PROJECT DETAILS

• Client: LODEPA
• Date: 2024
• Technology: Acoustic Simulation. CFD Simulation and Optimisation. Acoustic Testing.

INTRODUCTION

This project aims to improve the efficiency of a hospital air purification unit and reduce its noise level. To do this, CFD simulation is first used to study the flow and uniformity of the velocity in the design proposals. This is followed by a study combining experimental methods and FEM simulation to optimise the acoustics of the equipment.

For this project, Cradle CFD is used for the fluid dynamic simulation part and Actran for the acoustic study by means of FEM simulation.

REACTION CHAMBER CFD OPTIMISATION

The purification equipment has a reaction chamber with ultraviolet light lamps and disinfectant compounds to remove micro-organisms and contaminants from the air. To maximise effectiveness, it is important that the flow remains in the reactor chamber as long as possible and makes contact with the surfaces covered with the compounds. In addition, a good circulation flow must be ensured to achieve disinfection of the rooms in an adequate time.

For CFD simulation studies, we introduce the fan curves provided by the manufacturer into the models. In this way we can calculate the flow rate we will obtain taking into account the pressure drop of the design we are analysing.

The study of the current chamber indicates that the flow uniformity has ample room for improvement, as shown in the following picture:

To achieve a more optimised design, we first considered a modification of the inlets and outlets to produce a more uniform flow in the chamber. This leads us to work with axial fans.

After several iterations, we obtain a lamp distribution and a configuration of inlets and outlets that greatly improves the uniformity of the flow in the chamber. This design with the selected fans also achieves an increase in the flow rate provided with the nominal power supply, and will allow a reduction in the power consumption of this equipment.

The following image shows the velocity field in a vertical section of one of the designs studied:

ACOUSTIC OPTIMISATION

In the next phase of the project, we sought to optimise the sound level of the machine by reducing the sound pressure level generated.

First, the sound sources of the original and proposed fans are experimentally characterised. This will allow them to be introduced into the numerical model to obtain realistic results at the different frequencies of the audible spectrum.

With the sources characterised, the simulation of the actual equipment is carried out. These studies consider the sound sources of the fans and the noise caused by the air passing through the reaction chamber, the latter evaluated with the SNGR aeroacoustic method. The results are obtained as a function of frequency and the overall results can then be calculated with the necessary weightings:

The results of the numerical model are compared with experimental measurements carried out in our offices with the real equipment in operation. The difference between the numerical and experimental results of the overall equipment loudness is less than 1dB. This allows us to validate the simulation methodology and provide confidence in the results obtained in the new designs.

Applying the validated methodology, different configurations of the elements surrounding the reaction chamber inside the machine are studied. This process concludes with a design that obtains significant reductions with respect to the original equipment in all its operating modes.

EXPERIMENTAL VALIDATION THROUGH PROTOTYPES

With the final design defined, a prototype is manufactured using 3D printing to experimentally test the improvements obtained. The results obtained in the tests reflect very similar improvements to those predicted by the numerical models, with an accuracy again close to 1dB in the comparison. Flow measurements are also made with a hot-wire anemometer to verify that the flow rate is appropriate.

The optimisation process concludes with very satisfactory results in terms of flow improvement and noise reduction, with noise values about 8 dBA lower than the original design.

El proceso de optimización concluye con resultados muy satisfactorios en cuanto a mejora del flujo y reducción de la sonoridad, con valores de ruido unos 8 dBA inferiores al diseño original.