Velocity and size characteristics of liquid-fuelled flames stabilized by a swirl burner

Y. Hardalupas, A. M. Taylor, J. Whitelaw
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引用次数: 61

Abstract

Velocity and droplet size characteristics of an unconfined quarl burner, of 16 mm quarl inlet diameter, have been measured with a phase-Doppler anemometer at a swirl number of about 0.29: the Reynolds number of the flow was 30000, based on the cold bulk velocity of 30.4 m s-1 and the hydraulic diameter. The atomization was achieved by shear between the swirling air and six radial kerosene jets and the resulting Sauter and arithmetic mean diameters were about 70 and 50 μm respectively after injection: velocity characteristics are presented for three 5 μm-wide size classes, 10, 30 and 60 μm. The flows correspond to no combustion and combustion of natural gas with a heat release of 8 kW supplemented by liquid kerosene flow rates sufficient to generate 21.6 and 37.2 kW : the gas equivalence ratio was 0.45 and atomized kerosene at two flow rates increased the overall ratios to 1.64 and 2.53. In non-reacting flow, droplets 30 μm and smaller are sufficiently small to be entrained by the mean air velocity towards the central part of the flow and into the swirl-induced recirculating air bubble. The 60 μm droplets are able to travel through the bubble uninfluenced by turbulent fluctuations in the air and are ‘centrifuged’ away from the centreline, through acquisition of a mean swirl velocity component, so that a large proportion of the kerosene volume flow rate lies at the edge of the swirling jet. Because larger droplets are centrifuged to the outer part of the flow, whereas the smaller are entrained towards the centreline, the Sauter and arithmetic mean diameters are, by 1.22 quarl exit diameters downstream of the quarl, approximately 65 and 36 μm at the outer part of the flow and 35 and 12 μm near the centreline in the inert flow. In reacting flow, droplets evaporate rapidly in regions of elevated temperatures and hence no droplets are found within the flame brush and recirculation region. The aerodynamic response of each size class to the air velocity is similar to inert flow so that the majority of the kerosene flow is centrifuged away from the flame. On exit from the quarl, the evaporation and burning rates cause the Sauter and arithmetic mean diameters to be about 70 and 50 μm and 60 and 30 μm at the inner and outer edges of the spray respectively. By 1.22 quarl exit-diameters from the exit of the quarl, the air motion entrains droplets smaller than about 30 μm towards the flame, at the inner edge of the spray, so that the Sauter and arithmetic mean diameters are 60 and 40 μm at the outer edge of the jet. There is comparatively little effect of changing the flow rate of kerosene because the combustion is controlled by the low available number of smaller droplets, although the Group combustion number corresponds to ‘cloud’ burning. The relative response of droplets to the mean and turbulent components of air motion, including the ‘centrifuging’ effect, can be scaled to other flows through dimensionless numbers defined in the text.
旋流燃烧器稳定液体燃料火焰的速度和尺寸特性
用相位多普勒风速仪测量了进口直径为16 mm的无约束争吵燃烧器的速度和液滴尺寸特性,在旋流数约为0.29的条件下,以30.4 m s-1的冷体速度和水力直径为基础,计算了流动的雷诺数为30000。通过旋转空气与6个径向煤油射流的剪切作用实现雾化,喷射后的直径和算术平均直径分别约为70 μm和50 μm,并给出了10、30和60 μm 3个5 μm宽的速度特性。两种流量分别对应不燃烧和燃烧8 kW放热的天然气,加上足以产生21.6和37.2 kW的液体煤油流量,气体当量比为0.45,两种流量下雾化煤油使总当量比增加到1.64和2.53。在非反应流中,30 μm及以下的液滴足够小,可以被平均气流速度带向流的中心部分并进入旋涡诱导的再循环气泡。60 μm的液滴能够在不受空气湍流波动影响的情况下穿过气泡,并通过获得平均涡流速度分量而被“离心”离开中心线,因此煤油体积流量的很大一部分位于涡流射流的边缘。由于较大的液滴被离心到流的外侧,而较小的液滴则被夹带到流的中心线,因此在流的下游,以1.22 μ l的出口直径计算,在流的外侧,Sauter直径和算法平均直径分别约为65 μm和36 μm,在靠近流的中心线处,分别为35 μm和12 μm。在反应流中,液滴在高温区域迅速蒸发,因此在火焰刷和再循环区域内没有液滴。每个尺寸级对空气速度的气动响应类似于惰性流动,因此大部分煤油流被离心离开火焰。在喷嘴出口处,蒸发速率和燃烧速率导致喷嘴内外边缘的Sauter直径和算术平均直径分别约为70 μm和50 μm, 60 μm和30 μm。在距离射流出口1.22夸克出口直径处,空气运动将小于30 μm的液滴带向射流内缘的火焰,因此射流外缘的Sauter直径和算术平均直径分别为60 μm和40 μm。改变煤油流速的影响相对较小,因为燃烧是由少量可用的小液滴控制的,尽管组燃烧数对应于“云”燃烧。液滴对空气运动的平均和湍流分量的相对响应,包括“离心”效应,可以通过文本中定义的无量纲数字缩放到其他流动。
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