Prototype Combustor Research Articles

Numerical modeling and suppression of combustion instabilities in a partially premixed combustor

To suppress the combustion instabilities faced in the lean premixed combustion, the impacts of swirler hub configurations on combustion instabilities under elevated pressure are investigated using large eddy simulation combined with a flamelet generation manifold model. Good agreement between the numerical predictions and experimental data is achieved. The flow fields of the combustors with three distinct swirler configurations are simulated: prototype, swirler with lobes on the hub of pilot stage, and with lobes on the hub of the first main stage. Furthermore, dynamic mode decomposition (DMD) is used to extract the dynamic characteristics, and a flame transfer function (FTF) is employed to characterize the fluctuation characteristics. The results show that the prototype combustor demonstrates a coupled fluctuation between flow and heat release. Influenced by the precessing vortex core (PVC), the flame angle varies between 70° and 90° and the first DMD modes of axial velocity, temperature, and heat release rate are all at a frequency of 470 Hz. The lobes on the hub of the pilot stage suppress the formation of PVC, making the combustion very stable. The flame angle remains constant at 80°, and the gain of FTF is lower than 1. However, adding lobes to the first main stage makes the combustion extremely unstable. The flow field structure undergoes drastic changes, mimicking a “breathe” process. The flame surface is highly distorted, and flashback phenomena occur.

Experimental and numerical comparisons of geometric scaling criteria for lean premixed swirl combustor

This study experimentally and numerically investigates the applicability of the DaI and Re criteria for scaling the geometry of a lean premixed swirl combustor during a reaction and in the absence of it. We first set up an experimental system to test the loss of pressure, the flow field, and NOx emissions in a prototype combustor and two models of it scaled to 3/5 of its size. The results showed that the friction in the flow in the prototype decreased with an increase in its intensity, and the corresponding constant DaI model (M-D) exhibited a similar trend, while the constant Re model (M-R) exhibited an adverse trend to that of the prototype. The results of particle image velocimetry (PIV) of the flow field in the non-reactive state showed that regardless of the criterion used and the state of the reaction, the flow fields of the prototype and the models were similar under flows of different strengths. However, a quantitative comparison of their distributions of velocity showed that the peak velocity of the rotating jet of M-R was significantly lower than that of the prototype. PIV results of the flow field in the reactive state exhibited similar phenomena. Moreover, the NOx emissions of M-D were consistent with those of the prototype, while emissions from M-R were significantly higher. The numerical results also showed that the shape of the flame and the pattern of flow of M-R were significantly different from those of the prototype.

Effects of scaling laws on flow and combustion characteristics of lean premixed swirl burners

Modern heavy gas turbine combustors are always huge, so it is difficult and costly to do experiment. Thus, geometry scaling method has come into sight. In this paper, based on a single lean premixed swirl burner, validated computational fluid dynamic (CFD) model was used to study the effects of different scaling laws on various scalling models from 1/2 to 1/10. Experimental study on prototype combustor and the 3/5 scale model under full operating condition is also carried out to verify the NOx emission under different laws. Results showed that DaI scaling law was able to maintain good similarity under combustion state when scaling factor = 1/2–1/5, while Re scaling law would bring significant changes on flow and flame characteristics. The emission of NOx is also similar to prototype by using Da<sc>i</sc> law. But Re law could keep flow characteristics under non-combustion state. It is suggested that Dai law is suitable for lean premixed swirl combustor geometry scaling.

Research on combustion performance of a micro-mixing combustor for methane-fueled gas turbine

This research proposes an H-class micro-mixing gas turbine combustor prototype with a thermal power of 1.15 MW, which employs 40 exterior jet-in-crossflow micro-mixing nozzles with a tapered round outlet to reduce pollutant emissions and prevent the flashback. These nozzles are located by a distribution arrangement in order to improve the temperature distribution uniformity of field flow. To respond to off-design operation of gas turbine, the combustion performance of the micro-mixing combustor prototype is investigated numerically at the rated fuel temperature of 473 K and other ones of 373, 423, 523 and 573 K. The results indicate that NO emission has a relatively high augment as the fuel temperature decreases, which is due to the rising local equivalence ratio. The variation trends of the temperature distribution uniformity, combustion efficiency and total pressure loss are not obvious as the fuel temperature changes. Based on the rated and off-design combustion performance at different radial planes, this research finally proposes an optimal relationship between the axial liner length and micro-mixing nozzle outlet diameter.

Effect of fuel flexibility on combustion performance of a micro-mixing gas turbine combustor at different fuel temperatures

An H-class gas turbine combustor prototype that employs the micro-mixing combustion technology is first proposed in the current research. It consists of 60 interior jet-in-crossflow micro-mixing nozzles with a distributed arrangement. To analyze the fuel flexibility, a blend of 50% H2 and 50% CO by volume and the pure H2 are considered for the micro-mixing combustor prototype in terms of the design and alternative fuels, respectively. Secondly, this research discusses the fuel temperature effects on the rated and off-design combustion performance for both fuels, because the fuel temperature is an important factor for the power generation efficiency in the gas turbine combined cycles. The results indicate that at the rated fuel temperature of 473 K and off-design ones of 423, 523 and 573 K, the micro-mixing combustor prototype has a great fuel flexibility, in which NO emission is lower than 15 μL/L at 15% O2, outlet temperature distribution factor is lower than 15%, combustion efficiency is higher than 99%, and total pressure recovery factor is higher than 95% for both fuels. Through the trends of combustion performance in different radial planes, it can be found that the high temperature zone of flue gas produced by H2 is longer than H2/CO (50%/50%), and far away the nozzles. Thus, the H2-fired micro-mixing combustor prototype needs a longer axial liner length than H2/CO (50%/50%), in order to realize the enhanced temperature distribution uniformity and combustion efficiency. Eventually, an optimal relationship between the axial liner length and fuel hole diameter is proposed for both fuels based on the trade-offs around the whole combustion performance from the perspective of engineering applications.

Reducing Uncertainty in the Onset of Combustion Instabilities Using Dynamic Pressure Information and Bayesian Neural Networks

Abstract Modern low-emission combustion systems with improved fuel-air mixing are more prone to combustion instabilities and, therefore, use advanced control methods to balance minimum NOx emissions and the presence of thermoacoustic combustion instabilities. The exact operating conditions at which the system encounters an instability are uncertain because of sources of stochasticity, such as turbulent combustion, and the influence of hidden variables, such as unmeasured wall temperatures or differences in machine geometry within manufacturing tolerances. Practical systems tend to be more elaborate than laboratory systems and tend to have less instrumentation, meaning that they suffer more from uncertainty induced by hidden variables. In many commercial systems, the only direct measurement of the combustor comes from a dynamic pressure sensor. In this study, we train a Bayesain Neural Network to predict the probability of onset of thermoacoustic instability at various times in the future, using only dynamic pressure measurements and the current operating condition. We show that on a practical system, the error in the onset time predicted by the Bayesain Neural Networks is 45% lower than the error when using the operating condition alone and more informative than the warning provided by commonly used precursor detection methods. This is demonstrated on two systems: (i) a premixed hydrogen/methane annular combustor, where the hidden variables are wall temperatures that depend on the rate of change of operating condition, and (ii) full-scale prototype combustion system, where the hidden variables arise from differences between the systems.

Design of a Fluidic Actuator with Independent Frequency and Amplitude Modulation for Control of Swirl Flame Dynamics

Fluidic actuators are designed to control the oscillatory helical mode, called a precessing vortex core (PVC), which is often observed in gas turbine combustors. The PVC induces large-scale hydrodynamic coherent structures, which can considerably affect flow and flame dynamics. Therefore, appropriate control of this structure can lead to a more stable and efficient combustion process. Currently available flow control systems are designed to control the PVC in laboratory-scale setups. To further develop these systems and find an approach applicable to the industrial scale, a new actuator design based on fluidic oscillators is presented and studied in this paper. This actuator allows for independently adjusting forcing frequency and amplitude, which is necessary to effectively target the dynamics of the PVC. The functionality and flow control of this actuator design are studied based on numerical simulations and experimental measurements. To verify the flow control authority, the actuator is built into a prototype combustor test rig, which allows for investigating the impact of the actuator’s forcing on the PVC at isothermal conditions. The studies conducted in this work prove the desired functionality and flow control authority of the 3D-printed actuator. Accordingly, a two-part stainless steel design is derived for future test conditions with flame.

LES Analysis of CO Emissions from a High Pressure Siemens Gas Turbine Prototype Combustor at Part Load

This work contributes to the understanding of mechanisms that lead to increased carbon monoxide (CO) concentrations in gas turbine combustion systems. Large-eddy simulations (LES) of a full scale high pressure prototype Siemens gas turbine combustor at three staged part load operating conditions are presented, demonstrating the ability to predict carbon monoxide pollutants from a complex technical system by investigating sources of incomplete CO oxidation. Analytically reduced chemistry is applied for the accurate pollutant prediction together with the dynamic thickened flame model. LES results show that carbon monoxide emissions at the probe location are predicted in good agreement with the available test data, indicating two operating points with moderate pollutant levels and one operating point with CO concentrations below 10 ppm. Large mixture inhomogeneities are identified in the combustion chamber for all operating points. The investigation of mixture formation indicates that fuel-rich mixtures mainly emerge from the pilot stage resulting in high equivalence ratio streaks that lead to large CO levels at the combustor outlet. Flame quenching due to flame-wall-interaction are found to be of no relevance for CO in the investigated combustion chamber. Post-processing with Lagrangian tracer particles shows that cold air—from effusion cooling or stages that are not being supplied with fuel—lead to significant flame quenching, as mixtures are shifted to leaner equivalence ratios and the oxidation of CO is inhibited.

Non-premixed combustion characteristics and thermal performance of a catalytic combustor for micro-thermophotovoltaic systems

Micro-combustors with reliable flame stability and high surface temperature are crucial for micro-thermophotovoltaic devices. Recently, we verified the feasibility of catalytic combustion for non-premixed CH4/air in an asymmetrical planar micro-combustor. The upper limit of operational average velocity is 1.2 m/s and the maximum combustion efficiency is 52.4%. In the present work, a symmetrical configuration was proposed to improve the performance of the prototype combustor. Numerically investigation shows that the symmetrical combustor can be operated at an average velocity above 9 m/s, and the maximum combustion efficiency and surface radiation efficiency are 98.5% and 25.9%, respectively. In addition, as the average velocity is increased, the combustion efficiency decreases monotonically while the radiation efficiency and radiant energy output increase first and then decrease. Moreover, the combustion efficiency achieves its peak value under stoichiometric ratio. The accumulation of O2 near the Pt surface under fuel lean condition causes a sharp increase in O(s) coverage and a decrease in heat release rate. Furthermore, a larger wall thermal conductivity leads to more uniform wall temperature distribution and better heat recirculation effect on the incoming gases. However, a medium thermal conductivity of 1–10 W/(m·K) favors a larger radiant energy output.

Experimental investigation on thermal and combustion performance of a combustor with microchannel cooling

Microchannel cooling provides an efficient thermal management method for regenerative cooling systems of advanced areo engines. In this study, a prototype combustor with microchannel cooling passages in the walls is experimentally developed. Combustion tests are systematically conducted in a range of air inlet temperature (30–70 °C), equivalence ratios (0.8–1.2) and flow rates of coolant (100–300 kg/m2s) using liquid fuel of ethanol and air mixture. Comparative studies against a combustor without microchannel cooling are also conducted to assess the feasibility of performance enhancement. The introduction of microchannel cooling was found to induce an 80% reduction of the maximum wall temperatures, and a larger than 90% reduction of the streamline wall temperature gradient. The combustor with microchannel cooling also induced a 1%–3.2% increase of the combustion efficiency and a 0.3%–1.7% reduction of the CO emissions in general compared to that without microchannel cooling. The wall temperatures and combustion efficiency of the combustor with microchannel cooling were found to increase with the increase in air inlet temperatures from 30 to 70 °C, whereas they tended to decrease when the equivalence ratio increased from 0.8 to 1.2. Nevertheless, the flow rate of coolant played a negligible role on the combustion performance of the combustor with microchannel cooling.

Design and setup of a low calorific SOFC off-gas combustion chamber for a pressurized MGT hybrid power plant test rig

A demonstrator system for a hybrid power plant is currently being built at DLR, designed for an electrical power output level of 30 kW. Since the very low energy dense exhaust gas of the fuel cell anode side represents the fuel for the combustion chamber in this application, a low calorific SOFC off-gas combustor was developed at DLR specifically for this use case. With thorough investigations on the atmospheric test rig, the expected operational range of the combustor was quantified in preceding works. Now, a novel machine design, including dilution air with an adjustable air split configuration is derived to validate the gathered information on the micro gas turbine test rig under pressurized machine conditions. This work explains the design of the combustion system and addresses the different design features specifically implemented for this use case. Since simplifications had to be made for the atmospheric combustor prototype, a significant positive influence on the operational envelope is expected with the transition to the machine configuration.

Design and commissioning of a multi-mode prototype for thermochemical conversion of human faeces

This article describes the design and commissioning of a micro-combustor for energy recovery from human faeces, which can operate both in updraft and downdraft modes. Energy recovery from faecal matter via thermochemical conversion has recently been identified as a feasible solution for sanitation problems in low income countries and locations of high income countries where access to sewage infrastructures is difficult or not possible. This technology can be applied to waterless toilets with the additional outcome of generating heat and power that can be used to pre-treat the faeces before their combustion and to ensure that the entire system is self-sustaining. The work presented here is framed within the Nano Membrane Toilet (NMT) project that is being carried out at Cranfield University, as part of the Reinvent the Toilet Challenge of the Bill and Melinda Gates Foundation.For this study, preliminary trials using simulant faeces pellets were first carried out to find out the optimum values for the main operating variables at the scale required by the process, i.e. a fuel flowrate between 0.4 and 1.2 g/min of dry faeces. Parameters such as ignition temperature, residence time, and maximum temperature reached, were determined and used for the final design of the bench-scale combustor prototype. The prototype was successfully commissioned and the first experimental results, using real human faeces, are discussed in the paper.

Active Combustion Control for a Low-Emissions Aircraft Engine Combustor Prototype: Experimental Results

Lean combustion concepts for aircraft engines are more susceptible to combustion instabilities. Prior activity has demonstrated active control to suppress combustion instabilities with a conventional rich-front-end combustor. The current effort is developing further understanding of the problem specifically as applied to future lean-burning, very low-emissions aircraft engine combustors. A prototype low-emissions aircraft engine combustor with a combustion instability has been identified and previous work has characterized the combustor’s dynamic behavior. The combustor exhibits thermoacoustic instabilities related to increasing fuel flow that prevent full-power operation. Active combustion instability suppression using the adaptive sliding phasor average control method has been demonstrated experimentally with this combustor prototype in a combustion test cell operating at engine pressures, temperatures, and flows. A high-frequency fuel valve perturbed the combustor fuel flow. Successful instability suppression was shown using a dynamic pressure sensor in the combustor for controller feedback. Instability control was also shown with a pressure feedback sensor in the lower temperature region, upstream of the combustor. It was also demonstrated that the controller can prevent the instability from occurring while combustor operation was transitioning from a stable low-power condition to a normally unstable high-power condition, thus enabling the high-power condition.

NOx Reduction Strategy by Staged Combustion with Plasma-Assisted Flame Stabilization

A plasma-assisted staged combustor that can reduce NOx generation is introduced. This combustor is based on a commercial combustor. Only the internal structure of the commercial combustor head was modified to introduce staged combustion and plasma into the combustor. The outer frame and configuration of the combustor remain unchanged. Staged combustion adopts successive fuel-rich and fuel-lean flames to achieve low NOx combustion. However, this can cause flame instability. In the proposed combustor, plasma was adopted for stable operation of the first-stage combustor. The proposed combustor recorded lower NOx generation than a prototype combustor, and three different mechanisms that affected this NOx reduction in the staged combustor were analyzed. Plasma-stabilized fuel-rich flame in the first-stage combustor and hydrogen from the rich flame played a key role for NOx reduction. Plasma appears to be a prominent tool for flame stabilization under the harsh conditions of a combustor for low NOx applications.

Gas Turbine Combustor Flow Structure Control Through Modification of the Chamber Geometry

As combustors are put in service, problems such as erosion, hot spots, and liner oxidation occur, and a solution based on lessons learned is essential to avoid similar problems in future combustor generations. In the present paper, a combustor flow structure control via combustor geometry alteration is investigated using laser Doppler velocimetry. Mainly, three configurations are studied. The first configuration is that of a swirl cup feeding a dump (rectangular cross section) combustor. The rectangular chamber is configured with a width to breadth (w/b) ratio of 85%. The second configuration is similar to the first one, but a combustion dome is installed. The dome is configured with a 9 deg difference in the expansion angle on both sides (asymmetric dome). The third configuration is that of a swirl cup and a combustion dome installed in a prototype combustor (single annular combustor (SAC) sector), with both primary and secondary dilution jets blocked. The SAC is configured with a cross sectional area that decreases toward the exit through the tilting of the inner combustor liner. The results show that the combustion dome eliminates the corner recirculation zone and the low velocity region close to the combustor walls. The combustion dome asymmetry results in a significant asymmetry in the velocity magnitude, as well as the turbulence activities and the tilting of the central recirculation zone (CRZ) toward the surface with the higher expansion angle. The liner tilting results in a 40% reduction in the length of the CRZ. However, once the primary jets are open, they define the termination point of the CRZ. The chamber w/b ratio results in a CRZ with the same diameter ratio (85%) in all configurations. Interestingly, the maximum reverse flow velocity is roughly constant in all measurement plans and configurations up to a downstream distance of 1R (R is the flare radius). However, with open primary jets, the CRZ strength increases appreciably. It appears that the confinement dictates both the flow field outside the CRZ and the size of the CRZ, while the swirl cup configuration mainly influences the strength of the CRZ. Regarding turbulence activities, the presence of the dome damps the fluctuations in the expanding swirling jet region. On the other hand, the primary jets increase the turbulence activities appreciably in the jet impingement region, as well as the upper portion of the CRZ (60% increase).

Blowoff mechanism of two dimensional bluff-body stabilized turbulent premixed flames in a prototypical combustor

Near blowoff dynamics and characteristics of turbulent premixed flames stabilized by a triangular flame holder in the midspan of a rectangular duct were studied using high speed imaging and simultaneous particle imaging velocimetry and OH planar laser-induced fluorescence. Near blowoff dynamics manifested by the onset of asymmetric vortex shedding and local extinction were observed. It has been proposed that a partial or total extinction of the flame sheet along the shear layers is the major factor that determines the final blowoff event. Observation of flame kernels within the recirculation zone, which under stable conditions contain only combustion products, is further evidence of the shear layer extinction. For a stably burning planar V flame away from blowoff, the flame front envelopes the Kelvin Helmholtz vortices. Near blowoff, the two flame fronts become more aligned with the flow direction due to reduction in flame speed and interact with the vortices emanating from the shear layer. This overlap induces high local stretch rates that exceed the extinction stretch rates, resulting in local flame extinction along the shear layers. Following extinction, fresh reactants are entrained into and react within the recirculation zone, with all other parts of the flame extinguished. This flame kernel within the recirculation zone may survive for time scales of about one hundred milliseconds, potentially reigniting the shear layers such that the entire flame is re-established for a short period. This shear layer extinction and re-ignition event can happen several times before final blowoff, which occurs when the flame kernel fails to reignite the shear layers before being extinguished itself, thus leading to global flame extinction.

E113 熱光発電システムに用いる高温作動マイクロセラミック触媒燃焼器の開発(OS1 マイクロエネルギー変換)

A micro-scale catalytic combustor using high-precision ceramic tape-casting technology has been developed for high-temperature applications such as thermophotovoltaic power generation. Pd catalyst/nanoporous alumina support fabricated through anodic oxidation was employed for the catalyst layer. Combustor structure that gives much smaller temperature gradient with high fuel-conversion efficiency has been proposed based on a series of CFD analysis of heat and fluid flow and surface reaction. In combustion experiments with a prototype combustor, the wall temperature as high as 700℃ has been achieved. It is also found that the wall temperature gradient is significantly reduced with the present combustor structure.

A08 スターリングエンジン用木質ペレット燃焼器に関する研究(スターリングエンジン及び関連要素と応用システム(2))

Gases burn more efficiently than solids, gasification is used for generating energy from wood pellets Electricity can be generated cleanly by a Stirling engine driven by the heat energy obtained through gasified combustion, along with a cogeneration system, without emitting surplus CO2 into the atmosphere. This study examined the basic combustion characteristics of a wood pellet gasified combustor for a Stirling engine and the performance of a Stirling engine cogeneration system with a wood pellet gasified combustor. A gasified combustor was built, and the combustor temperature and concentration was measured to study the combustion state. As a result, it was revealed that a gasified combustor was suitable for use with a Stirling engine because its exit temperature was higher than that of an ordinary updraft-type combustor. Our prototype combustor was capable of supplying a maximum 18.5KW to a Stirling engine using 23.8KW of fuel, resulting in a heat-to-fuel capacity of 78%.

Tabulation of NOx chemistry for Large-Eddy Simulation of non-premixed turbulent flames

Detailed chemistry tabulations are mostly based on reaction progress variables used to track species evolutions along trajectories in composition space, which in some cases are obtained from simulations of prototype combustion problems. For hydrocarbon chemistry, these progress variables are usually built from major species, as CO, CO2, H2O to which reactants are sometimes added. Prediction of NO mass fraction is first evaluated by comparing results from single progress variable and flamelet based chemical look-up tables against simulations of reference flames performed with the detailed chemistry mechanism that was used to generate the tabulations. Nitric oxides mainly formed in the burnt gases still evolve once major species have almost reached their equilibrium levels, then large errors in the predictions of NO mass fractions are unavoidable when using a progress variable based on major species only. It is concluded that the time scale separation inherent to NOx species must be included in the progress variable definition. A novel definition of progress variable is proposed for flame based tabulation and tested, which allows for reproducing both major species as well as NO, NO2 and N2O. Large-Eddy Simulation (LES) of a jet flame in a vitiated coflow is performed with the new tabulation and results are compared against experiments.

High-temperature micro catalytic combustor with Pd/nano-porous alumina

A micro-scale catalytic combustor using high-precision ceramic tape-casting technology has been developed for high-temperature applications such as thermophotovoltaic power generation. Pd catalyst/nano-porous alumina support fabricated through anodic oxidation was employed for the catalyst layer having good adhesion to the substrate. It is found that the ignition temperature of the present catalyst is around 225 °C, and its activity is as high as the catalyst prepared with the conventional sol–gel methods. In order to minimize thermal stress at high operation temperature, catalyst arrangement that gives minimum temperature gradient has been proposed based on a series of CFD analysis of heat and fluid flow and surface reaction in micro channels. In combustion experiments with a prototype combustor, the wall temperature as high as 850 °C and extremely high heat generation density of 2–5 × 10 8 W/m 3 have been achieved. It is also found that the wall temperature gradient is significantly reduced with the present catalyst arrangement.