Boiling Under Hele-Shaw Flow Conditions: The Occurrence of Viscous Fingering

2015 ◽  
Vol 138 (2) ◽  
Author(s):  
Felix Reinker ◽  
Marek Kapitz ◽  
Stefan aus der Wiesche
Keyword(s):  
High Speed ◽  
Bubble Dynamics ◽  
Pure Water ◽  
Phase Interface ◽  
Viscous Fingering ◽  
Working Fluid ◽  
Small Scale ◽  
Vapor Bubbles ◽  
Cell Parameters ◽  
Fingering Phenomena

Boiling and bubble dynamics were experimentally investigated in a Hele-Shaw flow cell using pure water at atmospheric pressure as the working fluid. The resulting vapor bubble shapes were recorded by means of a high-speed camera for several plate spacings and heating power levels. It was found that viscous fingering phenomena of vapor bubbles occurred only under very special boiling conditions and cell parameters. The evaporation front velocity was identified as a major parameter for the onset of viscous fingering. The observed basic viscous fingering dynamics was in reasonable agreement with theoretical analyses. In addition to that classical viscous large fingering, small-scale evaporation instability was observed leading to microscopic roughening of accelerating evaporation fronts. This instability might be explicitly related to evaporative heat and mass transfer effects across the fast-moving phase interface.

Boiling Under Hele-Shaw Flow Conditions: The Occurrence of Viscous Fingering

2013 ◽  
Author(s):  
Felix Reinker ◽  
Marek Kapitz ◽  
Stefan aus der Wiesche
Keyword(s):  
High Speed ◽  
Bubble Dynamics ◽  
Pure Water ◽  
Phase Interface ◽  
Viscous Fingering ◽  
Working Fluid ◽  
Small Scale ◽  
Vapor Bubbles ◽  
Cell Parameters ◽  
Fingering Phenomena

Boiling and bubble dynamics were experimentally investigated in a Hele-Shaw flow cell using pure water at atmospheric pressure as working fluid. The resulting vapor bubble shapes were recorded by means of a high-speed camera for several plate spacings and heating power levels. It was found that viscous fingering phenomena of vapor bubbles occurred only under very special boiling conditions and cell parameters. The evaporation front velocity was identified as a major parameter for the onset of viscous fingering. The observed basic viscous fingering dynamics was in reasonable agreement with theoretical analyses. In addition to that classical viscous large fingering, small-scale evaporation instability was observed leading to microscopic roughening of accelerating evaporation fronts. This instability might be explicitly related to evaporative heat and mass transfer effects across the fast-moving phase interface.

A Photographic Study on the Effects of Hydrophobic-Spot Size and Subcooling on Local Film Boiling

2013 ◽  
Author(s):  
Bambang Joko Suroto ◽  
Masahiro Tashiro ◽  
Sana Hirabayashi ◽  
Sumitomo Hidaka ◽  
Masamichi Kohno ◽  
...  
Keyword(s):  
Heat Flux ◽  
High Speed ◽  
Bubble Dynamics ◽  
Pure Water ◽  
Nucleate Boiling ◽  
Spot Size ◽  
Copper Surface ◽  
Film Boiling ◽  
Working Fluid ◽  
Boiling Regime

The effects of hydrophobic circle spot size and subcooling on local film boiling phenomenon from the copper surface with single PTFE (Polytetrafluoroethylene) hydrophobic circle spot at low heat flux has been investigated. The experiments were performed using pure water as the working fluid and subcooling ranging from 0 and 10K. The heat transfer surfaces are used polished copper block with single PTFE hydrophobic circle spot of diameters 2, 4 and 6 mm, respectively. A high-speed camera was used to capture bubble dynamics and disclosed the sequence of the process leading to local film boiling. The result shows that local films boiling occurs on the PTFE circle spot at low heat flux and was triggered by the merging of neighboring bubbles. The study also showed that transition time required for change from nucleate boiling regime to local film boiling regime depends on the diameter of the hydrophobic circle spot and the subcooling. A stable local film boiling occurs at the smallest diameter of hydrophobic spot. Subcooling cause the local film boiling occur at negative superheat and oscillation of bubble dome.

Experimental Investigation on a 3 kW Air Tesla Expander With High Speed Generator

2020 ◽  
Author(s):  
Avinash Renuke ◽  
Federico Reggio ◽  
Paolo Silvestri ◽  
Alberto Traverso ◽  
Matteo Pascenti
Keyword(s):  
High Speed ◽  
Experimental Tests ◽  
Working Fluid ◽  
Small Scale ◽  
Efficient Design ◽  
Electric Generator ◽  
Vibrational Response ◽  
Machine Maintenance ◽  
Performance Results

Abstract Tesla bladeless turbomachines are recently being investigated due to many advantages such as its simple design and ease of manufacturing. If an efficient design is achieved, this will be a promising machine in the area of small-scale power generation and energy harvesting. This paper focuses on the experimental performance investigation of 3 kW (rated power) Tesla bladeless expander. The Tesla expander and electric generator are housed in a single casing making it first of its kind being tested with such configuration. The expander is fed with air and operated at high rotational speeds up to 40000 rpm. The test is carried out with different number of nozzles to understand its effect on the performance. Results show that the peak efficiency for two nozzles is better than one nozzle and four nozzle configurations for the same inlet pressure conditions. Experimental tests revealed that this turbine is most efficient Tesla turbine till now with air as a working fluid. Furthermore, one of the most important losses in Tesla turbomachines, nozzle loss, is experimentally characterized. Specific vibrational tests were carried out to obtain complete machine dynamical characterization. The vibrational response characterization of the turbine enabled us to recognize a disk mode family solicited by the air flow and to perform a proper machine maintenance and balancing aiming to reduce the energy of its operational vibration.

Sound of Single Vapor Bubbles

2006 ◽  
Author(s):  
Ho Sung Lee ◽  
Danny M. Higgs
Keyword(s):  
Spherical Harmonics ◽  
Vapor Bubble ◽  
High Speed ◽  
Bubble Dynamics ◽  
Acoustic Emissions ◽  
Sound Intensity ◽  
Initial Growth ◽  
Surprising Result ◽  
Vapor Bubbles ◽  
Significant Information

The recent preliminary acoustic measurements of single vapor bubbles on a heated platinum wire, combined with high-speed digital photography, provided significant information for the vapor bubble dynamics such as growth, departure, collapse or coalescence with a previous bubble. Furthermore, under a given condition, the numerous consecutive single bubbles consistently showed almost identical waveforms, even at different times. This surprising result indicates that the phenomenon is not a chaotic process, but an orderly mathematical process. The deceleration of a growing bubble following the rapid initial growth was apparently detected by the acoustic emissions as a negative acoustic pressure. This is believed to be a new observation and not seen in gas bubbles. Some successive bubbles clearly underwent the spherical harmonics and compared well with a series of photographs. These results are in contrast with the previous indeterminate measurements on the sound intensity and frequency in boiling in the literature. The information for vapor bubble dynamics will be supplementary to the gas bubble dynamics such as cavitation, sonoluminescence, etc. Visual observations will be valuable for the mathematicians who study the spherical harmonics analytically. Also, the technique and information may be applicable to the fields of science and engineering associated with vapor bubbles motion including boiling.

Dynamic Modelling of Small Scale and High Temperature ORC System Using Simulink and CoolProp

2020 ◽  
Author(s):  
Radheesh Dhanasegaran ◽  
Antti Uusitalo ◽  
Teemu Turunen-Saaresti
Keyword(s):  
High Temperature ◽  
Dynamic Model ◽  
High Speed ◽  
Waste Heat ◽  
Fluid Pressure ◽  
Dynamic Modelling ◽  
Working Fluid ◽  
Small Scale ◽  
Condensation Process ◽  
Feed Pump

Abstract In the present work, a dynamic model has been developed for the small-scale high-temperature ORC experimental test rig at the LUT University that utilizes waste heat from a heavy-duty diesel engine exhaust. The experimental facility consists of a high-speed Turbogenerator, heat exchanger components such as recuperator, condenser, and evaporator with a pre-feed pump to boost the working fluid pressure after the condensation process constituting a cycle. The turbogenerator consists of a supersonic radial-inflow turbine, a barske type main-feed pump, and a permanent magnet type generator components connected on a single shaft. Octamethyltrisiloxane (MDM) is the chosen organic working fluid in this cycle. Matlab-Simulink environment along with the open-source thermodynamic and transport database Cool-Prop has been chosen for calculating the thermodynamic properties of the dynamic model. A functional parameter approach has been followed for modeling each block component by predefined input and output parameters, aimed at modeling the performance characteristics with a limited number of inputs for both design and off-design operations of the cycle. The dynamic model is validated with the experimental data in addition to the investigation of exhaust gas mass flow regulation that establishes a control strategy for the dynamic model.

Experimental Characterization of Losses in Bladeless Turbine Prototype

2021 ◽  
Author(s):  
Avinash Renuke ◽  
Federico Reggio ◽  
Alberto Traverso ◽  
Matteo Pascenti
Keyword(s):  
High Speed ◽  
Electrical Power ◽  
Working Fluid ◽  
Small Scale ◽  
Experimental Characterization ◽  
Scaling Effects ◽  
Low Performance ◽  
Low Sensitivity ◽  
First Time

Abstract Multi-disk bladeless turbines, also known as Tesla turbines, are promising in the field of small-scale power generation and energy harvesting due to their low sensitivity to down-scaling effects, retaining high rotor efficiency. However, low (less than 40%) overall isentropic efficiency has been recorded in the experimental literature. This article aims for the first time to a systematic experimental characterization of loss mechanisms in a 3-kW Tesla expander using compressed air as working fluid and producing electrical power through a high speed generator (40krpm). The sources of losses discussed are: stator losses, stator-rotor peripheral viscous losses, end wall ventilation losses and leakage losses. After description of experimental prototype, methodology and assessment of measurement accuracy, the article discusses such losses aiming at separating the effects that each loss has on the overall performance. Once effects are separated, their individual impact on the overall efficiency curves is presented. This experimental investigation, for the first time, gives the insight into the actual reasons of low performance of Tesla turbines, highlighting critical areas of improvement, and paving the way to next generation Tesla turbines, competitive with state of the art bladed expanders.

Experimental Investigation of a Small-Scale Orc Turbo-Generator Supported On Gas-Lubricated Bearings

2021 ◽  
Author(s):  
Kévin Rosset ◽  
Olivier Pajot ◽  
Jurg Schiffmann
Keyword(s):  
High Speed ◽  
Waste Heat ◽  
Electrical Power ◽  
Organic Rankine Cycle ◽  
Mechanical Efficiency ◽  
Load Level ◽  
Working Fluid ◽  
Small Scale ◽  
Electrical Machine ◽  
Turbo Generator

Abstract Waste heat recovery is expected to contribute to reducing CO2 emissions from trucks. Organic Rankine cycle (ORC) systems show the highest potential for this application, but still lack efficient small-scale expansion devices, in practice. A novel turbo-generator supported on gas-lubricated bearings is presented in this paper. The device combines a single-stage radial-inflow turbine and a permanent-magnet machine in a single rotating part supported on aerodynamic bearings, lubricated with the working fluid (R245fa). The oil-free expander was tested within a dedicated ORC test setup. It was driven up to its nominal speed of 100 kRPM, generated up to 2.3 kW of electrical power, and reached a peak overall efficiency of 67%. Although the prototype was not actively cooled, the mechanical losses of the rotor shaft and the iron loss of the electrical machine reached their nominal levels. Only the copper loss was at a part-load level. The electro-mechanical efficiency of the turbo-generator reached 91% and is expected to increase while testing the device at higher load. This proof of concept confirms the high-speed and low-loss potential of gas-lubricated bearings for small-scale dynamic expanders.

Nucleate Boiling Bubble Dynamics in a PDMS Microchannel With a Single Nucleation Site

2011 ◽  
Author(s):  
Haojie Wang ◽  
Xipeng Lin ◽  
David M. Christopher
Keyword(s):  
High Speed ◽  
Bubble Dynamics ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Nucleation Site ◽  
Working Fluid ◽  
Heater Surface ◽  
Short Period ◽  
Pdms Microchannel ◽  
Very High

The bubble dynamics for flow boiling in a single microchannel was experimentally studied in the present work. A platinum heater was used as the nucleation site in a 0.1 mm hydraulic diameter PDMS (Polydimethylsiloxane) microchannel with FC72 as the working fluid. A high speed camera was used to visualize the bubble dynamics. The results show that the bubbles grow much slower than predicted by standard correlations due to the very large convective heat transfer to the liquid flowing around the bubble in the microchannel. The results also show that the bubble departure frequency, heat flux and bubble departure diameter are well correlated by two dimensionless parameters that also include the effect of the properties. Finally, the results suggest very high speed dryout and rewetting of the heater surface during the bubble growth with a very short period of more complete rewetting of the heater surface when a bubble separates from the main vapor stem.

Effects of Liquid Viscosity on Bubble Growth From Submerged Orifice Plates

2017 ◽  
Author(s):  
Sanjivan Manoharan ◽  
Milind A. Jog ◽  
Raj M. Manglik
Keyword(s):  
Bubble Growth ◽  
High Speed ◽  
Bubble Dynamics ◽  
Pure Water ◽  
Air Flow ◽  
Low Flow ◽  
Glycerol Solution ◽  
Stainless Steel Capillary ◽  
Flow Rates ◽  
Water Glycerol

Experimental investigation of bubble growth from orifice plates submerged in pools of viscous liquids has been carried out using high speed videography. Conflicting effects of viscosity on ebullience have been reported in the literature. These are addressed in the present study and their range of applicability has been identified. Furthermore, the effects of chamber volume on bubble dynamics in viscous media are examined. Orifice plates made of Acrylic glass (a hydrophilic surface) with varying orifice diameters from 0.813 mm to 1.500 mm, have been utilized. Additionally, bubble dynamics from a stainless steel capillary nozzle was captured and compared with that from orifice plates. The six different liquid pools were used, viz., pure distilled water, ethylene glycol, propylene glycol, and three different aqueous glycerol solutions. The aqueous glycerol solutions varied in viscosity from 48 cP to 128 cP. The flow rate was regulated such that the isolated bubble regime was encountered. For the smaller orifices, viscosity effects were present at all flow rates and the bubbles in water-glycerol solutions were much larger than those in pure water. However, for the larger orifice sizes, water-glycerol solutions produced bubbles that were larger than those in water only at high air flow rates. For larger orifice sizes, at low flow rates, there was no increase in bubble size in highly viscous water-glycerol solutions compared to pure water. In fact, with 1.5 mm diameter plate orifice, the bubbles for 128 cP water-glycerol solution were smaller than those in pure water at low air flow rates. When chamber effects were present, the bubbles in the more viscous medium differed in shape and size from those in pure water.

Bubble Characteristics During Boiling in Microchannels

2005 ◽  
Author(s):  
Jiang-Tao Liu ◽  
Yong Tian ◽  
Xiao-Feng Peng
Keyword(s):  
Bubble Growth ◽  
High Speed ◽  
Bubble Dynamics ◽  
Solid Wall ◽  
Ccd Camera ◽  
Vapor Bubbles ◽  
Experimental Conditions ◽  
Boiling Process ◽  
Parallel Microchannels ◽  
Bubble Characteristics

A series of visualized experiments were conducted to investigate the boiling nucleation and bubble dynamics restricted within parallel microchannels on a silicon chip. The cross-section of each channel was 100 μm (W) × 100 μm (H). A high-speed CCD camera (up to 8,000 fps) was employed together with a microscope to record the boiling process. Under the present experimental conditions, the incipience of boiling was captured. The rates of bubble growth were measured at various flow and heating conditions. The interaction between vapor bubbles, vapor-liquid interface, and solid wall, was analyzed.

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