Determination of particle temperatures in a silica-generating counterflow flame via flame emission measurements

We propose a simple technique to measure particle temperatures in a particle generating counterflow flame. The silica particle temperature was derived from flame light emission measurements. This technique allows the non-intrusive measurement of particle temperatures over 2000 K. In addition, the OH concentration distribution in the hydrogen–oxygen flame was estimated from flame emission spectra in the ultraviolet region. A numerical model of the combustion processes, which included the reactions of SiCl₄ leading to the formation of silica particles, verified that the measured particle temperatures and OH concentration were close to the theoretical values.     

Model of flame dynamics of laminar premixed flame subject to the low frequency equivalence ratio oscillations

The effect of the non-uniform profile of scalar variables, such a fuel at the upstream and temperature at the downstream of the flame zone was discussed theoretically to elucidate; (1) the deviation of motion from the steady state case and (2) the hysteresis of premixed flames response to the equivalence ratio oscillations seen in an experimental and numerical works. One-dimensional integral model for the non-uniform scalar variable profile with low frequency equivalence ratio oscillation has been developed. Here, the wavelength of the oscillation is assumed to be larger than the nominal flame thickness. Through the integral analysis, we obtained the relation of the flame propagation speed for steady and unsteady cases depending on the non-uniform scalar profile at the upstream and downstream of the flame zone. Hysteresis of the flame propagation speed is found due to the transport of fuel and heat by the non-uniform scalar profile at the upstream and downstream of the flame zone. This result qualitatively agreed with the numerical results of a response of the stagnation laminar CH₄/air premixed flames for a low equivalence ratio oscillation frequency.     

Effect of compression ratio, equivalence ratio and engine speed on the performance and emission characteristics of a spark ignition engine using hydrogen as a fuel

The present energy situation has stimulated active research interest in non-petroleum and non-polluting fuels, particularly for transportation, power generation, and agricultural sectors. Researchers have found that hydrogen presents the best and an unprecedented solution to the energy crises and pollution problems, due to its superior combustion qualities and availability. This paper discusses analytically and provides data on the effect of compression ratio, equivalence ratio and engine speed on the engine performance, emissions and pre-ignition limits of a spark ignition engine operating on hydrogen fuel.These data are important in order to understand the interaction between engine performance and emission parameters, which will help engine designers when designing for hydrogen.     

Effects of hydrogen concentration on the emission and heat transfer of a premixed LPG-hydrogen flame

This paper describes an experimental study of the effect of hydrogen concentration on the emission and heat transfer characteristics of a laminar premixed LPG-hydrogen flame. The mole fraction of hydrogen in the fuel mixture was varied from 0% to 50%. The equivalence ratio of the fuel/air mixture was kept at stoichiometry and the mixture jet Reynolds number was fixed at Re = 1500 for most of the tests. The results show that upon varying hydrogen content in the fuel mixture, there is a corresponding change in the appearance, pollutant emissions and heat transfer characteristics of the flame.     

The synergistic effect of equivalence ratio and initial temperature on laminar flame speed of syngas

The laminar flame speed of syngas (CO:H₂ = 1:1)/air premixed gas in a wide equivalence ratio range (0.6–5) and initial temperature (298–423 K) was studied by Bunsen burner. The results show that the laminar flame speed first increases and then decreases as the equivalence ratio increasing, which is a maximum laminar flame speed at n = 2. The laminar flame speed increases exponentially with the increase of initial temperature. For different equivalent ratios, the initial temperature effects on the laminar flame speed is different. The initial temperature effects for n = 2 (the most violent point of the reaction) is lower than others. It is found that H, O and OH are affected more and more when the equivalence ratio increase. When the equivalence ratio is far from 2, the reaction path changes, and the influence of initial temperature on syngas combustion also changes. The laminar flame speed of syngas is more severely affected by H + O₂ = O + OH and CO + OH = CO₂ + H than others, which sensitivity coefficient is larger and change more greatly than others when the initial temperature and equivalence ratio change. Therefore, the laminar flame speed of syngas/air premixed gas is affected by the initial temperature and equivalence ratio. A new correlation is proposed to predict the laminar flame speed of syngas (CO:H₂ = 1:1)/air premixed gas under the synergistic effect of equivalence ratio and initial temperature (for equivalence ratios of 0.6–5, the initial temperature is 298–423 K).     

Heat release rate variations in high hydrogen content premixed syngas flames at elevated pressures: Effect of equivalence ratio

Three-dimensional direct numerical simulations with detailed chemistry were performed to investigate the effect of equivalence ratio on spatial variations of the heat release rate and flame markers of hydrogen/carbon monoxide syngas expanding spherical premixed flames under turbulent conditions at elevated pressures. The flame structures and the heat release rate were analysed and compared between fuel-lean, stoichiometric and fuel-rich centrally ignited spherical flames. The equivalence ratio changes the balance among thermo-diffusive effects, Darrieus–Landau instability and turbulence, leading to different flame dynamics and the heat release rate distribution, despite exhibiting similar cellular and wrinkling flames. The Darrieus–Landau instability is relatively insensitive to the equivalence ratio while the thermo-diffusive process is strongly affected by the equivalence ratio. As the thermo-diffusive effect increases as the equivalence ratio decreases, the fuel-lean flame is more unstable than the fuel-rich flame with the stoichiometric flame in between, under the joint effects of the thermo-diffusive instability and the Darrieus–Landau instability. The local heat release rate and curvature display a positive correlation for the lean flame, no correlation for the stoichiometric flame, and negative correlation for the rich flame. Furthermore, for the fuel-lean flame, the low and high heat release rate values are found in the negative and positive curvature zones, respectively, while for the fuel-rich flame, the opposite trends are found. It is found that heat release rate markers based on species concentrations vary strongly with changing equivalence ratio. The results suggest that the HCO, HO₂ concentrations and product of OH and CH₂O concentrations show good correlation with the local heat release rate for H₂/CO premixed syngas-air stoichiometric flame under turbulent conditions at elevated pressures.     

Heat release and flame structure measurements of self-excited acoustically-driven premixed methane flames

An open-open organ pipe burner (Rijke tube) with a bluff-body ring was used to create a self-excited, acoustically-driven, premixed methane-air conical flame, with equivalence ratios ranging from 0.85 to 1.05. The feed tube velocities corresponded to Re = 1780-4450. Coupled oscillations in pressure, velocity, and heat release from the flame are naturally encouraged at resonant frequencies in the Rijke tube combustor. This coupling creates sustainable self-exited oscillations in flame front area and shape. The period of the oscillations occur at the resonant frequency of the combustion chamber when the flame is placed ∼¼ of the distance from the bottom of the tube. In this investigation, the shape of these acoustically-driven flames is measured by employing both OH planar laser-induced fluorescence (PLIF) and chemiluminescence imaging and the images are correlated to simultaneously measured pressure in the combustor. Past research on acoustically perturbed flames has focused on qualitative flame area and heat release relationships under imposed velocity perturbations at imposed frequencies. This study reports quantitative empirical fits with respect to pressure or phase angle in a self-generated pressure oscillation. The OH-PLIF images were single temporal shots and the chemiluminescence images were phase averaged on chip, such that 15 exposures were used to create one image. Thus, both measurements were time resolved during the flame oscillation. Phase-resolved area and heat release variations throughout the pressure oscillation were computed. A relation between flame area and the phase angle before the pressure maximum was derived for all flames in order to quantitatively show that the Rayleigh criterion was satisfied in the combustor. Qualitative trends in oscillating flame area were found with respect to feed tube flow rates. A logarithmic relation was found between the RMS pressure and both the normalized average area and heat release rate for all flames.     

Effect of the equivalence ratio,Damköhler number,Lewis number and heat release on the stability of laminar premixed flames in microchannels

The effect of the equivalence ratio on the stability and dynamics of a premixed flame in a planar micro-channel with a step-wise wall temperature profile is numerically investigated using the thermo-diffusive approximation. To characterize the stability behavior of the flame, we construct the stability maps delineating the regions with different flame dynamics in the inlet mass flow rate m   vs. the equivalence ratio ?$?$ parametric space. The flame stability is analyzed for fuels with different diffusivity by changing the Lewis numbers in the range 0.3?_LeF?1.4$0.3?{\mathit{Le}}_F?1.4$. On the other hand, the Lewis number of the oxidizer is kept constant and equal to unity _LeO=1${\mathit{Le}}_O=1$. Our results show that, for very diffusive fuels, the stability of the flame varies significantly with the equivalence ratio, transitioning from stable flames for lean mixtures to highly unstable flames when ?>1$?>1$. As the fuel Lewis number approaches unity, the stability behavior of the flame for lean and rich mixtures becomes more similar to give, in the equidiffusional case _LeF=1${\mathit{Le}}_F=1$, a symmetric stability map around the stoichiometric mixture ?=1$?=1$. In all cases considered, the most stable flames are always found around the stoichiometric mixtures ?=1$?=1$, when the flame instabilities are completely suppressed for very diffusive fuels _LeF<1${\mathit{Le}}_F<1$, or are reduced to a narrow range of inflow velocities for fuel Lewis numbers equal or greater than unity.     

Effects of multi-component diffusion and heat release on laminar diffusion flame liftoff

Numerical simulations were conducted of the liftoff and stabilization phenomena of laminar jet diffusion flames of inert-diluted C₃H₈ and CH₄ fuels. Both non-reacting and reacting jets were investigated, including multi-component diffusivities and heat release effects (buoyancy and gas expansion). The role of Schmidt number for non-reacting jets was investigated, with no conclusive Schmidt number criterion for liftoff previously arrived at in similarity solutions. The cold-flow simulation for He-diluted CH₄ fuel does not predict flame liftoff; however, adding heat release reaction lead to the prediction of liftoff, which is consistent with experimental observations. Including reaction was also found to improve liftoff height prediction for C₃H₈ flames, with the flame base location differing from that in the similarity solution - the intersection of the stoichiometric and iso-velocity (equal to 1-D flame speed) is not necessary for flame stabilization (and thus liftoff). Possible mechanisms other than that proposed for similarity solution may better help to explain the stabilization and liftoff phenomena.     

Combustion and heat release characteristics of hydrogen/air diffusion flame on a micro-jet array burner

Hydrogen (H₂) is considered as a carbon-free alternative fuel. The heat release characteristics of H₂ flame as a key parameter in its combustion process are unclear. In this study, the combustion and heat release characteristics of H₂/air diffusion flame on a micro-jet array burner were experimentally and numerically investigated. It is shown that the OH distribution and flame length based on Bilger mechanism are in good agreement with the experimental results. Furthermore, the intensity and distribution of OH and heat release rate can be adjusted by the thermal power and equivalence ratio. A uniform flame with intensive heat release rate can be achieved at a thermal power of 0.1 kW. R41: H + O₂ = OH + O and R43: H + O₂ + M = HO₂ + M are the main reactions with oxidizer consumption to form reactive radicals. R40: OH + H₂ = H₂O + H and R47: OH + OH = O + H₂O with OH consumption are the main heat release reactions at the upstream and downstream of the flame.     

Influence of fuel type,dilution and equivalence ratio on the emission reduction from the auto-ignition in an Homogeneous Charge Compression Ignition engine

One technology that seems to be promising for automobile pollution reduction is the Homogeneous Charge Compression Ignition (HCCI). This technology still faces auto-ignition and emission-control problems. This paper focuses on the emission problem, since it is incumbent to realize engines that pollute less. For this purpose, this paper presents results concerning the measurement of the emissions of CO, NO_x, CO₂, O₂ and hydrocarbons. HCCI conditions are used, with equivalence ratios between 0.26 and 0.54, inlet temperatures of 70 °C and 120 °C and compression ratios of 10.2 and 13.5, with different fuel types: gasoline, gasoline surrogate, diesel, diesel surrogate and mixtures of n-heptane/toluene. The effect of dilution is considered for gasoline, while the effect of the equivalence ratio is considered for all the fuels. No significant amount of NO_x has been measured. It appeared that the CO, O₂ and hydrocarbon emissions were reduced by decreasing the toluene content of the fuel and by decreasing the dilution. The opposite holds for CO₂. The reduction of the hydrocarbon emission appears to compete with the reduction of the CO₂ emission. Diesel seemed to produce less CO and hydrocarbons than gasoline when auto-ignited. An example of emission reduction control is presented in this paper.     

Influence of swirl number and fuel equivalence ratio on NO emission in an experimental LPG-fired chamber

The well-known kinetic mechanism of nitrogen oxides formation by thermal fixation of atmospheric nitrogen suggests the control of these pollutants by modifying the equivalence ratio and/or the temperature in the combustion region. In this experimental investigation a burner was specifically designed to take advantages of these chemical kinetic aspects. In particular, it was used to investigate the influence of swirl number and fuel equivalence ratio on nitric oxide concentration formed in a cylindrical combustion chamber. A set of 14 experimental runs was carried out to examine the influence of these factors, which were varied at seven and two levels (7¹ × 2¹) in the range of 0.36–1.32 and between 0.61 and 0.84, respectively. Liquefied petroleum gas (LPG) was always fired in the experiments. Temperature, O₂ and NO concentration were monitored in 42 different radial and axial positions along the chamber at four different operating conditions in terms of NO formation. The effect of increasing the swirl number and reducing the fuel equivalence ratio was to reduce approximately 31% and 33% the nitric oxide emission, respectively.     

Enhancing natural convection in a cube using a strong magnetic field — Experimental heat transfer rate measurements and flow visualization

The effect of a strong magnetic field on the average heat transfer rate and flow profiles of joint gravitational and thermo-magnetic convection of a paramagnetic fluid in a cubic enclosure heated from below and cooled from above was experimentally investigated. The working fluid consisted of 80% mass glycerol aqueous solution with a concentration of 0.8 mol/kg gadolinium nitrate hexahydrate making it paramagnetic. The cubic enclosure of 32-mm sides was located in the 10-cm bore of a horizontally oriented 5-Tesla super-conducting magnet at a position where the magnetic force distribution was relatively uniform. Under this configuration, the magnetic field imposed in the horizontal direction acted perpendicularly to gravity. It was found that the heat transfer rate through the cube increased with the increase of the magnetic induction. Furthermore, steady and transient state flow visualizations were carried out with a 10-Tesla super-conducting magnet to show a change in the temperature field when magnet-thermo convection dominated. Visualization was made using thermo-chromic liquid crystal slurry added to the working fluid and illuminated in a vertical cross-section of the cube.     

Response of heat release to equivalence ratio variations in high Karlovitz premixed H2/air flames at 20 atm

This paper presents three-dimensional direct numerical simulations of lean premixed H₂/air flames with equivalence ratios 0.4, 0.5 and 0.6, respectively. The initial Karlovitz number is around 2335 and the pressure is 20 atm, which is relevant to gas turbine conditions. The heat release in reaction zones under different equivalence ratios is examined statistically with the aim to extend our understanding of lean combustion under high-pressure conditions. With increasing equivalence ratio, the relative thickness of reaction zone (δ_f/δ_L) is increasing for both laminar and turbulent flames, but the extent of increase is reduced under high equivalence ratio. By examining the local structures of flame fronts, it is found that trenches and plateaus of local equivalence ratio are located on separate sides of the reaction zone edge. Due to the decreased Lewis number under high equivalence ratio, the trench ‘depth’ and plateau ‘height’ are reduced. For the flame under ultra-lean conditions, there are some spots with temperatures above adiabatic temperature. This is attributed to the high-fraction of radicals in these regions, which will promote heat release. Furthermore, the heat release rates of elementary reactions are investigated with the analysis of radical fractions and rate constants. When the mixture equivalence ratio varies, the local heat release is changed in different temperature windows due to the combined effects of radical fractions and reaction rate constants.     

Estimation of the time-dependent heat flux using the temperature distribution at a point by conjugate gradient method

In this paper, the conjugate gradient method coupled with adjoint problem is used in order to solve the inverse heat conduction problem and estimation of the time-dependent heat flux using the temperature distribution at a point. Also, the effects of noisy data and position of measured temperature on final solution are studied. The numerical solution of the governing equations is obtained by employing a finite-difference technique. For solving this problem the general coordinate method is used. We solve the inverse heat conduction problem of estimating the transient heat flux, applied on part of the boundary of an irregular region. The irregular region in the physical domain (r,z) is transformed into a rectangle in the computational domain (ξ,η). The present formulation is general and can be applied to the solution of boundary inverse heat conduction problems over any region that can be mapped into a rectangle. The obtained results for few selected examples show the good accuracy of the presented method. Also the solutions have good stability even if the input data includes noise and that the results are nearly independent of sensor position.     

Local heat flux measurements in a hydrogen and methane spark ignition engine with a thermopile sensor

This paper describes an experimental investigation of heat transfer inside a CFR spark ignition engine operated at a constant engine speed of 600 rpm. The heat flux is directly measured under motored and fired conditions with a commercially available thermopile sensor. The heat transfer during hydrogen and methane combustion is compared examining the effects of the compression ratio, ignition timing and mixture richness. Less cyclic and spatial variation in the heat flux traces are observed when burning hydrogen, which can be correlated to the faster burn rate. The peak heat flux increases with the compression ratio, but the total cycle heat loss can decrease due to less heat transfer at the end of the expansion stroke. An advanced spark timing and increased mixture richness cause an increased and advanced peak in the heat flux trace. Hydrogen combustion gives a heat flux peak which is three times as high as the one of methane for the same engine power output.     

Effects of fuel compositions on the heat generation and emission of syngas/producer gas laminar diffusion flame

Demand for the clean and sustainable energy encourages the research and development in the efficient production and utilisation of syngas for low-carbon power and heating/cooling applications. However, diversity in the chemical composition of syngas, resulting due to its flexible production process and feedstock, often poses a significant challenge for the design and operation of an effective combustion system. To address this, the research presented in this paper is particularly focused on an in-depth understanding of the heat generation and emission formation of syngas/producer gas flames with an effect of the fuel compositions. The heat generated by flame not only depends on the flame temperature but also on the chemistry heat release of fuel and flame dimension. The study reports that the syngas/producer gas with a low H_2:CO maximises the heat generation, nevertheless the higher emission rate of CO₂ is inevitable. The generated heat flux at H_2:CO = 3:1, 1:1, and 1:3 is found to be 222, 432 and 538 W m^-2 respectively. At the same amount of heat generated, H₂ concentration in fuel dominates the emission of NO_x. The addition of CH₄ into the syngas/producer gas with H_2:CO = 1:1 also increases the heat generation significantly (e.g. 614 W m^-2 at 20%) while decreases the emission formation. In contrast, adding 20% CO₂ and N₂ to the syngas/producer gas composition reduces the heat generation from 432 W m^-2 to 364 and 290 W m^-2, respectively. The role of CO₂ on this aspect, which is weaker than N₂, thus suggests CO₂ is preferable than N₂. Along with the study, the significant role of CO₂ on the radiation of heat and the reduction of emission are examined.     

Non-invasive blood perfusion measurements using a combined temperature and heat flux surface probe

Non-invasive blood perfusion measurement systems have been developed and tested in a phantom tissue and an animal model. The probes use a small sensor with a laminated flat thermocouple to measure the heat transfer and temperature response to an arbitrary thermal event (convective or conductive) imposed on the tissue surface. Blood perfusion and thermal contact resistance are estimated by comparing heat flux data with a mathematical model of the tissue. The perfusion probes were evaluated for repeatability and sensitivity using both a phantom tissue test stand and exposed rat liver tests. Perfusion in the phantom tissue tests was varied by controlling the flow of water into the phantom tissue test section, and the perfusion in the exposed liver tests was varied by temporarily occluding blood flow through the portal vein. The phantom tissue tests indicated that the probes can be used to detect small changes in perfusion (0.005 ml/ml/s). The probes qualitatively tracked the changes in the perfusion of the liver model due to occlusion of the portal vein.