Research On Fluid Flow And Heat Transfer Of Film Cooling With Chemical Heat Sink

Increased turbine inlet temperature further is much need in order to improve the performance of gas turbines. However, so high operating temperature is far above the permissible metal temperature of turbine blades under current technical conditions. Therefore, cooling methods are employed for turbine blades to bear high turbine inlet temperature, and increase the components’ safety and durability. It has significant implications for improving the performance of gas turbines and prolonging their service lives. Nevertheless, the benefits by using the conventional methods are likely to approach their limits, and cooling stream is not likely to be supplied infinitely, so temperature drop for turbine blade cooling is hard to continue to be improved. At present, the developments of advanced cooling methods with high efficiency have become one of the main supporting technologies in high-performance gas turbines.In order to promote a jump-over development of the film cooling technology, and satisfy the need of turbine blade cooling for aero-engines, an efficient cooling scheme is proposed based on the concept of heat transfer enhancement breaking through the limitation of a single cooling medium in the traditional film cooling method. This project is under the support of the item of National Natural Science Foundation of China "Investigation on Flow and Heat Transfer Characteristics of Film Cooling with Chemical Heat Sink"(No.50976118). The basic model of this method is that cooling stream contains some kind of gaseous substance that can absorb heat and decompose. When cooling stream ejects through discrete holes on the blades’ exterior surfaces forming a protective layer, once the conditions, such as temperature et al., are satisfied, the endothermic chemical reaction will take place inside the protective film boundary layer, and the reaction products are gaseous substances. It has great effect on film cooling effectiveness through internal heat sink, volume changes and radiation absorption in this process.In this paper, a simple ideal model with heat sink was adopted, and the convective heat transfer model was set up. The volume changes were summed up to the corrected term before and after the chemical reaction, and the convective heat transfer model was corrected. The novel film cooling with chemical heat sink was simulated by using the corrected standard k-ε turbulent model. The results showed that the more reaction heat is, the more film cooling effectiveness is improved. The faster reaction rate is, the more film cooling effectiveness is improved. The proposed method not only has the higher cooling effectiveness in the stream wise direction, but also has the wider film coverage in the spanwise direction, which is significantly different from the conventional one.Further, it was proposed that NH3is the best cooling stream for realizing film cooling with chemical heat sink. There are some advantages for film cooling with chemical heat sink produced by the decomposition of NH3:(1) Reaction products N2and H2don’t have carbon deposition problem, but hydrocarbons have.(2) The decomposition temperature is close to blade materials creep one, and fast decomposition temperature range of NH3with no catalyst is between1100～1300℃.(3) It can absorb more heat during the chemical reaction process, and enthalpy change is+46.4kJ/mol in the standard condition. The process of the high temperature decomposition of NH3used for cooling enhancement was simulated in detail. The results showed that the film cooling effectiveness is maintained at a high level due to the existence of chemical heat sink produced by the decomposition of NH3. When the blowing ratio is0.5, the film cooling effectiveness is higher than0.78downstream of the film hole. Compared with the conventional film cooling, the average cooling effectiveness at X/D=30is improved by73.78%for this novel method when the blowing ratio is1.5. Volume expansion occurs after the chemical reaction of NH3, and the repulsive forces from cooling stream to mainstream are much stronger, which is helpful for improving the cooling effectiveness and protecting the blade surface. The property of radiation absorption of NH3contributing to cooling effectiveness was also considered.The activation energy of the decomposition reaction of NH3is so high that the conversion rate is very low under the condition of no catalyst and low temperature. Therefore, it is hard to experimentally research film cooling using the endothermic reaction of NH3as heat sink. For this reason, oxalic acid is chosen as a model to match. The endothermic reaction of oxalic acid can take place under low temperature, and the products are CO, H2O, and CO2. This substance is suitable for film cooling with chemical heat sink, and the experimental data obtained are fully capable of modeling the decomposition of NH3. The conventional film cooling using air as cooling stream was adopted in the experiment as reference, and the effect of chemical heat sink produced by the decomposition of oxalic acid on film cooling effectiveness was investigated under different blowing ratios, different Reynolds numbers, and different density ratios. The results showed that in the area of0<X/D<10, film cooling effectiveness is between0.28～0.52for the method with chemical heat sink when the blowing ratio is1.0, but0.15～0.3for the conventional one.In addition, the impingement/effusion compound cooling with chemical heat sink on the leading edge of turbine blade was simulated based on the above-mentioned research. Compared to the conventional one, the cooling effectiveness is highly improved, and is improved more downstream of the leading edge than in the other regions. It also demonstrated that compound cooling effectiveness is improved with the increase of temperature ratio due to the existence of chemical heat sink.