J.N. Chung

Formation of bubbles in a simple co-flowing micro-channel

Bubble generation in a simple co-flowing micro-channel with a cross-sectional area of 1.69 × 0.07 mm2 was experimentally and numerically investigated. Air and water were used as the gas and liquid, respectively. Mixtures of water–glycerol and water–Tween 20 were also used to obtain the effects of viscosity and surface tension. The experimental data show that the break-up process is periodic under certain operating conditions. The break-up dynamics are also examined using three-dimensional incompressible two-phase flow numerical simulation based on the volume of fluid (VOF) method. The simulation successfully predicts the flow behavior and provides a more detailed examination of the bubble shape. The physics can be further explained by the detailed micro-PIV measurements, which show that the bubble is formed due to the velocity component perpendicular to the gas flow created by the sudden change of the liquid velocity distribution around the barrier. The bubble length L is dependent on the liquid flow rate Ql and the gas flow rate Qg, and the ratio of L to the channel width w is a function of the ratio of gas and liquid flow rates Qg/Ql which is similar to that previously used in the T-junction case. The formulation of bubble frequency f is derived under current conditions and it shows a good agreement with the experimental data at the low frequency region. Different bubble shapes can be obtained at different liquid viscosities and surface tensions. The ratio L/w can still be predicted by a modified equation which uses the real bubble width wb or an equivalent bubble length Le.

An experimental study of the size effect on adiabatic gas-liquid two-phase flow patterns and void fraction in microchannels

Adiabatic gas-liquid flow patterns and void fractions in microchannels were experimentally investigated. Using nitrogen and water, experiments were conducted in rectangular microchannels with hydraulic diameters of 0.209mm, 0.412mm and 0.622mm, respectively. Gas and liquid superficial velocities were varied from 0.06–72.3m∕s and 0.02–7.13m∕s, respectively. The main objective is focused on the effects of microscale channel sizes on the flow regime map and void fraction. The instability of flow patterns was observed. Four groups of flow patterns including bubbly slug flow, slug-ring flow, dispersed-churn flow, and annular flow were observed in microchannels of 0.412mm and, 0.622mm. In the microchannel of 0.209mm, the bubbly slug flow became the slug flow and the dispersed-churn flow disappeared. The current flow regime maps showed the transition lines shifted to higher gas superficial velocity due to a dominant surface tension effect as the channel size was reduced. The regime maps presented by other authors for minichannels were found to not be applicable for microchannels. Time-averaged void fractions were measured by analyzing 8000 high speed video images for each flow condition. The void fractions hold a nonlinear relationship with the homogeneous void fraction as opposed to the relatively linear trend for the minichannels. A new correlation was developed to predict the nonlinear relationship that fits most of the current experimental data and those of the 0.1mm diameter tube reported by Kawahara et al. [Int. J. Multiphase Flow 28, 1411 (2002)] within ±15%.Adiabatic gas-liquid flow patterns and void fractions in microchannels were experimentally investigated. Using nitrogen and water, experiments were conducted in rectangular microchannels with hydraulic diameters of 0.209mm, 0.412mm and 0.622mm, respectively. Gas and liquid superficial velocities were varied from 0.06–72.3m∕s and 0.02–7.13m∕s, respectively. The main objective is focused on the effects of microscale channel sizes on the flow regime map and void fraction. The instability of flow patterns was observed. Four groups of flow patterns including bubbly slug flow, slug-ring flow, dispersed-churn flow, and annular flow were observed in microchannels of 0.412mm and, 0.622mm. In the microchannel of 0.209mm, the bubbly slug flow became the slug flow and the dispersed-churn flow disappeared. The current flow regime maps showed the transition lines shifted to higher gas superficial velocity due to a dominant surface tension effect as the channel size was reduced. The regime maps presented by other author...

Coalescence of bubbles in nucleate boiling on microheaters

Abstract In this paper, an experiment was performed which is based on a heating surface consisting of microheaters where the temperature of each heater can be individually controlled by an electronic feedback loop. The power consumed by the heaters throughout the cycle of individual bubble growth, coalescence, detachment and departure was measured at high frequencies, thus the heat flux and its variation were obtained. By a careful timing and control of two individual microheaters, we were able to produce two individual bubbles side-by-side. The coalescence would takes place when they grow to a certain size that allows them to touch each other. We have recorded two major heat flux spikes for a typical cycle of boiling with coalescence. The first one corresponds to the nucleation of bubbles; the second one is for the coalescence of the two bubbles. We found that the heat flux variation is closely related to the bubble dynamics and bubble–bubble interaction. By comparing with the single bubble results without coalescence, we also found that the heat transfer is highly enhanced due to the coalescence.

A study of bubble dynamics in reduced gravity forced-convection boiling

Abstract Experiments are reported in which a single vapor bubble was nucleated and grown in a flow field of FC-72 on a flat surface in terrestrial gravity and microgravity. A thin gold film semi-transparent heater was designed and used to generate single bubbles. Bubble nucleation, growth, and departure in microgravity with different flow rates were observed by a CCD camera. It was found that the transient bubble diameters during growth are proportional to the parameter of ( Re −1/3 t ∗1/3 ), where Re is the Reynolds number and t∗ is the dimensionless time. The forced-convection would affect the downstream edge of a bubble significantly in microgravity, while the upstream coordinate of a bubble seems to be independent of the flow rate. It was observed that the high flow rate would offset the buoyancy effects. As a result, the bubble generation frequency, Weber number, and bubble shape tend to be similar with those in normal gravity.

System characteristics and performance evaluation of a trailer-scale downdraft gasifier with different feedstock.

The main objective of this study is to investigate the thermal profiles of a trailer-scale gasifier in different zones during the course of gasification and also to elaborate on the design, characteristics and performance of the gasification system using different biomass feedstock. The purpose is to emphasize on the effectiveness of distributed power generation systems and demonstrate the feasibility of such gasification systems in real world scenarios, where the lingo-cellulosic biomass resources are widely available and distributed across the board. Experimental data on the thermal profiles with respect to five different zones in the gasifier and a comprehensive thermal-chemical equilibrium model to predict the syngas composition are presented in detail. Four different feedstock-pine wood, horse manure, red oak, and cardboard were evaluated. The effects of C, H, O content variations in the feedstock on the thermal profiles, and the efficiency and viability of the trailer-scale gasifier are also discussed.

A Possible Role of Nanostructured Ridges on Boiling Heat Transfer Enhancement

Evaporation of a nanoscale meniscus on a nanostructured heater surface is simulated using molecular dynamics. The nanostructures, evenly spaced on the surface, are ridges with a width and height of 0.55 nm and 0.96 nm, respectively. The simulation results show that the film breaks during the early stages of evaporation due to the presence of nanostructures and no nonevaporating film forms (unlike a previous simulation performed in the absence of nanostructures where nonevaporating film forms on the smooth surface). High heat transfer and evaporation rates are obtained. We conclude that heat transfer rates can be significantly increased during bubble nucleation and growth by the presence of nanostructure ridges on the surface as it can break the formation of nonevaporating film. This causes additional chaos and allows the surrounding cooler liquid to come in contact with the surface providing heat transfer enhancements.

Nanoscale liquid-vapor phase-change physics in nonevaporating region at the three-phase contact line

Nanoscale liquid film evaporation is usually associated with super-high heat transport rates and can be found in natural processes and in many industrial and advanced technologies. In this paper, thin film evaporation is simulated in a nanochannel using molecular dynamics to study the effect of varying nanochannel height and film thickness. Three nanochannel heights (16.32, 25.5, and 35.7 nm; constant liquid film thickness=3 nm) and three liquid film thicknesses (2, 4, and 6 nm; constant nanochannel height=25.5 nm) are simulated to study six cases. A nonevaporating film is obtained for all six cases. Hamaker constant, vapor pressure, film thickness, and net evaporation and heat fluxes are evaluated. An additional simulation (case 7) is run with simultaneous evaporation-condensation; no nonevaporating film is obtained. Thus, the creation of a nonevaporating film, and its thickness (if the film forms), depends on the combination of three factors, namely, vapor pressure, substrate temperature, and solid-liqu...

An Experimental Study of Miniature-Scale Pool Boiling

By generating single bubbles on a micro-heater at different wall superheats, an experimental study of miniature-scale pool boiling heat transfer has been performed to provide a fundamental understanding of the heater size effect. In this study, the constant-temperature microheater is set at different temperatures by an electronic feedback control system. The heat transfer history during the lifetime of a single bubble which includes nucleation, growth, detachment and departure has been measured. The boiling curve obtained from the microheater is composed of two regimes which are separated by a peak heat flux. It is suggested that in the lower superheat regime, the boiling is dominated by liquid rewetting and micro-layer evaporation, while in the higher superheat regime, conduction through the vapor film and micro-convection plays the key heat transfer role as the heater is covered by vapor all the time. In general, boiling on microheaters is characterized by larger bubble departure sizes, smaller bubble growth rates due to the dryout of microlayer as the bubble grows, and higher bubble incipience superheat. As the heater size decreases, the boiling curve shifts towards higher heat fluxes with corresponding higher superheats.

A direct numerical simulation of transition phenomena in a mixed convection channel flow

Abstract This study intends to provide an increased understanding of the laminar–turbulent transition phenomena for the buoyancy-assisted heated vertical channel flow during the early transient stage. The spectral method with weak formulation is applied in the direct numerical simulation. Initial disturbances consist of the finite-amplitude two-dimensional TS wave and a pair of three-dimensional oblique waves for the K-type disturbances. The results from the harmonic energy competitions of different wave modes show that for the buoyancy-assisted heated flow, the ( k x =1, k z =1) or (1,1) and (1,0) modes would gain energy immediately and start to rise at almost the same rate. This phenomenon is different from that of the buoyancy-opposed flow, where the (1,1) mode decays slowly in the beginning until other modes gain enough energy and then it begins to grow quickly and overtakes the (1,0) mode after a short time period. These different transition patterns match with the experimental results that the flow transition is supercritical and subcritical for the buoyancy-assisted and -opposed flows, respectively. Buoyancy-assisted heated flow transition follows the general trend of an isothermal flow in the beginning, but the thermal-buoyant force is crucial in accelerating the instability and also causing notable differences during the subsequent transition process. All of the results for the vortex structures development, kinetic energy budget of the disturbances, flow visualization by tagged fluid particles, and the local temperature fluctuations are consistent in pointing to a clear pattern for the buoyancy-assisted heated flow transition.

Grand Challenges in Bioenergy and Biofuel Research: Engineering and Technology Development, Environmental Impact, and Sustainability

of Energy (NSF-DOE) Workshop report (Huber, 2007) concluded that liquid biofuels produced from lignocellulosic biomass can significantly reduce our dependence on oil, create new jobs, improve the rural economy, reduce greenhouse emissions, and ensure energy security. The report also emphasized that the bottleneck for lignocellulosic biomass-derived fuels is the lack of technology for the efficient conversion of biomass into liquid fuels. New technologies are thus needed to replace fossil fuels with renewable and sustainable energy resources. For example, in the US reliable estimates of renewable and sustainable lignocellulosic forest, agricultural biomass, and MSW (likewise mostly biomass) range from 1.5 to 2 billion dry metric tons per year (Huber, 2007); these biomass resources thus could contribute 10 times more to the US primary energy supply (PES) than now. Another forecast in the same report (Huber, 2007) claims that all forms of biomass and MSW have the potential to replace up to 60% of US consumption of crude oil; and that lignocellulosic biomass resources are significantly cheaper (at the equivalent of US