Low Superheat Research Articles

Explosive boiling-up in a swirl jet of superheated ethanol

Based on the knowledge of the kinetics of liquid vaporization under conditions of high and attainable superheats, experimental data on the atomization of a jet of boiling ethanol at various speeds of a flow swirl are discussed. It has been stated that under conditions of low superheats and boiling-up on solitary non-interacting heterogeneous sites, the rotation of the flow is considered to be the decisive factor in the atomization process. At high superheats and the realization of the mechanism of intense heterogeneous vaporization, the effect of boiling-up on the jet disintegration becomes dominant. When establishing conditions for explosive boiling-up (homogeneous nucleation) under attainable superheats, a complete jet breakup was observed both without a swirl and at low swirl speeds. With an increase in the swirl speed, the temperature range of the breakup narrowed. With a strong swirl the breakup disappeared completely. In the initial states of ethanol close to the thermodynamic critical point, the jet shape was similar to a gas one with or without a swirl. The 1/f fluctuations revealed were caused by oscillations (nutation) of the precessional motion of the jet. They indicate a possibility of large-scale outbursts in the jet.

Single Bubble Boiling from an Artificial Cavity

Pool boiling heat transfer is an aggressive and complex phenomenon which needs to be simplified for a better understanding of the mechanism of bubble growth and departure and how boiling heat transfer can be enhanced. Single bubble boiling heat transfer is a simple version of boiling phenomenon which has been used to study the effective elements on pool boiling heat transfer. The purpose of the present review paper is to understand how to produce single bubble pool boiling on a heated substrate and investigate, how single bubble boiling phenomenon can be affected by geometry of cavities, cavity size, wettability, roughness, working fluid, subcooling, wall superheat, heat flux, gravity, etc. It was demonstrated that cylindrical cavities are capable to generate stable and continuous bubbling, small temperature fluctuation, low superheat with short waiting period. The cylindrical cavities can be manufactured very easily in small sizes which can be a good candidate to produce single bubble pool boiling. As heat flux increases, smaller cavities start becoming active. For a given depth, as cavity size increases, the bubble growth rate and departure volume increase. Surface wettability is another complex and important factor to modify the single bubble boiling heat transfer. Wettability depends mainly on force balance at the triple contact line which relies on solid–liquid–gas materials. In case of hydrophobic surfaces, the triple line has tendency to move toward liquid phase and expand the radius of triple line, so the initiation of nucleation is easier, the waiting time is shorter, the downward surface tension force becomes bigger since radius of triple line is larger, the bubble departure volume is higher and bubble growth period is longer. The effects of the rest of main parameters on single bubble boiling are discussed in this paper in details. In addition, a theoretical model is developed to predict the liquid-vapor interface for the single bubble boiling. The theoretical model is compared with single bubble boiling experimental data and good results observed.

Energetic Assessment of an Electronic and a Thermostatic Expansion Valve for a Variable Capacity Compressor

In this study, a chiller system with an electronic expansion valve (EEV) and a thermostatic expansion valve (TXV) were compared from an energy saving view under variable and fixed speed compressor. The experimental chiller system under study has a variable speed scroll compressor, EEV and TXV, air cooled condenser and auxiliary equipments. The R134a was used in the compressor as a refrigerant. Although in variable speed chiller systems usually EEVs are used, in this study a TXV was also used to compare its performance. The results of the experimental studies show that the chiller system with EEV has 30% lower superheat value and 6% lower power consumption than the chiller system with TXV under variable speed operation. From the fixed speed compressor view, the chiller system with TXV performs 1.7% lower superheat value than the chiller system with EEV and, the chiller system with EEV has 8.5% higher compressor power consumption than TXV.

Numerical Investigation of Shape Effect on Microdroplet Evaporation

Abstract As electronic devices continue to shrink in size and increase in functionality, effective thermal management has become a critical bottleneck that hinders continued advancement. Two-phase cooling technologies are of growing interest for electronics cooling due to their high heat removal capacity and small thermal resistance (<0.1 k cm2/W). One typical example of a two-phase cooling method is droplet evaporation, which can provide a high heat transfer coefficient with low superheat. While droplet evaporation has been studied extensively and used in many practical cooling applications (e.g.,, spray cooling), the relevant work has been confined to spherical droplets with axisymmetric geometries. A rationally designed evaporation platform that yields asymmetric meniscus droplets can potentially achieve larger meniscus curvatures, which gives rise to higher vapor concentration gradients along the contact line region, and therefore, yields higher evaporation rates. In this study, we develop a numerical model to investigate the evaporation behavior of asymmetrical microdroplets suspended on a porous micropillar structure. The equilibrium profiles and mass transport characteristics of droplets with circular, triangular, and square contact shapes are explored using the volume of fluid (VOF) method. The evaporative mass transport at the liquid–vapor interface is modeled using a simplified Schrage model. The results show highly nonuniform mass transport characteristics for asymmetrical microdroplets, where a higher local evaporation rate is observed near the locations where the meniscus has high curvature. This phenomenon is attributed to a higher local vapor concentration gradient that drives faster vapor diffusion at more curved regions, similar to a lightning rod exhibiting a strong electric field along a highly curved surface. By using contact line confinement to artificially tune the droplet into a more curved geometry, we find that the total evaporation rate from a triangular-based droplet is enhanced by 13% compared to a spherical droplet with the same perimeter and liquid–vapor interfacial area. Such a finding can guide the design and optimization of geometric features to improve evaporation in advanced microcooling devices.

CFD-Assisted model development for estimation of hole-ablation diameter of a pressure vessel during severe accidents

To evaluate the released radioactive materials to an environment under a nuclear severe accident, it is necessary to analyze the ex-vessel phenomena. The releasing diameter is one of the most essential parameters to estimate the ex-vessel phenomena. The diameter is enlarged because the wall of a reactor vessel melts due to the high temperature of corium flow. In this study, a hole-ablation model was developed based on the one-dimensional heat balance equations for molten corium flow, crust layer, and molten layer of the reactor vessel, respectively. Moreover, the heat transfer coefficient for the situation of discharging flow was obtained using computational-fluid-dynamics. As a result, we proposed the enhancement factor, 1.57, to be multiplied by the heat transfer coefficient for an external turbulent flow. We found out that the strong effect of the critical thickness of the molten wall layer on the thicknesses of the crust layer and the molten wall layer. We suggested the relationship between the critical thickness of the molten wall layer and the ratio of the superficial velocity of the molten wall layer to that of the corium discharging flow. The proposed model was compared with the Pilch model: the mean averaged error (MAE) of the present model was 9.51%, whereas that of the Pilch model was 36.13%. In the case of the low superheat of the corium where the crust can be produced, the Pilch model without the crust formation model predicts the much larger final-hole diameter than the present model does.

Heat Transfer Characteristics and Flow Pattern Visualization for Flow Boiling in a Vertical Narrow Microchannel

Recently, microchannel heat sinks have been emerged as a kind of high performance cooling scheme to meet the heat dissipation requirement of electronics packaging and integration. In this study, an experimental investigation of subcooled flow boiling in a high-aspect-ratio rectangular microchannel was conducted with de-ionized water as the working fluid. In the experimental operations, the mass flux was varied from 200 to 400 kg/m2s and the imposed heat flux from 3 to 20 W/cm2 while the fluid inlet temperature was regulated constantly at 90 °C. The boiling curves, onset of nucleate boiling (ONB), and flow patterns of subcooled flow boiling were investigated with the aid of instrumental measurements and a high-speed camera. The slope of the boiling curves increased sharply once the superheat needed to initiate the onset of nucleate boiling was attained, with lower superheat required of boiling incipience for lower mass fluxes. Meanwhile, the initiative superheat and heat flux of onset of nucleate boiling were compared with the existing correlations in the literature with good agreement. As for the flow visualization images, slug flow and reverse backflow were observed, where transient local dryout as well as rewetting occurred. A facile image processing tool was developed to profile the transient development and progression of the liquid–vapor interface and partial dryout patches in microchannels, which proved that the physical quantities of bubble dynamics for the elongation period during subcooled boiling could be well detected and calculated.

Thermodynamic analysis and optimization of a solar organic Rankine cycle operating with stable output

Solar organic Rankine cycle (SORC) is a promising approach for solar energy utilization under low and moderate temperatures due to low investment and small-to-medium scale. The stable output of the SORC system under variable solar irradiation conditions is necessary to weaken the impact on the power grid and provide a base load. A novel operating mode was designed to obtain the stable output of 1MWe for a SORC system operated under variable solar irradiation for 1 day. The parabolic trough collector was selected for solar heat reception, and a two-tank direct thermal storage unit was used for heat storage. Given the variations in solar irradiation, the mass flow rate of the heat transfer fluid (HTF) in the solar collectors was adjusted to guarantee a constant HTF inlet temperature for the ORC unit, and the mass flow rate of the HTF from the hot tank to the ORC unit was kept constant. Thus, the SORC system could operate under stable conditions. Four organic working fluids and two cycle types were subjected to thermodynamic analysis and parametric optimization. Results show that as the HTF inlet temperature increases, the optimal efficiency of system with non-recuperative cycle and toluene increases, whereas those of systems with non-recuperative cycles and cyclohexane, hexamethyldisiloxane, and pentane as working fluids first increase and then remain constant. The increment in the superheat degree increases the efficiency of the system with recuperation. By contrast, the system with non-recuperation demonstrates the maximum system efficiency in the absence of or at a low superheat degree. The maximum system efficiency reaches 17.9%. The system efficiency of this novel operating mode is 4.2% higher than that of an unstable output operating mode when operated for 1 day.

Area-scalable high-heat-flux dissipation at low thermal resistance using a capillary-fed two-layer evaporator wick

A two-layer sintered porous evaporator wick for use in vapor chambers is shown to offer very high performance in passive high-heat-flux dissipation over large areas at a low thermal resistance. The two-layer wick has an upper cap layer dedicated to capillary liquid feeding of a thin base layer below that supports boiling. An array of vertical posts bridges these two layers for liquid feeding, while vents in the cap layer provide an unimpeded pathway for vapor removal from the base wick. The two-layer wick is fabricated using a combination of sintering and laser machining processes. The thermal resistance of the wicks during boiling is characterized in a saturated environment that replicates the capillary-fed working conditions of a vapor chamber evaporator. Thermal characterization tests are first performed using conventional single-layer evaporator wicks to analyze the effect of sintered particle size on capillary-fed boiling of water. Of the particle size ranges tested, wicks sintered from 180 to 212 μm-diameter particles provided the best combination of high dryout heat flux and a low boiling resistance. A two-layer evaporator wick comprising particles of this optimal size and a 15 × 15 array of liquid feeding posts yielded a maximum heat flux dissipation of 485 W/cm2 over a 1 cm2 heat input area while also maintaining a low thermal resistance of only ∼0.052 K/W. The thermal performance of the two-layer wick is compared against various hybrid and biporous evaporator wicks previously investigated in the literature. While previous wick designs are typically restricted to small areas and low power levels or high surface superheats when dissipating such heat fluxes, the unique area-scalability of the two-layer wick design allows it to achieve an unprecedented combination of high total power and low-thermal-resistance heat dissipation over larger areas than were previously possible.

Experimental study of nucleate pool boiling heat transfer of self‐rewetting solution by surface functionalization with TiO2 nanostructure

A titanium plate with titanium dioxide nanotube was prepared with the anodic oxidation process. The heat transfer through this plate was studied with quantitative heptanol solution as self‐rewetting solution. The microstructure of this nano‐structure surface was characterized by SEM and the surface tension of the self‐rewetting solution on it was measured by the static contact angle. The results indicated that the heat transfer coefficient (HTC) and the critical heat flux (CHF) were significantly improved with the increasing concentration of heptanol. In the 0.1 mass% heptanol solution, the best heat transfer enhancement effect was obtained. The HTC is 22.46 kW · m−2 · K−1 and CHF is 536.64 kW · m−2, which were increased by 121 and 62.9 % more than those in the distilled water. According to the experimental phenomenon and mechanism analysis, the surface of the nanotube has a more effective vaporization core, greater roughness, and better wettability. Combined with the special double surface tension gradient characteristic of the self‐wetting solution, the hot and cold liquid forms a microcirculation and the synergistic superposition of the driving force strengthens. The system can produce tiny and fine bubbles under low superheat, detach quickly, transfer heat quickly, and perform secondary wetting in time to achieve synergistic enhanced boiling heat transfer.

Effect of Primary α-Al Morphology in Slurry on Segregation during 357 Semi-Solid Die Casting

Controlling the morphology of the microstructure of the slurry is important during semi-solid die casting. For this project, semi-solid slugs were produced using the SEED (Swirled Enthalpy Equilibrium Device) process, where a fully liquid metal is poured into a steel crucible and cooled into the semi-solid temperature range, and the crucible and slurry are then swirled and cooled to the appropriate temperature (and solid fraction) for semi-solid casting. The pouring temperature of the melt into the crucible during SEED processing has been shown to influence the morphology and size of the aluminum solid particles within the slurry, which can influence the distribution and segregation of the solid particles during die casting. In this study, a specially-designed die with a serpentine-shaped flow channel has been used to study the distribution of the solid particles during semi-solid die casting. The experimental results show that a dendritic structure is formed when the liquid is poured from a high temperature, while a globular semi-solid morphology is more easily formed when poured from a low superheat. The current results also show that a dendritic structure leads to severe segregation during die casting.

Using the Phase Field Method to Investigate Microstructural Evolution of Semi-Solid 357.0 Slurries

Semi-solid aluminum slurries have globular grains, different from traditional dendritic solidification microstructures. The mechanism responsible for the formation of these globular grains is still in dispute. Some researchers suggest that the globular grains are formed by fractured dendrites, while others report it is due to copious nucleation. This study will model the growth of the α-Al phase during the production of semi-solid slurries using the swirled equilibrium enthalpy device (SEED) process, where liquid alloy with a low superheat is poured into a steel crucible and swirled within the semi-solid region. Experimental observations have shown that the final microstructure of the slurry produced by the SEED process is relatively non-uniform, with large dendrites at surface and fine spherical particles at the center. Open source code for the phase field method has been adopted to simulate the different microstructural evolution for semi-solid alloy 357.0. The effect of localized temperatures under isothermal condition on the morphology of the α-Al particles has been investigated according to the special conditions of the SEED process. In addition, suggestions for modifying the slurry production to achieve more uniform microstructure are discussed.

Pool boiling visualization of Novec-649 and FC-72 on copper microchannels

Saturated pool boiling from copper microchannel surfaces was examined using Novec-649 and FC-72 as working fluids. Parallel microchannels fabricated by machining were about 0.2 mm wide, and 0.2 to 0.4 mm deep and spaced every 0.4 mm. The experiments were carried out at atmospheric pressure. The images of the growing and departing bubbles for the entire surface of the specimen were taken with a Photonfocus PHOT MV-D1024-160-CL camera and an EX-FH20 (Casio) camera. At low superheats, the bubbles departing from enhanced surfaces were spherical in shape and did not coalesce. At higher superheats, the bubbles interacted with each other intensively, forming flattened spherical, funnel-shaped bubbles at the bottom. The visualization study aimed at identifying nucleation sites of the departing bubbles and determining their diameters and frequency at various superheats.

Characterization of inclusions in axle steel by ingot casting

The composition, size distribution and number density of inclusions in the rolled billet of bottom-teemed axle steel have been investigated through the inclusion automatic analysis system (INCAFeature, Oxford Instruments). Additionally, the thermodynamics calculation of sulfide precipitation during the solidification and rolling processes has been carried out. Seven types of inclusions are found in rolled billets, which are almost originated from the refined molten steel. However, re-oxidation, sulfide precipitation and mold powder entrainment modify the composition of inclusions during the pouring, solidification and rolling processes. Precipitation of CaS is initiated during the solidification process of molten steel. The precipitation of MnS can only be found during the hot rolling process. Inclusions in the interior of the rolled billet are more and larger than those in the subsurface of rolled billet. The inclusion index increases from the rear to the head of the rolled billet (from the bottom to the top of the ingot). Furthermore, the inclusion index increases as the superheat increases. On the basis of the results, it is recommended that molten steel should be poured at a low superheat under the protection of argon shroud. Furthermore, attention may be paid to the way of adding the mold powder.

Distilled water and ethyl alcohol boiling heat transfer on selected meshed surfaces

The article deals with the problem of pool boiling heat transfer enhancement on metal wire mesh coatings made of copper and phosphor bronze at nucleate boiling of distilled water and high purity ethyl alcohol under ambient pressure. The tests have been performed on horizontal samples containing different microstructures produced with the sintering technology. The samples were attached to the heating block with soldering. As a result of the experiments, boiling curves were obtained, describing the relationship between the dissipated heat flux and the superheat values for each specimen. A considerable augmentation of heat flux has been recorded for the meshed surfaces in relation to the smooth reference surface without any coating. Generally, the highest enhancement was recorded for the low superheat values. The presented test results have been discussed and then compared with selected correlations available in literature for nucleate boiling heat transfer on microstructure coated surfaces.

Preparation of Al-Zn-Mg-Cu alloy semisolid slurry through a water-cooled serpentine pouring channel

The semisolid slurry of Al-Zn-Mg-Cu alloy was prepared through a self-designed water-cooled copper serpentine pouring channel (WSPC) machine. Influences of pouring temperature, the number of turns and the cooling water flow rate on the microstructure of the semisolid Al-Zn-Mg-Cu alloy slurry were investigated. The results show that the semisolid Al-Zn-Mg-Cu alloy slurry with satisfactory quality can be generated by the WSPC when the pouring temperature is in the range between 680 oC and 700 oC. At a given pouring temperature, the average grain size of primary α-Al decreases and the shape factor increases with the increase of the number of turns. When the cooling water flow rate is 450 L·h-1, the obtained semisolid slurry is optimal. During the preparation of the semisolid Al-Zn-Mg-Cu alloy slurry with low superheat pouring, the alloy melt has mixed inhibition and convection flow characteristics by “self-stirring”. When the alloy melt flows through the serpentine channel, the chilling effect of the inner wall of the channel, the convection and mixed inhibition of the alloy melt greatly promote the heterogeneous nucleation and grain segregation. This effect destroys the dendrite growth mode under traditional solidification conditions, and the primary nuclei gradually evolve into spherical or nearspherical grains.

Numerical study of the dehumidification structure optimization based on the modified model

Nowadays, the vast majority of electricity is extracted from the steam turbine, while the non-equilibrium condensation will take place because of the high steam velocity and low superheat temperature in steam low-pressure stage, which will not only reduce the efficiency but endanger the safety of the steam turbine by corroding the blade. The aim of this work is to optimize the novel dehumidification structure to reduce the wetness loss and increase the efficiency of steam turbine. Firstly, the modified model is developed to predict the non-equilibrium condensation flow. The modified model is validated in stator cascade, which shows a greater agreement with the experimental data compared with original model, especially the Wilson point and pressure rise caused by the non-equilibrium condensation. Secondly, two novel dehumidification structures are presented on the basis of my previous study, and the dehumidification effect is checked using the modified model. With the passage outlet diameter increasing in the PICS, the reduction of the outlet average liquid mass fraction and wetness loss in cascade has the same tendency, and the more the passage outlet diameter, the lower the outlet average liquid mass fraction, the less the wetness loss caused by the non-equilibrium condensation in cascade. Thirdly, compared with the original cascade, the PICS with the φ=5° having the divergence nozzle characteristic and larger outlet diameter presents the best dehumidification effect and succeeds in reducing the average liquid mass fraction and wetness loss to 3% and 3.06 kJ/kg, respectively.

3D simulations of pool boiling above smooth horizontal heated surfaces by a phase-change lattice Boltzmann method

Pool boiling heat transfer from smooth horizontal hydrophilic and hydrophobic heaters having a finite thickness under constant bottom wall temperature conditions is simulated numerically with a three-dimensional liquid-vapor phase-change lattice Boltzmann method. For a small heater, single bubble dynamics and temporal variations of heater’s top wall temperature in contact with the bubble are investigated. It is found that top wall temperature drops during initial bubble growth period at low wall superheats on a hydrophilic heater due to the microlayer evaporation process. Because of a residual bubble is left on the top of the heater after bubble departure, no wall temperature drop is found on a hydrophilic surface at high wall superheats nor on a hydrophobic surface at any wall superheats. 3D pool boiling curves in dimensionless forms for smooth horizontal heaters of infinite extent from nucleation to critical heat flux through transition boiling to stable film boiling are presented. It is found that 2D and 3D simulated heat fluxes are identical in nucleate boiling and stable film boiling regimes, although transition boiling heat fluxes differ greatly. It is also found that the 3D simulated CHF is in better agreement with existing theories and correlation equations than 2D simulated CHF. Detailed 3D multiple bubbles dynamics above hydrophobic and hydrophilic surfaces and temporal variations of dry spots distribution on these surfaces at CHF are presented. Effects of wettability and thickness of the heater on boiling curves for smooth heaters are investigated. It is founded that only wettability play predominate roles in nucleate boiling regime from smooth superheated surfaces while all factors influence the transition boiling regime (including maximum and minimum heat fluxes) greatly.

The Structure of the Smelting Cell Ledge Under Variable Sidewall Heat Flow Conditions

The power input to most smelters has become more variable and cell voltages have been continually reduced, giving rise to bath superheat in the 3-6°C range in many cases. The effect on the structure of the ledge is now being investigated since it also impacts heat balance shifting for smelter power flexibility, which is an inevitable consequence of continental network. Shell Heat Exchanger technology has been created to achieve the heat balance shifting by regulating sidewall heat loss with low superheat and variable cell power input. This article provides new information about the structure of the ledge material under the influence of these factors and discusses the implications for future smelter operation.

A Semi-analytical Method for Computing the Dynamics of Bubble Growth: The Effect of Superheat and Operating Pressure

In this work, we propose a semi-analytical method for computing the dynamics of a spherical-bubble growth in an infinite pool of superheated water. The dynamics shows the temporal variation of bubble radius in superheat of 1.4–17.89 K. The method accurately computes the bubble growth controlled by surface tension, particularly in very low superheats, in which the existing theories are not adequate. The present method consistent with the existing theories computes the bubble growth controlled by heat transfer in high superheats. The semi-analytical method with a modification of heat transfer at the interface is capable of predicting the bubble growth in pool boiling experiments and numerical simulations. The effect of increasing operating pressure on the bubble growth decreases the growth constant following a power law. The variations in energy components due to pressure difference, inertia, surface tension, and thermal diffusion reveal some interesting insights of the bubble-growth mechanism.

Saturated film boiling of water over a finite size heater

A numerical investigation was carried out on pool film boiling of water over a finite size heater at near critical conditions using a Coupled Level Set and Volume of Fluid (CLSVOF) method. For the modelling of moving contact line, a dynamic contact angle approach has been considered to capture the flow characteristics near the contact line region accurately. The effect of Jacob number on flow and heat transfer characteristics was investigated for Jacob number ranging from Jav = 1.274 to 38.205. At lower Jacob number, Jav = 1.274 (ΔTSup = 1K), discrete bubbles are released from vapour layer periodically. At the medium Jacob numbers, discrete vapour bubbles are released from the vapour layer as well as from the tips of the vapour columns. At higher Jacob number, Jav = 38.205 (ΔTSup = 30K), discrete bubbles are released predominantly from the tips of long vapour columns. Over the heater surface, the rate of vapour bubble release is low at the edges of the heaters where the contact points exist compared to other regions of the heater surface. The comparison of the time averaged Nusselt number obtained from the finite and infinite size heaters show that the heat transfer rate is the same for the low wall superheats. However, at higher wall superheat conditions, heat transfer rate is higher in the case of finite size heaters compared to that of infinite size heaters.