K. Vasudeva Karanth

CFD Analysis on the Effect of Radial Gap on Impeller-Diffuser Flow Interaction as well as on the Flow Characteristics of a Centrifugal Fan

The flow between the impeller exit and the diffuser entry (i.e., in the radial gap is generally considered to be complex). With the development of PIV and CFD tools such as moving mesh techniques, it is now possible to arrive at a prudent solution compatible with the physical nature of flow. In this work, numerical methodology involving moving mesh technique is used in predicting the real flow behavior, as exhibited when a target blade of the impeller is made to move past corresponding vane on the diffuser. Many research works have been undertaken using experimental and numerical methods on the impeller-diffuser interactive phenomenon. It is found from the literature that the effect of radial gap between impeller and diffuser on the interaction and on the performance of the fan has not been the focus of attention. Hence numerical analysis is undertaken in this work to explore and predict the flow behavior due to the radial gap. This has revealed the presence of an optimum radial gap which could provide better design characteristics or lower loss coefficient. It is found that there is a better energy conversion by the impeller and enhanced energy transformation by the diffuser, corresponding to optimum radial gap. The overall efficiency also found to increase for relatively larger gap.

Numerical Analysis of the Whole Field Flow in a Centrifugal Fan for Performance Enhancement - The Effect of Boundary Layer Fences of Different Configurations

Generally the fluid flows within the centrifugal impeller passage as a decelerating flow with an adverse pressure gradient along the stream wise path. This flow tends to be in a state of instability with flow separation zones on the suction surface and on the front shroud. Hence several experimental attempts were earlier made to assess the efficacy of using boundary layer fences to trip the flow in the regions of separation and to make the flow align itself into stream wise direction so that the losses could be minimized and overall efficiency of the diffusion process in the fan could be increased. With the development of CFD, an extensive numerical whole field analysis of the effect of boundary layer fences in discrete regions of suspected separation points is possible. But it is found from the literature that there have been no significant attempts to use this tool to explore numerically the utility of the fences on the flow field. This paper attempts to explore the effect of boundary layer fences corresponding to various geometrical configurations on the impeller as well as on the diffuser. It is shown from the analysis that the fences located on the impellers near the trailing edge on pressure side and suction side improves the static pressure recovery across the fan. Fences provided at the radial mid-span on the pressure side of the diffuser vane and near the leading edge and trailing edge of the suction side of diffuser vanes also improve the static pressure recovery across the fan.

Effect of innovative circular shroud fences on a centrifugal fan for augmented performance - A numerical analysis

This paper presents the influence of circular front and back shroud fences on the performance of a backward swept centrifugal fan impeller handling air. The analysis is carried out to determine the main characteristics of the fan i.e. the variation of head and theoretical efficiency for various flow coefficients. The circular fence size is optimized parametrically by varying its diameter and the location on the shroud surface. The effect of the shroud fences on the flow field was analyzed for the fences placed on both front and back shroud surfaces of the impeller separately. The numerical simulation is carried out using an appropriate k - ε turbulence model. The relative flow inside the impeller passages is modeled using the standard sliding mesh technique. The numerical results for the base model (i.e. without the fence) are validated against experimental results obtained from the test rig built specially for validation study and are found to have good agreement. It is found from the analysis that the average percentage increase in head coefficient is significant and is about 2 % higher as compared to the base model for the optimized geometrical configuration of diameter ratio and radial fence location as determined in the study. The optimized geometrical configuration also yields a higher theoretical efficiency of about 2.3 % corroborating the improvement in head coefficient with respect to the base model. Hence the efficacy of optimally placed circular fence on the performance of centrifugal fan is established in this numerical study.

Investigations into the Flow Behavior in a Non-parallel Shrouded Diffuser of a Centrifugal Fan for Augmented Performance

It is a well-known fact that the diffuser of a centrifugal fan plays a vital role in the energy transformation leading to better static pressure rise and efficiency. Many researchers have worked on modified geometry with respect to both impeller and diffuser so as to extract better efficiency of the fan. This paper highlights a unique numerical study on the performance of a centrifugal fan, which has a diffuser having nonparallel shrouds. The shroud geometry is parametrically varied by adopting various convergence ratios (CR) for the nonparallel shrouds encompassing the diffuser passage. It is revealed in the study that there exists an optimal CR for which the performance is improved over the regular parallel shrouded diffuser passage (base model). It is observed from the numerical analysis that for a nonparallel convergent shroud corresponding to a CR of 0.35, a relatively higher head coefficient of 3.6% is obtained when compared to that of the base model. This configuration also yields a higher theoretical efficiency of about 2.1% corroborating the improvement in head coefficient. This study predicts a design prescription for nonparallel diffuser shrouds of a centrifugal fan for augmented performance due to the fact that the converging region accelerates and guides the flow efficiently by establishing radial pressure equilibrium.

Numerical analysis of a centrifugal fan for performance enhancement using boundary layer suction slots

Abstract Flow in centrifugal fans tends to be in a state of instability with flow separation zones on both the suction surface and the front shroud. The overall efficiency of the diffusion process in a centrifugal fan could be enhanced by judiciously introducing the boundary layer suction slots. With easy accessibility of computational fluid dynamics (CFD) as an analytical tool, an extensive numerical whole field analysis of the effect of boundary layer suction slots in discrete regions of suspected separation points is possible. This article attempts to explore the effect of boundary layer suction slots corresponding to various geometrical locations on the impeller as well as on the diffuser. The analysis shows that the suction slots located on the impeller blade near to its trailing edge appreciably improves the static pressure recovery across the fan. Slots provided at a radial distance of 30 per cent from the leading edge of the diffuser vane also significantly contribute to the static pressure recovery across the fan.Flow in centrifugal fans tends to be in a state of instability with flow separation zones on both the suction surface and the front shroud. The overall efficiency of the diffusion process in a centrifugal fan could be enhanced by judiciously introducing the boundary layer suction slots. With easy accessibility of computational fluid dynamics (CFD) as an analytical tool, an extensive numerical whole field analysis of the effect of boundary layer suction slots in discrete regions of suspected separation points is possible. This article attempts to explore the effect of boundary layer suction slots corresponding to various geometrical locations on the impeller as well as on the diffuser. The analysis shows that the suction slots located on the impeller blade near to its trailing edge appreciably improves the static pressure recovery across the fan. Slots provided at a radial distance of 30 per cent from the leading edge of the diffuser vane also significantly contribute to the static pressure recovery across the fan.