Subcooled critical heat flow

In channel flow it is usually necessary to locate the transducer upstream or downstream of the test channel, with the result that the sound field is greatly attenuated. Tests with 80-Hz vibrations [319] indicate no improvement of subcooled boiling heat transfer or critical heat flux. Romie and Aronson [324], using ultrasonic vibrations, found that subcooled critical heat flux was unaffected. Even where intense ultrasonic vibrations were applied to the fluid in the immediate vicinity of the heated surface, boiling heat transfer was unaffected [325]. The severe attenuation of the acoustic energy by the two-phase coolant appears to render this technique ineffective for flow boiling systems. 

Available data sets for flow boiling critical heat flux (CHF) of water in small-diameter tubes are shown in Table 6.9. There are 13 collected data sets in all. Only taking data for tube diameters less than 6.22 mm, and then eliminating duplicate data and those not meeting the heat balance calculation, the collected database included a total of 3,837 data points (2,539 points for saturated CHF, and 1,298 points for subcooled CHF), covering a wide range of parameters, such as outlet pressures from 0.101 to 19.0 MPa, mass fluxes from 5.33 to 1.34 x lO kg/m s, critical heat fluxes from 0.094 to 276 MW/m, hydraulic diameters of channels from 0.330 to 6.22 mm, length-to-diameter ratios from 1.00 to 975, inlet qualities from —2.35 to 0, and outlet thermal equilibrium qualities from -1.75 to 1.00. 

Inasaka F (1993) Critical heat flux of subcooled flow boiling in water under uniform heating conditions. Papers Ship Res Inst 30(4) 1-69... 

Inasaka F, Nariai H (1987) Critical heat flux and flow characteristics of subcooled flow boihng in narrow tubes. JSME Int J 30 1595-1600... 

Mudawar I, Bowers MB (1999) Ultra-high critical heat flux (CHF) for subcooled water flow boiling. I CHF data and parametric effects for small diameter tubes. Int J Heat Mass Transfer 42 1405-1428... 

Effect of surface roughness. CHF for rough surfaces was measured on vertical annular tubes cooled by a downward flow of subcooled water by Durant and Mirshak (1959, 1960). An increase in the apparent critical heat flux of as much as 100% over a smooth surface was obtained at the same coolant velocity, temperature, and pressure. The heated surfaces were 304 SS and Zircaloy-2 tubes about... 

Dean, R A., R. S. Dougall, and L. S. Tong, 1971, Effect of Vapor Injection on Critical Heat Flux in a Subcooled R-l 13 (Freon) Flow, Proc. Int. Symp. on Two-Phase Flow Systems, Haifa, Israel. (6) Deane, C. W., and W. M. Rohsenow, 1969, Mechanism and Behavior of Nucleate Boiling Heat Transfer to the Alkali Liquid Metals, USAEC Rep. DSR 76303-65, Massachusetts Institute of Technology, Cambridge, MA Also in 1970, Liquid Metal Heat Transfer and Fluid Dynamics J. C. Chen and A. A. Bishop, Eds., ASME Winter Annual Meeting, New York. (4)... 

Kutateladze, S. S., 1959, Critical Heat Flux in Subcooled Liquid Flow, Energetika 7 229-239. (2) Kutateladze, S. S., 1963, Fundamentals of Heat Transfer, Academic Press, New York. (2)... 

Vliet, G. C., and G. Leppert, 1962, Critical Heat Flux for Subcooled Water Flowing Normal to a Cylinder, ASME Paper 62-WA-174. Winter Annual Meeting, ASME, New York. (5)... 

V. I. Gomelauri and T. S. Magrakvelidze, Mechanism of Influence of Two Dimensional Artificial Roughness on Critical Heat Flux in Subcooled Water Flow, Therm. Eng. (25/2) 1-3,1978. 

FIGURE 15.117 Effect of tube length in critical heat flux and power input at the CHF condition (calculated from the correlation of Bowring [293] for water for a mass flow of 3000 kg/m2s, a tube diameter of 0.01 m, a pressure of 6 MPa, and zero inlet subcooling) (from Hewitt [291], with permission from The McGraw-Hill Companies). 

C. H. Lee, I. Mudawar, A mechanistic critical heat flux model for subcooled flow boiling based on local bulk flow conditions, Int. J. Multiphase Flow, 1988, 34, 711-728. 

H. Zhang, 1. Mudawar, M. M. Hasan, Photographic study of high-flux subcooled flow boiling and critical heat flux, Int. Com. Heat Mass Transfer, 2008, 35, 793-799. 

D. D. HaD, I. Mudawar, Critical heat flux (CHF) for water flow in tubes - 11. Subcooled CHF correlations, Int. J. Heat Mass Tranfer, 2000, 43, 2605-2640. 

The phenomenon of flow instability is a result of interaction between pressure drop and coolant flow in heated channels. For the heated channels, the pressure drop as a function of mass flow deviates from m -dependency at low flow rates and shows a minimum. Before the minimum, any decrease in the flow rate results in an increase of the pressure drop with the consequence of low local pressure and saturation temperature. The minimum in the pressure drop/mass flow kurve depends on flow characteristics and heat flux. The determination of critical heat flux at the onset of flow instability has been experimentally inverstigated by whittle and Forgan /6/ for the coolant chaimels conditions exiting in MTR. They measured the mass flow, exit temperature and pressure drop corresponding to minima in the pressure drop -vs- flow rate kurve for subcooled water flowing (upward and downward) in norrow heated channels (width 2.54 cm, thickness 0.14 to 0.32 cm, and length 40 to 61 cm) under the followong conditions ... 

All modules use the 2-fluid model to describe steam-water flows and four non-condensable gases may be transported. The thermal and mechanical non-equilibrium are described. All kinds of two-phase flow patterns are modelled co-current and counter-current flows are modelled with prediction of the counter-current flow limitation. Heat transfer with wall structures and with fuel rods are calculated taking into account all heat transfer processes ( natural and forced convection with liquid, with gas, sub-cooled and saturated nucleate boiling, critical heat flux, film boiling, film condensation). The interfacial heat and mass transfers describe not only the vaporization due to superheated steam and the direct condensation due to sub-cooled liquid, but also the steam condensation or liquid flashing due to meta-stable subcooled steam or superheated liquid. 

The static limit is the non-linear limit of conditionally instability, where departure from nucleate boiling or critical heat flux will occur at low and high qualities, respectively. There are sufficient data in the literature which show that instability in multiple channels precedes the limit of classic single channel (mass-flow controlled) dryout (Mathison, 1967) (D Arcy, 1967). This differs from the result for the zero frequency condition, which can only be written as a cubic in, (Ns / Np), and does not give a critical subcooling number. The condition of static instability in parallel channels is the Ledinegg condition (Saha et al, 1976) (Duffey and Hughes, 1991),... 

---