Contact Heat Transfer Coefficient Research Articles

Experimental investigation of contact heat transfer coefficients in nonisothermal glass molding by infrared thermography

AbstractNonisothermal glass molding has recently become a promising technology solution for the cost‐efficient production of complex precision glass optical components. During the molding process, the glass temperature and its temperature distribution have crucial effects on the accuracy of molded optics. In nonisothermal molding, the glass temperature is greatly influenced by thermal contact conductance because there is a large temperature difference between the glass and mold parts. Though widely agreed to be varied during the molding process, the contact conductance was usually assumed as constant coefficients in most early works without sufficient experimental justifications. This paper presents an experiment approach to determine the thermal contact coefficient derived from transient temperature measurements by using infrared thermographic camera. The transient method demonstrates a beneficially short processing time and the adequate measurement at desirable molding temperature without glass sticking. Particularly, this method promises the avoidance of the overestimated contact coefficients derived from steady‐state approach due to the viscoelastic deformation of glass during the inevitably long period of holding force. Based on this method, the dependency of thermal contact conductance on mold surface roughness, contact pressure, and interfacial temperature ranging from slightly below‐to‐above glass transition temperature was investigated. The results reveal the dominance of interfacial temperature on the contact conductance while the linear pressure‐dependent conductance with an identical slope observed for all roughness and mold temperatures. The accurate determination of the contact heat transfer coefficients will eventually improve the predictions of the form accuracy, the optical properties, and possible defects such as chill ripples or glass breakage of molded lenses by the nonisothermal glass molding process.

Continuous measurement of contact heat flux during minced meat grilling

Few studies concerning contact heating of food products were found in the literature despite the importance of this mode of heat transfer in many operations (grilling, pan-frying) and its drastic impact on product quality change during heating. An original heating device to measure continuously contact heat flux and heating surface temperature was therefore developed and applied to the contact heating of turkey meat with a heating surface at 128°C, 215°C and 255°C. Based on the experimental results obtained, a simplified heat transfer model was developed and allowed the calculation of the parts of the total energy received by the product (i) used to raise its temperature (ii) used to evaporate water exuding from its lower surface during heating and (iii) lost by heat exchange with the surroundings. In the experimental conditions covered by our experiments, the part of energy used to evaporate water exuding from the product was found to vary between 55% and 67% of the total energy received. The precise evaluation of the kinetics of variation of the contact heat transfer coefficient (quantifying the quality of thermal contact between the lower surface of the meat sample and the heating plate) was also made possible. At 215°C and 255°C, this coefficient ranged from 500 to 100 W⋅m−2⋅K−1 between the start and the end of the cooking, these values being in the same order of magnitude as those measured in previous studies concerning single-faced grilling of meat.

The contact heat transfer between the heating plate and granular materials in rotary heat exchanger under overloaded condition

Abstract In the present work, the contact heat transfer between the granular materials and heating plates inside plate rotary heat exchanger (PRHE) was investigated. The heat transfer coefficient is dominated by the contact heat transfer coefficient at hot wall surface of the heating plates and the heat penetration inside the solid bed. A plot scale PRHE with a diameter of Do = 273 mm and a length of L = 1000 mm has been established. Quartz sand with dp = 2 mm was employed as the experimental material. The operational parameters were in the range of ω = 1 – 8 rpm, and F = 15, 20, 25, 30%, and the effect of these parameters on the time-average contact heat transfer coefficient was analyzed. The time-average contact heat transfer coefficient increases with the increase of rotary speed, but decreases with the increase of the filling degree. The measured data of time-average heat transfer coefficients were compared with theoretical calculations from Schlunder’s model, a good agreement between the measurements and the model could be achieved, especially at a lower rotary speed and filling degree level. The maximum deviation between the calculated data and the experimental data is approximate 10%.

Modeling of heterogeneous heat transfer in fabrics

Thermal comfort, an important factor when designing fabric, is strongly related to heat transfer in the fabric. Fabric is not homogeneous because the constituent yarns are interlaced at a certain weave angle, and the contact area between yarns varies. Therefore, heat transfer along the yarn in the fabric is not yet well understood. In this study, we developed a heterogeneous fabric model in which heat transfer along the longitudinal and transverse directions of the yarn is treated independently, and proposed a method for estimating the contact heat transfer coefficients for yarns. The geometrical fabric structure was constructed based on the cross section of three fabrics observed with a three-dimensional microscope. The parameters used were the mass density, specific heat, and thermal conductivity of the fibers and air. The heat flow calculated using the model was compared with that measured experimentally. Fairly good agreement was observed, verifying the validity of this heterogeneous model. The simulation results indicated that when the anisotropy of the fiber thermal conductivity is high, heat is transferred significantly faster along the longitudinal direction than along the transverse direction of the yarn, and the equilibrium temperature distribution is strongly influenced by heat transfer in the longitudinal direction.

Transient contact heat transfer measurements based on high-speed IR-thermography

This paper presents measurements of transient contact heat transfer between specimens made out of several ferrous alloys for varying loads. The temporal resolution of the results allows for a deeper insight into the contact behaviour of the paired surfaces. At the same time, the application of high-speed infrared thermography increases the accuracy of the temperature measurements compared to the K-type thermocouples commonly used.The experimental data show a positive correlation between applied load and contact heat transfer, and the significance of the softer material's hardness, both agreeing with existing literature. With increasing load, the load-related gradient of contact heat transfer approaches zero, suggesting a limiting load, at which the maximum area of contact and therefore the maximum contact heat transfer coefficient is reached. In addition, the results suggest, that the transient approach allows for a post-experimental classification of the investigated contact.

Inverse evaluation of equivalent contact heat transfer coefficient in hot stamping of boron steel

It is of significance to evaluate the equivalent contact heat transfer coefficient (ECHTC) at the sheet-die interface in hot stamping of boron steel (HSBS) for controlling the local cooling rate of the sheet and hence obtaining components with tailored microstructure and properties. Moreover, accurate evaluation of the ECHTC can provide a reliable thermal boundary condition for numerical simulation of the HSBS process. In the study, a simple and effective physical simulation setup for die quenching of boron steel (DQBS) was developed, where the temperatures at measurement points are related only to the ECHTC so as to improve the evaluation accuracy. A FE-based optimization model for inverse evaluating the ECHTC was established. The ECHTCs at the B1500HS sheet-H13 die steel interface were identified under various contact pressures, and it was found that the ECHTC increases exponentially with the pressure increasing. Both the physical simulation of DQBS and the HSBS experiment were performed to verify the ECHTC evaluated, and the maximum relative errors of 9.6 and 19.7 % were observed, respectively, between the predicted and measured temperature histories.

Analytical solution of non-stationary heat conduction problem for two sliding layers with time-dependent friction conditions

In this article we conduct an overview of various types of thermal contact conditions at the sliding interface. We formulate a problem of non-stationary heat conduction in two sliding layers with generalized thermal contact conditions allowing for dependence of the heat-generation coefficient and contact heat transfer coefficient on time. We then derive an analytical solution of the problem by constructing a special coordinate integral transform. In contrast to the commonly used transforms, e.g. Laplace or Fourier transforms, the one proposed is applicable to a product of two functions dependent on time. The solution is validated by a series of test problems with parameters corresponding to those of real tribosystems. Analysis shows an essential influence of both time-dependent heat-generation coefficient and contact heat transfer coefficient on the partition of the friction heat between the layers. The solution can be used for simulating temperature fields in sliding components with account of this influence.

中間熱転写印刷のヘッド-ベルト間の接触伝熱状態の解明

An indirect thermal transfer printing is capable of printing on both flat and uneven surfaces and is generally used for IC card printer. At present, a transfer unit of the indirect thermal transfer printer has a problem with large power consumption. To overcome this problem a technique of a thermal print head in place of a heating roller currently used has been proposed as power saving technology of the transfer unit of the indirect thermal transfer printer. However, it is difficult to design it because contact heat transfer characteristics between the thermal print head and an indirect transfer belt are not clear. In this study, we simulated the transfer process, and revealed the contact heat transfer characteristics of the thermal print head through evaluation of the temperature distribution and the pressure distribution. First, contact heat transfer coefficients which are dominant parameters in the contact heat transfer characteristics were determined by inverse analysis method. Next, to find out the suitable shape of the thermal print head, three different types of thermal print heads were prepared. The flat thermal print head has shown the most sufficient heat transference to the indirect transfer belt and the heat transfer coefficient decreased in the order of the convex and the concave. However, the manufacturing cost of the concave thermal print head is about one half of those of the other two types. Then, the possibility of improving the heat transference of the concave thermal print head was considered. The heat transference of the concave thermal print head was able to be improved by spreading the concave width.

F-02 油膜を挟んだ金属-金属界面における熱伝達係数の評価

F-02 油膜を挟んだ金属-金属界面における熱伝達係数の評価

Extraction of heat transfer parameters in active carbon–ammonia large temperature jump experiments

Experimental and theoretical analyses were carried out with the aim of developing a method of extracting heat transfer parameters from active carbon–ammonia large temperature jump data. The approach presented in this paper extracts two thermo-physical properties from the data as opposed to one overall property in previously employed techniques. Active carbon samples were tested using the large temperature jump technique with a step temperature increase from 40 °C to 70 °C. For a loose packed sample, which is about 8 mm thick, the obtained thermal conductivity (k) and heat transfer coefficient (h) were 0.2 W m−1 K−1 and 250 W m−2 K−1 respectively. The effect of changing bulk density through compression was also investigated. The thermal conductivity and contact heat transfer coefficient were found to increase with bulk density. The two physical properties obtained were combined into one single heat transfer figure and were compared to what was obtained using a previous method on the same set of data. Considering the assumptions made in the analytical process, these numbers were in reasonable agreement. A dominance of the thermal conductivity was observed in thicker beds, while contact heat transfer coefficient dominated in thin beds.

Determination of time-dependent thermal contact conductance through IR-thermography

Values of contact heat transfer coefficients between two solid bodies proposed in literature differ, depending on the experimental method employed for their determination. This paper presents a method to measure contact heat transfer coefficients by means of transient temperature measurements, which are conducted using a high-speed infrared camera. Time-dependent contact heat transfer mechanisms can be visualized and quantified. This enables the technique to be utilized for describing transient heat transfer phenomena, which, among others, emerge within the field of thermo-energetic management of machine tools. Within this work, the experimental method and setup is explained. Subsequently, results of transient measurements are presented, showing a direct relation between the instantaneous load and contact heat transfer coefficient.

A computational study of low-head direct chill slab casting of aluminum alloy AA2024

The steady state casting of an industrial-sized AA2024 slab has been modeled for a vertical low-head direct chill caster. The previously verified 3-D CFD code is used to investigate the solidification phenomena of the said long-range alloy by varying the pouring temperature, casting speed and the metal-mold contact heat transfer coefficient from 654 to 702 °C, 60–180 mm/min, and 1.0–4.0 kW/(m2 K), respectively. The important predicted results are presented and thoroughly discussed.

Heat Transfer Experiments in a Rotary Drum for a Variety of Granular Materials

The influence of material properties on the contact heat transfer coefficient between the covered wall surface and the solid bed was investigated. The contact heat transfer coefficients were calculated from the measured radial and circumferential temperature profiles. Experiments were carried out with six different materials, including steel spheres, animal powder, cement clinker, quartz sand, glass beads, and expanded clay. The rotational speeds were varied from 1 to 6 rpm to evaluate the influence of rotational speed on the contact heat transfer coefficient. The measured contact heat transfer coefficients were compared with four models from the literature.

Evaluation of Contact Heat Transfer Coefficient and Phase Transformation during Hot Stamping of a Hat-Type Part.

Using an inverse analysis technique, the heat transfer coefficient on the die-workpiece contact surface of a hot stamping process was evaluated as a power law function of contact pressure. This evaluation was to determine whether the heat transfer coefficient on the contact surface could be used for finite element analysis of the entire hot stamping process. By comparing results of the finite element analysis and experimental measurements of the phase transformation, an evaluation was performed to determine whether the obtained heat transfer coefficient function could provide reasonable finite element prediction for workpiece properties affected by the hot stamping process.

On numerical modeling of low-head direct chill ingot caster for magnesium alloy AZ31

Abstract A comprehensive 3D turbulent CFD study has been carried out to simulate a Low-Head (LH) vertical Direct Chill (DC) rolling ingot caster for the common magnesium alloy AZ31. The model used in this study takes into account the coupled laminar/turbulent melt flow and solidification aspects of the process and is based on the control-volume finite-difference approach. Following the aluminum/magnesium DC casting industrial practices, the LH mold is taken as 30 mm with a hot top of 60 mm. The previously verified in-house code has been modified to model the present casting process. Important quantitative results are obtained for four casting speeds, for three inlet melt pouring temperatures (superheats) and for three metal-mold contact heat transfer coefficients for the steady state operational phase of the caster. The variable cooling water temperatures reported by the industry are considered for the primary and secondary cooling zones during the simulations. Specifically, the temperature and velocity fields, sump depth and sump profiles, mushy region thickness, solid shell thickness at the exit of the mold and axial temperature profiles at the center and at three strategic locations at the surface of the slab are presented and discussed.

A 3-D numerical study of turbulent flow and solidification of a direct chill caster fitted with a channel bag

3-D CFD simulations were carried out for a vertical direct chill slab caster for an aluminum-alloy AA-1050. The code was verified with an experimental study and reasonably good agreements were obtained. The casting speed and the metal-mold contact heat transfer coefficient were varied from 40 to 100 mm/min and from 750 to 3,000 W/m2 K), respectively. The velocity field, temperature contours and important quantities for different casting speeds are predicted.

Numerical Simulation of Temperature and Effective Strain Distribution of Aluminum Alloy in the Whole Rough Rolling Process

A two-dimensional finite element mathematical model of rough rolling in "1+4" hot continuous rolling of 5052 aluminum alloy was developed by using finite element software Deform, The temperature and effective strain distribution of the strip in different process parameters has been investigated in by simulating the mathematical model in different simulation conditions. The process parameters such as rolling speed, initial temperature, contact heat transfer coefficient between work roll and strip have been considered. The simulation conditions were built by the means of orthogonal experiment. The process parameters, which can make the temperature and effective strain of the strip in a relatively uniformity distribution, were achieved by analyzing the simulation results under different simulation conditions. To judge the uniformity of the temperature and effective strain distribution of the strip quantitative in different simulation conditions, standard deviation has been used as a criterion.

Theoretical and experimental analysis of bed-to-wall heat transfer in heat recovery processing

The accurate prediction of heat transfer behavior in packed bed is of great importance to the design and operation of this kind of facility. In this paper, heat transfer between packed bed and adjacent wall in heat recovery processing is investigated theoretically and experimentally. A bed-to-wall heat transfer model is presented with consideration of two essential components including particle-to-wall contact heat transfer and heat conduction through the thermal penetration layer. Details of model parameter determination are provided. Through comprehensive sensitivity analysis of those parameters, it is shown that particle temperature, particle residence time and particle diameter are critical parameters to heat transfer coefficient. Besides, surface roughness, solid thermal conductivity and environmental gas species are all influencing factors of the overall heat transfer coefficient. In particular, a dimensionless particle residence time is introduced to characterize the combining effect of residence time, effective thermal conductivity and wall contact heat transfer. Meanwhile, a revised simplified formula for calculating particle contact heat transfer coefficient is correlated as well, which provides better prediction than the one widely adopted in previous studies. Furthermore, experimental heat transfer test on an industrial packed bed char cooler is performed for validating the model predicting capability in heat recovery process. The systematical test on a bench scale packed bed heat exchanger has also been carried out. Parametric analysis of the experimental data is consistent with the conclusions drawn from the model analysis section. The model provides good predicting accuracy on the overall heat transfer coefficient from the bench scale facility as well as data from the field test by correlating 95% of the measured data within ±20%. The implementation of the proposed model shows good prediction of heat transfer coefficient in packed bed for heat recovery purpose. It is recommended that the model can be applied in industrial design.

Estimation of thermal contact resistance in fin–tube heat exchanger using inverse heat transfer methods

Thermal contact resistance (TCR) is a very important phenomenon in heat transfer problems, such as power generation, air conditioning, refrigeration, aerospace, etc. due to the complex mechanism of heat transfer in the contact surfaces; measurements and calculations of the TCR have many difficulties and problems. Today, one of the effective methods to investigate these problems is the use of inverse heat transfer. The objective of this work is to estimate the contact heat transfer coefficient (reverse of TCR) between the tube and its fin in heat exchanger. Two different methods, consisting of Levenberg–Marquardt for parameter estimation and conjugate gradient with adjoint problem for function estimation conjugate gradient method (CGM), are used. Results show that the CGM is successfully applied for the solution of the inverse problem to determine the unknown time-dependent TCR.

Analytical Modelling of a Spray Column Three-Phase Direct Contact Heat Exchanger

An analytical model for the temperature distribution of a spray column, three-phase direct contact heat exchanger is developed. So far there were only numerical models available for this process; however to understand the dynamic behaviour of these systems, characteristic models are required. In this work, using cell model configuration and irrotational potential flow approximation characteristic models has been developed for the relative velocity and the drag coefficient of the evaporation swarm of drops in an immiscible liquid, using a convective heat transfer coefficient of those drops included the drop interaction effect, which derived by authors already. Moreover, one-dimensional energy equation was formulated involving the direct contact heat transfer coefficient, the holdup ratio, the drop radius, the relative velocity, and the physical phases properties. In addition, time-dependent drops sizes were taken into account as a function of vaporization ratio inside the drops, while a constant holdup ratio along the column was assumed. Furthermore, the model correlated well against experimental data.