Multi-Jet Flows Underneath Small Bubble in Nucleate Boiling of Subcooled Self-Rewetting Fluid

2013 ◽  
Author(s):  
Leping Zhou ◽  
Yuanyuan Li ◽  
Longting Wei ◽  
Xiaoze Du
Keyword(s):  
High Speed ◽  
Marangoni Convection ◽  
Heated Surface ◽  
Ccd Camera ◽  
Jet Flow ◽  
Nucleate Boiling ◽  
Jet Flows ◽  
Wall Region ◽  
Heating Wire ◽  
Butanol Solution

Jet flow phenomenon is important in enhanced nucleate boiling heat transfer processes and applications. When heater sizes scale down, jet flow can be observed due to the Marangoni convection around bubbles staying on microscale heated surface. In this paper, two fluids were employed for comparing and demonstrating the effect of Marangoni convection on bubble behaviors on micro heating wire. One was ultrapure water and the other was aqueous n-butanol solution, a self-rewetting fluid. Bubble-top jet flow for water and multi-jet flows for n-butanol solution were observed around a platinum micro heating wire by high speed CCD camera. Corresponding numerical simulation proved that it is the Marangoni convection that attracts the sub-cooled water to flow from the super-heated microlayer at the bottom to the top of a stationary bubble. For n-butanol solution, however, the Marangoni convection can induce it to flow oppositely, which causes the subcooled solution to flow onto the heated surface. The simulation for the solution is in good agreement with the experiment where the subcooled liquid nears the bubble-top flow towards the bottom of bubble or the heated surface and hence the multi-jet flows occur. The multi-jet flows can sustain for a long period and cause transient chaos at the super-heated thin liquid layer near the heated surface. The temperature around the bubble presented sharp temperature gradient and the velocity in the near-wall region is almost vertical to the wall. The experimental and numerical studies on the effect of surface tension and thus Marangoni convection are crucial to the mechanisms of subcooled nucleate boiling of fluids.

Subcooled Nucleate Boiling of Alumina Nanofluid With/Without n-Butanol as Surfactant

2013 ◽  
Author(s):  
Leping Zhou ◽  
Longting Wei ◽  
Xiaoze Du
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
High Speed ◽  
Heated Surface ◽  
Jet Flow ◽  
Nucleate Boiling ◽  
Wall Region ◽  
Al2o3 Nanoparticles ◽  
Nanoparticle Deposition ◽  
Alumina Nanofluid

Nucleate boiling process in nanofluids is important because of its potential in enhanced heat transfer. However, it is difficult to observe the boiling phenomenon due to the indistinct image. In this investigation, stable nanofluids was prepared by α-Al2O3 nanoparticles, 30 nm in diameter, and ultrapure water. The bubble behaviors in water were observed by high-speed CCD camera. Unique bubble sweeping phenomenon, existing in the upper and/or lower part of the heated wire, emerged due to the existence of nanoparticles. The experiment shows that the bubble-top jet flow phenomenon only exists when the small bubble returned to the heated surface, which demonstrates that it was the vertical Marangoni convection along the bubble interface that induced the jet flow. Meanwhile, flocculent clustering of nanoparticles can be observed to swirl at the bubble-bottom for low-concentration nanofluid, when the heat flux was relatively small. The SEM images of the nanoparticle deposition layers indicated increased thermocapillarity, but it seemed to delay the detachment of small bubbles from the heated surface. While n-butanol was included as surfactant, it promoted the nanoparticle deposition for low heat flux condition. The bubble behaviors were consistent with those of pure fluids and no bubble circling phenomenon was observed. The boiling curves were then depicted for alumina nanofluid with or without n-butanol. The boiling heat transfer in water was enhanced with increasing nanoparticle concentration. The boiling curves shifted right when increased the surfactant concentration in the nanofluid. It appeared that the surfactant-induced inhibited bubble growth and enhanced nanoparticle clustering in the near-wall region were the main reason for the shifting.

Nucleate Boiling From Micro-Machined Surfaces

2003 ◽  
Author(s):  
Adrian M. Holland ◽  
Colin P. Garner
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Imaging System ◽  
Laser System ◽  
Ccd Camera ◽  
Nucleate Boiling ◽  
Heat Fluxes ◽  
Frame Rate ◽  
High Speed Imaging ◽  
Machined Surfaces

This paper discusses the production and use of laser-machined surfaces that provide enhanced nucleate boiling and heat transfer characteristics. The surface features of heated plates are known to have a significant effect on nucleate boiling heat transfer and bubble growth dynamics. Nucleate boiling starts from discrete bubbles that form on surface imperfections, such as cavities or scratches. The gas or vapours trapped in these imperfections serve as nuclei for the bubbles. After inception, the bubbles grow to a certain size and depart from the surface. In this work, special heated surfaces were manufactured by laser machining cavities into polished aluminium plates. This was accomplished with a Nd:YAG laser system, which allowed drilling of cavities of a known diameter. The size range of cavities was 20 to 250 micrometers. The resulting nucleate pool boiling was analysed using a novel high-speed imaging system comprising an infrared laser and high resolution CCD camera. This system was operated up to a 2 kHz frame rate and digital image processing allowed bubbles to be analysed statistically in terms of departure diameter, departure frequency, growth rate, shape and velocity. Data was obtained for heat fluxes up to 60 kW.m−2. Bubble measurements were obtained working with water at atmospheric pressure. The surface cavity diameters were selected to control the temperature at which vapour bubbles started to grow on the surface. The selected size and spacing of the cavities was also explored to provide optimal heat transfer.

Numerical Study of Marangoni Convection around a Single Vapor Bubble during Pool Boiling

2014 ◽  
Vol 624 ◽  
pp. 262-266
Author(s):  
Chun Long Xu ◽  
Yong Chao Shi ◽  
Xiao Xing Jin ◽  
Zuo Sheng Lei ◽  
Yun Bo Zhong ◽  
...  
Keyword(s):  
Vapor Bubble ◽  
Marangoni Convection ◽  
Numerical Study ◽  
Heated Surface ◽  
Nucleate Boiling ◽  
Field Method ◽  
Phase Field Method ◽  
Heated Wall ◽  
Bubble Behavior ◽  
Earth Gravity

Boiling is known to be a very efficient mode of heat transfer in earth gravity, however, in microgravity bubble behavior is different because the buoyancy effects are replaced by surface tension effects such as Marangoni convection. The modeling of nucleate boiling with the effect of Marangoni convection in 0 g is accomplished by using Phase Field Method. Numerical simulation is carried out of single nucleating vapor bubble on a heated wall with and without Marangoni convection. The results show that the flow field consists of a major vortex that recirculates colder fluid from the upper region, pulling it toward the hot surface to the point where the bubble meets the heated surface. This type of flow pattern has been observed in various experiments.

Highly Transient High Speed Jet Flow From Different Orifice Geometries

2020 ◽  
Author(s):  
Nicholas Findanis
Keyword(s):  
High Speed ◽  
Jet Flow ◽  
Compressed Air ◽  
Jet Flows ◽  
Reference Case ◽  
Transient Nature ◽  
Cleaning System ◽  
Type Flow ◽  
Base Reference ◽  
Pulse Jet

Abstract The majority of the many types of different industrial flows are not in the laminar flow regime, but rather these flows are well beyond laminar and continue to exceed the turbulent flow transition criterion for internal and free shear flows to be fully turbulent and highly unsteady involving the transfer of fluid through circular conduits or round pipes as well as other conduit geometries including the issuing of jet flows into some type of ambient environment. High speed jet flows have a wide range of applications in many areas of engineering. The understanding of jet flow theory has progressed substantially. However, there is a very little investigation into the transient nature of the high-speed jet flows and how the structure of these jet flows differs according to the geometry of the orifice out of which the jet flow emanates. The transient nature of these jet flows allows the applications into which they are installed to be optimized according to the characteristics of the jet flow and the configuration of the system. The focus of the present work is to characterize transient high speed jet flows from the differing orifice nozzle geometries and the introduction of a swirling motion into the jet flow and how this affects the characteristics of the jet flow from the reference jet flow that is free from swirling motion momentum. Another important but niche or specialised jet flow application is in reverse pulse-jet (RPJ) cleaning systems, of which is the focus application of the present work. A typical RPJ cleaning system consists of three main components: compressed air supply, valve and blowtube. The blowtube is the name given to the pipe connected to the valve and configured with a number of outlets or orifices where the flow exits into a plenum thereby entraining and inducting flow into a filter that is aligned with the orifice from which the jet flow issues. In the present work, improvements are sought from the blowtube or more specifically the exit pipe orifices for a more efficient operation of a well-designed cleaning system. The present paper will discuss and compare the flow through a number of different orifice geometries for the type flow that is typically experienced in this type of application. The operation of a single event or an actuation of the pulse-jet valve, is extremely rapid; typically approximately 300 ms. The valve is actuated and the diaphragm moves and allows the compressed air to travel from the pressure vessel or header tank through the valve past the valve seat into the blowtube and exits through plain orifices or nozzles. The extremely rapid event generates highly transient, highly turbulent free shear jet type flow from either the plain orifice or nozzle with a circular orifice geometry through which the flow exits. Advancements made by the author in subsonic flows and high-speed gas dynamic flows could provide not only improvements to the flow but further insight to the physics of high speed flows in particular around pipe exit orifices. This investigative study of the jet flow was based upon a computational analysis. It was shown that the base reference case of the jet flow that was solely a non-swirling flow although the jet flow was highly transient in nature that the centreline velocity of the jet flow had variability from the inner core to the outer extremities of the jet flow. The comparison of the base reference case with swirling jet flow will produce a longer coherent jet flow using the different orifice geometries. The stability of the jet flow was improved with the introduction of the swirling motion to the jet flow. Future developments of the transient nature of the jet flow will include experimental studies to verify the flow control methods that were used in the swirling jet flow cases.

Dynamics of a Water Droplet on the Heated Surface of Nano- and Micro-Structures

2012 ◽  
Author(s):  
Seol Ha Kim ◽  
Ho Seon Ahn ◽  
Joonwon Kim ◽  
Moo Hwan Kim
Keyword(s):  
Water Droplet ◽  
High Speed ◽  
Zirconium Alloy ◽  
Liquid Droplet ◽  
Heated Surface ◽  
Nucleate Boiling ◽  
High Speed Photography ◽  
Complete Wetting ◽  
Structured Surfaces ◽  
Bare Surface

In this study, we investigated the dynamic behavior of a water droplet near the Leidenfrost point (LFP) of bare and modified zirconium alloy surfaces with bundles of nanotubes (∼10–100 nm) or micro mountain-like structures using high-speed photography. A deionized water droplet (6 μL) was dropped onto the sample surfaces (20 × 25 × 0.7 mm) that were heated to temperatures ranging from 250°C to the LFP. The modified zirconium alloy surfaces showed complete wetting and well-spread features at room temperature due to strong liquid spreading by the structure. The meniscus of the liquid droplet on the structured surface experienced more vigorous dynamics with intensive nucleate boiling, compared with the clean, bare surface. The cutback phenomenon was observed on the bare surface; however, the structured surfaces showed a water droplet “burst”. We observed that the LFPs were 449°C, 522°C, and 570°C, corresponding to the bare, micro-, and nano-structures, respectively.

Experimental Study of the Vapor Film Behavior on a Highly Heated Surface Immersed Into Subcooled Water

2010 ◽  
Author(s):  
Kirill I. Belov ◽  
Yury P. Ivochkin ◽  
Konstantin G. Kubrikov ◽  
Alexandr A. Oksman ◽  
Sergei N. Vavilov ◽  
...  
Keyword(s):  
High Speed ◽  
Heated Surface ◽  
Time Lag ◽  
Experimental Studies ◽  
Saturation Temperature ◽  
Heating Surface ◽  
Nucleate Boiling ◽  
Vapor Film ◽  
Pressure Pulses ◽  
Film Collapse

Results of the experimental studies of the vapor film behavior on a highly heated hemispherical surface immersed into water subcooled relative to the saturation temperature are presented. Transition from film to nucleate boiling was studied with the use of high-speed videocamera. Temperature characteristics of the vapor-film collapse, pressure pulses, acoustic effects, and vapor-film thickness were measured, as well. The decisive influence of the material and condition of the heating surface and the degree of water subcooling on the mode of transition (explosion-like, quiet, or intermediate one) was confirmed. The explosion-like mode of the vapor-film collapse is accompanied by ejection of vapor jets (single or multiple) and pressure pulses with an amplitude of up to 1 MPa. A structure of the pulse packs under multiple jets ejection was investigated. Synchronous measurements of the pressure pulses and an area of the direct cold liquid contact with a hot surface made it possible to determine a time delay between an instant of the contact and the pressure pulse. Typical value of this time lag was several tens microseconds. The dependence of the pulse frequency and the number of pulses on the hemisphere temperature was obtained.

Multi-jet flows and bubble emission during subcooled nucleate boiling of aqueous n-butanol solution on thin wire

2014 ◽  
Vol 58 ◽  
pp. 1-8 ◽  
Author(s):  
Leping Zhou ◽  
Yuanyuan Li ◽  
Longting Wei ◽  
Xiaoze Du ◽  
Yongping Yang ◽  
...  
Keyword(s):  
Nucleate Boiling ◽  
Jet Flows ◽  
Thin Wire ◽  
Butanol Solution

Flow Visualization in Boiling Water Flows Leading Up to Departure From Nucleate Boiling in an Electrically-Heated Duct

2009 ◽  
Author(s):  
Helene A. Krenitsky ◽  
Evan T. Hurlburt ◽  
Larry B. Fore ◽  
Paul T. McKeown ◽  
Richard B. Williams
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Flow Visualization ◽  
Test Section ◽  
High Speed ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
High Heat Flux ◽  
Wall Region ◽  
Data Set

A fundamental departure from nucleate boiling (DNB) flow visualization experiment was designed to obtain a better understanding of flow boiling by visually capturing the mechanisms leading up to and including DNB for subcooled vertical flow boiling. At the critical heat flux (CHF) the heat transfer coefficient between the wall and fluid is greatly reduced, entering an inefficient heat transfer region that can cause a rapid increase in wall temperature. Most of the visual data on DNB in the open literature comes from experiments conducted with refrigerants or with water at relatively low pressure. One goal of this test was to capture high-quality photographs leading up to DNB for water at higher pressures, higher mass fluxes, and larger inlet subcooling than most of the data in the open literature. The fundamental DNB experiment consisted of three different run stages: incipient boiling, subcooled boiling, and CHF runs, which were intended to capture the behavior leading up to and including a departure from nucleate boiling crisis. At high heat flux conditions, the steep temperature and refractive-index gradients in the water near the wall act like lenses and bend the light away from the wall, which is the region of interest for discerning the DNB mechanism. By frosting the inner surface of the window on the light source side, the nearly collimated light was diffused as it entered the test section and enabled better visualization near the wall region. A high speed camera was used in testing. A typical run consisted of a 2 second image data set, with a resolution of 512 by 160 pixels, at 10,000 frames per second. Three excursive CHF runs were achieved, the last of which melted the test section. The trigger function on the camera captured images from before and after the power trip for the last CHF run. A trend can be seen of an increasing two-phase friction factor with power that begins to increase more rapidly at test section powers greater than 64.5kW. The 1995 Groenevel, et al. (1996) look-up table proved to be a good estimate of the heat flux at DNB.

Experimental Observation of Jet Flow and Liquid Exploding Emission From Mini/Micro Tubes During Boiling

2008 ◽  
Author(s):  
D. Wu ◽  
X. F. Peng ◽  
B. X. Wang
Keyword(s):  
Thin Liquid Film ◽  
Temperature Drop ◽  
Nucleate Boiling ◽  
High Heat ◽  
Heat Fluxes ◽  
Experimental Investigations ◽  
Jet Flows ◽  
Bubble Generation ◽  
Heating Wire

A series of experimental investigations were conducted to visually observe the fundamental features of boiling nucleation of water in micro capillary tubes covered with transparent metal film and ethanol in mini tubes winded with a heating wire as a heater. Normally, boiling nucleation and/or bubble generation were not observed even at very high heat fluxes compared with usual nucleate boiling situations. In mini tubes violent jet flows formed and played an important role of triggering the emission, while in micro tubes the liquid and/or vapor was instantaneously emitted as the applied heat flux reached a certain high value, rather than initialing nucleate boiling and jet flows which was much weaker than exploding emission. After the exploding emission, an ultra thin liquid film formed on the tube wall and evaporated intensely, causing a perceptible wall temperature drop.

Highly Transient High Speed Jet Flow From Different Orifice Geometries

2021 ◽  
Author(s):  
Nicholas Findanis
Keyword(s):  
High Speed ◽  
Jet Flow ◽  
Compressed Air ◽  
Jet Flows ◽  
Reference Case ◽  
Transient Nature ◽  
Cleaning System ◽  
Type Flow ◽  
Base Reference ◽  
Pulse Jet

Abstract The majority of the many types of different industrial flows are not in the laminar flow regime, but rather these flows are well beyond laminar and continue to exceed the turbulent flow transition criterion for internal and free shear flows to be fully turbulent and highly unsteady involving the transfer of fluid through circular conduits or round pipes as well as other conduit geometries including the issuing of jet flows into some type of ambient environment. High speed jet flows have a wide range of applications in many areas of engineering. The understanding of jet flow theory has progressed substantially. However, there is a very little investigation into the transient nature of the high-speed jet flows and how the structure of these jet flows differs according to the geometry of the orifice out of which the jet flow emanates. The transient nature of these jet flows allows the applications into which they are installed to be optimized according to the characteristics of the jet flow and the configuration of the system. The focus of the present work is to characterize transient high speed jet flows from the differing orifice nozzle geometries and the introduction of a swirling motion into the jet flow and how this affects the characteristics of the jet flow from the reference jet flow that is free from swirling motion momentum. Another important but niche or specialised jet flow application is in reverse pulse-jet (RPJ) cleaning systems, of which is the focus application of the present work. A typical RPJ cleaning system consists of three main components: compressed air supply, valve and blowtube. The blowtube is the name given to the pipe connected to the valve and configured with a number of outlets or orifices where the flow exits into a plenum thereby entraining and inducting flow into a filter that is aligned with the orifice from which the jet flow issues. In the present work, improvements are sought from the blowtube or more specifically the exit pipe orifices for a more efficient operation of a well-designed cleaning system. The present paper will discuss and compare the flow through a number of different orifice geometries for the type flow that is typically experienced in this type of application. The operation of a single event or an actuation of the pulse-jet valve, is extremely rapid; typically approximately 300 ms. The valve is actuated and the diaphragm moves and allows the compressed air to travel from the pressure vessel or header tank through the valve past the valve seat into the blowtube and exits through plain orifices or nozzles. The extremely rapid event generates highly transient, highly turbulent free shear jet type flow from either the plain orifice or nozzle with a circular orifice geometry through which the flow exits. Advancements made by the author in subsonic flows and high-speed gas dynamic flows could provide not only improvements to the flow but further insight to the physics of high speed flows in particular around pipe exit orifices. This investigative study of the jet flow was based upon a computational analysis. It was shown that the base reference case of the jet flow that was solely a non-swirling flow although the jet flow was highly transient in nature that the centreline velocity of the jet flow had variability from the inner core to the outer extremities of the jet flow. The comparison of the base reference case with swirling jet flow will produce a longer coherent jet flow using the different orifice geometries. The stability of the jet flow was improved with the introduction of the swirling motion to the jet flow. Future developments of the transient nature of the jet flow will include experimental studies to verify the flow control methods that were used in the swirling jet flow cases.

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