Threshold analysis of a chirped-grating distributed-feedback laser with the power series method

A new and efficient method for analyzing a chirped-grating distributed-feedback (DFB) laser is presented. We show that coupled-wave equations can be solved by use of the power series method. The single-mode gain margin of a linearly chirped-grating DFB laser is calculated for different chirping factors and coupling constants. We found that clearly optimum chirping exists for which the single-mode gain margin is maximum. The gain margins were also calculated for different positions of the cavity center. The effect of facet reflectivities and their phases on the gain margin was investigated. We found that the gain margin is maximum and the spatial hole burning is minimum for the cavity center at the middle of the laser cavity.     

Elastic moduli of liquid crystalline copolymers

Dynamic tensile and shear moduli measurements have been carried out on highly oriented thermotropic liquid crystalline copolymers formed by random copolymerization of 2-hydroxy-6-naphthoic acid, terephthalic acid and p-aminophenol. This liquid crystalline copolymer is known as HNATA. Application of the aggregate model of units of structure gives rise to an estimation of the orientation angle θ = 5° and a chain modulus E_c = 230 GPa. The fall in tensile modulus with increasing temperature is related to two factors: a decrease in the intrinsic chain modulus and a reduction in the shear modulus.     

Properties of Highly-Dense Iron‐Base Powder Metallurgy Materials Pressed without Zinc Stearate

The possibility of obtaining highlydense compacts by single pressing and sintering and the use of heatresistant solid lubricants (graphite, talc) instead of conventional lubricant was studied. It was shown that the addition of graphite is most effective to obtain highly-dense powder metallurgy materials since it substantially facilitates ejection of the part from the die and improves the mechanical properties.     

Oil entrainment in vertical refrigerant pipingEntraînement de l''huile dans les tuyauteries frigorifiques verticales

The aim of this study is to investigate the required refrigerant speed, hence minimum refrigeration load, for carrying the lubricating oil up in vertical sections of refrigerant lines. It is assumed that the downward flow of the thin oil layer over the inner surface of the riser due to gravity is to be balanced with the upward flow of the oil film due to the shear force created by the upward flow of the refrigerant vapor. Velocities are converted to refrigeration capacities by considering a saturated cycle between specified condenser and evaporator pressures. General relationships thus developed are enumerated for R134a by preparing minimum capacity tables for copper suction and discharge risers.     

Reduced myofibroblast differentiation on femtosecond laser treated 316LS stainless steel

In-stent restenosis is a common complication after stent surgery which leads to a dangerous wall narrowing of a blood vessel. Laser assisted patterning is one of the effective methods to modify the stent surface to control cell–surface interactions which play a major role in the restenosis. In this current study, 316LS stainless steel substrates are structured by focusing a femtosecond laser beam down to a spot size of 50 μm. By altering the laser induced spot density three distinct surfaces (low density (LD), medium density (MD) and high density (HD)) were prepared. While such surfaces are composed of primary microstructures, due to fast melting and re-solidification by ultra-short laser pulses, nanofeatures are also observed as secondary structures. Following a detailed surface characterization (chemical and physical properties of the surface), we used a well-established co-culture assay of human microvascular endothelial cells and human fibroblasts to check the cell compatibility of the prepared surfaces. The surfaces were analyzed in terms of cell adherence, proliferation, cell morphology and the differentiation of the fibroblast into the myofibroblast, which is a process indicating a general fibrotic shift within a certain tissue. It is observed that myofibroblast proliferation decreases significantly on laser treated samples in comparison to non-treated ones. On the other hand endothelial cell proliferation is not affected by the surface topography which is composed of micro- and nanostructures. Such surfaces may be used to modify stent surfaces for prevention or at least reduction of restenosis.     

Monte Carlo simulation of extrusion die life

Due to the high cost of metal forming tools (especially in hot extrusion), one of the major goals in tool design is a longer service life. Estimation and prediction of tool life thus becomes critically important for performance evaluation of the tools. The two most dominant failure mechanisms for extrusion dies (solid, hollow, and semi-hollow dies all taken together) are fracture and wear. In the first part of the paper, a fracture mechanics based fatigue life prediction model is described. A similar treatment is then presented for wear-related failures. Fracture and wear usually coexist as failure modes, and final die breakdown occurs due to the mechanism that becomes dominant. Therefore, a competing fracture–wear model has been later developed to represent the complete die failure situation. Attempt has been made to correlate the stochastic nature of various fatigue and wear related die parameters to die life. Monte Carlo simulation has been used to predict the life distribution of a die for a given set of manufacturing conditions and mechanical properties. In comparison with actual life data from the industry, the simulated life yields very realistic predictions.     

A review of gas engine driven heat pumps (GEHPs) for residential and industrial applications

This study reviews gas engine-driven heat pump (GEHP) systems for residential and industrial applications in terms of energetic and exergetic aspects for the first time to the best of the authors’ knowledge. These systems are novel heat pump systems (one of today's promising new technologies). Although the first investigations had been performed at late 1970s, the first merchandized GEHP was produced and introduced in the market in 1985. Gradually, it has become widespread all over the world for various purposes. Main application of GEHPs are for space and water heating/cooling purposes. However, they can be integrated to industrial applications, especially to drying processes.In this study, historical development of GEHP systems was briefly given first. Next, the operation of these systems was described, while studies conducted on them were reviewed and presented in tabulated forms. GEHPs were then modeled for performance evaluation purposes by using energy and exergy analysis methods. Finally, an illustrative example was given, while the results obtained were discussed. In addition, a new project on integration of GEHP systems to food drying processes in Turkey initiated by the authors was introduced. It is expected that this comprehensive study will be very beneficial to everyone involved or interested in the energetic and exergetic design, simulation, analysis and performance of assessment of GEHP systems.