High Flow Coefficient Research Articles

Influence of distorted inflows on the performance of a contra-rotating fan

AbstractA contra-rotating fan offers several aerodynamic advantages that make it a potential candidate for future aircraft engine configurations. Stall in a contra-rotating axial fan is interesting since instabilities could arise from either or both of the rotors. In this experimental study, a contra-rotating axial fan is analysed under clean or distorted inflow conditions to understand its performance and stall inception characteristics. The steady and unsteady measurements identified the relative contribution of each rotor towards the performance of the stage. The tip of rotor-1 is identified to be the most critical region of the contra-rotating fan. The contribution of rotor-2 to the overall loading of the stage is observed to be relatively less than rotor-1. The penalty due to distortion in the stage pressure rise is mostly felt by rotor-1, while rotor-2 also shows a reduction in performance for distorted inflows. Rotor-2 stalls at a high flow coefficient marking the initiation of partial stall of the stage, and the stall of the whole stage occurs once rotor-1 stalls. A fluid phenomenon that is attached to the blade surface marks the stall of rotor-1, and this fluid phenomenon initially rotates at a speed close to the speed of rotation of the blade. As the stage moves towards the fully developed stall, this fluid phenomenon sheds from the blade surface. The fluid phenomenon thus propagates at a speed much lower than the rotational speed of the blade during fully developed stall.

Mean-Line Design of a Supercritical CO2 Micro Axial Turbine

Supercritical carbon dioxide (sCO2) power cycles are promising candidates for concentrated-solar power and waste-heat recovery applications, having advantages of compact turbomachinery and high cycle efficiencies at heat-source temperature in the range of 400 to 800 ∘C. However, for distributed-scale systems (0.1–1.0 MW) the choice of turbomachinery type is unclear. Radial turbines are known to be an effective machine for micro-scale applications. Alternatively, feasible single-stage axial turbine designs could be achieved allowing for better heat transfer control and improved bearing life. Thus, the aim of this study is to investigate the design of a single-stage 100 kW sCO2 axial turbine through the identification of optimal turbine design parameters from both mechanical and aerodynamic performance perspectives. For this purpose, a preliminary design tool has been developed and refined by accounting for passage losses using loss models that are widely used for the design of turbomachinery operating with fluids such as air or steam. The designs were assessed for a turbine that runs at inlet conditions of 923 K, 170 bar, expansion ratio of 3 and shaft speeds of 150k, 200k and 250k RPM respectively. It was found that feasible single-stage designs could be achieved if the turbine is designed with a high loading coefficient and low flow coefficient. Moreover, a turbine with the lowest degree of reaction, over a specified range from 0 to 0.5, was found to achieve the highest efficiency and highest inlet rotor angles.

Experimental study on atomization characteristics and dust-reduction performance of four common types of pressure nozzles in underground coal mines

Pressure nozzle is commonly used in the dust-reduction techniques by spraying of underground coal mines. Based on the internal structure, the pressure nozzle can be divided into the following types: spiral channel nozzle, tangential flow-guided nozzle and X-swirl nozzle. In order to provide better guidance on the selection of nozzles for the coal mine dust-reduction systems by spraying, we designed comparing experiments to study the atomization characteristics and dust-reduction performance of four commonly used nozzles in the coal mine underground with different internal structures. From the experimental results on the atomization characteristics, both the tangential flow-guided nozzle and the X-swirl nozzle have high flow coefficients. The atomization angle is the largest in the spiral non-porous nozzle, and smallest in both the X-swirl nozzle and the spiral porous nozzle. The spraying range and the droplet velocity are inversely proportional to the atomization angle. When the water pressure is low, the atomization performance of the spiral non-porous nozzle is the best among the four types of nozzles. The atomization performance of the X-swirl nozzle is superior to other types when the water pressure is high. Under the high water pressure, the particle size of the atomized droplets is smallest in the X-swirl nozzle. Through the experiments on the dust-reduction performance of the four types of nozzles and the comprehensive analysis, the X-swirl nozzle is recommended for the coal mine application site with low water pressure in the dust-reduction system, while at the sites with high water pressure, the spiral non-porous nozzle is recommended, which has the lowest water consumption and obvious economic advantages.

Vibration caused by swing check valve closure

Check valve or non-return valve is a commonly used component in fluid control systems, the function of the check valve is to allow the fluid to flow in one direction and prevent it from flowing backward in the opposite direction. There are many types of check valve that differs in structure, material, theory of operation, applications, installations constraints, etc. The swing check valve type has many advantages like lower pressure drop, high valve flow coefficient, ease of maintenance, etc. In this paper, the swing check valve type will be investigated using a new mathematical model that avoids the inaccuracy in explicit models and time consumption of evaluating different coefficients in implicit models. The present model is validated by comparing to experimental data. Investigation of the swing check valve is carried out in order to determine the vibration produced by its closing action and closing response. Also the effect of equipping the swing check valve with a torsion spring is investigated to figure out the spring effect on the valve induced vibration.

Calculation of flow in incompressible regenerative turbo-machines with bucket form blades based on the geometry of flow path

Purpose Regenerative flow pumps are dynamic machines with the ability to develop high heads at low flow rates. Simplicity, compactness, stable features and low manufacturing costs make them interesting for many applications in industries. The purpose of this study is to present a new method for calculating the flow through regenerative pumps with bucket form blades to predict the performance curves by a cheap and easy-to-use way. Design/methodology/approach The analysis was carried out based on the geometric shape of a fluid particle trajectory in a regenerative turbomachine. The fluid particle path was assumed to be a helix wrapped into a torus. Loss models were considered and the results of predictions were compared with computational fluid dynamics (CFD) data. Findings The overall trend of performance curves resulted from presented model looked consistent with CFD data. However, there were slight differences in high and low flow coefficients. The results showed that the predicted geometric shape of the flow path with the presented model (a helix wrapped into a torus) was not consistent with CFD results at high flow coefficients. Due to the complexity and turbulence of the fluid flow and errors in the calculation of losses, as well as slip factor, there was a discrepancy between the results of the presented model and numerical simulation, especially in high and low flow coefficients. Originality/value The analysis was carried out based on the geometric shape of a fluid particle trajectory in a regenerative turbomachine with bucket form blades. The fluid particle path was assumed to be a helix wrapped into a torus.

Effects of forest on seasonal runoff in North China

The relationship of forest-water is a hot and important issue in ecohydrology studies. The relations between forest and water had been widely investigated, whereas information on the impact of forest on seasonal runoff is still lacking. In this study, the statistical and graphic analyses based on the long-term hydrological data of the 20 watersheds in the rocky mountain area of North China were made to analyse the impact trend of forest on seasonal runoff. The results showed that the low flow coefficient decreases by 3.3 % with a 10 % enhancement of the forest coverage. However, the effects of forest on high flow are much more complex and no obvious changing trends can be observed. With increasing forest coverage (f), the high flow show a trend of “increasing (f<20 %)—stable (f is 20-60 %)—decreasing (f>60 %)”, and there has the largest high flow when f is 20%∼60%, the largest high flow coefficient is about 30%.

A study of counter-rotating impulse turbine for wave energy conversion - Effect of middle vane on the performance -

In an oscillating water column (OWC) wave energy converter, the oscillating water column caused by the sea wave motion is used to drive an air column in the air chamber. An air turbine for this bi-directional airflow is used to convert the pneumatic energy into the mechanical energy. A counter-rotating impulse turbine for wave energy conversion has been proposed by M. E. McCormick in 1978. In a previous study, authors investigated the effect of turbine geometry on the performance of the counter-rotating impulse turbine and clarified that the efficiency of the turbine is higher than a single rotor impulse turbine in a range of high flow coefficients. However, the counter-rotating impulse turbine has a disadvantage. Its efficiency in a range of low flow coefficients is remarkably lower due to the deterioration of flows between two rotors. In this study, middle vanes were installed between two rotors in order to achieve a further improvement in the performance, and its effect was investigated by using the computation fluid dynamic (CFD) analysis. As a result, it was found that the efficiency of the turbine was greatly improved by installing the middle vanes, and a favourable turbine geometry was clarified.

One-Dimensional and Three-Dimensional Design Strategies for Pressure Slope Optimization of High-Flow Transonic Centrifugal Compressor Impellers

This paper deals with the numerical and theoretical investigations of the effect of geometrical dimensions and one-dimensional (1D)-design parameters on the impeller pressure slope of a transonic centrifugal compressor stage for industrial process application. A database being generated during the multi-objective and multipoint design process of a high flow coefficient impeller, comprising 545 computational fluid dynamics (CFD) designs is investigated in off-design and design conditions by means of Reynolds-averaged Navier–Stokes (RANS) simulation of an impeller with vaneless diffuser. For high flow coefficients of 0.16 &lt; ϕdes &lt; 0.18, the CFD-setup has been validated against measurement data regarding stage and impeller performance taken from MAN test rig experimental data for a centrifugal compressor stage of similar flow coefficient. This paper aims at answering the question how classical design parameter, such as the impeller blade angle distribution, impeller suction diameter, and camber line length affect the local and total relative diffusion and pressure slope toward impeller stall operation. A second-order analysis of the CFD database is performed by cross-correlating the CFD data with results from impeller two-zone 1D modeling and a rapid loading calculation process by Stanitz and Prian. The statistical covariance of first-order 1D-analysis parameters such as the mixing loss of the impeller secondary flow, the slip factor, impeller flow incidence is analyzed, thereby showing strong correlation with the design and off-design point efficiency and pressure slope. Finally, guide lines are derived in order to achieve either optimized design point efficiency or maximum negative pressure slope characteristics toward impeller stall operation.

Flow throttling in surge tanks using porous structures

Well-designed local loss throttles at inlet of pressurized and open surge tanks are beneficial in terms of reducing tank size and faster dampening of pressure oscillation. This research investigates porous structures experimentally as a possibility to increase the head loss at surge tanks throttle. Both scenarios of valve closure from upstream and downstream of the pipeline are considered to model pump trip and load rejection in hydropower plants. Performance of porous structure is examined and compared with the case of using simple and differential orifice throttles. Layered porous media is also studied to examine feasibility of achieving variable throttling effect for inflow and outflow from the surge tank. Results confirm considerable advantages of porous throttling over customary orifice throttling for faster transient dampening. This is due to lower head loss coefficient of porous throttle in high velocity flow and higher head loss coefficient in low velocity flow in comparison with the orifice throttle. In a layered porous structure, flow from smaller to larger spheres showed to have an average of 15% lower head loss than the flow from larger to smaller spheres. The Reynolds similarity of the porous spheres is also shown to be governing parameter in up and downscaling of the modeling results in practice.

Performance evaluation of a regenerative pump under fluid viscosity enhancement

Regenerative flow pumps are kinds of dynamic pumps with the ability to develop high heads at low flow rates. Despite the disadvantage of having low hydraulic efficiency, regenerative flow pumps have found many applications in industries. In this paper, the effect of fluid viscosity on the performance of regenerative pump has been investigated experimentally. The primary fluid was water with viscosity of 1 centipoise. Polyacrylamide (PAM) which is a soluble polymer in water was used to increase the viscosity of fluid. Solutions prepared from dry powder PAM at two concentrations of 1600 ppm and 2400 ppm. Solutions were pumped more than 30 times to have homogeneous fluids. The measured viscosities for these two solutions were 180 and 300 centipoise respectively. Results showed that as flow coefficient reduces, the decline in head coefficient gradually increases. Similarly, it was found that viscosity enhancement reduces the power coefficient. There was not any significant change in efficiency curves at low flow coefficients. In contrast, around design flow coefficient and at high flow coefficients, the hydraulic efficiency of the pump reduced by increasing viscosity of working fluid. DOI: http://dx.doi.org/10.5755/j01.mech.23.6.16204

A Counter-Rotating Impulse Turbine for Wave Energy Conversion

Wave energy can be converted to the electrical energy by using a wave energy converter. The wave energy converter with oscillating water column (OWC) is one of the most promising devices because of its simple structure and easy maintenance. In this device, an oscillating water column due to the wave motion is used to drive an air column. An air turbine is used to convert the pneumatic energy of this bi-directional airflow into the mechanical energy. The counter-rotating impulse turbine for wave energy conversion has been proposed and tested so far, and the average efficiency has been shown to about 0.3. On the contrary, in another offshore experiment, it has been reported that the power generation efficiency of this turbine is larger than Wells turbine in case of small waves. However, there is a scarcity of the detailed characteristics data of counter-rotating impulse turbine. In a previous study, the authors investigated the effect of rotor blade solidity and setting angle of guide vane on the performance of this turbine, and they clarified that the efficiency of this turbine is higher than impulse turbine with single rotor in the range of high flow coefficients. The present study aimed to investigate the effect of rotor blade profile on the turbine performance by using the computation fluid dynamic (CFD) analysis. The inner and outer angles of turbine rotor blade are changed in the range of 50° to 70°. The commercial CFD software of SCRYU/Tetra of Cradle Co. Ltd. was used in the present work. The Reynolds averaged Navier-Stokes (RANS) equations were used as the governing equations and the low Reynold’s number SST k-ω model was used to predict the turbulent stresses. As a result, it was found that the inner angle of γ = 70° and the outer angle of γ = 60° of the turbine rotor blades can give the best turbine efficiency and it shows the efficiency close to the impulse turbine with single rotor, even in the range of low flow coefficients.

Computational study of radial gap effect between impeller and diffuser on the unsteadiness of vaned diffuser in a centrifugal compressor

Understanding the unsteady fluid dynamics inside the diffuser holds the key to improve the performance of centrifugal compressor. A detailed computational study has been conducted in a low-speed centrifugal compressor to understand the unsteady flow mechanisms that govern the static pressure recovery inside the vaned diffusers. Simulations are carried out for three different leading edge locations at design and off-design conditions. The study is carried out using Reynolds-averaged Navier-Stokes simulations. This study revealed that the unsteady fluctuations exhibit contrasting behavior at different radial gaps and flow coefficients. An optimum radial gap is strictly a function of the stage loading. A high radial gap helps contain the fluctuations at low flow coefficients, but it enhances the fluctuations at high flow coefficients. If the leading edge is kept close to the impeller blade, then the above design flow coefficient of the vaned passage facilitates a reduction in the unsteady fluctuations. On the contrary, keeping the leading edge close to the impeller blade can accelerate the unsteady fluctuations at low flow coefficients.

Cavitation instabilities of an inducer in a cryogenic pump

Inducers assist cryogenic pumps to operate safely under cavitation conditions by increasing the pressure of the impeller inlet, but create cavitation instabilities. The use of cryogenic fluids requires special attention because of safety and handling concerns. To examine the cavitation instabilities of a cryogenic pump, two kinds of working fluids, water and liquid oxygen, were employed. The cavitation instabilities were measured with an accelerometer installed on the pump casing. The flow coefficient and the head slightly decrease with decreases in the cavitation number before the cavitation breakdown. These trends are true of both fluids. Several cavitation instabilities were identified with the accelerometer. At lower flow coefficients, super-synchronous rotating cavitation was found in a similar cavitation number range for both fluids. At higher flow coefficients, the cavitation numbers of the cavitation instabilities in the liquid oxygen test are smaller than those of the water test.

Effects of Impeller Squealer Tip on Centrifugal Compressor Performance

This paper summarizes the main results sorted out from a design of experiment (DoE) based on a validated computational fluid dynamics (CFD). Several tip recessed geometries applied to an unshrouded impeller were considered in conjunction with two tip clearance levels. The computations show that recessed tip geometries have positive effects when considering high-flow coefficient values, while in part-load conditions the gain is reduced. Starting from the results obtained when studying tip cavities, a single rim tip squealer geometry was then analyzed: the proposed geometry leads to performance improvements for all the tested conditions considered in this work.

Effects of Blade Deformation on the Performance of a High Flow Coefficient Mixed Flow Impeller

The effects of blade deformation under running conditions on the performance of a highly loaded transonic mixed flow impeller were investigated. Two impellers were manufactured, one using the “running” blade profiles as designed and one using the converted “unrunning” or “cold” geometry. Both impellers were tested experimentally and investigated numerically. The test data taken with smooth casing showed that at maximum speed, the isentropic efficiency and pressure ratio of the running geometry was higher than the unrunning geometry by about 0.4% and 1.4%, respectively. However, the difference in performance diminished in the presence of recirculating casing treatment. Numerical calculations suggested that the differences at high speeds were mainly due to the variation in the impeller tip clearance. The calculations using deformed blade profiles under centrifugal load only, predicted performance differences which were about twice as high as the measured values. However, closer predictions were obtained when the effects of pressure loads on blade deformation were included using closely coupled fluid-structural analyses.

Pneumatic performance of staggered impulse turbine for OWC wave energy convertor

A staggered impulse turbine is proposed for asymmetric air flows in Oscillating Water Column wave energy plants, which is expected to enhance the pneumatic power output in a wave cycle. The setting angle of rotor blades is set as 5°. The 3D numerical simulations were conducted under steady conditions using MRF and Mixing Plane model based on CFD software Fluent 12.0. Its mean efficiencies under different velocity amplitude ratios are studied using quasi-steady analysis, which derive corresponding data from the numerical simulation. It is found that the staggered turbine shows better performance than the conventional one under the asymmetrical air flows. Furthermore, its mean efficiency and output-work in a wave period are compared with another unsymmetrical twin impulse turbine system. The results show that the staggered turbine shows better output-work performance than the twin turbine system over the high flow coefficient domain (φ>0.7), which provides more choices to future research on turbine’s optimization.

An Investigation of Real Gas Effects in Supercritical CO2 Centrifugal Compressors

This paper presents a comprehensive assessment of real gas effects on the performance and matching of centrifugal compressors operating in supercritical CO2. The analytical framework combines first principles based modeling with targeted numerical simulations to characterize the internal flow behavior of supercritical fluids with implications for radial turbomachinery design and analysis. Trends in gas dynamic behavior, not observed for ideal fluids, are investigated using influence coefficients for compressible channel flow derived for real gas. The variation in the properties of CO2 and the expansion through the vapor-pressure curve due to local flow acceleration are identified as possible mechanisms for performance and operability issues observed near the critical point. The performance of a centrifugal compressor stage is assessed at different thermodynamic conditions relative to the critical point using computational fluid dynamics (CFD) calculations. The results indicate a reduction of 9% in the choke margin of the stage compared to its performance at ideal gas conditions due to variations in real gas properties. Compressor stage matching is also impacted by real gas effects as the excursion in corrected mass flow per unit area from inlet to outlet increases by 5%. Investigation of the flow field near the impeller leading edge at high flow coefficients shows that local flow acceleration causes the thermodynamic conditions to reach the vapor-pressure curve. The significance of two-phase flow effects is determined through a nondimensional parameter that relates the time required for liquid droplet formation to the residence time of the flow under saturation conditions. Applying this criterion to the candidate compressor stage shows that condensation is not a concern at the investigated operating conditions. In the immediate vicinity of the critical point however, this effect is expected to become more prominent. While the focus of this analysis is on supercritical CO2 compressors for carbon capture and sequestration (CCS), the methodology is directly applicable to other nonconventional fluids and applications.

A new method for reliable performance prediction of multi-stage industrial centrifugal compressors based on stage stacking technique: Part II–New integrated model verification

This is the second part of a conducted study to develop a new integrated model for reliable performance prediction of multi-stage industrial centrifugal compressors. The conducted evaluation in part 1 revealed a high deviation between the predicted surge flows using Casey–Robinson approach and measured values especially at low rotational speeds. Besides, a significant difference between the estimated and the experimental characteristics was observed at choke and surge conditions and this deviation is growing as the Mach number increases. One of the main disadvantages of this model is the dependency on the peak and design efficiencies and associated flow coefficients at both normal and low Mach number operations which are obviously not available at early preliminary stage. In contrast, Lüdtke method fails to detect the instable flow regions and with lower degree of accuracy in the predicted efficiencies. However, the predicted design efficiency was found very close to the experimental value. Therefore, this paper introduces a new approach to estimate the performance map of multistage industrial centrifugal compressors based on stage staking principle and by incorporating the advantages of both methods. The new model has been tested at both low and high flow coefficients applications and for vaned and vaneless diffusers. Comparing with the existing models, the developed method was proven to generate more accurate estimation for performance characteristics of multistage centrifugal compressors with less dependency on the geometrical features. One of the unique advantages of the new method is the fact that it does not require a prior knowledge of the peak and design efficiencies and associated flow coefficients.

A Performance Prediction Model for Low-Speed Centrifugal Fans

A generalized model for mapping the trend of the performance characteristics of a double-discharge centrifugal fan is developed based on the work by Casey and Robinson (C&amp;R), which formulated compressor performance maps for tip-speed Mach numbers ranging from 0.4 to 2 using test data obtained from turbochargers with vaneless diffusers. The current paper focuses on low-speed applications for Mach number below 0.4. The C&amp;R model uses four nondimensional parameters at the design condition including the flow coefficient, the work input coefficient, the tip-speed Mach number, and the polytropic efficiency, in developing a prediction model that requires limited geometrical knowledge of the centrifugal turbomachine. For the low-speed fan case, the C&amp;R formulas are further extended to a low-speed, incompressible analysis. The effort described in this paper begins by comparing generalized results using efficiency data obtained from a series of fan measurements to that using the C&amp;R model. For the efficiency map, the C&amp;R model is found to heavily depend on the ratio of the flow coefficient at peak efficiency to that at the choke flow condition. Since choke flow is generally not applicable in the low-speed centrifugal fan operational environment, an alternate, but accurate estimation method based on fan free delivery derived from the fan test data is presented. Using this new estimation procedure, the modified C&amp;R model predicts reasonably well using the double-discharge centrifugal fan data for high-flow coefficients, but fails to correlate with the data for low-flow coefficients. To address this undesirable characteristic, additional modifications to the C&amp;R model are also presented for the fan application at low flow conditions. A Reynolds number correction is implemented in the work input prediction of the C&amp;R model to account for low-speed test conditions. The new model provides reasonable prediction with the current fan data in both work input and pressure rise coefficients. Along with the developments for the efficiency and work input coefficient maps, the use of fan shut-off and free delivery conditions are also discussed for low-speed applications.

Casing Treatment of a Wave Energy Extracting Turbine

Performance of wave energy extracting turbine used in oscillating water column wave energy system depends on the turbine blade tip leakage flow. Computational study of flow through the turbine by solving Reynolds averaged Navier-Stokes equations were carried out using ANSYS CFX® v14.5 commercial code. The main objective of this paper was to study the effect casing treatment and the vortex pattern change which leads to stall limit change. It was found that the turbine performance in terms of operating range enlargement is possible through the casing treatment. The leakage flow interacted with the main stream and changed the flow separation profile as well as the turbine efficiency. It was found that the pick efficiency is decreased and it is shifted toward the higher flow coefficient as groove depth is increased, while the stall margin becomes wider.