The effect of the entrance hub geometry on the efficiency in an axial flow fan

doi:10.1016/j.ijrefrig.2019.02.026

International Journal of Refrigeration

Volume 101, May 2019, Pages 90-97

https://doi.org/10.1016/j.ijrefrig.2019.02.026 Get rights and content

Highlights

•
The loss of the axial fan in an outdoor unit occurs mainly in the end-wall region.
•
The interaction between hub entrance vortex and a blade affects the fan efficiency.
•
The certain condition of cleaved hub vortex by a blade increases the fan efficiency.

Abstract

The improvement in efficiency of axial flow fans is major concern in turbomachinery industries including outdoor units of air conditioners. The end wall region is responsible for losses in axial flow fan. Researches on the relationship between the efficiency and the vortex occurring on the hub with a short length have been little found in the literature. Variations of the vortical flow field and the effects on the efficiency have been investigated with the entrance hub length and corner shape of the axial flow fan. The losses and the efficiency were evaluated by the stagnation pressure loss coefficient distribution calculated using commercial CFD solver SC/Tetra v12. The results show the efficiency was improved at specific entrance hub length due to the interaction between the hub vortex and the blade.

Introduction

An axial flow fan in an outdoor unit of an air conditioner is used for cooling the condenser installed in the unit. Numbers of researches on axial flow fans for outdoor units are ongoing in order to improve the performance and efficiency of air conditioners and researches including a detailed analysis on flow field have been conducted owing to the development of experiment technique and computational fluid dynamics since 2000s (Jang et al., 2001a, Jang et al., 2001b).

Denton (1993) systematically classified the losses in turbomachinery, for example an axial flow fan. Of all, tip leakage flow and end-wall loss have been pointed as the main loss factors. The relationship between the fan efficiency and the tip leakage vortex depending on tip clearance, winglets, and shroud height of an axial flow fan have been studied (Jung et al., 2018); and the authors aimed to analyze the vortical flow near the hub, which is one of the causes of end-wall loss in axial flow fan in this study.

The end-wall loss of axial turbomachinery has been known to occur by the secondary flow that becomes stronger when the pressure gradient is large; hence, numerous studies have been performed in the field of axial flow compressors. The end-wall loss in an axial compressor is caused by corner vortex near the hub in most cases. Such corner vortex can occur even in the absence of adverse pressure gradient and is a phenomenon occurring due to the flow turning and wall boundary layer. The corner vortex is a complicated 3-dimensional flow phenomenon and various studies on the corner vortex have been conducted by adopting experiment and numerical analysis in the field of fluid mechanics (Lei et al., 2008). However, there are few researches on the end-wall loss occurring near the hub in axial flow fans, which is a low-pressure fluid machinery. In cases of axial flow fans, the strength of corner vortex is not strong and hub is not continuous unlike axial compressors; therefore, the influence of the vortex that occurs due to the shape of the hub at the entrance of the hub is thought to be dominant on the main flow compared to that of the corner vortex. In case of a high pressure axial flow fan, in particular, the front end of the hub is in cone shape and the influence of the loss occurring by the front end is relatively weak; hence, blade profile (Panigrahi et al., 2014) or the flow near the tip (Pogorelov et al., 2016; Li et al., 2014; Ye et al., 2015) rather than the flow near the hub have studied in most researches. Majority of the studies on the flow near the hub of an axial flow fan are focused on the corner stall or horseshoe vortex (Chu et al., 2016).

Occurrence of the disturbance including a vortex and a separation in the hub of an axial flow fan indicates the inflow of non-uniform flow on the blade row. The non-uniform inlet flow can be caused by the influence of the hub and various shape of the inlet. The influence of the non-uniform inlet flow on the fan performance has been studied in many researches. Salta et al. (1995) conducted experimental research on inflow distribution that varies by the installation location of a fan, which is applied to air-cooled heat exchangers. They found that as the distance between the ground and the fan inlet gets closer, the system efficiency decreases due to the non-uniform inlet flow. Duvenhage et al. (1996) studied about the identical subject by using CFD and compared the fan performance depending on the shape of the inlet shroud such as cylindrical, conical, and bell-mouth shape. As a result, they derived the critical length for each shroud type for optimal fan performance. Charalambous et al. (2004) used CFD to analyze the influence of the non-uniform flow on the inlet flow distortion of an axial compressor. They compared the influence on the performance characteristics of the axial compressor focusing on the various inlet distortions in axial, radial, and circumferential direction. Lee et al. (1998) studied the influence of the non-uniform inlet flow on the tonal noise of a fan. They reported that large discrete noise was observed when the inlet flow of the fan had a non-uniform radial velocity distribution. There was a case of attaching a cone-shaped hub cap in the anterior portion of the hub in order to introduce uniform flow by reducing the disturbance caused by the hub. Jang et al. (2008) studied the inflow distortion depending on the hub cap shape and hub entrance length. They found that the inflow characteristics were determined by the distance between the hub cap and the blade leading edge, and that a non-uniform axial inlet velocity profile near the hub caused a change in inlet flow angle. However, a hub of an axial flow fan applied to an outdoor unit of an air conditioner is difficult to be designed in a less-loss form (cone shape) due to spatial and cost issues. Therefore, the front end of the hub takes blunt shape in general. Separation occurs abrupt change of the flow in front end in blunt shape and the edge of the hub, and the vortex occurring there dominantly affects the flow near the hub of the axial flow fan. However, few researches studied this phenomenon since the phenomenon did not attract attention as a research subject. Jang et al., 2001a, Jang et al., 2001b) confirmed strong pressure fluctuations occurring in the region near the hub of the leading edge while studying tip leakage flow of an axial flow fan for an outdoor unit of an air conditioner; however, they determined that this was also the influence of the tip leakage vortex. In addition, Park et al. (2017) analyzed the flow field of the noise produced by an automotive cooling fan by using CFD. Despite that the CFD result presented a strong vortex produced at the front end of the hub, they only concentrated on the vortex presented near the tip. Besides, only a few articles have studied the relationship between the efficiency and the vortex occurring near the hub of the axial flow fan.

Meanwhile, defining the loss occurring in turbomachinery and analyzing the cause of the loss is important for conducting researches to improve the efficiency of an axial flow fan. Here, Denton (1993) introduced the stagnation pressure loss coefficient as a way of expressing loss that is the most commonly used. However, the stagnation pressure loss coefficient can only present the overall value of the aerodynamic loss occurring from the blade row of turbomachinery and 3-dimensional distribution of the loss has seldom been studied. While Zhang et al. (2016) was studying the nozzle guide vane applied to turbines, they have distinguished the properties of the loss occurring from the blade row by tracing the changes in entropy in the direction of axis; however, detailed distribution of the loss was not described. In this study, 3-D distribution of the stagnation pressure loss coefficient was drawn from the results of the numerical analysis on the axial flow fan for outdoor units of air conditioners and the loss was distinguished according to the occurring location and the cause on the basis of the 3-D distribution. Subsequently, the contribution of each kind of the loss occurring in the axial flow fan to the reduction in efficiency of the fan was analyzed. In addition, the characteristic of vortical flow that was changed with the alterations in the shape of the hub was analyzed and consequent changes in efficiency and the distribution of the loss coefficient were studied.

Section snippets

The model fan

The detailed geometry of the axial flow fan is shown in Fig. 1. The height of the shroud was 30% of the fan's axial length and the shroud was divided into round-shaped inlet, straight line part, and diffuser part of the outlet. In the blade tip of the axial flow fan, swept-back tip winglet was applied. The tip clearance was 3.2% of the fan's radius. The fan's hub was in cylindrical shape and its diameter was approximately 40.5% of the fan's diameter. The height of the hub is determined by how

Validations for the numerical method

In this study, tetrahedral mesh was applied to entire calculation domain. In addition, the mesh aligned in a shape of 3-layer-prism was applied to the surface of the wall of the fan and shroud. Mesh sensitivity test was performed in order to secure the reliability of the calculated results, and it was found that the appropriate number of mesh was approximately 26 million and y⁺ on the blade and hub surface was approximately 1∼2.

To validate the accuracy or numerical calculation, the results were

Definition of hub vortex

An axial flow fan used in an air conditioner usually has a blunt-ended hub. As shown in Fig. 5(b), flow is separated at the hub corner edge and reattached to create a separation bubble. Then, separation bubble is shed from the hub edge to form vortex due to the interaction with the potential field upstream of the blade. In this study, this vortex is called as ‘hub vortex’, and the effects of the hub vortex were investigated with the entrance hub length and shape of the hub on the

Conclusion

Using stagnation pressure loss coefficient obtained from calculations the loss in the hub of the axial flow fan and region proportion of losses was investigated. In addition, the trend of change in pressure loss coefficient and that of efficiency were confirmed to be similar. We also investigated the effect of the changes of entrance hub length and shape on hub region loss. Causes of changes in efficiency were able to be understood by observing the changes in distribution of the pressure loss

Acknowledgement

This work was supported by the Korea Evaluation Institute of Industrial Technology (KEIT) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 10084659). Author is enrolled in Ph.D. course as dispatched from LG electronics with tuition provided by LG electronics.

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The effect of the entrance hub geometry on the efficiency in an axial flow fanL’effet de la géométrie du moyeu d’entrée sur le rendement d’un ventilateur hélicoïde

Highlights

Abstract

Introduction

Section snippets

The model fan

Validations for the numerical method

Definition of hub vortex

Conclusion

Acknowledgement

Aerosp. Sci. Technol.

Appl. Therm. Eng.

Int. J. Refrig.

Energy

J. Sustain. Min.

Appl. Acoustics

Int. J. Heat Fluid Flow.

Energy