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c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

A*A A AD1*4311

AIAA-2001-3613Rotary Ejector Enhanced Pulsed DetonationSystem

M. Razi NalimZuhair A. Izzy

Indiana University - Purdue University IndianapolisIndianapolis, IN

37th AIAA/ASME/SAE/ASEEJoint Propulsion Conference & Exhibit

July 8-10,2001/Salt Lake City, UT

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ROTARY EJECTOR ENHANCED PULSED DETONATION SYSTEM

M. Razi Nalim *Zuhair A. Izzy f

Purdue School of Engineering and TechnologyIndiana University Purdue University Indianapolis

Indianapolis, Indiana, USA.

Abstract

A new type of non-steady ejector is proposed forpulsed detonation combustors, based on wave rotortechnology. It integrates a pulsed detonation processwith an efficient momentum transfer process inspecially shaped rotating channels of a single wave-rotor device. The detonation and non-steady flowprocesses are simulated using a quasi-one-dimensional computational gasdynamics code thatallows for transient transverse mass injection.Preliminary selection of geometric design parametersand example simulations are presented. The conceptappears to have good potential for enhancedperformance of pulsed detonation systems.

Introduction

Applications of pulsed detonation engines (PDE) arebeing studied that include both direct thrustdetonation devices as well as gas generation devicesthat drive various 'hybrid' engines. In most of theseapplications the highly concentrated and intermittentenergy of the PDE exhaust compromises thefundamental thermodynamic superiority of near-constant-volume combustion.

A simple PDE produces intermittent hightemperature high-momentum jets of exhaust,separated by longer periods of dribbling or nooutflow. This is a result of the fundamentalmechanics of detonation and the mixture detonabilitylimits. The detonation-induced velocities andtemperature are excessive for many applications,while the high gas pressure is desirable. Some PDEconfigurations have evolved to address theunsteadiness problem by using multiple detonationtubes that breathe and fire sequentially. A rotaryvalve or other type of valving is provided to achievethis.

Nevertheless, these configurations retain limitations:• Excessive velocity and excessive temperature in

Assistant Professor, AIAA Senior Member.Research Associate.

Copyright ©2001 by M. Razi Nalim.Published by AIAA with permission.

the outflow, resulting in low propulsive efficiencyand reduced thrust, and limited life of downstreamcomponents.

• Inlet and nozzle non-steadiness and flow lossesremain, with flow stagnation in individual feeddistribution and exhaust collection ducts.

• Limited number of tubes, with dedicated feed andignition hardware of significant weight and volumeper tube.

• Needs multiple high-repetition detonation initiationdevices, and complex, fast-cycling valving forpurge gas, fuel, oxidant (or enrichment); otherwisefrequency is limited and tube length excessive.

• Cyclically loaded valve parts or bearings transmitpressure and thrust, and have reduced durability,while creating vibration and noise.

The PDE configuration determines the significanceof these limitations. In a gas generator devicedesigned to power a turbine, it is undesirable to havea non-steady or non-uniform velocity turbine flow,and the gas must be diluted to the acceptable turbinetemperature. In a thrust device, a non-steady or non-uniform jet or a high-velocity jet has lowerpropulsive efficiency than a steady uniform jet orlower-velocity jet with the same total kinetic energy.

In order to realize their impressive thermodynamicpotential, it is necessary to control and deliver thePDE outflow within constraints of downstreamcomponents while minimizing losses. In addition,the non-steady flow process allows uniquepossibilities for hybrid systems that includeturbomachinery to maximize the benefits of eachtechnology. In particular, a wave rotor device canexploit the pressure variations of the cycle toefficiently join flow streams at different pressures.

The possible applications of a pulsed detonationcombustor include aircraft and missile propulsionwhen used for direct thrust and as a turbine powerplant when used as a gas generator. The conceptsdescribed here are of relevance to hybrid PDEsystems involving downstream components thatimpose temperature and uniformity requirements onthe PDE. They are also of relevance to direct thrustaugmentation at moderate flight speeds, when thepropulsive efficiency falls with high-velocity andnon-uniform jet properties.


Ejectors for Thrust AugmentationThe use of ejectors of various types to transfer energyand momentum from a high speed or high-pressureflow stream to another stream is known for thrustaugmentation in aeropropulsion. This actiondistributes energy and momentum to a larger mass,resulting in lower overall velocity and greaterpropulsive efficiency and thrust. A simple exampleof an ejector is a jet (high momentum) emerging intoa duct with low-momentum fluid flowing generally inparallel with the jet.

There is evidence that ejector devices using a non-steady work transfer process are more efficient thatsteady ejectors. This is because work transfer can beaccomplished by the action of pressure forces in non-steady flows, whereas work exchange betweenstreams in a steady flow can only occur through thedissipative mechanism of viscous forces. Such aneffect could be realized by placing a duct of largerdiameter to accept the intermittent PDE exhaust andentrain flow from a bypass duct or the atmosphere, inan alternating fashion. A difficulty with this methodis that the strong shock waves driven out of the PDEtube will tend to propagate upstream into the bypass.

Carlton (1994) examined the fundamental limits ofperformance of non-steady work-exchange devices.Using availability analysis to seek the optimalamount of isentropic work exchange under differentconditions for the most efficient energy transfer to auniform mixed outflow, he found that optimal workexchange occurs when both pressure and velocity oftwo streams become equal. Steady ejectors with nowork exchange have the greatest availability loss, asexpected, but non-optimal work-exchange alsoresults in significant availability loss upon mixing.

Wave Rotor PDEA new concept that addresses many of the stated PDEchallenges proposed to rotate the detonation tubesand keep all other parts stationary (Nalim & Jules,1998). Such a device, called a wave rotor pulseddetonation engine (WRPDE) could have low-loss,essentially steady inlet and nozzle flows, highfrequency operation without pulsed ignition, nomoving parts that transmit thrust, and simplevalveless purging and mixture stratification foroptimal detonation.

Of itself, the simplest WRPDE obtains internally thesame fundamental detonation process andcombustion stoichiometry as other PDE's and thusretains the problems of excessive outflowtemperature and velocity. Time-unsteadiness is

replaced by spatial non-uniformity of outflowproperties. However, when combined with an ejectordevice as described below, these drawbacks can beovercome, and the energy and momentum of thedetonation harnessed to maximize performance. Anintegrated ejector can improve the propulsiveefficiency of a direct-thrust WRPDE, and control thetemperature of gas supplied by a gas-generatorWRPDE.

Wave Ejector and Wave Fan

The required ejector device could be designed as anon-steady process in a separate wave rotor. This iscalled a "joiner" or "equalizer" cycle in wave rotorterminology. In the simplest case, it may be assumedthat several PDE tubes operating in a phased mannerprovide a steady stream of high-pressure gas, whichmust be joined with low-pressure bypass air. Ajoiner wave rotor would receive these two energystreams, which undergo wave compression and waveexpansion processes in the wave rotor, and exit thewave rotor with equalized intermediate pressure. Forgiven inlet conditions and flow rates of the twostreams, the most efficient work transfer willmaximize the exit total pressure. Kentfield (1969)studied the performance of wave rotor equalizers.His experimental study of the performance of waverotor (pressure-exchanger) equalizer cycles providesefficiency maps over a range of pressure and flowparameters for fixed geometry (timing) and rotorspeed.

This paper introduces two variations of a concept thatcombines a rotary ejector with a wave rotordetonation device. The "wave ejector" and "wavefan" variations differ in application and pressurelevels, but have the same hardware as describedbelow, and the principle of operation is similar to thatof a non-steady ejector or joiner cycle. The rotor andits housing are illustrated in disassembled view inFig. 1 A, and assembled (rear) view in Fig. IB (not toscale). The single rotor is made of a smaller-diameter forward section that has relatively narrowshrouded passages, and a rear section made of widerand taller passages that are either partially shroudedor unshrouded. There is a transition region ofgradually varying height that is mostly or completelyunshrouded. The rotor rotates within a housing,which provides for seals, bearings, and ducting.

A rotary ejector is similar to a joiner cycle that islongitudinally integrated with a detonation waverotor to create an elegant and efficient combustion


Fig. 1A Rotary Ejector Sketch: Exploded View

system with a single rotating part. The primary andbypass flows discussed below are admitted throughthe partial-annular ducts shown, which are helical toprovide the necessary rotational velocity. There aretwo sets of ducts of each type in the illustratedhousing, indicating that in each rotation the rotor isdesigned to undergo two identical gasdynamic cycles,as a preferred design option.

As the rotor rotates through one cycle, each passageexperiences a periodic sequence of events. The samesequence occurs in all passages, but at differenttimes. A detonation is produced in each forwardpassage, in a primary fuel-air mixture admittedthrough partial-annular ports in a forward end plateof the housing. During a portion of the cycle, theunshrouded part of each rear passage admits air flowradially and axially from a partial-annular bypass-airduct. During the remaining portion of the cycle, therear passage is disconnected from the bypass duct,and the detonation in the forward passage istransmitted as a shock wave to the confined admittedair, thus energizing it and achieving a non-steadyejector effect.

In the wave ejector case, the primary air andsecondary air flows are usually induced fromatmospheric conditions and are thus at the samepressure. The intent is to enhance thrust and specificimpulse of a direct thrust PDE device. A wave fandiffers from a wave ejector in that the primary flow issupplied from a compressor at higher pressure thatthe secondary flow, which may be atmospheric or arelatively lower pressure. This system may be

Fig. IB Rotary Ejector Sketch:.

utilized within a gas turbine engine to exploit thebenefits of pulsed combustion, while addressing thelimitations of combustion stoichiometry andexploiting the momentary pressure depression toenergize a low-pressure stream.

This qualitative description is elaborated in the eventdiagrams illustrated in Fig. 2 for an individualpassage of a wave ejector viewed in thecircumferential direction. Going clockwise from topleft:(a) The forward (leftward) narrow passage containsa quiescent detonable mixture, while the rear(rightward) contains a quiescent residual mixture ofcombusted gas and in-mixed air from the previouscycle.(b) The detonation is initiated at the left wall andmoves rapidly to the right, pressurizing andaccelerating the gas until it reaches non-combustiblemixture and converts to a shock wave.(c) As the shock transits the transition passage, thelarge area change causes an expansion wave to bereflected to the left, while a compression or shockwave is transmitted to the right.


(d) The expansion wave arrives and reflects at theleft wall, depressing the pressure locally. Meanwhilethe shock wave reflects at the open right end asanother expansion wave. When this wave arrives atthe primary inlet the pressure is depressed further.The primary inlet at this location may be opened nowand fresh detonable mixture admitted as shown. Anoptional buffer layer of unfueled air may precede themixture. Depending on the primary inlet pressure, itsopening may be delayed until a certain pressure isreached.(e) The transition region comes into communicationwith the bypass air duct just as the reflectedexpansion wave depresses the pressure in this region,thus admitting bypass air into the rear passage. Theforward passage may continue to admit detonablemixture.(f) The expansion wave is reflected back to the rightend. Depending on the relative wave strengths,inflow at the primary inlet may continue or slowdown. •(g) The right end reflects a compression wave andthe outflow terminates. At this point the outflow portmay be closed, again as an option depending onpressures and velocities. When the reflected orgenerated compression wave arrives at the transitionregion and slows the bypass inflow, the passage

moves out of communication with the bypass duct,preventing backflow.(h) The compression wave forms a shock wave andis transmitted into the forward passage, where itslows or terminates the mixture inflow. At this timethe primary inflow valve is closed.

A generalized illustration of a wave fan cycle ispresented in Fig. 3, which shows a developed view ofall the passages participating in one cycle, viewed ina radial direction. As the passages rotate upward inthe direction indicated "R", an individual passageexperiences events that are very similar to a waveejector, as indicated by the letters corresponding toparagraphs (a)-(h) above. The reader may visualizeeach of these cycles in both types of illustration.

Computer simulations, as discussed later, are helpingto discover the preferred relative timings of the portsof a wave ejector and a wave fan:• In a wave ejector, where the two inlet pressures

are the same, the primary inlet opens at theapproximately the same time or earlier than thebypass inlet. Typically, but not necessarily,these two inlet periods overlap closely, and theexhaust port remains open the entire cycle. .

• In a wave fan where the ratio of primary and

Fig. 2. Wave Ejector PDE Cycle


bypass inlet pressures is small (less than about2), the primary inlet opens after the bypass inletopens, and there may be overlap between the twoinlet periods. The exhaust valve may closeduring the time the primary inlet is open. This isthe case illustrated in Fig. 3.

• In wave fan cycles that have relatively higherpressure primary inlet, the primary inlet opensafter the bypass inlet, and there is no overlapbetween the two. The exhaust valve closesduring the primary inlet period.

The cycle may be modified to accommodate desiredmass flows and mixtures, and the available portpressure conditions, such that the wave timings setlimits to subsequent port timings, rather thandetermine them exactly. The waves and pressurefluctuations in the ejector section to the right arerelatively weak except for the transmitted shock, andthis represents an essentially constant-pressureboundary condition to the detonation and large-amplitude waves in the combustion section.

With the use of the rotary ejector geometry,integration of the pulsed detonation and joinerprocesses is simplified, and several PDE challengesare overcome. Several geometric parameters can be

UnshroudedSection

Fuel Heighttransition

Oxidant

BypassAir

Primary Air

Ignitor

Fig. 3. Wave Fan PDE Cycle

varied to provide the correct matching between thenon-steady wave processes of the pulsed detonationand joiner cycles.

Quasi 1-D Simulation of Wave Ejector

Preliminary analysis of the proposed concepts hasbeen performed using a quasi one-dimensional,uniform-grid numerical model of a pulsedcombustion process. It employs a code originallydeveloped and validated for wave rotors (Paxson,1993, Wilson & Paxson, 1996) with non-reactingflow in uniform passages, and later extended toreacting flow (Nalim & Paxson, 1997), and non-uniform passages.

The code uses an Euler solver to integrate thegoverning equations of mass, momentum, energy,and species. It has the capability to accommodatemultiple port boundary conditions at each end percycle. It can model mixing, opening time andviscous losses, as well as those losses from leakage,heat transfer, and flow turning effects. It has nowbeen modified to allow mass addition at any locationand include the associated momentum and energysources due to the presence of such mass sources.The governing equations are integrated for anyspecified time with cyclic boundary conditions. Inletconditions are specified by appropriate stagnationquantities, and an outflow static pressure isprescribed. Net thrust or pressure gain is calculatedbased on the averaged outflow stagnation pressure.A stoichiometric hydrogen-air mixture withdetonation initiated at the inlet end is considered.

Rotary Ejector ModelFigure 4 is a sketch of the rotary ejector model andgeometric variables. Although the transition sectionis shown as linear for simplicity, the model actuallyprovides a smooth sinusoidal transition of passageheight from the small to the larger diameter. A rangeof spatial (geometry) and temporal (timing) designparameters was simulated in order to test themodified code, including the case with no massaddition and no area variation to verify simple pulseddetonation cycle simulation. For the simulationsreported here, a part of the tested range of cyclefrequency and geometric parameters is used toprovide an exposition and illustration of the possiblebenefits. Based on a initial investigation over a rangeof parameters, a preliminary design was evaluated asgiven below. The selected geometric parameters are


provided below, where L is the total rotor length andHI is the primary passage inlet height.

Bypass Air

Prima

Fig. 4 Rotary Ejector Model for Simulation

Passage Outflow Height, H2 = 2.0 HIArea Transition Start Location, XI = 0.2 LArea Transition End Location, X2 = 0.5 LSecondary Duct Start Location, SX1 = 0.3 LSecondary Duct End Location, SX2 = 0.6 LSecondary Duct Start Angle, ccl = 30°Secondary Duct End Angle, oc2 = 30°

The timings of the secondary air (bypass) port arevaried to study the effect of changing the entrainedmass ratio of secondary air to primary air, as the ratioof primary to bypass pressure varied. Ideally, thetiming is selected to best exploit the sub-atmosphericpressure in the passage induced as the exitingdetonation is followed by discharge of high-momentum gas, and a reflected expansion wave.Care is taken to avoid significant backflow into thebypass duct. In general, as the open duration isincreased, the entrained secondary air mass increases,and the primary air mixture mass decreases as theinternal pressures return more rapidly to atmosphericconditions. Visual inspection for temperature,pressure and flow parameters in the contour diagramswere used to confirm the combustion mechanism,wave patterns, and the periodicity of the finalsolution.

The primary inlet port is partitioned into five sectorsof selected circumferential width, to allow non-uniform mixtures. Typically, the first sector was leftunfueled to provide a non-combustible buffer, andhad a width of 15% or 20% of the inlet. There is alsoanother very small port on the inlet side that isopened briefly to inject a small amount of hot, high-pressure gas to initiate the detonation. Typically, themass of gas injected is less than 1% of the totalthroughput. The actual initiation method is notinvestigated here; this is simply an energy input,representing ignition.

Example "Wave Ejector" PDE CycleFig 5 is an illustration of a typical hydrogen-air waveejector-PDE cycle that shows the computed flowproperties for a single cycle of operation. The figureis a set of plots of the cycle properties, presented ascolor contour diagrams of temperature, pressure (logscale) and fuel concentration on a relative scale, andline plots of Mach number and pressure at selectedlocations. The illustration shows the relationbetween location, timings, and output for any typicalsingle cycle. The x-axis is the location along the tube(x/L), and the y-axis is the elapsed time. The non-dimensional time is shown on the vertical axis, basedon a reference wave transit time L/a*, where is a* isthe reference sound speed (atmospheric). The y-values correlate nominally with the number oftransits over the full length of the rotor for reflectionsof the detonation wave. Qualitatively, the dark shade(blue in color) represents the lowest values, whilelight shade (red) is the highest values; gray (green) isintermediate values.

The primary and bypass inlets are both at oneatmosphere total pressure, and standard atmospherictemperature. The outlet port remains open for theentire cycle at one atmosphere. The non-dimensionalcycle time was set at 2.95, based again on a referencetransit time for the full rotor length. This frequencywas selected after some experimentation to allowsufficient time for inflow and ignition delay

From the pressure-time diagram, the channel pressureis below atmospheric at primary inlet port opening,therefore the fuel mixture will flow into the forwardpassage with ejector and the inlet is timed to be openat 1.0 through 2.3. The temperature-position plotclearly shows the initial injection of colder(dark/blue) secondary air beginning at time 1.0 alongthe length of the secondary inlet. The secondary flowterminates at time 2.6, but this is less evident as theflow rate diminishes and the primary air flow sweepsalong the passage. The fuel fraction-position graphshows the fuel concentration, green indicatingdilution by secondary air, from the full strengthmixture (red). The entrained air is 50% of totaloutflow. The specific impulse of the cycle iscalculated to be 1.23 times that of a simple PDE withno ejector.

Example "Wave Fan" PDE CycleThe primary inlet total pressure was set higher thanthe atmospheric pressure (the bypass pressure), andwith a correspondingly higher total temperature. Thebypass duct pressure was set equal to oneatmosphere, and temperature equal to the standard


atmospheric temperature. Other conditions such aspartitioning for purge air, and mixture are verysimilar to the wave ejector case, as is the ignitionmethod. The outlet port was not open for the entirecycle if the primary inlet pressure exceeds 1.2. Theexhaust was closed for a timed period to preventbackflow, avoid pressure loss during the primaryinlet period, and ensure the proper discharge of thedetonation traveling waves.

Fig 6 is an illustration of a typical Wave Fan PDECycle that shows the computed flow properties for asingle cycle of operation. The primary inlet totalpressure is 4.0 atmospheres. The non-dimensionalcycle time was set at 2.95, based again on a referencetransit time for the full rotor length. The primary inletwas open from 1.6 to 2.0. The bypass was open from0.1 to 0.5. The exhaust outflow port was open from

0.2 to 0.7. The entrainment mass is 13% of the total.The outflow total pressure of the cycle is computed tobe 1.16 times the primary inlet pressure or 4.6 timesthe secondary pressure.

Preliminary Port Design Rules

The pressure gain, thrust, and specific impulseaugmentation were calculated for a limited range ofdesign parameters. The outflow stagnation pressureis first computed by averaging the highly skewedoutflow properties on a constant-area basis, whileconserving mass, momentum and energy. Thisinvolves a loss as evidenced by an entropy increaseupon transition to the subsonic solution for thiscalculation. The stagnation pressure is obtained forthe average condition, and thrust is then computed byassuming an isentropic expansion of the product gas

0,5 1 0

Fig. 5 Example Wave Ejector TOE Cycle

2.5

1.5

0.5 U

0 1 2Mach No

o 1Log Pressure

0.5 1 0 0.5 I D 0.5 1tio^Length Position/length Posstmn/lersglh

Fig. 6 Example Wave Fan PDE Cycle


to atmospheric pressure. This approach results in anoverly conservative estimate of thrust; the directthrust force as measured by a momentum balancewill always be higher. Specific impulse is computedfrom the thrust and fuel mass flow rate.

The preliminary simulations explored outlet timing,outlet static pressure, and timing sequence of bypassand primary inlet ports to evaluate the impact ondetonation and enhance pressure gain or thrust.Based on primary inlet port pressure, tentativeexploration of the design space has begun byconcentrating on the choices of bypass and exhaustport timing as given in Table 1. The second columnindicates whether the secondary port is opened beforethe primary port. The third column indicates whetherthe exhaust port is a partial annulus, or a full annuluswith no end plate.

Table 1. Summary of port designPrimary Inlet

PortPressure

11.21.31.524

Bypass/Primary

PrecedenceOverlapsOverlaps

BeforeBeforeBeforeTBD

PartialExhaust Port

NoNo

YesYesYesYes

Summary

The concept of a wave rotor pulsed detonation enginewith an integrated rotary ejector has been developedand evaluated. Preliminary steps for modeling andanalysis of the rotary ejector concept have beencompleted: development of a basic thermodynamicmodel, and quasi-one-dimensional CFD calculations.The inclusion of mass addition to the original waverotor/PDE code allows a new range of possibleapplications.

Wave ejector and wave fan models are created andpreliminary simulations are reported for a stationaryhydrogen-air PDE in a sea-level atmosphericenvironment. It is operated at near-stoichiometricconditions in the combustible mixture, with purge asdescribed. No attempt has been made to optimize thegeometrical parameters of the model. Therefore,conclusions cannot be made yet about the preferreddesign parameters or operation conditions at thistime.

The simulated process results are sensitive tofrequency, secondary duct geometric configuration,and entrainment ratio. The buffer gas layer iseffective and necessary to avoid pre-ignition. It isplanned to perform more simulations with betterhandling of backflow and mistimed situations.Future simulations are also intended to explore andoptimize the design parameters. The concept of awave ejector has considerable merit and warrantsfurther detailed investigation as a candidatetechnology to enhance pulsed detonation technologyfor propulsion applications.

AcknowledgementsThis work was supported by grant NAG3-2325 fromthe NASA Glenn Research Center. The assistance ofDr. Daniel Paxson in modification of the code isgratefully acknowledged.

References

1. Bussing, T. R. A., 1995, "A Rotary ValveMultiple Pulse Detonation Engine (RVMPDE)",AIAA paper 95-2577.

2. Carlton R.A., "Fundamental Limits ofPerformance of Nonsteady Work ExchangeDevices", M.S. Thesis, University of Florida,1994.

3. Eidelman, S., "Pulsed Detonation Engines: KeyIssues", 1995, AIAA paper 95-2754.

4. Foa, J.V., Elements of Flight Propulsion, J.Wiley, 1960.

5. Fong, K.K. and Nalim, M.R., "Gas DynamicLimits and Optimization of Pulsed DetonationStatic Thrust", AIAA paper 2000-3471.

6. Kentfield J. A.C., "The Performance of Pressure-Exchanger Dividers and Equalizers", Journal ofBasic engineering, Sept. 1969, ASME.

7. Nalim, M. R. and K. Jules, "Pulse Combustionand Wave Rotors for High Speed PropulsionEngines", AIAA paper 98-1614, 1998.

8. Nalim, M.R. & D.E. Paxson, "A NumericalInvestigation of Premixed Combustion in WaveRotors". ASME J. of Engineering for GasTurbines and Power, v. 119, p. 668, July, 1997.

9. Paxson, D. E., "A Comparison BetweenNumerically Modeled and ExperimentallyDetermined Wave-Rotor Loss Mechanisms,"Journal of Propulsion and Power, Vol. 11, No. 5,1995.pp.908-914.

10. Wilson, J., and Paxson, D. E., "Wave RotorOptimization for Gas Turbine Engine ToppingCycles," Journal of Propulsion and Power, Vol.12, No. 4, 1996, pp. 778-785.

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