Thermal Contact

Thermal Contact Conductance (TCC) between an exhaust valve and its seat is one of the important parameters to be estimated in an internal combustion engine. An experimental study presented here to acquire temperature in some interior points to be used as inputs to an inverse analysis. An actual exhaust valve and its seat are utilized in a designed and constructed setup. Conjugate Gradient Method (CGM) with adjoin problem for function estimation is used for estimation of TCC. The method converges very rapidly and is not so sensitive to the measurement errors. Contact frequency is one the factors which have a significant influence on TCC. The results obtained from current inverse method as well as those obtained from linear extrapolation method show that the thermal contact conductance decreases as the contact frequency increases. The results obtained from both sets of results are also in good agreement.

A BSTRACT : Based on a dynamic frequency study, the thermal behavior of a wall consisting of a concrete slab contiguous to a panel of compressed rice straw is presented. The thermal behavior at the interface of the two materials is modeled by a thermal contact resistance. The insulating nature wall thus made is studied according the order magnitude of contact resistance: the perfect contact (R c =0) extreme contact defects (R c very high). The analysis of temperature and heat flow density curves show

A large number of input parameters are necessary for a successful modeling of the metal cutting by the finite element method such as material properties, the tool-chip friction model and thermal contact conductance of the tool-chip interface. Several constitutive material models are derived such as Johnson-Cook [17] and Zerrili- Armstrong [18] models with the material constants obtained from experiments. These material models determine the work piece material behavior in high strain, wide range of strain rates and high temperatures of cutting zones. The effect of the material model constants on the orthogonal cutting simulation accuracy is also investigated in the literature [19]. Tool-chip friction model is another important input parameter for the modeling of the cutting process. Many investigations are performed to study the influence of the friction modeling on metal cutting simulation results [20-22]. In these investigations different friction models are presented and numerical simulation results are compared with experimental data.

The use of thermoplastic composites has seen an important growth in the transportation sector in the past few years, partly thanks to shorter manufacturing time achievable compared to thermosetting composites. In order to reach the production rates required by the automotive industry, the processing techniques used such as thermoforming [1] must be optimized, for example by forming directly a non-consolidated stack. It is therefore critical to predict accurately the heat-up time and temperature distribution of the stack, given the effect on the mechanical properties of the final part [2]. The thermal contact resistances that exist in this case between the plies lower the heating efficiency and have to be carefully characterized.

relatively very small compared to the apparent contact area. Macro-contact is created due to the surface curvature of the contacting bodies. The relatively high temperature dierence occurs between the interfaces, because following the ow of heat through the macro contact, it must pass through the micro-contacts to conduct from one surface to another [2]. A constriction on the contact surface in heat transfer is created by this phenomenon called Thermal Contact Resistance (TCR) and is dened as follows [1,2]:

In internal combustion engines, exhaust valve and its seat gain considerable temperature as the hot gases exit through them. So, the rate of heat transfer should be under control. In this study, the contact heat transfer coefficient has been estimated. An experimental study on an Air-Cooled internal combustion engine cylinder head has been considered. Using the measured temperatures of sensors located in specific locations of the exhaust valve and the seat and the method of linear extrapolation, the surface contact temperatures, and constant and periodic contact heat transfer coefficient were calculated. Also, a sensitivity analysis has been done to study the effects of different parameters of contact pressure, contact frequency, heat flux and cooling air speed on thermal contact conductance. The results show that between the major four considered parameters, the thermal contact conductance is more sensitive to the contact pressure, then the contact frequency, heat flux, and the cooling air speed are the most affecting parameters on thermal contact resistance.

Abstract. Heat transfer has considerable applications in dierent industries such as designing of heat exchangers, nuclear reactor cooling, control system for spacecraft, and microelectronics cooling. As the surfaces of two metals make contact with each other, this issue becomes so crucial. Thermal Contact Resistance (TCR) is one of the key physical parameters in heat transfer of the mentioned surfaces. Measuring the experimental value of TCR in laboratory is highly expensive and dicult. 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 TCR value. To that end, three dierent congurations, namely, at-at, at-cylinder, and cylinder-cylinder, were utilized in two steady and unsteady state conditions to predict the value of TCR. A comparison between the measured and obtained values from the simulation shows that the errors for at- at, at-cylinder, and cylinder{cylinder congurations after 10 minutes from starting the experiment are 4.6074%, 0.1662%, and 0.5622%, respectively. For steady state condition, the corresponding errors are 6.06e-3%, 1.506%, and 0.846%, respectively. In conclusion, the nal results establish the fact that the inverse problem method solution can predict TCR values between contacting surfaces.

In this paper, the method of calculating thermal contact resistance of wavy surfaces is discussed completely. The results of experiments and analysis had good agreement with each other. For determining the thermal contact resistance of real contacts, a new method had been introduced. In this method some assumptions had been used. Changing these assumptions into more real conditions, one may obtain some better results. There is an interesting result obtained from the analysis: the extent of contact depends on the direction of heat flux and materials of the surfaces. Therefor, thermal contact conductance of wavy surfaces depends on the order of surface location. This point is seen in the experiments’ results too. At last, it may be noted that by knowing thermal contact resistance, exact determination of total conductivity of multi-layer isolators will be available.

It is well known that the contact between two solids is not perfect particularly because of the irregularities of contact surfaces due to the presence of asperities. For this reason, the heat transfer between the two solids occurs mainly through the contact points, and the assumption of thermal contact resistance is present because of the unevenness of interfaces in contact. This thermal contact resistance (TCR) is a barrier to heat flow resulting in dramatic temperature drop. This phenomenon results from the disturbance of the heat flux at the contact. The (TCR) is influenced by the size of surface roughness “Ra” and by the interstitial medium, which is generally a bad conductor. Therefore improving heat transfer between two contact materials means reducing the TCR, which requires installing a gasket between the two solids in contact. This gasket can be a thermal paste, or rubber.

When interpreting the predicted stress intensity factors it is useful to remember that the highest level of heat applied to the contact is expected to be present only briefly, not in every contact pass. Literature on temperature of wheels with tread brakes indicates that the surface reaches temperatures around 50-350C in normal operation [22][23]. Assuming that the rail within the contact patch is raised to the same temperature as the wheel when they come into contact, thermally assisted fatigue crack growth at these lower temperatures would be reasonable, but assuming repeated 1000C contact would be misleading. Where the very high temperatures may have more influence is in fracture of the rail material that can be investigated by comparing peak stress intensities with the fracture toughness of the rail. This can identify if the combined thermal and mechanical load would be capable of generating immediate crack growth in a single application, although the extent of this growth and the likelihood of continuing crack extension would depend on the temperature and mechanical load in subsequent contacts.

The submersible thermocouples which are applying to temperature measurement of different environments, as a rule, possess rather small duration of establishment of indications. Time of r[r]

The commercial success of sorption refrigeration and heat pump systems depend on a good heat and mass transfer in the adsorbent bed which allows higher coefficients of performance and greater specific heating or cooling power that reduce capital costs. In this study the thermal conductivity and thermal contact resistance of vibrated and compressed granular active carbon and binary mixtures of active carbon are investigated using two types of conductivity measurements: a steady state measurement between flat plates and a transient hot tube measurement. With these results is possible to draw conclusions on how the wall geometry, particle size distribution and bulk density affect the overall thermal performance. Results show that using binary mixtures of grains and powder give superior results to either grains or powder alone. The conductivity of the binary mixtures increases roughly linearly with bulk density and the 2/3 grain mixture achieves the highest densities. The method used to achieve compaction (vibration or compression) did not seem to affect the result. Thermal contact resistances reduce with increasing density but do vary with the mixture ratio, also appearing to be best with a 2/3 grain - 1/3 powder mixture.

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Non-contact measuring of temperatures of differ- ent objects and materials is based on the principle of energy radiation in the invisible infrared spectrum. The volume of emi�ed energy is proportionate to the temperature and “emissivity” of the object. The emissivity depends on the material from which the object is made and on its surface adjustment. The quality of emission and reflection of the material is determined by the emissivity that can reach the values from 0.30 to 1.00.

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In the current work EN 31 steel alloy was taken as a work piece material. This type of steel alloy has got lot of application in industrial sectors. This steel is widely used in Ball and Roller Bearing,spinning tools, Beading Rolls, Punches and Dies. In this 3D analysis of orthogonal turning operation was considered. Here work piece was of C.S (4x2 mm 2 ) with a length of (20 mm) was modelled. Mechanical and thermal properties of work piece is shown in the below table 4

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waiting room chairs and table, mylar balloons, hand sanitizer dispenser, troffer lights, manufactured light covers, stuffed bear, hospital gown, yarn, plastic canvas, tissue box, hospi[r]

Socie [10] to calculate rolling contact fatigue damage. Sraml et al [12] used the Hertz contact theory to calculate the stress response and treat the multi-axial fatigue problem as a uniaxial fatigue problem. Ekberg et al [13] used the Hertz contact theory for stress calculation and multi- axial fatigue model proposed by Dang Van et al [14] to present a fatigue life prediction model for rolling contact fatigue problem (RCF). Liu et al [15, 16] developed a general subsurface crack propagation analysis methodology for the wheel/rail rolling contact fatigue (RCF) problem. Then the fatigue damage in the wheel is calculated by using a previously developed mixed-mode fatigue crack propagation model [17]. In this model, a new mixed-mode threshold stress intensity factor is developed using a critical plane-based multi-axial fatigue theory and the Kitagawa diagram [18]. For this purpose, an equivalent stress intensity factor defined on the critical plane is proposed to predict the fatigue crack growth rate under mixed-mode loading. The railroad wheel has the initial residual stress created by the manufacturing process, and this residual stress changes due to the mechanical stress caused by service conditions. The residual stresses of railroad wheels are influenced by the heat treatment during manufacture processing.

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