Perforated nozzle arrays

By feed of a fluid through a nozzle array, which is a plate with many tiny holes, so-called micro-plume injection into a micro channel can be achieved [51, 147]. Typically, the micro channel s floor is perforated in a section in this way and a closed-channel fraction follows for completion of mixing. Large specific interfaces can in principle be achieved depending on the nozzle diameter. This mixing concept benefits from conceptual simplicity and fits well to existing MEMS techniques. Furthermore, it consumes less footprint area and therefore does not create much dead space, which is one of the prime requirements during pTAS developments. 

There are basically three possible designs of nozzle arrays which differ with regard to the spent flow of the air (Fig. 1.2). In a field of individual nozzles the aft-can flow unimpeded between almost all nozzles however, in a hole channel the air can flow only between those above. In a perforated plate the air can only continue to flow laterally and then escape. Hole channels and perforated plates are easier to produce than single nozzles, as they only require holes to be perforated. However, the heat transfer is the highest for nozzle fields and the lowest for perforated plates, as will be subsequently shown. 

Fig. 1.2 Types of nozzle arrays, (a) Single-nozzle array (b) Hole channel (c) Perforated plate.

The fields of nozzles can be made from single nozzles, or hole channels, or from perforated plates with aligned or staggered arrangements, permitting a variety of geometric parameters. The heat transfer coefficient of nozzle arrays is therefore considered in more detail in the following. 

Fig. 1.24 Comparison of single-nozzle array and perforated plate for different d ing rates.

Arrays with multiple oblique impinging jets are generated via 3-D channel networks which feed fluid from a reservoir via the outlet nozzles of the network into a mixing chamber (see Figures 1.196 and 1.197) [54], Perforated plates contain such arrays. 

Slot nozzles are convenient and easy to fabricate for smaller width dryers (1-2 m). For larger widths and for dryers employing elevated temperature jets, it is difficult to maintain a uniform jet width without developing excessive thermal stresses. The fabrication costs are high and are not justifiable. Most large-scale units (e.g., Yankee dryer for tissue) therefore use perforated plates to generate arrays of jets. They are easier and cheaper to fabricate. 

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