Some Aspects of Critical-Heat-Flux Enhancement in Tubes

2000 ◽  
Author(s):  
S. S. Doerffer ◽  
D. C. Groeneveld ◽  
K. F. Rudzinski ◽  
I. L. Pioro ◽  
J. W. Martin
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Cumulative Effect ◽  
Fluid Type ◽  
Flow Conditions ◽  
Local Flow ◽  
Flow Parameters ◽  
A Value ◽  
Geometric Factors ◽  
The Impact

Abstract This paper summarizes the effects of various types or numbers of critical-heat-flux (CHF)-enhancing inserts in tubular geometries. The impact of inserts on CHF is frequently expressed by an enhancement ratio K: the ratio of CHF with an insert to the CHF in a bare tube for the same local flow conditions. The impact on K of the following parameters was investigated: (i) fluid type (Freon-134a, water), (ii) axial spacing between inserts, (iii) shape of the insert, (iv) flow blockage of the insert, (v) number of similar/dissimilar insert planes upstream, and (vi) impact of flow conditions. The spacing and flow-obstruction area were found to be the major geometric factors that affected K: by decreasing the relative spacing, L/D, to 16, K can reach a value of from 2 to 3, depending on the flow-obstruction area. Among flow parameters, the critical quality, xc, usually has a strong effect on K: K can increase from a value of 1 to 3, when xc increases from 0 to 0.4 for a mass flux G ≥ 2 Mg/m2s. For G < 2 Mg/m2s, CHF enhancement can disappear or become negative (K < 1). No cumulative effect was found on K for a series of upstream insert planes. CHF enhancement does not depend on fluid type, provided that the conditions in the fluids meet the CHF fluid-to-fluid modelling requirements.

Analysis of the Critical Heat-Flux Condition in High-Pressure Boiling Water Flows

1964 ◽  
Vol 86 (1) ◽  
pp. 23-33 ◽  
Author(s):  
F. E. Tippets
Keyword(s):  
Heat Flux ◽  
High Pressure ◽  
Critical Heat Flux ◽  
High Speed ◽  
Boiling Water ◽  
Two Phase ◽  
Local Flow ◽  
Flow Parameters ◽  
Fluid Properties ◽  
Data Points

Based on the two-phase flow patterns shown in high-speed motion pictures of the process, a general working equation is derived which relates the critical heat flux for high-pressure bulk boiling water in forced convection to the significant local flow parameters and fluid properties. The equation is applied to a representative selection of several hundred data points from the major available sources for the purpose of investigating trends in the data and to test the validity of the equation.

Study on the Prediction of DNB-Type Critical Heat Flux in Rod Bundle Under Motion Conditions

2018 ◽  
Author(s):  
Siyang Huang ◽  
Xiaoyan Wang ◽  
Wenxi Tian ◽  
Ronghua Chen ◽  
Junmei Wu ◽  
...  
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Nuclear Reactor ◽  
Cross Flow ◽  
Nucleate Boiling ◽  
Heated Wall ◽  
Coupled Analysis ◽  
Flow Conditions ◽  
Local Flow ◽  
Rod Bundle

In the nuclear reactor design, the critical heat flux (CHF) is one of the most important parameters for the reactor safety analysis. The occurrence of CHF will cause a sharp increase in the fuel rod surface temperature, which will result in the failure of fuel claddings and damage of the core. The CHF depends on the local flow conditions and the geometry of the flow channels, which makes the prediction of CHF in a fuel assembly more difficult when considering the cross flow between neighboring channels, spacer grids and mixing vanes. In this paper, the departure from nucleate boiling (DNB) type CHF in rod bundles under motion conditions is investigated based on the coupled analysis of the subchannel method and a CHF mechanism model, namely the liquid sublayer dryout model. The liquid sublayer dryout model assumes that there is a thin liquid sublayer underneath a vapor blanket formed by the coalescence of small bubbles near the heated wall. The dryout of this sublayer is considered as the CHF occurrence. In the liquid sublayer dryout model, sublayer thickness, velocity and length of the vapor blanket are three crucial parameters. In present research, the subchannel code calculates the local flow conditions for the rod bundle and provides input parameters for the liquid sublayer dryout model to predict CHF. In order to verify the method above, the predicted results are compared with the CHF Look-Up Table 2006 (LUT-2006) and a reasonable agreement can be achieved. In addition, the effects of rod bundle inlet coolant mass flow rate, subcooling and motion conditions on the CHF are analyzed.

Analysis of Local Flow Structure and Global Compressor Performance During Rotating Stall

2004 ◽  
Author(s):  
Chiara Palomba
Keyword(s):  
Flow Field ◽  
Flow Structure ◽  
Rotational Speed ◽  
Axial Flow ◽  
Rotating Stall ◽  
Performance Parameters ◽  
Flow Conditions ◽  
Local Flow ◽  
Flow Parameters ◽  
Compressor Performance

Rotating stall is an instability phenomenon that arises in axial flow compressors when the flow is reduced at constant rotational speed. It is characterised by the onset of rotating perturbations in the flow field accompanied by either an abrupt or gradual decrease of performances. Although the flow field is unsteady and non axisymmetric, the global operating point is stable and a stalled branch of performance curve may be experimentally determined. The number, rotational speed, circumferential extension of the rotating perturbed flow regions named rotating cells may vary from one compressor to another and may depend on the throttle position. The present work focuses on the interaction between local flow parameters and global compressor performance parameters with the aim of reaching a better understanding of the phenomenon. Starting from the Day, Greitzer and Cumpsty [1] model the detailed flow conditions during rotating stall are studied and related to the global performance parameters. This is done both to verify if the compressor under examination fits to the model and if the detailed flow structure may highlight the physics that in the simple model may hide behind the correlation’s used.

Critical heat flux under zero flow conditions in vertical annulus with uniformly and non-uniformly heated sections

2001 ◽  
Vol 205 (3) ◽  
pp. 265-279 ◽  
Author(s):  
Se-Young Chun ◽  
Sang-Ki Moon ◽  
Heung-June Chung ◽  
Sun-Kyu Yang ◽  
Moon-Ki Chung ◽  
...  
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Flow Conditions ◽  
Vertical Annulus ◽  
Zero Flow

Effect of Flow Orientation on Critical Heat Flux in Subcooled Flow Boiling

Volume 3 ◽  
2004 ◽  
Author(s):  
Jason S. Bower ◽  
James F. Klausner
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Heat Exchangers ◽  
Flow Boiling ◽  
Bulk Flow ◽  
Flow Conditions ◽  
Convective Velocity ◽  
Subcooled Flow Boiling ◽  
Subcooled Flow ◽  
Significant Interest

Recent work has demonstrated that as the bulk convective velocity in subcooled nucleate flow boiling increases, the heat transfer tends to become independent of flow orientation with respect to gravity. There is significant interest in developing heat exchangers for next generation spacecraft that operate in the gravity-independent flow boiling regime. In order to develop such heat exchangers it is important to understand the effect of gravity on the critical heat flux and to determine whether a gravity on the critical heat flux and to determine whether a gravity-independent flow boiling critical heat flux regime exists. This work describes subcooled flow boiling experiments where the critical heat flux is measured over a range of flow orientations with respect to gravity: 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°. It has been found that at low bulk flow velocities there is a large variation of critical heat flux with different flow orientations. At large convective velocities, the variation of critical heat flux with different flow orientations is significantly diminished. It appears that with further increases in bulk flow velocity, a gravity-independent critical heat flux regime exists, although the current experimental facility was not capable of operating at those flow conditions.

Critical heat flux under choking flow conditions

2001 ◽  
Vol 205 (1-2) ◽  
pp. 175-190 ◽  
Author(s):  
A. Olekhnovitch ◽  
A. Teyssedou ◽  
P. Tye
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Flow Conditions

scholarly journals EXPERIMENTAL INVESTIGATION OF CRITICAL HEAT FLUX IN ANNULUS AT LOW PRESSURE AND LOW FLOW PARAMETERS

2020 ◽  
Vol 28 ◽  
pp. 50-58
Author(s):  
Daniel Vlček ◽  
Ladislav Suk ◽  
Kamil Števanka ◽  
Taron Petrosyan
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Flow Boiling ◽  
Low Pressure ◽  
Low Flow ◽  
Inconel 625 ◽  
Outer Diameter ◽  
Flow Parameters ◽  
Low Mass ◽  
Vertically Aligned

Steady state flow boiling experiments were conducted on a technically smooth Inconel 625 tube with outer diameter 9.1 mm at inlet pressures 131, 220 and 323 kPa, inlet temperatures 62, 78 and 94 °C and approximately 400, 600 and 1000 kg/(m2.s) mass flow. Water of these parameters was entering into the vertically aligned annulus, where the uniformly heated tube was placed until the critical heat flux (CHF) appeared. The experimental data were compared to estimations of CHF by local PGT tube correlation and Groeneveld’s look-up tables for tubes. The results imply that in the region of low pressure and low mass flux, the differences between calculations and experiments are substantial (more than 50 % of CHF). The calculations further imply that look-up tables and tube correlations should be corrected to the annulus geometry. Here, the Doerffer’s approach was chosen and led to a substantial enhancement of CHF estimation. Yet, a new correlation for the region of low pressure and flow is needed.

CFD Analyses for Water-Air Flow With the Euler-Euler Two-Phase Model in the Fluent4 CFD Code

2002 ◽  
Author(s):  
Jaakko Miettinen ◽  
Holger Schmidt
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Void Fraction ◽  
Bubble Diameter ◽  
Velocity Profiles ◽  
Global Model ◽  
Model Parameters ◽  
Fluid Viscosity ◽  
Flow Conditions ◽  
Two Phase

Framatome ANP develops a new boiling water reactor called SWR 1000. For the case of a hypothetical core melt accident it is designed in such a way that the core melt is retained in the Reactor Pressure Vessel (RPV) at low pressure owing to cooling of the RPV exterior and high reliable depressurization devices. Framatome ANP performs — in co-operation with VTT — tests to quantify the safety margins of the exterior cooling concept for the SWR 1000, for determining the limits to avoid the critical heat fluxes (CHFs). The three step procedure has been set up to investigate the phenomenon: 1. Water-air study for a 1:10 scaled global model, with the aim to investigate the global flow conditions. 2. Water-air study for a 1:10 scaled, 10% sector model, with the aim to find a flow sector with almost similar flow conditions as in the global model. 3. Final CHF experiments for a 1:1-scaled, 10% sector., the boarders of this model have been selected based on the first two steps. The instrumentation for the water/air experiments included velocity profiles, the vertically averaged average void fraction and void fraction profiles in selected positions. The experimental results from the air-water experiments have been analyzed at VTT using the Fluent-4.5.2 code with its Eulerian multiphase flow modeling capability. The aim of the calculations was to learn how to model complex two-phase flow conditions. The structural mesh required by Fluent-4 is a strong limitation in the complex geometry, but modeling of the 1/4 sector from the facility was possible, when the GAMBIT pre-processor was used for the mesh generation. The experiments were analyzed with the 150 × 150 × 18 grid for the geometry. In the analysis the fluid viscosity was the main dials for adjusting the vertical liquid velocity profiles and the bubble diameter for adjusting the phase separation. The viscosity ranged between 1 to 10000 times the molecular viscosity, and bubble diameter between 3 to 100 mm, when the calculation results were adjusted for a good agreement with the experimental data. The analysis results were very valuable for designing the final water/steam facility for final CHF tests. The validation against data from the air-water experiments proved that the present CFD codes approach to the state where they can be used for simulating such two-phase experiments, where the fraction of both phases is essential and the flow is strongly affected by the density differences. It is still too early to predict, if the CFD calculation of the 1:1 scale critical heat flux experiments is successful, could the result be used for formulating a new type of a critical heat flux correlation, where the effects of CRD’s on the flow patterns and gap dimensions are model parameters.

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