An Investigation of Ultra Low NOx Natural Gas Burners

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8 secondary air/feed gas from the two first rows of holes and the third row helps to create a low pressure zone in the burner head end region. The pressure difference with the feed air duct provokes a high flow rate into the small holes, which then joins the diluted gas mixture at right angle and a the top of the enclosure. (iii) The central part of the burner head acts as a bluff body in the low pressure, low vel oc i ty zone 1 eft in the wake of the vert i ca 1 flow emanating from the small holes. The velocity directions and magnitudes at the exit of the three rows of holes are highly dependent on the balance of pressures of the two main flows (secondary air and feed gas mixture) at their entrance in the burner head. It is their control which ensures that (i) the primary methane-air mi xture is d i st ri buted ina balanced way between the fi rst two and the th i rd rows of holes, and (i i) the secondary air flow is 1 arge enough to entrain the fuel-air mixtures toward the divergent but low enough to let the feed gas emerge from the two first rows of holes. The Reynolds Numbers suggest that the flow is intermediate between laminar and turbulent flow. Both cases have been considered previously [2] but the experimental evidence is that the flow is turbulent and only that case is considered here. Figure 3 shows a computation to illustrate the effect of the geometry of the burner head and of the appropriate choice of pressures at inlets on the flow field. The small region of low velocity created by the pressure di stri but i on after the thi rd row of hal es and before the burner head end enables the flame to hold to the burner walls and to develop. The reci rcul at i on zone near the x axi s start i ng at the burner head end and finishing 41 mm away from it has the effect of directing part of the flow toward the axis of symmetry x where the flame can stabilize. 3.3 The combustion process Three cases have been investigated in the present work: one in which the burner is fed with a lean ((j) = 0.77) methane-air mixture (mode A), a second with a stoichiometry of (j) = 0.885 (mode B), and a third where stoichiometric concentrations of methane-air were used (mode C). The first case represents the normal conditions of use as an ultra-low NOx burner for