Transient Thermal Conditions Research Articles

Analysis of heavy-duty turbocharged diesel engine response under cold transient operation with a pre-turbo aftertreatment exhaust manifold configuration

Diesel particulate filters are the most useful technology to reduce particulate matter from the exhaust gas of internal combustion engines. Although these devices have suffered an intense development in terms of the management of filtration and regeneration, the effect of the system location on the engine performance is still a key issue that needs to be properly addressed. The present work is focused on a computational study regarding the effects of a pre-turbo aftertreatment placement under full and partial load transient operation at constant engine speed and low wall temperature along the exhaust line. The aim of the paper is to provide a comprehensive understanding of the engine response to define the guidelines of a control strategy that is able to get the standards of engine driveability during sudden accelerations under restraining thermal transient conditions governed by the aftertreatment thermal inertia. The proposed strategy overcomes the lack of temperature at the inlet of the turbine caused by the thermal transient by means of the boost and EGR control. It leads to a proper management of the power in the exhaust gas for the expansion in the turbine.

Heat transfer modelling in honeycomb wall-flow diesel particulate filters

Heat transfer in wall-flow monoliths has gained in interest because of the widespread adoption of these systems by automotive industry to fulfil soot emission regulations and the importance of heat exchange on the regeneration process control to avoid damaging the monolith. This paper presents a heat transfer model for wall-flow diesel particulate filters coupled with an unsteady compressible flow solver. The heat exchange between the gas and the solid phase is based on a bi-dimensional discretisation of the porous medium both in axial and tangential directions. The monolith can be discretised in the radial direction to account for the heat fluxes towards the environment through the monolith and the canister, which is also coupled with the inlet and outlet ducts of the filter. The model is validated against experimental data obtained in a flow test rig. A test campaign under non-reacting conditions has been conducted to show the capability for thermal response prediction. Tests cover clean and soot loaded monolith, continuous flow under steady and transient thermal conditions, and pulsating flow. In this case, the characteristics of the pressure waves in amplitude and frequency are similar to those that the monolith can undergo depending on its location along the exhaust line.

Global time method for inverse heat conduction problem

Traditional space-marching techniques for solving the inverse heat conduction problem are highly susceptible to both measurement and round-off error. This problem is exacerbated if the problem requires small time steps to resolve rapid changes in the surface condition, since this can cause instability. The inverse technique presented in this article utilizes a global time approach which eliminates the instability usually observed when using small time steps. It is demonstrated that a higher sampling rate (smaller time steps) in fact improves the inverse prediction. This is accomplished using a functional representation of the time derivative in the heat equation, and a physically based regularization scheme. A Gaussian low-pass filter is used with an analytically determined optimum cut-off frequency. The filter delivers an analytical function which has smooth, bounded derivatives. The inverse technique is demonstrated to accurately resolve the transient surface thermal condition in the presence of noise.

A NARX damper model for virtual tuning of automotive suspension systems with high-frequency loading

A computationally efficient NARX-type neural network model is developed to characterise highly nonlinear frequency-dependent thermally sensitive hydraulic dampers for use in the virtual tuning of passive suspension systems with high-frequency loading. Three input variables are chosen to account for high-frequency kinematics and temperature variations arising from continuous vehicle operation over non-smooth surfaces such as stone-covered streets, rough or off-road conditions. Two additional input variables are chosen to represent tuneable valve parameters. To assist in the development of the NARX model, a highly accurate but computationally excessive physical damper model [originally proposed by S. Duym and K. Reybrouck, Physical characterization of non-linear shock absorber dynamics, Eur. J. Mech. Eng. M 43(4) (1998), pp. 181–188] is extended to allow for high-frequency input kinematics. Experimental verification of this extended version uses measured damper data obtained from an industrial damper test machine under near-isothermal conditions for fixed valve settings, with input kinematics corresponding to harmonic and random road profiles. The extended model is then used only for simulating data for training and testing the NARX model with specified temperature profiles and different valve parameters, both in isolation and within quarter-car vehicle simulations. A heat generation and dissipation model is also developed and experimentally verified for use within the simulations. Virtual tuning using the quarter-car simulation model then exploits the NARX damper to achieve a compromise between ride and handling under transient thermal conditions with harmonic and random road profiles. For quarter-car simulations, the paper shows that a single tuneable NARX damper makes virtual tuning computationally very attractive.

Damage characterization in non-isothermal stretching of acrylics. Part II: Experimental validation

Influence of temperature and strain rate on damage accumulation and large deformation behavior of acrylics was investigated under conditions similar to actual polymer processing. Poly(methyl methacrylate) (PMMA) samples were stretched to large strains at different rates under transient thermal conditions. During testing, specimens were cooled down from temperatures above glass transition temperature ( θ g ) to temperatures well-below θ g inducing a transition from rubbery state to solid state. Contrary to common practice of studying thermo-mechanical coupling in terms of adiabatic heating; in proposed experimental study, temperature effect on mechanical response of material was emphasized by externally intervening temperature variation within specimen. An improved version of dual-mechanism constitutive model presented in Part I [Gunel, E.M., Basaran, C., 2010. Damage characterization in non-isothermal stretching of acrylics. Part I: Theory. Mechanics of Materials] was proposed to predict thermo-mechanical response of amorphous polymer below and above θ g . Applicability of proposed constitutive model for the specific case of non-isothermal stretching of PMMA at different test conditions was demonstrated by incorporating it into a finite element scheme. Constitutive model was reasonably accurate to capture observed temperature–displacement–force history in experimental study. Damage evolution under different testing conditions was studied in terms of irreversible thermal and mechanical entropy production.

Thermal comfort models: A review and numerical investigation

Thermal comfort may be achieved more energy-efficiently in non-uniform thermal environments than in uniform ones, and such environments are also frequently transient, so developing a thermal comfort model to evaluate thermal comfort asymmetrical environments or transient conditions has being an hotspot of recent study. This paper first reviews several thermal comfort models that address local thermal sensations and attempts to distinguish these models by their advantages, limitations and suitable ranges of applications. Then, two typical thermal comfort models, the simple ISO 14505 standard method and the comprehensive UC Berkeley thermal comfort model (UCB model), were coupled to computational fluid dynamic (CFD) numerical simulation with different process to evaluate thermal environment of a small office. The results indicated that compared with the UCB model, the ISO 14505 index could be applied with caution as a convenient method to evaluate thermal comfort in non-uniform, overall thermally neutral environments.

Transient solution of temperature field in functionally graded hollow cylinder with finite length using multi layered approach

In this paper by using a multi-layered approach based on the theory of laminated composites, the solution of temperature in functionally graded circular hollow cylinders subjected to transient thermal boundary conditions are presented. The material properties are assumed to be temperature-independent and radially dependent. The cylinder has finite length and is subjected to axisymmetric thermal loads. It is assumed that the functionally graded circular hollow cylinder is composed of N fictitious layers and the properties of each layer are assumed to be homogeneous and isotropic. Employing Laplace transform techniques and series solving method for two-dimensional heat conduction equation in the cylinder, solutions for the variation of temperature with time as well as temperature distribution through the cylinder are obtained.

An experimental investigation on interior thermal conditions and human body temperatures during cooling period in automobile

Determining the thermal conditions in an automobile and their effects on the driver is an important issue from both thermal comfort and driving safety points of view. Especially in hot summer season, the interior thermal conditions in automobile change rapidly when the air conditioning unit runs. In this study, standard air conditioning system is switched in an automobile parked in the sun and then the interior thermal conditions of the automobile are determined in detail during the 1-h cooling period. During the period, relative humidity, air velocity, air and surface temperature measurements are taken at numerous locations in the automobile. Moreover, in order to evaluate the effects of transient interior thermal conditions on the occupant, the skin temperatures of human body are measured at nine points. In addition to this, the thermal sensation of the human subject is also questioned during the cooling period. Subjective thermal comfort data is recorded using a questionnaire. The series of tests are conducted on two different automobiles, and the experimental results for both automobiles are presented and scrutinized.

Experimental study on mechanical behaviors of Al-alloys under transient aerodynamic heating

Determination of mechanical parameters such as the ultimate strength of aerospace materials under the condition of transient heating is critical for reliability evaluation, fatigue life prediction and safety design of high-speed aircrafts including missiles. Al-alloys are widely used as aeronautical materials due to their advantages such as light weight, easy manufacturability and relatively low cost. However, their mechanical properties under transient thermal conditions are not available in the handbooks on material strength, mainly due to the difficulty of simulating the transient thermal conditions for thermal strength test. To solve this problem, a joint thermal–mechanical loading system has been developed in the present investigation to simulate the aerodynamic thermal conditions. The mechanical behaviors of some Al-alloys have been examined with this system. The ultimate strengths and durations of two different types of Al-alloys, i.e. Al7178 and Al2024, under joint thermal–mechanical loading conditions have been tested. Furthermore, their different failure modes and mechanisms under various temperatures have been investigated and compared. These data provide a reliable basis for the study of load bearing capacity and weight reduction of aerospace materials and structures under transient aerodynamic thermal conditions.

Thermal comfort: research and practice

Thermal comfort--the state of mind, which expresses satisfaction with the thermal environment--is an important aspect of the building design process as modern man spends most of the day indoors. This paper reviews the developments in indoor thermal comfort research and practice since the second half of the 1990s, and groups these developments around two main themes; (i) thermal comfort models and standards, and (ii) advances in computerization. Within the first theme, the PMV-model (Predicted Mean Vote), created by Fanger in the late 1960s is discussed in the light of the emergence of models of adaptive thermal comfort. The adaptive models are based on adaptive opportunities of occupants and are related to options of personal control of the indoor climate and psychology and performance. Both models have been considered in the latest round of thermal comfort standard revisions. The second theme focuses on the ever increasing role played by computerization in thermal comfort research and practice, including sophisticated multi-segmental modeling and building performance simulation, transient thermal conditions and interactions, thermal manikins.

Mechanical Characterization of Fibre Reinforced Polymers Materials at High Temperature

One of the greatest impediments to using fibre reinforced polymer (FRP) composites in buildings and parking garages is their susceptibility to degradation when exposed to elevated temperatures and the limited knowledge on the thermal and mechanical properties of these composites at such temperatures. Glass FRP (GFRP) tensile coupons and single lap-splice coupons were tested in tension to study the mechanical properties under steady-state and transient thermal conditions. Tests were conducted at a range of temperatures between room temperature and +200°C. In terms of tensile strength, approximately half of the strength of the FRP was lost near the glass transition temperature of the epoxy resin matrix. However, 40% of the room temperature strength of the GFRP was still retained at 200°C. The lap-splice tests showed that the FRP-to-FRP bond strength was affected even more by high temperature exposure with 90% loss in lap-splice near the glass transition temperature. An analytical model is also presented in this paper characterizing the mechanical properties at elevated temperature, which in turn will be used in numerical fire endurance models developed by the authors.

Infinite Row of Parallel Cracks in a Functionally Graded Piezoelectric Material Strip Under Mechanical and Transient Thermal Loading Conditions

In this paper, the problem of an infinite row of parallel cracks in a functionally graded piezoelectric material strip (FGPM strip) is analyzed under static mechanical and transient thermal loading conditions. The crack faces are supposed to be completely insulated. Material properties are assumed to be exponentially dependent on the distance from the bottom surface. By using the Laplace and Fourier transforms, the thermoelectromechanical problem is reduced to a singular integral equation, which is solved numerically. The stress intensity factors for both the embedded and edge cracks are computed. The results for the crack contact problem are also included.

Transient heat conduction in two-dimensional functionally graded hollow cylinder with finite length

In this paper a thick hollow cylinder with finite length made of two dimensional functionally graded material (2D-FGM) subjected to transient thermal boundary conditions is considered. The volume fraction distribution of materials, geometry and thermal boundary conditions are assumed to be axisymmetric but not uniform along the axial direction. The finite element method with graded material properties within each element is used to model the structure and the Crank–Nicolson finite difference method is implemented to solve time dependent equations of the heat transfer problem. Two-dimensional heat conduction in the cylinder is considered and variation of temperature with time as well as temperature distribution through the cylinder are investigated. Effects of variation of material distribution in two radial and axial directions on the temperature distribution and time response are studied. The achieved results show that using two-dimensional FGM leads to a more flexible design so that transient temperature, maximum amplitude and uniformity of temperature distributions can be modified to achieve required specifications by selecting a suitable material distribution profile in two directions.

平行き裂群を有する傾斜機能圧電厚板の非定常電気熱弾性応答

In this paper, the problem of an infinite row of parallel cracks in a functionally graded piezoelectric material strip (FGPM strip) is analyzed under transient thermal loading condition. The crack faces are supposed to be completely insulated. Material properties are assumed to be exponentially dependent on the distance from the bottom surface. The superposition technique is used to solve the governing equations. The transient temperature and thermal stress in an uncracked strip are the same as the previous results. This thermal stress is used as the crack surface traction with opposite sign to formulate the mixed boundary value problem. By using the Fourier transform, the thermoelectromechanical problem is reduced to a singular integral equation. The singular integral equation is solved by using the Gauss-Jacobi integration formula. The stress intensity factors for both the embedded and edge cracks are computed. The results for the crack contact problem are also included.

Re-evaluation of Stolwijk's 25-node human thermal model under thermal-transient conditions: Prediction of skin temperature in low-activity conditions

The performance of Stolwijk's 25-node thermal model of the human body was evaluated for the prediction of the skin temperature of a sedentary person in a thermal-transient state. The skin temperature calculated by the original Stolwijk model was compared to experimental data obtained systematically from a large number of subjects exposed to stepwise changes in environmental conditions, including neutral (29.4 °C), low (19.5 °C), and high (38.9 °C) ambient temperatures. The results show that the original Stolwijk model accurately predicts both the absolute value and the tendency in the transient mean skin temperature. This suggests that the Stolwijk model is valid for the prediction of the transient mean skin temperature for the “average” person under low-activity conditions. Discrepancies are observed in the local skin temperature for some segments. However, these discrepancies can be significantly reduced through modification of the basal skin blood flow distributions and the distributions of vasoconstriction and workload in the model.

Implications of subsurface thermal–hydraulic–mechanical processes associated with glaciation on Shield flow system evolution and performance assessment

A deep geologic repository (DGR) situated on the Canadian Shield will be subject to long-term climate change that will markedly alter surface conditions as a result of glaciation and permafrost penetration. Systematic, two-dimensional and three-dimensional coupled thermal–hydraulic–mechanical finite-element simulations with varying degrees of coupling, including depth-dependent salinity (represented as a change in groundwater density) and temperature-dependent density and viscosity, were undertaken to address the implications of glaciation on groundwater flow system dynamics as it could affect DGR performance. The modelling domain consisted of a 1.6-km deep sub-regional scale (≈100 km2) fractured Shield flow system. Initial and transient thermal, hydraulic and mechanical boundary conditions were developed from two realizations of the University of Toronto Glacial Systems Model of the last Laurentide glaciation. Results indicate that during the glacial loading/unloading cycle, for this particular conceptual model, there is limited penetration of glacial meltwaters to depth and small residual anomalous hydraulic head. During glacial coverage, the mechanical factor of safety increases in the moderately fractured and sparsely fractured rock mass, but principal effective stress reorientation also occurs. Given the assumed nonglacial in situ state of stress and mechanical properties, the fracture zones were predicted to be less stable under glacial conditions.

Thermal Shock Response of a Parallel Crack in a Functionally Graded Piezoelectric Material Plate

A crack in a plate of a functionally graded piezoelectric material mathematically modeled by a nonhomogeneous solid is studied under transient thermal loading conditions. It-is assumed that initially the medium is at the uniform temperature and is suddenly subjected to a uniform temperature rise along the one of the traction-free boundaries. The crack faces are supposed to be completely insulated. Some material properties are assumed to be exponentially dependent on the distance from the crack line parallel to the boundaries of the plate. By using both the Laplace transform and Fourier transform, the thermal and electromechanical problems are reduced to a singular integral equation and a system of singular integral equations. The singular integral equations are solved numerically, and a numerical method is then employed to obtain the time dependent solutions by way of a Laplace inversion technique. The intensity factors vs. time for various material constants and geometric parameters are calculated.

Visualizations of the unsteady interaction of opposite buoyancy induced flows

Although the fluid dynamics around an isolated heated circular cylinder is now reasonably well understood, this is not the case when the cylinder is positioned close to a free convection plane wall boundary layer under transient thermal conditions. One mechanism by which transitional boundary layer flows become turbulent is the instability of vortex interaction. The core of this contribution is to present flow visualizations showing the formation and development of resulting vortical complex structures in the interaction of opposite naturally induced flows. To the author’s knowledge, these complex phenomena have not been clearly investigated and documented. Therefore, the purpose of the study is to provide a detailed description of the resulting transient physical mechanisms. The effect of changing the gap between the cylinder and the wall has clearly shown the growth in time of both dipolar vortex and shear layer instabilities and complex vortical separations of the wall boundary layer.

An Equivalent Domain Integral Method for Fracture Analysis of Functionally Graded Materials Under Thermal Stresses

This paper presents the formulation and finite element implementation of the equivalent domain integral (EDI) for fracture analysis of functionally graded materials (FGMs) under thermal stresses. By carrying out the neccesary modifications resulting from material nonhomogeneity and thermal strains, the generalized J-integral is converted to an equivalent domain integral around the crack tip for both plane stress and plane strain problems of thermoelasticity. The developed procedure is integrated in a fracture analysis code FRAC2D using graded and cubic finite elements in order to calculate the stress intensity factor under mode I steady-state and transient thermal loading conditions. Temperature distribution profiles in FGMs are calculated using the finite elements based heat transfer analysis code HEAT2D. Comparisons of the computed thermal stress intensity factors to the results available in the literature and to those calculated by an enriched finite element method show that developed EDI approach produces highly accurate results and possesses the required domain independence. Detailed parametric analyses are performed in order to examine the influences of material property variation profiles and geometrical parameters on the mode I stress intensity factors. It is shown that variation profiles of the thermomechanical parameters such as Poisson's ratio, thermal expansion coefficient and thermal conductivity significantly influence both the amplitude of the stress intensity factors and the transient crack closure behavior.

The effect of thermal cycling on the properties of a carbon fibre reinforced magnesium composite

Effects of thermal cycling on mechanical properties of two unidirectional ultra-high modulus carbon fibre reinforced magnesium composites (P100S/AZ91D) have been investigated. Composite specimens reinforced with ∼50% or 60% fibre volume fraction were thermally cycled between the temperatures of 100 °C and −100 °C, in a vacuum, for up to 100 cycles. Mechanical testing showed there was no change in the bending strength of samples or the associated failure mode during cycling. Composites did, however, exhibit a moderate reduction in bending modulus and interlaminar shear strength during the first few thermal cycles. Little further degradation was observed thereafter. Modifications to the matrix microstructure were examined and related to a change in matrix properties measured via microhardness testing. Composite property degradation was attributed to the formation of microvoids and interfacial sliding in the early stages of cycling. The implications these processes may have on composite application are discussed and it is concluded that based on the limited dependence of mechanical properties to thermal cycling carbon fibre reinforced magnesium shows potential for space applications operating under transient thermal conditions.