Diffuser Area Ratio Research Articles

Physically based heat exchanger sizing method for the thermal management system of all-electric regional aircraft

Fully electric propulsion systems integrating hydrogen-powered fuel cells and batteries are promising options to reduce the overall climate impact of regional aircraft. However, the increase in low-temperature heat sources aboard the aircraft calls for advanced thermal management system solutions. To address this challenge, this study presents a sizing methodology for ram air heat exchangers in the nacelle-integrated cooling loop of an all-electric regional aircraft based on the ATR-72 platform. Different discretization schemes are compared to identify an optimal sizing method. The results highlight the simplicity and efficiency of the 0D ε-NTU model. Geometric design variables are optimized with respect to drag and mass during a hot-day take-off. The resulting Pareto front reveals a tendency for low airflow outlet temperatures and large diffuser area ratios to result in lightweight designs but in turn, induce high drag and require a large installation space. Comparative analyses of specific optimal ram air duct designs and equivalent skin heat exchangers demonstrate the potential of a second heat sink over a flight mission. The limited heat transfer area of the skin heat exchanger proves insufficient for hot-day take-off and climb but offers advantages during cruise and descent thanks to the reduced drag.

Experimental and comprehensive investigation of second throat diffuser area effect on ground test of a thrust optimized parabolic nozzle with different expansion ratios

This research examines the allowable second throat cross-sectional area of exhaust diffuser in ground tests of a thrust-optimized parabolic nozzle with different expansion ratios. First, theoretical methods used to determine the minimum second throat area and the minimum starting pressure of the exhaust diffuser have been reviewed. Next, the results of small-scale experimental tests with compressed air at diffuser inlet area to second throat area ratios (Ad/Ast) of 1.6, 1.7, 1.8, 1.85, 1.9, and 2 have been presented. To describe the physical phenomena during the start and breakdown conditions, numerical simulation of the flow has been used. The results reveal that with 11% narrowing of the second throat from area ratio 1.6 to 1.8 at all nozzle expansion ratios (34.7–65), the minimum starting pressure has grown by 7–9%. Examination of the further narrowing of the second throat up to area ratio 1.85 indicates that due to temporary flow choking in the second throat, the starting pressure has grown by up to 40%. Also, at diffuser area ratios 1.9 and above, due to severe flow choking in the second throat, the diffuser never starts. Present examinations show that with a large increase in the diffuser area ratio parameter, the nozzle pressure at which the shock pattern transition occurs in the nozzle does not change significantly. Also, diffuser breakdown pressure in the test of nozzles with different expansion ratios has shown very little dependence on the diffuser area ratio parameter. Our examinations indicate that the maximum permissible area ratio reaches the upper limit of 1.8, which is very different from the values calculated by conventional theoretical methods. Further, in this research, it has been shown that the suitable correction factor in estimating the minimum starting pressure with normal shock method in the second throat has lied within 1.4–1.7. The results indicate that the correction factor has heavily depended on the diffuser area ratio parameter. Nevertheless, this factor falls within a close range using a constant diffuser area ratio in testing of a thrust optimized parabolic nozzle with various expansion ratios. Thus, for a simpler application, the area ratio-dependent mean and maximum correction factors have been introduced which have maximum error of 3.2 and 5.6%, respectively.

Investigation of the Operating Principle of Diffuser Augmented Hydrokinetic Turbines

Alternative sustainable technologies for energy production are currently receiving increased attention, one of them being hydrokinetic turbines. For some applications an augmentation of the turbine rotor through a diffuser is a reasonable option offering some advantages. In order to get a better understanding of diffuser augmented turbines an extensive literature study is performed also including relevant publications on ducted wind turbines. The published geometry and performance data are collected and reevaluated applying a new procedure, which references the turbine power on the diffuser outlet area instead of the rotor area. Additionally, geometries are generated, manually optimized and compared to the data gathered in literature. Dominant diffuser attributes are identified independent from the exact diffuser concept such as the shape of the diffuser close to the exit plane and the diffuser area ratio. By using the new evaluation approach a strong correlation between the area ratio and the overall turbine power coefficient is observed. Observations suggest that an increase of the exit area is only reasonable up to a certain limit. To a some extent, this constraint can be overcome through boundary layer control, e.g. by applying multi-stage diffuser concepts. The presented work gives a guide line for effective design of diffusers for hydrokinetic turbines and generally improves the understanding of the operating principle of diffuser augmented turbines.

Multi-Objective Optimization of Jet Pump Based on RBF Neural Network Model

In this study, an annular jet pump optimization method is proposed based on an RBF (Radial Basis Function) neural network model and NSGA-II (Non-Dominated Sorting Genetic Algorithm) optimization algorithm to improve the hydraulic performance of the annular jet pump applied in submarine trenching and dredging. Suction angle, diffusion angle, area ratio and flow ratio were selected as design variables. The computational fluid dynamics (CFD) model was used for numerical simulation to obtain the corresponding performance, and an accurate RBF neural network approximate model was established. Finally, the NSGA-II algorithm was selected to carry out multi-objective optimization and obtain the optimal design variable combination. The results show that the determination coefficient R2 of the two objective functions (jet pump efficiency and head ratio) of the approximate model of the RBF neural network were greater than 0.97. Compared with the original model, the optimized model’s suction angle increased, and the diffusion angle, flow ratio and area ratio decreased. In terms of performance, the head ratio increased by 30.46% after the optimization of the jet pump, and efficiency increased slightly. The proposed jet pump performance optimization method provides a reference for improving the performance of other pumps.

Diffuser Optimization Using Computational Fluid Dynamics and Micro-Genetic Algorithms.(Dept.M)

An approach for the optimization of turbulent flow in diffusers is presented. A methodology is developed to integrate a finite volume-based computational fluid dynamics (CFD) model and an optimization tool uses micro-genetic algorithms (u GA). The CFD model is based on the Reynolds-averaged Navier-Stokes equations, with the standard k-e closure turbulence model. This methodology is tested on two cases. The first is the estimation of the conical diffuser length, which gives the maximum pressure recovery coefficient, for a given diffuser area ratio. Good agreement between the computational and experimental results is obtained. The second case is the optimization through wall contouring of a given two-dimensional diffuser area ratio and length ratio. The results indicate that the diffuser performance can be improved by this method.

Comparison of effectiveness of Vesselplasty and percutaneous kyphoplasty for Kümmell disease

To investigate the effectiveness of Vesselplasty and percutaneous kyphoplasty (PKP) in treatment of Kümmell disease. Between January 2015 and December 2018, 63 patients with Kümmell disease were treated. Among them, 28 cases were treated with Vesselplasty (Vesselplasty group) and 35 cases were treated with PKP (PKP group). There was no significant difference in gender, age, disease duration, bone mineral density (T value), fracture distribution, and preoperative pain visual analogue scale (VAS) score, Oswestry Disability Index (ODI), anterior height of injured vertebrae, and kyphosis Cobb angle between the two groups ( P>0.05). The operation time, intraoperative fluoroscopy time, bone cement injection volume, the leakage rate of bone cement, the diffusion area ratio of bone cement, and the complications of the two groups were recorded. VAS score, ODI, anterior height of injured vertebrae, and kyphosis Cobb angle were compared between the two groups before operation and at 1 day after operation and last follow-up. All patients of the two groups were followed up 12-36 months, with an average of 24.2 months. The operation time, intraoperative fluoroscopy time, bone cement injection volume, and diffusion area ratio of bone cement were significantly lower in the Vesselplasty group than in the PKP group ( P<0.05). The leakage rate of bone cement was significantly lower in the Vesselplasty group (7.14%) than in the PKP group (34.29%) ( χ 2=5.153, P=0.023). At 1 day after operation and last follow-up, the VAS score, ODI, anterior height of injured vertebrae, and kyphosis Cobb angle of the two groups were superior to those before operation ( P<0.05), and no significant difference between the two groups ( P>0.05). During the follow-up, there was no re-collapse of vertebrae, and the adjacent vertebrae fracture occurred in 2 cases of the Vesselplasty group and 5 cases of PKP group. There was no significant difference in the incidence of adjacent vertebrae fracture between the Vesselplasty group (7.14%) and the PKP group (14.29%) ( χ 2=0.243, P=0.622). Vesselplasty and PKP have similar effectiveness in the treatment of Kümmell disease. They can effectively relieve the pain symptoms, improve the quality of life, partially restore the height of injured vertebrae, and correct kyphosis. But the Vesselplasty has the advantages of shorter operation time, less intraoperative fluoroscopy time, and less bone cement leakage.

Effects of casing angle on the performance of parallel hub axial annular diffuser

AbstractIn this paper, the effects of non-swirling and swirling flow on the performance of parallel hub axial annular diffuser has been investigated. The study was conducted on a fully developed swirling flow and non-swirling flow to predict the separation of the flow from the wall. Three different annular diffusers were used with casing wall angles of 3°, 6°, and 9°. Furthermore, various swirl angles (0–25°) at the inlet of diffusers have been investigated to analyze the performance across the length. It was found that parallel hub axial annular diffuser performance increases up to a certain length as the inlet swirl angle increases. However, the performance also improves as the diffuser area ratio (AR) increases. The performance is evaluated based on the static pressure recovery coefficient (Cp) and the total pressure loss coefficient (CTL). The highest possible pressure recovery is achieved by the 12° swirl angle with a casing angle of 6°.

Effects of casing angle on the performance of parallel hub axial annular diffuser

Abstract In this paper, the effects of non-swirling and swirling flow on the performance of parallel hub axial annular diffuser has been investigated. The study was conducted on a fully developed swirling flow and non-swirling flow to predict the separation of the flow from the wall. Three different annular diffusers were used with casing wall angles of 3°, 6°, and 9°. Furthermore, various swirl angles (0–25°) at the inlet of diffusers have been investigated to analyze the performance across the length. It was found that parallel hub axial annular diffuser performance increases up to a certain length as the inlet swirl angle increases. However, the performance also improves as the diffuser area ratio (AR) increases. The performance is evaluated based on the static pressure recovery coefficient (Cp) and the total pressure loss coefficient (CTL). The highest possible pressure recovery is achieved by the 12° swirl angle with a casing angle of 6°.

Low-inertia centrifugal compressor wheels: Aerodynamic and mechanical design:

A new centrifugal compressor impeller design approach is presented, focusing on electrically driven compressors for gasoline engine and fuel cell applications. The performance and mechanical integrity are evaluated based on numerical simulations. Additionally, the numerical model is applied to several variations of the diffuser and volute geometries, in order to evaluate stage characteristics for diffuser area ratios of 110% and 150%, volute area ratios from 60% to 90%, and diffuser pinch ratios from 60% to 80%. The preliminary performance results show the capability to achieve a flow range comparable to a larger state-of-the-art impeller, with minor penalties regarding stage efficiency and near surge operating stability.

Low-inertia centrifugal compressor wheels: Aerodynamic and mechanical design

A new centrifugal compressor impeller design approach is presented, focusing on electrically driven compressors for gasoline engine and fuel cell applications. The performance and mechanical integrity are evaluated based on numerical simulations. Additionally, the numerical model is applied to several variations of the diffuser and volute geometries, in order to evaluate stage characteristics for diffuser area ratios of 110% and 150%, volute area ratios from 60% to 90%, and diffuser pinch ratios from 60% to 80%. The preliminary performance results show the capability to achieve a flow range comparable to a larger state-of-the-art impeller, with minor penalties regarding stage efficiency and near surge operating stability.

Coupled Reynolds-Averaged Navier–Stokes/Large-Eddy Simulation for Outlet Guide Vane and Prediffuser Flows

The objective of the present study was the development/testing of a novel hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation methodology for predicting the flow at the compressor/combustor interface of a gas turbine engine. The coupling between the flow through the outlet guide vanes and the downstream combustor prediffuser represents the first key component of this interface. Important flow characteristics include three-dimensional turbulent outlet guide-vane wakes, end-wall boundary layers, and both favorable and adverse pressure gradient effects. There is a risk of flow separation if diffuser length and area ratio are not appropriately chosen; alternatively, a significant opportunity exists to reduce system flow losses if this risk is managed properly. Outlet guide-vane/prediffuser flow thus represents a significant modeling challenge. In the present approach to Reynolds-averaged Navier–Stokes/large-eddy simulation computational fluid dynamics, the Reynolds-averaged Navier–Stokes solver is applied to the (attached) outlet guide-vane flow and the large-eddy simulation solver to the (close to separation) prediffuser. Interfacing a large-eddy simulation prediction with an upstream Reynolds-averaged Navier–Stokes prediction introduces the problem of generating a spatially and temporally correlated unsteady velocity field for large-eddy simulation inlet conditions, which is also consistent with the time-averaged Reynolds-averaged Navier–Stokes solution. A new Reynolds-averaged Navier–Stokes/large-eddy simulation coupling procedure has been developed. Experimental data for conventional and advanced outlet guide vane/prediffuser geometries are used to assess the success of the new coupled approach. The results show that the Reynolds-averaged Navier–Stokes/large-eddy simulation approach is able to provide a good balance of predictive capability, improving prediction of velocity profiles at prediffuser exit compared to Reynolds-averaged Navier–Stokes only solution, and computational cost; it offers an attractive option for outlet guide-vane/prediffuser flow prediction.

Flow analysis of shrouded small wind turbine with a simple frustum diffuser with computational fluid dynamics simulations

Reported experimental and computational fluid dynamic (CFD) studies have demonstrated significant power augmentation of diffuser shrouded horizontal axis wind turbine compared to bare one with the same swept area of the diffuser. These studies also found the degree of the augmentation is strongly dependent on the shape and geometry of the diffuser such as the length and the expansion angle. In this paper, CFD simulations of a small commercial wind turbine have been carried out with a simple frustum diffuser shrouding. The diffuser has been modeled with different shapes with the aim to understand the effect of length and area ratio on power augmentation. The simulations provide some effective frustum diffuser geometries for the small commercial wind turbine. From the analysis, sub-atmospheric back pressure is found to be the most influential factor in power augmentation. This factor is significantly affected by diffuser area ratio. Meanwhile flow separation in the diffuser can lead to significant lowering of the pressure recovery coefficient which reduces the overall power coefficient. This phenomenon can be mitigated by adjusting the length of the diffuser. The results of this study present a method on how to determine effective frustum diffuser geometries for a small wind turbine. These results can be extended for any types of wind turbines designed for nominal wind speed.

Hydrodynamic performance enhancement of a mixed-flow pump

This paper presents an optimization procedure based on a radial basis neural network surrogate model for design of a vaned diffuser in a mixed-flow pump. Numerical analysis of fluid flow in a mixed-flow pump has been carried out by solving three-dimensional Reynolds-averaged Navier-Stokes equations with the shear stress transport turbulence model. The optimization processes have been performed twice to investigate the coupled effects of diverse variables. The first optimization process has been conducted with two design variables defining the straight vane length ratio and the diffusion area ratio, and the second one has been conducted with four design variables, i.e., the angle at the diffuser vane tip, the distance between the impeller blade trailing edge and the diffuser vane leading edge, and the two design variables used in the first optimization. The efficiency as a hydrodynamic performance parameter has been selected as the objective function for optimizations. The objective function values have been assessed through three-dimensional flow analysis at design points sampled by Latin hypercube sampling in the design space. The first and second optimizations with the coupled effects of diverse variables have yielded maximum increases in efficiency of 7.16% and 9.75%, respectively, compared to the reference shape. The off-design performance has been also improved in most of the optimum shapes except in the shut-off flow region.

Analysis and Optimization of a Vaned Diffuser in a Mixed Flow Pump to Improve Hydrodynamic Performance

Hydrodynamic analysis and an optimization of a vaned diffuser in a mixed-flow pump are performed in this work. Numerical analysis is carried out by solving three-dimensional Reynolds-averaged Navier-Stokes equations using the shear stress transport turbulence model. A validation of numerical results is conducted by comparison with experimental data for the head, power, and efficiency. An optimization process based on a radial basis neural network model is performed with four design variables that define the straight vane length ratio, the diffusion area ratio, the angle at the diffuser vane tip, and the distance ratio between the impeller blade trailing edge and the diffuser vane leading edge. Efficiency as a hydrodynamic performance parameter is selected as the objective function for optimization. The objective function is numerically assessed at design points selected by Latin hypercube sampling in the design space. The optimization yielded a maximum increase in efficiency of 9.75% at the design flow coefficient compared to a reference design. The performance curve for efficiency was also enhanced in the high flow rate region. Detailed internal flow fields between the reference and optimum designs are analyzed and discussed.

Optimization of Vane Diffuser in a Mixed-Flow Pump for High Efficiency Design

This paper presents an optimization procedure for high-efficiency design of a mixed-flow pump. Optimization techniques based on a weighted-average surrogate model are used to optimize a vane diffuser of a mixed-flow pump. Validation of the numerical results is performed through experimental data for head, power and efficiency. Three-level full factorial design is used to generate nine design points within the design space. Three-dimensional Reynoldsaveraged Navier-Stokes equations with the shear stress transport turbulence model are discretized by using finite volume approximation and solved on hexahedral grids to evaluate the efficiency as the objective function. In order to reduce pressure loss in the vane diffuser, two variables defining the straight vane length ratio and the diffusion area ratio are selected as design variables in the present optimization. As the results of the design optimization, the efficiency at the design flow coefficient is improved by 7.05% and the off-design efficiencies are also improved in comparison with the reference design.

Investigation of Rocket Exhaust Diffusers for Altitude Simulation

The design and operational parameters of rocket exhaust diffusers equipped to simulate high-altitude rocket performance on the ground were investigated and characterized using a comprehensive approach (theoretical, numerical, and experimental). The physical model of concern includes a rocket motor, a vacuum chamber, and a diffuser, which have axisymmetric configurations. Further, the operational characteristics of a rocket exhaust diffuserwereanalyzed froma flowdevelopmentpointof view.Emphasiswasplacedondetailed flowstructure inthe diffuser, to observe the pressure oscillation in both the vacuum chamber and diffuser, which determines the minimum rocket-motor pressure required to start the diffuser. Numerical simulations were compared with experimental data on startup and in operational conditions to understand the effects of major design parameters, including the area ratio of diffuser to rocket-motor nozzle throat, the rocket-motor pressure, and the vacuumchamber size. Nomenclature Ad = inner cross-sectional area of diffuser Ade = exit cross-sectional area of diffuser Ae = exit cross-sectional area of rocket nozzle At = throat cross-sectional area of rocket nozzle

Performance analysis of the power conversion unit of a solar chimney power plant

The power conversion unit (PCU) of a large solar chimney power plant consists of one or several turbogenerators, power electronics, a grid interface and the flow passage from collector exit to chimney inlet. The main goals of this paper are to analyze the performance of the PCU and its interaction with the plant as well as to compare three configurations from an efficiency and energy yield point of view. First, a reference plant is defined and the plant performance data taken from simulations with a model found in the literature are analyzed, and the matching of the turbine(s) to the characteristic of the plant is discussed. It was found that a well designed turbine can be run at high efficiency over the entire operating range, as the plant performance data can be fitted using the ellipse law of Stodola. Loss models for all components of the power conversion unit are then defined, and the impact of the various losses on the overall performance is assessed. Three configurations of the PCU are compared, i.e. the single vertical axis, the multiple vertical axis and the multiple horizontal axis turbine configuration. It is found that the single vertical axis turbine has a slight advantage with regards to efficiency and energy yield because certain loss mechanisms are not present. But its output torque is tremendous, making its feasibility questionable. It is shown that with designing the flow passage in an appropriate manner the aerodynamic losses can be kept low. The assumption made by many other researchers that the total-to-total efficiency of the PCU is 80 % has been confirmed with the present model. Further, it has been shown that the PCU efficiency deteriorates significantly with increasing diffuser area ratio but improves only slightly with reducing the diffuser area ratio below unity.

Steady and Unsteady Flow Investigations on the Effect of Different Number of Diffuser Vanes

This paper presents a computational analysis of a high-speed centrifugal compressor stage where emphasis is focused on both the steady and unsteady flow investigations and on the effect of different number of diffuser vanes on the stage operating range. The simulations were carried out for a centrifugal stage with a backswept impeller and several diffusers with 11, 22 and 33 vanes. For the steady simulations, an averaging approach was used at the interface between the impeller and the diffuser. For the unsteady simulations, geometry scaling was used to deal with the problem of unequal pitch. As the number of diffuser vanes was increased, the pressure recovery coefficient in the semi-vaneless space at surge condition was found to reduce. Also, as the number of diffuser vanes was increased, the flow distortion became more pronounced in the vaneless and semi-vaneless space. It is proposed that this more distorted flow is harmful to the pressure recovery in the semi-vaneless space giving stages with more diffuser vanes narrower flow ranges. It was also found that the diffuser area ratio is not the dominant factor influencing the flow range when the number of diffuser vanes changes.

The science of making more torque from wind: Diffuser experiments and theory revisited.

History of the development of DAWT's stretches a period of more than 50 years. So far without any commercial success. In the initial years of development the conversion process was not understood very well. Experimentalists strived at maximising the pressure drop over the rotor disk, but lacked theoretical insight into optimising the performance. Increasing the diffuser area as well as the negative back pressure at the diffuser exit was found profitable in the experiments. Claims were made that performance augmentations with a factor of 4 or more were feasible, but these claims were not confirmed experimentally. With a simple momentum theory, developed along the lines of momentum theory for bare windturbines, it was shown that power augmentation is proportional to the mass flow increase generated at the nozzle of the DAWT. Such mass flow augmentation can be achieved through two basic principles: increase in the diffuser exit ratio and/or by decreasing the negative back pressure at the exit. The theory predicts an optimal pressure drop of 8/9 equal to the pressure drop for bare windturbines independent from the mass flow augmentation obtained. The maximum amount of energy that can be extracted per unit of volume with a DAWT is also the same as for a bare wind turbine. Performance predictions with this theory show good agreement with a CFD calculation. Comparison with a large amount of experimental data found in literature shows that in practice power augmentation factors above 3 have never been achieved. Referred to rotor power coefficients values of CP,rotort= 2.5 might be achievable according to theory, but to the cost of fairly large diffuser area ratio's, typically values of β>4.5.

Modelling of Ram-Accelerator Flow Fields

Dynamic phenomena in 'ram-accelerator', a ramjet-in-tube concept for accelerating projectiles to ultra high velocities, have been investigated analytically and compared with the experimental investigations reported in open literature. The projectile resembles the centrebody of a conventional ramjet, but travels through a stationary tube filled with a mixture of gaseous fuel and oxidizer. The energy release process travels with a projectile inside the accelerator tube. The characteristics of subsonic combustion, thermally-choked mode of propulsion, which is capable of increasing the velocity up to Chapman-Jouguet (C-J) detonation velocity of the propellant mixture used in ram-accelerator tube, have been studied. The ram-accelerator with a fixed diffuser area ratio operates with different initial velocities for different propellant mixtures. Propellant mixture with CO/sub 2/ as diluent is used for velocity range ~770-1150 m/S; propellant mixture with nitrogen as diluent is used for velocity range ~ 925-1450 m/s and that with helium as diluent is used for velocity range ~ 1500-2000 m/s. Mixtures of propellants with different diluents in varying degree of proportions, giving rise to different acoustic and C-J detonation speeds, have been investigated to evaluate their suitability in the ram-accelerator divided into several segments.