General Thermal Model Research Articles

Discretized blank holding force driven by electromagnetics: Mechanism of thermal effects and deformation

Electromagnetic blank holding for forming complex parts has received considerable attention due to the flexibility in die design and force control. However, electromagnetics experience overheating with continuous current loading for traditional variable blank holding force, which restricts widespread and long-term applications. The discretization of the blank holding force was first proposed to solve this issue, where mechanisms of thermal effects and the deformation of the part with the discrete current loading, which is significant for high-quality forming, remain to be further clarified. This study established a general thermal model for multi-poles in electromagnetics using Fourier's law of heat transfer. The temperature variations in different discretization levels of force were revealed by simulation and experiments. The appropriate discrete level of the force was applied to the forming process of the sheet material, reflecting the law of its influence on the deformation. With optimized discretization blank holding force, high forming accuracy was observed in forming thickness and microstructure tests. Results indicate that the discretization blank holding force can keep the die at a much lower temperature and is promising for improving the forming accuracy of the part. This work assists in designing the blank holding force and electromagnetic die for better forming and efficiency in long-term production.

Hybrid surrogate model for online temperature and pressure predictions in data centers

The increase in cloud computing and big data storage has led to significant growth in data center (DC) infrastructure that is now estimated to consume more than 1.5% of the world’s electricity. Due to suboptimal DC design and operation, a significant fraction of this energy is wasted because of the cooling systems inability to effectively distribute cold air to servers. Consequently, additional cooling air must be circulated inside a DC to prevent local hot spots, which leads to undercooling at other locations. Row-based cooling is an emerging architecture that provides more effective airflow distribution, which lowers energy consumption. Since available methods are unsuitable for accurate online predictions, a general thermal model is required to predict spatiotemporal temperature changes inside a DC and hence optimize airflow distribution for this architecture. Typical approaches include physical models, computational fluid dynamics (CFD) simulations, and black-box data-driven models (DDMs). All three approaches are limited because they do not encapsulate the entirety of relevant operational parameters, are time-consuming and can provide unacceptable errors during extrapolative predictions. We address these deficiencies by developing a fast, adaptive, and accurate hybrid surrogate model by combining a DDM and the thermofluid transport relations to predict temperatures in a DC. Training data for the DDM is obtained from CFD simulations. An artificial neural network (ANN) with the Rectified Linear Unit (ReLU) activation function is shown to predict pressure distributions accurately in a row-based cooling DC. These predicted pressures are inputs for thermofluid transport equations to determine the temperature distribution. The applicability of the model is demonstrated by comparing predictions with experimental measurements that characterize the influence of varying server workload distribution and cooling unit operational conditions, i.e., temperature set-point, airflow rate, and fan locations, on the temperature distribution. The model can be used to (1) improve cooling configuration design, (2) facilitate thermally aware workload management, and (3) test “what if” scenarios to characterize the influence of operating conditions on the temperature distribution.

Lifetime extension through Tj equalisation by use of intelligent gate driver with multi-chip power module

Typical high-current power modules used in traction and wind generation contain several dies in parallel. Non-ideal geometries and electrical parameter variations cause an unequal aging of each die which limits the effectiveness of using parallel devices. This paper presents an intelligent power module with a TSEP-based junction temperature measurement and internal temperature equalisation for extending its operational lifetime. As the magnitude and number of junction temperature cycles strongly dictate the power module lifetime, this paper focuses on the quantification of the lifetime gain via the application of common automotive drive cycles. Thus, a general thermal model is first deduced from the experimental results. The lifetime is then estimated using a stress-counting rainflow algorithm, a pre-determined damage law and a linear damage accumulation rule. Finally, in order to illustrate the lifetime extension, 160 drive cycles, including the legislatives drive cycles, are simulated. A lifetime gain of up to two times can be expected with the temperature equalisation method. Therefore, the initial investment on the developed intelligent power module is justified by the benefit of the large gain in lifetime and of the end-of life indication functions.

A Lumped Network Approach to a Coupled Electromagnetic-Thermal Analysis of Cable-in-Conduit Conductors

The paper presents a new thermal model of a forced-flow supercritical helium cooling channel, used in cable in conduit conductors. This model is based on a lumped network approach, is integrated in a more general thermal model for the description of the cable strands, and is coupled with the existing cable and joint electromagnetic models. The model is applicable when quasi-steady-state thermal conditions are present for the cooling helium and no phase transitions take place. The results of the model application to a SULTAN sample current sharing temperature test are presented, showing a good agreement between the measured and computed thermal and electrical variables.

General bidirectional thermal characterization via the 3ω technique.

The 3ω technique has become a popular method for determining the thermophysical properties of microscale and bulk materials. The prerequisite fabrication of a highly linear metal line a few hundred nanometers thick on the sample can be a failing point in specific material systems. This difficulty can be overcome by utilizing a bidirectional experimental geometry that employs a contact resistance between the sample and heating wire, which also allows for data collection under varying axial pressure loads. In this work, such a system is demonstrated with an emphasis on developing a thermal mount that will optimize sensitivity to the thermophysical parameters of interest: the sample's thermal conductivity, volumetric heat capacity, and the contact resistance between the sample and mount. A general thermal model is presented that can be simplified to analyze nearly any similar system. This model is then employed to analyze a sample in the mounting scheme described with varying applied pressures to demonstrate the general feasibility of the system.

Linear Approach to the Orbiting Spacecraft Thermal Problem

We develop a linear method for solving the nonlinear differential equations of a lumped-parameter thermal model of a spacecraft moving in a closed orbit. Our method, based on perturbation theory, is compared with heuristic linearizations of the same equations. The essential feature of the linear approach is that it provides a decomposition in thermal modes, like the decomposition of mechanical vibrations in normal modes. The stationary periodic solution of the linear equations can be alternately expressed as an explicit integral or as a Fourier series. We apply our method to a minimal thermal model of a satellite with ten isothermal parts (nodes) and we compare the method with direct numerical integration of the nonlinear equations. We briefly study the computational complexity of our method for general thermal models of orbiting spacecraft and conclude that it is certainly useful for reduced models and conceptual design but it can also be more efficient than the direct integration of the equations for large models. The results of the Fourier series computations for the ten-node satellite model show that the periodic solution at the second perturbative order is sufficiently accurate.

Coalescence and breakup of contact areas: Effects on surface temperatures

In a study of the mechanisms by which thin polymeric films can prevent or delay the onset of fretting corrosion, experimental observations were made of the apparent real area of contact and temperatures generated by friction in a dry-sliding system consisting of stationary polymer-coated steel balls loaded against a vibrating sapphire disk. Five different polymers were used in the original study at vibrating frequencies ranging from 100 to 200 Hz and amplitudes from 20 to 100 μm; but this paper focuses on only one of these—polystyrene coated steel balls in contact with sapphire. Surface temperatures generated by friction were measured using an infrared microscope system. A photomacro/video technique was developed to view the fretting contact interface and to measure the size and distribution of the real areas of contact. The experiments revealed several complex patterns and unusual phenomena. In one example of behavior, a number of small contact patches would suddenly coalesce into one larger contact patch and then break up again into a similar collection of separate patches. This coalescence and breakup occurred at a regular frequency which was much lower than the oscillating frequency. In addition, significant surface temperature spikes corresponding to the occurrence of coalesced areas were observed. A general thermal model previously developed was used to theoretically predict the temperatures corresponding to the experimental conditions [Furey MJ, Vick B, Foo SJ, Weick BL. A theoretical and experimental study of surface temperatures generated during fretting. In: Proceedings of Japan international tribology conference, Nagoya, Japan, 29 October 1990–1 November 1990, vol. II, pp. 809–14; Vick B, Furey MJ. A basic theoretical study of the temperature rise in sliding contact with multiple contacts. Tribology International 2001;34:823–29]. The thermal model consists of a sliding pair of any material combination with three-dimensional and transient conditions. The key feature is the contact area, which is modeled as a collection of rectangular patches in which each patch can have any specified size, shape, position, and time duration. In this way, each contact has a unique start and finish time and the entire collection of contacts can evolve with time in any specified manner. This provides the flexibility to model everything from hard, brittle surfaces such as ceramics to softer, deformable surfaces such as polymers. Using the changes in the apparent real area of contact as observed in the experiments, the theoretical model predicted surface temperatures in close agreement with experimental values. The results of this study show not only that the area of contact is complex and dynamically changing, but that the surface temperatures produced are extremely sensitive to the real area of contact. Although the fundamental mechanisms for the observed phenomena of breakup, coalescence and motion of contact areas are unknown, the study is important since it illustrates the connection between areas of contact and surface temperature—a key unknown which influences physical and chemical behavior in tribological processes.

Reduced-order transient thermal modeling for SOFC heating and cooling

Transient thermal analysis plays a central role in the design and optimization of high temperature solid oxide fuel cells (SOFCs) during startup/shutdown, because of the potential for damaging thermal gradients to develop within the SOFC components. The optimal design of a heating/cooling process is one that minimizes the total time required to reach a prescribed final operating temperature, while not exceeding given thresholds of maximum allowable temperature gradients. To this end, we consider the SOFC unit cell, which is heated by hot air supplied into the oxidizer channel at a specified, time-dependent inlet temperature. Beginning with a general thermal model of the cell, we develop and evaluate limiting cases that allow closed-form analytical solutions of the time-varying temperature fields, from which heating time and maximum temperature gradient are calculated. The results are generalized by presentation in terms of a modified effective Peclet number and dimensionless inlet temperature function. Finally, the accuracy of these predictions is evaluated by comparison to results of 3-D CFD modeling in Fluent, and design maps for optimizing the transient heating process are presented. Results indicate that the reduced-order models’ simplicity, computational savings, and ability to capture the essential physics of the transient process justify their use in design calculations over more complex, highly detailed, numerical/CFD schemes.

Large-scale tests on friction and wear of engineering polymers for material selection in highly loaded sliding systems

Tribological properties of polymer materials are traditionally tested on small-scale test rigs because of cost-efficiency and flexibility. For highly loaded applications (e.g., sea lock doors, retaining walls, positioning systems) however, large-scale tests should be performed for estimating the frictional behaviour and wear lifetime of different engineering polymers more accurately under nearly working conditions. In present paper, two filled polymer grades covering a broad range of mechanical properties polyethylene terephtalate (PET)/teflon (PTFE) and ultra high molecular weight polyethylene (UHMWPE)/carbon are slid against high alloy steel, stainless steel and epoxy resin, in order to make a selection for its use as bearing material under dry and lubricated conditions. A general thermal model is evaluated for present test rig. The coefficients of friction on large-scale tests are lower than those obtained from small-scale testing. Polymer wear rates are calculated from weight loss and compared to dimensional measurements (thickness reduction) including elastic deformation, thermal expansion and creep. Microscopic observation of the worn surfaces shows different wear mechanisms of PET/PTFE after sliding against stainless steel, developing a transfer layer onto the polymer surface, and after sliding against high alloy steel, causing polymer transfer onto the steel surface. The better adherence of a polymer transfer film onto a steel counterface is explained by higher attractive forces resulting from its high surface energy, while there is little adherence on stainless steel counterfaces in accordance with its lower surface energy and lower friction. Wear of PET/PTFE after sliding against stainless steel is partly caused by the incorporation of steel particles. After sliding of UHMWPE/carbon, no wear debris was observed as its higher toughness allows for tearing of the surface without particle detachment. The high surface toughness and abscence of a transfer film results however in higher friction.

Overview of Satellite Thermal Analytical Model

Thermal analysis is the major engineering work throughout the entire satellite development process, with some crucial stages such as design, test, and ground operations simulation. In the formal design and verification (by test) phases, a general thermal mathematical model for the entire satellite is constructed from a combined conduction and radiation heat transfer equation with environmental heating and cooling as boundary conditions. Some representative numerical schemes with constraints used in satellite thermal analysis, as well as an introduction to the thermal model for the thermal balance test, are presented. However, the general thermal model may be too complicated or inefficient for conceptual design, test monitoring, and ground-operation simulation while developing a satellite. Therefore, simpler governing equations for pure radiation heating and cooling with exact mathematical solutions are developed to fulfill this objective at the expense of analysis accuracy. The linear approximation and exact formulation for solving this simplified problem, as well as the corresponding results shown in graphical form, are also discussed.

An investigation into the influence of frictionally generated surface temperatures on thermionic emission

Tribochemistry—including chemical reactions at sliding surfaces in contact—is an important factor in boundary lubrication and the formation of protective films. It is a complex subject quite different from conventional glassware studies and may involve effects of high temperatures produced by friction, the emission of charged particles and high pressure. In previous theoretical studies based on a general thermal model of sliding contact, results were presented for surface temperatures generated with pure metallic elements as well as ceramics. More recently, this model was extended to include thermionic emission of electrons. The current paper is motivated by the need to understand the connection between high surface temperatures and electron emission in sliding systems. The objectives are to examine theoretically the temperatures and resulting thermionic emission at the surface of selected sliding contact systems. Results are presented and discussed for 20 pure elements including 19 metals and 1 semi-conductor. The results of this investigation may be of value in examining and testing various film-forming hypotheses. For example, in the tribopolymerization concept of boundary lubrication, it is postulated that high surface temperatures are controlling for the surface polymerization of condensation-type monomers while the emission of low-energy electrons initiates polymerization of vinyl-type addition monomers.

A General Thermal Model for Grinding with Slotted or Segmented Wheel

Summary Grinding with slotted or segmented wheel as one of effective ways to reduce grinding thermal damage, has been utilized in many field. In this paper two thermal models for periphery grinding with slotted wheel and vertical spindle face grinding with segmented wheel are established respectively in order to choose the slotting or segmental parameters more reasonably and predict grinding temperature. Further more a general thermal model is developed, which not only unifies the above theoretical models but also contains the J.C. Jaeger's and Des. Ruisseaux's thermal models. Experimental results arc also given to verify the models developed in this paper.

Modelling Thermal Resistance of Power Modules Having Solder Voids With Finite Elements

A general thermal model to calculate the thermal resistance of a power module having rectangular die and layers has been constructed. The model incorporates a finite element computer program to solve for three-dimensional heat conduction. Effects of voids in the solder regions are included. A sample case is analyzed, and a comparison is made to a recent study.

’’Vacuum’’ layer double‐shell cryogenic inertial fusion targets

A double‐shell cryogenic target has been suggested as a possible design for commercial Inertial Fusion (IF) reactors. Fabrication techniques have been developed to the point where these targets will soon be available for experimental evaluation. Heat transfer from the fuel container to the outer shell can be provided by a low‐pressure exchange gas between the two shells. The pressure and mass of gas is controlled by both temperature and the selection of the exchange gas. In the temperature range of interest (10–35 K), helium, a noncondensable gas, permits only a variation of pressure. A condensable gas, however, allows a variation in both the pressure and the mass density within the gas between the shells. A general thermal model has been developed as a framework to evaluate the thermal characteristics of proposed candidate target designs and estimate their expected fuel‐layer uniformity.