Saturated chf in horizontal eccentric annuli for low mass fluxes: measurement and modelling

1994 ◽  
Vol 20 (5) ◽  
pp. 901-913 ◽  
Author(s):  
J.L. Balino ◽  
J. Converti
Keyword(s):  
Mass Fluxes ◽  
Low Mass ◽  
Eccentric Annuli

Thermal-Hydraulic Performance of R-134a Boiling at Low Mass Fluxes in a Small Vertical Brazed Plate Heat Exchanger

2017 ◽  
Author(s):  
Hyun Jin Kim ◽  
Leon Liebenberg ◽  
Anthony M. Jacobi
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Mass Flux ◽  
Nucleate Boiling ◽  
Plate Heat Exchanger ◽  
Two Phase ◽  
Mass Fluxes ◽  
Low Mass ◽  
Brazed Plate Heat Exchanger ◽  
R 134A

An experimental investigation was performed to study the heat transfer and pressure drop characteristics of refrigerant R-134a boiling in a chevron-patterned brazed plate heat exchanger (BPHE) at low mass flux. The heat transfer coefficient and pressure drop characteristics are analyzed in relation to varying mass flux (30–50 kgm−2s−1), saturation pressure (675 kPa and 833 kPa), heat flux (0.8 and 2.5 kWm−2), and vapor quality (0.1–0.9). The two-phase pressure drop shows a strong dependence on mass flux and significant saturation temperature drop at high mass flux. The two-phase heat transfer coefficient was both strongly dependent on heat flux (at vapor qualities below 0.4) and on mass flux (at vapor qualities above 0.4). There was also apparent dryout, as depicted by decreased heat transfer at high vapor qualities. These observations suggest that both nucleate and convective boiling mechanisms prevailed. Existing transition correlations however suggest that the experimental data is rather convection-dominant and not a mix of convection and nucleate boiling. The experimental data further strongly suggest the prevalence of both macrochannel and minichannel type flows. Several acknowledged semi-empirical transition criteria were employed to verify our observations. These criteria mostly support our observations that R-134a evaporating at low mass fluxes in a BPHE with a hydraulic diameter of 3.4 mm, has heat transfer and pressure drop characteristics typically indicative of macrochannel as well as minichannel flows. Disagreement however exists with accepted correlations regarding the prevalence of convective or nucleate boiling.

scholarly journals Radiation-hydrodynamic Models of the Accretion Spots in Magnetic Cataclysmic Variables

2004 ◽  
Vol 190 ◽  
pp. 187-200 ◽  
Author(s):  
K. Beuermann
Keyword(s):  
Mass Flux ◽  
White Dwarf ◽  
Cataclysmic Variables ◽  
Strong Shock ◽  
Hydrodynamic Models ◽  
Mass Fluxes ◽  
Open Questions ◽  
Low Mass ◽  
Magnetic Cataclysmic Variables ◽  
Theoretical Foundations

AbstractThe structure of the near-polar accretion spots on accreting magnetic white dwarfs has been studied theoretically and observationally in numerous papers over the last decade. Detailed treatments are available for the regime of low mass flux, usually termed the bombardment case, and for higher mass fluxes which create a strong shock standing above the photosphere of the white dwarf. No general treatment is so far available for the case of shocks buried deep in the photosphere. I review the theoretical foundations, present some applications of theory, and discuss in short the open questions which still need to be addressed.

Effect of Inlet Restriction on Flow Boiling Heat Transfer in a Horizontal Microtube

2013 ◽  
Vol 135 (2) ◽  
Author(s):  
YanFeng Fan ◽  
Ibrahim Hassan
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Pressure Drop ◽  
Critical Heat Flux ◽  
Mass Flux ◽  
Boiling Heat Transfer ◽  
Flow Boiling ◽  
Mass Fluxes ◽  
Flow Boiling Heat Transfer ◽  
Low Mass

Flow boiling heat transfer in a horizontal microtube with inlet restriction (orifice) under uniform heating condition is experimentally investigated using FC-72 as working fluid. A stainless steel microtube with an inner diameter of 889 μm is selected as main microtube. Two microtubes with smaller diameters are assembled at the inlet of main microtube to achieve the restriction ratios of 50% and 20%. The experimental measurement is carried out at mass fluxes ranging from 160 to 870 kg/m2·s, heat fluxes varying from 6 to 170 kW/m2, inlet temperatures of 23 and 35 °C, and saturation pressures of 10 and 45 kPa. The effects of the orifices on two-phase pressure drop, critical heat flux (CHF), and flow boiling heat transfer coefficient are studied. The results show that the pressure drop caused by the orifice takes a considerable portion in the total pressure drop at low mass fluxes. This ratio decreases as the vapor quality or mass flux increases. The difference of normal critical heat flux in the microtubes with different orifice sizes is negligible. In the aspect of flow boiling heat transfer, the orifice is able to enhance the heat transfer at low mass flux and high saturation pressure, which indicates the contribution of orifice in the nucleate boiling dominated regime. However, the effect of orifice on flow boiling heat transfer is negligible in the forced convective boiling dominated regime.

Experimental Investigation of Flow Boiling Instability in a Single Horizontal Microtube With and Without Inlet Restriction

2012 ◽  
Vol 134 (8) ◽  
Author(s):  
YanFeng Fan ◽  
Ibrahim Hassan
Keyword(s):  
Heat Flux ◽  
Flow Instability ◽  
Flow Boiling ◽  
Area Ratio ◽  
High Mass ◽  
Working Fluid ◽  
Stainless Steel Tube ◽  
Operating Condition ◽  
Mass Fluxes ◽  
Low Mass

An experimental study is conducted to investigate the effects of inlet restriction (orifice) on flow boiling instability in a single horizontal microtube. The test-section is composed of a stainless steel tube with an inner diameter of 889 μm, and a length of 150 mm. Experiments are performed for three different orifice configurations with 20%, 35%, and 50% area ratio. Mass flux is varied from 700 to 3000 kg/m2 · s, whereas the heat flux is varied from 6 to 27 W/cm2. The dielectric coolant FC-72 is selected as the working fluid. In the absence of an orifice at the inlet, four oscillation types are observed at the onset of flow instability; it is also noticed that the frequency of the oscillations increases with increasing heat flux, while the amplitude remains constant. The addition of an orifice at the inlet helps stabilizing the flow without generating significant pressure drop at the same operating condition as the microtube without orifice. The 20% area ratio orifice shows better performance at low mass fluxes (<1000 kg/m2 · s). Whereas, at high mass fluxes (>2000 kg/m2 · s), 50% and 35% area ratio orifices are efficient in stabilizing the flow or delaying the onset of flow instability. Therefore, selecting the area ratio of the orifice depends on the operating condition. A small area ratio orifice is preferably used at low mass fluxes, whereas a large area ratio orifice is more suitable for high mass fluxes.

Critical Heat Flux in Horizontal Tube Bundles in Vertical Crossflow of R113

1992 ◽  
Vol 114 (1) ◽  
pp. 179-184 ◽  
Author(s):  
K. M. Leroux ◽  
M. K. Jensen
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Mass Flux ◽  
Horizontal Tube ◽  
High Mass ◽  
Intermediate Mass ◽  
Average Value ◽  
Mass Fluxes ◽  
Local Quality ◽  
Low Mass

The critical heat flux (CHF) on a single tube in a horizontal bundle subject to an upward crossflow of R113 has been studied in three bundle geometries. Effects of local quality, mass flux, pressure, and bundle geometry on the CHF were investigated. The shapes of the CHF-quality curves display three distinct patterns, which progress from one to another as mass flux increases. At low mass fluxes, the CHF data monotonically decreased with increasing quality. At intermediate mass fluxes with increasing quality, the CHF data initially decreased to a relative minimum, then increased to a relative maximum, and finally began to decrease again as the higher qualities were reached. At high mass fluxes, as quality increased, the CHF rose gradually from the zero quality value to a maximum and then began to decrease. For all mass fluxes, the zero-quality CHF points clustered around an average value, which varied slightly with test section geometry. Mechanisms for the CHF condition are suggested.

Shell-Side Flow Condensation of R410A on Horizontal Tubes at Low-Mass Fluxes

2016 ◽  
Vol 139 (1) ◽  
Author(s):  
Wei Li ◽  
Xu Chen ◽  
Jing-Xiang Chen ◽  
Zhi-Chuan Sun ◽  
Terrence W. Simon
Keyword(s):  
Saturation Temperature ◽  
Superior Performance ◽  
Smooth Tube ◽  
Outer Diameter ◽  
Vapor Quality ◽  
Mass Fluxes ◽  
Enhanced Tube ◽  
Low Mass ◽  
Side Flow

An investigation of refrigerant R410A condensation on a shell and tube heat exchanger simulation is conducted. Tests are on the outside of a horizontal smooth tube, a herringbone tube, and a newly developed three-dimensional-enhanced tube, called the enhanced tube (EHT) tube, all of the same outer diameter. Experiments were conducted at a constant saturation temperature of 45 °C, a constant inlet vapor quality of 0.8, a constant outlet vapor quality of 0.1, and mass fluxes ranging from 5 kg/(m2 s) to 50 kg/(m2 s). At low-mass velocities, the smooth tube shows superior performance over the herringbone tube and the EHT tube. The cause might lie in surface tension effects that result in liquid inundation at the lower portion of the tube, thickening the film on the tube and deteriorating the heat transfer performance. Analyses were conducted to find a suitable correlation of the experimental data.

Condensation Heat Transfer and Pressure Drop of R-404A in 7.0 mm O.D. Smooth and Microfin Tube at Low Mass Fluxes

2018 ◽  
Vol 26 (01) ◽  
pp. 1850005 ◽  
Author(s):  
Nae-Hyun Kim ◽  
Hyung-Ho Gook ◽  
Byung-Moo Lee
Keyword(s):  
Heat Transfer ◽  
Pressure Drop ◽  
Heat Transfer Enhancement ◽  
Mass Flux ◽  
Enhancement Factor ◽  
Condensation Heat Transfer ◽  
Transfer Enhancement ◽  
Mass Fluxes ◽  
Penalty Factor ◽  
Low Mass

R-404A condensation heat transfer and pressure drop data are provided for 7.0[Formula: see text]mm O.D. smooth and microfin tubes. Tests were conducted for a range of mass fluxes (from 80 to 200[Formula: see text]kg/m2s) and quality (from 0.2 to 0.8). The heat flux was 6[Formula: see text]kW/m2 and saturation temperature was 45[Formula: see text]C. It was found that both the heat transfer enhancement factor and the pressure drop penalty factor increase as mass flux increases. The range of pressure drop penalty factor (0.99–1.27) was smaller than that of heat transfer enhancement factor (1.21–1.96). Smooth tube heat transfer coefficients and pressure drops are reasonably predicted by Shah [An improved and extended general correlation for heat transfer during condensation in plain tubes, Int. J. HVAC&R Res. 15 (2009) 889–913] and Jung and Radermacher [Prediction of pressure drop during horizontal annular flow boiling of pure and mixed refrigerants, Int. J. Heat Mass Transfer 32 (1989) 2435–2446] correlation, respectively. For the microfin tube, however, all the existing correlations do not adequately predict the present data. Poor predictions may be attributed to the lack of R-404A and low mass flux data in their database.

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