Arbitrary Temperature Variations Research Articles

Study and Design of a Multi-range Programmable Sensor for Temperature Measurement

In this paper, a wide-range high-precision sensor has been designed in order to accurately measure the temperature in a medium with arbitrary temperature variation and the implementation of a wide-spectrum temperature measurement system with a self-selected multi-sensor has been realized. This multi-sensor core is made up of different sensors combined to measure different temperature ranges. This concept can be used for high-precision temperature measurement in electrical capacitance tomography applications. The proposed technique is well suited for temperatures in boilers, industries, and everywhere high temperature measurement sensitivity is needed by using different combined temperature sensors of high precision.

Evaluation of activation energy of thermally stimulated solid-state reactions under arbitrary variation of temperature

The thermal effect of a reaction makes the temperature inside the reaction system deviate from a prescribed heating program. To take into account the effect of such temperature deviations on kinetic evaluations, a computational method applicable to an arbitrary variation in temperature has been developed. The method combines the isoconversional principle of evaluating the activation energy with numerical integration of the equation, dα/dt = k[T(t)]f(α), over the actual variation of the temperature with the time, T(t). Details of the numerical algorithm are reported. A model example has been used to verify the reliability of this method as compared to an analogous method which does not account for the deviations of the temperature from a prescribed program. The method has been tested for tolerance for noise in the temperature. © 1997 by John Wiley & Sons, Inc.

Mixed convection in an axisymmetric stagnation flow of micropolar fluid on a vertical cylinder

An analysis is presented to study the effects of buoyancy on forced convection in an axisymmetric stagnation flow of micropolar fluids over a vertical cylinder with arbitrary temperature variations. Numerical solutions are given for the governing momentum, angular momentum, and energy equations. Two flow regions, namely the buoyancy assisted and buoyancy opposed cases, are analyzed. It is observed that the wall shear stress and surface heat transfer rate increase or decrease with the buoyancy force parameter depending upon which flow regime is being considered. The effect of Prandtl number on the flow field in both the flow regimes is discussed.

An experimental system for the transient, non-periodic thermal analysis of structural elements

An experimental set-up is described for measurement of the dynamic behaviour of structural elements under transient, non-periodic conditions. It is composed of two insulated test chambers O and I separated by the wall to be tested. The desired outdoor conditions, i.e. any arbitrary temperature variation, are obtained within test chamber O by the use of heating and cooling coils regulated by a computer. Constant temperature is retained within test chamber I, where the indoor conditions are simulated. The computer receives measurements from thermocouples and heat-flow meters and gives correction instructions with 1 min time step. An experiment may last many days. A specially developed computer program controls the whole system. Characteristic measurements of temperatures and heat flows are provided and compared with predictions obtained by solving the same problem numerically.

Mixed convection in an axisymmetric stagnation flow on a vertical cylinder

An analysis is presented to study the mixed convection in an axisymmetric stagnation flow over a vertical cylinder with arbitrary temperature variations. Numerical solutions are given for the governing momentum and energy equations Two flow regions, namely, the buoyancy-assisted and buoyancy-opposed cases are analyzed. It is observed that the friction factor and Nusselt number increase or decrease with the buoyancy force parameter depending upon which flow regime is being considered. The effect of the Prandtl number on the flow field in both the flow regimes is discussed.

Dynamics of shell‐and‐tube heat exchangers to arbitrary temperature and step flow variations

AbstractA versatile and efficient method is developed for predicting dynamic performances of parallel and counterflow heat exchangers subject to arbitrary temperature variations and step flow disturbances, including the effect of flow maldistribution and the influence of heat capacities of both fluids, shell wall and tube bank as well as nonzero initial temperatures. Two algorithms of numerical inversion of the Laplace transform are introduced to determine the final temperature profiles in the realtime domain and some examples are calculated with nonuniform initial conditions. The accuracy of the proposed method is demonstrated with the calculated results at new steady states. Experiments are carried out on a labor‐sized heat exchanger to further examine the feasibility of this method and the comparison between calculated and measured temperature profiles is illustrated and discussed.

Calculation of the Effective Emissivities of Specular-Diffuse Cavities by the Monte Carlo Method

An algorithm of the Monte Carlo method is described which allows evaluation of the effective emissivities of isothermal and nonisothermal specular-diffuse black-body cavities for use in radiometry, photometry and optical pyrometry. The calculation provides estimates of normal spectral effective emissivity for black-body cavities, formed by cone surfaces and a cylinder. It does this for an isothermal cavity and for a cavity having an arbitrary variation of temperature along the cavity length.

Transient Thermal Stress Analysis of a Rectangular Orthotropic Slab

This paper investigates the transient thermal stress problem of a rectangular slab, which possesses both thermal and elastic orthotropy. The slab initially held at a con stant temperature is suddenly subjected to an arbitrary temperature variation along one of its edges. Based upon the solution of transient temperature field, the thermal stress analysis is performed based upon the further development of a displacement- potential approach. The complete stress field has been obtained in closed forms. Numerical results for a typical unidirectional fiber composite have demonstrated high concentration of thermal stresses far exceeding those generated in steady state thermal environment.

THERMAL STRESSES IN AN ORTHOTROPIC SLAB CONTAINING A PLANE FLAW

This paper examines the stationary temperature and stress fields in an orthotropic elastic slab containing a plane interior region which resists heat flow. Over this region of imperfection the heat flux is assumed to be proportional to the local temperature difference, whereas the stress and displacement components are taken to be continuous. Each face of the slab is subject to an arbitrary temperature variation. Numerical results are given for isotropic and orthotropic slabs with flaws of various lengths and heat-flow resistances.

Linear viscoelastic creep under arbitrary temperature variations

We discuss various temperature jump methods for the accurate determination of the activation energy ΔH of creep for a linear viscoelastic material, and carry out a numerical computation in a discrete relaxation model to show that the effect arising from the temperature dependence of the compliances ( αΔT effect) quickly decays after the temperature jump. A general and compact expression is then derived for the creep rate of a viscoelastic solid under constant stress and subject to arbitrary temperature variations. Application of the expression to various experimental situations is discussed, and the double T-jump method is extended to cases in which a temperature pulse of arbitrary shape is imposed on the sample. A systematic analysis shows that the αΔT effect makes a significant contribution to the expression only during a short period immediately after a T-jump and that it is otherwise totally negligible for any reasonable distribution function in relaxation time. Measurements of ΔH for polypropylene at 40°C by single and double T-jump are in good agreement and support this theoretical prediction. The double T-jump technique is so precise that observations of variation of ΔH with retardation time is now possible.

Thermoelastic Analysis of a Finite Orthotropic Slab

Stationary two-dimensional temperature and stress distributions are examined for an orthotropic elastic body having a rectangular boundary. Temperature fields are determined for situations in which one edge of the rectangle experiences an arbitrary variation of temperature, heat flow or convection, while each of the remaining edges is either insulated or maintained at the initial temperature. Displacement potentials are then used to find the thermal stresses for those cases in which the temperature remains constant over two opposite ends of the rectangle. As an illustrative example, the temperature and stresses resulting from a uniform temperature rise over a portion of one edge of the rectangle are computed; numerical results are given for a fibre-reinforced composite material.

Some gas flows which obey Charles’ Law

The effects of substantial temperature and hence density changes on a low-speed ‘incompressible’ flow can be modelled by adopting Charles’ Law as one of the equations of state. It is found that planar radial inflows or outflows constitute a group of solutions which are self-similar for arbitrary temperature variations. When the temperature is written as a separable function of radius and polar angle, ordinary differential equations result. Permissible solutions include some with discontinuities in the temperature gradient across a radial line (streamline); this is a rough model of a diffusion flame and it is used to illustrate some features of a variable-density flow in a channel with radial walls in the presence of such a ‘flame’.Exact analytical solutions are given for the situation in which temperature increases linearly with radius; no boundary layers appear for either outflow or inflow. Approximate analytical solutions are presented for the case of a relatively rapid inflow with temperature independent of the radius; a velocity boundary layer exists at the walls and in the neigh bourhood of the ‘flame’, although the latter is of small velocity amplitude.

Laminar Free Convection on a Curved Wall

Laminar free convection of a Newtonian fluid passing over a curved wall having arbitrary temperature variation is considered. The governing equations are presented and the method of free parameters is used to investigate the existence of similarity solutions. It is found that similarity solutions do exist when the wall inclination and temperature are required to be certain functions of the coordinate parallel to the wall. Numerical solutions to several example cases are presented which indicate that higher heat-transfer rates are possible on a wall which is concave with respect to the fluid.

Thermal bending of plates with circular holes

The thermoelastic problem of a simply supported rectangular thin elastic plate with a circular hole and subjected to heat flow is solved by the method of point matching. The heat flow consists of an arbitrary temperature variation across the thickness of the plate but with no variation over the plate surfaces. The solution is derived in form of an infinite series in terms of circular cylindrical coordinates. Deflection and moment curves for various ratios of the diameter of the hole to the size of the plate are presented for the case of Poisson's ratio v = 0.3.

Thermal properties of a soil near the surface

The thermal properties, heat conductivity λ and volumetric heat capacity C or thermal diffusivity a = λ/ C, have been determined for soils in the field using a newly developed method. In this method a heat-flux meter is placed on the surface and a “heat pulse” is given by irradiating the heat-flux plate by an incandescant lamp for a short period 5–31 min. Instead of irradiating, shading of the soil can be applied on bright days. The use of Laplace transforms for temperature records permits calculation of the thermal properties in cases of a non-periodic, arbitrary temperature variation. The method has been developed particularly for application to the soil close to the surface, but it can be used for the determination of λ/ C in deeper layers also. Examples for a clay, sandy and peat soil are given.