thermal simulation of the nasa glacier vacuum jacket...
TRANSCRIPT
Thermal Simulation of the NASA GLACIER Vacuum Jacket
Brandon Kirkland
1/29/2012
University of Alabama at Birmingham
PROBLEM INTRODUCTION
NASA’s GLACIER is a cryogenic freezer used aboard space shuttles and the
international space station to preserve samples requiring temperatures between +4oC and -160
oC.
The vacuum jacket houses the heat exchanger within the GLACIER unit, and has two CryoTel
CT CryoCoolers to remove heat. The cooling lines containing air or liquid are currently insulated
by Aerogel blankets surrounded by a very low pressure vacuum inside a steel housing. Because
the Aerogel blankets must be wrapped around the complex geometry of the heat exchanger prior
to installation inside the vacuum sealed housing, voids of un-insulated space are impossible to
avoid. The low pressure (10^-6 torr) of the vacuum jacket is difficult to maintain as it tends to
leak, decreasing the efficiency of the insulation and requiring more power to cool the module.
Due to the supply limitations of the space station, the system needs to be sustainable for two
years without maintenance. To solve this issue, glass microspheres are being considered as an
alternative to Aerogel blankets. The spheres can be poured like a liquid into the insulating
region, potentially eliminating the voids left by the Aerogel blanket. A vacuum will still be
necessary, but will not have to be maintained at such low levels since the increased efficiency of
the insulation is expected to make up for the decreased vacuum.
Research is currently ongoing at the UAB Center for Biomedical Sciences and
Engineering to study the effect of glass microspheres and Aerogel on thermal efficiency. This
research entails constructing a physical model of the GLACIER vacuum jacket assembly and
directly measuring the temperatures of the heat exchanger, while insulated by glass microspheres
or Aerogel at varying pressures. However, the need was realized for a separate computational
thermal simulation of the assembly which was performed through CD-Adapco’s STAR CCM+
CFD software.
GEOMETRY CREATION
The geometry created for this thermal simulation pertains to the apparatus design for
testing of the insulation materials. As such, the heat exchanger is a solid non-fuctioning region,
from which heat will be removed. The geometry was created in ProEngineer and imported into
STAR CCM+ as an IGES file format with surfaces. This resulted in several free edges and
pierced mesh faces which were identified with STAR’s mesh repair tool and fixed through a
combination of filled holes (specified by feature curves) and interactive mesh triangle
generation.
Figure 1 shows the steel vacuum housing which encases the insulation and heat
exchanger. Two regions, the housing and exchanger, were created when the part was imported
from the IGES file. To facilitate thermal modeling conditions later, the boundary highlighted in
Figure 2 was created to model the attachment point for the CryoTel CT Cryocooler. Finally, the
third insulating region was obtained by combining the housing and exchanger regions then
splitting by topography.
MESHING PARAMETERS
Meshing models selected for this simulation included polyhedral, surface remesher, and
embedded thin mesher. The thin mesher was applied to the thin walls of the steel housing with 4
layers. Base mesh size was set as 0.1 meters and curvature was set to 120 points per circle.
Minimum mesh size was constrained to 0.0025 meters and maximum target size was 100% of
the base mesh size, 0.1 meters. This yielded 463 831total cells. To indicate the scale of the
model, the length of the exchanger is approximately 760 mm. The surface mesh of all three
regions can be viewed in figures 3, 4, and 5
PHYSICS CONTINUA
Three physics regions were established for the housing, insulation, and exchanger
regions. All three regions were modeled as a solid to simplify simulations and reduce CPU
runtime. However in application the insulation region will be a porous media with a vacuum
pressure regardless of Aerogel or glass microspheres. The physics models selected are listed
below:
Solid
Three Dimensional
Implicit Unsteady
Constant Density
Segregated Solid Energy
Radiation
Participating Media Radiation
Gray Thermal Radiation
Materials selected for the housing and exchanger were stainless steel and copper from the STAR
CCM materials database. Aluminum was selected for the insulation region, obviously not
because of its insulating properties but because the high thermal conductivity would quickly
spread heat through the iterations. This would prove the validity of the simulation for future
insulation studies and potentially identify locations of heat leaks. In future studies when the
insulation is modeled as a porous region, the effect of the vacuum pressure on thermal
conductivity will have to be considered. As shown in Graph 1, vacuum pressure strongly
influences thermal conductivity. One of the glass microspheres advantages, is it’s lower thermal
conductivity at relatively higher pressures when compared to Aerogel.
Finally, the boundary highlighted in Figure 2 is the attachment location for the
CryoCooler and was set at a constant 100 Kelvin. Initial conditions were set as 300 Kelvin for
the housing and insulation, and 100 K for the exchanger. Time step for the implicit unsteady
solver was set to 0.1 seconds.
RESULTS
The simulation was performed over 3500 iterations. The decline in residual energy is
shown in graph 2 below. Visualization of the temperature data was performed through scalar
scenes corresponding to cell surface temperatures and cross sections.
DISCUSSION
Figure 3 shows the surface temperature of the outer housing. The lowest figures in the
temperature scale correspond to the inside surface of the cross members. Lighter yellow colors
indicate temperatures less than ambient and therefore more heat conduction. However, it should
be noted the temperature difference over the outer surface of the housing is approximately only 1
Kelvin.
Figures 7, 8, and 9 show horizontal and vertical cross-sections of the assembly. The
vertical YZ plane section shows relatively warmer temperatures BEYOND the 90 degree bends.
The cold heads can be seen to be the locations of coldest temperatures in both plane section
figures. Which is optimal since this is where heat is removed from the circulating GLACIER air.
CONCLUSION AND SUGGESTIONS FOR FUTURE WORK
This use of aluminum for the insulating material in this case is an effective proof of
concept and may help indicate sources of heat leak. Future simulations will not only create new
insulating materials in STAR’s materials database, but will also consider the insulating region is
a porous media under vacuum pressure. Further, glass microspheres are anticipated to yield
improvements in efficiency because they will fill the entire volume of the insulating region. Any
simulations with Aerogel insulation will demonstrate the ideal condition in which the insulating
blanket fills all the available space. Additionally, both Aerogel and Glass Microspheres are
known to have thermal conductivities which vary as a function of pressure. In future simulations,
STAR CCM+ can easily allow for this by using a table and interpolating a given pressure to a
thermal conductivity.
Figure 1: Vacuum jacket assembly with the CryoCooler on top. Insulation is applied to the void
space between the exchanger and the housing. Overall length of the exchanger is approximately
760 mm.
Figure 2: Imported model to STAR CCM+ with the CryoCooler boundary highlighted.
Figure 3: Surface mesh of the vacuum jacket housing, 268141 cells in volume mesh.
Figure 4: Surface mesh of the insulating region, 174619 cells in volume mesh.
Figure 5: Surface mesh of the exchanger, 21071 cells are present in the volume mesh.
Graph 1: At relatively higher pressures, glass microspheres have lower thermal conductivity
than Aerogel.
Graph 2: Residual Energy over 3500 iterations quickly reached a steady state.
Figure 6: Surface temperature of the stainless steel housing.
Figure 7: Isometric view of horizontal and vertical cutting planes through the assembly. Planes
depict a scalar temperature scene.
Figure 8: Horizontal [XY] temperature cutting plane.
Figure 9: Vertical [YZ] temperature cutting plane.