bubbly turbulence with heat transfer
Biljana Gvozdić is a PhD student in the research group Physics of Fluids. His supervisors are prof.dr. D. Lohse and prof.dr. C. Sun from the Faculty of Science and Technology.
Bubble injection in convective flows has proven to be a useful method of enhancing heat transfer. In order to understand the underlying physics behind bubble induced heat transfer enhancement, in this thesis we study the global and local flow statistics in a canonical vertical natural convection flow with bubble injection. Additionally, we have designed and built a new experimental facility Twente Mass and Heat Transfer Water Tunnel, which allows for global temperature control, bubble injection and local heat/mass injection. This setup will be used for studying of heat and mass transfer in turbulent multiphase flows.
In §2 we perform experiments in a rectangular bubble column heated from one side and cooled from the other with homogeneous bubble injection from the bottom. We independently control the Rayleigh number and the gas volume fraction and study the effect on the global heat transfer. The gas volume fraction ranges from 0% to 5%, and the bubble diameters are around 2.5 mm. The Rayleigh number is in the range 4.0 109 - 1.21011. We find that two completely different mechanisms govern the heat transport in flows with and without bubbles. In the single-phase case, the vertical natural convection is driven solely by the imposed difference between the mean wall temperatures. In this configuration the temperature acts as an active scalar driving the flow. The Nusselt number which quantifies the overall heat transport increases with increasing Rayleigh number, and as expected effectively scales as: . However, in the case of homogeneous bubbly flow the heat transfer comes from two different contributions: natural convection driven by the horizontal temperature gradient and the bubble induced diffusion, where the latter dominates. This is substantiated by our observations that the Nusselt number in bubbly flow is nearly independent of the Rayleigh number and depends solely on the gas volume fraction, scaling as: . We thus find nearly the same scaling as in the case of the mixing of a passive tracer in a homogeneous bubbly flow for a low gas volume fraction, which implies that the bubble-induced mixing is indeed limiting the efficiency of the heat transfer. For single-phase flow, the mean temperature remains constant in the bulk at mid-height which is completely obstructed by the mixing induced by bubbles in two-phase flow. Injection of bubbles induces up to 200 times stronger temperature fluctuations. These fluctuations cover a wide spectrum of frequencies and are thus the signature of the heat transport enhancement due to bubble injection. A clear slope of -1.4 at the scales » 0.1 Hz – 3 Hz was also observed.
In §3, we study the effect of inhomogeneous injection of bubbles on the overall heat transport. The experiments were performed in the same setup as in §2, with millimetric bubbles injected only through one half of the injection section, either close to the cold wall or close to the hot wall. Two parameters were varied: the gas volume fraction (from 0.4% to 5.1%) and the Rayleigh number (from 4.0 109 - 2.21010). By characterising the global heat transfer we find that in the case of bubbles injected only through one half of the injection section, just as for homogeneous bubble injection, the Nusselt number is nearly independent on the Rayleigh number and increases with increasing gas volume fraction. However, the heat transfer enhancement is more prominent with inhomogeneously injected bubbles when compared to the same gas volume fraction and same range of RaH of homogeneous injection, provided .This finding can be explained by the multiple mixing mechanisms present in the setup, once a gradient of gas volume fraction is imposed. Namely, besides the bubble induced turbulence (BIT), the large-scale circulation of the liquid phase induced by inhomogeneous bubble injection leads to the occurrence of a shear layer between the fluid region injected with bubbles and its opposite side. The different superimposed mixing mechanisms lead to enhancement of mixing, which results in up to 1.5 times larger heat transport as compared to homogeneous bubble injection. For the inhomogeneous injection causes lower heat transport enhancement than the homogeneous one. We visually observe that with increasing gas volume fraction the instability of the bubble stream increases as well as the contribution of the shear-induced turbulence (SIT). The velocity measurements show that the large-scale circulation gets stronger with increasing as well. Therefore the competition between BIT, SIT and the advection reduces the heat transport enhancement.
In order to accurately study heat and mass transfer in turbulent multiphase flow we built an unique experimental setup Twente Mass and Heat Transfer Water Tunnel, presented in §4. The new vertical water tunnel has global temperature control, bubble injection and local heat/mass injection. The total tunnel volume is 300 liters. Three interchangeable measurement sections of 1 m height but of different cross sections (, , m2) span a Reynolds-number range from 1.5 104 to 3 105 in the case of water at room temperature. The glass vertical measurement sections allow for optical access to the flow, enabling techniques such as laser Doppler anemometry, particle image velocimetry, particle tracking velocimetry and laser-induced fluorescent imaging. Thermistors mounted on a built-in traverse provide local temperature information at a few milli-Kelvin accuracy. Combined with simultaneous local velocity measurements the local heat flux in single phase and two phase turbulent flow can be studied. A largely unexplored area of research is the fundamentals of heat transport in bubbly flows in the presence of salt. Heat transfer in salt solutions (e.g. brine) is highly relevant for certain industrial applications such as chlorate processes, heat exchangers etc. Taking this into account, we built the tunnel with high-grade stainless steel, making it suitable for studying previously unexplored heat transfer in salt solution in excess of 15% mass fraction. Future studies using this facility will focus on understanding the dependency of heat efficiency on control parameters such as gas volume fraction, bubble size, Taylor Reynolds number and salt concentration. By quantifying the spatial distribution of the local heat flux due to bubble injection we will gain more insight into the fundamentals of heat transport in bubbly flows in the presence of grid generated turbulence.
