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.