Multilayer Insulation Venting During Payload Depressurization

Multilayer Insulation (MLI) blankets consist of closely spaced aluminum coated shields that are spaced apart to reduce heat transfer between the payload and the environment, particularly in vacuum. In space application, satellite systems and sub-systems are wrapped in MLI blankets to thermally isolate them from the environment and achieve thermal control requirements. During spacecraft launch, the payload undergoes a rapid depressurization before reaching steady state condition. The MLI blankets are usually perforated and/or connected at the boundaries with Velcro strips to allow out-gassing. The blankets can lose their integrity and functionality if the depressurization process is too rapid: the out-gassing flow can tear the perforations, and the pressure differential built-up across the blanket can pull the Velcro strips apart. This paper describes the design and modeling of depressurization through X-slits cut into the blanket and Velcro strips taped along the sides. A methodology is developed, and a model for quantifying the pressure differential build-up is described and applied to a payload enclosure aboard a Delta II rocket.

Droplet and Bubble Dynamics in Saturated FC-72 Spray Cooling

Droplet and bubble dynamics and nucleate heat transfer in saturated FC-72 spray cooling were studied using a simulation model. Using the experimentally observed bubble growth rate, submodels were assumed based on physical reasoning for the number of secondary nuclei entrained by the impinging droplets, bubble puncturing by the impinging droplets, bubble merging and the spatial distribution of secondary nuclei. The predicted nucleate heat transfer was in agreement with experimental findings. Dynamic aspects of the droplets and bubbles, which had been difficult to observe experimentally, and their ability in enhancing nucleate heat transfer were then discussed based on the results of the simulation. These aspects include bubble merging, bubble puncturing by impinging droplets, secondary nucleation, bubble size distribution and bubble diameter at puncture. Simply increasing the number of secondary nuclei is not as effective in enhancing nucleate heat transfer as when it is also combined with increased bubble puncturing frequency by the impinging droplets. For heat transfer enhancement, it is desirable to have as many small bubbles and as high a bubble density as possible.

Calculation of Thermal Conductivity in Nanofluids From Atomic-Scale Simulations

Nanofluids that consist of nanometer sized particles and fibers dispersed in base liquids have shown the potential to enhance the heat transfer performance. Although three features of nanofluids including anomalously high thermal conductivities at very low nanoparticle concentrations, strongly temperature dependent thermal conductivity and significant increases in critical heat flux have been studied widely, and layering of liquid molecules at the particle-liquid interface, ballistic nature of heat transport in nanoparticles, and nanoparticle clustering are considered as the possible causations responsible for such kind of heat transfer enhancement, few research work from atomic-scale has been done to verify or explain those fascinating features of nanofluids. In this paper, a molecular dynamic model, which incorporates the atomic interactions for silica by BKS potential with a SPC/E model for water, has been established. To ensure the authenticity of our model, the position of each atom in the nanoparticle is derived by the crystallographic method. The interfacial interactions between the nanoparticle and water are simplified as the sum of interaction between many ions. Due to the electrostatic interaction, the ions on the nanoparticle’s surface can attract a certain number of water molecules, therefore, the effect of interaction between the nanoparticle and water on heat transfer enhancement in nanofluids is studied. By using Green-Kubo equations which set a bridge between thermal conductivity and time autocorrelation function of the heat current, a model which may derive thermal conductivity of dilute nanofluids that consist of silica nanoparticles and pure water is built. Several simulation results have been provided which can reveal the possible mechanism of heat enhancement in nanofluids.

Numerical Simulation of Steam Reforming of Methanol in Micro-Channel Reactor

On-board hydrogen generation from hydrocarbon fuels, such as methanol, natural gas, gasoline and diesel, etc., will be technically feasible in the near future for fuel cell powered vehicles. Among all the fuel processing methods, steam reforming is considered as the most widely used method of hydrogen reforming for the lower reactive temperature, pressure and higher hydrogen ratio in reformate. A laminate micro-channel catalytic reactor was designed for the purpose of hydrogen generation from hydrocarbons. The depth of the reaction channel is 0.5 mm, and the length and width are 50 mm and 40 mm, respectively. The same geometry is designed for the heating channels. A metal sheet is placed between reacting and heating channels to separate them. Piling up alternately the two channels is to buildup the laminate microchannel reactor. Numerical simulation has been conducted in one reactive unit, i.e., one reacting channel and one heating channel. The reactant is the solution of methanol and water mixing with a certain ratio. And the reaction heat is provided by hot air flow with a temperature of 600K. A 2D steady model of the reforming reactive processes was developed and solved numerically. The ratio of water and methanol is set to be at 1.3. The conversion rate of methanol was nearly 100% at the outlet of reactor, while the volume ratio of hydrogen is 51.4% with the selectivity of CO2 reaches 49.2%. Detail results showed that the 50 mm long reacting channel could be divided into four different regimes along with the reacting course. In the first regime (0-5mm), methanol in the reactants is almost completely converted and CO is mainly generated in the third one (15-20mm), while reactions in the other two regimes are indiscoverable. The reasons leading to such phenomena are clarified in this paper.

Enhancement of Heat Transfer Due to Plasma Flow in Material Processing Applications

Convective heating is used in materials processing industry for heat treatment and melting applications. Only recently, a new plasma device for convective heating at atmospheric pressure has become commercially available. In this paper, we have investigated heating of an aluminum sprue by conventional convective heating by air and by plasma flow. Transient temperature measurements were made in the sprue interior and the overall heat transfer coefficient was computationally predicted in the two cases. Results show that there is significant enhancement of heat transfer in convective plasma heating compared to heating due to unionized gas under identical flow and temperature conditions. For the cases considered in this study, close to a 60% increase in the heat transfer rate was obtained. The key finding is that even small amount of ionization (~ < 1%) can lead to significant increase in heat transfer coefficient.

Thermal Conductivity Measurements of Nylon 11-Carbon Nanofiber Nanocomposites

Nylon 11, a popular material for commercial use, has been combined with low-percent loads of carbon nanofibers (CNFs) to tailor mechanical, fire retardancy, and thermal properties. Transmission electron microscopy images show that the CNFs are randomly aligned in the polymer matrix. We show that the thermal conductivity is minimized at a certain percent loading of CNFs due to a large thermal contact resistance between the CNFs and the medium.

Building Vertical Walls by Deposition of Molten Metal Droplets

Experiments were conducted to determine conditions under which good metallurgical bonding was achieved in vertical walls composed of multiple layers of droplets that were fabricated by depositing tin droplets layer by layer. Molten tin droplets (0.75 mm diameter) were deposited using a pneumatic droplet generator on an aluminum substrate. The primary parameters varied in experiments were those found to most affect bonding between droplets on different layers: droplet temperature (varied from 250°C to 325°C) and substrate temperature (varied from 100°C to 190°C). Considering the cooling rate of droplet is much faster than the deposition rate previous deposition layer cooled down too much that impinging droplets could only remelt a thin surface layer after impact. Assuming that remelting between impacting droplets and the previous deposition layer is a one-dimensional Stefan problem with phase change an analytical solution can be found and applied to predict the minimum droplet temperature and substrate temperature required for local remelting. It was experimentally confirmed that good bonding at the interface of two adjacent layers could be achieved when the experimental parameters were such that the model predicted remelting.

Dual Role of Nanoparticles in the Thermal Conductivity Enhancement of Nanoparticle Suspensions

Experimental evidence exists that the addition of a small quantity of nanoparticles to a base fluid, can have a significant impact on the effective thermal conductivity of the resulting suspension. The causes for this are currently thought to be due to a combination of two distinct mechanisms. The first is due to the change in the thermophysical properties of the suspension, resulting from the difference in the thermal conductivity of the fluid and the particles, and the second is thought to be due to the transport of thermal energy by the particles, due to the Brownian motion of the particles. In order to better understand these phenomena, a theoretical model has been developed that examines the effect of the Brownian motion. In this model, the well-known approach first presented by Maxwell, is combined with a new expression that incorporates the effect of the Brownian motion and describes the physical phenomena that occurs because of it. The results indicate that the enhanced thermal conductivity may not in fact be due to the transport of energy by the particles, but rather, due to the stirring motion caused by the movement of the nanoparticles which enhances the heat transfer within the fluid. The resulting model shows good agreement when compared with the existing experimental data and perhaps more importantly helps to explain the trends observed from a fundamental physical perspective. In addition, it provides a possible explanation for the differences that have been observed between the previously obtained experimental data, the predictions obtained from Maxwell’s equation and the theoretical models developed by other investigators.

Change in Radiative Optical Properties of Tantalum Pentoxide Thin-Films Due to Material Compositional Changes at High Temperature

Thin films (0.85μm, 3μm) of Ta2O5 deposited on Si and SiO2 were heated to 900°C. Their reflectance in the infrared was measured using an FTIR (Fourier Transform Infrared Spectrometer) equipped with a multiple angle reflectometer before and after exposure to high temperature. An interfacial layer (TaSixOy) formed by the diffusion of Si from the substrate into the deposited film was observed using Auger depth profiling, and the effect of this interfacial layer on the reflectance was measured. Using a least squares optimization technique coupled with an optical admittance algorithm, the multiple angle reflectance data was used to calculate the optical constants of the as deposited Ta2O5 film, crystalline Ta2O5, and the interfacial layer in the 1.6μm to 10μm range. The interfacial layer formed due to exposure to high temperature was found to be more absorptive than the crystalline Ta2O5.

Double Laser Crystallization (DLC) of 50 Nanometer and 20 Nanometer Amorphous Silicon Film for Thin Film Transistors (TFTS) Application

Ultra-large grain poly-crystalline silicon has been formed in 20 nm and 50 nm amorphous silicon films by the double laser crystallization (DLC) method. Surface reflection properties of such thin films upon laser irradiation were calculated. In-situ images were captured to monitor the transient melting and solidification process of 50 nm silicon film in order to understand the crystallization induced by steep laser intensity gradients. SEM (scanning electron microscope) images of crystallized 50 nm film after Secco etch revealed grain size up to 10 m while plane-view TEM (transmission electron microscope) images of 50 nm film also showed perfect crystalline structure inside the grains. AFM (atomic force microscope) images were also taken to show the topology of the grain structure and RMS of 20 nm film.