The dynamics of the coupled Kelvin–Helmholtz (KH) and Rayleigh–Taylor (RT) instability (referred to as KHRT instability or KHRTI) is studied using statistically steady experiments performed in a multi-layer gas tunnel. Experiments are performed at four density ratios ranging in Atwood number $A_{t}$ from 0.035 to 0.159, with varying amounts of shear and $\unicode[STIX]{x0394}U/U$ ranging from 0 to 0.48, where $\unicode[STIX]{x0394}U$ is the speed difference between the two flow streams being investigated and $U$ is the mean velocity of these two streams. Three types of diagnostics – back-lit visualization, hot-wire anemometry and particle image velocimetry (PIV) – are employed to obtain the mixing widths, velocity field and density field. The flow is found to be governed by KH dynamics at early times and RT dynamics at late times. This transition from KH-instability-like to RT-instability-like behaviour is quantified using the Richardson number. Transitional Richardson number magnitudes obtained for the present KHRT flows are found to be in the range 0.17–0.56 similar to the critical Richardson numbers for stably stratified free shear flows. Comparing the evolution of density and velocity mixing widths, the density mixing layer is found to be approximately two times as thick as the velocity mixing layer. Scaling of velocity fluctuations is attempted using combinations of KH and RT scales. It is found that the proposed KHRT velocity scale, obtained using the combined mixing-layer growth equation, is appropriate for intermediate stages of the flow when both KH and RT dynamics are comparable. Probability density functions (p.d.f.s) for different fluctuating quantities are presented. Multiple peaks in p.d.f.s are qualitatively explained from the development of coherent KH roll-ups and their subsequent transition into turbulent pockets. The evolution of energy spectra indicates that density fluctuations start to show an inertial subrange from earlier times compared to velocity fluctuations. The spectra exhibit a slightly steeper slope than the Kolmogorov–Obukhov five-thirds law.

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Experiments were performed to investigate the effects of buoyancy on heat transfer characteristics of supercritical carbon dioxide in heating mode. Turbulent flows with Reynolds numbers up to 60,000, at operating pressures of 7.5, 8.1, and 10.2 MPa, were tested in a round tube. Local heat transfer coefficients were obtained from measured wall temperatures over a large set of experimental parameters that varied inlet temperature from 20 to 55°C, mass flux from 150 to 350  kg/m2s, and a maximum heat flux of 65  kW/m2. Horizontal, upward, and downward flows were tested to investigate the unusual heat transfer characteristics due to the effect of buoyancy and flow acceleration caused by large variation in density. In the case of upward flow, severe localized deterioration in heat transfer was observed due to reduction in the turbulent shear stress and is characterized by a sharp increase in wall temperature. In the case of downward flow, turbulent shear stress is enhanced by buoyancy forces, leading to an enhancement in heat transfer. In the case of horizontal flow, flow stratification occurred, leading to a circumferential variation in wall temperature. Thermocouples mounted 180° apart on the tube revealed that the wall temperatures on the top side are significantly higher than the bottom side of the tube. Buoyancy factor calculations for all the test cases indicated that buoyancy effects cannot be ignored even for horizontal flow at Reynolds numbers as high as 20,000. Experimentally determined Nusselt numbers are compared to existing correlations available in the literature. Existing correlations predicted the experimental data within ±30%, with maximum deviation around the pseudocritical point.