Heat transfer surface enhancement through the use of longitudinal vortices: a review of recent progress

Longitudinal vortices are more efficient for heat transfer enhancement than transverse vortices.A survey is given on triangular and rectangular protrusions from a heat transfer surface which generate mainly longitudinal vortex systems. Wings and winglets are considered in boundary layer and channel flow, either by themselves, or in a single row transverse to the flow direction, or in a two dimensional array. Local and global heat transfer are studied as a function of the major parameters. For channel flows also the pressure losses are given. Winglets are superior to wings, but winglet form is of minor importance. In laminar flow, heat transfer enhancement increases with Reynolds number. Heat transfer enhancement increases for constant winglet aspect ratio with angle of attack up to a maximum angle of attack. But it increases also up to limiting values with winglet height relative to transverse and streamwise winglet spacing and relative to channel height or boundary layer thickness. The nonlinear character of fluid mechanics does not allow simple predictions.

Vortices, Generators and Heat Transfer

The mean streamwise development of pairs of longitudinal vortices embedded in an otherwise two-dimensional turbulent boundary layer was studied. Planes of closely spaced measurements of the three components of mean velocity were obtained at several streamwise locations, and the vorticity and circulation were calculated. Skin-friction measurements were also made. It was found that the rate of vorticity spreading in a vortex was greatly increased by close proximity of other vortices. The rate of streamwise circulation decrease was significantly greater for corotating vortices than for counter rotating vortices. Boundary-layer thinning and increased skin friction occured in regions where the secondary flow induced by the pairs was directed toward the wall; the boundary layer was thickened and skin friction reduced where the secondary flow was directed away from the wall.

Experimental study of the development of longitudinal vortex pairs embedded in a turbulent boundary layer

Heat transfer enhancement and drag by longitudinal vortex generators in channel flow

Numerical and experimental determination of flow structure and heat transfer effects of longitudinal vortices in a channel flow

The heat transfer effects of an isolated longitudinal vortex embedded in a turbulent boundary layer were examined experimentally for vortex circulations ranging from GAMMA/U/sub infinity/delta/sub 99/ = 0.12 to 0.86. The test facility consisted of a two-dimensional boundary layer wind tunnel, with a vortex introduced into the flow by a half-delta wing protruding from the surface. In all cases, the vortex size was of the same order as the boundary-layer thickness. Heat transfer measurements were made using a constant-heat-flux surface with 160 embedded thermocouples to provide high resolution of the surface-temperature distribution. Three-component mean-velocity measurements were made using a four-hole pressure probe. Spanwise profiles of the Stanton number showed local increases as large as 24 percent and decreases of approximately 14 percent. The perturbation to the Stanton number was persistent to the end of the test section, a length of over 100 initial boundary-layer thicknesses. The weakest vortices examined showed smaller heat transfer effects, but the Stanton number profiles were nearly identical for the three cases with circulation greater than GAMMA/U/sub infinity/delta/sub 99/ = 0.53 cm. The local increase in the Stanton number is attributed to a thinning of the boundary layer on the downwash side of the vortex.

Heat Transfer Effects of a Longitudinal Vortex Embedded in a Turbulent Boundary Layer

The objective of this research program has been to determine experimentally the extent to which horseshoe vortices modify turbulent convective heat transfer along a flat plate downstream of an appendage. The importance of appendage shape on the heat transfer behavior was evaluated by taking Stanton-number measurements downstream of both a cylindrical body and a streamlined body. The results indicate that a region of augmented heat transfer occurs near the centerline downstream of both obstacles, with Stanton numbers 10 to 50% over the undisturbed values. The streamlined cylinder produces the strongest modifications in heat transfer.

The Influence of a Horseshoe Vortex on Local Convective Heat Transfer

Measurements of the turbulent velocity and temperature fields were made in a heated boundary layer 14 diameters downstream of a junction of a tapered cylinder and a wall (ReD=24,700). The boundary layer is strongly affected by the presence of large-scale unsteadiness arising from vortex shedding, which appears in the measurements as [open quotes]turbulence[close quotes] with a strong spectral component at the shedding frequency. The boundary layer exhibits three distinct spanwise regions: (1) the inner-wake region, z/D<0.8, where vortex shedding effects are observed only in the spanwise component of the fluctuations; (2) the middle-wake region, 0.8 1.6, where the flow is approaching a two-dimensional boundary-layer flow. Cross-spectra of [und v0] indicate that vortex shedding increases the turbulent heat flux in the middle-wake region. However, peak values of the Stanton number- and the eddy diffusivity are observed in the inner-wake region, where the cross-spectra of turbulent heat flux do not exhibit a peak near the shedding frequency, but do show an increase compared to a two-dimensional boundary layer over a much broader frequency range. 27 refs., 12 figs.

P. A. Eibeck

Papers

Heat Transfer Effects of a Longitudinal Vortex Embedded in a Turbulent Boundary Layer

The Influence of a Horseshoe Vortex on Local Convective Heat Transfer

Turbulent Heat Transport in a Boundary Layer Behind a Junction of a Streamlined Cylinder and a Wall