Turbomachinery can be divided into two main groups. Group one consists of machines that perform work on the fluid, requiring energy and increasing its pressure, such as compressors, pumps, and fans. Group two consists of those that extracts energy from the fluid flowing through it – for example, wind, hydro, steam, and gas turbines.
Pumps specifically are devices whose purpose is to move fluid at a constant density, increasing its kinetic energy and its pressure while consuming energy in the process. We are quite used to seeing centrifugal and axial pumps, as they are the most common configurations. However, more exotic designs have been tested and developed throughout the history of fluid machinery.
Viscous disk pumps are one such design – inspired by the concept of Tesla Disc Turbine, and conceived and developed in 1913. The key feature of these turbines is the bladeless design, consisting of multiple parallel rotating discs within a casing. Additionally, its operating principle is based in the so-called boundary layer effect. The main difference from a conventional turbine design is that there is no impingement of the rotating parts on the fluid.
As shown in figure 1, the nozzle is inserting the fluid to the discs’ edge, which is dragged by the moving fluid through its viscosity and fluid adherence to the discs’ surface. After energy is transferred to the discs, the fluid gets extracted by the center exhaust.
The same set of discs and a slightly different shape of casing/volute can be used as a pumping device, which is also called a boundary layer disc pump.
Even if Tesla’s turbine design didn’t undergo further development, the disc pump design found its purpose in highly demanding pumping applications.
In the last 30 years this type of pump has found its niche in high viscosity fluids and low Reynolds number flows. For instance, common applications include the pumping of crude oil, sludge, food pulps and wastewaters. They offer excellent pumping capacity for fluids with enthralled gases or delicate solid particles.
Due to its operating principles, flow is similar to an ordinary pipe with parabolic velocity profiles and stationary layers of fluid (relative to the rotating discs). This allows enthralled gases or solid particles to remain located in the core of the flow with no contact to the discs. As such, the reliability and lifetime for these pumps is very high, as the moving parts show little to no deterioration and require minimal maintenance.
In these conditions, viscous pumps show much higher efficiency despite the lower level of complexity when compared to their conventional counterparts with centrifugal design. Less complexity means lower maintenance and lower costs.
Simplicity of the design in these pumps also shows promising application in microfluidics with different miniaturized designs for drugs transportation and biomedical applications, such as blood transportation with no damage to plasma particles. Also in these operating conditions with low Reynolds flow, disc pumps have shown efficiencies as high as 95%, much higher than a centrifugal or displacement counterpart. Different designs have been investigated and proposed, involving 1, 2 or more bladed discs, depending on the application and size.
Despite of the advancements in the design of turbomachinery, and the increase in computing power allowing more and more complex fluid dynamics analyses, viscous disc pumps remain a unique case that shows that complexity and over-development do not necessarily equal higher efficiency and performance.
“Single-Disk and Double-Disk Viscous Micropump” – D. Blanchard, P. Ligrani and B. Gal – Sensors and Actuators A-Physical, 122, 149-158, 2005
“Analytical and experimental modeling of a viscous disc pump for MEMS applications” – Marco D. C. Oliveira and José C. Páscoa