Heat transfer and friction factor correlations of solar air heater ducts artificially roughened with discrete V-down ribs

Large-eddy simulation of turbulent flow in a channel with rib roughness

A practical model is developed for the vertical flux of a scalar, such as heat, from an urban street canyon that accounts for variations of the flow and turbulence with canyon geometry. The model gives the magnitude and geometric dependence of the flux from each facet of the urban street canyon, and is shown to agree well with wind-tunnel measurements described in Part I. The geometric dependence of the flux from an urban street canyon is shown to be determined by two physical processes. Firstly, as the height-to-width ratio of the street canyon increases, so does the roughness length and displacement height of the surface. This increase leads to a reduction in the wind speed in the inertial sublayer above the street canyons. Since the speed of the circulations in the street are proportional to this inertial sublayer wind speed, the flux then reduces with the inertial sublayer wind speed. This process is dominant at low height-to-width ratios. Secondly, the character of the circulations within the street canyon also varies as the height-to-width ratio increases. The flow in the street is partitioned into a recirculation region and a ventilated region. When the street canyon has high height-to-width ratios the recirculation region occupies the whole street canyon and the wind speeds within the street are low. This tendency decreases the flux at high height-to-width ratios. These processes tend to reduce the flux density from the individual facets of the street canyon, when compared to the flux density from a horizontal surface of the same material. But the street canyon has an increased total surface area, which means that the total flux from the street canyon is larger than from a horizontal surface. The variations in scalar flux from an urban street canyon with geometry is over a factor of two, which means that the physical mechanisms responsible should be incorporated into energy balance models for urban areas.

Scalar fluxes from urban street canyons. Part II: Model

Thermal characteristics of turbulent rib-grooved channel flows

Heat transfer and friction factor correlations for a solar air heater duct roughened artificially with broken arc ribs

This paper describes the flow structure over the repeated two-dimensional square ribs of side length D, placed at a pitch S on a ground plane. The value of S/D which most augments the turbulence of the free stream and, hence the heat transfer is calculated. The region of interest in this investigation is far downstream where the velocity and temperature distributions follow similarity rules. The time-mean velocity, static pressure, and the velocity vectors were measured by Pitot- and static pressure tubes and a three hole cylindrical yawmeter. The turbulence intensities and integral scale were obtained using a hot wire anemometer. The mean temperature distribution was measured by thermocouples and the local heat transfer coefficient was then calculated. It is found that at S/D = 9 the turbulence intensity is maximized. As a result of this effect and the fact that for S/D = 9 the flow reattaches within a groove, the heat transfer is also maximized. The measurements show how the location of reattachment depends on S/D and that high local heat transfer coefficient coincides with the reattachment point. The average heat transfer coefficient and the pressure drop correlation is quantified.

Turbulent Shear Flow and Heat Transfer Over the Repeated Two-Dimensional Square Ribs on Ground Plane

This paper describes the detailed study of the flow structure over rows of two-dimensional square ribs on a ground plane for various values of S/D, and the optimum value of S/D to augment the turbulence of the free stream. The pitch between the centers of two adjoining square ribs was varied at S/D=2, 3, 4, 5, 7, 9, 13 and 17. The time-mean velocity, static pressure and the velocity vectors were measured by Pitot and static pressure tubes, and a cylindrical yawmeter of 6mm in diameter having three pressure holes. The turbulence intensities and autocorrelation were obtained using the data processing system and the F. F. T. analyzer connected to the hot-wire anemometer. As a result, it is concluded that the pitch ratio S/D=9 is optimum for augmenting the turbulence intensity.

Kouichirou Nakaso

Papers

Turbulent Shear Flow and Heat Transfer Over the Repeated Two-Dimensional Square Ribs on Ground Plane

Effect of Rows of Two-Dimensional Square Ribs on Flow Property along Plane Wall

Turbulent shear flow over rows of two-dimensional square ribs on ground plane