Research Of External Film Cooling Performance Of Turbine Blade With Different Internal Cooling Structures

Most recent turbine rotor blades employ internal convection cooling and external filmcooling in order to reduce both surface temperature and heat flux. After flowing in internalchannel and absorbing the heat from the blade, the coolant is ejected and formed film toprotect outer surface, the outer film cooling performance may be strongly affected by thedifferent inlet velocity profiles, which are caused by different internal cooling structures.However, because of the internal and outer heat mechanism is quite different, extensivestudies have been carried out about internal cooling and outer cooling individually. Recently,there is a trend to design blade cooling structures elaborately as a result of the enhanceddesign request. It is necessary to find out the different film cooling performances withdifferent internal cooling structures and then exactly analysis the heat transfer in turbine blade,so that it is feasible to design the advanced and higher efficient gas turbine.In order to find out the effect of internal cooling structures on outer film coolingperformance, the experimental system with and without the internal cooling structures havebeen established. For every experimental structure, similar CFD simulation model has beenbuilt, and the flow mechanism of every model has been analyzed. The internal effect studiedin this paper includes different internal crossflow of the smooth channel and different ribangles or rib positions of the ribbed channel.The smooth channel case and the supply plenum case have been studied to research theeffect of the internal crossflow. The results show that the flow structures in the smoothchannel case have a big difference with the plenum case. At the entrance of film hole, aportion of coolant turn down and flow out of the cooling hole after impacting onto the wall,what means by this is a block occurs. In the hole, helical motion of secondary flow isobserved, the stream line closed to the wall has bigger screw-pitch and the stream in themiddle of the hole has smaller screw-pitch. As a result of the special stream line, a littlecoolant swirls into the hole on the exit with the negative Z-velocity component, which is theexit block. The cooling-air jet is split into two lumps after being blown out of the hole, andthe two lamps form to be a pair of vortices respectively, which affects the distribution of filmcooling effectiveness and heat transfer coefficient directly.At smaller blowing ratio, the filmcooling effectiveness decreases with the increase of crossflow; in the contrary, the filmcooling effectiveness is enhanced at larger crossflow at bigger blowing ratio. And the heattransfer coefficient adds with the enhanced crossflow. As to the discharge coefficient, in thesmooth case, because the coolant has to overcome the momentum and turn over a large angle to flow into the film cooling hole, large loss appears and then the discharge coefficient issmaller.The135°ribs case and the45°ribs case have been studied to research the effect of the ribangles. It can be observed that strong secondary vortices appear in the secondary crossflowchannel when the walls are roughened by rib, and the secondary vortices swirl in differentdirection at the different rib angle. In the135°ribs case, the vortex in the upper half region ofthe secondary flow channel rotates clockwise, which is in the same direction with the holeinclination direction. In the45°ribs case, the vortex close to the film hole rotatescounterclockwise, which is in the opposite direction with the hole inclination angle. Theinternal vortices affect the velocity distribution at the hole entrance, in the135°ribs case, highZ-velocity appears at the left side of the entrance and at the right side is the negative part,which is contrary in the45°ribs case. In the hole, stream lines of the135°ribs case arestraight at small blowing ratio, and some helical lines appear at bigger blowing ratio alonewith two lumps after blown out of the hole; the stream lines of the45°ribs case are muchsimilar with the smooth case. The different flow structures of the jet after being blown out ofthe hole result of different vortices structures, and then the distributions of film coolingeffectiveness and heat transfer coefficient are different too. In the based cylindrical hole, the135°ribs case has the highest film cooling effectiveness at small blowing ratio; however, atbigger blowing ratio, ribs reduce the film cooling effectiveness. In the lateral inclinedcylindrical hole, the45°ribs case has the highest film cooling effectiveness at every blowingratio. As to the heat transfer, the135°ribs case has the lowest heat transfer coefficient andthat of the45°ribs case is the highest. And the highest discharge coefficient appears in the135°ribs case and that of the45°ribs case is the lowest.Three different positions of the rib to film hole have been analyzed to research the effectof the rib positions. Changing the relative position of hole and rib, moving the hole from closeto the previous rib to close to the back rib, the coolant flowing into the hole forms muchsmaller swirls in the channel, correspondingly, the swirls on the entrance decrease too. As aresult, the inlet block reduces and the discharge coefficient decreases. At each rib angles, thechange of rib position has less influence on jet flow structures, and then the distributions offilm cooling effectiveness and heat transfer coefficient are much similar with each other.The film cooling performances of the based cylindrical hole and the lateral inclinedcylindrical hole have been studied to analyze the effect of the internal crossflow and the ribangles. Compared the results of two film holes, it can be concluded that: the dischargecoefficients of the lateral inclined cylindrical hole with four internal structures are all much higher than these of the based cylindrical hole; the film cooling effectiveness of the lateralinclined cylindrical hole is higher than the based cylindrical hole in the supply plenum caseand45°ribs case, however that is smaller in the smooth case and135°ribs case. In order toacquire higher film cooling performance with different internal structures, three advancedshape holes have been proposed based on the lateral inclined cylindrical hole:(1) expandingthe hole exit with a column along the lateral direction;(2) expanding the hole exit with acolumn along the X-coordinate;(3) integrate expansion (1) and (2). Results show that theadvanced hole (3) can enhance the lateral film coverage and prevent the decrease of filmcooling effectiveness along the flow direction, therefore significant enhancement of filmcooling effectiveness can be acquired. The film cooling performance of the best advancedhole (Advanced hole3) of the smooth case has been experimental measured. Results showthat: the film is quite well in every case with more than100%of the film coolingeffectiveness enhanced than the lateral inclined cylindrical hole; and the hole dischargecoefficient is a litter higher than the lateral inclined cylindrical hole.