Tohru Fukano

Characteristics of gas-liquid two-phase flow in a capillary tube

Abstract Gas-liquid two-phase phenomena in capillary tubes were investigated with special attention on the flow patterns, the time varying void fraction and pressure loss. The directions of flow were vertical upward, horizontal and vertical downward. Pipe inner diameters used were 1 mm, 2.4 mm and 4.9 mm. As a result it is made clear that due to the strong effects of surface tension the flow pattern is not severely affected by the direction of flow, the smaller the pipe inner diameter, the easier the formation of liquid slug and the pressure loss in a unit length takes much larger than the estimated value by the Chisholm equation.

Prediction of the circumferential distribution of film thickness in horizontal and near-horizontal gas-liquid annular flows

Abstract A theoretical model for predicting the circumferential film thickness distribution in horizontal and near-horizontal annular two-phase flows is presented. It is based upon a disturbance wave flow model which consists of disturbance waves and a base film. It differs from Laurinats and Lins models in tbat liquid is transferred in the circumferential direction by the pumping action of disturbance waves, which counteracts the drainage due to gravity, and that the effects of the induced secondary flow in the gas flow and the surface tension force bave minor effects on the formation of the liquid film near the top of the tube cross section. The film thickness distribution predicted by the present model agrees with the experimental data much better than those predicted by Laurinats model.

The effects of tip clearance on the noise of low pressure axial and mixed flow fans

Abstract Results are presented of a systematic experimental investigation of the effects of tip clearance on both noise and performance of four commercially representative fans (three low pressure axial flow and one mixed flow) in a circular duct, both with and without coincidence of the fan and duct axes (fan/duct eccentricity). It is shown that with eccentricity significant pure tone noise is generated due to blade tip/duct wall interaction, with a plane wave mode component. Reducing tip clearance both improves performance and reduces noise, not only at the maximum efficiency operating point but also in an appreciable low flow rate operating region. With tip clearances small enough to optimize performance and minimize noise fan/duct eccentricity must be kept to a very small value, implying strict precautions in fan and duct manufacture, installation and service conditions.

Tip clearance noise of axial flow fans operating at design and off-design condition

The noise due to tip clearance (TC) flow in axial flow fans operating at a design and off-design conditions is analyzed by an experimental measurement using two hot-wire probes rotating with the fan blades. The unsteady nature of the spectra of the real-time velocities measured by two hot-wire sensors in a vortical flow region is investigated by using cross-correlation coefficient and retarded time of the two fluctuating velocities. The results show that the noise due to TC flow consists of a discrete frequency noise due to periodic velocity fluctuation and a broadband noise due to velocity fluctuation in the blade passage. The peak frequencies in a vortical flow are mainly observed below at four harmonic blade passing frequency. The discrete frequency component of velocity fluctuation at the off-design operating conditions is generated in vortical flow region as well as in reverse flow region. The peak frequency can be an important noise source when the fans are rotated with a high rotational speed. The authors propose a spiral pattern of velocity fluctuation in the vortical flow to describe the generation mechanism of the peak frequency in the vortical flow. In addition, noise increase due to TC flow at low flow rate condition is analyzed with relation to the distribution of velocity fluctuation due to the interference between the tip leakage vortex and the adjacent pressure surface of the blade.

Measurement of time varying thickness of liquid film flowing with high speed gas flow by a constant electric current method (CECM)

Abstract A constant-electric-current method (CECM) developed by the present author is a kind of conductance method. The characteristics of the CECM are (1) a constant-current power source is used for supplying the electric power and (2) two kinds of electrodes are installed. One is used for supplying electric power and the other is for detecting the information of hold-up or film thickness. The main merits of the CECM are (1) the output from the sensor electrode is independent of the location of gas phase, for example radial location in a tube cross-section, (2) the sensitivity of detecting the change in the hold-up is higher in the case of the thinner film thickness, and (3) the interaction among the electrodes is negligible. The basic idea, calibration and examples of the application of the CECM will be discussed in the present paper.

Effects of liquid viscosity on flow patterns in vertical upward gas–liquid two-phase flow

Abstract The purpose of the present experimental study is to investigate the effects of liquid viscosity on the flow patterns of upward air–liquid two-phase flow in a vertical tube of 19.2 mm in inner diameter and about 5.4 m in length. Three different liquids, including water and aqueous glycerol solutions, were employed. Kinematic viscosity of these liquids varied from 1.0×10 −6 to 14.7×10 −6 m 2 /s. The flow patterns were observed using a video recorder and still photography. The time-spatial characteristic maps of gas–liquid interfaces which were drawn using the mean liquid holdup signals detected by 70 pairs of holdup sensors arranged with the axial spacing of 15 mm over the length of 1.035 m were also used. In this report, we first defined the flow pattern of each flow. Next, the effects of liquid viscosity on the overall flow pattern and interfacial structures, or the interfacial waves, were discussed. Finally, based on those results we proposed flow pattern maps for each liquid viscosity. It is found that the flow pattern transitions strongly depend on the liquid viscosity.

Noise generated by low pressure axial flow fans, III: Effects of rotational frequency, blade thickness and outer blade profile

In a previous paper [1], practical formulae which correlate the noise sound power of a low pressure axial flow fan to the design parameters of the impeller were derived analytically by introducing a simple model of the flow past the fan blade. The validity of these formulae was examined experimentally in respect to the effects of such parameters as number of blades and chord length on sound power [2]. In this report, the experimental results in respect to the effects of the fan rotational frequency and blade thickness at the trailing edge are described and these results are compared with the theory. The agreement of the theoretical and the experimental results is generally quite satisfactory. Finally, it is shown that the outer profile of the blade has considerable effects on both noise and aerodynamic characteristics of fans: that is impellers with blades swept forward are much superior to those with blades swept backward.

Noise generated by low pressure axial flow fans, II: Effects of number of blades, chord length and camber of blade

Abstract In a previous paper [1], a practical formula which correlates the sound pressure level of turbulent noise originating from an axial flow fan to the design parameters of the impeller was derived analytically by introducing a simple flow model. The validity of this formula is examined in respect to the effects of some of these parameters, such as number of blades and chord length, on the noise level. Experiments are also described on the effect of blade camber, to examine the applicability of the formula adopted for the evaluation of the wake width of the blade in the calculation of theoretical values, because the wake width plays a very important role in estimating the sound power. The agreement between the experimental and theoretical results is good unless separation occurs on the blade surface. Finally, it is shown that the measured sound pressure level data for most of the test impellers fall on a single curve when a dimensionless parameter, named the DT ratio (which represents the ratio of the wake width to the pitch of the blade row), is used as the abscissa.

Analysis of liquid film formation in a horizontal annular flow by DNS

Abstract The role of the disturbance waves for transferring liquid toward the top of a horizontal tube wall to cope with the drainage due to gravity is investigated by the direct numerical simulation (DNS) which is based on the continuity equation and the Navier–Stokes equations in three-dimensional cylindrical coordinate system. The level set method is used for capturing the interface between gas–liquid two fluids. Developing flow from a separated to an annular flow is simulated by using this numerical technique, and the liquid film formation is reproduced. It is demonstrated by this calculation result that liquid is transferred in the circumferential direction as the liquid film by the pumping action of disturbance waves which has been proposed by one of the present authors. That is, the pressure gradient formed within a disturbance wave in the circumferential direction plays an important role for the liquid film formation in a horizontal annular flow.

Relation between temperature fluctuation of a heating surface and generation of drypatch caused by a cylindrical spacer in a vertical boiling two-phase upward flow in a narrow annular channel

When a flow obstruction such as a cylindrical spacer is set in a boiling two-phase flow within an annular channel, the inner tube of which is used as a heater, the temperature on the surface of the heater tube is severely affected by the existence of the cylindrical spacer. In some case the cylindrical spacer has a cooling effect, and in the other case it causes the dryout of the cooling water film on the heating surface resulting in the burnout of the tube. In the present paper we will discuss the temperature distributions near the cylindrical spacer and its fluctuation characteristics in relation to the change of the flow configuration near the cylindrical spacer.