Investigation Of Thermal Contact Conductance Research Articles

Investigation of thermal contact conductance of bare copper-copper and aluminum 1070-aluminum 1070 interfaces at low temperatures between 10 and 70 K

The prediction model of thermal contact conductance (TCC) for bare OFHC-OFHC and Al 1070-Al 1070 flat interfaces is presented. OFHC (Oxygen free high conductivity copper) and Aluminum are widely used in cryogenic systems. Since the thermal conductivity of pure copper or aluminum materials has a highly non-linear characteristic with its peak value at around 20 K, it is difficult to predict TCC of those materials at this temperature range. The TCC model in this paper proposes the dimensionless correlation through 16 experiments considering various conditions such as surface roughness, materials, pressure, and temperature. The model has been established by the modification on the previous CMY (Cooper, Mikic, Yovanovich) model with additional parameters. The first important feature of the model is to adopt the contact thermal conductivity (kc). It can be calculated by using the size effect theory at the micro-scale contact single spot. The second feature is the dimensionless temperature (T/TD). An experimental setup is constructed inside of the conduction-cooled cryostat to measure the thermal conductivity of bulk material and the TCC simultaneously for the temperature range between 10 K and 70 K. OFHC and Al 1070 specimens with various surface roughness are prepared and tested with contact pressure of 6 to 13.5 MPa in vacuum environment. The predicted TCC by the proposed model shows a reasonable agreement with the experimental results in low temperature.

Fractal model of thermal contact conductance of rough surfaces based on elliptical asperity

PurposeThe purpose of this study is to present a fractal model of thermal contact conductance of rough surfaces based on elliptical asperity.Design/methodology/approachThe effects of contact load, fractal dimensional, fractal roughness and eccentricity on thermal contact conductance of rough surfaces were investigated by using numerical simulation.FindingsThe results indicate that the thermal contact conductance of rough surfaces increases with the increase of the contact load, increases with the increase of the fractal dimension and decreases with the increase of the fractal roughness. The thermal contact conductance of rough surfaces increases with the increase of eccentricity. The shape of the asperity of rough surfaces has an important influence on the thermal contact conductance of rough surfaces.Originality/valueA fractal model of thermal contact conductance of rough surfaces based on elliptical asperity was established in this study. The achievements of this study provide some theoretical basis for the investigation of thermal contact conductance of bolted joint surfaces.

Experimental study of contacting surfaces microgeometry and gas gap impact on thermal contact conductance of metallic joints

Thermal contact resistance is an important phenomenon in heat transfer and impacts many fields of technology. The interrelated influences of many factors—such as geometry and microgeometry of contacting surfaces, their mechanical and thermal properties, apparent contact pressure, type and properties of interstitial fluid—make prediction of this resistance problematic and necessitate both theoretical and experimental research. The paper reports an experimental study of thermal contact conductance (the reciprocal of thermal contact resistance) of joints created by flat, conforming metallic surfaces in static contact. The measurements were performed for samples made of mild steel and stainless steel with relatively low roughness (σ = 0.5 - 2.14 µm) and large slope of asperities (m = 0.4 – 1.3), resulting in values of σef/mef = 1.25 - 1.96 µm that are distinctly lower than reported in the majority of experimental research on thermal contact conductance. The apparent contact pressure was changed from 1.5 to 11 MPa and for some cases up to 19 MPa. Analysis of the results obtained showed that the correlations available in the literature, for greater values of roughness and smaller asperity slope, mostly overestimated the predicted thermal contact conductance. The elasticity indices values for samples used in this study indicated plastic deformation of asperities, but it was concluded that the smoothest surfaces of both kinds of steel underwent elastic deformation. A new experimental correlation for contact of mild steel surfaces (σ = 0.5 - 2.14 µm, σef/mef = 1.25 - 1.96) was proposed. The results of this study indicate the need for further investigation of thermal contact conductance, especially for this range of surface microgeometry parameters, which is not dealt with in the correlations available in the literature. There is also a need to study the impact of asperity curvature on contact conductance. The results of this study proved the important role that air gaps play in the transfer of heat across a joint. The gap conductance values were predicted correctly for low mechanical loads and rough surfaces.

Experimental Investigation of Thermal Contact Conductance in a Bundle of Flat Steel Bars

The phenomenon of thermal contact conduction in two-phase (fluid-solid) media determines many technological processes. An example of such a process is heat treatment of steel bars, when a heated charge has a form of a packed bundle. In order to determine the optimal heating curve it is necessary to have knowledge about the intensity of transfer through contact areas of the bars. This phenomenon is quantified by the thermal contact conductance hct. The article describes the methodology of determining the hct coefficient for bundles of flat steel bars. The starting point for the analysis is the measurement of the effective thermal conductivity kef performed for 5 × 20 mm and 10 × 20 mm bars. Individual samples of the same bars differed in arrangement. The analytical investigation used the concept of an elementary cell. This approach consisted in analysing resistances for individual heat transfer types: conduction, contact conduction and radiation. Based on the performed calculations it has been established that the value of the hct coefficient for the analysed samples is within the range 128–472 W/(m2 K). Changes of the hct coefficient in the temperature range 25–700 °C can be described with a second degree polynomial. It has been established that hct assumes maximum values in the temperature range from 300 °C to 400 °C.

Numerical investigation of thermal contact conductance between linear and curvilinear contacts

Heat transfer has considerable applications in different industries such as designing of heat exchanger, nuclear reactor cooling, control system for spacecraft and designing of microelectronics cooling. As the surfaces of two metals contact each other, this issue becomes so crucial. Thermal contact resistance is one of the key physical parameters in heat transfer of mentioned surfaces. Measuring the experimental value of thermal contact resistance in laboratory is highly expensive and difficult. As an alternative, numerical modeling methods could be engaged. In this study, Inverse problem method solution is utilized as a proper method for estimation of thermal contact resistance value. In this order, three different configurations (flat-flat, flat-cylinder, and cylinder-cylinder) were utilized in two steady and unsteady state conditions to predict the value of thermal contact resistance. In conclusion, the final results establish the fact that the inverse problem method solution can predict thermal contact resistance values between contacting surfaces.

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.

Experimental investigation of thermal contact conductance for nominally flat metallic contact

A unique experimental set-up was fabricated to carry out axial heat flow steady state experiments for the assessment of thermal contact conductance (TCC) at the interface of two materials. Three different materials (copper, brass and stainless steel) were selected for the experiments considering their mechanical and thermal properties. Heat transfer experiments were performed in vacuum environment (0.045 torr) to find out solid spot contact conductance for nominally flat surfaces with different surface roughness (1–5 μm) for each specimen under several load conditions (0.6–15 MPa). A precise estimation of TCC for the interface of sets of similar materials was one of the most important results of this research. The effects of the surface roughness, the material properties and the load conditions (nominal interface pressure) have been studied and documented. Furthermore, the experimental results of solid spot contact conductance were compared with four theoretical models, showing their limitations to make a precise estimation of the TCC in the range of the used parameters.

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.

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.

An experimental investigation of thermal contact conductance of Hastelloy C-276 based on steady-state heat flux method

In this study, measurements of the thermal contact conductance (TCC) at the interfaces of Hastelloy C-276/Hastelloy C-276 and Hastelloy C-276/SS302 test pairs were conducted by a self-developed experimental setup based on the steady-state heat flux method. The reliability of experimental setup and method was validated by the repeated measurement of the thermal contact conductance between Hastelloy C-276 and Hastelloy C-276. In addition, the effects of contact pressure and interfacial temperature on thermal contact conductance were also discussed. This study not only provides a necessary database for the accurate numerical simulation but also lays a foundation for the optimal design of the shrink fitting process of rotor can.

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.

Experimental Study of Thermal Contact Conductance with Compensation Heater

This article describes an experimental setup to measure the TCC between interfaces of two contact solid materials. A new kind of design scheme of the compensation heater is put forward. An experimental investigation of thermal contact conductance is conducted with pressed pairs of GH4169 and K417 contacts in the range 130~280°C. The results show that TCC over this condition increases with temperature, but it is not monotonic. Moreover, the thermal contact conductance measured by experiment with compensation heater at the interface is higher than that without compensation heater.

An experimental investigation of thermal contact conductance of stainless steel at low temperatures

An investigation is carried out to study the effects of surface topography and interfacial temperature on the thermal contact conductance of the pressed stainless steel 304 contacts in the range 125–210 K and 1–7 MPa. The surface roughness of test specimens is between 1.5 μm and 17.6 μm. A theoretical model is used to predict the results and gives a reasonable agreement. The combined effect of phonon conductivity and electronic conductivity to thermal conductivity at low temperatures is described using a proposed correlation. The small contact conductance at low temperatures can ascribe to the decreased thermal conductivity and the increased hardness. It is demonstrated that rms roughness and mean absolute slope are both needed for evaluating contact heat transfer.

An experimental investigation of thermal contact conductance across bolted joints

Abstract An experimental study of thermal contact conductance was conducted with pairs of aluminum alloy (6061-T6) specimens jointed by bolts. The individual aluminum samples have a square cross-section ( 63.5 mm × 63.5 mm ) and a height of 50 mm. Three different bolt patterns were adopted in this study, including single-bolt, 4-bolt, and 8-bolt configurations. The bolt–shaft diameters were 3, 5, and 8 mm, and the torque applied on each bolt was between 1 and 10 N m. The heat flux through the test specimens ranged from 4 to 20 kW/m 2 . The interfacial contact pressure of bolt-jointed specimens was determined by a pressure-measuring film inserted between samples. Results show that the interfacial contact pressure increases with an increase of either the applied torque or the number of bolts. The interfacial temperature difference across the junction was substantially reduced for bolt-jointed specimens, when compared with two superimposed samples without bolts. With the same bolt number, the variation of bolt–shaft diameter from 5 to 8 mm yields nearly no influence on the thermal contact conductance. However, when the size of bolt was kept constant the thermal contact conductance of samples jointed by 8 bolts was greatly larger than that of 4-bolt samples. The increase of contact surface roughness of test specimens leads to a decrease of the thermal contact conductance. When an RTV silicon layer was used as the interstitial material, the total joint conductance of Al/RTV/Al was much lower than the contact conductance of bare aluminum contact. The total joint conductance of Al/RTV/Al was increased with a decrease of the thickness of RTV silicon layers.

Experimental Investigation of Thermal Contact Conductance: Variations of Surface Microhardness and Roughness

Surface roughness parameters were measured using nine different sampling intervals. The results showed that the roughness of profile was nearly a constant at all sampling intervals; the slope, peak curvature and density of the profile decreased with increasing sampling interval; and the distribution of the peak height was different from the profile height distribution and was not Gaussian. It was found that the surface microhardness value, under small loads, varied with the indentation size and the surface preparation procedure. A model was developed which used a discretization method to obtain the contact radius and area in each section of the surface summit height distribution curve. This model eliminated some of the assumptions made by previous models and can be used to predict the thermal contact conductance without assuming the asperity deformation mode and surface summit height distribution. The predicted thermal contact conductance values were in good agreement with experimental results.

A theoretical investigation of thermal contact conductance

The flow of heat between two nominally flat rough surfaces is considered. The variation of thermal conductance with applied load is determined for elastic and plastic modes of surface deformation.