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05-09-2016, 09:29 AM
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PULSE DETONATION ENGINE TECHNOLOGY: AN OVERVIEW


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ABSTRACT
Pulse detonation is a propulsion technology that involves detonation of fuel to produce
thrust more efficiently than current engine systems. By library research and an interview
with Dr. Roger Reed of the Metals and Materials Engineering Department of the
University of British Columbia, it is shown that Pulse Detonation Engine (PDE)
technology is more efficient than current engine types by virtue of its mechanical
simplicity and thermodynamic efficiency. As the PDE produces a higher specific thrust
than comparable ramjet engines at speeds of up to approximately Mach 2.3, it is suitable
for use as part of a multi-stage propulsion system. The PDE can provide static thrust for
a ramjet or scramjet engine, or operate in combination with turbofan systems. As such, it
sees potential applications in many sectors of the aerospace, aeronautic, and military
industries. However, there remain engineering challenges that must be overcome before
the PDE can see practical use. Current methods for initiating the detonation process need
refinement. To this end, both Pratt & Whitney and General Electric have developed
different processes to accomplish this. Also, current materials used in jet engines, such
as Nickel-based super-alloys, are inadequate to withstand the extreme heat and pressure
generated by the detonation cycle. Therefore, new materials must be developed for this
purpose.


Introduction
Imagine aircraft crossing continents and oceans at higher speeds and efficiencies.
Imagine spacecraft launching with higher safety factors and lower costs. Imagine
military craft operating in many flight conditions with increased performance. This is the
potential for Pulse Detonation Engine (PDE) technology; it has the capability to
revolutionize the aviation industry.
As PDEs are an extension of pulse-jet engines, they share many similarities. However,
there is one important difference between them: PDEs detonate, rather than deflagrate,
their fuel. Based on library research and an interview with Dr. Roger Reed (Metals and
Materials Engineering, UBC), this report provides an overview of issues associated with
air-breathing PDE technology.
PDEs are currently not in production, although research has been ongoing for several
years and systems should be put to use in the near future. This report provides an
introduction to the principle of pulse detonation propulsion and describes its advantages
over current technology. Additionally, this report presents potential practical applications
for the PDE, and discusses engineering problems that must be overcome.


Description of Pulse Detonation Technology
The main objective of pulse detonation research is to develop an efficient engine that is
primarily used for high-speeds (potentially Mach* 5), as well as high-altitudes. The basic
concept behind the technology is to detonate*, rather than deflagrate*, the fuel. The
detonation of fuel results in immense pressure, which in turn is used as thrust.
2.1. The Beginnings
An exact history of pulse detonation technology is not easily determined. The history
is unclear for reasons such as the secrecy involved in research as this technology
could prove to be very profitable. What is known is that the technology is derived
from pulse jet engines, and many organizations within the past five to ten years have
produced test-bed engines. No engines have currently been put into production
(Wikipedia 2004). However, some people disagree and believe the military used
pulse detonation in the speculated Aurora* aircraft as early as 1980. The existence of
this aircraft is in question, and therefore it is not possible to conclude the type of
propulsion used.
2.2. Classifications
Pulse detonation engines (PDEs) can be classified in numerous ways. The type of
fuel used, whether it is air breathing, or the number of detonation chambers, for
example, are all ways in which the engines can be classified. Three broad categories
can be established: Pure, combined-cycle and hybrid. Pure PDEs, as the name
implies, rely only on a PDE, consisting of detonation tubes, an inlet, and a nozzle.
Combined-cycle engines use different cycles at different speed ranges. These cycles
include those of the PDE as well as a ramjet* or scramjet* flowpath among others.
Hybrid engines are a combination of a PDE and turbofan or turbojet engines (NASA
Glenn Research Center 2004, 3rd paragraph).


Description
Before describing the wave cycle of a PDE, it is beneficial to understand the
difference between detonation and deflagration.
2.3.1. Detonation Versus Deflagration
Deflagration is the relatively gentle process of burning fuel rapidly with flames.
One of the main characteristics of deflagration is that the flame travels at
subsonic* speeds. Detonation, on the other hand, can be thought of as a violent
reaction that travels at supersonic* speeds. Detonation produces a much higher
amount of pressure compared to deflagration (Aardvark, Pulse Detonation
Engines 2004).
2.3.2. Wave Cycle
The wave cycle of a PDE can be broken down into certain stages (Figure 1). In
the first stage of the cycle, air and fuel are drawn in through individual inlets.
Once the two have been combined to create a flammable mixture, the
combination is passed to the front of the detonation chamber, where it is then
detonated. Upon detonation, the pressure of the mixture increases tremendously,
which creates a shockwave* that travels the length of the chamber. There is no
need for a series of shutters within the chamber, as in pulse jet engines, to ensure
the explosion moves toward the rear; the shockwave instead serves the purpose of
the shutters. When the shock wave reaches the end of the chamber all of the
combustion products are discharged at once. As soon as the products exit, the
pressure inside the chamber drops suddenly resulting in air entering its inlet
(Wikipedia 2004). The cycle is then repeated up to hundreds of times a second.
Figure 2 clearly shows the inlets as well as other significant features of a pulse
detonation engine.



Advantages of Pulse Detonation Propulsion
Pulse detonation engines (PDE’s) exhibit several performance advantages in comparison
with current steady-deflagration jets. The inherent mechanical design simplicity of the
PDE results in smaller packaging volumes and lower part counts, aiding in integration
and maintenance. The thermodynamic efficiency of the pulse detonation cycle results in
higher theoretical performance across a wide speed range.
3.4. Mechanical Design Simplicity
Despite the apparent difficulties regarding the design of the PDE, its underlying
mechanical principle is simple. Since the nature of the detonation process
substantially increases the pressure within the detonation tube, fuel does not have to
be injected at the high pressures that are necessary for significant thrust with a
conventional engine. This eliminates the necessity for robust fuel injection pumps
(Ebrahimi 2003). Additionally, this pressure compresses the intake air, thus
mitigating the need for compressors, turbines or other heavy components typical of
current liquid-fuelled engine types. Indeed, the PDE was specifically designed to
avoid the mechanical complexity of spinning compressors or other rotating machinery
in its air-flow path (Povinelli 2002). With the absence of a separate compression
stage and the consolidation of the other stages of the detonation cycle into a single
component, the PDE generally demonstrates a lower part count than other engine
types (Mawid et al. 2003; Ebrahimi 2003; Povinelli 2002).
The PDE’s mechanical simplicity offers many benefits to propulsion systems. The
lower part count makes for simpler maintenance procedures. It also contributes to an
overall lighter engine, improving the thrust-to-mass ratio of equivalent engine
systems (Coleman 2004; Mawid et al. 2003). The simplicity as well as the number of
the parts involved will decrease the cost of PDE propulsion systems (Ebrahimi 2003).
It is clear that the PDE will therefore prove useful in propulsion systems when its
engineering challenges – outlined in Section Five of this document – are surmounted.
6
3.5. Thermodynamic Efficiency
The thermodynamic efficiency of the pulse detonation process results in higher
theoretical performance of the PDE across a wide range of flight conditions. Under
ideal situations, the PDE has a theoretical performance advantage in both specific
thrust and fuel consumption over conventional jet engines from the speed range of
Mach 0-5. This is attributed to the near-constant-volume nature of the pulse
detonation thermodynamic process (Coleman 2004). Figure 3 shows the theoretical
thrust advantage of the PDE employed in a hybrid configuration with a turbofan
engine, as opposed to the standard afterburner assembly.
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