Effect of inlet swirl on the flow behavior inside annular diffuser.

Kumar, Manoj ; Arora, B.B. ; Maji, Subhashish 等

Introduction

The task of a diffuser is to decelerate the flow and to regain pressure. It is more difficult to arrange for an efficient deceleration of flow than it is to obtain an efficient acceleration. There is a natural tendency in a diffusing process for the flow to break away from the walls of the diverging passage, reverse its direction, and flow back in direction of the pressure gradient. If the divergence is too rapid, this may result in the formation of eddies with consequent transfer of some kinetic energy into internal energy and a reduction in useful pressure rise. A small angle of divergence, however, implies a long diffuser and a high value of skin friction loss. Usually, flow separation in a diffuser is sought to be avoided due to the invoked additional pressure loss. Other than in many strongly separated flows, such as the flow over a backward facing step, the point of flow separation, in diffuser, is not defined by the geometry but entirely by the pressure gradient.

It was well demonstrated that diffuser of annular type are complex in nature as besides other parameters the inner wall of the diffuser comes into existence, which enhances the complexity. Flow through annular diffusers is characterized by a rapid growth of the boundary layer, leading to various degrees of irregularity in the flow pattern, non-uniformity of the velocity profile, total pressure loss, instability and recirculation if the flow separates. Experimental studies help the researchers to minimize the undesirable effects thereby optimizing the retrieval of the static pressure rise. Experimental studies combined with the empirical relations or analytical studies help in improving the diffuser performance.

The swirling component of velocity may arise either from the presence of inlet guide vanes or any other components preceding the diffuser e.g., a compressor, or from rotation of the central shaft through the diffuser. The introduction of presence of swirl alters the flow field considerably and this affects the overall performance of a system. The energy transfer in these turbo machineries involves the exchange of significant levels of kinetic energy in order to accomplish the intended purpose.

The research of extensive nature to define optimum geometrical characteristics of diffuser has been carried out by various researchers such as Agrawal et al. [1], Ali Pinarbasi. [2], Anderson M.G. [3], Arora et al.[4,5], Colodipietro et al. [10], Hoadley [11], Japikse, D [13], Klomp [14], Kochevsky A. N.[15], Kumar D.S. [16], Lohmann et al. [17], Mohan et al. [18], Sapre et al. [19], Singh et al.[21,22], Sovarn et al.[23] Shaalan et al.[20] Shrinath [24], Yeung et. al [27]. These investigators found improved diffuser performance with swirl up to ascertain point after that it deteriorated. The performance of an annular diffuser apart from swirl is dependent on a large number of geometrical and dynamical parameters. The effectiveness of annular axial diffusers worsens with flow separation. The separation of the flow can be suppressed or shifted from one location to another with the help of swirl. The efforts have been made to design an annular diffuser for no flow separation [12, 25,26], however little success has been achieved. Experimental studies on annular diffuser [7] require sophisticated instrumentation and complicated time consuming procedures which is not economically viable and thus has limited the research activity in the field of annular diffusers [8].

In the present study, CFD has been applied to the annular diffuser with fully developed inlet velocity profile. The analysis was accomplished with different inlet swirling intensity (0[degrees], 10[degrees], 15[degrees], 20[degrees] and 25[degrees]) to visualize the effect of swirl on the performance of annular diffuser in terms of pressure recovery.

Mathematical Formulation

Analyzing swirling flow in diffuser reveals that swirling flow helps in relocation of turbulent profile into laminar profile of axial velocity component with reduced hydraulic loss. The swirling flow is highly influenced by geometric properties and other quantative and qualitative changes in flow parameters in particular.

The tool used in the present study are GAMBIT for meshing and for computational fluid dynamics (CFD) analysis is FLUENT, which is a finite element/volume analysis program for solving fluid flow and conjugate heat transfer problems. In the pre study k- [epsilon] turbulence models such as standard, RNG and realizable were attempted for the same geometry as used for experimental investigation and was validated with the results obtained experimentally. The grid independence tests were also carried for mesh sizes varying from 100000 to 500000 mesh size. It was found that the model which approached more closely to the experimental results was 2D, double precision ax symmetric RNG, k- [epsilon] turbulence model. The same model was used for predicting the performance at various inlet swirls. The governing equations for 2D ax symmetric geometries Arora et. al {4,5}.

Results and discussion

Velocity Profile

Figure 1 show the longitudinal velocity profiles. These profiles are represented as non-dimensional longitudinal velocity u/Um as a function of diffuser passage height y/Ym for the area ratios 3. The velocity profiles are shown for various inlet swirl angles 0[degrees], 10[degrees], 15[degrees], 20[degrees]and 25[degrees]. All the velocity profiles have been shown in terms of non-dimensional velocity as the ratio of local longitudinal velocity to the local maximum velocity of the transverse, where velocity is required. The non-dimensional velocity has been shown as a function of non-dimensional diffuser passage height of the particular traverse (y/Ym). The hub position of the traverse is represented by y/Ym =0, whereas y/Ym =1 represents the casing position The graphs are shown at various traverses (in terms of non dimensional number x/L) of the diffuser passage at x/L= 0.1, 0.3, 0.5, 0.7 and 0.9 for all the area ratios and inlet swirl angles. Further Figure 1 illustrates that the flow is hub generated for no swirl condition and there is shift in the flow from hub towards casing when the inlet swirl is introduced. The peak of the velocity at y/Ym = 0. 41, With the introduction of swirl, the flow is pushed towards the casing. The separation or reversal of flow is neither observed on the hub nor on the casing wall even with the introduction of 25[degrees] inlet swirl. It is quite significant that the he peak velocity shifts towards the casing side as the inlet swirl increases.

[FIGURE 1 OMITTED]

Pressure Recovery Coefficient

Figure 2 indicates pressure recovery coefficient (Cp) at casing wall and Hub side for diffuser for area ratios 3 as a function of non-dimensional diffuser passage x/L for various inlet swirl angles 0[degrees], 10[degrees], 15[degrees], 20[degrees] and 25[degrees]. It is observed that Cp increases with the diffuser passage. The marginal increase in Cp is sharp in the beginning of the diffuser passage and later on it decreases with the diffuser passage. It is also that Cp is lower than the flow without swirl beyond x/L =0.64 and 0.30 for 10[degrees] and 25[degrees] inlet swirl respectively. Up to 0.2 of diffuser passage length, Cp is highest for 25[degrees] inlet swirls, from 0.2 to 0.4, it is for 20[degrees] inlet swirl and beyond that it is for 15[degrees] inlet swirl. In the case of hub the lower value of swirl i.e. 10[degrees] and 15[degrees] gain in Cp is observed up to x/L =0.60 where as there is decrease in magnitude of Cp for inlet angle of 20[degrees]and 25[degrees] to the flow without swirl.

[FIGURE 2 OMITTED]

Conclusions

Following inferences have been drawn from the predicted computational results for area ratios 3 for various inlet swirl angles.

1. The longitudinal velocity decreases downstream continuously irrespective of whether the inlet flow is swirling or non-swirling.

2. Due to boundary layer growth Velocity profiles have distinct shape.

3. Shifting is observed for maximum non-dimensional value of flow velocity with the introduction of swirl.

4. With the introduction of swirl, the flow is pushed towards casing wall thus making the flow stronger towards casing than hub wall.

5. With the introduction of swirl the recovery is faster towards the casing wall. The effect of swirl appears to gradually decay as the flow proceeds downstream and the recovery is negligible or nil towards the diffuser exit.

6. Pressure recovery coefficient increases with the diffuser passage for all values of inlet swirl. However, at higher values of swirl, the marginal recovery decreases with the diffuser passage.

Nomenclature

A Area

AR Area ratio

[C.sub.P] Pressure recovery co-efficient

k Turbulent kinetic energy

Re Reynolds number

S Swirl Number of flow

U Velocity

w Swirl velocity

x,y,z Cartesian coordinate system.

x/L non-dimensional diffuser passage.

y/Ym non-dimensional diffuser passage height of the particular traverse

Symbols

[??] Stress tensor

[mu] Laminar viscosity

[[mu].sub.t] Turbulent viscosity

2[theta] Equivalent cone angle

[epsilon] Turbulent kinetic energy dissipation rate

[eta] Diffuser effectiveness

[theta] Wall angle

v Kinematics viscosity

References

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Manoj Kumar (1), B.B. Arora, (2),* Subhashish Maji (3) and S. Maji (4)

(1) Research Scholar, IGNOU Delhi, India (2,4) Department of Mechanical Engineering, Delhi College of Engineering, Delhi, India (3) School of Engineering & Technology, IGNOU, Delhi, India * Corresponding Author E-mail: [email protected]