Thermal Steady State Research Articles

Numerical investigations of heat transfer and pressure drop in packed bed duct exposed to forced convection boundaries under local thermal non-equilibrium conditions

The present study is a 2-D numerical study which discusses the thermohydraulic performance of packed bed duct under local thermal non-equilibrium and steady state conditions exposed to forced convection boundaries (BC-6). The numerical model is well validated with the experimental work reported in the literature and found to be accurate enough to perform further parametric investigations. The analysis is done to see the effect of different ball diameter (5 mm, 9 mm and 11 mm), bed porosity, heat transfer fluid (air, water and engine oil - Pr = 0.70–645) and ball material (EPS, steel and bronze) on heat transfer and fluid flow characteristics in turbulent flow region of Reynolds number ranging from 900 to 14,320. The numerical results obtained using commercial CFD software COMSOL shows that, the heat transfer coefficient and pressure drop in packed bed increases with increase in ball diameter, thermal conductivity of ball and bed porosity which is exactly reverse as reported in literature for BC-1 to BC-5. The maximum thermal performance factor with bronze particles is 2.45 and 24.5 times more than stainless steel and EPS particles respectively for larger particle size and bed porosity. Engine oil exhibits significantly higher heat transfer coefficient (9.7 × 105 W/m2K) and pressure drop (3.3 × 106 Pa) compared to water (40,126 W/m2K and 2028 Pa) and air (600 W/m2K and 250 Pa) respectively. Overall, the combination of water as heat transfer fluid along with bronze particles of larger diameter and larger bed porosity emerges as the optimal choice for enhancing heat transfer in the packed duct exposed to forced convection boundary condition BC-6.

Accelerating Transient Aerothermal Simulations of the BOLT II Flight Experiment

Aircraft with short flight durations may not reach thermal steady state due to limited time for aeroheating to conduct throughout their internal structure. Consequently, predictions of the solid temperature field require time-accurate simulation of the transient heat conduction. In this work, the use of an explicit super-time-stepping scheme to accelerate the time-accurate solution of the heat equation has been explored. The experience with this technique is documented here because, to the authors’ knowledge, it is not widely used in the hypersonics simulation community. The performance of the super-time-stepping scheme was investigated by comparison to a standard explicit scheme and an implicit time-differencing scheme. Both the super-time-stepping scheme and the implicit time-differencing scheme demonstrate significant speedup over the standard explicit scheme, with comparable wall-clock performance. The solid solver has been coupled with an existing open-source fluid solver to perform conjugate heat transfer simulations. The efficacy of the developed solver in simulating hypersonic aerothermal problems is demonstrated through verification and validation studies. By applying the coupled solver to the aerothermal analysis of the BOLT II flight experiment, the effectiveness of the super-time-stepping scheme in mitigating the computational expense associated with transient conjugate heat transfer simulations is demonstrated.

Wetsuit Thermal Resistivity Measurements.

In recent years, attention to the realization and characterization of wetsuits for scuba diving and other sea sports or activities has increased. The research has aimed to establish reliable and standardized measurement methods to objectively assess wetsuit quality, particularly focusing on their mechanical and thermal properties. In this work, we describe and compare two different measurement methods for the characterization of neoprene wetsuit thermal resistivity. The first method follows the existing regulations in the field, while the second one, which we are originally proposing in this paper, offers an alternative yet accurate way based on a simplified experimental set-up and easier measurements. In both cases, the wetsuit sample under testing was shaped in the form of a cylindrical sleeve of proper dimensions and wrapped around a phantom containing water at a higher temperature and surrounded by water at a lower temperature. The wetsuit's cylindrical surface allows heat flow from the warmer water on the inside to the colder water on the outside through the wetsuit area. In the first case, a thermal steady state was achieved, with constant heat flow from the phantom to the exterior. This was obtained with a power balance between two homogenous quantities. Electrically supplied thermal heating within the phantom was used to balance the thermal energy naturally flowing through the wetsuit's surface. In this first case, a stable and fixed temperature difference was obtained between the inner and the outer surfaces of the wetsuit sample. In the second case, a thermal transient was analyzed during the cooling process of the phantom, and the thermal time constant was measured, providing the sample thermal resistance once the phantom thermal capacity was known. In both cases and methods, the heat flow and thermal resistance of other elements than the wetsuit must be evaluated and compensated for if they are not negligible. Finally, the thermal resistivity per unit area of the wetsuit material was obtained with the product of the wetsuit sample's thermal resistance and the wetsuit area. The measurements, conducted until now by immersing the phantom in a free surface tank, show that both methods-under stationary and under transient temperature conditions-were valid to assess the wetsuit's thermal resistivity. The stationary method somehow provided better accuracy while involving less well-known parameters but at the expense of a more complicated experimental set-up and additional energy consumption. The transitory method, on the other hand, is quite easy to implement and, after careful characterization of the phantom's parameters, it provided similar results to the stationary one. An uncertainty budget was evaluated for both methods, and they did provide highly compatible measurement results, with resistivity values of 0.104(9) m2·K/W (stationary method) and 0.095(9) K·m2/W (transient method) for the same wetsuit sample under testing, which is also consistent with the values in the literature. We finally propose that the novel method is a valid alternative for characterization of the thermal insulation properties of a scuba diving wetsuit.

Analysis and Experiment of Thermal Field Distribution and Thermal Deformation of Nut Rotary Ball Screw Transmission Mechanism

This study designs a differential dual-drive micro-feed mechanism, superposing the two “macro feed motions” (“motor drive screw” and “motor drive nut”) using the same transmission of “the nut rotary ball screw pair” structure. These two motions are almost equal in terms of speed and turning direction, thus the “micro feed” can be obtained. (1) Background: Thermal deformation is the primary factor that can restrict the high-precision micro-feed mechanism and the distribution of heat sources differs from that of the conventional screw single-drive system owing to the structure and motion features of the transmission components. (2) Discussion: This study explores the thermal field distribution and thermal deformation of the differentially driven micro-feed mechanism when two driving motors are combined at different speeds. (3) Methods: Based on the theory of heat transfer, the differential dual-drive system can be used as the research object. The thermal equilibrium equations of the micro-feed transmission system are established using the thermal resistance network method, and a thermal field distribution model is obtained. (4) Results: Combined with the mechanism of thermal deformation theory, the established thermal field model is used to predict the axial thermal deformation of the differential dual-drive ball screw. (5) Conclusions: Under the dual-drive condition, the steady-state thermal error of the nut-rotating ball screw transmission mechanism increases with the increase in nut speed and composite speed and is greater than the steady-state thermal error under the single screw drive condition. After reaching the thermal steady state, the measured thermal elongation at the end of the screw in the experiment is approximately 10.5 μm and the simulation result is 11.98 μm. The experimental measurement result demonstrates the accuracy of the theoretical analysis model for thermal error at the end of the screw.

Effect of thermal fluctuations on mold coating delamination during continuous casting of high-alloy (Mn, Al) steel

During the continuous casting of high-alloy (Mn, Al) steels, mold coating delamination occurs specifically at the narrow face (NF) mold corner immediately below the meniscus, different from where mold surface problems are generally known to occur. Most mechanical analyses have been performed assuming negligible thermal fluctuations and steady state interfacial heat fluxes. However, high-alloy steels exhibit larger thermal fluctuations than general steels during continuous casting, attributable to the reaction between the high-alloy steels and mold flux. Therefore, in this study, thermo-mechanical analyses are performed under both steady and fluctuating interfacial heat fluxes to compare the effects of thermal fluctuations on the continuous casting mold, with the results indicating that monotonic stresses are generated under steady interfacial heat fluxes at the corner of the NF mold, whereas cyclic stresses are induced by thermal fluctuations under fluctuating interfacial heat fluxes. Accordingly, the cyclic stresses at the corner of the NF mold, induced by thermal fluctuations, are determined as the major cause of mold coating delamination. These results can be applied to develop methods to reduce mold coating delamination, prevent the degradation of strand quality, and improve mold life.

Thermal Analysis of a Fast Charger for Public Service Electric Vehicles Based on Supercapacitors

The aging of supercapacitors (SCs) depends on several factors, with temperature being one of the most important. When this is high, degradation of the electrolyte occurs. The impurities generated in its decomposition reduce the accessibility of the ions to the porous structure on the surface of the electrode, which reduces its capacity and increases its internal resistance. In some applications, such as electric vehicles whose storage system consists of SCs, fast chargers, which supply very high power, are used. This can lead to an increase in temperature and accelerated aging of the cells. Therefore, it is important to know how the temperature of the SCs evolves in these cases and what parameters it depends on, both electrical and thermal. In this contribution, mathematical formulae have been developed to determine the evolution of the temperature in time and its maximum value during the transient state. The formulae for obtaining the mean and maximum temperature, once the thermal steady state (TSS) has been reached, are also shown, considering that the charger cells are recharged from the grid at a constant current. Based on this formulation, the thermal analysis of a specific case is determined.

Pyroelectric infrared device with multilayer compensation structure

The compensation structure can help the pyroelectric device to suppress noise caused by environment effects. However, the traditional compensation structure is not helpful for device miniaturization. In addition, although the traditional compensation structure almost has no problem with thermal crosstalk, the reflective layer is necessary for the traditional compensation structure to restrain the optical crosstalk. Obviously, the reflective layer will increase device costs and process complexity. In this paper, we propose the multilayer compensation structure. The proposed structure utilizes the detection element to screen the compensation element, so that the compensation element is not subject to the input radiation flux, thereby eliminating optical crosstalk. And the intermediate isolation layer can suppress the heat conduction from detection element to compensation element, so that the thermal crosstalk can be suppressed. In order to verify the proposed structure, the thermal steady state analysis and transient state analysis have been done. Furthermore, the devices based on P(VDF-TrFE)/PZT composites were prepared and characterized to further verify the proposed structure.

Application and evaluation of classic and demand oriented cooling strategies in context of machine tools

AbstractEnergy consumption is an important factor in the operation of machine tools. Recent studies showed that in average, $${1}/{5}$$ 1 / 5 of the machine tools total energy demand is caused by internal cooling systems. The continually rising performance of machine tools, together with the importance of thermal stability, reasons the requirement for demand oriented cooling approaches, which could significantly reduce the energy consumption for internal machine cooling. This study applied classic and demand oriented cooling strategies on a machine tool frame, which is one of the most complex machine tool components from a thermal point of view. The demand oriented cooling is based on volume flow control and can either be configured close to the environmental temperature or close to the thermal steady state, which can be obtained by loss forecast in combination with an FE model or measurement. The impact of the strategies on the thermal reaction as well as the energy consumption is compared, showing a significant reduction of energy demand without significant impairment of the thermal stability.

Recommendations for improving the experimental protocol for the determination of photocatalytic activity by nitric oxide oxidation measurements.

The photocatalytic nitric oxide (NO) oxidation reaction is used as a standard diagnostic tool for photocatalytic activity according to the well-defined protocol described by ISO Standard 22197-1-2007. This protocol identifies the negative peak showing a NOx concentration drop during a gas flow switch from the calibration bypass to the reactor as adsorption of NOx on the surface. Evidence is provided for this first transient to be due to a dilution effect in the gas phase within the reactor. With proper models of residence time distribution analysis, this transient revealed the internal hydrodynamics and it can be used to determine the internal volumes of the system. The second transient occurs immediately after the light is switched on. The conversions strongly depend on the time constant of this transient. Controlled measurements of the effect of illumination intensity revealed that at higher light intensities the transient takes longer to reach steady state. The longer transient was attributed to the time needed to reach a thermal steady state of the hot spots generated by the recombination of excess charge carriers. When the catalyst amount was investigated as a parameter, a saturation effect was observed. This saturation effect was correlated with the gas phase concentrations of NOx and moisture and their ratios to the available specific surface area. Hence, additional constraints with respect to the illumination intensity and catalyst amounts are recommended for accurate measurements of photocatalytic activity by NO oxidation.

In-Plane Behavior of Steel-Plate Composite Wall Elements under Intermediate High Temperature

To explore how steel-plate composite (SC) walls response to in-plane loading is essential for SC structural design, especially combined with thermal loading. This work focuses on the intermediate high temperature (≤400°C) that is often considered during design process. Individual mechanical behavior of steel plate and concrete in SC walls under intermediate high temperature are studied. Thermoplasticity is applied to describe steel plate as 2D element, by which plastic flow combined with mechanical property degradation can be considered. High-temperature mechanical behavior of concrete is very complicated. In this study, strain of concrete at high temperature is decomposed into transient thermal strain, free thermal strain, and mechanical strain. By elaborating the physical meaning of transient thermal strain and selecting mathematical models for every strain component, deformation of high-temperature concrete can be calculated. To study the in-plane behavior experimentally, an SC panel test with in-plane shear and thermal loading was conducted. Steady thermal state and transient thermal state were considered in two specimens, respectively. Comparison between test data and theoretical calculation results shows good agreement. Verifications from other researchers’ tests show that the model can be applied to general situations of in-plane loading combined with intermediate high temperature.

Characterization of local non-equilibrium phonons in bulk MoS2 probed by temperature-dependent Raman scattering

This study presents an experimental implementation to determine the temperature dependence of Raman spectra of bulk 2H-MoS2 in the temperature range of 300–650 K. In addition, we have measured according to the anti-Stokes to Stokes integrated intensity ratio (). The results indicate that Raman shift of both the vibration modes and exhibits a linear temperature-dependent behavior in the heating and cooling processes. It is confirmed that the sample did not deteriorate by oxidization and decomposition at high temperatures. However, above 500 K, the measured phonon temperature deviated from the heat bath temperature. It is suggested that the does not correspond to the equilibrium population of optical phonons, thus indicating that the measurement was not in thermal equilibrium but in a thermal steady state under laser irradiation. We conclude that this is caused by the difference in relaxation times in the thermal steady state.

Study of the effect of hole defects on wood heat transfer based on infrared thermography

From a quantitative perspective using infrared thermography, this study examines the impact of various surface defect sizes on the heat transfer of wood under the condition of wood along-grain excitation. Firstly, the correlation between the defect size and the temperature change ΔT in the thermal steady state phase and the cooling coefficient Ve in the unsteady state phase were built by theory analyzed. And then, a theoretical analysis conducted under wood along-grain excitation conditions examines the theoretical relationship between the diameter D and depth H of the hole defect and the two thermodynamic properties ΔT and Ve. Second, using larch (Larix gmelinii) as the study's object, thermal excitation experiments and natural cooling experiments were conducted on wood specimens in the down-grain direction. The temperature field changes on the specimens' surfaces were recorded using a thermal imaging camera in order to obtain ΔT and Ve for various hole defect sizes. Then, using finite element modeling, the prediction models of ΔT and Ve are built, respectively, for the case of hole defects of various sizes. The model demonstrates that: A very strong negative correlation between the defect diameter D and the thermal steady-state temperature change ΔT (R2 > 0.946) and a very strong positive correlation with the cooling coefficient Ve (R2 > 0.985); The defect depth H has an extremely strong positive correlation with Ve (R2 > 0.946). In addition, as H increases ΔT increases first and then decreases, which is in line with the quadratic curve distribution (R2 > 0.991). Finally, the prediction model was compared with the experimental data for validation. The validation results revealed that the established model's ΔT was similar to the experimental ΔT variation pattern (R2 = 0.864) and that it was able to more accurately predict the pattern and value of Ve (R2 = 0.846, MAPE = 5.44%). The research results could lay some preliminary theoretical foundation for the formation of wood defect nondestructive testing technology based on infrared thermography.

Accelerating the Heat Diffusion: Fast Thermal Relaxation of a Microcantilever

In most systems, thermal diffusion is intrinsically slow with respect to mechanical relaxation. We devise here a generic approach to accelerate the relaxation of the temperature field of a one-dimensional object, in order to beat the mechanical time scales. This approach is applied to a micrometer-sized silicon cantilever, locally heated by a laser beam. A tailored driving protocol for the laser power is derived to quickly reach the thermal stationary state. The model is implemented experimentally yielding a significant acceleration of the thermal relaxation, up to a factor 30. An excellent agreement with the theoretical predictions is reported. This strategy allows a thermal steady state to be reached significantly faster than the natural mechanical relaxation.

Multiphysics Computing of Challenging Antenna Arrays Under a Supercomputer Framework

A parallel multiphysics simulation solver is developed to solve electromagnetic-thermal-mechanical coupling for some challenging large-scale antenna arrays. To achieve high scalability of supercomputer architectures, we reconstruct the preconditioned BiCGSTAB method and the non-overlapping domain decomposition method, so that the most resource-intensive matrix factorization steps can be performed in parallel independently within subdomains. The electromagnetic and thermal fields are solved separately, while coupled through the dissipated power and the temperature-dependent material parameters; after thermal steady state is reached, the mechanical simulation is stimulated subject to the temperature rise. The accuracy of electromagnetic-thermal coupling and thermal stress solution are first validated, and then the strong/weak parallel scalability experiments of the developed multiphysics solver are performed on supercomputer. Finally, an extremely challenging antenna array is simulated using the proposed solver, where to our best knowledge we bring the scale of multiphysics simulations excited by frequency-domain electromagnetic fields to the order of billion unknowns for the first time.

Experimental investigation of airflow and heat transfer by natural convection in an insulated box with a Phase Change Material using a Particle Image Velocimetry technique

Airflow and heat transfer via natural convection in an insulated box with Phase Change Material (PCM) were experimentally investigated using Particle Image Velocimetry (PIV) and temperature measurements. The effects of PCM positions (side wall and lid) on flow pattern and temperature distribution were studied under empty and loaded conditions. Two loads were considered to study the obstacle effect and the influence of heat exchange with air. When PCM was either at the side wall or at the lid of the box, laminar flow was observed and the corresponding Rayleigh number was about 10 7 . Upward flow was always observed near the side walls of the box. When PCM was on the side, downward flow occurred along the PCM; in the empty case, flow was almost 2D but became 3D when the load was added. When PCM was on the lid, the air cooled in contact with PCM, detached from it and flowed downwards. In the empty case, downward flow was unstable, and with the load, it followed preferential pathways. The type of load exerted little effect on flow patterns at thermal steady state. Thus, a simpler load (extruded polystyrene) can be used in the first approach. The maximum velocity was about 0.1 m s −1 , so free convection cannot be neglected compared with conduction. Regarding temperature performance, PCM on the side and on the lid showed no substantial difference if gaps were left between the load and the walls or PCM. • Airflow and heat transfer due to natural convection in an insulated box with PCM were studied. • Effect of PCM positions on flow and temperature fields was investigated. • The highest observed air velocities in the box were around 0.1 m s −1 . • PCM position exerted no significant effect on maximum product temperature. • Gaps should be left between the product and the box walls or PCM.

THE METHOD OF THERMAL ENERGY RECOVERY OF EXHAUST GASES OF INTERNAL COMBUSTION ENGINE

In modern internal combustion engines, the part of the fuel energy that is consumed for useful energy is on average 35 percent. Up to 30 percent is spent during the combustion of fuel, which is removed with the exhaust gases of the engine. There are many ways to increase the efficiency of an internal combustion engine, one of which is a way to increase the efficiency by recuperating heat losses, implying the use of a device for converting thermal energy into electrical energy, operating on the Seebeck effect. (Research purpose) The research purpose is developing an experimental installation for the recovery of thermal energy from exhaust gases of an internal combustion engine into electrical energy and deducing the regularities of the thermoelectric generator in the power supply system in automotive equipment. (Materials and methods) Applied methods of numerical analysis and computer modeling using computer programs ANSYS Thermal Steady State and Solidworks FlowSimulation. A laboratory stand operating from the D-65 engine was used as a power plant (Results and discussion) The dependencies of the generated voltage and current of the thermoelectric generator on the temperature of the exhaust gases and coolant of the internal combustion engine were obtained. The maximum generated power of the developed thermoelectric generator was 54.5 watts. The maximum current of the thermoelectric generator during operation of the experimental installation was 3.7 amperes with an output voltage of 22 volts. (Conclusions) The use of the developed thermoelectric generator for the recovery of the energy of the exhaust gases of the internal combustion engine makes it possible to increase the use of the heat released during the combustion of fuel and provides an additional source of electrical energy.

Steady State Free Convection through some Variable Fin Geometries

Thermal performances on some typical variable geometries were conducted in this work which can be reported into two phases. In the first phase of this work, fins of rectangular, cylindrical, and conical geometries having the same total surface area were studied. It is found that the cylindrical fin is the least efficient and least effective among the three. In the second phase, design modification in the form of grooves is made on cylindrical fins. Cutting grooves on cylindrical fins increase its surface area. The three grooved cylindrical fins are namely- fully grooved cylindrical fins, bottom half grooved cylindrical fins, and top half grooved cylindrical fins. All three grooved fins have the same volume. It was noticed hat, on cutting groove the on fin the quality of convective heat transfer, as well as the effectiveness, improves, but the efficiency degrades. Keywords: Finite Element Method, thermal steady state, fin effectiveness, fin efficiency, free convection

Feedforward Model-Based Intelligent Compensation System for Spindle Dynamic Thermal Error

Among spindle dynamic errors, thermal error is an important factor affecting machining precision, the most convenient and economical solution for which is dynamic compensation of thermal error. In view of defects of complicated system, complex debugging, poor stability, high cost, etc., in the existing compensation approaches, thermal error compensation technology is investigated with compensation model established to complete the system design. The system is tested on the spindle of precise horizontal machining center. In the experiment, multiple linear regression, BP neural network, etc. are taken for technical analysis with dynamic feedforward model established. The experimental results indicate that the compensation system under study has dynamic compensation rate of over 90%, while the spindle has greatly shorter warm-up time, which can operate normally before entering the thermal steady state. The finally completed feedforward model-based intelligent compensation control system has good steady-state and dynamic compensation performance. In particular, its self-correction function greatly improves system adaptability and simplifies operation in use.

Damage integral and other predictive formulas for nonisothermal heating during laser exposure.

.Significance: Physics-based models supply simulated temperature rises to photothermal damage rate models and provide comprehensive risk assessments for laser-induced damage. As the physics-based models continue to be refined, the damage rate models have not advanced. This peculiar lack of improvement is counterintuitive considering the damage integral (), originally derived for isothermal heating events, and fails to accurately represent the nonisothermal heating from short laser exposures.Aim: Derive a nonisothermal form of the damage integral and predict more accurately the damage induced by short laser exposures, as well as identify the role of heating rate in laser damage.Approach: From first principles, we derived a version of the damage integral specific to the shape of thermal profiles rather than the square function provided by Arrhenius plots. We used previously published threshold thermal profiles, where all nonisothermal frequency factors () solved all values to unity. Nonisothermal correction factors correct isothermal values.Results: The values were identical for both the isothermal and nonisothermal conventions. Correction factor values for ranged from 0.0 (20-s exposures at thermal steady state) to (0.05-s exposures). Based on empirical results, we have derived a two-dimensional empirical formula that predicts the heating rate as a function of exposure duration and ambient temperature. Threshold peak temperatures () and threshold critical temperatures are mathematically determined without thermal profiles when appropriate and values are established.Conclusions: We have identified a modified damage integral that does not rely on the Arrhenius plot and provides a value for the frequency factor () that accounts for the nonisothermal nature of short laser exposures. The method, validated in our in vitro retinal model, requires thermal profiles recorded under threshold conditions, such as at minimum visible lesions or the boundary of cell death. The method is a new option for laser damage modelers.

Investigation of Eddy Current Loss and Structure Design with Magnetic-Thermal Coupling for Toothless BLDC High-Speed PM Motor

Brushless direct current (BLDC) permanent magnet (PM) synchronous motors are in high demand for ventilator applications owing to their high speed, high efficiency, and other significant features. However, it has become an important problem in eddy current loss calculations with high-speed motors, which leads to low motor (ventilator) life and PM demagnetization. This paper focuses on the eddy current loss calculation and the structure improvement design for the two-pole 90 W, 47,000 r/min toothless BLDC motor. First, the influencing factors of eddy current loss are comprehensively investigated, and a multiparameter improvement methodology is proposed accordingly. Second, by finite element analysis (FEA), the effective winding length ratio and the number of parallel wires were mainly researched for the winding, and the influence on the eddy current loss and the efficiency was determined, providing a reference for BLDC high-speed motors. This study has resulted in a 34.75% reduction in the winding losses, and a 4.6% increase in the efficiency of the improved model compared with the original design. Third, the new rotor structure is proposed, saving PM volume 15% more than original. THD of gap flux density is decreased 20.97%; the eddy current loss in the new rotor is decreased 22% more than original. Furthermore, by coupling simulation of the magnetic–thermal field, the maximum temperature of winding of the improved model is 13.4% lower than that of the original model at the thermal steady state. Finally, the electromagnetic and thermal properties simulation results were verified by testing the prototype. It is of great significance to the structure design and efficiency improvement of the BLDC high-speed motor.