E.R.G. Eckert

Effects of hole geometry and density on three-dimensional film cooling

Abstract Film cooling downstream of secondary gas injection through discrete holes has been studied experimentally. The influences of hole geometry, secondary fluid density, and mainstream boundary layer thickness are described. Significant improvements in the film cooling effectiveness are observed by having the coolant passages widened before the exit of the secondary fluid. The use of a relatively dense secondary fluid, as might be encountered in many applications, requires a significantly higher blowing rate to cause jet separation from the surface than when the densities of the freestream and secondary stream are the same. This results in considerably better film cooling over an important range of density ratios.

Application of rough surfaces to heat exchanger design

Abstract Equations are developed to define the performance advantage of roughened tubes in heat exchanger design, relative to smooth tubes of equal diameter. Three rough tube applications are presented: 1. To obtain increased heat exchange capacity; 2. To reduce the friction power; and 3. To permit a reduction of heat-transfer surface area. A heat exchanger design procedure is developed for each application and is based on the use of previously developed generalized heat transfer and friction correlations for rough surfaces. The graphical results of example solutions are presented for the case of “repeated-rib” roughness. These graphs show the heat exchanger performance improvements offered by this roughness type, relative to smooth tubes. Because the heat transfer and friction correlations used for the “repeated-rib” roughness can be used to correlate the data of other types of geometrically similar roughness, the design procedure is applicable to other types of roughness. In addition to flow thru tubes, the design procedure and the calculated results are equally applicable to the problem of parallel flow along the outside surfaces of tubes arranged in a tube or rod bundle.

Heat transfer-A review of current literature

Results of research on heat transfer published during the foregoing year are reviewed. The review is divided into sections dealing with following subjects: donduction, channel flow, boundary-layer flow, flow with separated regions, transfer mechanisms, natural convection, convection from rotating surfaces, combined heat and mass transfer, phase changes, radiation, liquid metals, low-density heat transfer, measurement techniques, heat-transfer applications, and thermodynamic and transport properties. A bibliography of 508 references, arranged according to the above subjects, is given. (D.L.C.)

Film cooling with air and helium injection through a rearward-facingslot into a supersonic air flow

The effect of injecting a secondary gas stream through a rear ward-facing slot in a direction parallel to a wall into a supersonic air flow has been examined in a small Mach 3 wind tunnel. The experiments were carried out for three step heights, 0.121, 0.180, and 0.239 in. with slot openings of 0.064, 0.123, and 0.182 in., respectively. Air was used as the secondary fluid with these three different slot openings. The total temperature of the main flow was equal approximately to the room temperature, and the temperature of the wall at the point of injection varied between 418° and 655°R as determined by the temperature of the secondary air. The ratio M of the mass velocity in the secondary stream to that of the mainstream was in the range of 0 to 0.408. Helium also was injected through a slot opening of 0.063 in. for two different injection parameters M = 0.01 and 0.02 and a temperature of the wall at the point of injection ranging between 562.8° and 663.7°R. Schlieren photographs are presented and adiabatic wall temperature measurements are described.

The steady and transient free convection boundary layer on a uniformly heated vertical plate

Abstract A Zehnder-Mach interferometer was used to study the free convection thermal boundary layer about a uniformly heated vertical plate and to derive the heat transfer coefficients connected with this situation. The experiments were performed when the plate was immersed in water and the steady state boundary layer, as well as its transient development from an initial state at rest and with uniform temperature to steady state condition, was investigated when a step function in the power input to the plate was applied. Results for the steady state runs agree very well with the results of an analysis by Sparrow and Gregg. The transient runs indicate that the temperature field in the fluid develops initially in the same way as for heat conduction into a semi-infinite solid. After a short transition period, the steady state condition is reached. The boundary layer grows with time in such a way that it increases at first with increasing time, reaches a maximum, and decreases again until it settles to its steady state value. The wall temperature and the local heat transfer coefficient can be predicted for the whole period from start to steady state by the solution for one-dimensional unsteady conduction or for the steady state boundary layer.

RADIATIVE HEAT EXCHANGE BETWEEN SURFACES WITH SPECULAR REFLECTION

Abstract A method has been developed for calculating the radiant interchange in an enclosure containing specularly reflecting surfaces. Consideration is given to systems composed of two specular surfaces and an unrestricted number of black surfaces. The method is illustrated by numerical examples and comparisons are made with the heat transfer results for diffusely reflecting surfaces.

Film cooling with injection through holes - Adiabatic wall temperatures downstream of a circular hole.

Experiments to determine film cooling with air injection through holes into turbulent air boundary layer on flat plate

A general analysis of moisture migration caused by temperature differences in an unsaturated porous medium

Abstract An analysis was performed which describes the moisture migration in a slab of an unsaturated porous material for the condition that the temperature of one surface is suddenly increased to a higher value whereas the temperature of the other surface is maintained constant. The two surfaces are assumed impermeable to mass flow. The thermodynamic and transport properties occurring in the equations describing the temperature and moisture transport are assumed constant. In this way, very general relations are obtained for the moisture field as it changes in time. Before dry-out, a dimensionless parameter describing the moisture field is a function of equivalent Luikov and Fourier numbers only. The dependence on the Luikov number becomes noticeable solely for the early periods and for small values of the Luikov number. After the onset of dry-out, the moisture ratio depends in addition on the thermal mass diffusion coefficient. The results of the analysis presented in the Figs 1–7, should describe the actual moisture migration with good accuracy where the variation of the properties is small in the range of the independent variables occurring in a specific situation and they should be useful as a first approximation for other situations as well.

The effect of turbulence parameters and support position on the heat transfer from spheres

Abstract Measurements are reported which show the influence of the turbulence intensity, the scale of turbulence, and the position of the support, on the average heat transfer from spheres to an air stream. The Reynolds number range was from about 3·6 × 10 3 to 5·2 × 10 4 . The turbulence intensity and the scale of turbulence distributions downstream from three grids, behind which the heat-transfer measurements were later made, were measured for velocities from 4·6 to 18·3 m/s. The heat-transfer measurements showed that the average Nusselt number increased with the turbulence intensity, and with the ratio of the sphere diameter to the scale of turbulence for values up to at least five. Nusselt numbers, obtained using a crossflow support, were found to be about 10 per cent higher than those obtained using a rear support. Equations relating the average Nusselt number to the Reynolds number, for a low-turbulence free stream, are given.

Effect of surface roughness on the total hemispherical and specular reflectance of metallic surfaces.

Measurements have been made of the hemispherical and specular reflectance of metallic surfaces of controlled roughness. The surfaces, which were ground nickel rectangles, were irradiated at various angles of incidence by a beam of black-body radiation, the temperature of which was also varied. The instrumentation which was devised to perform the experiments is described. The measurements show that beyond a certain surface roughness, the hemispherical reflectance is virtually independent of further increases in roughness. On the other hand, the specular reflectance decreases steadily with increasing roughness. Additionally, the hemispherical reflectance is found to be quite insensitive to the angle of incidence, while the specular reflectance increases with angle of incidence for the rougher surfaces.