Investigation of a thermomechanical improved tangential fan geometry

In convection-dominated thermoprocessing plants, the current state of the art is to use centrifugal and axial fans in conjunction with flow straighteners to ensure a homogeneous inflow of the fluid and thus a homogeneous temperature distribution. However, flow straighteners lead to an additional pressure loss in the system that must be overcome. Tangential fans can deliver a homogeneous volume flow over the entire width of the fan even without flow straighteners. Therefore, they offer a possibility to increase the energy efficiency of these systems. Based on this motivation, the possibilities of increasing the thermomechanical long-term stability of tangential fans are being evaluated as part of a current research and development project. To this end, the effects of design changes on the generated flow and the mechanical loads are being investigated with the help of FEM and CFD simulations. The aim is to design a tangential fan that can withstand the mechanical and dynamic loads and deliver a consistently high and homogenous fluid flow at the same time. A geometry based on the results of the simulations was manufactured as a functional sample. This sample was investigated at the hot test stand of the IOB and is compared to a reference tangential fan.


Introduction
Precise temperature control to achieve optimum material properties is the main requirement of a modern thermoprocessing plant.To achieve this goal, high convection furnaces with forced circulation are used for the temperature range up to approx.t = 800 °C.These plants are used for heat treatment, especially in the aluminium and copper industries.Axial or centrifugal fans are used for forced circulation, which generate an inhomogeneous volume flow due to their mode of operation.For this reason, flow straighteners are used to ensure a homogeneous flow through the material and thus a homogeneous temperature distribution within the product.The flow straighteners reduce the cross-sectional area through which the flow passes.The resulting pressure losses lead to a reduced energy efficiency of the overall system [1][2][3][4][5].
Currently tangential fans are mainly used in room air-conditioning technology at ambient temperatures, in the food industry or in thermo processes with temperatures below t = 300 °C.They draw in the air radially and blow it out radially across the entire width.The resulting homogeneous volume flow appears optimal for the use in industrial furnaces without flow straighteners [6,7].
As part of a former research project, the possibility of using a tangential fan in thermoprocessing plants was investigated at the Department for Industrial Furnaces and Heat Engineering (IOB).For this purpose, a hot test stand was set up and the flow behaviour of a tangential fan was investigated up to a temperature of t = 500 °C.The results of the investigation showed that a tangential fan can be used without a flow straightener and the required homogeneous velocity distribution can be generated at a sufficiently high volume flow.Therefore, the investigation showed that a tangential fan can be used in a thermoprocessing plant overcoming the current limitations of t = 300 °C.After a short period of operation, the tangential fan failed due to dynamic loads such as vibrations and oscillations.This resulted in the need for design modifications to ensure thermomechanically stable long-term operation in thermoprocessing plants [8,9].

Simulation setup
All simulations are carried out with commercial simulation software.ANSYS Mechanical is used for the FEM simulations and ANSYS Fluent for the CFD simulations.With the help of FEM simulations, the effect of design changes on the static load situation of the tangential fan at n = 1500 rpm are investigated.An isolated 3D model of the cross-flow rotor is used for this purpose.The effects of the design changes are then investigated for their fluid mechanical influence using CFD.For this purpose, a 3D model is used that depicts the tangential fan as well as the guide housing and an inlet and outlet area.The boundary conditions for these simulations are a rotational velocity of n = 1500 rpm, a free pressure outlet and a pressure inlet with variable inlet pressure.The k-ω shear stress transport (SST) model is used for the simulation as it is less susceptible to turbulence at the pressure inlet than the standard k-ω model.Additionally, it is less prone to turbulence overestimation in the exit zone of the fan blades than the k-ε model [10].

Experimental setup
The hot test stand at the IOB, as shown in figure 1, was used for the investigations of two different types of tangential fans.The hot test stand is a thermally insulated closed flow channel.A motor controlled by a frequency inverter drives the fan.The resulting flow, shown here as a blue arrow, flows through a heating register that allows for heating the fluid flow to temperatures of up to t = 550 °C.There are five flow measuring points each in the lower as well as in the upper part of the flow channel.With the help of S-Pitot tubes, velocity fields can be recorded at these points.For this purpose, the S-Pitot tubes are moved in height by a linear traverse.By adjusting throttle valves by means of a linear actuator, different operating points of the fan can be investigated and fan characteristics can be recorded.To determine the pressure loss through the heating register and the pressure increase through the fan, the static pressure at the relevant positions is measured.

Results
This chapter presents selected results of the FEM and CFD simulations that led to the manufacture of a functional sample.Afterwards, the results obtained with this functional sample on the hot test stand are presented and compared with a reference tangential fan.

FEM Simulation
The reference tangential fan consists of a total of six support discs and 52 blades, with a thickness of 1.5 mm.This is driven on one side and mounted friction-free on both sides.Due to the free path length between blades and supporting disks, deformations of the blades occur between them during operation.This in turn leads to stress maxima at the clamping points in the support discs, as shown in figure 2. These stress maxima occur at all clamping points and amount to up to σ V,max = 119 MPa at the peak.Due to the equal distribution of the support discs, a maximum of deformation of the blades results between the outer support discs.To reduce this, the distance between the outer support discs can be reduced to 0.93 times the length of the inner support discs, resulting in a decrease in stress to σ V,max = 107 MPa.The correlation between the number of support discs and the maximum equivalent stress is shown in figure 3 a).In this case the thickness of the support discs and blades was kept constant at 1.5 mm.It can be seen that the maximum equivalent stress decreases exponentially with increasing number of support discs.Increasing by four the six support discs of the reference fan to ten discs lowers the maximum equivalent stress by two-thirds to approx. , = 35 MPa.Based on a new model with ten support discs, the influence of the support disc and blade sheet thickness was investigated, shown in figure 3 b).With a support disc thickness of s = 1.5 mm or more, an optimum appears when both sheet thicknesses are identical.With lower support disc thicknesses, the optimum shifts to the next higher blade thickness.Overall, it can be seen that the combination of identical sheet thicknesses for supporting discs and blades is the best solution.

CFD Simulation
The averaged velocity distribution over the width of the reference tangential fan shown in figure 4 shows the typical S-shaped flow through the fan.The air is sucked in through the pressure inlet flows through the tangential fan and is blown out through the pressure outlet after a 90° deflection.Thereby two turbulence zones are formed.One in the upper area of the fan and one in the lower area which prevents a backflow into the inlet area due to the "nose" of the guide housing.The maximum flow velocity is reached in the lower area of the cross-flow fan at approx.v = 150 m/s and the flow is directed towards the flow channel ceiling.A volume flow rate of  ̇= 8.72 m³/s is achieved here.As a new approach, a through-flow capability of such a centre shaft was examined.For this purpose, a centre shaft was designed as a tube and provided with holes.This should help to utilise the positive effects on the static and dynamic stability of a centre shaft on the tangential fan and at the same time reduce the flow-mechanical disadvantages.The result of this simulation is shown in figure 5b).The maximum flow velocity drops to approx.v = 130 m/s.Compared to that of the reference fan, the volume flow rate is approx.18.5 % lower for a centre shaft and only 8 % lower for a perforated centre shaft.Based on these results, a functional model was designed and built that, in addition to increasing the number of support discs and the sheet thickness of support discs and blades, includes a flow-through centre shaft.The fan characteristics of this functional model in comparison with the reference tangential fan and the numerical model of the functional model are shown in figure 7.All changes combined lead to a significant deterioration of the fan curve at all rotational velocities.This is mainly explained by the significantly higher sheet thickness of the blades, as these significantly increase the area blocked in the direction of rotation.Unfortunately, this had to be accepted for manufacturing reasons.The comparison between the numerical model and the experimental results at n = 1500rpm again shows sufficient congruence, but the numerical solution slightly overestimates the overall fan curve.

Conclusion
With the help of the results presented here, the influence of various reinforcing elements on the stability of a tangential fan could be shown.In addition, the influence of these reinforcing elements on the fluid flow was investigated and validated by means of experimental tests.The greatest innovation here was the use of a flow-through centre shaft.In order to be able to conclusively prove its effectiveness the centre shaft is to be blocked and investigated in further studies.

Figure 1 .
Figure 1.Hot test stand at the IOB Figure 2. von Mises equivalent stress distribution with over scaled deformation at the reference tangential fan

Figure 3
Figure 3 Maximum von Mises equivalent stress with a) different number of support discs and b) different steel sheet parameters

Figure 4 .
Figure 4. Velocity distribution of the reference tagential fan from the numerical model

Figure 5 .Figure 6 .
Figure 5. Velocity distribution of a tangential fan with a) a 50mm diameter centre shaft and b) a 50mm diameter centre shaft with punch holes