Experimental Testing Setups

As already in Chapter 5, the modular filter media test rig (MFP 3000) is used for all experimental measurements with wired weaves. The fractional collection efficiency ηwwof the respective wired weave is calculated according to Eq. (2.3) from the ratio of the number of collected particles (Ninlet(dP)−Noutlet(dP)) of the size classdP

to the respective number of particles in the inlet distribution (Ninlet(dP)). In this case, the variablesNinlet(dP)andNoutlet(dP)refer conventionally to the respective number of particles in the raw and the clean gas. The test conditions for analyzing the filtration performance of wired weaves are summarized in Tab. 6.1.

The different variations of the modular testing units used to analyze the influence of the sensor position, the particle measurement technique and the electrostatic particle charge are introduced below.

6.2 Experimental Analysis

Parameter Value

Test dust ISO fine dust

Dust concentration c 75 mg/m³ Volume flow rate V˙ 25.2 l/min

Media velocity v 4.2 cm/s

Table 6.1: Constant test conditions used for measuring the filtration efficiencies of wired weaves. The volume flow rate and media velocity correspond to the flow conditions between the two measuring positions, i.e., at the height of the wired weave. Due to the continuous particle removal at both measuring points, the flow conditions upstream the first and downstream the second sampling point deviate from the mean flow.

Sensor Positions. Analogous to the empty test channel examined in Sec. 5.3, the effect of different aerosol sensor arrangements is first investigated. Since the measurement results using the exchanged sensor setup have already excluded an influence of the two (ideally identical) sensors, the focus with regard to wired weaves is on the measuring position of the raw gas particle distribution. The following two configurations are used for this purpose:

Theupstream & downstream setupintroduced in Sec. 2.2.1 was already used for the analysis of particle dynamics in the empty test channel in Sec. 5.3.1. Therefore, only reference is made to those two sections at this point. The fractional separation efficiency of wired weaves can directly calculated from the quasi-simultaneous measurement of raw and clean gas distribution. Since no substantial increase in pressure is expected in the case of the open-pored weaves, stepwise measurements during dust loading are carried out. This means that the filter medium is continuously loaded with dust over a period of time (20 min) and the fractional efficiency is determined after every minute. This loading measurement is performed twice for each weave geometry, using a new, clean sample each time.

In thedownstream-only setup, the particle size distributions of raw and clean gas are carried out at the same sample collection point, i.e., downstream the filter medium, with the same sensor. This requires an intervention in the test facility between the two measurements according to the following

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procedure: The raw gas distribution is determined in a pre-measurement in the empty test channel. Afterwards the filter medium or wired weave is inserted into the test bench and the clean gas distribution is subsequently measured in a second operation. For raw and clean gas distributions, three measurements are consecutively carried out. In each measurement, the particles arriving in the measuring volume during one minute are accumulated.

The clean gas distribution is measured at the same position, i.e., downstream the wired weave, in both setups. The intention of using the two different setups is to verify a potential influence of the raw gas sensor position on the separation efficiency.

For an empty test channel, the measurements in Sec. 5.3 revealed a similar particle size distributions at both measuring points. However, since an inserted wired weave changes the flow conditions, the results may not be transferable. Therefore, the influence of the sensor position is examined again in this chapter under the modified conditions.

Particle Measurement Techniques. In addition to the sensor configurations, the effects of different particle measurement techniques on the collection efficiency are examined in this section. Two particle sizing instruments, which are already introduced in Sec. 2.2.4, are applied:

In the case of theoptical particle counter (OPC), the results obtained from the sensor position analysis (downstream-only setup) are reused. As mentioned above, the optical diameter of particles is measured using a light-scattering spectrometer.

Theaerodynamic particle sizer (APS)is also used in combination with the downstream-only setup as only one measuring device is available. In this case, the aerodynamic diameter of particles is determined.

Due to the non-spherical shape of the dust particles, certain differences are expected between optically and aerodynamically measured particle size distributions. An approximate conversion formula proposed in the standard DIN 71460-1 [27] is given in Eq. (2.5). This equation is applied to the measured diameters of this section and the obtained results are additionally evaluated. The investigations with different

6.2 Experimental Analysis

particles are modeled in both simulation approaches. Therefore, the question arises whether one of the measurement methods is better suited for comparison with simulation results. For direct comparability of the measuring instruments, the same, downstream-only setup is used in both cases.

Particle Charging States. Finally, the influence of electrostatically charged particles on the separation behavior of wired weaves is analyzed. The corona-discharge unit already applied to particles in the empty test channel in Sec. 5.3.2, is used to obtain modified charging states. Again, the behavior of untreated (charged) particles and discharged particles is examined.

Untreated (charged) particles have already been used for the above mentioned analysis of sensor positions and particle measurement techniques.

The respective measurement results are reused to determine the influence of the particle charging state.

Discharged particlesare examined with the same sensor setups for this purpose. For the basic principles of discharging particles, reference is made to Sec. 2.2.3 at this point.

The measurements with different charging states for particles are carried out using the optical particle counter in combination with the upstream & downstream setup and using the aerodynamic particle sizer in combination with the downstream-only setup. Since the wired weaves are grounded in the test facility, as described in Sec. 6.1, any differences in the experimental results can directly be attributed to the particle charging state. The aim of the experiments is to quantify the influence of electrostatic effects such as mirror charges on the collection efficiency.

In order to additionally take the influence of different pore sizes and fiber diameters into account, all experiments in this chapter are carried out for the three wired weave geometries depicted in Fig. 6.1. A detailed evaluation of the experimental results is provided below.

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