Yasuo Hattori

Effects of freestream on turbulent combined-convection boundary layer along a vertical heated plate

Abstract A turbulent combined-convection boundary layer, created by imposing an aiding freestream on a turbulent natural-convection boundary layer along a vertical heated plate, was examined with normal hot- and cold-wires and a particle image velocimetry (PIV), with attention to the laminarization process of the boundary layer due to the freestream effects. As the freestream velocity increases, the wall shear stress monotonously increases, whereas the local heat transfer rate suddenly decreases. The reduction in heat transfer rates and the decays in velocity and temperature fluctuations, showing the transition from turbulent to laminar, arise at Grx / Rex ≃3×10 6 . With the laminarization of the boundary layer, a similar change in the turbulent quantities appears independently of Grx . For instantaneous velocity vectors obtained with PIV, large-scale fluid motions, which play a predominant role in the generation of turbulence, are frequently observed in the outer layer, while quasi-coherent structures do not exist in the near-wall region. The increasing freestream then restricts large-scale fluid motions in the outer layer, and consequently the generation of turbulence is suppressed and the boundary layer becomes laminar.

Characteristics of turbulent combined-convection boundary layer along a vertical heated plate

Abstract Fluid flow and heat transfer characteristics in a turbulent combined-convection boundary layer in air along a vertical heated plate have been investigated with normal hot and cold wires. The measured heat transfer rates and turbulent quantities show that the turbulent transition moves downstream with a slight increase in freestream velocity. Then, the heat transfer rate rapidly decreases to about 40% of that obtained in the turbulent natural-convection boundary layer, and velocity and temperature fluctuations become smaller in amplitude and change from random to harmonic at a specific frequency. Thus, the characteristics of the turbulent combined-convection boundary layer differ in several respects from those observed in both natural and forced convections. Based on the experimental results, the regimes of boundary layer flows are classified as a function of local Reynolds and Grashof numbers.

Heat removal characteristics of vault storage system with cross flow for spent fuel

Abstract Since the amount of spent fuel to be stored is expected to steadily increase in Japan, a use of large-scale dry storage facilities is considered to be a promising method in practice at reasonable economic cost. The present study is concerned with the heat removal experiment of vault storage system adopting cross flow with passive cooling, using a 1/5 scale model. The results show that the flow pattern of air in the storage module strongly depends on the ratio of the buoyancy to the inertia force. A simple method to estimate air flow patterns in the storage module was proposed, where the Ri (Richardson) number was considered as the most representative parameter. Then the heat transfer rate from a storage tube to cooling air was estimated, which could apply to the design of a full-scale vault storage system with cross flow, in which dozens of storage tubes were placed. The acquired information was also used to optimize heat removal design of the vault storage system in the present study.

Computational fluid dynamics simulation and statistical procedure for estimating wide-area distributions of airborne sea salt considering local ground conditions

Abstract Steel corrosion under atmospheric conditions is a critical issue in the maintenance of structures such as electric transmission towers and bridges during their long-term operation, which are generally located at many places over a wide area. Since a major factor causing corrosion is airborne salt particles coming from the sea, wide-area distributions of the long-term cumulative amount of sea salt deposited on surfaces are needed. Moreover, since the amount of airborne sea salt varies locally with the topography, it is also important to consider the effects of topography. In this paper, a method combining a computational fluid dynamics model and a statistical procedure is proposed to efficiently estimate wide-area distributions of the cumulative amount of airborne sea salt by considering the local topography. The predicted amount of airborne sea salt decreases with increasing distance from the coast and varies with the topography and the offshore wind. A comparison between predicted and observed amounts revealed that: (1) this method appropriately estimates topographical effects on sea-salt transport and enables the estimation of deposited sea salt on structure surfaces, and (2) consideration of the trapping efficiency of sea-salt particles on structure surfaces improves the prediction accuracy.

Experimental Study on Fire Behavior in a Compartment Under Mechanical Ventilated Conditions: The Effects of Air Inlet Position

The purpose of this research is to understand the fire behavior expected in a mechanically ventilated compartment. To date, some experimental studies have been conducted for investigation of fire behavior under mechanical forced ventilation; however, it might be not enough to understand everything. We therefore carried out a series of experiments on fire behavior focused on the effect of air inlet position in a compartment with same size as an ISO 9705 room (width 2.4 m × length 3.6 m × height 2.4 m) under conditions of mechanical ventilation using a pool fire. In this paper, the effects of ventilation conditions such as air inlet position and flow rate were studied. We found that differences in the air inlet position and flow rate were one of the principal factors for determining the burning behavior.

Interaction of a Pool Fire in a Compartment with Negative Pressure Generated by Mechanical Ventilation

We have experimentally investigated a medium-size pool fire in a compartment, the dimensions of which corresponded to the ISO 9705 room. Airflow rates in ducts, pressure, mole fraction, and temperature of air in the compartment and a mass loss rate of the fuel were measured. The liquid fuel and pool diameter were ethanol and 600 mm, respectively, which rapidly increased the compartment pressure just after ignition. The compartment was ventilated at inlet and outlet ducts with natural and mechanical ventilation systems, which initially gave negative compartment pressures in the range of −2 to −85 Pa. The negative pressure was much weaker than the pressure increase, which restrained the air supply with natural ventilation and resulted in extinction due to lack of oxygen. On the other hand, the negative pressure with the stronger mechanical ventilation sustained the air supply and yielded the transition to a ventilation-controlled fire without extinction. The ventilation-controlled fire led to two kinds of oscillating flame: one was caused by poor oxygen supply, which is similar to that reported by previous studies, and the other was caused by repetition of ignition and extinction, which was attributed to the change in the flow rate and direction of fresh air at the inlet duct. This oscillation generated large pressure fluctuations but did not yield thermal energy with combustion.

Large-Eddy Simulation of a Buoyant Plume Past a Bluff Body

A large-eddy simulation (LES) of a buoyant plume past a bluff body is performed. A round heat source is placed at the center of a horizontal flat wall, and a bluff body in the shape of a thick round plate is floating right above the heat source. The modified Rayleigh number based on the total heat input is set at 1.2×1010 . On the basis of past studies, the Smagorinsky model is adopted as a subgrid-scale (SGS) model, and a partial slip boundary condition based on the wall law is applied to a horizontal flat wall and a disk surface. The validity of numerical results is ascertained by comparison with theoretical solution and experimental data. The blocking of upward flow and imparting turbulence through a bluff body vary the process of developing a buoyant plume, while properties of a fully developed plume rarely vary. With heat from a bluff body, another buoyant plume is formed near the center, piled with upward flow passing around the bluff body. Moreover, main positions of buoyant production of turbulent kinetic energy move a point from near the side of the bluff body to a point near the central axis. This affects the transition to a fully developed plume in turbulence statistics.Copyright

Turbulent Structure in Stably Stratified Flow over a Wavy Surface

Large eddy simulation was performed for flow and heat transfer under stable density stratification in a wavy wall channel to reveal the effect of stratification on turbulent structures. Low-Mach-number approximation was applied to treat density variation. The iso-surface of instantaneous temperature in the middle of the channel shows that the effect of stable stratification is strong and greatly reduces turbulence there. On the other hand, instantaneous structures and two-point correlations near the wavy wall suggest that characteristic near-wall structures and their scales are not strongly affected by the extent of stratification in spite of considerable reduction of turbulent intensity.