การศึกษาการถ่ายเทความร้อนของเจ็ทพุ่งชนในช่องการไหลแบบหมุน

Abstract

Gas turbine engines are widely used in electric generation in power plants and jet propulsion in an aircraft engine. In general, the gas turbine blade is the most critical part, because it must operate under high temperatures and high-rotational speed conditions. This leads to damage from high thermal loads. Nowadays, the turbine inlet temperature (TTT) is higher than the melting point temperature of the blade material. Therefore, the blades of a gas turbine must have a cooling system to protect its damage. Impinging jet is one of the methods for internal turbine blade cooling by using an array of jets impinging on the internal wall of the blade, which gives high heat transfer rate. The main objective of this research is to study flow and heat transfer characteristics of impinging jets in the rotational channel to simulate the cooling system of gas turbine blades under rotation conditions. In this study, a row of 13 orifice jet holes with diameter, D = 5 mm were used to impinge on the wall in a channel having width and pitch distance between orifices at 5 = 4D. The study parameters included jet Reynolds number Re = 6,000, 7,500, and 9,000, jet-to-impingement distance H = 2D, 4D, and 6D, rotational speed of the channel N = 0, 50, 100, 150, 200, and 250 rpm (under rotation number R-0 to 0.0069). Besides, the effect of the rotation direction was studied for two types: same with jet flow direction and opposite to jet flow direction. And, three patterns of flow channel outlet were investigated: one-way flow exit and the flow direction toward the rotation axis, one-way flow exit and the flow direction outwards the rotation axis, and two-way flow channel exits and the flow direction towards and outwards the rotation axis. For heat transfer measurement, thermochromic liquid crystals (TLC) sheet which has a property of changing colour according to temperature was applied with image processing technique to evaluate temperature on the impingement surface with constant heat flux. The flow characteristics were also investigated by the computational fluid dynamics software, ANSYS ver. 15.0 (Fluent). The results show that the heat transfer characteristic of impinging jets in the case of a two-way flow exit pattern at the jet-to-impingement distance H = 2D under stationary case (R,-0.0) gave the highest of average Nusselt number when compared to other flow exit patterns and the different jet-to-impingement distances. Besides, it gave the most uniform Nusselt number distribution on the impingement surface. The average Nusselt number in the case of Re = 9,000 and impingement distance H = 2D was higher than the case of one-way flow exit with flow direction towards the rotation axis and the case of one-way flow channel exit with flow direction outwards the rotation axis about 10.17% and 10.56%, respectively. Because the case of two- way flow exits gained well airflow and the jet flows were less influenced by the crossflow. This can be observed from the flow, and jet flow velocity at the exit had a similar velocity for each jet holes. On the other hand, the case of one-way flow exit at the jet position near the channel exit, the cross-flow had a high velocity and also gained the strong influence of the crossflow, causing the jet flow was bent significantly. In the case of rotation, it was found that the two-way flow exits at the jet-to- impingement distance H = 2D, the Coriolis force acting on crossflow in the flow channel, was cancelled. Only the centrifugal force did not interfere with heat transfer in the flow channel at low impingement distance. For case of rotation number R, = 0.0037, the average Nusselt number for rotation direction at the same direction and the opposite direction of the jet flow was the higher than about 7.87% and 8.40% when compared to the stationary case.