Articles written in Sadhana

    • Experimental investigation of spray characteristics of kerosene and ethanol-blended kerosene using a gas turbine hybrid atomizer


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      Gas turbines have wide application as prime movers in transportation and power generating sectors,most of which are driven by fossil fuels like kerosene. The conventional fuels are associated with problems of air pollution, and the fuel reserves are getting depleted gradually. Addition of ethanol in kerosene leads to better spraying characteristics. The present work deals with the spray characteristics of pure kerosene and 10%-ethanol-blended (by volume) kerosene using a novel gas-turbine hybrid atomizer. Here the inner air and outer airenter in the same and opposite directions, respectively, with respect to the fuel flow direction into the atomizer and a high swirling effect occurs outside the nozzle. The fuel stream is sandwiched between two annular air streams and the flow rate of inner and outer air is varied continuously. Various spray stages like distorted pencil,onion, tulip and fully developed spray regimes have been observed. The breakup length, cone angle and sheet width of the fuel stream are analysed directly from backlit imaging for different fuel and air flow rates. From the image processing, it is observed that breakup occurs at an early stage for 10%-ethanol-blended kerosene due to low viscosity of ethanol. It is also observed that at higher air flow rate, breakup occurs at an early stage due toturbulent nature of the fuel stream.

    • Pressure dependence of dryout in a heat-generating porous debris bed


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      The primary objective of this work is to numerically study the effect of system pressure on the occurrence of dryout phenomena in heap-shaped, heat-generating porous debris beds. This is achieved using a multiphase computational fluid dynamics (CFD) model that is implemented in the framework of the commercial CFD platform ANSYS FLUENT. The model is extensively validated with available experimental data on two different aspects: pressure drop in two-phase flow through porous media and prediction of dryout in typicalporous debris beds. A wide range of system pressure, relevant to severe accident conditions, is considered in this analysis in order to obtain a thorough understanding of its impact on multiphase flow and dryout occurrence inporous debris beds. This analysis is performed for different subcoolings of flooding water, which gives additional knowledge on the effects of coolant subcooling on dryout occurrence. Results indicate that dryout in debris beds occurs at progressively higher power densities as the system pressure is raised. Similar effects are observed with increase in liquid subcooling as well.

    • Identification and early prediction of lean blowout in premixed flames


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      Lean premixed combustion is a state-of-the-art technology to meet the stringent emission norms, especially to reduce the NOx emission. In the present study, we focus on the transition to lean blowout of premixed flames. The main objective of the study is to explore some important measures for prediction of leanblowout as the existing methods are not always suitable in early detection of blowout in case of premixed flames. We further consider different cases through which lean condition can be attained. For such cases, we first discussthe flame dynamics prior to blowout. Among different statistical measures, mean, median, root mean square and coefficient of variance of heat release rate fluctuations are found to be useful in blowout prediction. On the otherhand, skewness and excess of kurtosis do not show any predictive quality. We also observe that the flickering frequency defined in a specific manner also shows a significant variation near lean blowout. Further, the meanfrequency evaluated using Hilbert transform is found to show indication of LBO well before its occurrence. Therefore, the frequency analysis along with statistical analysis of the heat release rate oscillations is found to be very relevant in the context of blowout prediction and can be applied in the practical combustors.

    • Flow reversal prediction of a single-phase square natural circulation loop using symbolic time series analysis


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      In the field of thermal engineering, one of the biggest concerns is the cooling of heat producing systems. For this purpose, today’s world is encouraging to use such cooling systems which are free from any active components (passive systems) for its high reliability and compact size. For this reason, to establishcooling by transferring heat from one place (source) to another (sink) passive system like natural circulation loop (NCL) is highly used. Fluid flow dynamics of the NCL is changing with the increase in heater power which is used as the source for the simulation. We found steady flow dynamics for the comparatively low power of heat, and with the rise in the power first, we saw the oscillatory flow dynamics and then found flow reversal characteristics. This paper presents a novel strategy for the early prediction of flow reversal phenomenon in NCLusing symbolic analysis of time series data. This time series data is found from the numerical simulation, and for the proper study, we are considering data after the initial transient part is overcome. Total time series data is transformed into a symbol string by partitioning into a finite number of specified symbolised groups. The state probability vector is calculated based on the number of occurrences of each symbol group. Present work is a single-phase study, and according to our geometry, we can provide a maximum 800 W heater power to stay in the single-phase. Therefore, for the early prediction of flow reversal in NCL, state probability vector evaluated at 800 W heater power which is the most undesirable state (chaotic data), and this is considered as the reference vector. The difference of the reference state vector from the current state vector is used as a parameter for early detection of flow reversal. It can be observed from the results that this difference changes significantly when the system is sufficiently away from the flow reversal.

    • Characterization of laminar flame using high speed camera and spectrometer


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      In order to improve flame stabilization in small scale combustors, there have been many research works going on non-premixed as well as on premixed flame. The present experimental study characterizes colour and shape of the premixed and non-premixed flames through a laminar co-flow burner. Various flamebehaviors are observed and their stabilization limits are investigated at different fuel–air ratios. The spectroscopic approach becomes suitable method for the measurement of chemiluminescensce for both premixed and diffusion flames. Thus, we can predict the behavior of a combustion process. We observe also the intensity variation of chemiluminescence species, OH*, CH*, and C2* for different fuel-air flow combinations. Apart from that, we have an analysis of colour variance of the premixed flame for different flame locations

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