Heat Recirculation Research Articles

Thermal performances investigation on an ammonia-fuelled heat-recirculating micro-combustor with reduced chemical mechanism

This study addresses the challenges in utilizing ammonia as a CO2-free fuel. However, the drawbacks of ammonia are—low burning speed and nitrogen oxide emissions. To enhance ammonia's flammability in atmospheric micro-combustion, hydrogen is blended in the fuel. First, a simplified chemical reaction mechanism of ammonia consisting of 44-step reactions and 19 species specifically for ammonia is developed and validated using experimental data to reduce computational cost and time. Then, we proposed a heat-recirculating micro-combustor fueled by premixed hydrogen/ammonia/air. Five key parameters are identical to numerically studying the thermal performance, entropy generation, and NO emissions. The present findings confirm that higher inlet velocity boosts thermal performance (power output achieving 15.8 W at 7 m/s) and NO emissions peak at 3 m/s (0.0169). Unity equivalence ratio optimizes thermal performance, rich-fuel combustion reduces NO emissions. At the stoichiometric ratio, the power output is 8.34 W, with the highest NO emissions at 0.9 (0.168). Hydrogen blending has a small effect on the performance (8.5 W at ξm = 0.7, 0.51 W above ξm = 0.4), but effectively reduces NO emissions (ξm = 0.7 NO emissions around 30 % lower than ξm = 0.4). Furthermore, changing the material from steel to Corundum enhances power output by approximately 6 %, while longer heat recirculation improves thermal performance.

Improving volume-averaged simulations of matrix-stabilized combustion through direct X-ray µCT characterization: Application to NH3/H2-air combustion

Porous media combustion (PMC) relies on internal heat recirculation in an open-cell ceramic foam matrix to enhance the flame speed of fuels with poor combustion properties. Volume-averaged simulations are often used to study the combustion performance and pollutant emissions of such systems. However, due to the varying complexity of matrix geometries found in practical burners, as well as the wide range of closure models for the constitutive relations of the solid phase, contradicting statements about the predictive accuracy of these volume-averaged models can be found in the literature. In this work, we propose an open-source modeling framework for accurate volume-averaged PMC simulations by using first-principles methods to determine effective properties used in closure models. This framework relies on adequately characterizing the topology of the solid matrix, using commonly available X-ray computed microtomography. With this approach, significant improvements in accuracy are reported compared to empirical models from the literature. The framework based on first-principle evaluations of constitutive relations is compared against experimental measurements conducted on an interface-stabilized burner operated with premixed NH3/H2-air. The model shows good agreement for exhaust gas composition and stability limits. The proposed simulation framework performs significantly better than state-of-the-art techniques that employ commonly used empirical correlations for effective matrix properties. Statement of SignificanceWe present a new open-source simulation framework for improved characterization of porous media combustion. By utilizing µCT techniques, accurate effective matrix properties can be determined from first-principle simulations. These effective properties are used in closure models for 1D volume-averaged reacting flow simulations using appropriate sub-models for heat recirculation. This modeling framework is able to reliably predict stability limits while conventional closure models yield erroneous trends. Assessment of the resulting modeling framework is performed using experiments with exhaust gas characterization performed on a NH3/H2-air porous media burner.

Orthogonal analysis of multi-factors of hydrogen-fueled combustion in a micro recuperative planar with porous media

To improve the energy conversion and efficiency of hydrogen-fueled combustion, a micro recuperative planar with porous media is proposed. Numerical simulation model of premixed H2/air combustion is verified by experimental test, then orthogonal analysis of multi-factors, i.e., porous media setting and flow rate, on the combustion characteristics and heat transmission are compared and analyzed. The results indicate that combustion characteristics and heat release are strongly impacted by porous media, that is, preheating of unburnt reactants and heat transmission of gases and gas-solid are improved via the heat recirculation in/around porous media. For instance, the difference of mean wall temperatures of the combustor with/without porous media reaches 4.5% at mf = 6.67 × 10−7 kg/s. So, the inserted porous media boosts the burner radiation temperature, and porous media dimensions and porosity affect the thermal performance of the burner, reducing exhaust gas temperature and improving energy efficiency. Besides, orthogonal analysis is also employed to understand the coupling effects of multi-factors on burning and heat transmission, the effect degree on Tm is ranked by mf, L, H and φ. Hence, an optimal burning setting is obtained, i.e., combustor C4.8–6.8, where Tm is increased by 4.6% compared to the combustor C3-3 at mf = 9.17 × 10−7 kg/s. It achieves the maximum radiation efficiency 35.26% at mf = 6.34 × 10−7 kg/s.

Co-partial oxidation of acetic acid, methanol, and ethanol in an integrated porous media with efficient heat recirculation: Green hydrogen production and its expansion method

Green energy from biomass contributed to substituting the traditional fossil fuels and environmental protection. To explore the hydrogen production from bioethanol with the industrial by-products, a porous media burner integrated with a heat-recirculating tube was designed to realize the efficient vaporization of ethanol. Then the partial oxidation characteristics of ethanol were investigated with the acetic acid and methanol addition at different operating conditions. The results indicated that the acetic acid and methanol addition inhibited hydrogen production but promoted methane production. Compared with pure ethanol, the heating values of syngas were decreased by 3.41 and 0.64 MJ/Nm3 with the acetic acid or methanol content of 20%. The highest yields of hydrogen and carbon monoxide were achieved at 43.94% and 66.45% respectively when the ratio of acetic acid content to methanol was 5:15. Nevertheless, blended fuels achieved more syngas production at higher equivalence ratios and velocities. The hydrogen concentration of the blended fuel reached the maximum of 21%, exceeding that of pure ethanol by 1.6% when the velocity was 22 cm/s. The corresponding results provided a theoretical reference for the industrial application of bioethanol.

Numerical investigation on thermal performance improvement of hydrogen-fueled tube with rib/fins for micro thermophotovoltaic

Combustion characteristics and thermal performance of pre-mixed hydrogen/air in micro tubes for micro-thermophotovoltaic are conducted under various operating conditions. To improve flame stability and energy efficiency, a cylindrical stepped tube with rib/fins is proposed. Effects of burner optimization on hydrogen-fueled flame regime, heat transmission and thermal performance are numerically conducted, which is verified by experimental tests. The results illustrate that the inserted rib and its location strongly modify the anchoring and propagation of premixed H2/air flame, resulting in the improvement of heat transfer and radiation temperature. Furthermore, the employment of outlet-fins modifies the flow field and enhances heat recirculation by utilization of hot exhausted gases. There exists a balance between the radiation temperature reduction and the radiation surface augment via the increment of chamber length. Meanwhile, the influence grades of chamber length, flow rate, outlet-fins number and rib position are ranked in turn based on the grey relational analysis. So, electrical power of combustor C33-7-10 is 0.77 W higher than that of stepped tube for a micro-TPV with InGaAsSb at mf = 5.10 × 10−5 kg/s, and it obtains a higher power output of 2.22 W at mf = 8.16 × 10−5 kg/s. It can also be further improved by the flow rate increment and employment of high efficiency PV cells.

Effect of hetero-/homogenous combustion on energy conversion performances and flame stability of methane/air-fueled micro-combustors with heat-recirculating structure and platinum-coated

In this work, the joint effect of applying Pt catalyst and heat-recirculating structure on the flame stability and thermal performance has been numerically investigated in methane-air-fueled micro-combustors by a detailed three-dimensional computational model. Numerical results indicate that such strategies contribute to improving outer wall temperature by 14.4% and extending the blowout limit by a factor of 0.6 compared to those in the straight non-catalytic combustor. Such enhancements are due to the intensified heat transfer, thus highlighting the contributions of heterogeneous combustion chemistry and heat-recirculating effect. Homogeneous chemistry in the presence of a Pt catalyst is shown to be self-sustained over wide-ranging conditions. Meanwhile, the effectiveness of heat recirculation structure on combustion performance is more significant as compared to that of the catalytic combustion effect, as evidenced by a 4% and 17.6% improvement in the wall mean temperature and blowout limit respectively. Further analysis of the heat recirculation catalytic combustors is conducted to obtain the optimal geometry. There exists a critical flow rate corresponding to the maximum outer wall temperature and radiation energy. In general, this work demonstrates the viability of applying a Pt catalyst and heat-recirculating structure to extend the flame stability limit and energy conversion performance in micro-power systems.

Characteristics of Butane Combustion in Mesoscale–Combustor Due to Stainless Steel Flame Holder Type

Mesoscale–combustor is one component of the micropower generator. The mesoscale–combustor function is to produce heat energy from the combustion of chemical hydrocarbon compounds. The small dimensions of the meso-combustor cause large losses and short residence times for the reactants. These two factors lead to narrow flame flammability limits. This study aims to increase flammability limits by lengthening the reactant residence time and increasing reactant mixing with inserts of flow retardants called flame holders. There are two kinds of flame holders used in this study, namely holes and lines. Lines type produces flames at an equivalent ratio of Φ0.83–1.76 and speeds of 17.01 cm/s to 72.93 cm/s, while holes type produces flames at an equivalent ratio of Φ0.91–1.71 and a flow rate of 24.94 cm/s to 54.95 cm/s. This phenomenon shows that the line type flame holder is able to inhibit flow better and form vortices better, thereby increasing heat recirculation, flame stability, and widening flammability limits.

A Review on Flame Stabilization Technologies for UAV Engine Micro-Meso Scale Combustors: Progress and Challenges

Unmanned aerial vehicles (UAV)s have unique requirements that demand engines with high power-to-weight ratios, fuel efficiency, and reliability. As such, combustion engines used in UAVs are specialized to meet these requirements. There are several types of combustion engines used in UAVs, including reciprocating engines, turbine engines, and Wankel engines. Recent advancements in engine design, such as the use of ceramic materials and microscale combustion, have the potential to enhance engine performance and durability. This article explores the potential use of combustion-based engines, particularly microjet engines, as an alternative to electrically powered unmanned aerial vehicle (UAV) systems. It provides a review of recent developments in UAV engines and micro combustors, as well as studies on flame stabilization techniques aimed at enhancing engine performance. Heat recirculation methods have been proposed to minimize heat loss to the combustor walls. It has been demonstrated that employing both bluff-body stabilization and heat recirculation methods in narrow channels can significantly improve combustion efficiency. The combination of flame stabilization and heat recirculation methods has been observed to significantly improve the performance of micro and mesoscale combustors. As a result, these technologies hold great promise for enhancing the performance of UAV engines.

A numerical study on heat transfer characteristics of a mesoscale combustor partially filled with wire meshes

Porous media are often inserted into micro-/meso-scale combustors for flame stabilization. The heat transfer processes involved are very complicated which still need to be comprehensively investigated. In the present study, the coupled heat transfer processes between gas phases and both the porous media and tube wall of a meso-scale combustor partially filled with wire meshes were numerically analyzed. First, it is found that there exists a heat loss segment (i.e., heat is transferred from near-wall gas to wall) on the inner wall of the porous zone when the inlet mixture velocity is not high enough (≤0.4 m/s), but this phenomenon vanishes as the inlet velocity is further increased. Meanwhile, the porous zone can be divided into two sub-regions, i.e., a heat recirculation zone and a heat loss zone. Interestingly, the boundary shape of these two zones will be inversed with the increase of inlet velocity. Furthermore, the heat recirculation efficiency of inner wall changes non-monotonically with an increasing inlet velocity, which differs from the variation trend of the scenario with a full insertion of porous media reported in the literature. However, the heat recirculation efficiency of the porous media decreases monotonically versus the inlet velocity. In summary, this study unraveled that the flame location and heat release rate vary with the inlet velocity, which affects the temperature distributions of both porous media and tube wall, and this in turn acts on the two pathways of heat recirculation of the combustion system.

Improvement of thermal performance and energy efficiency of micro-thermophotovoltaic with the dual-inlet combustor at appropriate equivalence ratio

Micro-thermophotovoltaic is facing challenge caused by small size, large area-volume proportion and heat loss ratio, reducing the burning stability and output power. So, the dual-inlet combustors with exit-step and porous medium are proposed for micro-thermophotovoltaic system. Effects of equivalence ratio, exit-step and porous medium setting on combustion characteristic and thermal performance are conducted and analyzed. Results demonstrate that 1% air in the second-inlet is conducive to releasing heat of combustion and optimizing wall temperature distribution, where the total equivalence ratio of H2/air keeps 1.0. Furthermore, the exit-step with applicable length and channel height can generate heat recirculation and enhance working performance of micro-combustor, while the employment of porous medium can further enhance heat transmission of gas-wall and boost the radiation temperature Tw. Combustor CP13 with porosity ε = 0.85 obtains the highest average Tw of 1273.9 K at mf = 3.376 × 10-5 kg/s. With the increase of mf, system efficiency of the combustor with porous medium is increased, while that with exit-step reaches the maximum at mf = 4.501 × 10-5 kg/s. The highest output electrical power and efficiency are appeared in combustor CP15 with ε = 0.95 at mf = 5.626 × 10-5 kg/s, that is, 4.2 W and 2.2%.

The Influence of Heat Recirculation Segment of Meso-Scale Combustor o Gas and Liquid Fuel Inside Microscale Combustor

This study aims to determine the effect of heat recirculation segments configuration on the stability of the flame. Mesoscale combustor used is made from duralumin-quart glass tube, and Ø 3.5 mm diameter combustion chamber. The heat recirculation segments part is made of duralumin as a segment of evaporation and air-fuel mixing. Combustor A has heat recirculation segments length of 7 mm, while combustor B has a heat recirculation segments length of 10 mm. The fuel used is liquid hexane and butane gas fuel. The results showed, the stability of hexane flame in combustor A between the equivalent ratio of ɸ 0.73-1.28 temperature can reach 803.8oC. Combustor B is stable at an equivalent ratio of ɸ 0.76-0.98 to a temperature of 580.9oC. Furthermore, for butane flames; combustor A, stable in the range of ɸ 0.68-1.31 temperature to 931.3oC. Combustor B has a stability range of ɸ 0.79-1.41 temperature reaches 857.2oC. Combustor A with a 7 mm heat recirculation segments length is proven to produce wider flame stability, using both liquid fuel and gas. So, combustor A can be recommended to be applied to micro power generators. It should be noted that butane flames appear to have more flame stability than hexane flames. This is because gaseous fuels are more homogeneous (gaseous) so they are more quickly mixed with air.

Modulation of Heat Transfer in a Porous Burner Based on Triply Periodic Minimal Surface

Abstract The list of reacting flows in porous media applications is quite long, including porous media combustion, syngas production, and fuel cells. Porous media combustion is recognized as a cutting-edge combustion technique for increasing flammability. In this process, heat is transferred from the exothermic reaction zone to the incoming reactants through porous media. This role of porous media distinguishes reacting flows in porous media from free combustion processes. Local heat transfer, such as solid conduction, solid–solid radiation, and solid–gas convection, as well as the response behavior, are affected by the topology of the porous material. Theoretical studies indicate that continuously graded porous materials can significantly enhance the performance benefits of heat transfer. However, topology design is challenging for smooth graded porous media, and thus investigations of combustion within graded porous media are still required. In this study, we constructed a porous structure of type W/P/D/G (porosity ε = 0.3–0.5, hydraulic diameter dh = 1.33–3.86 mm) using a triply periodic minimal surface (TPMS), and a computational model of the combustion reaction in porous media was established to compare the range of flame stability within different pore types. In addition, topology gradation was achieved via TPMS to modulate the heat transfer to ensure the dependable functioning of premixed flames and improved heat recirculation. Heat transfer in the graded TPMS-based porous structure was analyzed numerically. The conclusions obtained from this study can effectively address the aforementioned challenges related to porous media burner design.

Hydrogen-enriched flames in a novel miniature double-layer disc-combustor with annular step and Swiss-roll preheated channel

A novel miniature double-layer disc-combustor with annular step and Swiss-roll preheated channel is developed for the first time, which can act as a heater or the miniature heat source for a miniature power source. The premixed flame dynamics of 80%H2-20%CH4-air are experimentally studied in this combustor. The results show that the flame blow-off limit increases first and then sharply decreases with the decreased value of Reynolds number Re. It is the first time to find the anomalous blow-off phenomenon in the radial channel. The analysis indicates that the decreases of flow recirculation effect, heat recirculation effect, and preferential diffusion effect lead to the decreased blow-off limit with the decreased Re value. The flames can remain stable in a wide operating range. At the large Re value, the flame diameter obviously increases with the decreased value of equivalence ratio ϕ, and the flame anchors right behind the annular step at the small ϕ value. At the small Re value, the flames all reside around the exit of Swiss-roll preheated channel, which results in the small flame diameters. The magnitude of the upper wall temperature decreases with the decreased ϕ value. At large ϕ values, the distribution characteristics of high temperature zone of the upper wall are similar, and the peak value of temperature appears at its center. At small ϕ values, the peak value and magnitude of wall temperature are both larger for a bigger Re value, and the location of peak value slightly shifts upstream with the decreased Re value. The upper wall temperature of the combustor can be expediently managed by adjusting the operating conditions. In addition, the mean temperature of the upper wall and heat transferred from the upper wall to the environment almost linearly decrease with the decreased Re value. The present study expands our insights into the flame dynamics in the radial channel.

친환경 바이오연료를 사용하는 난방시스템 디자인설계 및 환경시험평가

Research and development for alternatives to fossil fuels such as coal and petroleum have progressed considerably worldwide. Among clean fuels, bio-diesel has been a representative fuel and it was used for the following advantages; renewable, ready to use with low fine dust emission and non-toxicity. But it is expensive and cause strain on local resources. In our study, to resolve these issues, we developed two types of heating systems by direct input of waste cooking oil, and the heating system was optimized through mechanical design and design process. In addition, we conducted an environmental test to evaluate the reliability and environmental characteristics of the developed device. Firstly, through the design modification of heat recirculation system, more efficient combustion for the complete combustion of bio-fuel(waste cooking oil), was possible. As for the heating system, two products (tube heating system, agricultural hot air blower) were optimized for management convenience, space utilization, and efficiency improvement by 6-step design. From the environmental test result, the combustion efficiency of the heating system was up to 99%, the average oxygen concentration was 8.3%, and the CO concentration was 7.9 ppm for a stable combustion, NOx was up to 67 ppm, and SOx was not detected.

Towards Thermal-Aware Workload Distribution in Cloud Data Centers Based on Failure Models

Increasing workload conditions lead to a significant surge in power consumption and computing node failures in data centers. The existing workload distribution strategies focused on either thermal awareness or failure mitigation, overlooking the impact of node failures on the energy efficiency of cloud data centers. To address this issue, a new holistic model is built to characterize the impacts of workloads, computing and cooling costs, heat recirculation, and node failure on the energy efficiency of cloud data centers. Leveraging such a holistic model, we propose a novel thermal-aware workload distribution strategy called <i>HGSA</i> that takes node failure into accountand can improve the energy efficiency of cloud data centers. Our empirical findings confirm that (i) faulty nodes lead to a large rise in power consumption, and (ii) failure locations play a vital role in the power consumption of data centers. Experimental results unveil that HGSA is adroit at making near-optimal decisions in workload distribution strategies. In particular, HGSA cuts down the minimum inlet temperature by 5.2 <inline-formula><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> -15 <inline-formula><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> , improves the maximum air temperature of a Computer Room Air Conditioner (CRAC) model by 4.2 <inline-formula><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> -26.5 <inline-formula><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> , lowers the cooling cost by 15.4 <inline-formula><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> -50 <inline-formula><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> compared to the existing solutions. Furthermore, HGSA cuts back the total power consumption by 0.65 <inline-formula><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> -78 <inline-formula><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> .

Investigation of CH4 and porous media addition on thermal and working performance in premixed H2/air combustion for micro thermophotovoltaic

Primary obstacles to the development of micro-thermoelectric generators are the gigantic heat loss ratio and flame instability. Premixed H2/air combustion in the micro-planar combustor with porous media (PM) and CH4 blended is experimentally and numerically conducted to boost the generator working performance. Effects of porosities, PM length and CH4 blending on combustion characteristics and thermal performance are detailedly studied at various operating conditions. Results indicate that flame stability is enhanced in PM, and the burner radiation temperature is also significantly boosted because of heat recirculation and heat transmission from flame. PM porosity ε = 0.9 balances the effects of solid skeleton on flame propagation and thermal improvement, which is conducive to flame stabilization. The coupling of PM setting and 25 % CH4 blending strongly improves the combustor thermal performance via altering the flame location and enhancing heat transmission. It also ameliorates the uniformity of radiation wall. Moreover, prolonging porous matrix has a positive effect on the improvement of thermal performance under a high chemical energy input. Particularly, combustor #C-Ls13 obtains an electrical output of 2.5 W and a system efficiency of 2.1 % at 25 %CH4-75 %H2 fueled combustion.

THERMAL ASPECTS OF HEATING BOILER PLANTS OPERATION WITH HEAT-RECOVERY AND EXHAUST GAS RECIRCULATION

The feature analysis of application for heating boiler plants of relatively small heating capacity, which are not equipped with air heaters, of the method of reducing the emission of nitrogen oxides due to decrease the temperature in the boiler furnace by recirculating exhaust gases into the blown air are presented. At the same time, heating gas-consuming boiler plants with traditional heat recovery systems of exhaust gases and implementation of this method with the selection of recycled gases after the boiler and heat-recovery exchanger are considered. The study results for a water-heating boiler with a heating capacity of 2 MW of the change regularities of the combustion temperature in the boiler furnace, the its exhaust gases temperature and water heated in the boiler as well as the boiler efficiency, depending on its relative heat load changed (from 30 to 100%) and with different parts of recirculated gases (from 10 to 20%) in the mixture with the blown air are presented. The studies were performed with and without them recirculation and heat recovery of flue gases. The research results show that the use of waste gas recirculation causes a noticeable decrease (by 150 – 250°C) in the combustion adiabatic temperature and a slight (by 0.5 – 4.7°C) decrease in the boiler waste gases temperature. Recirculation also leads to a decrease in boiler efficiency by 1.3 – 3.1% during the heating period, and the use of the considered heat recovery system of boiler waste gases with heating of return heat-network water by cooling them provides an increase in boiler efficiency by 2.5 – 8.1%. The resulting increase in boiler efficiency with a heat recovery and recirculation system is 0.1–4.8%.

Experimental and Computational Investigation upon Combustion Characteristics of Liquid Fuel in a Novel Combustor with Hybrid Swirl and Recirculation Bowl.

In the present study, a novel hybrid swirl combustor is designed to reduce the size and complexity of a conventional gas turbine combustor. In this combustor, a dual-swirl pattern is adopted by providing the central vane swirler (45° vane angle) and circumferential tangential injection scheme to achieve higher recirculation of heat and combustion products inside the combustor. Numerical and experimental studies are carried out to understand the flow patterns and combustion characteristics in this high mass-heat interacting environment. Initially, computational studies were carried out to find the optimum geometry for greater recirculation and interaction among the reacting species inside the combustor. Liquid fuel (kerosene) is sprayed into the combustor for two thermal inputs of 25 and 50 kW. Three cases were studied to analyze the effect of bowl recirculation and tangential air inputs in addition to the swirlers. The hybrid swirl, formed by the counter-flow pattern, helps in achieving low and uniform temperature throughout and assists in flame anchoring. The tangential air flow provides a push to the combustion products from the downstream to the central recirculation zone of the combustor. The combined effect of central and tangential swirlers helps in attaining a more distributed combustion. The CO and NO emissions reduced with the use of hybrid swirl.

A NUMERICAL STUDY ON THE TRANSITION CHARACTERISTICS OF AN INVERSE MICRO-SCALE DIFFUSION FLAME TO MICRO-SCALE PREMIXED FLAME UNDER HEAT RECIRCULATION CONDITIONS

In this investigation, the gaseous hydrogen peroxide (H2O2) is issued at the inner port, and methane (CH4) is injected at the outer port of the sub-millimeter scale burner in order to establish an inverse CH4/H2O2 micro-scale diffusion flame numerically under heat recirculation conditions. The velocity of gaseous H2O2 is considered only as a numerical parameter in this study and the velocity of CH4 is kept constant to examine the flame structure and the transition behavior of an inverse micro-scale diffusion flame. The numerical simulation has been conducted using ANSYS Fluent 14.5 based on Finite Volume Method (FVM) under normal gravity (1G) conditions with low thermally conductive burners. The semi-detailed reaction model, which consists of 58 chemical reactions and 17 chemical species, has been applied in this study. It is found that when the momentum of the gaseous H2O2 is high, then a CH4 /H2O2 inverse micro-scale diffusion flame is formed, and this flame is attached just on the top of the burner by which one can easily analysis an active flame-wall interaction phenomenon. On the other hand, when the momentum of the gaseous H2O2 has been reduced significantly, then H2 /O2 micro-scale premixed flame is established entirely inside the micro-burner. Furthermore, it is found that when the momentum of H2O2 reduces remarkably, then CH4 does not diffuse inside the micro-burner at the flame zone, and consequently only H2 /O2 micro-scale premixed flame is established there as H2O2 is playing the role of a monopropellant, where no extra oxidizer is needed for the survival of flames. This type of flame mode transition phenomenon under micro-burner systems is the first ever reported event in the literature of reacting fluid dynamics (i.e., combustion dynamics). Besides, this is the primary findings of our ongoing research, and hence, the further numerical calculation adopting different pertinent parameters is now under progress with the aim of extracting more profound physical and chemical insights that are associated with the aforesaid transition phenomena.

Hollow bluff body-stabilized natural gas-air premixed flames

The effects of hollow bluff body geometry, surface curvature, blockage ratio, mixture residence time, impingement strength, heat recirculation eddy size, Reynolds and Froude numbers were all investigated via 10 bluff body shapes. It was found that duplicating the wake flow zone and increasing the mixture flow velocity inside the bluff body cavity via a disc configuration provide a maximum blow-off velocity of 49 m/s. Enlarging the bluff body exterior surface area and the flow wake width in addition to elongating the mixture residence time inside the bluff body cavity via a trapezoidal configuration yield a minimum lean limit of 0.42. Having grooves on the bluff body exterior surface or providing a concave surface part at the wake region favorably develops flow turbulence and intensive heat recirculation. The bluff body cavity height should be adjusted at an optimum value which keeps a high impingement velocity onto the combustor hot surface via a large vortex and simultaneously allows enough mixture preheating length. The stability limits were thus correlated to the geometrical and thermal effective parameters in addition to Reynolds and Froude numbers.