An overview of the Lewis Research Center CSTI thermal management program

An integrated multi-element project effort, currently being carried out at NASA LeRC, for the development of space heat rejection subsystems, with special emphasis on lightweight radiators, in support of SEI power system technology, and in particular the SP-100 program, is reported. Principal project elements include both contracted and in-house efforts. Included in the first category are two contracts aimed at the development of advanced radiator concepts, and demonstration of a flexible fabric heat pipe radiator concept. In-house work is designed to guide and support the overall program by system integration studies, heat pipe testing and analytical code development, radiator surface morphology alteration for emissivity enhancement, and composite materials research focused on the development of lightweight high conductivity fins

Particle size and surface texturing effects on friction of magnetorheological fluids

MR fluids are produced by dispersing micron-sized ferrimagnetic particles in oil. Generally, the presence of foreign particles in a lubricant affects the sliding behavior of the lubricated surface. Accordingly, the dispersed particles in MR fluids should significantly affect the lubrication properties of the fluids. In this study, we investigated the lubrication properties of MR fluids through sliding tests and in situ observations, while focusing on the behavior of the dispersed particles. MR fluids containing dispersed particles of various diameters were tested. The results of the sliding tests suggested that the MR fluid with the smallest particles exhibited the best lubrication characteristics. Surface observations showed the presence of grooves on the lubricated surfaces after wear. It is assumed that the grooves are formed by the abrasive action of the dispersed particles and that they affect the behavior of the particles. In situ observations proved that, in the case of flat surfaces, the dispersed particles remained stationary during sliding. On the other hand, in case of grooved surfaces, they moved along the grooves. Given these results, it can be surmised that the grooves on the worn surface changed the behavior of the particles and improved the lubricity of MR fluids

Multi-scale analysis of the roughness effect on lubricated rough contact

Determining friction is as equally essential as determining the film thickness in the lubricated contact, and is an important research subject. Indeed, reduction of friction in the automotive industry is important for both the minimization of fuel consumption as well as the decrease in the emissions of greenhouse gases. However, the progress in friction reduction has been limited by the difficulty in understanding the mechanism of roughness effects on friction. It was observed that micro-surface geometry or roughness was one of the major factors that affected the friction coefficient. In the present study, a new methodology coupling the multi-scale decomposition of the surface and the prediction of the friction coefficient by numerical simulation was developed to understand the influence of the scale of roughness in the friction coefficient. In particular, the real surface decomposed in different roughness scale by multi-scale decomposition, based on ridgelets transform was used as input into the model. This model predicts the effect of scale on mixed elastohydroynamic point contact. The results indicate a good influence of the fine scale of surface roughness on the friction coefficient for full-film lubrication as well as a beginning of improvement for mixed lubrication

Influence of laser texturing on the wettability of PTFE

Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer showing excellent thermal and electrical insulation properties and a low coefficient of friction. Due to its large stability, and hydrophobic nature, the wettability of PTFE surfaces can be reduced to transform them into superhydrophobic. In this regard, laser texturing is a fast, simple and versatile method to produce superhydrophobic PTFE surfaces in one-step, and over large areas. In this work, we used a CO2 laser to modify the surface of PTFE samples. We studied the effect of the processing parameters (laser power or irradiance, scanning speed, and spacing -overlapping- between scan lines) on the wettability of textured surfaces using water, mineral oil and ethanol/water solutions as test fluids. Laser-treated surfaces showed a hierarchical micro- and nanotopography with a cotton-like appearance. The higher roughness and large quantity of air pockets make these laser-treated surfaces superhydrophobic, and highly oleophobic. Furthermore, they remain unaltered after being in contact with strong alkali and acid solutions or after slight friction. The self-cleaning performance of these surfaces was also demonstrated. The present findings suggest that CO2 laser texturing of PTFE is suitable for the large-scale preparation of surfaces with low-wettability to different liquids.Agencia Estatal de Investigación | Ref. RTI2018-095490-J-I00Xunta de Galicia | Ref. ED431C 2019/23Xunta de Galicia | Ref. ED481D 2017/010Xunta de Galicia | Ref. ED481B 2016/047-0Xunta de Galicia | Ref. POS-A/2013/16

Mutual influence of cross hatch angle and superficial roughness of honed surfaces on friction in ring-pack tribo-system

The cylinder bore surface texture, widely produced by the honing technique, is an essential factor for a good engine performance (friction, oil consumption, running-in, wear etc.). This explains the improvement and development of various new honing techniques. These different honing processes generate surfaces with various texture features characteristics (roughness, valleys depth, cross hatch angle, etc.). This paper addresses a comparison of ring-pack friction for cylinder surfaces produced by plateau honing and helical slide honing. It takes in consideration the mutual effect of superficial plateau roughness amplitude and honing angle. A numerical model is developed to predict friction within the cylinder ring-pack system in mixed lubrication regime. The results show the effectiveness of helical slide honed surface texture in comparison to plateau honed bore surface

Observation of superspin glass state in magnetically textured ferrofluid (gamma-Fe2O3)

Magnetic properties in a magnetically textured ferrofluid made out of interacting maghemite (gamma-Fe2O3) nanoparticles suspended in glycerin have been investigated. Despite the loss of uniform distribution of anisotropy axes, a superspin glass state exists at low temperature in a concentrated, textured ferrofluid as in the case of its non-textured counterpart. The onset of superspin glass state was verified from the sample's AC susceptibility. The influence of the anisotropy axis orientation on the aging behavior in the glassy states is also discussed