Microturbine

INTRODUCTION

One is emerging from perhaps the most deliberate and least colourful engineering fields

of all: gas turbine engineering. Gas turbines are internal combustion engines,

like the ones that drive cars, except that they use a rotating shaft or rotor instead

of pistons "reciprocating" in cylinders. This makes their operation smooth and

steady, which lowers maintenance costs and increases reliability. Though they

became practical only sixty years ago, today gas turbines are one of the keystone

technologies of the civilization. As jet engines, they deliver most of our air

transport, while stationary gas turbines are responsible for an increasing fraction

of our electrical power generation.Partly because of this critical role, gas turbine

engineers tend to innovate one tiny step at a time. In a field where liability

exposures and development costs both can run into nine and ten figures, any

kind of sweeping enthusiasm makes people nervous. Still, that doesn't mean

engineers can't dream on their own time. In the spring of 1994, when a MIT

turbine engineer named Alan Epstein found himself sitting in a jury pool, he

started to think about what it would take to build the smallest possible jet engine.

He concluded that in theory the device could be shrunk a lot, perhaps to the size

of a collar button.

If you attached a microgenerator to the turbine, essentially creating a tiny power plant, the

combination would act like a battery, making power at twenty to fifty times the rate of

anything you could get at the hardware store. (Because there is much more energy per

gram in burning hydrocarbons than in the electrochemicals that usually go in batteries.)

Depending on how much fuel came with the turbine, a laptop might run for months on a

single charge; a cellphone, for half a year. Given the insatiable appetite our portable

gizmos have for batteries, the microturbine project suddenly became very interesting. The

U.S. Army, which badly wants to reduce the weight carried by their "soldier systems",

agreed to write the checks.

By 1995 the microturbine project was humming along. Unlike a conventional gas turbine

design job, where each member is a world-class expert on one (but only one) phase of the

process, all the researchers on this project were starting from the same place: how to

make engines less than a hundreth the size of a conventional turbine design. For instance,

for a gas turbine to work well, the tips of its rotors have to turn at about the speed of

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Microturbine

sound, or five hundred meters a second. The smaller the diameter of a turbine, the faster

the rotor has to spin to move its tips at that speed. A conventional jet engine can get there

with a few tens of thousands of revolutions a minute. The microturbine had to do much

better: closer to two million rpm, or twenty thousand revolutions a second.

This awe-inspiring number raised all kinds of questions. For one: How was the rotor

going to be attached? The usual solution to this problem would be some sort of bearing,

but what material could handle that level of abuse? And even if such a substance existed,

how would you make the bearings or keep them in place? Eventually, after many failures,

the team discovered clever ways for the rotor to use its blistering speed to lift itself up

during operation, essentially making it fly in place, so that no material bearings were

needed. The project required such innovations constantly, radical ideas too new for

anyone to be expert on them.

Over the next seven years the project made amazing progress, considering that designing

a conventional jet engine usually takes five years. Today actual working models exist,

though the microturbine is not quite ready to be handed over to a manufacturer. (One of

the remaining problems is exactly how to cool the exhaust to a level comfortable for

consumer use .The success of the microturbine project has inspired a whole R&D sector

in micropower devices. The Defense Department alone is funding well over a dozen

projects, from microfuel cells and micropiston engines to microrockets. The University of

Wisconsin is even looking at a micronuclear reactor. (One of the attractions is that tiny jet

engines deliver ten times the thrust per unit weight of a conventional turbine, which

means the huge cost airplanes now pay to haul their engines around might be radically

reduced.)

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Microturbine

LITERATURE REVIEW

THERMODYNAMIC CONSIDERATIONS

It is influenced by fluid and structural mechanics, and by material, electrical and Thermal

power systems encompass multitude of technical disciplines. The architecture of the

overall system is determined by thermodynamics while the design of the system’s

components fabrication concerns, the physical constraints on the design of the mechanical

and electrical components are often different at micro scale than at more familiar sizes so

that the optimal component and system designs are different as well. Most

thermodynamic systems in common use today are variations of the Brayton (air), Rankine

(vapour, Otto, or Diesel cycles. The Brayton power cycle (gas turbine) was selected for

the initial investigation based on relative considerations of power density, simplicity of

fabrication, ease of initial demonstration, ultimate efficiency, and thermal anisotropy. A

conventional, macroscopic gas turbine engine consists of a compressor, a combustion

chamber, and turbine (driven by the combustion exhaust) that powers the compressor, and

can drive machinery such as an electric generator. The residual enthalpy in the exhaust

stream provides thrust. A macro scale gas turbine with a meter diameter air intake

generates power on the order of 100 MW. Thus, tens of watts would be produced when

such a device is scaled to millimeter size if the power per unit of airflow is maintained.

When based on rotating machinery, such power density requires (1) combustor exit

temperatures of 1300-1700 K; (2) rotor peripheral speeds of 300-600 m/s and thus

rotating structures centrifugally stressed to several hundred MPa (the power density of

both fluid and electrical machines scales with the square of the speed, as does the rotor

material centrifugal stress); low friction bearings; high geometric tolerances and tight

clearances between rotating and static parts; and thermal isolation of the hot and cold

sections. These thermodynamic considerations are no different at micro- than at

macroscale. But, the physics influencing the design of the components does change with

scale, so that the optimal detailed designs can be quite different. Examples include the

viscous forces in the fluid (larger at microscale), usable strength of materials (larger),

surface area to volume ratios (larger), chemical reaction times (invariant), realizable

electric field strength (higher), and manufacturing constraints (planar geometries).

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Microturbine

ENGINE DESIGN

There are many thermodynamic and architectural design choices in a device as complex

as a gas turbine engine. These involve trade-offs among fabrication difficulty, structural

design, heat transfer, fluid mechanics, and electrical performance. Given that the primary

goal is to demonstrate – that a high power density MEMS heat engine physically reliable,

the design philosophy adopted is that the first engine will be as simple as possible,

trading performance for simplicity. For Example, the addition of a heat exchanger

transferring heat from the turbine exhaust to the compressor discharge fluid (a

recuperated cycle) offers many benefits including reduced fuel consumption and relaxed

turbo machinery performance requirements, but it introduces additional design and

fabrication complexity. Thus, the baseline design is a simple cycle gas turbine generator.

While this engine is the simplest of gas turbines, it is an extremely complex and

sophisticated MEMS device. Arriving at a satisfactory design requires heavy dependence

on simulation of the mechanical, thermo fluid, and electrical behavior to achieve the

required levels of component performance and integration .The baseline engine design is

illustrated in Figure 1.The engine consists of a supersonic radial flow compressor and

turbine connected by a hollow shaft. Gaseous H2 fuel is injected at the compressor exit

and mixes with air as it flows radially outward to the flame holders. The combustor

discharges radially inward to the turbine whose exhaust turns 90 degrees to exit the

engine nozzle. A thin film electric induction starter-generator is mounted on a shroud

over the compressor blades and is cooled by compressor discharge air. Cooling air is also

used to thermally isolate the compressor from the combustor and turbine. The rotor is

supported on air bearings. The following sections briefly discuss component design

considerations.

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Microturbine

MATERIALS AND MECHANICAL DESIGN

Conventionally sized engines, constructed from titanium and heavily cooled nickel and

cobalt-based super alloys, are stress-limited in the rotating components. Nonmetallic such

as silicon (Si), silicon carbide (SiC), and silicon nitride (Si3N4) offer substantial

improvement in strength-to-density ratio and temperature capability, but large parts with

acceptable properties have proven difficult to manufacture from these materials.

However, they are readily available in essentially flaw-free form for micro scale

fabrication so that significantly superior material performance is available for micro-heat

engines than can now be realized in conventionally- sized devices. In addition, because of

the small length scales required here, material which are unsuitable for a large heat engine

due to thermal shock considerations (e.g. aluminum oxide), would be usable in a micro

engine given a fabrication technology [1]. Silicon is suitable for the compressor (600 K)

but cannot operate at the combustor discharge temperature needed (1300-1700 K) without

cooling. SiC can operate uncooled but SiC fabrication technology is much less developed

than that for Si. The baseline design assumes uncooled SiC for simplicity but a cooled Si

design is also under study. The individual components are being demonstrated in Si while

SiC manufacturing technology is being developed. Since the properties of such materials

are a strongly influenced by the details of their fabrication, material testing is an integral

part of this program .g/sec implies airfoil and passage heights on the order of 200-300

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Microturbine

microns as in figure 2.Deep reactive ion etching was used to produce the turbine shown in

Figure 3, which has a 4mm rotor diameter and 200 micron span blades. The rim of the

300-micron thick disk serves as a journal bearing. This unit is a rotor dynamics test piece.

With the addition of a generator on the back surface of the disc, it becomes an 80-watt

turbine generator. Also, using only known process steps, a “strawman” process simulation

yields wafers of completed engines, including a freely turning rotor, without additional

assembly. It is a complex and aggressive process requiring 7 aligned wafer bonds, 20

lithography steps, and the deposition of 9 thin film layers.

TURBO MACHINERY AND FLUID MECHANICS

Considerations of engine thermodynamic efficiency, combustor performance, and turbine

viscous losses suggest that compressor pressure ratio should be relatively high. Since both

the pressure ratio and the centrifugal stress in the rotor scale with the square of the

peripheral Mach number, the pressure ratio per stage of compression is set by the

allowable material stress. Material property values in the literature are consistent with a

500 m/s rotor tip speed, which was therefore adopted as a baseline. A 4:1 pressure ratio

compressor has been designed to operate at this speed. Current fabrication technology

largely restricts complex curvatures to in plane, which inhibits the use of the high degree

of three-dimensionality typically employed in centrifugal turbomachinery to improve

efficiency and reduce material stresses. However, the usable material strength is higher at

microscale. Also, this flow regime is unusual in that it is supersonic (Mach 1.4) but

laminar (Reynolds number 20,000). Three-dimensional fluid calculations suggest that this

machine should achieve an adiabatic efficiency of about 70%. To facilitate detailed

measurement of the turbomachinery fluid mechanics, a 75:1 geometrically scaled up test

rig has been built. It operates at the same Mach and Reynolds numbers as the

microturbomachinery.

COMBUSTION

Air breathing combustion requires fuel injection (and evaporation if a liquid), fuel-air

mixing, and chemical reaction of the mixed reactants. The time required for these

processes (the combustor residence time) sets the combustor volume. In large engines, the

residence time is typically 5-10 ms. Most of this is for fuel mixing; chemical reaction

times are a few hundred microseconds or less. In order to expedite the engine

development process, hydrogen was selected as the baseline engine’s fuel. Hydrogen

offers rapid mixing and chemical reaction times, and flammability over a wide range of

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Microturbine

fuel-to-air ratios. By operating at a low fuel-to-air ratio, the peak combustor temperature

can be reduced to levels compatible with uncooled SiC construction (1600 K),

eliminating the requirement for the complicated cooling geometries needed on large

engines. A combustor with the geometry of Figure 1 has been built and tested. It has

demonstrated the predicted levels of performance over a wide range of temperatures and

mixture ratios. The data agree with numerical simulations that suggest that complete

combustion can still be achieved with a factor of two reductions in combustor volume [2].

Work is now beginning on a hydrocarbon fueled catalytic combustor

BEARINGS AND ROTOR DYNAMICS

Low friction bearings are required to support the rotor against fluid and electrical forces,

rotor dynamics, and externally applied accelerations while operating at speeds of over

two million rpm. Gas film, electrical, and hybrid gas electrical bearing concepts were

examined. Gas bearings were selected for the baseline engine based on superior load

bearing capability and relative ease of fabrication. A journal bearing supports the radial

loads and thrust plates support the axial loads. The physical regime that the microgas

bearings operate in is unusual in several regards: the peripheral speed of the bearing is

transonic so compressibility effects are important; the ratio of inertial to viscous forces

(Reynolds number) is high; the surface area of the bearing is very large compared to the

mass of the rotor; and the journal length-to-diameter ratio is quite low. The net effect of

these influences is a journal bearing well outside existing theory and empirical design

practice. Magnitude higher than the critical frequency (spring-mass damper equivalent) of

the rotating system. Sub critical operation would require submicron-operating clearances,

which are difficult to fabricate and incur viscous losses greater than the engine power

output. The design adopted uses a ten-micron journal gap to reduce losses to a few watts

but is linearly unstable at some speeds. Numerical simulations indicated, however, that

this design would operate satisfactorily in a nonlinear limit cycle. Turbine-driven rotor

dynamic test rigs have been constructed both at 1:1 microscale (Figure 3) and at 26:1

macroscale (to facilitate detailed instrumentation). Preliminary data confirm that the rotor

does operate in a stable limit cycle. As a precaution, an electric damper is being designed

to augment the bearing stability should it prove desirable.

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Microturbine

ELECTRICAL MACHINERY

A motor-generator starts the gas turbine and produces the electrical power output.

Integrating the motor-generator within the engine offers the advantages of mechanical

simplicity since no additional bearings or structure are required over that needed for the

engine and cooling air is available. Either electric or magnetic machines could be used.

Here, an electric machine was chosen due to considerations of power density, ease of

microfabrication, and high-temperature and high-speed operation. The baseline design is

a 180-pole planar electric induction machine mounted on the shroud of the compressor

rotor. Simulations suggest that such a machine can produce on the order of 20-40 watts

with an electrical efficiency in excess of 80%. The major source of loss in the machine is

viscous drag in the rotor-stator gap.

In a first phase of the project, the problem has been scaled down to a turbine powered by

compressed air. Compressor, combustion chamber, and generator have been left out and

will be addressed in a later phase. The micro turbine is a single-stage axial impulse

turbine. Expansion of the gas takes place in the stationary nozzles and not between the

rotor blades. This type of turbine has been chosen because of its simple construction.

Figure 1 shows an exploded view and an assembly of the microturbine design. The

compressed air enters via a standard pneumatic connector (1) and expands over the

stationary nozzles (3) where it is deflected in a direction tangential to the turbine rotor (5).

After the air has passed the rotor blades, it leaves the device through the openings in the

outlet disc (6). Screwing the pneumatic connector in the housing (8) presses the stationary

nozzle disc against a shoulder in the housing. The rotor blades, wheel and axis are one

monolithic part. The rotor is supported by two ball bearings (4), one mounted in the

stationary nozzle disc and one mounted in the outlet disc. The outlet disc is locked in the

housing by a circlip (7).

Figure 2: Microturbine design.

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Microturbine

The diameter of the turbine rotor is 10 mm. The housing has a diameter of 15 mm and is

25 mm long. All parts, except pneumatic connector and circlip, are made of stainless

steel. The nozzles are designed for subsonic flow, so have a converging cross-section.

Sonic speed is reached for a relative supply pressure of 1 bar. The exit losses are minimal

when turbine is designed for a u/c1 ratio of 0.5, with u the circumferential speed and c1

the absolute speed at the nozzle exit. At 1 bar, c1 reaches sonic speed resulting in optimal

turbine speed of 420,000 rpm. As this is too high for the bearings, the turbine has been

designed for a u/c1 ratio of 0.25, and is operated below its optimal speed of 210,000 rpm.

TURBINE PRODUCTION

The different parts of the turbine are produced by turning and EDM .The nozzle disc and

rotor are the most complex parts. In a first step, their cylindrical surfaces are machined on

a lathe. In a second step, the nozzles and blades are created by die-sinking EDM as

illustrated for the rotor in figure 3. The rotor is clamped in a rotary head, which is indexed

with steps of 30º. A prismatic copper electrode with a cross-section having the shape of

the air channels between the blades is sunk into the turbine wheel by EDM. The electrode

is produced by wire-EDM. One of the problems during the production of the turbine

blades is electrode wear. This wear is difficult to predict and not uniform across the

electrode. This problem has been solved by cutting away the lower edge of the electrode

by wire-EDM at regular intervals. As the electrode is prismatic, the shape after shortening

remains the same. Figure 4 shows a subassembly of nozzle disc, rotor, and bearings.

Fig 3: Machining of the rotor blades by EDM. Fig4:Subassembly of nozzle disc, turbine

rotor, and bearings.

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Microturbine

MECHANICAL OUTPUT

Torque and power of the turbine have been tested up to a speed of 100,000 rpm. For this

purpose, a 30 mm diameter brass wheel has been fixed to the turbine axis. An optical

sensor measures the rotation of the wheel in a contact less way: two vanes on the wheel

interrupt the optical path of a photo sensor. The turbine is tested by switching on the

pressure and accelerating the turbine until it reaches its maximum speed. The torque is

then derived from the acceleration and the moment of inertia of the wheel and turbine

rotor. As the turbine passes through the whole speed range, acceleration, torque and

power are know as a function of speed.

When the turbine is rotating at full speed, the pressure is switched off and a new

measurement is done while the turbine slows down. This gives the friction torque as a

function of speed. Friction mainly occurs between the wheel with vanes and the

surrounding air. The friction torque and power are added to the results of the acceleration

test to obtain the total torque and power of the turbine.

Fig 5 and 6 show torque and mechanical power as a function of speed for different supply

pressures up to 1 bar. The maximum torque and power are respectively 3.7 Nmm and 28

W. The dashed lines represent the friction losses determined with the deceleration test.

Figure 5: Torque as a function of speed and

supply pressure.

Figure 6: Mechanical power of the turbine.

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Microturbine

At 1 bar, the turbine consumes 8 Nm3/h of

compressed air, which corresponds to a power

consumption of 152 W when assuming an

ideal isentropic expansion. This means that

the mechanical efficiency of the turbine lies

around 18 %. Figure 7 shows the turbine

efficiency as a function of speed for different

supply pressures.

The ‘dips’ in the characteristics at high speed

are caused by the measurement method as

they always occur at the maximal speed, even

for different loads and pressures. In reality,

power and efficiency increase further with

speed to reach their maxima theoretically at

210,000 rpm (for 1 bar). These speeds can be

reached using a smaller load.

Figure 7: Efficiency of the turbine

(compressed air to mechanical power).

ELECTRICAL OUTPUT

To measure the electrical power output of the system, the generator is connected to a

variable 3-phase load consisting of 3 potentiometers (range 2 kW, 10 turns). In contrast

with the mechanical tests, the electrical tests are performed at constant speed. The speed

of the turbine, which is measured from the frequency of the generator voltage, is

controlled by varying the load. Figure 8 shows the electrical power measured for different

supply pressures and speeds. At a pressure of 1 bar, the maximal electrical power is 16 W

and is reached at a speed of 100,000 rpm. Measurements show that the airflow and input

power depend only on the supply pressure and not on speed or load. Therefore, the input

power is the same as in the mechanical test at 1 bar, i.e. 152 W. Figure 9 shows the total

efficiency (compressed air to electricity) as a function of speed and for different supply

pressures. The maximal total efficiency is 10.5 % and is reached at a speed of 100,000

rpm.

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Microturbine

Figure 8: Electrical power generated by the

total system (turbine plus generator).

Figure 9: Total efficiency (compressed air to

electricity).

SANKEYS DIAGRAM

The energy flow and the different losses are illustrated in the Sankey diagram shown in

figure 10. The diagram is generated for a supply pressure of 1 bar and a speed of 100,000

rpm. This corresponds to the working point at which the maximal electrical power and

maximal total efficiency are reached. Input power, mechanical power, electrical power

and the combination of ventilation losses (6) and bearing friction (7) are measured values.

This last value (6 + 7) is obtained with a deceleration test of the turbine without generator

and without external load. The loss associated with the leak flow around the turbine

wheel (2) and the exit losses (8) are calculated from the known air speeds. The expansion

losses (1), incidence losses (4) and blade profile losses (5) are calculated using friction

and loss coefficients known from large turbines and may be less accurate. The generator

losses (10) are derived from the manufacturer’s data sheets. The obstruction losses (3)

and the losses in the coupling (9) are derived as the difference between the calculated and

measured values.

The major losses are the blade profile losses and the exit losses. The large blade profile

losses can be explained by the increased friction in miniature systems (large surface-to-

volume ratio and low Reynolds numbers). The high exit losses can be explained by the

low u/c1 ratio (0.25 instead of 0.5 in the optimal case). Additionally, the turbine operates

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below its optimal speed because the ball bearings limit the speed. Both factors result in

higher air speeds at the turbine exit, and thus higher exit losses.

Figure 10: Sankey diagram for a supply pressure of 1 bar and a speed of 100,000 rpm.

CHALLENGES IN DESIGN AND FABRICATION

The Microturbine presents challenges in the mechanical and electrical engineering

disciplines of fluid dynamics, structural mechanics, bearing and rotor dynamics,

combustion and electrical machinery design. Then comes difficulties involved in

fabrication, heat transfer, structural design and electrical performance .The challenges

also includes the need of small bearings and in manufacturing components .the turbine

blades may come across with hydrogen burning, so it should be ceramic blade with micro

ion etching .The challenges also there for cooling the systems. There will be a chance of

high centrifugal stress, which will effect the life of micro turbine during high speed of

rotation. Here there is the chance of heat losses due to the high surface to volume ratio,

which will effects the efficiency and performance of microturbine.

APPLICATIONS OF MICROTURBINE

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Microturbine

Microturbines are suited to meet the energy needs of small users such as schools,

apartments, restaurants, offices and small businesses. The Microturbine coupled with

solid oxide fuel cell can be used in supermarkets, factories, and military developing

nations. This can be applied for heating, drying, cooling, desalination and several others.

They include space vehicles, electronic devices, unmanned aircrafts. The microturbines

can be used in remote areas. This is because of small size. Microturbines are used when a

high quality, energy density energy is needed.

CONCLUSION

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Microturbine

Microturbines and miniature thermal devices pose unique challenges and opportunities

for combustion in small volume. The principal difficulties are associated with limited

residual time and heat transfer losses due to high surface to volume ratio. This paper

addresses a preliminary analysis of Microturbine .The microturbine is in early stages of

pre-production and is still in the developmental phase .The coupling of microturbine with

a high temperature fuel cell (SOFC – solid oxide fuel cell) is one of them .If the waste

heat is used the overall fuel utilization efficiency can be increased. Major features,

parameters and performance of the microturbine are discussed here. Fully understanding

these and identifying the solutions, it is key to the future establishing of an optimum

overall system. In the case of the microturbine changes will be minor as they enter

production on a large scale within the next year or so, there is an extensive efforts are

expanded to reduce unit cost .It is reasonable to project that a high performance and cost

effective hybrid plant, with high reliability, will be ready for commercial service in the

middle of the first decade of the twenty century

FUTURE WORK

The first goal is to increase the efficiency of the turbine, mainly by decreasing the exit

losses. This can be reached in two ways: introducing air bearings, which allow much

higher speeds, or decreasing the speed by using a multiple-stage design. In the long term,

a compressor and a combustion chamber will be added to finally come to a micro-

generator

REFERENCES

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Microturbine

1. A.F. Massardo and C.F.McDonald, “Microturbine for high-efficiency electrical

generation”, Transactions of ASME for gas turbine and power, January 2002.

2. E.Utrainen and B.Sunden,”Evaluation of the heat transfer surfaces for

Microturbine Recuperator”, Transactions of ASME for Gas turbine and power,

July 2002.

3. Lue Frrchette, Stuart A.Jacobson, Kenneth S.Breuer, Fedric F.Ehric, Reza

ghodssi, Ravi khanna, Martin A.Schmidt and Alan H.Epstein,”Demonstration of

microfabricated high speed turbine supported on gas bearings”, solid- state Sensor

and actuator workshop, Hilton Head, June 4-8,200

4. w.w.w.microturbine.com

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