Research On Round-to-Slot Hole Film Cooling Performances In Pressure-Gradient Primary Flow

As an efficient cooling scheme,film cooling plays a dominant role in improving the overall cooling effectiveness of turbine blades.The innovation and application of shaped holes has shown their potentials on improving the film coverage both in the streamwise and spanwise direction and resulting in better cooling uniformity of blades,which is of great significance for reducing the cold air consumption and thermal stress of blades.The converging slot-hole is a novel and efficient shaped hole for film cooling which is generated from a circular shape at the inlet to a narrow slot at the exit.It has been demonstrated to be able to effectively improve film covering uniformity relative to the conventional cylindrical hole and some diffused holes.However,this is at the expense of greater pressure drop of coolant jets across the film hole and hence the discharge coefficient is reduced.For this case,the concept of round-to-slot hole(RTSH)is proposed in the current study,which is similar to the converging slot-hole.The exit area of the round-to-slot hole may be smaller,comparable or larger relative to the inlet area by means of changing the geometrical parameters of the exit slot so that the round-to-slot holes are correspondingly generated with converging,equivalent-area and diffusing configurations.In the current study,the cooling characteristics of some typical round-to-slot holes subjected to different primary flow circumstances with pressure gradients were investigated and compared with the conventional cylindrical hole and the fan-shaped hole.The main contents and conclusions are summarized as the following.Firstly,experimental tests and numerical simulations were both performed to investigate the film cooling performances of single-row round-to-slot holes on a flat plate in the streamwise pressure gradient flow,including four different round-to-slot holes.In addition,the cylindrical hole and the fan-shaped hole are also studied as the benchmarks for comparisons with the round-to-slot holes.The results show that,apart from the conventional kidney-shaped vortices,another pair of anti-kidney vortices with the opposite sense of rotation is formed downstream of the round-to-slot holes,which effectively prevents the high temperature mainstream from intruding the coolant-protected wall from both sides of the coolant jet,enhancing the spreading ability of the coolant along the spanwise direction,and effectively suppressing the normal penetration of coolant jet into the primary flow.With the increase of the exit-slot length,the vortex scale and vorticity of the anti-kidney vortices downstream of the round-to-slot holes increase gradually,which play a leading role in the flow mixing process and are beneficial to the enhancement of the lateral film coverage uniformity.Thus,the film cooling effectiveess in the region between adjacent holes is remarkably improved.However,the flow spreading ability of the coolant jet along the streamwise direction is relatively reduced at the same time.The advantage of the round-to-slot holes over the cylindrical hole and the fan-shaped hole behaves more significantly under high blowing ratios.For the cylindrical hole,the favorable pressure gradient is beneficial to restraining the coolant jet lift-off and improving the film cooling effectiveness,and the adverse pressure gradient is the opposite.For the fan-shaped hole,the average film cooling effectiveness is slightly improved subjected to the adverse mainstream pressure gradient under low blowing ratios while the favorable mainstream pressure gradient is beneficial to the suppression of normal penetration of the coolant jets and improvement of mean film cooling performance.For the round-to-slot holes,the film coverage is improved along the spanwise and streamwise direction in the adverse pressure gradient primary flow,and hence the film cooling effectiveness increases over the entire region.However,the favorable mainstream pressure gradient leads to reduced film cooling area and thus the mean cooling performance decreases.As a whole,the cooling advantage of the round-to-slot holes over the cylindrical hole and the fan-shaped hole behaves more mildly under the favorable pressure gradient relative to the zero pressure gradient while more obviously under the adverse pressure gradient.The favorable mainstream pressure gradient leads to reduced discharge coefficients under small blowing ratios and the opposite tendency is obtained under the adverse pressure gradient.Under high blowing ratios,the mainstream pressure gradient has weak influence on the discharge coefficient of the film cooling holes.The discharge coefficient of the converging RTSH-1 is lower than that of the cylindrical hole and the discharge coefficient of the equivalent-area RTSH-2 is comparable.By contrast,the discharge coefficients of the diffusing RTSH-3 and RTSH-4 are higher than that of the cylindrical hole,indicating that the pressure loss inside the holes is well reduced with good cooling cooling improvement.Secondly,the film cooling characteristics of single-row round-to-slot holes on the guide vane surface in the turbine cascade flow were explored by means of experimental and numerical methods and compared with the cylindrical hole and the fan-shaped hole.The results show that,under the same nominal blowing ratio,the relatively high mainstream velocity at the suction side has obvious restraining effect on the scale of the jet vortex and the coolant jet lift-off relative to the pressure side.The jet penetration into the primary flow is obviously suppressed at the suction side,producing better film adherence and thinner film layer protection.Compared with the balde pressure side,the film jet separation for the cylindrical hole is obviously delayed on the suction side and thus the entire film cooling effectiveness is generally higher than that on the pressure side.With regard to the fan-shaped hole and round-to-slot holes,the mean cooling effectiveness on the suction side is generally lower than that on the pressure side except for the vicinity of the hole-exit.In the scope of currently investigated blowing ratios,all the round-to-slot holes are capable of improving the film cooling performance obviously on the pressure side and the equivalent-area RTSH-2 is suggested to be the most favorable hole-shape for achieving the optimal film cooling effectiveness.On the suction side,the benefit of round-to-slot holes only behaves obviously under high blowing ratios and the diffusing RTSH-3 shows the best mean film cooling performance.It is also demonstrated that the exit-inlet area ratio is a dominant factor on the bleed air pressure and the discharge coefficient.The discharge coefficient of each film hole is generally higher on the pressure side than that on the suction side.Thirdly,a numerical investigation was conducted on the multi-row film cooling of the turbine guide vane under high-temperature and high-pressure primary flows.Six rows of shower-head holes were arranged at the blade leading edge and five and three rows of film cooling holes were arranged on the pressure side and the suction side,respectively.The influence of changing the film-hole shapes on the pressure side and the suction side were emphasically discussed on the film flow and cooing performance.The results show that,under the same air bleed pressure,changing the film-hole shapes on the pressure side and the suction side has little influence on the discharge flow from the shower-head holes at the leading edge.Once the film cooling holes on the pressure surface and the suction surface are replaced by the converging round-to-slot holes,the coolant massflow rate of each film-hole row is less than that of the cylindrical holes and the energy loss coefficient of the cascade channel is reduced by about 4.2% relative to the cylindrical hole.When the fan-shaped holes or the diffusing round-to-slot holes are arranged on the pressure surface and the suction surface,the coolant massflow rate is more than that of the cylindrical holes.The energy loss coefficients of the cascade channel increase by about 3.2% and 5%,respectively.The equivalent-area round-to-slot hole produces approximately the same coolant massflow rate as the cylindrical hole while the energy loss coefficient increases by about 1.7% relative to the cylindrical hole.The development of the upstream film layer promotes the significant cooling effectiveness enhancement of each downstream film-hole row.However,the overall cooling superiority of the fan-shaped hole and round-to-slot holes relative to the cylindrical hole is also reduced compared with the single-row film cooling cases.The film cooling distribution of multi-row film cooling on the balde surface derived from the superposition algorithm is consistent with the numerical results,showing smaller deviations on the pressure surface.Considering the thermal conduction inside the turbine blade,the overall cooling effectiveness distribution on the blade surface is more uniform.Particularly,the overall cooling effectiveness at the leading edge is obviously enlarged relative to the adiabatic film cooling cases.On the pressure side and the suction side,the adiabatic cooling effectiveness and overall cooling effectiveness are very close to each other at the vicinity of the hole-exits for each film-hole row and the deviation increases gradually downstream of the film cooling rows.Finally,the film cooling characteristics of single-row round-to-slot holes on the pressure surface of a turbine rotor blade were numerically investigated under rotating conditions and compared with the cylindrical hole and the fan-shaped hole.The results show that a radial pressure gradient is clearly induced in the rotating cascade channel.The primary flow at the suction side remains the lateral flow while forms radial flow towards the blade tip at the pressure side.Under the joint action of the centrifugal force and Coriolis force,the coolant jets ejecting from the pressure side deflect to the balde tip so that the kidney-shaped vortices show asymmetric distributions.The cooling effectiveness on the blade surface reveals an obvious staircase distribution which increases gradually from the blade hub to the blade tip.For the cylindrical hole and the fan-shaped hole,the cooling effectiveness at the vicinity of the hole-exit is improved under the rotating state relative to the stationary case while leads to a slight reduction downstream with the increase of rotational speed.With regard to the round-to-slot holes,the mean film cooling effectiveness decreases monotonically with the increase of rotational speed and is more sensitive to the rotational speed under high blowing ratios.The round-to-slot holes can effectively suppress the normal penetration of the coolant jet into the mainstream either under the stationary state or under the rotational condition and perform notably better film cooling enhancement under higher blowing ratios.Relative to the stationary case,the film cooling improvement by the shaped holes is degraded a little under the rotational condition.