Flow Instability Study in Rectangular Channel

2010 ◽  
Author(s):  
Donghua Lu
Keyword(s):  
Mass Velocity ◽  
Flow Instability ◽  
Rectangular Channel ◽  
Density Wave ◽  
High Mass ◽  
Channel System ◽  
System Pressure ◽  
Rectangular Channels ◽  
Test Parameters ◽  
C 50

The density wave oscillation (DWO) was investigated with parallel rectangular channel test sections, which have 2mm*25mm flow cross section. Test parameters are pressure 1MPa–10MPa, mass velocity 200kg/m2s–800kg/m2, and inlet subcooling 10°C–50°C. The experimental data show that heat flux rises with high mass velocity, inlet subcooling or system pressure at the stable/unstable boundary. In them, increasing mass velocity can greatly improve stability of this two parallel channel system. Period of the oscillation becomes shorter while mass velocity rises, but when inlet subcooling is increased, it becomes longer. The comparison between data from rectangular channels and round tubes indicates they have the same trend based on the dimensionless phase change number and subcooling number.

One Dimensional Thermal-Hydraulic Stability Analysis of Supercritical Fluid Cooled Reactors

2004 ◽  
Author(s):  
Jiyun Zhao ◽  
Pradip Saha ◽  
Mujid S. Kazimi
Keyword(s):  
Flow Rate ◽  
Single Channel ◽  
Flow Instability ◽  
Density Wave ◽  
Reactor Power ◽  
Rate System ◽  
One Dimensional ◽  
Velocity Amplitude ◽  
System Pressure ◽  
Induced Velocity

A one-dimensional single-channel thermal-hydraulics model has been developed to investigate possible occurrence of density-wave instability in two U. S. Gen-IV reactors cooled by supercritical fluids, i. e., the Supercritical Water-cooled Reactor (SCWR) and Gas-cooled Fast Reactor (GFR). Water density in the SCWR core changes from 780 kg/m3, to 90 kg/m3, whereas the density of supercritical carbon-dioxide in the reference GFR changes from 155 kg/m3 to around 110 kg/m3. The standard frequency domain approach with a decay ratio of induced velocity amplitude of 0.5 has been used to determine the onset of flow instability. With suitable inlet orificing, the hot channel of SCWR has been found to be stable. Sensitivity studies show that the hot channel decay ratio reaches the critical value of 0.5 when either the reactor power is raised to 118% of full power or the core flow rate is reduced to 86% of nominal flow rate. System pressure has only a moderate effect. Detailed 3-D studies, preferably with neutronic feedback, should be carried out for the SCWR design because of its sensitivity to various important parameters. The GFR reference design has been found to be very stable since the density change in the GFR core is rather small compared to that in the SCWR design.

Theoretical Research on Two-Phase Flow Instability in Parallel Rectangular Channels Under Periodic Perturbation

2020 ◽  
Author(s):  
Libo Qian ◽  
Jian Deng ◽  
Tao Huang ◽  
Rong Cai
Keyword(s):  
Pressure Drop ◽  
Channel Model ◽  
Flow Instability ◽  
Rectangular Channel ◽  
Static Condition ◽  
Periodic Perturbation ◽  
Theoretical Research ◽  
Threshold Power ◽  
Additional Pressure ◽  
Rectangular Channels

Abstract A theoretical model for Density Wave Oscillations (DWOs) flow instability in parallel rectangular channels under periodic heaving motion is established with a lumped mathematical model based on homogenous hypothesis. The parallel rectangular channels comprise of the entrance section, the heating section, the riser section and the upper- and lower plenums, which guarantee the isobaric pressure drop condition between channels and the model consists of boiling channel model, pressure drop model, parallel channel model, additional pressure drop model generated by heaving motions, the constitutive and numerical models. The effect of periodic perturbation is introduced through additional pressure drop in the momentum equation. The model is validated with experimental data of a twin-rectangular-channel flow instability experiment under static condition. Then the flow instability in parallel-rectangular-channel system is studied under periodic perturbation and the margin of flow instability and the threshold power of the system under static condition is calculated as basis condition for comparison. The effect of the amplitude and period of perturbation is analyzed analytically and the results show that the amplitude and period of perturbation shows little effect on flow instability. While when the additional pressure difference introduced by heaving motion is comparable with that under static condition, the effect of amplitude becomes stronger. And the period of perturbation strongly effects the threshold power when it is identical to that of natural period of the system, which can be explained by resonance between the perturbation and the system. And this effect is even stronger when the asymmetric heating condition is introduced.

Experimental Study on Two-Phase Natural Circulation Flow Instability in Rod Bundle Channel Under Low Pressure Condition

2018 ◽  
Author(s):  
Kun Cheng ◽  
Sichao Tan ◽  
Zheng Liu ◽  
Tao Meng
Keyword(s):  
Flow Instability ◽  
Natural Circulation ◽  
Oscillation Amplitude ◽  
Density Wave ◽  
Low Pressure ◽  
Heating Power ◽  
Pressure Condition ◽  
Two Phase ◽  
Rod Bundle ◽  
System Pressure

An experimental investigation was conducted in a natural circulation (NC) loop to study the characteristics of two-phase flow instability under low pressure condition. A 3 × 3 rod bundle channel was used as the test section. The effects of heating power, inlet subcooling degree and system pressure on the two-phase NC flow instability types and stable boundaries were studied. The experimental results show that three typical flow conditions can occur in rod bundle channel under NC condition, which are single-phase NC flow, subcooled boiling NC flow oscillation and density wave oscillations (DWO). The oscillation amplitude and period of DWO can be enlarged by increasing the heat flux. Increasing the inlet subcooling degree can increase the marginal heating power of flow instability in NC system. The occurrence of DWO can be suppressed by increasing the system pressure. The flow instability boundary presented by the subcooling number and phase change number was also obtained in present work.

Research on Natural Circulation Flow Oscillation Experiment in Narrow Rectangular Channel

2016 ◽  
Author(s):  
Tao Zhou ◽  
Xiaolu Fang ◽  
Liang Liu ◽  
Yaoxin Wang ◽  
Zejun Xiao
Keyword(s):  
Mass Velocity ◽  
Natural Circulation ◽  
Rectangular Channel ◽  
Density Wave ◽  
Low Flow ◽  
Circulation Flow ◽  
Flow Oscillation ◽  
Flow Pulsation ◽  
Wave Instability ◽  
Oscillation Phenomenon

A narrow rectangular natural circulation experimental system is used to research flow pulsation occurred during the experiment. During the experiment, the flow pulsation phenomenon is found in narrow rectangular channels. When compared with density wave instability, it is found that the flow pulsation has certain characteristics of density wave instability, but at the same time, there will be a transient low flow phenomenon, which is a unique flow oscillation phenomenon in the narrow rectangular channel. The mechanism of flow oscillation phenomenon is analyzed, on this basis, a forecast equation of mass velocity is put forward. Forecast equation of mass velocity is used to calculate the natural circulation mass flow rate. Calculation results are in good agreement with the experimental results, which proves that the forecast equation of mass velocity is correct, provides control and prevention theory basis for natural circulation flow instability in narrow rectangular channel.

Analysis of Flow Stability Boundaries of ERVC System

2013 ◽  
Author(s):  
Fei Li ◽  
Di Jin ◽  
Xiao Jing Liu ◽  
Xu Cheng
Keyword(s):  
Flow Instability ◽  
Natural Circulation ◽  
Thermal Power ◽  
Management Strategies ◽  
Density Wave ◽  
Flow Stability ◽  
Severe Accident ◽  
Two Phase ◽  
System Pressure ◽  
Calculation Results

ERVC is widely adopted as a part of in-vessel retention (IVR) in severe accident management strategies. In this paper, two-phase flow instability in natural circulation loops of external reactor vessel cooling (ERVC) system in a large size power PWR (CAP 1700 with a thermal power 5000 MWt) is simulated and evaluated by the RELAP5 code. Under certain conditions, flow instability of ERVC system are obtained. It is a kind of density wave oscillation that occurs in non-equilibrium boiling in the heat section and void flashing in the riser at low equilibrium quality and low system pressure. The calculation results show such oscillation course clearly. And several parameters affecting the flow stability are discussed.

Experimental Research of Flow Instability Onset Conditions in a Natural Circulation System With Subcooled Boiling

2013 ◽  
Author(s):  
Liang-ming Pan ◽  
Jia Li ◽  
De-qi Chen
Keyword(s):  
Flow Instability ◽  
Natural Circulation ◽  
Stability Boundary ◽  
Density Wave ◽  
System Stability ◽  
Circulation System ◽  
Subcooled Boiling ◽  
Boundary Area ◽  
Wave Oscillation ◽  
System Pressure

With water as the working medium, this paper has investigated into the factors which affect the onset points of flow instability in the natural circulation system with subcooled boiling. Study finds that the increase of entrance subcoolling and pressure in the system can improve system stability. Increasing threshold heat flux can make the subcooling boundary area of density wave oscillation bigger. Both entrance subcooling and system pressure have significant influence on the position where ONB appears at stability boundary. According to dimensionless analysis of two-phase flow differential equations, this paper has put forward an semi-empirical formulae that can forecast the onset conditions of density wave oscillation.

scholarly journals Study of Mini Channel Heat Sink with Different Internal Configuration

2020 ◽  
Vol 319 ◽  
pp. 02004
Author(s):  
Muhammad Akif Rahman ◽  
Md Badrath Tamam ◽  
Md Sadman Faruque ◽  
A.K.M. Monjur Morshed
Keyword(s):  
Nusselt Number ◽  
Thermal Performance ◽  
Heat Sink ◽  
Rectangular Channel ◽  
Three Dimensional ◽  
Heat Sinks ◽  
Maximum Temperature ◽  
Rectangular Channels ◽  
Flow Through ◽  
Internal Configuration

In this paper a numerical analysis of three-dimensional laminar flow through rectangular channel heat sinks of different geometric configuration is presented and a comparison of thermal performance among the heat sinks is discussed. Liquid water was used as coolant in the aluminum made heat sink with a glass cover above it. The aspect ratio (section height to width) of rectangular channels of the mini-channel heat sink was 0.33. A heat flux of 20 W/cm2 was continuously applied at the bottom of the channel with different inlet velocity for Reynold’s number ranging from 150 to 1044. Interconnectors and obstacles at different positions and numbers inside the channel were introduced in order to enhance the thermal performance. These modifications cause secondary flow between the parallel channels and the obstacles disrupt the boundary layer formation of the flow inside the channel which leads to the increase in heat transfer rate. Finally, Nusselt number, overall thermal resistance and maximum temperature of the heat sink were calculated to compare the performances of the modified heat sinks with the conventional mini channel heat sink and it was observed that the heat sink with both interconnectors and obstacles enhanced the thermal performance more significantly than other configurations. A maximum of 36% increase in Nusselt number was observed (for Re =1044).

An Experimental Investigation of Flow Boiling Characteristics in a Single Transparent Heated Microtube

2009 ◽  
Author(s):  
Osamu Kawanami ◽  
Shih-Che Huang ◽  
Kazunari Kawakami ◽  
Itsuro Honda ◽  
Yousuke Kawashima ◽  
...  
Keyword(s):  
Heat Flux ◽  
Wall Temperature ◽  
Mass Velocity ◽  
Tube Wall ◽  
Flow Boiling ◽  
High Mass ◽  
Test Fluid ◽  
High Heat Flux ◽  
Gold Plating ◽  
Heated Tube

In the present study, flow boiling in a transparent heated microtube having a diameter of 1 mm was investigated in detail. The transparent heated tube was manufactured by the electroless gold plating method. The enclosed gas-liquid interface could be clearly recognized through the tube wall, and the inner wall temperature measurement and direct heating of the film were simultaneously conducted by using the tube. Deaerated and deionized water that was subcooled temperature of 15 K was used as a test fluid, and constant and stable mass velocities of 50, 100, and 200 kg/m2s were provided by using a twin plunger pump. Among our experimental results, a vapor bubble grew up in a direction opposite the flow at a low heat flux and low mass velocities; however, this flow pattern was not observed at a high mass velocity of 200 kg/m2s. Under the conditions of G = 50 kg/m2s and high heat flux, the liquid film surrounding an elongated bubble near the heated tube wall occasionally thickened partially. The inner wall temperature exhibited large random oscillations in this regime; however, the visual observation revealed that dry-patches did not occur. The mass velocity had a negligible effect on the boiling heat transfer except in the counter-growth bubble flow regime.

Research on Two-Phase Flow Instability in Parallel Multi-Channel Under Rolling Motion Condition

2008 ◽  
Author(s):  
Y. J. Zhang ◽  
G. H. Su ◽  
S. Z. Qiu ◽  
X. B. Yang
Keyword(s):  
Two Phase Flow ◽  
Flow Instability ◽  
Stability Boundary ◽  
Channel Number ◽  
Rolling Motion ◽  
Phase Flow ◽  
Two Phase ◽  
Channel System ◽  
Motion Condition ◽  
High Equilibrium

Two-phase flow instability of the parallel multi-channel system has been studied under rolling motion condition in this paper. Based on the homogeneous flow model with considering the rolling motion condition, the parallel multi-channel model is established by using the control volume integrating method. Gear method is used to solve the system equations. The influences of the inlet and upward sections and the heating power on the flow instability under rolling motion condition have been analyzed. The marginal stability boundary (MSB) under rolling motion condition is obtained and the unstable regions occur in both low and high equilibrium quality regions. The region with low inlet subcooling is also instable. In high equilibrium quality region, the multiplied period phenomenon is found and the chaotic phenomenon appears at the MSB. The oscillation part of mass flow rate (amplitude) may be averaged into other channels so that the influence of rolling motion is weakened. But the stability of multi-channel system is independent of the channel number and the increase of the channel number could only make the amplitude more uniformity in channels.

scholarly journals Spectro-interferometric observations of a sample of Be stars

2019 ◽  
Vol 621 ◽  
pp. A123 ◽  
Author(s):  
Y. R. Cochetti ◽  
C. Arcos ◽  
S. Kanaan ◽  
A. Meilland ◽  
L. S. Cidale ◽  
...  
Keyword(s):  
Critical Velocity ◽  
Velocity Ratio ◽  
Density Wave ◽  
Wave Structure ◽  
Maximum Size ◽  
Rotation Velocity ◽  
Variable Stars ◽  
High Mass ◽  
Be Stars ◽  
Infrared Excess

Context. Be stars are rapid rotators surrounded by a gaseous disk envelope whose origin is still under debate. This envelope is responsible for observed emission lines and large infrared excess. Aims. To progress in the understanding of the physical processes involved in the disk formation, we estimate the disk parameters for a sample of Be stars and search for correlations between these parameters and stellar properties. Methods. We performed spectro-interferometric observations of 26 Be stars in the region of the Brγ line to study the kinematical properties of their disks through the Doppler effect. Observations were performed at the Paranal observatory with the VLTI/AMBER interferometer. This instrument provides high spectral (R ≃ 12 000) and high spatial (θmin = 4 mas) resolutions. Results. We modeled 18 Be stars with emission in the Brγ line. The disk kinematic is described by a quasi-Keplerian rotation law, with the exception of HD 28497 that presents a one-arm density-wave structure. Using a combined sample, we derived a mean value for the velocity ratio V̅/V̅c = 0.75 (where Vc is the critical velocity), and found that rotation axes are probably randomly distributed in the sky. Disk sizes in the line component model are in the range of 2–13 stellar radii and do not correlate with the effective temperature or spectral type. However, we found that the maximum size of a stable disk correlates with the rotation velocity at the inner part of the disk and the stellar mass. Conclusions. We found that, on average, the Be stars of our combined sample do not rotate at their critical velocity. However, the centrifugal force and mass of the star defines an upper limit size for a stable disk configuration. For a given rotation, high-mass Be stars tend to have more compact disks than their low-mass counterparts. It would be interesting to follow up the evolution of the disk size in variable stars to better understand the formation and dissipation processes of their circumstellar disks.

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