STUDY OF EFFECT OF GAS TURBINE AND COMPRESSOR BLADES DETERIORATION ON THE PERFORMANCE OF GAS TURBINE POWER PLANT

Abstract

The efficiency of gas turbines used in power plants is largely dependent on their aerodynamic performance. The components like stators and rotors of the turbines and compressor are subject to abrasive and erosive wear. There are various contaminants which get deposited over the blades and produce roughnesses. The roughness magnitude both in turbines & compressors varies along the height and chord of blade and also over different stages of turbine. In actual turbines and compressors, roughness is not only found over entire surfaces of the blades but over a small portion of the surface of the blades also. The roughness is found, in the form of bands also on leading edge, middle chord and trailing edge of pressure and suction surfaces of blades of turbines and compressors. The flow through turbines and compressors is inherently three dimensional due to the vane/blade passage geometry and other variety of reasons. The blade profile continuously changes in the span wise direction. The flow structure, along the end walls, is strongly three dimensional. This effect is tremendous in case of lower span to chord ratio. These characteristics of the flow lead to increase profile losses which in turn adversely affect the efficiency of turbomachines. The secondary flows cause to generate a non-uniform flow at exit of the blade row thereby efficiency of the blade row downstream gets further reduced. This research work in fact is an attempt to capture complex three- dimensional secondary flow vortices near end wall region along with getting results for total losses. The total losses are segregated to obtain profile & secondary loss i.e. ends losses numerically. The Computational fluid dynamics (CFD) uses numerical methods and algorithms to solve and analyze problems that involve fluid flows. The present research work is carried out using the CFD, commercial softwares, Gambit 2.4.6® and FLUENT 6.2.16®. These softwares are used for designing stage and working on it without any actual manufacturing and installation of such cascade in real working situation. The cascades of turbine and compressor are simulated to carry out study of effect of blade deterioration on various losses using three different blade profiles for turbines and one profile for compressor. Total three number blade profiles titled 6030, 5530 and 3525 as selected by Samsher [2002], from impulse and reaction turbine are chosen. Of the three profiles selected, blade profile 3525 was nearly impulse type and blade profiles titled 5530 and 6030 iv were of reaction type with different degree of reactions. The study is conducted for a number of cascades. There are total 13 numbers of cascades to be simulated separately for each of combinations of roughness magnitude, location of roughness and the given single blade profile for application of roughness on entire surface. In addition study of localised roughnesses of varying magnitudes is also conducted. There are 6 numbers of locations over the suction and pressure surfaces of given cascade for application of localised roughness. The total pressure at inlet and total pressure and static pressure at exit measurement planes for numerous number of cascades are measured with the help of 'Fluent' software. These parameters are required to calculate local loss coefficients relative to nondimensional distance in the pitch wise direction along the measurement plane. The mass averaged loss coefficients is representative loss coefficient for selected pitch wise positions and calculated using the pitch wise local loss coefficients. The total loss, secondary loss and profile loss, for a cascade, are calculated on the basis of mass averaged loss coefficients for all selected span wise positions. The mass averaged loss coefficients near the end walls at both ends are higher than their values at mid span of the blade for each of cascade for all blade profiles i.e. 6030, 5530 and 3525. The local increase in mass averaged loss coefficients is observed due to the secondary flow cores near the hub and casing for all cascades. The results with regard to the total, profile and secondary losses for BSR and PSR cascade based on mass averaged loss coefficients show that the magnitudes of total loss for these cascades for all roughness values are higher than that of the smooth cascade for all blade profiles i.e. 6030, 5530 and 3525. The total loss increases as roughness increases on the blade surfaces of each cascade for all blade profiles. The losses increase in the same order when roughness was increased from lower roughness value to high roughness value for all type of cascades. It is observed that change in respective losses with the increase in magnitude of roughness at high roughness values (such as 750 μm) are negligibly small. The increasing of roughness on suction surface is found to be more detrimental than the same for pressure surface in terms of generation of total loss. It is found that the total loss increases as the roughness is increased on suction surfaces. The effect is combined when roughness is applied on both the surface of the blades. The v contribution of the profile loss in the total loss increases as roughness increases on the blade surfaces. The secondary loss increases with the increase of roughness on pressure surfaces of blades of cascades. The same is decreased with the increase of roughness on suction surfaces of blades of cascades. The combined effect of increase of roughness on both the surfaces of blades is seen. The effect of roughness on secondary loss is more pronounced in blade profile 6030. The results also show that shape or geometry of the blades of the cascades significantly affect the losses. The localised roughness and the effect of the same on losses vary from one blade profile to other blade profile. The application of localised roughness on leading edge on pressure surfaces of the cascade employing blade profile 3525 leads to more generation of total and profile losses than localised roughness on middle chord and trailing edge of same surfaces for the same cascade. On the other hand, the localised roughness on trailing edge of the pressure surfaces for blade profile 6030 and 5530 leads to more generation of total and profile losses. The application of localised roughness on blade profile 6030 and 5530 effect the phenomena of losses generation in same way. The localised roughness on various surfaces of blade profile 3525 effect the phenomenon of losses generation differently comparing with that of blade profile 6030 and 5530. The trend of increase in total loss with the increase of surface roughness on various surfaces for compressor cascade is similar to that of turbine cascades. It is noticeable that the profile loss contributes very significantly in the total loss for all type of compressor cascades and that the roughness magnitudes do not affect secondary loss very appreciably for all type of cascades.