scholarly journals Capturing the Swirling Vortex and the Impact of Ventilation Conditions on Small-Scale Fire Whirls

2020 ◽  
Vol 10 (10) ◽  
pp. 3428
Author(s):  
Xiang Fang ◽  
Anthony Chun Yin Yuen ◽  
Guan Heng Yeoh ◽  
Eric Wai Ming Lee ◽  
Sherman Chi P. Cheung
Keyword(s):  
Field Model ◽  
Lower Boundary ◽  
Air Entrainment ◽  
Small Scale ◽  
Combustion Dynamics ◽  
Eddy Simulation ◽  
Numerical Predictions ◽  
Fire Whirl ◽  
The Stability ◽  
The Impact

The fundamental flow structure and temperature distribution of small-scale fire whirls, including tangential and axial velocities, temperature variation, and air entrainment in the lower boundary layer, were successfully captured using a generic fire field model with large eddy simulation (LES) turbulence closure. Numerical predictions were validated thoroughly against two small-scale experimental measurements, where detailed temperature and velocity distributions were recorded. Good agreement between numerical and experimental results was achieved. Normalization was also performed to compare the numerical predictions with the empirical correlations by Lei et al. (2015) developed from medium-scale fire whirl measurements. The transient development stages of small-scale fire whirls and the impact of air entrainment on the stability of the fire whirls were also investigated based on the validated numerical results. The numerical validations showed the potential of the current LES fire field model in capturing the dynamic behaviour of the fire whirl plume and performing a quantitative analysis on its onset criteria and combustion dynamics in future.

Describing the Mechanism of Instability Suppression Using a Central Pilot Flame With Coupled Experiments and Simulations

2021 ◽  
Author(s):  
Jihang Li ◽  
Hyunguk Kwon ◽  
Drue Seksinsky ◽  
Daniel Doleiden ◽  
Jacqueline O’Connor ◽  
...  
Keyword(s):  
Gas Turbines ◽  
Equivalence Ratio ◽  
Vortex Breakdown ◽  
Eddy Simulation ◽  
Breakdown Region ◽  
Large Eddy ◽  
Rate Oscillations ◽  
Combustion Oscillation ◽  
The Stability ◽  
The Impact

Abstract Pilot flames are commonly used to extend combustor operability limits and suppress combustion oscillations in low-emissions gas turbines. Combustion oscillations, a coupling between heat release rate oscillations and combustor acoustics, can arise at the operability limits of low-emissions combustors where the flame is more susceptible to perturbations. While the use of pilot flames is common in land-based gas turbine combustors, the mechanism by which they suppress instability is still unclear. In this study, we consider the impact of a central jet pilot on the stability of a swirl-stabilized flame in a variable-length, single-nozzle combustor. Previously, the pilot flame was found to suppress the instability for a range of equivalence ratios and combustor lengths. We hypothesize that combustion oscillation suppression by the pilot occurs because the pilot provides hot gases to the vortex breakdown region of the flow that recirculate and improve the static, and hence dynamic, stability of the main flame. This hypothesis is based on a series of experimental results that show that pilot efficacy is a strong function of pilot equivalence ratio but not pilot flow rate, which would indicate that the temperature of the pilot gases as well as the combustion intensity of the pilot flame play more of a role in oscillation stabilization than the length of the pilot flame relative to the main flame. Further, the pilot flame efficacy increases with pilot flame equivalence ratio until it matches the main flame equivalence ratio; at pilot equivalence ratios greater than the main equivalence ratio, the pilot flame efficacy does not change significantly with pilot equivalence ratio. To understand these results, we use large-eddy simulation to provide a detailed analysis of the flow in the region of the pilot flame and the transport of radical species in the region between the main flame and pilot flame. The simulation, using a flamelet/progress variable-based chemistry tabulation approach and standard eddy viscosity/diffusivity turbulence closure models, provides detailed information that is inaccessible through experimental measurements.

Numerical Study on the Effect of Seal Leakage Flow on Low Pressure Axial Compressor Performance

2013 ◽  
Author(s):  
Balazs Farkas ◽  
Nicolas Van de Wyer ◽  
Jean-Francois Brouckaert
Keyword(s):  
Numerical Study ◽  
Axial Compressor ◽  
Test Facility ◽  
Leakage Flow ◽  
Stability Range ◽  
Compressor Rotor ◽  
Numerical Predictions ◽  
Aero Engines ◽  
The Stability ◽  
The Impact

This paper presents the extended numerical studies of a one and a half stage axial compressor designed for the LP compressor of a contra-rotating fan engine architecture. The essence of this architecture is given by the fact that the LP compressor rotor is mounted on the same shaft as the second fan stage which results in a lower rotational speed and therefore a much higher loading than in conventional high bypass-ratio aero-engines. The compressor itself was designed at VKI and subsequently tested in the closed loop test facility (VKI-R4) which allowed to compare numerical predictions with experimental data. In this study, particular interest was given to investigate the effect of the seal-leakage flow around the stator hub platform on the performance. To study the effect of the seal-leakage flow three different seal cavity configurations with different seal-tooth gaps sizes were simulated in comparison with no-cavity configuration. This set of investigations allowed to assess the different models by comparison with the results obtained experimentally. This comparison was made on the global performance of the stage, including the impact on the stability range, as well as on the flow field itself in particular in the rotor and stator exit planes. The computations were performed by using the Numeca developed code FINE™/Turbo with steady RANS solver.

scholarly journals Investigating the impact of atmospheric stability on thunderstorm outflow winds and turbulence

2018 ◽  
Vol 3 (1) ◽  
pp. 203-219 ◽  
Author(s):  
Patrick Hawbecker ◽  
Sukanta Basu ◽  
Lance Manuel
Keyword(s):  
Large Scale ◽  
Atmospheric Stability ◽  
Temperature Inversion ◽  
Ring Vortex ◽  
Large Eddy Simulation Model ◽  
Eddy Simulation ◽  
Large Scale Flow ◽  
The Stability ◽  
Thunderstorm Outflow ◽  
The Impact

Abstract. Downburst events initialized at various hours during the evening transition (ET) period are simulated to determine the effects of ambient stability on the outflow of downburst winds. The simulations are performed using a pseudo-spectral large eddy simulation model at high resolution to capture both the large-scale flow and turbulence characteristics of downburst winds. First, a simulation of the ET is performed to generate realistic initial and boundary conditions for the subsequent downburst simulations. At each hour in the ET, an ensemble of downburst simulations is initialized separately from the ET simulation in which an elevated cooling source within the model domain generates negatively buoyant air to mimic downburst formation. The simulations show that while the stability regime changes, the ensemble mean of the peak wind speed remains fairly constant (between 35 and 38 m s−1) and occurs at the lowest model level for each simulation. However, there is a slight increase in intensity and decrease in the spread of the maximum outflow winds as stability increases as well as an increase in the duration over which these strongest winds persist. This appears to be due to the enhanced maintenance of the ring vortex that results from the low-level temperature inversion, increased ambient shear, and a lack of turbulence within the stable cases. Coherent turbulent kinetic energy and wavelet spectral analysis generally show increased energy in the convective cases and that energy increases across all scales as the downburst passes.

Backlit Imaging of a Circular Plunging Jet With Floor Interactions

2020 ◽  
Author(s):  
Roy A. Pillers ◽  
Theodore J. Heindel
Keyword(s):  
High Speed ◽  
Large Scale ◽  
Air Entrainment ◽  
Ocean Waves ◽  
Jet Flows ◽  
Small Scale ◽  
Bubble Plume ◽  
Plunging Jets ◽  
Compression Effects ◽  
The Impact

Abstract Plunging jets occur when a liquid stream enters a slower moving or stationary liquid body after first passing through a gaseous region. The most commonly studied plunging jet structure is that of water entering water. Plunging jets have been studied in order to understand and model mixing and transport from the atmosphere into the liquid. Shear forces at the edge of the jet cause air entrainment both in the free jet and at the impact point on the pool surface. Plunging jet applications range from large scale environments, such as ocean waves, waterfalls, wastewater treatment, and dams, to small scale environments, such as liquid-gas fuel mixing, mineral separation, and molten metal pouring. The majority of the literature today involve facilities designed to approximate an infinite liquid pool; few of these studies take into account the compression effects prevalent in several of the real systems. Therefore, a tank has been developed for the visualization of plunging jet flows with varying pool depth. This study involved the creation of a 32 cm by 32 cm, 91.4 cm deep rectangular acrylic tank with an interior adjustable acrylic bottom for the visualization of plunging jet flows with bottom compression effects. The pool height was held constant using a secondary tank with an overflow weir. In this study high-speed backlit images were taken of the plunging jet region. Preliminary results indicate that there is a significant change in both the shape and estimated entrained air volume when the plunging jet is subjected to compression effects. This is attributed to the plate spreading the bubble plume and allowing for easier bubble rise.

scholarly journals RESPONSE OF SEADYKES DUE TO WAVE IMPACTS

1976 ◽  
Vol 1 (15) ◽  
pp. 149 ◽  
Author(s):  
A. Fuhrboter ◽  
H.H. Dette ◽  
J. Grune
Keyword(s):  
Laboratory Data ◽  
Breaking Waves ◽  
Scale Model ◽  
Small Scale ◽  
Air Entrapment ◽  
Small Areas ◽  
Impact Forces ◽  
The Stability ◽  
Short Time ◽  
The Impact

Damages on seadykes and revetments are mainly caused by wave impacts due to breaking waves. These impact forces act on small areas for a very short time and cause crater-like formations, when the forces are transmitted instantaneously to the side-walls of cracks in the cover of dykes or through joints into and below revetments. In this paper the results of investigations on impact forces are presented. A comparison of field data and laboratory data proves considerable differences, which must be explained mainly by the different air entrapment for prototype and small-scale conditions in the breaking waves. Both the data from field and small-scale model emphasize, that the slope of the dyke or revetment is responsible at first for frequency and magnitude of the impact forces. Furthermore the effect of impact forces is demonstrated by the results of investigations on the stability of stone revetments with joints.

Underwater noise emitted during small-scale air entrainment events

2018 ◽  
Vol 47 (1) ◽  
pp. 87-97 ◽  
Author(s):  
Justyna Szuszkiewicz ◽  
Zygmunt Klusek
Keyword(s):  
Bubble Formation ◽  
Gas Bubbles ◽  
Air Entrainment ◽  
Small Scale ◽  
Breaking Wave ◽  
Underwater Sound ◽  
Underwater Noise ◽  
Noise Emission ◽  
Water Properties ◽  
The Impact

Abstract The breaking wave phenomenon significantly takes part in the mechanisms of mass, heat and gas exchange at the air-water boundary and depends on the wind velocity. Some of the energy dissipated during this process is converted into underwater sound emitted by oscillating gas bubbles and bubble plumes. However, the underwater noise accompanying the lowest wind speed conditions has received only little attention. This report describes a study aimed at advancing the knowledge of underwater noise emission from air bubbles injected during small-scale breaking events occurring on the water surface. Results of model experiments performed in a small tank are presented. The object of the research is the relationship between the generated noise and the dissipated potential energy of water poured into a tank filled with water of varying physical water properties. Additionally, the impact of various water properties such as salinity, surface tension or microscale gas bubbles was examined. The experiment revealed that noise spectra are affected by different water properties and most likely reflect the varying efficiency of bubble formation and bubble size spectra.

CFD Study of Generation Process and Stability of a Fire Whirl in Large-Scale Fires

2019 ◽  
Author(s):  
Koyu Satoh ◽  
Domingos Viegas ◽  
Claudia Pinto ◽  
Ran Tu
Keyword(s):  
Forest Fires ◽  
Large Scale ◽  
Small Scale ◽  
Channel Height ◽  
Generation Process ◽  
Strong Wind ◽  
Cfd Simulations ◽  
Swirling Flame ◽  
Fire Whirl ◽  
The Stability

Abstract Large-scale urban and forest fires, especially earthquake-induced fires may produce huge fire whirls and cause serious damage, due to the involved tornado-like strong wind, together with radiation and swirling flame. If fire whirls are generated, the danger to firefighters increases extremely. Many small-scale experiments and CFD simulations on fire whirls have already been conducted and also our previous numerical studies examined the generation of a large fire whirl in an oil tank. However, details of large-scale fire whirls have not been clarified yet. In this study, developing the previous works, additional CFD simulations are conducted to examine the generation process and particularly the stability of fire whirls. Three schemes to generate fire whirls are employed, using the 15 × 15 PMMA fuel array in windy conditions and n-heptane burning in a steel pan placed centrally on the floor in a tall channel with staggered four corner gaps, also using a channel with a single corner gap. The numerical results showed that the relationship between the fire area and the wind blowing area is important on the fire whirl generation in the PMMA scheme and n-heptane fire burning scheme in a channel. In addition to the channel gap size to produce a maximum fire whirl, the effects of channel height and horizontal channel area upon the fire whirl are examined. The wall temperatures of the channel are important to keep the swirling motion stably, particularly the wall temperature at about 300°C can stabilize the fire whirl in a channel. Also multiple small fires placed surrounding the central swirling fire can increase the stability of the fire whirl, although too strong multiple fires may destroy the stability. These phenomena may be related to the real fire whirl generation in the natural environment.

Describing the Mechanism of Instability Suppression Using a Central Pilot Flame with Coupled Experiments and Simulations

2021 ◽  
Author(s):  
Jihang Li ◽  
Hyunguk Kwon ◽  
Drue Seksinsky ◽  
Daniel G Doleiden ◽  
Jacqueline O'Connor ◽  
...  
Keyword(s):  
Gas Turbines ◽  
Vortex Breakdown ◽  
Strong Function ◽  
Eddy Simulation ◽  
Breakdown Region ◽  
Large Eddy ◽  
Rate Oscillations ◽  
Combustion Oscillation ◽  
The Stability ◽  
The Impact

Abstract Pilot flames are commonly used to extend combustor operability limits and suppress combustion oscillations in low-emissions gas turbines. Combustion oscillations, a coupling between heat release rate oscillations and combustor acoustics, can arise at the operability limits of low-emissions combustors where the flame is more susceptible to perturbations. In this study, we consider the impact of a central jet pilot on the stability of a swirl-stabilized flame in a variable-length, single-nozzle combustor. Previously, the pilot flame was found to suppress the instability for a range of equivalence ratios and combustor lengths. We hypothesize that combustion oscillation suppression by the pilot occurs because the pilot provides hot gases to the vortex breakdown region of the flow that recirculate and improve the static, and hence dynamic, stability of the main flame. This hypothesis is based on a series of experimental results that show that pilot efficacy is a strong function of pilot equivalence ratio but not pilot flow rate, which would indicate that the temperature of the pilot gases as well as the combustion intensity of the pilot flame play more of a role in oscillation stabilization than the length of the pilot flame relative to the main flame. To understand these results, we use large-eddy simulation to provide a detailed analysis of the flow in the region of the pilot flame and the transport of radical species in the region between the main flame and pilot flame.

Rapid Accelerometer Measurements of Vehicle Bumps at Bridge Approaches

2015 ◽  
Vol 2528 (1) ◽  
pp. 1-8
Author(s):  
Yuchuan Du ◽  
Chenglong Liu ◽  
Xi Zhang ◽  
Lijun Sun
Keyword(s):  
Linear Time ◽  
Evaluation Model ◽  
Field Tests ◽  
Small Scale ◽  
Linear Time Invariant ◽  
Structural Deterioration ◽  
Power Spectral ◽  
Bridge Structures ◽  
The Stability ◽  
The Impact

Bridge approach settlement (BAS) is one of the most common problems in highway and bridge construction. The settlement can result in unsafe driving conditions, rider discomfort, structural deterioration of bridges, and long-term maintenance costs. BAS research has focused mainly on causes, hazards, and countermeasures, with little emphasis on BAS measurement. This paper presents a rapid vehicle-mounted accelerometer method for BAS measurement and proposes a comprehensive BAS measure consisting of three indexes that measure user physical and psychological comfort as well as the impact on bridge structures. A wavelet transform is used to identify the reaction position. Power spectral density analysis and one-third octave band filtering are applied to calculate the weighted root mean square acceleration as an index of comfort. Adopting the annoyance rate concept from experimental psychology, an annoyance rate model was developed that includes a membership function and the distribution of user comfort levels. A quarter-car model and a linear time invariant (LTI) system were used to estimate the equivalent impact coefficient as an index of durability. Furthermore, a collective system equipped with z-axis accelerometers and a GPS device was created, and small-scale field tests were conducted in Shanghai, China. A velocity correction equation was generated to correct the evaluation model. The test results demonstrate the stability and efficiency of the rapid measurement system.

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