Subcooled Flow Boiling

The same conclusion is evident from results obtained by Hino and Ueda (1975) and presented above in Fig. 6.4. The conclusion that A7s is almost unaffected by inlet flow velocity as at 7) -C 1 as at Z) < 1 was established from experiments carried out in the channels of diameters about d = 1—10 mm. What has been commonly observed at incipient boiling for subcooled flow in channels of this size is that small bubbles nucleate, grow and collapse while still attached to the wall, as a thin bubble layer formed along the channel wall. 

Figure 3.19 Configuration of bubble layer as affected by flow rate at high subcooling (Freon-118) (a) low-velocity boiling flow (b) high-velocity boiling flow. (From Tong et al., 1966b. Copyright 5 1966 by American Society of Mechanical Engineers, New York. Reprinted with permission.)...

Figure 5.5 Vertical upward, high-velocity boiling flow of Freon-113 mass flux 1.06 x 106 lbn/hr ft2 (1,432 kg/m2 s) bulk subcooling 66°F (37°C) pressure 41 psig (0.38 MPa) heat flux 14,300 Btu/hr ft2 (45,000 W/m2). (From Tong, 1965. Reprinted with permission.)...

Jeglic, F. A., and T. M. Grace, 1965, Onset of Flow Oscillations in Forced Flow Subcooled Boiling, NASA-TN-D 2821, NASA Lewis Res. Ctr., Cleveland, OH. (6)... 

Sekoguchi, K., O. Tanaka, T. Ueno, M. Yamashita, and S. Esaki, 1982, Heat Transfer Characteristics of Boiling Flow in Subcooled and Low Quality Regions, 7th Int. Heat Transfer Conf. Paper FBI 2, Munich, Germany, Hemisphere, Washington, DC. (4)... 

Thom, J. R. S., W. M. Walker, T. A. Fallon, and G. F. S. Reising, 1966, Boiling in Subcooled Water during Flow up Heated Tubes or Annuli, Proc. Inst. Mech. Eng. 180(Part 3C). (3)... 

Tong, L. S., L. E. Efferding, and A. A. Bishop, 1966b, A Photographic Study of Subcooled Boiling Flow and DNB of Freon-113 in a Vertical Channel, ASME Paper 66-WA/HT-39, Winter Annual Meeting, ASME, New York. (3)... 

Figure 7 shows the variation of the two-phase heat transfer coeffient (htp) as a function of the local subcooling parameter (Sc), for subcooled flow boiling. The subcooling parameter is defined as. 

Single-phase flow region At the inlet, the liquid is below its boiling point (subcooled), and heat is transferred by forced convection. The equations for forced convection can be used to estimate the heat transfer coefficient in this region. 

Regarding two pliase flow, pressurized liquid above its noniial boiling point will start to flash when released to aUiiospheric pressure, and two pliase flow will result. Two-pliase flow is also likely to occur from depressurization of tlie vapor space above a volatile liquid, especially if the liquid is viscous (e.g., greater tlian 500 cP) or has a tendency to foam. Fauske and Epstein liave provided tlie following practical calculation guidelines for two-phase flashing flows. The discharge of subcooled or saturated liquids is described by... 

Figure 8 shows that increasing the heat flux at constant mass velocity causes the peak in wall temperature to increase and to move towards lower enthalpy or steam quality values. The increase in peak temperature is thus due not only to a higher heat flux, which demands a higher temperature difference across the vapor film at the wall, but to a lower flow velocity in the tube as the peaks move into regions of reduced quality. The latter effect of lower flow velocity is probably the dominant factor in giving fast burn-out its characteristically rapid and often destructive temperature rise, for, as stated earlier, fast burn-out is usually observed at conditions of subcooled or low quality boiling. 

During the subcooled nucleate flow boiling of a liquid in a channel the bulk temperature of the liquid at ONB, 7b, is less than the saturation temperature, and at a given value of heat flux the difference ATsub.oNB = 7s - 7b depends on L/d. The experimental parameters are presented in Table 6.2. 

The observed ratio / = Lp/Ln, is quite different from that reported for subcooled flow boiling of water in tubes of 17-22 mm inner diameter. Prodanovic et al. (2002) reported that this ratio was typically around 0.8 for experiments at 1.05-3 bar. The situation considered in experiments carried out by Hetsroni et al. (2003) is however different as the bubbles undergo a significant volume change and the flow is unstable. Ory et al. (2000) studied numerically the growth and collapse of a bubble in a narrow tube filled wifh a viscous fluid. The situation considered in that study is also quite different from experiments by Hetsroni et al. (2003) as, in that case, heat was added to the system impulsively, rather than continuously as we do here. 

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