No abstract

In centrifugal compressor design, the volute plays a key role in defining the overall efficiency and operating range of the stage. The flow at the impeller outlet is indeed characterized by a high kinetic energy content, which is first converted to potential energy in the diffuser downstream. The compressed gas is then collected by the volute at the cylindrical outlet section of the diffuser and directed to the intake piping, possibly with a further pressure recovery to enhance the stage performance. Due to the high flow speed at the volute inlet, the capability of ensuring the lowest amount of total pressure loss is pivotal to prevent a detriment of the machine efficiency. Moreover, the flow conditions change when the volute operates far from its design point: at mass flow rates lower than the design one, the flow becomes diffusive, while at higher mass flow rates the fluid is accelerated, thus leading to different loss-generation mechanisms. These phenomena are particularly relevant in turbocharger applications, where the compressor needs to cover a wide functioning range; moreover, in these applications, the definition of the volute shape is often driven also by space limitations imposed by the vehicle layout, leading to a variety of volute types. The present paper reports an analysis on the sources of thermodynamic irreversibilities occurring inside different volutes applied to a centrifugal compressor for turbocharging applications. Three demonstrative geometrical configurations are analyzed by means of three-dimensional (3D) numerical simulations using common boundary conditions to assess the overall volute performance and different loss mechanisms, which are evaluated in terms of the local entropy generation rate. The modification of the loss mechanisms in off-design conditions is also accounted for by investigating different mass flow rates. It is finally shown that the use of the entropy generation rate for the assessment of the irreversibilities is helpful to understand and localize the sources of loss in relation to the various flow structures.

No abstract

In centrifugal compressor design, the volute plays a key role in defining the overall efficiency and operating range of the stage. The flow at the impeller outlet is indeed characterized by a high kinetic energy content, which is first converted to potential energy in the diffuser downstream. The compressed gas is then collected by the volute at the cylindrical outlet section of the diffuser and directed to the intake piping, possibly with a further pressure recovery to enhance the stage performance. Due to the high flow speed at the volute inlet, the capability of ensuring the lowest amount of total pressure loss is pivotal to prevent a detriment of the machine efficiency. Moreover, the flow conditions change when the volute operates far from its design point: at mass flow rates lower than the design one, the flow becomes diffusive, while at higher mass flow rates the fluid is accelerated, thus leading to different loss-generation mechanisms. These phenomena are particularly relevant in turbocharger applications, where the compressor needs to cover a wide functioning range; moreover, in these applications the definition of the volute shape is often driven also by space limitations imposed by the vehicle layout, leading to a variety of volute types. The present paper reports an analysis on the sources of thermodynamic irreversibilities occurring inside different volutes applied to a centrifugal compressor for turbocharging applications. Three demonstrative geometrical configurations are analyzed by means of 3D numerical simulations using common boundary conditions to assess the overall volute performance and the different loss mechanisms, which are evaluated in terms of the local entropy generation rate. The modification of the loss mechanisms in off-design conditions is also accounted for by investigating different mass flow rates. It is finally shown that the use of the entropy generation rate for the assessment of the irreversibilities is helpful to understand and localize the sources of loss in relation to the various flow structures.

The design of the volute (in terms of area distribution and shape of the cross section) has a relevant impact on the efficiency and the operating range of a centrifugal compressor. This latter aspect is even more relevant in turbochargers, where the compressor has to cover a functioning range much wider than that of industrial applications. In addition, beyond conventional aerodynamic requirements, the design of the cross section shape is driven often in these applications also by space constraints imposed by the vehicle layout, leading to a variety of volute layouts. In a previous study, some of the authors highlighted the prospects of using the entropy generation rate to evaluate the losses within a volute, since this parameter allows an exact localization of irreversibilities. Starting from these results, the present study shows the suitability of this parameter as an indicator for the fine design optimization of the volute shape. A methodology is presented, which, based on the CFD computed contours of both the entropy generation rate and the total pressure, is able to drive the fine optimization of the volute cross-section at different azimuthal positions in order to maximize its efficiency. Multiple volute shapes are analysed in the paper and the effect of the operating conditions is accounted for by investigating different mass flow rates. The proposed approach indeed lead to a maximization of the volute efficiency with only a few trials and it could indeed provide room for future automatized fine optimization strategies.