Secondary Air Research Articles

Turbine cooling air estimation in thermodynamic simulations

AbstractModern gas turbines utilize a high amount of the core mass flow rate for component cooling. Thus, a coherent thermodynamic gas turbine representation demands a well-modeled secondary air system, which is able to estimate mass flow rates depending on respective design decisions. In this paper, the focus is set on the estimation of turbine blade cooling air. For this purpose, five different methods are presented and analyzed. The described concepts can be split into empirical and semi-empirical approaches. The semi-empirical approaches, the Horlock [1], Jonsson [2] and the Halliwell [3] method, are able to predict the blade temperature based on a given cooling mass flow rate or the needed cooling air based on a given blade temperature. In contrast, the empirical methods, the Grieb [4] and the Walsh [5] method, can only predict the cooling air consumption. Due to the fully empirical approaches the field of application is limited to the considered engine structures. On the other hand, the empirical methods lead to a better convergence behavior in comparison to the semi-empirical approaches due to their relatively simple calculation methods. The selected cooling air estimations are implemented in the performance code DLRp2 [6–8]. Therefore, processes and methods are deployed that allow to estimate turbines with unlimited cooled stages. Additionally, an off-design procedure is proposed to consider the occurring stagnation temperature drop between stator and rotor based on a reference temperature offset. A simplified thermodynamic gas turbine model is used to analyze the different cooling air estimation methods. For this purpose, sensitivity analyses for the main cooling air parameters are carried out. Moreover, all methods that were developed for the most stressed operating point are compared. Finally, the simplified model is calibrated to NASA’s energy efficient engine [9].

Experimentation of Grate Combustion of Municipal Solid Waste in Ouagadougou: Influence of Primary and Secondary Air Flow on Thermal Potential

The present work concerns the study of the influence of combustion air flows on the thermal potential during the combustion of a 2 kg mixture of solid model waste from the city of Ouagadougou. This mixture consists primarily of wood, cardboard and plastic. The prototype model is a natural laterite internal-walled grate furnace. During these investigations, we were interested in the temperature variation measured by 4 thermocouples K1 to K4 placed respectively from the waste bed to the top of the furnace. These variations are considered according to a variable primary and secondary air flow. The maximum primary and secondary airflow is 3000 L/min. Measurements are taken throughout the experiment at a step of 18 seconds and a maximum duration of 3 minutes for each case of flow considered. Maximum temperatures reaching 450oC are observed following the injection of secondary air only. This is explained by the supply of oxygen making the combustion more complete. The results show that the injection of primary and secondary air at a flow rate of 1500 L/min gives a more stable temperature result, i.e. 362,18oC 186,82oC 109,27oC 72,41oC respectively for the thermocouples K1, K2, K3, K4. An increase in the primary and secondary air flow to a maximum of 3000 L/min leads to a drop in temperatures due to the dilute internal atmosphere. These results allow to set the optimal operating parameters of the furnace in order to produce more heat for its recovery into electricity through a heat exchanger and a steam turbine.

FLOW AND HEAT TRANSFER IN ROTATING COMPRESSOR CAVITIES WITH INVERTED SHROUD-THROUGHFLOW TEMPERATURE DIFFERENCES

Abstract In an aero-engine compressor, co-rotating discs form cavities that interact with an axial throughflow of secondary air at low radius. In the high-pressure (HP) compressor the shroud is hotter than the throughflow (directed downstream to the turbine) and the radial temperature gradient creates buoyancy-induced flow at Grashof numbers ∼ 1013. Such flows can be unstable and typically take the form of counter-rotating vortex pairs separated by radial hot and cold plumes. However, in low pressure (LP) and intermediate pressure (IP) compressors the secondary air is directed upstream. In this inverse scenario the axial throughflow is hotter than the compressor discs, reversing the disc temperature gradient and eliminating the fundamental driver for buoyancy. Despite its practical application and importance, this inverse scenario has not been previously investigated. The University of Bath Compressor Cavity Rig has been uniquely designed to simulate such flows, measuring temperature and unsteady pressure in the frame of reference of the rotating discs. Bayesian and spectral analysis have determined the radial distribution of disc heat flux, as well as the asymmetry of the rotating vortex structures and their slip relative to the discs. Unexpectedly, the new data reveal the flow structure in cavities with positive and inverted temperature differences are fundamentally similar (albeit with reversed radial-temperature profiles). Isothermal cases identified a critical Rossby number (Ro), above which the flow structure in the cavity was dominated by a toroidal vortex. At sub-critical Ro, the flow structure for the inverted temperature gradient continued

Adjustment algorithm of damper openings of tangentially fired boilers based on secondary air distribution

Adjustment algorithm of damper openings of tangentially fired boilers based on secondary air distribution

Predictions of windage heating and flow characteristics in a high-speed rotating seal of aero-engine

The labyrinth seal plays a crucial role within the secondary air system of an aeroengine, significantly impacting its safety and reliability. This paper presents a comprehensive evaluation of the complex windage heating phenomenon within a high-speed rotating labyrinth seal system. Employing similarity criteria and dimensional analysis, the multifactorial influences on the seal system are thoroughly characterized. A dedicated high-speed rotating labyrinth seal test rig was constructed to conduct an experimental investigation into the windage heating characteristics of a four-tooth, stepped-slanted labyrinth seal. The results demonstrate a peak windage heating of 11.8 K and windage heating coefficient of 1.14 when the inlet temperature rose from ambient to 100 °C, under the rotating Mach number of 0.418 and pressure ratio of 1.1. Conversely, as the seal clearance increased from 0.2 mm to 0.6 mm, with a rotating Mach number of 0.487 and a pressure ratio of 2.0, the windage heating coefficient decreased to 0.57. Furthermore, five dimensionless operating cases were analyzed using similarity principles to elucidate the correlation between system performance and the combined effects of strong rotating flow and windage heating characteristics. The analysis revealed that the windage heating coefficient exhibited a decreasing trend with increasing flow Reynolds number, effective pressure ratio, inlet swirl ratio, and dimensionless clearance, as the effective pressure ratio increase from 1.1 to 5.0, with the rotating Mach number of 0.624 and flowing Reynolds number of 9600, the windage heating coefficient decrease from 1.831 to 0.797, representing a 56 % reduction. Conversely, the windage heating coefficient increased with a rising rotating Mach number, while the flowing Reynolds number demonstrated a minor influence.

Recovering Low-Grade Heat from Flue Gas in a Coal-Fired Thermal Power Unit

To achieve the goals of carbon peaking and carbon neutrality, the retrofitting of existing coal-fired power plants is crucial to achieving energy-saving and emission reduction goals. A conventional recovery system of waste heat typically occurs downstream of the air preheater, where the energy quality in flue gas is low, resulting in limited coal-saving benefits. This study proposes a scheme involving a flue gas exchanger bypassing the air preheater and low-temperature economizers, which is used to transfer the waste heat from flue gas to primary and secondary air (System I). Additionally, a heat pump can be introduced to provide supplementary energy for primary and secondary air, as well as the condensate from the steam turbine (System II). The coal consumption rate and exergy efficiency are used to evaluate the two schemes. The results show that both waste heat recovery systems can increase the power output of the coal-fired unit by recovering waste heat. System II can boost power output by approximately 13.98 MW. The power increase in both waste heat recovery systems show a declining trend as the unit load decreases. This increased power is primarily attributed to the medium- and low-pressure cylinders, while the contributions from ultra-high-pressure and high-pressure cylinders are negligible. The increased power output for the medium-pressure cylinder ranges from approximately 3.49 to 3.58 MW, while the low-pressure cylinder has an increased power output of around 10.10 to 10.19 MW. The coal consumption rate is decreased from 250.3 g/(kW·h) to 247.5 g/(kW·h) under a full load condition for both systems, which can be augmented at lower load conditions. System II outperforms System I at 30% load condition, achieving a reduced coal consumption rate of 3.36 g/(kW·h). System I has an exergy efficiency of 40%, while System II shows a higher efficiency of 44%.

Horizontal Distribution of Liquid in an Over-Row Sprayer with a Secondary Air Blower

The aim of this study was to determine the influence of boom height above a crop stand and the spacing between nozzles and diffusers in an over-row sprayer on the uniformity of the horizontal spray distribution and the uniformity of the air velocity distribution. The experimental setup involved a prototype over-row sprayer equipped with a boom with a working width of 8 m and ten air diffusers with spray nozzles. Air diffusers were connected to one or two nozzles each, and they were installed on the boom at intervals of 60, 80, and 90 cm. Terminal airflow velocity at a canopy is determined by the height of a sprayer boom and the diffuser spacing, ranging from around 2 m s–1 to around 27 m s–1. The sprayer boom should be positioned at a height of 50 cm above a crop stand due to the difference between the minimum and maximum airflow velocities. The horizontal spray distribution was more uniform when the sprayer was equipped with hollow-cone nozzles instead of flat-fan nozzles; hollow-cone nozzles should be applied if the distance between nozzles needs to be adjusted to the row width and row spacing. The analyzed coefficients did not exceed 10% when the boom was positioned 50 cm above the crop stand and when the nozzles were spaced 80 cm apart, which suggests that, in this configuration, sprayers equipped with hollow-cone nozzles can also be applied to close-grown crops.

Performance analysis of a conventional two-stage indirect evaporative cooler

With the aim of saving a portion of the high-quality energy consumed in mechanical vapor compression systems for air conditioning applications, the present work focuses on utilization of low-quality energy conversion processes due to fluid evaporation. The conventional indirect evaporative cooler is selected based on its preference for further modification compared to a regenerative indirect evaporative cooler. It is configured to be operated with two identical stages in series. This task is achieved by developing a thermodynamic model to analyze the performance of the cooling unit and to indicate the extent of its development compared to the cooler in one stage. Preliminary results show that the best operating condition is when the ratio of primary air to secondary air is 1. The cooling unit produces fairly comfortable air within limited climatic conditions. It delivers air at 26 °C and lower even at climatic temperatures as high as 48 °C, but within a very low relative humidity range. The relative humidity range extends to less than 58% as the outdoor air temperature decreases. At high humidity levels (64 to 80% depending on climate temperature), cool air cannot be obtained at 26 °C, and the cooling unit performance is limited only by what the first stage produces. The average performance of the cooling unit exceeds that of the first stage by 42.6%. Increasing air flow rate leads to an increase in cooling capacity, but it also has a negative impact on supply temperature and effectiveness.

Assisting denitrification and strengthening combustion by using ammonia/coal binary fuel gasification-combustion: Effects of injecting position and air distribution

The direct co-firing of ammonia (NH3) with coal is an effective approach for reducing carbon emissions from coal-fired plants. However, the limitations of NH3 combustion, such as its high ignition energy requirement and high NOx emissions, restrict its large-scale co-combustion in coal-fired units. In this study, a self-sustaining gasification–combustion experimental system was employed, and its denitrification performance and combustion strengthening capability for NH3/coal binary fuel under different NH3 injection positions and air distribution methods were evaluated. The results of co-firing 20% NH3 in the gasifier revealed that its operating temperature can be flexibly controlled within the optimal reaction temperature window of SNCR by adjusting the primary air ratio (λ1). Increasing λ1 was beneficial for NH3 and coal conversion and denitrification in the gasifier; at λ1 = 0.53, the conversion rate of NH3 and N2 reached 86.37% and 83.59%, respectively, with no NOx being detected at the gasifier outlet. Furthermore, increasing λ1 promoted the development of gasified char pore structure, thereby improving reactivity. After entering the down-fired combustor (DFC), increasing λ1 promoted NH3 and coal burnout and effectively controlled NOx emissions, reaching a level similar with that of pure coal under λ1 = 0.53. Co-firing 20% NH3 in the DFC also demonstrated that the introduction of inner secondary air was conducive to char burnout, however, increased NH3 slip. The outer secondary air promoted NH3 combustion, however, exacerbated NOx emissions and inhibited gasified char combustion. The uniform air distribution method balanced the combustion of NH3 and gasified char, and effectively controlled NOx emissions. When the same air distribution method was used, co-firing NH3 in the gasifier was more favourable for NOx control, whereas co-firing NH3 in the DFC benefited coal burnout. This study provides innovative ideas and serves as a reference for developing enhanced combustion and low NOx emission technologies for large-scale NH3 co-firing in coal-fired units.

Comparative analysis on pulverized coal combustion preheated by self-sustained purifying burner with coaxial and centrosymmetric air nozzle structures: Purification, combustion and NOx emission characteristics

To optimize secondary air nozzle structure in purifying burner, this study focused on the comparison of purification, combustion and NOx emission characteristics of pulverized coal preheated by a 30 kW purifying burner with coaxial and centrosymmetric structures. Centrosymmetric structure shifted the position of main burning region down in high-temperature reduction unit (HTRU), and the number of branches differently influenced the temperature in different regions with this structure. For reductive gas components, CO concentration with centrosymmetric structure was higher compared to coaxial structure, while the differences in H2 and CH4 concentrations were smaller. Centrosymmetric structure was more disadvantageous to improve physicochemical properties of pulverized coal compared to coaxial structure, and this structure with four branches further deteriorated its properties compared to two branches. In mild combustion unit (MCU), the temperature at top was lower with centrosymmetric structure, while was higher in the rest. Centrosymmetric structure more effectively reduced NOx emission compared to coaxial structure, but with slight sacrifice of combustion efficiency (η). Moreover, both two-branch and four-branch centrosymmetric structures realized ultra-low NOx emission (<50 mg·m–3) with high η of over 98.50%, and the former was more advantageous. With this optimal structure, η and NOx emission were 99.25% and 40.42 mg·m–3.

Research on the Effect of Deflection Secondary Air on Slagging in Coal Fired Boilers

Abstract In response to the slagging phenomenon in coal-fired boilers, the combustion characteristics of pulverized coal and the effect of deflected secondary air on slagging in a 350MW Corner-tangentially-fired boiler are numerically investigated by the commercial software. The research results indicate that when the deflection secondary air (CFS air) volume increases, the tangential diameter will significantly increase, and the degree of influence is higher than the angle, which has a significant impact on wall temperature and smoke wall brushing. Only adjusting the CFS wind angle or wind volume has a relatively small impact on particle wall brushing; When CFS wind is superimposed with large Angle and high air volume, the temperature and the degree of smoke brushing near the water wall will be significantly increased, and the risk of slag formation on the water wall will be magnified.

Comprehensive Assessment and Optimization of a Middle-Arch Dual-Channel Municipal Solid Waste Incinerator Using Numerical Simulation Methods.

The present study focuses on a middle-arch dual-channel municipal solid waste (MSW) incinerator facing issues of high NO x emission and overheating. To address these problems and optimize the incinerator, an advanced numerical simulation method was employed to comprehensively assess its bed combustion, freeboard combustion, and NO x emission characteristics. A multiphase fuel bed model considering large-particle characteristics of MSW was developed, coupled with a three-dimensional (3D) model for combustion in freeboard. The analysis revealed that the observed issues stem from multiple factors, including primary-to-secondary air ratio, flame propagation in bed, release of volatiles from bed, and distribution and mixing of components in freeboard. Reducing the proportion of primary air and correspondingly increasing secondary air effectively alleviated the localized overheating in the furnace and reduced NO x emission. Further adjustments to the distribution of primary air in three stages delaying air supply toward the burnout stage, together with the decrease in the grate movement speed, can better control the amount and speciation of N released from the bed. Implementing a counterflow mixing strategy with NH3 in the front channel and NO in the rear channel can greatly reduce the original NO x emission concentration to 95.94 mg/(N·m3), as predicted by a numerical simulation. Subsequent practical adjustments to an actual incinerator led to notable improvements, clearly optimizing the localized high-temperature issues at various locations, especially the front channel suffering severe slagging problems, with the temperature reduced from 1118 to 957 °C. Meanwhile, NO x emission concentration decreased from 200 mg/(N·m3) to around 50 mg/(N·m3), with no negative effect on the boiler load.

Teaching Aero-Engine Performance: From Analytics to Hands-On Exercises Using Gas Turbine Performance Software

Abstract In this paper, we present an approach for teaching turbojet engine performance that combines mathematical analysis with the use of performance software to provide students with a comprehensive and in-depth understanding of engine performance and how it is affected by design parameter choices. In the first part, we present the derivation of a universally valid analytical model for turbojet engine cycle design, which is the basis of the propulsion courses at RWTH Aachen University. The analytical model allows for the calculation of specific thrust as well as thermal and propulsive efficiencies as a function of pressure ratios and burner exit temperature. These are all expressed as differentiable functions. Their mathematical extremes form a basis for examining the intricate relationships in turbojet engine design. In the second part, we move from theory to practice. We utilize the GasTurb software, computing turbojet engine cycles for a wide range of cycle design parameters. The analytically derived trends are validated and errors introduced by simplifications are assessed. Differences between the analytical approach and software-based performance simulations of real engines, like temperature-dependent gas properties and secondary air systems, are emphasized and discussed, thus providing a bridge to real-world applications. In summary, this paper describes a two-step approach to teaching turbojet engine design using analytics and performance software. Each approach alone can be used to enrich and extend existing performance courses. Combining the approaches has significant benefits, improving understanding and inspiring students to pursue further research in the field of propulsion.

Effect of Separating Air into Primary and Secondary in an Integrated Burner Housing on Biomass Combustion

Pellet burners, although they are commonly used devices, require high-quality fuels and yet are characterized by relatively high levels of CO and NO emissions and their variability. This article presents a combustion study of an original biomass burner that separates air into primary for biomass gasification and secondary for oxidizing the gasification products, with ducts placed in the housing of the burner. This study introduces a new burner design that separates air into primary and secondary streams within an integrated burner housing, aiming to optimize biomass combustion efficiency and reduce harmful emissions. Two burner designs were proposed, with a high secondary air nozzle (HCrown) and a low secondary air nozzle (LCrown). These two burners were compared with a typical retort burner (Ret). The LCrown burner reduced particulate matter emissions by 36% and CO emissions by 74% with respect to a typical retort burner. This study showed that the distance of the secondary air nozzles from the gasifying part has a significant impact on the operation of the burner and the possibility of reducing emissions of CO and NO. These results highlight the potential of the innovation to significantly improve combustion quality while simultaneously reducing environmental impact.

Monthly Characteristics and Source–Receptor Relationships of Anthropogenic Total Nitrate in Northeast Asia

The complex nonlinear characteristics of atmospheric chemistry necessitate the development of new methods for calculating source–receptor (S–R) relationships for secondary air pollutants. In this study, the monthly characteristics and S–R relationships of anthropogenic total nitrate (i.e., the sum of N from nitric acid, inorganic nitrate, and peroxyacetyl nitrate) in Northeast Asia were simulated and analyzed. The Community Multiscale Air Quality (CMAQ), Fifth-Generation NCAR/Penn State Mesoscale (MM5), and Sparse Matrix Operator Kernel Emissions (SMOKE) models were employed for air quality modeling, meteorological fields, and emissions processing, respectively. The study area encompassed Republic of Korea, Japan, and most of China. Five source/receptor regions were defined to derive the S–R relationships: three in China, one in Republic of Korea, and one in Japan. To produce data for the calculation of the S–R relationship, several experiments were conducted with a 20% reduction in NOx emission sources. As a result of the S–R relationships, China was rarely impacted by the other two countries. The total depositions in other countries were significantly dominated by China (i.e., 43.5% and 40.7% in Republic of Korea and Japan, respectively, and up to 82.3% in December for Republic of Korea).

Influence of secondary air opening on the performance of gasifier-based improved biomass cookstove

Influence of secondary air opening on the performance of gasifier-based improved biomass cookstove

Distinguishing the vertical air-staging low-NOx function of secondary air for suspension combustion within an industrial-scale coal-fired reversal grate furnace

The numerous coal-fired grate furnaces in China, which were designed to partition primary air on the grate and arrange secondary air in the freeboard for air-staging combustion, suffered from poor burnout and high NOx. However, secondary air was conventionally closed or set at marginal openings in real operations and no investigation was reported to explain why. Upon these off-design secondary-air circumstances, industrial-scale tests and modeling investigations were performed for a 75 t/h coal-fired reversal grate furnace at secondary-air ratios of 0 %, 3 %, 6 %, 10 %, and 16 %, respectively. The primary aims were to (i) uncover its combustion and NOx formation characteristics with varying secondary air and (ii) confirm whether secondary air acted on a real air-staging function in reducing NOx. Flow-field deflection and asymmetric combustion were found in the furnace, with the upward gas entirely deflecting towards the rear side. The front-half furnace was filled with a large but weak recirculation zone without effective combustion, acting on a huge dead zone to waste the furnace-space utilization. With increasing the secondary-air ratio, the upward gas deflection deteriorated continuously. Layer combustion worsened while suspension combustion first strengthened and then weakened. NOx emissions displayed an ascent-to-descent trend. Carbon in slag raised while carbon in fly ash decreased. Among the five case setups, the 0 % setting (i.e., fully closing secondary air) exhibited the lowest NOx emissions and burnout loss. Compared with the habitual 6 % setup, fully closing secondary air not only reduced NOx emissions by 22 % but also improved a little burnout. These trends meant that opening secondary air only strengthened suspension combustion for burning well fly ash while raised both NOx and total burnout loss. Secondary air thus failed to role as an air-staging function in reducing NOx.

Robust Optimization of the Secondary Air System Axial Bearing Loads with the Labyrinth Clearance Uncertainty

Robust Optimization of the Secondary Air System Axial Bearing Loads with the Labyrinth Clearance Uncertainty

Numerical study of NOx emission reduction by MILD and air staged synergistic combustion in a new type of coke oven flue

In order to address the issue of high emissions in the coke industry, we require innovative solutions. This article designs a coke oven flue with an array type secondary air nozzle, achieving the synergistic occurrence of MILD combustion and air staged combustion, which can significantly reduce nitrogen oxide emissions. However, further research is needed on the optimal control methods for synergistic combustion effects. First, Analyzed the mixing state of air and fuel under different air mass flow ratios. Next, by taking into account the oxygen distribution law, analyze the temperature distribution in both the primary and secondary combustion zones. Finally, analyze the distribution of NOx concentration. Study the impact of temperature differences caused by air mass flow ratio in synergistic effects on NOx emissions. The results indicate that by adjusting the air flow ratio, the air vortex intensity in the collaborative combustion process can be enhanced. This method can reduces flame temperature, achieving the objective of reducing NOx concentration. In the new type of coke oven flue, when the air mass flow ratio is 0.4, the combustion state in the primary and secondary combustion zones is optimal, and the NOx emission concentration is 312.88 mg/m3, representing a reduction of 38 %.

A Study of an Integrated Analysis Model with Secondary Flow for Assessing the Performance of a Micro Turbojet Engine

The objective of this study is to implement a more realistic integrated analysis model for micro gas turbines by incorporating secondary flow and combustion efficiency into the existing model, which includes main engine components such as the compressor and turbine, and to validate this model by comparing it with test results. The study was based on the JetCat P300-RX, which has a maximum thrust level of 300 N. Simulations were performed using ANSYS CFX, employing the κ-ω SST turbulence model and a mixing plane interface between individual components. The eddy dissipation model (EDM), with a combustion efficiency of 90%, was used as the combustion model. A user subroutine was also applied for the power matching of the compressor and turbine to calculate the fuel flow rate in each iteration. For secondary flow, it was assumed that 3% of the total air flow rate would flow through the secondary path and be applied to the compressor and turbine. Simulations were conducted over a range of 30,000 to 104,000 RPM, with ground conditions evaluated, including altitude-simulated conditions. To validate the analysis model, engine performance metrics such as pressure ratio, air flow rate, fuel flow rate, and exhaust gas temperature (EGT) were compared with test results. The results demonstrated that errors were less than 5% for most engine performance metrics, except for EGT and fuel flow. The discrepancy in EGT was attributed to differences in the sensing methods, while the variation in fuel flow was found to be due to the lubrication system and losses due to the secondary air flow. Consequently, this study confirmed that the integrated simulation model accurately predicts engine performance. The results indicate that the integrated simulation model provides a more realistic prediction of overall engine performance compared to previous studies. Therefore, it can evaluate detailed thermo-fluid properties without the need for component performance maps, enhancing performance evaluation and analysis.