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Axial, diag­onal, centrifugal: which fan fits which appli­ca­tion?

There are axial, diag­onal and radial fans, which can bring their specific advan­tages to bear in special appli­ca­tions. Which fan type is best suited for which appli­ca­tion is deter­mined by the pres­sure build-up.

Moving air and creating pres­sure at best effi­ciency and with least noise emis­sions – this is one way to describe the objec­tive of fans. For many appli­ca­tions, they are the best option also because they create a contin­uous flow, have small space require­ments and have only few moving parts. To under­stand which fan design, centrifugal, axial or mixed, is the best for a specific appli­ca­tion, one needs to look at the basic mech­a­nism of fan oper­a­tion.

Pres­sure deter­mines which fan fits

In prin­ciple, the fan blades of an axial-flow fan deflect the airflow from an axial inflow direc­tion into a helical flow pattern, thereby increasing the (total) pres­sure across the rotor. To get higher pres­sure, larger flow angles to the rotating blades are required. However, this prin­ciple has natural limits. When the rela­tive flow angle becomes too large, the aero­dy­namics become less effi­cient and the detached flow creates increasing noise.

Axial fans

In axial fans, the incoming air is deflected onto spiral paths by the blades. (Graphic | ebm-papst)

Diag­onal fans

In diag­onal fans, the air is also drawn in axially but discharged diag­o­nally. (Graphic | ebm-papst)

Centrifugal fans

Centrifugal fans rely on the centrifugal force of the impeller. (Graphic | ebm-papst)

In cases where more pres­sure rise is required, centrifugal forces are used in addi­tion to the blade aero­dy­namics. Since every fan is a rotating system, the air is always exposed to centrifugal forces. Beyond their best oper­a­tion point, the flow in axial fans develops a strong centrifugal compo­nent, while large portions of the flow path are aero­dy­nam­i­cally blocked due to recir­cu­lating air. Effec­tively, the fan oper­ates like a smaller but poorly designed centrifugal fan in this regime.

For those appli­ca­tions, a dedi­cated centrifugal design is the best choice. In centrifugal fans, centrifugal forces contribute signif­i­cantly to the overall pres­sure build-up, in some cases they are the domi­nant mech­a­nism. Axial fans are best applied to cases where a fairly small pres­sure rise and large flow rates are required, while centrifugal fans are able to deliver higher pres­sure together with smaller flow rates for similar external dimen­sions and speeds.

Optimal oper­ating ranges of the three fan designs. (Graphic | ebm-papst)

Opti­mizing fans with CFD

With these basic consid­er­a­tions the actual design can be started. In the past, fans were designed mainly exper­i­men­tally and a powerful expe­ri­ence base has been estab­lished over the years. Today, compu­ta­tional fluid dynamics (CFD) methods are applied to the aero­dy­namic design process. CFD is used in all kinds of systems where heat and mass transfer is impor­tant, for entirely new fans as well as for opti­mising compo­nents like blades or blade-tips. In prin­ciple, the aero­dy­nam­i­cist can design a new fan without prior knowl­edge of the fan type because the CFD method repre­sents a compre­hen­sive math­e­mat­ical flow model. It is often easier to perform so-called numer­ical exper­i­ments than conducting an exper­i­mental campaign, espe­cially since the CFD results allow for very detailed analyses of the flow field.

Visu­al­iza­tion of air flow through a venti­lation unit. ( Photo | ebm-papst)

With CFD, blade perfor­mance, loss and noise mech­a­nisms or the inter­ac­tion of the flow with walls can be under­stood and improved for every oper­ating point of interest. In times where high effi­ciency and cost-effec­tive solu­tions are key drivers, the aero­dy­nam­i­cist tries to squeeze out every possible percent of the avail­able design space. The curved and twisted blades of modern fans or the specific blade tips are exam­ples of such efforts. With state-of-the art design tools it is possible to design the fan for a specific appli­ca­tion rather than trying to tailor an existing fan to a new appli­ca­tion.

Together with the success of CFD, the focus of the exper­i­ments has changed. From being a basic design method to a neces­sary veri­fi­ca­tion and vali­da­tion method: on one hand, of the overall perfor­mance and on the other hand to verify or dismiss the impact of features like guide vanes or down­stream diffusers.

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