Volute Design in AxSTREAM®

Volutes are a tangential part, resembling the volute of a snail’s shell, which collects the fluids emerging from the periphery of a pump/compressor impeller. As such, they are utilized ubiquitously in turbomachinery applications. The words “volute”, “scroll”, “spiral collector”, “housing”, “casing”, “collector chamber”, and “collector” are used interchangeably across different industries. This elegant geometry is also found in nature – the snail is just one example.

Volute in AxSTREAM with Snail Example
Figure 1 – Volute in AxSTREAM® on the left; on the right – Snail – Natural Occurrence of Such Shape

There is a large number of different volute types and applications: centrifugal pumps, axial pumps, centrifugal compressors, axial-flow compressors, radial-inflow turbines, radial fans, and multi-stage blowers, to name a few. Within each group, there is a narrower division on volute types and every application has its own unique features as well as specific properties that can be shared among the group members. The purpose of this post is not to have a detailed discussion of every possible scenario, but rather to show a robust and proven method of volute geometry construction working as a part of aerodynamic design and analysis in a system such as AxSTREAM®.

Classification

Let’s classify this broad range of applications into categories and briefly explore the underlying physical phenomena, main affecting factors, and geometry construction main steps.

Volute underlying physics can be broadly divided into two groups:

  1. volutes that are part of diffusing systems such as in compressors, pumps, or fans; and
  2. radial-inflow turbines where volutes direct flow into the next flow path component axisymmetrically

 

From here, let’s move on to volutes commonly found in compressor applications. The main purpose of the volute is to collect axisymmetric flow to further deliver it to the discharge flange as well as recover static pressure. Velocities at the entrance of the scroll are relatively small as the gas was previously diffused in the diffuser. However, the later statement can not be generalized for all applications and turbomachines. For example, to reduce size and due to some assembly limitation, a volute can go right after small vaneless gap and impeller discharge – very common for centrifugal pumps and centrifugal fans.

Even though the losses can be evaluated for a volute, the author rarely considers performance of a standalone volute – a better indicator is the performance of the diffusing system, or even better, stage performance overall. A typical diffusing system consists of a diffuser, volute and discharge diffuser.

Manufacturing considerations and geometric limitations are important, nonetheless. Examples of manufacturing limitations for cast volutes include minimum wall thickness and minimum radii of fillets. Geometric limitations are typically set on the overall compressor/turbomachine size to fit into the enclosed space or save on material. The cross-section and volute type are predetermined by these important considerations as well as aerodynamic performance. Depending on a cross-section, the volutes can be:

  • – Circular cross-section;
  • – Trapezoidal (with round ages);
  • – Rectangular (commonly fabricated);
  • – Arbitrary shape cross-sectional shape; or
  • – Symmetric vs Asymmetric.

 

Based on geometrical arrangement to the diffuser, volutes are classified as:

  • – External;
  • – Internal; or
  • – Semi-external.

Based on geometric construction diameter, volutes can be:

  • – Constant mean diameter;
  • – Constant inner diameter; or
  • – Constant outer diameters.

 

Volute cross-sections
Figure 2 – Volute Cross-Sections

Volute Geometry Constriction as Part of Integrated Platform AxSTREAM®

Another aspect of volute design is area scheduling as shown in the Figure 3 (Bottom Right) below. The Volute throat is determined by area at the full collection plane. Full collection plane is at 0/360 degree location as shown below. The Volute throat is typically determined during the preliminary stage based on diffuser size and flow kinematics. Additionally, diffuser cone length and area ratio is determined so there are no large separation zones at the discharge. The right bottom corner of Figure 3 shows area distribution as a function of polar angle for constant external diameter volute. By default, AxSTREAM® schedules the areas right after preliminary design according to fundamental aerodynamic principles such as conservation laws. However, the user always has the ability to modify the area distribution as required.

AxSTREAM Volute Design Module
Figure 3 – AxSTREAM Volute Design Module (Area Scheduling in the Right Bottom Corner)

Typically, there is a straight diffuser cone following immediately after full collection plane which is part of the diffusing system. If the discharge cone orientation is different from tangential direction, one can design a transition to properly fit into the discharge flange as shown in the Figure 4.

Discharge diffuser orientation
Figure 4 – Discharge Diffuser Orientation

As noted earlier, volute design consists of many aspects such as aerodynamic performance, manufacturing details, geometric constraint. Some of the main steps during volute geometry construction are summarized below:

  • – Preliminary stage sizing
  • – Iterations on aerodynamic design, structural, manufacturability, etc.
  • – Determination of volute type (cross-section, internal vs external, etc.)
  • – Area scheduling
  • – Diffuser cone geometry
  • – Orientation of discharge flange details
  • – Construction of base surfaces
  • – Adding features such as tongue and fillets (ref. to figure below)
  • – Construction of 3D gas path
  • – Construction of 3D part solid model and detailed drawing based on 3D gas path (in CAD)
  • – Final manufacturing evaluation (by vendor)
  • – Final stage aerodynamic evaluation in design and off-design

 

In AxSTREAM®, one can iterate between geometric construction and stage performance evaluation within one platform. Highlighted in the list above are activities that can be accomplished in AxSTREAM® platform. Yes, AxSTREAM® constructs volutes with tongues! Below, the AxSTREAM® output of different volute shapes is provided. The model is parameterized, so the designer works with only few parameters and can generate complex shapes in the matter of seconds. This is the reason why our clients select AxSTREAM® – for its accurate aerodynamic design and prediction algorithms as well as robust geometric construction modules.

Volute

 

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