I wish to find performance (say effectiveness) of cellulose media in a set up. I have put an axial fan at around 1 feet distance from this media to force air on to it. Should I put straighners in between to remove turbulence from flow ?
If I do this, will the results remain relevant to practical situations?
If you are testing the effectivenes of cellulose as filter media, then the axial fan should be downstream of the media. If you select a cylindrical duct with suitable inlet length then you need not instal any flow straightener at all. It will also make dusting of the air easy. Your main concern is to observe how much dust could be captured by the media and also the poressure drop across the media. Ifact ehen we tested a filter media for a local manufacturer we installed a rectangular filter across the working section of a suction type wind tunnel. What is most important here is to determine the weight of dust released and weight of dust captured...walla we got excellent results.
It depends on the depth of the medium and the size of the pores, neither of which you have specified. Is it for dust capture? Any upstream turbulence will be filtered (suppressed by relaminarization of the flow) by the fine pores of the medium and no flow straighteners will be needed. It is unlikely that freestream turbulence will affect the performance. But, as Mohamed says, the fan is best placed in the suction mode for convenience, if it is for filtration.
More importantly, axial fans are not suitable for the test as they have low Δp and may not be able to overcome the media resistance unless it is has short length and large pores. Therefore I strongly suggest that you make an estimate of the pressure drop for your case (from data for materials of similar pore size) and then decide if the axial fan Δp is adequate. Otherwise go for a centrifugal fan in suction mode with smooth change in the flow area of the transition sections.
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Thank you Prof. Raghavan and Prof. Afifi.
Actually the set up is for testing the effectiveness of media for evaporative cooling. The purpose is not for dust collection.
The flute size is 7 mm, so I feel that turbulence may not affect performance much. But the fan still may be best placed downsteam of the media than upstream as suggested. Media thickness is also less (3"), so pressure drop is not much of a problem.
If it is for evaporative cooling, then the cellulose porous media you refer to is not a good choice. You would have seen that modern cooling towers use what is called 'film fill'. The 'Munters fill' was one of the earliest cross-corrugated fills. They have a better performance in evaporative cooling.
The kind of media you are talking about can be seen to be used in domestic "desert coolers" where high efficiency is not the aim, but low cost and compactness are. As such units are generally used in dry climates where the RH of the air is quite low, desert coolers give an impressive performance.
In evaporative cooling, it is all about mass transfer at the water-air interface. Wet air needs to be stripped from the interface and fresh dry air needs to be brought into contact. This is aided by turbulence in the air.
Therefore, the answer to your first question is "do not put any flow straighteners" so as to take advantage of any free stream turbulence you have in the flow. Since you propose to use a fan in the suction mode, all you need upstream is a bell mouth entry. That should ensure a fairly uniform air distribution at inlet.
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