The Effect of Hydraulic Diameter on Flow Boiling within Single Rectangular Microchannels and Comparison of Heat Sink Configuration of a Single and Multiple Microchannels

Phase change heat transfer within microchannels is considered one of the most promising cooling methods for the efficient cooling of high-performance electronic devices. However, there are still fundamental parameters, such as the effect of channel hydraulic diameter Dh whose effects on fluid flow and heat transfer characteristics are not clearly defined yet. The objective of the present work is to numerically investigate the first transient flow boiling characteristics from the bubble inception up to the first stages of the flow boiling regime development, in rectangular microchannels of varying hydraulic diameters, utilising an enhanced custom VOF-based solver. The solver accounts for conjugate heat transfer effects, implemented in OpenFOAM and validated in the literature through experimental results and analytical solutions. The numerical study was conducted through two different sets of simulations. In the first set, flow boiling characteristics in four single microchannels of Dh = 50, 100, 150, and 200 μm with constant channel aspect ratio of 0.5 and length of 2.4 mm were examined. Due to the different Dh, the applied heat and mass flux values varied between 20 to 200 kW/m2 and 150 to 2400 kg/m2s, respectively. The results of the two-phase simulations were compared with the corresponding initial single-phase stage of the simulations, and an increase of up to 37.4% on the global Nu number Nuglob was revealed. In the second set of simulations, the effectiveness of having microchannel evaporators of single versus multiple parallel microchannels was investigated by performing and comparing simulations of a single rectangular microchannel with Dh of 200 μm and four-parallel rectangular microchannels, each having a hydraulic diameter Dh of 50 μm. By comparing the local time-averaged thermal resistance along the channels, it is found that the parallel microchannels configuration resulted in a 23.3% decrease in the average thermal resistance R¯l compared to the corresponding single-phase simulation stage, while the flow boiling process reduced the R¯l by only 5.4% for the single microchannel case. As for the developed flow regimes, churn and slug flow dominated, whereas liquid film evaporation and, for some cases, contact line evaporation were the main contributing flow boiling mechanisms

The effect of channel aspect ratio on flow boiling characteristics within rectangular micro-passages

In the present paper, a fundamental analysis on the effect of the channel aspect ratio on the bubble dynamics and heat transfer characteristics for the early transient stages of the bubble growth within confined microchannels of rectangular cross-section, under saturated flow boiling conditions, is conducted, utilising high resolution, 3D, transient, conjugate heat transfer simulations. The opensource toolbox OpenFOAM is applied for the simulations, utilising a custom, user-enhanced, diabatic Volume OF Fluid (VOF) solver. Two different series of numerical simulations are performed, focused on a single nucleation event from a single nucleation site and a single nucleation event from multiple, arbitrarily located, nucleation sites, respectively. In each series, three different values of channel aspect ratio are considered, corresponding to a narrow, a square, and a wide microchannel. For the first series, the simulations are performed for a low, a medium, and a high value of applied heat flux and mass flux. For the second series, only the lower values of applied heat flux and mass flux are used for each channel aspect ratio, since this constitutes the worst-case scenario from the overall heat transfer performance point of view, among the cases examined in the first series of simulations. The micro-passage aspect ratio has a significant effect in the generated bubble dynamics during the onset of the nucleate boiling regime, as the bubbles grow within the confined liquid crossflow. This alteration of the generated interfacial dynamics, in effect, regulates the size and position of the contact areas of the generated bubbles with the microchannel walls, with a direct effect in the individual contribution and therefore, the balance between the contact line and the liquid film evaporation mechanisms. Moreover, the work presents the quantification of the effect of the solid domain thermal inertia on the whole process and in particular on the local Nusselt numbers. It is evident that considering conjugate heat transfer in numerical simulations of flow boiling is compulsory in order to predict the physical processes in a correct form

Infrared analysis and pressure measurements on a single loop pulsating heat pipe at different gravity levels

A Single Loop Pulsating Heat Pipe (SLPHP) with an inner diameter of 2 mm is tested in hyper/micro gravity conditions during the 68th ESA Parabolic Flight Campaign. The system is designed with two sapphire tubes that connect the heated and the cooled section, allowing simultaneous fluid flow high-speed visualization, and a direct to fluid IR analysis by using respectively a high-speed camera and a Medium-Wave Infrared Camera (MWIR). Three independent heaters are positioned at the evaporator in order to vary the power distribution and to promote different flow motions with specific heating configurations. Furthermore, two highly accurate pressure transducers measure the pressure drop between the condenser and the evaporator. Additionally, twelve thermocouples mounted on the external tube wall record local temperatures during parabolic flight tests. Such a complete thermo-fluid dynamic analysis at different gravity levels, coupled with the acquisition of high-speed and infrared images in the transparent section of the SLPHP, has the main objective of providing a better understanding on the relationship between the fluid flow motion and the thermal response of the device. Infrared Time-space temperature maps of the flow are correlated with pressure measurements, the external wall tube temperatures, the liquid slug velocity and the local void fraction; providing an exhaustive overview of such a PHP transparent tube both in microgravity and hyper-gravity conditions. Additionally, for the first time in microgravity, the effect of the condenser temperature on PHPs is explored. When the condenser temperature is set at a higher value than the environment, results highlight that the possibility to invert the flow motion direction by means of non-symmetrical heating configurations is hindered. These experimental data could assist the development of improved numerical models of Pulsating Heat Pipes at different gravity levels

Infrared analysis and pressure measurements on a single loop pulsating heat pipe at different gravity levels

A Single Loop Pulsating Heat Pipe (SLPHP) with an inner diameter of 2 mm is tested in hyper/micro gravity conditions during the 68th ESA Parabolic Flight Campaign. The system is designed with two sapphire tubes that connect the heated and the cooled section, allowing simultaneous fluid flow high-speed visualization, and a direct to fluid IR analysis by using respectively a high-speed camera and a Medium-Wave Infrared Camera (MWIR). Three independent heaters are positioned at the evaporator in order to vary the power distribution and to promote different flow motions with specific heating configurations. Furthermore, two highly accurate pressure transducers measure the pressure drop between the condenser and the evaporator. Additionally, twelve thermocouples mounted on the external tube wall record local temperatures during parabolic flight tests. Such a complete thermo-fluid dynamic analysis at different gravity levels, coupled with the acquisition of high-speed and infrared images in the transparent section of the SLPHP, has the main objective of providing a better understanding on the relationship between the fluid flow motion and the thermal response of the device. Infrared Time-space temperature maps of the flow are correlated with pressure measurements, the external wall tube temperatures, the liquid slug velocity and the local void fraction; providing an exhaustive overview of such a PHP transparent tube both in microgravity and hyper-gravity conditions. Additionally, for the first time in microgravity, the effect of the condenser temperature on PHPs is explored. When the condenser temperature is set at a higher value than the environment, results highlight that the possibility to invert the flow motion direction by means of non-symmetrical heating configurations is hindered. These experimental data could assist the development of improved numerical models of Pulsating Heat Pipes at different gravity levels

Rhizobium-legume symbiosis in the absence of Nod factors: two possible scenarios with or without the T3SS

International audienceThe occurrence of alternative Nod factor (NF)-independent symbiosis between legumes and rhizobia was first demonstrated in some Aeschynomene species that are nodulated by photosynthetic bradyrhizobia lacking the canonical nodABC genes. In this study, we revealed that a large diversity of non-photosynthetic bradyrhizobia, including B. elkanii, was also able to induce nodules on the NF-independent Aeschynomene species, A. indica. Using cytological analysis of the nodules and the nitrogenase enzyme activity as markers, a gradient in the symbiotic interaction between bradyrhizobial strains and A. indica could be distinguished. This ranged from strains that induced nodules that were only infected intercellularly to rhizobial strains that formed nodules in which the host cells were invaded intracellularly and that displayed a weak nitrogenase activity. In all non-photosynthetic bradyrhizobia, the type III secretion system (T3SS) appears required to trigger nodule organogenesis. In contrast, genome sequence analysis revealed that apart from a few exceptions, like the Bradyrhizobium ORS285 strain, photosynthetic bradyrhizobia strains lack a T3SS. Furthermore, analysis of the symbiotic properties of an ORS285 T3SS mutant revealed that the T3SS could have a positive or negative role for the interaction with NF-dependent Aeschynomene species, but that it is dispensable for the interaction with all NF-independent Aeschynomene species tested. Taken together, these data indicate that two NF-independent symbiotic processes are possible between legumes and rhizobia: one dependent on a T3SS and one using a so far unknown mechanism