180 research outputs found
Some Basic Properties of Low Quality Two-Phase Turbulent Flow
This Is an exploratory study dealing with the effect of gas bubbles on the turbulence Intensity of the liquid in a two-phase flow.
Distributions of time-mean and fluctuating velocities were measured in a two - dimensional channel. These were measured for single-phase flow and on the two sides of a bubble layer, which simulated the two-phase flow.
In single-phase flow the distribution of the time-mean velocities agrees well with other data, while the distribution of the Intensity of turbulence Is lower than expected.
The measurements of the time-mean velocity and turbulence Intensity across the bubble layer revealed that they are discontinuous at that point. This supports the hypothesis that the presence of bubbles in liquid flows tends to lower the intensity of turbulence
Experimental investigation of non-uniform heating effect on flow boiling instabilities in a microchannel-based heat sink
Copyright @ 2011 ElsevierTwo-phase flow boiling in microchannels is one of the most promising cooling technologies for coping with high heat fluxes produced by the next generation of central processor units (CPUs). If flow boiling is to be used as a thermal management method for high heat flux electronics it is necessary to understand the behaviour of a non-uniform heat distribution, which is typically the case observed in a real operating CPU. The work presented is an experimental study of two-phase boiling in a multi-channel silicon heat sink with non-uniform heating, using water as the cooling liquid. Thin nickel film sensors, integrated on the back side of the heat sinks were used in order to gain insight related to temperature fluctuations caused by two-phase flow instabilities under non-uniform heating. The effect of various hotspot locations on the temperature profile and pressure drop has been investigated. It was observed that boiling inside microchannels with axially non-uniform heating leads to high temperature non-uniformity in the transverse direction.This research was supported by the UK Engineering and Physical Sciences Research Council through grant EP/D500109/1
FOOT PRONATION AND STRESS FRACTURES OF THE FEMUR AND TIBIA: A PROSPECTIVE BIOMECHANICAL STUDY
The relation between foot pronation and stress fractures has been suggested. However, evidence based literature is lacking and contradictory. The purpose of this study was to
examine whether dynamic parameters of foot pronation are related to the development of stress fractures of the femur and tibia. 2 weeks prior to beginning of 14 weeks of basic
military training, 473 infantry recruits were inrolled into the study. 2D analysis was performed to measure foot pronation during treadmill walking. The soldiers were examined during the training course at two weeks intervals for stress fractures. The odds ratio was calculated for each dynamic pronation parameter in relation to the stress fractures. 10% of the 405 soldiers who finished the training were diagnosed with stress
fractures of the femur and tibia. Longer pronation time was related to risk reduction for the development of stress fractures and may have a protective effect during an extended
period of training
Effect of wire diameter on ultrasonic enhancement of subcooled pool boiling
Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.New methods for cooling of microelectronic elements have been recently developed, including application of ultrasonic fields. Ultrasonic fields enhance the heat transfer in two-phase cooling. The present work deals with ultrasonic enhancement of heat transfer from wires in sub-cooled pool boiling. The experiments have been carried out using three wires of different diameters: 0.05, 0.09, 0.2mm, submerged into a bath with water. The applied ultrasonic field was of frequency of 40 kHz and intensity of 0.5 W/cm2. The wire wall temperature was measured as a function of wire surface heat flux. When the ultrasonic field was applied, the wall temperature reduced in the range of measured heat fluxes. The temperature difference increased with the heat flux. It also increased with the wire diameter. At the smallest diameter only a small decrease of the wall temperature, about 10-15 degrees, was observed, while at larger diameters the decrease of the wall temperature was about 30 - 35 degrees.dc201
Pin fin two-phase micro gap coolers for concentrating photovoltaic arrays
Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.Concentrating photovoltaic (CPV) systems are among the most promising renewable power generation options but will require aggressive thermal management to prevent elevated solar cell temperatures and to achieve the conversion efficiency, reliability, and cost needed to compete with alternative techniques. Two-phase, evaporative cooling of
CPV modules has been shown to provide significant advantages relative to single-phase cooling but, to date, the available two-phase data has been insufficient for the design and optimization of such CPV systems. This Keynote lecture will begin with a brief review of CPV technology and the solar cell cooling techniques described in the literature. Energy modeling, relating the harvested solar energy to the âparasiticâ work expended to provide the requisite cooling, will be used to support the efficacy of twophase cooling for CPV applications. Attention will then turn to the available correlations for pin-finned microgap coolers and the gaps which must be addressed to enable such thermal management for CPV arrays. This will be followed by a detailed description of an experimental study of 3 pin-finned
microgap coolers for CPV systems and the derived heat transfer and pressure drop correlations. The data spans a large parametric range, with heat fluxes of 1 - 170 W/cm2, mass fluxes of 10.7 - 1300 kg/m2-s, subcooled (single phase) flow as well as exit qualities up to 90%, and 3 heat transfer fluids
(water, HFC-134a, HFE-7200). The lecture will close with a brief case study of two-phase CPV cooling, demonstrating that the application of this thermal management mode can lead to a highly energy efficient CPV system.dc201
Investigation of fluidized bed behaviour using electrical capacitance tomography
The temporal and crossâsectional distributions of particles in a 127âmm diameter fluidized bed have been obtained using a new generation, highâspeed electrical capacitance tomography. Two planes of eight electrodes were used and mounted at 160âmm and 660âmm from the gas distributor which was a 3âmm thick porous plastic plate (maximum pore size of 50 ÎŒmâ70 ÎŒm). 3âmm diameter, nearlyâspherical polyethylene granules made up the bed. Experiments at sampling frequencies of 200â2000 crossâsections per second and gas superficial velocities from just below the minimum fluidization to 83% above minimum fluidization velocities were used. The time series of the crossâsectional average void fractions have been examined both directly and in amplitude and frequency space. The last two used probability density functions and power spectral densities. The information gathered shows that the fluidized bed was operating in the slugging mode, which is not surprising given the size of the particles. It has been found that an increase in the excess gas velocity above the minimum fluidization velocity resulted in an increase in the mean void fraction, an increase in the length and velocity of the slug bubbles as well as the bed height, and a slight decrease in the slug frequency. The results are presented in a level of detail suitable for comparison with later numerical simulation
Direct Numerical Simulation of Turbulent Heat Transfer Modulation in Micro-Dispersed Channel Flow
The object of this paper is to study the influence of dispersed micrometer
size particles on turbulent heat transfer mechanisms in wall-bounded flows. The
strategic target of the current research is to set up a methodology to size and
design new-concept heat transfer fluids with properties given by those of the
base fluid modulated by the presence of dynamically-interacting,
suitably-chosen, discrete micro- and nano- particles. We run Direct Numerical
Simulation (DNS) for hydrodynamically fully-developed, thermally-developing
turbulent channel flow at shear Reynolds number Re=150 and Prandtl number Pr=3,
and we tracked two large swarms of particles, characterized by different
inertia and thermal inertia. Preliminary results on velocity and temperature
statistics for both phases show that, with respect to single-phase flow, heat
transfer fluxes at the walls increase by roughly 2% when the flow is laden with
the smaller particles, which exhibit a rather persistent stability against
non-homogeneous distribution and near-wall concentration. An opposite trend
(slight heat transfer flux decrease) is observed when the larger particles are
dispersed into the flow. These results are consistent with previous
experimental findings and are discussed in the frame of the current research
activities in the field. Future developments are also outlined.Comment: Pages: 305-32
Hybrid Approach in Microscale Transport Phenomena: Application to Biodiesel Synthesis in Micro-reactors
A hybrid engineering approach to the study of transport phenomena, based on the
synergy among computational, analytical, and experimental methodologies is
reviewed. The focus of the chapter is on fundamental analysis and proof of concept
developments in the use of nano- and micro-technologies for energy efficiency and
heat and mass transfer enhancement applications. The hybrid approach described
herein combines improved lumped-differential modeling, hybrid numericalanalytical solution methods, mixed symbolic-numerical computations, and
advanced experimental techniques for micro-scale transport phenomena. An
application dealing with micro-reactors for continuous synthesis of biodiesel is
selected to demonstrate the instrumental role of the hybrid approach in achieving
improved design and enhanced performance
Thermal Transport in Micro- and Nanoscale Systems
Small-scale (micro-/nanoscale) heat transfer has broad and exciting range of applications. Heat transfer at small scale quite naturally is influenced â sometimes dramatically â with high surface area-to-volume ratios. This in effect means that heat transfer in small-scale devices and systems is influenced by surface treatment and surface morphology. Importantly, interfacial dynamic effects are at least non-negligible, and there is a strong potential to engineer the performance of such devices using the progress in micro- and nanomanufacturing technologies. With this motivation, the emphasis here is on heat conduction and convection. The chapter starts with a broad introduction to Boltzmann transport equation which captures the physics of small-scale heat transport, while also outlining the differences between small-scale transport and classical macroscale heat transport. Among applications, examples are thermoelectric and thermal interface materials where micro- and nanofabrication have led to impressive figure of merits and thermal management performance. Basic of phonon transport and its manipulation through nanostructuring materials are discussed in detail.
Small-scale single-phase convection and the crucial role it has played in developing the thermal management solutions for the next generation of electronics and energy-harvesting devices are discussed as the next topic. Features of microcooling platforms and physics of optimized thermal transport using microchannel manifold heat sinks are discussed in detail along with a discussion of how such systems also facilitate use of low-grade, waste heat from data centers and photovoltaic modules.
Phase change process and their control using surface micro-/nanostructure are discussed next. Among the feature considered, the first are microscale heat pipes where capillary effects play an important role. Next the role of nanostructures in controlling nucleation and mobility of the discrete phase in two-phase processes, such as boiling, condensation, and icing is explained in great detail. Special emphasis is placed on the limitations of current surface and device manufacture technologies while also outlining the potential ways to overcome them. Lastly, the chapter is concluded with a summary and perspective on future trends and, more importantly, the opportunities for new research and applications in this exciting field
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