Primary Flow Research Articles

Influence of Pulsatility and Inflow Waveforms on Tracheal Airflow Dynamics in Healthy Older Adults.

Tracheal collapsibility is a dynamic process altering local airflow dynamics. Patient-specific simulation is a powerful technique to explore the physiological and pathological characteristics of human airways. One of the key considerations in implementing airway computations is choosing the right inlet boundary conditions that can act as a surrogate model for understanding realistic airflow simulations. To this end, we numerically examine airflow patterns under the influence of different profiles, i.e., flat, parabolic, and Womersley, and compare these with a realistic inlet obtained from experiments. Simulations are performed in ten patient-specific cases with normal and rapid breathing rates during the inhalation phase of the respiration cycle. At normal breathing, velocity and vorticity contours reveal primary flow structures on the sagittal plane that impart strength to cross-plane vortices. Rapid breathing, however, encounters small recirculation zones. Quantitative flow metrics are evaluated using time-averaged wall shear stress (TAWSS) and oscillatory shear index (OSI). Overall, the flow metrics encountered in a real velocity profile are in close agreement with parabolic and Womersley profiles for normal conditions, however, the Womersley inlet alone conforms to a realistic profile under rapid breathing conditions.

Fluoride removal in batch and column systems using bonechar produced in a top-lit updraft drum gasifier and furnace

Hundreds of millions of people are exposed to excessive levels of fluoride in drinking water, predominately in low-resource communities. Activated alumina is recognized as the best available technology for fluoride removal from drinking water by the United States Environmental Protection Agency, but it has substantial economic and environmental costs. Bonechar is a more environmentally friendly and potentially lower cost alternative adsorbent. Here, fluoride adsorption from groundwater (pH 8.1 ± 0.2) by activated alumina was compared with bonechar primarily produced from bovine bones at peak heating temperatures between 400 and 1100 °C in a modular top-lit updraft drum (TLUD) stove (using a bone-wood mixture) and furnace. TLUD and furnace bonechar produced at peak heating temperatures 650–1000 °C and 400–800 °C, respectively, outperformed activated alumina in batch tests (i.e., required smaller doses to achieve 90% fluoride removal). The impact of using bovine versus swine bones to produce bonechar had a negligible impact on fluoride adsorption. A wide range of peak heating temperatures in the TLUD achieved by varying primary air flow rates and fuel selection (e.g., bone-to-wood mass ratios) produced efficient fluoride adsorbents. This finding demonstrates that a TLUD can be a robust, operationally flexible production system. Fluoride removal by TLUD and furnace bonechars showed strong, negative correlations (R2 ≥0.88) with organic matter content. Bonechar pilot column tests indicated that the mass transfer zone was captured (i.e., immediate fluoride breakthrough was not observed) at an empty bed contact time (EBCT) of 5 min, increasing EBCT to 30 min had a minimal impact on adsorption efficiency, and intermittent operation (3–10 d shut-off periods) decreased effluent fluoride concentrations. Furnace bonechars produced at peak heating temperatures 400–700 °C outperformed activated alumina in pilot columns. Differences in adsorption efficiencies in batch and column tests were associated with the linearity of fluoride adsorption. A theoretical model quantifying adsorption linearity with Freundlich 1/n values was able to predict adsorber performance solely based on batch test data.

Working condition expansion and performance optimization of two-stage ejector based on optimal switching strategy

Ejectors are widely used as steam power cycle components to recover superfluous water vapor. However, in a system for preparing distilled water used for pharmaceutical injection, the ejector periodically deviates from the design condition owing to fluctuations in steam source pressure. In particular, when the pressure is reduced to approximately 75% of the design value, performance is significantly degraded, affecting water production efficiency. To solve this problem, a two-stage ejector with control switching strategy is proposed in this paper. The performance of a series of two-stage ejector structures with different scale ratios was analyzed, and the optimum scale ratio was determined. Furthermore, an optimal switching strategy is devised to ensure that the two-stage ejector maintains stable performance under different primary flow pressures. The numerical results indicate that the entrainment ratio of the two-stage ejector is 79.4% higher than that of the single-stage ejector when the pressure is 60% of that of the designed primary flow. Moreover, the two-stage ejector can maintain satisfactory entrainment performance when the traditional ejector enters reversed mode under 50% of the designed pressure. The experimental results show that the entrainment performance of the proposed two-stage ejector can be improved significantly when power fluid pressure is insufficient.

A mass rate-of-rise model for additively manufactured wick structures

The hydraulic performance of heat pipes is typically characterized by the ratio of the wick's permeability to effective pore radius, K/reff. To experimentally quantify these values, mass rate-of-rise (mROR) testing is performed by dipping the tip of the wick into a liquid reservoir to track liquid uptake; the experimental data is then fitted to an ROR model. For conventional wicks, the gravity-based m-t model is free of user decisions and the most reliable. However, here we show that this model poorly fits ROR data for additively manufactured (AM) wicks, resulting in underprediction of K/reff. Experimental tests were conducted using 2 AM wicks—grooved and simple cubic—fabricated by laser powder bed fusion. Visual observations revealed that AM wicks exhibited a superposition of two wicking behaviors: a primary flow and microgroove/corner flow. The primary flow quickly fills the innermost area of the wicking structure. In contrast, the secondary flow gradually creeps up the 3D-printed strut surface through the printed fine scale features. A third term was introduced to improve the data fitting, based on previous research on capillary rise in corners. The revised model measured the AM grooved and simple cubic wicks' K/reff values as 1.23 μm and 1.02 μm, respectively.

An active mixing enhancement technique in supersonic ejectors using pulsed streamwise injections

The current study numerically investigates an active mixing enhancement technique for a supersonic ejector with a constant area mixing chamber operating under the critical regime. Streamwise control jet injections are alternatively pulsed at the top and bottom sides of the mixing chamber entrance. The induced oscillation of the primary jet enhances the bulk mixing between the primary and secondary streams. The secondary stream penetrates the primary core flow much upstream with the control strategy compared to the no-injection case, improving the onset of mixing by 65.36%. A higher spread of the primary jet along the mixing chamber height is observed with the control strategy indicating an enhanced mixing between the two streams. Dynamic mode decomposition analysis of the fluctuations of the velocity magnitude revealed that the dominant dynamic structures are determined by the pulsation frequency and a dominant flapping mode can be observed. The frequency spectrum of the primary jet oscillation revealed that the dominant frequency of oscillation is dictated by the pulsation frequency of the injection. With an increase in the control jet pulsation frequency, the amplitude of primary jet oscillation reduces near the entrance region of the mixing chamber, whereas the amplitude of oscillation far downstream reaches nearly the same value for all the cases. The power spectral analysis of the unsteady pressure fluctuations along the mixing chamber wall revealed that the wall pressure oscillations are contributed by the control jet pulsation frequency as well as the shock wave reflections produced by the supersonic jet–jet interaction within the mixing chamber.

Seasonal Flow Types of Glaciers in Sermilik Fjord, Greenland, Over 2016–2021

AbstractGreenland glaciers have three primary seasonal ice flow patterns, or “types”: terminus‐driven, runoff‐driven, and runoff‐adapting. To date, glacier types have been identified by analyzing flow at a single location near the terminus; information at all other locations is discarded. Here, we use principal component (PC)/empirical orthogonal function (EOF) analysis to decompose multi‐year time series of glacier speed, combined from three satellite‐derived products at four glaciers feeding Sermilik Fjord, Greenland. This improves on single‐point methods by yielding spatial patterns (EOFs), which ensure the result reflects data at all locations on the glacier, and associated temporal patterns (PCs), which allow identification of glacier type. We find that the leading EOF is uniformly signed over the entire glacier domain, that this mode explains the majority of the variance in speed, and therefore that glacier type can be inferred from the leading PC. We find that Helheim Glacier was terminus‐driven, Fenris Glacier and Midgard Glacier were runoff‐adapting, and Pourquoi Pas Glacier was runoff‐driven over 2016–2021. Our classification agrees with previous work for Helheim and Midgard Glaciers, but differs at the other two. At all but Fenris Glacier, the leading PC correlates significantly with the speed pattern observed at the single point used in previous analyses. Thus, Fenris Glacier has more complex flow patterns than single‐point analysis can capture, and wider spatial analysis techniques such as EOF/PC are required. We suggest that, due to its low computational cost and inclusion in standard analysis packages, EOF/PC analysis should be used for assessing glacier type.

Experimental and numerical study on the ejector containing condensable species in the secondary flow for PEM fuel cell applications

Phase transitions in two-phase ejectors have gained attention in various applications, including refrigeration, desalination, and fuel cell systems. The phase transition of condensable components in the primary flow significantly affects the ejector performance. However, studies on the phase transition of condensable species in the secondary flow are scarce. Consequently, this study aims to investigate two-phase flow characteristics of the ejector containing condensable species in the secondary flow. A schlieren experimental system with a visual rectangular ejector was constructed, and a transient Eulerian-Lagrangian two-phase flow model incorporating non-equilibrium condensation was developed. The numerical simulations achieved an average deviation of 4.7% in the entrainment ratio compared to experimental measurements, with a maximum deviation of 11.6%. The schlieren experiment revealed that the condensable species in the secondary flow can condense when the primary pressure exceeds 0.30 MPa. The numerical results showed that the condensed droplets reach a minimum proximity of approximately 3 mm from the primary nozzle exit and exhibit a divergence angle ranging from 7° to 12° as they traverse with the gas flow. Additionally, increasing the primary flow pressure from 0.2 MPa to 0.5 MPa resulted in a noticeable lengthening of the first shock wave, from 1.45 mm to 3.00 mm.

Investigation into the operation of an autothermal two-section subbituminous coal fluidized bed gasifier

Using a newly developed experimental setup, the features and advantages of an autothermal single-casing atmospheric subbituminous coal fluidized bed air-blown gasifier, combining a combustion and gasification section, and mixing the dispersed phase (inert material, char) and heat exchange between them through an annular transfer device, have been revealed. To increase the efficiency of the gasifier, an experimental-computational method was developed find the conditions for optimal operation, combining changing the annular flow’s geometry and regulating the primary air for gasification. A simple and reliable multizone thermodynamic calculation model makes it possible to predict the composition of char and syngas in the gasification section with acceptable accuracy. This method confirmed that a two-section fluidized bed gasifier can provide efficient gasification of solid fuels and is suitable for use in small-scale cogeneration plants. Syngas with a heating value of 3.6–4.5 MJ/m3 and CGE of 38.2%–42.3% was obtained in the experimental setup without optimizing the primary air flow rate. With optimization, the indicators increased to the heating value of syngas of 5.20–5.34 MJ/m3 and CGE of 42.5%–50.0%. With heat regeneration of 0.8, CGE increases to 70%.

EXPLORING THE DYNAMICS OF AN EXOTHERMIC CHEMICAL REACTION CONTROLLED BY ARRHENIUS KINETICS WITH HALL EFFECT IN A MICROCHANNEL

The consequences of chemically reactive fluids have provoked many researchers due to their potential to enhance the behavior of heat. Hence, this article discussed a theoretical investigation to explore the actions of Arrhenius chemical reaction and Hall current on hydro-magnetic free convection of a viscous fluid flowing along an upstanding micro-channel. Subject to the required boundary conditions, the governing coupled equations representing the flow pattern in non-dimensional form were solved using the homotopy perturbation method (HPM). Line graphs are also used to generate expressions for energy, momentum, volume flow rate, drag force, and Nusselt number in both primary and secondary flow directions as a function of regulating parameters like chemical reaction, Hall current, rarefaction, and wall ambient temperature difference ratio. It is worthy of note that the fluid temperature and the fluid flow are substantially propelled by the viscous heating term in the Arrhenius chemical reaction of the system for growing values of the wall ambient temperature difference ratio parameter. Additionally, it is noticeable that volume flow rate performance is seen as a growing influence of viscous heating and rarefaction parameters. The findings of this study can be applied to a wide range of electrically controlled devices, thermal and petro chemical engineering, and the serviceability of industrial products.

Fuel ejector design and optimization for solid oxide fuel cells using response surface methodology and multi-objective genetic algorithm

Ejectors are the main devices used to circulate high-temperature anode exhaust gases for solid oxide fuel cells (SOFCs) for recovering and utilizing water vapor and heat. To optimize the performance of a high-temperature fuel ejector for SOFCs, we established a numerical model of the ejector and then validated it using the experimentally obtained primary flow and secondary flow mass flow rates. The influence of the primary nozzle exit position (NXP), length (Lm) and radius (Rm) of the mixing chamber, and other key structures, on ejector performance was examined via Response Surface Methodology (RSM). The performance of the ejector gradually decreased with increasing NXP and Lm and first increased and then decreased with increases in Rm. The sensitivity of the ejector performance to Rm, Lm, and NXP decreased sequentially, and its structure was optimized using a multi-objective genetic algorithm. The entrainment ratio of the optimized ejector reached 6.66, representing an increase of 23.37 % compared with the original ejector. This work proposes a design and optimization method for optimizing the ejector to meet the high entrainment ratio required by SOFCs. The response surface methodology was used to realize the research on the influence of multi-structural parameter coupling on the performance of the ejector, which significantly improved the optimization efficiency and accuracy. This study also introduced multi-objective genetic algorithm on the basis of response surface methodology to optimize the performance of the ejector. Through continuous gene mutation and iteration, the ejector has the best performance. This provides a new direction for future SOFC fuel ejector performance optimization, that is, introducing various optimization algorithms on the basis of multi-parameter coupling effects, and finally solving an optimal performance through continuous permutation, combination and iterative calculation.

Multiple solutions of Hiemenz flow of CNTs hybrid base [formula omitted] nanofluid and heat transfer over stretching/shrinking surface: Stability analysis

The purpose of the current article is to numerically and theoretically examine the flow of two-dimensional (2D) steady Hiemenz with the transfer of heat of carbon nanotubes (CNTs) hybrid base C2H6O2+H2O (Ethylene glycol + water) nanofluid across a linear shrinking/stretching surface. The equations of Navier–Stokes have been converted into equations of self-similar applying suitable transformations of similarity variables, and then numerically resolved using the three-stage Labatto-three-A formula. In addition, an endeavor is made to extend the behavior of asymptotic of the solution to massive stretching. The comparison between the found asymptotic solutions and previously reported numerical results is rather impressive. Observations indicate that equations of self-similar display double solutions within the restricted shrinking parameter range. There exists one solution for every case of stretching. In the first solution, the impacts of nanoparticle solid volume fraction and shrinking parameters on velocity and thermal fields exhibit an increasing trend. Consequently, the linear analysis of temporal stability has been performed to establish the most fundamentally viable option. The smallest eigenvalue sign determines whether a solution is unstable or stable for the purposes of stability analysis. The analysis of stability demonstrates that the first solution describing the primary flow is stable.

Investigation of the Kinetics of Pressure Coal Char Hydrogasification

The authors of the study focused on the problem of hydrogasifying coal extracted from a particular location. Since hydrogen is transparent to radiation, it can only be heated by convection. To achieve this, we developed a swirler and utilized Fluent software (version 19.0) to simulate the primary flow vectors and the temperature distribution of hydrogen in the hydrogasification reactor. The process was carried out under varying conditions, including temperatures ranging up to 1173 K, pressures of up to 8 MPa, and gas flow rates between 0.5 and 5 dmn3 min−1. The results showed that the carbon reactivity of the char was high up to a certain level of carbon conversion. In this study, the kinetic equations of the hydrogasification process were developed based on the theory of active centers. The researchers also evaluated the kinetic constants at the maximum reaction rate for the analyzed chars. The analysis was conducted for four extreme cases of process parameters, which included temperatures of 973 and 1173 K as well as pressures of 6 and 8 MPa. The results showed that the maximum hydrogasification reactivity of chars could be accurately described using equations for both the first- and second-order reactions toward hydrogen. This was likely due to the use of a narrow pressure range of 6–8 MPa during the experiments. The kinetic equations developed in the study could be used to model the process on a technical scale.

A Machine Learning Approach for Gas Kick Identification

Summary Warning signs of a possible kick during drilling operations can either be primary (flow rate increase and pit gain) or secondary (drilling break and pump pressure decrease). Drillers rely on pressure data at the surface to determine in-situ downhole conditions while drilling. The surface pressure reading is always available and accessible. However, understanding or interpretation of this data is often ambiguous. This study analyzes significant kick symptoms in the wellbore annulus both under static (shut in) and dynamic (drilling/circulating) conditions. We used both supervised and unsupervised learning techniques for flow regime identification and kick prognosis. These include an artificial neural network (ANN), support vector machine (SVM), K-nearest neighbor (KNN), decision trees, K-means clustering, and agglomerative clustering. We trained these machine learning models to detect kick symptoms from the gas evolution data collected between the point of kick initiation and the wellhead. All the machine learning techniques used in this work made excellent predictions with accuracy greater than or equal to 90%. For the supervised learning, the decision tree gave the overall best results, with an accuracy of 96% for air influx cases and 98% for carbon dioxide influx cases in both static and dynamic scenarios. For unsupervised learning, K-means clustering was the best, with Silhouette scores ranging from about 0.4 to 0.8. The mass rate per hydraulic diameter and the mixture viscosity yielded the best types of clusters. This is because they account for the fluid properties, flow rate, and flow geometry. Although computationally demanding, the machine learning models can use the surface/downhole pressure data to relay annular flow patterns while drilling. There have been several recent advances in drilling automation. However, this is still limited to gas kick identification and handling. This work provides an alternative and easily accessible primary kick detection tool for drillers based on data at the surface. It also relates this surface data to certain annular flow regime patterns to better tell the downhole story while drilling.

Performance and mixing process investigation of a novel mixing-enhanced ejector

Ejector refrigeration systems rarely require electrical energy, satisfying the requirements of sustainable development and low carbon emissions. However, owing to the limitations of the operating conditions, the ejector performance is not sufficient, and there is an urgent need to improve the entrainment performance of the ejector. In this study, experiments and numerical simulations were performed on a conventional ejector, and the numerical simulation results were fitted with the experimental data. A numerical simulation was used to investigate the mixing process and entrainment performance of the novel mixing-enhanced supersonic steam ejector. The optimal values of lm = 120 mm and dc = 57.5 mm achieved the best entrainment ratio (0.778) for the novel ejector under 5%-tab blockage ratio. The results showed that the entrainment performance of the novel ejector improved by 51.1–54.7% and 10.5–12.3% compared to those of the conventional ejector and the conventional ejector with an adjusted mixing chamber, respectively. This was because the generated streamwise vortices entrained more secondary flow into the mixing chamber, and the mixing process between the primary and secondary flow avoided unnecessary kinetic energy loss, increasing the mixing efficiency and accelerating the secondary flow crashing out of the diffuser.

Mathematical Modelling and Fluidic Thrust Vectoring Control of a Delta Wing UAV

Pitch control of an unmanned aerial vehicle (UAV) using fluidic thrust vectoring (FTV) is a relatively novel technique requiring no moving control surfaces, such as elevators. In this paper, the authors’ previous work on the characterization of a static co-flow FTV rig is further validated using the free to pitch dynamic test bench. The deflection of a primary jet after injection of a high-velocity secondary jet was captured using the tuft flow visualization technique, along with the experimental recording of subsequent normal force impinged on the Coanda surface resulting in the pitching moment. The effect of primary and secondary flow velocities on exhaust jet deflection, as well as on the pitch angle of the aircraft, is examined. Aerodynamic gain as well as the inertia of a delta wing UAV test bench are computed through experiments and fed into the equation of motion (e.o.m). The e.o.m developed aided in the design of a model-based PID controller for pitch motion control using the multi-parameter root locus technique. The root locus tuned controller serves as a benchmark controller for performance evaluation of the genetic algorithm (GA) and particle swarm optimization (PSO) tuned controllers. Furthermore, the frequency domain metric of gain and phase margins were also employed to reach a robust control design. Experiments conducted in a simulation environment reveal that PSO-PID results in a better response of the UAV in comparison to the baseline pitch controller.

Experimental investigation of jet pump performance used for high flow amplification in nuclear applications

The jet pump can be used in a test device of a nuclear reactor for high flow amplification as it reduces inlet flow requirement and thereby size of the process components. In the present work, a miniature jet pump was designed to meet high flow amplification greater than 3. Subsequently, experiments were carried out using a test setup for design validation and performance evaluation of the jet pump for different parameters. It was observed that a minimum pressure of 0.6 bar (g) was required for the secondary fluid inside the jet pump to ensure cavitation free performance at high amplification. Spacing between the nozzle tip and the mixing chamber entry point had significant effect on the performance of the jet pump. Variation in primary flow, temperature and area ratio also affected the performance. It was observed that at high flow amplification, the analytical solution differed significantly from experimental results due to very large velocities encountered in the miniature size jet pump.

A novel UNet segmentation method based on deep learning for preferential flow in soil

Accurate binary images based on dye-tracing experiments are fundamental for understanding preferential flow in soil. Nevertheless, general image-processing software, such as Photoshop, Image-Pro Plus, and ImageJ, cannot automatically and accurately binarize dye-strained regions for the areas with uneven staining and blurred boundaries, causing errors in the subsequent analysis of preferential flow characteristics. Therefore, this study aimed to develop a novel UNet segmentation method based on a dual-scale attention residual module (DARM-UNet), to improve the segmentation accuracy of preferential flow images and assist in digital soil descriptions. The proposed method is a two-level nested U-structure, i.e., a dual-scale attention residual module (DARM) on the lowest level and the typical architecture of UNet at the top level. On the lowest level, the DARM obtained improved contextual information and expand the receptive field based on the attention mechanism and the dual-scale convolution, which was used to improve the recognition ability of uneven dye and low-contrast regions. At the top level, the typical architecture of the UNet model was followed to fuse feature maps of different resolutions, in which each stage is filled by a DARM block. Compared with general image processing software and deep learning methods, the DARM-UNet method showed higher preferential flow segmentation accuracies (95.71% and 93.29%), recall rates (91.91% and 89.29%), and harmonic means (92.72% and 91.82%) for the natural secondary and hazelnut shrub forests, respectively. The four preferential flow indicators indicated that the flow patterns of the natural secondary and hazelnut shrub forests varied with soil depth. In natural secondary forests, the preferential flow started from the surface and was of the primary flow type, whereas in hazelnut shrub forests, it occurred only in the deeper layer, following the matrix flow and concentrating in the 0–16 cm soil layer. This study demonstrated that the proposed DARM-UNet method could effectively identify and segment preferential flow of different forests, especially the areas with uneven staining and blurred boundaries.

Water management and performance enhancement in a proton exchange membrane fuel cell system using optimized gas recirculation devices

Fuel gas utilization and water management are particularly challenging integrated engineering problems in hydrogen-oxygen proton exchange membrane fuel cell (PEMFC) systems. Ejectors are promising fuel cell exhaust gas recirculation devices that can be used to address both challenges. In this study, hydrogen and oxygen recirculation ejectors are designed and manufactured using three-dimensional (3D) printing technology. An experimental investigation on a 1 kW PEMFC system with anodic and cathodic dual-ejector- based gas recirculation is presented. Key parameters such as stack current, stack voltage, cell voltage, operating pressure, and mass flow rate at the primary flow and secondary flow of the anode ejector are measured. The performance of the PEMFC stack equipped with 3D-printed ejectors is compared to that of commercial ejectors. The experimental results reveal that the 3D-printed ejector significantly outperforms the commercial ejector in terms of entrainment ratio, with an improvement rate of up to 31.3%. The performance of the PEMFC stack in the dual-ejector recirculation mode increased by 4.75% with a current density of 320 mA cm−2 compared with dead ended mode in 130 kPa. Furthermore, the dual-ejector recirculation mode of the PEMFC stack outperforms that of the dead-end anode and cathode mode during dynamic loading by alleviating the gas shortage problem of the PEMFC stack.

Modelling of a passive hydraulic lock concept for the mitigation transfer tubes in a SFR core

To improve the safety of the integrated Sodium-cooled Fast Reactor, recent safety complementary devices can prevent or mitigate the consequences of a hypothetical severe accident consecutive to a loss of reactor cooling sequence. In particular, the M−TT device is set by empty transfer tubes, in place of some fuel sub-assemblies present in the reactor core, using an innovative passive hydraulic lock. It is based on the concept of hydraulic diode controlled by the flowrate of primary pumps. This system is dedicated to provide an easy way to discharge corium towards the core catcher in the hypothetical case of severe accident. In bonus, concerning the prevention of severe accident, it opens a direct flow path between the hot pool, equipped with decay heat removal exchangers, and the cold pool. This can lead to an easier passage of colder sodium towards the cold pool to promote core cooling by natural convection in case of the loss of the primary flow. This paper is devoted to the evaluation of the assessment of the hydraulic lock function and of the small magnitudes of the perturbations induced by our device on the thermal–hydraulic behavior of the reactor during normal operations. Through an analytical analysis and an up-scaling numerical approach, ranging from the local scale (CFD) to the system one, we claim that the lock function is kept during the normal situations at various power regimes. The leak flow represents no more than 1% of the primary pump flow. In addition, no important reactor thermal–hydraulic perturbation is brought by adding transfer tubes in the core. This is true for normal steady states, whatever the power is, but also for a full-power loss of forced flow accidental transient leading to the natural convection cooling.

The effects of fuel distribution on exit temperature distribution in a centrally staged combustor

Fuel injection and distribution play an important role in the formation and migration of hot streaks in gas turbine combustors and determine the exit temperature uniformity especially for modern low emission combustors that have little or no primary and dilution air flows. To better understand the underlying mechanism of the formation and evolution of hot streak, both experimental and numerical investigations have been carried out in a centrally staged tri-sector model combustor. Flow, spray, and flame have been investigated by particle image velocimetry and planar laser-induced fluorescence. The exit temperature distributions are also measured and analyzed with both laser diagnostics and simulation results. The results show that increasing fuel-to-air ratio (FAR) or fuel staging ratio enhances the formation of high temperature hot streak and leads to high overall temperature distribution factor and radial temperature distribution factor at the combustor exit. It is found that the high temperature zone is formed through the non-premixed combustion of pilot stage and resides in the shear layer of pilot stage and primary circulation zone. Increasing FAR at low power conditions and increasing SR at high power conditions promote the formation of a large hot streak area through pilot stage combustion and thus deteriorate the uniformity of temperature distribution at the combustor exit.