28-07-2014, 11:58 AM
PULSE DETONATION ENGINE
PULSE.doc (Size: 867.5 KB / Downloads: 39)
General description
All regular jet engines and most rocket engines operate on the deflagration of fuel, that is, the rapid but subsonic combustion of fuel. The pulse detonation engine is a concept currently in active development to create a jet engine that operates on the supersonic detonation of fuel. Because the combustion takes place so rapidly, the charge (fuel/air mix) does not have time to expand during this process, so it takes place under almost constant volume. Constant volume combustion is more efficient than open-cycle designs like gas turbines, which leads to greater fuel efficiency.
As the combustion process is so rapid, mechanical shutters are difficult to arrange with the required performance. Instead, PDE's generally use a series of valves to carefully time the process. In some PDE designs from General Electric, the shutters are eliminated through careful timing, using the pressure differences between the different areas of the engine to ensure the "shot" is ejected rearward.
LITERATURE REVIEW
To evaluate the relative advantages of different combustion modes for propulsion, Jian-ling Li and his colleagues studied engine in a given flight situation for a fixed amount of energy release during the combustion. Preliminary analysis suggests that compared with conventional subsonic combustion ramjet and Scramjet, PWDE has superiority in theory, even though steady normal detonation engine is much inferior to the deflagration-based engine, for a given flight condition and a given amount of heat release. Pulse detonation engine utilizes repetitive detonations to produce thrust or power. It differs from conventional systems in two major ways: unsteady operation and detonation combustion. The flow of detonable mixture is subsonic in combustion chamber of PDE. Although a series of challenging fundamental and engineering difficulties must be overcome before PDE can be considered as a feasible choice for practical propulsion, a vast number of applications for the PDE have been proposed over the years.
The entropy rise of steady normal detonation is much larger than that of steady deflagration. It is indicated that the performance of steady normal detonation engine is much inferior to that of deflagration-based engine, for a given flight condition and a given amount of heat release. PWDE has superiority in theory, compared with conventional subsonic combustion ramjet and Scramjet. The Mach number before unsteady detonation wave should carefully be selected for a given amount of heat release [1].
Wei Fan and his colleague tested three PDE models with different sizes: 30mm-I.D. by 2 m-length; 56 mm-I.D. by 2 m-length and 50 mm by 1m, which were operated over a repetition frequency range from 1 Hz to 36 Hz. One-way valves were used to adaptively control intermittent supplies of air and fuel flows. The results of detonation velocity over-pressure and impulse measurements were presented. The measured pressure ratio of detonation wave was close to that of C-J detonation. The effects of equivalence ratio, PDE diameter, length, and detonation frequency on its performance were experimentally investigated. PDE offers the potential to provide increased performance while simultaneously reducing engine weight, cost, and complexity relative to conventional propulsion systems currently in service. Due to its obvious advantages, worldwide attention has been paid to the scientific and technical issues concerning PDE. Efforts were focused on initiation and propagation of detonation waves by means of one-step detonation initiation method, low-energy ignition system, and effective Schelkin spiral.
The effects of equivalence ratio, PDE diameter, length, and detonation frequency on its performance were experimentally investigated. The obtained results have demonstrated that the averaged thrust of PDE is approximately proportional to the volume of detonation chamber and detonation frequency. The maximum averaged thrust for 56 mm diameters PDE at frequency 20 Hz was 143.8 N. For liquid C8H16/air mixture, the PDE operation with as short a length as 1000 mm and detonation frequency up to 36 Hz was successfully realized, which had made an important step to practical PDE [2].
V.F. Nikitin find the results of theoretical investigations of basic operating cycle in PDE deflagration-to-detonation transition (DDT) processes in combustible gaseous mixtures and transmission of detonation into large chambers are presented. The paper investigates the effect of implosion shock waves on the onset of detonation in gases, and suggests the scheme of detonation transmission from a narrow gap into a wide chamber, which makes it possible to reduce the predetonation length thus shortening the necessary length of the engine. The rates of energy release in detonation modes of combustion of gases are three orders of magnitude higher than in deflagration combustion modes that could make the use of detonation combustion modes more efficient for creating high power energy converters. The rate of combustible mixture supply is usually lower thus requiring the pulsed operation mode of such an energy converter.
The results of investigations show that for successful onset of the detonation with mild initiator it is necessary for DDT to appear in a narrow gap with a successive transmission of detonation into a wider section by implosion. Furthermore to promote DDT wider cavities should be used in the narrow gap [3].
G.D. Roy focused in this paper on recent accomplishments in basic and applied research on pulse detonation engines (PDE) and various PDE design concepts. Current understanding of gas and sprary detonations, thermodynamic grounds for detonation based propulsion, principles of practical implementation of the detonation-based thermodynamic cycle, and various operational constraints of PDEs are discussed. In a PDE, detonation is initiated in a tube that serves as the combustor. The detonation wave rapidly traverses the chamber resulting in a nearly constant-volume heat addition process that produces a high pressure in the combustor and provides the thrust. The operation of multi-tube PDE configurations at high detonation-initiation frequency (about 100 Hz and over) can produce a near-constant thrust. In general, the near-constant-volume operational cycle of PDE provides a higher thermodynamic efficiency as compared to the conventional constant-pressure (Brayton) cycle used in gas turbines and ramjets.
The results found by G.D. Roy were the thermodynamic efficiency of pulse detonation thrusters is considerably higher than that of other conventional thrusters based on combustion, particularly at subsonic flight at relatively low altitudes. In view of it, both air-breathing and rocket propulsion seem to receive a chance of getting a long-expected breakthrough in efficiency, and, as a consequence, in increased range, payloads, etc. The additional benefits of an ideal PDE are: simplicity of design and low weight [4].
The PDRE test model used by LI Qiang and his colleagues in their experiments utilized kerosene as the fuel, oxygen as oxidizer, and nitrogen as purge gas. The solenoid valves were employed to control intermittent supplies of kerosene, oxygen and purge gas. PDRE test model was 50 mm in inner diameter by 1.2 m long. The DDT (deflagration to detonation transition) enhancement device Shchelkin spiral was used in the test model. The effects of detonation frequency on its time-averaged thrust and specific impulse were experimentally investigated. PDREs utilize the high-energy release rate and thermodynamic characteristics of detonation waves to produce thrust. This enables PDREs to operate at higher thermodynamic efficiencies. Furthermore, since the reactants are injected into a PDRE at relatively low pressures, the need for1massive turbomachinary, as used in conventional steady-state liquid rocket engines, is eliminated.
LI Qiang and his colleagues obtained results showed that the time-averaged thrust of PDRE test model was approximately proportional to the detonation frequency. The results demonstrated that all of those nozzles could augment the thrust and specific impulse. Among those three nozzles, the convergent nozzle had the largest thrust and specific impulse augmentations, which were approximately 18%, under the specific conditions of the experiments [5].
The research done by Zhen Cen Fan and colleagues was to study the beneficial effects of the fuel pretreatments on PDRE performance, which comprised preheating and adding additives. Firstly, five concentric-counter-flow heat exchangers based on active cooling were tested to investigate the effect of geometric dimension on the heating efficiency. The ignition time and DDT time are nearly an order of magnitude longer for complex liquid hydrocarbon fuels than for gaseous fuels. A reduction in DDT or initiation time would shorten the PDRE cycle time, allowing for higher operation frequency and higher average thrust.
With the aid of kerosene preheating, the detonation initiation time for liquid kerosene was noticeably reduced and a fully-developed detonation wave was achieved. By adding additives to liquid kerosene, the detonation initiation time can be significantly reduced and the detonability of fuel also be dramatically improved. All the catalysts used in the experiment can reduce the detonation initiation time, and the performance of using TEA was best [6].
Four injectors were tested by Yu Yan and his colleagues to improve the atomization of liquid fuel and mixing process of reactants for performance enhancement of PDRE. Injector with small fuel exit area and large gas exit area was found to be effective for liquid fuel atomization and reactants mixing process. Nozzles are often considered as a method to enhance the performance that can be attained from a straight tube configuration. Therefore, there have been many research efforts about effects of nozzles on the performance of a pulse detonation engine.
The PDRE performed the best with injector having small fuel exit area and large gas exit area was found to be effective for liquid fuel atomization and reactants mixing process. The role of oxygen was very important in liquid fuel atomization and reactants mixing. The geometry of injector must be well designed to reach considerable mixing requirement. The effects of various bell-shaped converging diverging nozzles on the performance of PDRE were also experimentally investigated here to seek the optimum nozzle configuration. In our study, among four tested nozzles the highest thrust augmentation could be up to 27.3%. It was found that nozzles with high contraction ratio and high expansion ratio may be better [7].
Jian-ling Li and his colleagues based on Constant Volume Cycle (CVC) model did a systematic analysis of PDRE’s performance. Combined CVC model with Chemical Equilibrium and Applications (CEA) code, the specific impulse of PDRE using different propellants could be obtained. Utilizing this model, the specific impulse of kerosene/oxygen PDRE in vacuum and at finite backpressures is estimated. When the back pressure exists, there is an optimum nozzle area expansion ratio in the nozzle design of PDRE, similar to conventional steady-state rocket. The performance of PDRE and conventional steady rocket is compared respectively, under the two conditions of the same supply system and the same maximum material stress.
They found that in vacuum, PDRE has a comparable performance to steady rocket engine. With the same filling system, the specific impulse of PDRE significantly exceeds that of steady rocket engine, when the ambient pressure exists. With the same material stress, the specific impulse of steady rocket engine exceeds that of PDRE, when the ambient pressure exists. When PDRE and steady rocket engine have a comparable performance, the filling pressure of PDRE is much less than that of rocket engine [8].
CONCLUSIONS
• The successful implementation of a PDE requires effective detonation initiation techniques, such as a pre-detonator or DDT devices, a proper ignition system, air-fuel flow and mixing control techniques including fast-acting valves, and a fast and capable, closed-loop control system.
• Once the geometry of the detonation chamber, including the pre-detonator or DDT devices, has been set the only variables that can be controlled are the initial temperature of the flow within the detonation chamber, the initial pressure, the fuel-oxidizer ratio and the energy and timing of the ignition.
• The C-J detonation model is reliable enough to predict the properties of the detonation wave to a fairly accurate level based on the initial conditions. In addition, the post detonation flow properties can be expressed as a function of the wave Mach number, obtained from the wave speed. The C-J model can be used to control a PDE reliably by employing a fast-acting closed-loop control system to control the initial conditions within the detonation chamber.
• In the detonation chamber, combustion occurs at a very small distance and the combustion chamber is compact. Since the pressure in detonation is more, the thermal efficiency of the system is also more.