Fluid Thermal Diffusivity Research Articles

A High-Temperature Transient Hot-Wire Thermal Conductivity Apparatus for Fluids.

A new apparatus for measuring both the thermal conductivity and thermal diffusivity of fluids at temperatures from 220 to 775 K at pressures to 70 MPa is described. The instrument is based on the step-power-forced transient hot-wire technique. Two hot wires are arranged in different arms of a Wheatstone bridge such that the response of the shorter compensating wire is subtracted from the response of the primary wire. Both hot wires are 12.7 µm diameter platinum wire and are simultaneously used as electrical heat sources and as resistance thermometers. A microcomputer controls bridge nulling, applies the power pulse, monitors the bridge response, and stores the results. Performance of the instrument was verified with measurements on liquid toluene as well as argon and nitrogen gas. In particular, new data for the thermal conductivity of liquid toluene near the saturation line, between 298 and 550 K, are presented. These new data can be used to illustrate the importance of radiative heat transfer in transient hot-wire measurements. Thermal conductivity data for liquid toluene, which are corrected for radiation, are reported. The precision of the thermal conductivity data is ± 0.3% and the accuracy is about ±1%. The accuracy of the thermal diffusivity data is about ± 5%. From the measured thermal conductivity and thermal diffusivity, we can calculate the specific heat, Cp, of the fluid, provided that the density is measured, or available through an equation of state.

Analysis of the conjugate transient natural convection-conduction heat transfer of a fin array in a cavity

This article is concerned with the conjugate transient natural convection-conduction heat transfer for finned packages in a cavity. The situation is commonly found in the cooling of very large scalar integration (VLSI) electronic packages in a closed space. The present study uses a numerical integration technique with a cubic spline to solve the conjugate coupled nonlinear partial differential equations. Results are obtained for a range of values of Prandtl and Rayleigh numbers and the fin and fluid thermal diffusivity ratio. The transient and steady-state phenomena of the velocity and temperature field and average Nusselt numbers on the hot surface, cold surface, and fin are presented.

A method for determining the thermal diffusivity of fluids

A method for determining the thermal diffusivity of fluids

Thermal diffusivity of fluids in a broad region around the critical point

Dynamic light scattering is a suitable method for the investigation of transport properties such as the thermal diffusivity of optically transparent fluids. The main advantages of the method are its quickness, the fact of the thermodynamic state of equilibrium of the sample (gradients are not required), and the relatively simple evaluation of data without the necessity of calibration. However, an insufficient production of intensity of scattered light may be a limiting effect. For that reason the vicinity of the gas-liquid critical point represents the classical range of application. In this paper, it is shown that by means of an appropriate choice of experimental apparatus, measurements are also feasible in an extended range of states. Broad regions around critical points of three pure fluids (sulfur hexafluoride, SF6; ethane, C2H6; nitrous oxide, N2O) over temperature ranges ¦T-Tc¦ of 0.02 to 50 K and density ranges (ρ/ρc) of 0.2 to 2 were investigated. In this region the thermal diffusivity shows great variations with temperature and density and cannot be described by means of ideal-gas behavior or relations for liquids. The measurements were carried out along the coexistence curve for both phases, along the critical isochore and along some isotherms with T≶Tc. The measured or calculated density, pressure, and thermal diffusivity data as well as some correlations are presented.

Knudsen-effect errors with transient line-source measurement in fluids

This paper decribes the Knudsen-effect errors of the transient line-source method used for accurate measurements of the thermal conductivity and thermal diffusivity of fluids. The analysis demonstrates that the instrument can be used with a good accuracy (>0.5%) to lower densities than previously thought. The principal errors are illustrated by measurements on propane in the temperature range 250–300 K at densities less than 9 kg · m−3.

ON THE OVERSTABILITY CONVECTION

The oscillatory mode of instability in a rotating fluid layer heated from below is theoret-ically investigated with a combination of Eulerian and Lagrangian analyses. An exact analysisis not only nmde for the case of two free bounding surfaces, but for the case of one free andone rigid bounding surfaces. The physics of overstability convection is attributable to the dual role of the thermaldiffusion in this rotating fluid Thermal diffusion ordinarily only destroys the buoyancy offluid parcels. But if the rotation rate is sufficiently large and if the viscous dissipation iscomparatively weak, the heat exchanges alternately produce overcooling and overheating of theparcels and thereby give rise to a net positive feedback mechanism for the instability. The parcels near a rigid bounding surface (they are different from those near a freebounding surface), however, do not have a net horizontal drift and penetrate more and moreinto the interior of the layer, and they just progressively drift towards the rigid surface. Andthe vertical velocity near the rigid surface layer lags behind that in the bulk of the interiorlayer by a quarter of the oscillation period. These differences stem from the dynamical effectsof the frictionally induced secondary circulation known as Ekman pumping and suction. The spatially averaged energetics of oscillatory unstable modes are examined and discussed. On the basis of the theoretical results, it is suggested that the observod oscillatory vortexin a laboratory simulation of the flow over a heated Qinghai-Xizang model plateau (Researchon Experimental Simulation, 1978) may be interpreted in terms of the notion of overstabilityconvection.

Internal waves in a fluid of finite Prandtl number

Abstract It is shown, through calculation and physical arguments, that in a smoothly stratified fluid thermal diffusion plays a damping role comparable to that of viscosity only in the interior of the fluid, and not in the boundary layers.

On the principle of exchange of stabilities for the magnetohydrodynamic thermal stability problem in completely confined fluids

The principle of exchange of stabilities (exchange principle) for the thermal stability problem has been proved by Pellew & Southwell (1940) for fluids bounded by two infinite, horizontal parallel planes. Chandrasekhar (1952) discussed the establishment of the exchange principle for the same geometry when the fluid is an electrical conductor and when an arbitrary oriented, uniform, external magnetic field is applied in the vertical direction.In this paper, the exchange principle is examined for fluids completely confined in an arbitrary region with rigid bounding surfaces that are good electrical conductors with respect to the fluid. The uniform magnetic field is applied in an arbitrary direction. A generalized thermal boundary condition is imposed which includes the fixed temperature and prescribed heat-flux conditions as special cases.If no magnetic field is applied to the fluid, the present work reduces to a generalization (for completely confined fluids) of the Pellew & Southwell proof of the exchange principle. In the magnetohydrodynamic (MHD) thermal stability problem, the exchange principle is found to be valid if the total kinetic energy associated with an arbitrary disturbance is greater than or equal to its total magnetic energy. In a special case it is demonstrated that a sufficient condition which will establish the exchange principle is k ≤ η, where k is the fluid thermal diffusivity and η is the fluid electrical resistivity.