Takeyuki Ami

CHF in a Circumferentially Nonuniformly Heated Tube Under Low-Pressure and Low-Mass-Flux Condition (Influence of Inclined Angles Under High-Heat-Flux Condition)

Critical heat flux (CHF) is an important design factor for boiling two-phase flow equipment, such as boilers and others. In actual boiling systems, the water tube suffers from the nonuniform heating and/or tube inclinations. The objective of this investigation is to understand the influence of tube inclination on CHF characteristics under such high-heat-flux conditions. The experimental investigation was conducted with a forced convective boiling system by using a uniformly heated tube and a nonuniformly heated tube set at arbitrary inclination angles ϕ. The obtained CHF was strongly influenced by the circumferential location of local maximum heat flux point and tube inclination. In the case of the normal tube, the CHF always occurred by the liquid film dryout at the top of the tube. In the case of the nonuniformly heated tube, the influence of the inclination on the CHF characteristics strongly depended on the circumferential heat flux distribution. When the the heat flux at the bottom was higher than that at the top, two types of CHF mechanism, namely, low-quality CHF upstream of the test section under high-mass-flux condition, and liquid film dryout at the tube exit under low-mass-flux condition, were observed. When the heat flux at the top was higher than that at the bottom, intermittent dryout was observed as the dryout mechanism. These CHF characteristics could be categorized by using the CHF ratio against the value of the vertical upward flow with the modified Froude number, which corresponded to the influence factor of disturbance wave.

Flow Pattern and Boiling Heat Transfer of CO2 at High Pressure in Horizontal Mini-Channels

Experimental investigation was conducted with CO2 at high pressure, ranging 5.0 to 6.5 MPa, in horizontal mini-channels of 0.51, 1.0 and 2.0 mm in diameters. In smaller bores tube, e.g. 0.51 and 1.0 mm, the phase stratification is not serious and the flow pattern becomes quasi-axi-symmetric, while in 2.0 mm or larger tubes phase stratification becomes significant as in conventional sized tubes. This phase stratification, together with the intermittent flow behavior, causes the heat transfer deterioration at the upper wall. Existing criteria as a whole were insufficient in predicting flow patterns. In order to predict complicated two-phase flow behavior, including inherent void fraction fluctuation and flow patterns, the discrete bubble model based on a pattern dynamics approach was developed, so as to be applicable to the horizontal mini-channels. The simulated void fraction fluctuations successfully provided not only the flow pattern maps in terms of statistical properties, but also quantitative estimation of the heat transfer deterioration at the upper wall of the tube of 2.0 mm in diameter. Existing correlations were evaluated through the comparison with the present experimental data, and a nucleate boiling mode was found a dominant mode over the boiling heat transfer at high pressure in mini-channels.Copyright