MINIATURE ENGINEERING SYSTEMS GROUP

1
MINIATURE ENGINEERING SYSTEMS GROUP

• Two-Stage CryoCooler Development for Liquid
  Hydrogen Systems

This work is supported by NASA Hydrogen Research
at Florida Universities.
2
List 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.
3
Outline of the presentation

• Introduction
• Complete design of the proposed system
• Compressor design and CFD analysis
• Conceptual design of Gas Foil Bearings
• Motor design
• Development of tribological coatings
• Conclusion

4
Introduction
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.
5
Complete design of the proposed system
6
Two Stage CryoCoolerNeon RTBC and Helium RTBC
7
Advantages Over Existing CryoCoolers for Liquid
Hydrogen Systems

• High COP for the overall system due to high
  efficiency values of compressors and motors
  compared to what is available commercially.
• High system reliability because of a DC
  cryo-cycle, rotary components, gas foil bearings
  and advanced tribological coatings.

8
Thermodynamics schematic
9
System Performance

• Top cycle is capable of removing heat at liquid
  Nitrogen temperature with cooling power 1000 W
• 2-stage RTBC cycle is capable of removing heat at
  liquid Hydrogen temperature with cooling power
  50W
• COP 0.007

10
Design Features

• Top cycle can work separately as a liquid
  nitrogen cryocooler or it can work with bottom
  cycle as a liquid hydrogen cryocooler.
• State-of-the-art aerodynamics design of the
  2-stage intercooled neon centrifugal compressor
  and the 4-stage intercooled helium centrifugal
  compressor.
• Integrated motor and oil-free non-contact
  bearings for high speed and efficiency.
• Hard, low friction and durable coatings at
  cryogenic temperature.
• Innovative micro-channel high effectiveness heat
  exchanger.

11
Schematic of the bottom cycle showing the four
stage Helium compressor
12
Compressor Design and CFD Analysis
13
Single Stage Compressor

• Single stage compressor is being developed first
  to aid the design of more complex two and four
  stage compressors.
• Plastic models have been created showing the
  conceptual idea. They indicate the small size of
  the parts.

14
Existing Single Stage Design
15
Single Stage Centrifugal Compressor Development
Motor
Coupler
Compressor
16
Impeller
Diffuser
Inlet Guide Vane
17
Experimental Set-up
18
CFD Simulation of IGV
Fully Structured 3D Grid (Created in GAMBIT, 330K)
19
Reverse flow occurs at outlet of IGV. (Solved by
Fluent 6.0)
20
CFD-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.
21
Conceptual Design of the Gas Foil Bearings
22
Schematic of the conceptual design
23
Conceptual Design Configuration

• It contains an outer hollow cylinder to which the
  foils are attached.
• An inner hollow cylinder would have long cut
  grooves extending to about 90 of its length
  through which the foils would pass and hold the
  shaft in position during start-up and at stop.
• The outer hollow cylinder can be rotated about
  the shaft center axis of rotation and the
  rotation of which would cause the foils to lose
  contact with the shaft thus making the same
  bearing as Gas Bearing and also as a Gas Foil
  Bearing.

24
(No Transcript)
25
Motor Design
26
Specifications of the Motor
Output Shaft Power 2000W
Shaft Speed 200krpm
Shaft Diameter 10-20mm
Max. Length 100mm
Max. Outer Diameter 44mm
DC Power Supply 28V

• The motor efficiency needs to be as high as
  possible.
• Size and weight are also important issues.

27
Some Popular Motor Types

• Induction motor (IM) low cost, but low
  efficiency at high speed due to higher iron loss.
• Switched reluctance motor (SRM) high
  reliability, but iron loss is very critical at
  high speed.
• Permanent magnet synchronous motor (PMSM) very
  high efficiency due to no exciting copper loss in
  the rotor. High power density with high energy
  density permanent magnet Nd-Fe-B.
• Brushless DC motor (BLDC) high power density as
  PMSM, but the large harmonics will reduce
  efficiency significantly at high speed.

28
Radial Flux PMSM Structure
Shaft
Stator Outer Diameter 36mm Stator Inner
Diameter 26.3mm Rotor Diameter 16mm PM Width
7mm PM Height 15mm Motor Active Length
25.4mm
PM
Laminated low loss core
Winding
29
Shaft Structure
30
Winding Method

• 2-pole, 3-phase.
• 5 coils/phase/pole.
• Two layer lap winding.
• Pitch factor 23/30.
• First coil Top1 -gt Btm12.
• Rectangular Litz wire.
• 50 strands AWG30.
• Outer dimensions 1.78x2.27mm2 .

31
Wire Selection

• Solid copper wire
• AWG13(Do75mil, R2mO/ft)
• AWG14(Do67mil,R2.6mO/ft)
• Litz wire (multi-strand configuration)
• Round Litz Wire
• Rectangular
• Benefit of using Litz-wire
• Easy shape.
• Reduce skin effect, proximity effect.
• Each strand tends to take all possible positions
  in the cross-section of the entire conductor.

32
Simulated Back EMF-Litz Rectangular Wire
Configuration
33
Simulated Current-Litz Rectangular Wire
Configuration
34
Efficiency
Copper Loss 16.9W
Iron Loss 16.4W
Windage Loss 21W
Motor Efficiency 97.3
Control Efficiency 95
Total Efficiency 92.5
The bearing loss is not considered here, since
we will use gas foil bearing later.
35
Development of Tribological Coatings
36
Objective

• The goal is to develop tribological coatings
  having extremely high hardness, extremely low
  coefficient of friction, and high durability at
  temperatures lt60 K
• To fulfill these functional demands, an adequate
  adhesion between coating and substrate as well as
  an adequate load carrying capacity are both
  essential.
• Extremely hard and extremely low friction
  coatings of titanium nitride (TiN) and molybdenum
  disulphide (MoS2) as well as diamond-like-carbon
  (DLC) were chosen for this research based on
  literature for friction behavior and wear
  resistance under cryogenic temperatures .

37
Titanium Nitride (TiN) Coatings - By DC
Magnetron Sputtering

• DC Magnetron Sputtering was used to achieve film
  thicknesses of approximately 1 micron.
• Number of Depositions have been carried out by DC
  Magnetron Sputtering under varying conditions.
• Achieved film thickness of gt 1 micron.
• Peel test has shown good adhesion of TiN coatings
  with glass substrates.
• Dektak Profilometer have shown good uniformity of
  TiN films.
• Analysis by Energy Dispersive Spectroscopy (EDS)
  and microhardness measurements have been carried
  out.
• Results for three samples are shown in the
  following slides.

38
Titanium Nitride (TiN) Coatings

• EDS analysis and results of microhardness
  measurement

Sample ID N2 Ar Ratio Atomic Percent Nitrogen Argon Average Hardness Average Hardness Average Elastic Modulus (GPa)
Sample ID N2 Ar Ratio Atomic Percent Nitrogen Argon GPa HV (Kgf/mm2) Average Elastic Modulus (GPa)
1 0.5 6 N2 Ti 50.349.7 9.32 878.47 144.20
2 0.5 4 N2Ti 53.0546.95 ----- ----- -----
3 1 4 N2Ti 5248 16.62 1567.02 200.21
HV Vickers Hardness

• Films have shown good stoichiometric ratio of Ti
  N

39
Titanium Nitride (TiN) Coatings

• 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 reached as the films indicated greater
  porosity and signs of peeling off.
• Characteristic golden color of TiN films was
  achieved.
• XRD analysis of the above samples indicated fully
  reacted microcrystalline TiN nature that may
  provide excellent hardness.
• Additional samples on aluminum substrates will be
  prepared using optimized parameters based on the
  above observations for XRD, microhardness, wear
  and coefficient of friction analysis.
• 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
  samples and providing more accurate coefficient
  of friction and wear measurements has been
  designed and procured.

40
Titanium Nitride (TiN) Coatings
TiN coatings deposited on Aluminum substrates
41
Molybdenum Disulphide (MoS2) Coatings

• Depositions of MoS2 by RF magnetron sputtering
  were carried out.
• XRD analysis of the samples indicated fully
  reactive microcrystalline MoS2 nature.
• Deposition of bilayer coatings of TiN and MoS2 on
  a glass substrate have been carried out.
• Testing of the above film will be carried out for
  satisfying requirements of good wear resistance
  and low coefficient of friction coatings.
• Hard Coatings at Cryogenic Temperatures
• Cryogenic environment leads to increase in the
  coefficient of friction and DLC coatings have
  lower coefficient of friction and good wear
  resistance as compared to hard coatings of
  nitrides at cryogenic temperatures.
• A special cryogenic tribometer is required for
  the study of friction and wear at cryogenic
  temperatures

42
Microwave CVD Setup
Microwave assisted plasma chemical vapor
deposition system (MWCVD) has been ordered for
depositions of diamond-like carbon (DLC) coatings.
43
Conclusion

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