective and Radiative Heat Losses of a Cylindrical Accumulator for Solar Water Heating Collectors

Convective and Radiative Heat Losses of a Cylindrical Accumulator for Solar Water Heating Collectors

Construction of a ternary channel efficient passive cooling composites with solar-reflective, thermoemissive, and thermoconductive properties

Current passive radiative cooling is a promising alternative to electrical cooling systems, but it is still difficult to be used in cooling devices with an internal heat source. Hence, we here present a ternary channel efficient passive cooling composites by embedding hexagonal boron nitride (h-BN) into high-density polyethylene (HDPE). Like the traditional passive radiative cooling materials, the h-BN/HDPE composite possesses a high solar reflectance (~87%) that minimizes solar heat gain, and a high thermal emittance (~0.71) that maximizes radiative heat loss, but the composites also have a high thermal conductivity (~1.47 W/(m·K)) that maximizes the internal heat conduction. Therefore, the combination of the optical and thermal properties allows for the LED's temperature drops of 5 °C, compared with the bare LED, under the average solar intensity of 1100 W/m2. The notably cooling effect provides the guidance for the future application of the ternary channel efficient passive cooling composites in cooling the devices with internal heat.

Modelling laminar diffusion flames using a fast convergence three-dimensional CVFEM code

Laminar diffusion flames have been employed extensively to investigate the effects of physical and chemical-related parameters on combustion phenomena. Attention has been given to improve combustion models while keeping the two-dimensional (2D) axisymmetric approach and simplified boundary conditions. The flow structure due to the discrete jet nature of the reactants claims a more realistic inlet boundary condition for the burner plate which can only be accomplished if the solution domain is 3D. In this work, we present a novel 3D simulation code for the analysis of non-premixed laminar flames of methane and air, considering detailed inlet boundary conditions. The code is based on the control volume finite element method, written in MATLAB environment. To accomplish that a novel flow-oriented upwind scheme was implemented for the advection terms of the conservation equations. Combustion modelling can be carried out by either an enhanced flame sheet model in which an enthalpy equation is integrated in combination with the mixture fraction equation or detailed reaction schemes. Radiative heat losses were accounted for in the added energy equation. Three-dimensional numerical predictions, based on the flame sheet model, were able to simulate the near-wall gas temperature and inflow velocity field of a set of perforated plate burner with different porosities. It was found that the discrete nature of the injection of fuel and oxidiser, inflow boundary condition, plays a significant role on flame shape and length. Much better agreement between published experimental data and numerical predictions was obtained, particularly regarding flame structure, species mass fractions and temperature distribution of non-premixed laminar flames regardless of the adopted simplified combustion model. It was observed that flame length decreased as the burner porosity increased, up to a certain limit. The modified flow-oriented scheme greatly improved solution stability and computational time of the reacting system predictions.

Acoustic parametric instability, its suppression and a beating instability in a mesoscale combustion tube

The present work reports thermo-acoustic instability in a mesoscale tube of diameter 8 mm (> quenching diameter) and length 702 mm. Mixtures with Lewis number, Le~ 0.8 (rich C2H4/O2/CO2), 1.05 (lean C2H4/O2/CO2) and 1.34 (lean C2H4/O2/N2) are used. Several new flame responses are observed. For lower burning velocity mixtures, flame extinction is observed due to heat loss when primary instability transforms to secondary instability for all Le mixtures. However, parametric cellular structures which are characteristic of parametric instability are observed only for Le of 0.8. It is proposed based on calculations and experiments that parametric structures will be observed only when diameter of tube is two times larger than the characteristic wavelength of parametric instability. If the tube diameter doesn't allow formation of parametric structure whirling and counter-rotating flames are observed instead of parametric structures. Suppression of acoustic parametric instability is observed for higher burning velocity mixtures with Le>1 and its mechanism is discussed. For a range of SL, a beating instability is observed for CO2 diluted mixtures of Le>1, where pressure oscillation and flame motion show beating oscillations of frequency around 15 Hz. This beating instability is believed to be caused by non-linear interaction of acoustic instability with pulsating instability of flame front which is caused due to combined radiative and convective heat loss. Due to CO2 dilution, the radiative heat losses could play a significant role in inducing pulsating instability.

All-Cold Evaporation under One Sun with Zero Energy Loss by Using a Heatsink Inspired Solar Evaporator.

Interfacial solar steam generation is a highly efficient and sustainable technology for clean water production and wastewater treatment. Although great progress has been achieved in improving evaporation rate and energy efficiency, it's still challenging to fully eliminate the energy loss to the surrounding environment during solar steam generation. To achieve this, a novel heatsink‐like evaporator (HSE) is developed herein. During solar evaporation, the temperature on the top solar evaporation surface can be regulated by the fin structures of the HSE. For the evaporators with 5 to 7 heatsink fins, the temperature of the solar evaporation surface is decreased to be lower than the ambient temperature, which fully eliminates the radiation, convection, and conduction heat losses, leading to the absolute cold evaporation over the entire evaporator under 1.0 sun irradiation. As a result, massive energy (4.26 W), which is over 170% of the received light energy, is harvested from the environment due to the temperature deficit, significantly enhancing the energy efficiency of solar steam generation. An extremely high evaporation rate of 4.10 kg m−2 h−1 is realized with a 6‐fin photothermal HSE, corresponding to an energy conversion efficiency far beyond the theoretical limit, assuming 100% light‐to‐vapor energy conversion.

Experimental investigation on the effect of carbon chain length to the droplet combustion characteristic of fatty acid methyl ester

Biodiesel which produces from vegetable oil consists of various fatty acids as fatty acid methyl ester constituent. Each fatty acid has a specific combustion characteristic due to the difference in physicochemical characteristics. This study was done with a single fatty acid methyl ester from various saturated fatty acids to analyze the effects of carbon chain length on the droplet combustion during the evaporation and combustion stages in ambient temperature and atmospheric pressure. Results show that the ignition delay time increase with the longer carbon chain due to the higher viscosity and boiling point. The higher oxygen content in the fatty acid methyl ester molecule promotes the faster combustion, gives a higher burning rate, and cause the flame dimension shorter. Furthermore, oxygen content results in higher radiation caused a brighter flame. The high droplet temperature occurs in the long carbon chain due to the higher of combustible matter gives an increase to the heating value. Low radiation heat loss in a long carbon chain which indicates by the flameless bright also causes the droplet temperature higher. The higher droplet temperature gives the lower gas density which causes the flame dimension higher due to the natural convection.

Limitations of flamelet formulation for modeling turbulent pool fires

A well-resolved database of a small-scale heptane pool fire is analyzed to assess the assumptions of flamelet models in fire simulations. The pool fire database features well-resolved turbulence fields, finite-rate chemistry for heptane using a 33-species skeletal mechanism, and a coupled Monte Carlo ray-tracing radiation solver with a line-by-line spectral model. A budget analysis of the flamelet equation is performed by transforming temperature and species to the mixture fraction space and extracting instantaneous flamelet solutions along flame-aligned directions. Five flame-aligned budget terms and one unsteady term are identified through transformation of the temperature equation. Three conditions are examined, including an adiabatic case, a case involving only gas radiation, and a case involving both gas and soot radiation. The budget analysis shows that preferential diffusion is non-negligible near the base of the pool. Radiation is found to be appreciable in the transition region of the fire, resulting in a reduction of flame temperature by as much as 270 K due to radiative heat losses. The curvature of the flame surface is found to have a minor impact on the temperature dynamics near the flame front. The extracted flame structures deviate from those predicted from a steady-state adiabatic laminar flamelet model, particularly in the profiles of CO, CO2, and OH. A time scale analysis is performed, indicating that steady state assumptions for chemical species are reasonable for this flame, although radiation is much slower compared to chemistry and requires consideration to correctly capture the temperature evolution. The source terms from radiative absorption and emission show pronounced non-proportionality between the two, where absorption can exceed emission, thereby serving as a heating source for the fuel and oxidizer streams. The controlling factors for radiative absorption are analyzed, based on which a non-local model is proposed to approximate absorption in pool fires. A priori assessment of this model shows good agreement with the pool fire database, demonstrating the feasibility of enhancing the flamelet formulation to account for radiative emission and absorption effects.