Large Superheat Research Articles

Measurement of heat flux in dense air-mist cooling: Part I – A novel steady-state technique

This work presents a technique for characterizing water air-mist cooling of surfaces at temperatures from 200–1200°C while subjected to intense spraying conditions, so that the heat flux could reach values in excess of 10MW/m2. In this new steady-state method, a local region of an air-mist/spray impinges upon the exposed surface of a hot Pt-disk (8mm ∅ by 2.5mm thickness) which is surrounded by an induction coil while both are embedded in a cast ceramic monolith. A digital controller adjusts the output power of a high-frequency generator to balance the induction heating of the Pt-specimen with the heat removed by the boiling of the spray droplets impinging on its active surface, in order to maintain the control temperature of the sample at the predetermined set-point. Measurement of the RMS current flowing through the coil to maintain this temperature, together with the solution of a two-dimensional axi-symmetric computational model of the electromagnetic field and heat conduction equations, enabled estimation of the heat extracted locally by the mist. A rigorous experimental procedure was performed to determine the boiling curves from 200°C to 1200°C and then back to 200°C. The curves for both trajectories revealed that strong boiling hysteresis occurred in the nucleate and transition boiling regimes and that this was almost absent in the stable film boiling regime. On the other hand, hysteresis was not present in thermal loops involving surface temperatures only between 600 and 1200°C. The existence of hysteresis, in a certain temperature interval, points out the importance of considering the thermal history of actual cooling processes when simulating them in the laboratory. The ability of the new technique to maintain the sample temperature for prolonged periods, e.g., hours, enables future studies to improve the understanding of boiling phenomena during intense heat-removal processes. In Part II of this article, the effects that the mist droplet size, droplet velocity and water impact density have on the heat extracted under steady-state conditions from a surface with large superheat are investigated for a wide variety of those characteristics.

Bubble coalescence at constant wall temperatures during subcooled nucleate pool boiling

Bubble coalescence during subcooled nucleate pool boiling was investigated experimentally using constant wall temperature boundary conditions while the wall heat flux was measured at various locations to understand the effects of coalescence on the heat transfer. The effects of the subcooling on the coalesced bubble shape, size and departure time were also investigated. The observations showed that the coalesced bubble moved and oscillated on the heater surface with significant heat transfer variations prior to departure. The coalesced (or departure) bubble size and departure frequency decreased as the subcooling increased. The heat flux for boiling with coalescence fluctuated much more than for single bubble boiling with the heat flux fluctuations due to oscillations of the coalesced bubble on the heater surface resulting in frequent rewetting of the heated surfaces. In some cases, many additional small bubbles nucleated very near a larger bubble and then rapidly coalesced with the larger bubble which enhanced the overall heat transfer. The observations also showed coalescence with no increase in the heat transfer rate for very fast bubble coalescence such as would occur for large surface superheats as the boiling conditions approach CHF. A mechanistic model comparing the inertia and heat flux controlled bubble growth phases for inhomogeneous bubble growth shows that this coalescence without increased heat transfer corresponds to the inertial growth phase.

Experimental analysis of the low-GWP refrigerant R1234yf as a drop-in replacement for R134a in a typical mobile air conditioning system

This study investigated the performance of a typical mobile air conditioning system using R134a and R1234yf as the working fluids under different working conditions. The system is composed of a microchannel parallel flow condenser, a laminated plate evaporator, a variable displacement compressor, and a thermal expansion valve. The different working cycles of each refrigerant were comprehensively compared. The optimum refrigerant charge amount of the R1234yf system was approximately 95% of the R134a system during drop-in tests. The performance of the R134a system was a little higher than that of the R1234yf system. The cooling capacity and system coefficient of performance of the R134a system were 12.4% and 9% larger, respectively. This result is mainly because of the thermophysical property differences between the two refrigerants and the improper expansion valve opening of R1234yf. Analysis on the whole cycle revealed that the R1234yf system could obtain a higher evaporating pressure and a larger superheat and subcooling. Redesigning the expansion valve for R1234yf could improve performance of the system.

Experiments on metal–silicate plumes and core formation

Short-lived isotope systematics, mantle siderophile abundances and the power requirements of the geodynamo favour an early and high-temperature core-formation process, in which metals concentrate and partially equilibrate with silicates in a deep magma ocean before descending to the core. We report results of laboratory experiments on liquid metal dynamics in a two-layer stratified viscous fluid, using sucrose solutions to represent the magma ocean and the crystalline, more primitive mantle and liquid gallium to represent the core-forming metals. Single gallium drop experiments and experiments on Rayleigh-Taylor instabilities with gallium layers and gallium mixtures produce metal diapirs that entrain the less viscous upper layer fluid and produce trailing plume conduits in the high-viscosity lower layer. Calculations indicate that viscous dissipation in metal-silicate plumes in the early Earth would result in a large initial core superheat. Our experiments suggest that metal-silicate mantle plumes facilitate high-pressure metal-silicate interaction and may later evolve into buoyant thermal plumes, connecting core formation to ancient hotspot activity on the Earth and possibly on other terrestrial planets.

Pool boiling heat transfer of ultra-light copper foam with open cells

High speed visualizations and thermal performance studies of pool boiling heat transfer on copper foam covers were performed at atmospheric pressure, with the heating surface area of 12.0 mm by 12.0 mm, using acetone as the working fluid. The foam covers have ppi (pores per inch) from 30 to 90, cover thickness from 2.0 to 5.0 mm, and porosity of 0.88 and 0.95. The surface superheats are from −20 to 190 K, and the heat fluxes reach 140 W/cm 2. The 30 and 60 ppi foam covers show the periodic single bubble generation and departure pattern at low surface superheats. With continuous increases in surface superheats, they show the periodic bubble coalescence and/or re-coalescence pattern. Cage bubbles were observed to be those with liquid filled inside and vented to the pool liquid. For the 90 ppi foam covers, the bubble coalescence takes place at low surface superheats. At moderate or large surface superheats, vapor fragments continuously escape to the pool liquid. Boiling curves of copper foams show three distinct regions. Region I and II are those of natural convection heat transfer, and nucleate boiling heat transfer for all the foam covers. Region III is that of either a resistance to vapor release for the 30 and 60 ppi foam covers, or a capillary-assist liquid flow towards foam cells for the 90 ppi foam covers. The value of ppi has an important effect on the thermal performance. Boiling curves are crossed between the high and low ppi foam covers. Low ppi foams have better thermal performance at low surface superheats, but high ppi foams have better one at moderate or large surface superheats and extend the operation range of surface superheats. The effects of other factors such as pool liquid temperature, foam cover thickness on the thermal performance are also discussed.

Effects of grain refiner additions on the grain structures in HPDC A356 castings

In the cold chamber high pressure die casting process (HPDC) solidification begins when the metal is poured into the shot sleeve and impinges on the relatively cold shot sleeve wall and plunger. Therefore, a mixture of liquid and externally solidified crystals (ESCs) is injected into the die cavity. The mechanisms that control the formation of ESCs are not fully understood. In the work presented here, the microstructures of thin walled A356 aluminium alloy die castings have been investigated. The castings were produced by varying the melt superheat and constitutional conditions. It was found that the area fraction of ESCs (fsESC) increases when decreasing the melt superheat; a low superheat generates coarser, more globular ESCs, whilst a larger superheat results in branched, dendritic crystals; additions of Ti in solution increase the fsESC and additions of Al–5Ti–1B grain refiner increase the number of globular, coarse ESCs and generate a finer grain size in the casting. The results are discussed with special emphasis on the shot sleeve solidification conditions and the mechanisms that control the formation of the ESCs.

Convective Instability of a Mushy Layer - I: Uniform Permeability

Mushy layers arise and are significant in a number of geophysical contexts, including freezing of sea ice, solidification of magma chambers and inner-core solidification. A mushy layer is a region of solid and liquid in phase equilibrium which commonly forms between the liquid and solid regions of a solidifying system composed of two or more constituents. We consider the convective instability of a plane mushy layer which advances steadily upwards as heat is withdrawn at a uniform rate from the bottom of a eutectic binary alloy. The solid which forms is assumed to be composed entirely of the denser constituent, making the residual liquid within the mush compositionally buoyant and thus prone to convective motion. In this article we focus on the large-scale mush mode of instability, arguing that the 'boundary-layer' mode is not amenable to the standard stability analysis, because convective motions occur on that scale for any non-zero value of the Rayleigh number. We quantify the minimum critical Rayleigh number and determine the structure of the convective modes of motion within the mush and the associated deflections of the mush-melt and mush-solid boundaries. This study of convective perturbations differs from previous analyses in two ways; the inhibition of motion and deformation of the mush-melt interface by the stable stratification of the overlying melt is properly quantified and deformation of the mush-solid interface is permitted and quantified. We find that the mush-melt interface is almost unaffected by convection while significant deformation of the mush-solid interface occurs. We show that each of these effects causes significant (unit-order) changes in the predicted critical Rayleigh number. The marginal modes depend on three dimensionless parameters: a scaled eutectic temperature, τ e (which characterizes the eutectic temperature relative to the depression of the liquidus), a scaled superheat, τ∞ (which measures the amount by which the temperature of the incoming melt exceeds the liquidus temperature) and the Stefan number, S (which measures the latent heat of crystallization). To survey parameter space, we focus on seven cases, a standard case having S = τ∞ = τ e = 1, and six others in which one of the parameters is either large or small compared with unity: a nearly pure case (τ e = 100; having little of the light constituent), the large superheat limit (τ∞→ ∞), a case of large latent heat (S = 100), the near eutectic limit (τ e → 0), a case of small superheat (τ∞ = 0.01) and the case of zero latent heat (S = 0). The critical Rayleigh number and the associated wavelength of the convection pattern are determined in each case. The eigenvector for each case is presented in terms of the streamlines and the isolines of the perturbation temperature and solid fraction.

Local heat transfer from a hot plate to a water jet

Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Because of its importance to many cooling procedures, investigations on basic mechanisms of jet impingement boiling heat transfer are needed. Until now, most of the experimental studies, carried out under steady-state conditions, used a heat flux controlled system and were limited by the critical heat flux (CHF). The present study focuses on steady-state experiments along the entire boiling curve for hot plate temperatures of up to 700°C. A test section has been built up simulating a hot plate. It is divided into 8 independently heated modules of 10 mm length to enable local heat transfer measurements. By means of temperature controlled systems for each module local steady-state experiments in the whole range between single phase heat transfer and film boiling are possible. By solving the two dimensional inverse heat conduction problem, the local heat flux and the corresponding wall temperature on the surface of each module can be computed. The measurements show important differences between boiling curves measured at the stagnation line and those obtained in the parallel flow region. At the stagnation line, the transition boiling regime is characterised by very high heat fluxes, extended to large wall superheats. Inversely, boiling curves in the parallel flow region are very near to classical ones obtained for forced convection boiling. The analysis of temperature fluctuations measured at a depth of 0.8 mm from the boiling surface enables some conclusions on the boiling mechanism in the different boiling regimes.

Thermoacoustic microengines

The high rate of heat transfer (up to 500 MW/m2) which can be achieved by explosive bubbles has long been recognized. This process finds a widening range of application from microsurgery to computer peripherals design. The best-known example is the thermal ink-jet printer, where an ultrahigh heat flux applied for short duration of about 6 μs induces rapid bubble generation and growth, which drives an adequate mass of ink forward through the exit nozzle. Explosive boiling is a phenomenon in which boiling is developed in an explosive manner at large superheat. Superheat conditions can be realized by absorption of a strong laser beam or by quick heating of a surface. Reversible phase transitions near the fluid’s critical temperature by ‘‘acoustic explosive bubble’’ may provide a new way to continuously transfer the high thermal power associated with explosive boiling in a variety of heat exchange processes. For example, explosive bubbles could form the basis for fluid micropumping devices to be used in microrefrigerators. The present paper concentrates mainly on estimating the necessary conditions for realizing explosive boiling in liquids and how this boiling can be used to create a stable acoustic oscillation involving growth and collapse of the bubble.

CONSTRAINED VAPOR BUBBLE

The equilibrium and nonequilibrium phenomena associated with a very thin liquid film formed between a vapor bubble and a container are discussed. Numerical examples of the effects of temperature and intermolecular forces on the film thickness and transport processes are presented. For relatively thick films, the sensitivity of experimental measurements to extremely small temperature gradients is found to be large. For extremely thin films, the sensitivity is small. Large superheats and heat fluxes are possible near the contact line. A small amount of adsorbed contamination has a large effect on the film profile. The constrained vapor bubble has both basic uses in the study of thermophysical properties and transport processes, and has applied uses as a heat exchanger.

Effects of Dissolved Gas Content on Pool Boiling of a Highly Wetting Fluid

Experimental results on pool boiling heat transfer from a horizontal cylinder in an electronic cooling fluid (FC-72) are presented. The effects on the boiling curve of having air dissolved in the fluid are documented, showing that fluid in the vicinity of the heating element is apparently liberated of dissolved gas during boiling. Dissolved gas was found to influence boiling incipience only with high gas concentrations (>0.005 moles/mole). For low-to-moderate concentrations, a larger superheat is required to initiate boiling and a hysteresis is observed between boiling curves taken with increasing and decreasing heat flux steps. Boiling, a very effective mode of heat transfer, is attractive for electronics cooling. The present experiment provides further documentation of the role of dissolved gas on the incipience process and shows similarities with subcooled boiling of a gas-free fluid.

An analysis of freeze-up phenomena during gas atomization of metals

A numerical model is developed to describe the flow and heat transfer behavior of molten metals during flow in the delivery tube in gas atomization and spray deposition. Numerical simulations for Al, Cu, Mg, Ni, Ti and W melt are conducted to investigate the influence of processing parameters and material properties on the minimum melt superheat that is necessary to prevent the tube from premature solidification during delivery of the molten metal prior to atomization. Processing maps are developed to provide direct insight into the complex relationship among the minimum melt superheat, processing parameters and material properties. A quantitative correlation is obtained by means of a regression analysis of the numerical results, which facilitates application of the numerical model. The calculated results demonstrate that for the materials studied, the minimum melt superheat ranges from 0.005 T m to 0.19 T m, depending on processing parameters and material properties. The dependence can be expressed using a correlation derived from the regression analysis such as ΔT T m =0.15 μU 2 kc pρΔP T m T g 0.1 L D 0.3 Increasing the overpressure can effectively decrease the minimum melt superheat, especially for a large tube-length: diameter ratio and for materials possessing low densities. This effect diminishes with increasing overpressure. The minimum melt superheat can also be decreased by reducing the tube length: diameter ratio, by selecting a smooth delivery tube with low thermal conductivity and thick tube wall, and/or by enhancing the ambient gas temperature. Materials with high thermal conductivity, high thermal capacity and/or large density allow a small melt superheat to prevent the delivery, tube from freeze-up, while materials with high melting temperature and/or high viscosity require a large melt superheat.

Modeling of liquid metal flow and heat transfer in delivery tube during gas atomization

A numerical model is developed to describe the flow and heat transfer behavior of liquid metals in a cold delivery tube during gas atomization. Numerical calculations for liquid In, Sn, Bi, Pb, Zn and Sb are performed to investigate the influence of processing parameters and material properties on the flow and cooling of the liquid metals, and to predict the minimum melt superheat that is necessary to prevent the liquid metals from premature solidification during delivery. Processing maps are developed to provide direct insight into the complex relationship between the minimum melt superheat, processing parameters and material properties. A quantitative correlation is derived from the numerical results by means of a regression analysis, which facilitates application of the numerical model. The calculated results reveal that the overpressure, tube length/diameter ratio, ambient temperature and thermal properties of the tube and liquid metals are the important factors influencing liquid metal flow and heat transfer. For the materials studied, the minimum melt superheat ranges from 0.001 T m to 0.17 T m, depending on the processing parameters and material properties. The dependence can be expressed usnig a correlation derived from the regression analysis such as ΔT T m =0.15 μU 2 kc pρΔP T m T g 0.1 L D 0.3 Increasing the overpressure can effectively decrease the minimum melt superheat, especially for a large tube length/diameter ratio and for materials possessing low densities. The minimum melt superheat asymptotically approaches its final steady value with increasing overpressure. The minimum melt superheat can also be decreased by reducing the tube length/diameter ratio, by selecting a smooth delivery tube with low thermal conductivity and a thick tube wall, and/or by enhancing the ambient temperature. Materials with high thermal conductivity, high thermal capacity and/or large density require a small melt superheat to prevent the liquid metals from premature solidification, while materials with high melting temperature and/or high viscosity require a large melt superheat.

Experimental study of cryogenic liquids in the metastable superheated state

The results of an experimental investigation of cryogenic liquids in a metastable (superheated) state are considered. Data are presented on the maximum attainable superheat temperature of typical cryogenic liquids from atmospheric to close to critical pressures. The nucleation mechanism and the influence of factors that induce boiling on the attainable superheat of liquids are discussed. The results of experiments are compared with theory. Data on the thermodynamic properties ( P, ϱ, T properties, heat capacity, ultrasonic velocity and attenuation, volume viscosity) of cryogenic liquid in a metastable state are presented. The construction of the equation of state of metastable liquids is discussed. Explosive boiling phenomena connected with large superheats of cryogenic liquids are considered, and the discharge of cryogenic liquids through short tubes and the heat interaction of cold liquid droplets with a hot surface are investigated.

Film Boiling Heat Transfer to Large Superheats From a Horizontal Flat Surface

A recent investigation reported steady-state measurements of film boiling heat transfer from a flat horizontal circular surface to an overlying pool of water at saturation and atmospheric pressure. Measurements were made from near the Minimum Film Boiling (MFB) point to a superheat of approximately 570 K. These data substantiate that the Berenson model is applicable at temperatures larger than previously observed, as long as a correction is applied to account for the effect of thermal radiation to the overall heat transfer. The objective of this work is to show quantitatively the validity of the Berenson film boiling model at surface superheats well above the MFB to approximately 600 K.

Bubble growth in superheated He II

Bubble growth in superheated He II is controlled by the transfer of heat to the surface of the growing bubble by nonlinear Gorter-Mellink counterflow. The present work presents analytic formulae for the bubble radius as a function of time in the limiting cases of small and large superheats. The formulae include the effect of the inertial reaction of the surrounding liquid to the expansion of the bubble. A numerical example shows that bubble velocities of the order of metres per second are possible. A related problem, involving only heat transfer but no movement of the liquid, is the motion of the free surface of superheated He II in a very long tube. This problem has a similarity solution. The interfacial velocity in the tube is much smaller than the bubble growth velocity.

Oscillation of a liquid-vapor interface: experimental observations

The effects of liquid subcooling, velocity, and vapor superheat on the wavy nature of film boiling from a sphere in Freon-113 were studied. Experiments for both pool and flow film boiling were performed for a heater surface superheat of around 100 to 300°C, liquid velocity from 0- to 2.10 m/s, and Freon-113 subcooling from 0 to 25°C. Photographs taken in the film boiling regime show that the nature of the liquid-vapor interface is a function of the above-mentioned parameters. For low liquid subcooling ripples were present on the liquid-vapor interface. At greater subcooling these ripples disappear. For very large vapor superheat and liquid velocity ripples are always present on the liquid-vapor interface.

Freezing of saturated and superheated liquid in porous media

Freezing of saturated and superheated liquid-porous media contained in a rectangular test cell has been studied experimentally. Water and different diameter glass beads constituted the liquid and porous media, respectively. The effects of different size glass beads, imposed temperature difference and liquid superheat were investigated. A one-dimensional conduction-based model was used to examine the importance of non-equilibrium effects. These effects were found to be unimportant for both water-aluminum balls and water-glass bead systems. For water initially at the freezing temperature, the model predictions agreed well with the experimental temperature and solidification front position data. When the superheat was sufficiently high to induce buoyancy driven convective flow, the local freezing rates varied with vertical location along the cooled wall. For low superheat, due to the density inversion of water, the rate of freezing at the top was higher than at the bottom ; however, for large superheat, the freezing rate near the bottom was higher as in freezing of ordinary liquids.

Solidification of a Steel Billet in the Mold of a Synchronized Rotary Type Continuous Caster

A high speed continuous billet caster where casting speed matches the rolling speed has been developed. The mold of the caster consists of a copper wheel with a groove and an endless belt, both strongly cooled by water. There is almost no friction between the mold and a cast billet because of the mold moving simultaneously with a billet. In the course of development two studies were performed to establish the optimum casting and operating conditions. The first was to study the effects of casting conditions on the shell thickness in the mold. The second was on the heat transfer characteristics of the rotating wheel. The results are summarized as follows:(1) The solid shell thickness of the belt side was smaller than that of the wheel side when the molten steel with large superheat was poured.(2) Water modeling experiment revealed that flow with high velocity went downward along the belt side, while there was upward flow with low velocity along the wheel side.(3) The allowable superheat was estimated to be 50°C by the casting experiments and computer calculation taking account of fluid flow.(4) Good contact between a cast billet and the wheel was obtained by smooth nozzle stream, tension casting where the pinch rollers were driven at a slightly higher rate than the wheel and appropriate guide roller configuration.(5) The solidification rate constant was 27mm/min1/2, which was somewhat larger than that of an average conventional caster.Based upon these results the belt wheel type continuous caster was put into practical use.

Boiling of Potassium in Vertical Tube under Transverse Magnetic Field

The experimental study was conducted to examine the applicability of boiling potassium two-phase flow to the blanket cooling of fusion power reactors. A high flux heater pin of 44mm heating length and 6.5mm O. D. with eight thermocouples of 0.5mm O. D. was inserted from the bottom of a vertical channel which was made of a 4m long, 14.9mm I.D. and 17.5mm O. D. stainless steel tube and placed in a D.C. magnet of 50cm long poles. The experimental conditions were the heat flux: 0~67W/cm2, the magnetic field strength: 0~1.8T, the Ar cover gas pressure: 1.0bar, the potassium level above heater: 1.8m, and the temperature of upper unheated section: 400°C. In the absence of magnetic field, boiling occurred intermittently, repeating the cycles between superheating with moderate temperature fluctuation and desuperheating with condensing shock pulses. When a weak magnetic field was applied, the temperature fluctuation was enhanced by natural convection, the incipient boiling superheat was reduced, and the boiling pattern became continuous. With increasing the magnetic field strength, the fluctuation was suppressed and the incipient boiling superheat increased but tended to level off around 1.5 T. Violent incipient boiling caused by a large superheat inherent in liquid metals was mitigated by magnetohydrodynamic interaction under a transverse magnetic field of 1.0T or larger, and subsequently followed by continuous saturation boiling with small fluctuation. No burn-out of the heater pin occurred in spite of symptom of dryout within the experimental range: q=67W/cm2 and B=1.5T.