Convection Film Research Articles

Pressure gradient and variable wall temperature effects during filmwise condensation from downward flowing vapors onto a horizontal tube

A simple model is developed to study the process of combined forced and natural convection film condensation from downward flowing vapors onto a horizontal tube with variable wall temperature, including effects of the pressure gradient and vapor shear stress. In the present work, the result of mean heat transfer shows that, for forced-convection film condensation, as the wall temperature variation amplitude, A, increases, the value of Nu Re −1/2, with inclusion of the pressure gradient effect, goes down appreciably. However, the value of Nu Re −1/2 when ignoring the pressure gradient effect will increase with A at a smaller pace. As for natural-convection film condensation, the mean heat transfer coefficients remain almost uniform for varying A, which are in good agreement with the previous work.

Effect of the Behavior of Generated Vapor on the Minimum-Heat-Flux Point During Rapid Quenching of a Thin Horizontal Wire.

Heat transfer characteristics and minumum heat flux point (MHF point) condition during rapid quenching of a thin horizontal wire were studied both experimentally and theoretically. Experiments were conducted with ethanol as a quenching liquid. The wire was either fixed or moving downward at a constant speed in an ethanol bath. When the wire was heated instantly to a high temperature, bead like bubbles were formed along the wire. This was in contrast to the previous results for a heated wire that was dipped into the ethanol bath at a relatively high velocity, where a vapor sheet was formed in the wake of the wire. Consideration was given to the effect of film boiling mode on the MHF point condition. A linear stability analysis of vapor film for natural convection film boiling on a horizontal cylinder was presented. The theoretical prediction of the critical vapor film thickness compared well with the measured value at the MHF point for the fixed wire.

Effect of Liquid Subcooling on Film Boiling Heat Transfer from Finite-Size Horizontal Surfaces Facing Downward.

The effect of liquid subcooling on the convective film boiling heat transfer from a finite-size horizontal downward-facing surface was investigated both theoretically and experimentally. The approximate solution with some simplified assumptions was obtained by solving the two-phase boundary layer equations including the energy equation for the liquid boundary layer. The average Nusselt number, Nuco, derived from the solution may be expressed in a simple form with the parameters of Grashof number, density ratio, (density-viscosity) ratio, liquid Prandtl number, dimensionless degree of wall superheating and dimensionless degree of liquid subcooling. The theoretical results were compared in terms of a ratio of the average Nusselt number for the case of subcooled liquid to that for the case of the saturated liquid, Nuco/Nuco, sat, with the experimental data obtained by Nishio et al. under the steady-state condition and those obtained by the present authors using a quenching methed. For the case of subcooled liquids, a correlation equation of heat transfer was proposed to effectively correlate the experimental data obtained under both quenching and steady-state conditions within ±15 percent.

Heat Transfer Performance of Annular Type Rotating Heat Pipes with Horizontal Axis. 3rd Report. Evaporation Characteristics of Working Fluid Film.

The evaporation patterns of methanol liquid film feeded on heated rotating tube inner wall is visualized by optical image de-rotation method, while the heat flux is measured under the same condition. It is shown that there are several flow patterns of natural convection and that the evaporation heat transfer coefficient of the natural convection film increase with the rotational acceleration but the nuclear boiling is restrained within the conditions of 15 to 325 for the ratio of rotational acceleration and gravitational one, and 0.8 to 4.2 mm for film thickness, 2500 to 75000 W/m2 for heat flux, 5 to 40 K for wall super heat.

Superheated Laminar Film Condensation on a Nonisothermal Horizontal Tube

A theoretical model is developed for the study of combined forced and natural convection film condensation from downward-flowing vapors onto a horizontal tube with variable wall temperature, including effects of superheated vapor, the pressure gradient, and vapor shear stress. The characteristics of condensate film flow and its corresponding critical angle Φ c , where the film separates from the wall, have been investigated. The mean heat transfer result shows that, as the wall temperature variation amplitude A increases, the value of Nu Re -1/2 with the inclusion of the pressure gradient effect goes down appreciably; however, the value of Nu Re -1/2 ignoring the pressure gradient effect will remain almost uniform. Furthermore, as the vapor superheating parameter Sp increases, the mean condensing heat transfer coefficient is significantly increased about 3.4 to 18.5%, depending on A.

Mixed convection film condensation from downward flowing vapors onto a sphere with variable wall temperature

A model is developed for the study of mixed- convection film condensation from downward flowing vapors onto a sphere with variable wall temperature. The model combined natural convection dominated and forced convection dominated film condensation, concerning effects of pressure gradient (P), interfacial vapor shear drag and non-uniform wall temperature variation (A), has been investigated and solved numerically. The effect of pressure gradient on the dimensionless mean heat transfer, Nu¯Re−1/2 will remain almost uniform with increasing P until \(P={2 \over 9} F \) for various corresponding available values of F. Meanwhile, the dimensionless mean heat transfer, Nu¯Re−1/2 is increasing significantly with F for its corresponding available values of P. Although the non-uniform wall temperature variation has an appreciable influence on the local film flow and heat transfer; however, the dependence of mean heat transfer on A can be almost negligible.

Mixed convection film condensation from downward flowing vapors onto a sphere with uniform wall heat flux

A model is developed for the study of mixed convection film condensation from downward flowing vapors onto a sphere with uniform wall heat flux. The model combined natural convection dominated and forced convection dominated film condensation, including effects of pressure gradient and interfacial vapor shear drag has been investigated and solved numerically. The separation angle of the condensate film layer, φs is also obtained for various pressure gradient parameters, P* and their corresponding dimensionless Grashof 's parameters, Gr*. Besides, the effect of P* on the dimensionless mean heat transfer, \(\) will remain almost uniform with increasing P* until \(\) for various corresponding available values of Gr*. Meanwhile, the dimensionless mean heat transfer, \(\) is increasing significantly with Gr* for its corresponding available values of P*. For pure natural-convection film condensation, \(\) is obtained.

Analysis of the conjugation effect during convective flow film boiling conditions in an eccentric annular test section

The objective of this investigation was to present a technique for estimating the conjugation effect on post-dryout heat transfer. To this aim, an experimental study was conducted to measure the conjugation effect on the post-dryout heat transfer using an internally heated eccentric annular test section (outside diameter (O.D.), 13.51 mm; inside diameter (I.D.), 11.54 mm). The experiments were carried out at 7 and 9 MPa with the mass flux varying from 2.0 to 5.0 Mg m −2 s −1 and the inlet vapour quality from 0.01 to 0.20. Five different minimum gap sizes between 0.06 and 1.92 mm were tested. The two-dimensional well temperature distribution on the inside surface of the heated tube was measured using a unique sliding thermocouple technique. A data reduction method was developed to determine the radial heat flux variation on the boiling surface from the temperature measurements. A numerical smoothing procedure was used to minimize the effect of the random temperature measurement error of the sliding thermocouple on the estimated radial heat flux variation. The results show that the conjugation effect can cause the local radial heat flux to deviate by as much as 17% from the average value.

An analysis of natural convection film boiling from spheres using the spherical coordinate system

The problem of natural convection film boiling on a sphere was analysed by solving the momentum and energy equations in spherical coordinates. These solutions were compared to the analytical model of Frederking and Clark based on the Cartesian coordinate system, empirical correlation of Frederking and Clark and recent experimental data of Tso et al. for boiling in various refrigerants and liquid nitrogen. For the average Nusselt number, good agreement with Frederking and Clark's model was obtained. Results using spherical coordinates yield a limiting value of 2 for the average Nusselt number near a modified Rayleigh number of 1 which could not be extracted from Frederking and Clark's model.

Projectile fin damage from propellant combustion

Thermal ablation of anodized (hard coated) aluminum fins on high-velocity , gun-launched kinetic energy penetrators is a serious problem at long ranges. It degrades flight performance by reducing stability and roll. Additionally, uneven ablation creates aerodynamic asymmetries that further increase projectile dispersion. The problem is known to begin in-bore. A fin-testing procedure is used to create in-bore heating conditions similar to those of a normal launch, without actually launching the fin. Postfired examination of the fins is then used to ascertain the fin damage mechanism for the nonlaunched fins as well as to infer the fin damage mechanism for normally launched fins. A two-phase flow interior ballistic code is also used to model the fin temperature profile for both the nonlaunch and the normal launch configuration. Qualitative comparison is made between the effects expected from modeling and those actually observed. Experimentally, the nonlaunched fins sustained mostly leading-edge damage, entailing loss of hard coat followed by the uneven downstream ablation. Modeling appears to underpredict the observed effects, but this is thought to be a consequence of neglecting, a priori, the possibility of rapid fin surface oxidation (burning). Nomenclature H = q82/l2, solution to the given heat conduction equation h = heat transfer coefficient ^con = convective film coefficient /irad = radiative film coefficient k = thermal conductivity of fin material

Forced Convection Film Condensation on a Horizontal Tube—Influence of Vapor Boundary-Layer Separation

Forced Convection Film Condensation on a Horizontal Tube—Influence of Vapor Boundary-Layer Separation

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.

Experimental Evaluation of Sector-Shaped Hydrodynamic Thrust Bearings Under Translation and Transverse Vibration

The bearing load of a plane inclined sector-shaped hydrodynamic thrust bearing, under simultaneous translation and transverse vibration, is measured experimentally. The results are used to evaluate the lubrication theory solutions. Consequently, both the influences of the unsteady film inertia, measured by the squeeze Reynolds number Res, and the convective film inertia, measured by the modified Reynolds number Re*, on load amplitude and phase are investigated. It is found that the inertia-neglected lubrication solutions underestimate: (1) the oscillatory component of the bearing load by 6.5 percent at Res = 1.0 and by 1.4 percent at Re* = 1.0, and (2) the mean component of the bearing load by 0.7 percent at Res = 1.0 and by 2.0 percent at Re* = 1.0 Moreover, the fluid inertia induces an equivalent negative spring force component which shifts the phase of the bearing load by 9.5 deg at Res =1.0 and by 4 deg at Re* = 1.0 as compared to the lubrication theory predictions. Hence it can be an important consideration when designing bearings for vibration control purposes.

Forced convective film condensation inside vertical tubes

A theoretical study of forced convective film condensation inside vertical tubes is presented. We propose a unified procedure for predicting the pressure gradient and condensation heat transfer coefficient of a vapor flowing turbulently in the core and associated with laminar or turbulent film on the tube wall. The analysis for the vapor flows is performed under the condition that the velocity profiles are locally self-similar. The laminar and turbulent film models equate the gravity, pressure and viscous forces, and consider the effect of interfacial shear. The transition from laminar to turbulent film depends not only on the liquid Reynolds number but also on the interfacial shear stress. In this work we also proposed a new eddy viscosity model which is divided into three regions: the inner region in liquid condensate near the wall; the interface region including both liquid and vapor; and the outer region for the vapor core. Comparisons of the theory with some published experimental data showed good agreement.

Forced convection film condensation on a horizontal tube—effect of surface temperature variation

In free and forced convection condensation on a horizontal tube, measurements show that the surface temperature variation with angle approximately follows a cosine distribution. For forced convection condensation of steam, relatively low (with respect to isothermal-surface theoretical values) observed vapour-side heat-transfer coefficients have been attributed to surface temperature non-uniformity. A recent theoretical study of the effect of wall temperature variation in free convection condensation shows negligible effect on the average heat-transfer coefficient. In the present paper it is shown that, when a simple surface shear stress approximation, which gives satisfactory values for the mean heat-transfer coefficient for an isothermal tube, is adopted with variable wall temperature, higher mean surface heat-transfer coefficients are obtained. This seems to be in contradiction with results of conjugate (vapour-to-coolant) solutions which predict variable wall temperature and lower mean surface heat-transfer coefficients. Reasons for this apparent anomaly are advanced.

Forced convection film boiling in the stagnation region of a molten drop and its application to vapour explosions

This work was undertaken as part of an investigation into the heat transfer mechanism related to thermal explosion. The theoretical model of forced convection film boiling in the stagnation point region of an axi-symmetrical molten drop is presented. The model is particularly relevant to the coarse pre-mixing stage of vapour explosion: it takes account of the fact that, at this stage, the hot substance is molten. Stagnation point momentum (Falkner-Skan) and energy equations, for cold and hot liquids and for the vapour layer, were solved numerically, with a wide range of boundary-matching conditions at the interfaces. The model presented, by covering a wide range of the conditions and parameters, is more general than the cases discussed in other publications. For this reason the results can be applied not only to the coarse pre-mixing stage of vapour explosion, but also to other situations, where three-phase forced convection occurs on an axi-symmetrical body.

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.

An analytical model for free convection film boiling on immersed solids

An analytical model has been developed for the film boiling of immersed slabs with free convection. The heat flows calculated with the model agree well with the numerically calculated and experimentally determined values. For liquid temperatures significantly below the saturation, the Nusselt number is dependent on the Galilei number, the Prandtl number and on the ratio of the mean heat conductivities and the temperature differences in the vapour and the liquid, respectively. The Nusselt function also applies to other solid geometries, provided the flow length is inserted as a characteristic dimension. Under consideration of the limiting value T ∞ = T s, the analytical model yields the known Nusselt function for saturated liquids. Furthermore, it is possible to calculate the vapour film thicknesses using this model.

Subcooled forced convection film boiling drag and heat transfer of awedge

Subcooled laminar forced convection film boiling flow on a wedge is analyzed considering the streamwise pressure gradient imposed on the flow and the streamwise buoyancy force acting on the vapor film. A twophase boundary-layer model is proposed, and the local similarity concept is applied to obtain an approximate solution of the governing equations. For a water-steam system at atmospheric pressure considered within this study, wall skin friction results display a strong dependency on the streamwise buoyancy force driving the vapor film and the external pressure gradient. Adverse streamwise buoyancy force acting on the vapor film, which is the case on the lower surface of a horizontally aligned wedge, may cause vapor flow separation. In contrast to wall skin friction dependency, the wall heat transfer parameter shows a secondary dependence on the streamwise pressure gradient and the buoyancy force.

A simple forced convection film boiling model

A simple analysis of the steady laminar forced convection film boiling flow over a horizontal flat plate is presented. A novel analogy drawn between the mobile liquid-vapor interface in forced convection film boiling and a moving surface in a flowing single-phase fluid enables a simple analysis of this complex flow. As a result, easy to apply closed-form expressions to predict theoretical wall heat transfer and skin friction are obtained, circumventing the need for previous complex analyses.