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Continuous detonation combustion of ternary “hydrogen–liquid propane–air” mixture in annular combustor
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Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor S.M. Frolov a,b,c,*, V.S. Aksenov a,b, V.S. Ivanov a, I.O. Shamshin a,b a
Semenov Institute of Chemical Physics of the Russian Academy of Sciences, Moscow 119991, Russia National Research Nuclear University MEPhI, Moscow 115409, Russia c Federal Scientific Center Institute of System Research of the Russian Academy of Sciences, Moscow 117218, Russia b
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abstract
Article history:
Experiments are performed on continuous detonation combustion of ternary hydrogeneliquid
Received 2 February 2017
propaneeair mixture in a large-scale annular combustor 406 mm in outer diameter with an
Received in revised form
annular gap of 25 mm. Liquid propane is fed into the combustor at the time when sustained
16 May 2017
continuous-detonation combustion of hydrogeneair mixture is attained therein. Mass flow
Accepted 19 May 2017
rates of hydrogen, propane and air in the experiments ranged from 0.1 to 0.5 kg/s (hydrogen),
Available online xxx
0.1 to 0.5 kg/s (propane), and 5 to 12 kg/s (air). Continuous-detonation combustion of liquid propane in air is obtained for the first time due to addition of hydrogen rather than due to
Keywords:
enrichment of air with oxygen. Combustor operation with a single continuously rotating
Continuous-detonation combustion
detonation wave (DW) for about 0.1 s has been obtained when the flow rates of propane and air
Hydrogen
remained constant while the flow rate of hydrogen was rapidly decreasing.
Liquid propane
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Air Experiment Detonation wave
Introduction Supersonic detonative combustion has several advantages over a subsonic deflagrative combustion. It is accompanied by an increase in the total fluid pressure in the combustion chamber and exhibits a much higher power of energy deposition. Since chemical conversion in a DW occurs in the autoignition mode, it provides high combustion efficiency, and emissions of environmentally hazardous substances are minimized due to the high propagation velocity of detonations. Zel'dovich [1] was the first who pointed out the prospects of using detonative combustion in energy and transport sectors
[1]. In the same year, 1940, Hoffman [2] proposed to organize controlled detonative combustion for creating jet thrust by periodically generated DWs in a channel-like combustor. In 1959, Voitsekhovskii [3] proposed another scheme of detonative combustion in a continuous spinning DW circulating in an annular combustion chamber with a continuous supply of a fuel mixture. Ideas [1e3] obtained the rapid development since the 1990s, in particular towards the creation of practical devices utilizing detonative combustion. Thus, devices implementing the ideas of Hoffman and Voitsekhovskii have got special names: pulse-detonation and continuousdetonation (or rotating-detonation) combustors or engines.
* Corresponding author. Semenov Institute of Chemical Physics of the Russian Academy of Sciences, Moscow 119991, Russia. E-mail address: [email protected] (S.M. Frolov). http://dx.doi.org/10.1016/j.ijhydene.2017.05.138 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Detailed reviews of the state-of-the-art in this field of science and technology are given elsewhere [4e6]. These references contain up-to-date information on various aspects of pulse- and continuous-detonation rocket engines operating on oxygen as oxidizer and various fuels (hydrogen, methane, ethylene, acetylene, propane, kerosene, etc.). As for airbreathing engines utilizing air as oxidizer the relevant information is much less available. The general outcome of the reviews [4e6] is that the major current problem standing in the way of practical application of detonative combustion is how to manage a sustainable operation process with regular liquid motor fuels and air. The purpose of this work is experimental study of continuous-detonation combustion of ternary “hydrogeneliquid propaneeair” mixture. It should be noted that so far continuous detonation of gaseous propane could be obtained only when the air was enriched with oxygen at least to 50% by mass [5]. Experiments in Ref. [5] were carried out in the cylindrical annular combustor with the outer wall diameter of 306 mm and the annular gap of 23 mm. The continuousdetonation mode with a single DW rotating in the annulus with the velocity of ~1600 m/s has been obtained at mass flow rates of gaseous propane and oxidizer (O2:N2 ¼ 1:1) equal to 0.24 and 1.75 kg/s, respectively. Continuous-detonation combustion of hydrogeneair mixtures was studied experimentally elsewhere [5e12] in annular combustors of different scale and design. Various modes of self-sustained detonative combustion are reported including modes with three, two and one DW(s) simultaneously rotating in the combustor annulus in the same circumferential direction, as well as the longitudinal pulse-detonation mode arising at certain limiting conditions of hydrogen and air supply. In the longitudinal pulse-detonation mode, the detonation is reinitiated in a position close to the combustor outlet and propagates axially upstream as a supersonic reaction front occupying the entire cross section of the combustor without regular rotation [8,10]. Other gaseous fuels used so far in experiments on continuous-detonation combustion with air are ethylene, acetylene, and syngas (H2 þ CO) (see, e.g., Ref. [5]). As for liquid fuels, reported in Ref. [5] are the results of experiments on continuous-detonation combustion of aviation kerosene in oxygen-enriched air with O2:N2 ¼ 1:1 (by mass). The experiments were carried out in the cylindrical annular combustor with the outer wall diameter of 306 mm and the annular gap of 23 mm. The continuous-detonation mode with two DWs simultaneously rotating in the same circumferential direction in the annulus with the velocity of ~1500 m/s has been obtained at mass flow rates of liquid fuel and oxidizer equal to 0.9 and 6.3 kg/s, respectively. In this work, for the experiments with liquid propane we used the large-scale experimental water-cooled cylindrical annular combustor which we recently developed for the experiments with hydrogeneair mixture [7,8].
Experimental setup The schematic of the experimental setup is shown in Fig. 1. The current setup differs from that used in Refs. [7,8] by the presence of the system for liquid propane supply. The main
element of the setup e combustor (Fig. 2a) e has the following dimensions: outer diameter 406 mm, height 310 mm and the width of the annular gap 25 mm. It is mounted on the thrustmeasuring table vertically. The upper end of the combustor communicates with the outdoor environment through the outlet nozzle with a removable central body in the form of a cone with a half-angle of 23 (Fig. 2b). To ensure controlled delivery of liquid propane in the combustor we designed and constructed a new injector head and the fuel supply system of displacement type. The injector head was designed based on multivariant three-dimensional gas dynamic calculations using the computational technology described in Refs. [13,14]. The injector head (see Fig. 2c) is made of steel and has a belt of 80 radial nozzles 0.46 mm in diameter equally spaced on the inner wall of the annular combustor along a circle 386 mm in diameter. Liquid propane is supplied to the propane plenum of the combustor through two 10-mm diameter tubes with the overpressure ranging from 0.5 to maximum 10 MPa. In addition to the system of liquid propane supply, the combustor is equipped with a system of hydrogen supply. Pressurized hydrogen (with the overpressure ranging from 0.5 to 4 MPa) is supplied to the hydrogen plenum attached to the outer wall of the combustor and enters the combustor through the belt of 240 radial holes of 1-mm diameter evenly distributed along the outer circumferential wall at a distance of 30 mm (can be varied) downstream from the belt of liquid fuel nozzles. Pressurized air (with the overpressure ranging from 0.2 to 0.7 MPa) is supplied from the air reservoir to the air plenum through four side tubes of round cross section connected to the outer combustor wall tangentially, so that the butt end of the combustor is closed. From the plenum, air flows into the combustor through the annular gap of 10 mm width between the injector head and the combustor outer wall. The tangential supply of air leads to a strong swirling flow in the combustor annulus with the tangential velocity of about 200 m/s. The liquid propane supply system (Fig. 3) has a fuel tank of 5-L volume equipped with an electrical heater of 6 kW power. The heater allows heating the liquid fuel up to a temperature of 200 C. Prior to experiments the system of liquid propane supply was operated with water to assess the discharge coefficient of the injector head. The discharge coefficient was found to be equal to 0.76. The combustor is equipped with a detonation initiator (Fig. 4), a tube 26 mm in diameter and 600 mm long with inlet ports for fuel (hydrogen) and oxidizer (air), two independent automotive spark plugs and 400-mm long Shchelkin spiral ensuring reliable deflagration-to-detonation transition inside the tube and detonation transmission into the annular gap of the combustor. Two spark plugs are used to ensure reliable ignition in case of misfire. The energy deposited by each spark plug is 0.1 J. The initiator tube is attached to the combustor tangentially at a distance of 150 mm downstream from the belt of holes for hydrogen supply and has its own feed system for the supply of fuel mixture components. The detonation velocity in the initiator is monitored by two ionization probes IP1 and IP2 developed based on standard automotive spark plugs. The experimental setup is equipped with a remote-control system. The control system provides digital control over the
Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Fig. 1 e Schematic of the experimental setup.
operation of air, hydrogen and liquid propane valves, detonation initiator, as well as data acquisition and safety systems. The control system consists of a laptop, a digital control module and a digital signal amplifier. Similar to Refs. [7,8], the data acquisition system comprises 16 short-response-time (~5 ms) ionization probes with power supply unit and several low-frequency pressure transducers (response time ~100 ms) all connected to the personal computer via an analog-to-digital converter. The ionization probe is designed to detect and measure the electrical conductivity of the medium in the combustor. The probe is the needle isolated from the housing and introduced by the exposed end into the annular gap of the combustor. The other end of the needle is soldered to a shielded cable connected to the data acquisition system. The ionization probes are installed in the outer combustor wall in two lines (Fig. 5): axial line with 9 equidistant (step 25 mm) probes and circumferential line with 7 equidistant probes plus one common probe belonging to both axial and circumferential lines. For processing and rapid analysis of digital probe records shown in Fig. 5 and recorded in a PGC file we have developed a special computer program [15]. Using this program, the signals of the probes are “visualized” both in the axial direction of the combustor (for probes in the axial line), and in the
circumferential direction (for probes in the circumferential line). Low-frequency CURANT DI 6 MPa pressure transducers are mounted in the supply lines of air, hydrogen and liquid propane upstream the corresponding valves, as well as in the air, hydrogen and liquid propane plenums of the combustor and are used for monitoring the corresponding pressureetime histories. The valves used are the fast-response pneumatic valves coupled with electromagnetic valves. The actuation air pressure in the pneumatic valves is 70 atm. Contrary to Refs. [7,8], high-frequency pressure transducers were generally not used in this work because of their vulnerability due to intense thermal loads. As a matter of fact, experiments in Refs. [7,8] proved that ionization probes are reliable sensors of detonations. Nevertheless, in some selected experiments a high-frequency PCB 113B24 pressure transducer (response time ~5 ms) was used to measure local instantaneous static pressure in one position of the combustor located in the cross section of a circumferential line of ionization probes at a distance of 25 mm from ionization probe 1 in Fig. 5 (denoted as PT in Fig. 5). Due to hightemperature conditions in the combustion chamber this transducer was mounted outside the combustor outer wall and communicated with the combustion chamber through a
Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Fig. 2 e (a) A photograph of the combustor, (b) the schematic of installing a new injector head for liquid fuel supply in the combustor of [7,8], and (c) a photograph of the injector head. Dimensions are in millimeters.
short branch tube 2 mm in diameter. Thus, this transducer was used only for detecting wave fronts and relative pressure fluctuations rather than for measuring pressure itself. The experimental procedure is as follows. After activation by an analog switch, the control system activates the air and hydrogen valves of the combustor and the valves in the initiator feed system. Thereafter, the control system activates ignition in the initiator tube resulting in deflagration-todetonation transition and transmission of a DW from the tube to the annular gap of the combustor followed by the
establishment of continuous-detonation combustion with a single DW or several DWs continuously rotating in the annulus downstream from the circumferential belt of hydrogen injection holes. The liquid propane valve is normally opened 100 ms after opening the air and hydrogen valves. This time delay results in liquid propane entering the annular gap of the combustor only in 250e300 ms after opening the air and hydrogen valves due to the finite time of filling the propane line and plenum. As soon as liquid propane begins to bleed into the combustor annulus, the hydrogen
Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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below when discussing Fig. 12). The operation is terminated by successive deactivation of liquid propane and air valves. All experiments were conducted at atmospheric pressure and at 22 C ambient air temperature.
Experiments with hydrogeneair mixture
Fig. 3 e Photograph of the liquid fuel supply system.
valve is turned off and the combustor is fed only with air and liquid propane but some residual hydrogen is still present for some time. The combustor operation time is preset in the control system and is usually limited by 1 s (without water cooling) and results in several hundred rotations of the DWs in the annular gap at a frequency of about 1400 Hz (see details
Due to the replacement of the injector head in the combustor of Refs. [7,8], at the first stage of the research reported herein we carried out experiments with hydrogeneair mixture to ensure the feasibility of the operation process with continuous-detonation combustion. In a series of experiments several modes of stable continuous-detonation combustion with a single or several DWs rotating simultaneously in one direction in the combustor annulus were obtained. Fig. 6a shows a photograph of the exhaust plume at continuous-detonation combustion of hydrogeneair mixture in one of the experiments in this series. The number of rotating DWs depended on the pressure of air supply. Despite the direction of detonation rotation could change from run to run, in most experiments the rotation direction was opposite to the swirling flow. At air overpressure above approximately 0.5 MPa the continuous-detonation combustion process with two DWs running in the same direction was registered. Fig. 7 shows the example of overpressure records in the air and hydrogen plenums in one of experiments with the initial (at ignition) pressures of 0.52 and 1.95 MPa, respectively (at mass flow rates of 11 and 0.32 kg/s, respectively). The overall equivalence ratio of hydrogeneair mixture in this experiment
Fig. 4 e Schematic of detonation initiator.
Fig. 5 e Arrangement of (a) axial and (b) circumferential lines of ionization probes in the combustor and example of ionization current records provided by the probes. PT stands for high-frequency pressure transducer. Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Fig. 6 e Photographs of exhaust plumes at (a) continuous-detonation combustion of hydrogeneair mixture and (b) hydrogeneliquid propaneeair mixture, and (c) at conventional deflagrative combustion of hydrogeneliquid propaneeair mixture (at detonation failure).
is estimated as 1.0. It is seen that the pressures virtually level out in the plenums in a time interval from about 180 to about 300 ms. Next, at time 300 ms, the hydrogen supply pressure begins to drop rapidly due to closing of the hydrogen valve. Following the rapid decrease in hydrogen pressure, the air pressure starts to gradually decrease until ~400 ms. The decrease in the air plenum overpressure starting from ~300 ms (0.52 MPa) and ending at ~400 ms (0.4 MPa, see solid curve in Fig. 7) could be treated as the indication of the gradual drop in the back pressure in the combustor due to gradual decrease in the mean rate of energy release in the combustion
Fig. 7 e Example of overpressure records in the air and hydrogen plenums in one of experiments with hydrogeneair mixture at air and hydrogen supply pressures of 0.52 and 1.95 MPa, respectively.
process. At a time of 400 ms, the air pressure also starts to drop rapidly due to closing of the air valve. Fig. 8 shows the results of processing the fragments of records of ionization probes of the axial line (9 probes, Fig. 8a) and circumferential line (8 probes, Fig. 8b) with the procedure described in Ref. [15]. This procedure attributes the brightest color (“max” in the scale of Fig. 8a) to the highest ionization current recorded by a probe in an area with the highest temperature and the darkest color (“min” in the scale of Fig. 8a) to the lowest ionization current recorded by a probe in an area with the lowest temperature. Thus, plotted along the Y-axes in Fig. 8 are pixels corresponding to each consecutive ionization probe: 9 pixels for the axial line and 8 pixels for the circumferential line. Plotted along the X-axes in Fig. 8 is time in milliseconds. The white color (“max”) of the pixels in Fig. 8 depicts high ionization current registered by the corresponding probe caused by high temperature in a DW. Black color (“min”) of the pixels denotes the absence of ionization current in the cold fresh reactants. Such visualization of ionization probe records allows determining the number of DWs, their apparent propagation velocity, direction of motion, rotation frequency, their height and many other specific features of the phenomenon, and allows studying the dynamics of various transient processes. Each fragment in Fig. 8 corresponds to the time interval of 30 ms. The same visualization procedure is adopted below for other experiments discussed herein. The fact that Fig. 8 depicts a DW propagation at least in the time interval from 290 to 302 ms is substantiated by the fragment of records of ionization probe No. 1 (top record) and the neighboring pressure transducer (bottom record) in Fig. 9: the pressure record exhibits periodic sharp pressure fronts inherent in a leading shock of the rotating detonation wave. The sharp peaks in both records appear simultaneously with each other, thus indicating that the reaction front is led by a
Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Fig. 8 e Results of processing the fragments of records of ionization probes of (a) axial and (b) circumferential lines at continuous-detonation combustion of hydrogeneair mixture at air and hydrogen supply pressures of 0.52 and 1.95 MPa, respectively.
strong shock. Moreover, the duration of peaks at the halfheight of ionization probe and pressure signals is nearly the same and equals to ~300 ms. In Fig. 7, during a time interval from 150 to 300 ms (shown only partly in Fig. 8) the continuous-detonation combustion process with two DWs rotating with an apparent velocity of ~1200 m/s in the direction opposite to the direction of air flow swirling was detected in this experiment. The apparent detonation velocity is determined as the slope of white bands in Fig. 8b (the larger the slope, the higher is the apparent detonation velocity). Thus, the distance between ionization probes No. 1 and 8 in the circumferential line is 7P/8, where P
is the perimeter of the outer combustor wall (1.275 m). In the time interval, say, from 290 to 300 ms in Fig. 8b, the time taken for the DW to travel between these probes is 1.06 ms. Therefore the apparent detonation velocity is ~1200 m/s. Taking into account the air flow swirling velocity (~200 m/s), the absolute speed of detonation rotation is estimated as ~1400 m/s (~0.7DCJ, where DCJ is the ChapmaneJouguet detonation velocity in homogeneous stoichiometric hydrogeneair mixture). The number of DWs simultaneously rotating in the combustor annulus is determined by dividing the actual detonation rotation frequency by the estimated rotation frequency of a single DW, other conditions been equal. The actual detonation
Fig. 9 e The fragments of records of an ionization probe (top) and high-frequency pressure transducer (bottom) and its exploded view. The inclination of the “zero” line of the pressure record is caused by thermal effect of detonation products on the sensitive element of pressure transducer. Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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rotation frequency in the same time interval from 290 to 300 ms is the number of white bands (19) divided by time (0.01 s) and is equal to 19/0.01 ¼ 1900 Hz. The rotation frequency of a single DW is estimated as the apparent detonation velocity (~1200 m/s) divided by the perimeter of the outer combustor wall (1.275 m), and is 1200/1.275 ¼ 940 Hz. Clearly, the actual frequency is twice as large, which means that in a time interval between 290 and 300 ms there exist two DWs in the combustor. Small error (about 1%) accompanying such a definition of a number of DWs in the combustor is caused by the error in determining the apparent detonation velocity. Note that the pressure peaks in Fig. 9 appear with the same frequency as that obtained from Fig. 8. After switching off the supply of hydrogen (at 290 ms) the flow of hydrogen through the combustor decreases rapidly causing changes in the overall mixture composition (the mixture becomes progressively leaner in average), in the reactive flow structure and in the propagation velocity of the DWs (the slope of white bands in Fig. 8b gradually decreases). By 340e345 ms the combustion process in the combustor virtually ceases, despite there still exists a reaction front rotating in the annular combustor at the apparent velocity of about 400e450 m/s (see several faint bands in Fig. 8b at 340e345 ms). Taking into account that this reaction front propagates in the direction opposite to the direction of air flow swirling, its absolute propagation velocity is 600e650 m/s. The complete failure of combustion occurs (see black background in Fig. 8 at about 350 ms) when the pressure in the hydrogen plenum drops down to about 0.52 MPa (see Fig. 7). The absence of ionization currents at all ionization probes indicates that the reaction front is completely blown-off from the combustor by the mixture of residual hydrogen and air which exhibits low reactivity insufficient for further support of the reaction fronts in the combustor. Fig. 10 presents the fragments of records of an ionization probe (top) and high-frequency pressure transducer (bottom) shortly prior to completion of the combustion process. It is seen that the sharp pressure and reaction fronts appear simultaneously in both records until the ionization probe shows reaction quenching. This means that the faint reaction front registered in Fig. 8b at 340e345 ms is still coupled with a shock wave or, by other words, this reaction wave still resembles a detonation despite a very low (but supersonic) propagation velocity on the level of 600e650 m/s. Such propagation velocities of the reaction front are only possible if mixture ignition is virtually insensitive to shock temperature, which is too low for autoignition of fuel-lean (in average) hydrogeneair mixture behind the incident shock wave. At these conditions, the detonation-like structure can
only be preserved in steady-state supersonic reaction waves which are supported by turbulent combustion ignited by a reflected rather than incident shock wave (see, e.g., Ref. [16]). Thus, according to [16], in tubes with regular orifice plates or obstructions of other shape like Shchelkin spiral the reaction waves in hydrogeneair mixtures can propagate at supersonic velocities starting from the hydrogen volume fraction in the mixture exceeding ~13%. The local pressure build up in such waves is dispersed at speeds of the order of the reaction wave speed and the pressure front closely coincides with the reaction front. In the annular combustor, shock reflection from the outer compressive wall is the intrinsic feature of the continuous-detonation operation process [13,14]. As for the turbulence, its intensity in the recirculation zone downstream the injector head is quite high [13]. This hypothesis is indirectly confirmed by the pressure profiles in Fig. 9b: the pressure wave is composed of the lead shock followed by a gradual pressure rise with the characteristic maximum. Very similar pressure waves are detected in experiments and calculations on nonideal detonations in tubes with regular obstacles [17]. If this hypothesis is valid for the supersonic reaction fronts in an annular combustor, then the reaction decay in Fig. 10 is explained by the failure of mixture ignition in a lead shock wave reflected from the outer combustor wall. At air overpressure below approximately 0.5 MPa continuous-detonation combustion with a single rotating DW was registered. As an example, Fig. 11 shows overpressure records in the air and hydrogen plenums in one of experiments with the initial (at ignition) pressures of 0.2 and 1.1 MPa, respectively. It is seen that the pressures virtually level out in the plenums in a time interval from about 180 to about 250 ms. Next, at time 250 ms, the hydrogen supply pressure begins to drop rapidly due to closing of the hydrogen valve, whereas air pressure starts to decrease only at time ~400 ms after closing of the air valve. Fig. 12 shows the results of processing the fragments of records of ionization probes of the axial line (9 probes, Fig. 12a) and circumferential line (8 probes, Fig. 12b). Each section in Fig. 12 corresponds to the time interval of 30 ms. In this experiment, up to time of about 220 ms a stable continuousdetonation combustion process with a single DW rotating with an apparent velocity of ~1850 m/s in the direction of flow swirling (the slope of white bands in Fig. 12 is opposite to the slope of similar bands in Fig. 8b) was detected. The actual frequency of detonation rotation in time interval from 170 to 200 ms is 43/0.03 ¼ 1433 Hz, whereas the estimated rotation frequency of a single DW is 1850/1.275 ¼ 1450 Hz, i.e., there exists only single DW in the combustor.
Fig. 10 e The fragments of records of an ionization probe (top) and high-frequency pressure transducer (bottom) prior to completion of the combustion process. Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Experiments with triple “hydrogeneliquid propaneeair” mixture
Fig. 11 e Example of overpressure records in the air and hydrogen supply plenums in one of experiments with hydrogeneair mixture at air and hydrogen supply pressures of 0.2 and 1.1 MPa, respectively.
After switching off the supply of hydrogen (at 230 ms) the flow of hydrogen through the combustor decreases rapidly causing changes in the reactive flow structure and in the mode of DW propagation. Here, we observed a transition mode with a single DW propagating periodically either faster (~2200 m/s) or slower (~1700 m/s) as compared to the detonation velocity value at time before ~230 ms (see successive white bands with two different slopes in the time interval from 230 to 260 ms in Fig. 12b). By about 260e270 ms this latter continuous-detonation mode in the combustor transforms to another combustion mode which is accompanied by the periodic appearance of reaction fronts and their nearly simultaneous arrival at the circumferential line of ionization probes at a frequency 2e3-fold lower than the rotation frequency of a single DW. In Refs. [7,8], this combustion mode attributed to the longitudinal pulse-detonation mode was observed in tests with hydrogeneair mixture at limiting conditions of fuel and air supply. In the longitudinal pulsedetonation mode, the detonation is reinitiated in a position close to the combustor nozzle and propagates upstream as a reaction front occupying the entire cross section of the combustor either without evident rotation or with sporadic rotations of arising reaction fronts in different directions. In the experiment of Figs. 11 and 12, the longitudinal pulsedetonation mode ceases at about 350 ms when the pressure in the hydrogen plenum drops down to about 0.03 MPa. Contrary to the continuous-detonation mode in Fig. 8, the longitudinal pulse detonation mode fails abruptly without exhibiting slowly decaying reaction fronts. This finding indirectly supplements the hypothesis that the limiting continuous-detonation mode is a supersonic low-velocity mode with mixture ignition in a reflected shock wave. In fact, in the longitudinal pulse-detonation mode there seems to exist no mechanism for transition between normal detonation and nonideal detonation (“quasidetonation” [16]) due to the absence of regular lead shock reflections during axial propagation of the lead shock.
At the second stage of the research reported herein we carried out experiments with a ternary “hydrogeneliquid propaneeair” mixture, with hydrogen being used as initiating fuel. As mentioned above, the supply valve of liquid propane was opened 100 ms after opening the air and hydrogen valves. This time delay resulted in liquid propane entering the annular gap of the combustor only in 250e300 ms after opening the air and hydrogen valves due to the finite time of filling the propane line and plenum. As soon as liquid propane began to bleed into the combustor annulus, the hydrogen valve was turned off and the combustor was fed only with air and liquid propane but some residual hydrogen was still present for some time. In experiments with the supply pressure of liquid propane up to 0.7 MPa (at a mass flow rate of up to 0.27 kg/s) and with the mass flow rate of air up to 10.4 kg/s we have registered the continuous-detonation combustion process with a single rotating DW. Fig. 5b shows a photograph of the exhaust plume at continuous-detonation combustion of liquid propane in one of the experiments in this series. Fig. 13 shows the example of overpressure records in the air, hydrogen, and liquid propane plenums in one of experiments of this series, which is very close by conditions to the experiment relevant to Figs. 7 and 8 in Section 3 for hydrogeneair mixture. Here, the initial pressures (at ignition) of air and hydrogen are 0.53 and 1.9 MPa, respectively (mass flow rates 10.4 kg/s and 0.32 kg/s (at the stage of feeding with air) and 0.15 kg/s (at the stage of feeding with air and liquid propane)). The pressure in the propane plenum is initially produced by expanding propane vapor. Liquid propane enters the combustor only at about 420 ms when the pressure in the propane plenum exhibits a local maximum due to hydraulic shock. The overpressure in the propane plenum at hydrogen valve turning-off is 0.7 MPa (mass flow rate 0.27 kg/s). It is seen that by about 600 ms the hydrogen supply to the combustor is virtually ceased whereas the supply pressures of air and liquid propane are constant and equal to 0.7 and 0.4 MPa, respectively, until the closing of the liquid propane valve at 800 ms. Fig. 14 shows the results of processing the fragments of records of ionization probes in the experiment of Fig. 13. Like in Figs. 8 and 12, in Fig. 14 each fragment corresponds to the time interval of 30 ms. In a time interval between 330 and 370 ms two DWs in a hydrogeneair mixture propagating at an apparent velocity of ~1200 m/s in the direction opposite to the direction of air flow swirling are registered in this experiment. The actual detonation rotation frequency in the upper fragment of Fig. 14b is the number of white bands (57) divided by time (0.03 s) and is equal to 1900 Hz. As mentioned above, after the opening of the liquid propane valve the combustor is first filled with propane vapor. Therefore at a subsequent time interval between 370 and 420 ms the two DWs existing in the combustor are replaced by a single DW propagating in a “hydrogenegaseous propaneeair” mixture at an apparent velocity of ~1290 m/s in the same direction. As a matter of fact, the actual frequency of detonation rotation in the time interval from 390 to 420 ms is 31/ 0.03 ¼ 1033 Hz, whereas the estimated rotation frequency of a
Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Fig. 12 e Results of processing the fragments of records of ionization probes of (a) axial and (b) circumferential lines at continuous-detonation combustion of hydrogeneair mixture at air and hydrogen supply pressures of 0.2 and 1.1 MPa, respectively.
single DW is 1290/1.275 ¼ 1012 Hz, i.e., there exist only a single DW in the combustor. At time 420 ms the combustor begins to be fed with liquid propane (this time corresponds to a local maximum of the pressure curve in the propane plenum, see. Fig. 13). Due to injection of relatively cold liquid in the combustor, in a time interval between 420 and 450 ms a transition period with the longitudinal pulse-detonation mode is observed which is accompanied by periodic appearance of reaction fronts in the combustor and their nearly simultaneous arrival at the circumferential line of ionization probes at a frequency 2e3-fold lower than the rotation frequency of a single DW. After the transition period, at time ~450 ms, the operation process in the combustor is transformed to the continuousdetonation combustion mode with a single DW of ~150 mm height propagating at an apparent velocity of ~1390 m/s in the direction opposite to the direction of air flow swirling. Thus, the supply of gaseous and thereafter liquid propane to the combustor led to transition from the continuous-detonation combustion mode with two DWs to the mode with a single
Fig. 13 e Example of overpressure records in the air, hydrogen, and liquid propane plenums in one of experiments with hydrogeneliquid propaneeair mixture at air and hydrogen supply pressures of 0.5 and 1.9 MPa, respectively.
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DW via the longitudinal pulse-detonation mode. At 500 ms, hydrogen supply is switched off resulting in a rapid decrease of hydrogen flow rate, but the reactive flow structure in the combustor and the DW velocity remain virtually unchanged. For example, at time 550 ms, when the hydrogen supply pressure is reduced more than twice and approximately equals to the pressure of liquid propane supply (see Fig. 13), the DW structure and propagation velocity are almost the same as at time 500 ms, i.e., propane participates in the reaction. The overall equivalence ratio of “hydrogeneliquid propaneeair” mixture in this experiment is estimated as 0.93. Further evidences of propane participation in the reaction can
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be found by comparing Fig. 14 with Fig. 8, the analog of Fig. 14 for the experiment with pure hydrogeneair mixture. If in Fig. 8 the detonation failure occurs when hydrogen pressure drops down to 0.52 MPa, the detonation failure in Fig. 14 occurs at a time interval from 570 to 580 ms when hydrogen pressure is reduced to 0.24 MPa. At this condition, combustion of fuel mixture in the combustor ceases completely (solid black background in Fig. 14). Later on, in this experiment external diffusion combustion of propane has been established at the combustor outlet. Careful examination of Figs. 13 and 14 allows finding a possible reason of detonation decay. The decrease in the air
Fig. 14 e Results of processing the fragments of records of ionization probes mounted (a) in axial and (b) in circumferential lines of the combustor outer wall: the overpressure of liquid propane supply is 0.7 MPa. Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Fig. 15 e Decay of continuous-detonation combustion at liquid propane supply with the overpressure of 0.8 MPa.
plenum overpressure starting from ~540 ms (0.53 MPa) and ending at ~590 ms (0.4 MPa, see solid curve in Fig. 13) could be treated as the indication of the gradual drop in the back pressure in the combustor due to gradual decrease in the mean rate of energy release from the value inherent in the flow with a DW rotating in the annual gap at the apparent velocity of ~1390 m/s to the value inherent in the flow with external diffusion combustion of propane at the combustor outlet. The gradual decrease in the mean rate of energy release in the combustor is caused by the gradual decrease in the detonation velocity which is seen in Fig. 14b as the gradual decrease in the slope of white bands in the time interval between 540 and 590 ms. The apparent velocity of the reaction front corresponding to the last white band appearing at time ~590 ms in Fig. 14b is quite low and close to 400e450 m/s. Taking into account that this reaction front propagates in the direction opposite to the direction of air flow swirling, its absolute propagation velocity is about 600e650 m/s, which is the same as the limiting velocity value found in Section 3 for the hydrogeneair mixture. Thus, similar to the statement in Section 3 we come to the hypothesis that the combustion process ceases presumably due to the failure of mixture ignition in the lead shock wave reflected from the compressive outer wall of the annular combustor. When the supply pressure of liquid propane was increased to 0.8 MPa, the continuous-detonation combustion mode was not observed even when hydrogen valve was not turned off (Fig. 15). In this case, the operation mode with external diffusion combustion of the triple mixture was observed in experiments (see Fig. 6c). It is seen in Fig. 15 that the evolution of the operation process in the time interval from 390 to 410 ms is very similar to that shown in Fig. 14, i.e., a single DW is rotating in the combustor annulus filled with the “hydrogenegaseous propaneeair” mixture at an apparent velocity of ~1290 m/s. However, at a time exceeding 410 ms the height of the DW (the height of the black triangular in the upper fragment of Fig. 15) starts to increase, which is accompanied by the decrease in the detonation propagation velocity (the slope of white bands decreases), and detonation decays in several milliseconds. It is interesting that while decaying, the continuous-detonation mode transforms temporarily to the longitudinal pulse-detonation mode, discussed above. However, contrary to Fig. 14, only one pulse of this kind is registered in Fig. 15. The failure of the continuous-detonation combustion mode in this case occurs just at arrival of liquid propane to the combustor annulus. This means that supply of relatively cold liquid propane at 0.8 MPa greatly affects
formation of reactive heterogeneous mixture and leads to fast detonation decay.
Concluding remarks Experiments were carried out on continuous-detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in a large-scale annular combustor. Liquid propane was injected into the combustor from the injector head at a time when a sustained continuous-detonation combustion of hydrogeneair mixture was attained therein. In the experiments, the mass flow rates of hydrogen, liquid propane and air ranged from 0.1 to 0.5 kg/s, from 0.1 to 0.5 kg/s, and from 5 to 12 kg/s, respectively. In the experiments with a liquid propane supply pressure of up to 0.7 MPa (with a mass flow rate of up to 0.27 kg/s) and with air mass flow rate of about 10 kg/s the continuousdetonation combustion process with a single rotating DW has been registered after shutoff of hydrogen supply. Evidences are obtained that propane was participating in the detonation process in the combustor. If without propane supply the failure of continuous-detonation combustion of hydrogeneair mixture after the shutdown of hydrogen supply occurred at reduction of hydrogen pressure down to 0.52 MPa, the failure of continuous-detonation combustion of the triple mixture occurred at a lower hydrogen pressure (0.24 MPa). The failure of continuous-detonation combustion of hydrogeneair and hydrogeneliquid propaneeair mixtures was found to occur when the absolute velocity of reaction wave rotation in the combustor annular gap was as low as 600e650 m/s. In view of it, the hypothesis has been put forward that the mechanism of steady-state supersonic reaction front propagation in the annular gap is similar to the mechanism of flame propagation in tubes with regular obstacles: the reaction is ignited by the lead shock wave reflected from the compressive outer wall of the annulus and propagates due to turbulent mixing of hot combustion products with fresh reactants in the recirculation zone behind the injector head. According to this hypothesis, failure of continuous-detonation combustion occurs due to the failure of mixture ignition in the lead shock wave reflected from the compressive outer wall of the annular combustor. Thus, the continuous-detonation combustion of liquid propane has been obtained for the first time due to addition of hydrogen rather than due to air enrichment with oxygen. It is planned to continue this research to obtain continuousdetonation combustion of propaneeair mixture without addition of hydrogen.
Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Acknowledgments This work was supported by the Russian Ministry of Education and Science under the State contract 14.609.21.0002 (contract ID RFMEFI60914X0002 contract).
references
[1] Zel'dovich YB. On utilizing detonative combustion in power engineering. Sov J Tech Phys 1940;10(17):1453e61. [2] Hoffman H. Reaction propulsion by intermittent detonative combustion. Ministry of Supply; 1940. Volkenrode Translation. [3] Voitsekhovskii BV. Stationary detonation. Dokl USSR Acad Sci 1959;129:1254e6. [4] Roy GD, Frolov SM, Borisov AA, Netzer DW. Pulse detonation propulsion: challenges, current status, and future perspective. Prog Energy Combust Sci 2004;30(6):545e672. [5] Bykovskii FA, Zhdan SA. Continuous spinning detonation. Novosibirsk: Siberian Branch of the Russ Acad Sci Publ; 2013. p. 423. [6] Wolanski P. Detonation propulsion. Proc Combust Inst 2013;34:125e58. [7] Frolov SM, Aksenov VS, Dubrovskii AV, Ivanov VS, Shamshin IO. Energy efficiency of a continuous-detonation combustion chamber. Combust Explos Shock Waves 2015;51(2):232e45. [8] Frolov SM, Aksenov VS, Ivanov VS, Shamshin IO. Large-scale hydrogen-air continuous detonation combustor. Int J Hydrogen Energy 2015;40:1616e23.
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[9] Lin W, Zhou J, Liu S, Lin Z, Zhuang F. Experimental study on propagation mode of H2/air continuously rotating detonation wave. Int J Hydrogen Energy 2015;40:1980e93. [10] Anand V, St. George A, Driscoll R, Gutmark E. Investigation of rotating detonation combustor operation with H2-air mixtures. Int J Hydrogen Energy 2015;41(2):1281e92. [11] Wang C, Liu W, Liu S, Jiang L, Lin Z. Experimental investigation on detonation combustion patterns of hydrogen/vitiated air with annular combustor. Exp Therm Fluid Sci 2015;66:269e78. [12] Fotia ML, Schauer F, Kaemming T, Hoke JL. Experimental study of the performance of a rotating detonation engine with nozzle. J Propuls Power 2016;32:674e81. [13] Frolov SM, Dubrovskii AV, Ivanov VS. Three-dimensional numerical simulation of the operation of a rotatingdetonation chamber with separate supply of fuel and oxidizer. Russ J Phys Chem B 2013;7(1):35e43. [14] Dubrovskii AV, Ivanov VS, Frolov SM. Three-dimensional numerical simulation of the operation process in a continuous detonation combustor with separate feeding of hydrogen and air. Russ J Phys Chem B 2015;9(1):104e19. [15] Frolov SM, Aksenov VS, Dubrovskii AV, Zangiev AE, Ivanov VS, Medvedev SN, et al. Chemiionization and acoustic diagnostics of the process in continuous- and pulsedetonation combustors. Dokl Phys Chem 2015;465(1):273e8. [16] Lee JHS, Knystautas R, Freiman A. High speed turbulent deflagrations and transition to detonation in H2eair mixtures. Combust Flame 1984;56:227e39. [17] Zel'dovich YB, Borisov AA, Gelfand BE, Frolov SM, Mailkov AE. Nonideal detonation waves in rough tubes. In: Kuhl AL, Bowen JR, Leyer J-C, Borisov AA, editors. Progr astron aeron, dynamics of explosions, vol. 114. Washington DC: AIAA Inc; 1988. p. 211e31.
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Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor S.M. Frolov a,b,c,*, V.S. Aksenov a,b, V.S. Ivanov a, I.O. Shamshin a,b a
Semenov Institute of Chemical Physics of the Russian Academy of Sciences, Moscow 119991, Russia National Research Nuclear University MEPhI, Moscow 115409, Russia c Federal Scientific Center Institute of System Research of the Russian Academy of Sciences, Moscow 117218, Russia b
article info
abstract
Article history:
Experiments are performed on continuous detonation combustion of ternary hydrogeneliquid
Received 2 February 2017
propaneeair mixture in a large-scale annular combustor 406 mm in outer diameter with an
Received in revised form
annular gap of 25 mm. Liquid propane is fed into the combustor at the time when sustained
16 May 2017
continuous-detonation combustion of hydrogeneair mixture is attained therein. Mass flow
Accepted 19 May 2017
rates of hydrogen, propane and air in the experiments ranged from 0.1 to 0.5 kg/s (hydrogen),
Available online xxx
0.1 to 0.5 kg/s (propane), and 5 to 12 kg/s (air). Continuous-detonation combustion of liquid propane in air is obtained for the first time due to addition of hydrogen rather than due to
Keywords:
enrichment of air with oxygen. Combustor operation with a single continuously rotating
Continuous-detonation combustion
detonation wave (DW) for about 0.1 s has been obtained when the flow rates of propane and air
Hydrogen
remained constant while the flow rate of hydrogen was rapidly decreasing.
Liquid propane
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Air Experiment Detonation wave
Introduction Supersonic detonative combustion has several advantages over a subsonic deflagrative combustion. It is accompanied by an increase in the total fluid pressure in the combustion chamber and exhibits a much higher power of energy deposition. Since chemical conversion in a DW occurs in the autoignition mode, it provides high combustion efficiency, and emissions of environmentally hazardous substances are minimized due to the high propagation velocity of detonations. Zel'dovich [1] was the first who pointed out the prospects of using detonative combustion in energy and transport sectors
[1]. In the same year, 1940, Hoffman [2] proposed to organize controlled detonative combustion for creating jet thrust by periodically generated DWs in a channel-like combustor. In 1959, Voitsekhovskii [3] proposed another scheme of detonative combustion in a continuous spinning DW circulating in an annular combustion chamber with a continuous supply of a fuel mixture. Ideas [1e3] obtained the rapid development since the 1990s, in particular towards the creation of practical devices utilizing detonative combustion. Thus, devices implementing the ideas of Hoffman and Voitsekhovskii have got special names: pulse-detonation and continuousdetonation (or rotating-detonation) combustors or engines.
* Corresponding author. Semenov Institute of Chemical Physics of the Russian Academy of Sciences, Moscow 119991, Russia. E-mail address: [email protected] (S.M. Frolov). http://dx.doi.org/10.1016/j.ijhydene.2017.05.138 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Detailed reviews of the state-of-the-art in this field of science and technology are given elsewhere [4e6]. These references contain up-to-date information on various aspects of pulse- and continuous-detonation rocket engines operating on oxygen as oxidizer and various fuels (hydrogen, methane, ethylene, acetylene, propane, kerosene, etc.). As for airbreathing engines utilizing air as oxidizer the relevant information is much less available. The general outcome of the reviews [4e6] is that the major current problem standing in the way of practical application of detonative combustion is how to manage a sustainable operation process with regular liquid motor fuels and air. The purpose of this work is experimental study of continuous-detonation combustion of ternary “hydrogeneliquid propaneeair” mixture. It should be noted that so far continuous detonation of gaseous propane could be obtained only when the air was enriched with oxygen at least to 50% by mass [5]. Experiments in Ref. [5] were carried out in the cylindrical annular combustor with the outer wall diameter of 306 mm and the annular gap of 23 mm. The continuousdetonation mode with a single DW rotating in the annulus with the velocity of ~1600 m/s has been obtained at mass flow rates of gaseous propane and oxidizer (O2:N2 ¼ 1:1) equal to 0.24 and 1.75 kg/s, respectively. Continuous-detonation combustion of hydrogeneair mixtures was studied experimentally elsewhere [5e12] in annular combustors of different scale and design. Various modes of self-sustained detonative combustion are reported including modes with three, two and one DW(s) simultaneously rotating in the combustor annulus in the same circumferential direction, as well as the longitudinal pulse-detonation mode arising at certain limiting conditions of hydrogen and air supply. In the longitudinal pulse-detonation mode, the detonation is reinitiated in a position close to the combustor outlet and propagates axially upstream as a supersonic reaction front occupying the entire cross section of the combustor without regular rotation [8,10]. Other gaseous fuels used so far in experiments on continuous-detonation combustion with air are ethylene, acetylene, and syngas (H2 þ CO) (see, e.g., Ref. [5]). As for liquid fuels, reported in Ref. [5] are the results of experiments on continuous-detonation combustion of aviation kerosene in oxygen-enriched air with O2:N2 ¼ 1:1 (by mass). The experiments were carried out in the cylindrical annular combustor with the outer wall diameter of 306 mm and the annular gap of 23 mm. The continuous-detonation mode with two DWs simultaneously rotating in the same circumferential direction in the annulus with the velocity of ~1500 m/s has been obtained at mass flow rates of liquid fuel and oxidizer equal to 0.9 and 6.3 kg/s, respectively. In this work, for the experiments with liquid propane we used the large-scale experimental water-cooled cylindrical annular combustor which we recently developed for the experiments with hydrogeneair mixture [7,8].
Experimental setup The schematic of the experimental setup is shown in Fig. 1. The current setup differs from that used in Refs. [7,8] by the presence of the system for liquid propane supply. The main
element of the setup e combustor (Fig. 2a) e has the following dimensions: outer diameter 406 mm, height 310 mm and the width of the annular gap 25 mm. It is mounted on the thrustmeasuring table vertically. The upper end of the combustor communicates with the outdoor environment through the outlet nozzle with a removable central body in the form of a cone with a half-angle of 23 (Fig. 2b). To ensure controlled delivery of liquid propane in the combustor we designed and constructed a new injector head and the fuel supply system of displacement type. The injector head was designed based on multivariant three-dimensional gas dynamic calculations using the computational technology described in Refs. [13,14]. The injector head (see Fig. 2c) is made of steel and has a belt of 80 radial nozzles 0.46 mm in diameter equally spaced on the inner wall of the annular combustor along a circle 386 mm in diameter. Liquid propane is supplied to the propane plenum of the combustor through two 10-mm diameter tubes with the overpressure ranging from 0.5 to maximum 10 MPa. In addition to the system of liquid propane supply, the combustor is equipped with a system of hydrogen supply. Pressurized hydrogen (with the overpressure ranging from 0.5 to 4 MPa) is supplied to the hydrogen plenum attached to the outer wall of the combustor and enters the combustor through the belt of 240 radial holes of 1-mm diameter evenly distributed along the outer circumferential wall at a distance of 30 mm (can be varied) downstream from the belt of liquid fuel nozzles. Pressurized air (with the overpressure ranging from 0.2 to 0.7 MPa) is supplied from the air reservoir to the air plenum through four side tubes of round cross section connected to the outer combustor wall tangentially, so that the butt end of the combustor is closed. From the plenum, air flows into the combustor through the annular gap of 10 mm width between the injector head and the combustor outer wall. The tangential supply of air leads to a strong swirling flow in the combustor annulus with the tangential velocity of about 200 m/s. The liquid propane supply system (Fig. 3) has a fuel tank of 5-L volume equipped with an electrical heater of 6 kW power. The heater allows heating the liquid fuel up to a temperature of 200 C. Prior to experiments the system of liquid propane supply was operated with water to assess the discharge coefficient of the injector head. The discharge coefficient was found to be equal to 0.76. The combustor is equipped with a detonation initiator (Fig. 4), a tube 26 mm in diameter and 600 mm long with inlet ports for fuel (hydrogen) and oxidizer (air), two independent automotive spark plugs and 400-mm long Shchelkin spiral ensuring reliable deflagration-to-detonation transition inside the tube and detonation transmission into the annular gap of the combustor. Two spark plugs are used to ensure reliable ignition in case of misfire. The energy deposited by each spark plug is 0.1 J. The initiator tube is attached to the combustor tangentially at a distance of 150 mm downstream from the belt of holes for hydrogen supply and has its own feed system for the supply of fuel mixture components. The detonation velocity in the initiator is monitored by two ionization probes IP1 and IP2 developed based on standard automotive spark plugs. The experimental setup is equipped with a remote-control system. The control system provides digital control over the
Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Fig. 1 e Schematic of the experimental setup.
operation of air, hydrogen and liquid propane valves, detonation initiator, as well as data acquisition and safety systems. The control system consists of a laptop, a digital control module and a digital signal amplifier. Similar to Refs. [7,8], the data acquisition system comprises 16 short-response-time (~5 ms) ionization probes with power supply unit and several low-frequency pressure transducers (response time ~100 ms) all connected to the personal computer via an analog-to-digital converter. The ionization probe is designed to detect and measure the electrical conductivity of the medium in the combustor. The probe is the needle isolated from the housing and introduced by the exposed end into the annular gap of the combustor. The other end of the needle is soldered to a shielded cable connected to the data acquisition system. The ionization probes are installed in the outer combustor wall in two lines (Fig. 5): axial line with 9 equidistant (step 25 mm) probes and circumferential line with 7 equidistant probes plus one common probe belonging to both axial and circumferential lines. For processing and rapid analysis of digital probe records shown in Fig. 5 and recorded in a PGC file we have developed a special computer program [15]. Using this program, the signals of the probes are “visualized” both in the axial direction of the combustor (for probes in the axial line), and in the
circumferential direction (for probes in the circumferential line). Low-frequency CURANT DI 6 MPa pressure transducers are mounted in the supply lines of air, hydrogen and liquid propane upstream the corresponding valves, as well as in the air, hydrogen and liquid propane plenums of the combustor and are used for monitoring the corresponding pressureetime histories. The valves used are the fast-response pneumatic valves coupled with electromagnetic valves. The actuation air pressure in the pneumatic valves is 70 atm. Contrary to Refs. [7,8], high-frequency pressure transducers were generally not used in this work because of their vulnerability due to intense thermal loads. As a matter of fact, experiments in Refs. [7,8] proved that ionization probes are reliable sensors of detonations. Nevertheless, in some selected experiments a high-frequency PCB 113B24 pressure transducer (response time ~5 ms) was used to measure local instantaneous static pressure in one position of the combustor located in the cross section of a circumferential line of ionization probes at a distance of 25 mm from ionization probe 1 in Fig. 5 (denoted as PT in Fig. 5). Due to hightemperature conditions in the combustion chamber this transducer was mounted outside the combustor outer wall and communicated with the combustion chamber through a
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Fig. 2 e (a) A photograph of the combustor, (b) the schematic of installing a new injector head for liquid fuel supply in the combustor of [7,8], and (c) a photograph of the injector head. Dimensions are in millimeters.
short branch tube 2 mm in diameter. Thus, this transducer was used only for detecting wave fronts and relative pressure fluctuations rather than for measuring pressure itself. The experimental procedure is as follows. After activation by an analog switch, the control system activates the air and hydrogen valves of the combustor and the valves in the initiator feed system. Thereafter, the control system activates ignition in the initiator tube resulting in deflagration-todetonation transition and transmission of a DW from the tube to the annular gap of the combustor followed by the
establishment of continuous-detonation combustion with a single DW or several DWs continuously rotating in the annulus downstream from the circumferential belt of hydrogen injection holes. The liquid propane valve is normally opened 100 ms after opening the air and hydrogen valves. This time delay results in liquid propane entering the annular gap of the combustor only in 250e300 ms after opening the air and hydrogen valves due to the finite time of filling the propane line and plenum. As soon as liquid propane begins to bleed into the combustor annulus, the hydrogen
Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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below when discussing Fig. 12). The operation is terminated by successive deactivation of liquid propane and air valves. All experiments were conducted at atmospheric pressure and at 22 C ambient air temperature.
Experiments with hydrogeneair mixture
Fig. 3 e Photograph of the liquid fuel supply system.
valve is turned off and the combustor is fed only with air and liquid propane but some residual hydrogen is still present for some time. The combustor operation time is preset in the control system and is usually limited by 1 s (without water cooling) and results in several hundred rotations of the DWs in the annular gap at a frequency of about 1400 Hz (see details
Due to the replacement of the injector head in the combustor of Refs. [7,8], at the first stage of the research reported herein we carried out experiments with hydrogeneair mixture to ensure the feasibility of the operation process with continuous-detonation combustion. In a series of experiments several modes of stable continuous-detonation combustion with a single or several DWs rotating simultaneously in one direction in the combustor annulus were obtained. Fig. 6a shows a photograph of the exhaust plume at continuous-detonation combustion of hydrogeneair mixture in one of the experiments in this series. The number of rotating DWs depended on the pressure of air supply. Despite the direction of detonation rotation could change from run to run, in most experiments the rotation direction was opposite to the swirling flow. At air overpressure above approximately 0.5 MPa the continuous-detonation combustion process with two DWs running in the same direction was registered. Fig. 7 shows the example of overpressure records in the air and hydrogen plenums in one of experiments with the initial (at ignition) pressures of 0.52 and 1.95 MPa, respectively (at mass flow rates of 11 and 0.32 kg/s, respectively). The overall equivalence ratio of hydrogeneair mixture in this experiment
Fig. 4 e Schematic of detonation initiator.
Fig. 5 e Arrangement of (a) axial and (b) circumferential lines of ionization probes in the combustor and example of ionization current records provided by the probes. PT stands for high-frequency pressure transducer. Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Fig. 6 e Photographs of exhaust plumes at (a) continuous-detonation combustion of hydrogeneair mixture and (b) hydrogeneliquid propaneeair mixture, and (c) at conventional deflagrative combustion of hydrogeneliquid propaneeair mixture (at detonation failure).
is estimated as 1.0. It is seen that the pressures virtually level out in the plenums in a time interval from about 180 to about 300 ms. Next, at time 300 ms, the hydrogen supply pressure begins to drop rapidly due to closing of the hydrogen valve. Following the rapid decrease in hydrogen pressure, the air pressure starts to gradually decrease until ~400 ms. The decrease in the air plenum overpressure starting from ~300 ms (0.52 MPa) and ending at ~400 ms (0.4 MPa, see solid curve in Fig. 7) could be treated as the indication of the gradual drop in the back pressure in the combustor due to gradual decrease in the mean rate of energy release in the combustion
Fig. 7 e Example of overpressure records in the air and hydrogen plenums in one of experiments with hydrogeneair mixture at air and hydrogen supply pressures of 0.52 and 1.95 MPa, respectively.
process. At a time of 400 ms, the air pressure also starts to drop rapidly due to closing of the air valve. Fig. 8 shows the results of processing the fragments of records of ionization probes of the axial line (9 probes, Fig. 8a) and circumferential line (8 probes, Fig. 8b) with the procedure described in Ref. [15]. This procedure attributes the brightest color (“max” in the scale of Fig. 8a) to the highest ionization current recorded by a probe in an area with the highest temperature and the darkest color (“min” in the scale of Fig. 8a) to the lowest ionization current recorded by a probe in an area with the lowest temperature. Thus, plotted along the Y-axes in Fig. 8 are pixels corresponding to each consecutive ionization probe: 9 pixels for the axial line and 8 pixels for the circumferential line. Plotted along the X-axes in Fig. 8 is time in milliseconds. The white color (“max”) of the pixels in Fig. 8 depicts high ionization current registered by the corresponding probe caused by high temperature in a DW. Black color (“min”) of the pixels denotes the absence of ionization current in the cold fresh reactants. Such visualization of ionization probe records allows determining the number of DWs, their apparent propagation velocity, direction of motion, rotation frequency, their height and many other specific features of the phenomenon, and allows studying the dynamics of various transient processes. Each fragment in Fig. 8 corresponds to the time interval of 30 ms. The same visualization procedure is adopted below for other experiments discussed herein. The fact that Fig. 8 depicts a DW propagation at least in the time interval from 290 to 302 ms is substantiated by the fragment of records of ionization probe No. 1 (top record) and the neighboring pressure transducer (bottom record) in Fig. 9: the pressure record exhibits periodic sharp pressure fronts inherent in a leading shock of the rotating detonation wave. The sharp peaks in both records appear simultaneously with each other, thus indicating that the reaction front is led by a
Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Fig. 8 e Results of processing the fragments of records of ionization probes of (a) axial and (b) circumferential lines at continuous-detonation combustion of hydrogeneair mixture at air and hydrogen supply pressures of 0.52 and 1.95 MPa, respectively.
strong shock. Moreover, the duration of peaks at the halfheight of ionization probe and pressure signals is nearly the same and equals to ~300 ms. In Fig. 7, during a time interval from 150 to 300 ms (shown only partly in Fig. 8) the continuous-detonation combustion process with two DWs rotating with an apparent velocity of ~1200 m/s in the direction opposite to the direction of air flow swirling was detected in this experiment. The apparent detonation velocity is determined as the slope of white bands in Fig. 8b (the larger the slope, the higher is the apparent detonation velocity). Thus, the distance between ionization probes No. 1 and 8 in the circumferential line is 7P/8, where P
is the perimeter of the outer combustor wall (1.275 m). In the time interval, say, from 290 to 300 ms in Fig. 8b, the time taken for the DW to travel between these probes is 1.06 ms. Therefore the apparent detonation velocity is ~1200 m/s. Taking into account the air flow swirling velocity (~200 m/s), the absolute speed of detonation rotation is estimated as ~1400 m/s (~0.7DCJ, where DCJ is the ChapmaneJouguet detonation velocity in homogeneous stoichiometric hydrogeneair mixture). The number of DWs simultaneously rotating in the combustor annulus is determined by dividing the actual detonation rotation frequency by the estimated rotation frequency of a single DW, other conditions been equal. The actual detonation
Fig. 9 e The fragments of records of an ionization probe (top) and high-frequency pressure transducer (bottom) and its exploded view. The inclination of the “zero” line of the pressure record is caused by thermal effect of detonation products on the sensitive element of pressure transducer. Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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rotation frequency in the same time interval from 290 to 300 ms is the number of white bands (19) divided by time (0.01 s) and is equal to 19/0.01 ¼ 1900 Hz. The rotation frequency of a single DW is estimated as the apparent detonation velocity (~1200 m/s) divided by the perimeter of the outer combustor wall (1.275 m), and is 1200/1.275 ¼ 940 Hz. Clearly, the actual frequency is twice as large, which means that in a time interval between 290 and 300 ms there exist two DWs in the combustor. Small error (about 1%) accompanying such a definition of a number of DWs in the combustor is caused by the error in determining the apparent detonation velocity. Note that the pressure peaks in Fig. 9 appear with the same frequency as that obtained from Fig. 8. After switching off the supply of hydrogen (at 290 ms) the flow of hydrogen through the combustor decreases rapidly causing changes in the overall mixture composition (the mixture becomes progressively leaner in average), in the reactive flow structure and in the propagation velocity of the DWs (the slope of white bands in Fig. 8b gradually decreases). By 340e345 ms the combustion process in the combustor virtually ceases, despite there still exists a reaction front rotating in the annular combustor at the apparent velocity of about 400e450 m/s (see several faint bands in Fig. 8b at 340e345 ms). Taking into account that this reaction front propagates in the direction opposite to the direction of air flow swirling, its absolute propagation velocity is 600e650 m/s. The complete failure of combustion occurs (see black background in Fig. 8 at about 350 ms) when the pressure in the hydrogen plenum drops down to about 0.52 MPa (see Fig. 7). The absence of ionization currents at all ionization probes indicates that the reaction front is completely blown-off from the combustor by the mixture of residual hydrogen and air which exhibits low reactivity insufficient for further support of the reaction fronts in the combustor. Fig. 10 presents the fragments of records of an ionization probe (top) and high-frequency pressure transducer (bottom) shortly prior to completion of the combustion process. It is seen that the sharp pressure and reaction fronts appear simultaneously in both records until the ionization probe shows reaction quenching. This means that the faint reaction front registered in Fig. 8b at 340e345 ms is still coupled with a shock wave or, by other words, this reaction wave still resembles a detonation despite a very low (but supersonic) propagation velocity on the level of 600e650 m/s. Such propagation velocities of the reaction front are only possible if mixture ignition is virtually insensitive to shock temperature, which is too low for autoignition of fuel-lean (in average) hydrogeneair mixture behind the incident shock wave. At these conditions, the detonation-like structure can
only be preserved in steady-state supersonic reaction waves which are supported by turbulent combustion ignited by a reflected rather than incident shock wave (see, e.g., Ref. [16]). Thus, according to [16], in tubes with regular orifice plates or obstructions of other shape like Shchelkin spiral the reaction waves in hydrogeneair mixtures can propagate at supersonic velocities starting from the hydrogen volume fraction in the mixture exceeding ~13%. The local pressure build up in such waves is dispersed at speeds of the order of the reaction wave speed and the pressure front closely coincides with the reaction front. In the annular combustor, shock reflection from the outer compressive wall is the intrinsic feature of the continuous-detonation operation process [13,14]. As for the turbulence, its intensity in the recirculation zone downstream the injector head is quite high [13]. This hypothesis is indirectly confirmed by the pressure profiles in Fig. 9b: the pressure wave is composed of the lead shock followed by a gradual pressure rise with the characteristic maximum. Very similar pressure waves are detected in experiments and calculations on nonideal detonations in tubes with regular obstacles [17]. If this hypothesis is valid for the supersonic reaction fronts in an annular combustor, then the reaction decay in Fig. 10 is explained by the failure of mixture ignition in a lead shock wave reflected from the outer combustor wall. At air overpressure below approximately 0.5 MPa continuous-detonation combustion with a single rotating DW was registered. As an example, Fig. 11 shows overpressure records in the air and hydrogen plenums in one of experiments with the initial (at ignition) pressures of 0.2 and 1.1 MPa, respectively. It is seen that the pressures virtually level out in the plenums in a time interval from about 180 to about 250 ms. Next, at time 250 ms, the hydrogen supply pressure begins to drop rapidly due to closing of the hydrogen valve, whereas air pressure starts to decrease only at time ~400 ms after closing of the air valve. Fig. 12 shows the results of processing the fragments of records of ionization probes of the axial line (9 probes, Fig. 12a) and circumferential line (8 probes, Fig. 12b). Each section in Fig. 12 corresponds to the time interval of 30 ms. In this experiment, up to time of about 220 ms a stable continuousdetonation combustion process with a single DW rotating with an apparent velocity of ~1850 m/s in the direction of flow swirling (the slope of white bands in Fig. 12 is opposite to the slope of similar bands in Fig. 8b) was detected. The actual frequency of detonation rotation in time interval from 170 to 200 ms is 43/0.03 ¼ 1433 Hz, whereas the estimated rotation frequency of a single DW is 1850/1.275 ¼ 1450 Hz, i.e., there exists only single DW in the combustor.
Fig. 10 e The fragments of records of an ionization probe (top) and high-frequency pressure transducer (bottom) prior to completion of the combustion process. Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Experiments with triple “hydrogeneliquid propaneeair” mixture
Fig. 11 e Example of overpressure records in the air and hydrogen supply plenums in one of experiments with hydrogeneair mixture at air and hydrogen supply pressures of 0.2 and 1.1 MPa, respectively.
After switching off the supply of hydrogen (at 230 ms) the flow of hydrogen through the combustor decreases rapidly causing changes in the reactive flow structure and in the mode of DW propagation. Here, we observed a transition mode with a single DW propagating periodically either faster (~2200 m/s) or slower (~1700 m/s) as compared to the detonation velocity value at time before ~230 ms (see successive white bands with two different slopes in the time interval from 230 to 260 ms in Fig. 12b). By about 260e270 ms this latter continuous-detonation mode in the combustor transforms to another combustion mode which is accompanied by the periodic appearance of reaction fronts and their nearly simultaneous arrival at the circumferential line of ionization probes at a frequency 2e3-fold lower than the rotation frequency of a single DW. In Refs. [7,8], this combustion mode attributed to the longitudinal pulse-detonation mode was observed in tests with hydrogeneair mixture at limiting conditions of fuel and air supply. In the longitudinal pulsedetonation mode, the detonation is reinitiated in a position close to the combustor nozzle and propagates upstream as a reaction front occupying the entire cross section of the combustor either without evident rotation or with sporadic rotations of arising reaction fronts in different directions. In the experiment of Figs. 11 and 12, the longitudinal pulsedetonation mode ceases at about 350 ms when the pressure in the hydrogen plenum drops down to about 0.03 MPa. Contrary to the continuous-detonation mode in Fig. 8, the longitudinal pulse detonation mode fails abruptly without exhibiting slowly decaying reaction fronts. This finding indirectly supplements the hypothesis that the limiting continuous-detonation mode is a supersonic low-velocity mode with mixture ignition in a reflected shock wave. In fact, in the longitudinal pulse-detonation mode there seems to exist no mechanism for transition between normal detonation and nonideal detonation (“quasidetonation” [16]) due to the absence of regular lead shock reflections during axial propagation of the lead shock.
At the second stage of the research reported herein we carried out experiments with a ternary “hydrogeneliquid propaneeair” mixture, with hydrogen being used as initiating fuel. As mentioned above, the supply valve of liquid propane was opened 100 ms after opening the air and hydrogen valves. This time delay resulted in liquid propane entering the annular gap of the combustor only in 250e300 ms after opening the air and hydrogen valves due to the finite time of filling the propane line and plenum. As soon as liquid propane began to bleed into the combustor annulus, the hydrogen valve was turned off and the combustor was fed only with air and liquid propane but some residual hydrogen was still present for some time. In experiments with the supply pressure of liquid propane up to 0.7 MPa (at a mass flow rate of up to 0.27 kg/s) and with the mass flow rate of air up to 10.4 kg/s we have registered the continuous-detonation combustion process with a single rotating DW. Fig. 5b shows a photograph of the exhaust plume at continuous-detonation combustion of liquid propane in one of the experiments in this series. Fig. 13 shows the example of overpressure records in the air, hydrogen, and liquid propane plenums in one of experiments of this series, which is very close by conditions to the experiment relevant to Figs. 7 and 8 in Section 3 for hydrogeneair mixture. Here, the initial pressures (at ignition) of air and hydrogen are 0.53 and 1.9 MPa, respectively (mass flow rates 10.4 kg/s and 0.32 kg/s (at the stage of feeding with air) and 0.15 kg/s (at the stage of feeding with air and liquid propane)). The pressure in the propane plenum is initially produced by expanding propane vapor. Liquid propane enters the combustor only at about 420 ms when the pressure in the propane plenum exhibits a local maximum due to hydraulic shock. The overpressure in the propane plenum at hydrogen valve turning-off is 0.7 MPa (mass flow rate 0.27 kg/s). It is seen that by about 600 ms the hydrogen supply to the combustor is virtually ceased whereas the supply pressures of air and liquid propane are constant and equal to 0.7 and 0.4 MPa, respectively, until the closing of the liquid propane valve at 800 ms. Fig. 14 shows the results of processing the fragments of records of ionization probes in the experiment of Fig. 13. Like in Figs. 8 and 12, in Fig. 14 each fragment corresponds to the time interval of 30 ms. In a time interval between 330 and 370 ms two DWs in a hydrogeneair mixture propagating at an apparent velocity of ~1200 m/s in the direction opposite to the direction of air flow swirling are registered in this experiment. The actual detonation rotation frequency in the upper fragment of Fig. 14b is the number of white bands (57) divided by time (0.03 s) and is equal to 1900 Hz. As mentioned above, after the opening of the liquid propane valve the combustor is first filled with propane vapor. Therefore at a subsequent time interval between 370 and 420 ms the two DWs existing in the combustor are replaced by a single DW propagating in a “hydrogenegaseous propaneeair” mixture at an apparent velocity of ~1290 m/s in the same direction. As a matter of fact, the actual frequency of detonation rotation in the time interval from 390 to 420 ms is 31/ 0.03 ¼ 1033 Hz, whereas the estimated rotation frequency of a
Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Fig. 12 e Results of processing the fragments of records of ionization probes of (a) axial and (b) circumferential lines at continuous-detonation combustion of hydrogeneair mixture at air and hydrogen supply pressures of 0.2 and 1.1 MPa, respectively.
single DW is 1290/1.275 ¼ 1012 Hz, i.e., there exist only a single DW in the combustor. At time 420 ms the combustor begins to be fed with liquid propane (this time corresponds to a local maximum of the pressure curve in the propane plenum, see. Fig. 13). Due to injection of relatively cold liquid in the combustor, in a time interval between 420 and 450 ms a transition period with the longitudinal pulse-detonation mode is observed which is accompanied by periodic appearance of reaction fronts in the combustor and their nearly simultaneous arrival at the circumferential line of ionization probes at a frequency 2e3-fold lower than the rotation frequency of a single DW. After the transition period, at time ~450 ms, the operation process in the combustor is transformed to the continuousdetonation combustion mode with a single DW of ~150 mm height propagating at an apparent velocity of ~1390 m/s in the direction opposite to the direction of air flow swirling. Thus, the supply of gaseous and thereafter liquid propane to the combustor led to transition from the continuous-detonation combustion mode with two DWs to the mode with a single
Fig. 13 e Example of overpressure records in the air, hydrogen, and liquid propane plenums in one of experiments with hydrogeneliquid propaneeair mixture at air and hydrogen supply pressures of 0.5 and 1.9 MPa, respectively.
Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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DW via the longitudinal pulse-detonation mode. At 500 ms, hydrogen supply is switched off resulting in a rapid decrease of hydrogen flow rate, but the reactive flow structure in the combustor and the DW velocity remain virtually unchanged. For example, at time 550 ms, when the hydrogen supply pressure is reduced more than twice and approximately equals to the pressure of liquid propane supply (see Fig. 13), the DW structure and propagation velocity are almost the same as at time 500 ms, i.e., propane participates in the reaction. The overall equivalence ratio of “hydrogeneliquid propaneeair” mixture in this experiment is estimated as 0.93. Further evidences of propane participation in the reaction can
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be found by comparing Fig. 14 with Fig. 8, the analog of Fig. 14 for the experiment with pure hydrogeneair mixture. If in Fig. 8 the detonation failure occurs when hydrogen pressure drops down to 0.52 MPa, the detonation failure in Fig. 14 occurs at a time interval from 570 to 580 ms when hydrogen pressure is reduced to 0.24 MPa. At this condition, combustion of fuel mixture in the combustor ceases completely (solid black background in Fig. 14). Later on, in this experiment external diffusion combustion of propane has been established at the combustor outlet. Careful examination of Figs. 13 and 14 allows finding a possible reason of detonation decay. The decrease in the air
Fig. 14 e Results of processing the fragments of records of ionization probes mounted (a) in axial and (b) in circumferential lines of the combustor outer wall: the overpressure of liquid propane supply is 0.7 MPa. Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Fig. 15 e Decay of continuous-detonation combustion at liquid propane supply with the overpressure of 0.8 MPa.
plenum overpressure starting from ~540 ms (0.53 MPa) and ending at ~590 ms (0.4 MPa, see solid curve in Fig. 13) could be treated as the indication of the gradual drop in the back pressure in the combustor due to gradual decrease in the mean rate of energy release from the value inherent in the flow with a DW rotating in the annual gap at the apparent velocity of ~1390 m/s to the value inherent in the flow with external diffusion combustion of propane at the combustor outlet. The gradual decrease in the mean rate of energy release in the combustor is caused by the gradual decrease in the detonation velocity which is seen in Fig. 14b as the gradual decrease in the slope of white bands in the time interval between 540 and 590 ms. The apparent velocity of the reaction front corresponding to the last white band appearing at time ~590 ms in Fig. 14b is quite low and close to 400e450 m/s. Taking into account that this reaction front propagates in the direction opposite to the direction of air flow swirling, its absolute propagation velocity is about 600e650 m/s, which is the same as the limiting velocity value found in Section 3 for the hydrogeneair mixture. Thus, similar to the statement in Section 3 we come to the hypothesis that the combustion process ceases presumably due to the failure of mixture ignition in the lead shock wave reflected from the compressive outer wall of the annular combustor. When the supply pressure of liquid propane was increased to 0.8 MPa, the continuous-detonation combustion mode was not observed even when hydrogen valve was not turned off (Fig. 15). In this case, the operation mode with external diffusion combustion of the triple mixture was observed in experiments (see Fig. 6c). It is seen in Fig. 15 that the evolution of the operation process in the time interval from 390 to 410 ms is very similar to that shown in Fig. 14, i.e., a single DW is rotating in the combustor annulus filled with the “hydrogenegaseous propaneeair” mixture at an apparent velocity of ~1290 m/s. However, at a time exceeding 410 ms the height of the DW (the height of the black triangular in the upper fragment of Fig. 15) starts to increase, which is accompanied by the decrease in the detonation propagation velocity (the slope of white bands decreases), and detonation decays in several milliseconds. It is interesting that while decaying, the continuous-detonation mode transforms temporarily to the longitudinal pulse-detonation mode, discussed above. However, contrary to Fig. 14, only one pulse of this kind is registered in Fig. 15. The failure of the continuous-detonation combustion mode in this case occurs just at arrival of liquid propane to the combustor annulus. This means that supply of relatively cold liquid propane at 0.8 MPa greatly affects
formation of reactive heterogeneous mixture and leads to fast detonation decay.
Concluding remarks Experiments were carried out on continuous-detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in a large-scale annular combustor. Liquid propane was injected into the combustor from the injector head at a time when a sustained continuous-detonation combustion of hydrogeneair mixture was attained therein. In the experiments, the mass flow rates of hydrogen, liquid propane and air ranged from 0.1 to 0.5 kg/s, from 0.1 to 0.5 kg/s, and from 5 to 12 kg/s, respectively. In the experiments with a liquid propane supply pressure of up to 0.7 MPa (with a mass flow rate of up to 0.27 kg/s) and with air mass flow rate of about 10 kg/s the continuousdetonation combustion process with a single rotating DW has been registered after shutoff of hydrogen supply. Evidences are obtained that propane was participating in the detonation process in the combustor. If without propane supply the failure of continuous-detonation combustion of hydrogeneair mixture after the shutdown of hydrogen supply occurred at reduction of hydrogen pressure down to 0.52 MPa, the failure of continuous-detonation combustion of the triple mixture occurred at a lower hydrogen pressure (0.24 MPa). The failure of continuous-detonation combustion of hydrogeneair and hydrogeneliquid propaneeair mixtures was found to occur when the absolute velocity of reaction wave rotation in the combustor annular gap was as low as 600e650 m/s. In view of it, the hypothesis has been put forward that the mechanism of steady-state supersonic reaction front propagation in the annular gap is similar to the mechanism of flame propagation in tubes with regular obstacles: the reaction is ignited by the lead shock wave reflected from the compressive outer wall of the annulus and propagates due to turbulent mixing of hot combustion products with fresh reactants in the recirculation zone behind the injector head. According to this hypothesis, failure of continuous-detonation combustion occurs due to the failure of mixture ignition in the lead shock wave reflected from the compressive outer wall of the annular combustor. Thus, the continuous-detonation combustion of liquid propane has been obtained for the first time due to addition of hydrogen rather than due to air enrichment with oxygen. It is planned to continue this research to obtain continuousdetonation combustion of propaneeair mixture without addition of hydrogen.
Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138
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Acknowledgments This work was supported by the Russian Ministry of Education and Science under the State contract 14.609.21.0002 (contract ID RFMEFI60914X0002 contract).
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[9] Lin W, Zhou J, Liu S, Lin Z, Zhuang F. Experimental study on propagation mode of H2/air continuously rotating detonation wave. Int J Hydrogen Energy 2015;40:1980e93. [10] Anand V, St. George A, Driscoll R, Gutmark E. Investigation of rotating detonation combustor operation with H2-air mixtures. Int J Hydrogen Energy 2015;41(2):1281e92. [11] Wang C, Liu W, Liu S, Jiang L, Lin Z. Experimental investigation on detonation combustion patterns of hydrogen/vitiated air with annular combustor. Exp Therm Fluid Sci 2015;66:269e78. [12] Fotia ML, Schauer F, Kaemming T, Hoke JL. Experimental study of the performance of a rotating detonation engine with nozzle. J Propuls Power 2016;32:674e81. [13] Frolov SM, Dubrovskii AV, Ivanov VS. Three-dimensional numerical simulation of the operation of a rotatingdetonation chamber with separate supply of fuel and oxidizer. Russ J Phys Chem B 2013;7(1):35e43. [14] Dubrovskii AV, Ivanov VS, Frolov SM. Three-dimensional numerical simulation of the operation process in a continuous detonation combustor with separate feeding of hydrogen and air. Russ J Phys Chem B 2015;9(1):104e19. [15] Frolov SM, Aksenov VS, Dubrovskii AV, Zangiev AE, Ivanov VS, Medvedev SN, et al. Chemiionization and acoustic diagnostics of the process in continuous- and pulsedetonation combustors. Dokl Phys Chem 2015;465(1):273e8. [16] Lee JHS, Knystautas R, Freiman A. High speed turbulent deflagrations and transition to detonation in H2eair mixtures. Combust Flame 1984;56:227e39. [17] Zel'dovich YB, Borisov AA, Gelfand BE, Frolov SM, Mailkov AE. Nonideal detonation waves in rough tubes. In: Kuhl AL, Bowen JR, Leyer J-C, Borisov AA, editors. Progr astron aeron, dynamics of explosions, vol. 114. Washington DC: AIAA Inc; 1988. p. 211e31.
Please cite this article in press as: Frolov SM, et al., Continuous detonation combustion of ternary “hydrogeneliquid propaneeair” mixture in annular combustor, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.138