A recent mathematical technique of homotopy perturbation method (HPM) for solving nonlinear differential equations has been applied in this paper for the analysis of steady-state heat transfer in an annular fin with temperature-dependent thermal conductivity and with the variation of thermogeometric fin parameters. Excellent benchmark agreement indicates that this method is a very simple but powerful technique and practical for solving nonlinear heat transfer equations and does not require large memory space that arises out of discretization of equations in numerical computations, particularly for multidimensional problems. Three conditions of heat transfer, namely, convection, radiation, and combined convection and radiation, are considered. Dimensionless parameters pertinent to design optimization are identified and their effects on fin heat transfer and efficiency are studied. Results indicate that the heat dissipation under combined mode from the fin surface is a convection-dominant phenomenon. However, it is also found that, at relatively high base temperature, radiation heat transfer becomes comparable to pure convection. It is worth noting that, for pure radiation condition, the dimensionless parameter of aspect ratio (AR) of a fin is a more desirable controlling parameter compared to other parameters in augmenting heat transfer rate without much compromise on fin efficiency.
Lean blowoff in distributed combustion was investigated at moderate heat release intensities of 5.72, 7.63, and 9.53 MW/m3-atm to characterize the blowoff phenomenon. Distributed combustion conditions were established from a conventional swirl flame at an equivalence ratio 0.9 using carbon dioxide as the diluent to the inlet airstream. A gradual increase in the air flowrate provided reduction of equivalence ratio that eventually resulted in the lean blowoff limit. Blowoff occurred at relatively higher equivalence ratios for higher heat release intensities, which was attributed to higher inlet turbulence leading to early introduction of flame instabilities and blowoff. High-speed chemiluminescence imaging (at 500 frames/second) performed near blowoff moments demonstrated transition of distributed reaction zone to a near V-shape zone due to quenching of flame surface along the sides. A closer examination of the reduction in equivalence ratio in small steps near the global blowoff showed the presence of a very thin thread-like rotating reaction zone. The observations of blowoff were further supported by the analysis of chemiluminescence signals in each case. The effect of inlet air preheats on blowoff was also investigated. Air preheats broadened the lean blowoff to a lower equivalence ratio which was attributed to enhanced flame speed, providing additional flame stability and reduction of flowfield instabilities. The laminar flame speeds obtained at each preheats case using Chemkin-Pro simulation with GRI-Mech 3.0 reaction mechanisms supported such hypothesis of gradually enhanced flame speed, providing additional flame stability.
One of the major challenges in the development of micro-combustors is heat losses that results in flame quenching, and reduced combustion efficiency and performance. In this work, a novel thermal barrier coating (TBC) using hexagonal boron nitride (h-BN) nanosheets as building blocks was developed and applied to a Swiss roll micro-combustor for determining its heat losses with increased temperatures inside the combustor that contributes to improved performance. It was found that by using the h-BN TBC, the combustion temperature of the micro-combustor increased from 850K to 970K under the same thermal loading and operational conditions. This remarkable temperature increase using the BN TBC originated from its low cross-plane thermal conductivity of 0.4 W m-1 K-1to mitigate the heat loss from the micro-combustor plates. Such a low thermal conductivity in the h-BN TBC is attributed to its interfacial resistance between the nanosheets. The development of h-BN TBC provides an effective approach to improve thermal management for performance improvements of gas turbine engines, rocket engines and all various kinds of micro-combustors.
Swirl-assisted distributed combustion was examined using a deep learning framework. High intensity distributed combustion was fostered from a 5.72 MW/m3-atm thermal intensity swirl combustor (with methane fuel at equivalence ratio 0.9) by diluting the flowfield with carbon dioxide. Dilution of the flowfield caused reduction of global oxygen (%) content of the inlet mixture from 21% to 16% (in distributed combustion). The adiabatic flame temperature gradually reduced, resulting in decreased flame luminosity and increased flame thermal field uniformity. Gradual reduction of flame chemiluminescence was captured using high-speed imaging without any spectral filtering at different oxygen concentration (%) levels to gather the data input. Convolutional neural network (CNN) was developed from these images (with 85% of total data used for training and 15% for testing) for flames at O2 = 16, 18, 19, and 21%. Hyperparameters were varied to optimize the model. New flame images at O2 = 20% and 17% were introduced to verify the image recognition capability of the trained model in terms of training image data. The results showed good promise of developed deep learning-based convolutional neural network or machine learning neural network for efficient and effective recognition of distributed combustion regime.
Swirl-assisted distributed combustion was investigated with hydrogen-enriched methane. Distributed reaction zones were fostered from a conventional swirl-flame at a heat release intensity of 5.72 MW/m3-atm by diluting the main airstream with either carbon dioxide or nitrogen. The effect of hydrogen addition to the fuel mixture on the performance of distributed combustion was studied for reaction zone stability, variation of blowoff equivalence ratio, and emissions of nitrogen oxide, carbon monoxide, and carbon dioxide. High-speed imaging of reaction zone chemiluminescence was performed for different cases without any spectral filtering. Gradual increase of %H2 in the fuel mixture increased the chemiluminescence intensity in both the swirl and distributed combustion cases. The standoff distance was gradually reduced with hydrogen enrichment along with the appearance of a narrow flame shape from increased reactivity in the flame brush. Fluctuation of pressure (p′) and heat release (q′) was qualitatively measured from the microphone and photomultiplier (fitted with CH* filter) signals at different %H2 enrichments. The amplitude of fluctuation of p′ and q′ showed the existence of a common peak in swirl combustion indicating the possibility of thermo-acoustic coupling. This peak diminished in distributed combustion for H2 enrichment between 0–20% providing enhanced stability compared to swirl combustion. However, a small peak common to p′ and q′ appeared at 40% H2-enrichment indicating the departure of this reaction zone from its distributed nature. Such fluctuations of reaction zones were further investigated with the proper orthogonal decomposition to verify if the vortex shedding influenced these fluctuations. The appearance of vortex shedding characteristics for the distributed combustion with 40% H2-enrichment was found to be responsible for the fluctuations of reaction zones resulting in a departure from the purely distributed behavior. Measurement of lean blowoff equivalence ratios (ϕLBO) at different combustion conditions showed extension of ϕLBO in distributed combustion indicating wider operational limits in distributed combustion. The performance of distributed reaction zones was analyzed from the exhaust emission characteristics of NO, CO, and CO2. The NO levels (ppm) gradually increased in conventional swirl combustion while it consistently decreased in distributed combustion with the increase of %H2. The increase in NO in normal swirl combustion was attributed to the increase in flame temperature. The overall exhaust CO (ppm) decreased with hydrogen enrichment. The exhaust CO2 gradually decreased with %H2-enrichment for both swirl and distributed reaction zones. The higher CO2 observed with CO2 dilution (compared to N2 dilution) is attributed to the usage of CO2 as the diluent. Emission characteristics were also investigated with preheating of inlet airstream (in the range 373–573 K) to study the performance of distributed combustion relevant to actual gas turbines. The results of reduced pollutant emission with hydrogen enrichment at any preheats temperature were consistent with the non-preheated case. However, some increase in pollutants concentration was observed with gradual preheating that was attributed to higher flame temperature and high-temperature dissociation of CO2. The decreased CO2 emission observed in this research further signifies the favorable potential of distributed combustion with hydrogen-enriched methane to support the decarbonization goal worldwide.
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