Small Diameter Effects on Internal Flow Boiling

2001 ◽  
Author(s):  
Gail E. Kendall ◽  
Peter Griffith ◽  
Arthur E. Bergles ◽  
John H. Lienhard
Keyword(s):  
Heat Transfer ◽  
Thermal Management ◽  
Nuclear Power ◽  
Power Plants ◽  
Flow Instability ◽  
Flow Boiling ◽  
Small Diameter ◽  
Internal Flow ◽  
Two Phase ◽  
Wide Range

Abstract Since the 1950’s, the research and industrial communities have developed a body of experimental data and set of analytical tools and correlations for two-phase flow and heat transfer in passages having hydraulic diameter greater than 6 mm or so. These tools include flow regime maps, pressure drop and heat transfer correlations, and critical heat flux limits, as well as strategies for robust thermal management of HVAC systems, electronics, and nuclear power plants. Designers of small systems with thermal management by phase change will need analogous tools to predict and optimize thermal behavior in the mesoscale and smaller sizes. Such systems include a wide range of devices for computation, measurement, and actuation in environments that range from office space to outer space and living systems. This paper examines important proceses that must be considered when channel diameters decrease, including flow distribution issues in single, parallel, and split flows; flow instability in parallel passages; manufacturing tolerances effects; nucleation processes; and wall conductance effects. The discussion focuses on engineering issues for the design of practical systems.

Experimental Investigation on Heat Transfer for Two-Phase Flow Under Natural Convection

2012 ◽  
Author(s):  
Milan Amižić ◽  
Estelle Guyez ◽  
Jean-Marie Seiler
Keyword(s):  
Heat Transfer ◽  
Experimental Investigation ◽  
Nuclear Power ◽  
Power Plants ◽  
Liquid Pool ◽  
Severe Accident ◽  
Bottom Plate ◽  
Interfacial Region ◽  
Two Phase ◽  
Wide Range

In the frame of severe accident research for the second and the third generation of nuclear power plants, some aspects of the concrete cavity ablation during the molten corium–concrete interaction are still remaining issues. The determination of heat transfer along the interfacial region between the molten corium pool and the ablating basemat concrete is crucial for the assessment of concrete ablation progression and eventually the basemat melt-through. For the purpose of experimental investigation of thermal-hydraulics inside a liquid pool agitated by gas bubbles, the CLARA project has been launched jointly by CEA, EDF, IRSN, GDF-Suez and SARNET. The CLARA experiments are performed using simulant materials and they reveal the influence of superficial gas velocity, liquid viscosity and pool geometry on the heat transfer coefficient between the internally heated liquid pool and vertical and horizontal pool walls maintained at uniform temperature. The first test campaign has been conducted with the smallest pool configuration (50 cm × 25 cm × 25 cm). The tests have been performed with liquids covering a wide range of dynamic viscosity from approximately 1 mPa s to 10000 mPa s. This paper presents some preliminary conclusions deduced from the experiments which involve a liquid pool with the gas injection only from the bottom plate. A comparison with existing models for the assessment of heat transfer has also been carried out.

A Scaling Analysis Inquiring Into Two-Phase Flow Reversal

2008 ◽  
Author(s):  
C. M. Rops ◽  
R. Lindken ◽  
L. F. G. Geers ◽  
J. Westerweel
Keyword(s):  
Heat Transfer ◽  
Flow Boiling ◽  
Pressure Fluctuations ◽  
Small Diameter ◽  
Flow Instabilities ◽  
Scaling Analysis ◽  
Two Phase ◽  
Large Pressure ◽  
Small Channels ◽  
Physical Phenomena

Physical processes limit the maximum achievable heat flux when miniaturising heat transfer equipment. In case of boiling heat transfer literature reports large pressure fluctuations, flow instabilities, and possible vapour backflow. The occurrence of the flow instabilities during boiling in small channels (defined by the Confinement Number, Co > 0.5) are explained by the formation of slug bubbles blocking the entire channel. These particular bubbles are likely to emerge during nucleate flow boiling in small diameter channels. Slug bubble blockage during flow boiling is investigated experimentally by creating a single hotspot in a small-diameter channel (Co∼5). For different liquid flow rates the detachment length of such a blocking slug bubble is determined. A scaling analysis offers to insight into the physical phenomena causing the flow instabilities. The position of the bubble caps as a function of time is identified as an important parameter.

Flow Boiling Heat Transfer and Two-Phase Flow Instability of Nanofluids in a Minichannel

2015 ◽  
Vol 137 (5) ◽  
Author(s):  
Leyuan Yu ◽  
Aritra Sur ◽  
Dong Liu
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
Two Phase Flow ◽  
Flow Instability ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Flow Instabilities ◽  
Phase Flow ◽  
Two Phase ◽  
Flow Boiling Heat Transfer

Single-phase convective heat transfer of nanofluids has been studied extensively, and different degrees of enhancement were observed over the base fluids, whereas there is still debate on the improvement in overall thermal performance when both heat transfer and hydrodynamic characteristics are considered. Meanwhile, very few studies have been devoted to investigating two-phase heat transfer of nanofluids, and it remains inconclusive whether the same pessimistic outlook should be expected. In this work, an experimental study of forced convective flow boiling and two-phase flow was conducted for Al2O3–water nanofluids through a minichannel. General flow boiling heat transfer characteristics were measured, and the effects of nanofluids on the onset of nucleate boiling (ONB) were studied. Two-phase flow instabilities were also explored with an emphasis on the transition boundaries of onset of flow instabilities (OFI). It was found that the presence of nanoparticles delays ONB and suppresses OFI, and the extent is correlated to the nanoparticle volume concentration. These effects were attributed to the changes in available nucleation sites and surface wettability as well as thinning of thermal boundary layers in nanofluid flow. Additionally, it was observed that the pressure-drop type flow instability prevails in two-phase flow of nanofluids, but with reduced amplitude in pressure, temperature, and mass flux oscillations.

The Effect of Nanoscale Surface Modification on Boiling Heat Transfer and Critical Heat Flux

2010 ◽  
Author(s):  
Moo Hwan Kim
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
Nuclear Power ◽  
Power Plants ◽  
Flow Boiling ◽  
Heated Surface ◽  
Pool Boiling ◽  
Working Fluid ◽  
Wetting Ability ◽  
Modified Surface

Recently, there were lots of researches about enormous CHF enhancement with the nanofluid in pool boiling and flow boiling. It is supposed the deposition of nanoparticles on the heated surface is one of main reasons. In a real application, nanofluid has a lot of problems to be used as the working fluid because of sedimentation and aggregation. The artificial surfaces on silicon and metal were developed to have the similar effect with nanoparticles deposited on the surface. The modified surface showed the enormous ability to increase CHF in pool boiling. Furthermore, under flow boiling, it had also good results to increase CHF. In these studies, we concluded that wetting ability of surface; e.g. wettability and liquid spreading ability (hydrophilic property of surface) was a key parameter to increase CHF under both pool and flow boiling. In addition, using wettability difference of surface; e.g. hydrophilic and hydrophobic, we conducted some tests of BHT (boiling heat transfer) enhancement using the oxide silicon which have micro-sized hydrophobic islands on hydrophilic surface. By using both of these techniques, we propose an optimized surface to increase both CHF and BHT. Also, the fuel surface of nuclear power plants is modified to have same effect and the results shows a good enhancement of CHF, too.

scholarly journals CFD for Subcooled Flow Boiling: Parametric Variations

2013 ◽  
Vol 2013 ◽  
pp. 1-22 ◽  
Author(s):  
Roland Rzehak ◽  
Eckhard Krepper
Keyword(s):  
Heat Flux ◽  
Power Systems ◽  
Nuclear Power ◽  
Volume Fraction ◽  
Flow Boiling ◽  
Working Fluid ◽  
Model Parameters ◽  
Two Phase ◽  
Radial Profiles ◽  
Wide Range

We investigate the present capabilities of CFD for wall boiling. The computational model used combines the Euler/Euler two-phase flow description with heat flux partitioning. Very similar modeling was previously applied to boiling water under high pressure conditions relevant to nuclear power systems. Similar conditions in terms of the relevant nondimensional numbers have been realized in the DEBORA tests using dichlorodifluoromethane (R12) as the working fluid. This facilitated measurements of radial profiles for gas volume fraction, gas velocity, liquid temperature, and bubble size. Robust predictive capabilities of the modeling require that it is validated for a wide range of parameters. It is known that a careful calibration of correlations used in the wall boiling model is necessary to obtain agreement with the measured data. We here consider tests under a variety of conditions concerning liquid subcooling, flow rate, and heat flux. It is investigated to which extent a set of calibrated model parameters suffices to cover at least a certain parameter range.

Periodic Oscillations in Natural Circulation Boiling System

2017 ◽  
Author(s):  
Xianbing Chen ◽  
Puzhen Gao ◽  
Qiang Wang ◽  
Yinxing Zhang ◽  
Jiawei Liu
Keyword(s):  
Mass Flux ◽  
Nuclear Power ◽  
Power Plants ◽  
Two Phase Flow ◽  
Flow Instability ◽  
Natural Circulation ◽  
Flow Reversal ◽  
Phase Flow ◽  
Two Phase ◽  
Periodic Oscillations

Natural circulation has been widely used in some evolutionary and innovative nuclear power plants. Natural circulation systems are susceptible to flow instabilities which are undesirable in the nuclear power devices. An experimentally investigation of two phase flow instability in up-flow boing channel under natural circulation is presented in this paper. Flow instability with and without flow reversal have been found. A pulse signal of water temperature at the inlet of the test section can be detected when the channel suffers from flow reversal. Single phase and two phase flow alternate in the channel regardless of the occurrence of flow reversal. Periodic oscillations with multiple high-order harmonic waves are confirmed by applying Fast Fourier Transform to the time traces of flow rates. Period of flow instability which is the reciprocal of the frequency with the largest amplitude in the amplitude-frequency plane are obtained. Period of flow oscillation presents a nonlinear change with the increase of mass flux. Period of flow instability increases rapidly with the increase of mass flux and decreases slowly when it reaches the maximum value.

Flow Boiling Heat Transfer and Two-Phase Flow Instability of Nanofluids in a Minichannel

2013 ◽  
Author(s):  
Leyuan Yu ◽  
Dong Liu
Keyword(s):  
Heat Transfer ◽  
Boiling Heat Transfer ◽  
Two Phase Flow ◽  
Flow Instability ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Flow Instabilities ◽  
Phase Flow ◽  
Two Phase ◽  
Flow Boiling Heat Transfer

Recent studies of single-phase convective heat transfer of nanofluids reveal that, unlike the promising hypohesis in the early works, there is no significant improvement in the overall thermal performance of nanofluids over that of the base fluids when both heat transfer and hydrodynamic characteristics are considered. Meanwhile, very few studies have been devoted to investigating two-phase heat transfer of nanofluids, and it remains inconclusive whether the same pessimistic outlook should be expected. In this work, an experimental study of forced convective flow boiling and two-phase flow was conducted for Al2O3-water nanofluids through a minichannel. General flow boiling heat transfer characteristics were measured, and the effects of nanofluids on the onset of nucleate boiling (ONB) were studied. Two-phase flow instabilities were also explored with an emphasis on the transition boundaries of onset of flow instabilities (OFI). It was found that the presence of nanoparticles delays ONB and suppresses OFI, and the extent is correlated to the nanoparticle volume concentration. These effects were attributed to the change of surface wettability and the thinning of thermal boundary layer in the nanofluid flow. Additionally, it was observed that the pressure-drop type flow instability prevails in nanofluid two-phase flow, however, the oscillation amplitudes of the pressure, temperature and mass flux measurements are reduced.

Experimental Study of Heat Transfer, Pressure Drop, and Dryout for Flow Boiling of Water in an Oil Heated Minichannel

2004 ◽  
Author(s):  
Levi A. Campbell ◽  
Satish Kandlikar
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Test Section ◽  
Two Phase Flow ◽  
Flow Boiling ◽  
Small Diameter ◽  
Constant Heat Flux ◽  
Phase Flow ◽  
Two Phase ◽  
Mass Fluxes

Heat transfer and pressure drop, are experimentally recorded for flow boiling water in a single 706 μm circular copper channel 158.75 mm long. Heat is supplied by heat transfer oil at specified temperatures to a helical channel in the test section. In contrast to other current experimental techniques for flow boiling in small diameter tubes, a uniform temperature boundary condition is employed rather than a constant heat flux condition. The principal results of these experiments are two-phase flow boiling heat transfer rates and an analysis of the time-dependent pressure drop signature during two-phase flow in a minichannel. The range of experiments includes mass fluxes of 43.8–3070 kg/m2s and wall temperatures of 100°C–171.2°C. In all cases the test section water inlet is subcooled to between 72.9°C and 99.6°C. The inlet pressures used are 1.1–230.5 kPa (gage).

Two-Phase Heat Transfer and Flow Regimes in Pin Fin-Enhanced Microgaps—Effect of Pin Spacing

2020 ◽  
Vol 143 (2) ◽  
Author(s):  
Pouya Asrar ◽  
S. Mostafa Ghiaasiaan ◽  
Yogendra K. Joshi
Keyword(s):  
Heat Transfer ◽  
High Speed ◽  
Flow Boiling ◽  
Frame Rate ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Pin Fin ◽  
Speed Flow ◽  
Wide Range ◽  
Two Phase Heat Transfer

Abstract An experimental investigation of the flow boiling of dielectric refrigerant R245fa is conducted in microgaps with enhancement features. A silicon microgap of height 200 μm populated with pin fin arrays of diameter 150 μm with spacing 200 μm in both horizontal and vertical directions is examined. For five different test conditions and in a wide range of mass flux from 781 to 5210 kg/m2s, and inlet temperatures in the range of 13–18 °C, average single-phase and two-phase heat transfer coefficients, pressure drop, and exit vapor quality are reported. Three major flow patterns are observed in the pin finned area using high-speed flow visualization at frame rate of 2229 fps: foggy, bubbly, and slug flow. Based on the experimental data, a flow regime map is constructed.

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