Combustion characteristics and thermal enhancement of premixed hydrogen/air in micro combustor with pin fin arrays

Targeted at improving the combustion stability and enhancing heat transfer in micro combustor, the combustion characteristics and thermal performance of micro combustor with pin fin arrays are numerically investigated by employing detail H₂/O₂ reaction mechanism. It is shown that the micro combustor with staggered pin fin arrays exhibits the highest average temperature and heat flux of external wall, while the micro combustor with in-line pin fin arrays displays the most uniform temperature distribution of external wall. When the equivalence ratio is 1.1, all micro combustors exhibit the highest mean temperature and heat flux of external wall. The micro combustor materials with high thermal conductivity can not only improve the average temperature and heat flux of external wall, but also enhance heat transfer to the upstream which can preheat the mixed gas. Therefore, the materials with high thermal conductivity, such as red copper and aluminum, can make up for the nonuniform temperature distribution of micro combustor with staggered pin fin arrays, so as to realize uniform high heat flux output of external wall.     

Enhancement of thermal performance by converging-diverging channel in a micro tube combustor fueled by premixed hydrogen/air

As the core component of the micro thermophotovoltaic (MTPV) system, the micro combustor with a high and uniform wall temperature distribution is beneficial to improve the energy conversion efficiency. In this paper, a micro tube combustor with converging-diverging channel is proposed and the thermal performance is numerically investigated, compared with that of the micro combustor with cylindrical channel. The effects of inlet velocity of H₂/air mixture, dimensionless position and diameter of throat, and solid material on the thermal performance are widely analyzed. Results show that the outer wall temperature and emitter efficiency of the micro combustor with converging-diverging channel are higher than that of the micro combustor with cylindrical channel, and the converging-diverging channel has more uniform temperature distribution. The converging-diverging micro combustor with dimensionless throat position l = 0.375 and dimensionless throat diameter β = 0.4 is more suitable for the application of MTPV system. When H₂/air inlet velocity is 11 m/s and H₂/air equivalence ratio is 1.0, the mean wall temperature is increased by 82.39 K and the emitter efficiency is increased by 6.59%, while the normalized temperature standard deviation is reduced by 65.85%. Additionally, the use of SiC as wall material can improve the thermal performance of the micro combustor. It is worth noting that this work will offer us significant guidelines for the optimized work of micro tube combustor.     

Development of 1D model for the analysis of heat transport in cylindrical micro combustors

A 1D flame model was developed to analyze the heat transport occurring in the cylindrical micro combustors. The one-step global reaction mechanisms were employed for three fuel–air mixtures (H₂–air, CH₄–air and C₃H₈–air) to account for the difference of fuel property in terms of the kinetics. The effects of various parameters such as the combustor size, fuel property, fuel–air equivalence ratio and unburned mixture temperature on the heat loss ratio (defined as Q_l/Q_in) and the heat recirculation ratio (defined as Q_recir/Q_l) were investigated. The results indicated that these parameters have significant effects on the two ratios, and therefore should be carefully managed in order to achieve efficient and stable combustion. After comparing the results of different fuel–air mixtures, it is concluded that hydrogen is superior to methane and propane as the fuel for micro combustion engines owing to its higher flame temperature and thinner flame thickness, which favors the reduction of heat loss from the flame zone.     

Investigation of combustion and emission performance of a micro combustor: Effects of bluff body insertion and oxygen enriched combustion conditions

Major challenges for micro combustors are high heat losses and inappropriate residence time. In this study, it was aimed to eliminate these challenges via placing bluff bodies into the combustion zone and combusting fuel with oxygen enriched air. To this end, micro combustor models with different geometries were constructed and in these models, premixed H₂/air combustion was simulated by using ANSYS/Fluent CFD code to investigate effects of bluff body shape, location and thickness, and low level O₂ enhancement on performance determining parameters such as rate of conversion of fuel to useable heat, temperature uniformity, pollutant emissions etc. To further analyze effects of micro combustor geometry, a perforated plate was also placed into the combustion zone. Thermal performance of the micro combustor with perforated plate insertion in O₂ enriched conditions was found to be highest in terms of increased reaction kinetics and heat transfer characteristics. The trade-offs of respective design are increased NO_x emissions and slightly decreased temperature uniformity.     

Effects of rectangular rib on exergy efficiency of a hydrogen-fueled micro combustor

In order to further optimize the working performance of micro-cylindrical combustor, the combustion chamber of the micro-cylindrical combustor is inserted in a rectangular rib. Extensive numerical investigations are conducted to compare the exergy efficiency of non-ribbed and rectangular-ribbed micro combustors under various hydrogen mass flow rates and hydrogen/air equivalence ratios. Moreover, the effects of dimensionless rib positions and heights on the exergy efficiency of micro-cylindrical combustor are also widely investigated. Results suggest that the exergy efficiency of the rectangular-ribbed micro combustor is significantly higher than that of the non-ribbed micro combustor under different inlet conditions. Moreover, the exergy efficiency of the rectangular-ribbed micro combustor is significantly affected by the dimensionless positions l. The optimum dimensionless l is increased with the increase of hydrogen mass flow rate. This work offers us significant reference for optimizing the micro combustor in energy utilization.     

Combustion in micro-cylindrical combustors with and without a backward facing step

Micro-combustors are critical components for micro-power systems using hydrogen and hydrocarbon fuels as an energy source. The micro-thermophotovoltaic (TPV) power system requires an output of high and uniform temperature from the wall of the combustor. This paper presents the experimental results of three types of stainless cylindrical micro-combustor with or without a backward facing step. Hydrogen was used as the fuel. The temperatures at exit and along the wall of the combustors were measured. The results show that the backward facing step provides a simple yet effective solution to enhance the mixing of fuel mixture and prolong the residence time. In addition, the step is very useful in controlling the position of flame and widening the operational range of the flow rate and H₂/air ratio. A high and uniform temperature distribution has been achieved for micro-combustors with a backward facing step. This result is relevant to the application of micro-TPV power systems we are currently pursuing.     

Flame stabilization and emission of small Swiss-roll combustors as heaters

The characteristics of small Swiss-roll combustors were investigated experimentally in detail. Three types of Swiss-roll combustors of different designs and two cases of heat transfer conditions were studied. The effects of design parameters on the performance of these combustors were examined. Each combustor consisted of a combustion region at the center (called the combustion room) and double spiral-shaped channels, the widths of which were smaller than the minimum quenching distance of a propane premixed flame at a normal state. Flames could be stabilized successfully for a wide range of equivalence ratios and mean velocities by using the recirculated heat from the burned gas, and blow-off was not observed. Temperature distributions of the combustors, variation of gas temperature, and the concentrations of the exhaust gas from the combustors were also investigated. Mean temperatures of the combustors were found to be governed by both the radiant heat loss from the combustors and the total chemical energy liberated by the combustors. Efficiencies of the combustors as heaters were evaluated. As a combustor became smaller, its thermal efficiency as a heater increased and its NO_x emission decreased, while the emission of CO increased. By adding a catalytic reactor at the exhaust port, it was found that the emission of CO could be eliminated. This study provides new experimental results for a small Swiss-roll combustor, which represents an essential step toward the development of a microcombustor.     

Combustion characteristics of a lean premixed LPG–air combustor

Fuel/air mixing effects in a premixer have been examined to investigate the combustion characteristics, such as the emission of NO_x and CO, under simulated lean premixed gas turbine combustor conditions at normal and elevated pressures of up to 3.5 bar with air preheat temperature of 450 K. The results obtained have been compared with a diffusion flame type gas turbine combustor for emission characteristics. The results show that the NO_x emission is profoundly affected by the mixing between fuel and air in the combustor. NO_x emission is lowered by supplying uniform fuel/air gas mixture to the combustor and the NO_x emission reduces with decrease in residence time of the hot gases in the combustor. The NO_x emission level of the lean premixed combustor is a strong function of equivalence ratio and the dependency is smaller for a traditional diffusion flame combustor under the examined experimental conditions. Furthermore, the recirculation flow, affected by dome angle of combustor, reduces the high temperature reaction zone or hot spot in the combustor, thus reducing the NO_x emission levels.     

Water and steam injection in micro gas turbine supplied by hydrogen enriched fuels: Numerical investigation and performance analysis

This paper investigates the energetic and environmental performance of micro gas turbine plant with two proposed concurrent improvements: the methane-based fuel enriched by hydrogen and the humidification of the plant cycle. The energetic and environmental benefits of both features are well-know, and the aim of this work is the analysis of their combined impact on the micro gas turbine operation. Despite enhancing fuel with H₂ involves significant advantages like greenhouse emission reduction and a better combustion in case of low LHV fuels, most of commercial micro gas turbine combustors are not able to burn fuels with high hydrogen content unless structurally modified. On the contrary, has been demonstrated that humidified gas turbines (i.e., gas turbines with water injection, humid air turbine (HAT) and steam injection gas turbine (STIG) cycles) improve the combustion stability as well as electric power delivered and plant efficiency. Hence, in order to investigate the feasibility of the concurrent two features, the first step of this work was the thermodynamic analysis of a micro gas turbine supplied by methane-based fuels enriched with H₂ up to 20%_vol, considering both dry and humidified cycles. Since a combustion anomaly was detected, i.e., flashback, in the CFD study on the combustion chamber, a steam injection in the combustor has been added in the plant layout with the aim of overcoming the anomaly, and its effect on the combustion process has been analyzed also raising the hydrogen content up to 30%_vol. The main outcome of this paper is the assessment of the feasibility of supplying the combustor of the proposed HGT-STIG micro gas turbine with a hydrogen enrichment up to 30%_vol, achieving a safe and regular combustion mainly owing to a steam injection mass flow equal up to 125% of fuel flow.     

Numerical investigations on thermal performance and flame stability of hydrogen-fueled micro tube combustor with injector for thermophotovoltaic applications

In order to design a micro tube combustor with good thermal performance and flame stability for thermophotovoltaic applications, in this work, thermal performance and flame stability of hydrogen-fueled micro tube combustors with backward facing step and with injector are compared. It is found that the decrease of diameter ratio d₂/d₁ leading to expansion of the symmetrical recirculation zone, which is helpful for fluid and heat circulation, and higher flame locations, which is not helpful for flame stabilization. Furthermore, effects of diameter ratio d₂/d₁ on thermal performance and flame stability are analyzed and discussed. Results suggest that when the diameter ratio d₂/d₁ is decreased from 0.9 to 0.8, positive effects of injector on thermal performance are enhanced and flame stability is improved under lower hydrogen/air equivalence ratio. Finally, the applications conditions of the micro tube combustor with injector are achieved. This work will provides us significant reference for designing micro tube combustor with injector.     

Measurement of the mixing quality in premix combustors

The problem of mixing fuel and air is the essential point of low emission combustion in gas turbines. In premixed combustors and fuel staged combustors the quality of the fuel–air mixture is the determinant parameter for the amount of emissions of nitric oxides (NO_x). The nearly perfect preparation of the fuel–air mixture is also a condition for trouble-free operation in catalytical combustion. Prevaporization of liquid fuels hampers the process of mixing. So the investigative work at the Department of Steam and Gas Turbines at the University of Bochum concentrated on experiments with liquid fuels. The results show that there is a great potential of reducing NO_x emission even with liquid fuels and reveal the key role of prevaporization and mixing. The experiments were carried out at a premix combustion test rig at moderate pressure. By using the technique of planar-laser induced fluorescence (LIF), highly time and spatially resolved measurements of fuel concentrations were yielded from the experiments. The optical measurements showed the structure of the mixture field of fuel and air in the zone downstream of the flameholder. The pollutant emissions were simultaneously monitored with conventional gas analysers. As the main result, the strong dependence of the pollutant emissions on the mixture could be clearly revealed. On one hand the homogeneity over the cross-section of the combustor was the main condition for low emission combustion. On the other hand the time-resolved two-dimensional LIF images of the turbulent mixture field showed that the instationary distribution also had a considerable influence on the rate of emissions. Even the mixture of static mixers contained spatial and temporal inhomogeneities, which could be observed by using the LIF-technique but not with conventional methods.     

Performance of a micro-thermophotovoltaic power system using an ammonia-hydrogen blend-fueled micro-emitter

The potential of ammonia (NH₃)-hydrogen (H₂) blends as a carbon-free, green fuel in a 1–10 W micro-thermophotovoltaic (micro-TPV) device is evaluated experimentally. When NH₃–H₂ blends are used directly (without any modification) in a heat-recirculating micro-TPV configuration that has an installation of gallium antimonide (GaSb) photovoltaic cells and was developed for hydrocarbon fuel, low temperature on the micro-emitter outer surface is observed, generating a secondary flame at the micro-emitter outlet. Thus, the micro-TPV device has been modified to eliminate the secondary flame by enhancing the residence time of fed NH₃–H₂–air mixtures and uniform burning: a cyclone adapter for a fuel-air mixture supply system and a helical adapter for the fuel-air mixture upstream of the micro-emitter. Under optimized design and operating conditions, the micro-TPV device produces 5.2 W with an overall efficiency of 2.1% and an emitter efficiency of 37%, indicating the maximum temperature of the micro-emitter outer surface up to 1408 K. Thus, the feasibility of using NH₃–H₂ blends in practical micro power-generation devices has been demonstrated, implying the potential of partial NH₃ substitution to improve the safety of pure H₂ use with no carbon generation.     

Combustion and heat transfer at meso-scale with thermal recuperation

Results are presented from successfully designed and fabricated meso-scale ceramic combustors that incorporate internal thermal energy recirculation. The combustor provided sustained operation using propane and air as the reactants. Flames could be obtained well below the normal quenching distance. The development required examination of several different combustor designs and materials. Flammability limits of these combustors have been determined experimentally. Experimental investigations have been performed on the effects of flame holder geometry, material conductivity, equivalence ratio, and inlet Reynolds number on the combustor performance. Measurement of the reactant preheating and product exhaust temperatures was performed using K-type thermocouples which were installed with minimal intrusion to the flow. The reactant preheating temperatures were observed to be in the range 700 K–1000 K. However, the combustor suffered significant overall heat loss (50–85%) which was implied by the low exhaust temperatures (500 K–750 K). For a constant fuel flow rate, the exhaust temperature increased monotonously with decrease in equivalence ratio until the blow-off condition implying that the combustor’s maximum thermal efficiency occurs at its lean blow-off limit. Thermal imaging of the combustor walls was performed using infrared camera to obtain the temperature distribution within the combustor. Numerical simulations were performed with the aid of CFD software using a heat loss coefficient chosen so as to give best correlation with experimental results. These CFD simulations helped to obtain better insight of the dependence of combustor performance on thermal conductivity of the material and heat load.     

Numerical study on premixed hydrogen/air combustion characteristics and heat transfer enhancement of micro-combustor embedded with pin fins

Aimed at improving the energy output performance of the Microthermal Photovoltaic (MTPV) system, it is necessary to optimize the structure of the micro combustor. In this paper, micro combustor with in-line pin fins arrays (MCIPF) and micro combustor with both end-line pin fins arrays (MCEPF) were presented to realize the efficient combustion and heat transfer enhancement, and the influence of inlet velocity, equivalent ratio, and materials on thermal performance was investigated. The results showed that pin fins embedding is beneficial to improving combustion, and the combustion efficiency of MCIPF and MCEPF reaches 98.5% and 98.7%, which is significantly higher than that of the conventional cylindrical combustor (MCC). However, with the increase of inlet velocity from 8 m/s to 14 m/s, MCIPF exhibits the highest external wall temperature with a range of (1302–1386 K), while MCEPF maintains the best temperature uniformity. As the inlet velocity increases to 10 m/s, the external wall temperature and temperature uniformity reach the optimum. Besides, under the conditions of different equivalence ratios, both external wall temperature and heat flux increases first and then decreases, meanwhile the temperature uniformity of MCEPF is significantly improved compared with that of MCIPF, they all exhibit the highest external wall temperature with an equivalence ratio of 1.1, and the thermal performance is greatly enhanced. By comparing the heat transfer performance of combustors with different materials based on MCEPF, it is interesting to find that the application of high thermal conductivity materials can not only increase the external wall temperature, but also improve the temperature uniformity. Therefore, materials with high thermal conductivity such as Aluminum, Red Copper and Silicon Carbide should be selected for application in micro combustors and their components. The current work provides a new design method for the enhanced heat transfer of the micro combustor.     

Numerical study of hydrogen‐enriched methane–air combustion under ultra‐lean conditions

The demand for gas turbines that accept a variety of fuels has continuously increased over the last decade. Understanding the effects of varying fuel compositions on combustion characteristics and emissions is critical to designing fuel‐flexible combustors. In this study, the combustion characteristics and emissions of methane and hydrogen‐enriched methane were both experimentally and numerically investigated under ultra‐lean conditions (Ø ≤ 0.5). This study was performed using global mechanisms with a one‐step mechanism by Westbrook and Dryer and a two‐step mechanism with an irreversible and reversible CO/CO₂ step (2sCM1 and 2sCM2). Results show that the 2sCM2 mechanism under‐predicted the temperature, major species, and NO_x by more than 100% under ultra‐lean conditions; thus, we proposed a modified‐2sCM2 mechanism to better simulate the combustion characteristics. The mechanisms of Westbrook, 2sCM1, and modified 2sCM2 predicted the temperature and the CO₂ emission with an average deviation of about 5% from the experimental values. Westbrook and 2sCM1, however, over‐predicted the NO_x emission by approximately 81% and 152%, respectively, as compared with an average under‐prediction of 11% by the modified‐2sCM2 mechanism. The numerical results using the proposed modified‐2sCM2 mechanism shows that the presence of hydrogen in the fuel mixture inhibits the oxidation of methane that led to the formation of unburned hydrocarbons in the flame. We also showed that for any given fuel compositions of H₂/CH₄, there is an optimum equivalence ratio at which the pollutant emissions (CO and NO_x) from the combustor are minimal. Zero CO and 5 ppm NO_x emissions were observed at the optimal equivalence ratio of 0.45 for a fuel mixture containing 30% H₂. The present study provides a basis for ultra‐lean combustion toward achieving zero emissions from a fuel‐flexible combustor. Copyright © 2016 John Wiley & Sons, Ltd.     

A chemical reactor network for oxides of nitrogen emission prediction in gas turbine combustor

This study presents the use of a new chemical reactor network （CRN） model and non-uniform injectors to predict the NOx emission pollutant in gas turbine combustor. The CRN uses information from Computational Fluid Dy- namics （CFD） combustion analysis with two injectors of CH4-air mixture. The injectors of CH4-air mixture have different lean equivalence ratio, and they control fuel flow to stabilize combustion and adjust combustor＇s equiva- lence ratio, Non-uniform injector is applied to improve the burning process of the turbine combustor. The results of the new CRN for NOx prediction in the gas turbine combustor show very good agreement with the experimen- tal data from Korea Electric Power Research Institute.     

Numerical investigation on the mixing performance of H2 and air in curved micro-channel combustors

Curved micro-channels are frequently used in micro-Swiss roll combustors and other applications. The secondary flows (i.e., Dean vortices) play an important role in both the mixing performance of fuel and oxidant and the flame propagation characteristics in curved micro-channel combustors. In the present study, the helicity method was adopted and a mixing performance evaluation criterion (MPEC) was proposed to investigate the impacts of inlet velocity, nominal equivalence ratio (φ) and curvature radius on the Dean vortices and mixing performance of H₂ and air in curved micro-channels. First, it is found that with the increase of inlet velocity, the intensity of Dean vortices increases and their shape grow asymmetrical. When the inlet velocity is high enough, another pair of smaller vortices appear near the outer wall. Meanwhile, the mixing performance of H₂ and air becomes worse due to the reduced residence time of gas flow. Second, as the nominal equivalence ratio is decreased, the Dean vortices are intensified and the vortices shape become asymmetrical. Moreover, the mixing process is improved owing to the enhanced secondary flows. Thirdly, the intensity of Dean vortices increases significantly as the curvature radius is decreased. However, the mixing performance becomes worse due to the shortened length of the curved micro-channel. Finally, an empirical correlation between MPEC and the Reynolds number (Re), Dean number (De) and φ was obtained based on the numerical results, which may provide a guidance for the design and operation of curved micro-channel combustors.     

Swirling distributed combustion for clean energy conversion in gas turbine applications

Distributed combustion provides significant performance improvement of gas turbine combustors. Key features of distributed combustion includes uniform thermal field in the entire combustion chamber, thus avoiding hot-spot regions that promote NO_x emissions (from thermal NO_x) and significantly improved pattern factor. Rapid mixing between the injected fuel and hot oxidizer has been carefully explored for spontaneous ignition of the mixture to achieve distributed combustion reactions. Distributed reactions can be achieved in premixed, partially premixed or non-premixed modes of combustor operation with sufficient entrainment of hot and active species present in the flame and their rapid turbulent mixing with the reactants. Distributed combustion with swirl is investigated here for our quest to explore the beneficial aspects of such flows on clean combustion in simulated gas turbine combustion conditions. The goal is to develop high intensity combustor with ultra low emissions of NO and CO, and much improved pattern factor. Experimental results are reported from a cylindrical geometry combustor with different modes of fuel injection and gas exit stream location in the combustor. In all the configurations, air was injected tangentially to impart swirl to the flow inside the combustor. Ultra-low NO_x emissions were found for both the premixed and non-premixed combustion modes for the geometries investigated here. Swirling flow configuration, wherein the product gas exits axially resulted in characteristics closest to premixed combustion mode. Change in fuel injection location resulted in changing the combustion characteristics from traditional diffusion mode to distributed combustion regime. Results showed very low levels of NO (∼3 PPM) and CO (∼70 PPM) emissions even at rather high equivalence ratio of 0.7 at a high heat release intensity of 36 MW/m³-atm with non-premixed mode of combustion. Results are also reported on lean stability limit and OH^* chemiluminescence under both premixed and non-premixed conditions for determining the extent of distribution combustion conditions.     

贫预混预蒸发燃烧室振荡燃烧特性的实验研究

针对燃用航空煤油的贫预混预蒸发模型燃烧室的振荡燃烧特性开展了实验研究。实验表明:在相同的燃烧室入口空气燃料混合物流速下,随着当量比的增加,燃烧室振荡燃烧的振荡主频从132 Hz增加到144 Hz,但燃烧室的均方根脉动压力幅值却从1 464 Pa下降到342 Pa。在当量比不变情况下,入流空气燃料混合物流速较低时,容易引发振荡燃烧现象,而当入流空气燃料混合物流速较高时,则燃烧会变得稳定。分析了整个燃烧实验装置的前4阶轴向声学模态频率,发现实验中所激励出的振荡燃烧主频和第二阶轴向声学模态频率吻合的很好。     

The effect of premixed stratification on the wave dynamics of a rotating detonation combustor

Typical injection schemes of rotating detonation combustors inject fuel locally into the combustion channel, creating stratified fuel-rich and fuel-lean mixing regions. In this study, premixed hydrogen and air rotating detonations are explored in a rotating detonation combustor through premixing part of the fuel into the oxidizer flow. The objective is to investigate the effect of premixing on the operation of the combustor. Three premixing schemes are examined where the detonation wave speeds are analyzed. The results show that in premixing, the fuel-lean regions became more favorable for continuous detonation propagation when premixed with the bypass fuel, resulting in higher detonation wave speeds. This phenomenon is shown to be independent of the global fuel-air equivalence ratio and the amount of fuel premixed into the oxidizer. As such, combustor performance and the operational regime could be improved with lean hydrogen premixing amounts in the main flow oxidizer.