Magnitude Of Temperature Gradient Research Articles

Effect of thermal expansion on shape memory behavior of polyurethane and its nanocomposites

AbstractThe effects of thermal expansion on shape memory performance of shape memory polyurethanes and their nanocomposites with organoclay, carbon nanofiber (CNF), silicon carbide (SiC), and carbon black (CB) were evaluated. The shape memory test cycle involved tensile deformation at above the trigger temperature to initiate shape memory function, cooling to room temperature to fix the shape, and shape recovery induced by heating to above the trigger temperature. Phenomenological models were used to interpret the experimental data on coefficient of thermal expansion (CTE). It was found that Kerner model showed good fit for composites of SiC and CB, and Halpin model gave better fit for composites of organoclay and CNF. It was observed that thermal expansion exerts negative effect on recovered strain, the extent of which depends on the magnitude of temperature gradient, CTE, and the level of tensile strain. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1437–1449, 2008

X-ray diffraction in single crystals under the influence of temperature gradient

In Bragg condition and Laue geometry specific interest are presented coherent processes in optics of x-ray when external influences are applied to the monocrystal. As an example of such coherent processes may serve total pumping x-rays from the direction of propagation to the reflection direction [1], diffraction focusing and defocusing of x-rays under temperature gradient [2], the phenomenon of transparency of the crystal under influence of ultrasound [3], etc. The Laue diffraction of narrow collimated (collimation angle ≈3′′) polychromatic X-ray radiation in single quartz and KDP crystals under the influence of temperature gradient is studied in present work. It is shown, that the deformation field distribution in the crystal stimulated by the external fields can be determined from the polychromatic beam spectral distribution. Due to the relative layout of diffraction g and temperature gradient B vectors the geometrical focusing B ↑↓ g and defocusing B↑↑ g of the reflected beams is obtained. A principle of the new type of x-ray spectrometry is considered for which a linear and angular dispersion depends on the magnitude of temperature gradient.

Influences of a temperature gradient and fluid inertia on acoustic streaming in a standing wave

Following the experimental method of Thompson and Atchley [J. Acoust. Soc. Am. 117, 1828-1838 (2005)] laser Doppler anemometry (LDA) is used to investigate the influences of a thermoacoustically induced axial temperature gradient and of fluid inertia on the acoustic streaming generated in a cylindrical standing-wave resonator filled with air driven sinusoidally at a frequency of 308 Hz. The axial component of Lagrangian streaming velocity is measured along the resonator axis and across the diameter at acoustic-velocity amplitudes of 2.7, 4.3, 6.1, and 8.6 m/s at the velocity antinodes. The magnitude of the axial temperature gradient along the resonator wall is varied between approximately 0 and 8 K/m by repeating measurements with the resonator either surrounded by a water jacket, suspended within an air-filled tank, or wrapped in foam insulation. A significant correlation is observed between the temperature gradient and the behavior of the streaming: as the magnitude of the temperature gradient increases, the magnitude of the streaming decreases and the shape of the streaming cell becomes increasingly distorted. The observed steady-state streaming velocities are not in agreement with any available theory.

A Detailed Thermal Analysis of the Binospec Spectrograph

ABSTRACTRefractive optics in astronomical instruments are potentially sensitive to temperature gradients and temperature transients. This sensitivity arises from thermally dependent refractive indices, lens spacings, and lens dimensions. In addition, thermal gradients in the instrument structure can cause undesirable image shifts at the detector that degrade instrument calibration. We have therefore undertaken a detailed thermal analysis of Binospec, a wide‐field optical spectrograph under development for the converted Multiple Mirror Telescope (MMT). Our goals are to predict the temperature gradients that will be present in the Binospec optics and structure under realistic operating conditions and to determine how design choices affect these gradients. We begin our analysis by deriving thermal time constants for instrument subassemblies to estimate the magnitude of temperature gradients in the instrument and to determine where detailed thermal models are required. We then generate a low‐resolution finite‐difference model of the entire instrument and high‐resolution models of sensitive subassemblies. This approach to thermal analysis is applicable to a variety of other instruments. We use measurements of the ambient temperature in the converted MMT's dome to model Binospec's thermal environment. In moderate conditions, the external temperature changes by up to 8°C over 48 hr, while in extreme conditions the external temperature changes by up to 17°C in 24 hr. During moderate conditions, we find that the Binospec lens groups develop ∼0.14°C axial and radial temperature gradients and that lens groups of different mass develop ∼0.5°C temperature differences; these numbers are doubled for the extreme conditions. Internal heat sources do not significantly affect these results; heat flow from the environment dominates. The instrument must be periodically opened to insert new aperture masks, but we find that the resulting temperature gradients and thermal stresses in the optics are small. Image shifts at the detector caused by thermal deflections of the Binospec optical bench structure are ∼0.1 pixel hr-1. We conclude that the proposed Binospec design has acceptable thermal properties, and we briefly discuss design changes to further reduce temperature gradients.

Two-dimensional instability study of a thermoacoustic prime-mover

A numerical model aimed at investigating the development of instability in a thermoacoustic prime-mover is presented. It is based on a discretization of the two-dimensional compressible Navier–Stokes equations, and does not rest on the plane pressure wave assumption. The computational domain is made of a single stack interplate space extended up to both resonator ends. The working gas is assumed to be initially at rest, and a longitudinal temperature gradient is enforced. The linear instability properties of the system are then given by the eigenmodes of the discretized evolution operator linearized in the vicinity of the above basic state. Temporal growth rate and oscillation frequency are thus obtained for all thermoacoustic modes, and the prime-mover instability onset is determined as a function of the various parameters (mainly temperature gradient magnitude and mean pressure). As far as linear instability is concerned, this formulation is attractive, since it avoids any direct numerical simulation and any problem involved by the presence of two very different time scales.

Instability in a thermoacoustic prime-mover: A numerical approach

A numerical model aimed at investigating the development of instability in a thermoacoustic prime-mover is presented. It is based on a discretization of the two-dimensional compressible Navier–Stokes equations, and does not rest on the plane pressure wave assumption. The computational domain is made of a single stack interplate space extended up to both resonator ends. The working gas is assumed to be initially at rest, and a longitudinal temperature gradient is enforced. The linear instability properties of the system are then given by the eigenmodes of the discretized evolution operator linearized in the vicinity of the above basic state. Temporal growth-rate and oscillation frequency are thus obtained for all thermoacoustic modes, and the prime-mover instability onset is determined as a function of the various parameters (mainly temperature gradient magnitude and mean pressure). As far as linear instability is concerned, this formulation is attractive, since it avoids any direct numerical simulation and any problem involved by the presence of two very different time scales.

InGaAs mixed crystals grown by the temperature-difference method and the numerical analysis of solution convection

InxGa1 − xAs (x = 0.045) ternary mixed crystals were grown on GaAs seeds from an In-Ga-As solution by the temperature-difference method. The In-compositional profiles were very uniform in the crystals. The lateral profiles of the growth rate measured along the diameter changed from concave to convex with an increase in the temperature gradient of a GaAs seed near the In-Ga-As solution.To investigate the effect of temperature gradients on the growth rate, both flow patterns and compositional profiles in the solution were calculated by solving four equations: Navier-Stokes, continuity, energy, and solute diffusion. The magnitude of temperature gradient brought about changes of heat flux, flow patterns, and consequently growth rate. The calculated growth rates were qualitatively in agreement with the experimental ones.

Thermal conductivity measurements on saturated rocks at permafrost temperatures

A simple divided-bar apparatus for studying the thermal conductivity of rocks and soils at permafrost temperatures is described. The apparatus is intended also for use with permafrost specimens tested in their natural frozen state. It has been designed for low heat fluxes and consequent small temperature drops across the test specimen, while retaining a high precision of measurement.Different thicknesses of two water-saturated sandstones and a limestone have been tested as a function of changes in temperature, frozen and unfrozen. Significant corrections for the contact resistance between the specimen and divided bar were determined. The influence of the magnitude of temperature gradient across the specimen was not found to be important for the low gradients employed in these experiments.The measured thermal conductivities show a slight decrease in value with temperature increasing to 0 °C. In the neighbourhood of 0 °C, a sharp reduction in thermal conductivity was observed as the ice–water phase change occurred. It is clear that the magnitude of the thermal conductivity of the rocks depends on their mineral content; quartz, with its high thermal conductivity, plays the predominant role where it is present.

Effect of toroidal gradient drifts on the dissipative trapped-ion instability

Conditions for the onset of the dissipative trapped-ion instability in tokamaks with elliptic and circular cross sections are determined. Toroidal gradient drifts, ignored in earlier calculations, are included in the analysis and found to give rise to significant stabilizing effects. In particular, the results show that the ions trapped in regions of good gradient (dB/dρ > 0) contribute to the Landau damping of the unstable waves, and that ion collisional damping is also enhanced when the drifts are taken into account. Since vertical ellipticity increases the region of good gradient, these stabilizing effects are further enhanced in elliptical cross-section tokamaks. It is also shown that previously calculated restrictions on the allowable magnitude of temperature gradients for favorable damping can be relaxed. Results of the analysis are applied to expected parameters for the Princeton Large Torus experiment and growth rate estimates given for conditions violating the stability criteria.

Measuring And Modelling Thermal Distortion On CNC Machine Tools

Heat sources internal and external to a machme tool cause temperature gradients to arise due to resistance to heat flow in and around the machine tool and workpiece. This causes linear expansion and distortion of the structural elements from which the machine tool is constructed. The connectivity of the structural elements sets the effect of the expansion and distortion on the relative positions of the tool and workpiece, leading to thermal errors on the workpiece. Measurement on a wide range of machine tools confirms that temperature gradients are significant in their effect on machining accuracy and that they move and change shape during the machining process. An analysis of machine tool structures and connectivity shows that both the position and magnitude of temperature gradients is important in its effect upon the relative movement of the tool and workpiece. A bending model that estimates the effects of thermal distortion using knowledge of the position of the temperature gradient is derived. The performance of the bending model is compared with a finite element analysis model and a model that has no knowledge of the position of temperature gradients. Results obtained from a vertical machining centre show that knowledge of the position and magnitude of temperature gradients is an essential part of predicting thermal distortion accurately.