Bladeless Turbines

Overview

Turbines convert the energy of moving gas into shaft power. In nearly every machine in service, rows of precisely shaped airfoil blades do that work. As flow speeds rise into the supersonic range, blade rows become harder to design, build and start, because shock waves form at the blade surfaces and can choke the passage.

The bladeless turbine developed in our laboratory follows a different approach. A wavy, helically twisted hub surface replaces the blade rows. The surface generates a repeating sequence of shock waves and expansion fans, and the pressure loading that results on the hub produces torque. The concept turns the shock structure that conventional turbomachinery works to avoid into the working mechanism of the machine.

How it works

Supersonic flow passes over the contoured hub. Along the rise of each wave, the flow decelerates through compression and the static pressure increases. Along each descent, flow accelerates through an expansion and the pressure falls. Because the surface is inclined along a helix, this pressure difference acts as a tangential force on the hub, and the hub rotates.

The inflow enters along the axis and carries no swirl. Torque comes directly from pressure forces on the contour, so the design needs no guide vanes or pre-swirl nozzles to turn the flow before it reaches the rotor. The geometry is defined by a small set of parameters: wave amplitude, wave count, helix angle and hub radius. Each can be varied to tune the machine to a given flow condition.

Experimental Schlieren Imaging

Configurations

Axial-inlet design:

In the axial-inlet configuration, flow passes through a channel bounded by a wavy hub and a fixed shroud. Steady Reynolds-averaged Navier–Stokes (RANS) simulations of the baseline design, with a 45° helix angle and 20 waves, predict about 75 kW of power from Mach 1.25 inflow. A comparable bladeless design without the wavy surface extracted no power at that condition. This shows that the contour itself, and not the flow path alone, drives the power extraction.

Parametric studies map how the design responds to key variables:

  • Wave amplitude. At Mach 2.1, doubling the amplitude increased the predicted power by 210%.
  • Helix angle. Reduced torque peaks near a 53 degree helix angle at the design speed.
  • Wave count. Reduced torque and efficiency were highest for the 28-wave geometry in the study.
  • Inflow Mach number. Power peaks near Mach 1.4 along the constant reduced-speed line, with the optimum dependent on geometry.

A reduced-parameter formulation developed in this work predicts power across two different hub radii to within 7%. This gives engine designers a way to scale performance from one size to another.

Conical design:

Our experimental work centers on a compact conical configuration. A helically twisted, wavy-contoured cone replaces an earlier cylindrical layout that required a difficult annular supersonic flow path. The cone is axisymmetric and works directly with the exhaust of a standard de Laval nozzle. The reference prototype is 3 cm in diameter with 20 waves and a cone half-angle chosen to form an attached conical shock at the tip at Mach 1.6.

Results

Experimental validation at Mach 1.6:

A 3 cm additively manufactured prototype was tested in a Mach 1.6 free jet, with a dynamometer-instrumented motor/generator absorbing the shaft power. The turbine reached a peak electrical output of 62 W near 37,500 RPM. This is the first experimental characterization of the conical bladeless turbine concept. Torque falls approximately linearly with speed, with a measured stall torque of 0.0255 N·m and a no-load speed of roughly 70,000 RPM.

Steady RANS simulations with Metacomp iCFD++ predict a peak near 100 W at Mach 1.6. The simulations also show what limits performance. Both pressure-driven and shear-driven torque fall with speed, but the pressure-driven component falls much faster, because shock strength weakens as the rotor approaches the flow speed. Friction is not the limiting factor.

Extension to Mach 3:

Companion work takes the concept to Mach 3. Simulations predict peak power of about 78 W for the same 3 cm geometry, with a near-linear torque–speed relation. Static torque was measured experimentally across stagnation pressures from 4.1 to 6.25 bar using a reaction-torque sensor. Between Mach 1.6 and Mach 3, the specific power of the turbine, power per unit mass flow, does not change. The geometry performs consistently across a wide range of operating Mach numbers.

Applications and scaling

The bladeless concept targets high-speed, high-energy exhaust flows, such as those downstream of a rotating detonation combustor. In that environment, thin blades are exposed to extreme heat and strongly unsteady shocks.

Dimensional analysis based on the Buckingham Pi theorem gives a scaling law for power at matched tip Mach number, with rotational speed varying inversely with diameter. Applied to the laboratory result, the law projects more than 10 kW from a 15 cm turbine at conditions representative of a rotating detonation engine exhaust, about 10 bar stagnation pressure and 2,500 K stagnation temperature.

Our capabilities:

– Design. Parametric geometry generation for the wavy hub, with amplitude, wave count, helix angle and radius all variable along the axis.

– Simulation. Steady and unsteady RANS of rotating, shock-dominated flows in Metacomp iCFD++, with periodic-sector domains that keep high-resolution meshes tractable.

– Fabrication. Additive manufacturing of complex turbine geometries.

– Experiments. Supersonic free-jet and nozzle test facilities with dynamometer, reaction-torque, pressure and high-speed optical diagnostics.