Cross-flow fans and axial-flow fans

1. Through-flow fan
The cross-flow fan, also known as a through-flow fan, consists of an impeller, a volute, and a tongue. The impeller is cylindrical with multiple blades; part of it is open, while the other part is enclosed by the volute. Unlike centrifugal fans, the volute has no air inlets on either side. As the impeller rotates, airflow enters the blade grid from the open side, passes through the interior of the impeller, and then exits from the opposite side of the blade grid into the volute, forming the working airflow. The airflow within the impeller is quite complex, and the velocity field inside the impeller is unstable. Observations indicate that vortices exist within the impeller.

The center of the vortex is located near the blade tip. The presence of the vortex creates a circulating flow at the outlet of the impeller, while outside the vortex, the streamlines of airflow within the impeller take on an arc-shaped configuration. Consequently, the gas flow velocities at various points along the outer periphery of the impeller are not uniform: the closer one is to the center of the vortex, the higher the flow velocity; the closer one is to the volute, the lower the flow velocity. At the fan’s outlet, both the air velocity and pressure become uniform, and the fan’s flow coefficient and pressure coefficient represent average values.

The location of the vortex significantly affects the performance of a cross-flow fan. When the center of the vortex is close to the inner rim of the impeller and near the tongue, the fan exhibits better performance. However, if the vortex center moves farther away from the tongue, the area of recirculating flow increases, leading to reduced fan efficiency and unstable flow rates. The shape of the housing, the position of the tongue, and the pressure difference between the fan’s inlet and outlet all have a significant impact on the location of the vortex center. Currently, the optimal ranges for various dimensions are primarily determined through experimentation.

As shown in the cross-flow fan performance curves below, at higher flow rates, the ratio of dynamic to static pressure is greater. When the fan diameter is smaller, it can generate a larger flow rate. However, since the static pressure curve has a hump-shaped profile, unstable operation may occur at lower flow rates.

2. Axial-flow fan
In a centrifugal fan, the airflow within the impeller flows radially, whereas in an axial-flow fan, the airflow flows axially through the impeller. An axial-flow fan consists of a shroud, an impeller, guide vanes, a diffuser, a collector, and a duct. The impeller rotates to become the rotor, while the other components remain stationary. During operation, the airflow enters through the collector, gains energy as it passes through the impeller, then flows into the guide vanes, where it is converted into an axial flow. Finally, the airflow passes through the diffuser, during which part of its kinetic energy is transformed into static pressure energy. After exiting the diffuser, the airflow enters the duct.

By extending the profiles of the impeller and guide vanes along a certain radius R, a set of planar blade meshes can be obtained, as shown in the figure below. The shape of the blade mesh significantly influences the fan’s flow rate, pressure, and efficiency, making it a critical aspect of fan design.
The performance curves of axial-flow fans are similar to those of centrifugal fans and can also be represented by dimensional and dimensionless curves. However, even within the same series and with the same model number, the performance curves may differ due to variations in blade installation angles.

3. Fan Performance Testing
There are many types of fans used in agricultural machinery, some of which have not yet been standardized into series, and it may even be difficult to find a suitable model fan. In such cases, theoretical calculations are required for design purposes. Due to the inherent imprecision of theoretical designs, experiments are often necessary to make corrections, making fan testing an extremely important task.

The aerodynamic performance testing of fans can be divided into field measurements and tests conducted using experimental setups. Field measurements refer to the aerodynamic performance measured at the actual site of use; due to indoor conditions and measurement environments, such measurements often lack precision. However, performance measurements for fans that have already been installed are relatively convenient.

1. Fan performance testing equipment
Commonly used test equipment includes intake, exhaust, and combined intake-exhaust types, which can be selected according to the actual operating conditions of the fan. For example, when working with an intake duct, an intake test device can be used; when working with an exhaust duct, an exhaust test device can be used; and if the fan has long working ducts on both the intake and exhaust sides, a combined intake-exhaust test device should be chosen. Since intake test devices are simple and convenient, most experiments routinely use this type of test equipment for fan performance testing.

2. Method for Determining the Basic Parameters of a Fan
The basic parameters of the fan’s gas characteristic curves include flow rate, pressure, power consumption, and efficiency. The flow rate is measured using a flow collector, which can be either arc-shaped or conical, with holes in its walls for measuring static pressure. In the absence of losses, the dynamic pressure and static pressure at the j-j section are equal.