Abstract. In the oil and gas industries, understanding the behaviour of a flow through an annulus gap in a vertical position, whose outer wall is stationary whilst the inner wall rotates, is a significantly important issue in drilling wells. The main emphasis is placed on experimental (using an available rig) and computational (employing CFD software) investigations into the effects of the rotation speed of the inner pipe on the axial velocity profiles. The measured axial velocity profiles, in the cases of low axial flow, show that the axial velocity is influenced by the rotation speed of the inner pipe in the region of almost 33% of the annulus near the inner pipe, and influenced inversely in the rest of the annulus. The position of the maximum axial velocity is shifted from the centre to be nearer the inner pipe, by increasing the rotation speed. However, in the case of higher flow, as the rotation speed increases, the axial velocity is reduced and the position of the maximum axial velocity is skewed towards the centre of the annulus. There is a reduction of the swirl velocity corresponding to the rise of the volumetric flow rate.

In order to improve the efficiency of steam boilers in modern power plants, engineers have to use contemporary materials that enable exploitation at higher parameters, primarily operating temperature and pressure, with resistance to different forms of corrosion at the same time. For that purpose, contemporary 9-12Cr martensitic steels, designated P91, P92, E911, VM12-SHC, and SHM12, were developed. The paper analyzes experimental research on the behavior of commercial ferritic steel P91 samples. Behavior data on the heat affect zone (HAZ) of welded joints were obtained by testing smooth specimens produced by simulation of the new material welding. A survey of testing hardness and tensile properties is given and images of the fracture surfaces of the samples were processed. Image analysis with ImagePro Plus confirmed experimental testing results for the grain size measured by the circle method and a comparative relationship between the percentage of carbides, precipitates, and lamellar diameters with respect to temperature is provided.  The material that was PWHT (post-weld heat treatment) has the highest mean value of lamellar diameter, which indicates increased toughness and decreased hardness compared to BM and material without PWHT i.e (The higher lamellar diameter, the smaller material hardness).