Aritra Sur

Flow Boiling Heat Transfer and Two-Phase Flow Instability of Nanofluids in a Minichannel

Single-phase convective heat transfer of nanofluids has been studied extensively, and different degrees of enhancement were observed over the base fluids, whereas there is still debate on the improvement in overall thermal performance when both heat transfer and hydrodynamic characteristics are considered. Meanwhile, very few studies have been devoted to investigating two-phase heat transfer of nanofluids, and it remains inconclusive whether the same pessimistic outlook should be expected. In this work, an experimental study of forced convective flow boiling and two-phase flow was conducted for Al2O3–water nanofluids through a minichannel. General flow boiling heat transfer characteristics were measured, and the effects of nanofluids on the onset of nucleate boiling (ONB) were studied. Two-phase flow instabilities were also explored with an emphasis on the transition boundaries of onset of flow instabilities (OFI). It was found that the presence of nanoparticles delays ONB and suppresses OFI, and the extent is correlated to the nanoparticle volume concentration. These effects were attributed to the changes in available nucleation sites and surface wettability as well as thinning of thermal boundary layers in nanofluid flow. Additionally, it was observed that the pressure-drop type flow instability prevails in two-phase flow of nanofluids, but with reduced amplitude in pressure, temperature, and mass flux oscillations. [DOI: 10.1115/1.4029647]

Bubble Ebullition on a Hydrophilic Surface

Nucleate boiling heat transfer depends on various aspects of the bubble ebullition, such as the bubble nucleation, growth and departure. In this work, a synchronized high-speed optical imaging and infrared (IR) thermography approach was employed to study the ebullition process of a single bubble on a hydrophilic surface. The boiling experiments were conducted at saturated temperature and atmospheric pressure conditions. De-ionized (DI) water was used as the working fluid. The boiling device was made of a 385-um thick silicon wafer. A thin film heater was deposited on one side, and the other side was used as the boiling surface. The onset of nucleate boiling (ONB) occurs at a wall superheat of ΔTsup= 12 oC and an applied heat flux of q” = 35.9 kW/m2. The evolution of the wall heat flux distribution was obtained from the IR temperature measurements, which clearly depicts the existence of the microlayer near the three-phase contact line of the nucleate bubble. The results suggest that, during the bubble growth stage, the evaporation in the microlayer region contributes dominantly to the nucleate boiling heat transfer; however, once the bubble starts to depart from the boiling surface, the microlayer quickly vanishes, and the transient conduction and the microconvection become the prevailing heat transfer mechanisms. 6.027 mm 6.027 mm

Experimental and Numerical Investigation of Two-Phase Patterns in a Cross-Junction Microfluidic Chip

Two-phase microfluidic systems have been found in a wide range of engineering applications. Accurate determination of the two-phase flow patterns in microchannels is crucial to selecting appropriate predictive tools for pressure drop, heat and mass transfer in the microfluidic devices. Most of the prevailing two-phase flow maps developed using visualization techniques are unable to reveal the fundamental mechanisms responsible for the formation of specific flow pattern under given flow conditions. In this work, the high-speed photographic method is employed to study the liquid-gas two-phase flow in a cross-junction microfluidic chip with a rectangular cross section of 300 μm by 100 μm. The dynamics of bubbly, slug and annular flows are investigated. Numerical models using the VOF approach are developed to simulate the two-phase mixing and flow pattern formation in the microfluidic device. The roles of the inertia, viscous shear and surface tension forces in forming various two-phase flow patterns are discussed. The experimental results and the simulation data together provide a comprehensive phenomenological description of the key parameters and processes that govern the two-phase flow pattern formation in microfluidic devices.Copyright

Thermal Modeling of a Wireline Tool for Ultra-High Temperatures

The drive to obtain more accurate petrophysical information from deeper wells has led to the demand for operating various downhole tools at higher temperatures for longer time periods. If the borehole temperature reaches values higher than 175°C, it is considered a high temperature (HT) well. In HT wells, reliability of the electronic components of the logging tools is a major concern. One way to address the concern is through using a thermal flask to reduce the heat flow rate from the formation to the tool and evenly distribute the heat generated by the internal electronic components.Optimizing the design of the aforementioned thermal flask is very important in providing a longer operative time for the tool before the temperature of the sensitive electronic parts reaches a critical threshold. To obtain the sensitive parameters for designing the flask, the thermal transport inside the tool must be accurately modeled.In this work, high fidelity FEA and CFD-based transient thermal models are developed for thermal transport in a flask for an ultra-high temperature wireline tool. Two models with different levels of complexity are presented. The models are verified by experimental results and the physical insights obtained from them presented. The predictive capability of the models is used to provide recommendations for safe operating time for various environmental conditions which prevail in the formations. The results obtained from the models can also be used for optimizing the performance of the future generation of the tool and reducing the amount of time spent in unnecessary trip outs.Copyright

Adiabatic Air-Water Two-Phase Flow in Circular Microchannels

Gaseliquid two-phase flow in microchannels with hydraulic diameters of 100e500 mm exhibits drastically different flow behaviors from its counterpart in conventional macroscopic channels. Of particular interests are the two-phase flow patterns and the two-phase frictional pressure drop for given flow conditions in these microchannels. This paper presents an experimental study of the effects of channel size and superficial phasic velocity on the two-phase flow pattern and pressure drop of airewater mixture in circular microchannels with inner diameters of 100, 180 and 324 mm. Two-phase flow patterns were visualized using high-speed photographic technique. Four basic flow patterns, namely, bubbly flow, slug flow, ring flow and annular flow, were observed. The two-phase flow regime maps were constructed and the transition boundaries between different flow regimes identified. In an effort to unify the flow transition boundary in microchannels of different sizes, a new flow map was developed using the modified Weber numbers as the coordinates. The two-phase frictional pressure gradient in the microchannels was measured and the data were compared with predictions from the separated flow model, the homogeneous flow model and the flow pattern-based phenomenological models. Results show that the flow pattern-based models provide the best prediction of the two-phase pressure drop in the microchannels.

Two-Phase Flow With Surfactants in a Microchannel

Addition of surfactants to liquids helps to eliminate intermittent two-phase flow patterns and alleviate flow instability. These features are very desirable for two-phase microfluidic applications. However, very little information is available on two-phase flow patterns of surfactant solution in the microchannels. The present paper reports a study of adiabatic two-phase flow with surfactants in a circular microchannel of a 180-μm diameter. Air-water mixtures with trace quantities of sodium dodecyl sulfate (SDS) were used in the experiments. The maximum superficial velocities measured were 4 m/s for the liquid and 65 m/s for the gas. High-speed photographic technique was employed to visualize various two-phase flow patterns and to identify the transition boundaries between different flow regimes. The results were compared to data obtained from air-water flow without surfactants. It was found that addition of surfactants brings in significant modification to the two-phase flow regimes as well as their transition characteristics in microchannels; in particular, slug flow is effectively suppressed.Copyright