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Current Research by Paul Dellenback
Mixing and Heat Transfer in Confined Swirling Flows
Through a series of investigations, various aspects of the turbulent mixing and convective heat transfer in confined swirling flows have been examined. In particular, swirling flow through sudden expansions, co-axial jet flows with swirled inner jet, co-axial flow with counterswirled jets, and swirling flows with both high and low temperature thermal boundary conditions have been examined. The investigations entailed two-component laser Doppler velocimetry measurements of mean and fluctuating velocities, as well measurement of local convective heat transfer coefficients. Measurements associated with the counterswirled jets research have documented some of the highest turbulence levels known for any engineered flows. The goals of the investigations have been to model flows in combustors of gas turbine engines, to understand the effect of centrifugal instabilities on the convective heat transfer, and to understand the role of high turbulence levels on convective heat transfer.
The Effect of High Turbulence Levels on Convective Heat Transfer
As a consequence of having two unusually large data sets, one for swirling flows and another for flows through passages with ribbed walls, both of which entail highly turbulent flows, we have been interested in trying to characterize heat transfer coefficients as a function of turbulence level. Our work has been guided by that of Maciejewski and Moffatt who showed that relatively simple geometry-independent correlations could be developed, and that these correlations might not depend on some parameters that have been historically important, such as the Reynolds number. Rather, for some flows, the convective heat transfer appears to be dominated by the turbulent fluctuations, so that the resulting correlations are dominated by some appropriate characterization of the turbulence level. The current work is attempting to develop a geometry-independent correlation for internal flows that does not entail inclusion of a length scale, which appears to be an overly-complicated and unneeded parameter. Relative to this last goal, our measurements have shown that the heat transfer coefficient is directly proportional to the turbulence intensity for high levels of turbulence, but a general-purpose correlation for collapsing the data has been elusive. The most recent measurement focused on a new set of experiments intended to decouple the effects of Reynolds number and turbulence intensity, so that the effects of each can be clearly understood.
Enhanced Heat Transfer Downstream of Synthetic Jet Arrays
In recent years, various schemes for introducing longitudinal vorticity into boundary layer flows have been examined for their influence on convective heat transfer enhancement. The goal of the present project is to examine the relatively new mechanism of vortex generation by synthetic, zero-mass flux jets introduced normal to the boundary layer, with particular emphasis on the resulting heat transfer enhancement downstream of the jets. Synthetic jets are being proposed for a wide range of applications, most of which pertain to delaying flow separation, and in most of these the heat transfer enhancement will be very important to the jets' feasibility. A phase change surface coating technique is being used in conjunction with real-time video data acquisition, and a similar liquid-crystal technique with color-video data acquisition is under development. The heat transfer enhancement due to both a single jet and arrays of jets are to be examined as a function of a variety of variables, including jet velocity, jet angle, jet frequency, and jet mass flux.