Wrapped multilayer insulation design and testing

New vehicles need improved cryogenic propellant storage and transfer capabilities for long duration missions. Multilayer insulation (MLI) for cryogenic propellant feedlines is much less effective than MLI tank insulation, with heat leak into spiral wrapped MLI on pipes 3–10 times higher than conventional tank MLI. Better insulation for cryogenic feed lines is an important enabling technology that could help NASA reach cryogenic propellant storage and transfer requirements. Improved insulation for Ground Support Equipment could reduce cryogen losses during launch vehicle loading. Wrapped-MLI (WMLI) is a high performance multilayer insulation using innovative discrete spacer technology specifically designed for cryogenic transfer lines and Vacuum Jacketed Pipe (VJP) to reduce heat flux.The poor performance of traditional MLI wrapped on feed lines is due in part to compression of the MLI layers, with increased interlayer contact and heat conduction. WMLI uses discrete spacers that maintain precise layer spacing, with a unique design to reduce heat leak. A Triple Orthogonal Disk spacer was engineered to minimize contact area/length ratio and reduce solid heat conduction for use in concentric MLI configurations.A new insulation, WMLI, was developed and tested. Novel polymer spacers were designed, analyzed and fabricated; different installation techniques were examined; and rapid prototype nested shell components to speed installation on real world piping were designed and tested. Prototypes were installed on tubing set test fixtures and heat flux measured via calorimetry. WMLI offered superior performance to traditional MLI installed on cryogenic pipe, with 2.2 W/m² heat flux compared to 26.6 W/m² for traditional spiral wrapped MLI (5 layers, 77–295 K). WMLI as inner insulation in VJP can offer heat leaks as low as 0.09 W/m, compared to industry standard products with 0.31 W/m. WMLI could enable improved spacecraft cryogenic feedlines and industrial hot/cold transfer lines.     

Transmissivity testing of multilayer insulation at cryogenic temperatures

The problem of degraded emissivity of thin films at low temperatures has been a long observed phenomena. Previous efforts at measuring properties have suggested that transmission of energy through the films may play a key role in the thermal performance of multilayer insulation systems at low temperatures. Similarly, recent testing on tank applied systems has suggested a radiative degradation at low temperatures. Two different approaches were used to attempt to measure the transmission of energy through MLI at low temperatures. A laser based measurement system was set up to directly measure transmittance and a calorimetric based measurement system was used to measure relative emittance of a single layer between aluminum foil and double aluminized Mylar. Minimal transmission at long wavelengths were observed through standard MLI blanket materials at deposition thicknesses of even 35 nm. Where transmission was measured, it was too low to effect the performance of a multilayers system. Similarly, the calorimeter showed similar increases of emissivity for both standard blanket materials and aluminum foils. Multiple different methodologies of measurement have all yielded the same result: that there is no transmission through standard MLI blanket materials at wavelengths associated with temperatures as low as 2 K.     

Novel load responsive multilayer insulation with high in-atmosphere and on-orbit thermal performance

Aerospace cryogenic systems require lightweight, high performance thermal insulation to preserve cryopropellants both pre-launch and on-orbit. Current technologies have difficulty meeting all requirements, and advances in insulation would benefit cryogenic upper stage launch vehicles, LH₂ fueled aircraft and ground vehicles, and provide capabilities for sub-cooled cryogens for space-borne instruments and orbital fuel depots. This paper reports the further development of load responsive multilayer insulation (LRMLI) that has a lightweight integrated vacuum shell and provides high thermal performance both in-air and on-orbit.LRMLI is being developed by Quest Product Development and Ball Aerospace under NASA contract, with prototypes designed, built, installed and successfully tested. A 3-layer LRMLI blanket (0.63 cm thick, 77 K cold, 295 K hot) had a measured heat leak of 6.6 W/m² in vacuum and 40.6 W/m² in air at one atmosphere. In-air LRMLI has an 18× advantage over Spray On Foam Insulation (SOFI) in heat leak per thickness and a 16× advantage over aerogel. On-orbit LRMLI has a 78× lower heat leak than SOFI per thickness and 6× lower heat leak than aerogel.The Phase II development of LRMLI is reported with a modular, flexible, thin vacuum shell and improved on-orbit performance. Structural and thermal analysis and testing results are presented. LRMLI mass and thermal performance is compared to SOFI, aerogel and MLI over SOFI.     

Flight model performance test results of a helium dewar for the soft X-ray spectrometer onboard ASTRO-H

ASTRO-H is a Japanese X-ray astronomy satellite, scheduled to be launched in fiscal year 2015. The mission includes a soft X-ray spectrometer instrument (SXS), which contains an X-ray micro calorimeter operating at 50 mK by using an adiabatic demagnetization refrigerator (ADR). The heat sink of the ADR is superfluid liquid helium below 1.3 K. The required lifetime of the superfluid helium is 3 years or more. In order to realize this lifetime, we have improved the thermal performance from the engineering model (EM) while maintaining the mechanical performance. Then, we have performed a thermal test of the flight model (FM). The results were that the heat load to the helium tank was reduced to below 0.8 mW in the FM from 1.2 mW in the EM. Therefore, the lifetime of the superfluid helium is more than 3 years with 30 L of liquid helium.In this paper, the thermal design and thermal test results are described.     

Entropy generation in a heat exchanger working with a biological nanofluid considering heterogeneous particle distribution

Entropy generation rates considering particle migration are evaluated for a biologically produced nanofluid flow in a mini double-pipe heat exchanger. The nanofluid is used in tube side and hot water flows in annulus side. Silver nanoparticles synthesized through plant extract method from green tea leaves are utilized. Particle migration causes non-uniform concentration distribution, and non-uniformity intensifies by increase in Reynolds number and concentration. The results indicate that at high concentrations and Reynolds numbers, particle migration can have a great effect on entropy generation rates. For water inlet temperature of 308 K, the contribution of friction in nanofluid entropy generation is much more than that of heat transfer. However, as the water inlet temperature increases to 360 K, the heat transfer contribution increases such that at low Reynolds numbers, the thermal contribution exceeds the frictional one. For total heat exchanger, Bejan number is smaller than 0.2 at water inlet temperature of 308 K, while Bejan number has a large value at water inlet temperature of 360 K. Furthermore, entropy generation at the wall has an insignificant contribution, such that for Re = 1000 and φ_m = 1%, the total entropy generation rates for the nanofluid, wall, and water are 0.098810, 0.000133, and 0.041851 W/K, respectively.     

Effect of the geometry of flattened tube on the thermal performance of a high temperature generator

The present study numerically investigated the effect of the geometry of flattened tube on the thermal performance of a high temperature generator (HTG) with the pre-mixed surface flame burner of the double effect LiBr–water absorption system. The heat transfer tubes of the HTG were consisted with a set of circular and flattened tubes in series. FLUENT, as a commercial code, was applied for estimating the thermal performance of the HTG. Key parameters were the aspect ratio of flattened tubes, rib transversal length, and the rib pitch ratio on the flattened tube of the HTG. The maximum heat transfer rate of the HTG was obtained at the aspect ratio of 6.8 for the flattened tube. The heat transfer rate for the flattened tube was increased by 4.2% as the rib transversal length was increased from 2 mm to 3 mm. The heat transfer rate of the flattened tube with the rib pitch ratio of 15.3 was higher by the maximum 5.8% than that without rib. The heat transfer rate of the HTG with the rib of the rib transversal length of 3 mm and the rib pitch ratio of 15.3 was higher by 3.4% than that without the rib. It led that the exhaust gas temperature of the HTG with the rib was lower by 23 °C than that without the rib.     

Natural convection flow from an isothermal horizontal circular cylinder in presence of heat generation

Natural convection laminar boundary layer flow from a horizontal circular cylinder with a uniform surface temperature at presence of heat generation has been investigated. The governing boundary layer equations are transformed into a non-dimensional form and the resulting nonlinear systems of partial differential equations are solved numerically applying two distinct methods namely (i) implicit finite difference method together with the Keller box scheme and (ii) series solution technique. The results of the surface shear stress in terms of the local skin friction and the surface rate of heat transfer in terms of the local Nusselt number for a selection of the heat generation parameter γ (= 0.0, 0.2, 0.5, 0.8, 1.0) are obtained and presented in both tabular and graphical formats. Without effect of the internal heat generation inside the fluid domain for which we take γ = 0.0, the present numerical results show an excellent agreement with those of Merkin [J.H. Merkin, Free convection boundary layer on an isothermal horizontal circular cylinders, in: ASME/AIChE, Heat Transfer Conference, St. Louis, MO, August 9–11, 1976]. The effects of γ on the fluid velocity, temperature distribution, streamlines and isotherms are examined.     

Liquid helium free mechanical property test system with G-M cryocoolers

In the present work, a cryogenic mechanical property testing system conduction-cooled by two G-M cryocoolers was developed. The testing sample can be cooled from room temperature to 2.7 K within 7.5 h. The sample was first cooled down to 11.1 K directly by the two G-M cryocoolers and then cooled down to 2.7 K by decompressing the chamber. Instead of liquid helium, the cooling process is characterized by cooling with recycled helium gas as heat transfer medium. The heat load of the system was analyzed and optimizations were adopted in terms of material selections and design. The static load capacity of the system reaches 200 kN and the fatigue load capacity can reach 50 kN. This system can be installed onto an electronic universal testing machine or a fatigue testing machine to characterize static tension, fracture mechanics or fatigue properties at tunable low temperatures. Tensile properties of 316L austenitic stainless steels at 4.2 K were tested with the system and the results were compared with those obtained by cooled using liquid helium, which demonstrates high reliability.     

Fire retardant evaluation of carbon nanofiber/graphite nanoplatelets nanopaper-based coating under different heat fluxes

Three types of carbon nanofiber based nanopapers, namely, 1Clay/5CNF/9APP, 1xGnP/5CNF/9APP, and 3xGnP/1CNF/9APP were made and their flame retardant efficiency was compared with thermogravimetric analysis and cone calorimeter test with 50 kW/m² of heat flux. The nanopaper of 3xGnP/1CNF/9APP was selected for experimental study because of its relatively good bonding with underlying structure and flame resistance performance. The fire response of glass fiber reinforced polyester composites with and without the selected nanopaper coating was thoroughly examined with cone calorimeter test using varied heat fluxes. It was found that at higher heat flux, the nanopaper demonstrated better flame retardant efficiency. Specifically, at 100 kW/m² of heat flux, the 1st and 2nd PHRR of the nanopaper-coated sample were more than 32% and 47% lower than the PHRR of control sample, respectively. In order to gain an insight into the pyrolysis process and flame retardation mechanism, the temperature profiles at the middle and back of the samples subjected to different heat fluxes were recorded. At 100 kW/m² of heat flux, the final temperature within the nanopaper-coated sample was roughly 280 ^oC, which is lower than control sample. The degradation rates in flexural moduli of the samples with coupon shape were determined using three-point bending. The three-point bending test results showed when the sample was exposed to 25 kW/m² heat flux for 240 seconds, the flexural modulus of control sample almost reduced to zero, whereas the nanopaper-coated sample still retained a half of its original flexural modulus. Finally, flame retardation mechanism was proposed for the nanopaper-coated composites.     

Fire retardant evaluation of carbon nanofiber/graphite nanoplatelets nanopaper-based coating under different heat fluxes

Three types of carbon nanofiber based nanopapers, namely, 1Clay/5CNF/9APP, 1xGnP/5CNF/9APP, and 3xGnP/1CNF/9APP were made and their flame retardant efficiency was compared with thermogravimetric analysis and cone calorimeter test with 50 kW/m² of heat flux. The nanopaper of 3xGnP/1CNF/9APP was selected for experimental study because of its relatively good bonding with underlying structure and flame resistance performance. The fire response of glass fiber reinforced polyester composites with and without the selected nanopaper coating was thoroughly examined with cone calorimeter test using varied heat fluxes. It was found that at higher heat flux, the nanopaper demonstrated better flame retardant efficiency. Specifically, at 100 kW/m² of heat flux, the 1st and 2nd PHRR of the nanopaper-coated sample were more than 32% and 47% lower than the PHRR of control sample, respectively. In order to gain an insight into the pyrolysis process and flame retardation mechanism, the temperature profiles at the middle and back of the samples subjected to different heat fluxes were recorded. At 100 kW/m² of heat flux, the final temperature within the nanopaper-coated sample was roughly 280 ^oC, which is lower than control sample. The degradation rates in flexural moduli of the samples with coupon shape were determined using three-point bending. The three-point bending test results showed when the sample was exposed to 25 kW/m² heat flux for 240 seconds, the flexural modulus of control sample almost reduced to zero, whereas the nanopaper-coated sample still retained a half of its original flexural modulus. Finally, flame retardation mechanism was proposed for the nanopaper-coated composites.     

Cryogenic flat-panel gas-gap heat switch

A compact additive manufactured flat-panel gas-gap heat switch operating at cryogenic temperature is reported in this paper. A guarded-hot-plate apparatus has been developed to measure the thermal conductance of the heat switch with the heat sink temperature in the range of 100–180 K. The apparatus is cooled by a two-stage GM cooler and the temperature is controlled with a heater and a braided copper wire connection. A thermal guard is mounted on the hot side of the device to confine the heat flow axially through the sample. A gas handling system allows testing the device with different gas pressures in the heat switch. Experiments are performed at various heat sink temperatures, by varying gas pressure in the gas-gap and with helium, hydrogen and nitrogen gas. The measured off-conductance with a heat sink temperature of 115 K and the hot plate at 120 K is 0.134 W/K, the on-conductance with helium and hydrogen gases at the same temperatures is 4.80 W/K and 4.71 W/K, respectively. This results in an on/off conductance ratio of 37 ± 7 and 35 ± 6 for helium and hydrogen respectively. The experimental results matches fairly well with the predicted heat conductance at cryogenic temperatures.     

Pressure drop and heat transfer during two-phase flow vaporization of propane in horizontal smooth minichannels

This study examined the two-phase flow boiling pressure drop and heat transfer for propane, as a long term alternative refrigerant, in horizontal minichannels. The pressure drop and local heat transfer coefficients were obtained for heat fluxes ranging from 5–20 kW m^?2, mass fluxes ranging from 50–400 kg m^?2 s^?1, saturation temperatures of 10, 5 and 0 °C, and quality up to 1.0. The test section was made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and lengths of 1000 mm and 2000 mm, respectively. The present study showed the effect of mass flux, heat flux, inner tube diameter and saturation temperature on pressure drop and heat transfer coefficient. The experimental results were compared against several existing pressure drop and heat transfer coefficient prediction methods. Because the study on evaporation with propane in minichannels was limited, new correlations of pressure drop and boiling heat transfer coefficient were developed in this present study.     

Thermal resistance of indium coated sapphire–copper contacts below 0.1 K

High thermal resistances exist at ultra-low temperatures for solid–solid interfaces. This is especially true for pressed metal–sapphire joints, where the heat is transferred by phonons only. For such pressed joints it is difficult to achieve good physical, i.e. thermal contacts due to surface irregularities in the microscopic or larger scale. Applying ductile indium as an intermediate layer reduces the thermal resistance of such contacts. This could be proven by measurements of several researchers. However, the majority of the measurements were performed at temperatures higher than 1 K. Consequently, it is difficult to predict the thermal resistance of pressed metal–sapphire joints at temperatures below 1 K.In this paper the thermal resistances across four different copper–sapphire–copper sandwiches are presented in a temperature range between 30 mK and 100 mK. The investigated sandwiches feature either rough or polished sapphire discs (Ø 20 mm × 1.5 mm) to investigate the phonon scattering at the boundaries. All sandwiches apply indium foils as intermediate layers on both sides of the sapphire. Additionally to the indium foils, thin indium films are vapour deposited onto both sides of one rough and one polished sapphire in order to improve the contact to the sapphire.Significantly different thermal resistances have been found amongst the investigated sandwiches. The lowest total thermal resistivity (roughly 26 cm² K⁴/W at 30 mK helium temperature) is achieved across a sandwich consisting of a polished sapphire with indium vapour deposition. The thermal boundary resistance between indium and sapphire is estimated from the total thermal resistivity by assuming the scattering at only one boundary, which is the warm sapphire boundary where phonons impinge, and taking the scattering in the sapphire bulk into account. The so derived thermal boundary resistance agrees at low temperatures very well with the acoustic mismatch theory.     

Parametric study of unsteady forced convection with pressure gradient

By an extension of differential method, this paper has successfully examined the unsteady forced convection heat transfer from a flow over a flat plate. Transient state is inherent to a sudden change on the heat flux density at the surface of a plate. The general case where the pressure along the direction of flow is not constant is presented. The differential momentum and heat transfer equations are solved numerically. The results are given for different values of pressure gradient parameter m, in the cases of attached boundary layer, and for several values of Prandtl number corresponding to usual fluids (0.71 ⩽ Pr ⩽ 100). The dependences of transient behaviours with Pr number and parameter m are evidenced from the evolutions in time of temperature profile, Stanton number, and duration of unsteady process. Solutions given from the beginning of transient state to the ultimate steady state are discussed. Moreover, analytical solutions, as function of Pr and m, are deduced for Stanton number and duration of unsteady regime.     

Experimental study of the influence of cold heat exchanger geometry on the performance of a co-axial pulse tube cooler

Improving the performance of the pulse tube cooler is one of the important objectives of the current studies. Besides the phase shifters and regenerators, heat exchangers also play an important role in determining the system efficiency and cooling capacity. A series of experiments on a 10 W @ 77 K class co-axial type pulse tube cooler with different cold heat exchanger geometries are presented in this paper. The cold heat exchangers are made from a copper block with radial slots, cut through using electrical discharge machining. Different slot widths varying from 0.12 mm to 0.4 mm and different slot numbers varying from around 20–60 are investigated, while the length of cold heat exchangers are kept the same. The cold heat exchanger geometry is classified into three groups, namely, constant heat transfer area, constant porosity and constant slot width. The study reveals that a large channel width of 0.4 mm (about ten times the thermal penetration depth of helium gas at 77 K, 100 Hz and 3.5 MPa) shows poor performance, the other results show complicated interaction effects between slot width and slot number. These systematic comparison experiments provide a useful reference for selecting a cold heat exchanger geometry in a practical cooler.     

Effects of biofouling on air-side heat transfer and pressure drop for finned tube heat exchangers

Experimental investigations on the effects of biofouling on air-side heat transfer and pressure drop for three biofouled finned tube heat exchangers and one clean finned tube heat exchanger were performed. Artificial accelerated method of microorganism growth on the fin surface was used for simulating the biofouled finned tube heat exchangers. Experimental results indicate that the effects of biofouling on the air-side heat transfer coefficient decreases 7.2% at 2.0 m/s when the biofouled area ratio is 10%, while it decreases 15.9% at 2.0 m/s when the biofouled area ratio is 60%, and biofouling causes a 21.8% ～ 41.3% increase in pressure drop when the air velocity is between 0.5 and 2.0 m/s. The increase of inlet air velocity is helpful to improve the long-term performance of finned tube heat exchanger. Biofouling makes the hydrophilic coating failure, and the condensation water easily converges on the fin surface where biofouling grows.     

Modification of UHMWPE by ion,electron and γ-ray irradiation

Due to good wear resistance Ultrahigh Molecular Weight Polyethylene (UHMWPE) is the material of choice for the load bearing surfaces of total joint implants. In order to improve its performance polymer parts are often modified by the use of ionizing radiation. Here we report on the use of electron and ion beams and γ-rays for the purpose. UHMWPE samples were irradiated with 600 keV and 1.5 MeV electron beam with doses ranging from 50 to 500 kGy and bombarded with 1–10 MeV He- and 9 MeV Cl-ions to fluences ranging from 10¹² to 5 × 10¹⁶ ions/cm². Co-bomb was used for γ-ray irradiation. Polymer radiolysis due to the irradiations was studied by means of nuclear reaction analysis (NRA) using the ¹H(¹⁵N, αγ)¹²C reaction. Hydrogen release increases with the applied dose and was correlated to the linear energy transfer (LET). Irradiated polymers oxidize rapidly when exposed to the air. Oxygen uptake profiles were determined using RBS. Correlation between radiolysis and oxidation has been revealed. Enriched in oxygen region extends to the depth at which radiation induced hydrogen release took place. Once started oxidation proceeds until the saturation concentration of about 10 at.% was attained.     

Experimental measurements and modeling of transient heat transfer in forced flow of He II at high velocities

An experiment has been built to study heat transfer in forced flow of He II at flow velocities up to 22 m/s. The main part of this experiment is a 10 mm ID, 0.86 m long straight test section instrumented with a heater, thermometers and pressure transducers. The high flow velocities allow clear observation of the effects of the forced convection, counterflow heat transfer and the Joule–Thomson effect. A numerical model based on the He II energy conservation equation and including pressure effects has been developed to compare with the experimental results. The model works well for low flow velocities where the heat flux is primarily driven by the temperature gradient and for high flow velocities where the heat flux is primarily driven by the pressure gradients. In the intermediate velocity region, discrepancies between the model and experiment may result from an inappropriate representation of the heat flux by counterflow when the temperature and pressure gradients have an effect of similar magnitude on the heat flux.     

Thermal and mechanical properties of waste grass broom fiber-reinforced polyester composites

The main focus of this study is to utilize waste grass broom natural fibers as reinforcement and polyester resin as matrix for making partially biodegradable green composites. Thermal conductivity, specific heat capacity and thermal diffusivity of composites were investigated as a function of fiber content and temperature. The waste grass broom fiber has a tensile strength of 297.58 MPa, modulus of 18.28 GPa, and an effective density of 864 kg/m³. The volume fraction of fibers in the composites was varied from 0.163 to 0.358. Thermal conductivity of unidirectional composites was investigated experimentally by a guarded heat flow meter method. The results show that the thermal conductivity of composite decreased with increase in fiber content and the quite opposite trend was observed with respect to temperature. Moreover, the experimental results of thermal conductivity at different volume fractions were compared with two theoretical models. The specific heat capacity of the composite as measured by differential scanning calorimeter showed similar trend as that of the thermal conductivity. The variation in thermal diffusivity with respect to volume fraction of fiber and temperature was not so significant.The tensile strength and tensile modulus of the composites showed a maximum improvement of 222% and 173%, respectively over pure matrix. The work of fracture of the composites with maximum volume fraction of fibers was found to be 296 Jm⁻¹.     

Design and experimental investigation of a cryogenic system for environmental test applications

This paper is concerned with the design, development and performance testing of a cryogenic system for use in high cooling power instruments for ground-based environmental testing. The system provides a powerful tool for a combined environmental test that consists of high pressure and cryogenic temperatures. Typical cryogenic conditions are liquid hydrogen (LH2) and liquid oxygen (LO2), which are used in many fields. The cooling energy of liquid nitrogen (LN2) and liquid helium (LHe) is transferred to the specimen by a closed loop of helium cycle. In order to minimize the consumption of the LHe, the optimal design of heat recovery exchangers has been used in the system. The behavior of the system is discussed based on experimental data of temperature and pressure. The results show that the temperature range from room temperature to LN2 temperature can be achieved by using LN2, the pressurization process is stable and the high test pressure is maintained. Lower temperatures, below 77 K, can also be obtained with LHe cooling, the typical cooling time is 40 min from 90 K to 22 K. Stable temperatures of 22 K at the inlet of the specimen have been observed, and the system in this work can deliver to the load a cooling power of several hundred watts at a pressure of 0.58 MPa.