MINIATURE ENGINEERING SYSTEMS GROUPMINIATURE ENGINEERING SYSTEMS GROUP

Two-Stage CryoCooler Two-Stage CryoCooler Development for Liquid Hydrogen Development for Liquid Hydrogen

SystemsSystems

This work is supported by NASA Hydrogen Research at This work is supported by NASA Hydrogen Research at Florida Universities.Florida Universities.

List of AuthorsList of Authors

L.An, Q.Chen, J.Cho, L.Chow, N.Dhere, C.Ham, J.Kapat, K.B.Sundaram, T.Wu,

K.Finney, X.Y.Li, K.Murty, A.Pai, H.Seigneur, L.Zhao, L.Zheng, L.Zhou.

 

Dept. of Mechanical, Materials and Aerospace Engineering,

School of Electrical Engineering and Computer Science, and

Florida Solar Energy Center.

Outline of the presentationOutline of the presentation

IntroductionIntroduction Complete design of the proposed systemComplete design of the proposed system Compressor design and CFD analysisCompressor design and CFD analysis Conceptual design of Gas Foil BearingsConceptual design of Gas Foil Bearings Motor designMotor design Development of tribological coatingsDevelopment of tribological coatings ConclusionConclusion

IntroductionIntroduction

Spaceport of future will use large quantities of liquid hydrogen. Efficient storage and transfer of liquid hydrogen is necessary for reducing launch costs. An overall design of a two-stage cyrocooler for application in zero boil-off for long duration storage of liquid hydrogen systems is presented here. Primary focus of the presentation is on the design concept of centrifugal helium compressor for bottom stage of the working cycle, motor for driving the compressor, bearings and tribological coatings for the system.

Complete design of the Complete design of the proposed systemproposed system

Two Stage CryoCoolerTwo Stage CryoCoolerNeon RTBC and Helium RTBCNeon RTBC and Helium RTBC

Advantages Over Existing CryoCoolers Advantages Over Existing CryoCoolers for Liquid Hydrogen Systemsfor Liquid Hydrogen Systems

High COP for the overall system due to high High COP for the overall system due to high efficiency values of compressors and motors efficiency values of compressors and motors compared to what is available commercially.compared to what is available commercially.

High system reliability because of a “DC” cryo-High system reliability because of a “DC” cryo-cycle, rotary components, gas foil bearings and cycle, rotary components, gas foil bearings and advanced tribological coatings.advanced tribological coatings.

Thermodynamics schematicThermodynamics schematic

System PerformanceSystem Performance

Top cycle is capable of removing heat at Top cycle is capable of removing heat at liquid Nitrogen temperature with cooling liquid Nitrogen temperature with cooling power ~ 1000 W power ~ 1000 W

2-stage RTBC cycle is capable of removing 2-stage RTBC cycle is capable of removing heat at liquid Hydrogen temperature with heat at liquid Hydrogen temperature with cooling power ~ 50W cooling power ~ 50W

COP ~ 0.007COP ~ 0.007

Design FeaturesDesign Features

Top cycle can work separately as a liquid nitrogen Top cycle can work separately as a liquid nitrogen cryocooler; or it can work with bottom cycle as a cryocooler; or it can work with bottom cycle as a liquid hydrogen cryocooler.liquid hydrogen cryocooler.

State-of-the-art aerodynamics design of the 2-stage State-of-the-art aerodynamics design of the 2-stage intercooled neon centrifugal compressor and the 4-intercooled neon centrifugal compressor and the 4-stage intercooled helium centrifugal compressor.stage intercooled helium centrifugal compressor.

Integrated motor and oil-free non-contact bearings Integrated motor and oil-free non-contact bearings for high speed and efficiency.for high speed and efficiency.

Hard, low friction and durable coatings at cryogenic Hard, low friction and durable coatings at cryogenic temperature.temperature.

Innovative micro-channel high effectiveness heat Innovative micro-channel high effectiveness heat exchanger.exchanger.

Schematic of the bottom cycle Schematic of the bottom cycle showing the four stage Helium compressorshowing the four stage Helium compressor

Compressor Design and CFD Compressor Design and CFD AnalysisAnalysis

Single Stage CompressorSingle Stage Compressor

Single stage compressor is being developed Single stage compressor is being developed first to aid the design of more complex two first to aid the design of more complex two and four stage compressors.and four stage compressors.

Plastic models have been created showing Plastic models have been created showing the conceptual idea. They indicate the small the conceptual idea. They indicate the small size of the parts.size of the parts.

Existing Existing Single Stage Single Stage

DesignDesignflow inlet

Inlet guide vanes

contoured endwall

full blades and splitter blades

vaned diffuser with multiple vane segments

flow exit

Single Stage Centrifugal Single Stage Centrifugal Compressor DevelopmentCompressor Development

Compressor

MotorCoupler

Impeller

Diffuser Inlet Guide Vane

Experimental Set-upExperimental Set-up

Fully Structured 3D Grid

(Created in GAMBIT, 330K)

CFD Simulation of IGVCFD Simulation of IGV

Reverse flow occurs at outlet of IGV. (Solved by Fluent 6.0)

CFD-IGVCFD-IGV

CFD simulation results show that pressure loss through IGV is about 5000 Pa. As expected, IGV creates an acceptable flow angle at the eye of impeller. However, certain amount of reverse flow still exists in spite of careful design. This may be eliminated by the interaction of IGV and rotor, which would be simulated in the next stage. If the flow reversal still persists, IGV design will be modified by adjusting angle of IGV vanes.

Conceptual Design of the Gas Foil Conceptual Design of the Gas Foil BearingsBearings

Schematic of the conceptual designSchematic of the conceptual design

Conceptual Design ConfigurationConceptual Design Configuration

It contains an outer hollow cylinder to which the foils are It contains an outer hollow cylinder to which the foils are attached. attached.

An inner hollow cylinder would have long cut grooves An inner hollow cylinder would have long cut grooves extending to about 90% of its length through which the extending to about 90% of its length through which the foils would pass and hold the shaft in position during start-foils would pass and hold the shaft in position during start-up and at stop.up and at stop.

The outer hollow cylinder can be rotated about the shaft The outer hollow cylinder can be rotated about the shaft center axis of rotation and the rotation of which would center axis of rotation and the rotation of which would cause the foils to lose contact with the shaft thus making cause the foils to lose contact with the shaft thus making the same bearing as ‘Gas Bearing’ and also as a ‘Gas Foil the same bearing as ‘Gas Bearing’ and also as a ‘Gas Foil Bearing’.Bearing’.

Motor DesignMotor Design

Specifications of the MotorSpecifications of the Motor

The motor efficiency needs to be as The motor efficiency needs to be as high as possible.high as possible.

Size and weight are also important Size and weight are also important issues.issues.

Output Shaft PowerOutput Shaft Power 2000W2000W

Shaft SpeedShaft Speed 200krpm200krpm

Shaft DiameterShaft Diameter 10-20mm10-20mm

Max. LengthMax. Length 100mm100mm

Max. Outer DiameterMax. Outer Diameter 44mm44mm

DC Power SupplyDC Power Supply 28V28V

Some Popular Motor TypesSome Popular Motor Types

Induction motor (IM)Induction motor (IM) : low cost, but low efficiency : low cost, but low efficiency at high speed due to higher iron loss.at high speed due to higher iron loss.

Switched reluctance motor (SRM):Switched reluctance motor (SRM): high reliability, high reliability, but iron loss is very critical at high speed.but iron loss is very critical at high speed.

Permanent magnet synchronous motor (PMSM):Permanent magnet synchronous motor (PMSM): very high efficiency due to no exciting copper loss in very high efficiency due to no exciting copper loss in the rotor. High power density with high energy the rotor. High power density with high energy density permanent magnet Nd-Fe-B.density permanent magnet Nd-Fe-B.

Brushless DC motor (BLDC)Brushless DC motor (BLDC): high power density as : high power density as PMSM, but the large harmonics will reduce efficiency PMSM, but the large harmonics will reduce efficiency significantly at high speed.significantly at high speed.

Radial Flux PMSM StructureRadial Flux PMSM Structure

Stator Outer Diameter = 36mm

Stator Inner Diameter = 26.3mm

Rotor Diameter = 16mm

PM Width = 7mm

PM Height = 15mm

Motor Active Length = 25.4mm

Laminated low loss core

PM

Shaft

Winding

Shaft Shaft StructureStructure

WindingWinding MethodMethod

2-pole, 3-phase.2-pole, 3-phase. 5 coils/phase/pole.5 coils/phase/pole. Two layer lap winding.Two layer lap winding. Pitch factor: 23/30.Pitch factor: 23/30.

First coil: Top1 -> Btm12.First coil: Top1 -> Btm12. Rectangular Litz wire.Rectangular Litz wire.

50 strands AWG30.50 strands AWG30.Outer dimensions: 1.78x2.27mmOuter dimensions: 1.78x2.27mm22 . .

Wire SelectionWire Selection Solid copper wireSolid copper wire

AWG13(Do:75mil, R:2mAWG13(Do:75mil, R:2mΩΩ/ft)/ft) AWG14(Do:67mil,R:2.6mAWG14(Do:67mil,R:2.6mΩΩ/ft)/ft)

Litz wire (multi-strand configuration)Litz wire (multi-strand configuration) Round Litz WireRound Litz Wire RectangularRectangular

Benefit of using Litz-wireBenefit of using Litz-wire Easy shape.Easy shape. Reduce skin effect, proximity effect.Reduce skin effect, proximity effect. Each strand tends to take all possible positions in the Each strand tends to take all possible positions in the

cross-section of the entire conductor.cross-section of the entire conductor.

Simulated Back EMF-Simulated Back EMF-Litz Rectangular Wire ConfigurationLitz Rectangular Wire Configuration

Simulated Current-Simulated Current-Litz Rectangular Wire ConfigurationLitz Rectangular Wire Configuration

EfficiencyEfficiency

Copper LossCopper Loss 16.9W16.9W

Iron LossIron Loss 16.4W16.4W

Windage LossWindage Loss 21W21W

Motor EfficiencyMotor Efficiency 97.3%97.3%

Control EfficiencyControl Efficiency 95%95%

Total EfficiencyTotal Efficiency 92.5%92.5%

*The bearing loss is not considered here, since we will use gas foil bearing later.

Development of Tribological Development of Tribological CoatingsCoatings

ObjectiveObjective

The goal is to develop tribological coatings having extremely The goal is to develop tribological coatings having extremely high hardness, extremely low coefficient of friction, and high high hardness, extremely low coefficient of friction, and high durability at temperatures <60 Kdurability at temperatures <60 K

To fulfill these functional demands, an adequate adhesion To fulfill these functional demands, an adequate adhesion between coating and substrate as well as an adequate load between coating and substrate as well as an adequate load carrying capacity are both essential. carrying capacity are both essential.

Extremely hard and extremely low friction coatings of titanium Extremely hard and extremely low friction coatings of titanium nitride (TiN) and molybdenum disulphide (MoSnitride (TiN) and molybdenum disulphide (MoS22) as well as ) as well as

diamond-like-carbon (DLC) were chosen for this research based diamond-like-carbon (DLC) were chosen for this research based on literature for friction behavior and wear resistance under on literature for friction behavior and wear resistance under cryogenic temperatures .cryogenic temperatures .

Titanium Nitride (TiN) CoatingsTitanium Nitride (TiN) Coatings - By DC Magnetron Sputtering - By DC Magnetron Sputtering

DC Magnetron Sputtering was used to achieve film thicknesses of DC Magnetron Sputtering was used to achieve film thicknesses of approximately 1 micron.approximately 1 micron.

Number of Depositions have been carried out by DC Magnetron Number of Depositions have been carried out by DC Magnetron Sputtering under varying conditions.Sputtering under varying conditions.

Achieved film thickness of > 1 micron.Achieved film thickness of > 1 micron. Peel test has shown good adhesion of TiN coatings with glass Peel test has shown good adhesion of TiN coatings with glass

substrates.substrates. Dektak Profilometer have shown good uniformity of TiN films. Dektak Profilometer have shown good uniformity of TiN films. Analysis by Energy Dispersive Spectroscopy (EDS) and Analysis by Energy Dispersive Spectroscopy (EDS) and

microhardness measurements have been carried out. microhardness measurements have been carried out.

Results for three samples are shown in the following slidesResults for three samples are shown in the following slides ..

SampleSampleIDID

NN22 : Ar : Ar

RatioRatioAtomic Atomic PercentPercent

Nitrogen: Nitrogen: ArgonArgon

AverageAverageHardnessHardness

AverageAverageElasticElastic

ModulusModulus(GPa)(GPa)

GPaGPa HV (Kgf/mmHV (Kgf/mm22))

11 0.5: 60.5: 6 NN22 :Ti = 50.3:49.7 :Ti = 50.3:49.7 9.329.32 878.47878.47 144.20144.20

22 0.5 : 40.5 : 4 NN22:Ti = 53.05:46.95:Ti = 53.05:46.95 ---------- ---------- ----------

33 1: 41: 4 NN22:Ti = 52:48:Ti = 52:48 16.6216.62 1567.021567.02 200.21200.21

EDS analysis and results of microhardness measurement

HV –Vicker’s Hardness

Films have shown good stoichiometric ratio of Ti & N

Titanium Nitride (TiN) CoatingsTitanium Nitride (TiN) Coatings

Several more depositions of TiN films by DC magnetron sputtering were Several more depositions of TiN films by DC magnetron sputtering were carried out. The limit in terms of varying the argon to nitrogen ratio was carried out. The limit in terms of varying the argon to nitrogen ratio was reached as the films indicated greater porosity and signs of peeling off.reached as the films indicated greater porosity and signs of peeling off.

Characteristic golden color of TiN films was achieved. Characteristic golden color of TiN films was achieved. XRD analysis of the above samples indicated fully reacted XRD analysis of the above samples indicated fully reacted

microcrystalline TiN nature that may provide excellent hardness.microcrystalline TiN nature that may provide excellent hardness. Additional samples on aluminum substrates will be prepared using Additional samples on aluminum substrates will be prepared using

optimized parameters based on the above observations for XRD, optimized parameters based on the above observations for XRD, microhardness, wear and coefficient of friction analysis. microhardness, wear and coefficient of friction analysis.

Mask required for deposition of TiN coatings on three bump on 1cm x 1 Mask required for deposition of TiN coatings on three bump on 1cm x 1 cm silicon wafer to minimize the contact area between two rubbing cm silicon wafer to minimize the contact area between two rubbing samples and providing more accurate coefficient of friction and wear samples and providing more accurate coefficient of friction and wear measurements has been designed and procured.measurements has been designed and procured.

Titanium Nitride (TiN) CoatingsTitanium Nitride (TiN) Coatings

Titanium Nitride (TiN) CoatingsTitanium Nitride (TiN) Coatings

TiN coatings deposited on Aluminum substrates

Molybdenum Disulphide Molybdenum Disulphide

(MoS(MoS22) Coatings) Coatings Depositions of MoSDepositions of MoS22 by RF magnetron sputtering were carried out. by RF magnetron sputtering were carried out.

XRD analysis of the samples indicated fully reactive microcrystalline XRD analysis of the samples indicated fully reactive microcrystalline MoSMoS22 nature. nature.

Deposition of bilayer coatings of TiN and MoSDeposition of bilayer coatings of TiN and MoS22 on a glass substrate on a glass substrate

have been carried out.have been carried out. Testing of the above film will be carried out for satisfying requirements Testing of the above film will be carried out for satisfying requirements

of good wear resistance and low coefficient of friction coatings.of good wear resistance and low coefficient of friction coatings.

Hard Coatings at Cryogenic TemperaturesHard Coatings at Cryogenic Temperatures Cryogenic environment leads to increase in the coefficient of friction Cryogenic environment leads to increase in the coefficient of friction

and DLC coatings have lower coefficient of friction and good wear and DLC coatings have lower coefficient of friction and good wear resistance as compared to hard coatings of nitrides at cryogenic resistance as compared to hard coatings of nitrides at cryogenic temperatures.temperatures.

A special cryogenic tribometer is required for the study of friction and A special cryogenic tribometer is required for the study of friction and wear at cryogenic temperatureswear at cryogenic temperatures

Microwave CVD SetupMicrowave CVD Setup

Microwave assisted plasma chemical vapor deposition system (MWCVD) has been ordered for depositions of diamond-like

carbon (DLC) coatings.

5’

Vacuum Gauge Controller

Fw Pw Ref Pw

12”

3’

Microwave

Generator

12”

4 Stub TunerPlasma Applicator

Sliding Short Circuit

Dual Directional Coupler

RF Substrate Biasing

Baffle

MW Control Panel

Turbo- MolecularAssembly

Pump & Exhaust

(1.5 “ tube)

Mechanical

Pump

1“ feedthrough hole

for water inlet and

substrate biasing

Convectron Gauge

Ionization Gauge

Six-Way

Cross-Chamber

Fixed Flanges,

3 places

Rotatable Flanges,

3 places

ConclusionConclusion

An innovative concept of a Two Stage An innovative concept of a Two Stage CryoCooler for maintaining Hydrogen at CryoCooler for maintaining Hydrogen at liquid state is presented. The system is liquid state is presented. The system is highly efficient and reliable for manufacture highly efficient and reliable for manufacture and storage of liquid hydrogen.and storage of liquid hydrogen.