Conductivity Of Gas Research Articles

Improved heat-transfer models for fibrous insulations

We have developed three improved heat transfer models, which allow prediction of the thermal conductivity of evacuated and gas-filled fibrous insulations. These models take into account the coupling between the solid conduction of the fibre system and the gaseous conduction of the gas in the pore space. They use cell configurations, in which the contact resistance between two neighbouring fibres can be modelled by statistical considerations. Results from the derived models are compared with data obtained for 15 different fibre insulations, which have been investigated in evacuable, load-controlled guarded hot plate systems. Experimentally changed parameters were gas pressure, temperature and density of specimens.

Optimization of monolithic silica aerogel insulants

In this work we systematically investigate thermal transport in opacified monolithic silica aerogels by changing their density and the concentration of infrared opacifier. The goal is to minimize the thermal conductivity of these highly porous inorganic materials. The lowest achieved thermal conductivities at 300 K are about 0.013 W m −1 K −1 for non-evacuated specimens, which have to be compared with values of about 0.020–0.025 W m −1 K −1 for CFC-blown insulating polyurethane foams and with 0.035 W m −1 K −1 for the best fiber insulations. Our investigations allow us to quantitatively determine the gaseous, solid and radiative conductivities λ g, λ s andλ r, respectively. The derived variations with aerogel densityρ are: λ g ∝ ρ −0.6, λ s ∝ ρ 1.5 and λ r ∝ (ρ· e) −1 in the range 70 < ρ/kg m −3 < 230. The total conductivity shows a minimum around 120 kg m −3.

Thermal Conductivity of Monolithic Organic Aerogels

The total thermal conductivity lambda of resorcinol-formaldehyde aerogel monoliths has been measured as a function of density rho in the range from rho = 80 to 300 kilograms per cubic meter. A record-low conductivity value in air at 300 K of lambda approximately 0.012 watt per meter per kelvin was found for rho approximately 157 kilograms per cubic meter. Caloric measurements under variation of gas pressure as well as spectral infrared transmission measurements allowed the determination of solid conductivity, gaseous conductivity, and radiative conductivity as a function of density. The development of such low conductivity materials is of great interest with respect to the substitution of environmentally harmful insulating foams made from chlorofluorocarbons.

Natural Convection Heat Transfer From a Plate in a Semicircular Enclosure

In the subject geometry, a long thin plate at uniform temperature is contained coaxially and symmetrically in a long semicircular trough closed at the top and having a uniform but different temperature. Heat flows across the air-filled region between the two by both natural convection and gaseous conduction. The problem of characterizing the free convective component of this heat transfer—that is, the component caused by bulk fluid motion—is treated experimentally by using a heat balance technique, with the measurements being repeated at different pressures, in order to cover a wide Rayleigh number range, from Ra ≈ 10 to Ra ≈ 108. Nusselt number versus Rayleigh number plots are presented for each of several combinations of plate-to-trough spacing and tilt angle, and the plots are correlated by equations. The problem of characterizing the conductive component is treated by numerically solving the steady diffusion equation in the air-filled region, and the results are correlated as a function of the spacing and the plate thickness.

Role of Interfacial Carbon layer in the Thermal Diffusivity/Conductivity of Silicon Carbide Fiber‐Reinforced Reaction‐Bonded Silicon Nitride Matrix Composites

The role of an interfacial carbon coating in the heat conduction behavior of a uniaxial silicon carbide nitride was investigated. For such a composite without an interfacial carbon coating the values for the thermal conductivity transverse to the fiber direction agreed very well with the values calculated from composite theory using experimental data parallel to the fiber direction, regardless of the ambient atmosphere. However, for a composite made with carbon‐coated fibers the experimental values for the thermal conductivity transverse to the fiber direction under vacuum at room temperature were about a factor of 2 lower than those calculated from composite theory assuming perfect interfacial thermal contact. This discrepancy was attributed to the formation of an interfacial gap, resulting from the thermal expansion mismatch between the fibers and the matrix in combination with the low adhesive strength of the carbon coating. In nitrogen or helium the thermal conductivity was found to be higher because of the contribution of gaseous conduction across the interfacial gap. On switching from vacuum to nitrogen a transient effect in the thermal diffusivity was observed, attributed to the diffusion‐limited entry of the gas phase into the interfacial gap. These effects decreased with increasing temperature, due to gap closure, to be virtually absent at 1000°C.

Heat transfer in opacified aerogel powders

Thermal conductivities of opacified powderous silica aerogels with different density, grain size and concentration of IR-opacifier were investigated. Measurements were performed in a guarded hot-plate apparatus by variation of temperature, gas pressure and external load. The lowest achieved thermal conductivity in air was around 0.017 W/(m K) at room temperature and an external load of 0.1 bar. The measurements allow determination of the gaseous, solid and radiative contributions to the thermal conductivity. The solid conductivities derived are in the range 0.002–0.004 W/(m K), depending on density, mechanical load and grain size. The specific extinction was found to be dependent on the concentrations of IR-opacifier and grain size. Typical radiative conductivities were of the order of 0.001 W/(m K) at room temperature. Gaseous conductivities observed were about 0.011–0.017 W/(m K), depending on the inter-grain pore space.

Thermal transport in organic and opacified silica monolithic aerogels

The thermal properties of monolithic aerogels were investigated by changing their density, the gas pressure and the temperature. Results show that opacified SiO 2 -aerogels and organic aerogels at room temperature in air have a total thermal conductivity as low as 0.013 W/m K and 0.012 W/m K, respectively, and show a flat minimum on variation of density. Aerogels have a very low solid thermal conductivity due to their extremely high porosity. Nanosize pores are responsible for partial suppression of gaseous conduction. A low radiative conductivity caused mainly by absorption is observed for organic aerogels. In the case of SiO 2 -aerogels, the radiative transport can be reduced by integrating an opacifier such as carbon black in the SiO 2 -skeleton.

Role of the Interfacial Thermal Barrier in the Effective Thermal Diffusivity/Conductivity of SiC‐Fiber‐Reinforced Reaction‐Bonded Silicon Nitride

Experimental thermal diffusivity data transverse to the fiber direction for composites composed of a reaction bonded silicon nitride matrix reinforced with uniaxially aligned carbon‐coated silicon carbide fibers indicate the existence of a significant thermal barrier at the matrix‐fiber interface. Calculations of the interfacial thermal conductances indicate that at 300°C and 1‐atm N2, more than 90% of the heat conduction across the interface occurs by gaseous conduction. The magnitude of the interfacial conductance is decreased significantly under vacuum or by removal of the carbon surface layer from the fibers by selective oxidation. Good agreement is obtained between thermal conductance values for the oxidized composite at 1 atm calculated from the thermal conductivity of the N2 gas and those inferred from the data for the effective composite thermal conductivity.

COMPUTER-BASED, CHARGE TRANSFER DATA ACQUISITION FROM MULTIPLE TARGETS UNDERGOING ELECTROSTATIC SPRAYING

ABSTRACT An instrumentation system was designed to monitor the transient space-charge electric field and the low-level bipolar currents occurring at up to 15 earthed targets undergoing electrostatic spraying. Experimental measurements were made of the charge exchanges flowing between charged spray clouds and targets due to both charged-droplet deposition and gaseous conduction induced from earthed target points. Experimental results show up to 40% field reduction due to the electrical discharge induced at earthed points protruding prominently from nearby earthed targets. It was also verified that for such ionizing targets, up to 75% of the total charge exchanged between the spray cloud and the targets can be attributed to gaseous conduction from target points.

Effects of liquid conductivity upon gaseous discharge of droplets

The effect that liquid conductivity has upon gaseous breakdown and conduction between a droplet and a sharp grounded metal point was investigated as a function of the droplet charge level and point-to-droplet gap. A uniform stream of equally spaced 1210 mu m droplets was studied in passing the point at 2.05 m/s at a rate of 465/s. The negative droplet charge was set at 24, 40, 55, and 60% of the Rayleigh hydrodynamic instability limit (i.e. 3*10/sup -10/ C) for liquid conductivity values in the 10/sup -4/-10/sup 1/ S/m range characterizing electrostatic crop sprays. No significant conductivity effect was found for charges up to the 55% level; the most conductive liquid exhibited a significantly ( alpha <0.10) higher discharge current at the 60% charge level. For close gaps, droplets departed the grounded point region with a reversed charge, indicating they were overneutralized by a positive-ion flux from the grounded point.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Radiative heat transfer in ultra-fine powder insulations

This paper reports the first systematic study of the spectral radiation heat transfer through the ultra-fine powder insulation Aerosil 380. Experimental results are obtained for the spectralextinction coefficient from infra-red transmission measurements and for the effective thermal conductivity using a guarded-hot-plate apparatus. These results are compared and assessed with relevant theoretical calculations. In comparison to total effective conductivity experiments in the temperature range of 320–400 K, radiation heat transfer accounted for approximately 10% of the total heat leakage through the insulation, with the remaining by gaseous conduction. At high temperatures, the radiation contribution is expected to increase sharply, reaching over 50% at about 1000 K.

Extension of thermocouple gauge sensitivity to atmospheric pressure

The amount of heat lost from a heated thermocouple (TC) junction through gaseous conduction varies widely with pressure over the range from 1 mTorr to atmosphere and this has largely confined accurate measurement by this technique to the range of about 1 mTorr to 2 Torr. A new design, which matches the junction temperature to a reference temperature, creates a basic constant temperature gauge controller and is largely responsible for fast response to pressure changes. A second control function which adjusts the reference temperature in a nonlinear manner depending on the pressure, modifies the controller response to the TC gauge’s intrinsic pressure sensitivity in such a way as to enhance response in the 1–15 mTorr and 2–200 T ranges, while attenuating its high sensitivity in the 15 mTorr to 2 Torr and 200 Torr to atmosphere regime. This technique gives rise to an output signal which better approximates the log of pressure over six decades. By time multiplexing the junction heating with measurements of its thermal emf, contamination of the thermal signal by the heating current is eliminated, while requiring only two connections to the remote gauge tube. The inclusion of a baffle in the gauge tube reduces inaccuracies in the response during fast pressure changes.

A New Gaseous Gap Conductance Relationship

A new relationship for predicting the gaseous gap conductance between the fuel and clad of a nuclear fuel rod is derived. This relationship is derived from purely analytical considerations and represents a departure from approaches taken in the past. A comparison between the predictions from this new relationship and experimental measurements is presented and the agreement is very good. Predictions can be generated relatively quickly with this relationship making it attractive for fuel pin analysis codes.

Phytoluminographic Detection of Dynamic Variations in Leaf Gaseous Conductivity

Gas exchange and plant luminescence (delayed light emission) of a single red kidney bean leaf undergoing synchronous oscillations in gas exchange were recorded and analyzed. Introduction of 1.1 microliter per liter SO(2) during these oscillations produced increases in plant luminescence that, when averaged over a portion of the leaf, oscillated in phase with the gas exchange oscillations. However, examination of a video record of the plant luminescence showed not only that luminescence intensities tended to be localized within discrete areas of the leaf, but that the time-dependence of luminescence intensities within these regions varied considerably from the period, amplitude, and often phase of the overall gas exchange oscillations. The video recording also showed that changes in luminescence intensities appeared to migrate across the leaf in wave-like patterns. These data are interpreted in terms of localized fluctuations in gaseous conductances of the leaf.

Proposal for a compound-honeycomb collector

The performance of a flat plate solar collector combining a selective surface on the absorber plate and a plastic honeycomb in the air gap is not as high as might be expected because of a certain coupling between the radiative and (gaseous) conduction heat transfer modes. To decouple the modes and achieve the desired low heat loss coefficient, this paper suggests leaving an air gap of about 10 mm between the plate and the honeycomb—the air gap and the honeycomb together comprising a compound honeycomb air layer. The paper evaluates the suggestion through three basic studies. The first two experimentally and theoretically studied the heat transfer in the case where the air is artificially maintained stationary, and the third experimentally studied the effect of free convective air motion on this heat transfer. These studies demonstrated that leaving the air gap effectively decouples the modes but does not significantly alter the free convection at the design condition. It is concluded that the compound honeycomb collector should have very good performance at high temperature.

Turkey Egg Weight Losses and Embryonic Mortality During Incubation ,

Shell conductances (mg H2 O·day–1 ·torr–1) of fertile and infertile turkey eggs did not differ significantly after days 7, 14, 21, or 25 of incubation. Furthermore, conductances of eggs that subsequently hatched, contained early dead embryos, or contained late dead embryos did not differ significantly before 7 days of incubation. For the incubation periods 7 to 14, 14 to 21, and 21 to 25 days, eggs with embryos dying late in the course of incubation had significantly lower conductances than eggs with embryos that subsequently hatched or eggs that had died early during development. Fertile and infertile eggs did not differ significantly in egg weight, surface area, percentage of initial egg mass lost during incubation, or amount of water vapor lost per square centimeter of surface area. Similarly, no significant differences were observed between eggs with embryos that hatched normally and eggs with embryos dying early in incubation for any of the aforementioned parameters, but eggs with embryos dying late in incubation had significantly lower percentage of initial egg mass lost and lower amounts of water vapor lost per square centimeter of surface area. Eggs exhibiting late embryonic mortality had the same egg weights and surface areas as the other hatchability groups. It was concluded that gaseous conductance of some turkey eggshells is insufficient to meet embryonic demands late in incubation.

Multifoil Thermocouple Gauge for Measuring Pressures up to 1 atm

A thermocouple gauge is described which is capable of indicating pressure in the complete pressure range between 10−3 Torr and atmospheric pressure. The experimental results obtained from a test gauge are presented and found to be in good agreement with analysis. The gauge utilizes the free molecule and transition gaseous heat conduction that exists in a gas contained between the foils of multifoil thermal insulation up to pressures in the 100 Torr range. The resulting pressure variation of the thermal conductivity of multifoil insulation is used to indicate the pressure.

Gaseous Conduction in Low Pressure Supersonic Gas Flow

Gaseous Conduction in Low Pressure Supersonic Gas Flow

Constant-Temperature Magneto-Gasdynamic Channel Flow

In the course of investigating boundary-layer flow in continuous plasma accelerators with crossed electric and magnetic fields, it was found advantageous to have at hand simple closed-form solutions for the magneto-gas dynamic flow in the duct which could serve as free-stream conditions for the boundary layers. Nontrivial solutions of this sort are not available at present. and in fact, as in the work of Resler and Sears, the variation of conditions along the flow axis must be obtained through numerical integration. Consequently, some simple solutions of magneto-gasdynamic channel flow were sought, possessing sufficient algebraic simplicity to serve as free-stream boundary conditions for analytic investigations of the boundary layer in a physically reasonable accelerator. In particular, since the cooling of the accelerator tube is likely to be an important physical problem because of the high gas temperatures required to provide sufficient gaseous conductivity, channel flow with constant temperature appears interesting. Some simple algebraic solutions for the case of a constant temperature plasma are developed in the following paragraphs.

Gaseous heat conduction at low pressures and temperatures

Over the large temperature differences that can exist in vacuum insulation of cryogenic devices some of the assumptions on which the formulae for free molecule heat conduction are based are not strictly valid. The consequences of this are discussed, and some incorrect usages are pointed out. Working formulae and auxiliary data on mean free paths and accommodation coefficients at cryogenic temperatures are presented.