Flow and heat transfer regimes during quenching of hot surfaces
56
Citation
28
Reference
10
Related Paper
Citation Trend
Keywords:
Subcooling
Subcooled flow boiling in water is thought to be advantageous in removing high heat load of more than 10 MW/m2. Characteristics of the critical heat flux (CHF), which determines the upper limit of heat removal, are very important for the design of cooling systems. In this paper, studies on subcooled flow boiling CHF, which have been conducted by the authors, are reported. Experiments were conducted using direct current heating of stainless steel tube. For uniform heating conditions, CHF increment in small diameter tubes (1 - 3 mm inside diameter) and the CHF characteristics in tubes with internal twisted tapes were investigated, and also the existing CHF correlations for ordinary tubes (more than 3 mm inside diameter) were evaluated. For peripherally non-uniform heating conditions using the tube, whose wall thickness was partly reduced, the CHF for swirl flow was higher than the CHF under uniform heating conditions with an increase of the non-uniformity factor.
Subcooling
Cite
Citations (6)
Microbubble emission boiling, MEB has been observed in highly subcooled boiling. Liquid subcooling of 20K has been considered a threshold temperature for MEB generation under atmospheric condition. Subcooled quasi-pool boiling of binary mixture of Ethanol and water is performed with flat heating surface of 10mm in diameter in the field of ultrasonic vibration at liquid subcooling of 20K. Microbubble emission boiling is remarkably generated and the heat flux increases considerably higher than the critical heat flux as same case of highly subcooled boiling. Ultrasonic vibration is strongly effective on MEB acceleration at the threshold temperature. For the liquid with lower concentration of Ethanol, the instability of liquid-vapor interface is accelerated by Marangoni effect adding to the ultrasonic vibration.
Subcooling
Leidenfrost effect
Compressed fluid
Cite
Citations (0)
An investigation was conducted on the potential for altering the boiling curve through effects of high velocity and high subcooling. Experiments using water and Freon-113 flowing over cylindrical electrical heaters in crossflow were made to see how velocity and subcooling affect the boiling curve, especially the film and transition boiling regions. We sought subcooling levels down to near the freezing points of these two liquids to prove the concept that the critical heat flux and the minimum heat flux could be brought together, thereby averting the transition region altogether. Another emphasis was to gain insight into how the various boiling regions could be represented mathematically on various parts of heating surface. Motivation for the research grew out of a realization that the effects of very high subcooling and velocity might be to avert the transition boiling altogether so that the unstable part of the boiling curve would not limit the application of high flux devices to temperatures less than the burnout temperatures. Summaries of results from the study are described. It shows that the potential for averting, the transition region is good, and points the way to further research that is needed to demonstrate the potential.
Subcooling
Leidenfrost effect
Freon
Cite
Citations (0)
Subcooling
Vaporization
Leidenfrost effect
Cite
Citations (0)
Pool boiling from inclined and downward-facing flat and curved surfaces is of interest to the passive cooling of the pressure vessel lower head of a Light Water Reactor following a core meltdown accident. The effect of water subcooling on film boiling from a downward-facing curved surface was investigated experimentally. The local and surface average Nusselt numbers were correlated for both saturation and subcooling (5, 10, and 14 K) conditions. Video images of boiling surface and analysis of data demonstrated that surface rewetting in saturation boiling was hydrodynamic in nature, but thermally driven in subcooled boiling. Results confirmed that lower minimum film boiling heat flux values were associated with longer rewetting times and vice versa. Surface rewetting occurred when vapor film reached a critical thickness, {delta}{sub c}, first at the lowermost position, {theta} = 0{degree}, then sequentially at higher inclinations. For saturation boiling, {delta}{sub c} {approximately} 85 {micro}m and 180 {micro}m at {theta} = 0{degree} and 8.26{degree}, respectively. For subcooled boiling, however, {delta}{sub c} varied for {approximately}50 to 175 {micro}m, depending on the values of local inclination and water subcooling.
Subcooling
Leidenfrost effect
Boiling point
Cite
Citations (0)
The transition process to film pool boiling in microgravity is studied experimentally aboard the Chinese recoverable satellite SJ-8. A quasi-steady heating method is adopted, in which the heating voltage is controlled to increase exponentially with time. Small, primary bubbles are formed and slid on the surface, which coalesce with each other to form a large coalesced bubble. Two ways are observed for the transition from nucleate to film boiling at different subcoolings. At high subcooling, the coalesced bubble with a smooth surface grows slowly. It is then difficult for the coalesced bubble to cover the whole heater surface, resulting in a special region of transition boiling in which nucleate boiling and local dry areas can coexist. In contrast, strong oscillation of the coalesced bubble surface at low subcooling may cause rewetting of local dry areas and activation of more nucleate sites, resulting in an abrupt transition to film boiling.
Subcooling
Oscillation (cell signaling)
Leidenfrost effect
Cite
Citations (4)
Subcooling
Leidenfrost effect
Cite
Citations (11)
Subcooled quasi-pool boiling for water and for ethanol aqueous solutions of 10% by weight (10wt%) and 50wt% and ethanol in an ultrasonic field was experimentally performed for the upward flat heating surface of a copper block with 10 mm diameter under atmospheric conditions. Tested liquid subcooling was 15 K, 20 K, and 25 K for water and aqueous solutions of ethanol and 20 K, 30 K, and 40 K for 100wt% ethanol. At 20 K of liquid subcooling for water and ethanol aqueous solutions, no microbubble emission boiling (MEB) has been observed in quasi-pool boiling. Even if MEB occurs, the heat flux levels off and it turns easily to film boiling. In an ultrasonic field, MEB occurs remarkably. Then the heat flux increases to higher than the ordinary critical heat flux as observed in highly subcooled boiling. The experimental results show that the ultrasonic vibration introduces instability of the interface of liquid and vapor and accelerates MEB at 20 K of liquid subcooling for water and aqueous solutions of ethanol. At 15 K of liquid subcooling for water and aqueous solutions, no effect of ultrasonic vibration is observed. However, at 25K of liquid subcooling, the ultrasonic vibration extends MEB region to higher superheating of the heating surface for aqueous solutions of ethanol. The maximum heat flux in MEB decreases with increasing of ethanol concentration and becomes critical heat flux for 100wt% ethanol. No effect of ultrasonic vibration on boiling is observed for the 100wt% ethanol in these experiments.
Subcooling
Leidenfrost effect
Cite
Citations (24)
In highly-subcooled boiling, microbubble emitted from coalescing bubbles formed on the heating surface and the heat flux increases higher than the ordinary critical heat flux. This boiling regime is called microbubble emission boiling (MEB). In this study, the effect of ultrasonic wave on the MEB have been investigated in subcooled quasi-pool boiling condition for an upward flat copper heating surface of 10 mm in diameter. The liquid subcooling is kept at 15, 20, 25, 30K and the liquid is the aqueous solution of ethanol of 0, 10, 50 wt%. The high speed video imaging system is employed to observe the bubble behaviors on the heating surface. In the case of liquid subcooling of 20K without ultrasonic wave, no MEB occurs and it turns to film boiling immediately even if MEB occurred. On the other hand, in the case liquid subcooling of 20K and 25K with ultrasonic wave, the large coalescing bubbles are collapsed to many microbubbles and the heat flux increases surprisingly higher than the critical heat flux. Therefore, Heat transfer enhancement is remarkably introduced in the subcooled pool boiling with ultrasonic wave at liquid subcooling of 20K.
Subcooling
Leidenfrost effect
Cite
Citations (0)
An investigation was conducted on the potential for altering the boiling curve through effects of high velocity and high subcooling. Experiments using water and Freon-113 flowing over cylindrical electrical heaters in crossflow were made to see how velocity and subcooling affect the boiling curve, especially the film and transition boiling regions. We sought subcooling levels down to near the freezing points of these two liquids to prove the concept that the critical heat flux and the minimum heat flux could be brought together, thereby averting the transition region altogether. Another emphasis was to gain insight into how the various boiling regions could be represented mathematically on various parts of heating surface. Motivation for the research grew out of a realization that the effects of very high subcooling and velocity might be to avert the transition boiling altogether so that the unstable part of the boiling curve would not limit the application of high flux devices to temperatures less than the burnout temperatures. Summaries of results from the study are described. It shows that the potential for averting, the transition region is good, and points the way to further research that is needed to demonstrate the potential.
Subcooling
Leidenfrost effect
Freon
Boiling point
Cite
Citations (0)