Thermal stresses in radiant tubes: A comparison between recuperative and regenerative systems

A comparison was made between thermal stresses occurring in radiant tubes used in regenerative and recuperative systems. It was found that higher temperatures close to the annealing temperatures lead to stress relieving. The only stresses occurring were due to the non-linear axial temperature gradients along the length of the radiant tube in a recuperative system, whereas the radiant tube in a regenerative system remained essentially stress free due to a comparatively linear axial temperature gradient.     

Thermal stresses in helical tubes

Equations governing one-dimensional (circumferentially varying) temperature distribution and stress states in boiler tubes such as those used in solar power plants are derived. An approximate solution method relying on series expansions in powers of small parameters is proposed and detailed. Finally, in addition to treating the tube as a thin shell its rod-like characteristics are related to its shell deformations.     

Simple formulae for the approximate computation of axial stresses in pipes due to thermal stratification

Simple formulae for the computation of the axial stresses of pipes subjected to thermal stratification are presented. The formulae are based on conventional beam theory. The pipe cross-section is divided into temperature zones. A beam with its own material properties is attributed to each zone. Through coupling equations the beams are linked and the governing equation system developed. This system is solved analytically and expressions for the axial stresses are obtained. It is shown that the approximate formulae reproduce to a sufficiently accurate degree the real stresses.     

Fluid temperature distributions in a rotating cavity with an axial throughflow

Radial and circumferential variations of cavity air temperature are presented for a rotating cavity with an axial throughflow of cooling air. Results show that the air temperatures are influenced significantly by the cavity surface temperature distribution. Strong (around 5°C) circumferential air temperature variations are observed showing the flow to be three-dimensional. Temperature time traces are used to infer flow structure features and also the variation of the fluid to cavity angular velocity ratio with temperature. Features and trends are found to be consistent with flow visualisation evidence, Laser Doppler Anemometry data, and also computations of other workers.     

Time evolution of double-diffusive convection in a vertical cylinder with radial temperature and axial solutal gradients

The characteristics of transient double-diffusive convection in a vertical cylinder are numerically simulated using a finite element method. Initially the fluid in the cavity is at uniform temperature and solute concentration, then constant temperature and solute concentration, which are lower than their initial values, are imposed along the sidewall and bottom wall, respectively. The time evolution of the double-diffusive convection is investigated for specific parameters, which are the Prandtl number, Pr = 7, the Lewis number, Le = 5, the thermal Grashof number, Gr_T = 10⁷, and the aspect ratio, A = 2, of the enclosure. The objective of the work is to identify the effect of the buoyancy ratio (the ratio of solutal Grashof to thermal Grashof numbers: N = Gr_S/Gr_T) on the evolution of the flow field, temperature and solute field in the cavity. It is found that initially the fluid near the bottom wall is squeezed by the cold flow from the sidewall, a crest of the solute field forms and then pushed to the symmetry line. In the case of N > 0, a domain with higher temperature and weak flow (dead region) forms on the bottom wall near the symmetry line, and the area of dead region increases when N varies from 0.5 to 1.5. More crests of the solute field are formed and the flow near the bottom wall fluctuates continuously for N < 0. The frequency of the fluctuation increases when N varies from −0.5 to −1.5. Corresponding to the variety of the thermal and solutal boundary layers, the average rates of heat transfer (Nu) at the sidewall remain almost unchanged while the average rates of mass transfer (Sh) at the bottom wall change much in the cases of N = 1, 0, −1.     

Thermal impedance and transient temperature due to a spot of heat on a half-space

Analytical solutions are proposed in this paper to calculate the thermal impedance and transient temperature in a semi-infinite body subjected to a heat spot. Two cases of uniform and non-uniform heat flux are considered. These solutions are developed using integral transforms and are given in exact expression forms, without any restrictive hypothesis. They include special functions such as Bessel, Struve and hypergeometric functions. Many softwares to treat these functions are available (eg. Maple, Mathematica and others). The solutions are validated through comparisons with available models treating particular cases. The transient temperatures and impedances are presented for different spatial distributions of the heat flux dissipated by the spot.     

Mechanical stresses in fixed-speed wind turbines due to networkdisturbances

Electrical faults and other network disturbances generate transients of the generator electromagnetic torque which result in significant stresses for the wind turbine (WT) mechanical system and may also have a detrimental effect on the fatigue life of important drive train components, such as the gearbox. In this paper, the effect of faults and other disturbances, common in rural medium voltage (MV) distribution systems, is analyzed for a typical fixed speed WT, equipped with an induction generator. The resulting torque transients are evaluated and their propagation in the drive train is examined. Important conclusions are drawn with respect to the type of electrical faults to be considered in the design and certification of WTs according to the current IEC 61400-1 Standard. Although the emphasis is placed on the developed torque transients, the methodology presented is also suitable for analyzing electrical stresses in the generator, due to overcurrents or overvoltages     

Investigations on an Axial Flow Fan Stage subjected to Circumferential Inlet Flow Distortion and Swirl

INTRODUCTIONInletfiowdistortionisatermusedtodenotethewriationofflowpropertiesasafunctionofthespa-tialcoordinatesandtime.Thenonuniformityoc-cursinconunonflowpropertiessuchastotalpressure,staticpressure'velocity,temperature,flowangleandgasconstituency.Swirlisthetermusedtodescribetherotationoffiowinrelationtotherotationoftheforor.Swirlmaybegeneratedasaconsequenceofsec-ondaryflowsinducedduetoflowcurvaturesorduetothepresenceofInletGuideVanes(IGV).DistortionandSwirl(inducedduetocurvatureoffl…     

Uneven temperature and voltage distributions due to rapid discharge rates and different boundary conditions for series-connected LiFePO4 batteries

This paper presents the surface temperature and voltage distributions on a prismatic lithium-ion battery pack at 1C, 2C, 3C, and 4C discharge rates and 5 °C, 15 °C, 25 °C, and 35 °C boundary conditions (BCs) for water cooling and ~ 22 °C for air cooling methods. It provides quantitative data regarding thermal behaviour of lithium-ion batteries for designing thermal management systems and developing reliable thermal models. In this regard, three large LiFePO₄ 20 Ah capacity, prismatic batteries are connected in series with four cold plates used between cells and eighteen thermocouples are placed at distributed locations on the principle surface of all three cells: the first six for the first cell, the second six for the second cell, and the third six for the third cell, and the average and peak surface temperatures as well as voltage distributions are measured and presented in this study. In addition, the simulated heat generation rate, temperature and voltage distributions are validated with an experimental data for the above mentioned C-rates and BCs. The present study shows that increasing discharge rates and BCs results in increase in the maximum and average surface temperatures at the three locations (near the anode, cathode, and mid surface of the body). The highest value of the average surface temperature is obtained for 4C and 35 °C BC (36.36 °C) and the lowest value is obtained for 1C and 5 °C BC (7.22 °C) for water cooling method.     

Remarks on wind turbine power absorption increase by including the axial force due to the radial pressure gradient in the general momentum theory

It is shown that the general momentum theory, in the form that it has been published previously, underestimates the axial force and the absorbed power, possibly by a large amount if the expansion of the wake is large. Consequently, the Betz limit, theoretically, can probably be exceeded by a correspondingly large amount. Copyright © 2006 John Wiley &Sons, Ltd.     

Some design aspects related to stresses and strains in high temperature piping

This paper is concerned with the differences which exist between piping design procedures for normal temperature usage and those for high temperatures. Some additional modes of failure must be considered in high temperature design, such as creep and creep ratchet, creep relaxation and elastic follow up.The paper discusses possibilities of calculating the influence of the high temperature effects in relation to the analysis of stresses and strains in piping systems. Stress categories and strain limits are developed with respect to typical systems in a sodium-cooled reactor and to steam piping in a gas-cooled high temperature power plant. Rules for dealing with high temperature piping are proposed, the intention being to ensure that these provide the same level of safety as that provided for class 1 piping in water-cooled reactors.     

Thermal wave propagation in a bi-layered composite sphere due to a sudden temperature change on the outer surface

The dynamic thermal behavior of a bi-layered composite sphere due to a sudden temperature change on the outer surface is investigated. The analytical–numerical technique, which is based on the Laplace transformation and the Riemann-sum approximation, is employed to predict the temperature and heat flux histories in the composite sphere. The effects of different parameters such as the relaxation time, the thermal diffusivity ratio, the thermal conductivity ratio, the relaxation time ratio, and the radius ratio of the inner and outer layers of the composite sphere are studied and presented.     

Experimental investigation of stresses in cold-bent pipes due to internal pressure and in-plane bending

Experimental measurements have been made of stresses in out-of-round pipe bends under internal pressure and in-plane bending. This paper describes the results of tests on one new and one ex-service pipe bend under the two loadings and compares these results with various theoretical predictions. For the pressure case the original formula due to Haigh² with modifications to take into account thickness variations and pipe bend radius, is reasonably accurate and for bending loads the recent formulation by Spence and Boyle¹² is a reasonable approximation. The code method of combining peak stresses by addition is confirmed in this case. The results of the tests have assisted designers in reviewing allowable limits on the ovality of manufactured pipes and in placing realistic limits on the cold springing of pipes to overcome erection tolerances.     

Transient temperature distributions in spherical and cylindrical food products subjected to hydrocooling

Analytical models were proposed to estimate the dimensionless temperature distributions of spherically and cylindrically shaped objects during cooling. In order to test the analytical model, an experimental investigation was carried out to measure temperatures at the centre, half radius and surface positions of the spherical and cylindrical products (pears and eggplants) subjected to hydrocooling. The measured temperature distributions were compared with the computed temperature distributions and very good agreement was observed.     

Thermal and mechanical stresses in a functionally graded thick sphere

In this paper, a general solution for the one-dimensional steady-state thermal and mechanical stresses in a hollow thick sphere made of functionally graded material is presented. The temperature distribution is assumed to be a function of radius, with general thermal and mechanical boundary conditions on the inside and outside surfaces of the sphere. The material properties, except Poisson's ratio, are assumed to vary along the radius r according to a power law function. The analytical solution of the heat conduction equation and the Navier equation lead to the temperature profile, radial displacement, radial stress, and hoop stress as a function of radial direction.     

Thermal propagation in solids due to surface laser pulsation and oscillation

Most studies of heat transfer pertaining to a planar medium subjected to time-varying and spatially-decaying laser incidence along with external surface cooling are based on the diffusion theory (parabolic heat conduction equation), an approximation that implies a non-physical infinite speed of energy transport. In this study, temperature distributions within one-dimensional plates subjected to the aforementioned heating and cooling conditions, but with thermal energy propagation at a finite speed, are presented. Incident energy that is partially absorbed at the external surface is transferred through the plate by conduction, while the remaining energy is convectively cooled to the environment. The present investigation will examine three different time characteristics of the incident heat sources which include a continuously operating, a pulsed and an oscillatory laser source. The temperature results were obtained by using a simple and concise finite-difference algorithm based on the Godunov scheme, a method developed for the solution of resulting characteristic equations that govern thermal wave propagations within the solid.     

Mechanical and thermal stresses in a functionally graded hollow cylinder due to radially symmetric loads

In this paper, a general analysis of one-dimensional steady-state thermal stresses in a hollow thick cylinder made of functionally graded material is developed. The temperature distribution is assumed to be a function of radius, with general thermal and mechanical boundary conditions along the inside and outside surfaces. The material properties, except Poisson's ratio, are assumed to depend on variable the r and they are expressed as power functions of r. The direct method is used to solve the heat conduction and Navier equations.     

The effect of radial temperature gradient and axial magnetic field on the stability of couette flow: The narrow gap problem

A numerical solution to the MHD stability problem for dissipative Couette flow in a narrow gap is presented under the following conditions: (i) the inner cylinder rotating with the outer cylinder stationary, (ii) corotating cylinders, (iii) counter-rotating cylinders, (iv) an axially applied magnetic field, (v) conducting and nonconducting walls, and (vi) the presence of a radial temperature gradient. Results for the critical wave number a_c, and the critical Taylor number T_c, are presented. The variation of T_c is shown on graphs for both the conducting and nonconducting walls and for different values of ±μ (= Ω₂/Ω₁, where Ω₂ is the angular velocity of the outer cylinder, and Ω₁ is the angular velocity of the inner cylinder), the magnetic field parameter Q, which is the square of the Hartmann number and ± N (= Ra/Ta, where Ra is the Rayleigh number). The effects of ±μ, N and Q on the stability of flow are discussed. It is seen that the effect of the magnetic field is to inhibit the onset of instability, this being more so in the presence of conducting walls and a negative temperature gradient.     

Surface temperature calculations in radiant surroundings of arbitrary complexity—for gray, diffuse radiation

The question of calculating the temperatures of opaque and non-opaque surfaces subject to assigned net rates of radiant flux is considered. It is shown that, for gray surfaces, any opaque surface in radiant balance in an enclosure of arbitrary complexity achieves a steady state temperature which is independent of the emissivity (or absorptivity) of the surface. Relations are developed for the temperature of opaque and non-opaque surfaces subjected to an assigned net radiant flux.