Deflagration vent design should be based on the burning velocity in the pressure range below 3 bara, where vent relief devices normally operate, rather than the K deflagration index, which is typically measured at pressures >50% of the maximum deflagration pressure, P. Tests using a standardized 1000-L vessel showed that in the pressure range below about 3 bara, niacin and lycopodium burned faster than cornstarch, despite its larger K index. We attribute this behavior to the endothermic dehydration of cornstarch during the early stages of deflagration. Deflagration vents for cornstarch and other milled grains may have been oversized while vents for some other dusts may have been undersized. Burning velocities are most relevant in the region of the 2-bara midpoint overpressure and can theoretically be found from the “isothermal pressure rate gradient”, or IPRG, which we define as the gradient of (dP/dt) plotted against P (1-1/P). However, owing to irregular combustion caused primarily by two 5 kJ igniters, the IPRG is found indirectly from the “pressure rate gradient” or PRG, which we define as the gradient of (dP/dt) plotted against P. The PRG was found to be adequately linear for measurement at 2 bara and the experimental curves passed through the "origin". It was shown mathematically that at 2 bara the PRG is 1.05 times larger than IPRG, permitting the IPRG and turbulent burning velocity to be calculated. Since the calculation of burning velocity from the IPRG requires P, a general extrapolation technique was developed for correcting P values obtained in vessels smaller than 1000-L. We propose that, owing to the greater turbulence in the 20-L vessel, a calibration be made using methane. This would establish the turbulence factors for each vessel at 2 bara, allowing the underlying “reference” burning velocities to be calculated and compared. However, to measure turbulent dust burning velocities in 20-L vessels a smaller and more efficient igniter must be used. Two 5 kJ igniters not only obscure the pressure history but wastefully expend energy far from the vessel core, depleting the unburned mixture and depressing the subsequent pressure rate. An observed “double sigmoid” dependence of dust deflagration rate on particle diameter suggests that routine explosibility testing of organic dusts is usually carried out in a region where pressure rates are relatively insensitive to particle size.

中文翻译：

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测量粉尘燃烧速度以进行爆燃通风口设计

爆燃通风口设计应基于低于 3 bara 的压力范围内的燃烧速度（通风泄压装置通常在该范围内运行），而不是基于 K 爆燃指数，后者通常在 > 最大爆燃压力 P 的 50% 的压力下测量。 测试使用标准化的 1000 升容器表明，在低于约 3 bara 的压力范围内，烟酸和石松比玉米淀粉燃烧得更快，尽管其 K 指数更大。我们将这种行为归因于爆燃早期阶段玉米淀粉的吸热脱水。玉米淀粉和其他磨碎谷物的爆燃通风口可能尺寸过大，而其他一些粉尘的通风口可能尺寸过小。燃烧速度在 2 bara 中点超压区域最为相关，理论上可以从“等温压力速率梯度”或 IPRG 中找到，我们将其定义为 (dP/dt) 相对于 P (1- 1/P）。然而，由于主要由两个 5 kJ 点火器引起的不规则燃烧，IPRG 是从“压力速率梯度”或 PRG 间接找到的，我们将其定义为相对于 P 绘制的 (dP/dt) 梯度。对于 2 bara 下的测量具有足够的线性，并且实验曲线通过“原点”。数学显示，在 2 bara 压力下，PRG 比 IPRG 大 1.05 倍，从而可以计算 IPRG 和湍流燃烧速度。由于 IPRG 的燃烧速度计算需要 P，因此开发了一种通用外推技术来校正在小于 1000 L 的容器中获得的 P 值。我们建议，由于 20 升容器中的湍流较大，应使用甲烷进行校准。这将为每个容器建立 2 bara 的湍流系数，从而可以计算和比较基本的“参考”燃烧速度。然而，为了测量 20 升容器中的湍流粉尘燃烧速度，必须使用更小、更高效的点火器。两个 5 kJ 点火器不仅掩盖了压力历史，而且浪费了远离容器核心的能量，耗尽了未燃烧的混合物并降低了随后的压力速率。观察到的粉尘爆燃速率对粒径的“双 S 形”依赖性表明，有机粉尘的常规爆炸性测试通常在压力速率对粒径相对不敏感的区域进行。