Optimal Vaneless Diffuser Design For A High-End Centrifugal Compressor

Turbochargers are widely used in the automotive industry to reduce engine emissions and to increase the power. Centrifugal compressors are an integral part of turbochargers. Centrifugal compressors comprises primarily of inducer, impeller, diffuser and volute. The diffuser has an important role in the isentropic efficiency of the compressor. Over the past few decades, researchers have been trying to increase the total-to-total compressor stage efficiency by altering the diffuser’s geometries. Many different methods have been adopted for this purpose, like pinching the diffuser walls, tilting the diffuser walls etc. Pinching increases the outer width of the diffuser while tilting provides an increased radial length. In the present study, both these methods have been used simultaneously. The primary benefit of doing so is to make the turbocharger compressor stage more compact in design, which is the current requirement of the automotive market. In order to investigate the effect of pinching and tilting of diffuser walls, a Computational Fluid Dynamics based solver has been used to predict the flow phenomena within the compressor, especially in the vaneless diffuser. Design of Experiments, using Taguchi’s method, has been incorporated in this study to statistically define the scope of the numerical work, and to obtain the optimal configuration of pinching and tilting that leads to maximum total-to-total compressor stage efficiency. The results depict that the compressor stage efficiency increases up to a tilt angle of 6.25º, after which it starts to decrease. Furthermore, the stage efficiency increases with increase of diffuser outlet width i.e., pinching the diffuser passage, however, this increasing trend has been observed up to an outlet width ratio of 1.23, after which the stage efficiency starts to decrease. Hence, the optimal diffuser model, based on the combined tilting and pinching results obtained, which leads to the maximum total-to-total compressor stage efficiency, has been identified and analysed

Optimal vaneless diffuser design for a high-end centrifugal compressor.

Turbochargers are widely used in the automotive industry to reduce engine emissions and to increase the power. Centrifugal compressors are an integral part of turbochargers. Centrifugal compressors comprises primarily of inducer, impeller, diffuser and volute. The diffuser has an important role in the isentropic efficiency of the compressor. Over the past few decades, researchers have been trying to increase the total-to-total compressor stage efficiency by altering the diffuser’s geometries. Many different methods have been adopted for this purpose, like pinching the diffuser walls, tilting the diffuser walls etc. Pinching increases the outer width of the diffuser while tilting provides an increased radial length. In the present study, both these methods have been used simultaneously. The primary benefit of doing so is to make the turbocharger compressor stage more compact in design, which is the current requirement of the automotive market. In order to investigate the effect of pinching and tilting of diffuser walls, a Computational Fluid Dynamics based solver has been used to predict the flow phenomena within the compressor, especially in the vaneless diffuser. Design of Experiments, using Taguchi’s method, has been incorporated in this study to statistically define the scope of the numerical work, and to obtain the optimal configuration of pinching and tilting that leads to maximum total-to-total compressor stage efficiency. The results depict that the compressor stage efficiency increases up to a tilt angle of 6.25º, after which it starts to decrease. Furthermore, the stage efficiency increases with increase of diffuser outlet width i.e., pinching the diffuser passage, however, this increasing trend has been observed up to an outlet width ratio of 1.23, after which the stage efficiency starts to decrease. Hence, the optimal diffuser model, based on the combined tilting and pinching results obtained, which leads to the maximum total-to-total compressor stage efficiency, has been identified and analysed

Effects of asymmetric flow within vaneless diffuser on the performance characteristics of the compressor stage of a turbocharger.

Modern engines use turbocharger that provides the extra boost to the engines and hence helps in downsizing. Turbochargers comprise of the turbine stage, bearing housing and the compressor stage. Compressor Stage helps in providing compressed air to the engine resulting in possibility of increasing the fuel-to-air ratio, which may provide extra power to the engine. Diffuser is one of the major components within the compressor stage, which helps in increasing the pressure and hence the density of incoming air. The shape of the diffuser has a significant effect on the performance characteristics of the compressor stage. According to the studies found in the literatures, it has been found that the variations in velocity profiles within the diffusers have impact on total-to-total compressor stage performance. Therefore, it is essential to critically evaluate the effect of diffuser shape on the velocity profiles across the diffuser passage. Published literature is severely limited in establishing the effects of the velocity profile asymmetry across the diffuser on the performance characteristics of the compressor stage. Hence, the present study focuses on using a well-validated Computational Fluid Dynamics tool to numerically simulate the flow within the diffuser of various shapes quantified in form of an asymmetric effect on the performance of the compressor stage. Both straight wall diffuser and diverged wall straight diffuser have been investigated in the present study. A full factorial based DoE have been incorporated whereby two factors (L/Lmax and b2/b1) have been selected respectively. Variations in flow related parameters within the diffuser have been discussed in detail for a wide range of geometrical parameters associated with the diffuser shape. It has been found in the analysis of this paper that flow across diffuser is highly asymmetric. Therefore, asymmetry of velocity profiles values has been used to predict the performance of the compressor stage as a function of radial and circumferential velocities across the diffuser. Furthermore, a novel semi-empirical prediction model has been developed to predict diffuser performance as a function of geometric and flow variables of the diffuser. The resulting diffuser map can be used for inverse design of diffuser for compressor stage as well

Flow visualization within a ventricular inhaler device using open source CFD for performance enhancement.

A Ventricular Inhaler Device (VID) is used as a quick-relief medication for treating wheezing and shortness of breath. VID uses Albuterol (a bronchodilator) to relax muscles and open airways. The flow distribution of Albuterol within the VID is the primary parameter that dictates the performance of the device. Poorly designed VIDs can cause accumulation of Albuterol in outlet section of the device, causing flow blockages, which lead to inefficient performance of the device and inappropriate drug delivery. In the present study, Computational Fluid Dynamics (CFD) based advanced techniques have been used in order to visualize the complex flow behavior within a conventional VID, with a purpose to enhance the performance of the device. It has been observed that the pressure coefficient within the injector and outlet duct remains constant, while it decreases in the cyclone separator. Thus, the inlet flow velocity has been seen to have dominating effect on the performance of the VID compared to the rotational velocity of the blades. It has been further noticed that as the flow rate of the drug increases, the efficiency of VID also decreases to a minimum value, after which it remains constant. Moreover, lower rotational speeds of the blades increase the efficiency of the device at higher drug flow rates

Performance Evaluation and Optimisation of Vaneless Diffuser Of Various Shapes for a Centrifugal Compressor

In recent years, diesel engines with reduced emissions and low fuel consumption have been developed worldwide for the purpose of environmental protection and energy conservation. Turbochargers are playing an important role in these modern engines by providing power boost to the engine. A turbocharger comprises of three major parts i.e. the turbine stage, the bearing housing and the compressor stage. Turbocharger designers are continuously seeking for compact stage designs, while maintaining the stage performance. A turbocharger’s compressor stage comprises of various parts i.e. inlet, impeller, diffuser and volute. The diffuser is an important section of the turbocharger compressor stage that plays a key role in increasing the isentropic efficiency of the stage. The diffuser converts the kinetic energy imparted to the flow by the impeller, into static pressure rise, which inturn increases the isentropic efficiency of the stage. The shape of a diffuser is conventionally simple in design. Modifications to the diffuser geometry can lead to higher efficiencies and compact designs of the compressor stage. The present study focuses on the use of advanced computational techniques, such as Computational Fluid Dynamics (CFD), to analyse the effects of diffuser modifications on the local flow features, and the global performance parameters. A baseline diffuser configuration, consisting of a parallel wall diffuser, is numerically analysed to establish the accuracy of CFD based predictions. Various diffusers’ geometrical configurations have been analysed in the present study, both qualitatively and quantitatively. These geometrical configurations cover a wide range, such as diverging, tilting and curving of the diffuser walls. These parametric investigations aid to improve the compressor stage performance and make it more compact. The first aim of the study is to quantify the increase in the stage performance by diverging the straight wall vaneless diffuser passage. This is carried out by diverging the shroud wall (i.e. increasing the outlet-to-inlet width ratio) and varying the location of the divergence point on the shroud wall. The results obtained depict that the effect of increasing the diffuser’s outlet-to-inlet width ratio is dominant in comparison with the location of the wall divergence point. Moreover, increase in diffuser’s outlet-to-inlet width ratio increases the downstream area ratio of the diffuser, causing the flow to separate and creating flow recirculation near the hub wall. This creates restriction to the flow and causes air blockage. Furthermore, shifting the wall divergence point towards the outlet of the diffuser relocates the flow separation point closer to the diffuser exit. The second aim of this study is to analyse the effects of tilted diffuser walls on the flow variables within the compressor stage of the turbocharger. Tilting diffuser walls provides an increased streamwise length to the flow. Furthermore, divergence is applied to the diffuser hub wall in order to increase the outlet-to-inlet width ratio. This makes the turbocharger compressor stage compact in design, while maintaining the stage performance, which is the current requirement of the automotive sector. Design of Experiments, using Taguchi method, has been incorporated in this study to define the scope of the numerical work. The results obtained show that the diffuser with both titled and diverged walls together, performs optimally as compared to the other configurations considered. The third aim of this study is to use curved diffuser walls in order to make the design more compact. Divergence to the hub wall is also applied to enhance the performance of the compressor stage. Various configurations of curvilinear diffuser walls have been considered for numerical analysis. The local flow field analysis has been carried out, quantifying the effects of the geometrical parameters on the stage performance. The results depict that a curved diffuser model reduces the losses within the diffuser passage, but there is negligible effect on the stage efficiency. However, when the divergence is applied to the hub wall of the curved diffuser, there is significant increase in the stage efficiency. Based on these investigations, a turbocharger’s compressor stage can be designed for a compact design and optimal efficiency