Tsuneo Azuma

Heat-transfer enhancement and pressure loss by surface roughness in turbulent channel flows

Abstract A theoretical study of turbulent channel flows was conducted in order to investigate the relationship between the heat-transfer enhancement and the increase in drag by rough surfaces using time–space averaged momentum and energy equations. The results confirmed that the efficiency of heat-transfer surfaces is in general less than unity when the molecular Prandtl number of the working fluid is less than the turbulent Prandtl number. When the molecular Prandtl number is greater than the turbulent Prandtl number, on the other hand, it was shown that the efficiency could be greater than unity as long as the surface roughness is transitional. It is suggested that the breakdown of the Reynolds analogy being experimentally observed for the riblet surface seems to be due to the inhomogeneity of the heat flux over the micro-grooves at the initial stage of thermal boundary-layer development.

LAMINAR-TURBULENT TRANSITION OF THIN RADIAL LIQUID FILM FLOW

A remarkable transition from laminar to turbulent flow of a thin liquid film flow is investigated. The radial liquid film flow was generated by a water discharge to the atmosphere from a clearance between the lower end of a vertical pipe and a horizontal flat surface of a disk. The transition process was elucidated by high-speed photography. Characteristic properties of the disturbance in the transitional region were made clear by measurement of wall pressure fluctuation. A linear stability analysis of the flow was carried out. The measured values of the phase velocity, the frequency and the amplification factor of disturbance were in good agreement with the linear stability theory. Thus, it was found that this transition does not occur due to the shearing stress on the liquid surface, but rather to the amplification of disturbance inside the liquid film.

Effect of Surfactant Additives on the Transition in Pipe Flow

The effect of drag reducing surfactant (C14TABr) additives on the transition in pipe flow has been investigated through pressure drop measurement, LDV measurement and flow visualization. The experiment was carried out using the glass tube of diameter D=5.02 mm and D=2.61 mm. Two types of flow were experimented upon : the flow that decreased background -turbulences as much as possible and the flow that was disturbed by a ring-shaped roughness element. In the flow of the former, the following was understood : within the range from Re=3 000 to 4 500, a temporary rapid increase in the friction factor appeared although the flow was quasi-laminar. In a concentration that exceeds 400 ppm, a turbulent transition happened gradually and vaguely, not drastically as Re increased. In the flow of the latter, the following was understood : although in the case of water a turbulent patch generated in the initial stage of the transition in Re>2450 was a turbulent slug, the present study suggests that in 260 ppm, the turbulent puff was generated in the initial stage of the transition in Re=3 500. Furthermore, turbulent patches were not generated in the concentration above 400 ppm.

An Experiment on Transition of Three-Dimensional Boundary Layer

The laminar-turbulent transition of a three-dimensional boundary layer was studied experimentally using a thin radial liquid-film flow along a stationary and rotating disk, and with a linear stability theory. Because of the very small liquid-film thickness of the flow, a disturbance growing near the wall produces an uneven liquid surface. Hence, the process of transition and the radius of the point of transition could be easily observed through the features of the liquid surface. It was found that two different types of instability, viscous and inflectional, may appear, and that the weak three-dimensionality of the mean flow suppresses the transition due to viscous-type instability, whereas the strong one promotes it due to inflectional-type instability.