Interfacial Heat Transfer Coefficient Values Research Articles

Numerical Analysis of Flow and Heat Transfer Characteristics of Lattice-Based Compact Heat Sinks

Abstract Single unit cell thick lattice frame materials have applications in efficient heat exchangers. This study is focused on strut-based sandwich-type configurations obtained through reticulation of unit cell topologies of tetrakaidecahedron (TKD), octet, and rhombic dodecahedron (DDC) shapes at a porosity of 0.9 with water as the working fluid. Interfacial heat transfer coefficient values on struts and endwalls were determined by imposing constant temperature boundary condition. Averaged heat transfer coefficient on the endwall was the highest for tetrakaidecahedron lattice whereas rhombic dodecahedron lattice exhibited the highest average interfacial heat transfer coefficients on the struts. Flow analysis showed the presence of strong secondary flow features on planes normal to the mean flow direction that demonstrated the unique flow mixing capabilities of these lattices. Reported interfacial heat transfer coefficient at struts and endwall can be used in volume-averaged computations of metal foams (representative of lattices' flow and thermal properties) under local thermal nonequilibrium.

Inverse analysis of mould-casting interfacial heat transfer towards improved castings

The correct information about the interfacial heat transfer coefficient (IHTC) at the mould-casting interface is of utmost importance for the better castings. The quality of caste materials is essentially dependent on the rate of heat transfer across the mould-casting interface during solidification and cooling of the casting materials. The uniform heat transfer across the casting and mould would lead to reduce casting defects. The aim of the present investigation is to carry out the inverse heat transfer analysis on the interface of mold and casting. The effect of pouring temperature, mould material and the properties of the cast metal were investigated by evaluating interfacial heat flux and interfacial heat transfer coefficient (IHTC) using transient temperatures inside the mould and casting. During the experiments, transient temperatures inside the mould and casting are recorded by very precise thermocouples. From the results, both interfacial heat flux and interfacial heat transfer coefficient are found to be dependent on pouring temperature keeping all other parameters constant. At same pouring temperature, interfacial heat flux is found to be independent of casting material while IHTC is found to be dependent on the properties of casting material. Peak value of IHTC is found to be much higher in case of zinc metal casting as compared to aluminium metal casting. Novel and experimental based analytical interpretation of the results are provided. The results might be useful for metal forming industries to develop improved castings at lower costs.

Influence of carbon nanotube reinforcement on the heat transfer coefficient, microstructure, and mechanical properties of a die cast Al-7Si-0.35Mg alloy

Al-7Si-0.35Mg or A356 alloy is most widely used in aircraft and automobile industries owing to its high strength to weight ratio. This alloy has been reinforced with a 1 wt% carbon nanotube (CNT) to improve its properties in this investigation. First, A356/1 wt% CNT powders were ball milled in the presence of ethanol and subsequently consolidated using gravity die casting. Ball milling was effective in achieving homogeneous dispersion of CNT. The microstructural study revealed the segregation of the Al4C3 phase at the grain boundary. This mechanism is known as grain boundary precipitation. Also, the grain size has decreased by ~44%. Next, the casting-die interfacial heat transfer coefficient (IHTC) has been evaluated using Beck's inverse heat transfer algorithm. With the reinforcement, the IHTC has increased by ~2.5%, which indicates the rise in heat transfer rate during solidification. Then, the experimental and theoretical tensile properties of A356 were correlated using simulation software. The experimental results showed the synergistic effect of grain size, Al4C3, and IHTC improving yield strength by ~19.8%, ultimate tensile strength by ~14.13%, elongation by 7%, and hardness ~22%. Therefore, a meagre 1 wt% CNT has improved the heat transfer rate of the melt as indicated by IHTC values. This effect was further corroborated by evaluating the thermal conductivity of the sample. The thermal conductivity has improved by 10% that resulted in finer grain size of the sample. Therefore, such reinforced alloys are expected to display higher strength demanded in various industrial applications.

Influence of applied pressures and casting section thicknesses on interfacial heat transfer in squeeze casting of magnesium alloy AZ91

The estimation for interfacial heat transfer coefficient (IHTC) between casting and mold is paramount to predict solidification phenomena. In this study, three hydraulic pressures of 30, 60, and 90 MPa were applied to indirectly squeeze cast magnesium alloy AZ91 in a P20 steel mold with five casting section thicknesses of 2, 4, 8, 12, and 20 mm, named steps 1, 2, 3, 4 and 5, by using a 75-ton hydraulic press machine. For comparison, the AZ91 alloy was also cast without the applied pressure (0 MPa). K-type thermocouples were utilized to measure temperatures simultaneously on the casting surface and at three different depths from the mold surface inward. Based on the temperature measurements, the IHTCs at the five steps were calculated by the inverse method. Despite of the differences in their peak values, the IHTC variation for the three applied pressures at the five steps during the squeeze casting process was similar to that of 0 MPa, in which the IHTC values abruptly rose to an apex first, and then declined slowly. The overall IHTC values of step 5 were higher than those of the other four steps, while the lowest peak IHTC value was present at step 1. When the applied pressure was increased, the IHTC peak value of each step became large accordingly. The empirical equations relating the IHTCs to the pressures, section thicknesses, and temperatures at the casting/mold interface were derived based on the multivariate linear and polynomial regression.

Investigations of interfacial heat transfer efficiency in HFQ® process of high strength aluminum alloy

Hot form quench (HFQ®) is a novel hot stamping process to deform complex-shaped parts applied in automobile industry, and interfacial heat transfer coefficient (IHTC) play an important role in HFQ® process. In this research, the average IHTC values were compared under different forming conditions during die quenching based on the calculation and finite element (FE) simulation results. Beside, transmission electron microscope (TEM), differential scanning calorimeter (DSC) and tensile tests were performed to investigate the relationship among IHTC, microstuructures and mechanical properties. The results show that the experimental results agree well with those achieved by FE simulation. The value of IHTC for AA7075 increased with the increased contact pressure during HFQ® process. In contrast, the increasing temperature of forming die has a negative impact on IHTC. The critical quenching rate (Qc), inducing the formation of a large number of equilibrium phases η, is between 14.3 °C s−1–18.5 °C s−1, and the corresponding IHTC range is 2468 W·m2 k–2656 W·m2 k. With regard to the condition with cooling rate lower than Qc, the formation of a large amount of η is recognized as the reason for the strength decrease.

Characterisation of the interfacial heat transfer coefficient in hot stamping of titanium alloys

Abstract The interfacial heat transfer coefficient (IHTC) for titanium alloys is an important parameter in non-isothermal hot stamping processes to determine the temperature field as well as temperature-dependent material behaviours that consequently affect the post-form properties of the formed components. However, the IHTC for titanium alloys in hot stamping processes has seldom been studied before. In the present research, the effects of contact pressure, lubricant, surface roughness, tooling material and initial blank temperature on the IHTC for the titanium alloy Ti-6Al-4V were studied and modelled to characterise the IHTC values under various hot stamping conditions as well as identify the functional mechanisms affecting the IHTC. Furthermore, the results of hot stamping of Ti-6Al4V wing stiffener components were used to verify the simulation results of the temperature field of the formed component with an error of less than 5%.

Measuring the interfacial heat transfer coefficient between flowing molten alloy and sand mold using fluidity tests

Abstract This study specifically examined a method using fluidity test results to measure the interfacial heat transfer coefficient (IHTC) between a molten alloy and a mold during flow. Review of earlier studies revealed that conventional methods require the choking area length to calculate the IHTC during flow, but one earlier study used an assumed choking area length. Details of its length remain unclear. Using an arbitrarily chosen choking area length leads to inaccurate acquisition of the IHTC. For this study, we observed the microstructures and macrostructures around a fluidity test specimen tip to ascertain the choking area length. This value led to calculation of the IHTC between the flowing bronze melt and the shell sand mold as 1521 W/m2•K. This study also found a new method requiring no choking area length to ascertain the IHTC between the flowing melt and the sand mold. The IHTC obtained using this method was 1406 W/m2•K, representing a difference of about 8% from the value obtained using a conventional method. This study assessed experimental conditions under which the new method can produce accurate IHTC values.

Spatial interfacial heat transfer and surface characteristics during gravity casting of A356 alloy

Abstract As one of the key boundary conditions during casting solidification process, the interfacial heat transfer coefficient (IHTC) affects the temperature variation and distribution. Based on the improved nonlinear estimation method (NEM), thermal measurements near both bottom and lateral metal-mold interfaces throughout A356 gravity casting process were carried out and applied to solving the inverse heat conduction problem (IHCP). Finite element method (FEM) is employed for modeling transient thermal fields implementing a developed NEM interface program to quantify transient IHTCs. It is found that IHTCs at the lateral interface become stable after the volumetric shrinkage of casting while those of the bottom interface reach the steady period once a surface layer has solidified. The stable value of bottom IHTCs is 750 W/(m2·°C), which is approximately 3 times that at the lateral interface. Further analysis of the interplay between spatial IHTCs and observed surface morphology reveals that spatial heat transfer across casting-mold interfaces is the direct result of different interface evolution during solidification process.

Experimental study on interfacial heat-transfer coefficients at the casting/die interface in squeeze casting under transient condition

ABSTRACTBy using two-dimensional transient state inverse problems method, the interfacial heat-transfer coefficient (IHTC) was evaluated, based on experimentally measured temperature distributions of solidification of an Al alloy under various pressures (40, 50 and 60 MPa). The value of IHTC obtained from the mould and the casting varies with time subsequent to pouring the molten metal into a mould cavity. The impact of different applied pressure with different positions of mould insulation on IHTC was experimentally measured and calculated for the analysis. The calculated heat-transfer coefficients were used for solidification simulation in the compress casting process.

Effect of tool coating on interfacial heat transfer coefficient in hot stamping of AA7075 aluminium alloys

Chromium carbonitride coatings have been gaining importance in the hot stamping industry due to their excellent wear and corrosion resistances that extend the lifetime of tools they are applied to. However, their effect on the heat transfer between materials being formed and the tools in a hot stamping process has yet to be studied. In the present research, the interfacial heat transfer coefficient values between a hot AA7075 workpiece and cold chromium carbonitride-coated tools were experimentally determined under different contact pressure conditions. A mechanism-based interfacial heat transfer coefficient model was subsequently developed for precise finite element simulations of the temperature evolution and cooling rate of the components being formed. The model enables the selection of the tool coatings to be optimized based on the requirements of the hot stamping process, with the thermal conductivity of the coating material determining its thermal performance and effect on interfacial heat transfer coefficient.

A study of interfacial heat transfer and its effect on quenching when hot stamping AA7075

The aim of this research was to investigate the effects of contact pressure and lubrication on the interfacial heat transfer coefficient (IHTC) between AA7075 sheet and H13 tool steel and the in-die quenching performance in hot stamping. Firstly, a series of designed in-die quenching experiments were performed using different contact pressures, 0.05–30MPa and lubrication conditions to determine the IHTC values using an efficient methodology. Secondly, temperature evolution of the tools and blank during the in-die quenching was investigated. Mechanical properties of material in-die quenched under different process conditions, were measured to determine their relation to quench conditions. The results have shown that IHTC values increased with increasing contact pressure and use of lubricant. A strength level of T6 condition could be obtained using a contact pressure greater than 5MPa in the lubricated condition.

Determination of the interfacial heat transfer coefficient for a hot aluminium stamping process

The interfacial heat transfer coefficient (IHTC) is an important thermophysical parameter in hot stamping processes and must be identified not only to retain the full mechanical strength of formed components, but also to optimise the production rate. In this work, a novel experimental facility was developed and applied to measure the temperature evolutions of the specimens and tools in stamping processes. Simulated temperature evolutions obtained using the FE software PAM-STAMP were then fit to this data. The IHTC values between AA7075 and three different tool materials were characterized at different contact pressures under both dry and lubricated conditions. In addition, a mechanism based IHTC model was developed and validated as a function of contact pressure, tool material and lubricant thickness to predict the IHTC values under different conditions.

The Effects of Interfacial Heat Transfer Coefficient on the Microstructure of High-Pressure Die-Cast Magnesium Alloy AM60B

This paper describes the details of a quantitative experimental and numerical study on the influence of solidification conditions, including the apparent interfacial heat transfer coefficient (IHTC) between the die and solidifying metal, on the resulting local microstructure. Multiple runs of the commercial casting simulation package, ProCASTTM, are used to model the mold filling and solidification events employing a range of IHTC values. The simulation results are used to estimate the centreline cooling curve at various locations through the casting. The centreline cooling curve, together with the die temperature and the thermodynamic properties of the alloy are then used as inputs to compute the solution to the Stefan problem of a moving phase boundary, thereby providing the through-thickness cooling curves at each chosen location of the casting. Finally, the local cooling rate is used to calculate the resulting grain size and skin thickness via previously established relationships. A comparison of the predicted and experimentally determined grain size profiles enables the determination of the apparent IHTC, which, in this study, was approximately 12000 W/m2·K. Additional useful observations from the numerical study suggest that the IHTC has a significant influence on the skin thickness and grain size in both the skin and core regions of the casting, while the effect of die temperature is limited to influencing the skin grain size only.

Grain Size Distribution and Interfacial Heat Transfer Coefficient during Solidification of Magnesium Alloys Using High Pressure Die Casting Process

The objective of this study is to predict grain size and heat transfer coefficient at the metal–die interface during high pressure die casting process and solidification of the magnesium alloy AM60. Multiple runs of the commercial casting simulation package, ProCAST™, were used to model the mold filling and solidification events employing a range of interfacial heat transfer coefficient values. The simulation results were used to estimate the centerline cooling curve at various locations through the casting. The centerline cooling curves, together with the die temperature and the thermodynamic properties of the alloy, were then used as inputs to compute the solution to the Stefan problem of a moving phase boundary, thereby providing the through-thickness cooling curves at each chosen location of the casting. Finally, the local cooling rate was used to calculate the resulting grain size via previously established relationships. The effects of die temperature, filling time and heat transfer coefficient on the grain structure in skin region and core region were quantitatively characterized. It was observed that the grain size of skin region strongly depends on above three factors whereas the grain size of core region shows dependence on the interfacial heat transfer coefficient and thickness of the samples. The grain size distribution from surface to center was estimated from the relationship between grain size and the predicted cooling rate. The prediction of grain size matches well with experimental results. A comparison of the predicted and experimentally determined grain size profiles enables the determination of the apparent interfacial heat transfer coefficient for different locations.

Effect of deformation rate on interfacial heat transfer coefficient in the superalloy GH4169 hot forging process

To evaluate the effect of billet deformation rate on the interfacial heat transfer coefficient (IHTC) during hot forging process, the upsetting test of wrought superalloy GH4169 on the hot flat die was conducted. The inside temperature field data of the billet and lower die under the deformation rate of 0s−1, 0.04s−1, 0.08s−1, 0.1s−1, 0.2s−1, 0.4s−1 were respectively collected through using a multi – channel data acquisition system. Besides, Beck’s nonlinear inverse estimation method was adopted to calculate IHTC and analyze the effect of GH4169 deformation rate on IHTC. According to the research results, the deformation rate had a distinctive influence on IHTC. When the deformation rate φ̇z⩽0.08s-1, the IHTC is small due to the blocking effect of the oxide scale on heat transfer; but when φ̇z>0.1s-1, the oxide scale was exfoliated in layers under the action of shearing force and a great deal of deformation heat and friction heat have been generated in short time, which has made the heat transfer condition improve greatly. Finally, the peak value of IHTC correlates approximately with deformation rate by a Boltzmann function, and tends to be saturated when the deformation rate φ̇z>0.2s-1.

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.

Interfacial heat transfer during hot metal forming operations assuming scale failure effects

Analysis of real contact area and thermal resistance combined with experimentally derived interfacial heat transfer coefficient values led to the development of an advanced finite element based model to simulate the heat transfer at the oxidised tool/workpiece interface during hot steel rolling. An extensive progress review and building on the Sellars 1990's core assumptions are discussed. Today, oxide scale failure is predicted, taking into account the main physical phenomena such as stress directed diffusion, fracture and adhesion of the oxide scale. The separation loads within the scale metal/system are measured during testing. They are sensitive to the chemical composition of steel. The assumption of several parallel heat flow systems at the roll/stock interface remains the core model for today's research.

Determination of the Interfacial Heat Transfer Coefficient in the Hot Stamping of AA7075

The interfacial heat transfer coefficient (IHTC) is a key parameter in hot stamping processes, in which a hot blank is formed and quenched by cold dies simultaneously. The IHTC should therefore be identified and used in FE simulations to improve the accuracy of simulation results of hot stamping processes. In this work, a hot stamping simulator was designed and assembled in a Gleeble 3800 thermo-mechanical testing system and a FE model was built in PAM-STAMP to determine the IHTC values between a hot aluminium alloy 7075 blank and cold dies. The IHTC values were determined at different contact pressures under both dry and lubricated (Omega-35) conditions. In addition, a model to calculate the IHTC value at different contact pressures and area densities of lubricant was developed for the hot stamping process, which was proved to be working well with verification tests.

Determination of the heat transfer coefficient at the metal–sand mold interface of lost foam casting process

For modeling solidification process of casting accurately, a reliable heat transfer boundary condition data is required. In this paper, an inverse conduction model was established to determine the heat flux and heat transfer coefficient at the metal–sand mold interface for cylindrical casting in the lost foam process. The numerically calculated temperature was compared with analytic solution and simulation solution obtained by commercial software ProCAST to investigate the accuracy of heat conduction model. The instantaneous cast and sand mold temperatures were measured experimentally and these values were used to determine the interfacial heat transfer coefficient (IHTC). The IHTC values during lost foam casting were shown to vary from 20 to 800 W m−2 K−1. Additionally, the characteristics of the time-varying IHTC have also been discussed in this study.

AN INVERSE HEAT CONDUCTION MODEL FOR DETERMINING THE CASTING/CHILL INTERFACIAL HEAT TRANSFER COEFFICIENT

The interfacial heat transfer coefficient (IHTC) is required for accurate simulation of the casting process. However, it cannot be easily obtained simply by using either experimental or theoretical methods. Using measured temperature data, an inverse method can be used to predict the IHTC. However, the latent heat released during the solidification of a molten metal complicates the associated temperature and IHTC calculations. To overcome this difficulty, an equivalent specific heat method is applied in this study to calculate the IHTC for the casting process. It is found that the identified IHTC varies with time during castings solidification. The IHTC values calculated for an aluminum alloy casting of simple geometry cast on a steel chill vary from 1200 to 3500 W·m−2·K−1. Application of the indentified IHTC in the forward heat conduction model with the same boundary condition allows comparison between numerically calculated and experimentally obtained results for validation of the method. The results show that the numerically calculated temperatures are in good agreement with those measured experimentally. This confirms that the proposed method is a feasible and effective tool for determining the casting-mold IHTC. Additionally, the characteristics of the time−varying IHTC have also been discussed.