|
Three-dimensionality in turbomachine increases complexity in flow field so, determination of losses becomes difficult unlike two-dimensional losses where equation complexity is little. Three-dimensionality includes large pressure gradients in every direction, design/curvature of blade, shock wave, heat transfer, cavitation and viscous effects, which generates secondary flow, vortices, tip leakage vortices and other losses. Viscous effects in turbomachinery causes blockage to the flow by formation of viscous layers around blade profile which effect pressure rise/fall and reduce effective area of flow field. Interaction between these losses in rotor increase instability and decreases in efficiency of turbomachinery. In calculation of three-dimensional losses every parameter comes into picture of flow path like axial spacing between vane and blade rows, end wall curvature, radial distribution of pressure gradient, hup/tip ratio, dihedral, lean, tip clearance, flare, aspect ratio, skew, sweep, platform cooling holes, surface roughness, off take bleeds. Associated with blade profile like camber distribution, stagger angle, blade spacing, blade camber, chord, surface roughness, leading and trailing edge radii, maximum thickness. So, correlations are dependent on so many parameter it becomes difficult to correlate. Correlation based on geometric similarity has been developed by many industries in form of charts, graphs, data statistics and performance data. Two-dimensional losses are easily evaluated by Navier-Stokes code but three-dimensional losses are difficult to evaluate so, correlation is used. Three-dimensional losses are mainly classified as: # Three-dimensional profile losses # Three-dimensional shock losses # Secondary flow # Endwall losses in axial turbomachinery # Tip leakage flow losses == Three-dimensional profile losses == Main points are- * Profile Losses occurs due to curvature of blade. It includes span-wise mixing of flow field in addition to two-dimensional mixing losses (can be predicted from Navier-Stokes code). * In rotors major losses occur by radial pressure gradient from midspan to tip (flow ascending to tip). * Reduction in high loses between Annulus wall and tip clearance region which includes trailing edge of blade profile. This is due to flow mixing and flow redistribution at the inner radius as flow proceeds downstream. * Between hub and annulus wall, losses are prominent due to three-dimensionality. * In single stage turbomachinery, shows large radial pressure gradient loss at exit of rotor. * Platform cooling increases the endwall flow loss and coolant air increases profile loss. * Navier-Stokes identifies much of the losses when some assumption are made like unseparated flow, etc. Here correlation is no longer justified. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Three-dimensional losses and correlation in turbomachinery」の詳細全文を読む スポンサード リンク
|