Assessment of CHF Enhancement Mechanisms in a Curved, Rectangular Channel Subjected to Concave Heating

1999 ◽  
Vol 121 (2) ◽  
pp. 394-404 ◽  
Author(s):  
J. C. Sturgis ◽  
I. Mudawar
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Cross Sections ◽  
Rectangular Channel ◽  
Side Wall ◽  
Gravitational Acceleration ◽  
Centripetal Acceleration ◽  
Concave Wall ◽  
Buoyancy Forces ◽  
Flow Curvature

An experimental study was undertaken to examine the enhancement in critical heat flux (CHF) provided by streamwise curvature. Curved and straight rectangular flow channels were fabricated with identical 5.0 × 2.5 mm cross sections and heated lengths of 101.6 mm in which the heat was applied to only one wall—the concave wall (32.3 mm radius) in the curved channel and a side wall in the straight. Tests were conducted using FC-72 liquid with mean inlet velocity and outlet subcooling of 0.25 to 10 m s−1 and 3 to 29°C, respectively. Centripetal acceleration for curved flow reached 315 times earth’s gravitational acceleration. Critical heat flux was enhanced due to flow curvature at all conditions but the enhancement decreased with increasing subcooling. For near-saturated conditions, the enhancement was approximately 60 percent while for highly subcooled flow it was only 20 percent. The causes for the enhancement were identified as (1) increased pressure on the liquid-vapor interface at wetting fronts, (2) buoyancy forces and (3) increased subcooling at the concave wall. Flow visualization tests were conducted in transparent channels to explore the role of buoyancy forces in enhancing the critical heat flux. These forces were observed to remove vapor from the concave wall and distribute it throughout the cross section. Vapor removal was only effective at near-saturated conditions, yielding the observed substantial enhancement in CHF relative to the straight channel.

Buoyancy Effects in the Entrance Region of an Inclined Multirectangular-Channel Solar Collector

1983 ◽  
Vol 105 (2) ◽  
pp. 157-162 ◽  
Author(s):  
S. M. Morcos ◽  
M. M. M. Abou-Ellail
Keyword(s):  
Heat Flux ◽  
Nusselt Number ◽  
Solar Collector ◽  
Flux Distribution ◽  
Rectangular Channel ◽  
Numerical Procedure ◽  
Side Wall ◽  
Entrance Region ◽  
Heat Flux Distribution ◽  
Constant Property

A numerical procedure is presented for the entrance region of an inclined multirectangular-channel solar collector with significant buoyancy effects. The upper wall heat flux is taken to be uniform, while the lower wall is assumed to be insulated. The heat flux distribution on the side wall of the rectangular channel is obtained by coupling a heat-conduction numerical procedure in the metallic region surrounding the channel to the main numerical procedure which solves the hydrodynamic and energy equations of the flow inside the channel. Numerical results are presented for water flowing in a multirectangular-channel solar collector with an aspect ratio AR = 4 inclined at an angle α = 30 deg to the horizontal. The resulting variable heat flux distribution on the side wall enhances the intensity of the secondary flow. The effects of the nonuniform heat flux distribution and the spacing between the rectangular channels on the variation of Nusselt number in the entrance region are presented for different values of Rayleigh number. At a value of Ra = 5 × 105, Nusselt number is more than 300 percent above the constant property prediction.

scholarly journals Orientation Effects on Critical Heat Flux due to Flooding in Thin Rectangular Channel

2002 ◽  
Vol 39 (7) ◽  
pp. 736-742 ◽  
Author(s):  
Futoshi TANAKA ◽  
Kaichiro MISHIMA ◽  
Tamio KOHRIYAMA ◽  
Yukimitsu OKANO
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Rectangular Channel ◽  
Orientation Effects

Critical Heat Fluxes and Flow Patterns in High-Pressure Boiling Water Flows

1964 ◽  
Vol 86 (1) ◽  
pp. 12-22 ◽  
Author(s):  
F. E. Tippets
Keyword(s):  
Heat Flux ◽  
High Pressure ◽  
Critical Heat Flux ◽  
Rectangular Channel ◽  
Motion Pictures ◽  
Flow Patterns ◽  
Heat Fluxes ◽  
Boiling Water ◽  
Forced Flow ◽  
Flux Level

High-speed motion pictures (4300 pictures/sec) of boiling water flow patterns in conditions of forced flow at 1000 psia pressure in a vertical heated rectangular channel were taken over the range of mass velocities from 50 to 400 lb/sec-ft2, fluid states from bulk subcooled liquid flow to bulk boiling flow at 0.66 steam quality, and heat fluxes up to and including the critical heat flux level. Eighty critical heat flux determinations were made in the course of the experiment at 1000 psia in conditions of bulk boiling. The motion pictures provide photographic evidence of the general arrangement of the flow in conditions of bulk boiling at high pressure with heat fluxes near and including the critical heat flux level.

Application and Analysis of Wavelet Transform and Edge Detection in Critical Heat Flux of Natural Circulation

2011 ◽  
Vol 354-355 ◽  
pp. 333-337
Author(s):  
Cheng Sheng ◽  
Tao Zhou
Keyword(s):  
Heat Flux ◽  
Wavelet Transform ◽  
Edge Detection ◽  
Critical Heat Flux ◽  
Natural Circulation ◽  
Rectangular Channel ◽  
New Approach ◽  
One Dimensional ◽  
Temperature Signal ◽  
Dry Patch

Based on the experimental data obtained in natural circulation experiment in narrow rectangular channel, critical heat flux (CHF) was detected and analyzed through wavelet transform, and the technology of edge detection was applied in the analysis of the photos of CHF which were taken in the experiment. Results showed that the apply of wavelet transform using of db1 wavelet and edge detection using of Canny algorithm could both distinguish the singularity of CHF in one-dimensional temperature signal and regions of dry patch that represented CHF phenomena in two-dimensional photograph accurately, which can provide a new approach in the analysis of CHF experimental research of natural circulation.

Critical Heat Flux by High Velocity Liquid Flow in Narrow Rectangular Channel

2008 ◽  
Author(s):  
Hisashi Sakurai ◽  
Yasuo Koizumi ◽  
Hiroyasu Ohtake
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Critical Heat Flux ◽  
High Speed ◽  
Rectangular Channel ◽  
Video Camera ◽  
Jet Flow ◽  
Heat Transfer Surface ◽  
Bubble Behavior ◽  
Film Type

Experiments of critical heat flux of extremely thin-fast plate jet film sub-cooled flow were conducted. The extremely thin-fast film-type jet of sub-cooled water was erupted into a stagnant pool. The heat transfer is augmented by the fast jet flow on the heat transfer surface. Vapor generated on the surface is easily taken away from the surface by the fast jet flow and leaves upward from the surface. The static head of water in the pool depress down the fast film-type jet flow on to the heat transfer surface and may collapse the vapor film that is formed between the heat transfer surface and the fast film flow. All these combine to have the possibility to improve the critical heat flux. In the experiments, the liquid sub-cooling was in the range of 30 ∼ 70 K. The thickness of the jet film was 0.2 mm and 0.5 mm. The width of the jet film was 2 mm. The velocity of the erupting jet film was 5.0 ∼ 32 m/s. The heat transfer surface was 2.0 × 2.0 mm heated electrically. The heat transfer surface was placed on the bottom of the pool. The fast-thin film jet was erupted on the bottom of the pool parallel to the heat transfer surface. Bubble behavior generated on the heat transfer surface was recorded by a high speed video camera at 10,000 frames/s. The highest critical heat flux obtained up to now is 3.2 × 107 W/m2. The analytical model of the critical heat flux for the present flow system will be presented.

Theoretical Calculation of the Critical Heat Flux in Annular Upward Flow in Vertical Narrow Rectangular Channels

2009 ◽  
Author(s):  
Dongxiao Du ◽  
Guanghui Su ◽  
Suizheng Qiu
Keyword(s):  
Heat Flux ◽  
Liquid Film ◽  
Critical Heat Flux ◽  
Rectangular Channel ◽  
Interfacial Shear Stress ◽  
Momentum Conservation ◽  
Conservation Equation ◽  
Entrainment Rate ◽  
Upward Flow ◽  
Rectangular Channels

The present paper has developed a mathematical separated flow model for annular upward flow in vertical narrow rectangular channels to predict the critical heat flux. The theoretical model is based on fundamental conservation principles: the mass, momentum, and energy conservation equation of liquid film and the momentum conservation equation of vapor core together with a set of closure relationships (such as entrainment rate, deposition rate, interfacial shear stress and initial entrainment fraction at the onset of annular flow). The predicted results are compared with the experimental data and fairly good agreement between them is achieved. By numerically solving the equations, liquid film thickness, liquid film velocity in the liquid film and heat transfer coefficient are obtained. With the applications of the present model, the critical heat flux in the rectangular channel is calculated and analyzed.

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