Ahmet Z. Şahin

Plasma nitriding of Ti6Al4V alloy to improve some tribological properties

The present study was conducted to investigate the tribological and mechanical properties of plasma-nitrided Ti6Al4V alloy. Specimens were nitrided in an H2N2 (1:8 ratio) plasma. The nitrogen concentration along the nitrided zone was obtained using the nuclear reaction analysis technique. The workpiece temperature was varied from 450 to 520 °C during the nitriding process. Pin-on-disc wear tests were carried out to evaluate the wear properties of the resultant samples and a ball-on-disc experiment was conducted to measure the friction coefficient. Microhardness tests, Scanning electron microscopy and X-ray diffraction were carried out to investigate the phases developed in the nitrided zone. It was found that the wear resistance improved considerably after the nitriding process. Three distinct layers were identified: (i) an inner layer where δ-TiN + e-Ti2N phases formed, (ii) an intermediate layer where α-(TiN) with or without e phase developed and (iii) an outer layer where precipitations were dominant.

Friction welding of StAl and AlCu materials

Abstract The present study investigates the mechanical and metallurgical properties of friction-welded steel-aluminum and aluminium-copper bars, a friction welder having been designed and built for this purpose. The effects of the three main parameters: speed of rotation; friction load; and duration of welding; on the metallurgical and mechanical properties of the weld such as the yield, tensile and breaking strengths, are studied experimentally and statistically. The metallurgical properties of the weld are examined using electron and optical microscopy.

Laser heating mechanism including evaporation process initiating laser drilling

Abstract A laser heating mechanism initiating the drilling process is examined. The study includes conduction and convection effects and assumes a steady-state evaporation process. This enables estimation of the limits of the Fourier theory in laser drilling applications. Using the model described in the analysis, maximum temperature attained inside the material, nucleation, explosion process and drilling efficiency are predicted. The results obtained from the present work for the vapor front velocity are compared to those obtained from previous experiments. In addition, a laser drilling experiment is carried out while monitoring the surface evaporation process. This enables correlation of the theoretical results with the experiments.

Entropy generation in turbulent liquid flow through a smooth duct subjected to constant wall temperature

Abstract The entropy generation for a fully developed turbulent fluid flow in a smooth duct subjected to constant wall temperature is investigated analytically. The temperature dependence of the viscosity is taken into consideration in the analysis. The ratio of the pumping power to the total heat flux decreases considerably when fluid is heated and the entropy generation per unit heat flux attains a minimum along the duct length for viscous fluids. The results corresponding to the temperature dependent and constant viscosity cases are compared. It is found that constant viscosity assumption may yield a considerable amount of deviation in entropy generation and pumping power results from those of the temperature dependent viscosity cases, especially when highly viscous fluids are considered.

Wind power energy potential at the northeastern region of Saudi Arabia

In the present study the energy potential of wind for the Eastern Province of Saudi Arabia is investigated. A suitable Weibull distribution is generated based on the data obtained for a duration of one complete year at a costal location in northeastern Saudi Arabia. Comparison of this model is made with the Rayleigh distribution of wind power densities. Two horizontal-axis type of wind energy conversion systems which operate at fixed rpm are considered and a model of quadratic power output function is used. It is found that the error in using the Rayleigh approximation will be less than 10% of the full rated power density level.

Thermodynamics of laminar viscous flow through a duct subjected to constant heat flux

Entropy generation for a laminar viscous flow in a duct subjected to constant heat flux has been investigated. The temperature dependence of the viscosity is taken into consideration. The ratio of pumping power to total heat flux decreases considerably and entropy generation increases along the duct length for viscous fluids. Therefore, it may be shown that an optimum duct length may be obtained which minimizes total energy losses due to both entropy generation and pumping power. For low heat-flux conditions, entropy generation due to viscous friction becomes dominant and the dependence of viscosity on temperature must be considered in order to determine entropy generation accurately.

Effective thermal conductivity of frost during the crystal growth period

An effective frost thermal conductivity for the crystal growth period of frost formation was developed based on a simple model consisting of cylindrical frost columns surrounded by moist air. Being closely related to the frost layer density, the thermal conductivity of frost layer is found to be affected by the vapor diffusion inside the frost layer as well. The density of frost columns, on the other hand, depends on the sublimation temperature of ice crystals and the humidity ratio of air, therefore, the whole process of frost formation and the thermal conductivity of frost layer, in particular, is shown to depend on the controlling temperatures, i.e. plate temperature, air temperature, and air humidity ratio.

A statistical analysis of wind energy potential at the eastern region of Saudi Arabia

The energy potential of wind for the eastern region of Saudi Arabia is investigated based on measurements of a complete year data at a coastal location in eastern Saudi Arabia. A suitable Weibull distribution is generated and a comparison of this model is made with the Rayleigh distribution of wind power densities. Two horizontal-axis type of wind energy conversion systems which operate at fixed rpm are considered for the determination of the extractable wind power, and a model of quadratic power output function is used between the cut-in speed and rated speed. It is shown that small-scale wind energy systems are suitable in the eastern part of Saudi Arabia for power generation and irrigation purposes. Copyright

Transient heat conduction in semi-infinite solid with spatially decaying exponential heat generation

Abstract An analytical solution of transient temperature profile and heat flux variation for a semi-infinite body with a heat generation exponentially decaying with position is obtained. The initial temperature condition is assumed to be constant while the surface boundary either transfers heat to ambient by convection, is kept at constant temperature, or insulated.

Laser heating mechanism including evaporation process

Abstract A quasi-steady analytical solution is obtained for laser heating process. Conduction, convection and phase change processes are considered in developing the heat transfer equation. An iterative procedure is introduced to obtain the temperature rise and surface velocity variation with time. The study is extended to include the comparison of surface velocities predicted from the present study and obtained from the previous experiment.