At motor and fan manu­fac­turer ebm-papst, advanced numer­ical simu­la­tion tools for calcu­lating fluid mechanics are part of everyday devel­op­ment. For many years, the company’s engi­neers have been working on devel­oping their fans using powerful CFD tools, not only for opti­mizing complex impeller geome­tries, but also for motor or elec­tronics cooling concepts or in the context of acoustic testing for noise char­ac­ter­is­tics. Customers can also benefit from this exten­sive know-how if they seek support in the aero­dy­namic devel­op­ment of their own end devices in which ebm-papst fans are to be used.

RadiCal 2: High air flow and quiet oper­a­tion

The RadiCal 2 series shows how aero­dy­namic opti­miza­tions of this kind have an impact in prac­tice. The compact centrifugal fans have been designed to deliver high air flows at low back pres­sure while oper­ating as effi­ciently and quietly as possible (Fig. 1). Several design details contribute to this, making it possible to achieve a ‘wound’ blade geom­etry that helps signif­i­cantly increase effi­ciency and reduce noise. Not only the blade shape, but also the profile of the inlet and outlet edges has been revised. This improves the flow behavior while at the same time increasing the strength of the fan impeller, which is made of resis­tant plastic.

The wavy cover plate also improves the aero­dy­namic char­ac­ter­is­tics and there­fore air perfor­mance. With the centrifugal module, i.e. the housing box, the inclined struts further increase the air perfor­mance. The axial height of the struts is selected to fill the back­flow areas in the best possible way. There is also an enlarged intake diam­eter, which ensures a greater air flow rate through the impeller.

The opti­mized Flow­Grid air inlet grille also helps to prevent noise. Compared to the earlier series version, both the intake side distur­bance and the fan itself is quieter, by at least between 1 and 3 dB(A). As the rotor area had a major impact on noise, the air inlet grille was also given a cap over the area of the motor. This enables the disrup­tive blade passing noise in the lower frequency range in partic­ular to be signif­i­cantly reduced. With regard to the envi­ron­mental factor of “noise”, the fans there­fore meet the strictest require­ments.

Air guide moduls for greater effi­ciency

The centrifugal fans in the RadiPac product range were designed and engi­neered espe­cially for use in air condi­tioning and ambient air devices and have been constantly improved over the past few years. Higher speeds ensure greater air flow and higher pres­sures, meaning that even appli­ca­tions with more than 2000 Pa static pres­sure increase can be covered. (Fig. 2) In addi­tion, noise gener­a­tion has decreased further; depending on the oper­ating point, the noise level is reduced by between 3 and 7 dB(A) compared to the prede­cessor series.

Special air guide modules increase the effi­ciency again by up to five percent (Fig. 3). This is achieved by reducing the outlet losses. For this purpose, ebm-papst has devel­oped a housing for the RadiPac, consisting of four segments. The module segments are made of galva­nized sheet steel and have an aero­dy­namic shape. This special shape slows down the flow, which reduces the dynamic pres­sure compo­nent and increases the usable static pres­sure compo­nent (Fig. 4). At the same oper­ating point, the fans can run at a lower speed, which in turn has a lower energy consump­tion and lower noise emis­sions. The RadiPac C Perform has exactly the same mounting hole pattern as the industry stan­dard, making it easy to change over.

With CFD simu­la­tion for optimum fan use

To ensure that the fans really play out their strengths, unfa­vor­able inter­ac­tions cannot occur after instal­la­tion in venti­lation and air condi­tioning units. That is why it is advis­able to simu­late the instal­la­tion situ­a­tion for aero­dy­namics in the early devel­op­ment phases of a customer device. The fan’s energy consump­tion and sound depends on how the housing encour­ages air flow, whether it is drawn in axially from the front, centrifu­gally from all sides or on one side. In poor cases, this can double the power consump­tion or halve the effi­ciency, and can signif­i­cantly increase the noise level. CFD helps in under­standing the aero­dy­namic condi­tions in the device (Fig. 5).

Before the simu­la­tion begins, the objec­tive should first be defined and a few ques­tions answered. Does air have to flow through a heat exchanger as evenly as possible? Is the simu­la­tion supposed to detect pres­sure losses? Does the customer want to perform a general check on the choice of fan? Is the device in which the fan oper­ates supposed to be as quiet as possible? Once the goal has been clar­i­fied, the 3D CAD data and the expected oper­ating point (pres­sure, air flow, speed) of the fan provide the basis for simu­la­tion.

In addi­tion, there are also char­ac­ter­istic curves from the filters, heat exchangers or guard grills installed in the device, for example. The data are cleared up and unnec­es­sary details are removed (cleaning). Then the simu­la­tion is prepared in a prepro­cessing phase, meaning that certain boundary condi­tions are spec­i­fied and the geome­tries are networked. The grid struc­ture divides the space that the air passes through into many indi­vidual cells, which form the basis of the math­e­mat­ical calcu­la­tion. The simu­la­tions then run on high-perfor­mance computers.

Prac­tical exam­ples: Exploiting poten­tial for opti­miza­tion

Using the simu­la­tion results, the engi­neers then esti­mate the poten­tial for opti­miza­tion and develop concrete sugges­tions for improve­ment. For example, a customer devel­oped an air purifi­ca­tion device in which the fan was to be used for pushing air out. However, the simu­la­tion showed signif­i­cant turbu­lence (Fig. 6), which increased the power consump­tion at the oper­ating point. Using the fan to draw air in, i.e. changing its instal­la­tion posi­tion, proved to be helpful here. The air flow is now very uniform, the power consump­tion is decreased and there is also a posi­tive effect on oper­ating noise. With an air condi­tioner for ceiling mounting, the simu­la­tion proved that it is advan­ta­geous to use a centrifugal fan instead of the usual axial fan.

This was initially coun­ter­in­tu­itive, as the oper­ating point of the device indi­cated that an axial panel fan would be the best choice. However, switching to a large centrifugal fan made an air duct design according to the centrifugal impeller design possible, which meant that no addi­tional deflec­tion was required. This was reflected in a power saving of over 70% with a reduc­tion in sound power level by 20 dB(A). The extent of the pres­sure losses caused by the deflec­tion of the air towards the side outlet was previ­ously always severely under­es­ti­mated. 

Find and utilise poten­tial with CFD

Before

After

Fig 6: In an air purifi­ca­tion unit, the air flow could be signif­i­cantly improved by changing the fan instal­la­tion posi­tion. The power consump­tion dropped and the device oper­ates more quietly. (Graphic | ebm-papst)

Another example: Using simu­la­tion result once again, the effi­ciency of a heat pump could be signif­i­cantly improved. Thanks to geometric changes, the pres­sure losses in the device fell by a total of 17 percent. The indi­vidual sub-areas in the device were assessed sepa­rately and the product design was adapted accord­ingly. The user has also bene­fited from the options offered by CFD simu­la­tion.

These exam­ples show that it is worth­while for end device manu­fac­turers to rely on numer­ical flow simu­la­tion and to apply these at an early stage of devel­op­ment to avoid subse­quent revi­sion costs. ebm-papst supports this with its long-standing CFD expe­ri­ence and offers such eval­u­a­tions and calcu­la­tions to ensure that the fans operate as effi­ciently as possible in the appli­ca­tion. Even small opti­miza­tions to the instal­la­tion situ­a­tion can reduce pres­sure losses, increase effi­ciency, or mini­mize running noise.