Numerical analysis of a waterjet propulsion system
Numerical analysis of a waterjet propulsion system
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DOI：
10.6100/ir614907
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发表时间：
2006-11
影响因子：
8.2
通讯作者：
N. Bulten
中科院分区：
文献类型：
--
作者：
N. Bulten
水射流推进系统是用泵来产生高速射流来推进船舶的。一个标准的喷水装置可以分为一个入口，一个泵和一个喷嘴。为了操纵和倒车的目的，一个额外的转向装置可以集成到安装中。在过去的几十年里，水射流推进系统的发展取得了重大进展。现在，商业快速渡轮的速度可以达到50节，也就是每小时90公里。描述水射流推进系统的理论来源于开式螺旋桨理论。螺旋桨推力的预测是基于流管控制体积的动量平衡。这种推力然后通过螺旋桨的轴转移到船体上。相比之下，对于水射流推进系统，力不仅通过轴而且通过装置的固体表面传递到船体。一项重要的审查发现，对开放式螺旋桨所做的一些假设不适用于水射流。射流泵的流入是不均匀的。这导致叶片载荷在叶轮旋转过程中发生变化。研究了这种不均匀流入的原因和影响。确定了四个影响泵上游非均匀流速分布的因素。作为第一个原因，水是从船体下面吸收的，那里有一个速度分布不均匀的边界层。即使在正常运行条件下，水随后也会在入口受阻，从而导致不均匀性的增加。最后，流入流体通过入口弯曲和突出的轴，这增加了不均匀性。结果表明，这种不均匀性是由气流中累积涡量引起的。由于涡量的存在，找到了一个稳定的速度分布，而典型的速度分布或多或少与实际的进气道设计无关。这些研究是基于对整个水射流装置流动的数值分析。数值方法的选择是基于捕捉喷水装置中典型流动现象的能力：高雷诺数、时间依赖性和部分旋转参照系中的不可压缩流动。由于流入的高度非均匀性，在流动中产生和输送涡量的能力是一个重要的要求，同时考虑到旋转叶片和固定壳体之间的叶尖间隙区域的流动现象的可能性。采用Reynolds平均Navier-Stokes （RANS）方法进行所有数值分析。雷诺应力采用双方程k-e湍流模型得到。众所周知，这种湍流模型在停滞点附近会产生误差。对该误差对推力和扭矩预测的影响的估计表明，实际偏差是可以接受的。用已有的实验数据验证了水射流入口和混流泵的数值模型。将水射流进口流量计算结果与进口静压测量结果以及叶轮面总压和速度分布进行了比较。计算结果与实验数据吻合良好。射流入口的流动现象用入口速度比（IVR）来描述，它是船速与泵速的比值。通过浓度标量来确定进水流管的形状和位置。这使得流管的可视化和质量平均流入速度的计算成为可能。通过这种方法，可以准确地确定喷水装置的尾流分数。结果表明，流管的实际形状取决于IVR。结合泵扬程和轴功率的实验数据，验证了混流泵的CFD计算结果。采用准稳态多参照系法和全瞬态移动网格法进行计算。两种方法预测的头和功率之间的差异很小。均匀入流速度分布的全瞬态运动网格计算提供了转子-定子相互作用对叶轮的非定常激励力。结果表明，径向作用力的大小与泵的流量有关。研究了非均匀流速分布对泵的影响。对水射流推进系统的数值分析表明，在计算条件下，泵的性能偏差仅为191%。然而，对径向力的影响要大得多。发现了径向力的另一个平均分量，其大小和方向与流量和非均匀性水平有关。这个平均力的来源是叶轮叶片上的不平衡扭矩，由于在旋转期间攻角的变化。将进口和泵的验证数值模型结合起来，形成完整的水射流装置。将整个机组的计算结果与荷兰瓦锡兰推进公司的标准水射流性能预测和选择软件的计算结果进行了比较。对装置流量、推力和扭矩的预测结果吻合较好。确定推力的两种方法是：(i)对固体壁面上的轴向力分量进行积分，（ii）应用积分动量平衡方程的简化版本。造船公司一般采用后一种方法。对于较高的航速，两种方法之间存在明显的偏差。对垂直方向上的合力进行分析，揭示了高速时的显著升力。结果表明，基于动量平衡的流管控制容积方法存在一定的不足。船速越高，偏差越大。数值结果证实了用简化方法描述喷水装置的假设是不正确的。这可以部分归因于忽略了水射流入口附近船体的影响，部分归因于忽略了作用在流管上的压力分布的贡献。
A waterjet propulsion system is used to propel ships, using a pump which produces a high speed jet. A standard waterjet installation can be divided into an inlet, a pump and a nozzle. For manoeuvring and reversing purposes an additional steering device can be integrated into the installation. The development of waterjet propulsion systems has made significant progress over the last few decades. Nowadays, commercial fast-ferries reach velocities of 50 knots, which is about 90 km/h. The theory to describe waterjet propulsion systems is derived from open propeller theory. The prediction of the thrust of a propeller is based on the momentum balance of a streamtube control volume. This thrust is then transferred though the shaft of the propeller to the hull of the ship. In contrast, for a waterjet propulsion system, forces are transferred to the hull not only through the shaft but also through the solid surface of the installation. A critical review learns that some assumptions made for open propellers are not valid for waterjets. The inflow to the waterjet pump is non-uniform. This results in a blade loading that varies during an impeller revolution. The cause and the effects of this non-uniform inflow have been investigated. Four contributing factors are identified for the development of a non-uniform velocity distribution just upstream of the pump. As a first cause, the water is ingested from below the the hull of the ship, where a boundary layer with a non-uniform velocity distribution is present. Even at normal operating conditions, the water is subsequently retarded in the inlet, which results in an increase of the nonuniformity. Finally, the inflow passes the bend in the inlet and the protruding shaft which add to the increase in non-uniformity. It is concluded that the nonuniformity is the result of the accumulated vorticity in the flow. Due to this vorticity, a stable velocity distribution is found, and the typical velocity distribution is more or less independent of the actual design of the inlet. The investigations are based on numerical analyses of the flow through the complete waterjet installation. Selection of the numerical method is based on the capability to capture typical flow phenomena in a waterjet installation: high Reynolds number, time-dependency, and incompressible flow in a partially rotating frame of reference. Due to the high level of non-uniformity of the inflow, the ability to generate and transport vorticity in the flow is an important requirement, as well as the possibility to take into account the flow phenomena in the tip clearance region between the rotating blades and the stationary housing. A Reynolds averaged Navier-Stokes (RANS) method is chosen to perform all numerical analyses. The Reynolds-stresses are obtained using the twoequation k-e turbulence model. This turbulence model is known to produce an error near a stagnation point. An estimation of the influence of this error on the prediction of thrust and torque shows that the actual deviations are acceptable. The numerical models of both the waterjet inlet and the mixed-flow pump are validated with available experimental data. Results of calculations of the waterjet inlet flow are compared with measurements of static pressure along the inlet and with the total pressure and velocity distribution at the impeller plane. Agreement between the CFD results and the experimental data is good for all calculated conditions. The flow phenomena in a waterjet inlet are characterised by the inlet velocity ratio (IVR), which is the ratio of the ship speed and the pump speed. The shape and location of the streamtube of the ingested water is determined with aid of a concentration scalar. This enables the visualisation of the streamtube and the calculation of the mass averaged inflow velocity. In this way the wake fraction of the waterjet installation is determined accurately. It is shown that the actual shape of the streamtube depends on IVR. The CFD calculations of the mixed-flow pump are validated with experimental data for the pump head and the shaft power. The calculations are performed with a quasi-steady multiple frame of reference (MFR) method and a fully transient moving mesh method. Differences between predicted head and power in both methods are small. The fully transient moving mesh calculations with a uniform inflow velocity distribution provide the unsteady excitation forces on the impeller due to rotor-stator interaction. It is found that the magnitude of the radial interaction force depends on the flow rate though the pump. The influence of the non-uniform velocity distribution to the pump is investigated as well. The deviation in pump performance is limited to a few Numerical analysis of a waterjet propulsion system 191 percent for the calculated conditions. The influence on radial forces is far greater, however. An additional mean component of the radial force is found, the magnitude and direction of which are related to the flow rate and the level of non-uniformity. The origin of this mean force is an unbalanced torque on the impeller blades, due to a variation of the angle of attack during a revolution. Both validated numerical models of the inlet and the pump are combined to form the complete waterjet installation. Results of the calculations of the complete unit are compared with the results of the standard waterjet performance prediction and selection software of Wartsila Propulsion Netherlands BV. Good agreement is found for the prediction of flow rate, thrust and torque of the installation. Two methods to determine the thrust are used: (i) the integration of the axial force component on the solid wall and (ii) the application of a simplified version of the integral momentum balance equation. The latter method is generally applied by ship building companies. A clear deviation between the two methods is found for higher ship speeds. Analysis of the net force in vertical direction reveals a significant lift force at high speeds. It is concluded that the method based on the momentum balance for the streamtube control volume, has some short-comings. The deviation increases for higher ship speeds. The numerical results confirm the hypothesis that the simplified method to describe waterjet installations is not correct. This can be partly attributed to the neglect of the influence of the hull in the vicinity of the waterjet inlet and partly to the neglect of the contributions of the pressure distribution acting on the stream tube.