Conjugate Heat Transfer Characteristics Of Gas Turbine Combustor Effusion Cooling

High temperature rise and low emission is the significant trend for modern gas turbine combustor development.In this context,amount of air used for combustor cooling reduces greatly.So it is an urgent problem that how to provide better cooling protection with less cooling air consumption needs solved.Effusion cooling is realized via perforating numerous discrete holes on the combustor liner wall to form quasi-porous structure,which can remove a great amount of heat load on the liner by convection inside thousands of small holes as well as uniform film coverage on the main flow side in the combustor.It shows great advantage of performing high efficient cooling behavior with little cooling air.Besides,the development of the machining process nowadays,such as fast laser machining and additive manufacturing,makes real application in the industry,causing effusion cooling is regarded to be one of the most promising cooling method for gas turbine especially aero-engines combustors.The present study investigated the effusion cooling of gas turbine combustor,analyzed the conjugate heat transfer properties with both experimental and numerical implementation.Firstly,experimental work was carried out to assess the conjugate heat transfer characteristics at non-reacting/reacting flow conditions.IR temperature methodology was adopted to measure the overall cooling effectiveness on the liner.Effect of effusion jets injection direction(forward,normal and backward)on flow characteristics and cooling performance was examined by adjusting the inclination angle of the cooling holes(30°,90°and 150°).The studied range of pressure drop across the liner was 0-2.5%.The results show that mass flow rate as well as discharge coefficient is increased with pressure drop across the effusion cooling plate.No matter at non-reacting or reacting flow conditions,discharge coefficient for normal holes is higher than inclined holes.Expanding swirl main flow impinges on the liner wall somewhere,resulting cooling effectiveness drops largely in the impingement region.It is because that impinging itself owns high heat transfer coefficient,in addition,the stagnation pressure inhibits injection of the effusion flow leading to intensity of convection inside the holes decreasing with poor film coverage on the hot side.At non-reacting flow conditions,backward effusion cooling holes get the highest cooling effectiveness,while normal holes show worst cooling performance at the same pressure drop across the perforated liner.However,at the reacting flow conditions,forward and normal effusion holes characterize best and worst cooling behavior respectively at relatively lower effusion plate pressure drop.As the pressure drop is increased to 1.5%,the great increment of cooling air for normal holes makes the overall cooling effectiveness get higher than forward cooling holes configuration.Meanwhile,the backward effusion cooling scheme gets the lowest overall cooling effectiveness.Backward effusion injection can't show its advantage on improving cooling performance in corner recirculation zone and impinging zone,the enhancement on cooling effectiveness only shows up at more cooling air flow scenario.Secondly,deep going comparison analysis was conducted based on experimental results for non-reacting and reacting flow conditions.Aixal location and its shift of the lowest laterally averaged cooling effectiveness was characterized.Modification for cooling effectiveness at the reacting flow conditions was proposed based on the non-reacting results.Numerical simulation with Reynolds Averaged Navior Stokes method was also performed to predict the near-wall flow feature and thus to reveal the mechanism behind of difference between two conditions and the interaction between main flow and effusion jets.It can be concluded from the results that cooling efficiency evaluates the cooling contribution by every amount of cooling air,which goes down as effusion plate pressure drop increases for all three inclination configurations at both non-reacting and reacting flow conditions.Backward cooling holes configuration has the highest cooling efficiency while normal holes owns the lowest cooling efficiency.Forward effusion holes tend to form film well attached on the wall,so it shows limited detrimental effect on the main flow structure.Normal holes will uplift the main flow near the combustor outlet as the film accumulation there.It has to say backward effusion cooling influences the main flow most because upstreamly moving film and downstreamly developed main flow will encounter and strong shear effect between them helps to form a coherent vortex structure above the liner wall,which influences the main flow to a great extent.The competition between main flow and backward effusion jets in terms of momentum flux decides whether the reverse cooling jets can cover the impinging zone and corner recirculation zone,and improves the cooling performance locally as a consequence.It also determines whether the lowest laterally averaged overall cooling effectiveness point will move upwards.Finally effusion cooling performance on three-injetor annular combustor liner wall was experimentally examined to assess the different interaction effect of main flow and effusion flow on outer and inner liner wall because of their opposite curvature close to more realistic combustor configuration.IR detected the temperature distribution on the inner and outer liners,and overall cooling effectiveness was evaluated to compare.Influence of cooling air amount and holes array(inline and staggered arrangement)on overall cooling effectiveness were studied.It is concluded that difference of curvature results in different radiation load on the liner wall,to be specific,the inner liner gets higher temperature than the outer liner.Laterally averaged overall cooling effectiveness increases as moving to downstream due to superposition effect on the outer liner,but this effect is absent on the inner liner.It is believed that effusion film is easier to detach from the inner liner wall.In general,staggered effusion holes array performs better cooling performance in the circumferential direction.To sum up,the present study evaluate the effects of primary parameters on effusion cooling performance at reacting flow conditions,which is more similar to the realistic working conditions of combustor.Meanwhile,correction method is proposed to relate overall cooling effectiveness at non-reacting and reacting flow conditions.It is believed to be beneficial to support real gas turbine combustor effusion cooling design with non-reacting experimental data from the laboratory.Finally,backward effusion hole configuration is proved to be potential locally optimization options for combustor liner.