**Hsin-Hung Lin 1,2,\* and Jui-Hung Cheng <sup>3</sup>**


Received: 1 July 2019; Accepted: 21 July 2019; Published: 30 July 2019

**Abstract:** In this study, different designs of the opening pattern of computer fan grills were investigated. The objective of this study was to propose a simulation analysis and compare it to the experimental results for a set of optimized fan designs. The FLUENT computational fluid dynamics (CFD) simulation software was used to analyze the fan blade flow. The experimental results obtained by the simulation analysis of the optimized fan designs were analyzed and compared. The effect of different opening pattern designs on the resulting airflow rate was investigated. Six types of fans with different grills were analyzed. The airflow velocity distribution in the simulated flow channel indicated that the wind speed efficiency of the fan and its influence were comparable with the experimental model. The air was forced by the fan into the air duct. The flow path was separately measured by analog instruments. The three-dimensional flow field was determined by performing a wind speed comparison on nine planes containing the mainstream velocity vector. Moreover, the three-dimensional curved surface flow field at the outlet position and the highest fan rotation speed were investigated. The air velocity distribution at the inlet and the outlet of the fan indicated that among the air outlet opening designs, the honeycomb shaped air outlet displayed the optimal performance by investigating the fan characteristics and the estimated wind speed efficiency. These optimized designs were the most ideal configurations to compare these results. The air flow rate was evenly distributed at the fan inlet.

**Keywords:** fan design; numerical simulation; fan experiments; axial flow fan; electronics cooling

### **1. Introduction**

The opening pattern of an axial fan grill is one of the most important factors of the resulting airflow rate. The fan grill is provided with a number of significant characteristics. From the standpoint of aerodynamic performance, the pressure rise will decrease depending on the different types of opening patterns on the axial fan grill. The increased density of the boundary layer of the fan grill opening pattern affects the exhaust flow of an axial. The resulting phenomenon is the influence of the flowing vortex when the airflow passes the fan grill. On the other hand, the style of the fan grill also affects the overall efficiency of the intake airflow when the blade design remains unchanged. A smaller aspect ratio and a higher opening ratio allow for an axial fan to generate better outflow performance. The overall airflow rate can be increased by optimizing the opening pattern of the outflow when considering the complex aerodynamic parameters [1].

The influence of different fan grill patterns on the resulting airflow velocity has been investigated. In the simulation, the airflow velocity distribution indicated the efficiency of the air velocity. The three-dimensional flow field of the experimental model was also verified from the verification of the

airflow velocity. One of the most important factors that affect the resulting airflow velocity of an axial fan grill is its characteristics. Sergio Marinetti et al. 2001 carried out the investigation of the rotational speed of a fan by forcing airflow through an evaporator and two fans. The three-dimensional flow field was investigated by measuring the airflow distribution at different elevations at the evaporator inlet and outlet. The results indicated a uniform air velocity distribution at the evaporator inlet [1]. Gebrehiwot et al. 2010 applied the CFD approach to the investigation of the performance of a cross-flow fan by using three fans with similar geometry at the cross-flow opening. The fan load can be determined from the impedance curve of the perforated plates with different openings. The results of the wind tunnel testing indicated that the non-uniform airflow distribution at the air inlet along the width of a cross-flow fan was an important factor for the CFD simulation [2]. The findings from Chen's et al. 2009 investigation indicated that the design of a distorted stator of a transonic fan presented better aerodynamic characteristics [3]. Betta et al. 2010 studied the fluid dynamic performance by comparing it to the conventional axial ventilation system and by applying the CFD analysis to the system by the *k–e* model [4]. Li et al. 2008 carried out the analysis and experimental investigation of the aerodynamics of forward inclined impellers and radial low pressure axial impellers. The design of the forward-inclined blades was optimized by CFD techniques and by measuring the aerodynamics and aeroacoustics of the two blades. In comparison to typical radial blades, a forward-inclined blade was proven to be able to improve the efficiency by carrying out a detailed flow rate measurement and the calculated exhaust flow field. The results indicated that a forward-inclined blade could trigger the redistribution along the radial direction and reduce the tip overload [5]. Delele et al. 2005 A CFD model was developed for the investigation of the three-dimensional airflow pattern of the ground velocity of the cross-flow air sprayer. The researchers conducted a simulation of the rotational speed of two different fans, and the results of the instant cross-sectional velocity distribution and simulation of the maximum vertical exhaust velocity and the change in directions indicated good consistency. This is due to the fact that the magnitude of the exhaust velocity of the jet flow is larger and the change in direction is greater [6]. The flow allocation and performance investigation were carried out by CFD approaches and anemometers in order to determine the effect of the balanced split vortex walls between two grid points. The results indicated that a two-dimensional CFD model could be used to predict fan performance to an acceptable level, especially at the portion between two outlets. The modification indicated a more uniform air vortex distribution at the walls and the mass flow rate between the two outlets affected the rotor exhaust air to a greater extent [7]. Li et al. 2011 carried out the comparison between two impellers with diameters 5% and 10% larger than the original impeller by both numerical analysis and experimental study. The numerical simulation of the internal characteristics indicated that the flow speed and total pressure increased, so the axial power and sound pressure also increased. When the efficiency decreased, the impeller needed to have a larger diameter for a better operating point [8]. The fan assembly could keep the average level of the components of the heat transfer coefficient. The results indicated that if a fan design could resolve the problem of cross-flow environment, the heat transfer efficiency could be improved by 30% [9]. Other studies have implemented CFD approaches in the flow field of axial fans and their performance and characteristics. The deviation of the fan performance curve obtained by experimental results reached the horizontal DFR method, and the use of static meshes and moving meshes was less than 3% and 1.5%, respectively. This presents significant improvement to the conventional approach, which has a significant deviation of 26% [10,11]. Hu et al. 2013 the calculation includes three steps: firstly, the unsteady viscous flow around the blades is calculated using the CFD method to acquire the noise source information; secondly, the radiated sound pressure is calculated using the acoustic analogy Curle equation in the frequency domain; lastly, the scattering effect of the duct wall on the propagation of the sound wave is expressed using the thin-body BEM method [12]. Owen 2013 a comparison of test data collected at an existing ACC (cooling performance of an air-cooled condenser) and numerical data generated in a CFD analysis of the flow around the same ACC shows a discrepancy in the predicted effects of wind on fan inlet temperature. Careful analysis of the test data indicates the potential involvement of atmospheric temperature distributions in fan inlet

temperature deviations. A numerical case study is conducted considering four differing atmospheric temperature distributions [13]. Detailed flow measurement and computation were performed for outlet flow field for investigating the responsible flow mechanisms. Yang et al. 2008 The results show the forward-skewed blade can cause a spanwise redistribution of flow toward the blade mid-span and reduce tip loading [14]. Carolus et al. 2007 In the limits of the necessary assumptions the SEM (a simple semi-empirical noise prediction model) predicts the noise spectra and the overall sound power surprisingly well without any further tuning of parameters; the influence of the fan operating point and the nature of the inflow is obtained. Naturally, the predicted spectra appear unrealistically "smooth", since the empirical input data are averaged and modeled in the frequency domain. By way of contrast the LES (The numerical large eddy simulation) yields the fluctuating forces on the blades in the time domain. Details of the source characteristics and their origin are obtained rather clearly. The predicted effects of the ingested turbulence on the fluctuating blade forces and the fan noise compare favorably with experiments [15].

The objective of this study is to investigate the performance of different gap designs of fan grills. The flow field of an axial fan was investigated by CFD simulation and the result was compared to experimental one. The airflow velocity of the resulting airflow was determined in the three-dimensional flow field. The maximum rotating speed at the fan outlet was also measured. The distribution of the airflow velocity at the fan inlet and outlet was analyzed in order to determine the effect of different gap designs. The objective is to generate uniform airflow velocity at the fan inlet so that an optimal configuration can be created.
