This study investigates the influence of injector flare angle ( β ) on the flow dynamics and dynamic stability of a counter-rotating dual-radial swirl injector. The objective is to elucidate how geometric variation modifies both hydrodynamic and thermo-acoustic instability characteristics, thereby shaping flame topology and global stability limits in swirl-stabilized combustors. A series of non-reacting and reacting experiments were conducted for flare angles β = 0^∘ , 30^∘ , and 50^∘ . High-speed OH^* chemiluminescence, stereo-PIV, and dynamic pressure measurements were acquired simultaneously to resolve unsteady flow–flame interactions. Time-resolved and spectral analyses–including spectral POD–were employed to extract coherent structures, instability modes, and coupling mechanisms between pressure and velocity oscillations. Under non-reacting conditions, increasing the flare angle enhances the interaction between primary and secondary swirl streams, leading to stronger recirculation, a larger central recirculation zone (CRZ), and intensified precessing vortex core (PVC) activity. In reacting flows, β profoundly affects both static and dynamic stability. The β = 0^∘ and β = 30^∘ cases sustain attached V-flames dominated by longitudinal thermo-acoustic oscillations, whereas β = 50^∘ exhibits a transition from bubble-type to conical vortex breakdown (BVB → CVB), yielding lifted flames and intermittent low-frequency oscillations. This transition weakens acoustic coupling and produces a dynamically quieter yet stable flame. The flare angle is identified as a critical geometric control parameter dictating the balance between hydrodynamic and thermo-acoustic dominance. Optimizing β improves fuel–air mixing, extends the rich blow-off limit, and mitigates high-amplitude oscillations. These findings provide fundamental guidance for designing high-shear swirl injectors in next-generation low-emission gas turbine combustors with enhanced stability and reduced acoustic sensitivity.

The combustion characteristics of a batch of in-situ prepared high-ash coal char have been investigated under different fluidized bed operating conditions (i.e. O2 concentration, balance gas, bed temperature, particle size) in an optically accessible fluidized bed. A fast Universal Exhaust Gas Oxygen (UEGO) sensor measured the oxygen partial pressure at the exhaust to determine the reaction rate and burnout time of char particles, which demonstrates a novel application of this type of sensor in high H2O (wet) environments. A pre-calibrated two-colour pyrometry technique was employed to measure the temperature of char particles. The effect of the oxyfuel atmosphere on the burnout time of char particles was found to be prominent at higher bed temperatures, lower O2 concentrations, and larger particle sizes. The sensitivity of combustion behaviour to variations in the combustion environment was higher for larger particle size. Results show that the burnout time was the lowest in O2/H2O for both 1.2 mm and 3 mm particle sizes. This was attributed to the higher diffusivity of O2 in H2O and the H2O gasification reaction. The addition of H2O to O2/CO2 environment enhanced the diffusivity of O2, making it comparable to that in N2. In 10% O2/45% CO2/45% H2O environment, competitive interactions among O2, CO2 and H2O were more pronounced for smaller particles. The reactivity of char in 10% O2/90% H2O at higher bed temperatures became comparable to that in 20% O2/80% N2 environment making oxy steam combustion a potential advancement to oxyfuel combustion.

An experimental investigation in a sector ( 20^∘ ) of a full-scale annular gas turbine combustor is performed. The sector combustor is optically accessible for the flow and flame visualization of the primary and exit zones of the combustor. The distinctive feature of the experimental setup is that it preserves the geometrical details of an annular combustor, which includes the casing, dome, and combustor liner. The combustor design features a series of primary and secondary dilution holes with multiple film cooling strips on the outer and inner liner. In the present study, the time-resolved particle image velocimetry (PIV) experiments are conducted on the central longitudinal plane and two azimuthal planes to gain insight into the dynamics of the sector combustor. Proper orthogonal decomposition (POD) is applied to the data to obtain the dominant dynamics of the combustor. The major coherent structures of the swirl flow field, the primary dilution jets flow field, and the dominant interaction of swirl and dilution jets are elucidated here. The azimuthal plane data provide a three-dimensional explanation of dilution jet dynamics. The dynamics of the exit zone is found to be influenced by the secondary dilution jet dynamics. The spectral properties of dynamics are illustrated from the recorded acoustic ( p^' ) signal and the time coefficient of the POD eigenmodes. Further, the experiments are performed by blocking the dilution jets (without-DJ). These experimental data help to identify the source of the dominating frequency ( f_d ) within the combustor, which is found to be the swirl flow instabilities. Without-DJ data also showcases the role of dilution jets in convecting the swirl flow generated acoustics to the exit zone. The reconstructed flow field using POD provides physical insights into the dynamics occurring within the sector combustor.

The non-linear response of a swirl-stabilized, partially-premixed flame subjected to high-frequency transverse acoustic excitation is experimentally investigated. Motivated by azimuthal thermo-acoustic instabilities in gas turbine combustors, experiments are conducted in a model gas turbine combustor using simultaneous high-speed particle image velocimetry (PIV), OH* chemiluminescence, and unsteady pressure measurements. The flame is excited at 1500 Hz with varying forcing amplitudes, corresponding to 1.6–5.0% of chamber pressure. At low forcing amplitudes, the flame maintains a periodic V-shaped structure with stable, linear acoustic-flame coupling. Increasing the forcing amplitude induces amplitude modulations and spectral broadening, signaling the onset of non-linear interactions. Phase space reconstructions and recurrence analyses reveal a transition to Type-II intermittency, characterized by bursts of high-amplitude oscillations interspersed with low-amplitude states. At higher amplitudes, strong intermittency and chaotic behavior are observed, leading to flame blow-off. The flame dynamics are closely linked to transitions in the underlying flow structure, with the swirling jet evolving from a columnar vortex breakdown (V-flame) to a wall-jet (wall-flame) configuration under strong transverse excitation. This transition promotes heat loss to the combustor walls and destabilizes the flame. Cross wavelet transform analysis highlights a progressive loss of phase-locking between pressure and heat release rate fluctuations with increasing forcing amplitude. The results emphasize the critical role of non-linear flame dynamics and flow-flame-acoustic coupling in driving combustion instabilities under high-frequency excitation. This study provides new insights into the mechanisms governing intermittent behavior and blow-off in swirl-stabilized flames, with implications for the design of more robust, low-emission combustion systems.

An experimental investigation in a sector ( 20 deg ) of full-scale annular gas turbine combustor is performed. The sector combustor is optically accessible for the flow and flame visualization of the primary and exit zones of the combustor. The distinctive feature of the experimental setup is that it preserves the geometrical details of an annular combustor that includes the casing, dome and combustor liner. The combustor design features a series of primary and secondary dilution holes with multiple film cooling strips on the outer and inner liner. In the present study, the combustor is operated at inlet Mach numbers of 0.02-0.3 at operating absolute pressures of 1-5 bar. Static pressure measurements are performed at multiple locations in the rig to characterize the pressure drop across the combustor. Two-dimensional particle image velocimetry (PIV) is performed to measure the velocity fields of the primary and exit zones of the combustor simultaneously. The results show the presence of a central recirculation zone (CRZ), high-velocity annular jets, and a pair of dilution jets in the primary zone of the combustor. The steady-state flow structures are invariant of inlet Mach number and pressures. The relationship between the relative pressure drop across the combustor and the combustor inlet condition is obtained. Mass flowrate and momentum flux are calculated for the flow through the swirler, central recirculation zone, the primary dilution jets, and the exit zone. The paper shows how the flow structures in a realistic combustor change with variations in global combustor parameters.

Controlling the carbon dioxide (CO2) concentration in the atmosphere of chambers/rooms is a necessity for various applications. The adsorption-desorption using solid adsorbent is often considered the best technique to actively control CO2 concentration in closed space. Activated carbon is extensively used in CO2 adsorption due to its economic cost, high adsorption capacity and easy availability. This study presents an experimentally validated numerical approach to demonstrate the removal of CO2 using activated carbon through cyclic adsorption and desorption processes. The numerical model simulates the heat and mass transfer phenomena in the porous media to determine the maximum CO2 removal rate from solid activated carbon with varying operating parameters. The effective cycle time for maximum CO2 removal is determined based on CO2 removal per hour through a scrubber in daily application. The CO2 removal rate can be maximized in a cyclic adsorption-desorption process by operating in a partial capacity mode (based on the adsorption kinetics and uptake curve), instead of trying to utilize the nearly full adsorption-desorption capacity of the bed in each cycle. Increasing the thickness of activated carbon leads to greater CO2 removal per cycle with increasing pressure drop, while it also prolongs the adsorption and desorption cycle time.

Biomass combustion power generation has received significant attention as it is a carbon-neutral fuel. Countries with coal as the primary resources for power generation are adopting means of co-firing locally available biomass with coal to reduce carbon emissions. Depending upon the source of biomass particles, there are unique challenges associated with the combustion of biomass particles. However, emissions of alkali metals, chlorine, and sulfur-based gases during biomass combustion poses serious challenges in terms of the operability of power plants. In this work experimental investigations have been carried out to study the effect of co-firing different blends of high-ash content coal with biomass on sodium (Na) emissions in oxyfuel and non-oxyfuel environments. Two types of biomass have been studied: beechwood, a woody type of biomass, and paddy straw, an agroresidue-based biomass. Experiments on pellets composed of different blends of biomass and high-ash content coal have been conducted in an environment maintained at approximately 1110 K and 30% O2/N2/H2O or 30% O2/CO2/H2O. Temporal emission of Na has been measured quantitatively using the laser-induced breakdown spectroscopy technique. The effect of blending high-ash content coal with biomass, along with the impact of an oxyfuel environment in reducing Na emissions has been studied. A profound reduction in emissions of Na was found by blending high-ash content coal with biomass. High ash content coal was more effective in reducing Na emissions from coal-beechwood blends when compared with coal-paddy straw blends. Additionally, replacing N2 with CO2 in the combustion environment further reduces Na emissions from the coal-biomass blend pellets. As the concentration of high ash coal increased in the blend, the effective reduction in Na emissions due to CO2 was observed to decrease. The effect of grain size used to make the fuel pellets of different blending ratios of paddy straw and high-ash content coal has also been explored. A finer grain size of paddy straw in the blend was more effective in reducing Na emissions. A mathematical model has been developed to identify the dependence of peak Na emission on different parameters during the combustion of coal-biomass blended pellets of different blending ratios of biomass and high-ash content coal.

In this work, we investigate the effect of transverse acoustic excitation on a partially premixed swirling flame. Azimuthal instabilities in annular gas turbine combustors are one of the major challenges. Thermoacoustic instabilities are driven by azimuthal acoustic modes. We have designed and fabricated a multi-nozzle linear array combustor to simulate the flow conditions of an annular combustor. The nozzle features an axial swirler with a centerbody acting as a fuel injector. Simultaneous high-speed chemiluminescence (OH*) and high-frequency pressure measurements are conducted to measure the time-averaged flame shape and the chamber acoustics respectively. These measurements are taken at multiple forcing frequencies and amplitude under in-phase (IP) and out-of-phase (OP) conditions. It is observed that, at one of the combustor modes (580Hz), during OP forcing, the flame transitions from a V-shape to an M-shape as the forcing amplitude is increased. However, with IP forcing the flame continues to remain in V-shape with the increase in forcing amplitude. Spectral proper orthogonal decomposition (SPOD) analysis of OH* images show that for OP forcing dominant mode shapes are present mostly at 0.5D away from the center-line, but for IP forcing the dominant mode shapes indicate combined axial and radial oscillations in the OH* intensity field. Furthermore, cross-wavelet transform (XWT) analysis shows a strong correlation between pressure fluctuations and OH* fluctuations around the forcing frequency for IP forcing, and a weak or intermittent correlation for OP forcing. Based on the acoustic simulation, instantaneous time snapshots of OH* chemiluminescence and existing literature, we hypothesize that the transition from V-shape to M-shape can be due to acoustically induced velocity and vortical fluctuations near burner lip during OP forcing. This causes the V-flame to stretch, roll outward, and stabilize in the outer recirculation zone (ORZ).

A novel high-shear injector developed at the National Centre for Combustion Research and Development (NCCRD) is further investigated in the current study. The effect of flare angle on reacting and non-reacting flow field and stabilization of the flame is explored using high-speed OH* chemiluminescence, high-speed stereo particle image velocimetry and simultaneous pressure fluctuation measurements. Flare is the diverging part attached at the end of the injector and the angle of the diverging section is called the flare angle (beta). The flare angle is varied in three steps, 0 degrees, 30 degrees, 50 degrees. For each case of flare, 4 cases of inlet Reynolds's number (Re) and 8 cases of equivalence ratio(phi) totalling 32 cases are explored to elucidate the effect of beta on flame size and rich and lean blow-off limits. Central recirculation zone (CRZ) which plays a pivotal role in the stabilization of the flame is found to be altered with a change in beta. With an increase in beta, the CRZ size increases in non-reacting and reacting cases. For beta = 0 degrees, the effect of heat addition and the effect of increasing phi on CRZ size is very minimal. There is a slight change noticed in CRZ size with increasing phi for beta = 30 degrees case. The effect of increasing beta on CRZ size is found to be very pronounced for the beta = 50 degrees case. Non-reacting flow field provides insights into the CRZ formation mechanism, which also changes for different beta. Due to the counter-rotating nature of the primary and secondary swirling air, it is possible to qualitatively distinguish among them using the azimuthal velocity component. This distinction provides insights into the mixing distance and its variation with beta. Lean blow-off limit deteriorates with an increase in beta, and this is delineated using non-reacting flow field data. Reacting flow-field provides valuable insights into the effect of heat addition and the effect of phi on flow field at various beta. Rich blow-off limit improves with an increase in beta, which can be explained using reacting flow field data. Further, the dynamics of the flame and pressure data are reported indicating the presence of thermo-acoustic instability.

Nanosatellites are important for carrying out short-term and cost-effective communication and surveillance missions. Their small size necessitates the need for propulsion systems that are lightweight, compact, and capable of delivering accurate reaction and attitude control while allowing for seamless integration with the satellite. This paper reports on a numerical analysis to determine the performance of a micro-electromechanical system (MEMS)-based vaporizing liquid microthruster that utilizes microtextured substrates for passive feeding of the propellant (water) using capillary force and subsequent thin film evaporation by localized heating. The generated vapor flows through a converging nozzle to produce thrust. The performance of the propulsion device is evaluated in terms of the mass flow rate, thrust, and specific impulse. The model demonstrates a unique way of integrating the evaporation characteristics at the liquid-vapor interface to real nozzle flow dynamics. The evaporation phenomenon at the liquid-vapor interface is captured by utilizing kinetic theory of the gases, and real nozzle flow is analyzed by considering compressible-slip flow through the converging nozzle. It is shown that the microthruster can generate a thrust of similar to 60 mu N and an specific impulse of similar to 67 s with a power input of approximately 3 W. The thrust and specific impulse efficiencies, when compared to quasi-one-dimensional isentropic values, are determined to range between eta thrust similar to 12 and 40% and between eta ISP similar to 55 and 92%, respectively, for a power input of 0.2-3 W.

Gelled propellants are a propulsion system fuel whose rheological properties are modified by the addition of a gellant, usually a polymeric or colloidal substance, leading to the liquid fuel having a very viscous liquid, or gel-like consistency. Gelled propellants offer many advantages over conventional liquid propellants such as greater control over the energy density, as well as, ease of transportation and storage (like solid propellants). In this work, we study the combustion dynamics of a single polymer-infused fuel droplet, using both experiments and numerical simulation. The droplet is suspended over a flat flame and different stages of combustion are captured. Using the observations from the experimental data, we formulate a theoretical model for the internal dynamics of the polymer-infused fuel droplet till the end of the initial stage. The resulting partial differential equation is solved numerically and is compared against the experimental data.

Oxy-fuel biomass combustion in a circulating fluidized bed is a clean and sustainable biomass utilization technology. Woody forest biomass is considered a promising source of renewable energy because of its widespread availability across the globe. However, effective biomass utilization in thermal power plants is largely limited by severe ash deposition and corrosion of heat transfer surfaces due to alkali metal and chlorine emissions during biomass combustion. In this study, laser-induced breakdown spectroscopy (LIBS) was applied to detect in-situ gas phase sodium (Na) emission during single particle combustion of widely available forest biomass, namely, beech wood. The operating conditions simulate the thermo-chemical conditions relevant to a fluidized bed combustor system, where the operating temperature varies between 1073K–1273K. This work aims to investigate the effect of O2 concentration, CO2 (oxyfuel environment), and particle size on the emission of sodium during the combustion of a single beech wood particle. The unresolved sodium doublet was used as the LIBS signal to quantify sodium emission concentration.Oxygen concentration is the most critical parameter affecting Na emissions, and an increase in O2 concentration led to higher Na emissions. It is observed that there is a significant reduction in the emission of Na in an O2/CO2 environment as compared to an O2/N2 environment. It is hypothesized that this drop in Na emissions in O2/CO2 environment could be due to lower particle temperatures caused by the lower diffusivity of O2 in CO2 as well as the endothermic gasification reaction in high CO2 environments. In addition, it is reported that the dissociation of sodium carboxylates and carbonates in the biomass matrix to release gaseous Na due to thermal decomposition is retarded at higher CO2 concentrations which could explain the drop in Na emissions. The effect of biomass particle size is pronounced mainly at lower concentration of O2 where, larger particles keep Na trapped in its matrix, however, at higher O2 concentrations (30%), the effect of particle size on Na emission is negligible.

This paper discusses the effect of nitrogen (N2) and water vapor (H2O) dilution in oxidizer stream on the flame structure, and soot production and extinction of laminar ethylene (C2H4)-fueled co-flow diffusion flame (CFDF). The total oxidizer stream flow rate is kept constant while varying the flow rates of air, N2, and H2O to achieve the desired oxygen concentration called oxygen index (OI). As OI reduces, the flame starts to lift off from the burner surface and shows upward and downward oscillations before extinction through blow-off. The OI at which the flame blows off is called the limited oxygen index (LOI), and it depends on burner configuration, fuel, and oxidizer flow conditions. LOI value for C2H4-fueled laminar CFDF were found to be 12.4% and 14% when the diluent was N2 and H2O, respectively. The experiment and simulation results are presented from 21% O2 concentration in oxidizer to very low oxygen concentration in oxidizer close to extinction. Through quantitative measurements of soot volume fraction (SVF), the result shows a notable reduction in the peak concentrations of SVF as the OI decreases. This decrease is more pronounced with H2O than N2 dilution. One-dimensional (1D) opposed flow diffusion flame simulations are performed to understand various effects of the addition of diluents like H2O and N2. It is observed that in addition to inert, thermal and diffusion effect of H2O in reduction of flame temperature and temperature-dependent production rate of key species, H2O also affects flame chemistry through OH + H2 = H + H2O, 2OH = O + H2O. As a result there is a further drop in the concentration of active radicals (like H, O) which leads to quicker soot inhibition and flame extinction.

In this study, we discuss the flow and heat transfer instabilities that develop during the heating of n-heptane in a 3 x 3 mm helical regenerative cooling channel. The experiments are conducted for given pressure conditions and mass flow rates for two different flow directions, one is vertically downward and the other one is horizontal flow. The results show oscillation in the static pressure, and mass flow rate near the critical point for both the flow configuration at lower mass flux. The instabilities in the case of horizontal configuration shift towards the higher temperature value compared to the vertically downward flow configuration.

We propose a geometrical mechanism that generates an augmented swirling and round jet for impingement heat transfer from a heated flat plate which is predicted using 3-D RANS numerical simulations.Important parameters such as jet-plate distance (H/D), Reynolds number (Re) [1,2] and the split ratio (SR) which defines the percentage of flow through the axial and tangential ports each resulting in a single augmented jet (swirling and round jets) of diameter D = 30mm.Also, Numerical simulations for the conventional round jets and swirling jets [3] generated by the geometrical vane-swirler (at three different vane angles = 45 0 , 60 0 , and 30 0 ) each of jet diameter D = 30 mm is performed for the Reynolds number (Re = 6000 -15,000) and at a jet-plate distance (H = 1.5D -4D).A comparative study of their impingement heat transfer characteristics is studied with the proposed augmented jet.It is inferred that at a smaller jet-plate distance H =1.5D or H/D =1.5, the proposed augmented jet and vane swirler jets showed an improved heat transfer from the impingement surface (heated flat plate).The conventional round jets showed maximum heat transfer at H = 4D.From the numerical analysis, for the proposed augmented jet, at an optimized jet-plate distance H=1.5D and split ratio (SR-4), the average Nusselt number (Nu avg) is enhanced by 88% than the conventional round jet and 101% than the vane-swirler jet counterpart.Similarly, an enhancement in the stagnation Nusselt number (Nu stg) of 189% than the round jet is predicted for the proposed augmented jet at SR-4.

Jet impingement cooling techniques using conventional nozzles, round jets, and swirling jets are effective heat transfer methods extensively used for various industrial applications. This work proposes a geometrical mechanism that generates augmented swirling and round jets for enhanced heat transfer. The proposed geometry has an axial inlet port and three tangential inlet ports. The total flow is partitioned through these inlet ports using different split ratios (SR), resulting in a single augmented flow comprising swirling and round jets. The impingement heat transfer from a heated flat plate is modelled using steady 3-D RANS based numerical simulations. Also, computations for conventional round jets and swirling jets generated by an in-house developed vane-swirler are performed for the Reynolds number range of 6000-15,000, and for a range of jet-plate distance H = 1.5-4 times the jet diameter D. The optimum conditions for maximum heat transfer for various configurations of jets are presented. From this numerical study, the proposed augmented jet is found to enhance the impingement heat transfer rate at an optimal condition of split ratio (SR-4) and dimensionless jet-plate distance (H/D = 1.5) compared to conventional round jets as well as swirl jets generated by vane swirlers.

In this study, the effect of hydrodynamic cavitation has been realized as a potential application towards in-situ cracking into smaller hydrocarbons and reactive radicals. The cracked composition can potentially improve stability of flame in supersonic combustors, that could eliminate the use of flame holding devices or fuel additives. The energy and momentum transfer across coherent structures are seen as the regions of maximum chemical transformations and cracking to take place, which has been validated through the reported experimental studies. In order to validate the formation of large coherent structures the flow was studied by high-speed shadowgraph and acoustic measurements. The experiments have been conducted on the hydrocarbon fuels, heptane and decane, to better understand physics of turbulence induced by cavitation on fuel cracking and its impact on ignition delay.

This paper summarizes the design and development of a unique optically accessible sector of a full-scale annular gas turbine combustor. The distinctive feature of the experimental setup is that it preserves the geometrical details of an annular combustor that includes the casing, dome and combustor liner. The combustor design features a series of primary and secondary dilution holes with multiple film cooling strips on outer and inner liner. The details of the facility requirements and instrumentation and controls are provided in the paper. The methodology employed in the design of the optically accessible combustion chamber is elucidated, including quartz window considerations and thermal management of the experimental hardware under extremely high heat loads. The goal is to obtain the evolution of global parameters such as spray patternation, pattern factor, pressure drop and combustion efficiency through measurement of the local flow-flame interactions using advanced optical diagnostics. Rig contains multiple ports for measurements of temperature and pressure. For current isothermal study, simultaneous pressure and mass flow rate data of the rig is acquired during PIV measurements to ensure the state of the system. 2D two component PIV experiments are conducted to measure the velocity inside the combustor under isothermal conditions. Experiments are conducted at 0.1, 0.3 and 0.5 kg/s of inlet flow rate and time averaged flow structures inside the combustor are reported. The major structure of flow such as central recirculation zone (CRZ) and swirl jet deflection angle for the respective cases are elucidated here.