Toshitake Ando

Flow and Heat Transfer of Petal Shaped Double Tube—Effects of Pipe Geometry

Heat exchangers are widely used to transfer heat from one fluid flow to another. Over time, various types of heat exchangers have been developed to decrease energy consumption and increase efficiency, including compact fin tube and double tube with special inner pipe geometries (ribbed, spiral, and petal shapes) [1, 2]. In this study, we experimentally investigated the heat transfer efficiency of petal-shaped double tubes with a large wetted perimeter or heat transfer area. Moreover, the influence of the number of petals was studied. Our results show that a larger heat transfer area improves heat transfer rate in heat exchangers but the heat transfer efficiency of the five petal-shaped double tube (5P-tube) is much higher than that of the 6P-tube because of a smaller flow resistance of the 5P-tube.

Interaction Between Water or Air-Water Bubble Flow and Tube Bundle—Effects of Arrangement of Tube Bundle and Void Fraction

Water or air-water bubble flow passing through tube bundle can be seen in many industrial equipments such as heat exchanger and chemical equipment. In addition, tube bundle can be used as a flow-straightener, -mixer, -resistor, and -damping device. In this study, the effects of tube arrangement of equally or unequally spaced in-line and staggered tube bundle and of void fraction on the flow characteristics such as flow pattern and flow resistance of a tube or tube bundle are examined experimentally.

Loss Reduction of the Flow in the Reducing Elbow Duct by Using Weir Shaped Small Obstacle

Reducing elbow is \( 90^{\circ }\) bend or elbow pipe/duct which has smaller outlet than inlet. This type of element is frequently used in the industrial facilities. After the corner of this element separated flow from the corner flows away from the inner wall. And then, effective cross-sectional area of downstream pipe/duct near the corner is reduced. Reduction of effective cross-sectional area causes extremely large flow resistance. In this study, loss reduction method with weir-shaped obstacle for reducing elbow duct is newly proposed. To clarify the effects of obstacle on the flow in the reducing elbow, pressure measurement and flow visualization were carried out. As a result, it is made clear that the loss of reducing elbow is reduced by maximum of about 63 % by using obstacle.

Direct Numerical Simulation of Dynamic-Rotational Controlled Jet

In order to develop a new mixing procedure, we conduct DNS (direct numerical simulation) of dynamic controlled free jets. As the dynamic control, it is assumed that the inflow velocity of jet is alternately rotated around streamwise direction. To realize the high accurate computation, the discretization in space is performed with hybrid scheme in which sine or cosine series and 6th order compact scheme are used. From view of instantaneous vortex structures, it is found that the flow pattern considerably changes according to the rotating frequency, i.e., according to the increasing frequency, helical and entangled mode appear in turn. From the ensemble averaged flow properties such as jet width and entrainment, it is confirmed that the jet widely spreads perpendicular to the rotating axis and that the mixing of upstream jet is markedly enhanced due to the dynamic control. Further in order to quantify the mixing efficiency of the dynamic control, as another mixing measure, a statistical entropy is examined. Compared to the uncontrolled jet, it is confirmed that the mixing efficiency is markedly improved, suggesting that the dynamic control can be expected to be useful for the improvement of mixing performance.

Flow and heat transfer of petal shaped double tube

In this study, the flow and heat transfer characteristics of petal-shaped double tube with 6 petals are examined experimentally for a compact heat exchanger. As results, the heat transfer rate, Q, of the 6 petal shaped double tube (6-p tube) is much larger than that, Qp, of conventional circular double tube in all Reynolds number Rein,h (where, the reference length is hydraulic diameter) ranges. For example, at Rein,h =(0.5～1.0)× 104 it is about 4 times of Qp. The heat transfer enhancement of 6-p tube is by the increase of heat transfer area, wetting perimeter, and a highly fluctuating flow, and Q of the 6-p tube can be expressed by Q [kW/m] = 0.54Rein,h + 2245.

Jet Flow from Resonance Nozzle and Flow Control by Notched Nozzle

The jet flow from an orifice nozzle shows a particular velocity profile with a large velocity gradient at the edge of the jet. Setting an orifice nozzle, second one, with a resonance room just after the first orifice nozzle the jet causes resonance phenomena and the resonance frequency depends on the jet velocity and the volume of resonance room. In this study, a notched nozzle was used as the second one to enhance the flow disturbance or fluctuation, and the effects of the notched nozzle on the mean and fluctuating velocities and the mixing or diffusion characteristics of the jet itself and with the surroundings were examined experimentally using a hot wire measurements. As a result, it was made clear that under a same operation power the flow rate near the nozzle exit of a notched resonance jet is larger than those of an orifice and resonance jets, for example, at x/d = 0.2 (x: distance from the nozzle exit to the downstream, d: nozzle diameter) it is about 2.5 and 1.3 times of theirs, respectively, and others.

Direct numerical simulation of vector-controlled free jets

We conduct DNS (direct numerical simulation) of vector controlled free jets. The inflow velocity of jet is periodically oscillated perpendicular to the jet axis. In order to realize the high accurate computation, a discretization in space is performed with hybrid scheme in which Fourier spectral and 6th order compact scheme are adopted. From visualized instantaneous vortex structures, it is found that the flow pattern considerably changes according to the oscillating frequency, i.e., according to the increasing the frequency, wave, bifurcating and flapping modes appear in turn. In order to quantify mixing efficiency under the vector control, as the mixing measure, statistical entropy is investigated. Compared to the uncontrolled jet, the mixing efficiency is improved in order of wavy, flapping and bifurcating modes. Thus the vector control can be expected for the improvement of mixing efficiency. Further to make clear the reason for the mixing enhancement, Snapshot POD and DMD method are applied. The primary flow structures under the vector control are demonstrated.

DNS of turbulent flow in a channel with an elastic cantilever

In various scientific fields, numerical solving of fluid-structure interaction (FSI) problems are of importance. Therefore the establishment of stable numerical scheme is needed to analyze the details of flow phenomena. In the present paper, a weak-coupling method for FSI problem is proposed: the rigorous equations of motion for a elastic body are discretized with finite volume method; the elastic body is reproduced via immersed boundary method in the flow computation. In order to demonstrate the performance of proposed scheme, the 3D structure analysis and 3D FSI problems of a elastic body are solved. It is found that the computation is stably conducted using the proposed method, in spite of the occurrence of fairly large deformation of the object. Also it turns out that the realistic flow including turbulence phenomena is well reproduce using the proposed FSI scheme.

Effects of Reynolds Number on Drag Reduction in T-Junction Pipes due to Small Obstacles

T-junction pipes are used to distribute one flow into two flows or join two flows into one flow. Separated vortex flow regions near the corners of junctions are caused in these types of flows. They reduce the effective cross-sectional area of the pipe flows and then create large flow resistance or drag. The corners of junctions are generally rounded to avoid flow separation and reduce flow resistance. We tried to reduce the flow resistance of counter-flow T-junction pipes in which two flows in the opposite direction entered the junction, mixed, and then vertically flowed out together by using a simple method. We mounted two small weir-shaped obstacles on the walls of the two upstream pipes by the side around the junction corners, which is a new way we propose of controlling flow separation. The pressure distribution along the pipes was measured and the drag of the T-junction pipe was estimated. Additionally, the effects of the Reynolds number on the flow resistance and its rate of reduction by mounting small obstacles were clarified. The two major results we obtained were: (1) the flow resistance of T-junctions could be reduced by about a maximum of 30% by mounting small obstacles at heights of 0.30 D and 0.47 D (D: pipe diameter) from the upstream of the corner. We also found (2) the rate of reduction in flow resistance increased with decreasing Reynolds numbers between 5–10 × 104 , but this decreased rapidly between 2.5–5 × 104 .Copyright

Studies on Flow and Pipe Wall Thinning and the Reduction in the Downstream of Orifice Nozzle

In a piping system of power plant, pipe wall thinning by Flow Accelerated Corrosion, FAC, Liquid Droplet Impingement Erosion, LDI, and Cavitation Erosion, C/E, are very serious problems because they give a damage and lead to the destructtion of the piping system[1]–[6] . In this study, the pipe wall thinning by FAC in the downstream of orifice nozzle, flow meter, is examined. Namely, the characteristics of FAC, generation mechanism, and prediction of the thinning and the reduction are made clear by experimental analysis. As a results, it was made clear that (1) the thinning is occurred mainly according to the size of the pressure fluctuation p′ on the pipe wall and the thinning can be estimated by it, and (2) the suppression of p′ can be realized by replacing the orifice to a taper shaped one having an angle to the upstream.Copyright