Original Article Parametric study of injection from the casing in an axial turbine Sarallah Abbasi 1 and Afshin Gholamalipour 2 Abstract Tip leakage flow reduces both efficiency and performance of axial turbines and damages turbine blades as well. Therefore, it is of great importance to identify and control tip leakage flow. This study investigated the effect of flow injection (from the casing), alongside flow structure, on turbine performance. Additionally, the effect of different injection parameters, including injection mass flow rate, angle, location, and diameter on the turbine performance are evaluated. A numerical analysis of the flow in a two-stage axial turbine was employed by using CFX software. To ensure the accuracy of the results, turbine performance curves were compared with the experimental results, which are in good agreement. Analyses revealed that active control method reduces tip leakage flow, improves turbine performance, and increases the efficiency by 1% to 5% as well. A parametric investigation of the tip injection has sought to identify how various parameters affect the turbine performance. The cross-section diameter and the angle of injection had no significant increase on efficiency. Additionally, results showed that at a point 9mm further from the leading edge, the injection degree of effectiveness is optimum. Finally, analysis of the flow structure in the tip clearance region supported the tip leakage flow reduction. Keywords Axial turbine, tip leakage flow, injection, numerical simulation, parametric study Date received: 13 December 2018; accepted: 26 August 2019 Introduction Axial flow turbines are widely used in most gas tur- bines due to their high efficiency. Till date, tip leakage flow and its effect on the performance of turbines have attracted the attention of many researchers. The entry of hot gas into the tip clearance causes the formation of a boundary layer with high heat transfer rate, which leads to the deficiency of blades. This is con- ducted in a way that the edge of the blade high-pres- sure surface is gradually destroyed and the tip clearance is increased. On the other hand, the leakage flow along with the mainstream leads to the formation of a vortex flow at the suction side of the blades. This phenomenon has a significant effect both on the flow field of the blade tip and the blockage of the main- stream. This also reduces turbine efficiency. 1–4 Blockage and loss caused by tip leakage flow can even account for up to one-third of the total losses. 5 Therefore, controlling the leakage flow is necessary for the reduction of losses. Tip leakage flow control methods can be divided into two general categories, passive and active control methods. In the former, tip leakage flow is controlled by making changes in the tip geometry or tip clearance size. But, in the latter method, tip leakage flow is controlled by applying energy such as air injection. 6–8 On the other hand, the rotor and stator blades can be cooled internally by the compressor air. Therefore, higher temperatures for gas exiting from the combustion chamber can be observed and the efficiency can be increased as well. As a result, there are two important factors affecting the turbine efficiency including the leakage flow of the blades tip and the temperature of the flow entering the turbine. Till date, a number of studies have investigated gas turbines due to their wide application in various industries. Extensive experiments, thereby, are desig- nated to improve its efficiency and performance. For instance, Tang et al. 9 and Newton et al. 10 carried out 1 Department of Mechanical Engineering, Arak University of Technology, Arak, Iran 2 Faculty of Mechanical Engineering, K. N. Toosi University of Technology (KNTU), Tehran, Iran Corresponding author: Sarallah Abbasi, Department of Mechanical Engineering, Arak University of Technology, Arak, Iran. Email: s_abbasi@arakut.ac.ir Proc IMechE Part A: J Power and Energy 0(0) 1–12 ! IMechE 2019 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0957650919877276 journals.sagepub.com/home/pia