| As an important strategic equipment in the field of national defense,energy and power,the design and manufacturing level of gas turbines and aero-engines are the concentrated reflection for the technological level of national scientific and industrial might.As the core hot-end working parts,the turbine blades of gas turbines and aero-engines bear extremely high thermal loads due to the harsh working environment.According to statistics,the amount of air pumped by the compressor stage for turbine cooling has reached 20%,which will inevitably affect the performance of the engine.Therefore,in the face of the increasing cooling demand caused by the rising turbine inlet temperature,it is particularly important to develop cooling structure and cooling technology of high efficiency and low flow resistance to reduce the thermal load of turbine blades,decrease the consumption of cold air,and ensure the safe operation of the engine.In view of this,this thesis adopts numerical computation method to study the flow and heat transfer mechanism of combined internal cooling structures,including pin-finned channel for turbine blade trailing region,ribbed U-shaped channel for mid-chord region,and reflux impingement cooling structure for leading region,and analyze the heat transfer enhancement and influence mechanism of different structural characteristic parameters.Finally,the conjugate heat transfer computation method is used to verify the application of the mechanism cooling structures and research conclusions of simple model on real blades and under real operating conditions.Firstly,for the trailing region of turbine blade,a curved pin fin with smaller flow resistance is proposed.Compared to upright pin fin channel,the friction factor in curved pin fin channel is decreased by 20%～30%,which makes maximum improvement in comprehensive heat transfer performance exceed 10%.The influences of the curved angle and length of vertical section on friction factor and heat transfer performance are discussed.The effects of Coriolis force caused by rotation on heat transfer are also considered.By analyzing the flow structure,the action mechanism and effects of jet-like secondary flow in the channel with curved /inclined pin fin are found and expounded,as well as the balance mechanism of longitudinal secondary flow and jet like secondary flow under rotating conditions.At the same time,the comprehensive heat transfer performance of the spherical dimple after being introduced into the channel of the pin fin arrays is investigated.It is found that dimple arranged in front of pin fin arrays is conducive to the development of the horseshoe vortex,while dimple in the middle of pin fin arrays is conducive to the development of the counter-rotating secondary vortex inside the dimple,both of which show good heat transfer enhancement performance.Secondly,for ribbed U-shaped channel in the mid-chord region of turbine blade,the influence of channel geometric characteristics on the rib heat transfer effect is studied,including the inclined divider wall,guide vane in the bend and suction effects of bleed holes.The specific investigated variables include inclined angle of the divider wall,shape of the guide vane,the arrangement pattern of the ribs,arrangement pattern of the film holes,and so on.The heat transfer effects of various characteristic parameters are summarized,and the friction factor and comprehensive heat transfer performance are evaluated.The research results indicate that the inclined divider wall causes changes in the flow crosssectional area,changing airflow velocity and increasing channel resistance loss.It is shown that the increase of friction loss in the ribbed U-shaped channel with divider wall inclined by 3° in clockwise direction or counter-clockwise direction,has exceeds 30% and 15%,respectively.The introduction of guide vanes can suppress the separation of airflow after passing through the divider wall and improve the uniformity of downstream airflow flow.Therefore,the friction factor in the ribbed channel is decreased significantly(18.6% in maximum).Under stationary condition,the arrangement pattern of the bleed holes has little effect on the local heat transfer between the ribs and overall heat transfer performance of the channel.However,the flowrate of each bleed hole at different positions shows significant differences,which inevitably affects the film cooling characteristics outside the blade body.Under rotating condition,the influence of bleed hole pattern on the local heat transfer level is more evident on the opposite side of the surface where Coriolis force acts.Furthermore,for the leading region of the turbine blade,an impingement cooling structure with return channel is proposed to suppress the negative impacts of accumulated crossflow on jet cooling,and the study is carried out around the outflow schemes,the "swirling and rib" heat transfer in the return channel,and the suction effects of the bleed holes.It is found that scheme with single-outlet outflow scheme and the inline return holes pattern shows the best heat transfer enhancement on the target surface.It is proved that the area-averaged Nusselt number on the target surface is improved by 27.3%～33.4%.The research on the heat transfer in the return channel shows that the strong swirl formed after the return flow entering the reflux channel,is helpful to apply scraping effects on the side wall,reducing the boundary layer and promoting the heat transfer enhancement,with areaaveraged Nusselt number improved by 5.4%～10.4%.The introduction of bleed holes reconstructs the impingement flow field,inhibits the development of wall-jet flow to a certain extent,and weakens the heat transfer performance on the impingement target surface with a maximum reduction by 8.1%.Finally,the conclusions of the mechanism research are applied to a certain type of turbine blade to verify the heat transfer performance of the mechanism research conclusions in the real blade under the real working condition.Research findings proves that the heat transfer enhancement of the inclined pin fin on the trailing region of the blade is significantly stronger than that of the traditional upright pin fin,which not only improves the uniformity of the temperature distribution,but also reduces the average temperature of the blade trailing region,making the area-averaged temperature on external surface of trailing region decreased by 8 K.Importantly,the local high temperature region of the trailing edge has been removed,while the heat transfer enhancement effect of end-curved pin fin is small due to influences of the incoming flow direction.The heat transfer enhancement effect of dimple in the middle position is more obvious in vertical and inclined pin-finned channels,while the heat transfer enhancement effect of dimples in the front position is more obvious in curved pin-finned channels.The research on the heat transfer in the mid-chord region of the blade shows that the addition of the guide vanes significantly reduces the friction loss of the U-shaped channel with NP rib turbulators,the pressure loss of which has been reduced by 2.4%.Therefore,the velocity of cooling air entering the trailing edge channel is improved and the temperature close to blade root has been reduced.Compared to traditional impingement jet cooling schemes,the impingement cooling scheme with return channel for the leading region of the blade has improved the heat transfer performance of the internal target surface,decreased temperature of the blade external surface and improved distribution uniformity,which proves the value and potential of the impingement cooling scheme with return channel for enhancing the leading region cooling.The addition of ribs is of great significance in reducing local temperature distribution.However,the heat transfer advantage of impingement channel with return channel is slightly decreased when film holes are arranged on the leading edge. |
