Investigation of Critical Heat Flux in Single Fuel Pin With and Without Wire Spacer

2014 ◽  
Author(s):  
Tri Dan Le ◽  
Noriaki Inaba ◽  
Minoru Takahashi
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Inlet Temperature ◽  
Small Range ◽  
Two Phase ◽  
Mass Fluxes ◽  
Fuel Pin ◽  
High Conversion Ratio ◽  
Phase Water ◽  
Tight Lattice

Light water reactor could have fast neutron spectrum with high conversion ratio nearly equal unity by using tight lattice fuel assembly with wire spacer. There were some previous about critical heat flux for tight lattice but it were not focused on small range of qualities and also not directly using water as a coolant. We experimentally simulated vertical single fuel rod geometry with and without a wire spacer by using an electrically heated stainless steel rod. The rod is cooled by single or two-phase water in vertical up flow (from the bottom to the top) depending on the electrical input. We determined the critical heat flux for this system by varying the inlet temperature from 333 to 373 K and mass fluxes from 205 to 410 kg/m2s. The result show the critical heat flux (CHF) data in two phase flow condition base on inlet and outlet condition in both case of heater pin with and without wire. The CHF values were higher with wire than without wire due to the effect of wire and spiral flow.

Critical Heat Flux in Microchannels With an Adjustable Inlet Orifice

2012 ◽  
Author(s):  
Brent A. Odom ◽  
Carlos A. Ortiz ◽  
Patrick E. Phelan
Keyword(s):  
Heat Flux ◽  
Pressure Drop ◽  
Critical Heat Flux ◽  
Mass Flux ◽  
Cooling Capacity ◽  
Hydraulic Diameter ◽  
Low Flow ◽  
Two Phase ◽  
Mass Fluxes ◽  
Inlet Conditions

The benefits of eliminating instabilities in two-phase microchannel flow with inlet orifices come with costs. This study describes the tradeoffs between microchannels with and without inlet orifices, focusing on results from critical heat flux data obtained for various orifice sizes and mass fluxes. An adjustable inlet orifice controlled with a micrometer was placed in front of an array of 31 parallel microchannels each with a hydraulic diameter of 0.235 mm and a length of 1.33 cm. For mass fluxes ranging from 186 kg m−2 s−1 to 847 kg m−2 s−1, critical heat flux (CHF) data were obtained for 7 different orifice sizes. For low flow rates that provided a low quality saturated inlet condition, the difference in CHF values was found to be minimal between open and almost closed orifice conditions. The smallest orifice achieved a CHF value of 5 W cm−2 less than the largest orifice size for a mass flux of 186 kg m−2 s−1, and 7 W cm−2 less for a mass flux of 433 kg m−2 s−1. For mass fluxes higher than 433 kg m−2 s−1, subcooled conditions were present at the orifice inlet, and the highest CHF values occurred with an orifice hydraulic diameter of 35 percent of fully open. For the higher mass flux cases, orifice sizes in the range of 1.8 percent to 28 percent of fully open caused CHF to occur at lower values than less restrictive orifice sizes. This was due to loss of cooling capacity from rapid pressure drop through the orifice. Slightly higher average channel pressures also decrease the refrigerant’s latent heat of vaporization. For the orifice sizes from 35 to 70 percent of unrestricted flow, a very minimal increase in pressure drop over fully open inlet conditions occurred and the general trend was higher CHF values. Very small inlet orifices are beneficial for steady state conditions that do not approach CHF; however, overly restricting the flow at the inlet to microchannels reduces cooling capacity significantly and will cause early onset of CHF. A slightly restrictive inlet orifice will increase CHF.

Development of Critical Heat Flux Correlation in Circular Pipe Sections With Vertically Downward Two-Phase Flows by Numerical Analysis and Comparison With Experimental Correlation

2020 ◽  
Author(s):  
Rajeshwar Sripada ◽  
Vasudeva Rao Veeredhi ◽  
Siva Subrahmanyam Mendu
Keyword(s):  
Heat Flux ◽  
Numerical Analysis ◽  
Critical Heat Flux ◽  
Failure Modes ◽  
Experimental Investigations ◽  
Inlet Temperature ◽  
Two Phase Flows ◽  
Two Phase ◽  
Flow Rates ◽  
Cooling Effects

Abstract Critical heat flux (CHF) and premature tube burnout are the common failure modes observed in steam water two-phase flows. Unlike the vertically upward two-phase flows, the vertically downward two-phase flows pose significant challenges including two-phase flow instabilities and premature tube burnout arising due to competing behavior between the buoyancy effects on vapor bubble and momentum and gravitational force acting on the liquid. Experimental investigations were conducted previously to understand the CHF at atmospheric pressures. There were very limited number of numerical analysis conducted in vertically downward flows using commercially available software and at such low-pressure conditions. In the current investigations, numerical simulations were carried with commercially available computational fluid dynamics software Fluent for vertically downward two-phase flows up to pressures of 5 bar. The magnitude of CHF from numerical investigations was compared with the experimental results conducted in house up to 5 bar and including the sub-cooling effects. The numerical results tend to agree with the experimental data at lower flow rates and at all pressures considered, but tend to deviate significantly at higher flow rates and at all pressures. Finally, A CHF correlation is proposed as a function of mass flux, inlet temperature and pressure. The proposed CHF correlations fits in with an average deviation of 16% and a standard deviation of 21%.

scholarly journals Determination of Pool Boiling Critical Heat Flux Enhancement in Nanofluids

2007 ◽  
Author(s):  
Bao H. Truong
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Pool Boiling ◽  
Nucleate Boiling ◽  
Porous Coating ◽  
Transfer Coefficients ◽  
Two Phase ◽  
Sem Images ◽  
Heat Transfer Coefficients ◽  
Nucleate Boiling Heat Transfer

Nanofluids are engineered colloids composed of nano-size particles dispersed in common fluids such as water or refrigerants. Using an electrically controlled wire heater, pool boiling Critical Heat Flux (CHF) of Alumina and Silica water-based nanofluids of concentration less than or equal to 0.1 percent by volume were measured. Silica nanofluids showed a CHF enhancement up to 68% and there seems to be a monotonic relationship between the nanoparticle concentration and the magnitude of enhancement. Alumina nanofluids had a CHF enhancement up to 56% but the peak occurred at the intermediate concentration. The boiling curves in nanofluid were found to shift to the left of that of water and correspond to higher nucleate boiling heat transfer coefficients in the two-phase flow regime. Scanning Electron Microscopy (SEM) images show a porous coating layer of nanoparticles on wires subjected to nanofluid CHF tests. These coating layers change the morphology of the heater’s surface, and are responsible for the CHF enhancement. The thickness of the coating was estimated using SEM and was found ranging from 3.0 to 6.0 micrometers for Alumina, and 3.0 to 15.0 micrometers for Silica.

Heat Transfer Characteristics and Flow Pattern Visualization for Flow Boiling in a Vertical Narrow Microchannel

2018 ◽  
Author(s):  
Kan Zhou ◽  
Junye Li ◽  
Zhao-zan Feng ◽  
Wei Li ◽  
Hua Zhu ◽  
...  
Keyword(s):  
Heat Flux ◽  
Flow Pattern ◽  
Flow Boiling ◽  
Nucleate Boiling ◽  
Working Fluid ◽  
Inlet Temperature ◽  
Lower Mass ◽  
Subcooled Flow Boiling ◽  
Mass Fluxes ◽  
Subcooled Flow

For improving the functionality and signal speed of electronic devices, electronic components have been miniaturized and an increasing number of elements have been packaged in the device. As a result there has been a steady rise in the amount of heat necessitated to be dissipated from the electronic device. Recently microchannel heat sinks have been emerged as a kind of high performance cooling scheme to meet the heat dissipation requirement of electronics packaging, In the present study an experimental study of subcooled flow boiling in a high-aspect-ratio, one-sided heating rectangular microchannel with gap depth of 0.52 mm and width of 5 mm was conducted with deionized water as the working fluid. In the experimental operations, the mass flux was varied from 200 to 400 kg/m2s and imposed heat flux from 3 to 20 W/cm2 while the fluid inlet temperature was regulated constantly at 90 °C. The boiling curves, flow pattern and onset of nucleate boiling of subcooled flow boiling were investigated through instrumental measurements and a high speed camera. It was found that the slope of the boiling curves increased sharply once the superheat needed to initiate the onset of nucleate boiling was attained, and the slope was greater for lower mass fluxes, with lower superheat required for boiling incipience. As for the visualization images, for relatively lower mass fluxes the bubbles generated were larger and not easy to depart from the vertical upward placed narrow microchannel wall, giving elongated bubbly flow and reverse backflow. The thin film evaporation mechanism dominated the entire test section due to the elongated bubbles and transient local dryout as well as rewetting occurred. Meanwhile the initiative superheat and heat flux of onset of nucleate boiling were compared with existing correlations in the literature with good agreement.

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