NUMERICAL SIMULATION OF FLOW THROUGH TURBINE BLADE INTERNAL COOLING CHANNEL USING COMSOL MULTIPHYSICS

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1 International Journal of Emerging Technology and Innovative Engineering Volume 1, Issue 12, December 2015 (ISSN: ) NUMERICAL SIMULATION OF FLOW THROUGH TURBINE BLADE INTERNAL COOLING CHANNEL USING COMSOL MULTIPHYSICS C T Dheeraj Kumar Singh 1, Vipul Ram K P 2, Siva Prasanna 3,Ramesh Kumar Donga 4 [1, 2] M. Tech in Computational Fluid Dynamics, UPES, Dehradun [3] M. Tech in Pipeline Engineering, UPES, Dehradun [4] Assistant Professor, Department of Mechanical Engineering, UPES, Dehradun Corresponding Author dheeru310@gmail.com ABSTRACT:For any gas turbine (working on Brayton cycle) thermal efficiency and power output increases with increase in rotor inlet temperature, in new advance gas turbines this inlet temperature is far higher than the softening points of the blade material generating thermal stresses in the blade, so these blades must be cooled. The present age cooling strategy is leading edge is cooled by impingement cooling the trailing edge is cooled by pin film and the middle portion is cooled by rib roughened coolant passage of the turbine blade. The problem here is to numerically simulate the flow in a turbine blade cooling channel by using rib turbulators. The study extensively looks into effect of heat transfer with different P/E (pitch between two ribs to height of the rib) ratios. Geometry, grid generation and Simulation work is carried out using COMSOL Multiphysics. Keywords: Gas turbine blade cooling, Rib turbulators, Heat transfer, Cooling channel, Grid generation. 1. INTRODUCTION Modern gas turbines operate at very high temperatures for increasing the efficiency and performance of the turbines. But these high temperatures may exceed the material melting temperature of the turbine blades. Hence proper cooling system must be employed for the cooling of the turbine blades for their long life. The leading edge of the turbine blade airfoil is cooled by using impingement cooling with film cooling, the middle portion is cooled by using rib Turbulator cooling and the trailing edge is cooled by using pin fin cooling with ejection. The cooling of the turbine blades must include all the regions exposed to hot gases. One of such regions of a blade in case of high pressure turbines is the tip of the blade. For the mechanical and thermal expansion of the blades a clearance gap is provided between the turbine blade tip and the shroud/ fixed casing of the turbine. But due to this clearance gap, leakage of hot gases occurs mainly due to the difference between the pressure side and suction side of the blade [1]. And this gap cannot be completely eliminated since it will cause rubbing of the blades to the casing thereby causing damage to the turbine. Hence the cooling of the turbine blade tip must be given a special attention for safe and efficient working of the turbines. One very common method of cooling is by providing serpentine passages in the middle portion of theturbine blade. Fig 2 shows the typical serpentine passages that are commonly used for the internal cooling of turbine blades. Figure 1: Schematics of a turbine blade cooling channel. 2.INTERNAL COOLING IN A TURBINE BLADE COOLING CHANNEL RIB TURBULATORS 1

2 Rib Turbulators are the most regularly utilized strategy to upgrade the warmth move in the inward serpentine cooling sections. The rib turbulence promoters are ordinarily thrown on two inverse dividers of the cooling entry. Heat that directs from the weight and suction surfaces through the edge dividers is exchanged to the coolant going inside through the sharpened steel [4]. The warmth exchange execution of the ribbed channel relies on upon the channel viewpoint proportion, the rib arrangements, and the Reynolds number of the coolant stream. Numerous central studies have been led to comprehend the coolant move through a stationary ribbed channel. The studies demonstrate as the coolant disregards a rib arranged 90 to the standard stream, the stream close to the channel divider divides. Reattachment takes after the partition, and the limit layer reattaches to the channel divider; this more slender, reattached limit layer results in expandedwarmth move coefficients in the ribbed channel. improvement and less frictional misfortunes than the ribs inclining toward or far from the stream. Figure 2.a:Recirculation Zones inthe Fluid Channel afterthe rib. The ribs likewise make turbulent blending in the territories of stream detachment. With this extra blending, the warmth is all the more adequately dispersed from the divider, and subsequently extra warmth exchange upgrade. Since just the stream close the mass of the cooling channel is irritated by the ribs, the weight drop punishment by ribs moderate. LITERATURE SURVEY: J. C. Han and Park created relationships for both the weight punishment and warmth move upgrade in ribbed channels. Given the Reynolds number of the coolant stream and the rib geometry (E/D, P/E, W/H, and α), the normal erosion consider a channel with two inverse ribbed dividers, f, and the centerline normal Stanton number on the ribbed dividers, Str, can be resolved from the relationships. Exhibits the connections produced for cooling entries with 90 ribs. The rubbing harshness capacity, R, is just an element of rib dividing for the scope of the harshness Reynolds number, e+, indicated. Taking into account the rib dividing (P/E), R can be figured, and substituted into the accompanying comparison to focus f, the four ribbed divider erosion variable. The larger part of ribs utilized as a part of exploratory studies has a square cross-segment; then again, studies have researched the warmth exchange improvement of different profiled ribs. Delta-formed ribs were considered here. What s more, these ribs were additionally indicated to result in higher warmth exchange improvement than the customary calculated ribs. Explored the execution of ribs inclining into or far from the stream. They finished up the conventional square ribs give more prominent warmth exchange Figure 2.b:Turbine blade cooling channel with rib Turbulators. 3. GEOMETRY DETAILS In this, the considered geometry is 3-dimensional of square cross section with U shaped bend having dimensions of 25mm 25mm. The hydraulic diameter D is considered as 25mm. the length of the channel is 125mm long on both sides of the U-bend In this case the rib dimensions are considered with the relation of E = 0.1 D = = 2.5mm and the Pitch (P) value is 25mm. P/E ratio is calculated i.e.= 25/2.5 = 10. 2

3 Here for this geometry the P/E ratio is considered to be 10. Similarly different configuration of 6, 8 and 10 are constructed. This means the distance from first rib begin to second rib start. This is maintained same in case of all the ribs present in the geometry. The inlet of the geometry is elongated for the flow to become fully developed. Rib Turbulators. Orthogonal ribs i.e. ribs perpendicular to the flow (90 Degrees to flow) are placed and these ribs are places in two orientations both parallel and alternating to each other (top and bottom). P/E Number of Ribs Figure 4: Actual geometric model in COMSOL Table 1: Number of ribs for different P/E ratios Figure 3: Different geometric configuration for the classified on P/E ratios For an inlet Reynolds number different configurations are tested which are parallel and alternating ribs In alternating ribs when top and bottom ribs are compared the top rib is places in such a way that it lies at the mid of the pitch of the bottom blades But in parallel rib configuration each rib lies exactly parallel to one another 3. USE OF COMSOL MULTIPHYSICS As the problem is an internal incompressible flow dealing with heat transfer at a particular Reynolds number [3]. A non-isothermal flow will be used where the fluid flow interface (spf) is coupled with the heat transfer (ht) interface, and pressure forces and the viscous dissipation rate of the fluid are considered. The application builder is used to create an interface which could solve the model with different aspect ratios [5] of the cooling channel. The general governing equations of conservation of mass, momentum and energy and two other equations of (k) kinetic energy and (Ԑ) dissipation energy must be solved RANS equation ρ U. U +. μ T U + U T 2 3 μ T(. U)I = P + μ U + U T 2 3 μ. U I + F And. ρu = 0 U and P are the time-averaged velocity and pressure respectively. The term μ T represents the turbulent viscosity, i.e., the effects of the small-scale time-dependentvelocity fluctuations that are not solved for by the RANS equations. The turbulent viscosity; μ T, is evaluated using turbulence models. The most common one is the k-ε turbulence model (one of many RANS turbulence models). This model is often used in industrial applications because it is both robust and computationally inexpensive. It consists of solving two additional equations for the transport of turbulent kinetic energy k and turbulent dissipation ϵ This momentum equation is coupled with energy equation by using the Non-Isothermal flow module Coupling the pressure and temperature 3

4 The Non-Isothermal Flow interface solves for conservation of energy, mass and momentum in fluids. It synchronizes the features from the Heat Transfer and Fluid Flow interfaces when a turbulent flow regime is defined. Equations Here t represents time T represents temperature P is the coefficient of thermal diffusivity ρ is the density and P is pressure. The above equations couple the heat transfer in fluid interface and turbulent flow K-Ԑ Normal air is selected as the material as the model. The model is meshed in the COMSOL automatic mesh generation ability by putting the mesh density to be normal. Figure5:Isothermal temperature contour for P/E=6 4. BOUNDARY CONDITIONS The geometry has different zones Zone Boundary type Thermal value Momentum value 5.2: Parallel Ribs Figure 6:Heat flux for P/E= 6 wall wall Heat flux No slip 950 W/m 2 V=0 inlet Velocity inlet Temperat ure T= K Reynolds number Re=1000 Pressure outflow Pressure P=1 atm Figure 7:Isothermal temperature contour for P/E= 6 5. RESULTS AND OBSERVATIONS 5.1: P/E=6 4

5 Figure 8:Heat flux contour forp/e=6 Figure 11:Iso-thermal temperature contour for P/E=8 5.3: P/E=8 Figure 12: Heat flux contour for P/E=8 Figure 9:Iso-thermal temperature contour for P/E=8 5.4: P/E=10 Figure 10:Heat flux contour for P/E=8 Figure 13:Iso-thermal temperature contour for P/E=10 Parallel ribs 5

6 Figure 14:Heat flux contourfor P/E=10 independent of the rib configuration and is dependent on the Reynolds number. The inlet Reynolds number is All the figures show the different contour of the different P/E configurations.due to the turbulence effect, Heat transfer increased due to disorderness in the fluid flow. The heat transfer on the ribbed surfaces has been shown in this work, in form of temperature contour on surface. So convective cooling can be enhanced by this new design of the cooling channel, from the study we observe that as the pitch to eccentricity ratio (P/E) decreases the heat transfer rate increases. Parallel ribs p/e Temperature at Nusselt number Figure 15:Iso-thermal temperature contour for P/E=10 Table 2: Temperature At Outlet And Nusselt Number For Alternating Ribs p/e Temperature at Nusselt number Table 3: Temperature At Outlet And Nusselt Number For Parallel Ribs Figure 16:Heat flux contour for P/E=10 Different P/E ratios and different rib placement configurations are analyzed in this study for a flow of Reynolds number The temperature and heat flux contours are presented in the results. 6. CONCLUSIONS From the observed results the pressure drop is observed to be 1 Pa where shows the pressure drop is We could find high temperature at the for P/E ratio of 8 both in parallel and alternating ribs and parallel rib configuration gives more desirable results than alternating ribs. 7.REFERENCES [1]. Turbulence Modelling for Internal Cooling of Gas-Turbine Blades by JONASBREDBERG. [2]. Recent progress in the computation of flowand heat transfer in internal coolingpassages of turbine blades by H. Iacovides, M. 6

7 RaiseeDepartment of Mechanical Engineering,UMIST. [3]. Reynolds averaged simulation of flow andheat transfer in ribbed ducts by A. Ooi a,*, G. Iaccarino b, P.A. Durbin c, M. Behnia. [4]. Review on Heat Transfer Augmentation Techniques: Application in Gas TurbineBlade Internal Cooling by S. Gupta1,*, A.Chaube1 and P. Verma. [5]. Aspect ratio effect on heat transfer in rotating two-pass rectangular channels with smooth walls and ribbed walls by WENLUNG FU. [6]. Effect of rib spacing on heat transfer and friction in a rotating two-pass rectangular(ar=1:2) channel by YAO -HSIEN LIU. [7]. C T Dheeraj Kumar Singh, G BharathGoud CFD Analysis of Flow through Turbine Blade in 2D Cooling Channel with Square Rib Turbulators in IJETR ISSN: , Volume-3, Issue-6, June