Background: Micro-mixing is an innovative technique adopted in large-scale combustion to enhance flame stabilization. Flame stability enhancement is achieved as the flame structure is transformed from diffusion to partially premixed type. The present work is the first attempt to introduce micro-mixing in micro-scale combustion. A slot (micro-mixing slot) is provided in the centrally slotted bluff body to enable a small quantity of air gets mixed with fuel which subsequently enhances the flame stabilization in a micro-combustor. The angle of the micro-mixing slot is optimized by considering varied inlet velocity and equivalence ratio range. Moreover, the local mixing characteristics, variation in flame dynamics at different inlet conditions and lift-off dynamics were investigated in the present study. Methods: The analysis was conducted numerically by adopting steady state and transient (higher velocity and lift-off conditions) laminar simulations at different inlet velocities and equivalence ratios. Significant Findings: The local equivalence ratio at the central slot interface (phi(inter)) plays the key role in anchoring the flame to bluff body as well as in the transformation of flame structure. The effect of phi(inter) (cold flow) was investigated for different micro-mixing slot at varied inlet conditions and a new criterion to predict stable flames using cold flow itself, was developed. The range obtained is 2 < phi(inter) < 5 for cold flow to achieve stable flames. The transformation of flame structure from diffusion to partially premixed type and the subsequent flame stability enhancement was analyzed by comparing combustors with and without micro-mixing. The magnitude of micro-mixing slot angle was optimized as '60 degrees ' by analyzing the flame stabilization characteristics and thermal performance of the combustor at different inlet conditions. The trend in flame dynamics and wall temperature distribution by varying inlet velocity (at phi(g) = 1.0) and global equivalence ratio (at V-air = 1m/s) were analyzed and furthermore the lean stability limit was determined. The lift-off mechanism was investigated at stoichiometric and lean global equivalence ratios - phi(g) = 1.0, 0.8 and 0.5. Even at the lift-off condition, heat transfer from flame to bluff body tip was prominent, which enabled enhanced flame stabilization. As the mixture became leaner, the reaction zone expanded, and the flame shifted radially toward the fuel side.

This study investigates the shell hydroforming of 1.2 mm-thick AA5052 aluminum alloy sheets to produce hemispherical domes which possess inherent spatial symmetry about their central axis. Shell hydroforming is widely used in fabricating lightweight, high-strength components for aerospace, automotive, and energy applications. The forming process was driven by a spatially symmetrical internal pressure distribution applied uniformly across the blank to maintain balanced deformation and minimize geometrical distortion. Experimental trials aimed at achieving a dome depth of 50 mm revealed wrinkle formation at the blank periphery caused by circumferential compressive stresses symmetrical in nature with respect to the dome’s central axis. To better understand the forming behavior, a validated 3D finite element (FE) model was developed, capturing key phenomena such as material flow, strain rate evolution, hydrostatic stress distribution, and wrinkle development under symmetric boundary conditions. The effects of the internal pressure (IP), blank holding force (BHF), coefficient of friction (CoF), and flange radius (FR) were systematically studied. A strain rate of 0.1 s−1 in the final stage improved material flow, while a symmetric tensile hydrostatic stress of 160 MPa facilitated dome expansion. Although tensile stresses can induce void growth, the elevated strain rate helped suppress it. An optimized parameter set of IP = 5.43 MPa, BHF = 140 kN, CoF = 0.04, and FR = 5.42 mm led to successful formation of the 50 mm dome with 19.38% thinning at the apex. Internal pressure was identified as the most critical factor influencing symmetric formability. A process window was established to predict symmetric failure modes such as wrinkling and bursting.

This study aims to measure the laminar burning velocity (LBV) of ethyl valerate (EV) at elevated mixture temperature conditions and subsequently refine the reaction mechanism proposed by Li et al. to accurately model the flame propagation behaviour of ethyl valerate-air mixtures. Through experimental measurements conducted using externally heated diverging channel method (EHDC), precise LBV measurements were obtained for ethyl valerate-air mixtures across a range of equivalence ratios (phi = 0.7-1.4) and temperatures (300-610 K) at atmospheric pressure. Subsequently, modifications were carried out to the Li mechanism by adjusting the pre- exponential factors, particularly focusing on reactions R471 and R703, to improve the agreement between model predictions and experimental data at elevated temperatures. The refined mechanism offers a more accurate representation of EV combustion behaviour, showcasing enhanced agreement between predicted and experimental burning velocities.

Stepped micro-combustors can be reliably used for portable power generation applications and can be scaled up in size to meet higher power requirements. The previous studies in stepped micro-combustors did not focus on the use of liquid fuels and the effect of scaling up (in size). This work numerically investigates ethanol combustion in stepped micro-combustors across increasing sizes, examining key performance parameters such as heat recirculation, flame structure, exergy destruction, and entropy generation. The finite volume method (FVM) and detailed chemistry are used to study the effect of scaling on premixed ethanol-air micro-combustor for five different micro-combustors of surface area to volume (S/V) ratio ranging from 2000 (smallest) to 1000 (largest). With scaling up, i.e., reducing surface area to volume (S/V) ratio, flame stabilizes closer to the inlet tube, with reduced heat recirculation through combustor walls. The reduced preheating of the incoming air-fuel mixture affects the chain branching reaction, which reduces flame speed. Scaling up increases the exergy efficiency (reaching a maximum of 86% at S/V ratio of 1000 at 25 W) and reduces the entropy generation rate (73% decrease at 25 W). With the reduction in the S/V ratio (from 2000 to 1333), a significant enhancement (42% ) in the radiative power of the micro-combustor is observed (for 35 W thermal power). The combustor with an S/V ratio of 1600 consistently showed superior temperature uniformity at all power levels, with the flame stabilizing at step 2 for all the input power levels investigated here. The results will enable scaling up micro-combustor designs that can support higher input thermal power, enhance energy conversion efficiency, and minimize losses.

Thermal ignition of fuel-air mixtures due to hot particles can pose security risks and be hazardous under various circumstances, leading to their auto-ignition. The ignition characteristics of hydrogen-air mixtures with hot particles were investigated by performing 2-D numerical simulations using a detailed H2-air kinetic model. The simulations were performed with hot particles of various sizes and shapes with hydraulic diameters of 2, 4, and 6 mm. The effect of particle surface temperature on the ignition characteristics was studied by performing simulations at various particle surface temperatures ranging from 1000 to 1200 K. The results showed that for different particle shapes and sizes, the ignition delay depends strongly on the particle temperature. The location of the ignition point depends on the particle shape and temperature. A particle with a temperature of 1200 K ignites at the front stagnation point. The behavior significantly differs at 1000 K as the ignition location is shifted to a different point due to a competition between reaction kinetics and flow around the hot particles. Spherical particle showed the highest heat release rate compared to other particle shapes.

A novel micro-mixing micro-combustor was developed and the flame stabilization characteristics were investigated numerically. The Combustor exhibited excellent lean stability limits, with stability limits increasing as the fuel-air mixture became leaner. Stability limits achieved for the equivalence ratios 1.0, 0.8 and 0.6 were 8 m/s, 14 m/s and 16 m/s respectively (although the exact blow-off limit for each case is expected to be higher). The limits are higher than those reported in previous studies on hydrogen fueled combustors. As the Reynolds number exceeded 500, simulations above 6 m/s were performed using a turbulent model to account for the transition to turbulence. An all-new criterion to predict the flame stabilization was suggested by specifying the range of local equivalence ratio at central slot interface (phi inter) as 7 <= phi inter <= 21 (cold flow) for both laminar and turbulent regimes when 0.6 <= phi g <= 1.0. The uniformity of wall temperature demonstrated significant improvement at lean equivalence ratios in both the laminar and turbulent regimes, which is crucial for practical applications such as thermoelectric generators (TEGs) and similar systems. Furthermore, the introduction of micro-mixing resulted in the transformation of the diffusion flame into a partially premixed flame. The underlying mechanisms driving this transformation in flame structure were analyzed and discussed in detail. Regarding the enhancement of stability limits, the firing rates of existing combustors were compared with the that of the present combustor and the results revealed a significant 28 % hike at the lift-off condition, which would be even higher when the exact blow-off limits are determined.

An experimental study was performed to explore the flammability limits of ethanol fuelled stepped micro-combustor (three-step) at different scales (by changing surface area to volume ratio (S/V)). The combustors were made of quartz, stainless steel, and aluminium, each with a thermal conductivity that differed by order of magnitude. The study indicates that reducing the S/V, leads to upstream flame stabilization, with aluminium combustors having superior flame stabilization nearer to the combustor inlet followed by stainless steel and quartz combustors. It was observed that for all scales of combustors, better blowout limit and flashback limit were observed at cent = 1.1. Interestingly the blowout limit followed a trend similar to that of laminar burning velocity (SL) and the flashback limit followed a trend similar to temperature dependency (a) of SL with cent. The aluminium combustors at all scales showed better outer wall mean temperature (T-mean) and uniformity (sigma(T)), making it more suitable for Thermoelectric Generator (TEG) applications. The uniform wall temperature distribution in the aluminium combustor allows for the mounting of more TEG modules compared to Stainless steel combustors, where TEG modules can only be effectively mounted on the second and third steps due to non-uniform wall temperatures in the first step.

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.

The study involves in the design of a domestic two-layer self-aspirating porous burner that operates at a thermal load of 1 kW using liquefied petroleum gas (LPG). The porous burner is intended to function within India's standard domestic regulator fuel inlet pressure of 3000 Pa. For the same thermal load, this work also provides a numerical comparison between the domestic conventional burner and the designed porous burner. For both porous and conventional burners, a full-scale 3D model is developed to calculate the flow, combustion, heat transfer to the cooking vessel, thermal efficiency, and emissions. The combustion process of both burners is numerically computed using a detailed chemical kinetic mechanism of LPG combustion, the San Diego Mechanism (SDM) with 57 species and 268 reactions are used. The porous burner is simulated using a non-thermal equilibrium condition to better calculate the heat recirculation within the porous domain. The self-aspirated porous burner has an equivalence ratio phi of 0.75 at 1 kW and an efficiency of 84.2%; conventional burner at the same load had an efficiency of 68%. 10 and 6 parts per million (ppm), respectively, are the measured CO and NOx emissions from the domestic porous burner and 660 and 80 ppm for domestic conventional burner, respectively.

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.

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 H2-air and CH4/H2/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 H2-air mixtures. Lower ignition thresholds are observed in CH4/H2/air mixtures with high concentrations of H2. Mixtures having higher CH4 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.

Two-dimensional numerical simulations are performed to study the flame dynamics of DME/H2/air mixtures in a microchannel with a controlled wall temperature profile. The characteristics of premixed stoichiometric DME/H2/air flames at various H2 compositions (% by volume) are analyzed in a 1 mm diameter tube of 120 mm length for a mixture inlet velocity of 0.15 m/s at 300 K temperature and 1 bar pressure. For every mixture composition under investigation, flame repetitive extinction and ignition (FREI) instability is noted. When the hydrogen percentage in the mixture rises, the frequency of FREI considerably decreases. The effect of hydrogen addition is nonlinear when the H2 composition in the mixture is above 40 %. Throughout the FREI cycle, a stable, weak flame is observed at the upstream side of the combustor. When the H2 composition in the mixture is increased to 80 %, the hot flame interacts with the weak flame, resulting in an increased rate of weak flame reactions. The weak flame shifts further upstream in this condition. The CH2O and H2O2 produced at the weak flame region are being consumed downstream, resulting in another peak in heat release rate between the cool flame and hot flame regions. This intermediate peak disappears during the propagation phase.

Cryo-rolled aluminum alloys have a much higher strength-to-weight ratio than cold-rolled alloys, which makes them invaluable in the aerospace and automotive industries. However, this strength gain is frequently accompanied by a formability loss. When uniformly applied to the blank surface, hydroforming provides a solution by generating geometries with constant thickness, making it possible to produce complex structures with “near-net dimensions”, which are difficult to achieve with conventional approaches. This study delves into the cavity die sheet hydroforming (CDSHF) process for high-strength cryo-rolled AA5083 aluminum alloy, focusing on two primary research questions. Firstly, we explored the utilization of a nonlinear 3D finite-element (FE) model to understand its impact on the dimensional accuracy of hydroformed components within the CDSHF process. Specifically, we investigated how decreasing fluid pressure and increasing the holding time of peak fluid pressure can be quantitatively assessed. Secondly, we delved into the optimization of process parameters—fluid pressure (FP), blank holding force (BHF), coefficient of friction (CoF), and flange radius (FR)—to achieve dimensional accuracy in hydroformed square cups through the CDSHF process. Our findings reveal that our efforts, such as reducing peak fluid pressure to 22 MPa, implementing a 30 s holding period, and utilizing an unloading path, enhanced component quality. We demonstrated this with a 35 mm deep square cup exhibiting a 16.1 mm corner radius and reduced material thinning to 5.5%. Leveraging a sophisticated nonlinear 3D FE model coupled with response surface methodology (RSM) and multi-objective optimization techniques, we systematically identified the optimal process configurations, accounting for parameter interactions. Our results underscore the quantitative efficacy of these optimization strategies, as the optimized RSM model closely aligns with finite-element (FE) simulation results, predicting a thinning percentage of 5.27 and a corner radius of 18.64 mm. Overall, our study provides valuable insights into enhancing dimensional accuracy and process optimization in CDSHF, with far-reaching implications for advancing metal-forming technologies.

Micro-combustion based power generation devices can be considered as future alternatives to batteries in miniature electronic devices. Micro-combustors operating in non-premixed mode are free from flashback but face the challenge of properly mixing fuel and air within a small volume. In this work, the effect of a divergent fuel-air splitter design on the mixing performance and combustion characteristics of H-2-air fueled diffusion micro-combustor is studied. The laminar reacting flow is simulated using the finite volume method and a detailed hydrogen kinetic mechanism. Three divergent splitter designs are compared with the commonly used rectangular splitter to study the effect on radiation power, an essential parameter for thermophotovoltaic power generation. The best-performing divergent and base rectangular splitter designs are investigated in detail. The study shows that the micro-combustor with divergent splitter design reduces mixing distance (L-mix) by 5-23% depending on inlet velocity and channel height. With the divergent splitter, the peak value of the heat release rate also increases slightly, implying enhanced combustion. The divergent splitter increases the high-temperature surface area of the outer wall as compared to the rectangular splitter. This leads to the micro-combustor with divergent splitter producing significantly higher radiation power (>10%) than the rectangular splitter for larger channel heights and higher inlet velocities.

The combustion characteristics of n-decane+air mixtures are experimentally investigated through laminar burning velocity measurements at 1 atm pressure and higher initial temperatures using an externally heated diverging channel (EHDC) method. Up to 610 K mixture temperature over an equivalence ratio range of 0.7-1.4, laminar burning velocities are reported with an accuracy of +/- 5%. The current measurements exhibit a good match with existing experimental measurements, and agree closely with the predictions of Zhao, LLNL and PoliMi mechanisms at different mixture temperatures. The present measurements show an excellent match of temperature exponent (alpha) variation with equivalence ratio (phi) with the predictions of distinct kinetic models as well as experimental measurements. This study reveals that a substantial scatter exists among the predictions of different kinetic models. A variation of 20-30 cm/s in the burning velocity is observed at 610 K mixture temperature. Reaction R16 (H + HO2 = H-2 + O-2), which inherently reduces the burning velocity becomes insignificant at an elevated mixture temperature of 610 K, and the reaction R15 (H + HO2 = 2OH) plays a dominant role in accelerating the flame propagation. From reaction pathway diagrams, it is clear that a higher burning velocity at 610 K is associated with the increased reaction rate. The elemental-flux value associated with the formation of C2H3 from C2H4 at 610 K mixture temperature is approximately equal to 34% higher in comparison to the 470 K mixture temperature.

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.

The purpose of this numerical investigation is to characterize the longitudinal dispersion coefficients in open-cell reticulated porous structures. Open-cell foams are modelled using idealized Kelvin cell struc-tures. Using the conventional Navier-Stokes equation, airflow has been calculated through various por-ous structures. Along with the flow, the dispersion of a tracer fluid is traced across the structures and analyzed in terms of the effective dispersion coefficient. Using direct pore level simulations (DPLS), a parametric study is performed to understand the influence of geometrical parameters on the dispersion in porous media. To evaluate the longitudinal dispersion coefficient (LDC), the analytic solution gradients were fitted into the simulated gradients. From the results, a new characteristic length correlation is pro-posed to calculate the Peclet number, and it is compared with experimental and numerical data that are available in the literature.(c) 2023 Elsevier Ltd. All rights reserved.

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.

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.

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.