consider the turbine blade tip available in the laboratory of University and discuss the needs of...

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Mechanical Engineering

consider the turbine blade tip available in thelaboratory of University and discuss the needs of coolingtechnology specially specially to be used for the blood tree usingphoto and 77 develop a mathematical model and show the effect ofwearing the dimensions of the blood type on its performance discussalso the effect of film cooling and unsteady turbulence somethingpulling film hall and gauge arrangement to be made and enhancementin the cooling techniques which we can implement Jet impingement ofmultiple jets effect then pin fin cooling been array and partiallength in arrangement to be made effect of pension orientation pinfin dimple cooling to be used on the reap tabulated calling andalso discuss the f fusion cooling techniques of gas turbine

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Introduction to Cooling Gas turbines play a vital role in the todays industrialized society and as the demands for power increase the power output and thermal efficiency of gas turbines must also increase One method of increasing both the power output and thermal efficiency of the engine is to increase the inlet temperature of the gas entering the turbine In the advanced gas turbines of today the turbine inlet temperature can be as high as 1500C however this temperature exceeds the melting temperature of the metal airfoils The heat transferred to the turbine blade is substantially increased as the turbine inlet temperature is continuously increased Thus it is very important to cool the turbine blades for a long durability and safe operation Cooling the blade must include cooling of the key regions being exposed to the hot gas The blade tip region is such a critical area and is indeed difficult to cool This results from the tip clearance gap where the complex tip leakage flow occurs and thereby local high heat loads prevail Cooling air around 650C is extracted from the compressor and passes through the airfoils With the hot gases and cooling air the temperature of the blades can be lowered to approximately 1000C which is permissible for reliable operation of the engine It is widely accepted that the life of a turbine blade can be reduced by half if the temperature prediction of the metal blade is off by only 30C In order to avoid premature failure designers must accurately predict the local heat transfer coefficients and local airfoil metal temperatures By preventing local hot spots the life of the turbine blades and vanes will increase However due to the complex flow around the airfoils it is difficult for designers to accurately predict the metal temperature At the leading edge of the vane the heat transfer coefficients are very high As the flow splits and travels along the vane the heat fluxes decreases Along the suction side of the vane the flow transitions from laminar to turbulent and the heat transfer coefficients increase As the flow accelerates along the pressure surface the heat transfer coefficients also increase The trends are similar for the turbine blade The heat flux at the leading edge is very high and continuously decreases as the flow travels along the blade On the suction surface the flow transitions from laminar to turbulent and the heat flux sharply increases Types of Cooling There are three major types for cooling of gas turbine blades These are explained as follows Convection Cooling It works by passing cooling air through passages internal to the blade Heat is transferred by conduction through the blade and then by convection into the air flowing inside of the blade A large internal surface area is desirable for this method so the cooling paths tend to be serpentine and full of small fins A variation of convection cooling is impingement cooling It works by hitting the inner surface of the blade with high velocity air This allows more heat to be transferred by convection than regular convection cooling does Impingement cooling is often used on certain areas of a turbine blade like the leading edge with standard convection cooling used in the rest of the blade Film Cooling The second major type of cooling is film cooling This type of cooling works by pumping cool air out of the blade through small holes in the blade This air creates a thin layer the film of cool air on the surface of the blade protecting it from the high temperature air The air holes can be in many different blade locations but they are most often along the leading edge One consideration with film cooling is that injecting the cooler bleed into the flow reduces turbine efficiency That drop in efficiency also increases as the amount of cooling flow increases The drop in efficiency however is usually reduces the increase in overall performance produced by the higher turbine temperature Transpiration cooling This is third major type of cooling is similar to film cooling in this it creates a thin film of cooling air on the blade But it is different in that air is leaked through a porous shell rather than injected through holes This type of cooling is effective at high temperatures as it uniformly covers the entire blade with cool air Needs of Cooling Technology for Blade Tip To satisfy the fast developments of advanced gas turbines the operating temperature must be increased to improve the thermal efficiency and output work of the gas turbine engine However the heat transferred to the turbine blade is substantially increased as the turbine inlet temperature is continuously increased Thus it is very important to cool the turbine blades for a long durability and safe operation Due to an unavoidable gap clearance between the blade tip and casing the hot gas flowing through the gap results in a large thermal load on the blade tip The potential damage due to the large heat load will lead to blade oxidation Hence the blade tip is a key region that needs cooling The turbine blades are cooled by the use of extracted air from the compressor of the gas turbine This extraction results in a reduction of the thermodynamic efficiency and power output Too little coolant flow results in high blade temperature If a proper cooling system is designed the gain from high firing temperature is so significant that it can outweigh the losses in the efficiency and power output and offset the complexity and cost of the cooling technology The turbine blade tip and neartip regions are difficult to cool and are subjected to potential damage because of the high heat load caused by tip leakage flow A common way to cool the tip is to extract the cooling air from the internal coolant passages through some film holes that are located on the blade surface discretely This cooling is known as film cooling The relatively cool air passes these holes and forms a thin protective film to protect the tip surface from the highly hot mainstream A high and uniform cooling effectiveness will ensure overall performance of the blade surface cooling In general a higher blowing ratio at a specific temperature ratio gives a higher film cooling performance and thereby the heat is transferred to the blade surface and hence the protection of surface is improved It is important to optimize the amount of coolant for film cooling at the engine operating conditions For a better cooling performance it is necessary to study the film cooling holes pattern which affect the film cooling performance As the turbine inlet temperature increases the heat transferred to the turbine blade also increases The level and variation in the temperature within the blade material which cause thermal stresses must be limited to achieve reasonable durability The operating temperatures are far above the permissible metal temperatures Therefore there is a critical need to cool the blades for safe operation The blades are cooled with extracted air from the compressor of the engine Since this extraction incurs a penalty on the thermal efficiency and power output of the engine it is important to understand and optimize the cooling technology for a given turbine blade geometry under engine operating conditions Fig 21 shows the common cooling technology with three major internal cooling zones in a turbine blade with strategic film cooling in the leading edge pressure and suction surfaces and blade tip region The leading edge is cooled by jet impingement with film cooling the middle portion is cooled by serpentine ribroughened passages with local film cooling and the trailing edge is cooled by pin fins with trailing edge injection The blade tip internal cooling mostly focuses on the rotational effects inside the turbine blade with heat transfer passage Also on the unsteady high freestream turbulence effects on the turbine blade film cooling performance with standard and shaped filmhole geometry The details of these techniques are discussed in chapter 3 Comparison of various tip cooling configurations and their effects on film effectiveness and heat transfer coefficients were presented Above figure shows the clearance gap and tip film cooling configuration Four film cooling configurations were tested 1 discrete slot injection 2 round hole injection 3 pressure side flared hole injection and 4 groovedtip cavity injection These four configurations are as shown in figure 23 given below It was found that for case 4 the overall film cooling performance varied significantly with injection locations and that among the planetip injections the discrete slot injection provided better performance than the others As per review by B Sunden and G Xie 1 numerically study of various film holes configurations on plane and squealer tips of a turbine blade has been done From this three configurations were tested 1 the camber arrangement 2 the upstream arrangement and 3 the two rows arrangement The effects of rotation were also observed They found that at high blowing ratios the latter two cases provided better film cooling performance on the plane and squealer tips than the former one Higher blowing ratios resulted in a higher cooling effectiveness on the shroud for all cases They also found See Answer

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