Thermal Contact Conductance Values Research Articles

Estimation of thermal contact conductance of spacecraft heat sink bolted joints

Efficient thermal management is one of the key steps required to minimize failure and enhance the lifespan of electronic packages deployed in space applications. Precise and accurate knowledge of thermal contact conductance at the electronics package — heat sink interface is a key input required to achieve the efficient design of heat sinks for the mentioned application. In this study, a novel non-intrusive method for estimating the spatially varying thermal contact conductance for thin-plate bolted joints used in spacecraft thermal control systems is presented. A five bolt configuration joining the electronics package base plate to the heat sink plate is considered. Different bolt torques and their effect on the interface thermal contact conductance between the plates are investigated. Using pressure-sensitive films, the actual contact area and its spatial variation is determined for each torque level in the range of 1.5 N m to 9.0 N m and the contact interface is divided into three regions. Heat transfer experiments are carried out in vacuum at a pressure of 1×10−6 mbar for different combinations of bolt torques, heat load and heat sink temperatures. The thermal contact conductance values over the three regions are simultaneously estimated using an inverse parameter estimation technique. To improve the computational accuracy of the estimation process, a two stage stochastic optimization technique using the Monte-Carlo random sampling algorithm is used and the obtained estimates are compared. With an increase in applied bolt torque from 1.5 N m to 6.0 N m, the actual contact area and the area weighted interface thermal contact conductance are found to increase by 72.26% and 138.82%, respectively. Furthermore, the stochastic estimates of the spatially varying thermal contact conductance obtained using the developed methodology are found to follow a normal distribution with the average limit of standard deviation being ±7.34%, ±6.26% and ±13.22% of the mean values for the considered three regions respectively.

Characterization of the Transient Contact Heat Transfer between C276 Superalloy and 5CrMnMo

The thermal contact conductance (TCC) between billet and tool is a key parameter to determine the contact heat transfer in the thermal processing of alloys and metals. In this work, a self-developed transient contact heat transfer detective equipment was applied to determine the TCC between C276 superalloy and die steel. The temperature variations in specified regions of specimens during transient heat transfer process were measured and the temperatures at contacted interface were programed based on the reverse heat transfer algorithm. Then the values of TCC between C276 superalloy and die steel were calculated. The TCC shows high dependency on the interface temperature, contact pressure, lubricant, and surface oxidation conditions. The increasing interface temperature difference and contact pressure enhanced the thermal contact conductance, while the surface oxidation alleviated the heat transfer capability. In addition, the addition of lubricant was also found to enhance the thermal contact conductance. The determined values are supposed to be of significant importance for processing optimization and numerical simulation.

Contact area and thermal conductance estimation based on the actual surface roughness measurement

Thermal contact conductance (TCC) plays a vital role during any thermo-mechanical based analysis of the contacting surfaces. Inherently, it depends upon the actual contact-area at the interfaces, which is quite complex to estimate. This work aims to perform the micro-contact based structural, and thermal analysis of stochastically re-created rough surfaces, while using actual surface profile measurement, and proposes an experimentally validated FEM based approach for real contact area and TCC estimation. Different surface topography parameters estimation methods are compared, and a semi-deterministic based approach is evolved for the construction of surface topographies from actual roughness measurements. The asperities interactions and large deformation effects are incorporated during analysis. Close agreement between numerical and experimental TCC values shows the efficacy of the present approach.

Use of Galinstan as a Contact Agent for Additively Manufactured Components in Cryogenic Engineering

We introduce a new, demountable, brush-on thermal interface material for cryogenic applications. A process has been developed that allows to removably provide contact e.g. between an additively manufactured component and its interface. The use of Galinstan as an industrial material for cryogenic applications and in particular for those materials used in additive manufacture is reviewed and explained. Thermal contact conductance values are also presented for dissimilar materials, that have not been investigated before.

Estimation of thermal contact conductance using transient approach with inverse heat conduction problem

Precise evaluation of thermal contact conductance (TCC) is essential for the thermal and mechanical analysis of a thermal system found in different engineering applications. Inverse heat conduction method provides an effective tool for the estimation of thermal contact conductance at the interface of two bodies in contact. In this work, 1-D inverse heat conduction method has been employed for the estimation of thermal contact conductance for metallic contacts. Function estimation technique using conjugate gradient method of minimization with adjoint problem has been utilized. Transient temperature data at fixed axial locations in the contacting bodies, their thermophysical properties and boundary conditions at the two ends are provided as the input to the inverse problem. Initially, the inverse method has been applied with the simulated data for various boundary conditions and different test functions to check the validity of inverse problem. Subsequently, the experiment has been performed for various boundary conditions, under different environmental, and loading conditions for metallic specimens. Transient temperature data from the experiments are provided as an input in the inverse problem. Eventually, the effectiveness and reliability of the transient approach based on inverse analysis has been established by comparing the results with their corresponding steady state values of TCC. The accuracy of the inverse solutions has been demonstrated with the help of parity plots. Conclusively, the transient approach has been found to be successful in TCC estimation with short duration experimental runs as compared to long waiting period of steady state TCC run. In addition, the transient approach eliminates the need for the measurement of heat flux and temperature drop at the interface, encountered in steady state approach.

Non-intrusive measurement of thermal contact conductance at polymer-metal two dimensional annular interface

This work reports the development of a measurement technique for the estimation of thermal contact conductance at a polymer (Nylon)-metal(Copper) two dimensional interface having an annular contact under low external loading. The experimental setup consists of a vacuum chamber capable of achieving a pressure of 10−6 mbar. The chamber is fitted with electrical, thermocouple and fluid feed-throughs. Two dimensional temperature measurements are performed on the top surface of the polymer sample away from the boundary interface and are used to estimate the thermal contact conductance at the interface. The estimation process is accomplished by solving an inverse heat conduction problem using artificial neural networks coupled with Genetic algorithm. The actual pressure distribution at the annular Nylon-Copper interface is measured using a pressure sensitive film and the variation and the distribution of contact area between the mating surfaces is shown. The estimated values of thermal contact conductance obtained using the developed non-intrusive technique are found to be in good agreement with those reported in literature.

Quantifying the effect of asperity size and shape on thermal contact conductance of metal-metal contacts through direct numerical simulations

The thermal contact conductance (TCC) between two metallic rough surfaces is dictated both by the size and shape of the asperities on the surface. In this study, scale-resolved direct numerical simulation of thermal transport across the interface was conducted to isolate and quantify their effects. The mean roughness height (indicator of asperity size) and the mean absolute slope (indicator of asperity shape) of the asperities—two common surface topography descriptors—were treated as parameters, in addition to the contact pressure. Microscale models of the interface geometry were generated by statistically reconstructing the surface pairs from the aforementioned topography inputs, followed by generation of unstructured meshes that resolved all fine-scale features of the interface, including the air pockets. Steady state conjugate heat conduction computations were conducted, and the TCC values were extracted. The extracted TCC values were found to be in good agreement with experimental measurements. Examination of trends revealed the TCC to be a strong function of asperity size and a weak function of asperity shape. The extracted TCC data was finally reduced and curve-fitted by correlating it to two non-dimensional parameters: the number of contacts and the mean absolute slope. The accuracy of this correlation (curve-fit) was tested, and the TCC values predicted by it were found to be well within the statistical uncertainty of the raw DNS results. The curve-fit represents a simple, yet valuable tool for future researchers to estimate the TCC of metal-metal interfaces of any topography characterized by common topography descriptors.

Determination of thermal contact conductance of flat and curvilinear contacts by transient approach

Accurate knowledge of the thermal contact conductance (TCC) across fixed metallic contacts of flat-flat as well as curvilinear surfaces is becoming vital for the safety and performance in the critical fields like nuclear energy production, thermal management of electronics packaging and aviation sector. In this regard, a steady state methodology is well acceptable as a reliable approach for estimating the TCC for specimen under pressed contact conditions, which requires long hours of experimental run. Whereas, a transient methodology aims to determine the time varying estimate of TCC or the heat flux related with the specific application areas requires a very short duration experiment. Research works on the estimation of TCC through transient methods are available for only flat-flat contacts. Moreover transient methods have seldom been used for estimating the steady state TCC. Present investigation systematically explore and develop a better understanding of transient methodology towards estimating TCC between flat-flat as well as curvilinear metallic contacts under consistent loading conditions by using the inverse analysis. A sequential function estimation algorithm based on the analytical solution for a semi-infinite body has been used for solving inverse heat conduction problem (IHCP), and subsequently, the transient TCC is determined by using an estimated transient heat flux and an instantaneous temperature jump at the interface. Firstly, the inverse problem has been tested by a numerically generated measurement. Later a customized experimental facility has been used to conduct transient, as well as steady state experiments for flat-flat, cylinder-flat and cylinder-cylinder configurations under similar working parameters. Experimental results of transient heat flux show a quasi-static thermal equilibrium at the end of test run, which form the basis to successfully extract the steady-state TCC measurement with greater reliability. An under prediction in transient TCC up to 60% is found to be inevitable, if compared against the steady state TCC estimate based upon the linearly extrapolated temperature drop measurements of each contacting specimens. However, a close agreement between the stabilized transient TCC and the steady state TCC has been observed, when steady TCC values are directly based upon the actual interfacial temperature drop measurements without any extrapolation. Eventually, this work establishes the viability of present transient approach in suitably predicting the steady state TCC values of different contact configurations.

Steady state experimental investigation of thermal contact conductance between curvilinear contacts using liquid crystal thermography

Heat transfer components comprise of several types of metallic contacts. Commencing from the conventional conforming rough surface contact combinations, the situation might culminate in terms of the complex non-conforming rough curvilinear contacts of the real heat transfer devices. Available theoretical models fail to correctly predict thermal contact conductance (TCC) over the broader range of influencing parameters, even for the simplest geometry, and thus experiments plays a pivotal role in the field. Researchers are indefatigably working for developing accurate experimental methodologies to get precise estimate of TCC, and create broader database of results for upcoming theoretical models.In this regard, this paper presents steady state thermal contact conductance analysis on two solid bodies of brass, carrying flat and curvilinear contact combinations, under variable loading conditions ranging in between 0.27 and 4.0 kN. A customized and standardized experimental set up has been used to measure steady state TCC for three different types of geometrical configurations, which are flat-flat, cylinder-flat and cylinder-cylinder contacts. At the start, TCC has been evaluated on the basis of centrally placed high response, super accurate, ungrounded thermocouples, which are mounted axially across the contacting bodies. In the later part of the paper, an optical, non-invasive and inexpensive method, based upon liquid crystal thermography (LCT) has been implemented to get the precise estimate of TCC for different configurations under consideration. The region close to the interface, which has the profound effect on the axial temperature distributions, is identified. Eventually, the separation region, where dramatic variation in the thermal conductivity occurs and classical Fourier law tends to fail has been identified. The separation region is further segregated in sub-regions on the basis of distinct temperature zones, which allows estimating the effective thermal conductivity of the materials in gap. The precise temperature jump close to the interface is extrapolated, and consequentially used to predict the steady state TCC for all three geometrical configurations. The value of TCC is evaluated again on the basis of effective thermal conductivity concept, and the results have been compared together. The present investigation establish a unique methodology for TCC estimation on the basis of steady state liquid crystal (LC) measurements, and provide valuable insight of heat transfer across the curvilinear contacts, and can be treated as the base line measurements for any of the upcoming scale resolved numerical models.

Toward reliable photoconductive atomic force microscopy measurements

Photoconductive atomic force microscopy (PC-AFM) allows one to study the influence of illumination on the conductive properties of different samples at the nanoscale. However, for such measurements to be considered reliable, one has to be sure that illumination does not impact the tip–sample contact force. The change of the contact force may be caused by the influence of radiation on the atomic force microscope's position sensitive detector (PSD) as well as the probe and sample. There are certain approaches to reduce this illumination influence; however, they cannot always be implemented. Therefore, it is important to perform a detailed study of the influence of illumination on the PSD, the probe, and the sample during measurements in the PC-AFM mode. In this paper, this was done by studying the mechanism by which elevation is formed in topographic images of the cleaved GaAs substrate surface under lateral continuous wave (CW) laser illumination, with λ = 1064 and 404 nm. It was demonstrated that the illumination results, mainly, in the thermal expansion of the tip and the sample, as well as the generation of a thermal stress in the cantilever. The authors consider the cases in which these effects can increase the tip–sample contact force during PC-AFM measurements and suggest simple approaches by which the increase in force can be minimized. Also, the authors developed the model of thermal expansion of the GaAs sample fixed in the holder under CW laser illumination. Based on this model, the approach was suggested for estimation of the thermal contact conductance value for different solid–solid interfaces.

Heat Transfer Analysis of Slewing Ring Bearing for High Thermal Applications

The challenge in design and manufacturing tasks includes precise measurement of static friction, stiffness, deformation, raceway relative approach, and heat distribution of slewing ring bearings. These parameters affect the life and performance of the rolling elements bearing at elevated thermal environments. Toward that, sections of a complete bearing called linear model mock-up bearing (LMMB) have been designed and fabricated for the study of heat distribution. The physical significance of the heat distribution on bearing elements was studied using square-guarded hot-plate (SGHP) apparatus. The thermal contact conductance (TCC) experiments were carried out by varying surface roughnesses and interface temperatures under different loading conditions by using the combinations of steel plates and LMMB under dry and lubricated conditions. From this study, it is observed that the TCC values obtained from LMMB were significantly high across the contact surfaces of AISI 42100–AISI 52100–AISI 4140 steel plate's combination. Further, the finite element method (FEM) simulation has been carried out successfully to evaluate the temperature distribution in combination of bearing steel plates and LMMB by solidworks software, and the results are discussed.

Estimation of Thermal Contact Conductance between Blank and Tool Surface in Hot Stamping Process

In hot stamping, the determination of the thermal contact conductance values between the blank and tool surface during the process is crucial for the purpose of simulating the blank rapid cooling inside the tool using finite element analysis (FEA). The thermal contact conductance value represents the coefficient of the heat transfer at the surface of two solid bodies in contact and is known to be influenced greatly by the applied pressure. In order to estimate the value and its dependency on applied pressure, the process of hot stamping was replicated and simplified into a process of compression of heated flat blank in between the tool at different applied pressure. The temperature of the blank and tool surface were measured by means of thermocouples installed inside the tool. Based on the measured temperature, the thermal contact conductance between the surfaces was calculated using Newton's cooling law equation. The calculated value was then used to simulate the blank cooling inside the tool using FEA commercial software. This paper describes an experimental approach to estimate the thermal contact conductance between a blank made of Boron Steel (USIBOR 1500) and tool made of Tool Steel (STAVAX). Its dependency on applied pressure is also studied and the experimental results were then compared with FEA simulations.

Extraction of thermal contact conductance of metal–metal contacts from scale-resolved direct numerical simulation

The thermal contact conductance (TCC) between two conforming metallic rough surfaces was extracted from scale-resolved direct numerical simulation (DNS) of thermal transport across the interface. To compute thermal transport across the interface, microscale models of the interface geometry were created by stochastically reconstructing the topography of the two metallic surfaces, followed by generation of meshes that resolved all fine-scale features of the interface, including the air pockets. Steady state conjugate heat conduction computations were then conducted, and the TCC values were extracted and expressed as a function of the applied pressure (which translates into a mean separation distance) and the mean interface temperature. When compared with experimental data, the extracted TCC values were found to be in good agreement, thereby validating the approach. To lend practical value to the methodology and data presented in this paper, a relationship was also established between the extracted TCC and the number of contacts between the two surfaces. Further, it was shown that the number of contacts generated using surface reconstruction (numerically computed) correlates well with the theoretically calculated number of contacts using the joint probability distributions of the asperity heights on the two surfaces. The implication of these relationships is that irrespective of the actual topography of the surface, for a given pressure (separation distance) and metallic pair combination, the TCC can be estimated from the correlations presented in this paper (TCC versus number of contacts) without the need to conduct any additional numerical computations.

Heat Generation in Bond Wires

Equations for fast and exact calculation of a simple model for heat transfer from a bond wire to a cylindrical finite mold package including nonideal heat transfer from wire to mold are presented. These allow for a characterization of an arbitrary mold/bond wire combination. The real mold geometry is approximated using the mold model cylinder radius and the thermal contact conductance of the mold/bond wire interface. For changes in bond and mold material, wire length, diameter, and current transient profiles, the resulting temperature transients can then be predicted. As the method is based on numerical integration of differential equations, arbitrary pulse shapes, which are industrially relevant, can be calculated. Very high thermal contact conductance values (above 40000 W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> K heat transfer) have been detected in real package/bond systems. The method was validated by successful comparison with finite-element method simulations and alternative calculation methods and measurements.

Influencing factors of thermal contact conductance between TC4/30CrMnSi interfaces

Thermal contact conductance (TCC) between interfaces is an important parameter in the analysis of temperature field for structural components. Of great concern is the TCC between TC4/30CrMnSi interfaces, as TC4 and 30CrMnSi are the main structural materials used in aviation engines. This paper presents an experimental study of influencing factors of TCC. The effects of temperature, applied load, heat flux direction and surface roughness on TCC using a test device independently developed are detailed. The temperature of contacting surfaces ranges from 200°C to 350°C, and the pressure of consideration is from 0MPa to 150MPa. Results show that the temperature affects little on TCC. TCC reaches its peak value at approximately 120MPa in the processes of loading and unloading, but the variation trends are found to be different for the two processes. Compared with smooth surfaces, the rough surfaces exhibit higher TCC values under the same condition.

Experimental investigation of thermal contact conductance across GFRP–GFRP joint

An experimental apparatus was established for measurements of thermal contact conductance of glass fiber reinforced plastic (GFRP) to GFRP interface. The influence of several primary factors on thermal contact conductance was investigated. Thermal contact conductance across GFRP to GFRP joint increases with increasing contact pressure, while decreases with increasing surface roughness. Higher interfacial temperature causes higher thermal contact conductance due to temperature dependency of hardness of test materials. The values of thermal contact conductance across stainless steel/GFRP joint are 1.7 times of the ones for GFRP/GFRP junction under the same experimental conditions. A comparison of experimental data with Mikic and CMY models shows that existing models overestimate experimental values.

Modelling Phase Change in a 3D Thermal Transient Analysis

A 3D thermal transient analysis of a gap profiling technique which utilises phase change material (plasticine) is conducted in ANSYS. Phase change is modelled by assigning enthalpy of fusion over a wide temperature range based on Differential Scanning Calorimetry (DSC) results. Temperature dependent convection is approximated using Nusselt number correlations. A parametric study is conducted on the thermal contact conductance value between the profiling device (polymer) and adjacent (metal) surfaces. Initial temperatures are established using a liner extrapolation based on experimental data. Results yield good correlation with experimental data.

Determination of thermal contact conductance between thin metal sheets of battery tabs

A novel method combining experimental test and heat transfer modeling was developed to determine the thermal contact conductance (TCC) between thin metal sheets as a function of contact pressures. In the experiment, thin metal samples were sandwiched between one white light transparent and one infrared (IR) transparent glass disks pressed together under different pressure levels. The metal stack was then heated up from the white light transparent side by an intense short pulse of flash light. The temperature transient on the other side was measured by an IR camera. To obtain a value of TCC, two separate experiments having different layers of thin sheet materials were performed and the values of maximum temperature rise were measured. Numerical heat transfer modeling was used to calculate the temperature evolution in the stack-up comprised of metal layers sandwiched between two glass disks. The heat transfer calculation results showed that TCC had a strong correlation to the ratio of maximum temperature rise between the two experiment configurations, but it was insensitive to the variations of other thermal properties. Thus, for a given pair of metal sheets in contact, a unique correlation between the TCC and the ratio of temperature rise was established using the heat transfer calculation. Such correlation allows the direct determination of the TCC value from the ratio of the experimentally measured temperature rise. The TCC between three types of thin metal sheets (i.e., 0.2-mm-thick Al, 0.2-mm-thick Cu and 0.9-mm-thick Cu) were measured and compared with the available literature data.

Compensation Heating Technique for Experimental Investigation of Thermal Contact Conductance across GH4169/K417 Interface

Abstract An experimental apparatus to measure the thermal contact conductance (TCC) between interfaces of two contact solid materials was introduced, and a new compensating heater was proposed to better achieve the one-dimensional axial heat transfer. A practical engineering example was used with pressed pairs of GH4169/K417 contacts in the range 160∼280 °C and 60∼180 MPa to demonstrate the applicability of the experimental apparatus. The measured values of TCC with and without compensation heater were studied and compared. The results show that the measured TCC with compensation heater is larger than that without compensation heater under the same condition and the compensation heater works better at high temperature.

Investigation on Thermal Contact Conductance Based on Data Analysis Method of Reliability

The method of reliability is proposed for the investigation of thermal contact conductance (TCC) in this study. A new definition is introduced, namely reliability thermal contact conductance (RTCC), which is defined as the TCC value that meets the reliability design requirement of the structural materials under consideration. An experimental apparatus with the compensation heater to test the TCC is introduced here. A practical engineering example is utilized to demonstrate the applicability of the proposed approach. By using a statistical regression model along with experimental data obtained from the interfaces of the structural materials GH4169 and K417 used in aero-engine, the estimate values and the confidence level of TCC and RTCC values are studied and compared. The results show that the testing values of TCC increase with interface pressure and the proposed RTCC model matches the test results better at high interface pressure.