Convection Film Boiling Heat Transfer Research Articles

Forced convection film boiling heat transfer model for a sphere by scaling analysis

Forced convection heat transfer phenomenon over a sphere has been carried out based on scale analysis under film boiling condition for the high subcooled and low subcooled condition of liquid, respectively. Heat and mass transfer across the liquid–vapor interface has been studied incorporating appropriate vaporization criteria. The heat transfer characteristic is shown for relevant nondimensional numbers, and Nusselt number has been obtained separately for high subcooling and low subcooling of liquid situations, respectively. A wide set of physical parameters with ranges of wall superheat 280–1000 °C, liquid subcooling 0–30 °C and flow velocity 0.1–0.3 m/s have been utilized to form the model closure, and the model is validated with the analytical model from Epstein and Hauser (Int J Heat Mass Transf 23:179–189, 1980), Kobayasi (J Nucl Sci 2:62–67, 1965), Witte (Ind Eng Chem Fundam 7:517–518, 1968) and Jouhara and Axcell (Nucl Eng Des 239:1885–1900, 2009) experiment.

Forced Convection Film Boiling Heat Transfer Over a Vertical Cylinder

The knowledge of subcooled film boiling heat transfer is important as the basis of understanding the reflooding phenomenon during emergency cooling in a nuclear reactor under a loss-of-coolant accident. In this study, forced convection film boiling heat transfer from a vertical cylinder in Freon-113 flowing upward along the cylinder was measured for the flow velocities ranging from 0 to 1.3 m/s, and liquid subcoolings ranging from 0 to 20 K at pressures near atmospheric. A platinum heater with a diameter of 3 mm was heated by electric current. The heat transfer coefficients obtained are almost independent of vertical positions on the cylinder. The heat transfer coefficients are almost independent of velocity for the velocities lower than about 1 m/s and become higher for the velocities higher than 1 m/s. The heat transfer coefficients at each velocity are higher for higher liquid subcoolings. Improvement of film boiling heat transfer from the vertical cylinder with the increase in flow velocity is much less than that of horizontal cylinder in cross flow previously reported by the authors. This is mainly due to the difference of heat transfer enhancement mechanism; the former is the drag force on vapor flow acted by a liquid flow, and the latter is the pressure gradient near the front stagnation point caused by external potential flow.

垂直円柱発熱体における強制対流膜沸騰熱伝達の実験的研究

The knowledge of subcooled film boiling heat transfer is important as the basis of understanding the reflood-ing phenomenon during the emergency cooling in a nuclear reactor under a loss of coolant accident. In this study, forced convection film boiling heat transfer from a vertical cylinder in Freon-113 flowing upwards along the cylinder was measured for the flow velocities ranging from 0 to 1.3 m/s, and liquid subcoolings ranging from 0 to 20 K at pressures near atmospheric. A platinum heater with a diameter of 3 mm was heated by electric current. The heat transfer coefficients obtained are almost independent of vertical positions on the cylin-der. The heat transfer coefficients are almost independent of velocity for the velocities lower than about 1 m/s and become higher for the velocities higher than 1 m/s. The heat transfer coefficients at each velocity are higher for higher liquid subcoolings. Improvement of film boiling heat transfer from the vertical cylinder with the increase in flow velocity is much less than that of horizontal cylinder in cross flow previously reported by the authors. This is mainly due to the difference of heat transfer enhancement mechanism; the former is the drug force on vapor flow acted by a liquid flow and the latter is the pressure gradient near the front stagnation point caused by external potential flow.

Film boiling heat transfer from a vertical cylinder in forced flow of liquids under saturated and subcooled conditions at pressures

Forced convection film boiling heat transfer on a vertical 3-mm diameter and 180-mm length platinum test cylinder located in the center of the 40-mm inner diameter test channel was measured. Saturated water, and saturated and subcooled R113 were used as the test liquids that flowed upward along the cylinder in the test channel. Flow velocities ranged from 0 to 3 m s −1, pressures from 102 to 490 kPa, and liquid subcoolings for R113 from 0 to 60 K. The heat transfer coefficients for a certain pressure and liquid subcooling are almost independent of flow velocity and of a vertical position on the cylinder for the flow velocities lower than ≈1 m s −1 (the first range), and they become higher for the velocities higher than ≈1 m s −1 (the second range). Slight dependence on a vertical position being nearly proportional to z −1/4, where z is the height from the leading edge of the test cylinder, exists for the flow velocities in the second range. The heat transfer coefficients at each velocity in the first and second ranges are higher for higher pressure and liquid subcooling. Correlation for the forced convection film boiling heat transfer with radiation contribution on a vertical cylinder was derived by modifying an approximate analytical solution for a two-phase laminar boundary layer model to agree better with the experimental data. It was confirmed that the experimental data of film boiling heat transfer coefficients in water and R113 were described by the correlation within ±20% difference.

Numerical prediction of forced convection film boiling heat transfer from a sphere

Forced convection film boiling over a sphere is modeled by applying the laminar boundary-layer approximation for both the vapor and liquid flows. In the vapor momentum equation, a buoyancy term is included, which has often been neglected in past analyses. The solution is obtained numerically to the point of flow separation. From the analysis of forced convection film boiling over a sphere submerged in water, it is shown that the vapor film thickness is in the submillimeter range, and as the subcooling of liquid increases, the film thickness becomes thinner, down to the order of ten microns. It is also observed that the buoyance force may not be neglected in the liquid velocity range considered in this study (up to 7 m s −1). The heat transfer results of the present model agree qualitatively with available experimental data, although quantitatively the model generally underestimates the data. Using the results of the present analysis, an improved correlation for the convective heat transfer to the bulk liquid in subcooled film boiling of a sphere in water is proposed for use in predicting the vapor generation rate.

Free Convection Film Boiling Heat Transfer From a Rotating Surface

A boundary layer model of laminar, subcooled, free convection film boiling from a rotating sphere has been developed. The conservation equations for the vapor and liquid were simplified, transformed into ordinary differential equations using an integral approach, and solved numerically. The theoretical variation of vapor film thickness with heater temperature and the resulting boiling fluxes were investigated. An experimental facility was built for the purpose of verifying the validity of the theoretical model and good agreement was found between the model and the experimental data at low rpm. The instability of the vapor film near the minimum heat flux for a rotating surface flux was also investigated.

The Effect of Surface Motion on Forced Convection Film Boiling Heat Transfer

The growth in demand for high-quality metallic alloys has placed greater emphasis on the predictability of cooling methods used in manufacturing processes. Several methods involve forced convection film boiling, which can occur on metallic strips or plates cooled by water jet impingement or on strips inside cooling jackets of continuous annealing processes. Since surface temperatures are typically well above the boiling point of water, a substantial portion of the surface area can involve film boiling. The strip or plate speed often exceeds the water velocities and strongly influences boundary layer development in the vapor and liquid. The purpose of this paper is to estimate the effect of plate motion on heat transfer in the film boiling regime. Conservation equations for mass, momentum, and energy have been solved by the integral method for film boiling in forced convection boundary layer flow on a flat isothermal plate in motion parallel to the flow direction. Unlike previous studies, which have shown that heat transfer is chiefly governed by the plate and subcooled liquid temperatures, heat transfer is shown to also depend on the plate velocity. For large velocities, the importance of radiation heat transfer across the vapor layer is reduced. However, when the velocities of the plate and liquid are oppositely directed and of nearly equal magnitude, radiation across the vapor layer can become significant, even at low plate temperatures.

Film Boiling Heat Transfer on an Isothermal Vertical Surface

Natural convection film boiling heat transfer of saturated liquids on an isothermal vertical surface is investigated both experimentally and theoretically. Local heat transfer coefficients are obtained at steady-state conditions on a 6.3-cm-wide and 10.3-cm-high heated surface which was machined from a large block of copper. Effectively isothermal surface conditions are achieved even for wall superheats up to 450 K. Experiments conducted with water at 1 atm pressure show that predictions from existing theoretical models are inadequate and a detailed consideration of the liquid-vapor interface behavior is required. Still and motion pictures of the vapor film are taken and data on vapor film thickness, interfacial wave behavior, and bubble detachment characteristics are obtained to build an analytical model for predicting film boiling heat transfer. This time-dependent model based on laminar flow in the film incorporates the effects of both large-scale and small-scale waves. Results from analysis are compared with experimental data.

Forced Convection Film Boiling Heat Transfer from a Horizontal Cylinder to Liquid Cross-flowing Upward : 2nd Report, Subcooled Liquid

Forced Convection Film Boiling Heat Transfer from a Horizontal Cylinder to Liquid Cross-flowing Upward : 2nd Report, Subcooled Liquid

Forced Convection Film Boiling Heat Transfer Form a Horizontal Cylinder to Liquid Cross-Flowing Upward : 2nd Report, Subcooled Liquid

水平円柱の上向き直交流における過冷液の強制対流膜沸騰熱伝達に前報の飽和液の場合の解析を拡張して, 大気圧の水, エタノールおよびヘキサンに対するパラメータに対して数値解を決定した. 計算結果により, 過冷液の場合の速度分布境界層のはく離点, 境界層厚さおよび平均ヌセルト数に関して考察し, さらに近寄り速度の大きさとこれらの関係を検討した. また計算結果と測定値との比較を行った.

Natural convection film boiling on a vertical surface

Experimental results are presented for an investigation of free convection film boiling heat transfer to saturated ethanol, saturated Freon 113, and saturated distilled water from a vertical cylinder. High speed motion pictures of the vapor-liquid interface in film boiling have been obtained. All tests have been conducted at atmospheric pressure over a range of heat flux between 9000 and 37000 BTU/hr ft(2). A study of the stability of the vapor-liquid interface is performed and the results are compared to the photographic data. Parameters influencing the heat transfer process are developed and the experimental data is used to determine their proper relationship. The heat transfer coefficient in natural convection film boiling of saturated liquids on moderately long vertical surfaces at constant heat flux is shown to be insensitive to vertical position, and to be dependent only on the temperature differential and the fluid properties. Large scale waves have been observed on the vapor-liquid interface: however, their effect on the heat transfer is minimal. It appears that intense vaporization at the interface plays an important role in the generation of these waves, while surface tension does not. A strong possibility of liquid droplet entrainment in the vapor flow is indicated by the presence of a rough, wavy interface, a relatively high speed vapor-flow, and by the unexpectedly strong dependence of the heat transfer on the latent heat of vaporization.

Forced Convection Film Boiling Heat Transfer from a Horizontal Cylinder to Liquid Cross-flowing Upward : 1st Report, Saturated Liquid

Forced convection film boiling heat transfer from a horizontal cylinder to a subcooled liquid cross-flowing upward is analysed based on the two-phase boundary-layer theory. Numerical solution of the conservation equations is determined for subcooled water, ethanol and hexane under the atmospheric pressure by the method similar to that of the first report for saturated liquid. The velocity profile, the separation point in the vapor film, the thickness of the boundary-layer and the average Nusselt number are discussed in the same manner as in the previous report and the effects of the subcooling and the approaching velocity on them are also examined. The calculated results on heat transfer are compared with the experimental data.

Forced Convection Film Boiling Heat Transfer from a Horizonal Cylinder to Liquid Cross-Flowing Upward : 1st Report, Saturated Liquid

Forced Convection Film Boiling Heat Transfer from a Horizonal Cylinder to Liquid Cross-Flowing Upward : 1st Report, Saturated Liquid