Effect of a Heat Flux Spike on the Downstream Dryout Behavior

1974 ◽  
Vol 96 (2) ◽  
pp. 121-125 ◽  
Author(s):  
D. C. Groeneveld
Keyword(s):  
Heat Flux ◽  
Density Ratio ◽  
Interfacial Instability ◽  
Force Balance ◽  
Vapor Density ◽  
Uniform Heating ◽  
Axial Conduction ◽  
Dry Patch ◽  
Wall Interaction ◽  
Patch Boundary

The effect of a short hot patch on the downstream dryout behavior was investigated in heated tubes cooled by Freon-12 at conditions equivalent (on the basis of liquid to vapor density ratio) to 69 bar in steam-water. The hot patch, located well upstream from the heated end, generated a heat flux spike which resulted in a local dry patch. Following an increase in downstream surface heat flux the dry patch would often spread in the downstream direction even though the heat flux was only a small fraction of the measured critical heat flux with uniform heating. A force balance on the dry patch boundary showed that, for certain conditions, spreading of the dry patch in the downstream direction is inevitable. This spreading however can be limited by interfacial instability, droplet-wall interaction and axial conduction.

A Model for Fluid Flow During Saturated Boiling on a Horizontal Cylinder

1987 ◽  
Vol 109 (2) ◽  
pp. 485-490
Author(s):  
K. Kheyrandish ◽  
C. Dalton ◽  
J. H. Lienhard
Keyword(s):  
Heat Flux ◽  
Fluid Flow ◽  
Weber Number ◽  
Flow Boiling ◽  
Density Ratio ◽  
Horizontal Cylinder ◽  
Vapor Film ◽  
Vapor Density ◽  
Saturated Liquid ◽  
Saturated Boiling

A model has been developed to represent the vapor removal pattern in the vicinity of a cylinder during nucleate flow boiling across a horizontal cylinder. The model is based on a potential flow representation of the liquid and vapor regions and an estimate of the losses that should occur in the flow. Correlation of the losses shows a weak dependence on the Weber number and a slightly stronger dependence on the saturated liquid-to-vapor density ratio. The vapor jet thickness, which is crucial to the prediction of the burnout heat flux, and the shape of the vapor film are predicted. Both are verified by qualitative experimental observations.

Critical Heat Flux in Saturated Forced Convection Boiling on a Heated Disk With an Impinging Jet

1987 ◽  
Vol 109 (4) ◽  
pp. 991-996 ◽  
Author(s):  
M. Monde
Keyword(s):  
Heat Flux ◽  
High Pressure ◽  
Forced Convection ◽  
Critical Heat Flux ◽  
Weber Number ◽  
High Pressures ◽  
Density Ratio ◽  
Vapor Density ◽  
Heated Disk ◽  
Jet Velocity

Critical heat flux during forced convection boiling on an open heated disk being supplied with saturated liquids through a small round jet which impinges at the center of the disk has been studied experimentally employing refrigerant R12 at comparatively high pressures from 0.6 to 2.8 MPa. Generalized correlations, predicting the CHF within an experimental range of liquid-to-vapor density ratio 5.3–41.25 and the reciprocal of Weber number 2 × 10−3–2 × 10−7, are given for three different characteristic regimes: V-regime where the CHF increases with an increase in the jet velocity, I-regime where the CHF is nearly constant with jet velocity, and HP-regime where the CHF appears only at high pressure and again rises with an increase in the jet velocity.

scholarly journals Predicting Critical Heat Flux With Multiphase CFD: 4 Years in the Making

2017 ◽  
Author(s):  
Emilio Baglietto ◽  
Etienne Demarly ◽  
Ravikishore Kommajosyula
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Critical Heat Flux ◽  
Bubble Dynamics ◽  
Flow Boiling ◽  
Heated Surface ◽  
Force Balance ◽  
Modeling Framework ◽  
Lift Off ◽  
Limiting Condition

Advancement in the experimental techniques have brought new insights into the microscale boiling phenomena, and provide the base for a new physical interpretation of flow boiling heat transfer. A new modeling framework in Computational Fluid Dynamics has been assembled at MIT, and aims at introducing all necessary mechanisms, and explicitly tracks: (1) the size and dynamics of the bubbles on the surface; (2) the amount of microlayer and dry area under each bubble; (3) the amount of surface area influenced by sliding bubbles; (4) the quenching of the boiling surface following a bubble departure and (5) the statistical bubble interaction on the surface. The preliminary assessment of the new framework is used to further extend the portability of the model through an improved formulation of the force balance models for bubble departure and lift-off. Starting from this improved representation at the wall, the work concentrates on the bubble dynamics and dry spot quantification on the heated surface, which governs the Critical Heat Flux (CHF) limit. A new proposition is brought forward, where Critical Heat Flux is a natural limiting condition for the heat flux partitioning on the boiling surface. The first principle based CHF is qualitatively demonstrated, and has the potential to deliver a radically new simulation technique to support the design of advanced heat transfer systems.

Numerical Simulation Of A Microchannel For Microelectronic Cooling

2012 ◽  
Author(s):  
Wai Hing Wong ◽  
Normah Mohd. Ghazali
Keyword(s):  
Heat Transfer ◽  
Numerical Simulation ◽  
Heat Flux ◽  
Aspect Ratio ◽  
Three Dimensional ◽  
Large Temperature ◽  
Rectangular Microchannel ◽  
Numerical Work ◽  
Solid Region ◽  
Wall Interaction

Kertas kerja ini membincangkan simulasi berangka ke atas sinki haba saluran mikro dalam penyejukan alatan mikroelektronik. Model Dinamik Bendalir Berkomputer (CFD) tiga dimensi dibina menggunakan pakej komersil, FLUENT, untuk mengkaji fenomenon aliran bendalir dan pemindahan haba konjugat di dalam suatu sinki haba segi empat yang diperbuat daripada silikon. Model ditentusahkan dengan keputusan daripada uji kaji dan pengkajian berangka yang lepas untuk lingkungan nombor Reynolds kurang daripada 400 berdasarkan diameter hidraulik 86 mm. Kajian ini mengambil kira kesan kelikatan bendalir yang bersandaran dengan suhu dan keadaan aliran pra–membangun dari segi hidrodinamik dan haba. Model memberi maklumat tentang taburan suhu dan fluks haba yang terperinci di dalam sinki haba saluran mikro. Kecerunan suhu yang tinggi dicatat pada kawasan pepejal berdekatan dengan sumber. Fluks haba paling tinggi didapati pada dinding tepi saluran mikro diikuti oleh dinding atas dan bawah. Purata pekali pemindahan haba yang lebih tinggi bagi silikon menjadikan ia bahan binaan sinki haba saluran mikro yang lebih baik berbanding dengan kuprum dan aluminium. Peningkatan nisbah aspek saluran mikro yang bersegi empat memberi kecekapan penyejukan yang lebih tinggi kerana kelebaran saluran yang berkurangan memberi kecerunan halaju yang lebih tinggi dalam saluran. Nisbah aspek yang optimum yang diperoleh adalah dalam lingkungan 3.7 – 4.1. Kata kunci: Saluran mikro, CFD, FLUENT, simulasi berangka, penyejukan mikroelektron The paper discusses the numerical simulation of a micro–channel heat sink in microelectronics cooling. A three–dimensional Computational Fluid Dynamics (CFD) model was built using the commercial package, FLUENT, to investigate the conjugate fluid flow and heat transfer phenomena in a silicon–based rectangular microchannel heatsink. The model was validated with past experimental and numerical work for Reynolds numbers less than 400 based on a hydraulic diameter of 86 mm. The investigation was conducted with consideration of temperaturedependent viscosity and developing flow, both hydrodynamically and thermally. The model provided detailed temperature and heat flux distributions in the microchannel heatsink. The results indicate a large temperature gradient in the solid region near the heat source. The highest heat flux is found at the side walls of the microchannel, followed by top wall and bottom wall due to the wall interaction effects. Silicon is proven to be a better microchannel heatsink material compared to copper and aluminum, indicated by a higher average heat transfer. A higher aspect ratio in a rectangular microchannel gives higher cooling capability due to high velocity gradient around the channel when channel width decreases. Optimum aspect ratio obtained is in the range of 3.7 – 4.1. Key words: Microchannel, CFD, FLUENT, numerical simulation, microeletronics cooling

Experimental Study of the Endwall Heat Transfer of a Transonic Nozzle Guide Vane With Upstream Jet Purge Cooling: Part 1 — Effect of Density Ratio

2020 ◽  
Author(s):  
Shuo Mao ◽  
Ridge A. Sibold ◽  
Stephen Lash ◽  
Wing F. Ng ◽  
Hongzhou Xu ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Mass Flux ◽  
Guide Vane ◽  
Density Ratio ◽  
Momentum Ratio ◽  
Blowing Ratio ◽  
Nozzle Guide Vane ◽  
Nozzle Guide ◽  
The Impact

Abstract Nozzle guide vane platforms often employ complex cooling schemes to mitigate ever-increasing thermal loads on endwall. Understanding the impact of advanced cooling schemes amid the highly complex three-dimensional secondary flow is vital to engine efficiency and durability. This study analyzes and describes the effect of coolant to mainstream blowing ratio, momentum ratio and density ratio for a typical axisymmetric converging nozzle guide vane platform with an upstream doublet staggered, steep-injection, cylindrical hole jet purge cooling scheme. Nominal flow conditions were engine representative and as follows: Maexit = 0.85, Reexit/Cax = 1.5 × 106 and an inlet large-scale freestream turbulence intensity of 16%. Two blowing ratios were investigated, each corresponding to upper and lower engine extrema at M = 3.5 and 2.5, respectively. For each blowing ratio, the coolant to mainstream density ratio was varied between DR = 1.2, representing typical experimental neglect of coolant density, and DR = 1.95, representative of typical engine conditions. An optimal coolant momentum ratio between = 6.3 and 10.2 is identified for in-passage film effectiveness and net heat flux reduction, at which the coolant suppresses and overcomes secondary flows but imparts minimal turbulence and remains attached to endwall. Progression beyond this point leads to cooling effectiveness degradation and increased endwall heat flux. Endwall heat transfer does not scale well with one single parameter; increasing with increasing mass flux for the low density case but decreasing with increasing mass flux of high density coolant. From the results gathered, both coolant to mainstream density ratio and blowing ratio should be considered for accurate testing, analysis and prediction of purge jet cooling scheme performance.

Numerical Study of Microchannel Heat Sinks With Non-Uniform Heat Flux Conditions

2011 ◽  
Author(s):  
Ling Ling ◽  
Yanfeng Fan ◽  
Ibrahim Hassan
Keyword(s):  
Heat Flux ◽  
Heat Sink ◽  
Numerical Study ◽  
Heat Sinks ◽  
Maximum Temperature ◽  
Bottom Surface ◽  
Nonuniform Heating ◽  
Uniform Heat Flux ◽  
Computational Domain ◽  
Uniform Heating

Higher heat flux is produced by Micro-Electro-Mechanical Systems (MEMS) because of their reduced size and increased clock speed. At the mean time, studies of non-uniform heating conditions which are more practical than uniform heating conditions are inadequate and needed urgently. Four nonuniform heating conditions are simulated in the paper. Three heat sinks with different widths of cross-linked channels locating above the center of hotspots are studied and compared to conventional straight microchannel heat sink. Half of the module geometry is chosen to be the computational domain. Two hotspots are placed at the bottom surface. The coolant is water, whose properties are dependent on temperature. Two inlet velocities, 0.5 m/s and 1 m/s, are tested for each heat sink. Temperature profile at the hotspots, pressure drop and total thermal resistance are selected as criteria of evaluating heat sink performance. All heat sinks have better performance when there is an upstream hotspot or the upstream hotspot is subjected to a higher heat flux. Cross-linked channel width of 0.5 mm has the best benefit to obtain better temperature uniformity without increasing the maximum temperature on the bottom surface.

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