Low Wall Superheat Research Articles

Experiment on Steady Pool Transition Boiling of Water on a Horizontal Copper Heated Surface.

Transition boiling heat transfer is studied by carrying out an experiment on steady pool boiling of water under atmospheric pressure and saturated condition on a horizontal copper disc 10 mm in diameter. Bubble (vapor mass) departure diameter and frequency were measured using high-speed video movies and an impedance probe, and they are well correlated as a function of heat flux. It is found that in transition boiling, heat transfer characteristics at low and at high wall superheats are considerably different. The macrolayer evaporation model, which was first proposed by Katto and Yokoya in 1970, is revised on the basis of nucleation heat flux previously measured by the authors to explain the transition boiling heat transfer at low wall superheat.

An experimental study of saturated pool boiling from downward facing and inclined surfaces

Pool boiling curves for inclinations of 0° (downward facing), 5°, 10°, 15°, 30°, 45°, and 90° are obtained by quenching a 12.8-mm thick copper disk having a diameter of 50.8 mm in a pool of saturated water. Results show that nucleate boiling heat flux decreases as the angle of inclination is increased. However, the decrease in nucleate boiling heat flux with inclination is more pronounced at lower wall superheats. Conversely, the transition boiling heat flux and both qCHF and qmin, as well as the corresponding wall superheat, increase with surface inclination. The quenching time of a downward facing surface having an initial wall superheat of 160 K is about six times that for a 5° inclination and 23 times that for a 90° inclination. For all inclinations, quenching always begins at the lowermost position and propagates upward. The average quenching velocity of about 2.6 cm s−1, is almost constant for θ⩾ 45°, but increases rapidly with the decrease in surface inclination reaching approximately 6.3 cm s−1 at 0°. Des courbes d'ébullition en réservoir pour des inclinaisons de 0° (face vers le bas), 5°, 10°, 15°, 30°, 45° et 90° sont obtenues en trempant un disque de cuivre de 12,8 mm d'épaisseur ayant un diamètre de 50,8 mm dans une eau saturée en réservoir. Les résultats montrent que le flux thermique diminue quand l'angle d'inclinaison augmente. Néanmoins cette diminution est plus prononcée aux surchauffes de la paroi inférieure. Inversement, le flux thermique de transition en même temps qCHF et qmin aussi bien que la surchauffe de la paroi augmentent avec l'inclinaison de la surface. Le temps de trempe d'une surface tournée vers le bas, ayant une surchauffe initiale de 160 K, est environ six fois celui pour une inclinaison de 5° et 23 fois celui pour 90°. Pour toutes les inclinaisons, la trempe commence toujours à la position inférieure et se propage vers le haut. La vitesse moyenne de trempe, de 2,6 cm s−1 environ, est à peu près constante pour θ ⩽ 45° mais elle augmente rapidement quand l'inclinaison de la surface décroît, pour atteindre approximativement 6,3 cm s−1 à 0°. Durch Abkühlen einer 12,8 mm dicken Kupferscheibe mit einem Durchmesser von 50,8 mm in einem mit gesättigtem Wasser gefüllten Behälter ergeben sich Siedekurven für folgende Neigungswinkel: 0° (nach unten gerichtet), 5°, 10°, 15°, 30°, 45° und 90°. Die Ergebnisse zeigen, daβ die Wärmestromdichte beim Blasensieden mit steigendem Neigungswinkel abnimmt. Die Abnahme der Wärmestromdichte mit zunehmendem Winkel ist bei kleineren Wärmestromdichten ausgeprägter. Dagegen steigt die Wärmestromdichte beim Übergangssieden, wie auch die maximale und die minimale Wärmestromdichte und die zugehörige Wandüberhitzung, mit dem Neigungswinkel an. Die Abkühlzeit einer nach unten gerichteten Oberfläche mit einer anfänglichen Wandüberhitzung von 160 K beträgt ungefähr das 6-fache der um 5° geneigten und das 23-fache der um 90° geneigten Fläche. Bei allen Winkeln beginnt die Abkühlung immer an der untersten Stelle und schreitet nach oben hin fort. Die mittlere Abkühlgeschwindigkeit von ungefährt 2,6 cm s−1 ist für Winkel θ ⩾ 45° nahezu konstant; sie wächst jedoch für kleinere Neigungswinkel stark an und erreicht schlieβlich 6,3 cm s−1 bei 0°. Пpй зaкaлкe мeдиcкa тoлщиHoй 12,8 H дHaмeтpoи 50,8 мм V бoльщoм oбъeмe HacыщeHHoй Voды пoлyчeHы кpиVыe кипeHия для yблoV HaклoHa, cocтaVляющич 0° (иccлeдyeмaя пoVepчHocть oбpaщeHa VHиз), 5°, 10°, 15°, 30°, 45° и 90°. peзyльтaты пoкaзыVaют, чтo тeплoVoй пoтoк пpи пyзыpькoVoм кипeHии yмeHьшaeтcя c pocтoм yглa HaклoHa, пpичeм этa тeHдeHция бoлee VыpaзeHa пpи мaлыч тeплoVыч пoтoкaч. HecтaциoHapHый зe тeплoVoй пoтoк, a тaкзe qKTn, qMHH и cooтVeтcтVyющий пepeгpeV cтeHки Voзpacтaют c yVeличeHиeм HaклoHa пoVepчHocти. Vpeмя зaкaлки oбpaщeHHoй VHиз пoVepчHocти c HaчaльHым пepeгpeVoм cтeHки, paVHым 160 К, пoчти V шecть paз бoльшe, чeм пpи yглe HaклoHa, cocтaVляющeм 5°, и V 23 paзa бoльшe, чeм пpи yглe HaклoHa 90°. Для Vceч yглoV HaклoHa пpoцecc зaкaлки HaчиHaeтcя cHизy и pacпpocтpaHяeтcя VVepч. cpeдHяя cкopocть зaкaлки, cocтaVляющaя oкoлo 2,6 cм · c−1, пoчти пocтoяHHa пpи θ ⩾ 45°, Ho быcтpo пoVышaeтcя c yмeHьшeHиeм HaклoHa пoVepчHocти, дocтигaя пpимepHo 6,3 cм · c−1 пpи 0°.

Steady-state post-critical heat flux heat transfer to a refrigerant

Heat transfer in the post-critical heat flux (CHF) regime was studied experimentally under steady-state conditions producing low superheat temperature of the heated surface. Experiments were performed with the vertical flow of refrigerant-12 in a tube with inside diameter of 7.75 mm over the mass flux range of 182 to 808 kg/sq m s at a pressure of 1 MPa. Liquid heating produced the low wall-superheat in the post-CHF region at steady state, which is typical of heat exchanger operation. Superheated vapor measured at the test section exit in most tests ensured that the entire post-CHF region was included. Heat transfer results were compared to predictions from nine post-CHF correlations and models. The potential for thermodynamic nonequilibrium diminishes and direct wall-drop heat transfer is enhanced when wall-superheat is low. These two phenomena were considered in the comparisons, and the best predictions of independent data for two fluids, refrigerant-12 and water, were obtained from a single correlation that included both phenomena explicitly. 15 refs.

ENHANCED POOL BOILING HEAT TRANSFER WITH SURFACE ATTACHMENT

Abstract A pool boiling experiment with either water or Freon-113 was conducted to investigate nucleate boiling from a heated wall with either a spherical or a cylindrical attachment. The result revealed that nucleate boiling can be enhanced by applying a horizontal cylindrical attachment to a vertical heated wall, owing to the favorable thermal environment characterized by a small-gradient liquid temperature profile within the restricted regions between the attachment and the heated wait. Nucleate boiling is enhanced in terms of a lower wall superheat required for incipient boiling and more bubbles generated than from an open heated wall. As a result of the enhanced nucleate boiling, heal transfer of the vertical heated wall above the attachment was improved due to excessive bubbles moving upward along the heated wall, causing removal of the thermal layer near the wall and evaporation of the thin liquid film between the bubbles and the wall. The boiling curve hysteresis with Freon-113 was significantly r...

HEAT TRANSFER TO DISPERSED SWIRL FLOW OF HIGH-PRESSURE WATER WITH LOW WALL SUPERHEAT

Heat transfer was studied to two-phase flow in the post-CHF dispersed flow regime under swirl conditions. Data were used for post-CHF water flow at high pressure, 15.9 MPa, inside a vertical tube with twisted-tape inserts. Data used were obtained under test conditions typical of heat exchanger operation with low wall superheats of 5-40°C. The mass flux range of the data was 910-1878 kg/m2 s, and three tape-twist ratios were used: 2.55, 5.03, and 7.52. The low wall superheat and the relatively high mass fluxes resulted in higher than predicted drop-wall heat transfer. The only existing heat transfer correlation for this flow situation was modified to include this heat exchanger parameter range.

Post-CHF two-phase flow with low wall-superheat

Abstract Heat transfer was measured to a two-phase flow in the post-CHF region under liquid-heated conditions and low wall-superheat. The boiling fluid was water at high pressure, 7.0–15.3 MPa, and mass fluxes in the range of 720–3200 kg/m 2 s. Experiments were performed in a vertical tube with a 10 mm inside diameter and a 13.1 m heated length. In considering the effects of thermodynamic non-equilibrium and direct drop-wall heat transfer at the low wall-superheats of this investigation (25–100°C), 13 correlations, models and analysis were compared to the data. A review of these formulations is presented including development bases and approaches. The effects of both non-equilibrium and drop-wall heat transfer were evaluated, and recommendations are presented for post-CHF heat transfer predictions at low wall-superheat.

Turbulent forced convective heat transfer in two-phase evaporating droplet flow through a vertical pipe

Abstract A physical model was developed to study heat transfer in turbulent dispersed flow at very high vapor quality in a vertical pipe by numerically solving the coupling governing differential equations for both phases. Major heat transfer mechanisms included in the model were the thermal nonequilibrium effects, droplet vaporization, droplet deposition on the duct wall and thermal radiative transfer. The predicted results indicated that vapor superheating is dominant for the cases with high wall superheat, otherwise droplet vaporization dominates the energy transport processes. Heat transfer during the droplet-wall interaction only exists at low wall superheat but in small amounts.

Laminar film boiling on a porous vertical wall with uniform suction velocity

AbstractLaminar free convection film boiling on a porous vertical wall with uniform suction or blowing is analysed using boundary layer theory. The solutions are obtained assuming suction or blowing to be a disturbance superposed on the isothermal, impermeable wall case. Using a parameter involving the suction or blowing velocity, universal functions are derived for various values of Prandtl Number and cp(Tw — Tsat)/hfgPr. These universal functions can be used to estimate the heat transfer rate in the presence of suction or blowing. As expected, suction increases the heat transfer rate while blowing decreases the heat‐transfer. Even small velocities of suction or blowing could significantly affect the heat transfer. It is also found that the effects of suction or blowing are more pronounced at lower wall superheats.

The Determination of Forced-Convection Surface-Boiling Heat Transfer

The characteristics of the boiling curve for forced-convection surface boiling are examined in detail. In the region of low wall superheat, the heat transfer can be predicted by available correlations for forced convection. An analysis is presented for the inception of first significant boiling. Experimental results are in good agreement with analytical predictions. Pool-boiling data were taken under saturated and subcooled conditions for surfaces similar to those used in forced-convection surface boiling. These data indicate that the curves for forced-convection surface boiling cannot be based on data for saturated pool boiling but must rather be based on actual forced-convection data.