往复流动下预混合气体在多孔介质中超绝热燃烧的数值模拟

【作者】 马世虎；

【导师】 解茂昭；

【作者基本信息】 大连理工大学 ， 工程热物理， 2004， 硕士

【摘要】 多孔介质内的往复流动超绝热燃烧技术可以拓宽燃料可燃极限，实现超低热值预混合气的稳定燃烧，并可显著降低尾气中NO_x和CO的排放，因而是一项很有发展潜力的先进燃烧技术。本文对这项技术进行了数值研究。 本文认为多孔介质和气体之间处于局部热平衡状态，化学反应在一步内完成，气体为透明介质，多孔介质为吸收发射性的灰介质；在此基础上，文章分别进行了一维和二维数值计算，并系统地研究了各种参数对燃烧的影响。在一维和二维模型中，分别使用了Rosseland模型和离散坐标法来计算辐射传输。一维计算中，模拟了燃烧器中各物理量在一个半周期内的变化，以及半周期、当量比、流速、热损失、多孔介质的衰减系数、比热和孔隙率，以及燃烧器长度对燃烧的影响。计算的结果基本与实验的趋势相符；燃烧器内的温度分布基本呈梯形状；半周期对燃烧器的最高温度没有明显的影响，但是半周期越大，燃烧器的出口温度也越大；大的当量比或是流速条件下，最高温度和出口温度都明显升高，高温区域变宽；热损失对最高温度的影响不大，但是高温区域的宽度会随着热损失的加大不断减小；衰减系数增大，最高温度将升高，高温区域也会变宽；较大的比热容条件下，高温区域将变宽，同时出口温度下降；较小的孔隙率下高温区域将变宽，但最高温度基本不变；燃烧器长度的改变对燃烧的影响不大。利用二维模型，得到了与一维模型相类似的温度场和温度变化趋势；探讨了多孔介质中空气段，绝热层厚度以及燃烧器半径对燃烧的影响。当燃烧在多孔介质中发生时，空气段对燃烧器的影响很小；但是，当空气段中发生燃烧时，将会在此处产生较高的温度；燃烧器半径对燃烧的影响很小。本文的模拟计算结果表明，对于细长的隔热较好的燃烧器，一维模型可以给出较满意的预测结果；但是为了精确模拟气体与多孔介质间的换热有必要采用双温度模型。更多还原

【Abstract】 Reciprocating superadiabatic combustion in porous media(RSCP) is a advanced technique with great potential by which flammability limits of fuels can be significantly extended and premixed gases with extremely low heat content can combust steadily while the emission of NOX and CO can be reduced greatly. This thesis presents a numerical study on RSCP.To simplify simulations, assumptions are put up as follows: the porous media and the working gas are in local thermodynamic equilibrium; the combustion chemistry is simulated by a global reaction; the working gas is optically transparent and nonradiative; porous media are absorbing and emitting gray media. Numerical simulation is carried out with both 1-D and 2-D models. Effects of various operating parameters on combustion are detailedly studied. The Rosseland method is chosen to simulate radiation transfer in the 1-D analysis, while the discrete ordinate method in the 2-D simulation. Based on 1-D model, transient profiles of temperature, species mass fraction and reaction rate during one half cycle are calculated, and influences of half cycle, equivalence ratio, inlet velocity, heat loss, as well as porous media’s properties such as extinction coefficient, specific heat capacity and porosity, and the length of combustor on the combustion are studied. The results are qualitatively consistent with experimental results. The configuration of temperature profile is trapeziform-shaped; the half cycle has little influence on the peak of temperature, but when the half cycle become longer, the outlet temperature will become higher; both larger equivalence ratio and faster inlet velocity of gases can cause higher temperature peak and outlet temperature and wider high temperature zone; influence of heat loss on the peak of temperature is little, but with the heat loss becoming greater and greater, the zone of high temperature will be shorter and shorter; the peak of temperature is higher with the greater specific heat capacity of porous media while outlet temperature is lower; with the less porosity, the zone of high temperature is extended and the peak of temperature almost keeps changeless; the length of combustor has little effects on combustion. Profiles of temperature at each time step during one half cycle of the 2-D model are similar with those of 1-D model. With 2-D model, combustion in combustors with and without air gap and the influence of the thickness of insulation and radius of combustor are analyzed. It is found that when combustion happen in porous media, the air gap has little effect on combustion, but higher temperature will come out in air gap with combustion happening in the gap. The radius has insignificant effect on combustion. The numerical simulation indicate that combustion in slender combustors can be predicted satisfactorily with 1-D model; but to accurately predict heat exchange between working gas and porous media, separate energy equations for the solid and the gas are needed.更多还原