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Register a new userImage Convolution-Based Method and Statistical Modelling for Experimental Height Estimations of Laminar to Transition-to-Turbulent Regime Diffusion Flames
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Mauricio De la Cruz-\\'Avila Dr.
Publication date
2020
fields
Physics
and research's language is
English
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This paper presents an experimental methodology to measure the height of the flame using convolution image processing and statistical analysis. The experimental setup employs a burner with four circularly arranged nozzles. Six different volumetric fuel flows were employed, and flame images were captured from three different visualization planes utilizing a three high-definition camera array, a thermal imaging camera and an image-processing algorithm. The flame height was indirectly measured using pixel quantification and conversion through a reference length. Although the fuel flow was the most significant factor, the visualization plane and the image source were also found to be particularly relevant, since certain flame features were only perceivable depending on the approach. The measurements were compared to different existing theoretical correlations, yielding an overall adjustment ranging from 3.25 to 3.97cm. The present methodology yields an overall statistical tolerance of 1.27 cm and an expanded uncertainty of 0.599 cm. Furthermore, the thermal imaging has revealed a consistent difference in the overall luminous observable flame of 2.54 cm. For this particular burner configuration, correlations were derived by statistical modelling, which explain the flame height fluctuations with an average setting of 97.23%.
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This paper presents a flame-height correlation for laminar to transition-to-turbulent regime diffusion flames. Flame-height measurements are obtained by means of numerical and experimental studies in which three high definition cameras were employed to take frontal, lateral and 45{deg} angled images simultaneously. The images were analysed using an image-processing algorithm to determine the flame-height through indirect measurement. To locate an overall chemical-flame-length, numerical simulations were conducted with the unsteady Reynolds-Averaged Navier-Stokes technique. The eddy-dissipation model was also implemented to calculate chemical reaction rate. The experiments show that this proposed correlation has an adjustment variation of luminous flame-height for the laminar regime of 16.9%, which indicates that, without the use of the intermittent buoyant flame-height correlation, it globally best represents the flame-height in this regime. For the laminar and transition-to-turbulence regime the adjustment variations are 5.54% compared to the most accepted flame-height correlations, thus providing an acceptably good fitting. The numerical results show that the proposed range for the chemical-flame-length is located between the luminous and flickering flame zone compared to the experimental flame images. These results agree with the chemical length zone reported in the literature. Therefore, the correlation can be used for laminar and transition-to-turbulent combustion regimes.
The mixing process of multiple jets of liquefied petroleum gas and air in a diffusion flame is numerically analysed. The case study considers a four-port array burner where the fuel is injected by four peripheral nozzles and mixed with the surrounding air. Simulations are conducted with the Reynolds-Averaged Navier-Stokes technique, and the turbulence effect is modelled with the realizable k-e model. In addition, the eddy dissipation model is implemented to calculate the effect of the turbulent chemical reaction rate. Results show that the essential mixture mechanism occurs within a flame cone derived from the fuel jets interaction. However, the mixing process is driven by jets drag allowing an air/fuel smooth mixture to reach the flammability limits at two zones: one at a lower location or close to the burner surface and a second before the flame front development. The entire mixing mechanism culminates with the development of the flame necking cone. Any air concentration that falls into the cone radius will be entrained, contributing to the overall flame structure. Since the cone radius reach is limited only by radial distance of the jet array and the nozzles distance, the flame heights, consequently, depend solely on fuel mass flow.
In this visualisation, the transition from laminar to turbulent flow is characterised by the intermittent ejection of wall fluid into the outer stream. The normalised thickness of the viscous flow layer reaches an asymptotic value but the physical thickness drops exponentially after transition. The critical transition pipe Reynolds number can be obtained simply by equating it with the asymptotic value of the normalised thickness of viscous flow layer. Key words: Transition, critical stability Reynolds number, critical transition Reynolds number, non-Newtonian pipe flow
We investigate the capability of neural network-based model order reduction, i.e., autoencoder (AE), for fluid flows. As an example model, an AE which comprises of a convolutional neural network and multi-layer perceptrons is considered in this study. The AE model is assessed with four canonical fluid flows, namely: (1) two-dimensional cylinder wake, (2) its transient process, (3) NOAA sea surface temperature, and (4) $y-z$ sectional field of turbulent channel flow, in terms of a number of latent modes, a choice of nonlinear activation functions, and a number of weights contained in the AE model. We find that the AE models are sensitive against the choice of the aforementioned parameters depending on the target flows. Finally, we foresee the extensional applications and perspectives of machine learning based order reduction for numerical and experimental studies in fluid dynamics community.
This study concerns wavepackets in laminar turbulent transition in a Blasius boundary layer. While initial amplitude and frequency have well-recognized roles in the transition process, the current study on the combined effects of amplitude, frequency, and bandwidth on the propagation of wavepackets is believed to be new. Because of the complexity of the system, these joint variations in multiple parameters could give rise to effects not seen through the variation of any single parameter. Direct numerical simulations (DNS) are utilized in a full factorial (fully crossed) design to investigate both individual and joint effects of variation in the simulation parameters, with a special focus on three distinct variants of wavepacket transition {textemdash} the reverse Craik triad formation sequence, concurrent N-type and K-type transition and abrupt shifts in dominant frequency. From our factorial study, we can summarize the key transition trends of wavepackets as follows: 1. Broad bandwidth wavepackets predominantly transit to turbulence via the N-route. This tendency remains strong even as the frequency width is reduced. 2. Narrow bandwidth wavetrains exhibit predominantly K-type transition. The front broadband part of an emerging wavetrain may experience N-type transition, but this wavefront should not be considered as a part of truly narrow-bandwidth wavepackets. 3. K-type transition is the most likely for wavepackets that are initiated with high energy/amplitude and/or with the peak frequency at the lower branch of the neutral stability curve.
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