The present invention generally relates to cooling system of turbine blades. More particularly, it relates to jet impingement cooling of leading edge of turbine stator / rotor.

BACKGROUND:

Gas turbine plays a vital role in aerospace and power industry. Efficiency is the basic criteria for evaluation of performance of gas turbine system. An increase of turbine inlet temperature leads to higher efficiency, but turbine inlet temperature is limited by the safe working limits of turbine blade material. The leading edge of turbine is exposed to the highest temperature of hot gases. One of the effective ways of intensely cooling this region is jet impingement cooling. For improving the cooling performance of turbine airfoils, several proposals have been suggested.

United States Patent Number 8608430 to George Liang entitled "Turbine vane with near wall multiple impingement cooling" describes a multiple impingement cooling circuit extending from leading edge to the trailing edge of blade airfoil. Cooled air is supplied from the chamber located in leading edge through impingement cooling holes.

United States Patent Numbers 5538394, 9133717 and 6890153 relates to an impingement cooling of turbine blade in which projecting ribs/projections/fins are provided behind the leading edge of airfoil/ along the cooling flow passages to increase heat transfer rate.

United States Patent Number 8628292 to John David Maltson entitled "Eccentric chamfer at inlet of branches in a flow channel" provides a flow channel in which eccentric chamfer is provided at the upstream edge of the inlet opening till separate branch which reduces turbulence.

United States Patent Number 8348613 to Shawn J. Gregg et.al., entitled "Airflow influencing airfoil feature array" relates to an impingement cooling of turbine airfoil in which the leading- edge portions of the airfoil are provided with plurality of features to facilitate cooling. The showerhead arrangement of impingement holes are provided at the baffle. Similarly, United

States Patent Numbers 55538394, 7798776, 8851848 and 8087893 discuss impingement cooling of leading edge of turbine blade in which a showerhead arrangement of film cooling holes are provided with diffusion opening/tear drop shaped opening etc. These film holes are subjected to unwanted pressure loss resulting in lower film cooling effectiveness. Also increasing number of holes result in backward flow of cooled air. Consequently, addition of projections such as ribs along the internal surface of leading edge increases convective heat transfer area thus reducing required number of film cooling holes as discussed in United States Patent Number 9133717 and European Patent Application Number 0416542. However, these projections increase the weight of the blade.

Incorporating multiple impingement cooling holes/circuits demand large quantity of cooled air which indirectly reduces the efficiency of any turbine engine. Moreover, all the invention till now focus on cooling of local regions or intense cooling of selected area of hot spots of the leading edge. This non-uniform cooling of the blade may lead to thermal stresses.

Prior art discussed above discloses the use of multiple impingement cooling circuit with/without film cooling holes which do not result in improving the effectiveness of cooling with uniform temperature distribution. Hence, there is a need for a cooling system which enhances heat transfer with the use of less number of impingement cooling circuit at leading edge with improved shower head arrangement and modified jet impingement holes/tubes along with 3-D protrusions instead of 2-D protrusions as described in prior art.

SUMMARY:

The primary objective of the present invention is to enhance heat transfer with the use of less number of impingement cooling circuit at leading edge with improved shower head arrangement and modified jet impingement holes/tubes. Another objective of the present invention is to achieve effective cooling and uniform temperature distribution of turbine blade and to reduce the cooling air flow rate and weight of the blade. Yet another objective of the present invention is to increase the effectiveness of cooling without involving film holes and thus simplifying the design of turbine blade/vane.

The above objectives are achieved by directing cooled air through impingement inserts containing impingement holes. The inserts are conventionally supported or by inverted C- shaped support. The impingement holes are cylindrical with chamfer at the exit of hole facing the inner wall of the leading edge. Plurality of holes are arranged in the form of showerhead. Consequently, the inner wall of leading edge has 3-D protrusions acting as turbulent promoters and increasing minimal surface area resulting in lower weight of blade. The protrusions can be of any geometrical shape and size (e.g.: hemispherical). Apart from acting as turbulence promoters, these 3D protrusions obstruct cross flow and improve jet cooling effectiveness. The protrusions can also be incorporated with film cooling holes in turbine blade/vane such that it does not obstruct air flow.

In the present invention, impingement cooling inserts are provided which form an assembly of the blade. The impingement inserts have either impingement cooling holes or jet impingement tubes attached with the inserts. The purpose of these impingement holes/ tubes is to provide path for cooled air for impingement cooling of leading edge of turbine blade. Hot air after passing through leading edge goes to the trailing edge of the blade. A group of 5 holes (central hole surrounded by 4 neighbouring holes) forms the showerhead arrangement. The impingement holes/tubes are arranged at desired pitch depending on the desired range of cooling.

The holes/tubes have chamfer at one of its ends facing the leading edge. The purpose of the chamfer is to enhance cooling coverage area of the leading surface and bring uniformity of temperature to avoid thermal stresses. The chamfer of the impingement holes/tubes together with showerhead arrangement contribute to favourable modification in jet flow structure and jet-to-jet interaction promoting higher heat transfer rate. This is attributed to change in vena- contract and flow behaviour arising due to chamfering and arrangement of jets.

This showerhead arrangement together with inventive impingement holes/tubes reduces the number of impingement holes thus reducing the amount of coolant flow. The chamfering direction could be chamfer leading the flow or chamfer trailing the flow based on turbine operating conditions, requirements and jet-to-leading edge distance. The impingement holes/tubes may have any cross section, for example cylindrical, square or rectangular cross section. The chamfer is provided on jet impingement holes/tubes without disturbing its 105 neighbouring elements and hence do not affect structural integrity. Further, the chamfer in the present invention extends from one end to the other without changing the geometry of neighbouring elements.

Moreover, for further enhancement of heat transfer the inner leading edge surface is modified 110 by 3-D protrusions either in in-line or staggered arrays. These protrusions do not enhance heat transfer due to increase in surface area but because of acting as turbulent promoters and by reducing cross flow effect and improving structural rigidity. These 3-D protrusions increase minimal surface area and result in lowering the weight of the blade.

115 The improved configuration of shower head arrangement together with chamfered impingement holes/tubes and 3-D protrusions can also be implemented for combustor liners, electronics cooling application and wherever such arrangement could be incorporated.

According to the present invention, a turbine blade/vane consists of impingement insert which assembles an improved showerhead arrangement for gas turbine vane/blades using jet

120 impingement cooling system is disclosed. The shower head arrangement involves multiple number of jet impingement holes/tubes in a specific pattern. The jet impingement holes/tubes are modified by chamfering one of the ends of the holes/tubes facing the leading edge of the blade. The chamfering angle and dimensions of jet impingement tube is chosen such that it does not make significant changes in air inlet flow rate. The direction of chamfering is decided

125 by chamfer leading the flow and chamfer trailing the flow based on turbine operating condition and requirements. This new arrangement provides homogeneous flow along all directions. The modified jet impingement tubes configuration gives significant enhancement in heat transfer. This modification also ensures uniformity and extension in cooling coverage on leading edge of turbine blade/vane thus reducing the coolant requirement.

130

These objectives and advantages of the invention will become more evident from the following detailed description when taken in conjunction with the accompanying drawings.

135 BRIEF DESCRIPTION OF THE DRAWINGS: The detailed description is set forth with reference to the accompanying figures. FIG. 1 shows an assembly with impingement tubes fitted to the impingement inserts.

140

FIG. 2 shows impingement cooling hole in impingement inserts.

FIG. 3 shows the jet impingement inserts arranged in showerhead fashion.

145 FIG. 4a & 4b shows the isometric view of chamfered impingement tubes with chamfer leading the flow and chamfer trailing the flow respectively.

FIG. 5 shows isometric view of a turbine blade.

150 FIG. 6 (a & b) shows the different views of protrusions on leading edge of the blade.

FIG. 7 shows the enlarged view of inverted C-shaped section with branches.

FIG. 8 shows comparison of maximum Nusselt number variation between existing 155 impingement cooling technique and present invention.

FIG. 9 shows the layout of experimental setup.

REFERENCE NUMERALS:

160

1- Impingement insert

2- Leading edge of blade/vane

2a- outer wall of leading edge of blade/vane

2b- inner wall of leading edge of blade/vane

165 3- Outer wall of air supply chamber

4- Exit of hole/tube

5- Showerhead 6- Protrusions

7- Jet impingement tubes

170 8- Jet impingement holes

9- Outer wall of Impingement insert

10- Trailing edge

11- Central jet impingement hole

12- Neigboring jet impingement holes surrounding central jet impingement hole 175 13- Chamfer leading the flow

14- Chamfer trailing the flow

15- In-line arrangement of protrusion

16- Stagerred arrangement of protrusion

17- Inverted C-shaped support

180 18- Branches of inverted C-support

19- Air Compressor

20- Filter-Regulator cum moisture separator

21- Pressure gage

22- Needle valve

185 23- Rotameter

24- Wire screen mesh

25- Settling Chamber (long pipe)

26- Pressure transducer

27- Flange containing jet impingement tubes

190 28- Test section

29- Power supply and measuring unit

30- Traversing mechanism

31- Data logger

32- Personal computer

195 33 -Thermocouples

DETAILED DESCRIPTION OF THE INVENTION: 200 The present invention discloses an improved showerhead arrangement for gas turbine vane /blades using jet impingement cooling system in order to enhance heat transfer by cooling the leading edge of the blade with uniform temperature distribution.

According to the present invention, the shower head arrangement (5) assembled with

205 impingement insert (1) involves multiple number of impingement holes (8) /tubes (7) in a specific pattern arranged in the form of shower as shown in FIG. 3. The inserts are conventionally supported by inverted C-shaped support (17). One end of the inverted C-shaped support is fixed with the outer wall of air supply chamber (3) to which the impingement tubes (7) are assembled or with the outer wall of impingement insert (9) in the case of impingement

210 holes. The other end of inverted C-shaped support is always fixed to inner wall of the leading edge (2b). Further, referring to FIG. 7, the inverted C-shaped supports have several branches (18) to facilitate passage of air from leading edge (2b) to the trailing edge (10). All the branches extend from one end of the inverted C-shaped support to the other end of inverted C-shaped support. The purpose of inverted C-shaped support is to provide structural support to

215 impingement inserts and also to facilitate supply of air from one region to the other. The branches (18) of inverted C-shaped support can be tapered, angled or cylindrical in shape on the basis of availability of space and ease of fabrication. The jet impingement tubes (7) are modified by chamfering all neighbouring jet impingement holes/tubes except central jet (11) impingement tube facing the inner wall of leading edge (2b). The chamfering angle and

220 dimensions of jet impingement tube is chosen such that it does not make significant changes in air inlet flow rate. This new arrangement provides homogeneous flow along all direction. The modified jet impingement holes/tubes configuration gives significant enhancement in heat transfer. This modification also ensures uniformity and extension in cooling coverage on leading edge of turbine blade/vane (2) thus reducing the coolant requirement.

225

The direction of chamfering is decided by chamfer leading the flow (13) and chamfer trailing the flow (14) based on turbine operating condition and requirements. The chamfered end of impingement tubes help in achieving uniformity and extension in cooling coverage with less number of impingement jets and thus lower coolant flow rate.

230

The impingement holes (8) / tubes (7) are cylindrical with chamfer at the exit of hole/tube (4) facing the inner wall of the leading edge (2b). Plurality of holes (8) are arranged in the form of showerhead. The plurality of impingement holes (8) / tubes (7) can be of any shape and cross- section such as circular, rectangular etc., Consequently, the inner wall of leading edge has 3-D 235 protrusions (6) acting as turbulent promoters and increasing minimal surface area resulting in lower weight of blade. The protrusions (6) can be of any geometrical shape and size (e.g.: hemispherical). The protrusions (6) can also be incorporated with film cooling holes in turbine blade/vane such that it does not obstruct air flow.

240 Apart from acting as turbulence promoters, these 3D protrusions obstruct cross flow and improve jet cooling effectiveness. The jet impingement holes (8) / tubes (7) of the present invention can be applied as an impingement cooling hole in an impingement inserts as shown in FIG. 2 or it can be manufactured in an impingement tube and fitted to the impingement inserts to form an assembly as shown in FIG. 1. Set of 5 JIPs are arranged in shower head

245 fashion as shown in FIG. 3.

In the present invention, impingement cooling inserts are provided which form an assembly of the blade. The impingement inserts (1) have either impingement cooling holes or jet impingement tubes attached with the inserts. The purpose of these impingement holes/tubes is 250 to provide path for cooled air for impingement cooling of leading edge of turbine blade. Hot air after passing through leading edge goes to the trailing edge of the blade. A group of 5 holes (central hole surrounded by 4 neighbouring holes) forms the showerhead arrangement. The impingement holes/tubes are arranged at desired pitch depending on the desired range of cooling. The central hole of showerhead arrangement is always circular.

255

The holes (8) / tubes (7) have chamfer at one of its ends facing the leading edge. The holes (8) or tubes (7) can be arranged in any pattern depending on the availability of space and requirement of cooling. The purpose of the chamfer is to enhance cooling coverage area of the leading surface and bring uniformity of temperature to avoid thermal stresses. The chamfer of 260 the impingement holes/pipes together with showerhead arrangement contribute to favourable modification in jet flow structure and jet-to-jet interaction promoting higher heat transfer rate. This is attributed to change in vena-contracta and flow behaviour arising due to chamfering and arrangement of jets.

265 The exit of JIP facing impingement surface has chamfer of certain angle (Preferably, 25°-70°) provided it extends from one end to the other. The chamfer can be along leading the flow direction as shown in FIG. 4a or along trailing the flow as shown in FIG. 4b depending on jet- to-leading edge distance.

270 Referring to FIG. 5, 6(a) and 6(b), along the inner walls of leading edge 3-D protrusions are included in in-line or staggered arrangements to reduce cross flow effect by multi-jet impingement. The protrusions enhance jet impingement heat transfer not because of increase in surface area but by acting as turbulent promoters. Larger dimensions of protrusions (d/D > 0.025) do not necessarily indicate heat transfer augmentation. The offset distance of the 3-D

275 protrusions is symmetrical along the centre-line of said leading edge of said turbine blade.

The dimensions of protrusion along with pitch to diameter ratio is also critically investigated for various parameters like flow rate, Reynolds number, jet-to-target distance etc.

280 The improved configuration of shower head arrangement together with chamfered impingement holes/tubes and 3-D protrusions can also be implemented for combustor liners, electronics cooling application and wherever such arrangement could be incorporated.

Thus, the present invention provides a jet impingement cooling system in the form of shower 285 head for leading edge of gas turbine vane/blades. The improved arrangement provides significant increase in heat transfer by extending cooling coverage with less number of modified impingement jets.

Experimental analysis and results:

290

Numerical simulations and experiments are also carried out for the validation of the proposed arrangement at various curvature of turbine leading edge dimension along with variation in Reynolds number and jet-to-target plate distance commonly known as gap ratio.

295 The maximum difference in Nusselt number is calculated according to the following formula- Nu _max — Nu _m i _n

Nu _max

Unlike previous impingement results available in literature, the present experimental results give the maximum difference in Nusselt number in the range of 7-11% as shown in FIG. 8. This shows that always showerhead configuration is better for uniform distribution of Nusselt number over the leading edge of turbine blade and thus reducing thermal stresses arising due to non-uniformity in temperature.

The experiments are performed over semi-circular concave surface. In spite of the non- cylindrical shape of leading edge of a gas turbine, the cylindrical shape is considered as the 305 best model for characterizing the geometric parameters of the concave surface. Also, the small changes occurring because of changing cross-section of the airfoil along spanwise direction would not affect the result significantly. In this view, the experiments are performed by considering semi-circular concave surface as the impingement surface.

310 The layout of the experimental setup is shown in Fig. 9. As shown, high-pressure air from the compressor (19) passes through the filter-regulator cum moisture separator unit (20) which removes the moisture content in the air. The dial pressure gage (21) gives the pressure reading. The air flow rate is controlled with the help of a needle valve (22) and it is measured by a rotameter (23). Air at high pressure enters axially into a 50.8mm stainless steel cylinder

315 containing a wire screen mesh (24) to reduce flow fluctuations and make the flow uniform across the section. This pressurized air passes through the settling chamber which is a meter long pipe (25). The other end of settling chamber consists of a flange joint (27) with five holes in which the jet impingement tubes are fitted based on the particular arrangement. Jet comes out through the jet impingement tubes and impinges on target concave plate (28). The jet

320 impingement tube and impingement plate (semi-circular with D= 160mm) are made of Aluminium. The jet to target plate distance which is known as gap ratio is varied with the help of a traverse mechanism (30). A thin foil heater is attached to the convex side of the impingement surface. The power supply unit (29) supplies required power to the heater in connection with rheostat and ammeter clamp. Multimeter is also used to measure minute

325 fluctuations in voltage.

To reduce conduction heat losses, the target plate assembly is placed into an insulation chamber filled with glass wool (thermal conductivity = 0.04W/m-K) except the concave impingement surface. The temperature and pressure of impinging jets are calculated by a pre-calibrated K- 330 type thermocouple (33) and a pressure tap respectively. A digital differential pressure transducer (26) is connected to the pressure tap for measuring the pressure drop. The outcomes are also compared with ordinary shower head arrangement to determine the 335 effectiveness of the present invention. Also, combined effect of proposed shower head arrangement and protruded leading edge further enhances heat transfer. The effectiveness of cooling in new arrangement is likely to increase by at least 25%. This technology can be used in power as well as aircraft gas turbine engines where the cooled blades are used.

340 While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiment, method, and examples, but by all embodiments

345 and methods within the scope and spirit of the invention as claimed.