WHITE PAPER

THE FAN AND PROPELLER

DESIGN GUIDE

This paper addresses significant fan and propeller design considerations for
applications across a multitude of industries. Topics include types of each
component, regulatory and standard design factors, and how to use computational
fluid dynamics (CFD) to optimize designs.
Contents

2 Introduction
3 Types of Fans
5 Fan Design
6 Types of Propellers
7 Propeller Design
9 CFD for Turbomachinery
10 About SimScale
10 Why SimScale?

Introduction

Propellers and fans are major surrounding fluid. In turn, a

components in any rotating propeller’s fan blades are adapted
machinery or turbomachinery for use across many industries.
designs for applications across a Propellers and fans have the same
diverse set of industries. Fans have physics, yet a fan is generally
many crucial applications from wind stationary whereas a propeller
tunnels and cooling towers to causes linear motion to the object.
smaller electronic devices and The key mission in designing a
enclosures. Fan designs differ based hydrodynamic or aerodynamic
on power, flow rate, pressure rise, propeller is ensuring efficiency for
and efficiency. A propeller is a special the respective application. This
type of fan that converts rotational guide will explore how to improve
motion into thrust by producing a the design process and performance
pressure difference in the of both components.

2
 Types of Fans

In any application involving a fan For best performance, axial fans

component, 3 main types of fans will generally have the ability to work up
generally be employed; axial-flow, to about 800 Pa. While axial-flow
radial-flow, or cross-flow. fans can be used in a plethora of
applications from large to small scale
operations, they are most commonly
found in air conditioning and
industrial cooling systems. Other
everyday examples of this type of fan
include table fans, ceiling fans,
electro-mechanical fans, and
computer cooling fans.

The Axial-Flow Fan

Axial-flow fans are characterized by
large surface blades that push air to
move parallel to the shaft axis
around which the blades rotate. This
means the flow runs axially along
the shaft axis. Axial fans produce a
pressure difference which is
determined by the number and
shape of the blades ,as well as the
rotational speed, ultimately inducing
flow stream. These types of fans
have an average diameter of 300 to
400 mm up to 1800 to 2000 mm.

5
 When selecting fans for HVAC
applications, many engineers choose
centrifugal fans for their energy
efficiency (reaching up to 84% static
efficiency), durability even in
corrosive environments, ease of
maintenance, and suitability for
multiple airflows or pressure
combinations.
The Radial or Centrifugal Fan
The centrifugal fan (often called The Cross-Flow Fan
squirrel cage fan) is a mechanical The cross-flow fan (also called a
device specifically designed to move tangential fan) is used extensively in
air or other gases. Due to their the HVAC industry, as well as in
simple design, centrifugal fans are electronic applications. It produces
more easily manufactured than an even laminar airflow and allows
other types of fans, which renders horizontal or vertical mounting; in
them more time- and cost-effective addition, a cross-flow fan prevents
to produce. This makes centrifugal the overheating of active
fans a popular choice for components in a system.
air-handling applications.

Their rotating impellers utilize either

forward- or backward-curved blades
to increase the speed and volume of
an air stream. Additionally,
centrifugal fans have significantly
superior aerodynamic efficiency
than axial fans.

5
 Fan Design

High efficiency is an important

criterion in the fan design process, as
fans consume around 18% of
electricity in commercial and
industrial buildings and processes.
Fan energy consumption is receiving
much attention worldwide, with
emerging standards such as ASHRAE
90.1-2013 and Commission
Regulation (EU) No 327/2011. The
Air Movement and Control
Association (AMCA Intl.), as an
advisor to interested parties
worldwide, plays a major role in In addition to FEI, there are also
establishing appropriate fan requirements for fan efficiency
efficiency metrics for codes, based on metrics such as FEG (fan
standards, U.S. and European efficiency grade), FMEG (Fan motor
regulations, as well as efficiency grades). FEG is a
non-mandatory incentive programs. numerical rating that classifies fans
by their aerodynamic ability to
As part of a continuing effort to convert mechanical shaft power, or
create metrics that can be used to impeller power, into to air power.
reduce fan energy use and promote FMEG is similar, but also takes into
eco-friendly practices, AMCA has consideration the efficiency of the
developed the fan energy motor design operating the fan.
index, or FEI, which addresses both These standards greatly impact fan
fan design and fan application (sizing design conditions and thus their
and selection). This index is has been selection by engineers and
adopted across the globe. customers alike.

3
 Types of Propellers

Propellers are rotating fans that 4 Blade Propeller

transmit power through converting A 4 blade propeller has a higher
rotational motion to generate a manufacturing cost and is generally
thrust which imparts momentum to made from stainless steel alloys.
the fluid flow pushing the object These devices are characterized by
forward. These devices propel having steady low-speed, strength,
objects on the basis of Bernoulli’s durability, and general ability to
principle and Newton’s third law. A handle fluid conditions. These
pressure difference is created on the propellers also provide the best fuel
forward and aft side of the blade, economy.
and the fluid (air or water) is
accelerated behind the blades. 5 Blade Propeller
Propellers are generally classified by This type of propeller incurs the
the number of blades attached, and highest costs, but has the lowest
may vary from a 3 blade propeller to vibrations, and has the best holding
4 blade propeller and sometimes power in turbulent fluid flow.
even 5 blade propeller. However, the
most commonly used are 3 blade and
4 blade propellers.

3 Blade Propeller
The most basic type of propeller
contains 3 blades, and has lower
manufacturing costs than other
types of propellers. These devices
are normally made from aluminum
alloy, and have better acceleration
and high-speed performance
compared to other propellers.

5
 Propeller Design

Design parameters can impact the

performance of your propeller.
These variables can include the
number of blades, the size of the
outer diameter, the pitch-affecting
angle of attack, as well as the leading
and trailing edge blade angle along
with countless others.

The Number of Blades

Increasing the number of blades will
actually reduce the efficiency of the Lift and Drag Distributions
propeller but with a higher number Instead of the standard lift and drag
of blades there is a better coefficients, ensuring propeller
distribution of thrust helping to keep design efficiency requires specific
the propeller balanced, therefore a airfoils with prescribed angles of
trade-off must be established. attack at each radius. The
distribution of Cl (lift coefficient) and
The Diameter Cd (drag coefficient) along the radius
The diameter of the propeller has a can be examined by performing
significant impact on its efficiency. analysis for the design point. For
Larger propellers have the capacity maximum efficiency, the airfoils must
to create more power and thrust on operate at maximum L/D. If the
a larger fluid volume. Yet, most propeller should also work fairly well
designs face limitations when it under poor conditions, it is usually
comes to diameter, so optimization necessary to use a lower angle of
must occur elsewhere. attack for the design.

3
 Flow freestream velocity
The flow freestream velocity is
another important variable to
consider. This velocity, along with
the rotational speed (RPM)
determines the pitch distribution of
the system. Large propeller designs
can become less effective operating
at the axial velocity.

Fluid Flow Density

While the actual density of the fluid Similarly to fan design, propellers
has no effect on the efficiency of the must adhere to their own regulatory
system, it does play a role in defining conditions. Most notably, aircraft
the shape and size in the initial propellers have strict environmental
design process. For example, an air noise legislations to follow in order
propeller used for planes will have a to mitigate disturbance and noise
bigger diameter than its aquatic pollution. In turn, aquatic propellers
counterparts, as the fluid density is for ships and underwater operations
less. are held to efficiency standards just
as with stationary fans.

As both propellers and fans can be

modified to improve overall
performance, efficiency, and
compliance, design optimization
requires a multifaceted approach
that should include online
simulation.

4
 CFD for Turbomachinery

To meet efficiency goals, the classical engineering simulation from

approach to design – build – test – platforms such as SimScale.
redesign is time-consuming and Computational fluid dynamics (CFD)
expensive. Prototyping for different is the method that is used for the
design iterations is also not resource computerized calculation of fan and
efficient and not an environmentally propeller aerodynamic quality to
friendly method due to incurred accurately predict fan performance
waste. Commercial and industrial earlier in the design stage, analyze
fan and propeller designers alike are various designs, reduce the number
turning to a more effective method of physical prototypes and expensive
to virtually test, make accurate testing, optimize designs for
aerodynamic assessments, and maximum performance, shorten the
optimize using online design cycle and minimize costs.

Cross flow fan CFD analysis

7
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