Flame Volume Research Articles

Study on the flame structure and flow field of hydrogen-enriched combustion in array micro-tube

Addressing climate change and reducing greenhouse gas emissions are critical priorities. Utilizing hydrogen-rich methane or pure hydrogen as fuels within gas turbines, facilitated by array micro-tube premixed combustion technology, is anticipated to markedly accelerate the decarbonization process of the energy sector. In this study, the flame structure of the array micro-tube premixed burner under various fuel compositions was examined using OH-Planar Laser-Induced Fluorescence and Particle Image Velocimetry measurement techniques. The effects of the equivalence ratio (φ) and the hydrogen power ratio (HPR) on the characteristics of the flame front, including its curvature, density, volume, and the associated flow field properties, were discussed. As φ and HPR increase, the wrinkled structure of the flame front is significantly enhanced, with a more pronounced effect on smaller scales. This enhancement leads to the separation of the unburned pockets from the main flame. Concurrently, both the flame length and the flame area decrease with the augmentation of φ and HPR, indicating a more concentrated combustion process and increased combustion intensity under hydrogen-enriched and pure hydrogen conditions. The study also observed a slight increase in both the negative and positive curvatures of the flame front, with a more notable increase in the negative curvature. The increased negative curvature results in an elevated degree of wrinkling and a higher value of Σ (flame surface density), reaching a maximum of 0.876 mm−1 under the conditions where φ is 0.8 and ⟨c⟩ (mean progress variable) is 0.5, resulting in the smallest observed flame volume of 100.6 mm3. Upon coupling the flame with the flow field, it was discovered that the exit flow field of the array micro-tube exhibits symmetry and a characteristic conical flame shape. The burning velocity of the side flame brushes increases, and the velocity peak shifts upstream. The aforementioned findings confirm that the addition of hydrogen increases the laminar flame velocity, enabling the flame to stably anchor to the microtube outlet and thereby enhance the flame's robustness and stability.

Dispersion characteristics of large-scale liquid hydrogen spills in a real-world liquid hydrogen refueling station with various releasing and environmental conditions

The global expansion of hydrogen refueling stations (HRS) has heightened interest in the development of liquid hydrogen refueling stations (LHRS). This study presents a model for an LHRS, inspired by the Yilan HRS in Shanghai, to simulate large-scale liquid hydrogen (LH2) leakage scenarios. Utilizing the FLACS-CFD software, comprehensive simulations were performed to assess the impact of various environmental conditions and leakage parameters—such as wind speed, wind direction, spill rate, and spill duration—on critical outcomes including the size of the LH2 pool, evaporation rate, flammable volume of the vaporized hydrogen gas cloud, and maximum downwind distance/height. The findings establish a correlation between these factors and the predicted results, providing real-time guidance for managing LH2 leakage incidents at operational LHRS.

Influence of external wind on fire characteristics and smoke dynamics in a stairwell adjacent to a room

A range of fire experiments were conducted to explore fire characteristics and smoke dynamics in a 1/6-scale stairwell adjacent to a room at different external wind velocities. The results indicated that the flame volume during the initial stage and the time taken for combustion to enter the steady stage both decrease with the increasing wind velocity. There are two opposite effects of the external wind on the fire combustion in a burning room adjacent to a stairwell. The inhibition effect plays an important role at lower wind velocities, leading to a decrease in the mass loss rate (MLR) of the pool fire. Conversely, the promotion effect dominates at higher wind velocities, resulting in an increase in the MLR. Stack effect and external wind demonstrate a synergistic relationship during experiments, and a correlation of the airflow velocity, wind velocity and heat release rate (HRR) is established. There are intersections in temperature curves at different wind velocities due to the smaller temperature decay rate in the stairwell and the lower smoke temperature near the pool fire at a higher velocity. Furthermore, a prediction model is also proposed for the smoke temperature distribution inside the stairwell, taking into account the wind velocity and HRR.

Fuel properties and incineration behavior of poultry litter blended with sweet sorghum bagasse and pyrolysis oil

The incineration of poultry litter (PL) effectively reduces the volume of waste in line with the United Nations Sustainable Development Goal of “affordable and clean energy”. However, mono-incineration is associated with considerable challenges due to the varying moisture, structural and chemical composition and low energy yield. The aim of the present work was to investigate the influence of sweet sorghum bagasse (SS) and pyrolysis oil (PO) on improving the fuel properties of PL and mitigating ash related burdens during incineration. The different biomass feedstocks were produced by combining PL with SS at 0.0% (T0), 25% (T1), 50% (T2), 75% (T3) and compared with 100% SS (T4). In order to achieve high energy potential and low ash deposition, the parallel samples were additionally mixed with 10% PO to improve the energy value. The experimental results show that increasing the proportion of SS and adding PO to the mixtures increases the volatile matter and decreases the moisture and ash content. The addition of PO also increases the carbon and hydrogen content. The use of SS and PO thus increased the values of the ignitability index and apparently also the flammability by 30.0%–49.4% compared to pure PL. SS and PO shifted the HHV of the starting material from 16.90 to 18.78 MJ kg−1. In addition, SS + PO improved the flame volume and red color intensity of the PL blends based on the image analysis method. However, the presence of SS and PO did not sufficiently improve the ash-related index values, which requires further investigation.

Influence of velocity on the transient auto-ignition of ethylene jet in a hot coflow

The difficulty of ignition is one of the main problems in the combustion chamber of the air-breathing combined engine. Based on this, the transient auto-ignition in the turbulent non-premixed flames of ethylene in a high-speed jet in hot coflow was studied using a newly developed burner. High-speed hydroxyl (OH) planar laser-induced fluorescence (up to 10 kHz) and large eddy simulations (LES) were performed to investigate the auto-ignition process. The experimental conditions included injection velocities of 334–491 m/s, and the effect of jet velocity on the mixing process, ignition process, and flame stabilization mechanisms were investigated. The results showed that the numerical simulation results were consistent with the experimental data. The velocity could affect the auto-ignition position, whereas it had no effect on the auto-ignition time. The increase in fuel jet velocity is beneficial to the mixing of fuel and high-temperature gas and helps to increase the premixed combustion zone during the formation of fire kernels. Moreover, as the velocity increased, the flame status changed from a transitional flame to a lifted flame, the growth rates of the flame volume and total heat release increased, and the flame stabilization mechanism changed from flame propagation to auto-ignition.

Effect of ignition distance on deflagration characteristics of non-uniform oil mist in closed cabins

The deflagration of non-uniform oil mist in closed cabins can easily cause damage to ship equipment and casualties. In this work, the influence of ignition distances on the oil mist deflagration hazard is experimentally investigated, focusing on analyzing the oil mist deflagration propagation process, flame propagation speed, deflagration overpressure, and gas temperature, and exploring its influence mechanism. The results show that as the ignition distance decreases, the oil mist deflagration intensifies, flame propagation speed increases, deflagration overpressure improves, and gas temperature increases. Oil mist deflagration is divided into deflagration stage, turbulent combustion stage, and self-extinguishing stage. In deflagration stage, the center of the flame volume grows, and then gradually shrinks, moving in the direction of the nozzle. Flame propagation speed increases firstly and then decrease, accompanied by oscillatory changes in the process. The deflagration peak overpressure gradually decreases from the nozzle position to the right side of the cabin. With a nozzle diameter of 0.1 mm, the peak oil mist deflagration overpressure is around 0.25 MPa when the ignition distance is 10 cm away from the nozzle. However, when the ignition distance is 30 cm away from the nozzle, the oil mist deflagration peak overpressure is around 0.21 MPa. The high-temperature range of oil mist deflagration is mainly concentrated in the deflagration center area.

Numerical investigation on flammability of R32 leakage in heat pump outdoor unit with CFD simulation

R32 is receiving significant attention as an alternative refrigerant owing to its higher operating efficiency and lower environmental impact; however, it is classified as flammable refrigerants. There have been some studies on R32 leakages about indoor unit rooms but extremely limited studies about outdoor unit rooms. The outdoor unit room is more cramped than the indoor one, and leakage easily occurs due to high-temperature and high-pressure operating conditions. In this study, the flammability risk was evaluated through a comprehensive R32 leak study in the outdoor unit room. Critical factors including the leak diameter, unit installation height, refrigerant charge, air infiltration, window opening, and room size were carefully selected, and parameter studies and correlation analysis were conducted. As the leak diameter increased from 2 to 8 mm, the R32 volume fraction increased from 7.7 to 46.3%, indicating that the leak diameter is the most significant factor in increasing the R32 volume fraction. Considering that the leak diameter is less than 1 mm in a typical leak accident and the leak occurs with a diameter of 2 mm or more in a catastrophic case, the risk of R32 leak was found to be low under the conditions. By increasing the window opening and unit installation height, the concentration of R32 can be lowered below the LFL, thereby reducing the flammable volume to close to zero.

Phase change modeling of air at the liquid hydrogen release

Hydrogen energy has emerged as one of the most promising types of future energy. Accurately predicting the diffusion behavior of liquid hydrogen after an accidental release is one of the most important aspects of promoting the safe use of hydrogen energy. Considering the condensation and freezing of air, a three-dimensional unsteady leakage and diffusion model of liquid hydrogen is developed to predict the distribution of hydrogen vapor cloud and evaporation of liquid hydrogen pool in a large liquid hydrogen release experiment conducted by NASA. During the release of liquid hydrogen, the volume fractions of liquid air and solid air are quite small due to turbulence, and there is almost no liquid air and solid air present in the space directly opposite the leakage source. After the termination of the release, the flow field is more stable and the volume fractions of liquid air and solid air tend to increase, which on the ground can exceed 0.05 and 0.005 respectively at 50 s. This indicates that the ice layer observed in the former experiments may mainly be generated after the release. The evaporation coefficient of hydrogen has a significant effect on the hydrogen flammable volume, as the hydrogen evaporation coefficient increases from 0.006 to 0.06, the flammable volume increases from 1003.3 m3 to 7656.3 m3. As the evaporation coefficient continues to increase from 0.06, the volume growth rate of flammable hydrogen cloud tends to decrease. The flammable volume is not sensitive to the air condensation coefficient in the range of 1–1000, which increases from 6858.8 m3 to 7656.3 m3 as the air condensation coefficient increases from 1 to 1000. This study helps to refine the model for predicting the dispersion characteristics of accidental liquid hydrogen release considering the phase change of the air.

Investigation of the leakage and diffusion characteristics of hydrogen-addition natural gas from indoor pipelines

The hydrogen-addition natural gas pipeline is a promising technology for large-scale civilian use, revealing the indoor diffusion behaviors of flammable gas in a leakage accident is of great importance for formulating a safety warning. A three-dimensional unsteady model is established, the distribution characteristics of CH4 and H2 are studied and the dynamic flammable regions are investigated. Case studies are performed to evaluate the effects of hydrogen-addition ratios and floor levels on average concentrations and flammable volumes of CH4 and H2. The results show that the high-concentration region of H2 is always near the leakage hole. The increase rate of flammable gas mixture volume is larger than that of the volume of high-concentration CH4. Under a higher hydrogen-addition ratio, the high-concentration region of CH4 and the flammable region of gas mixture both float up while the high-concentration region of H2 keeps horizontal, the flammable volume of gas mixture increases faster but the final volume is smaller. When the room is ventilated, a bigger wind speed leads to a smaller flammable volume of gas mixture. The flammable volume can be constrained at a quite low value from the beginning as the wind speed surpasses 1.5 m/s. When the leakage of hydrogen-addition natural gas occurs, floor 1 takes the most serious ignition risk with the largest flammable volume of 9.87 m3, occupying 44% of the room space. By contrast, the upper floors have quite smaller flammable volumes of gas mixture (under 0.05 m3) and the flammable region is near the leakage hole. The high-concentration region of H2 is affected slightly by the floor height.

Flame extension lengths beneath a double slope roof induced by gaseous fuel jet flame

Due to the characteristics of good drainage and stable structure characteristics, the double slope roof is widely used in civil houses, industrial plants, and ancient buildings. However, double slope roof fire accidents occurred frequently. Studying the flame characteristics under the ceiling can provide scientific guidance for building fire protection. The present study investigated the flame extension lengths beneath a double slope roof induced by gaseous fuel jet flame, which had not been quantified in the literature. Results show that the length of flame extension was longer in the ridge direction compared to the slope direction, with a maximum difference of approximately two times. Moreover, their deviation increases with larger HRR and smaller θ, resulting from the fact that the structure of the double slope roof would restrict air entrainment and induce more unburned gaseous fuel to spread along the ridge. Comparing the experimental data with previous models proves that they cannot predict the ceiling flame extension length under the influence of roof angle. A new correlation incorporating the effect of θ was established based on the proposed flame volume assumption to estimate the equivalent HRR at the ceiling, whose accuracy and reliability were further validated by reference data. The model prediction agreed well with the literature data, with a maximum error of around 20%.

Experimental and numerical study of MILD combustion characteristics in the long-narrow confined space with different types of flue gas dilution

MILD combustion is extensively applied in wide confined spaces as a new ultralow NOx emission technology but is rarely reported in long-narrow confined spaces. To address this, as exemplified by a coke oven heat flue, a flue gas dual-dilution system in a long-narrow confined space was constructed to achieve MILD combustion, where combustion characteristics such as temperature distribution, flow field, flame morphology and NOx emission were assessed through experiments and numerical simulations. The results indicate that when the dilution rate reaches 25%, single-fuel dilution can achieve unstable MILD combustion, while single-air dilution is still in diffusion combustion, and the corresponding NOx emissions are reduced to 371 ppm and 325 ppm, respectively. When the fuel/air dilution ratio is 10/15 in flue gas dual-dilution, the flame front disappears completely, the flame volume ratio reaches 41.9%, and MILD combustion is established. The corresponding NOx emission is reduced to 58 ppm, which is 95.0% lower than conventional combustion and achieves ultralow NOx emission. An in-depth analysis proves that flue gas dual dilution is the key to achieving MILD combustion in a long-narrow confined space, which is irrelevant to the construction of the flue gas internal recirculation rate.

Flame expansion behavior induced by a water droplet impacting on burning and stratified oil-water pool

The dynamic process of a water droplet impacting stratified oil-water and the phenomena of instantaneous flame expansion were recorded using a high-speed digital camera. The flame expansion scales (flame height and flame volume) were calculated under various conditions. Moreover, the effects of the water droplet diameter and impact height on the flame expansion scales and flame expansion duration were quantitatively analyzed. For a water droplet impacting the burning oil pool, the flame expansion is mainly caused by the steam explosion. But for the water droplet impacting the stratified oil-water, many small daughter oil droplets splash to increase the contact area between the fuel and air, which is probably the main reason for the instantaneous expansion of the flame. The flame expansion duration, height, and volume increase with the water droplet diameter and the impact height. The frequency of flame pulsation decreases gradually with the increase of the impact height. For the burning diesel oil pool, the flame volume expands about 50–135 times, while for the burning diesel layer on the water surface, the flame volume expands only 6 to 16 times.

Influence of refrigerant charge volume on the flammability risk of an R32 rotary compressor

The use of R32 as an ideal alternative refrigerant has been widely considered, but its slight flammability cannot be ignored. Air intrusion into the compressor and diluting the R32 to a flammable concentration range will bring flammability risk. This paper aims to study the influence of different refrigerant charge quantities on the flammability risk of the R32 in a rotor compressor under certain air intrusion conditions. The exhaust pressure, temperature distribution, R32 flammable concentration area, and flammable volume time inside the compressor were studied by experiments and simulations. The results indicate that the R32 high-risk flammable area is near the compressor motor and exhaust port. Flammability risk keeps rising as air intrusion increases and rises substantially when refrigerant is insufficient. Special compressors whose motors and exhaust ports can withstand high temperatures and pressure should be developed for the R32. The minimum allowable R32 charging standard should be calculated to ensure that the refrigerant concentration in the compressor is higher than the flammable upper limit when a certain amount of air intrudes. The results provide the technical basis for the formulation of standards for the safe use of R32 rotor compressors, thus obtain the best application potential of the R32.

Simulation and risk assessment of hydrogen leakage in hydrogen production container

In this paper, the hydrogen leakage and diffusion characteristics analysis and risk assessment are carried out on the container where a 2 Nm3/h alkaline hydrogen production device is located. Firstly, the transient and steady process of hydrogen leakage from hydrogen production container is analyzed. Secondly, the dynamic balance of combustible hydrogen cloud is analyzed, the concept of critical ventilation flow is put forward. It was found that in order to reduce the flammable volume by 85%, the installed ventilation can only cover the leakage flow of 1.0 Nm3/min. Finally, TNT equivalent method is used to evaluate the hazard degree of hydrogen leakage. It is found that the existed hydrogen production container ventilation device can only exhaust the hydrogen-air cloud with small flow leakage, while the accumulation of gas cloud still exists in large flow leakage. Under the critical ventilation flow, the minor injury radius can be reduced from 4.8 m to 2.78 m. The effect of critical ventilation flow was verified.

A novel approach combining bootstrapped non-intrusive reduced order models and unscented transform for the robust and efficient CFD analysis of accidental gas releases in congested plants

The risk assessment for safety-critical, complex systems is a very challenging computational problem when it is performed with high-fidelity models, e.g. CFD, like in the case of accidental gas releases in congested systems. Within this framework, a novel CFD approach, named Source Box Accident Model, has been recently proposed to efficiently model such phenomena by splitting the simulation of the gas release and its subsequent dispersion in the system in two steps. In this view, the present paper proposes a non-intrusive, Proper Orthogonal Decomposition-Radial Basis Functions reduced order model that exploits the two-step nature of the SBAM approach, to mimic the behaviour of the original, long-running CFD model code at a significantly lower computational cost. Moreover, the paper presents a methodology combining the bootstrap and unscented transform approaches to efficiently assess the ROM uncertainty in the safety-critical simulation output quantities of interest, e.g. the flammable volume. The results obtained in a test case involving a high pressure, accidental gas release in an off-shore Oil & Gas plant are in very satisfactory agreement with those produced by CFD, with a relative error smaller than 10% and a reduction in the computational time of about three orders of magnitude.

An FV-EE model to predict lean blowout limits for gas turbine combustors with different structures and sprays

The occurrence of Lean Blowout (LBO) is a disadvantage that endangers a stable operation of gas turbines. A determination of LBO limits is essential in the design of gas turbine combustors. A semiempirical model is one of the most widely used methods to predict LBO limits. Among the existing semiempirical models for predicting LBO limits, Lefebvre’s LBO model and the Flame Volume (FV) model are particularly suitable for gas turbine combustors. On the basis of Lefebvre’s and FV models, the concept of effective evaporation efficiency is introduced in this paper, and a Flame Volume-Evaporation Efficiency (FV-EE) model is derived and validated. LBO experiments are carried out in a model combustor with 23 different structures and 10 different sprays. The prediction uncertainty of the FV-EE model is less than ± 13% for all of these 33 structures and sprays, compared with ± 50% for the FV model and ± 60% for Lefebvre’s model. Furthermore, the prediction uncertainty of the FV-EE model is also less than ± 13% for other combustors from available literature.

Flame front reconstruction and volume estimation through computational geometry: a case study on machine vision applied to combustion systems

A computationally-supported experimental procedure to estimate the primary dimensions of diffusion flames, using volume reconstruction from thermal imagery, is presented. The experimental setup uses a 4 × 16.94 mm radial distribution gas-burner, with a 0.8 mm nozzle diameter, a thermal imaging camera and a proprietary image processing algorithm. Flame thermal imagery was captured, using four different fuel loads, 350, 650, 950 and 1200 cc/min, from two different visualisation planes, 0° and 90°. The images were visually and qualitatively processed leaving aside the temperature measurement and favouring instead a non-dimensional temperature gradient, . Corresponding flame front structures were estimated and reconstructed employing computational geometry. The height and diameter magnitudes were measured indirectly through a reference length. The results show that at the flame front structure separates itself from the background noise. Furthermore, when compared against available benchmarks, at and , the resulting flame coincides with the luminous and continuous flame heights, respectively. This approach yields maximum relative error of 36.54% and 18.91% for both compared geometries. When compared to image convolution and spatial density clustering procedures, this approach reduces the maximum error obtained by 47%. Based on this information, the methodology presented is considered suitable for dimensioning diffusion flames, thus, proposed as an estimation tool for the design and manufacturing of gas-fuelled appliances/devices.

Visual Fire Power: An Algorithm for Measuring Heat Release Rate of Visible Flames in Camera Footage, with Applications in Facade Fire Experiments

This paper presents a novel algorithm, called Visual Fire Power, for measuring the heat release rate of a turbulent flame using video footage taken from two cameras, located at an approximate right angle to each other and at a known distance from the fire. By measuring the time-averaged volume of the fire, Visual Fire Power can measure heat release rate in situations where traditional calorimetry may be impractical (such as experiments outdoors), as well as uniquely providing a method for comparing the heat release rates of different flames in the same experiment, e.g. externally venting flames from different windows of the same compartment. The algorithm was benchmarked against synthetic data and calculated the volume of common solids with approximately 30% uncertainty. The relationship between volume and heat release rate was then calibrated from videos of burners at known heat release rates. These experiments were used to calculate the Orloff-DeRis constant gamma, which linearly relates flame volume and heat release rate. The value for gamma was found to be 1505pm 183 kW/m3. The algorithm was demonstrated on recordings of a standard Polish facade fire test. Improving the range of data measured in both fire testing and fire experiments could help to increase our knowledge of fire dynamics and provide better data for researchers and engineers in the future.

Experimental study of upward flame spread behaviors over thermally thick vertical PMMA of various sample widths under normal wind

The effect of wind normal to the fuel surface on vertically upward flame spread is investigated over thermally thick PMMA slabs of various widths. The 1.2 cm-thick PMMA slabs of three sample width (10, 15, 20 cm) are vertically arranged subject to a horizontal wind (normal to fuel surface) ranging from 0 to 1.8 m/s. Flame size parameters, heat flux distribution and flame spread rate are measured and analyzed quantitatively. It is found that flame shape is much different under the strike of wind and flame spread rate decreases with wind velocity. To interpret the effect of normal wind, mechanism analysis is conducted based on flame spread theory. It is concluded that because of the transverse fuel diffusion, flame extending width increases with wind velocity, flame thickness and sample width in a relation of we∼δf2u∞ws. This leads to a lower flame height with increasing wind velocity, from which flame preheating length is shorter, being negative to flame spread. Flame thickness or flame volume increases with wind velocity which indicates a larger fuel burning rate due to the increasing flame heat flux in pyrolysis region. Two different modes are found for pyrolysis heat flux, that heat flux is controlled by buoyancy (flame) induced flow at low wind conditions while it could change to be mainly influenced by forced flow with the increasing wind velocity. Based on the analysis of the controlling mechanisms of flame spread under this configuration, a theoretical model is proposed to predict vertically upward flame spread process in normal wind, from which flame spread rate increases as a power function of time, and decreases with wind velocity. This work provides basic observation on upward flame spread progress in normal wind conditions and a detailed analysis of wind effect mechanisms on vertically upward flame spread behaviors.

A simultaneous measurement technique for soot temperature and volume fraction of sooting flames considering self-absorption through hyperspectral imaging

Due to the complex optical properties of sooting flames, the accuracy of flame emission-based measurement techniques depends on an appropriate self-absorption correction strategy. Thus, this paper presents a novel iterative procedure to retrieve the soot temperature and volume fraction of sooting flames along with a self-absorption correction strategy through hyperspectral imaging. Numerical simulations were carried out on suppositional flames to investigate the performance of the proposed technique. Relative errors obtained from the simulations are below 1.5%, indicating a better accuracy can be achieved by the proposed reconstruction technique. A hybrid camera hyperspectral video imaging technique is designed and implemented with the concept of compressed sensing. A significant improvement in flame spectrum acquisition has been observed. Experiments were carried out under ethylene-air diffusion flames to validate the technique. The reconstructed flame soot temperature and volume fraction distributions demonstrated that the proposed hyperspectral video imaging technique improves the soot temperature and volume fraction reconstruction accuracy.