A corrugated heat shield in an aircraft engine combustion chamber

Abstract

The application discloses a corrugated heat shield in an aero-engine combustion chamber, comprising a corrugated heat shield and a row of film holes arranged thereon, the corrugated heat shield plate is in a cylindrical structure, a longitudinal corrugated cylinder is coaxially arranged inside a combustion chamber / nozzle cylinder, and an annular tapered gap between the outer surface of the corrugated cylinder and the inner surface of the combustion chamber / nozzle cylinder forms a cooling channel. The corrugated heat shield is in a periodic longitudinal asymmetric corrugated structure, each corrugation comprises a wave crest region and a wave trough region, the wave crest region is in a circular arc structure, and the wave trough region is in a flat structure, the corrugated heat shield is arranged with 4-6 rows of staggered film holes at equal intervals along the axial direction from the wave crest, and the row of film holes is unevenly distributed on the corrugated heat shield. The application can be used in an aero-engine combustion chamber, an afterburner or a nozzle, and can meet the cooling design requirements of the heat shield and improve the cooling efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to a combustion chamber in an aero-engine, in particular to a longitudinal corrugated heat shield arranged in the combustion chamber, and belongs to the technical field of combustion chamber cooling of an aero-engine. The longitudinal corrugated heat shield can be used in a combustion chamber, a reheat combustion chamber and a tail nozzle. BACKGROUND

[0002] Currently, a cylindrical heat shield with film cooling holes is usually used for cooling of a reheat combustion chamber of an aero-engine. The film cooling technology of a porous longitudinal corrugated heat shield on the surface of a cylinder is widely used in a reheat combustion chamber of a military aero-engine, which plays a role in suppressing oscillatory combustion and preventing high-temperature ablation of the combustion chamber cylinder. With the increase of the thrust of the engine, the gas temperature of the reheat combustion chamber is continuously rising, and the air flow for cooling is reduced. Therefore, it is necessary to develop a more advanced heat shield cooling structure to achieve better cooling effect with as little cooling air as possible, so as to improve the cooling efficiency of the engine combustion chamber.

[0003] Reference Figure 1 、 2 In early times, a flat plate structure was mostly used on the surface of the cylindrical heat shield, which has uniform cooling but poor thermal deformation adaptability. The periodic sinusoidal symmetrical longitudinal corrugated heat shield solves the problem of high-temperature thermal deformation of the heat shield, and the cooling air can stably stay in the valleys of the longitudinal corrugated plate, which plays a good protective role on the heat shield. However, at the same time, due to the enlarged cross section of the heat flow passage at the valley, a complex vortex structure is formed at the valley, and the mixing of cold and hot flows reduces the protection effect of the cooling film on the downstream wave crest. Existing researches have shown that increasing the length ratio of the windward side of the single period is beneficial to enhancing the protection effect of the cooling air at the valley on the downstream wave crest and improving the average comprehensive cooling efficiency of the corrugated heat shield.

[0004] In addition to the corrugated structure, different film hole arrangement modes also affect the cooling effect of the longitudinal corrugated heat shield. The traditional heat shield Figure 1 、 2 has uniformly distributed holes on the corrugated plate, but the structure characteristics of the corrugated plate will lead to uneven film jet. By arranging film holes with different spacings on the windward side and the leeward side of the corrugated plate, the cooling air flow can be more reasonably distributed. A suitable hole arrangement mode can further improve the cooling efficiency of the corrugated heat shield. Existing researches have shown that under the same conditions, increasing the number of film holes on the windward side of the main flow of the heat shield can effectively improve the cooling efficiency of the heat shield.

[0005] Most of the current researches are focused on the sinusoidal symmetric longitudinal corrugated heat shield, and less attention is paid to the influence of the change of corrugated structure and the arrangement of non-uniform hole on the cooling effect of the heat shield. The existing researches on the arrangement of non-uniform hole are only for the sinusoidal symmetric longitudinal corrugated heat shield structure, and the research on the corrugated structure is also limited to the change of the length ratio of the windward side and the leeward side in the half period. The corrugated structure and the overall arrangement of the film hole exist optimization space. SUMMARY

[0006] In order to overcome the deficiencies of the prior art, the present application proposes a corrugated heat shield in the combustion chamber of an aero-engine from the corrugated structure and the arrangement of the film hole, and optimizes the design of the corrugated structure and the arrangement of the film hole arranged on the heat shield, thereby improving the cooling efficiency and the cold gas use efficiency of the corrugated heat shield in the cooling channel of the combustion chamber / nozzle.

[0007] To achieve the above-mentioned purposes, the technical scheme adopted by the present application is as follows: a corrugated heat shield in the combustion chamber of an aero-engine, comprising a corrugated heat shield and a film hole row arranged on the corrugated heat shield, the corrugated heat shield is a cylindrical structure, and a continuous periodic corrugation is arranged on the cylinder wall in the axial direction. Each periodic corrugation is the same and includes two parts of a wave crest area and a wave trough area. The film hole row is uniformly arranged on the wave crest and the wave trough of each periodic corrugation in the circumferential direction of the cylinder. The axial spacing of the film hole row is equal. The positions and the number of the film holes of the odd rows are the same. The positions and the number of the film holes of the even rows are the same, but the film holes of the even rows are circumferentially staggered with the film holes of the odd rows. The hole diameters of all the film holes are the same. The corrugated heat shield is coaxially installed inside the combustion chamber / nozzle cylinder. The annular reduced gap between the outer surface of the corrugated heat shield cylinder and the inner surface of the combustion chamber / nozzle cylinder forms a cooling channel. The corrugated heat shield is characterized in that: the wave crest area is a circular arc structure, and the wave trough area is a flat structure.

[0008] Further, the curvature radius R of the wave crest area circular arc is 10-100 mm. The wave crest area of each corrugation period of the corrugated heat shield is a same concave wave circular arc from the first end axis direction, and the wave trough area is a same flat structure.

[0009] Further, the axial length of each period of the corrugated heat shield is L, and 4-6 rows of film holes are arranged thereon. The first row of film holes is arranged at the wave crest. The axial length of the wave crest area and the wave trough area of the corrugated heat shield is L / 2 respectively. The value of L is 30-120 mm. The thickness of the corrugated heat shield is t f , and the value is 0.5-1.5 mm. In each period of the corrugated heat shield, the flow direction spacing of the film hole row is s f , and the value is 0.05L-0.2L. The spanwise spacing of the film hole row is p f, the circumferential offset distance between two adjacent rows of the gas film holes is b f , and the value is 1 / 2p f .

[0010] Further, the corrugated heat shield is perpendicular to the corrugated heat shield at the position of the gas film hole in the opening direction of the gas film hole, and the diameter d f of the gas film hole is 0.5-2mm.

[0011] Preferably, the corrugated heat shield is arranged with 4 rows of misaligned gas film holes at equal intervals in the axial direction from the wave crest, and the 4 rows of gas film holes are arranged in the wave crest region, the thickness t f of the heat shield plate is 1.2mm, the period length L is 60mm, the curvature radius R of the wave crest region is 37.4mm, the axial length is L / 2, the axial length of the wave valley region is L / 2, the streamwise spacing s f of the gas film hole is 0.08L, i.e. 4.8mm, the spanwise spacing p f is 0.04L, i.e. 2.4mm, the ratio of the gas film hole spacing is defined as the ratio of the streamwise spacing s f of the gas film hole to the spanwise spacing p f of the gas film hole = 2; the circumferential offset distance between two adjacent rows of the gas film holes is b f = 1 / 2p f ; the diameter d f of the gas film hole is 1mm, the opening rate of the gas film hole, i.e. the ratio of the total area of all the gas film holes to the total area of the heat shield surface including the total area of the gas film holes = 2.18%.

[0012] Preferably, the corrugated heat shield is arranged with 5 rows of misaligned gas film holes at equal intervals in the axial direction from the wave crest, and the first 3 rows of gas film holes are arranged in the wave crest region, and the last 2 rows of gas film holes are arranged in the wave valley region, the thickness t f of the heat shield plate is 1.2mm, the period length L is 60mm, the curvature radius R of the wave crest region is 37.4mm, the axial length is L / 2, the axial length of the wave valley region is L / 2, the streamwise spacing s f of the gas film hole is 0.1L, i.e. 6mm, the spanwise spacing p f is 0.05L, i.e. 3mm, the ratio of the gas film hole spacing is defined as the ratio of the streamwise spacing s f of the gas film hole to the spanwise spacing p f of the gas film hole = 2; the circumferential offset distance between two adjacent rows of the gas film holes is b f = 1 / 2p f ; the diameter d f of the gas film hole is 1mm, the opening rate of the gas film hole, i.e. the ratio of the total area of all the gas film holes to the total area of the heat shield surface including the total area of the gas film holes = 2.18%.

[0013] The advantages and remarkable effects of the present application are as follows: the present application changes the traditional corrugated structure in the valley region into a flat structure by optimizing the overall arrangement of the corrugated structure and the gas film hole, relatively increases the length proportion of the windward side in a single period of the corrugated plate, and weakens the adverse pressure gradient generated by the expansion of the heat flow passage cross section in the valley region, so that the cold gas can be stably retained in the valley region, and the protection of the downstream wave peak by the valley cold gas is enhanced, and the cooling efficiency of the corrugated heat shield is improved. At the same time, by opening the gas film hole at the wave peak of the corrugated windward side, the gas film is attached to the corrugated heat shield plate by using the Coanda effect near the wave peak, so as to isolate the main flow heat exchange, so that the distribution of the cooling gas is more reasonable, and the use efficiency of the cooling gas of the corrugated heat shield is improved. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 、 2 is a cross-sectional structure schematic diagram of a prior art sinusoidal corrugated heat shield;

[0015] Figure 3 is a perspective view of the overall structure of the corrugated heat shield of the present application;

[0016] Figure 4 is a vertical half-section schematic diagram of Figure 3 ;

[0017] Figure 5 is a cross-sectional view of the wave peak region and the valley region on a periodic corrugated heat shield;

[0018] Figure 6 is a top view of Figure 5 ;

[0019] Figure 7 is a flattened projection schematic diagram of Figure 6 ;

[0020] Figure 8 is an embodiment one of the present application;

[0021] Figure 9 is an embodiment two of the present application;

[0022] Fig. 10 is a thermal side surface temperature distribution cloud diagram of a single period of the corrugated heat shield at the same position of the corrugated heat shield, that is, in the same x coordinate range along the axial direction, x is the axial distance of the corrugated heat shield, and z is the spanwise distance of the corrugated heat shield;

[0023] Figure 10a is a color scale diagram of the thermal side surface temperature of the corrugated heat shield;

[0024] Figure 10b is a thermal side surface temperature distribution cloud diagram of the corrugated heat shield corresponding to the prior art Figure 1 ; and

[0025] Figure 10c Corresponding to existing technology Figure 2 Temperature distribution cloud diagram of the hot side of the corrugated heat insulation screen;

[0026] Figure 10d Corresponding to the present invention Figure 8 Temperature distribution cloud diagram of the hot side of the corrugated heat insulation screen of Example 1;

[0027] Figure 10e Corresponding to the present invention Figure 9 Temperature distribution cloud diagram of the hot side of the corrugated heat insulation screen of Example 2;

[0028] Figure 11 It is a curve diagram of the span-wise average film cooling efficiency of the corrugated heat shield in the combustion chamber. DETAILED DESCRIPTION

[0029] The technical solution of the present invention is described in more detail below with reference to the accompanying drawings and embodiments.

[0030] See Figure 3 A cylindrical corrugated heat shield 1 of uniform diameter is provided with continuous periodic corrugations along the axial direction. Each periodic corrugation consists of a peak region 6 and a flat trough region 7. Film hole rows 2 are evenly distributed along the circumference of the cylinder in both the peak region 6 and the trough region 7 of each period. The film holes 3 are film holes. The peak region 6 is an arc structure, while the trough region 7 is a straight structure. Starting from the axial end, the peak region 6 of each consecutive corrugation cycle is the same concave arc, the peak region 4 is a concave peak, and the trough region 7 is the same straight structure 5.

[0031] See Figure 4 The corrugated heat insulation screen 1 with continuous periodic corrugations is coaxially installed inside the combustion chamber / nozzle barrel. The annular reduction gap between the outer surface of the corrugated heat insulation screen barrel and the inner surface of the combustion chamber / nozzle barrel 8 forms a cooling channel A for low-temperature cold air to flow through, and high-temperature combustion gas B flows inside the corrugated heat insulation screen barrel.

[0032] See Figure 5 The peaks of a corrugated heat shield 1 are concave, with five rows of film holes 2 staggered between them. The concave regions are equipped with three exhaust film holes, with the first row located at the concave peaks 4. The troughs are straight structures 5, each with two exhaust film holes. The five rows of circumferential cylindrical film holes 3 are identical in number and diameter throughout the entire circumference.

[0033] See Figure 6 The axial length of each cycle of the heat shield is L, which is 30-120 mm. The axial lengths of the peak area 6 and the trough area 7 are L / 2, and the thickness of the heat shield is t f , the value is 0.5-1.5mm; Hf R is the curvature radius of the concave arc of the wave peak region, and is 10-100 mm.

[0034] Referring to Figure 7 , the streamwise spacing (the spacing between the rows of film holes) of the rows of film holes is s f , and is 3-12 mm; the spanwise spacing of the rows of film holes is p f , and is 2-6 mm; the circumferential offset distance between the adjacent two rows of film holes is b f , and is 1 / 2p f . The opening direction of the film holes is perpendicular to the corrugated heat shield at the position of the film holes, and the diameter d f of the film holes is 0.5-2 mm.

[0035] Figure 8 is an embodiment of the present application. As shown in the figure, the corrugated heat shield is arranged with 4 rows of offset film holes at equal intervals along the axial direction from the wave peak, and the 4 rows of film holes are arranged in the wave peak region and are unevenly distributed. In this embodiment, the thickness t f of the heat shield plate is 1.2 mm, the period length L is 60 mm, the curvature radius R of the wave peak region is 37.4 mm, the axial length is L / 2, and the axial length of the wave valley region is L / 2. In this embodiment, the streamwise spacing s f of the film holes is 0.08L, i.e. 4.8 mm, the spanwise spacing p f is 0.04L, i.e. 2.4 mm, the ratio of the streamwise spacing s f of the film holes to the spanwise spacing p f of the film holes is 2, which is constant; the circumferential offset distance between the adjacent two rows of film holes is b f , which is 1 / 2p f ; the diameter d f of the film holes is 1 mm, the opening direction of the film holes is perpendicular to the corrugated heat shield at the position of the film holes, and the opening rate of the film holes, i.e. the ratio of the total area of all the film holes to the total area of the heat shield surface including the total area of the film holes, is 2.18%.

[0036] Figure 9 is an embodiment of the present application. As shown in the figure, the corrugated heat shield is arranged with 5 rows of offset film holes at equal intervals along the axial direction from the wave peak, wherein the first 3 rows of film holes are arranged in the wave peak region, and the last 2 rows of film holes are arranged in the wave valley region, and the rows of film holes are unevenly distributed on the corrugated heat shield. In this embodiment, the thickness t f of the corrugated heat shield plate is 1.2 mm, the period length L is 60 mm, the curvature radius R of the wave peak region is 37.4 mm, the axial length is L / 2, and the axial length of the wave valley region is L / 2. In this embodiment, the streamwise spacing s f of the film holes is 0.1L, i.e. 6 mm, the spanwise spacing p f0.05L, i.e. 3mm, the ratio of the pitch of the film holes in the flow direction s f to the spanwise pitch p of the film holes f = 2, the circumferential offset distance b of the two adjacent rows of film holes f = 1 / 2p f , the diameter d of the film holes f = 1mm, the perforation direction is perpendicular to the corrugated heat shield at the position of the film holes, the perforation rate of the film holes, i.e. the ratio of the total area of all the film holes to the total area of the surface of the heat shield including the total area of the film holes = 2.18%.

[0037] Table 1 shows the structural features and numerical calculation methods of the embodiments of the present application and the conventional models. The structural feature of the conventional model is a sinusoidal center-symmetrical corrugated plate, and the rows of film holes on the corrugated plate are uniformly distributed in the axial direction. Referring to Figure 1 , the structural feature of the conventional model one is a sinusoidal center-symmetrical corrugated plate, but the distribution mode of the rows of film holes on the corrugated plate adopts the non-uniform distribution mode of the rows of film holes in the present application. Referring to Figure 2 , the structural features of the embodiment one and the embodiment two are the novel non-symmetrical corrugated plate configuration and the non-uniform rows of film holes arranged thereon of the present application. The four kinds of corrugated heat shield models are simulated and calculated by using the same numerical calculation method. 1 / 72 of the circumferential direction of the heat shield is taken as the calculation domain, the calculation domain is periodically arranged, the Fluent meshing is used to divide the calculation domain into unstructured grids, the Realizable k-ε model is used to solve the steady compressible Reynolds time-averaged Navier-Stokes equation set, the implicit separation method is used for solving, the SIMPLE algorithm is used for pressure-velocity coupling, the second-order upwind format is used for discretizing the convection term, the enhanced wall treatment is used in the near-wall region, the energy equation has no heat source, the viscous dissipation term is ignored, the residual convergence accuracy is less than 1×10 -4 , and the monitored hot side wall temperature no longer changes. The cold efficiency of each model is evaluated by using the comprehensive cooling efficiency, which is defined as the difference between the high-temperature gas temperature and the heat shield hot side wall temperature, and the difference between the high-temperature gas temperature and the cold gas temperature. The average comprehensive cooling efficiency of the four kinds of corrugated heat shield models is shown in Table 1. It can be seen that the average comprehensive cooling efficiency of the conventional model is the lowest, and the cooling effect is the worst. The comprehensive cooling efficiency of the two embodiments of the present application is better than that of the two conventional models, and the average comprehensive cooling efficiency of the embodiment two is the largest, and the cooling effect is the best.

[0038] Table 1 shows the numerical calculation methods of the embodiments of the present application and the conventional models

[0039]

[0040]

[0041] Figure 10 is a cloud chart of the temperature distribution of the heat side of the corrugated heat shield according to the embodiment of the present application and the conventional model.

[0042] wherein, Figure 10a is a temperature color scale chart of the heat side of the corrugated heat shield, Figure 10b is a cloud chart of the temperature distribution of the heat side of the corrugated heat shield according to the conventional model, Figure 10c is a heat chart of the corrugated heat shield according to the conventional model one, Figure 10d is a cloud chart of the temperature distribution of the heat side of the corrugated heat shield according to the embodiment one of the present application, Figure 10e is a cloud chart of the temperature distribution of the heat side of the corrugated heat shield according to the embodiment two of the present application. As shown in the figure, the temperature of the heat side of the conventional model is the highest among the four corrugated heat shields at the same position of the corrugated heat shield, i.e. the same x coordinate range along the axial direction, and a high temperature area of nearly 600K appears at the axial distance x = 610mm, and the temperature distribution is also the most uneven, and the temperature difference of the heat shield in a single cycle is nearly 80K, which indicates that the cooling effect of the conventional model is the worst; the overall temperature of the conventional model one is lower than that of the conventional model, and the temperature difference of the heat shield in a single cycle is also reduced, and the cooling effect is improved; the maximum temperature of the heat side of the heat shield of the embodiment one of the present application is further reduced compared with the conventional model one, and the cooling uniformity is further improved; the temperature of the heat side of the heat shield of the embodiment two of the present application is the lowest among the four heat shields, the temperature distribution is the most uniform, and the cooling effect is the best.

[0043] Referring to Figure 11 , is Figure 1 (conventional model), Figure 2 (conventional model one), Figure 8 (embodiment one), and Figure 9 (embodiment two) are four kinds of corrugated heat shields along the corrugated heat shield of the combustion chamber. The average film cooling efficiency curve of the corrugated heat shield. There are 23 cycles along the axial direction. As shown in the figure, the cooling efficiency of the two conventional models is the lowest, the fluctuation range along the path is the largest, and the uniformity of the cooling effect distribution is the worst. The cooling efficiency of the conventional model one is improved, but the fluctuation range is still large, and the cooling uniformity is poor. The cooling efficiency of the embodiment one and the embodiment two of the present application is higher as a whole, and the cooling uniformity of the embodiment two is the best, and the cooling efficiency of the downstream along the path is the highest.

Claims

1. A corrugated heat shield in an aeroengine combustion chamber, comprising a corrugated heat shield and an array of film holes arranged on the corrugated heat shield, the corrugated heat shield is a cylindrical structure, and a continuous periodic corrugation is arranged on the cylindrical wall in the axial direction, each periodic corrugation is the same and includes two parts of a wave crest region and a wave trough region, the array of film holes is arranged on the wave crest and the wave trough of each periodic corrugation in the circumferential direction of the cylinder, the axial spacing of the array of film holes is equal, the position and number of the film holes of the odd-numbered array are the same, the position and number of the film holes of the even-numbered array are the same, but the circumferential position of the film holes of the even-numbered array is staggered with the film holes of the odd-numbered array, the diameters of all the film holes are the same, the corrugated heat shield is coaxially installed inside a combustion chamber / nozzle cylinder, and an annular reduced gap between the outer surface of the corrugated heat shield cylinder and the inner surface of the combustion chamber / nozzle cylinder forms a cooling channel, characterized in that: The crest region is a circular arc structure, and the trough region is a flat structure.

2. The corrugated heat shield in an aircraft engine combustion chamber of claim 1, wherein: The crest region of each wave period of the corrugated heat shield is a concave circular arc, and the trough region is a flat structure.

3. The corrugated heat shield in an aeroengine combustion chamber according to claim 1 or 2, characterized in that: The curvature radius R of the crest region circular arc is 10-100 mm.

4. The corrugated heat shield in an aircraft engine combustion chamber of claim 3, wherein: The corrugated heat shield has 4-6 rows of air film holes, and the first row of air film holes is arranged at the crest.

5. The corrugated heat shield in an aircraft engine combustion chamber of claim 4, wherein: The axial length of the crest region and the trough region of the corrugated heat shield is L / 2, and L is 30-120 mm.

6. The corrugated heat shield in an aircraft engine combustion chamber of claim 1 or 2 or 4 or 5, characterized by: The corrugated heat shield has a thickness t f , and the value is 0.5-1.5mm.

7. The corrugated heat shield in an aircraft engine combustion chamber of claim 6, wherein: The flow direction spacing of the gas film hole row in each period of the corrugated heat shield is s f , and the value is 0.05L-0.2L; the spanwise spacing of the gas film hole row is p f , and the value is 0.03L-0.1L; the circumferential offset distance of the adjacent two rows of gas film hole rows is b f , and the value is 1 / 2p f .

8. The corrugated heat shield in an aircraft engine combustion chamber of claim 7, wherein: The opening direction of the air film hole on the corrugated heat insulation screen is perpendicular to the corrugated heat insulation screen where the air film hole is located, and the air film hole diameter d f The value is 0.5-2mm.

9. The corrugated heat shield in an aircraft engine combustion chamber of claim 4 wherein: The corrugated heat insulation screen is provided with 4 rows of staggered air film holes at equal intervals along the axial direction starting from the wave crest. The 4 air film holes are all arranged in the wave crest area. The thickness of the heat insulation screen plate is t f The curvature radius R of the wave crest area is 37.4 mm, the axial length is L / 2, the axial length of the wave trough area is L / 2, and the flow spacing s of the air film hole is 1.2 mm. f is 0.08L, i.e. 4.8mm, and the spanwise spacing p f It is 0.04L or 2.4mm. The air film hole spacing ratio is defined as the flow direction spacing s of the air film holes. f Spacing p from the air film hole f The ratio is 2; the circumferential offset distance between two adjacent exhaust film holes is b f =1 / 2p f ; Diameter of air film hole d f The corrugated heat insulation screen has a diameter of 1 mm and a hole direction perpendicular to the location of the air film holes. The air film hole opening rate, that is, the ratio of the total area of ​​all air film holes to the total surface area of ​​the heat insulation screen including the total area of ​​the air film holes, is 2.18%.

10. The corrugated heat shield in an aircraft engine combustion chamber of claim 4 wherein: The corrugated heat shield is arranged with 5 rows of staggered air film holes at equal intervals in the axial direction from the wave crest, wherein the front 3 rows of air film holes are arranged in the wave crest area, the rear 2 rows of air film holes are arranged in the wave trough area, the thickness t of the corrugated heat shield plate is 1.2 mm, the period length L is 60 mm, the wave crest area curvature radius R is 37.4 mm, the axial length is L / 2, the wave trough area axial length is L / 2, the flow direction spacing s of the air film holes is 0.1L (6 mm) f , the spanwise spacing p of the air film holes is 0.05L (3 mm), and the air film hole spacing ratio is defined as the ratio of the flow direction spacing s of the air film holes to the spanwise spacing p of the air film holes f . f f f f f f The hole diameter d of the air film holes is 1 mm, the opening direction is perpendicular to the corrugated heat shield at the position of the air film holes, the air film hole opening rate, that is, the ratio of the total area of all air film holes to the total area of the heat shield surface including the total area of the air film holes, is 2.18%.​​​​​

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