Range Of Temperature Differences Research Articles

A Resistance Deviation-to-Pulsewidth Converter for Resistive Sensors

A resistance deviation-to-pulsewidth converter is presented for interfacing resistive sensors. It consists of a ramp integrator, two resistance-tunable Schmitt triggers, and two logic gates. A prototype circuit built using discrete components exhibits a resolution as high as 14 bits and a linearity error less than plusmn0.06% when the output pulse is counted by a 10-MHz clock signal. The proposed circuit is applied to measure the temperature difference with the platinum resistance temperature detectors. The measured conversion sensitivity of the temperature difference-to-pulsewidth converter is 5.74 mus/degC, and its linearity error is less than plusmn1% in the temperature difference range of 0degC to 100degC.

Linear mixing in thermal infrared temperature retrieval

Virtually all remotely sensed thermal infrared (IR) pixels are, to some degree, mixtures of different materials or temperatures: real pixels are rarely thermally homogeneous. As sensors improve and spectral thermal IR remote sensing becomes more quantitative, the concept of homogeneous pixels becomes inadequate. Quantitative thermal IR remote sensors measure radiance. Planck's Law defines a relationship between temperature and radiance that is more complex than linear proportionality and is strongly wavelength‐dependent. As a result, the area‐averaged temperature of a pixel is not the same as the temperature derived from the radiance averaged over the pixel footprint, even for blackbodies. This paper uses simple linear mixing of pixel elements (subpixels) to examine the impacts of pixel mixtures on temperature retrieval and ground leaving radiance. The results show that for a single material with one temperature distribution and with a subpixel temperature standard deviation of 6 K (daytime images), the effects of subpixel temperature variability are small but can exceed 0.5 K in the 3–5 µm band and about a third of that in the 8–12 µm band. For pixels with a 50 : 50 mixture of materials (two temperature distributions with different means) the impact of subpixel radiance variability on temperature retrieval can exceed 6 K in the 3–5 µm band and 2 K in the 8–12 µm band. Sub‐pixel temperatures determined from Gaussian distributions and also from high‐resolution thermal images are used as inputs to our linear mixing model. Model results are compared directly to these broadband thermal images of plowed soil and senesced barley. Finally, a theoretical framework for quantifying the effect of non‐homogeneous temperature distributions for the case of a binary combination of mixed pixels is derived, with results shown to be valid for the range of standard deviations and temperature differences examined herein.

A new experimental technique for measuring the dynamical free surface deformation in liquid bridges due to thermal convection

The dynamical free surface deformation due to thermal (Marangoni-buoyant) convection in liquid bridges of 5 cSt silicone oil was studied experimentally using optical imaging. The experiments were conducted under ground-based conditions for a wide range of temperature differences between the solid supports, and for several liquid bridge volumes. Oscillatory motion set in at a critical value of the temperature difference and caused oscillation of the free surface. The amplitude and fundamental frequency characterizing the magnitude and shape of that oscillation were obtained by optical imaging. The procedure allows one to describe the oscillation up to sub-micron order. The temperature oscillations at five points inside the liquid bridge were measured by five thermocouples. Their amplitude and fundamental frequency were compared with the corresponding values of the free surface oscillations. The optical imaging revealed the same dynamics as was obtained from the (more intrusive) temperature measurements. The spatial structure of the free surface oscillations along the liquid bridge axis was also analysed.

Initial smoothness of concrete pavements under environmental loads

In the current paper, the effect of curling and warping, caused by environmental ambient conditions, on the initial smoothness of concrete pavements is discussed. Surface profile measurements were made during the hours of the early morning and late afternoon on an instrumented jointed plain concrete pavement (JPCP) on highway US-30 near Marshalltown, Iowa during the first seven days after construction in summer 2005. Variations in temperature and moisture during this critical period were monitored using the temperature and relative humidity sensors installed within the test sections at the time of construction. Based on the measured surface profile data, it was observed that the initial pavement smoothness, in terms of international roughness index (IRI) and the ride number (RN), was not significantly influenced by the early-age curling and warping behaviour of the JPCP. Using finite-element modelling (FEM), sensitivity studies were conducted to investigate the influence of slab curvature on initial pavement smoothness for a range of equivalent temperature differences between the top and bottom of the slab. The results indicated that the initial JPCP smoothness is sensitive to changes in slab curvature resulting from environmental ambient conditions only at higher magnitudes. Although the FEM-based IRI predictions were higher than the surface profile-based IRI values, the differences were not significant.

Experimental and numerical study of Marangoni convection in shallow liquid layers

An experimental and numerical study of thermal Marangoni convection in shallow liquid layers was carried out for a range of temperature differences and layer depths. This was done to permit earth based experiments to be undertaken in situations where Marangoni convection dominated the flow. Particle image velocimetry (PIV) was used to obtain the flow patterns and velocity vectors. The experimental results were compared to numerical models created using FLUENT V6. Both results are in good agreement. The liquid free surface profile due to the presence of the menisci is shown to be critical for good quantitative validation. The layer depths are also proven to be shallow enough for Marangoni convection to dominate over buoyancy under normal gravity conditions.

The effect of condenser heat transfer on the energy performance of a plate heat pipe solar collector

For a novel prototype solar collector, using a plate heat pipe, condenser heat transfer was analysed in detail. The condenser has the shape of a rectangular channel. Flow and heat transfer of water in the rectangular channel was modelled and the heat transfer coefficient assessed, using the Fluent code. Under typical operating conditions a mixed convection situation occurs. The channel is inclined and heating is through one wall only (upper channel surface). The range of temperature differences considered was similar to the one verified under real operating conditions, covering a wide range of Grashof numbers. Results showed that the Nusselt number is significantly higher than the one for forced convection in a rectangular channel with fully developed boundary layers. In order to enhance heat transfer, a modification to the rectangular channel was analysed, using baffles to improve flow distribution and increase velocity. The effect of this modification on collector energy performance (efficiency) was assessed. Copyright © 2005 John Wiley & Sons, Ltd.

The growth and structure of double-diffusive cells adjacent to a cooled sidewall in a salt-stratified environment

Observations and measurements are reported on the patterns and rates of growth in time of the double-diffusive cells that form adjacent to a cooled sidewall in a salt-stratified environment. Fluid near the wall is cooled and sinks a distance h where its density, increased by cooling, matches that of the salt-stratified ambient. The fluid separates from the wall, moving outwards as a cool, fresher layer beneath a warmer, more saline region. This leads to growing double-diffusive cells that advance outward at a rate, found by dimensional reasoning, to initially be proportional to N(0)h, where N-0 is the initial buoyancy frequency in the ambient and h is the intrusion's vertical thickness. Near the wall at the top of each cell, the sinking colder fluid is continually replaced by selective withdrawal from the ambient 'far field'. The fluid being withdrawn from the ambient is always the least dense in the cell, and as the experiment proceeds, the straining of the fluid in the ambient region reduces the stratification. The vertical density gradient inside the cell relaxes by continuous hydrostatic adjustment (CHA) to match the ambient and the speed of advance reduces. Measurements of the rate of advance of the cell nose were made in tanks of different lengths L with a range of initial salinity gradients and temperature differences. A simple two-dimensional model is developed to describe the rate of extension of the cells and the internal density gradient as functions of time in which the tank length appears as an important variable. This effect does not seem to have been recognized previously. The rates of evolution in each run involve the time scale tau = L/(C(H)hN(0)), where C-H approximate to 10(-2) is a heat transfer coefficient. The mean length of the cells (l) over bar (t) and the internal buoyancy frequency as functions of time are given by(l) over bar (t)/L = t/tau - (t/2tau)(2), N = N-0(1 - t/2tau).Inversion of the first of these expressions results in t/tau = 2 - 2{1 - ((l) over bar (t)/L)}(1/2) from which a time scale tau(-1) can be estimated. The measurements from individual runs when plotted in this way generally produce accurate straight lines as the model predicts, from which C-H is found. This should be approximately the same for each run; the mean over all runs was found to be 9.3 x 10(-3) with standard deviation 2.4 x 10(-3). The velocity scale of the intrusions at the beginning of an experiment is of order 10(-2) cm s(-1), for typical parameters of water at temperature 20 degreesC, cooled wall temperature of 0 degreesC and mean salinity of 5%.

Effect of a temperature difference between the cylinders on the Taylor–Couette bifurcation in a gas

A gas in a time-independent state between rotating coaxial circular cylinders with different temperatures is considered. The effect of the temperature difference on the Taylor–Couette bifurcation in the continuum limit is studied on the basis of the fluid-dynamic-type equations and their boundary conditions derived from the Boltzmann system. The behavior of the bifurcation point for a wide range of temperature differences is presented, and the mechanism of the shift of the bifurcation point owing to a small temperature difference is clarified analytically. The effect of the temperature difference on the bifurcation cannot be explained correctly by the guess by the analogy with the Bénard problem.

Porous structure evolution of cellulose carbon fibres during heating in the initial activation stage

This paper is focused on the description of changes in the porous structure during fast heating to the activation temperature of the viscose fibres, pyrolysed to different final temperatures. Standard regenerated cellulose fibre structures were tested. Fabrics were subjected to pyrolysis, the samples being heated to final temperatures of 400, 600 and 850 °C. Carbon fibres were subsequently heated to activation temperature (850 °C) at a rate of 100 °C/min, and then the samples were cooled down. The characteristics of obtained carbon preparations were examined. We have defined a level of restructuring and internal ordering of fibres which originated during slow pyrolysis as well as the range of temperature differences of pyrolysis and activation where fast increase of carbon fibre temperature before activation is advantageous for the development of porous structure. It allows for partial release of pores and fast rebuilding of structure accompanied by a considerable number of defects in the carbon matrix with higher reactivity to oxidiser which, in turn, promotes the development of pores in active carbon during oxidation. Temperature difference for viscose carbon fibres is approximately 150–300 °C at pyrolysis temperature of 550–700 °C.

Modes of reaction front propagation from hot spots

The results of computations with detailed chemical kinetic schemes for the autoignition of stoichiometric H 2-CO-air and H 2-air mixtures at high pressure and high temperature are reported, with and without a single hot spot. Autoignition delay and excitation times first are computed in zero-dimensional, homogeneous mixture, simulations. Spherical hot spots of three different radii are then studied, for a range of temperature differences between the centre of the hot spot and the surrounding mixture. The effects of the resulting localised initial temperature gradients on the propagation modes of the ensuing reaction waves are examined, with particular regard to possible transitions to a developing detonation. Five modes of reaction front propagation are identified and demonstrated. One mode involves normal flame deflagration, the other four involve different types of hot spot autoignition. These modes depend upon the value of the initial hot spot temperature gradient normalised by the critical temperature gradient for a developing detonation. The latter is conveniently obtained from the homogeneous computations. Upper and lower limits of this normalised temperature gradient, ξ, are observed for a developing detonation. The bounds for this also depend upon the ratio of the hot spot acoustic time to the heat release rate excitation time. A tentative first attempt is described to quantify the bounds for all the modes, in terms of the two dimensionless groups.

Climate warming experiments: Design of a mesocosm heating system

Research into the impacts of climate change on lakes requires novel experimental methods that enable realistic tests of the effects of increased water temperatures on communities. This article describes the design of a heating system that has been used in situ to study the effects of an increase in lake surface temperatures on littoral communities. Water within four 700‐L enclosures was heated using a propane‐fuelled heat exchange system. Hot water was circulated through a network of heat exchange pipes nested in the bottom of enclosures and temperature within the enclosures was controlled electronically by regulating water flow through a series of valves. The system performed well, with temperatures within warmed enclosures paralleling diurnal fluctuations within control enclosures. The average temperature difference between warm and control enclosures of 4.5°C was close to our target temperature difference of 5°C. Strengths of the experimental system are discussed and potential improvements, including improved heat retention and a design adjustment to facilitate repair in the event of lightning damage are noted. The system is adaptable to larger and smaller volumes, different temperature regimes, and can be adapted for use in pelagic systems.

PRESSURE EFFECTS ON NATURAL CONVECTION FOR NON-BOUSSINESQ FLUID IN A RECTANGULAR ENCLOSURE

Numerical predictions of pressure effects on natural convection for a non-Boussinesq fluid in the rectangular enclosures are presented. A solution method based on a compressible flow model is employed to simultaneously determine the absolute pressure, density, temperature, and velocity distributions in the enclosures. Discretization equations are derived from the integral mass, momentum, and energy equations on a staggered grid. The fluid pressure in the enclosure is varied from 20 to 300 kPa such that the flow behavior in a vacuum or pressurized system can be observed. Physical situations investigated also include cases in a wide range of wall temperature difference associated with respective length scales, corresponding to an equivalent modified Rayleigh number ranging from 10 4 to 10 6 . The validity of the incompressible flow model coupled with the Boussinesq approximation for the fluid density, which is commonly used in the existing studies of the buoyant flows, is discussed.

Nonlinear stability of mixed convection flow under non-Boussinesq conditions. Part 1. Analysis and bifurcations

The weakly nonlinear theory for modelling flows away from the bifurcation point developed by the authors in their previous work (Suslov & Paolucci 1997) is generalized for flows of variable-density fluids in open systems. It is shown that special treatment of the continuity equation is necessary to perform the analysis of such flows and to account for the potential total fluid mass variation in the domain. The stability analysis of non-Boussinesq mixed convection flow of air in a vertical channel is then performed for a wide range of temperature differences between the walls, and Grashof and Reynolds numbers. A cubic Landau equation, which governs the evolution of a disturbance amplitude, is derived and used to identify regions of subcritical and supercritical bifurcations to periodic flows. Equilibrium disturbance amplitudes are computed for regions of supercritical bifurcations.

Nonlinear stability of mixed convection flow under non-Boussinesq conditions. Part 2. Mean flow characteristics

Based on amplitude expansions developed in Part 1 (Suslov & Paolucci 1999), we examine the mean flow characteristics of non-Boussinesq mixed convection flow of air in a vertical channel in the vicinity of bifurcation points for a wide range of temperature differences between the walls, and Grashof and Reynolds numbers. The constant mass flux and constant pressure gradient formulations are shown to lead to qualitatively similar, but quantitatively different, results. The physical nature of the distinct shear and buoyancy disturbances is investigated, and detailed mean flow and energy analyses are presented. The variation of the total mass of fluid in a flow domain as disturbances develop is discussed. The average Nusselt number and mass flux are estimated for supercritical regimes for a wide range of governing parameters.

Thermal membrane potential across anion-exchange membranes with different benzyltrialkylammonium groups

The thermal membrane potentials were measured across poly(styrene)-based copolymer anion-exchange membranes with 4.5 and 8% divinylbenzene having anion-exchange groups with different hydrophobicity for various electrolyte solutions. Linear relationships between the membrane potential (Δ ψ ) and the temperature difference (Δ T ) or the logarithmic temperature difference (Δ ln T ) were observed when the average temperature T av was kept at 308.2 K and Δ T varied from −10 to +10 K. The values of Δ ψ /Δ T against log a ± decreased with increasing hydrophobicity of the ion-exchange groups. The mean transported entropy of the counterions s ̄ ̄ − in the membrane was estimated by combining data for the thermal membrane potential, thermo-osmosis, and electro-osmosis. For hydrophilic IO − 3 -form membranes having relatively high water content, the water term (Δ ψ /Δ T ) 0 is predominant in Δ ψ /Δ T . The equation derived from a set of entropy and coupled absolute temperature difference was a good approximation in the range of small temperature differences.

Extended performance measurements for SETAC

The Shipboard Electronics ThermoAcoustic Cooler (SETAC) was built at the Naval Postgraduate School in Monterey, CA. In a demonstration at sea aboard the U.S.S. Deyo in April 1995, SETAC was used to provide 419 watts of useful cooling power across a stack temperature difference Thot−Tcold=30 °C with an acoustic input power of 227 watts. New data spanning a wider range of temperature difference and heat load will be presented and compared to a DeltaE model. [Work supported by the Office of Naval Research.]

Time-dependent natural convection in a glass melting furnace

The main purpose of this study is to determine bifurcation as the primary instability of a glass melting furnace. Steady-state and unsteady characteristics of natural convection in the partially open cavity as appeared in a glass melting furnace is investigated by using numerical analysis. Three types of convection, such as steady laminar, unsteady periodic or unsteady quasi-periodic convection may occur according to the temperature difference between upper two isothermal surfaces along the depth of cavity in a glass melting furnace. In the temperature difference of 150-900 K between batch and free surface, the larger the temperature difference, the weaker the convection strength and unsteadiness. Since the glass viscosity is increasing exponentially in the lower temperature, the batch freezes the thermofluidic field especially below the surface of it. If the depth of cavity is 0.5 m, the bifurcation to time-dependent natural convection may occur in the range of 60-650 K. If that is 1.0 m, it may occur in the whole range of temperature difference.

Mixed convection in straight and curved channels with buoyancy orthogonal to the forced flow

Mixed convection in straight and curved channels is investigated when buoyant forces are perpendicular to the bulk flow direction. Grashof numbers are based on temperature difference range from 0 to 50 × 10 6, and Reynolds numbers are based on channel width range from 343 to 2057 which gives laminar and transitional forced flow. Four difference regimes of behavior are apparent: pure natural convection, mixed convection with strong buoyancy, mixed convection with weak buoyancy and pure forced convection. Differences in behavior between these different regimes are discussed, along with curvature influences, the effects of buoyancy on forced flow behavior, and Nusselt number correlations for the two mixed convection regimes.

Measurements in buoyancy-opposing laminar flow over a vertical forward-facing step

Measurements are reported for buoyancy-opposing laminar mixed convection flow over a vertical, two-dimensional forward-facing step, in which the upstream wall of the step simulated an adiabatic surface, while both the step and the downstream wall from the step were heated to a uniform temperature that was higher than the approaching air temperature. Results presented in this paper are for a step height of 0.8 cm, over a range of free stream velocities 0.39 m s −1 ≤ u ∞≤ 0.71 m s −1 and a range of temperature differences 0°C ≤ ΔT ≤ 35°C between the heated surface and the free stream. Laser-Doppler velocimeter and cold-wire anemometer were used to measure, respectively, the air velocities and the temperature distributions simultaneously. Flow visualizations were also used to observe the nature of the flow and to measure the length of the recirculation region downstream of the forward-facing step. The results reveal that the buoyancy-opposing force due to the downstream wall heating affects significantly the velocity and temperature distributions, the local Nusselt number, and the location and the size of the recirculation region downstream of the step.

Different convection dynamics in mixtures with the same separation ratio.

We report on convection near threshold in circular samples of radial aspect ratio {Gamma}=11.5 using ethanol-water mixtures with two concentrations {ital x}, but with the same separation ratio {Psi}{approx_equal}{minus}0.08. For {ital x}=0.250, convection begins with transients that lead to a time-independent cell-filling state, typically in a day. For {ital x}=0.011, there is a range of temperature differences where repeated transients and aperiodically fluctuating localized regions of disordered convection persist for many days. In rare cases these lead to an apparently stable {open_quote}{open_quote}wall{close_quote}{close_quote} state of traveling waves surrounding pure conduction. {copyright} {ital 1996 The American Physical Society.}