The leading edge of turbine blades is one of the critical areas that need to be cooled effectively because of the high local heat transfer rate of the main flow. Film cooling with different shaped holes as well as internal cooling by impinging jets has successfully been applied in modern gas turbine applications. This paper numerically studies the cooling of the leading edge with a row of dual impinging jets — two jets close to each other. Heat transfer of the dual jets is compared to that of a single jet (in a row) based on the same flow rate or jet velocity. The effect of the distance between the dual jets and the jet inclination angle is examined to seek the best geometric parameters. In addition, the curvature of the leading edge surface is considered to examine the heat transfer difference between curved and flat walls. Various jet-to-target spacing and Reynolds numbers are also studied. Results show that the dual impinging jets generally produce two high heat transfer regions in the stagnation point, and the peak value is slightly higher than the single row of jets with the same Reynolds number. When the distance between two jets is 3d, the jet flow after bouncing back from the symmetry line affects the heat transfer as a crossflow. The target surface curvature has little effect on the overall heat transfer, but the peak heat transfer coefficient is lower on the curved surface than that on the flat surface. The dual impinging jets present a higher average heat transfer around the stagnation region.
Gas turbines have been widely used in power generation and aircraft propulsion. To improve the gas turbine performance, the turbine inlet temperature is usually elevated higher than the metal melting point. Therefore, cooling of gas turbines becomes very critical for engines' safety and lifetime. One of the effective methods is film cooling, in which the coolant air from the discrete holes blankets the surface from the hot gas flow. The major issues related to film cooling are its poor coverage, aerodynamic loss, and increase of heat transfer coefficient due to strong flow mixing. To improve the cooling performance, this paper examined film cooling with backward injection. It is observed that film cooling with backward injection can produce much more uniform cooling coverage under different conditions, which include cases on flat surface with low or high pressure and temperature. The backward injection also performs better in the presence of blade curvature. The effect of other parameters on the film cooling is also reported. The numerical results are validated by simple experimental test in this study.
Due to its high heat and mass transfer rate, especially near the stagnation region, jet impingement has been widely used in many applications, such as annealing of metal and plastic sheets, cooling of gas turbine blades and electronic components. However, the high heat transfer rate can be strongly affected by the crossflow coming from either the exhaust fluid in an array of jets or other sources. On the other hand, the inclination angle of injection also influences the jet impingement flow and heat transfer, and an inclined jet towards the crossflow may strengthen jet penetration. The combined effect of the crossflow and inclination angle on jet impingement heat transfer makes the problem even more complicated. This paper studied this effect numerically with various parameters, including the velocity ratio of jet to the crossflow, inclination angles, the crossflow temperature as well as the jet-to-target spacing. The heat transfer coefficient distribution and its peak value at the stagnation region are examined and compared. Results show that at small jet-to-target spacing the crossflow can enhance the heat transfer, while at large jet-to-target spacing the heat transfer is reduced due to the crossflow as well as inclination. In some cases jet impingement heat transfer can be higher with an inclined angle than that of a normal jet, given the same crossflow ratio.
The jet fire caused by the leakage of combustible materials is one of the biggest threats to the safety of chemical plants. Thermal radiation of the jet fire brings severe damage to nearby facilities and people's health. To evaluate the damage of jet fires, a precise model for the calculation of heat radiation is indispensable. Classical thermal radiation models of jet fires either have a lower prediction accuracy or a higher computation complexity. To overcome such deficiencies, this paper proposes a novel segmented line heat source (SLHS) model for jet fires. Because the length of the jet fire is often much larger than the width, the jet fire is viewed as a line heat source, with all the heat radiated from the centerline of the jet fire. The jet fire is divided into three segments along the flame length according to the temperature distribution and thermal radiation characteristics of the flame. Based on the SLHS model, three types of thermal radiation models, called cone-cylinder-cone, ellipsoid-cylinder-ellipsoid and ellipsoid-cylinder-cone models, are built for computing the radiant heat flux distribution around the jet fire. The effectiveness and advantages of the proposed models are illustrated with the experimental data and a numerical simulation.
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