Interfacial Heat Flux Research Articles

Dynamic heat transfer analysis of condensed droplets growing and coalescing on water repellent surfaces

We present the dynamic heat transfer analysis of condensed droplets growing and coalescing on hydrophobic (HPo) and superhydrophobic (SHPo) surfaces using a full 3D numerical simulation. In the model, two water droplets surrounded by fully-saturated water vapor grow on a horizontal surface through condensation until they coalesce together. The dynamic changes in the interfacial areas, temperature distributions and heat flux through each interface were analyzed. The effects of vapor phase temperature distribution, parasitic thermal resistance and surface flooding on the heat transfer rate are also quantified. The results show that a relatively high heat transfer rate through solid–vapor interface on SHPo partially compensates the low heat transfer rate through solid–liquid interface. The parasitic thermal resistance of the suggested SHPo may reduce the heat transfer performance over 30%. When the flooding occurs on HPo, the heat transfer rate rapidly decreases below a half of the value obtained at the beginning of coalescence. This work shows the importance of the heat transfer analysis considering dynamic changes in the interfacial area and resulting 3D temperature distributions, and will help develop the optimal condensation heat transfer surfaces.

Interfacial heat transfer behavior at metal/die in finger-plated casting during high pressure die casting process

Heat transfer at the metal-die interface has a great influence on the solidification process and casting structure. As thin-wall components are extensively produced by high pressure die casting process (HPDC), the B390 alloy finger-plate casting was cast against an H13 steel die on a cold-chamber HPDC machine. The interfacial heat transfer behavior at different positions of the die was carefully studied using an inverse approach based on the temperature measurements inside the die. Furthermore, the filling process and the solidification rate in different finger-plates were also given to explain the distribution of interfacial heat flux (q) and interfacial heat transfer coefficient (h). Measurement results at the side of sprue indicates that qmax and hmax could reach 9.2 MW∙m-2 and 64.3 kW∙m-2∙K-1, respectively. The simulation of melt flow in the die reveals that the thinnest (T1) finger plate could accelerate the melt flow from 50 m∙s-1 to 110 m∙s-1. Due to this high velocity, the interfacial heat flux at the end of T1 could firstly reach a highest value 7.92 MW∙m-2 among the ends of T n (n=2,3,4,5). In addition, the qmax and hmax values of T2, T4 and T5 finger-plates increase with the increasing thickness of the finger plate. Finally, at the rapid decreasing stage of interfacial heat transfer coefficient (h), the decreasing rate of h has an exponential relationship with the increasing rate of solid fraction (f).

Mixing rates and vertical heat fluxes north of Svalbard from Arctic winter to spring

AbstractMixing and heat flux rates collected in the Eurasian Basin north of Svalbard during the N‐ICE2015 drift expedition are presented. The observations cover the deep Nansen Basin, the Svalbard continental slope, and the shallow Yermak Plateau from winter to summer. Mean quiescent winter heat flux values in the Nansen Basin are 2 W m−2 at the ice‐ocean interface, 3 W m−2 in the pycnocline, and 1 W m−2 below the pycnocline. Large heat fluxes exceeding 300 W m−2 are observed in the late spring close to the surface over the Yermak Plateau. The data consisting of 588 microstructure profiles and 50 days of high‐resolution under‐ice turbulence measurements are used to quantify the impact of several forcing factors on turbulent dissipation and heat flux rates. Wind forcing increases turbulent dissipation seven times in the upper 50 m, and doubles heat fluxes at the ice‐ocean interface. The presence of warm Atlantic Water close to the surface increases the temperature gradient in the water column, leading to enhanced heat flux rates within the pycnocline. Steep topography consistently enhances dissipation rates by a factor of four and episodically increases heat flux at depth. It is, however, the combination of storms and shallow Atlantic Water that leads to the highest heat flux rates observed: ice‐ocean interface heat fluxes average 100 W m−2 during peak events and are associated with rapid basal sea ice melt, reaching 25 cm/d.

Thermocapillary convection due to imposed interfacial heating in the presence of magnetic field

Marangoni convection arises when the surface tension of the fluid varies spatially due to accompanying temperature gradients along the interface, which can occur in various ways. In the case of nonuniform surface heat fluxes imposed due to incident beams from power sources, Marangoni convection can be induced within heated liquid pools formed during melting in a number of material-processing applications. In this work, we develop a physical model and study the thermocapillary flow generated due to an imposed surface heat flux with a specified spatial variation. Furthermore, we incorporate the effect of applying a magnetic field to investigate its influence on the structure of such unique class of surface tension-driven flows. New similarity solutions are developed for the thermocapillary convection due to an imposed interfacial heat flux variation specified as $$\mathop q\nolimits _0 \mathop x\nolimits ^{n + 1} $$ , where x is the surface coordinate and $$ \mathop q\nolimits _0 $$ is a parameter representing the magnitude of the incident power. Numerical results are obtained and the structure of the Marangoni convection for various sets of the characteristic parameters, such as the power-law exponent n, Prandtl number Pr and a magnetic interaction parameter M, are studied. In particular, it is found that with the increasing n, i.e., considering sharper variations in the surface heat flux profiles, the Marangoni convection is promoted. Conversely, with the increasing M, the magnitudes of the thermocapillary convection velocity are decreased, thereby demonstrating the damping effect of the magnetic field.

Point heat sink induced by droplet train impingement

A point heat sink is produced by impinging a high frequency microscale droplet stream onto a superheated copper substrate. Although the overall target surface area is larger than the liquid-solid interface by two or three orders of magnitude, the thermal energy is mainly removed through the point heat sink rather than the rest dry area. Therefore, the spherical conduction patterns in the solid materials are observed with a “nozzle-shifting” method which requires only two temperature probes. The temperature gradient in the vicinity of the impingement stagnation point is tremendously high, suggesting that the liquid-solid interface temperature is significantly lower than the far-field bulk temperature of the substrate. Moreover, the liquid-to-solid heat transfer is measured, which agrees well with the theoretical prediction. The maximum interface heat flux can reach around 80 W/mm2. It is insensitive to the substrate temperature in a relatively wide temperature range, which brings conveniences to the potential industrial applications.

Analysis of the effects of package design on the rate and uniformity of cooling of stacked pomegranates: Numerical and experimental studies

Computational fluid dynamics (CFD) model was developed, validated and used to analyse cooling characteristics of two different package designs (CT1 and CT2) used for postharvest handling of pomegranate fruit. The model incorporated geometries of fruits, packaging box, tray and plastic liner. Thin layer of plastic material with conservative interface heat flux was used to model liners. The accuracy of the model to predict airflow and temperature distributions were validated against experimental data. The model predicted airflow through the stacks and cooling rates within experimental error. Stack design markedly affected the airflow profile, rate and uniformity of cooling. The cooling rate of the two package designs differed by 30% and plastic lining increased the average 7/8th cooling times from 4.0 and 2.5h to 9.5 and 8.0h for the CT1 and CT2 stacks, respectively. Profile of high and low temperature regions depended considerably on packaging box design.

Modeling of Interface Heat Flux and Thermal Field of Mold Materials during Gravity Die Casting

One of the key controllable and influential factors to obtain a casting simulation, representative of reality, is the choice of boundary condition. The thermal boundary condition to be specified at the metal-mold interface must account for complex heat transfer phenomena associated with solidifying casting. The present study aims at estimating the heat flux at the interface of the mold and the solidifying metal by Inverse Heat Conduction Problem (IHCP) approach. Solidification studies were conducted on casting of aluminum reinforced with boron carbide composite. Copper, cast iron and stainless steel were used as mold materials. The temperature data of the mold was recorded from the beginning to end of solidification using k-type thermocouples connected to temperature data logger. This time-temperature history was used as input to the IHCP algorithm to simulate the interface heat flux and thermal field of the mold. The results indicate that the interface heat flux is highly transient and varies with the variation in the thermo-physical properties of the mold materials. The study also demonstrates that heat conduction is one dimensional in copper mold and two dimensional in cast iron and stainless steel mold during phase change.

Role of Hydrogen Bonds in Thermal Transport across Hard/Soft Material Interfaces.

The nature of the bond is a dominant factor in determining the thermal transport across interfaces. In this paper, we study the role of the hydrogen bond in thermal transport across interfaces between hard and soft materials with different surface functionalizations around room temperature using molecular dynamics simulations. Gold (Au) is studied as the hard material, and four different types of organic liquids with different polarizations, including hexane (C5H11CH3), hexanamine (C6H13NH2), hexanol (C6H13OH), and hexanoic acid (C5H11COOH), are used to represent the soft materials. To study the hydrogen bonds at the Au/organic liquid interface, three types of thiol-terminated self-assembled monolayer (SAM) molecules, including 1-hexanethiol [HS(CH2)5CH3], 6-mercapto-1-hexanol [HS(CH2)6OH], and 6-mercaptohexanoic acid [HS(CH2)5COOH], are used to functionalize the Au surface. These SAM molecules form hydrogen bonds with the studied organic liquids with varying strengths, which are found to significantly improve efficient interfacial thermal transport. Detailed analyses on the molecular-level details reveal that such efficient thermal transport originates from the collaborative effects of the electrostatic and van der Waals portions in the hydrogen bonds. It is found that stronger hydrogen bonds will pull the organic molecules closer to the interface. This shorter intermolecular distance leads to increased interatomic forces across the interfaces, which result in larger interfacial heat flux and thus higher thermal conductance. These results can provide important insight into the design of hard/soft materials or structures for a wide range of applications.

Determination of Metal/Die Interfacial Heat Transfer Coefficients in Squeeze Casting of Wrought Aluminum Alloy 7075 With Variations in Section Thicknesses and Applied Pressures

Squeeze casting of wrought aluminum 7075 was carried out on a 75-ton hydraulic press. Metal/die interface heat transfer phenomena in squeeze casting of the alloy were investigated. To facilitate experimental measurements, a five-step casting mold was designed for the experiments. The five-step casting consisted of five different section thicknesses of 2, 4, 8, 12, and 20 mm. Squeeze casing experiments were performed under the applied hydraulic pressures of 30, 60, and 90 MPa. Temperatures were measured at the casting surface and at various specific locations inside the die. At each step, thermocouples were placed at 2, 4, and 6 mm away from the inside die face. Based on the measured temperature results, the interfacial heat transfer coefficients (IHTCs) and heat fluxes were determined by solving the one-dimensional transient heat conduction equation with the inverse method. With increasing the casting section thicknesses from 2 to 20 mm, the peak IHTC values varied from 1683.46 W/m2 K to 9473.23 W/m2 K, 2174.78 W/m2 K to 13,494.05 W/m2 K, and 3873.45 W/m2 K to 15,483.01 W/m2 K for the applied hydraulic pressures of 30, 60, and 90 MPa, respectively.

On numerical simulation of cavitating flows under thermal regime

In this work, we investigate closure laws for the description of interfacial mass transfer in cavitating flows under thermal regime. In a first part, we show that, if bubble resident time in the low pressure area of the flow is larger than the inertial/thermal regime transition time, bubble expansion are no longer monitored by Rayleigh equation, but by heat transfer in the liquid phase at bubbles surfaces. The modelling of interfacial heat transfer depends thus on a Nusselt number that is a function of the Jakob number and of the bubble thermal Péclet number. This original approach has the advantage to include the kinetic of phase change in the description of cavitating flow and thus to link interfacial heat flux to interfacial mass flux during vapour production. The behaviour of such a model is evaluated for the case of inviscid cavitating flow in expansion tubes for water and refrigerant R114 using a four equations mixture model. Compared with inertial regime (Rayleigh equation), results obtained considering thermal regime seem to predict lower local gas volume fraction maxima as well as lower gradients of velocity and gas volume fraction. It is observed that global vapour production is closely monitored by volumetric interfacial area (bubble size and gas volume fraction) and mainly by the Jakob number variations. It is found that, in contrast with phase change occurring in common boiling flow, Jakob number variation is influenced by phasic temperature difference but also by density ratio variation with pressure and temperature (Ja∝(ρL/ρG)ΔT).

Experimental study on phase change heat transfer characteristics of alloys

Solid–liquid phase change process widely exists in many applications and acts important roles. Facing the deficiency of existing research, an apparatus was designed to measure the motion of mushy zone inside alloys. In this study, the temperature distribution inside the alloy bar, the heat flux at the cooling interface, average interface heat transfer coefficient, evolutions of phase boundaries and mushy zone in the solidification process were obtained. The effects of cooling condition and physical properties of alloy on the motion of the mushy zone were discussed. The results suggest that the evolution of liquidus boundary is controlled by sensible heat in liquid zone and interface heat flux at the cooling interface. By contrast, the evolution of solidus boundary is governed by sensible and latent heat released by mushy zone as well as heat flux at the cooling interface. Moreover, the increase of thermal diffusion inside the alloy, Biot number and Stefan number lower the max-length of the mushy zone and shorten the solidification time. Finally, dimensionless correlations were developed to predict the motion of mushy zone in the solidification process.

Macro-scale conjugate heat transfer in periodically developed flow through solid structures

This paper treats the macro-scale description of the periodically developed conjugate heat transfer regime, in which heat transfer takes place between an incompressible viscous flow and spatially periodic solid structures through a spatially periodic interfacial heat flux. The macro-scale temperature of the fluid and the solid structures are defined through a spatial averaging operator with a specific weighting function. It is shown that a double volume average is necessary in order to have a linearly changing macro-scale temperature in response to a constant macro-scale heat flux. Furthermore, with the aid of a double volume average, the thermal dispersion source, the thermal tortuosity and the interfacial heat transfer coefficient all become spatially constant in the developed regime. That way, these closure terms of the macro-scale temperature equations can be exactly determined from the periodic temperature part on a unit cell of the solid structures without taking the spatial moments of the solid into account. The theoretical derivations of this paper are illustrated for a case study describing the heat transfer between a fluid flow and an array of solid squares with a uniform volumetric heat source.

Carbonated aqueous media for quench heat treatment of steels

Distilled water and polyalkylene glycol (PAG)-based aqueous quenchants of 5 and 10 vol.% with and without carbonation were prepared and used as heat transfer media during immersion quenching. Cooling curves were recorded during quenching of an inconel 600 cylindrical probe instrumented with multiple thermocouples. It was observed that the vapor stage duration was prolonged and the wetting front ascended uniformly for quenching with carbonated media. The cooling data were analyzed by determining the critical cooling parameters and by estimating the spatially dependent probe/quenchant interfacial heat flux transients. The study showed significantly reduced values of heat transfer rate for carbonated quenchants compared to quenchants without carbonation. Further, the reduction was more pronounced in the case of PAG-based carbonated quenchants than carbonated distilled water. The results also showed the dependence of heat transfer characteristics of the carbonated media on polymer concentration. The effect of quench uniformity on the microstructure of the material was assessed.

Experimental and Numerical Determination of Casting-Mold Interfacial Heat Transfer Coefficient in the High Pressure Die Casting of A-360 Aluminum Alloy

Although die casting is a near net shape manufacturing process, it mainly involves a thermal process. Therefore, in order to produce high quality parts, it is important to determine casting-mold interfacial heat transfer coefficient and heat flux. In this paper the effects of different injection parameters (second phase velocity, injection pressure, pouring and die temperature) on heat flux and interfacial heat transfer coefficient were investigated experimentally and numerically. Experiments were performed in cylindrical geometry using a cast aluminum alloy A360 against H13 steel mold. Selected injection parameters were 1.7–2.5 m/s for second phase velocity, 100–200 bar for third phase pressure, 983–1053 K for pouring temperature and 373, 433, 493, 553 K for the die temperature. These parameters were used for both non-vacuum and vacuum conditions in the cavity of the mold. The effects of the application under vacuum conditions were also studied. Temperatures were measured as functions of time, using 18 thermocouples, which were mounted at different depths of casting and mold material. Measured and calculated temperature values are found compatible. Interfacial heat transfer coefficient h and heat flux q depending on the experimentally measured temperature values were calculated with finite difference method using explicit technique in C# programming language. In addition to experiments, Flow-3D software simulations were performed using the same parameters. Interfacial heat transfer coefficient and heat flux results obtained from Flow-3D are also presented in the study. Interfacial heat transfer coefficient has decreased as a result of increasing of temperature of mold and pouring. In addition, interfacial heat transfer coefficient values have increased slightly with the increase of injection speed and pressure. It was observed that the values of interfacial heat transfer coefficient and heat flux have also increased when vacuum was applied inside the cavity of the mold. When all injection parameters are considered, it is seen that the interfacial heat transfer coefficient varies between 92–117 kW/m K.

Development and application of inverse heat transfer model between liquid metal and shot sleeve in high pressure die casting process under non-shooting condition

To predict the heat transfer behavior of A380 alloy in a shot sleeve, a numerical approach (inverse method) is used and validated by high pressure die casting (HPDC) experiment under non-shooting condition. The maximum difference between the measured and calculated temperature profiles is smaller than 3 °C, which suggests that the inverse method can be used to predict the heat transfer behavior of alloys in a shot sleeve. Furthermore, the results indicate an increase in maximum interfacial heat flux density (qmax) and heat transfer coefficient (hmax) with an increase in sleeve filling ratio, especially at the pouring zone (S2 zone). In addition, the values of initial temperature (TIDS) and maximum shot sleeve surface temperature (Tsimax) at the two end zones (S2 and S10) are higher than those at the middle zone (S5). Moreover, in comparison with fluctuations in heat transfer coefficient (h) with time at the two end zones (S2 and S10), 2.4-6.5 kW∙m-2∙K-1, 3.5-12.5 kW∙m-2∙K-1, respectively, more fluctuations are found at S5 zone, 2.1-14.7 kW∙m-2∙K-1. These differences could theoretically explain the formation of the three zones: smooth pouring zone, un-smooth middle zone and smooth zone, with different morphologies in the metal log under the non-shot casting condition. Finally, our calculations also reveal that the values of qmax and hmax cast at 680 °C are smaller than those cast at 660 °C and at 700 °C.

Improving a conjugate-heat-transfer immersed-boundary method

Purpose– The purpose of this paper is to provide the current state of the art in the development of a computer code combining an immersed boundary method with a conjugate heat transfer (CHT) approach, including some new findings. In particular, various treatments of the fluid-solid-interface conditions are compared in order to determine the most accurate one. Most importantly, the method is capable of computing a challenging three dimensional compressible turbulent flow past an air cooled turbine vane.Design/methodology/approach– The unsteady Reynolds-averaged Navier–Stokes (URANS) equations are solved within the fluid domain, whereas the heat conduction equation is solved within the solid one, using the same spatial discretization and time-marching scheme. At the interface boundary, the temperatures and heat fluxes within the fluid and the solid are set to be equal using three different approximations.Findings– This work provides an accurate and efficient code for solving three dimensional CHT problems, such as the flow through an air cooled gas turbine cascade, using a coupled immersed boundary (IB) CHT methodology. A one-to-one comparison of three different interface-condition approximations has shown that the two multidimensional ones are slightly superior to the early treatment based on a single direction and that the one based on a least square reconstruction of the solution near the IB minimizes the oscillations caused by the Cartesian grid. This last reconstruction is then used to compute a compressible turbulent flow of industrial interest, namely, that through an air cooled gas turbine cascade. Another interesting finding is that the very promising approach based on wall functions does not combine favourably with the interface conditions for the temperature and the heat flux. Therefore, current and future work aims at developing and testing appropriate temperature wall functions, in order to further improve the accuracy – for a given grid – or the efficiency – for a given accuracy – of the proposed methodology.Originality/value– An accurate and efficient IB CHT method, using a state of the art URANS parallel solver, has been developed and tested. In particular, a detailed study has elucidated the influence of different interface treatments of the fluid-solid boundary upon the accuracy of the computations. Last but not least, the method has been applied with success to solve the well-known CHT problem of compressible turbulent flow past the C3X turbine guide vane.

Experimental Study on the Distribution of Heat Flux along Contact Arc in Twin-Roll Continuous Strip Casting

Heat flux between molten metal and casting rolls plays a key role in improving the quality of the strip, studying on the interfacial heat flux has very important theoretical significance and practical value. According to the contact form and the heat flux characteristics of molten metal and casting rolls in twin-roll strip casting process, a set of measuring equipments has been developed, which are used to measure the heat flow of the interface between the molten metal and solid, and a relevant software system has been exploited to acquire and analyze the experiment data. Using the method of combining experimental and numerical computation, we analyze the influence of pouring temperature on the contact interface heat flux. The experimental results show that the higher the pouring temperature , the greater the summit of heat flux density and the shorter time to the summit heat flux are, but when the pouring temperature reaches a certain critical value, the summit of heat flux will decrease with the increase of pouring temperature.

Heat transfer analysis of PCM slurry flow between parallel plates

A CFD analysis of melting of solid particles during sedimentation in their own melt is presented. The motion of the solid particles is determined using a Lagrangian approach, while hydrodynamics and heat transfer throughout the fluid are determined using a finite volume scheme. Particle and fluid motions are two-way coupled through moving solid–liquid interfaces, whose morphologies are determined from the local interfacial heat fluxes and tracked using a deforming grid. The accuracy of the model is verified using benchmark solutions of a single particle undergoing simultaneous melting and settling. The results show that the presence of solid-phase particles within the liquid enhances the heat transfer between the bulk fluid and the heating surfaces due to improved mixing, as well as the latent heat associated with phase change. Particle loadings corresponding to solid volume fractions of 3–18% have been considered here, and it is found that the average wall Nusselt number increases linearly with volume fraction. An enhancement in the average wall Nusselt number of 100% as compared to a single-phase flow is achieved using a slurry with 18% solid particles by volume. Initial particle arrangement is found to have a minimal effect on the overall heat transfer. Additionally, for a fixed solid volume fraction, it is found that particle diameter does not strongly influence heat transfer enhancement.

Rational Micro/Nanostructuring for Thin-Film Evaporation

Heat management in electronics and photonics devices is a critical challenge impeding accelerated breakthrough in these fields. Among approaches for heat dissipation, thin-film evaporation with micro/nanostructures has been one of the most promising approaches that can address future technological demand. The geometry and dimension of these micro/nanostructures directly govern the interfacial heat flux. Here, through theoretical and experimental analysis, we find that there is an optimal dimension of micro/nanostructures that maximizes the interfacial heat flux by thin-film evaporation. This optimal criterion is a consequence of two opposing phenomena: nonuniform evaporation flux across a liquid meniscus (divergent mass flux near the three-phase contact line) and the total liquid area exposed for evaporation. In vertical micro/nanostructures, the optimal width-to-spacing ratio is 1.27 for square pillars and 1.5 for wires (e.g., nanowires). This general criterion is independent of the solid material and th...

Effect of cerium addition on casting/chill interfacial heat flux and casting surface profile during solidification of Al-14%Si alloy

In the present investigation, Al-14 wt. % Si alloy was solidified against copper, brass and cast iron chills, to study the effect of Ce melt treatment on casting/chill interfacial heat flux transients and casting surface profile. The heat flux across the casting/chill interface was estimated using inverse modelling technique. On addition of 1.5% Ce, the peak heat flux increased by about 38%, 42% and 43% for copper, brass and cast iron chills respectively. The effect of Ce addition on casting surface texture was analyzed using a surface profilometer. The surface profile of the casting and the chill surfaces clearly indicated the formation of an air gap at the periphery of the casting. The arithmetic average value of the profile departure from the mean line (Ra) and arithmetical mean of the absolute departures of the waviness profile from the centre line (Wa) were found to decrease on Ce addition. The interfacial gap width formed for the unmodified and Ce treated casting surfaces at the periphery were found to be about 35µm and 13µm respectively. The enhancement in heat transfer on addition of Ce addition was attributed to the lowering of the surface tension of the liquid melt. The gap width at the interface was used to determine the variation of heat transfer coefficient (HTC) across the chill surface after the formation of stable solid shell. It was found that the HTC decreased along the radial direction for copper and brass chills and increased along radial direction for cast iron chills.