علی ضیائی اصل

Due to the very high importance of gas turbine in electricity production and also its increasing development in the electricity industry, one of the most important methods to increase the efficiency of gas turbine blades is to add thermal barrier coating to their outer surface. These coatings improve the cooling of gas turbine blades, but they also have disadvantages such as roughness of their surface cause to the separation of the hot fluid flow around the turbine blades. The rough surfaces of thermal barrier coatings should be used optimally and purposefully. In this research, using commercial software of Fluent, this type of coating has been investigated and analyzed in order to find the most optimal possible mode for its use. The influence of surface parameters such as roughness, thickness and material of thermal barrier coatings have been investigated. The most optimal state for the mentioned parameters are: 0.1 mm, 0.38 mm and 2.7 W/m.K respectively.

In this paper, experimental data have been used to develop a semi empirical relationship for double-ellipsoidal heat source to model the welding process of a T-shape fillet weld of carbon steel AISI 1020 and stainless steel 304. This model is used in a finite element based computer code to simulate the three dimensional welding process and obtain the temperature profile around the weldment. Experimental data in the form of temperature for certain points have been recorded during the welding process using a computerized data processing system which has been designed for this purpose. Also, the thickness of the weldment layers has been compared by observing their hardness and crystallography. By comparing experimental data with numerical result, the coefficient of the model has been determined using “model updating” process. The effects of material properties and welding parameters have been studied to insure the generality of the model. This model can be used to evaluate the quality of the welding and thickness of the heat affected zone as well as the risks during the welding process such as burn-through and hot cracking. The main advantage of this model is that the number of coefficients is reduced to only one parameter and the rest have been related to the physical and geometrical characteristics of the weld. Results of the numerical simulation obtained using this model show that the major factors which affect the temperature distribution around the weldment are material conductivity, plate thickness, input heating and welding speed.