Experimental Thermal Performance Testing Of Cryogenic Tank .

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NASA KSC – Internship Final ReportExperimental Thermal Performance Testing of CryogenicTank Systems and MaterialsWesley C. MyersKennedy Space CenterMajor: BiologySpring Session 2018Date: 09 04 2018Kennedy Space Center121 May 2018

NASA KSC – Internship Final ReportExperimental Thermal Performance Testing of CryogenicTank Systems and MaterialsW. C. Myers1 and J. E. Fesmire2NASA Kennedy Space Center, Florida 32899A comparative study was conducted to collect and analyze thermal conductivity data on a wide variety oflow density materials, as well as thermal performance data on a number of vacuum-jacketed cryogenic tanksystems. Although a vast number of these types of materials and cryogenic tank systems exist, the thermalconductivity of insulation materials and the thermal performance of cryogenic tank systems is often difficult tocompare because many industrial methods and experimental conditions are available and utilized. Theavailability of a new thermal conductivity measurement device, the Macroflash Cup Cryostat, which isapplicable for assessing a variety of materials, is accessible at NASA’s Cryogenic Test Laboratory (CTL) at theKennedy Space Center (KSC). The convenience of this device has resulted in the ability to rapidly measure thethermal conductivity properties of these materials by using a flat-plate liquid nitrogen (LN2) boiloff techniquethat employs a guarded heat flow test methodology in order to determine the effective thermal conductivity (k e)of a test specimen. As the thermal conductivities are measured at cryogenic temperatures, materials suitablefor both future space missions and cryogenic tank systems can be identified and experimentally analyzed. Alsorecognizable are materials which may help increase energy efficiency by limiting the thermal lossesencountered under various environmental conditions. The overall focus of this work consisted of two parts.One part, was to produce and analyze thermal conductivity data on a wide variety of materials with suitableproperties conducive to those needed to aid in the production of a calibration curve for the “low end” of theMacroflash instrument. (Low end meaning materials with a thermal conductivity rating below 100 mW/m-K).The second part was to collect and analyze heat transfer data for a variety of small vacuum-jacketed vessels(cryogenic tank systems) in order to compare the thermal performance between them.NomenclatureQṁhfgqAekexΔTZρσc Heat flow rate (W)Mass flow rate (g/s)Heat of vaporization (J/g)Heat flux (W/m2)Effective heat transfer area (m2)Effective thermal conductivity (mW/m-K)Thickness of specimen (m)Temperature difference (K)Volumetric boiloff rate (W/L)Density (g/cm3)Compressive strengthI. IntroductionThe Cryogenics Test Laboratory at Kennedy Space Center was established to develop thermally efficienttechnologies for a wide range of below-ambient temperature applications. Its overall discipline areas include thestudy of heat management, energy efficiency, thermal insulation systems, and novel materials. Both novel andconventional means of refrigeration and heat control are being investigated to develop the most suitable thermalmanagement and control systems to meet the needs for propellant process systems, superconducting power devicerefrigeration, rocket vehicle protection systems, human exploration habitats, cold chain shipping, and biomedicalresearch platforms, to name a few.In general, one cannot measure how much heat is present in an object, but rather only how much energy istransferred between objects at different temperatures, hot and cold. Early attempts at temperature measurementinclude the experiments of Greek physician Galen in Ad 170, followed many centuries later by Fahrenheit’sdescription of the first modern temperature scale in 1724. A few decades later, J. Black devised an ice calorimeter12NIFS Intern, Cryogenics Test Laboratory, Kennedy Space Center, Central New Mexico Community College.Senior Principal Investigator, Cryogenics Test Laboratory, Mail Code UB-R1, Kennedy Space Center.Kennedy Space Center221 May 2018

NASA KSC – Internship Final Reportbased on his discovery of hidden heat. And by 1822, J. Fourier had published The Analytical Theory of Heat, a workthat remains the basis of our notions of heat, thermal energy and temperature. The technological development oflarge-scale liquid hydrogen in the US in the 1950’s gave rise to the demand for high performance thermal insulationsystems. To meet this demand and enable the development of multilayer insulation (MLI) and evacuated perlitepowder systems, engineers devised different apparatuses to directly measure heat flows from a few milliwatts and upusing evaporation— or “boiloff” calorimetry. The use of boiloff calorimetry to measure the effects of thermal energy(or heat) dates back to the early 1900’s [1, 2]. It has become a practical and useful tool to measure, in a direct way,the thermal insulating performances of materials and systems of materials. Gas flow rates measured using boiloffcalorimetry enable direct calculation of quantities such as heat flux and thermal conductivity. A particularly usefulapproach is to use nitrogen for the heat measurement fluid as it is readily available, inert and generally safe to use.Because heat does not flow through a material as a function of temperature but according to a temperature difference,the use of a cryogen such as liquid nitrogen also provides a convenient way to establish the sub-ambient test conditionsrepresented in the wide range of end-use applications. The temperature range from normal boiling point (77.4 K) toambient (approximately 300 K) represents a wide range of particular needs in construction, transportation, food andbeverage, pharmaceuticals, electrical power, electronics, medical imaging, aerospace, industrial processes and soforth, touching on virtually all aspects of modern life.[3]One of the greatest advantages of using liquid nitrogen boiloff calorimetry is its ultimate simplicity and provisionof a direct energy measurement. The liquid provides a stable cold boundary temperature and serves as a sort of powermeter. The approach also lends itself to testing under representative conditions (i.e., those that reflect the actual-useor field-installed conditions) afforded by the very large temperature difference established by the liquid nitrogen.Cryogenic boiloff methods provide the means to reliably test the thermal conductivity of materials and the thermalperformance of cryogenic tank systems. This method is a direct measurement of the flow of heat and enables thetesting of complex materials and systems over a very wide range of conditions. This type of testing will be used forexperimental laboratory investigations of both materials and cryogenic tanks. The thermal conductivities of materialswill be tested using a Macroflash (Cup Cryostat). The total system thermal performance of small tanks will be testedusing a custom developed laboratory methodology.II. Macroflash Testing of MaterialsA. Test ApparatusThe Macroflash Cup Cryostat instrument (shown in Figure 1) is a cryogenic boiloff calorimeter whose data isrecorded by a National Instruments LabVIEW Data Acquisition Program. The Macroflash houses a cold mass testchamber centered directly over the test specimen, and wrapped in multiple layers of aerogel blankets to ensure thethermal isolation and stability necessary for accurate steady-state boiloff measurements. The Macroflash is acomparative, flat-plate apparatus that tests at a large ΔT (such as 187K) and determines effective thermal conductivity(ke) in accordance with ASTM 1774-13 [4]. Boiloff calorimetry provides a way to directly measure the heat flow ratethrough the test specimen. In Figure 1, the Macroflash system can be seen positioned on a high-sensitivity scale usedfor measuring the mass change of cryogenic fluid as fluid boil off occurs. The Macroflash system is connected to aheat controller for the hot plate and nitrogen gas for purging of the specimen throughout testing. The nitrogen gas isused to maintain an inert and dry environment, which prevents any moisture condensation on the test specimen. Anadjustable compression loading system provides 0.5, 1.5, or 4.5 psi of applied force, ensuring full consolidation andKennedy Space Center321 May 2018

NASA KSC – Internship Final Reportcontact between the test specimen and cold/hot plate boundaries, preventing any void spots that would otherwise alterthermal conductivity results.Figure 1. (Left) Assembled Macroflash setup. (Right) Cross sectional schematic for the Macroflashthermal test instrument.B. Test MethodThe test method is comparative and therefore requires calibration with materials of known thermalconductivity data. However, finding these materials with the necessary standard reference data is a difficult challenge.This project builds on the previous five years of work by different colleagues of the NASA Cryogenics Test Laboratoryincluding scientists, engineers, and university researchers and brings in additional testing for a specific range of thespectrum of materials [5, 6].The test conditions are representative of actual-use cryogenic applications with a warm boundary temperature(WBT) of approximately 293 K and a cold boundary temperature (CBT) of approximately 78 K. The test measurementprinciple is liquid nitrogen boil-off calorimetry where the mass flow rate of nitrogen gas is directly related to the rateof heat energy transmitted through the material. All tests are performed at an ambient pressure gaseous nitrogen(GN2) condition for consistency. A boiloff calorimetry test is conducted by filling a test chamber with a liquid cryogenwhich then boils/evaporates at room temperature. A test specimen of pre-determined geometry is affixed to the bottomof the test chamber and put in an environmental apparatus that provides the desired test conditions. The flow of heatthrough the test sample is directly proportional to the cryogenic fluid boiloff flow rate, measured by a weight scale ormass flow meter, with energy transfer measured as the heat flow rate. This cryogen boiloff rate, or gas flow rate(which can also be measured as mass loss) is directly used for the calculation of heat flux and effective thermalconductivity. Under steady-state flow conditions, the rate of heat flow through the test specimen is constant at allpoints through the thickness of the specimen. The thermal conductivity can then be easily calculated using a series ofequations:𝑄 ṁℎ𝑓𝑔𝑞 𝑘𝑒 Kennedy Space Center𝑄𝐴𝑒𝑄𝑥(𝐴𝑒 𝛥𝑇)4(1)(2)(3)21 May 2018

NASA KSC – Internship Final ReportThe heat flow rate (Q) is the product of boiloff mass flow rate (ṁ) and enthalpy of vaporization of liquid nitrogen(hfg) as shown in the equation above. Knowing the boundary temperatures on either side of the specimen in a set testenvironment, the heat flux (q) can be calculated. Since the specimen area is also known (Ae), as well as the thicknessof the specimen (x) and the temperature differential between cold and hot boundaries (ΔT), the effective thermalconductivity (ke) can then be calculated.For this testing, liquid nitrogen (LN2) was used as the cryogen, allowing for a temperature differential from 77 K(nominal boiling point of liquid nitrogen) to 293 K (room temperature) to be created across the thickness of the testspecimen (Figure 2). The LN2 provides the cooling (refrigeration) required, acts as an “energy meter”, and producesthe temperature differential (i.e., the change in temperature fromone side of the specimen, chilled by LN2, to the other side,maintained at room temperature by a heat controller. Boiloffcalorimetry provides the ability to test both simple uniformmaterials and complicated non-homogeneous, anisotropic,composite materials with equal ease [7]. This method alsoinherently presents a temperature differential where thermalconductivity is calculated by the mass flow rate of cryogenic fluidboiloff, which is measured through either the system mass loss orboiloff flow rate. The extent of the temperature differential istherefore dependent on the cryogenic fluid and the h eat sourceFigure 2. Boundary temperatures on theused, providing high sensitivity for the accurate measurement offaces of a test specimen.highly thermally insulating materials or structural materials alike.Intermediate temperature sensors can also be employed forobtaining thermal conductivity data at different mean temperaturesup to the ambient.C. Materials (Test Specimens)The test specimens (examplesshown in Figure 3) are typically 3”diameter by ¼” thickness and shouldbe flat and smooth-faced or easilycompressible to insure good thermalcontact between the heater assemblyand the cold mass. The specimen isplaced in the test section betweentwo plates that are maintained atdifferent temperatures during thetest. For commercial samples, thecompressivepropertieswereobtained from the supplier technicaldata. For in-house research samples,compressionpropertieswereevaluated according to ASTMD695. A complete list of specimenstested and their properties is given inTable 1.Kennedy Space CenterFigure 3. Example test specimens prepared for Macroflash testing: avariety of different thermal insulation materials (left) and fiveaerogel blanket materials (right).521 May 2018

NASA KSC – Internship Final ReportTable 1. Summary and Properties of Materials TestedTestSeriesMatlCodeThickDia.MaterialmmmmMass gel x201Spaceloft WhiteSpaceloft GreySpaceloft SubseaPyrogel XT-EPolyglas SBS Asphalt Aerogel BlanketCryogel x201 2-layersAeroplastic Versify Sample-A 22-stackAeroplastic Versify Sample-B 13-stackAeroplastic Versify Sample-C 14-stackCryogel x201 #2 (1 Layer)Cryogel x201 #3 (1 Layer)Cryogel x201 (1 layer)SOFI #2Balsa (in-plane) 7.3mmBalsa (in-plane) 6.4mmPolyimide Foam SolimideFoamGlass #1 brokenULD AerogelFoamGlass #2FoamGlass #3Glass Bubbles, low density 0.038Glass Bubbles, nominal tap densityGlass Bubbles, nominal tap densityGlass Bubbles, nominal tap p.Y or N .47700.80.80.80.81.721.721.721.72D. Test ResultsThe thermal conductivities of a large number of test specimens have been evaluated using the Macroflash, asdescribed above. The specimens tested covered a wide variety of materials that met the thermal conductivityspecification for the “low end calibration”, and thus have a similar range in bulk density, but a varying range inmaterial composition. A total of 25 test specimens representing approximately 130 boiloff test runs were performedas part of the current project. The test results are listed in Table 2.Kennedy Space Center621 May 2018

NASA KSC – Internship Final ReportTable 2. Macroflash thermal conductivity test data summaryTestSeriesMatlCodeMaterialTest 158aZ1-158bZ1-158cAAAAALAPPPAAAFWWFFAFFBBBBCryogel x201Spaceloft WhiteSpaceloft GreySpaceloft SubseaPyrogel XT-EPolyglas SBS Asphalt Aerogel BlanketCryogel x201 2-layersAeroplastic Versify Sample-A 22-stackAeroplastic Versify Sample-B 13-stackAeroplastic Versify Sample-C 14-stackCryogel x201 #2 (1 Layer)Cryogel x201 #3 (1 Layer)Cryogel x201 (1 layer)SOFI #2Balsa (in-plane) 7.3mmBalsa (in-plane) 6.4mmPolyimide Foam SolimideFoamGlass #1 brokenULD AerogelFoamGlass #2FoamGlass #3Glass Bubbles, low density 0.038Glass Bubbles, nominal tap densityGlass Bubbles, nominal tap densityGlass Bubbles, nominal tap 5555Mass 167.0707.122qKe - comp2W/mmW/m-KHeat Flux 45.846.1StdkecalComp. Figure OfMeritDev mW/m-K L or H Strength% 1.72For example of a range of data for materials with exceptionally low thermal conductivity, Figure 4 shows acomparison of various commercial aerogel insulation materials, which have a thermal conductivity less than 50mW/m-K, which is specific to the needs of the work to accomplish a special “low end” calibration. The work of thecurrent project begins with test specimen Z1-148 and goes through Z1-158.to the low end calibration.Kennedy Space Center721 May 6

NASA KSC – Internship Final ReportMacroflash Cup Cryostat Various Aerogel MaterialsEffective Thermal Conductivity - calibrated keBoundary Temperatures 293 K / 78 K; Environment 760 torr GN23532.432.232.73027.827.8calbirated ke 17.84.754.23.20.30Aero Zero Aero Zero Primaloft Primaloft AerogelPI Flexcon PI FlexconThinThin paper 0.7Sheet (20 Sheet (20 Aerogel Aerogel mm whitestack)stack)(10 stack)X-aerogelYellowDisk GR C(repeatZ314)X-aerogelFXP TanDisk #1(Flexcon)X-aerogelFXP TanDisk #2(Flexcon)X-aerogel X-aerogel CryogelFXP Tan Yellowx201Disk #3 Disk GR C(Flexcon) (repeatZ315)Spaceloft Spaceloft Spaceloft PyrogelWhiteGreySubseaXT-ECryogel Cryogel Cryogel Cryogelx201 2- x201 #2 (1 x201 #3 (1 x201 (1layersLayer)Layer)layer)ULDAerogelFigure 4. Comparison of various commercial and research Aerogel insulation materialsAdditionally, the compressive strength and density were recorded for each material so that a “Figure of Merit”(FOM) could be derived as a representation of the overall performance. These results are summarized by variouscategories in Table 2, and the FOM was calculated according to the provided FOM equation:𝐹𝑂𝑀 (𝜎𝑐𝑘𝑒 𝜌) 103(4)Using the compressive strength (σc), effective thermal conductivity (ke), and density (ρ), the FOM equationgenerates values (with units of K m s/g) in which larger values are indicative of potential candidates for structuralthermal insulation materials. Since low values of thermal conductivity and density are desired, these parameters wereplaced in the denominator of the FOM equation. The compressive strength was placed in the numerator since largestrengths are desired. Combining the desired values in this way allows for a rapidly obtained, quantitative screeningparameter to identify potentially high performing structural thermal insulation candidates.III. Macroflash Analysis and CalibrationInitial Macroflash comparative ke measurements are reported as raw data that is then calibrated to give the effectiveke values reported in the tables above. The calibration is based off of a linear fit of measured material thermalconductivities

thermal conductivity properties of these materials by using a flat-plate liquid nitrogen (LN 2) boiloff technique that employs a guarded heat flow test methodology in order to determine the effective thermal conductivity (k e) of a test specimen. As the thermal conductivities are measured at cryogenic temperatures, materials suitable

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