Narrow Rectangular Duct Research Articles

Heat transfer in rotating narrow rectangular pin-fin ducts

Abstract Rotational effects on heat transfer in radial-outflow ducts have implications for the design of gas turbine blades. The turbine blade trailing edge convective cavity generally has a narrow cross-section and often utilizes pin-fins for heat transfer enhancement. Previous research on rotational effects considered cavity shapes quite different from those of typical trailing edge cavities. In this research, experiments were conducted to determine the effect of rotation on heat transfer in pin-fin ducts of narrow cross-section (height-to-width ratio of 1:10), oriented with the heated sides at 60° to the r – z plane. In the experiment, a high-molecular-weight gas (Refrigerant-134A) at ambient pressure and temperature conditions was used to match the dimensionless parameters at engine conditions. The pin-fins were arranged in a staggered array. Thin foil heaters were used to produce a constant heat flux at the long sides of the duct; the narrow sides were unheated. Duct Reynolds numbers were varied up to 17,200; rotation numbers were varied up to 0.082. The test results show the effect of rotation on duct leading and trailing side heat transfer. In addition, the results show the variation in heat transfer coefficient with transverse location in the duct. Results are compared to data obtained for a smooth duct of identical dimensions.

LOCAL FIN-SURFACE HEAT TRANSFER FOR FLOW AROUND A CIRCULAR CYLINDER WITH AND WITHOUT VORTEX-GENERATING WINGLETS

A direct comparison of local fin-surface heat transfer coefficients for flow around a circular tube without ~a! and with ~b! winglets at ReH;1200 is presented in Fig. 1. The comparison reveals that, for this winglet location, the horseshoe vortex produced by the interaction of the flow with the circular cylinder is disrupted by the winglets. There is a reduction in the width of the low-heat-transfer wake region, but heat transfer coefficients directly downstream of the cylinder are actually slightly reduced for the winglet case compared to the no-winglet case. Stagnationregion heat transfer coefficients are slightly higher for the winglet case compared to the no-winglet case. The experiments were performed in a narrow rectangular duct designed to simulate a single passage of a fin-tube heat exchanger. A transient heat transfer measurement technique was employed for obtaining detailed local heat transfer measurements on the model fin surface. Using this technique, the room-temperature fin/tube model is suddenly exposed to a uniformly heated airflow, initiating a heat conduction

Heat Transfer in Rotating Narrow Rectangular Ducts With Heated Sides Parallel to the r-z Plane

In gas turbine blade design, a variety of channel shapes and orientations are used in the cooling circuit. Most of the rotating channel heat transfer research to date has considered channels of square or round cross-sections. This research characterizes the effect of rotation on fully developed turbulent convective heat transfer in ducts of narrow cross-section (height-to-width aspect ratio of 1:10). Experiments were conducted using ducts of narrow cross-section, oriented such that the long sides of the duct cross-section are perpendicular to the direction of blade tangential velocity (parallel to the r-z plane). In the experiment, a high-molecular-weight gas (Refrigerant-134A) at ambient pressure and temperature conditions was used to simulate coolant-to-wall density ratios that match engine conditions. Thin foil heaters were used to produce a uniform heat flux at the long sides of the duct; the narrow sides were unheated. Duct Reynolds numbers were varied up to 31,000; rotation numbers were varied up to 0.11. The test results show the effect of rotation and aspect ratio on duct leading and trailing side heat transfer. The results provide insight into the effect of rotation (Coriolis) in the absence of buoyancy effects. Comparisons with previously reported results are presented to show the effect of cross-section shape on rotating channel heat transfer.

Heat Transfer in Rotating Narrow Rectangular Ducts With Heated Sides Oriented at 60° to the r-z Plane

Gas turbine blade life is often limited by the effectiveness of the cooling in the trailing edge convective cavity, which generally has a narrow cross-section. Previous research on rotational effects considered cavity shapes quite different from those of typical trailing edge cavities. In this research, experiments were conducted to determine the effect of rotation on heat transfer in ducts of narrow cross-section (height-to-width ratio of 1:10), oriented with the heated sides at 60° to the r-z plane. In the experiment, a high-molecular-weight gas (Refrigerant-134A) at ambient pressure and temperature conditions was used to match the dimensionless parameters at engine conditions. Thin foil heaters were used to produce a constant heat flux at the long sides of the duct; the narrow sides were unheated. Duct Reynolds numbers were varied up to 20,000; rotation numbers were varied up to 0.25. The test results show the effect of rotation and aspect ratio on duct leading and trailing side heat transfer. In addition, the results show the variation in heat transfer coefficient with transverse location in the duct, demonstrating the effect of rotation not only on lead and trail side heat transfer, but also on forward and aft end heat transfer.

Effects of spacer geometry on pressure drop, mass transfer, mixing behavior, and residence time distribution

Net-type turbulence promotors or grids, so-called spacers, play an important role in membrane systems referring to problems of mass transfer, homogenizing and mixing behavior. In spiral wound modules and electrodialysis stacks spacers have several functions as supporting nets (stabilisators) and as turbulence promotors to increase mass transfer rates and reduce fouling layers. Spacers show a number of geometric and fluid-dynamic parameters like, e.g., the distance between two grid rods (wavelength), the inclination angle of the grid rods and the Reynolds number. The effects of these parameters on the local flow and mass transport phenomena in narrow rectangular ducts including different spacers are investigated in gas flows in a wind tunnel. For this purpose spacers were enlarged geometrically similar and a fundamental study of the geometric parameters, based on the results of Schock [1], led to a new definition of the hydraulic diameter of spacers.

Visualization and Measurements of Two-Phase Flow in Metallic Ducts Using Neutrons as Microscopic Probes. 2nd Report. Measurements of Some Flow Characteristics by Image Processing Techniques.

In order to apply the neutron radiography technique to fluid research, quantitative analyses of high-frame-rate neutron radiographs of air-water flows in a narrow rectangular duct were demonstrated by using image processing techniques. From the geometrical information of the images, the flow regime, bubble rising velocity, water-layer thickness, and interfacial area calculated by multiplying the interfacial length by the channel gap and size distribution of bubbles with diameter greater than 1 mm could be measured by the image processing technique. From the gray levels of the images, the spatial and temporal averaged void fractions and the void profile across the duct could also be measured by the image processing technique. It was clarified that the limited value of the measurable liquid-film thickness with the neutron radiography technique was calculated by an error analysis. It was shown quantitatively that the measurement error decreased by integrating the image temporally.

Some characteristics of gas-liquid flow in narrow rectangular ducts

Flow regime, void fraction, slug bubble velocity and pressure loss were measured for rectangular ducts with a narrow gap and a large aspect ratio. The neutron radiography technique was used to visualize the flow and the void fraction was obtained by image processing. The void fraction was well-correlated by the drift flux model with the existing correlation for the distribution parameter, which was about 1.35. Similar results were obtained for the slug bubble velocity, however the distribution parameter was in the range 1.0–1.2. The frictional pressure loss was well-correlated by the Chisholm-Laird correlation. In collaboration with previously obtained data, it was found that the Chisholm's parameter C, however, changed from 21 to 0 as the gap decreased.

Laminar flow in narrow ducts and circular tubes: Entrance effects

Series solutions are presented for laminar flow heat transfer in narrow rectangular ducts and circular tubes when the hydrodynamic and thermal boundary layers are developing simultaneously. In addition, simple engineering relationships are presented which agree well with literature values over a wide range of Peclet numbers. It is demonstrated how simplified local heat flux calculations may be performed using the results obtained.

Fluctuating forces on a rigid circular cylinder in confined flow

The fluctuating lift and drag acting on a long, rigidly supported circular cylinder placed symmetrically in a narrow rectangular duct were investigated for various blockage percentages over a wide range of Reynolds numbers around the critical value. The data obtained permit a full assessment of the effect of confinement on the mean-drag coefficient, the root-mean-square values of both the drag and the lift fluctuations, the Strouhal number of the dominant vortex shedding, and the Reynolds number marking transition from laminar to turbulent flow separation. Besides experimental information on a subject on which little is known so far, the paper provides a basis for the deduction of better correction procedures concerning the effects of blockage.

Influence of Flow and High Sound Level on the Attenuation in a Lined Duct

The influence of turbulent flow on the sound attenuation in a narrow rectangular duct is calculated by a finite difference approximation of the differential equation for inviscid flow. The absorptive lining at one duct wall is characterized by a flow-independent wall impedance. Results are found in good agreement with experimental data from a duct with 1-in.2 open area lined on one side with a resistive layer of very fine metal fibers in front of a partitioned air backing. For resistive layers of fibers with larger diameter or of perforated plate used in the same configuration, the wall impedance depends greatly upon the flow velocity. A correlation is found between the nonlinearity of resistive layers due to turbulent flow and due to high sound-pressure levels in a duct. The influence of the nonlinearity due to high sound-pressure levels up to 160 dB is studied in the absence of flow. Measured attenuation data agree with those from a theoretical model, which correlates the varying attenuation along the duct with the varying sound-pressure difference across the resistive lining. By comparison of theoretical and experimental results, the influence of nonlinearity due to turbulent flow can be described by an equivalent level of sound-pressure difference across the resistive lining. A biasing effect of flow on nonlinear duct lining material linearizes the attenuation along the duct, if that equivalent level due to flow exceeds the actual sound level.

Impact Theory Calculations of the Lift and Drag of Ducted Bodies

SummaryNewtonian impact theory has been used to estimate the external pressure forces on ducted bodies of rectangular, circular and semi-circular cross section. With an allowance for skin friction (or other incidence-independent drag) it is shown that maximum lift/drag ratios of about three to four are possible with little effect of body geometry provided that short and very divergent, or tall and narrow rectangular ducts are avoided.Large changes occur in the separate lift and drag at maximum lift/drag ratio and these have to be made compatible with the weight and thrust capacity of the configurations. The maximum lift/drag ratio of a duct can be improved by the addition of wings, especially for a duct with poor lift/drag ratio, but the best overall performance is with a duct of good lift/drag ratio and low drag.It is anticipated that impact theory underestimates the pressure forces, and hence the lift/drag ratios, which would obtain at finite Mach numbers but the trends with geometrical changes should be reasonably reliable.