Enhancing thermal mixing in turbulent bubbly flow by adding salt

The presence of bubbles in a turbulent flow changes the flow drastically and enhances the mixing. Adding salt to the bubbly aqueous flow changes the bubble coalescence properties as compared to pure water. Here we provide direct experimental evidence that also the turbulent thermal energy spectra are strongly changed. Experiments were performed in the Twente Mass and Heat Transfer water tunnel,in which we can measure the thermal spectra in bubbly turbulence in salty water. We find that the mean bubble diameter decreases with increasing concentration of salt (NaCl), due to the inhibition of bubble coalescence. With increasing salinity, the transition frequency from the classical $-5/3$ scaling of the thermal energy spectrum to the bubble induced $-3$ scaling shifts to higher frequencies, thus enhancing the overall thermal energy. We relate this frequency shift to the smaller size of the bubbles for the salty bubbly flow. Finally we measure the heat transport in the bubbly flow, and show how it varies with changing void fraction and salinity: Increases in both result into increases in the number of extreme events.Comment: 18 pages, 10 figures, submitted to International Journal of Multiphase Flo

Enhancing thermal mixing in turbulent bubbly flow by inhibiting bubble coalescence

The presence of bubbles in a turbulent flow changes the flow drastically and enhances the mixing. Adding salt to the bubbly aqueous flow changes the bubble coalescence properties as compared to regular demineralized water. Here we provide direct experimental evidence that also the turbulent thermal energy spectra are strongly changed. Experiments were performed in the Twente Mass and Heat Transfer water tunnel, in which we can measure the thermal spectra in bubbly turbulence in salty water. We find that the mean bubble diameter decreases with increasing concentration of salt (NaCl), due to the inhibition of bubble coalescence. With increasing salinity, the transition frequency from the classical −5/3 scaling of the thermal energy spectrum to the bubble induced −3 scaling shifts to higher frequencies, thus enhancing the overall thermal energy. We relate this frequency shift to the smaller size of the bubbles for the salty bubbly flow. Finally we measure the heat transport in the bubbly flow, and show how it varies with changing void fraction and salinity: Increases in both result into increases in the number of extreme events.</p

Exploring the phase space of multiple states in highly turbulent Taylor-Couette flow

We investigate the existence of multiple turbulent states in highly turbulent Taylor-Couette flow in the range of $\mathrm{Ta}=10^{11}$ to $9\cdot10^{12}$, by measuring the global torques and the local velocities while probing the phase space spanned by the rotation rates of the inner and outer cylinder. The multiple states are found to be very robust and are expected to persist beyond $\mathrm{Ta}=10^{13}$. The rotation ratio is the parameter that most strongly controls the transitions between the flow states; the transitional values only weakly depend on the Taylor number. However, complex paths in the phase space are necessary to unlock the full region of multiple states. Lastly, by mapping the flow structures for various rotation ratios in a Taylor-Couette setup with an equal radius ratio but a larger aspect ratio than before, multiple states were again observed. Here, they are characterized by even richer roll structure phenomena, including, for the first time observed in highly turbulent TC flow, an antisymmetrical roll state.Comment: 9 pages, 7 figure

The emergence of bubble-induced scaling in thermal spectra in turbulence

We report on the modification of the spectrum of a passive scalar inside a turbulent flow by the injection of large bubbles. While the spectral modification through bubbles is well known and well analyzed for the velocity fluctuations, little is known on how bubbles change the fluctuations of a passive scalar. Here we uncover the thermal spectral scaling behavior of a turbulent multiphase thermal mixing layer. We trigger the development of a $-3$ spectral scaling by injecting large bubbles ($\text{Re}_{\text{bub}} = \mathcal{O}(10^2)$) with gas volume fractions up to 5\%. For these bubbly flows, the $-5/3$ scaling is still observed at intermediate frequencies but it is followed by a steeper slope for larger frequencies. This $-3$ scaling range extends with increasing gas volume fraction. The $-3$ scaling experiment coincides with the typical energy spectral scaling for the velocity fluctuations in high Reynolds number bubbly flow. We identify the frequency scale of the transition from the $-5/3$ scaling to the $-3$ scaling and show how it depends on the gas volume fraction

Twente mass and heat transfer water tunnel: Temperature controlled turbulent multiphase channel flow with heat and mass transfer

A new vertical water tunnel with global temperature control and the possibility for bubble and local heat and mass injection has been designed and constructed. The new facility offers the possibility to accurately study heat and mass transfer in turbulent multiphase flow (gas volume fraction up to 8%) with a Reynolds-number range from 1.5 × 104 to 3 × 105 in the case of water at room temperature. The tunnel is made of high-grade stainless steel permitting the use of salt solutions in excess of 15% mass fraction. The tunnel has a volume of 300 l. The tunnel has three interchangeable measurement sections of 1 m height but with different cross sections (0.3 × 0.04 m2, 0.3 × 0.06 m2, and 0.3 × 0.08 m2). 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. Local sensors can be introduced from the top and can be traversed using a built-in traverse system, allowing, for example, local temperature, hot-wire, or local phase measurements. Combined with simultaneous velocity measurements, the local heat flux in single phase and two phase turbulent flows can thus be studied quantitatively and precisel

The Helicobacter pylori Genome Project : insights into H. pylori population structure from analysis of a worldwide collection of complete genomes

Helicobacter pylori, a dominant member of the gastric microbiota, shares co-evolutionary history with humans. This has led to the development of genetically distinct H. pylori subpopulations associated with the geographic origin of the host and with differential gastric disease risk. Here, we provide insights into H. pylori population structure as a part of the Helicobacter pylori Genome Project (HpGP), a multi-disciplinary initiative aimed at elucidating H. pylori pathogenesis and identifying new therapeutic targets. We collected 1011 well-characterized clinical strains from 50 countries and generated high-quality genome sequences. We analysed core genome diversity and population structure of the HpGP dataset and 255 worldwide reference genomes to outline the ancestral contribution to Eurasian, African, and American populations. We found evidence of substantial contribution of population hpNorthAsia and subpopulation hspUral in Northern European H. pylori. The genomes of H. pylori isolated from northern and southern Indigenous Americans differed in that bacteria isolated in northern Indigenous communities were more similar to North Asian H. pylori while the southern had higher relatedness to hpEastAsia. Notably, we also found a highly clonal yet geographically dispersed North American subpopulation, which is negative for the cag pathogenicity island, and present in 7% of sequenced US genomes. We expect the HpGP dataset and the corresponding strains to become a major asset for H. pylori genomics

Scalars in bubbly turbulence

This thesis investigates the mixing in swarms of rising bubble in a flow with and without incident turbulence by studying the corresponding temperature statistics and heat transport. The temperature mixing in inhomogeneous bubbly flows was studied by measuring the global normalized heat transport (Nusselt number, Nu) in a vertical convection cell with millimetric bubbles introduced only through half of the injection section at the bottom (close to the hot or cold wall). For the void fraction ⍺ < 4.0%, Nu was enhanced as compared to homogenous bubbly flows, because of the presence of shear-induced turbulence (SIT) and the buoyancy-induced recirculation in addition to bubble-induced agitations (BIA). For 4.0% < ⍺ < 5.1%, SIT and the large-scale circulation started to compete with BIA, leading to a smaller Nu enhancement. To study the effect of incident turbulence, the Twente Mass and Heat Transfer Tunnel (TMHT) was built. It was demonstrated that temperature and mass transfer measurements in turbulent bubbly flows are possible in a controlled manner. By using TMHT, the temperature statistics in a turbulent bubbly thermal mixing layer were studied. The Kolmogorov -5/3 law in the temperature frequency spectra was followed by a steeper spectral slope when ⍺ increases. It saturated to a -3 scaling when ⍺ is large enough. We proposed that when the enhanced mixing of the small-scale temperature fluctuations due to the bubbles is strong enough, the local net spectral transfer of the temperature fluctuations is directly dissipated by diffusivity, leading to the -3 power law. Finally, we studied the integral statistics (the mean, variance and probability density functions) of the temperature and velocity and the small-scale statistics (temperature increment statistics) for different ⍺. The scaling properties of the structure function of the order p versus the second order structure function were investigated using extended self-similarity. Such scaling exponents for ⍺ = 0% and ⍺ = 4.7% are similar to each other up to p = 5. The exponents also resembled those in the boiling Rayleigh-Bénard convection (point bubbles simulations) reported before and were higher than other single-phase cases reported before

Data publication: 3D gas distribution in lab-scale zero-gap water electrolysers measured by 3D X-ray computed microtomography

Postprocessed 3D raw attenuation data of lab-scale zero-gap water electrolysers at different operating conditions.The data was collected within the project grant number KICH1.ED04.20.014 of the research programme ECCM KICkstart DE-NL of the Dutch Research Council (NWO). The responsibility for the content of this publication lies with the authors

Data publication: 3D gas distribution in lab-scale zero-gap water electrolysers measured by 3D X-ray computed microtomography

Postprocessed 3D raw attenuation data of lab-scale zero-gap water electrolysers at different operating conditions.The data was collected within the project grant number KICH1.ED04.20.014 of the research programme ECCM KICkstart DE-NL of the Dutch Research Council (NWO). The responsibility for the content of this publication lies with the authors