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Lighter-than-air (LTA) aerial vehicles such as airships and aerostats can be found in various strategic and commercial applications, primarily due to their capability to hover and stealth. The mathematical model of these vehicles helps in understanding their complex dynamics and designing and developing proper stabilisation systems for them. Stability derivatives have been used for developing mathematical models for heavier-than-air aerial vehicles since their introduction. This paper presents a methodology to develop a mathematical model of an aerostat based on stability derivatives. One of the major contributions of this study is the estimation of aerostat's added mass terms expressed as longitudinal stability derivatives due to acceleration of the longitudinal motion variables. A longitudinally decoupled linear mathematical model of a single-tethered aerostat using stability derivatives is investigated in this study. A computational fluid dynamics (CFD)-based analysis of the 3D model of the vehicle is used to obtain the stability derivatives. The methodology presented considers the aerostat and tether models separately before coupling them to create the full model. The stability derivative analysis is carried out using ANSYS Fluent, and the coupled tethered aerostat model is investigated using MATLAB 2020. The negative pitch angle of the aerostat is caused by the selection of the pitching centre as the aerostat centre of volume instead of the tether confluence point. The tension force on the tether, which is proportional to the wind velocity, and aerostat velocity components are found to be stabilised within 200–400 s. [ABSTRACT FROM AUTHOR]

An investigation into the non-premixed combustion characteristics of methane in a planar micro-combustor with a splitter was performed. The impact of blending methane with hydrogen on these characteristics was also analyzed. Additionally, the effects of inlet velocity and global equivalence ratio on flame location, flame temperature, combustion efficiency and outer wall temperature were studied for three different fuel compositions: pure methane (MH0), 60% methane with 40% hydrogen (MH40), and 40% methane with 60% hydrogen (MH60)). A heat recirculation analysis of the combustor wall was conducted to determine the amount of heat recirculated into the unburnt gas at various inlet velocities for all three fuel compositions. The results demonstrated that the stability limit of methane in terms of inlet velocity (1–2 m/s) and global equivalence ratio (1.0–1.2) was significantly enhanced to 1–3 m/s and 0.8–1.2, respectively, with the addition of hydrogen. At an inlet velocity of 2 m/s, the flame location of 3.6 mm for MH0 was significantly improved to 2.2 mm for MH60. Additionally, outer wall temperature exhibited a rise of 100 K for MH60 compared to MH0. Furthermore, from heat recirculation analysis, when the ratio of heat recirculated to heat loss exceeded unity, the flame started exhibiting the lift-off phenomenon for all the fuel compositions. [ABSTRACT FROM AUTHOR]

Gaseous mixtures ignite when they come in contact with a surface with the ignition threshold temperature of the mixture. The hot surface ignition of fuels like hydrogen poses a safety threat due to its wide flammability range and high diffusivity. The present study numerically investigates hot surface ignition of H 2 -air and CH 4 /H 2 /air mixtures using detailed chemistry. The effects of the equivalence ratio (φ), composition and heating rate on the ignition threshold temperature of the mixture are studied. At rich mixture conditions, the ignition occurs at the side of the heated surface (glow plug). On the other hand, the lean mixtures ignite at the top of the heated surface. The variations in heating rates significantly affect the ignition temperature of rich mixtures. In contrast, the ignition temperature of the lean mixture does not show any considerable deviation concerning the variation in heating rates. The phenomenon of puffing flame is observed at lean extinction limits of H 2 -air mixtures. Lower ignition thresholds are observed in CH 4 /H 2 /air mixtures with high concentrations of H 2. Mixtures having higher CH 4 concentrations show higher ignition thresholds. The role of local equivalence ratio and heat release rate on the ignition characteristics has been studied to predict the behaviour of ignition kernel at various mixture conditions. • Hot surface ignition of H 2 -air and CH 4 –H 2 -air mixtures were investigated. • For rich mixtures, ignition starts at the side of the heated surface. • At lean mixtures condition, ignition starts at the top of the heated surface. • Heating rate has significant effect on the ignition temperature of the rich mixture. [ABSTRACT FROM AUTHOR]

Hot surface ignition of combustible gas mixtures poses a safety threat in many engineering applications. Gaseous mixtures ignite when hot surface temperature reaches the ignition threshold. In the present work, two-dimensional numerical simulations with detailed reaction mechanism are performed to simulate the flow of stoichiometric hydrogen-air mixture over a stationary hot wire. The effect of heating rates, wire diameters, mixture inlet velocities, and mixture equivalence ratios on the ignition threshold is investigated. In all the cases investigated, ignition is found to occur at the rear stagnation point, and wire heating rate did not influence the ignition phenomenon significantly. With an increase in mixture inlet velocity and mixture equivalence ratio, the ignition threshold increases, whereas the threshold has been found to decrease with increasing wire diameters. The role of the local equivalence ratios at the ignition point and reaction rates prior to the ignition process has been studied to help better understand the ignition phenomenon under different conditions. • Wire heating rate does not influence the ignition threshold significantly. • The ignition threshold values increased with an increase in mixture inlet velocity. • With an increase in wire diameter, the ignition threshold value decreases. • At lower velocities in the range of 0.1 m/s, buoyancy forces also affect the flow. [ABSTRACT FROM AUTHOR]

Radiative furnaces pose significant thermal inertia and single impinging flames have been observed to cause occurrence of hotspots on the target surface. Multiple burners arranged in suitable array configuration represent one of the plausible solutions for more uniform heat transfer. In this study, computational fluid dynamics (CFD) simulations have been carried out for multiple swirling impinging flames arranged in a hexagonal array configuration. The turbulence chemistry interactions in the flame field are solved numerically using renormalization group (RNG) based k-e/eddy dissipation model (EDM) framework. Comparison of co-and-counter-swirling configurations has been studied for interactions and spent gas release mechanism. Multiple swirling impinging flames undergo strong interactions resulting in distortions of recirculation zones (RCZ) for all the surrounding except central flame. Co-swirling flames result in development of higher turbulence in the interaction regions as compared to counter-swirl case. Results indicate that some flames in counter-swirl case are underutilized due to the fluid dynamics developed in the system and co-swirling hexagonal array configuration is a better arrangement for effective heating of target surface. Effect of interjet spacing (S/Dh=5, 7, and 9) and separation distance (H/Dh=3, 5, 7, and 9) studied for co-swirl case revealed that peak heat fluxes decreased with increasing interjet spacing and separation distance. Central flame represented a region of low heat flux and this region has been noticed to expand in size for increasing interjet spacings. Suppression of central flame has been observed to be maximum for minimum separation distance. [ABSTRACT FROM AUTHOR]

Radiative furnaces pose significant thermal inertia and single impinging flames have been observed to cause occurrence of hotspots on the target surface. Multiple burners arranged in suitable array configuration represent one of the plausible solutions for more uniform heat transfer. In this study, computational fluid dynamics (CFD) simulations have been carried out for multiple swirling impinging flames arranged in a hexagonal array configuration. The turbulence chemistry interactions in the flame field are solved numerically using renormalization group (RNG) based k-e/eddy dissipation model (EDM) framework. Comparison of co-and-counter-swirling configurations has been studied for interactions and spent gas release mechanism. Multiple swirling impinging flames undergo strong interactions resulting in distortions of recirculation zones (RCZ) for all the surrounding except central flame. Co-swirling flames result in development of higher turbulence in the interaction regions as compared to counter-swirl case. Results indicate that some flames in counter-swirl case are underutilized due to the fluid dynamics developed in the system and co-swirling hexagonal array configuration is a better arrangement for effective heating of target surface. Effect of interjet spacing (S/Dh=5, 7, and 9) and separation distance (H/Dh=3, 5, 7, and 9) studied for co-swirl case revealed that peak heat fluxes decreased with increasing interjet spacing and separation distance. Central flame represented a region of low heat flux and this region has been noticed to expand in size for increasing interjet spacings. Suppression of central flame has been observed to be maximum for minimum separation distance. [ABSTRACT FROM AUTHOR]

Detailed flow field and heat transfer characteristics developed in a row of three co-rotating dual swirling impinging flames have been investigated experimentally as well as numerically. Impingement heat transfer and pressure distribution has been studied experimentally at different separation distances and inter-jet spacings. Inverse heat conduction procedure (IHCP) has been used for estimating heat fluxes on the front side of impingement plate. Turbulence induced mixing results in strong interactions amongst adjacent flames causing deflections of inner flames of the burners situated at sides of central burner. Numerical simulation predicted formation of asymmetric recirculation zones for side flames. Symmetric interactions taking place for central flame produced two equal recirculation lobes for central flame. Behavior of central inner flame has been observed to be dependent on the value of inter-jet spacing used. Suppression of central inner flame tend to occur at higher inter-jet spacings due to recirculating products. Impingement pressure distribution is observed to be consistent with the heat flux distribution. Averaged heat fluxes registered at the impingement plate due to the central flame are higher in magnitude to those pertaining to each of the side flames. [ABSTRACT FROM AUTHOR]

Flame jet impingement has been studied as an indispensable technique for direct heating applications suitable for metal processing industries. The current experimental and numerical investigation pursues to unfold the fluid dynamics and heat transfer characteristics for an obliquely impinging co-axial swirling flame jet. Co-axial flame used in the present work contains outer swirling flame together with an inner non-swirling flame. Steady state heat balance technique using one dimensional energy balance across the impingement plate has been employed to determine the local heat flux distribution. Due to obliquely impinging swirling jet, development of an asymmetric thermal boundary layer takes place on the target surface. The geometry being studied here develops a complex flow field which has also been investigated numerically using RANS based 'k-e' RNG turbulence model. The effect of various operating parameters such as inclination (θ = 0°, 15°, 30° and 45°), impingement distance (Z/D h = 3, 5, 7 and 9), Reynolds number (Re(s) = 2500, 4000, 5500, 7000 and 8500) and equivalence ratio (ϕ = 0.9, 1.0, 1.1 and 1.2) have been studied on the flow field development and resulting heat transfer distribution. • Obliquely impinging co-axial swirling flame jet with inner non-swirling flame. • Asymmetric heat flux patterns have been observed for inclined impingement plate. • Deflection of inner flame does not only depend on the plate inclination. • Deflection of inner flame is a stronger function of co-axial swirling flame behavior. • Structure of recirculation zone depends on inclination angle of plate. • Asymmetric recirculation lobes have been noticed for co-axial swirling flame. [ABSTRACT FROM AUTHOR]

The present work numerically investigated the effect of variation in the inlet Mach number and stagnation temperature on the mixing of fuel with the oxidizer and the subsequent stabilization of a flame in a combustor at supersonic conditions. Dimensions of the studied combustor were taken from literature. It had a 10° wedge located at the top wall of the combustor. The combustor was modeled and analyzed using ANSYS FLUENT software. Three-dimensional, compressible, reacting flow calculations with a detailed chemistry model were performed. Turbulence was modeled using SST k-ω model. Necessary grid refinement was done to capture the incident oblique shock formed at the 10° wedge. Hydrogen was injected through the fuel inlet port. The computations were performed for Mach numbers of 2.0, 2.5 and 3.0 at the combustor inlet for a combustion inlet stagnation temperature of 1500 K. Later, the combustor inlet Mach number was kept constant at 2.5 and the combustor inlet stagnation temperature was varied as follows: 1500 K, 1700 K, and 1900 K. The results indicated that as the combustor inlet Mach number increased, the location of incidence of the oblique shock shifted to the downstream of the fuel inlet and it resulted in the better mixing of the fuel with cross flow stream of air and led to better degree of combustion of hydrogen. The contours of mole fraction of OH radical and hydrogen also corroborated the improvement in the mixing of fuel with the cross flow air and the subsequent flame stabilization at higher Mach numbers. The flow pattern, mixing of fuel with air and flame stabilization was not affected significantly till 1700 K whereas for 1900 K, combustion of hydrogen was more uniform. [ABSTRACT FROM AUTHOR]

The present experimental study reports the effect of CO 2 /N 2 dilution on laminar burning velocity of premixed LPG (liquid-petroleum-gas)-air mixtures at elevated temperatures using a preheated mesoscale diverging channel technique. The experiments were carried out for a range of equivalence ratios varying from 0.8 < Φ < 1.3 with percentage dilution of the fuel component by volume (β) for CO 2 varying from 10% < β < 30% and N 2 varying from 10% < β < 40%. A power–law correlation has been obtained for the present experimental data as a function of percentage dilution, mixture temperature and equivalence ratio. It has been observed that an increase in dilution with CO 2 /N 2 leads to an increase in temperature exponent (α). The increase in temperature exponent due to CO 2 dilution is more pronounced as compared to N 2 dilution case. A flame structure study has been carried out to understand the effect of mixture temperature and diluent using USC (University of Southern California) Mech II reaction mechanism. [ABSTRACT FROM AUTHOR]

Meeting future world energy needs while addressing climatic changes has led to greater strain on conventional power sources. One of the viable sustainable energy sources is wind. But the installation large scale wind farms has a potential impact on the climatic conditions, hence a decentralized small scale wind turbines is a sustainable option. This paper presents review of on different types of small scale wind turbines i.e., horizontal axis and vertical axis wind turbines. The performance, blade design, control and manufacturing of horizontal axis wind turbines were reviewed. Vertical axis wind turbines were categorized based on experimental and numerical studies. Also, the positioning of wind turbines and aero-acoustic aspects were presented. Additionally, lessons learnt from various studies/countries on actual installation of small wind turbines were presented. [ABSTRACT FROM AUTHOR]

Experimental and numerical study of linear arrays of co-axial flames impinging on a flat quartz plate have been performed and interactions and heat transfer characteristics are analyzed. High resolution inverse heat conduction procedure (IHCP) has been employed for determining local heat flux distribution. Strong interactions developed amidst adjacent flames led to asymmetric interactions. Combustion simulations revealed differences in the flow structures of four and five element arrays. Asymmetric interactions resulted in bending of all of the inner flames in four element array whereas central inner flame remains undeflected in five element array. Same has also been observed for six and seven element arrays too. Tilted columnar flows have been identified in even as well as odd numbered linear array of co-axial flames. These tilted flow structures resulted in distortion of recirculation zones (RCZ) of all the flames. Impingement heat flux contours are studied for various impingement spacings (H/D h = 3, 5 and 7). At smallest impingement spacing, these contour maps revealed footprints corresponding to central inner flame due to symmetrical interactions. Thermal efficiency and heat transfer uniformity of four element array has been observed to be higher than five element array at all impingement spacings. [ABSTRACT FROM AUTHOR]

An analysis of the effect of wind gusts on the aerodynamic characteristics of an aerostat is presented in this study. A three dimensional computational fluid dynamics based flow analysis of an aerostat using the realizable k − ϵ model is presented. The performance of the aerostat is analyzed by varying the angle of attack, gust amplitude, gust duration, gust velocity profile and the aerostat shape. Actual wind data is used to formulate the gust parameters used for the study. It is found that the aerostat drag is unaffected by the angle of attack because of the streamlined structure. A trade-off between the drag and lift values suggests the use of a slight positive angle of attack for the stable operation of the aerostat. A gust with a short rising or falling time with small amplitude is severe than the symmetric gust with larger amplitude and a slower rising or falling profile. A scaling methodology for the aerodynamic characteristics of the aerostat for different angles of attack, gust amplitudes, gust duration and aerostat shapes is presented. The scaling obtained for different angles of attack and gust amplitudes is linear, whereas for different gust duration the scaling is not linear. [ABSTRACT FROM AUTHOR]

• Interactions result in clustering of high heat flux regions. • Flow circulation observed along the inner periphery of annulus. • Significant thermal dilution due to entrainment along the outer periphery of annulus. • Asymmetric interactions under impinging conditions deflected the inner flames. • Optimum performance observed at S/D h of 4 and H/D h of 6. Performance of gas burners can be optimized by improvements in their design and proper selection of operating parameters associated with combustion and heat transfer mechanisms. Array of multiple swirl burners is one such configuration where uniformity in heat transfer can be increased significantly. The current study presents experimental and numerical investigations conducted for flow patterns and impingement heat transfer developed in an annular arrangement of eight co-rotating dual swirling impinging flames. Impingement heat flux distribution has been studied experimentally using analytical inverse heat conduction procedure (IHCP). Effect of change in separation distance and inter-jet spacing has been studied for interactions and subsequent impingement heat transfer characteristics. Turbulent co-swirling flames portray intense mixing developed at the interaction regions due to strong interactions. Presence of impingement plate causes large scale changes in the flow-field. Numerical simulation conducted under reacting conditions predicted formation of asymmetric and distorted recirculation zones. Inner flames have been observed to deflect from their straight upright positions. Adjacent co-swirling flows merge together and develop circulation of flow at the inner periphery of the annulus. High heat fluxes observed at the target surface are corresponding to interaction regions and tend to cluster around the geometric centre of the array. Reversed downward flow developed at the geometric centre represents region devoid of heating. Averaged heat fluxes registered at the impingement plate decreased with increasing inter-jet spacing and separation distance. Maximum uniformity in the heat transfer has been observed at inter-jet spacing, S/D h of 4 and separation distance, H/D h of 6. [ABSTRACT FROM AUTHOR]

An experimental and numerical study on the combined effect of N 2 /CO 2 dilution and hydrogen addition on the laminar burning velocity (LBV) of methane-oxygen mixtures was conducted. The experiments were performed at atmospheric conditions using the heat flux method for effective equivalence ratios (ϕ F) varying from 0.7 to 1.3. The results show that the hydrogen addition causes an increase in LBV for all the mixture conditions. The variation in LBV based on hydrogen addition parameter (R H) for all N 2 dilution conditions were following a linearly increasing trend. The strong effect of hydrogen addition on LBV is observed at lean and rich mixtures compared to that at near stoichiometric mixture conditions. The experimental results show that the percentage variation in LBV with R H at rich mixture is more substantial at 75% N 2 dilution compared to that at 65% N 2 dilution. • Effect of N 2 /CO 2 dilution and hydrogen addition on LBV of methane-oxygen mixtures was studied. • Mixture diluent approach is used to study the diluent effect by isolating the diluent. • Strong effect of hydrogen addition on LBV is observed at lean and rich mixtures. • R H effect on LBV is less significant at 75% N 2, ϕ F = 1. • LBV increases linearly with R H for all N 2 dilution mixture conditions. [ABSTRACT FROM AUTHOR]

As the demand for powerful, light energy sources continues to grow, traditional electrochemical batteries are no longer sufficient and combustion-based power generation devices have become an attractive alternative due to their high energy density, compact size, fast recharging time and long service life. While most research on miniature-scale combustors has focused on gaseous fuels, the use of commonly available liquid fuels has the potential to be highly portable and economical. However, the complexity of droplet atomization, evaporation, mixing and burning in a limited volume and short residence time has presented significant challenges for researchers. This review focuses on various methodologies proposed by researchers (like flow burring injector, fuel film injection, injecting into porous media, electrospray and some self-aspirating designs) to overcome these challenges, the combustion behaviour and different instabilities associated with liquid fuels at small scales. The current review intends to present a clear direction to channel the efforts made by researchers to overcome the difficulties associated with liquid fuel combustion at small scales for power generation applications. Additionally, this review aims to give an overview of power systems at the micro and meso scales that operate using liquid fuels. The methodologies introduced like electrospray requires external power, which again makes the system complex. Towards the development of standalone type power generators, the self-aspirating design which makes use of hydrostatic pressure, fuel film injection or taking advantage of exhaust gas enthalpy to preheat and evaporate the liquid fuel are the promising methodologies. [ABSTRACT FROM AUTHOR]

Vertical Axis Wind Turbines (VAWTs) have proven to be suitable for changing wind conditions, particularly in urban settings. In this paper, a 2D URANS (Unsteady Reynolds-Averaged Navier Stokes) numerical analysis is employed for an H-Darrieus VAWT. A turbulent domain is created through systemically randomising the inlet velocity to create macro-turbulence in front of the VAWT. The parameters for spatial and temporal randomisation of velocity and its effects on the turbine performance are studied for a mean free stream velocity, U∞ = 10 m/s, and a tip speed ratio (TSR) of 4.1. The mean Coefficient of power (Cp) for randomised fluctuation of 2 m/s and half-cycle randomisation update frequency is 0.411 and for uniform inlet velocity is 0.400. The Cp vs. Tip Speed ratio plot suggests that the optimal tip speed ratio for operation is around 4.1 for this particular wind turbine of diameter 1 m, chord 0.06 m, and NACA 0018 airfoils. The effect of randomisation for tip speed ratio λ = 2.5, 3.3, 4.1, and 5.3 on the performance of the turbine is studied. Turbine wake recovers at a faster rate for macro-turbulent conditions and is symmetric when compared to wake generated by uniform velocity inlet. The maximum velocity deficit for a distance behind the turbine, x/d = 8 at TSR (λ) = 4.1 is 46% for randomised inlet and 64% for uniform inlet. The effect of randomisation for λ = 2.5 to 5.3 on the performance of the turbine is analysed. A time-varying gust based on International Electrotechnical Commission (IEC) Extreme Operating Gust is used to study the effect of fluctuating wind conditions in a turbulent environment. Since real-time conditions often exceed gust factors mentioned by IEC, winds with large gust factors such as 1.50, 1.64, and 1.80 are analysed. With an increase in gust amplitude, Ugust = 6 m/s to Ugust = 12 m/s on a free stream velocity of U∞ = 10 m/s, the mean Cp decreases from 0.41 to 0.35 since the wind turbine operates under tip speed ratios outside optimal range due to large fluctuations in incoming velocity. [ABSTRACT FROM AUTHOR]

Film cooling as applied to rocket nozzles is analyzed numerically with emphasis on the assessment of the effect of the mixing of coolant with the hot stream. Cooling performance, as characterized by cooling effectiveness, is studied for three different coolants in the three-dimensional, turbulent flow field of a supersonic convergent-divergent nozzle operating with a hot stream temperature of 2500 K over a range of blowing ratios. The coolant stream is injected tangentially into the mainstream using a diffuser-type injector. Parameters influencing the effectiveness, such as coolant injector configuration and mixing layer, are analyzed. Thermal and species mixing between the coolant and the mainstream are investigated with regard to their impact on cooling effectiveness. The results obtained provide insight into the film cooling performance of the gases and the heat transfer characteristics associated with these three gases. An injector taper angle of 30° results in the most effective cooling among the configurations considered (0°, 15°, 30° and 45°). Mixing of the coolant with the hot stream is examined based on the distributions of velocity, temperature and species. The higher values of cooling effectiveness for Helium are attributed to its thermophysical properties and the reduced rate of mixing with the hot stream. The results further indicate that through optimization of the blowing ratio and the coolant injector configuration, the film cooling effectiveness can be substantially improved. [ABSTRACT FROM AUTHOR]

In this study, numerical simulations have been carried out to analyze the effect of convective heat transfer on flow separation occurring in a DLP-PAR nozzle. Heat transfer coefficient (0, 200 and 1000 w/m2K) was applied to the nozzle wall to incorporate the cooling effect for different gas inlet temperatures ranging from 1000 to 1500 K. The impact of the cooling effect was analyzed based on nozzle wall temperature and wall static pressure. The wall static pressure distribution also characterizes movement of the separation point. For an inlet temperature of 1000 K, a detailed heat transfer study was carried out for four different nozzle pressure ratios (14, 22, 30 and 40). Significant amount of heat transfer was observed for pressure ratio 14, which in turn had an impact on flow separation. The wall cooling resulted in a shift of the point of separation towards the nozzle exit. For the nozzle pressure ratio of 14, this shift was by about 8.8%, indicating that the flow separation can be delayed by way of cooling for the considered inlet temperature. For higher inlet temperatures, the effect of heat transfer on flow separation seems to be negligible. The current study concludes that the separation point can be controlled by convective cooling for inlet gas temperatures below 1500 K so that the optimal performance of the nozzle can be achieved. [ABSTRACT FROM AUTHOR]

Numerical investigations of steady, laminar premixed Syngas-air flames are presented in this paper. Three-dimensional simulations were performed to examine the impact of operating conditions on steady state characteristics of perforated burner flame. A detailed H 2 CO reaction mechanism having 12 species and 38 reactions was used for combustion modelling. The three dimensional simulation results are validated against the 1D flat flame result using PREMIX. Effects of inlet velocity, fuel composition and equivalence ratio on flame stability were examined. A clearly identified recirculation zone was present above the top surface of the burner plate in the case of 50% H 2 -50% CO Syngas mixture. The strength recirculation zone was diminishing with the increase in percentage of hydrogen in the Syngas mixture, and the flame has stabilized closer to the top surface of the burner plate. The flame stand-off distance is found to decrease with increase in inlet velocity. Effect of increase in H 2 fraction in Syngas has less effect on flame height at higher H 2 fractions. [ABSTRACT FROM AUTHOR]

The effect of hydrogen (H2) addition on the flame dynamics of premixed methane/air mixtures in a microchannel was investigated through two-dimensional numerical computations using a detailed chemistry model. Detailed numerical simulations were performed in a 2 mm diameter tube with 120 mm length and a hyperbolic wall temperature gradient condition. All numerical computations were performed at stoichiometric mixture conditions with a fixed inlet velocity of 15 cm/s. Flame repetitive extinction and ignition (FREI) has been observed to appear at various mixture conditions. The frequency of FREI shows a non-monotonic variation for CH4/air mixtures with H2 addition. The time taken for completion of one FREI cycle increases with H2 addition. The maximum temperature and heat of the reaction were observed to decrease with hydrogen addition. The effect of diameter on the FREI cycle was studied by comparing the numerical results for 1, 1.5, and 2 mm diameter tubes. As the diameter is reduced from 2 mm to 1 mm, the FREI frequency increased, and the maximum temperature decreased owing to increased heat loss through channel walls. The location of the ignition and extinction shifted downstream for 1 mm tube, as compared to a 2 mm diameter tube. [ABSTRACT FROM AUTHOR]

Chevron nozzles, which are characterized by the serrations at the nozzle exit, are widely used for suppressing jet noise in aircraft engines. The noise suppression is accomplished by the enhanced mixing of the exhaust streams, which, in turn, is a result of the streamwise vorticity induced by the serrations. The present study focuses on the numerical modeling of the acoustic field in a compressible jet issuing from a chevron nozzle at a Mach number of 0.8. The study evaluates the effectiveness of turbulence modeling approaches of Large Eddy Simulation and Detached Eddy Simulation methods and compares them with the less computationally intensive Unsteady Reynolds Averaged Navier–Stokes (URANS) formulation. The Ffowcs Williams–Hawkings noise model was used to predict the overall sound pressure level in the far field. The LES predictions of the acoustic signature were found to match well with the experimental data, whereas the URANS model grossly underpredicted the sound pressure levels in the compressible jet flow field. [ABSTRACT FROM AUTHOR]

We numerically investigate the buoyancy-induced laminar flow in an adiabatic vertical chimney with straight and converging walls. The study is done with air as the fluid for different radii of the chimney, R = 15–50 mm, aspect ratios, L/R = 5–25, and inlet temperature, T i = 350–550 K. In addition to these parameters, the converging chimney analysis are performed with different converging angles, θ = 1–5 degrees. The commercial CFD code Fluent is used to solve the equations governing the flow taking temperature effects on fluid properties into consideration. The effect of the above parameters on the buoyancy-induced mass flow rate has been presented. We see that the mass flow rate increases with an increase in T i due to increased buoyancy with both straight and converging chimneys. With an increase in L/R , the mass flow rate increases for straight chimney and this is due to the frictional pressure drop characteristics of the chimney. On the other hand, the converging chimney shows a varying trend with L/R depending on the converging angle: with the increase in L/R the mass flow rate initially increases and then decreases for lower θ (<1o) and continuously decreases for higher θ (>1 o). The varying combined effect of the frictional and acceleration pressure drop characteristics of the chimney causes this change in trend. For the given inlet conditions and aspect ratio, the straight chimney would drive the highest mass flow rate. Based on the simulated data, we present correlations to predict the induced mass flow rate. For generality, we present correlations for the non-dimensional mass flow rate in terms of the relevant non-dimensional parameters i.e., the modified Gr number, the temperature ratio, and the area ratio. The correlations we present are applicable over a range that can be useful in designing chimneys for bio-mass gasification, household utilities, and medium-scale industries. [ABSTRACT FROM AUTHOR]

The effect of hydrogen (H 2) addition on the flame dynamics of premixed C 1 –C 4 alkane/air mixtures in a microchannel is investigated using a detailed-chemistry model through two-dimensional numerical computations. A detailed computational study have been performed in a 2 mm diameter tube with 120 mm length and a wall temperature gradient along the axial direction of the channel. The numerical simulations are carried out for various stoichiometric hydrocarbon (HC)/H 2 mixtures at 0.15 m/s mixture inlet velocity. Flame repetitive extinction and ignition (FREI) flame pattern has been identified for all the fuel mixtures at these channel wall and mixture flow conditions. CH 4 /air mixture shows a higher HRR than C 3 –C 4 alkane/air mixtures. Flame residence time in microchannel increases with increase in hydrogen addition percentage for all the three hydrocarbon/air mixtures considered in the present study. A non-monotonic behavior of FREI frequency is identified for CH 4 /air mixture, whereas it decreases monotonically for C 3 H 8 /air and C 4 H 10 /air mixtures with H 2 addition. The amount of HRR and flame propagation velocity decreases with increase in H 2 addition for lower-alkanes/air mixtures. The flame bifurcation effect is observed for CH 4 /air mixture, which disappears due to H 2 addition in the mixture. The bifurcation effect is not present for other hydrocarbon/air mixtures investigated in the present study. The addition of H 2 in the mixture enhances the flame stability of hydrocarbon/air mixtures in the microchannel. • FREI flame pattern is observed for all the HC-H2 mixtures studied. • CH 4 /air mixture shows a higher HRR than C 3 –C 4 alkane/air mixtures. • FREI frequency shows a non-monotonic behavior for CH 4 /air mixture with H 2 addition. • FREI frequency decreases monotonically for C 3 H 8 /air and C 4 H 10 /air mixtures with H 2 addition. • The flame bifurcation effect is observed only for CH 4 /air mixture. [ABSTRACT FROM AUTHOR]

The effect of hydrocarbon addition on tip opening of lean and stoichiometric hydrogen-air flames is studied computationally by performing two-dimensional numerical simulations. The numerical study reveals that the flame tip of the H 2 -air burner stabilized flame is open at lean and stoichiometric mixture conditions. The flame tip closes upon hydrocarbon addition. The tip closing is mainly affected by preferential diffusion of the multi-component mixture and the stretch effects. In the addition of light hydrocarbon (CH 4), the tip closing starts at a higher percentage of hydrocarbon addition in H 2 -air flames. Whereas, upon the addition of heavy hydrocarbons such as propane and butane in H 2 -air flames, tip closing starts with a lesser amount of hydrocarbon addition. Temperature, OH mole fraction and heat release rate have been investigated, focusing on the flame structure at the tip. The tip opening regime diagram for H 2 –CH 4 -air, H 2 –C 3 H 8 -air and H 2 –C 4 H 10 -air mixtures are presented. • Effect of hydrocarbon addition on the tip opening of H 2 -air flames is studied. • Addition of hydrocarbon increases the intensity of burning at the tip. • Tip opening is strongly depended on preferential diffusion for HC-H 2 -air flames. • Tip closing occurs earlier by adding higher hydrocarbons compared to CH 4 addition. • Tip opening regime is presented for lean and stoichiometric HC-H 2 -air flames. [ABSTRACT FROM AUTHOR]

The effect of hydrogen (H 2) addition in the laminar burning velocity (LBV) of n-butane-air at elevated temperatures is described in this paper. For various equivalence ratios (ϕ), ranging from 0.7 to 1.3, LBV was measured for 20%, 40% and 60% H 2 addition to n-butane using a preheated mesoscale diverging channel technique. Using this experimental technique, LBV measurements were conducted for unburnt mixture temperature up to 450 K. The maximum burning velocity has been obtained at equivalence ratio 1.1 for all the mixture conditions. The LBV results at atmospheric condition for n-butane-hydrogen-air mixture were obtained by extrapolating the experimental data at elevated temperatures. "Heat flux method" experimental setup was used for measuring the LBV of n-butane-hydrogen-air mixture at atmospheric condition. The results obtained for LBV at atmospheric conditions with the two different methods at 0%, 20%, 40% and 60% H 2 composition in n-butane were found to be in good agreement. The experimental results of LBV for n-butane were compared with the numerical predictions using USC mech II, Aramco mech 2.0 and LLNL reaction mechanisms. The numerical predictions of LBV using Aramco mech 2.0 shows good agreement with experimental data at rich, lean and stoichiometric mixture conditions. • The effect of H 2 addition in the LBV of n-butane is studied. • LBV at elevated temperatures is measured using diverging channel setup. • Temperature exponent decreases with addition of hydrogen. [ABSTRACT FROM AUTHOR]

Abstract For the past few decades flapping wing aerodynamics has attracted a great deal of research interest from both the aeronautical and biological communities pertaining to the development of MAVs. The objective of this study is to examine and understand the effect of non-dimensional plunge amplitude and reduced frequency on propulsive performance of NACA 4-digit airfoil series and to examine the performance characteristics of square plunge motion and trapezoidal plunge motion. Two dimensional flow simulations around plunging symmetric aerofoils were performed using FLUENT. The simulations were carried out at Reynolds number of 20000 using incompressible laminar, NS solver. The reduced frequency (k) was varied from 0.5–5 and the plunging amplitude (h) was varied from 0.25–1.5. The plunging motions to the aerofoils were provided through UDFs. The effect of variation of k and h on the thrust coefficient (C T), power-input coefficient (C P) and propulsive efficiency (η) is studied. C T value is maximum for square plunge profile for all the airfoils. However, for a given value of h , with the increase in k , C T increases with increasing thickness of the airfoil and reaches a maximum value for airfoil thickness of NACA0018 and then starts decreasing. With varying h and k , it was observed that the propulsive efficiency reached a peak value and the peak shifts to higher h and k with increasing airfoil thickness. From the above study, it was concluded that airfoil thickness played a major part in influencing the thrust generation at low Strouhal number. However, at high Strouhal numbers airfoils showed diverse trends with respect to thrust generation. Sinusoidal plunging motion was more efficient but generated less thrust when compared to square and trapezoidal plunging motions. [ABSTRACT FROM AUTHOR]

This study is focused on the impact of certain important geometric parameters on cooling effectiveness and coolant consumption for effusion cooling of aircraft combustor liner. The three dimensional turbulent flow field in a domain representing the combustor with several rows of effusion coolant injection is considered for the analysis. The geometric parameters considered are: angle of injection of the coolant, axial and transverse pitch of the injection holes, hole spacing and hole diameter. Also, based on the analysis of the temperature field within the chamber, a novel concept of ‘ variable hole diameter ’ has been introduced to reduce coolant consumption. A symmetric 3D computational model including the combustion chamber, coolant chamber and the effusion plate was used for the study. Conjugate heat transfer was modeled between the effusion-cooled wall and the two chambers. A detailed mass flow rate analysis has been performed for the various cases in order to study the impact of parameters on coolant consumption. The proposed approach of using an effusion plate with variable hole diameters is found to be effective in reducing the net coolant consumption significantly while maintaining a given level of cooling effectiveness. [ABSTRACT FROM AUTHOR]

A numerical study on the structure of the laminar premixed flames established on perforated plate burners is performed. Three dimensional numerical simulations are performed on different perforated plate geometries, under various operating conditions, using methane reaction mechanism consisting 36 species and 253 reactions. The comparison of numerical results of inline and staggered configurations of perforated plate burner models show that the flame interaction has significant role in the structure and stabilization of perforated burner flame. The flame structure, flame height and the standoff distances are studied for both inline and staggered configurations at fuel lean and rich conditions. The flame standoff distance and the flame height found to be higher for staggered configuration in comparison to the inline configuration. The flame structure is studied for different hole-to-hole distances. The numerical results show an increase in flame base curvature with an increase in hole-to-hole distance for both inline and staggered configurations. The flame thickness reduces with an increase in hole-to-hole distance for both inline and staggered cases. The effect of recirculation and flame base curvature causes the flame base to get stabilized at a higher distance in staggered configuration in comparison to the inline configuration. [ABSTRACT FROM AUTHOR]

This paper presents a study on the effect of solidity and airfoil profile on the performance of Vertical Axis Wind Turbines (VAWTs). A 1.1 kW commercially viable Darrieus VAWT was studied using ANSYS Fluent. Four different airfoils – NACA 0012, NACA 0015, NACA 0030 and AIR 001 – were considered in the analysis. The tip speed ratios (λ) were varied from 1 to 2.5 with an incoming wind velocity of 10 m/s. It was observed that NACA 0030 performed better at lower values of λ due to long duration of attached flow; while NACA 0012 performed better at λ > 1.8 with a wider range of λ. The shed vortex dissipates much faster for thinner airfoils than for thicker airfoils at higher values of λ. Two bladed VAWTs generated more power than the three bladed turbines. This indicated that turbines with lower solidity perform better at high λ. [ABSTRACT FROM AUTHOR]

Characteristics of microjet hydrogen diffusion flames stabilized near extinction are investigated numerically. Two-dimensional simulations are carried out using a detailed reaction mechanism. The effect of burner wall material, thickness, and thermal radiation on flame characteristics such as flame height and maximum flame temperature are studied. Results show that the flame stabilizes at lower fuel jet velocities for quartz burner than steel or aluminum. Higher flame temperatures are observed for low conductive burners, whereas the flame length increases with an increase in thermal conductivity of the burner. Even though thermal radiation has a minor effect on flame characteristics like flame temperature and flame height, it significantly influences the flame structure for low conductive burner materials. The burner tip and its vicinity are substantially heated for low conductive burners. The effect of burner wall thickness on flame height is significant, whereas it has a more negligible effect on maximum flame temperature. Variation in wall thickness also affects the distribution of H and HO2 radicals in the flame region. Although the variation in wall thickness has the least effect on the overall flame shape and temperature distribution, the structure near the burner port differs. [ABSTRACT FROM AUTHOR]

Syngas is a promising alternative fuel due to clean combustion with lower greenhouse gas emissions. The multi-component fuel mixture primarily consists of hydrogen, carbon monoxide, nitrogen, carbon dioxide and traces of moisture. The composition of syngas strongly depends on the feedstock and the choice of production method. The wide compositional variability of syngas poses hurdles in developing appliances such as burners and combustion chambers. This review summarizes the recent research on syngas' fundamental combustion characteristics, such as laminar and turbulent burning velocity. The burning velocity prediction capabilities of various reaction mechanisms were analyzed. Comparing the laminar burning velocity predictions using different kinetic schemes with available experimental data in the literature establishes the validity of kinetic schemes. A considerable discrepancy is observed between the experimental data and the present numerical predictions for elevated temperatures at different equivalence ratios. The recent developments in syngas burners, stability regimes, and the need for laminar/turbulent burning velocity data at high temperatures and pressure to improve computational modeling of industrial syngas burners are emphasized. The existing research gap in burners to accommodate syngas with the higher mole fraction of hydrogen is also explored. • The article summarizes research in understanding the fundamental combustion characteristics of syngas. • This review highlights the recent developments in syngas burners and stability regimes. • The need for burning velocity data in industrial syngas burner development is discussed. • Existing research gap in using syngas with high H 2 content in burners is highlighted. [ABSTRACT FROM AUTHOR]

The energy crisis has forced researchers to look for various non-conventional energy sources. Wind energy is one of the potential sources, and researchers have invested resources in developing different kinds of wind turbines. Vertical axis wind turbines (VAWT) have received less attention than their horizontal-axis counterparts. A helical-bladed VAWT is preferred because it makes perfect sense as an improvement in design, as they have higher azimuth angles of power generation capabilities. This paper studies the effects of the helix angle of blades in the aerodynamic performance of VAWT using 3D numerical simulations. Three different helix angles of 60°, 90°, and 120° of a three-bladed VAWT operating across different tip speed ratios were studied. Turbulence is modelled using a four-equation transition SST k-ω model (shear stress transport). The 60° helical-bladed VAWT was found to be better performing in comparison with all other helical-bladed and straight-bladed VAWT. The ripple effects on the shaft are also analysed using a standard deviation plot of the moment coefficient generated by a single blade over one complete cycle of its rotation. It was observed that the greater the helix angle, the lower the standard deviation. The paper also tries to analyse the percentage of power generated by each quartile of flow and the contribution of each section of the blade. Ansys FLUENT was employed for the entire study. A comparative study between different helical-bladed VAWT and straight-bladed VAWT was carried out along with wake structure analysis and flow contours for a better understanding of the flow field. [ABSTRACT FROM AUTHOR]