李枫 毕设翻译

翻译

3.1 Introduction

3.1介绍

In axial-flow compressors, the stage pressure rise is very dependent on the axial flow velocity. To achieve the design pressure ratio in the minimum number of stages, a high axial velocity is essential; in many aircraft engines, compressor outlet velocities may reach 170 m/s or higher. It is,of course, impractical to attempt to burn fuels in air flowing at such high velocities. Quite apart from the formidable combustion problems involved,the fundamental pressure loss would be excessive. For example, for an air velocity of 170 m/s and a combustor temperature ratio of 2.5, the pressure loss incurred in combustion would be approximately 25% of the pressure rise

achieved in the compressor. Thus, before combustion can proceed,the air velocity must be greatly reduced, usually to about one-fifth of the compressor outlet velocity. This reduction in velocity is accomplished by fitting a diffuser between the compressor outlet and the upstream end of the liner.

在轴流式压缩机、压力上升阶段非常依赖于轴流速度。达到设计压力比最低数量的阶段,高轴向速度是至关重要的,在许多飞机发动机,压缩机出口速度可以达到170米/秒或更高。当然,它是不切实际的试图燃烧燃料在空气流动在如此高的速度。除了强大的燃烧问题,基本将过多的压力损失。例如,170米/秒的气流速度和燃烧室的温度比为2.5,燃烧产生的压力损失将大约25%的增长实现压缩机的压力。因此,在燃烧过程,必须大大降低气流速度,通常约五分之一的压缩机出口速度。这减少速度是通过合适的压缩机之间的扩散器出口和上游的班轮。

In its simplest form, a diffuser is merely a diverging passage in which the flow is decelerated and the reduction in velocity head is converted to a rise in static pressure. The efficiency of this conversion process is of considerable importance because any losses that occur are manifested as a fall in total pressure across the diffuser. In long diffusers of low divergence angle, the pressure loss is high due to skin friction along the walls,as shown in Figure 3.1. Such diffusers are, in any case, impractical

because of their extreme length. On all aircraft engines, and also on many industrial engines, length is crucial, and it is essential, therefore, that diffusion is accomplished in the shortest possible distance. With an increase in divergence angle, both diffuser length and friction losses are reduced, but stall

losses arising from boundary-layer separation become more significant.Clearly, for any given area ratio, there is an optimum angle of divergence at which the pressure loss is a minimum. Usually this angle lies between 6° and 12°.

在其最简单的形式,一个扩散器只是一个分叉通道流动的减速,减少速度转换为静压。这个转换过程的效率是相当重要的,因为发生的任何损失表现为扩散器总压力下降。在长低发散角的扩散器,压力损失沿墙高是由于皮肤摩擦,如图3.1所示。这样的扩散器,在任何情况下,不切实际,因为他们的极端的长度。在所有飞机引擎,也在许多工业引擎,长度是至关重要的,它是至关重要的,因此,扩散实现在最短的距离。发散角的增加,扩散器长度和降低摩擦损失,但摊位边界层分离所产生的损失变得更重要。显然,对于任何给定的面积比,有一个最佳的发散角的压力损失最小。通常这个角度位于6°和12°之间。

翻译

From a designer’s viewpoint, an ideal diffuser is one that achieves the required velocity reduction in the shortest possible length, with minimum loss in total pressure, and with uniform and stable flow conditions at its outlet.Sufficient experimental data are now available to design such a

diffuser,provided that the inlet velocity profile is symmetrical and not too peaked.Unfortunately, on many engines, the compressor outlet velocity profile is both peaked and asymmetric and is also subject to appreciable variation with changes in engine operating conditions. Under these circumstances,stable flow conditions cannot always be achieved, with the result that some engines are plagued by various deficiencies, such as a lack of consistency in the temperature distribution at the combustor exit and an increase in exhaust gas pollutants.

从设计师的角度来看,一个理想的扩散,达到所需的速度在最短的长度,减少与最低总压损失和出口均匀和稳定的流动条件。现在可以使用足够的实验数据来设计这样一个扩散器,提供入口速度剖面是对称和不太见顶。不幸的是,在许多发动机,压缩机出口速度剖面是和非对称达到顶峰,也受到明显的变化与引擎操作条件的变化。在这种情况下,并非总是可以达到稳定流动条件下,其结果是一些引擎饱受各种缺陷,如缺乏一致性在燃烧室出口温度分布和废气污染物的增加。

There is no lack of reliable experimental data on the performance of conventional conical diffusers. Available data on two-dimensional and annular diffusers are less comprehensive, and nearly all these data are summarized in a few important papers. Unfortunately, the performance charts presented in these papers are for boundary-layer-type inlet flows, developed in approach sections, which differ appreciably from the compressor-generated flows encountered in combustor diffusers. Moreover, in comparison with conventional diffusers, there are a number of additional geometric parameters that strongly affect the performance of combustor diffusers, such as the size and shape of the liner and its position relative to the diffuser exit. This complex interaction between the liner and diffuser explains why there are no general performance charts for combustor diffusers comparable to those for conventional diffusers.

没有缺乏可靠的实验数据常规锥形扩散器的性能。可用二维和环形扩散器的数据不全面,而且几乎所有这些数据在几个重要的文件进行了总结。不幸的是,这些论文中给出的性能图表为

boundary-layer-type进气流动,方法开发的部分,这明显区别生成的压缩机中遇到流燃烧室扩散器。此外,与传统的扩散器相比,有许多额外的几何参数,强烈影响燃烧室扩散器的性能,如衬管的大小和形状及其位置相对于扩散器出口。这之间的复杂交互班轮和扩散器解释了为什么没有总体性能图表燃烧室扩散器与传统的扩散器。

At the present time, there is no completely general and accurate method for predicting

combustor-diffuser performance. However, much useful progress has been achieved with numerical modeling techniques, which can now successfully predict the gross features of flow fields in combustor diffusers.

目前,没有完全通用和精确的方法预测燃烧室扩散性能。然而,很多有用的进展与数值模拟技术,现在可以成功地预测总燃烧室扩散器的流场特性。

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3.2 Diffuser Geometry

扩散器几何

The geometry of straight-walled diffusers may be defined in terms of three geometric parameters, as shown in Figure 3.2. Area ratio, AR, is an obvious choice as a major parameter because it is directly related to the primary function of the diffuser in achieving a prescribed reduction in velocity. Some form of nondimensional length is a logical selection for another because, as pointed out by Sovran and Klomp [1], in combination with the area ratio,such a length defines the overall pressure gradient; the principal factor in boundary-layer development. Usually, either the wall length, L, or the axial length, N, is used as a characteristic length; it is expressed in nondimensional form by dividing by a

representative inlet dimension.

直壁的几何扩散器可以定义三个几何参数,如图3.2所示。面积比,基于“增大化现实”技术,是一个显而易见的选择作为一个主要参数,因为它直接关系到扩散器的主要功能在实现规定的减少速度。某种形式的对另一个无量纲长度是一个合乎逻辑的选择,因为所指出的君主和Klomp[1],结合面积比,这样的长度定义总体压力梯度;边界层发展的主要因素。通常,要么墙长度L或轴向长度N,作为特征长度,它是用无量纲形式表示的入口尺寸除以一个代表。

A third parameter is the divergence angle, 2θ, which is not an independent variable, but is related to the other parameters by for two-dimensional units, and for conical units.

第三个参数是发散角、2θ,这并不是一个独立的变量,但与其他参数的二维单元,和锥形单位。

Sovran and Klomp [1] recommend the use of L/ΔR1 as the characteristic dimension for annular diffusers, where L is the average wall length, and ΔR1 is the annulus height at the diffuser inlet. This gives an expression for area ratio that is similar to the expression for conical units when the inlet radius ratio approaches zero, and similar to the expression for two-dimensional units when it approaches unity. Thus, the performance characteristics of all three types of diffuser may be plotted on a single set of coordinate axes as,for example, in Figure 3.3.

Sovran和Klomp建议使用L/ΔR1作为环形扩散器的特征维度,L是墙的平均长度,ΔR1是扩散器入口的环的高度。这给了一个表达式的面积比为锥形单位类似于表达式时,进气半径比接近零,和类似于表达为二维单元方法统一。因此,所有三种类型的扩散器的性能特征可以绘制一组坐标轴上,例如,在图3.3。

3.3 Flow Regimes

3.3流动机制

The first systematic study of flow patterns in diffusers was carried out by Kline et al. [2]. Tests were conducted on two-dimensional diffusers with straight walls, and the inlet flow conditions, the wall length, and the throat width were held constant. They observed that as the divergence angle is progressively increased from zero, a number of different flow regimes are found, which can be described as follows:

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在扩散器方面的第一个系统研究模式是由克莱恩等提出来的。测试了二维扩散器与直墙,在进气流,墙的长度,和喉部宽度保持不变的条件下。他们观察到从零发散角逐渐增加,找到许多不同的流动机制,它可以描述如下:

1. No ―appreciable‖ stall, with the main flow well behaved and apparently unseparated.

2. Transitory stall, whereby eddies are formed that run along the diffuser,some in close proximity to the wall. These eddies assist the diffusion process by transporting lethargic air away from the boundary layer and replacing it with more energetic air from the main core of the flow. This is a region of pulsating flow.

3. Fully developed stall, where the major portion of the diffuser is filled with a large triangular-shaped recirculation region, extending from the diffuser exit to a position close to the diffuser throat.

4. Jet flow, in which the main flow is separated from both walls. The separation begins slightly

downstream from the throat, and the flow does not reattach until well downstream from the diffuser. Jet flow occurs only at high angles of divergence.

1.没有“明显的”摊位,主要流表现好,显然是分不开的。

2.暂时的停滞,漩涡形成扩散器,运行一些接近城墙。这些涡流协助扩散过程运输昏睡的空气从边界层,代之以更有活力的空气流的主要核心。这是一个地区的脉动流。

3.充分发展停滞,扩散器的主要部分是满一个大三角形循环地区,从扩散器出口扩展到喉咙位置靠近扩散器。

4.射流,主要流程是分开两个墙。分离略下游喉咙开始,流不再植到下游扩散。射流只发生在高角度的差异。

3.4 Performance Criteria

3.4性能标准

The function of a diffuser is to reduce velocity and to convert kinetic energy or dynamic pressure into a rise in static pressure, as shown schematically in Figure 3.4. To assess the efficiency of conversion, it is necessary to define the quantity of available dynamic pressure. This is usually based on a mean velocity, u, which is obtained directly from the continuity equation as The dynamic pressure is then obtained as The pressure loss in the diffuser is defined as

扩散器的作用是降低速度和动能或动态压力转换为静压,示意图见图3.4。评估转换的效率,有必要定义可用的数量动态压力。这通常是基于平均速度,你是直接从连续性方程获得一样然后获得动态压力扩散器的压力损失被定义为

where ΔPdiff includes both the internal energy loss and the effects of redistribution of velocity between inlet and outlet.

在ΔPdiff包括内部能量损失进出口之间的速度再分配的影响。

For one-dimensional incompressible flow we have

对于一维不可压缩流

Continuity

连续性

Hence

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因此

Bernoulli:

伯努利:

From Equations 3.7 and 3.8, the rise in static pressure is given by Several useful parameters for expressing diffuser performance can be derived from this equation.

从方程3.7和3.8,静态压力的增加是由几个来自这个方程有用的表达扩散性能的参数。

3.4.1 Pressure-Recovery Coefficient

3.4.1压力恢复系数

The pressure-recovery coefficient Cp is calculated as A nondimensional coefficient of ideal static pressure rise, Cpideal, is derived directly from this equation as

压恢复系数Cp作为无因次系数理想的静态压力上升计算, Cpideal,直接从这个方程推导

This equation shows that Cpideal is dependent solely on area ratio to which it is related by a law of diminishing returns.

这个方程表明, Cpideal仅仅与依赖面积比的收益递减规律有关。

3.4.3 Overall Effectiveness

3.4.3整体效果

This is the ratio η of the actual pressure rise to the maximum theoretically obtainable, i.e., Thus, η is related to Cp by the equation Typically η varies between 0.5 and 0.9, depending on the geometry and flow conditions [6].

这是比η上升到理论上最大的实际压力的获得,。因此,η与Cp的方程有关.通常η在0.5和0.9之间,这取决于几何和流条件[6]。

3.4.4 Loss Coefficient

3.4.4损失系数

This is usually defined as where the over bar denotes a mass-flow weighted value derived from a detailed traverse across the duct.

这是通常定义为在障碍外表示来自整个管道质量加权值。

The value of λ depends largely on the type of diffuser employed. Typical values of λ for

combustor diffusers range from around 0.15 for ―aerodynamically clean‖ faired diffusers to around 0.45 for dump diffusers of high liner/depth ratio (DL/h1) containing a normal complement of support struts and fuel injectors. For vortex-controlled diffusers (VCD), reported values of λrange from 0.05 to 0.15.

λ的值在很大程度上取决于扩散器的类型。典型值λ的燃烧室柔光镜范围从约0.15“空气清洁”流线型的扩散器0.45左右转储柔光镜高班轮/深度比(DL / h1)包含一个支持层和喷油嘴的正常补充。为涡控制扩散器(VCD)报道的λ值从0.05到0.15不等。