A moving‐front transition boiling model

Abstract

In accordance with visual observations, a transition boiling model that assumes a quench front which alternately advances and retreats in the water flow direction, is proposed. With appropriate choice of a quench zone width and heat transfer rate when the surface is wet, the model predictions are in the same range as the observed temperature oscillations. SCOPEHeater wall temperature oscillations are characteristic of transition boiling. In flow boiling, the mechanism governing transition boiling depends on the volume fraction of vapor present. At high vapor volume fractions, annular flow occurs and a thin film of liquid is present on the wall during nucleate boiling. Transition boiling behavior at high volume fractions is governed by behavior in the region where this liquid film disappears.In accordance with visual observations and temperature measurements, it was postulated that at high vapor fractions the characteristic temperature fluctuations are caused by the vertical oscillation of the edge of the liquid film on the heater surface. The temperature oscillations computed from this moving front model were compared with experimental observations taken in a test section heated by hot mercury.The observed temperature oscillations were also used to estimate wetted area fractions during transition boiling. By relating these wetted area fractions to the aforementioned analytical model, a means for prediction of transition boiling heat transfer rates in the mercury-heated test section was developed. CONCLUSIONS AND SIGNIFICANCEThe present work strongly supports the assumption that transition boiling temperature fluctuations in flowing systems at high vapor fractions are caused by vertical oscillations of the wetting front. Not only is this model in accord with recent visual observations, but it leads to predictions of wall temperature oscillations whose magnitudes are in general agreement with measurements. Further, it explains the high crosscorrelation coefficient between temperature variations at the same axial elevation but different circumferential locations. The model also leads to heat transfer coefficient predictions which are in

Benetzungsvorgänge beim partiellen Filmsieden und in Austrocknungszonen beheizter Zweiphasenströmungen

“…Messungen von Kao und Weisman [4,8] mit siedendem Wasser bei niedrigem Druck best/itigen im wesentlichen die hier bei einem Druck yon 5 bar erzielten Ergebnisse. Eine am ganzen Umfang auf gleicher H6he vorrfickende Flfissigkeitsfront mfigte daher positive Laufzeiten liefern.…”

Transition boiling heat transfer during reflooding transients

“…After a brief review of the steady state model, this note shows how the results of the steady state analysis for vertical surfaces may be used to obtain heat transfer rates during reflooding transients. Kao and Weisman (1985) concluded that during steady state transition boiling at high vapor fractions, intermittent wetting of a vertically oriented heater surface is primarily due to the vertical oscillation of the liquid film on the surface. That is, the upper edge of the liquid film moves up and down in a direction parallel to the vertical heater surface.…”