小川 和彦

In this study, the measurement of the number and size of cavitation bubbles in the biased velocity distribution were performed at nine positions on the downstream side of a butterfly valve. In practical engineering, more than two valves are installed in a series or bends are installed just ahead of a valve to save space. In such cases, the cavitation occurrence will be affected by the biased velocity distribution in front of a valve. In the biased velocity distribution in which the velocity was larger on the nozzle side, there were very small differences in the number of the cavitation bubbles at each position. If a cavitation number was small to some extent, the number of cavitation bubbles at the orifice side also increased due to vena contracta. Moreover, the cavitation bubble occurrence on the nozzle side was more intense than in the biased velocity distribution in which the velocity was larger on the orifice side. Therefore, the installation of a valve where the flow velocity on the nozzle side becomes large should be avoided in actual piping.

バタフライ弁のキャビテーションは、弁後部の大きな流れの剥離と、ノズル側とオリフィス側からの流れの干渉によって発生する。本研究ではこのキャビテーションを避けるために、弁下流側に多孔プレートを挿入することを提案し、実験から騒音を抑制できることを明らかにした。また数値解析によりプレートの多数の孔が弁の圧力回復に役立つことが明らかとなった。バタフライ弁がコパンクトかつ簡単な構造であることから産業において数多く用いられている。多くの耐キャビテーション弁が提案されてきたが、高価で複雑な構造である。多孔プレートは簡単な構造で安価であり、実用上非常に有益である。

従来キャビテーション気泡の大きさと個数の分布は明らかでなく、定量的な考察もなかったが、本研究では初めてこれを明らかにした。弁付近に発生するキャビテーション気泡の直径は、弁開度45度の場合20μmから500μmの範囲であった。弁前縁でのキャビテーション気泡の直径は、弁下流側1dia.での気泡の直径の数倍であり、縮流による前縁での発生が支配的であること、下流へと流れるにしたがって気泡径が分裂、縮小により増加することが明らかになった。

Cavitation occurs intensely around the butterfly valve because of the interference of the flow from the nozzle side with the flow from the orifice side. To avoid this interference, semicircular fins were attached to the valve body. In this paper, it was confirmed based on the experimental results that the attachment of the fins was very effective in reducing cavitation noise. In past studies, photographs focusing on the aspect of the butterfly valve cavitation have been reported in great numbers. However,since the measurements of number and size of the cavitation bubbles were not carried out, the details of the cavitation growth were not clear. In this study, close-up photographs of cavitation bubbles were taken and their number and size were analyzed. The difference between the normal valve and the valve with the fins is reported.

本研究ではバタフライ弁に半円形フィンを付加し，キャビテーション騒音抑制を図った．フィンの大きさは，弁体中央での高さが管内径の約1/2程度、弁体中央から弁直径の1/4程度の距離に取り付けると、従来弁に比べて5dB程度騒音を小さくすることができた．可視化実験により，弁体前縁側からの流れと弁体後縁側からの流れの干渉がフィンによって緩和されたことにより低騒音化を図ることができたと考えられる．

産業用プラントではバタフライ弁は流体制御に構造が簡単でコンパクトであることから広く使われているが、キャビテーションにより騒音と壊食が配管に発生し、メインテナンスの上で大きな問題となっている。本研究ではフィンを付加して弁体のノズル側とオリフィス側を通過する流れの干渉を抑制することにより、騒音の低減を試みた。高速度カメラと騒音測定の結果から、半円状のフィンを弁体の背面に取り付けると、従来の弁に比べて初生は若干低い流速で発生するものの最大音圧を低下させることができることがわかった。

本研究ではバタフライ弁のキャビテーション抑制に関し、配管の管径拡大部を設け、弁体後方の剥離領域が低圧になるのを緩和し、弁体後方の圧力を上昇させることによりキャビテーション抑制が可能であることを明らかにした。溝部の大きさは、実験・解析から管壁の間隔Dに対し1/5Dの深さ、1Dの長さが望ましいことがわかった。実際の配管における実験結果では、中間開度において同一の圧力損失係数のもとで初生のキャビテーション係数が約60％に抑制され、最大音圧も約5dB抑制されることがわかった。構造が簡単なため、汚泥などが混じった流体でも用いることがき、 プラントでのメンテナンスが容易になると考えられる。

Butterfly valves are widely used in industry because of their compactness and simple construction. Many kinds of valves have been proposed to prevent cavitation. However, those valves are fairly expensive and the structures of the valve are complicated. In this paper, the attachment of fins to the valve body is proposed to reduce cavitation noise. The cavitation around the butterfly valve occurs because of the interference of the flow from the nozzle side and the flow from the orifice side. To avoid this interference, semicircular fins were attached to the valve body. From the experimental results, it was confirmed that the attachment of the fins was very effective in reducing the cavitation noise.

高圧ガス噴射で直接駆動させる船舶推進用の二次元半開ノズル内の流れを可視化し、圧力測定を行って非定常流の考察を行った。実験から、噴射した気相と水流の相は完全に分離し、二相の界面は干渉波のように挙動することがわかった。また高速度カメラの撮影から、水流の速度により干渉波の形状が変化し、水流の速度が増加するにつれて波長が長くなることが明らかになった。

高圧ガスで駆動する船舶推進用の二次元半開ノズルにおける内部流れについて、可視化実験を行った結果、流れは非定常であり、かつ気相と液相がはっきりと分離していることを確認した。高圧ガスを連続的に噴出すると、その気相と液相の界面波の伝播が見られた。この界面波の発生周期は高圧ガスの噴出を間欠的にしても変わらなかったが、推力自体は低下したが、推進効率は増大することが明らかになり、界面波の存在がノズル性能に大きな影響を及ぼすことがわかった。

クヌッセンポンプの工学的応用について考察を行った。希薄気体の場合、温度勾配のある平板が平行に置かれた場合、気体分子は低温側から高温側へと移動する現象が知られており、この現象を利用したものがクヌッセンポンプで、真空装置やコンプレッサとして使用できる。本論文では人工衛星への適用を検討し、数値解析の結果から高度150km程度では使用でき、また高度200km程度になると適用が困難との結果を得た。

The authors proposed the method of performance improvement by injecting moist air as the new propulsion system. The authors investigated on the effect by injecting dry air into the nozzle in former papers but there was the limit because the average fluid density decreased in the nozzle by air. In this study, the method of injecting moist air was investigated to obtain more thrust. Steam of the moist air injected into the nozzle condensed and the average density of fluid increased. By the increase of the density, the thrust increased about 20% comparing with the result of using only dry air and the propulsion efficiency was improved. There are some sources of exhausted heat such as engines and boilers in actual ships and obtaining the steam is relatively easy to make use of them. Our propulsion method is applicable for practical use and desirable for the energy efficiency.

The vortex method is advantageous to high-Reynolds number flows and its algorithm is relatively simple. However, the number of vortices increases with time and the calculation load of velocties by the Biot-Savart law becomes very large. The authors propesed Lattice Vortex Method (LVM) to accelerate the calculation and to perform numerical analysis stably. The vorticity is not always conserved in the calculation by the conventional finite difference method and the conservation of vorticity is one of the crucial points in numerical flow analysis. In our lattice vortex method (LVM), the vorticity and the density are conserved because those are moved and distributed on cells by Lagrangian scheme. In this paper, lattice cortex method was extended for three-dimensional flow analysis, the vorticities were expressed by vortex rings arranged on the cell sides of the grid to avoid the treatment on the end of vortex lines and Lagrangian scheme was used to preserve the mass and vorticities. The propagation of internal waves was grasped fairly good in three-dimensional stratified flow by LVM.

Spark technique was applied to qualitative velocity profile visualization in hypersonic flows over compression and expansion corners. For comparison, Schlieren measurement and numerical simulation based on the two-dimensional full Navier-Stokes equations were performed. The computational results did not contradict with the Holden criterion. For shock wave visualization, quantitative agreement among the spark, the Schlieren, and the computational results was good. Further, we found that the spark column shapes observed photographically and the computational velocity profiles correlate to each other. The results demonstrate the capability of the technique.

The authors proposed the method of performance improvement by blowing moist air for a water jet ship. The authors investigated on the performance of the water jet ship by blowing air into the nozzle in former papers but there was the limit by blowing only air. In this study, the method of moist air blowing was investigated to obtain more thrust. Steam blown into the nozzle condenses just after the outlet and the average density of fluid increases. By the increase of density, the fluid in downstream can be easily accelerated by expanding bubbles of air. The experiment on this method was performed in a towing tank and the result showed that the thrust increased about 20% in comparison with the conventional method. There are many sources for supplying moist air like exhaust engines and boilers in actual ships. Our propulsion method is promising for practical use.

As is well known, shock wave shapes over a flat plate with a slightly blunt leading edge in hypersonic flow exhibit a similarity for large Reynolds numbers. However, deviation from the similarity for smaller Reynolds numbers has not been investigated systematically. It is the purpose of this paper to investigate the characteristic of the deviation experimentally. Shock wave shapes over a flat plate model with a slightly blunt leading edge and those over a cylinder model are compared with blast wave theory. The results show that, at high Reynolds numbers, a similarity shock wave shape emerges not only for the flat plate but also for the cylinder, because of the Mach- and Reynolds-number independence. The deviation from the similarity shape for low Reynolds number regime is due to the variation of nose drag coefficient, which becomes larger with a decreasing Reynolds number.

This study has focused on the prediction of torque characteristics of butterfly valves and we propose a prediction equation derived from theoretical investigation and experimental results. The free-streamline theory is applied to the prediction of the torque characteristics because the flow separation was confirmed on the rear surface by flow visualization. Actual valves are affected by the duct wall. These effects are considered as the change of attack angles and approaching velocities of the flows. Thus, the correction for these effects is added to the theoretical torque equation obtained by the free-streamline theory. The results of our prediction are shown to be successful comparing with the experimental results.

Investigation on fluidmechanic performance on a butterfly valve has been carried out. A practical valve model of a given thickness and a hub is used for the loss coefficient theory. A theoretical loss coefficient has been formulated from a contraction factor obtained by applying the generalized Borda mouthpiece theory. Cavitation stages (such as cavitation inception, supercavitation inception, cavitation damage inception, choking cavitation) have been theoretically predicted from the valve loss coefficient. The cavitation prediction has been carried out by applying the free streamline theory, where the relation between the loss coefficient and the critical cavitation factor has been formulated. The results of the theoretical prediction equations agree well with the experimental results.

There are several stages of Cavitation phenomena in the flow around butterfly valves. The cavitation stage such as cavitation inception, supercavitation inception, cavitation damage inception, choking (or flashing) cavitation occurs for the various valve opening angles. In this paper, these cavitation stages were theoretically predicted for valve loss coefficient. The cavitation prediction was performed by applying the free streamline theory, where the relation between the loss coefficient and the critical cavitation factor were formulated. As for the experiment, the cavitation inception and critical supercavition were obtained from the vibration intensity rising and the noise level rising, and the flashing cavitation was observed by means of flow visuarization. The theoretical predicted characteristic curves and these experimental results were compared, and agreed very well with each other.

The purposes of this sutdy are to clarify the characteristics of cavitation erosion in piping line behind a butterfly valve and to propose the method of predicting the erosion resistance.<BR>One of the major obstructions in an experiment of cavitation erosion is taking a long time with the use of metal specimen. Therefore, the use of a substitutive material which had the fragile property to anti-cavitation erosion was proposed in this study and the similarity of the erosion characteristics was confirmed between the substitutive material and the real material.<BR>In the second stage of this study, the substitutive material was installed as pipe wall at a bend behind a butterfly valve. It was confirmed from observations and measuring the position of erosion that the erosion of the material was due to cavitation occured at the butterfly valve. Therefore the prediction of the erosion resistance in a real condition is possible from the result of the similarities of erosion characteristics of a substitutive material.

In order to clarify the effects of frost formation on the performance of an air-cooled heat exchanger, analyses by a uniform frost formation model and experiments were performed. Using this model, frost thickness and frost density can be estimated from the increase of the pressure drop of the heat exchangers. It is found that the heat transfer characteristics under frost formation can be grasped in the same manner as the case of no frost formation and that the analogy of heat and mass transfer holds in the practical application.

The heat-mass transfer and pressure drop performances during the dehumidification of moist air flowing between plate fin and tube heat exchangers which are mainly used in air conditioners were investigated experimentally. The experimental results revealed the following. The heat transfer performance with condensation was equal to that without condensation, in consideration of dehumidified water on the surface of a heat exchager. The analogy between heat and mass transfer under dehumidification was not completely substantiated. The mass transfer coefficient is smaller than that of theoretical analogy.

記事分類: 応用流体力学 ; 機械工学--流体機械

When cavitation around a butterfly valve becomes very intense, cavitation bubbles flow downstream without collapses. As the flow velocity is increased further, a cave whose shape is like a wedge is formed behind the back surface of the valve. This phenomenon is called flashing. In this condition, the place occurring cavitation changes to the clearance between the edge of the valve and pipe walls from the back surface of the valve. Changing the place of cavitation occurrence is an interesting phenomenon from the standpoint of hydrodynamics. From the visualization of flashing phenomena, it can be said that flashing is different from choking and supercavitation since cavitation bubbles can occur from the clearance between the edge of the valve and pipe walls in flashing condition.