A WAVE DISC ENGINE CONCEPT FOR MICRO POWER GENERATION
M. Vagani1, L. Pohořelský2, G. Sun1, D. Alemu3, J.R. Lee1, R.T. Kelly1, T.J. Qualman1,
S.A. Bonner1, D.E. Park1, F. Iancu1, P. Akbari1, J. Piechna4, and N. Müller1
1
Michigan State University, East Lansing, USA
2
Czech Technical University in Prague, Prague, Czech Republic
3
Addis Ababa University, Addis Ababa, Ethiopia
4
Warsaw University of Technology, Warsaw, Poland
Abstract: Wave disc technology is emerging as a possible replacement for compressors and turbines in engines.
This paper presents a new engine concept, the Wave Disc Engine with internal combustion. This new engine
concept combines the advantages of (a) higher efficiency confined combustion with (b) the high power density
and low maintenance of continuous flow but (c) at a much lower unit cost due to its physical simplicity and
compactness. Furthermore, the geometry of the Wave Disc Engine is especially suited for microfabrication and
micro power generation. Theoretical validation of the engine has been performed using four simulation models. 1D and 2-D validations were completed with both in-house and commercial software.
Keywords: Wave Disc Engine, internal combustion, Humphrey cycle, power generation
cycle peak temperature of 1070 K. Accidentally, the
engine was destroyed due to over-speeding from an
improperly connected fuel line, and the project was
canceled.
INTRODUCTION
The idea of direct energy exchange between two
media without using mechanical components such as
pistons or vaned impellers started in the early 1900s
[1]. The first functional device was developed in the
1940s when the Brown Boveri Company (now ABB)
designed a pressure-exchange wave rotor.
The wave rotor is a non-steady flow device that
uses shock waves to pressurize fluids by transferring
energy from a high-pressure flow to a low-pressure
flow in a series of channels. The wave rotor consists of
many axial channels in a rotating cylindrical drum,
usually driven by an external motor. For gas turbine
engine applications, the wave rotor employs the hot,
high pressure exhaust gas from combustion to generate
a shock wave that compresses the cooler, lower
pressure air received from the compressor. For car
engine applications, the wave rotor has been used as a
supercharger and was successfully commercialized by
Mazda Company in 1980s and 90s in the serial
production of diesel passenger cars [2].
Figure 1. Assembly of successfully working wave
engine in axial wave rotor configuration [1](left) and
ABB wave rotor combustor cross section [3] (right)
The Wave Combustor
The wave combustor is a wave rotor with
combustion occurring inside the rotor channels. This
produces a pressure rise during combustion, unlike a
typical steady-flow combustor used in gas turbines.
ABB and ETH Zurich commenced the design of a full
wave rotor with 36 axial combustor channels in 1992
[3]. Each channel had 165 mm length and 15 x 15 mm
cross section, shown in Figure 1 (right). The 200 mm
inner-diameter rotor was driven by an electric motor
capable of up to 5000 RPM. Spark-plug electric
ignition was used for initial start up. Auto ignition, for
continuous operation, was achieved by injecting hot
gas from the neighboring channels. The fuel injection
and ignition worked well and pressures of 9 bar were
obtained. The project was canceled in 1994, due to the
perception at the time that there was no profitable
market for a 100kW gas turbine unit.
The Wave Engine
The wave engine consists of a wave rotor with
curved blades, used for compression of air and
expansion of exhaust gas, while producing shaft work.
It uses a steady-flow gas turbine type combustor. The
Ruston-Hornsby Turbine Company, in the UK,
developed one such engine in the mid-1950s [1]. This
wave engine, shown in Figure 1 (left), had a 230 mm
diameter and a 76 mm length. The engine worked
successfully for several hundred hours in a wide range
of operating conditions (from 3000 to 18,000 RPM)
and produced up to 26 kW at its design point with a
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PowerMEMS 2009, Washington DC, USA, December 1-4, 2009
tangential momentum at the outlet (jet propulsion) and
by the change in angular momentum in the channel
(turbomachinery principle).
CONCEPT OF THE WAVE DISC ENGINE
WITH INTERNAL COMBUSTION
The Wave Disc Engine (WDE) combines the
principles of the wave engine and wave combustor.
While wave rotors use axial flow, the WDE uses a
radial-flow wave rotor, or wave disc [4]. The wave
disc is particularly suited for microfabrication as it has
a simple extruded 2D geometry with a short depth.
The wave disc also allows for curved channels, which
will allow the rotor to extract energy from the flow.
The mechanical simplicity of the engine concept,
with only one rotating part allows it to be miniaturized
without many of the technical issues faced by other
small-scale gas turbine engines [4]. A starter-generator
could be built into the engine similar to that designed
by MIT for micro gas turbines [5].
The WDE, shown in Figure 2 utilizes a typical
engine cycle consisting of compression, combustion,
expansion with work extraction, and heat rejection to
the ambient atmosphere. The cycle however, occurs
completely in the curved disc channels. Using
shockwaves for compression reduces the inertia of the
hardware and promotes a rapid response to load
changes. The compression and expansion are achieved
through shockwaves and expansion waves and the
momentum of the flow is harnessed to drive the rotor.
Fresh Air-Fuel Mixture
Burnt Exhaust Gas
Rotational
Direction
Loading
Figure 3. Schematic for the internal combustion WDE,
incorporating combustion within the shock channels
3) When the inlet port opens the ingestion of fresh
fuel/air mixture into the channels begins. The
expansion wave created in step 2 draws in this mixture
and completes the scavenging process. Centrifugal
fluid forces on the rotating disk assist the scavenging
and loading processes.
4) After the outlet port is suddenly closed, a
“hammer shock” is generated by the deceleration of
the flow to zero velocity. The hammer shock
compresses the fresh air-fuel mixture. It is favorable to
close the inlet port when the hammer shock reaches it.
Another operating cycle starts with the ignition of the
air-fuel mixture (step 1).
Expansion
Wave
Compression
Shock Wave
Compression
Shock Wave
Constant
Volume
Combustion
Reflected
Expansion
Jet
Wave
Propulsion
Scavenging
Figure 2. Schematic engine model and cycle for a twocycle WDE
Figure 4. Thermodynamic cycle of the WDE in
comparison with the gas turbine engine
The schematic in Figure 3 depicts the working
cycle of the internal combustion WDE, briefly
described in the following four steps:
1) The cycle begins with the channel closed on both
sides and filled with a compressed air/fuel mixture.
Constant volume combustion takes place within the
channel, producing a pressure and temperature rise
during the combustion process.
2) As the disk rotates, the exhaust side of the channel
opens to ambient conditions. This sudden opening of
the channel creates an expansion wave propagating
towards the air fuel mixture inlet and scavenging
begins. Torque generation is produced by the fluid
Thermodynamically, the WDE operates on the
Humphrey cycle, where combustion ideally occurs at
constant volume, as shown in Figure 4. In the ideal
Brayton cycle for gas turbines, the heat addition occurs
at constant pressure. Additionally, the WDE is
periodically filled with fresh air, cooling the channel
wall temperature. Thus, the WDE can achieve a higher
cycle peak temperature than a typical gas turbine. As a
result, a WDE can be more efficient than a gas turbine
for comparable compression pressure ratios.
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MATHEMATICAL MODELING AND
PRELIMINARY RESULTS
Quasi 1-D Algebraic Algorithm
An analytical algorithm was established to develop
an initial WDE porting geometry and a first estimate
of the flow velocities and wave propagation pattern in
the rotor, shown in Figure 6. It uses the gasdynamic
wave equations and the method of characteristics for
specified boundary conditions, including postcombustion pressures and temperatures [6]. The
algorithm accounts for the effects of centrifugal force,
channel curvature, and work generated from angular
momentum.
Ideal Cycle Efficiency of WDE
The ideal cycle efficiency does not consider heat
losses across the channel walls, pressure losses during
admission and exhaust, and mechanical friction. It is
an efficiency that can be achieved in an adiabatic
frictionless engine operating with an ideal gas.
0.8
Humphrey
0.7
Brayton
0.6
η
0.5
Otto
0.4
1-D WDE Model in GT-POWER
A 1-D model of the WDE was developed in the
commercially available GT-POWER, which has
successfully completed a 1-D wave rotor model [7].
The model describes pressure waves, combustion heat
release, work extraction, friction, heat exchange, and
throttling in the distribution ports during the opening
and closing phases of an individual rotor channel. The
1-D model wave disc engine has 18 channels, a disc
outer diameter of 15 cm, an inlet diameter of 6 cm and
an engine height of 3 cm. Combustion inside the
channel was modeled using the Wiebe heat release
function [8]. The GT-POWER model enables
optimization of the WDE geometry and power output,
using the Quasi 1-D code results as a starting point.
The Euler turbomachinery equation was used to
compute the power generation and modeled by
extracting the same amount from the energy
conservation equation. For this model engine, the
maximal predicted power output was 1.2 kW at 18,000
rpm.
Diesel
0.3
0.2
0.1
0
0
5
10
15
20
25
P2/P1
Figure
5.
Thermal
efficiencies
of
ideal
Humphrey, Brayton, Otto, and Diesel cycles as a
function of engine compression pressure ratios
As can be seen in Figure 5, the Humphrey cycle
has the highest efficiency when compared with all gas
power cycles. For instance, a WDE operating on the
Humphrey cycle can have an overall efficiency of 45%
for a compression pressure ratio of 10, assuming 10%
heat losses through the wall, 5% scavenging losses,
and 5% mechanical losses. The mechanical losses are
minimal, as it has only one rotating part.
To evaluate the concept and support the design of
a working WDE prototype, the engine was modeled
and analyzed using four numerical approaches. The
results are presented in the following sections.
Quasi 2-D CFD Code
A Quasi 2-D CFD code was additionally
developed to confirm and improve on the results from
the Quasi 1-D algebraic algorithm. For this code, it is
assumed that the streamlines follow the profile of the
channel wall, so no parameters change over the
channel cross-section. The Wiebe function was again
used to model the heat generation due to combustion
[8].
The 4th order MacCormack Scheme was
employed to solve the continuity equation, NavierStokes equations and the energy equation. The code
calculates the port timing by tracing the waves and the
exhaust scavenging. In addition, it yields velocity,
pressure, density and temperature profiles in the
channel, power generation and engine efficiency and
draws the p-v and T-s diagrams for thermodynamic
analysis.
Velocity
Velocity
patterns
patterns
Gas/Air
Gas/Air
Interface
Interface
t
x
Figure 6. WDE wave pattern computed by the
developed Quasi 1-D algebraic algorithm
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The
simulation
results are presented for
a disc of 10 mm outer
diameter, 6 mm inner
diameter, rotating at
200,000 rpm. Figure 7
shows the shape and
locations of the ports,
and Figure 8 the p-v
and T-s diagrams. The
power
generation was
Figure 7. Geometric design
found
to
be 1.189 kW
of the WDE calculated by
and
the
efficiency
the Quasi 2-D code
0.2647.
3
x 10
p-v diagram
Temperature (K)
Pressure (Pa)
1
Air/Fuel
Mix Inlet
Figure 9: FLUENT temperature distribution results
for two cycle WDE
Exhaust
Outlet
Pressure [Pa]
Start of
Combustion
T-s diagram
4000
1.5
Scavenging
Rotational
Direction
5000
2
Rotational
Direction
3000
2000
Compression
Shockwave
Air/Fuel
Mix Inlet
Expansion
Wave
1000
0.5
0
0
Start of
Combustion
6
2.5
Exhaust
Outlet
Temperature [K]
2
4
6
Specific volume (kg/m3)
8
0
-500
0
500
1000
1500
Figure 10: FLUENT pressure distribution results for
two cycle WDE
2000
Specific entropy (kJ/(kg*K))
Figure 8. p-v and T-s diagrams given by Q2D code
REFERENCES
2-D CFD FLUENT Code
Numerical CFD simulations were also performed
using FLUENT 6.2. The 2-D solver was used to model
the flow within the wave disc. In order to model the
wave disc engine, the heat addition in the channels
was achieved by patching a small area of the channel,
the “combustion zone” to the pressure and temperature
expected after combustion. The resulting pressure and
temperature profiles are presented in Figure 9 and
Figure 10.
The FLUENT pressure profiles show that the
pressure and temperature in the channel equalizes
before the exhaust port opens. They confirm that the
wave pattern in the rotor matches the predicted wave
pattern. It also shows that the exhaust gases are
completely scavenged before the beginning of each
cycle. The scavenging actually happens fast enough
that it may be beneficial to close the exhaust port
sooner. This will increase the pressure created by the
“hammer shock” after the closing of the port.
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Review of Wave Rotor Technology and Its
Applications.” ASME J. of Enineering. for Gas
Turbines and Power, 128-4, 717-735
[2] Pohořelský L., Obernesser P., et al., 2007, “1-D
Model and Experimental Tests of Pressure Wave
Supercharger.” ASME IMECE2007-43427
[3] Akbari P. and Nalim M. R., 2009, “Review of
Recent Developments in Wave Rotor
Combustion Technology.” J. of Propulsion and
Power, 25-4, 833-844
[4] Iancu F., Akbari P., and Müller N., 2004,
"Feasibility Study of Integrating Four-Port Wave
Rotors into Ultra-Micro Gas Turbines." 40th
Joint Propulsion Conference, AIAA 2004-3581
[5] Epstein A.H., 2003, “Millimeter-scale, MEMS
gas turbine engines.” Proc. of ASME Turbo
Expo, GT-2003-38866
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Rotor Design Procedure for Gas Turbine Engine
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2008, GT-2008-51354
[7] Pohořelský L., Macek J., Polašek M., Vítek, O.,
2004, “Simulation of a COMPREX Pressure
Exchanger in a 1-D Code.” SAE International
Paper 2004-01-1000, 13
[8] Heywood J.B., 1988, Internal Combustion
Engine Fundamentals, McGraw-Hill
CONCLUSIONS
Four separate numerical methods were used to
model the Wave Disc Engine. Each of the preliminary
results confirms that the concept of the engine is
feasible. Further investigations will determine possible
improvements to the engine before the concept is
tested with an experimental prototype.
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