Showing papers in "Experimental Heat Transfer in 1998"

Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles

Abstract: Turbulent friction and heat transfer behaviors of dispersed fluids (i.e., uttrafine metallic oxide particles suspended in water) in a circular pipe were investigated experimentally. Viscosity measurements were also conducted using a Brookfield rotating viscometer. Two different metallic oxide particles, γ-alumina (Al2O3) and titanium dioxide (TiO2), with mean diameters of 13 and 27 nm, respectively, were used as suspended particles. The Reynolds and Prandtl numbers varied in the ranges l04-I05 and 6.5-12.3, respectively. The viscosities of the dispersed fluids with γ-Al2O3 and TiO2 particles at a 10% volume concentration were approximately 200 and 3 times greater than that of water, respectively. These viscosity results were significantly larger than the predictions from the classical theory of suspension rheology. Darcy friction factors for the dispersed fluids of the volume concentration ranging from 1% to 3% coincided well with Kays' correlation for turbulent flow of a single-phase fluid. The Nusselt n...

Experimental study of marangoni-benard convection in a liquid layer induced by evaporation

Abstract: Marangoni-Benard instability and convection in evaporating liquid layers have been studied experimentally through flow visualization and temperature profile measurement. Benard cells have been observed in an evaporating thin liquid layer whether it is heated, adiabatic, or cooled from below. This experimental study has revealed a different mechanism from the traditional Rayleigh-Benard and Marangoni-Benard instabilities and convections, which require a negative temperature gradient in the thin liquid layer. Evaporation rate and enthalpy of evaporation have been found to be important parameters of instability and convection in an evaporating liquid layer. A modified form of Marangoni number, Ma*, is proposed and its critical values, Ma* c, for alcohol and Freon-113 evaporating layers are determined experimentally. A quantitative comparison between Ma* and the traditional Marangoni number, Ma, shows that Ma* is an adequate indicator of the stability status in evaporating liquid layers.

High-heat-flux resistance heaters from vps and hvof thermal spraying

Abstract: This article describes the application of thermal spray techniques to produce resistance heating elements suitable for applying very large heat flaxes to solid surfaces. The surface to be heated is electrically insulated by deposition of a ceramic layer onto which a thin metallic layer is deposited; the metallic layer serves as the heating element. Each layer has a thickness in the range of 75 to 300 μm. Design considerations for the heaters are described. Previous efforts have produced the films using air plasma spraying. In the present work, we applied vacuum plasma spraying and high-velocity oxygen fuel spraying, which result in considerable improvements in performance and reliability. Heaters have been tested at fluxes up to 17 MW/m2, The heaters generally fail by fracture once the thermal stresses in the system exceed a level that depends on the process by which the films have been deposited. These heaters are useful for the experimental development of high-heat-flux cooling systems.

Flow boiling through v-shape microchannels

Abstract: An experimental facility was set up to study the flow and boiling heat transfer characteristics of water and methanol flowing through V-shape microchannels. The microchannels have hydraulic diameters ranging from 0.2 to 0.6 mm and V-shape groove angles 0 of 30 to 60°. Both the heat transfer and the pressure drop were affected by the thermofluid and geometric parameters such as liquid flow velocity, subcooling, and the hydraulic diameter and groove angle of the microchannels. The experiments indicted that there exists both an optimum hydraulic diameter and an optimum groove angle. The visualization experiments showed that, if there was a good seal between the glass cover and the microchanneled test plate, no bubbles were observed in the microchannels for flow boiling with heat fluxes as high as of the order of 106 W/m2, at which fully nucleate boiling with a large number of bubbles would be expected in conventional situations. Fluctuating liquid flow was induced in the microchannels when many bubbles forme...

Effects of Surface Roughness on Forced Convection and Friction in Triangular Ducts

Abstract: Experimental investigations had been conducted to study the forced convective heat transfer and pressure drop characteristics of the hydrodynamic fully developed turbulent flow in horizontal equilateral triangular ducts fabricated with Ike same length and hydraulic diameter but different surface roughness of 1.2, 3.0, and 11.5 μm. The experiments were performed with hydraulic diameter-based Reynolds number ranging from 7,000 to 20,000. The entire inner wall of the duct was heated uniformly, while the outer surface was thermally insulated. It was found that the variation of Stanton number (St) with friction factor (f) can be expressed by a relationship of St = C * f, where the constant (C) increases from 0.41 to 0.50 when the surface roughness is increased from 1,2 to 11.5 μm. It was also concluded that the duct with a higher surface roughness will have better heat transfer performance. Nondimensional expressions for the determination of the heat transfer coefficient and friction factor of the equilateral ...

A radiation efficiency measurement procedure for gas-fired radiant burners

Abstract: Radiant burners are used in drying, preheating, curing, and baking processes in the manufacturing industry. High radiation efficiency is one of the most important performance criteria for these burners. A wide range of radiation efficiencies (varying by more than 200% ) has been reported in the past for similar burners under similar operating conditions. The wide variation in reported radiation efficiencies appears to be due partly to the differences in the measurement techniques. If appropriate calibration source and procedures are used, the differences in data result from (1) nondiffuse emission from the burner and (2) nonuniform burner surface. The present article reports a technique for radiation efficiency measurements of diffuse and directional radiant burner surfaces. The technique is not sensitive to surface nonuniformities, which are common in commercial radiant burners. A calorimetric approach is used as a consistency check on the efficiency data obtained using the present technique. Companion m...

Direct liquid cooling of electronic chips by single-phase forced convection of fc-72

Abstract: Experimental studies of single-phase heat transfer with the working fluid FC-72 from a single chip and a four-in-line chip module mounted on one wall of a vertical rectangular channel are carried out. Both flush-mounted and protruding chip configurations are investigated with the channel height being equal to the ship length. The Reynolds number based on chip length ranged from 2.2 × 103 to 6.3 × 104 (3.0 × 103 < ReD < 8.4 × 104) for an inlet temperature of 25°C. Results for a single flush-mounted chip are in good agreement with those obtained for the most upstream chip of the multichip module. Heat transfer data of a single heated protruding chip located downstream agree well with the results from the single flush-mounted chip. Heat transfer coefficients from flush-mounted chips are less than those from protruding chips.

Experimental study of melting heat transfer in an enclosure with three discrete protruding heat sources

Abstract: To explore the possibility of using solid-liquid phase-change cooling for periodically or intermittently operated electronic equipment, details of an experimental study of time-dependent melting heat transfer in a rectangular enclosure with three discrete protruding heat sources is presented. A series of experimental measurements in an enclosure with phase-change materials (PCM) of n-octadecane heated by three discrete protruding heat sources at a constant rate on one of vertical wall is described. The opposite wall of the enclosure is cooled at a constant temperature, and all other walls of the enclosure are insulted. The time-dependent solid-liquid shapes of the interface during the melting process, the variation of surface temperature and Nusselt number of the heat sources, and the effects of subcooling are investigated. An empirical correlation predicting the relationship between the liquid-phase fraction and subcooling is given.

Temperature gradients and heat flux measurements in hot pressing of paper

Abstract: Impulse drying was simulated with a platen press equipped with a heated pressing head. The heat transferred to the wet paper sheet and the temperature gradient achieved in the thickness direction of the sheet during the press pulse were measured. The influence of the basis weight, the applied pressure, and the hot surface temperature on the temperature gradient and the total amount of heat transferred was investigated. It was found that at or below 0.5 MPa peak pressure, the wet paper sheet temperature never reaches temperatures significantly above 100°C even if the applied hot surface temperature is as high as 300°C. This is because almost all the load in this case is carried by the fiber network, leading to hydraulic pressures close to atmospheric. At higher press loads only the part of the wet paper sheet closest to the hot surface was heated to temperatures above 100°C. The peak heat fluxes were in the range 2-8 MW/m2, with the time-average heat flux being 0.8-3 MW/m2 at surface temperatures ranging f...

Laboratory Testing of Radiant Gas Burners and Electric Infrared Emitters

Abstract: A project focusing on industrial infrared technology was carried out at Hydro-Quebec's LTEE laboratory [1], The project required the optimization and use of a special test facility. Seven electric emitters and seven radiant gas burners typically used in the textile and pulp and paper industries were evaluated. Three types of experiments were carried out, providing four types of information: radiant flux maps, radiant efficiency evaluation, transient behavior characterization, and spectral emission characterization. The apparatus and the experimental procedures are described. Results are discussed and are presented in the form of tables and graphs. As the procedure and instruments used within each test were identical, this work furnishes rigorous data and objective information on both electric and gas IR technologies.

Measurements of the temperature field in an axisymmetric thermal pure plume

Abstract: An experimental study of the temperature field of a thermal pure plume rising from a heated disk immersed in an air-filled enclosure is presented. The measurements were carried out with a fine thermocouple. The centerline temperature distribution verifies the law —5/3 corresponding to a zone of self-similarity. The profiles of temperature, which follow Gaussian behavior, allow one to define two zones. The half-width of temperature bT and the spread rate are determined and compared with those of other authors. The centerline temperature distribution, according to the dimensionless form proposed by Bill and Gebhart [122], defines the limit of the two zones around Grz = 2.7 × 107 and places the beginning of transition at Grz = 4.7 X 107, following a law Grz −1.14. The flow does not reach a fully developed turbulent regime.

Fragmentation of dendritic crystals during solidification of aqueous ammonium chloride

Abstract: To obtain an improved understanding of the production, development, and transport of crystal fragments during directional solidification of binary alloys, experiments have been performed for an aqueous ammonium chloride solution. For vertical directional solidification (VDS) induced by cooling from below, fragmentation occurred in two stages during the solidification process, each requiring conditions conducive to remelting of dendrite arms. Production increased with increasing and decreasing values of the initial NH 4 Cl composition and chill wall temperature, respectively. For horizontal directional solidification (HDS) induced by cooling from the side, fragmentation occurred in three stages, the first depending exclusively on mechanical fracture and the others involving remelting as a required or dominant production mechanism. Production increased with decreasing values of both the initial NH 4 Cl composition and chill wall temperature. While solid volume fractions of approximately 5% are representative of slurries created by HDS, solid volume fractions are less than 0.5% for VDS. Hence, although fragmentation can have a significant influence on flow and species redistribution in HDS, its effect is likely to be negligible for VDS

Experimental study of the natural-convection plume from a heated vertical cylinder

Abstract: This article describes an experimental study of the natural convection from a vertical cylinder, and gives new data for the velocity and temperature distributions of a heat source with real structure and high heating capacity. Two different instruments, a laser Doppler anemometer and 3D ultrasonic anemometer, were used for velocity measurements. Thermistors were used for temperature measurements. Velocity data were used to evaluate volume flow rates. Velocity fluctuations were used to describe the turbulent behavior of the convection plume. Verticle mean velocity profiles and volume fluxes were compared with values obtained by using the approximate solution for the round turbulent plume associated with a point source. Three important results were obtained. First, the present study produced significantly wider velocity profiles than found in previous studies with buoyant jets. Second, the round turbulent model produced significantly higher volume flow rates than real velocity distributions, which was expla...

Impinging jet atomization at elevated and supercritical ambient temperature and pressure conditions

Abstract: A study of a spray formed by a pair of liquid nitrogen jets impinging on one another at elevated up to above-critical ambient temperature and pressure conditions has been conducted using double-pulse, two-reference-beam holography and high-speed photography. Qualitative observations as well as quantitative measurements on droplet size distribution and its dependence on pressure and temperature were obtained. Droplet size measurements showed that, at subcritical conditions, the increase of the ambient pressure initially makes the atomization quality deteriorate. Further increase in pressure (above 50% of the critical pressure) improves the atomization quality. The spray pattern changed drastically as the ambient pressure approached and exceeded the critical pressure in an environment already at supercritrical temperature. At supercritical ambient conditions, the usual mechanisms of droplet formation are no longer in place. The liquid nitrogen spray undergoes a mixing process with the nitrogen environment.

Heat transfer to a falling viscoelastic fluid film

Abstract: Steady heat transfer to water and viscoelastic fluid films falling down a vertical tube, for the case of uniform wall heat flux thermal boundary condition, was measured experimentally. The viscoelastic fluids studied were aqueous solutions ofPraestol at 100, 200, and 500 ppm (by weight) concentration. The heat transfer to viscoelastic film was lower than the Newtonian prediction, at any given Prandtl number. A new correlation is proposed for dimensionless laminar and turbulent heat transfer coefficient in terms of ike Weissenberg number, over a range of 300 ≤ Reo ≤ 6,500, and 6 ≤Pro ≤ 30.

An experimental investigation of the use of an auxiliary heater to promote more vapor generation during flash boiling of HCFC-22

Abstract: An experimental investigation was conducted to study the influence of a stainless steel, flat spiral, immersion heater on the mass flow during flash boiling in a glass pressure vessel. The data from these experiments were compared with the baseline experimental results primarily with regard to the mass flow. As with the baseline experiments, each test was run for 60 s using varied orifice diameters (1.59 and 5.56 mm), initial refrigerant amounts (0.45 and 0.68 kg), initial pressures (575 and 840 kPa), and vessel geometries (665 and 1,110 ml). Pressures, temperatures, and mass flow rates, along with calculated saturation temperatures, amount of superheat, mass flux, and total mass flashed, were used to compare the baseline experiments with the enhanced boiling method. Results showed a consistent increase in the total mass flashed (or liquid inventory decrease) at each test condition, ranging from an average of 47% to 111% with respect to baseline experiments.

a Study of Liquid Microlayer during Boiling in Narrow Vertical Slot Channels

Abstract: This article presents the results obtained during a study of microlayer evaporation and its role in heat transfer during boiling in narrow slot channels. Experiments were run in a flat vertical channel. The distribution of microlayer thickness along the vertical radius was obtained from interferograms taken using a one-beam laser interferometer and high-speed photography. For the first time, the “nonevaporating” scenario of microlayer evolution, proposed by Moore and Mesler in 1961, was proved experimentally. Analytical examination of the microlayer formation process was carried out using the approach of Katto and Shoji, and taking into account the body force. The comparison of the results obtained, and published previously, is also presented in the article.

The dual beam picosecond continuum technique for measurement of short-time-scale transmission spectra

Abstract: In this work, a dual-beam measurement technique is presented to measure the short-time-scale optical properties of novel materials. It is shown that the picosecond continuum probe source obtained from nonlinear interaction with H2O/D2O is highly variable and does not permit detection of small changes in absorption /transmission spectra. The dual-beam approach is proposed to allow simultaneous measurement of a sample's properties in both the excited and steady states. As an example, the technique is applied to the picosecond transmission spectrum of light-emitting porous silicon. Photo-induced absorption is observed at a level that could not be detected without the new technique.

Double-diffusive convection in an annular enclosure with a hot inner cylinder

Abstract: Consideration is given to the convection in an electrochemical system described, which enables both opposing or aiding temperature and concentration gradients to be imposed. The flows have double-diffusive characteristics because of the large difference between the thermal and solutal diffusion rates. Complex flow patterns are observed as the parameters are varied. Under certain conditions the flow is found to become a fingering and multilayer structure in an annular enclosure. The dimensionless mass transfer rate Sh increases with increasing G,, and fixed Gr m, and Sh is higher in the aiding case than in the opposing case for the same Gr, and Gr " in the present experimental ranges.

Correction of ambient temperature measurement by thermocouple

Abstract: This aim of this study is to assess the error caused by solar radiation in real-site ambient temperature measurements obtained with a thermocouple and to determine a correction method. The results show that the high values of solar radiation that may occur in highly glazed spaces can have a significant influence on thermocouple measurements. The authors therefore decided to validate and develop a corrective model of the measurement error based on an energy balance of the sensor. The influence of the various parameters was studied and the most important error was found to occur when solar fluxes were neglected. They therefore propose a simplified equation to correct the error measurement.