Cooling rate measurements on pure iron rapidly solidified by piston quenching

Cooling curves have been monitored during rapid solidification of pure iron, using a rapid response thermocouple embedded in one of the quenching pistons. Cooling rates are found to be typically 10⁶ to 10⁷ K sec^–1 in the vicinity of the solidification point at 1500° C, falling to 2 × 10⁴ to 3 × 10⁵ K sec^–1 at 500° C. Heat-flow analysis shows that cooling conditions during rapid solidification are clearly non-Newtonian, with heat transfer coefficients of 3 × 10⁵ to 6 × 10⁵Wm^–2 K^–1 and Nusselt numbers of 0.5 to 1.0. Cooling rates, heat transfer coefficients and Nusselt numbers are higher for piston quenching than for other rapid solidification processes such as melt spinning. Piston-quenched iron microstructures can be ferritic or martensitic depending on the cooling rate during rapid solidification.     

Microstructure — cooling rate correlations in melt-spun alloys

Photocalorimetric techniques have been used to measure top surface temperatures during melt spinning of Ni-Al and 316L stainless steel ribbons, in order to investigate the effect of cooling rate on the melt-spun alloy microstructures. Cooling conditions during melt-spinning are found to be near-Newtonian, with mean cooling rates, heat transfer coefficients and Nusselt numbers in the range 4×10⁴ to 5×10⁵ K sec^–1, 5×10⁴ to 3× 10⁵ Wm^–2K^–1 and 0.07 to 0.22, respectively, for wheel speeds in the range 4 to 36 m sec^–1. The cooling rate during melt-spinning is directly proportional to the wheel speed and inversely proportional to the square of the ribbon thickness. Melt-spun Ni-Al and 316L stainless steel ribbons exhibit a columnar through-thickness solidification microstructure, with a segregation-free region adjacent to the wheel surface. Solidification takes place by heterogeneous nucleation of the undercooled liquid on the wheel surface, followed by partitionless solidification during recalescence, and finally cellular breakdown and segregated solidification. The columnar grain size decreases and the fractional segregation-free thickness increases with increasing wheel speed and cooling rate, indicating that the nucleation undercooling in the liquid is proportional to the cooling rate.     

Production of Al-(12–25) wt% Si alloys by rapid solidification: melt spinning versus centrifugal atomization

Al-Si-x(x=Cu, Mg, Ni, Co or Sr) alloys with Si content in the range 12–25 wt% were quenched from the liquid state using two methods: melt spinning and centrifugal atomization. The powders obtained were degassed followed by hot extrusion. Effects of chemical composition, quenching conditions, hot extrusion and heat treatment on the variation in the microstructure were examined. The present results show the necessary conditions for supersaturated solid solution, and those required for mechanism of solute trapping by moving the solid/liquid interface. Also, the mechanical properties of the products obtained were evaluated. It is observed that melt spun ribbons with Si concentrations of more than 12% possess high yield strength with low ductility. These materials undergo softening on ageing at temperatures above 150 °C. The properties of extruded alloy powders are markedly improved as compared to those made by ingot metallurgy. This effect is mainly due to the silicon particle refinement brought about by rapid solidification with cooling rates higher than 10⁵ Ks^–1.     

Thermal and Electrical Properties of Gadolinium Sulfides at High Temperatures

A study is made into the temperature dependence of the thermal conductivity, electrical conductivity, thermoelectromotive force, thermal expansion coefficient, and heat capacity in the temperature range from 300 to 1200 K for polycrystalline gadolinium sulfides GdS _y (y= 1.495–1.487) produced both by recrystallization pressing and by crystallization from a melt. The role of the mechanisms of heat and charge transfer is estimated depending on the composition. The reasons for changes in their electrical and thermal properties are analyzed. The thermoelectric efficiency is calculated. It is demonstrated that Z 0.6 × 10^–3K^–1at T 1000 K.     

Studies of combustion and heat-transfer processes in a combustion flame in a melt

With a specially developed zonal mathematical model theoretical studies of heat transfer processes in a combustion flame immersed into a melt were carried out in steady-state conditions. As a result of this analysis, including the main phenomena which take place in gaseous fuel burning in the melt, the temperature and heat flux distributions between different phases over the melt height were found. The effect of convective heat transfer on the heat-transfer processes between the melt and the immersed combustion flame was determined.Notation convective heat-transfer coefficient W/(m²·K) - F interface area, m² S. M. Kirov Urals Polytechnical Institute, Ekaterinburg, Russia. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 64, No. 3, pp. 293–296, March, 1993.     

Two-phase heat transfer to a refrigerant in a 1 mm diameter tubeTransfert de chaleur vers un frigorigène circulant dans un tube d'un diamètre d'un mm

A study of two-phase flow and heat transfer in a small tube of 1 mm internal diameter has been conducted experimentally as part of a wider study of boiling in small channels. R141b has been used as the working fluid. The boiling heat transfer in the small tube has been measured over a mass flux range of 300–2000 kg/m² s and heat flux range of 10–1150 kW/m². In this paper the boiling map for a mass velocity of 510 kg/m² s and heat flux of 18–72 kW/m² is discussed and the problems of determining heat transfer coefficients in small channels are highlighted.     

Optimization of the Parameters of a Porous Coating under Conditions of Pool Boiling of Propane

Results of an investigation of the influence of the parameters of a sintered porous copper coating on heat transfer under conditions of propane boiling are reported. The experiments were carried out within the ranges of specific heat fluxes of q = 10²–6.4·10⁴ W/m² and of saturation vapor pressures of p _s = 0.48–1.08 MPa (T _s = 0–30°C). Optimum parameters of the porous coating were determined within the investigated saturation pressure range. A dimensionless equation is suggested for determination of the heattransfer rate in propane boiling on sintered porous copper coatings.     

An investigation of amorphous phase separation, leachability and surface area of an ionomer glass system and a sodium-boro-silicate glass system

Glasses from a complex SiO₂-Al₂O₃-P₂O₅-CaO-CaF₂ glass series, known as 'ionomer glasses' were investigated. For comparison, a sodium-boro-silicate (s-b-s) glass system, which is known to undergo amorphous phase separation was also investigated. Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Analysis (EDX) and BET surface area and pore distribution analysis were the principal analytical techniques used in this study. SEM analysis of the ionomer glass compositions revealed smooth spherical droplets of 2–15 nm while the background morphology appeared rough and speckled. A classic interconnected structure was observed for the s-b-s glass. EDX analysis of the s-b-s glass confirmed that the sodium-borate phase was removed by leaching with 0.3 M HNO₃, leaving behind a silica-rich structure. EDX analysis of ionomer glasses leached with 10% NaOH showed that the calcium and phosphate phases were being removed, although not to completion. For the base s-b-s glass a surface area of m 8² g^–1 was recorded. However, the base glass after extraction with 0.3 M HNO₃ of the sodium borate rich phase gave a BET surface area of 330 m² g^–1 indicating that it had already undergone phase separation on quenching from the melt, giving rise to a fine scale interconnected structure. The leached s-b-s glasses exhibited type 4 adsorption/desorption isotherms characteristic of mesoporous materials. Glasses which had been heat treated at 580°C for 4 h exhibited a surface area of 62 m² g^–1. This indicates that the as-quenched glass is already phase separated and that the phase separated microstructure is coarsening on heat treatment. A surface area of 4 m² g^–1 was measured for the base ionomer glasses. After leaching with 10% NaOH this value rose 10-fold with a maximum surface area of 44.1 m² g^–1 being recorded. The ionomer glasses also exhibited adsorption/desorption isotherms characteristic of mesoporous materials.     

The effect of chloride ions on the optical properties of TeO2-CuO-CuCl2 glasses

The glasses with composition 65TeO₂-(35–x)CuO-xCuCl₂ (x=0, 1, 2, 3, 4, 5 mol%) were prepared by a melt quenching technique and thin films of different thicknesses were made by blowing. The optical energy gap was studied and its variation with composition is discussed in terms of the effective role played by chloride ions which reduce the non-bridging oxygen ions and modify the structure of the network. The infrared spectra of all these glass samples at room temperature, recorded between 200 and 2400 cm^–1, are discussed in terms of anti-symmetric vibrations of the heteronuclear atoms.     

Local heat transfer on the entrance segment of a tube with a sharp inlet edge. 1. Nature and behavior of heat transfer intensity extrema

Experimental results on local heat transfer on the entrance segment of a round smooth tube with a sharp leading edge when Re _d =(13–110)·10³, X/d=0.1–13, which support the previously proposed model for separated flow, are examined. It is shown that under separated flow conditions the usual sequence of flow transitions in a boundary layer for mixed nonseparated flows and Re _d >10⁴ is still retained. Formulas are obtained for calculating local heat transfer coefficients on a segment where a laminar boundary layer occurs, including cross sections where these coefficients acquire extreme values.Kiev Polytechnic Institute, Ukraine. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 65, No. 2, pp. 131–138, August, 1993.     

Estimation of the unknown parameters in the melt-spinning process

The free-surface temperature history of the melt spinning of copper measured by Tenwick and Davies [3] is compared with those calculated using a thermokinetic model assuming different parameters. The heat-transfer coefficient, nucleation temperature and the crystal-growth kinetics were thus estimated for the melt spinning of copper at a wheel speed of 35 ms^–1 as follows: heat-transfer coefficient during liquid cooling stage HL=1.0 × 10⁷ W m^–2K^–1, heat-transfer coefficient after solidification finished HS=1.0 × 10⁵ W m^–2K^–1, heat-transfer coefficient during solidificationH= 1.0 x 10⁷- 1.2 x 10¹¹ (t-t _n) (W m^–2K^–1), the nucleation temperatureT _n 1233 K and the crystal-growth kinetic lawV=4.0 × 10^–3 T^1.1 (ms^–1).     

Air-side thermal hydraulic performance of multi-louvered fin aluminum heat exchangers

An experimental study on the air-side heat transfer and pressure drop characteristics for multi-louvered fin and flat tube heat exchangers has been performed. For 45 heat exchangers with different louver angles (15–29°), fin pitches (1.0, 1.2, 1.4 mm) and flow depths (16, 20, 24 mm), a series of tests were conducted for the air-side Reynolds numbers of 100–600, at a constant tube-side water flow rate of 0.32 m³/h. The inlet temperatures of the air and water for heat exchangers were 21 and 45°C, respectively. The air-side thermal performance data were analyzed using effectiveness-NTU method for cross-flow heat exchanger with both fluid unmixed conditions. The heat transfer coefficient and pressure drop data for heat exchangers with different geometrical configurations were reported in terms of Colburn j-factor and Fanning friction factor f, as functions of Reynolds number based on louver pitch. The general correlations for j and f factors are developed and compared to other correlations. The f correlation indicates that the flow depth is one of the important parameters for the pressure drop.     

Microstructure and mechanical properties of TiC–WC–(Ti,W)C–Ni cermets

(Ti_0.93W_0.07)C solid-solution powder is synthesized via high-energy milling and carbothermal reduction. When the solid-solution carbide is sintered after blending with Ni and commercial single-phase carbides such as TiC and WC, a complex microstructure is developed, typically featuring a core–rim structure along with the initial solid-solution phase. Furthermore, an increase in total volume of the solid-solution phase with (Ti_0.93W_0.07)C in the cermets results in significantly enhanced mechanical properties: fracture toughness (K_1C) is 7.5–12.7 MPa m^1/2 and hardness (H_v) is 11.9–14.1 GPa. These properties are in contrast with those of conventional cermets, which yield K_1C = 6 MPa m^1/2 and H_v = 12.6 GPa.     

Extraction of water from alkali germanosilicate glasses for optical fibres

A study is reported on the extraction of water from alkali germanosilicate glasses, used for making graded index optical fibres. It is shown that the extraction rate is largely determined by the gas-glass melt interface created during the bubbling of dried O₂ through the melt. Both increased gas stream rates and the use of O₂, containing D₂O, resulted in a significant decrease in water concentration in the glass, thereby reducing the loss of the optical fibres produced from these glasses, e.g. a decrease at 850 nm from 15 to 10 dB km^–1, at 1100 nm from 28 to 10 dB km^–1.     

Thermodynamic properties by levitation calorimetry — V. High-temperature heat content of liquid gallium

The heat content (enthalpy) of liquid gallium relative to the supercooled liquid state at 298.15 K has been measured by levitation calorimetry over the temperature range 1412–1630 K. Thermal energy increments were determined using an aluminum block calorimeter of conventional design. The sharp decrease of C _p with increasing temperature observed just above the melting point does not persist up to the high temperatures of the present work. When combined with recent laser-flash calorimetry results from the literature, the present work indicates that C _p is 26.46 ± 0.71 J · g-atom^–1 · K^–1 over the temperature range 587–1630 K.Paper presented at the Japan-United States Joint Seminar on Thermophysical Properties, October 24–26, 1983, Tokyo, Japan.     

Controlling temperatures in free convection for a high-viscosity fluid

The self-similarity problem with consideration of the relationship between viscosity and temperature in the range of Prandtl numbers 1–10 ⁴ has been solved for laminar free-convective flow of a viscous fluid about an isothermal vertical plate. We have calculated the controlling temperatures for local heat transfer, as well as the magnitudes and positions for the maximum of longitudinal velocity, the force of friction, and the thickness of the boundary layer.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 60, No. 3, pp. 386–390, March, 1991.     

Flow boiling of ammonia in a plain and a low finned horizontal tubeEbullition en éncoulement d'ammoniac dans un tube lisse ou un tube ailetté

Experimental data of the local heat transfer coeffcient of flow boiling ammonia in dependence of vapor fraction, mass flux and local heat flux is presented. Two horizontal test sections of 450 mm length and an inner diameter of 10 mm have been used, one being a plain tube, one being a spirally low finned tube. A constant wall temperature boundary has been aimed for the test section by heating with a fluid condensing on the tube outside. Local heat transfer coeffcients and pressure drops have been measured in the range −40 < T_sat < 4°C, 0 < x< 0.9, 50 < < 150 kg/m² s and 2 < ΔT_w < 15 K with resulting heat fluxes of 17 < < 75 kW/m². The vapor quality is denoted as x, is the mass flux and ΔT_w the wall superheat. The measured data is carefully evaluated using a finite element model of the tube with regard to the circumferential heat flow distribution. The smooth tube results are compared with recently published data and the correlation from Zürcher (Zürcher, O., Thome, J.R., Favrat, D. Evaporation of ammonia in a smooth horizontal tube: heat transfer measurements and predictions. Journal of Heat Transfer, 1999;121:89–101), and with the correlations of Steiner (Steiner D. Strömungssieden gesättigter Flüssigkeiten. VDI-Wärmeatlas, vol. 8. VDI-Verlag, 1997) and Kattan (Kattan N, Thome JR, Favrat D. Flow boiling in horizontal tubes: part 3 — development of a new heat transfer model based on flow pattern. Transactions of the ASME, 1998;120). The results of the low finned tube are not matched by any known correlation.     

The structural degradation and lamellar to rod transition in the Bi-Cd eutectic during unidirectional growth

The Bi-Cd eutectic system is a prototype quasi-regular eutectic in which the bismuth-rich phase has a volume fraction of 57%. It shows a high degree of regularity but, clearly, is not a normal (regular) eutectic. Microstructural observations of unidirectionally-grown specimens show that the minor cadmium-rich phase degrades at small temperature gradients and small growth rates. However, the structural refinement resulting from rapid freezing or chemical addition is found to be analogous to that of the F/NF eutectics. A lamellar rod transition has been achieved at intermediate growth rates by adding 2.0 wt % Sn as a modifier to the eutectic alloy. However, this was accompanied by the bismuth phase showing cellular facets of the solid-liquid interface.Nomenclature G _L temperature gradient in the melt ahead of the solid/liquid interface (° C cm^–1) - G _S temperature gradient in the solid behind the solid-liquid interface (° C cm^–1) - R growth rate of solid (cm sec^–1) - S cooling rate (° C sec^–1, ° C h^–1) - K _S thermal conductivity in the solid (W m^–1 K^–1) - K _L thermal conductivity in the melt (W m^–1 K^–1) - L latent heat of fusion (J mol^–1) - T temperature difference, undercooling (° C) - K ₁ constant in Equation 2 - K ₂ constant in Equation 3 - D diffusion coefficient of solute in solid (m² sec^–1) - C solubility in solid (wt %, at %) - M molecular weight (g mol^–1) - density (g cm^–3) - interfacial energy, surface tension (J mm^–2) - R gas constant, 8.314J mol^–1 K^–1 - r radius of curvature (m) - T temperature (K) - t time (sec) - F faceted - NF non-faceted     

Related surface and heat of immersion characterization of some alumina samples

MnCl₂- and CuCl₂-loaded aluminas were thermally treatedin vacuo in the temperature range 20–500 °C. The water loss, specific surface area from nitrogen adsorption measurements and the integral heat of immersion in water were determined for all the samples. The high-pressure hysteresis loop displayed in the adsorption isotherms, together with the limited increase in adsorption values near saturation indicate the dominance of mesopores in the tested samples, as also detected by theV _a -t method of isotherm analysis. Changes in the slope ofV _a -t plots suggests that the mesopores rather belong to narrow-ranged mesopore sizes, which are thus considered responsible for adsorption in thet-ranges of 0.5–0.7 and above 0.7 nm. The constancy ofV _a(mlg^–1) values abovet=1.2 nm points out the absence of effective capillary condensation in the high relative pressure region. Variations of BET-surface area and the reciprocal of average pore radius, ¯r^–1 (nm^–1), as functions of the heat-treatment temperature indicate that MnCl₂-loaded samples display a higher thermal stability than CuCl₂-loaded samples; the latter exhibit some aggregation cementation behaviour. At temperatures below 300 °C, the heat of immersion data are not as predicted from changes in water loss and surface-area parameters. Thus, MnCl₂-loaded samples display lower heat of immersion values than the respective CuCl₂-loaded samples, despite the higher BET areas of the former samples; an effect which may be tentatively interpreted in terms of the capability of the smaller sized Mn²⁺ ions to preserve more water molecules via such processes as complex formation, ion hydration and crystallization, which contribute to lowering the integral heat measured. This apparent discrepancy in the behaviour of integral heat of immersion (Jg^–1) data with water loss (gg^–1) at temperatures below 300°C is clarified by considering the change in the heat of immersion per unit area,h _i (J m^–2) with the temperature of treatment.     

Heat transfer enhancement by Marangoni convection in the NH3–H2O absorption process

The objectives of this paper are to quantify the effect of Marangini convection on the absorption performance for the ammonia–water absorption process, and to visualize Marangoni convection that is induced by adding a heat transfer additive, n-octanol. A real-time single-wavelength holographic interferometer is used for the visualization using a He–Ne gas laser. The interface temperature is always the highest due to the absorption heat release near the interface. It was found that the thermal boundary layer (TBL) increased faster than the diffusion boundary layer (DBL), and the DBL thickness increased by adding the heat transfer additive. At 5 s after absorption started, the DBL thickness for 5 mass% NH₃ without and with the heat transfer additive was 3.0 and 4.5 mm, respectively. Marangoni convection was observed near the interface only in the cases with heat transfer additive. The Marangoni convection was very strong just after the absorption started and it weakened as time elapsed. It was concluded that the absorption performance could be improved by increasing the absorption driving potential (x_vb−x_vi) and by increasing the heat transfer additive concentration. The absorption heat transfer was enhanced as high as 3.0–4.6 times by adding the heat transfer additive that generated Marangoni convection.