Effects of wetted wall on laminar natural convection in vertical annular ducts

Abstract

A detailed numerical analysis is performed to investigate the effects of latent heat exchange, in connection with evaporation of the liquid film on the wall, on the natural convection heat transfer in vertical concentric annuli. Major governing parameters identified are Gr_T, Gr_M, Pr, Sc, and N. Results are specifically presented for an air‐water system under various heating conditions to illustrate the latent heat transport during the evaporation process. The effects of the channel length, ratio of radii N and wetted wall temperature on the momentum, heat and mass transfer are examined in detail. Tremendous enhancement in heat transfer due to the exchange of latent heat was clearly demonstrated.     

Specific Heat and Thermal Transport Measurements of Reactive Metallic Alloys by Noncontact Calorimetry in Reduced Gravity

Experimental concepts, application, and recent results of noncontact calorimetry inculding ac-specific heat and thermal transport property measurement of reactive metallic specimens are described. The method is based on induction heating in an electromagnetic levitation device. Experiments have been performed in reduced gravity onboard Spacelab. A heat flow model is discussed regarding conditions for precise specific heat determination from the temperature response to modulated heating power input as well as determination of thermal transport properties from the transient temperature response. Modulation techniques were developed for application in the metastable regime of the undercooled melt, where processing time is a critical issue. Results to be discussed include the total hemispherical emissivity of a bulk metallic glass-forming alloy revealing effects of surface segregation, thermal conductivity measurements, and application of ac calorimetry in the two-phase region.     

Experimental Study of Nucleate Pool Boiling of FC-72 on Smooth Surface under Microgravity

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were studied both in short-term microgravity condition utilizing the drop tower Beijing and in normal gravity conditions. The bubble behavior and heat transfer of air-dissolved FC-72 on a small scale silicon chip (10 × 10 × 0.5 mm³) were obtained at the bulk liquid subcooling of 41 K and nominal pressure of 102 kPa. The boiling heat transfer performance in low heat flux region in microgravity is similar to that in normal gravity condition, while vapor bubbles increase in size but little coalescence occurs among bubbles, and then forms a large bubble remains attached to the heater surface during the whole microgravity period. Thermocapillary convection may be an important mechanism of boiling heat transfer in this case. With further increasing in heat flux to the fully developed nucleate boiling region, the vapor bubbles number as well as their size significantly increase in microgravity. Rapid coalescence occurs among adjacent bubbles and then the coalesced large bubble can depart from the heating surface during the microgravity period. The reason of the large bubble departure is mainly attributed to the momentum effects caused by the coalescence of small bubbles with the large one. Hence, the steady-state pool boiling can still be obtained in microgravity. In the high heat flux regime near the critical heat flux, significant deterioration of heat transfer was observed, and a large coalesced bubble forms quickly and almost covers the whole heater surface, leading to the occurrence of the critical heat flux in microgravity condition.     

Rupture of thin ductile tubes by oblique impact of blunt missiles: experiments

Impact tests at both normal and oblique angles of incidence were conducted on thin mild steel tubes using a moderate size of blunt missile (D_m/h=4.33) at various angles of obliquity (0°⩽φ₀⩽60°) from normal. The minimum impact speed that generated cracks through the thickness of the wall, termed the speed for rupture, was measured, and various types of rupture were identified. For a thin tube hit by a flat-nosed missile at a large angle of obliquity, the flat-nose initially cuts into the surface of the tube wall; through thickness rupture is due to tensile tearing that occurs in a region just below the crater. The speed for rupture of the tube is found to be a minimum at an angle of incidence equal to 45°; this speed is about 41% less than that required to rupture a tube of equal thickness by impact at a normal angle of obliquity. If the missile nose has a radius of curvature of the same order as the missile radius, the deformation is more diffuse in the region immediately adjacent to the contact surface; consequently the mode of failure changes from predominately shear at the edge of the missile to more uniform stretching under the contact surface. For oblique impact of missile with more rounded noses, this causes the observed speed for rupture to increase with increasing angle of obliquity.     

Neutron radiography study of pulsed boiling in a water-filled heat pipe

The boiling and condensation processes in a water-filled industrial heat pipe have been investigated by dynamic neutron radiography. Pulsed boiling was visualized and analysed up to a characteristic temperature depending on the filling quantity. Three typical regions were distinguished in the heat pipe: that constantly filled by the liquid; a periodically wetted zone; and a region constantly filled by the vapour. Pulsation was found to be not strictly periodic and its mean amplitude and frequency were determined. It is concluded that any models based on the equilibrium state are incorrect.     

The Effect of Gravity and Shear Stress on a Liquid Film Driven in a Horizontal Minichannel at Local Heating

The present study is focused on the investigation of gravity effect on thermocapillary deformations in a film flowing under action of co-current gas flow, which creates the tangential force on the gas–liquid interface. The influence of local heating intensity on the heater at a substrate is also investigated. Effects of surface tension, temperature dependent viscosity and thermocapillarity are taken into account. Investigations have shown that gravity has a significant effect on the film deformations and pattern. Decreasing of gravity level leads to a flow destabilization. 3D liquid film pattern noticeably changes in spanwise direction. Increasing of heat flux leads to increasing of liquid film deformations. Dependence of film thinning on heat flux is strongly nonlinear. The most dangerous deformations (regions of minimum film thickness with possible disruption of liquid) take place behind the downstream edge of the heater at any gravity conditions.     

Ammonia-water desorption heat and mass transfer in microchannel devices

This paper presents the results of an experimentally validated model for the prediction of local heat and mass transfer rates in a microchannel ammonia-water desorber. The desorber is an extremely compact 178 mm × 178 mm × 0.508 m tall component capable of transferring the required heat load (∼17.5 kW) for a residential heat pump system. The model predicts temperature, concentration and mass flow rate profiles through the desorber, as well as the effective wetted area of the heat transfer surface. Previous experimental and analytical research by the authors demonstrated the performance of this same microchannel geometry as an absorber. Together, these studies show that this compact geometry is suitable for all components in an absorption heat pump, which would enable the increased use of absorption technology in the small-capacity heat pump market.     

Calculation of the wetted area of a planing hull with a chine

Summary The extent to which a low-aspect-ratio flat ship with a chined hull is wetted when planing at infinite Froude number is investigated. A numerical method of solution for the wetted area, which is applicable to more general planing problems, is presented. The results obtained by this method are compared with those found by solving the inverse problem of determining the hull shape which produces a given waterplane shape and are shown to be in excellent agreement. Results are also presented which indicate that a vertical chine may be used to fix the shape of the wetted region.     

Gravity Influence on Heat Transfer Rate in Flow Boiling

The aim of the present paper is to describe the results of flow boiling heat transfer at low gravity and compare them with those obtained at earth gravity, evaluating possible differences. The experimental campaigns at low gravity have been performed with parabolic flights. The paper will show the analysis of differences between the heat transfer coefficients at normal and at zero gravity, and the study of the effects of mass flux, heat flux, and tube diameter on boiling phenomena at microgravity. Three tube diameters are tested: 6.0, 4.0, and 2.0?mm. With respect to terrestrial gravity, both heat transfer rate enhancement (up to 15?C20%) and deterioration (up to 35%) have been observed. Heat transfer differences for the two gravity conditions may be related to the different bubble size in each of them. The size of a bubble in flow boiling is generally affected by the gravity level, being larger at low gravity, unless inertial forces are largely predominant over buoyancy and other forces acting on the bubble itself when detaching from a heating wall. Heat transfer enhancements at low gravity, are observed in those conditions where the flow pattern is bubbly flow at normal gravity and intermittent flow at low gravity. The results are presented in a flow boiling gravity influence map, which can be considered a useful tool for designing boiling systems for space applications.     

Microwave Heating of an Emulsion Drop

The results of numerical modeling of the microwave heating of a water-in-oil emulsion drop in a gravity field with consideration of the empirical temperature dependence of the viscosity of the liquid surrounding the drop are presented. The system of thermal convection equations is considered in the Boussinesq approximation. The solution is obtained by the control volume method with the SIMPLE algorithm and by the VOF method. It is established that the emerging convective structures result in nonuniform heating of the drop predominantly near the surface, which can lead to a local rupture of the armor envelope and the formation of fine-dispersed phase. It is shown that there is an optimal range of power of microwave field in which an intensive deposition of water drops occurs, which leads to water-oil emulsion breakdown.     

Test method for evaluating fabric flammability and predicted skin burn injury in microgravity

To address fire safety concerns associated with the use of flammable fabrics during space travel, an apparatus was designed to be flown on low-gravity parabolic aircraft flights in order to assess the flammability of cotton and 50% cotton/50% polyester fabrics, and the resulting skin burn injury that would occur if these fabrics were to ignite. The apparatus, modelled after a standard fabric flammability test, was also used on the ground for experiments under earth’s gravity. Variables examined in the tests include gravity level, fabric type, air gap size, and orientation of the fabric. Flame spread rates, heat fluxes, and skin burn predictions determined from test results were compared under the two gravity levels. The orientation of the fabric had a large effect on flame spread rates, heat fluxes and predicted skin burn times for tests conducted under earth’s gravity. Flame spread rates and heat fluxes were highest when the fabric was held in the vertical orientation, which resulted in the lowest predicted times to produce skin burns. Flame spread rates and heat fluxes were considerably lower in microgravity than under earth’s gravity, which resulted in longer predicted times to produce skin burns.     

Cancellation of the Heating Piston Effect by Convective Enhancement of a Cooling Piston Effect

This work brings new insight to the question of the piston effect, which has been found to be the main cause of temperature equilibration in the vicinity of the liquid–vapor critical point under weightlessness conditions. The thermalization process of a near-critical fluid confined in a cavity and submitted to local heating is modeled with special emphasis on the role of gravity and boundary conditions. The solution of the unsteady Navier–Stokes equations written for a hypercom-pressible low-heat-diffusing van der Waals gas is obtained in a 2-D configuration by means of a finite-volume numerical code. Under Earth gravity conditions, the results show that the thermal plume rising from a heat source strongly decreases and rapidly cancels bulk fluid heating when it strikes the top thermo-stated wall. It is proved that convection does not prevent heat transfer by the piston effect but that it causes a sudden enhancement of the cooling piston effect generated at the thermostated top boundary, which leads to an early equilibrium between the cooling and heating piston effects.     

Sessile Drop Wettability in Normal and Reduced Gravity

This paper presents initial work performed to develop a database of contact angles of sessile drops in reduced gravity. Currently, there is no database of wettability of sessile drops in reduced gravity. The creation of such a database is imperative for continued investigations of heat and/or mass transfer in reduced gravity and future engineering designs. In this research, liquid drops of water and ethanol were created on aluminum and PTFE substrates. The formed drops were characterized by their dimensions including contact angle, wetted perimeter and droplet shape in both normal gravity and reduced gravity. The droplets were recorded during testing with high definition video and the images obtained digitally analyzed, post-test, to determine their characteristics as a function of the experimental parameters. The Queensland University of Technology (QUT) Drop Tower Facility was utilized for the reduced gravity experimentation. For droplets with diameters above their capillary length, the changes in drop dimensions and/or wettability was observed. The Young-Laplace equation was validated to accurately predict the contact angle in reduced gravity for small droplets, however it was not adequate to describe the contact angle for larger drops (above the drops associated capillary length).     

Bubble Behavior and Heat Transfer in Quasi-Steady Pool Boiling in Microgravity

Pool boiling of degassed FC-72 on a plane plate heater has been studied experimentally in microgravity. A quasi-steady heating method is adopted, in which the heating voltage is controlled to increase exponentially with time. Compared with terrestrial experiments, bubble behaviors are very different, and have direct effect on heat transfer. Small, primary bubbles attached on the surface seem to be able to suppress the activation of the cavities in the neighborhoods, resulting in a slow increase of the wall temperature with the heat flux. For the high subcooling, the coalesced bubble has a smooth surface and a small size. It is difficult to cover the whole heater surface, resulting in a special region of gradual transitional boiling in which nucleate boiling and local dry area can co-exist. No turning point corresponding to the transition from nucleate boiling to film boiling can be observed. On the contrary, the surface oscillation of the coalesced bubble at low subcooling may cause more activated nucleate sites, and then the surface temperature may keep constant or even fall down with the increasing heat flux. Furthermore, an abrupt transition to film boiling can also be observed. It is shown that heat transfer coefficient and CHF increase with the subcooling or pressure in microgravity, as observed in normal gravity. But the value of CHF is quite lower in microgravity, which may be only about one third of that at the similar pressure and subcooling in terrestrial condition.     

Effect of tandem submerged arc welding process and parameters of Gleeble simulator thermal cycles on properties of the intercritically reheated heat affected zone

The effects of real and Gleeble simulated double pass thermal cycles on the properties of the intercritically reheated coarse grained heat affected zones in X80 microalloyed pipeline steel has been investigated. The Gleeble simulated process involved heating the X80 steel specimens to the first peak temperature of 1400 °C and then reheating to the second peak temperature of 800 °C, with different cooling rates. The size and area fraction of martensite/austenite (M/A) constituents were obtained by a combination of field emission scanning electron microscopes and image analysis software. In addition, misorientation was characterized by electron back-scatter diffraction analysis. It is clear that the intercritically thermal cycles have a significant effect on morphology of M/A constituents. The M/A constituent’s size, such as mean diameter and length, are important factors influencing Charpy impact properties of thermally simulated intercritically reheated heat affected zones. The simulated thermal cycles of the intercritically reheated region in the high heat input tandem submerged arc welding processes, showed extremely poor Charpy impact absorbed energy. The intercritical reheated thermal cycles with lower heat input value showed higher Charpy impact absorbed energy due to a decrease in the prior-austenite grain and M/A particle size.     

Bubble behaviors in subcooled boiling of wetting surface under low gravity

Stainless steel plate with 30mm in length, 1 mm in width and 0.1 mm in thickness is employed for a heating surface in subcooled quasi-pool boiling of water under low gravity performed by a parabolic flight. Testing liquid subcooling is about 10K at atmospheric pressure. The wetting heating surfaces are coated with ceramics materials which have been developed by a certain glass company. DC power is applied directly into the test heating surface and the bubble behaviors are observed by a high-speed video camera. Contact angle of water droplet is about 77–96 degree for the stainless surface and 30 degree or less for the wetting surface. In the ground experiment, the size of detaching bubbles from the wetting surface is smaller than those of stainless surface and the detaching period is shorter at same heating power. The burnout heat fluxes of wetting surfaces are about 50 percent higher those of stainless surfaces. In the low gravity experiment, DC power is applied into the surface at 10 second before start of low gravity and increases slightly until burnout. A single large bubble grows on the stainless surface and finally, the surface is burned out in a short period. For wetting surface, several large coalescing bubbles appear and they move rapidly on the surface, then one of the large bubbles grows and the burnout occurs. The burnout heat fluxes are higher than those of stainless surface. The wetting ceramics surface is considered to accelerate the liquid supply and the bubble moving.     

Investigation of the thermophysical properties of zirconium by subsecond pulsed heating technique

The heat capacity, enthalpy, and spectral (for the wavelength of 0.65 μm) emissivity of zirconium are investigated in the temperature range from 1000 to 2100 K by the method of single subsecond resistance pulsed heating. The heat of the α-β phase transition is measured. A number of singularities are discovered in the variation of the properties of material in the region of α-β transition and in the β phase, which are associated with variations in the oxide film and with the specific features of volume heat release during heating in the two-phase region.     

液氢空间贮存过程膜态沸腾数值模拟

为实现液氢在空间中安全高效应用,针对微重力条件下液氢膜态沸腾现象,建立了加热细丝浸没在过冷液氢池中的数值计算模型.采用VOF方法捕捉相界面,相变模型选取Lee模型,利用文献中的实验数据验证了模型的准确性.从气泡运动行为和换热特性两方面开展研究,结果发现液体过冷度和重力水平是影响换热机理的两个重要因素.在高重力水平、低液...     

加热方位对流动沸腾临界热流密度影响

为了研究重力场对流动沸腾临界热流密度的影响,搭建了两相沸腾换热实验系统。以蒸馏水为工质,采用单侧加热的窄缝通道,通过改变质量流速、入口过冷度和重力场与加热方位的夹角,考察不同加热方位临界热流密度特性和实验段流阻特性。分析了质量流速、入口过冷度、加热方位对流动沸腾临界热流密度的影响,并将实验数据与Ivey-Morris模型、Sudo模型和Wojtan模型的计算值进行了验证对比。结果表明:加热面呈0°放置时的临界热流密度最大,呈180°放置时最小,质量流速和入口过冷度的增大会加大临界热流密度。Sudo模型对本实验条件不适用;Ivey-Morris模型和Wojtan模型在加热面呈0°放置时与实验值符合情况良好,相对误差约在30%以内,其他加热方位时,计算值均大于实验值。     

Combined momentum, heat and mass transfer in vertical slug flow absorbers

Numerical solutions have been obtained for the system of equations of momentum, heat and mass transfer describing the absorption of a refrigerant vapour from a Taylor bubble into the refrigerant-absorbent solution film around the bubble. The numerical results are compared with Nusselt's solution of the energy equation and with the penetration theory solution of the mass diffusion variation. Experimental data have been collected in vertical tubular absorbers in the slug flow region with the systems ammonia-lithium nitrate and ammonia-sodium thiocyanate. Four different absorber tubes have been tested with internal diameters of 10, 15, 20, and 25 mm. These data are compared with the numerical and theoretical results. The effect of the bubble nose on mass transfer is studied. Typical temperature profiles during the absorption process in absorption cooling/heating systems are shown.