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本次研究是用球形火焰法来测量层流火焰燃烧速度,图1给出了实验装置原理图。主要由定容燃烧弹(CVCC)、高速摄像机、点火系统、数据采集系统和进排气系统组成。CVCC为立方体结构,容量为2.067 L,内部半径为79.02 mm,燃烧室两侧装有石英玻璃窗,具有耐高温和高透光的优点,并可提供直径90 mm的光学通道。在CVCC系统中,PID控制加热电极加热,型号为WRNK-234的K型热电偶(精度在±0.75%范围内)和压力表(
0.0001 MPa)测定初始温度和压力。PID控制器与加热器均匀分布于筒壁上(总功率为2.160 kW),确保CVCC内部温度均匀分布。混合物在CVCC中使用两个直径0.4 mm的对置电极点燃,连接至点火控制系统和点火线圈,协同实现点火过程。点火后,形成的球形火焰向外扩散,其传播过程通过纹影系统记录。球形火焰传播图像由高速摄像机拍摄(记录频率为12800 FPS,分辨率为1024 ×1024 Pixels)。 -
本实验研究内容的总化学方程式为:
$$ \mathrm{C}_x\mathrm{H}_y\mathrm{O}_z+\left(x+\frac{y}{4}-\frac{z}{2}\right)\mathrm{O}_2-x\mathrm{C}\mathrm{O}_2-\left(\frac{y}{2}\right)\mathrm{H}_2\mathrm{O}=0 $$ (1) 由公式(2)~(5)可计算得到氢气和乙醇的物质的量和体积。
$$ {{n}}_{\mathrm{H}}={{X}}_{\mathrm{H}}\left({{n}}_{\mathrm{H}}+{{n}}_{\mathrm{E}}\right)={{X}}_{\mathrm{H}}\frac{0.21{\varphi }}{0.21{\varphi}+0.5{{X}}_{\mathrm{H}}+3{{X}}_{\mathrm{E}}}\frac{{{P}}_{0}{{V}}_{0}}{\mathrm{R}{T}_{0}} $$ (2) $$ {{n}}_{\mathrm{E}}={{X}}_{\mathrm{E}}\left({{n}}_{\mathrm{H}}+{{n}}_{\mathrm{E}}\right)={{X}}_{\mathrm{E}}\frac{0.21{\varphi }}{0.21{\varphi }+0.5{{X}}_{\mathrm{H}}+3{{X}}_{\mathrm{E}}}\frac{{{P}}_{0}{{V}}_{0}}{\mathrm{R}{T}_{0}} $$ (3) $$ {{V}}_{\mathrm{H}}=\frac{{{n}}_{\mathrm{H}}{M}_{\mathrm{H}}}{{{\rho }}_{\mathrm{H}}} $$ (4) $$ {{V}}_{\mathrm{E}}=\frac{{{n}}_{\mathrm{E}}{{M}}_{\mathrm{E}}}{{{\rho }}_{\mathrm{E}}} $$ (5) 式中:
nH ——氢气物质的量(mol);
nE ——乙醇物质的量(mol);
XH ——氢气的比例(%);
XE ——乙醇的比例(%);
MH ——氢气的摩尔质量(g/mol);
ME ——乙醇的摩尔质量(g/mol);
ρH ——氢气的密度(g/cm³);
ρE ——乙醇的密度(g/cm³);
VH ——氢气的体积(cm³);
VE ——乙醇的体积(cm³)。
图2为使用HALCON软件对火焰图像的处理结果,步骤如下:(1)对导入的图像进行预处理,包括去噪、增强对比度等,以便更好地提取火焰前锋面;(2)利用HALCON中的边缘检测算子检测火焰前锋面的边缘;(3)对检测到的边缘进行形态学处理以去除噪声并平滑边界;(4)提取火焰前锋面轮廓;(5)通过拟合圆形的方法,拟合火焰前锋面的轮廓,从而得到火焰前锋的半径;(6)根据拟合得到的圆形,测量其直径或半径,即为火焰前锋的半径。
在球形扩散火焰中,拉伸火焰传播速度Sb可以由公式(6)得[20]:
$$ \mathrm{\mathit{S}_{\text{b}}}=\frac{\mathrm{d}r_{\mathrm{u}}}{\mathrm{d}t} $$ (6) 式中:
ru ——实际火焰半径(mm);
t ——时间(s);
Sb ——拉伸火焰传播速度(mm/s)。
对于球形膨胀火焰来说,火焰拉伸率α可由公式(7)得:
$$ \alpha =\frac{1}{{A}}\frac{{A}}{\mathrm{d}{t}}=\frac{2}{{{r}}_{\mathrm{u}}}{{S}}_{\mathrm{b}} $$ (7) 根据非线性拟合后得出的无拉伸火焰传播速度[21],如公式(8)所示:
$$ {\left(\frac{{S}_{\mathrm{b}}}{{S}_{\mathrm{b}}^{0}}\right)}^{2}\mathrm{l}\mathrm{n}{\left(\frac{{S}_{\mathrm{b}}}{{S}_{\mathrm{b}}^{0}}\right)}^{2}=\frac{-2{L}_{\mathrm{b}}\mathrm{\alpha }}{{S}_{\mathrm{u}}^{0}} $$ (8) 预混燃气的层流燃烧速度由公式(9)得:
$$ {{u}}_{\mathrm{L}}={{S}}_{\mathrm{b}}^{0}\frac{{\mathrm{\rho }}_{\mathrm{b}}}{{\mathrm{\rho }}_{\mathrm{u}}} $$ (9) 式中:
UL ——层流燃烧速度(mm/s);
Ρb和Ρu ——已燃气与未燃气的密度(mol);
$ {S}_{\mathrm{b}}^{0} $ ——无拉伸火焰传播速度(mm/s)。
Laminar Combustion Characteristics of Ethanol-Hydrogen Premixed Fuel
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摘要:
目的 随着化石燃料的日渐枯竭,生物燃料逐渐进入大众视野,乙醇氢气混合燃料作为新一代可再生清洁燃料引起了广泛关注,开展乙醇氢气预混火焰燃烧特性的影响研究非常有必要。 方法 文章基于定容燃烧系统,结合高速纹影技术,在初始温度为370 K,氢气比例为50%,当量比为0.7~1.4,初始压力为1、2、4 bar条件下研究了当量比和压力对乙醇氢气预混火焰的层流燃烧特性的影响。聚焦火焰的传播燃烧特性,计算得到了层流燃烧速度并分析了其影响因素,借助Chemkin仿真平台建立相关反应模型,采用Marinov的乙醇氧化反应机理,对其层流燃烧特性进行详细的化学动力学分析。 结果 研究表明,层流燃烧速度与绝热火焰温度具有正相关性且均在φ=1.1达到最大值;压力对净放热速率影响显著,当量比越大其峰值出现在更高温度区域;R1:H+O2⇔O+OH是促进火焰层流燃烧速度最敏感的反应。随着压力的增大,H、OH、O自由基的峰值摩尔分数均逐渐减小且向上游移动,随着当量比的增大H、O自由基摩尔分数逐渐下移,OH自由基摩尔分数先增加后减少。 结论 当量比、压力以及活性自由基对乙醇氢气预混层流燃烧特性的影响显著,可为后续研究提供理论依据。 Abstract:Introduction With the depletion of fossil fuels and bio-fuels' emergence, ethanol-hydrogen hybrid fuel as a new generation of renewable clean fuel has attracted wide attention, so it is necessary to study the effect of ethanol-hydrogen premixed flame combustion characteristics. Method Based on the constant volume combustion system and combined with high-speed schlieren technology, the effects of the equivalent ratio and pressure on the laminar combustion characteristics of ethanol-hydrogen premixed flame were studied under the conditions of initial temperature of 370 K, hydrogen ratio of 50%, equivalent ratio of 0.7~1.4 and initial pressure of 1, 2 and 4 bar. Focusing on the propagation combustion characteristics of the flame, the laminar combustion velocity was calculated and its influencing factors were analyzed. The relevant reaction model was established with the help of Chemkin simulation platform, and the chemical dynamics of the laminar combustion characteristics were analyzed in detail by using Marinov's ethanol oxidation reaction mechanism. Result The results showed that the laminar combustion velocity was positively correlated with the adiabatic flame temperature and reached the maximum value around φ=1.1. The pressure significantly affects the net heat release rate, and the peak value occurs in the higher temperature region with a greater equivalent ratio. R1:H+O2⇔O+OH represents the most sensitive reaction which promotes the laminar combustion velocity of the flame. With the increase of pressure, the peak molar fraction of H, OH, and O free radicals gradually decreased and moved upstream. With the increase of the equivalent ratio, the molar fraction of H and O free radicals gradually decreased, and the molar fraction of OH free radicals first increased and then decreased. Conclusion The equivalent ratio, pressure and active free radicals have significant effects on the laminar combustion characteristics of ethanol-hydrogen premixed fuel, which can provide theoretical basis for subsequent studies. -
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