Investigation of design methods for forward curved radial fan using CFD

Fan is a type of turbomachine which transfers energy to air. Axial fans and centrifugal fans are main categories of fans. In axial fans, flow enters the impeller in axial direction and leaves in axial direction. But in centrifugal fans air enters the impeller axially and leaves radially. Centrifugal fans or in other name radial fans can be categorized in three different groups which are backward curved (BC) radial fans, straight radial and forward curved (FC) radial fans. BC radial fans are the most popular type in these three. Their blades are curved opposite direction to impeller's rotation direction so their blades' outlet angle's are less than 90°. Most theoretical and experimental studies have been made by using them. Theoretical design parameters were also derived for this type of fans. Efficiency of this type fans is higher than FC ones but dimension of this fans is larger than FC radial fans for same flow rate. Their theoretical pressure rise is decreasing with increasing flow rate. Straight radial fans' blade outlet angle is 90°. They have been used for their easier and cheaper construction. Their theoretical pressure rise is constant with increasing flow rate and their theoretical power consumption is increasing linearly with increasing flow rate. FC radial fans in other names Sirocco fans or squirrel cage fans have been used since 19th Century mostly in air conditioning systems, automobile industry and cooling of electronic devices. Blades are curved in the same direction with impeller's rotation direction so their blade outlet angle's are larger than 90°. Their theoretical pressure rise is increasing with increasing flow rate. Their main characteristics are low noise level and smaller size comparing to same flow rate with other type of radial fans. As a result of they pump same flow rate at low rotational speed, low noise is produced in this type. Disadvantage of FC radial fans is their efficiency which is not higher than %50-60. In FC radial fans number of blades is higher and blade lenght is smaller than other types. In low flow rates stall region is seen and it is main characteristic of this fans and generally flow separation zones are seen in blades. Although FC radial fans are widely used, their flow characteristics and design parameters are less studied comparing to BC radial fans. If we compare these three types of radial fans for the same flow rate, same pressure rise, same rotational speed and same blade inlet angle, it can be said that backward curved radial fans have lower number of blades and longer blade length than other types and forward curved radial fans have higher number of blades and shorter blade length than other types. Studies about the forward curved radial fans can be diveded four categories which are, studies done to understand flow field inside the FC radial fans, studies done to improve efficiency of FC radial fans, studies done to obtain characteristic curves and improve fan performance and studies done to understand noise sources of FC radial fans. The most recent study on forward curved radial fans is a doctorate thesis which is done in Germany in 2015. Numerical and experimental studies are done to understand noise and performance characteristics of forward curved radial fans. Design method isn't studied in that thesis. Two different analysis methods are used for CFD analyses which are rigid body motion (RBM) and moving referance frame (MRF). Rigid body motion method is an unsteady method which use sliding mesh for modeling rotation. Moving referance frame method gives steady state solutions by modeling the rotation adding Coriolis and centrifugal terms to Navier Stokes equations. It is offered that in overload region of fans MRF can be used for modeling of the flow inside the fan but in throttle region rigid body motion method gives more accurate results. In this thesis main purpose is to understand the parameters which mostly affect the design of a FC radial fan. In literature some design methods have been used for fan design but these methods are not capable enough and mostly do not yield correct results for FC radial fans. The most popular design method is combination of Euler's one dimensional turbomachinery equations and Pfleiderer's experimental corelations which is named in this thesis as classical method. This method mostly used for BC radial fans but it also has some modifications for FC radial fans. According to literature this method can not use without modification for FC radial fans. Other design method is Peng's method which based on theoretical derivation for critical design parameters like blade inlet angle, number of blades and blade width. It offers constant blade inlet angle for all FC radial fans. All design methods uses Cordier diagram for first step of design. Cordier diagram shows the relation between non-dimensional speed and non-dimensional diameter. It is derived from datas of high efficient fans. It shows the range of axial fans, mixed-flow fans and radial fans on its curve. FC radial fans stand in different and narrow region on the diagram and in this thesis designs are made using this region. Different diagrams can be seen in literature. The most important thing in here is the unit of the rotational speed. Some old diagrams use rotational speed as rad/s while some recent studies use the unit of rotational speed as rpm. In all designs, volute designs are made by using same method for back to back comparison. Firstly, experimental results which we found in literature is verified by using the most populer commercial codes ANSYS FLUENT and ANSYS CFX. Mass flow inlet and zero pascal pressure outlet boundary conditions are used during these analyses. Also mesh independency study is done using five different number of elements and it is seen that 3.1 million mesh is enough for analyses. In turbomachinery analyses for modeling the rotation two different methods are mostly used which are sliding mesh method and moving reference frame method (MRF). MRF method in other name frozen rotor method is used in all analyses because of the low analysis time and low cost. In sliding mesh method time dependent solution is needed and it is time consuming method for radial fans, but in MRF method steady state solutions are made by adding some terms to Navier-Stokes equations. In FLUENT k-epsilon realizable, k-omega SST and Spalart Allmaras and in CFX k-epsilon RNG, k-omega SST and Spalart Allmaras turbulance models are used for verification. As a result of these analyses CFX gave more accurate results when we compare with experimental data. Also in these three different turbulance models all models predict the trend of the fan but k-omega SST gave more accurate solution comparing to experimental data. Then analyses' parameters which we used in CFX like turbulance model and others are determined to use our fans' CFD analyses. Other analyses are made by using CFX. Five different fan designs are made by using different design methods for 216 m3/hour flow rate, 420 Pa pressure rise at 2900 rpm rotational speed. Prelimary design is made by using combination of Euler's turbomachinery equations which is a one dimensional design method and Pfeleiderer's experimental corelations. It is a one dimensional design method and mostly used for BC radial fans and named as classical method in this thesis. Even good results are achieved for backward curved radial fans, this method is not suitable for forward curved radial fan design. This method gives low number of blades and low blade inlet angle comparing to our expectation. According to results it is seen that this design does not work at design point so this method is not reccommended for FC fan design without modification. Second design is made by using Peng's parameters which is offered in his book. After some theorical calculation Peng offers 37° for blade inlet angle. Impeller width is smaller than other methods and expactations. After CFD analyses, this impeller does not work at our design point. These parameters can not used for forward curved radial fan design. In the third design we used classical design method with genarally used parameter for FC radial fans in literature like blade inlet angle, blade outlet angle and number of blades. With this corrections it is seen that this fan works at our design point. It can be said that these three parameters are most important ones for FC radial fan design and parametric studies should be done. In the fourth design we changed blade inlet angle to 90° which we used in third design to see if it work on Peng's method. According to results blade inlet angle doesn't affect fan characretistic alone and this impeller does not work at our design point. In the fifth design we changed impeller width to 40 mm which we used in third design. Fan characteristics become better than Peng's method but it is not close to our design point again. As a result of this five designs we see that Peng's theorical design parameters are not realistic and can not use for forward curved radial fan design. Classical design method also can not use without modification. Also blade inlet angle doesn't affect fan characteristic alone. Impeller width is important parameter for fan design but not enough alone. Parametric studies should made for combination of blade inlet angle, blade outlet angle and blade width.