Volutes - Did Nature or Need Inspire Turbo-Machines?

Figure 1: Volutes: Structured multi-block grid using GridPro.

1192 words / 6 minute read

One of the most fascinating geometrical shapes seen in nature is volute spirals. They are everywhere, from the unrolling of the fern leaves to the nautilus shell to the swirling hurricanes and galaxies. It is still not known exactly as to why nature has a penchant for this shape and repeats them in various forms at different scales. However, for at least a select few, there are some scientific trailing thoughts.

Take a spiral galaxy like our Milky Way for example. Scientists believe that they are formed when giant molecular clouds come under the influence of gravity and other astrophysical forces. The gravitational differences along with rotational motion reinforce the stellar clouds and give the spiral shapes to galaxies.

In another case of a nautilus shell, the spiral takes on the pattern of mathematical logarithmic spiral with the repeated shape at various scales. Life science experts believe that such shapes are the most efficient way to grow and develop. By self-replicating itself at every turn, a very small amount of energy is needed to construct the entire nautilus structure.

Figure 2: Volute spirals in nature and human architecture.

From times past, we humans who constantly learn from nature, fascinated by the shape, made it a part of the culture, art, and architectural designs. In the ancient world, they were considered as a symbol of growth and evolution. Spiral designs can be seen in many historical monuments ranging from the ancient temples of the Zapotec civilization in Mexico and India to monuments in Europe, South America, and central Asia. The double helix staircase in the Vatican museum is one such example. Built by architect Giuseppe Momo in 1932, the design facilitates uninterrupted traffic by allowing ascending people to completely avoid people descending.

Interestingly, in the present era of science and technology, human appreciation of spiral volutes has moved from being a piece of visual and architectural beauty to a more functional role in turbomachines. It is not clearly known how they got introduced into the world of turbomachines. Whether it was an act of biomimicry or the design/functional needs which drove the early Engineers to come up with this shape. However, having made an entry, with the objective of delivering fluid efficiently into or out of a turbomachine, they have become one of the essential components of pumps and compressors.

With this introduction, let’s take a deeper look into volutes as used in turbomachines, right from the need for volutes, to understanding the fluid flow pattern and design aspects in detail, in the following sections.

Criticality of Volute Designing

In mechanical engineering field, volutes are the spiral-shaped stationary hydraulic parts that aid in converting the kinetic energy of the fluid to pressure energy and vice-versa. In pumps, for example, the volutes, sand-witched between a set of rotating blades called impeller and an exit pipe, aid in the energy conversion, by reducing the fluid speed and increasing the pressure. In turbines, they do the opposite i.e. convert pressure energy to kinetic energy.

Volutes play a critical role in the effective functioning of turbomachinery equipment and the unsteady swirl nature of flow gives little margin for design errors. The flow pattern which sets in a volute is mainly influenced by the volute geometry and the impeller exit flow conditions. Various geometrical aspects like cross-section area, shape, radial location, throat placement, tongue shape and position, exit conical diffuser layout, etc. influence the fluid flow pattern in the volute casing, which in turn affects the overall performance of the volute.

The fluid motion in a volute is usually unsteady in nature, dominated by swirls. The interaction between the rotating impeller and volute makes the flow unsteady. Further, if the volutes are asymmetric, there is a circumferential distortional component added to the flow parameters like pressure, velocity, and flow angles. These distortional flow causes extra losses, reduced operating range, increased radial forces on the shaft, blade vibration, noise, and a higher risk of cavitation.

Figure 2: Graphical representation of the volute flow. Image source – Ref [1], Ref [2]

This implies, even a small deviation from the optimal geometric shape could easily derail the performance of the entire turbomachinery. This shows the criticality of volute design and the need for an accurate volute optimization design cycle.

Wading Through a Volute Spiral

For the effective design of volutes, it is essential to understand the flow pattern in them. The swirl nature of volute flow is mainly due to the tubular spiral shape. Fluid entering at a small radius close to the tongue, occupies the central portion of the volute, while the fluid particles entering at a larger radius, further downstream, start to rotate around the central core. This sets up concentric vortex tubes of different radius, with each fluid particle maintaining an almost constant radius during their flow inside the volute tube.

Figure 3: CFD simulation results showing the static pressure, total pressure fill contour, and streamlines in an asymmetric volute. Image source – Ref [2]

The swirl velocity of a fluid particle at a given location depends on the radial velocity with which it had entered the volute. Other properties like total pressure depend on the incoming flow and on the changes in vortex structure inside the volute. Theoretically, the constant radial velocity results in a vortex with constant swirl velocity. The swirl nature of the flow introduces large shear forces in the core. The shear forces generate losses resulting in a drop in total pressure in the central core region. With time, the kinetic energy of the fluid in the center gets dissipated out and a friction-free solid body rotation like forced vortex structure gets established. Figures 2 and 3 show typical flow patterns in an asymmetric volute.

Figure 4: Cross-sectional variations used in volutes for various applications. Image source – Ref [2]

The Era of Simulation-Based Optimization

Volute designing is a complex and involved task. The flow nature and the interference effects brought by various components demand a closed-coupled impeller-volute design. Since the last decade, with rapid improvement in computational power, there has been a steady shift of turbomachine design from traditional to mathematical model-based approaches.

This new trend can be attributed to the increasing demand for better performance turbomachines with higher efficiency and overall performance. Modern-day turbomachines are a lot more complex and it will not be possible to achieve the expected high level of improvements with a limited number of design variants outputted by conventional design cycles. Being fast, less expensive, and having the potential to create infinite variants, numerical model-based optimization is the right tool that can meet the requirement efficiently and effectively.

Figure 5: A typical optimization loop in picking the best geometry. Image source Ref [2].

The Sparsity of Research Work on Volute Optimizations

The volume of research work in CFD-based turbomachinery component design is immense. However, interestingly, if one skims through the body of published research work on turbomachinery optimization, based on numerical simulations, it can be observed that most works are focused on improvements of impeller blades, diffuser guide vanes and hardly much research work seems to have been conducted related to volute optimization.

The primary reason for this could be that it is easy to generate parameterized geometries for blades and vanes with the help of dedicated software available in the market and also automate the entire geometry building and gridding process. In addition, these components allow for simplification of the flow field by allowing 2D and rotational/translational periodicity-based simulations without compromising on the quality of flow field prediction and solution accuracy.

On the other hand, automating the geometry building and meshing steps is difficult for volutes because of the complex nature of their geometric shape. Also, volutes are not amenable for geometric simplifications and the entire geometry has to be considered for the simulation. This makes the CFD simulation of volutes a time-consuming and non-trivial task.

Concluding Remarks

The volute design has a significant bearing on the overall performance of a turbomachinery device. Even in a basic turbo device with a volute and an impeller, volutes can turn out to be the main cause for the drop in efficiency and performance, especially during off-design conditions. Proper volute design is critical to turbomachinery performance. Its design cannot happen in isolation but should be tightly coupled with that of impeller blade and diffuser vane design. And this demands a no-fuss parametric geometry building, grid-generation optimization tools coupled with an accurate CFD solver working automated environment. Only with a reliable geometry building and grid-generation combination can a CFD-based optimization of compute intense turbomachinery component like volute be possible.

References

1. “Flow and Loss Mechanisms in Volutes of Centrifugal Pumps”, R.A. Van den Braembussche, von Kàrmàn Institute for Fluid Dynamics. EN-AVT-143-12.pdf.
2. “Genetic Optimization of Turbomachinery Components using the Volute of a Transonic Centrifugal Compressor as a Case Study”, Martin Heinrich, Thesis, Nov 22, 2016.
3.” Research on Pump Volute Design Method Using CFD”, Sunsheng Yang, Fanyu Kong, and Bin Chen, 31st March, 2011.
4. “Wrapping around the mystery of spiral galaxy arms“,Tammy Plotner, Universe Today.