VALUATION OF TURBULENCE MODELLING ON LOW SPEED …€¦ · VALUATION OF TURBULENCE MODELLING ON LOW SPEED CENTRIFUGAL COMPRESSOR USING COMPUTATIONAL FLUID DYNAMICS G. Sravan Kumar,

http://www.iaeme.com/IJMET/index.asp 634 [email protected]

International Journal of Mechanical Engineering and Technology (IJMET) Volume 7, Issue 6, November–December 2016, pp.634–641, Article ID: IJMET_07_06_063

Available online at

http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=7&IType=6

ISSN Print: 0976-6340 and ISSN Online: 0976-6359

© IAEME Publication

VALUATION OF TURBULENCE MODELLING ON

LOW SPEED CENTRIFUGAL COMPRESSOR USING

COMPUTATIONAL FLUID DYNAMICS

G. Sravan Kumar, Dr. D. Azad and K. Mohan Laxmi

Department of Mechanical Engineering,

AITAM Engineering College, Tekkali - 532201, Andhra Pradesh, India

ABSTRACT

An enhanced framework for various turbulence models study is exercised in a rotating and curved

flow channels present in the centrifugal compressor of a micro gas turbine. This study is to evaluate

the suitable turbulence model which asses the close behaviour of the internal flows obtaining in the

present geometry since as it is believed from the previous research a turbulence model plays a major

role in disparity between various approaches (i.e. Experimental, Analytical and Numerical).

Presently a steady state numerical analysis is carried out by using Navier-Strokes equations coupled

with standard k-ε, Realizable k-ε and k-kl-ω models were used in simulating the flow field around

the geometry by means of commercial software ANSYS 15.0. On accumulation of curvature

correction and compressibility effects to the corresponding models shows good match in numerical

predictions of density profiles, Mach number profiles, Pressure profiles, temperature profiles etc.

Overall Realizable k-ε model shows good agreement rather to standard k-ε, k-kl-ω models for the

present geometry.

Key words: Navier-Strokes Equation, Turbulence, Centrifugal Compressor, Highly Swirled Flows,

Curvature Effects.

Cite this Article: G. Sravan Kumar, Dr. D. Azad and K. Mohan Laxmi, Valuation of Turbulence

Modelling on Low Speed Centrifugal Compressor Using Computational Fluid Dynamics.

International Journal of Mechanical Engineering and Technology, 7(6), 2016, pp. 634–641.

http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=7&IType=6

1. INTRODUCTION

Computational fluid dynamics is a requisite approach for the design and investigation of flow phenomena in

turbo machinery. From mid-2 to 3 decades onwards, three dimensional numerical investigations have been

gained an immense usage in the development field. One of the components of turbo machinery is centrifugal

compressor. Due to its wide range of applications causes the need to simulation also. Generally the procedure

begins with the analysis and design of the single components like impellers, vaned diffusers; return channels

in multi stage machines, and volutes (1-3). Numerical approach is authentic to some extent for turbo

machinery applications but it is commercial, in-house or academic. Regardless, there is a well-defined

prediction enhancement of results obtained when computing either subsonic or transonic compressors. On

considering previous results in literature survey, closer values obtained between numerical and experimental

results, hence giving me a path to carry out with simulations. For the compressors, the contours are well



captured, but there is no possibility of getting accurate results in crucial points like choking or vibrations

onset. In these cases high pressure ratio compressors shown better results than subsonic ones since

investigations is going on for inconsistency’s like geometry when compared between real and the virtual

model. Let us considered fillet radii at the blade root, while in experiments causes severe deformation due

to its higher rotational speed which are usually not modelled in numerically (4) therefore it requires

additional modelling.

Another important aspect in the present study is evaluating of sharp turbulence model for aerodynamic

designs in simulations of complex turbulent flows. During the last 3 decades an immense growth has been

seen in development of improved turbulence models due to sophisticated technology in the present

aerodynamic systems and advancement in computers on performing numerical simulation capabilities.

Marcelo R. Simões et.al (5) in his work shows the use of three different turbulence models namely k-ε , k-ω

and SST models applied to CFD simulation of turbulent flow inside a rotor of an axial flow compressor of

which flow field has been obtained experimentally in laboratory test. The simulation results shows good

agreement with experimental data conclude that SST model is the most appropriate for the simulation of an

axial flow compressor rotor may be used in the design of new axial flow compressors. The compressor

dimension for the present study is taken from the literature (12).

2. TURBULENCE MODELS

The three eddy-viscosity models included in the present study are the two-equation standard k-ε model, two-

equation Realizable k-ε and three equation k-kl-ω models. A brief description of these models is presented

below

The standard k-ε model (curvature correction and compressibility effects) is a model based on model

transport equations for the turbulence kinetic energy (k) and its dissipation rate (ε). The model transport

equation for k is derived from the exact equation, while the model transport equation for ε was obtained

using physical reasoning and bears little resemblance to its mathematically exact counterpart. In the

derivation of the standard k-ε model, the assumption is that the flow is fully turbulent, and the effects of

molecular viscosity are negligible. The standard k-ε model is therefore valid only for fully turbulent flows.

The realizable k-ε model (curvature correction and compressibility effects) differs from the standard k-ε

model in two important ways:

• The realizable k-ε model contains an alternative formulation for the turbulent viscosity.

• A modified transport equation for the dissipation rate, ε has been derived from an exact equation for the

transport of the mean-square vorticity fluctuation.

The term “realizable” means that the model satisfies certain mathematical constraints on the Reynolds

stresses, consistent with the physics of turbulent flows. Standard k-ε model is not realizable. Realizable k-ε

model have shown substantial improvements over the standard k-ε model where the flow features include

strong streamline curvature, vortices, and rotation

The k-kl-ω transition model (compressibility effects) is used to predict boundary layer development and

calculate transition onset. This model can be used to effectively address the transition of the boundary layer

from a laminar to a turbulent regime.

3. MODELLING AND MESHING

The impeller geometry was designed in INVENTOR software where it is sliced to 18 degree sector in order

to reduce the number of elements which causes less computational time to converge the solution as shown

in figure 1. This 18 degree sector is made into complete geometry by mentioning the number of blades to

twenty with the option of cyclic symmetry present in the INVENTOR software to generate the full visual

3D model is shown in figure 2.

Valuation of Turbulence Modelling on Low Speed Centrifugal Compressor Using Computational Fluid Dynamics


Figure 1: 18 Degree Sector Compressor Blade Figure 2: Compressor with 20 Blades

(INVENTOR) Software

Figure 3: Complete Centrifugal Compressor Obtained In ANSYS 15.0 Workbench Software

Figure 4: Mesh Model of Compressor Blades Figure 5: Quality of Mesh Obtained

The geometry is imported to ANSYS workbench where we can found the actual frozen geometry of the

compressor as shown in figure 3 now onset of mesh is generated. The mesh obtained in the present case is

hexahedral with 28000 elements as shown in figure 4 & 5.

4. BOUNDARY CONDITIONS AND SOLVER SETTINGS

While coming to boundary conditions default scalable wall function setting were used and all the walls in

the simulation such as shroud, hub, wall blade suction, wall blade pressure are set to be smooth, adiabatic

and non-slip and are considered as wall type. Pressure is acting as inlet and outlet conditions and ten percent

of viscosity ratio and intensity in turbulence is preferred. In order to get the appropriate results, usage of



double precision is required in fluent solver. Since the fluid flow region in compressor are regarding to high

compressible flows, density based coupler solver is preferred and process is carried out in steady state

analysis. The present work is simulated under atmospheric conditions where inlet total pressure is set to 1

bar (equals to 101325 Pa) and the inlet total temperature is 288.1K and rotational speed is taken at 14000

rpm. A close tolerance of 0.1 to 0.2 mm is preferred between the impeller eye tip and the shroud in the

geometry. Second-order upwind is used in discretisation scheme due to high realistic geometry section. Thus

it should be provided with correct time step for the solution convergence from initial condition to the steady

condition ranging from 0.5/ω to 1/ω were within limit prescribed by ANSYS. The convergence criteria for

all the residuals such as x, y, z direction velocity profiles, mass, momentum, energy, k values etc. are set to

10℮-3 where under this criteria solutions are chosen. Hybrid initialisation is assumed rather to standard and

run calculation up to the solution is converged.

5. RESULTS AND DISCUSSION

All the three turbulence models, standard k-ε, realizable k-ε and k-kl-ω models shows good convergence

criteria as they converged with less computational time with iterations of 587, 800, 505 respectively. The

important step is to choose the accurate model that is suitable to the given geometry which is explained

below. The contours of static pressure, static temperature, density, Mach number for three turbulence models

are exposed in the figure 6, 7, 8, 9. The efficiency graph were acquired from all these models are revealed,

a good agreement is obtained from all these results which are presented here

• From the theoretical study it is found that velocity of the fluid is converted to pressure partially in the impeller

and partially in the stationary diffusers, most of the velocity leaving the impeller is converted into pressure

energy in the diffuser from the figure 6 it is observed that Realizable k-ε turbulence model follows a

concentration of stagnation pressure increases and distributes evenly throughout the span in entire flow field

of geometry and it meet the theoretical argument.

• From equation of state temperature raises with increase in pressure, in the figure 7 a uniform coincidence

would be seen in realizable k-ε rather than the other two turbulence models.

• The value of density is depends on the mass flow rate from the continuity equation. In figure 8 change

in density is obtained in the flow regime is well predicted by the realizable k-ε turbulent model.

• In the impeller the change in velocity is variable along the flow field due to the aerodynamic design

of blades from the figure 9 it sounds good in case of realizable k-ε turbulence model compared to

others.



Figure 6 Contours of Static Pressure for Standard K-ε, realizable K-ε and K-Kl-ɷ Turbulence Models

Figure 7 Contours of Static Temperature for Standard K-ε, realizable k-ε and k-kl-ω Turbulence Models



Figure 8 Contours of Density for Standard k-ε, realizable k-ε and k-kl-ω Turbulence Models

Figure 9 Contours of Static Pressure for Standard K-ε, realizable k-ε and k-kl-ω Turbulence Models



Figure 10 Efficiencies obtained for Standard k-ε, realizable k-ε and k-kl-ω Turbulence Models to the 180 Sector

Compressor Blade

6. CONCLUSION

The aim of the present work is to examine the various turbulence models which is suitable for the flow

structure of centrifugal compressor by commercial CFD simulation practises. As earlier research recognised

the inconsistency was due to the turbulence model paved a way to evaluate advanced models. Three

turbulence models, offering with increasing in firmness and complication has been taken from the finite

volume solver ANSYS 15.0 namely standard k-ε, realizable k-ε and k-kl-ω turbulence models. Among them

realizable k-ε turbulence model with curvature correction and compressibility effects is giving better than

standard k-ε and k-kl-ω turbulence models based on the comparison of obtained results. The overall results

obtained in detailed turbulence models present slight dissimilarities from each other even though these minor

variances in the flow field do not affect the overall behaviour of the geometry.

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Documents

VALUATION OF TURBULENCE MODELLING ON LOW SPEED …€¦ · VALUATION OF TURBULENCE MODELLING ON LOW SPEED CENTRIFUGAL COMPRESSOR USING COMPUTATIONAL FLUID DYNAMICS G. Sravan Kumar,