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.

Study on onset of nucleate boiling with screw tube for fusion reactor application

2021 ◽  
Author(s):  
Ji Hwan Lim ◽  
Minkyu Park
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
High Heat ◽  
Transfer Mechanism ◽  
High Heat Flux ◽  
Heat Transfer Mechanism ◽  
System Parameters ◽  
Smooth Tubes

Abstract The onset of nucleate boiling (ONB) is the point at which the heat transfer mechanism in fluids changes and is one of the thermo-hydraulic factors that must be considered when establishing a cooling system operation strategy. Because the high heat flux of several MW/m2, which is loaded within a tokamak, is applied under a one-side heating condition, it is necessary to determine a correlative relation that can predict ONB under special heating conditions. In this study, the ONB of a one-side-heated screw tube was experimentally analyzed via a subcooled flow boiling experiment. The helical nut structure of the screw tube flow path wall allows for improved heat transfer performance relative to smooth tubes, providing a screw tube with a 53.98% higher ONB than a smooth tube. The effects of the system parameters on the ONB heat flux were analyzed based on the changes in the heat transfer mechanism, with the results indicating that the flow rate and degree of subcooling are proportional to the ONB heat flux because increasing these factors improves the forced convection heat transfer and increases the condensation rate, respectively. However, it was observed that the liquid surface tension and latent heat decrease as the pressure increases, leading to a decrease in the ONB heat flux. An evaluation of the predictive performance of existing ONB correlations revealed that most have high error rates because they were developed based on ONB experiments on micro-channels or smooth tubes and not under one-side high heat load conditions. To address this, we used dimensional analysis based on Python code to develop new ONB correlations that reflect the influence of system parameters.

Flow Boiling Enhancement in Microchannels With Diffusion Bonded Wire Mesh

2007 ◽  
Author(s):  
Hailei Wang ◽  
Richard Peterson
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
High Heat ◽  
Wire Mesh ◽  
Working Fluid ◽  
High Heat Flux ◽  
Vapor Quality ◽  
Heating Wall

Flow boiling and heat transfer enhancement in four parallel microchannels using a dielectric working fluid, HFE 7000, was investigated. Each channel was 1000 μm wide and 510 μm high. A unique channel surface enhancement technique via diffusion bonding a layer of conductive fine wire mesh onto the heating wall was developed. According to the obtained flow boiling curves for both the bare and mesh channels, the amount of wall superheat was significantly reduced for the mesh channel at all stream-wise locations. This indicated that the nucleate boiling in the mesh channel was enhanced due to the increase of nucleation sites the mesh introduced. Both the nucleate boiling dominated and convective evaporation dominated regimes were identified. In addition, the overall trend for the flow boiling heat transfer coefficient, with respect to vapor quality, was increasing until the vapor quality reached approximately 0.4. The critical heat flux (CHF) for the mesh channel was also significantly higher than that of the bare channel in the low vapor quality region. Due to the fact of how the mesh was incorporated into the channels, no pressure drop penalty was identified for the mesh channels. Potential applications for this kind of mesh channel include high heat-flux electronic cooling systems and various energy conversion systems.

Enhanced Flow Boiling Over Open Microchannels With Uniform and Tapered Gap Manifolds

2013 ◽  
Vol 135 (6) ◽  
Author(s):  
Satish G. Kandlikar ◽  
Theodore Widger ◽  
Ankit Kalani ◽  
Valentina Mejia
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Performance ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
High Heat ◽  
High Heat Flux ◽  
Transfer Performance ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients

Flow boiling in microchannels has been extensively studied in the past decade. Instabilities, low critical heat flux (CHF) values, and low heat transfer coefficients have been identified as the major shortcomings preventing its implementation in practical high heat flux removal systems. A novel open microchannel design with uniform and tapered manifolds (OMM) is presented to provide stable and highly enhanced heat transfer performance. The effects of the gap height and flow rate on the heat transfer performance have been experimentally studied with water. The critical heat fluxes (CHFs) and heat transfer coefficients obtained with the OMM are significantly higher than the values reported by previous researchers for flow boiling with water in microchannels. A record heat flux of 506 W/cm2 with a wall superheat of 26.2 °C was obtained for a gap size of 0.127 mm. The CHF was not reached due to heater power limitation in the current design. A maximum effective heat transfer coefficient of 290,000 W/m2 °C was obtained at an intermediate heat flux of 319 W/cm2 with a gap of 0.254 mm at 225 mL/min. The flow boiling heat transfer was found to be insensitive to flow rates between 40–333 mL/min and gap sizes between 0.127–1.016 mm, indicating the dominance of nucleate boiling. The OMM geometry is promising to provide exceptional performance that is particularly attractive in meeting the challenges of high heat flux removal in electronics cooling applications.

An Experimental Investigation of Sub-Cooled Flow Boiling in Hypervapotron Cooling Configuration

2004 ◽  
Author(s):  
Peipei Chen ◽  
Barclay G. Jones ◽  
Ty A. Newell
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Flow Boiling ◽  
Experimental Studies ◽  
Nucleate Boiling ◽  
High Heat ◽  
Heat Fluxes ◽  
High Heat Flux ◽  
Subcooled Boiling ◽  
Reduced Pressure

This work reports on experimental studies to visualize nucleate boiling on the enhanced heat transfer surface of the hypervapotron for with application in the International Thermonuclear Experiment Reactor [ITER]. This research uses the simulant fluid Freon (R134A) instead of prototypic water to model the system performance. This results in much lower thermophysical conditions to represent the prototypic phenomena. By using reduced pressure, temperatures, etc, based on the critical physical properties of both working fluids, Freon and water, the dramatic drop in the level of these quantities with Freon allows the use of modest test conditions. The experiment was conducted for both saturated and subcooled boiling with different heat fluxes (from 50 to 300 kW/m2). A comparison of the heat transfer performance of finned structures and flat surfaces were examined under particular fluid conditions. The uniqueness of this work is the visualization method that allows direct observation of the subcooled boiling process of the Hypervapotron surfaces. Working with a high speed (12,000 frames per second), high fidelity digital camera with variable magnifications (from 1×–25×), the sub-cooled boiling phenomena was observed in detail. A major conclusion of this work is the existence of two separate zones linked to different energy removal efficiency in hypervapotron. Under high heat flux condition, enhanced boiling heat transfer (about 20–30% higher than flat surface) was observed for hypervapotron effect, while saturated boiling happened in the cavity, and a large portion of the region was vapor filled. The process of vapor bubble rotation in the slot appeared to be helpful to enhance energy transfer, as evidenced by an improved wetting condition on the heating surfaces.

Subcooled Flow Boiling in a Minichannel

2009 ◽  
Author(s):  
Akira Oshima ◽  
Koichi Suzuki ◽  
Chungpyo Hong ◽  
Masataka Mochizuki
Keyword(s):  
Heat Flux ◽  
High Speed ◽  
Flow Boiling ◽  
Heating Surface ◽  
Nucleate Boiling ◽  
High Heat Flux ◽  
Subcooled Flow Boiling ◽  
Maximum Heat ◽  
High Speed Video ◽  
Subcooled Flow

It has been considered that the dry-out is easy to occur in boiling heat transfer for a small channel, a mini or microchannel because the channel was easily filled with coalescing vapor bubbles. In the present study, the experiments of subcooled flow boiling of water were performed under atmospheric condition for a horizontal rectangular channel of which size is 1mm in height and 1mm in width with a flat heating surface of 10mm in length and 1mm in width placed on the bottom of the channel. The heating surface is a top of copper heating block and heated by ceramics heaters. In the high heat flux region of nucleate boiling, about 70 ∼ 80 percent of heating surface was covered with a large coalescing bubble and the boiling reached critical heat flux (CHF) by a high speed video observation. In the beginning of transition boiling, coalescing bubbles were collapsed to many fine bubbles and microbubble emission boiling was observed at higher liquid subcooling than 30K. The maximum heat flux obtained was 8MW/m2 (800W/cm2) at liquid subcooling of higher than 40K and the liquid velocity of 0.5m/s. However, the surface temperature was extremely higher than that of centimeter scale channel. The high speed video photographs indicated that microbubble emission boiling occurs in the deep transition boiling region.

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.

scholarly journals Boiling Heat Transfer Performance of Parallel Porous Microchannels

Energies ◽  
2020 ◽  
Vol 13 (11) ◽  
pp. 2970
Author(s):  
Donghui Zhang ◽  
Haiyang Xu ◽  
Yi Chen ◽  
Leiqing Wang ◽  
Jian Qu ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
High Heat ◽  
High Heat Flux ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Boiling Process

Flow boiling in microporous layers has attracted a great deal of attention in the enhanced heat transfer field due to its high heat dissipation potential. In this study, flow boiling experiments were performed on both porous microchannels and a copper-based microchannel, using water as the coolant. As the heat flux was less than 80 W/cm2, the porous microchannels presented significantly higher boiling heat transfer coefficients than the copper-based microchannel. This was closely associated with the promotion of the nucleation site density of the porous coating. With the further increase in heat flux, the heat transfer coefficients of the porous microchannels were close to those of the copper-based sample. The boiling process in the porous microchannel was found to be dominated by the nucleate boiling mechanism from low to moderate heat flux (<80 W/cm2).This switched to the convection boiling mode at high heat flux. The porous samples were able to mitigate flow instability greatly. A visual observation revealed that porous microchannels could suppress the flow fluctuation due to the establishment of a stable nucleate boiling process. Porous microchannels showed no advantage over the copper-based sample in the critical heat flux. The optimal thickness-to-particle-size ratio (δ/d) for the porous microchannel was confirmed to be between 2–5. In this range, the maximum enhanced effect on boiling heat transfer could be achieved.

scholarly journals High heat flux flow boiling of refrigerant R236fa in parallel microchannels

2019 ◽  
Vol 196 ◽  
pp. 00062
Author(s):  
Vladimir Kuznetsov ◽  
Alisher Shamirzaev ◽  
Alexander Mordovskoy
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
High Heat ◽  
Heat Fluxes ◽  
High Heat Flux ◽  
Flux Flow ◽  
Microchannel Heat Exchanger ◽  
Parallel Microchannels

This paper presents the results of an experimental study of the heat transfer during flow boiling of refrigerant R236fa in a horizontal microchannel heat sink. The experiments were performed using closed loop that re-circulates coolant. Microchannel heat exchanger that contains two microchannels with 2x0.4 mm cross-section was used as the test section. The dependence of average heat flux on wall superheat and critical heat flux were measured in the range of mass fluxes from 600 to 1600 kg/m2s and in the range of heat fluxes from 5 to 120 W/cm2. For heat flux greater than 60 W/cm2, nucleate boiling suppression has significant effect on the flow boiling heat transfer, and this leads to decrease of the heat transfer coefficient with heat flux grows.

Correlation of the Flow Pattern and Flow Boiling Heat Transfer in Microchannels

2011 ◽  
Author(s):  
Vladmir V. Kuznetsov ◽  
Alisher S. Shamirzaev ◽  
Igor A. Kozulin ◽  
Stanislav P. Kozlov
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Flow Pattern ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
High Heat Flux ◽  
Reduced Pressure ◽  
Heat Transfer Deterioration ◽  
Flow Boiling Heat Transfer

Flow boiling in microchannels is characterized by the considerable influence of capillary forces and constraint effects on the flow pattern and heat transfer. In this paper we used the flow patterns of gas-liquid flow in rectangular microchannel to explain the regularities of refrigerants flow boiling heat transfer. The characteristics of the flow such as frequency of elongated bubbles, their length, velocity of liquid and gas phases were determined by dual laser flow scanning for the upward and horizontal nitrogen-water flow in microchannels with the size of 1500×720 μm. The flow pattern boundaries were determined also and compared with extended Mishima and Ishii correlation. Flow boiling heat transfer data were obtained for vertical and horizontal microchannel heat sink with similar channels using refrigerants R21 and R134a. The data on local heat transfer coefficients were obtained in the range of mass flow rate from 33 to 190 kg/m2s, reduced pressure from 0.03 to 0.25 and heat flux from 10 to 160 kW/m2. The flow boiling modes with nucleate and convective boiling were observed as far as heat transfer deterioration at high vapor quality and high heat flux. It was found that deterioration occurs for the annular flow when nucleate boiling was suppressed in a thin liquid film, and for elongated bubble flow pattern. The mechanism of heat transfer deterioration was discussed. The model of heat transfer deterioration was used to predict the experimental data.

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