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Abstract:
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The successful implementation of a PDE running on fuel and air mixtures will require fast -acting fuel -air injection and mixing techniques , detonation initiation techniques such as DDT enhancing devices or a pre -detonator , an effective ignition system that can sustain repeated firing at high rates and a fast and capable , closed -loop control system . The control system requires high -speed transducers for real -time monitoring of the PDE and the detection of the detonation wave speed . It is widely accepted that the detonation properties predicted by C -J detonation relations are fairly accurate in comparison to experimental values . The post -detonation flow properties can also be expressed as a function of wave speed or Mach number . Therefore , the PDE control system can use C -J relations to predict the post -detonation flow properties based on measured initial conditions and compare the values with those obtained from using the wave speed . The controller can then vary the initial conditions within the combustor for the subsequent cycle , by modulating the frequency and duty cycle of the valves , to obtain optimum air and fuel flow rates , as well as modulate the energy and timing of the ignition to achieve the required detonation properties . Five different PDE ground demonstrators were designed , built and tested to study a number of the required sub -systems . This work presents a review of all the systems that were tested , along with suggestions for their improvement . The PDE setups , ranged from a compact PDE with a 19 mm (3 /4 in . ) i .d . , to two 25 mm (1 in . ) i .d . setups , to a 101 mm (4 in . ) i .d . dual -stage PDE setup with a pre -detonator . Propane -oxygen mixtures were used in the smaller PDEs . In the dual -stage PDE , propane -oxygen was used in the pre -detonator , while propane -air mixtures were used in the main combustor . Both rotary valves and solenoid valve injectors were studied . The rotary valves setups were tested at 10 Hz , while the solenoid valves were tested at up to 30 Hz on a 25 mm i .d . PDE . The dual -stage PDE was run at both 1 Hz and 10 Hz using solenoid valves . The two types of valves have their drawbacks and advantages which are discussed , along with ways to enhance their functionality . Rotary valves with stepper motor drives are recommended to be used for air flow control , while an array of solenoid injectors may be used for liquid or gaseous fuel injection . Various DDT enhancing devices were tested , including Shchelkin spirals (with varying thicknesses , lengths and pitches ) , grooved sleeves and converging -diverging nozzles . The Shchelkin spirals are found to be the most effective of all , at blockage ratios in the region of 50 to 55 % . To improve the durability of Shchelkin spirals , it is recommended that they be grooved into the inside of tubes or inserted as replaceable sleeves . Orifice plates with high blockage ratios , in the region of 50 to 80 % , are also recommended due to their simple and rugged design . All these devices along with the PDE combustor will require a strong cooling system to prevent damage from the extreme detonation temperatures . High energy (HE ) and low energy (LE ) ignition systems were tested and compared along with various designs of igniters and automotive spark plugs . It is concluded that while HE ignition may help unsensitized fuel -air mixtures to achieve detonations faster than LE systems , the former have severe drawbacks . The HE igniters get damaged quickly , and require large and heavy power supplies . While the HE ignition is able to reduce ignition delay in a propane -oxygen pre -detonator , it did not show a significant improvement in bringing about DDT in the main combustor using propane -air mixtures . The compact pre -detonator design with a gradual area change transitioning to a larger combustor is found to be effective for detonation initiation , but the pre -detonator concept is recommended for high -speed applications only , since higher speeds requires more sensitive , easily detonable fuels that have short ignition delays and DDT run -up distances . Dynamic pressure transducers , ion detectors and photo -detectors were compared for the diagnostics of the detonation wave . The ion detector is found to be a safe , cheap and effective choice for obtaining detonation or flame velocities , and better than the optical detector , which is not practical for long -duration PDE operations . The piezoelectric dynamic pressure transducer has problems with heating and requires an effective cooling system to enable it to function in a PDE . Other diagnostics studied include thrust measurement and mass flow rate measurement techniques . Additionally , fuel sensitizing techniques , such as hydrogen blending , along with the DDT devices can ensure that detonations are produced successfully . |