I.N.Gusev, A.A.Karachev, P.V.Kopyl, V.M.Shibkov, L.V.Shibkova, O.S.Surkont
Faculty of Physics, Moscow State University, 119991, Moscow, Russia, shibkov <shibkov@phys.msu.ru>
One of the major practical problems arising by development of the hypersonic direct-flow air- breathing engine is reduction of its longitudinal size. For this purpose it is necessary to apply new unconventional ways of maintenance of fast volumetric ignition and control of process of combustion of hydrocarbon fuel in conditions of a high-speed stream. One of new decisions of the given problem is use of various types of gas discharges. For stationary combustion of hydrocarbon-air mixture at use of non-stationary low-temperature plasma it is necessary to optimize a mode of initiation of the pulsed discharge, i.e. the value of energy put into plasma, pulse duration and frequency of following of the microwave pulses. It can be carried out at creation of the discharge in a mode of a programmable pulse [1]. In a programmable pulse mode gas breakdown and creation of plasma is carried out with the help of a short powerful pulse, or a train of short powerful pulses, and maintenance of the formed plasma and energy input occurs during the long low-power pulse following with some time delay after the first pulse, or after a pack of short powerful pulses.
In the report the combined discharge created in a programmable mode in a high-speed air stream is considered. Influence of low temperature plasma of a programmable microwave discharge on efficiency of combustion of liquid hydrocarbon fuel, injected in a drop phase into transonic (flow Mach number of M ≈ 1) air stream is investigated.
These researches are a first step to revealing an opportunity of application of a programmable discharge in hypersonic aerodynamics. The programmable discharge represents a combination of the self-sustained pulse-periodic microwave discharge and the non-self-sustained transversal electrode discharge created on a surface of a dielectric plate. Thus surface microwave discharge serves for the several purposes. First, it facilitates initiation of the direct current discharge. Second, effective energy contribution to plasma takes place under conditions of the microwave discharge. It leads to effective creation of the active radicals, excited and charged particles, and also to intensive volumetric illumination of gas flow by ultra-violet radiation. These factors can result in fast ignition of
a gas mixture. The direct current discharge serves for the contribution of a thermal energy to gas and stabilization of combustion of a high speed stream of hydrocarbon fuel. Without use of the microwave discharge the electrode direct current discharge could not exists in a free high-speed air stream.
In the work the programmable discharge was created in a mode when the low-power pulse is switched during time, equal to duration of a pack of short powerful pulses. The number of microwave pulses N in a train was possible to change from 1 up to 100, frequency of recurrence of pulses f = 50 Hz. The microwave source is a pulsed magnetron generator with parameters: the wavelength is λ = 2.4 cm, the pulse duration is τ = 20 μs, the pulsed microwave power is W = 50 kW, and the period-to-pulse duration ratio is Q = 1000. As a direct current source for creation of the transversal electrode discharge the power supply with a voltage up to 5 kV was used. The discharge current could change from 1 up to 20 A. In experiments duration of a programmable pulse changed from 0.8 up to 1.2 s. At the first stage technical alcohol was used as liquid hydrocarbon fuel. The delay time of injection of liquid alcohol changed from 0.2 up to 0.3 s.
For optimization of process of combustion of liquid hydrocarbon fuel in a high-speed air stream it is necessary to carry out full diagnostics of the investigated phenomenon. With this purpose in laboratory the automated diagnostic complex consisting from optical refracting gauges, photo electronic multipliers, pulse shadow installation, system for measurement of conductivity of a flame, electric probes, thermocouples, gauges of pressure, a two-wire line, digital monochromaters, spectroscopes, oscillographs, photo and video cameras has been created.
Under conditions of the programmable discharge stabilization of plasma-stimulated combustion of alcohol, injected in drop phase (as a spray) in transonic (M ~ 1) air stream is realized.
Measurements of parameters of plasma and gas in conditions of stationary combustion of alcohol have been carried out. It was shown, that at ignition of liquid hydrocarbon fuel sharply change common view and characteristics of the discharge (the volt-
ampere characteristic of the discharge, time dependences of a discharge current and it is especial voltage on a discharge gap, a spectrum of radiation and integrated intensity of a luminescence of a flame, the thermal stream, registered by
thermocouple located on distance of 15 cm downwards on an air stream).
The general view of process of combustion is submitted in Fig. 1. The stream is propagated from left to right. The separate Fig. 1
Fig. 2
Fig. 3
fragments of dynamics of development of process of ignition and combustion of liquid alcohol are submitted in Fig. 2. An exposition time of one fragment is 4 μs, frequency of shooting is f = 5000 Hz/s. At the left – the discharge in an air stream, in the middle – transient of ignition, on the right – combustion stabilization. It was shown, that at injection of alcohol intensity of the thermal streams registered by the thermocouple sharply grows.
Process of stabilization of liquid hydrocarbon fuel combustion can be observed on change of a time dependences of a pulse of a discharge current and especially a pulse of a voltage on a discharge gap.
Fluctuations of amplitude of a voltage sharply
decrease at combustion stabilization. Completeness of combustion of liquid alcohol reaches of 80 % depending on microwave power. Combustion occurs at a flame temperature ~2000 K (look Fig. 3). Integrated intensity of a flame luminescence increases only on 25-30 % at combustion whereas intensity of a hydroxyl luminescence grows in 30 times. The same results are received at high-speed combustion others liquid hydrocarbon fuels (kerosene, gasoline).
1. A.S.Zarin, A.A.Kuzovnikov, V.M.Shibkov.
Freely localized microwave discharge in air.
Moscow: Oil and gas. 1996.