Microwave attenuation in forest fuel flames

The flames of forest fuels form a weakly ionized gas. Assuming a Maxwellian velocity distribution of flame particle and collision frequencies much higher than plasma frequencies, the propagation of microwaves through forest fuel flames is predicted to have attenuation and phase shift. A controlled fire burner was constructed where various natural vegetation materials could be used as fuel. The burner was equipped with thermocouples and used as a cavity for microwaves with a laboratory quality network analyzer to measure phase and attenuation. The controlled fires had temperatures in the range of 500-1000 K and microwave attenuation of 1.0-4.5 dB m⁻¹ was observed across the 0.5 m diameter cavity. Attenuations of this magnitude could affect active remote sensing systems signals at microwave frequencies in forest fire environments where flame depths of up to 50 m are possible. In the experiment, temperature was not the only controlling parameter for the ionisation; type of fuel burnt also influenced it. Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) analysis of the composition of the fuel confirmed that a higher content of alkali (with low ionization potential) lead to higher electron densities. Electron densities in the range of 0.32-3.21×¹⁰¹⁶ m⁻³ and collision frequencies of 1.1-4.0×¹⁰¹⁰ s⁻¹ were observed for flames with temperature in the range of 730-1000 K.     

Intensity calibrated hydrogen flame spectrum

The detection of hydrogen fires is important to the aerospace community. The National Aeronautics and Space Administration (NASA) has devoted significant effort to the development, testing, and installation of hydrogen fire detectors based on ultraviolet, near-infrared, mid-infrared, and/or far-infrared flame emission bands. Yet, there is no intensity calibrated hydrogen-air flame spectrum over this range in the literature and consequently, it can be difficult to compare the merits of different radiation-based hydrogen fire detectors. In this paper we present an intensity calibrated irradiance spectrum for a low pressure hydrogen flame burning in air from 200 nm to 13.5 microns that varies by more than six orders of magnitude. The results resolve relative intensity errors between spectral bands that appear within the literature. The impact of the measured spectrum on the choice of radiation-based hydrogen fire detectors is discussed.     

A simple method for measuring positive ion concentrations in flames and the calibration of a nebulizer/atomizer

A simple and accurate (<±10%) method has been developed for measuring absolute concentrations of positive ions in a flame. The method involves only the measurement of a saturation current (∼μA) when collecting the total flux of cations in a flame when it impinges on a conducting plate or electrode perpendicular to the flame axis. A fixed negative bias (∼50 or 100 V) is applied to this electrode with respect to the metallic burner; such a voltage is more than adequate to stop all negative charges from reaching the electrode. The required bias voltage for this separation of positive and negative charges varies roughly as the one-third power of the concentration of positive ions. One application of the method involves the accurate (<±10%) calibration of the delivery factor f for a nebulizer or pneumatic atomizer used to dope a flame with a metal (<5 × 10⁻⁶ mole fraction); the metal is introduced by spraying an aerosol of an aqueous salt solution into the gas mixture feeding the burner. When a dilute solution (∼10⁻⁴ molar) of a Cs salt is sprayed into a fairly hot flame (>2300 K), the Cs added is almost completely ionized (>99%); accordingly, f can be determined by measuring the absolute ion density of Cs⁺. Calibration difficulties can be encountered if a more concentrated solution is employed, because Cs⁺ can, quite surprisingly, achieve a superequilibrium concentration early in the flame. However, the method was used to show that the delivery of a pneumatic atomizer is essentially linear when the strength of the salt solution varies by at least three orders of magnitude. Of great utility is the ease with which ion concentration profiles can be measured along a flat flame for studying the kinetics of reactions involving ions. Such ion profiles have revealed the very rapid production of Cs⁺ ions and free electrons in the reaction zone of a flame doped with Cs; two possible chemi-ionization reactions are discussed to explain this phenomenon. In addition, the rate constant of the recombination reaction H₃O⁺ + e^- → H + H + OH is confirmed to be (3.2 ± 0.3)×10⁻⁷ cm³ ion⁻¹ s⁻¹ at 2400 K. Confirmation of this rate coefficient provides verification of the simple method presented here for the measurement of absolute concentrations of positive ions in flat flames.     

Development of high intensity CDC combustor for gas turbine engines

Colorless distributed combustion (CDC) has been demonstrated to provide ultra-low emission of NOx and CO, improved pattern factor and reduced combustion noise in high intensity gas turbine combustors. The key feature to achieve CDC is the controlled flow distribution, reduce ignition delay, and high speed injection of air and fuel jets and their controlled mixing to promote distributed reaction zone in the entire combustion volume without any flame stabilizer. Large gas recirculation and high turbulent mixing rates are desirable to achieve distributed reactions thus avoiding hot spot zones in the flame. The high temperature air combustion (HiTAC) technology has been successfully demonstrated in industrial furnaces which inherently possess low heat release intensity. However, gas turbine combustors operate at high heat release intensity and this result in many challenges for combustor design, which include lower residence time, high flow velocity and difficulty to contain the flame within a given volume. The focus here is on colorless distributed combustion for stationary gas turbine applications. In the first part of investigation effect of fuel injection diameter and air injection diameter is investigated in detail to elucidate the effect fuel/air mixing and gas recirculation on characteristics of CDC at relatively lower heat release intensity of 5 MW/m³ atm. Based on favorable conditions at lower heat release intensity the effect of confinement size (reduction in combustor volume at same heat load) is investigated to examine heat release intensity up to 40 MW/m³ atm. Three confinement sizes with same length and different diameters resulting in heat release intensity of 20 MW/m³ atm, 30 MW/m³ atm and 40 MW/m³ atm have been investigated. Both non-premixed and premixed modes were examined for the range of heat release intensities. The heat load for the combustor was 25 kW with methane fuel. The air and fuel injection temperature was at normal 300 K. The combustor was operated at 1 atm pressure. The results were evaluated for flow field, fuel/air mixing and gas recirculation from numerical simulations and global flame images, and emissions of NO, CO from experiments. It was observed that the larger air injection diameter resulted in significantly higher levels of NO and CO whereas increase in fuel injection diameter had minimal effect on the NO and resulted in small increase of CO emissions. Increase in heat release intensity had minimal effect on NO emissions, however it resulted in significantly higher CO emissions. The premixed combustion mode resulted in ultra-low NO levels (<1 ppm) and NO emission as low as 5 ppm was obtained with the non-premixed flame mode.     

A theoretical framework for calculating full-scale jet fires induced by high-pressure hydrogen/natural gas transient leakage

Previous experimental results on full-scale jet fires induced by high-pressure hydrogen/natural gas transient leakage can only be suitable for solving practical engineering problems, or testing the limitation of previous models. Thus, this paper presents a theoretical framework for the high-pressure hydrogen/natural gas leakage and the subsequent jet fire. The proposed framework consists of a transient leakage model, a notional nozzle model, a jet flame size model, a radiative fraction correlation and a line source radiation model. The framework is validated by comparing the model predictions and experimental measurements of mass flow rate, total flame height and thermal radiation field of hydrogen, natural gas, hydrogen/natural gas mixture jet fires with a flame height up to 100 m. The comparison shows that the theoretical framework can give considerable predictions to properties of full-scale jet fires induced by high-pressure hydrogen/natural gas transient leakage.     

Modeling a fire whirl generated over a 5-cm-diameter methanol pool fire

We conducted a series of laboratory-scale fire whirl experiments spinning 5-cm-diameter methanol pool fires and observed elongated flame height compared with the pool fire without spin. A simple scaling analysis was conducted to obtain dependency of the axial flame height on the momentum-controlled circulation and the effect of buoyancy. To obtain a specific functional relationship for the parameters obtained by the scaling analysis, we developed an analytical model consisting of coupled species and energy equations and Burgers vortex for circulation generated by a fire whirl. The solution of the coupling equations shows that the average rate of heat transfer from the flame to the fuel surface is a function of the vortex core radius; a smaller vortex core radius provides more heat to the fuel surface enhancing evaporation thereby producing the longer flame height. This new model predicts both flame height and flame shape. The flame height prediction compare favorably with results from the scaling analysis and experiment.     

Cooling strategies,summer comfort and energy performance of a rehabilitated passive standard office building

One of the first rehabilitated passive energy standard office buildings in Europe was extensively monitored over two years to analyse the cooling performance of a ground heat exchanger and mechanical night ventilation together with the summer comfort in the building. To increase the storage mass in the light weight top floor, phase change materials (PCM) were used in the ceiling and wall construction. The earth heat exchanger installed at a low depth of 1.2 m has an excellent electrical cooling coefficient of performance of 18, but with an average cooling power of about 1.5 kW does not contribute significantly to cooling load removal. Mechanical night ventilation with 2 air changes also delivered cold at a good coefficient of performance of 6 with 14 kW maximum power. However, the night air exchange was too low to completely discharge the ceilings, so that the PCM material was not effective in a warm period of several days. In the ground floor offices the heat removal through the floor to ground of 2–3 W m⁻² K⁻¹ was in the same order of magnitude than the charging heat flux of the ceilings. The number of hours above 26 °C was about 10% of all office hours. The energy performance of the building is excellent with a total primary energy consumption for heating and electricity of 107–115 kW h m⁻² a⁻¹, without computing equipment only 40–45 kW h m⁻² a⁻¹.     

The effects of liquid-fuel thermophysical properties, carrier-gas composition, and pressure, on strained opposed-flow non-premixed flames

A computational model is developed to study flame–droplet interactions in non-premixed opposed-flow flames. Although the stagnation-flow configuration is idealized, the operating conditions are chosen to be relevant to practical combustion. By considering droplet thermophysical properties ranging from heptane to diesel fuel, the results provide quantitative insight about the expected behavior of synthetic fuels. Initial droplet diameters range from 10 μm to 20 μm, the loading densities range between 10⁵ and 10⁷ cm⁻³, strain rates are on the order of 1000 s⁻¹, and pressure ranges from atmospheric pressure to 10 atm. The gas-phase chemistry for all cases is represented using detailed heptane kinetics. Increasing pressure significantly narrows the flame zone. Heptane is highly volatile and small droplets evaporate well before entering the flame zone. Diesel fuel is significantly less volatile and small droplets can survive up to the edge of the flame. However, for the conditions studied, the flame structure is only weakly affected by the droplet thermophysical properties. Including oxygen in the droplet carrier gas permits some premixed chemistry, which slightly broadens the flame structure and produces more stable flames.     

Numerical analysis of the combustion characteristics of graphitic hydrogen-chlorine synthesis combustor with different intake strategies

This study investigates the effect of intake strategies on the combustion and flows characteristics of hydrogen-chlorine synthesis combustors via numerical methods. A crucial issue of hydrogen-chlorine synthesis combustor is to have a sufficiently low flame height and high conversion efficiency. In this study, the combustion performance of combustors equipped with the annular tube, plum nozzle, and porous-bullet nozzle has been thoroughly analyzed. The temperature distribution and gas flow are analyzed using the method of fluid-solid coupling, which indicates that the combustor with porous-bullet nozzle had the best gas distribution, the maximum HCl mole fraction at outlet is 97.24%, and the lowest flame height is 3.4 m, which is 27.15% lower than the combustor with the annular tube. Furthermore, the nozzle structure has a great influence on the fluid velocity in the recirculation zone of the combustor. Finally, the effect of hydrogen/chlorine equivalence ratio (?) and inlet volume flow rate were analyzed, and it can be concluded that with the increase of inlet volume flow, the high-temperature area inside the combustor gradually increases. As the equivalent ratio increases, the combustor outlet's mole fraction changes with a normal distribution trend. It is the most appropriate when the chlorine gas flow rate is 1,100 m³/h and ? = 1.05. The research can be applied to the field of high-purity hydrogen chlorine production, providing researchers with some solutions.     

The quantitative risk assessment of a hydrogen generation unit

The safety of hydrogen generation process is a major concern. This paper discusses the quantitative analyzes of the risk imposed on neighborhood from the operation of a hydrogen generator using natural gas reforming process. For this purpose, after hazard identification, the frequency of scenarios was estimated using generic data. Quantitative risk assessment was applied for consequence modeling and risk estimation. The results revealed that, jet fire caused by a full bore rupture in Desulphurization reactor has the highest fatality (26person) and affects the largest area of 5102 m². The lethality radius, maximum radiation and safe distance of this incident were 140 m, 370 kW/m² and 225 m respectively. A full bore rupture in Reformer can lead to the most dangerous flash fire. In this incident the concentration of released material in LFL zone (area of 1483.17 m²) and ½ LEL zone (area of 1970.74 m²) were 61,125 ppm and 40,000 ppm respectively. QRA is a credible method to assess the risks of hydrogen generation process.     

Hydrogen dispersion phenomenon in nominally closed spaces

The hydrogen dispersion phenomenon in an enclosure depends on the ratio of the gas buoyancy-induced momentum and diffusive motions. Random diffusive motions of individual gas particles become dominative when the release momentum is low, and a uniform hydrogen concentration appears in the enclosure instead of the gas cumulation below the ceiling. The expected hydrogen behavior could be projected by the Froude number, which value ~1 predicts a decline of buoyancy. This paper justifies this hypothesis by demonstrating full-scale experimental results of hydrogen dispersion within a confined space under six different release variations. During the experiments, hydrogen was released into the test room of 60 m³ volume in two methods: through a nozzle and through 21 points evenly distributed on the emission box cover (multi-point release). Each release method was tested with three volume flow rates (3.2 × 10⁻³ m³/s, 1.6 × 10⁻³ m³/s, 3.3 × 10⁻⁴ m³/s). The tests confirm the decrease of hydrogen buoyancy and its stratification tendencies when the Mach, Reynolds, and Froud number values decrease. Because the hydrogen dispersion phenomenon would impact fire and explosive hazards, the presented experimental results could help fire protection systems be in an enclosure designed, allowing their effectiveness optimization.     

Copper oxide nanoparticle made by flame spray pyrolysis for photoelectrochemical water splitting – Part I. CuO nanoparticle preparation

Copper oxide (CuO) semiconductor nanoparticles are of interest because of their promising use for electronic and optoelectronic devices, and the size of the CuO particles for these applications is important. In this work, near spherical CuO nanoparticles with aspect ratio of 1.2–1.3 were made by a flame spray pyrolysis (FSP) method. In FPS, flame temperature, residence time, precursor concentration can be used to control particle size. As the precursor concentration increased from 0.5% to 35% w/w, primary particle diameter increased from 7 ± 2 to 20 ± 11 nm. Larger primary particle diameters were observed in the low gas flow system (set B) due to the long residence time in the high temperature zone. For the dependence of temperature on particle diameter, particles grew to similar diameter, i.e. ∼11 nm, in both flame conditions within the hot temperature zone (80% of melting point of CuO) but for particles having longer residence time, i.e. 550 ms in set B, the standard deviation of particle diameter is 45% larger than for particles with 66 ms as residence time in set A. Modeling gave a result for CuO final particle diameter, based on collision/sintering theory with sintering by solid state diffusion, of 6.7 and 9.0 nm for set A and set B, respectively, with surface tension assumed to be 0.5 J/m².Comparison with the experiment results, 11 ± 4 nm diameter for both flame conditions, indicates the simulations were reasonable.     

Experimental study on elevated fires in a ceiling vented compartment

The impacts of elevation on fires in a ceiling vented compartment were investigated experimentally. The flame behavior of elevated fires was recorded. Various parameters including the fuel mass loss rate, the light extinction coefficient, the oxygen concentration and the gas temperature were measured. Results indicated that the variations of the flame behavior were consistent with that of the fuel mass loss rate. The fire location significantly impacted the light extinction coefficient, the oxygen concentration and the gas temperature, which all showed distinct stratification phenomena. For a higher elevated fire, the average fuel loss rate and the overall light extinction coefficient were smaller, the oxygen concentration was higher and the gas temperature was lower. In addition, the smoke descending was slower. From the perspective of those parameters the fire was less hazardous if the fire was elevated higher, which was totally different from the elevated fires in closed compartments.     

Determination of characteristic parameters for the thermal decomposition of epoxy resin/carbon fibre composites in cone calorimeter

Present study concerns to the thermal degradation of two carbon fibre/epoxy composites, which differ by their volume fractions in carbon fibre (56 and 59 vol%), investigated in cone calorimeter (under atmospheric condition with a piloted ignition). In order to study the influence of the carbon fibre amount on the composite thermal decomposition, the cone calorimeter external heat flux was varied up to 75 kW m⁻². Thus, main parameters of the thermal decomposition of two different composites determined were: mass loss, mass loss rate, ignition time, thermal response parameter, ignition temperature, critical heat flux, thermal inertia and heat of gasification. As a result, when carbon fibre fraction decreases from 59 to 56 vol%, an increase of the thermal parameters was observed: 14–18 kW m⁻² for critical heat flux, 370–435 kW s^1/2 m⁻² for thermal response parameter, 2.25–5.07 kW² s m⁻⁴ K⁻² for thermal inertia and 16–18 kJ g⁻¹ for gasification heat. By analysing the mass loss rate evolutions, a four phases thermal decomposition mechanism is proposed. In the first phase, epoxy resin is cracked to form low molecular weight gaseous species and epoxy-derived compounds. For two next phases, the combustion of epoxy resin and liquor monomer solvent is observed that induces the formation of carbon char. In the last phase, char oxidation and carbon fibre decomposition are identified. Further, during the composite decomposition process, thermal behaviour of solid matrix is changed from a thermally thick material to a thermally thin one when sample is exposed at high external heat flux above 20 kW m⁻².     

Effect of void fraction models on the film thickness of R134a during downward condensation in a vertical smooth tube

In the present study, the void fraction and film thickness of pure R-134a flowing downwards in a vertical condenser tube are indirectly determined using relevant measured data together with an annular flow model and various void fraction models reported in the open literature. The vertical test section is a countercurrent flow double tube heat exchanger with refrigerant flowing down in the inner tube and cooling water flowing upward in the annulus. The inner tube is made from smooth copper tubing of 9.52 mm outer diameter with a length of 0.5 m. The experimental runs are carried out at average saturated condensing temperatures of 40 and 50 °C, and mass velocities are around 456 kg m^− 2 s^− 1, over the vapour quality range 0.82–0.93, while the heat fluxes are between 45.60 and 50.90 kW m^− 2. Analysis based on simple void fraction models of the annular flow pattern are presented for forced convection condensation of pure R134a, taking into account the effect of the different saturation temperatures at high mass flux conditions. The comparisons of calculated film thickness show that the void fraction models of Spedding and Chen, and Chisholm and Armand are the most accurate ones with the experimental data due to their low deviation with Whalley's annular flow model over 35 void fraction models presented in this paper.     

H2 production via water gas shift in a composite Pd membrane reactor prepared by the pore-plating method

In this work, H₂ production via catalytic water gas shift reaction in a composite Pd membrane reactor prepared by the ELP “pore-plating” method has been carried out. A completely dense membrane with a Pd thickness of about 10.2 μm over oxidized porous stainless steel support has been prepared. Firstly, permeation measurements with pure gases (H₂ and N₂) and mixtures (H₂ with N_2, CO or CO₂) at four different temperatures (ranging from 350 to 450 °C) and trans-membrane pressure differences up to 2.5 bar have been carried out. The hydrogen permeance when feeding pure hydrogen is within the range 2.68–3.96·10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5, while it decreases until 0.66–1.35·10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5 for gas mixtures. Furthermore, the membrane has been also tested in a WGS membrane reactor packed with a commercial oxide Fe–Cr catalyst by using a typical methane reformer outlet (dry basis: 70%H₂–18%CO–12%CO₂) and a stoichiometric H₂O/CO ratio. The performance of the reactor was evaluated in terms of CO conversion at different temperatures (ranging from 350 °C to 400 °C) and trans-membrane pressures (from 2.0 to 3.0 bar), at fixed gas hourly space velocity (GHSV) of 5000 h⁻¹. At these conditions, the membrane maintained its integrity and the membrane reactor was able to achieve up to the 59% of CO conversion as compared with 32% of CO conversion reached with conventional packed-bed reactor at the same operating conditions.     

Investigation of flow phenomena in a kettle reboiler

Two-phase flow phenomena were investigated while boiling R113 and n-pentane in a 241-tube thin slice kettle reboiler. For heat fluxes between 10 and 40 kW/m², row pressure drop measurements were made in three columns and visual observations of the flow patterns were recorded by a video camera. The height of the two-phase mixture above the tube bundle was also varied. The results revealed that the height of the mixture had little effect on the row pressure drop distribution in each column. At heat fluxes below 10 kW/m², the pressure drops were reasonably constant. However, at heat fluxes greater than this, the row pressure drop continuously declined.Two one-point-five-dimensional models were developed, one to aid the investigation of static liquid driven lateral flow in the tube bundle, and another to aid the investigation of the cause of the change from reasonably constant to continually declining row pressure drop. The data and the analysis showed that the flow within the tube bundle was always two-dimensional and that the flow pattern was dominated by the static liquid at the tube bundle edge when the heat flux was less than 10 kW/m². This corresponded to the bubbly flow regime. At larger heat fluxes, the flow pattern changed to intermittent flow. The change occurred when the Kutateladze number was 1.09. Declining row pressure drops occurred in this latter flow regime.     

Two-phase friction factor in vertical downward flow in high mass flux region of refrigerant HFC-134a during condensation

The two-phase pressure drop of the pure refrigerant HFC-134a during condensation inside a vertical tube-in-tube heat exchanger was investigated. The double tube test section was 0.5 m long with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube was constructed from smooth copper tubing of 8.1 mm inner diameter and 9.52 mm outer diameter. The test runs were performed at average condensing temperatures of 40–50 °C. The mass fluxes were between 260 and 515 kg m^− 2 s^− 1 and the heat fluxes between 11.3 and 55.3 kW m^− 2. The quality of the refrigerant in the test section was calculated using the temperature and pressure obtained from the experiment. The pressure drop across the test section was directly measured by a differential pressure transducer. A new correlation for the two-phase friction factor of R134a flow is proposed by means of the equivalent Reynolds number model. The effects of heat flux, mass flux and condensation temperature on the pressure drop are also discussed.     

Experimental study of hydrogen production and soot particulate matter emissions from methane rich-combustion in inert porous media

A detailed experimental study of stationary Thermal Partial Oxidation (TPOX) within inert porous media has been conducted. The reaction zone of the tested TPOX reformer is designed so as to enable stationary conversion of fuel/air mixtures for a wide range of operational conditions. Operating characteristics of the process have been examined for two different porous matrices, with different thermal and transport properties, namely SiSiC open foam structure and a packed bed of pure Al₂O₃ packing material in the form of cylindrical rings. The influence of reactants preheating was also examined since the reformer is meant for integration within high temperature fuel cell systems. The operating regime was scanned for reactants' inlet temperature of 400 °C and 550 °C, varying the thermal load in a range from 350 kW/m² up to 2600 kW/m² and the equivalence ratio from 1.9 up to 2.9. Temperature profiles within the reaction region of the reformer were recorded for all tested conditions while gas samples were on-line analyzed for the major species H₂, CO, CO₂, and minor species CH₄, C₂H₂. At reactants' inlet temperatures of 400 °C and 600 °C, for a fixed thermal load of 1540 kW/m² and for selected equivalence ratios around the sooting limit of the process (φ = 2.2–2.6), soot particle size distributions were measured in the exhaust gas with a Scanning Mobility Particle Sizer (SMPS). The results show that the better thermal properties and the higher porosity in the case of the SiSiC matrix enables longer residence times for slow reforming reactions to evolve towards equilibrium and yields syngas with significantly less soot in terms of particle numbers and mass concentration.     

Hydrogen jet flames

A critical review and rethinking of hydrogen jet flame research is carried out. Froude number only based correlations are shown to be deficient for under-expanded jet fires. The novel dimensionless flame length correlation is developed accounting for effects of Froude, Reynolds, and Mach numbers. The correlation is validated for pressures 0.1–90.0 MPa, temperatures 80–300 K, and leak diameters 0.4–51.7 mm. Three distinct jet flame regimes are identified: traditional buoyancy-controlled, momentum-dominated “plateau” for expanded jets, and momentum-dominated “slope” for under-expanded jets. The statement “calculated flame length may be obtained by substitution the concentration corresponding to the stoichiometric mixture in equation of axial concentration decay for non-reacting jet” is shown to be incorrect. The correct average value for non-premixed turbulent flames is 11% by volume of hydrogen in air (range 8%–16%) not stoichiometric 29.5%. All three conservative separation distances for jet fire are shown to be longer than separation distance for non-reacting jet.