Central recirculation

Typically, when central recirculation is used the contaminant in the supply air is the main source. This is not the case for industrial use, where the main source is in the ventilated room. This usually results in the concentration being somewhat higher when using recirculation than when not using it. Figure 8.1 outlines the ventilation system, the contaminant source, and the cleaning system. 

FIGURE 8.1 Model of a central recirculating system used for calculating the connection between contaminant concentrations, airflow rates, contaminant source strength, q, and air cleaner efficiency, rj. Cj p is the concentration in the supply (outside) air, c is the concentration in the room, c is the concentration in the returned air, (JaMot the total flow rate through the room, ic is the ratio between recirculated airflow rate and total air flow rate, T is the time constant for the room, and V is the room volume. 

Centrally recirculated Exhaust air from one or more treated spaces that is reintroduced through a central unit before air treatment occurs. 

The results of Fig. 19.8 for a swirl number of 1.35 show that the attenuation increased to 10 dB with the velocity of the axial jet up to 42 m/s, and further increase to 47 m/s caused the amplitude to fall from around 6 kPa to less than 1.5 kPa and the attenuation to decrease from 10 dB to almost zero. Similar results were observed with the swirl number of 0.6 the attenuation improved with axial jet velocity up to 60 m/s, after which the amplitude and attenuation decreased. The decline in the amplitude of oscillation and its attenuation by active control was due to the interaction between the axial jet with a large velocity and the central recirculation zone, which caused the flame to move further downstream of the swirler and heat release to occur further from the pressure antinode. The consequent increase in the distance between the point of entry of the oscillated fuel and the active burning zone reduced the effectiveness of the oscillated input due to increased fluid dynamic damping and development of a large difference in phase between different parts of the oscillated flow, especially with swirl surrounding the oscillated axial jet. 

When the outlet is non-reflecting, the flame is stable. The mean axial velocity and fuel mass fraction fields are displayed in Fig. 9.15 and 9.16 [320] the central recirculation zone (marked by a white line in Fig. 9.15) is filled by burnt gases which provide flame stabilization. Fuel staging is also apparent in Fig. 9.16. 

Measurements of the species concentration (CO2, H2, O2, and CO) with and without a ring at the nozzle exit and at two diameters downstream of the exit are shown in Figure 11. The main features of the measurements are consistent with other results (e.g., size of the recirculation zone and mean temperatures), and the profiles are similar to those for conventional swirl-stabilized flames. The central recirculation zone consists largely of hot burnt products (i.e., CO2). There are, however, significant differences in the two modes, e.g., in the ring mode, com-... 

They found fhat the same swirl generator can provide different velocity patterns as the relative furnace size dj/ di, is changed. At the same time, the size and the shape of the internal or central recirculation zone (CRZ) is primarily dependent on furnace diameter and not vane swirler diameter. Cristea [15,16] in 1984 and 1987, respectively, introduced, relative to a complete system swirler-quarl-furnace, a new modified swirl number in which the characteristic length is the exit diameter of the burner quad. 

Cristea, E. D. "Prediction of the Central Recirculation Zone Size for a Complete Bumer-Quarl-Fumace System." Institute of Aeronautics and Astronautics Journal 25, no. 3 (1987) 457-463. 

Figure 3.5 Schematic diagram of the effect of swirl on jet aerodynamics. CRZ, central recirculation zone ERZ, external recirculation zone.

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