Debasish Biswas

A High-Order LES Turbulent Model to Study Unsteady Flow Characteristics in a High Pressure Turbine Cascade

In this work, unsteady viscous flow analysis around turbine blade cascade using a High-Order LES turbulent model is carried out to investigate basic physical process involved in the pressure loss mechanism. This numerical analysis is assessed to the wind tunnel cascade test. Basically, all the physical phenomena occurring in nature are the effect of some cause, and the effect can somehow be measured. However, to understand the cause, detail information regarding the visualization of the phenomena, which are difficult to measure, are necessary. Therefore, in our work, firstly the computed results are compared with the measured data, which are the final outcome of the cause (of the phenomena under investigation), to verify whether our physics-based model could qualitatively predict the measured facts or not. It was found that the present model could well predict measured data. Therefore, the rest of the computed information, which were difficult to measure, were used to visualize the overall flow behavior for acquiring some knowledge of the physical process associated with the pressure loss mechanism. Our study led to an understanding that the interaction of the vortex generated on the suction and pressure surface of the blade and the secondary vortex generated on the end-wall, downstream the trailing edge resulted in the formation of a large vortex structure in this region. This unsteady three-dimensional flow characteristic is expected to play an important role in the pressure loss mechanism.Copyright

Three-Dimensional Thermo Fluid Analysis of Large Scale Electric Motor

In the present work, the flow and temperature fields in large scale rotating electric motor are studied by solving the Navier–Stokes equations along with the temperature equation on the basis of finite difference method. All the equations are written in terms of relative velocity with respect to the rotating frame of reference. Generalized coordinate system is used so that sufficient grid resolution could be achieved in the body surface boundary layer region. Differential terms with respect to time are approximated by forward differences, diffusion terms are approximated by the implicit Euler form, convection terms in the Navier–Stokes equations are approximated by the third order upwind difference scheme. The results of calculation led to a good understanding of the flow behavior, namely, the rotating cavity flow in between the supporting bar of the motor, the flow stagnation and region of temperature rise due to flow stagnation, etc. Also the measured average temperature of the motor coil wall is predicted quite satisfactorily.

Numerical Analyses of Flow around Airfoils Subjected to Flow Induced Vibration

This paper describes extensive computer-based analytical studies on the details of unsteady flow behavior around airfoils subjected to flow induced vibration in turbo-machinery. To consider the time-dependent motions of airfoils, a complete Navier-Stokes solver incorporating a moving mesh based on an analytic solution of motion equation for airfoil translation and rotation was applied. The drag and lift coefficients for the cases of stationary airfoils and airfoils subjected to flow induced vibration were examined. From the numerical results in non-coupling case as out of consideration of the airfoil motion, it was found that the separation vortex consisted of large-scale rolls with axes in the span direction, and rib substructures with axes in the stream direction. In the coupling simulation including the airfoil motion, both the translation and the rotation displacement were gradually increased when the airfoil translation and rotation natural frequencies synchronize exactly with the oscillation frequency of the fluid force. In addition, the transformation from complex structure with rolls and ribs to two-dimensional aspect of only rolls could be visualized in three-dimensional simulation.

Studies on Characteristic Frequency and Length Scale of Shock Induced Motion in Transonic Diffuser Using a High Order LES Approach

Unsteady transonic flows in diffuser have become increasingly important, because of its application in new propulsion systems. In the development of supersonic inlet, air breathing propulsion systems of aircraft and missiles, detail investigations of these types of flow behavior are very much essential. In these propulsion systems, naturally present self-sustaining oscillations, believed to be equivalent to dynamically distorted flow fields in operational inlets, were found under all operating conditions. The investigations are also relevant to pressure oscillations known to occur in ramjet inlets in response to combustor instabilities. The unsteady aspects of these flows are important because the appearance of undesirable fluctuations generally impose limitation on the inlet performance. Test results of ramjet propulsion systems have shown undesirable high amplitude pressure fluctuations caused by the combustion instability. The pressure fluctuations originated from the combustor extend forward into the inlet and interact with the diffuser flow-field. Depending on different parameters such as the diffuser geometry, the inlet/exit pressure ratio, the flow Mach number, different complicated phenomena may occur. The most important characteristics are the occurrence of shock induced separation, the length of separation region downstream of the shock location, and the oscillation of shock location as well as the oscillation of the whole downstream flow. Sajben experimentally investigated in detail the time mean and unsteady flow characteristics of supercritical transonic diffuser as a function of flow Mach number upstream the shock location and diffuser length. The flows exhibited features similar to those in supersonic inlets of air-breathing propulsion systems of aircraft. A High-order LES turbulence model developed by the author is assessed with experimental data of Sajben on the self-excited shock oscillation phenomena. The whole diffuser model configuration including the suction slot located at certain axial location around the bottom and side walls to remove boundary layer, are included in the present computation model. The time-mean and unsteady flow characteristics in this transonic diffuser as a function of flow Mach number and diffuser length are investigated in detail. The results of study showed that in the case of shock-induced separation flow, the length and thickness of the reverse flow region of the separation-bubble change, as the shock passed through its cycle. The instabilities in the separated layer, the shock /boundary layer interaction, the dynamics of entrainment in the separation bubble, and the interaction of the travelling pressure wave with the pressure fluctuation region caused by the step-like structure of the suction slot play very important role in the shock-oscillation frequency.Copyright

Studies on Transitional Heat Transfer Characteristics Over Turbine Vane Surface Using a High Order LES Approach

The boundary layer developing on a turbo-machinery blade usually starts as a laminar layer but in most situations it inevitably becomes turbulent. The transition from laminar to turbulent in the boundary layer, which often causes a significant change in operational performance of the machinery, is generally influenced by the free-stream turbulence level, the pressure gradient, and surface curvature, etc. Therefore, boundary layer transition is an important phenomenon experienced by the flow through gas turbine engines. A substantial fraction of the boundary layer on both sides of a gas turbine airfoil may be transitional. The extended transition zone exist due to strong favorable pressure gradients, found on both near the leading edge portion of the suction side and the pressure side, which serve to stabilize the boundary layer and consequently delay the transition process, even under high free-stream turbulence intensity (FSTI) in practical gas turbine. It is very important to properly model and predict the high FSTI transition mechanism, since boundary layer transition leads to substantial increase in friction coefficients and heat transfer rate. Boundary layer separation, which is expected to be a significant problem on the suction side of some high pressure turbine airfoils due to shock-boundary layer interaction, also depends strongly on the state of boundary layer with respect to transition. Acceleration rates, Reynolds numbers and FSTI play very important role in controlling the boundary layer transition on the pressure side of gas turbine airfoils. The main objective of the present work is to study the performance of a high order LES turbulence model in predicting the transitional heat transfer characteristics over turbine vane surface under high pressure turbine flow conditions. In this regard the model is assessed to the precise experimental data where measurements were carried out in moderate temperature using three-vane cascades under steady state conditions. Two types of vane configurations were used in the experiment. The aerodynamic configurations of the two vanes were carefully selected to emphasize fundamental differences in the character of suction surface pressure distributions and the consequent effect on surface heat transfer distributions. In both the experiments and the computations, principle independent parameters (Mach number, Reynolds number, turbulence intensity, and wall-to-gas temperature ratio) were varied over ranges consistent with actual engine operation. The computed results explained measured data very satisfactorily and helped to have a very good understanding of basic mechanism involved in the complex flow behavior and transition from laminar to turbulent flow.© 2014 ASME

Application of a High Order LES Approach to the Redistribution of Inlet Temperature Distortion in a Turbine

The demand of an increase in the cycle performance of today’s gas turbines creates severe heat loads in the first turbine stage, since higher operating temperatures are required. The mean flow temperature is usually well above the limit supported by the surrounding material. Cooling of both end-walls and the blades of the first stage is thus usually necessary. Consequently, mid-span streaks of hot gas pass through the first stator row and become hot jets of fluid. Also, the exit flow from a gas turbine combustor entering a turbine stage can have a wide variation in temperature. These variations may be both spatial and temporal. The implementation of cooling method requires a clear understanding of the aerodynamics involved. Both qualitative and quantitative assessments of the redistribution of inlet temperature distortions can be used to considerable advantage by the turbine designer. Experimentally it has been demonstrated that the rotor actually separates the hotter and cooler streams of fluid so that a hotter fluid migrates toward the pressure surface and cooler fluid migrates towards the suction surface. The main purpose of this study is to test the performance of a high-order LES model in terms of predicting this type of highly complicated unsteady flow and heat transfer phenomena. This work describes the performance of a high-order Large Eddy Simulation (LES) turbulent model (developed by the first author) related to the prediction of above mentioned redistribution of inlet temperature distortion in an experimental turbine. Because the understanding of the physical phenomena associated with this temperature redistribution behavior is a very challenging computational fluid dynamic problem. If the numerical method could predict the precisely measured data satisfactorily, then the fluid dynamic variables which are difficult to measure (but obtained as computed results) could be used to visualize the flow characteristics. This technique will also help to get rid off indirect measurement techniques with large measurement uncertainty. In our study emphasis is put to predict the unsteady turbulence characteristics. In this work 3-D unsteady Navier-Stokes analysis of a turbine stage (satisfying the experimental stator-rotor blade ratio) is carried out to study the above mentioned phenomena. The numerical results predicted the experimentally observed phenomena very well. The fact that the streamlines in the stator row remain unaffected was demonstrated by the numerical results. The measured characteristics of the streamline patterns in the rotor row resulted from the secondary flow effect and consequently the inlet temperature distortion effect is also very well predicted.Copyright

Three Dimensional Thermo-Fluid Analyses on Convective Heat Transfer and Friction Loss in Micro/Mini Channel Based on High Order LES Model

Several researches dealing with the single-phase forced convection heat transfer in micro tubes have been published in the past years. Most of their tests results are significantly departed from those of the traditional forced convection heat transfer coefficients in larger tubes. Some recent work reported that measurement accuracy is one of the most important factors that may cause this discrepancy. Since the diameter of the sensor for measuring micro tubes surface temperature is comparable to the size of the micro-tubes, the tubes surface temperature can not be accurately measured due to the effect of sensor wire thermal shunt. In this work some recent experimental results on heat transfer and frictional losses in mini/micro channel of semi-circular configurations using water as working fluid are investigated numerically by comparing the measured and predicted data. The flow conditions considered here cover a wide range of Reynolds number (300–4000), which corresponds to laminar, transitional and turbulent flow. Since the flow considered here is turbulent in nature emphasis is put on the physics based turbulent model. In this study, a high order LES turbulent model in which in a dynamic eddy viscosity model, transfer of information between the sub-grid and large scale eddies is improved by solving an additional transport equation for turbulent kinetic energy in the grid scale level. Here, sub-grid-scale turbulent stresses are closed using a dynamic turbulent kinetic energy transport model. The sub-grid scale length scale is represented by the minimum of the universal length scale lu and the grid scale. The universal length scale lu, which represents the blending of the length scales of cascade of eddies starting from the near wall small scale all the way to the sub-grid scale, is defined on the basis of turbulent Reynolds number Ret. A test filter was used for the dynamic procedure, which is applicable to stretched grid near the body surface. Also the thermal convection problem is coupled with thermal conduction within the material to obtain the overall solution. Predicted results agreed well with the measured data. The results helped to have a good understanding of how the flow and thermal phenomena attributed to the overall heat transfer and frictional loss mechanism. The comparison of measured and predicted data based on single phase N-S equations showed a very good agreement and the visualization of the three-dimensional results of computation led to a good understanding of the physics based mechanism associated with the laminar to turbulent transitional phenomena inside the micro/mini channels.Copyright

Studies on Flow Transition Under Simulated Low Pressure Turbine Conditions Based on a High Order LES Model

Boundary layer transition is an important phenomenon experienced by the flow through gas turbine engines. A substantial fraction of the boundary layer on both sides of a gas turbine airfoil may be transitional. The extended transition zone exist due to strong favorable pressure gradients, found on both near the leading edge portion of the suction side and the pressure side, which serve to stabilize the boundary layer and consequently delay the transition process, even under high free-stream turbulence intensity (FSTI) in practical gas turbine. It is very important to properly model and predict the high FSTI transition mechanism, since boundary layer transition leads to substantial increase in friction coefficients and heat transfer rate. Near wall turbulence production is thought to be largely absent in the non-turbulent zone. The intermittent nature of transition need to be taken into account in developing improved transition model. Much has been learned from the to date, but the nature of separated flow transition is still not completely clear, and existing models are still not robust as needed for accurate prediction. Therefore, in the present work a high order LES turbulent model proposed by the author is used to predict the separated flow transition. The experimental data of Volino is chosen for this comparison purpose. In his experimental work, the flow through a single-passage cascade simulator is documented under both high and low FSTI conditions at several different Reynolds numbers. The geometry of the passage (in Volino’s work) corresponds to that of the “Pak-B” airfoil, which is an industry supplied research airfoil that is representative of a modern, aggressive LP turbine design. Volino’s data included a complete documentation of cases with Re as low as 25,000 and also the documentation of turbulent shear stress in the boundary layer under both high and low FSTI.Copyright

Unsteady Viscous Flow Simulation around Turbine Blade

In this study, 3-D unsteady viscous flow analysis around turbine blade cascade is carried out to investigate basic physical process involved in the pressure loss mechanism. In this regard, the strategy of the present study is first to compare the predicted results with the experimentally measurable data to check the prediction ability of numerical method. As a consequence of that the computed results agree with the experimental results. Therefore computed results could be used for visualization of the overall flow behavior to gather knowledge about what physical phenomena are associated with the mechanism of pressure loss. Because all the experimentally results compared so far with the computed results are the final outcome of the cause. From computed results, it turned that structure of vorticity from suction side and pressure side of turbine blade is a factor of pressure loss mechanism.

Unsteady 3-D Navier-Stokes Simulations on Characteristic Frequency and Length Scales in Transonic Diffuser

An improved low-Reynolds k-τ turbulent model developed by the author is assessed against experimental data of Sajben on the selfexcited shock oscillation phenomena. The whole diffuser model configuration including the suction slot located at certain axial location around the bottom and side walls to remove boundary layer, are included in the present computation model. The time-mean and unsteady flow characteristics in this transonic diffuser as a function of flow Mach number and diffuser length are investigated in detail. The results of study showed, that in the case of shock-induced separation flow, the length and thickness of the reverse flow region of the separation-bubble change, as the shock passed through its cycle. The instabilities in the separated layer, the shock /boundary layer interaction, the dynamics of entrainment in the separation bubble, and the interaction of the travelling pressure wave with the pressure fluctuation region caused by the step-like structure of the suction slot play very important role in the shock-oscillation frequency.