Convective Heat Transfer Experiments Research Articles

Convection Heat Transfer Measurements to Fine Stationary Wires with Application to Fiber Dry Spinning

Convection heat transfer experiments were performed on a single fine wire in parallel flow. By successively varying the wire length tested, it was determined that entrance effects were negligible. Data were also obtained on a wire bundle in parallel flow to more closely duplicate the conditions typical of dry spinning applications. In that case, it was determined that an individual wire was unaffected by the proximity to adjacent wires, provided those adjacent wires were not heated. Significant differences were observed between heat transfer from a single fine wire and a wire bundle, if all the wires in the bundle were heated simultaneously.

초음파 진동에 의해 발생된 음향유동을 활용한 급속냉각 메카니즘

Acoustic streaming Induced by longitudinal vibration at 30 kHz is visualized for a test fluid flow between the stationary glass plate and ultrasonic vibrating surface with particle imaging velocimetry (PIV) To measure an increase in the velocity of air flow due to acoustic streaming, the velocity of air flow in a gap between the heat source and ultrasonic vibrator is obtained quantitatively using PIV. The ultrasonic wave propagating into air in the gap generates steady-state secondary vortex called acoustic streaming which enhances convective cooling of the stationary heat source. Heat transfer through air in the gap is represented by experimental convective heat transfer coefficient with respect to the gap. Theoretical analysis shows that gaps for maximum heat transfer enhancement are the multiple of half wavelength. Optimal gaps for the actual design are experimentally found to be half wavelength and one wavelength. A drastic temperature variation exists for the local axial direction of the vibrator according to the measurement of the temperature distribution in the gap. The acoustic streaming velocity of the test fluid in the gap is at maximum when the gap agrees with the multiples of half wavelength of the ultrasonic wave, which are specifically 6 mm and 12 mm.

Transient temperature analysis of potato during drying: theoretical modelling and experimental validation

SYNOPSIS A mathematical model describing the heat transfer diffusion equation with boundary conditions incorporating the effect of surface moisture transfer during drying is proposed for transient temperature prediction in cylinders and slices of potato. Lap-lace transform was applied to the solution of the governing equations. The experimental drying constants and convective heat transfer coefficients obtained with and without sample shrinkage were used in the prediction model. Comparison between the predicted and measured mean potato temperature indicates that the predicted values using shrink-age-dependent parameters fit the experimental data better than those obtained without considering shrinkage. The present investigation indicates that the redvoditelev number, Fourier number and convective heat transfer coefficient have significant influence on the temperature prediction. Results of error analysis reveal that the accuracy of predicted values of sample temperature is within acceptable limits.

Heat Transfer Enhancement by Electric Fields in Several Heat Exchange Regimes

With the present article, the authors summarize over 15 years of work dedicated to studying the effects of the electrical and the gravitational force fields on two-phase and single-phase thermofluid dynamics. Results obtained on several microgravity platforms are presented and the role played by the electric field in the different heat transfer processes is analyzed. In particular, the regimes of nucleate boiling and film boiling are treated. Also, the parameters controlling the liquid-vapor interface instability and the main consequences regarding the critical heat flux (CHF) are outlined. In the final section, the promising technique of ion injection for efficient heat transfer enhancement in single-phase liquids is described, together with the main results obtained under different flow regimes and geometries. The following dielectrics were compared: R113, Vertrel XF, and FC-72 for the pool boiling research; again FC-72 and HFE-7100 for the single-phase convection heat transfer experiments.

Intercomparisons of Experimental Convective Heat Transfer Coefficients and Mass Transfer Coefficients of Urban Surfaces

The convective heat transfer coefficient (CHTC) of an urban canopy is a crucial parameter for estimating the turbulent heat flux in an urban area. We compared recent experimental research on the CHTC and the mass transfer coefficient (MTC) of urban surfaces in the field and in wind tunnels. Our findings are summarised as follows. Although there is some agreement in the measured values, our overall understanding of the CHTC remains too low for accurate modelling of urban climate.

Simulation of mixed convection heat transfer to carbon dioxide at supercritical pressure

Computational simulations of turbulent mixed convection heat transfer experiments using carbon dioxide at supercritical pressure have been performed by solving the Reynolds averaged transport equations using an elliptic formulation. A number of two-equation low Reynolds number turbulence models have been used and the results have been compared directly with the experimental data. It has been shown that most of the models were to some extent able to reproduce the effects of the very strong influences of buoyancy on heat transfer in these experiments. However, the performance of the models varied significantly from one to another in terms of the predicted onset of such effects.

Experimental study on convective coefficients in a slurry bubble column

The main objective of this work was to determine the influence of gas and slurry velocities and solid load concentration on experimental convective heat transfer coefficients (h), for a three-phase suspension in cross flow around a square in-line tube array. Experiments were performed in a pilot scale equipment conformed by a column of square cross section. In the central part of this equipment there is a bank of 49 tubes with a S T =S L =4.4 cm. The system used was air, tap water and sieved sand at average particle diameter of 505 μm. The velocities of liquid and gas phases were varied from 0.28 to 0.60 cm/s and from 1.36 to 2.45 cm/s, respectively; and the solid load concentration from 0 to 30% w/w. The results allow to conclude that h for a three-phase system is larger than either for a two-phase or one-phase system. The presence of solid phase enhances the heat transfer, however, this increase is less evident for high values of solid Concentration.

Variable Property Mixed Convection Heat Transfer to Air Flowing Through a Vertical Passage of Annular Cross Section: Part 1

In this paper, an experimental study is reported of variable property mixed convection heat transfer to air flowing through a vertical passage of annular cross-section of diameter ratio 2.1, having a uniformly heated core and a thermally insulated outer casing. This is the first of three related papers each of which is concerned with buoyancy-influenced flow and heat transfer in vertical passages with one surface heated and the opposite one unheated. The second paper also reports mixed convection heat transfer experiments with a vertical passage of annular cross-section. The diameter ratio of 1.9 is similar. However, in that case the working fluid was water rather than air and effects of fluid property variations were negligibly small. The third paper reports mixed convection heat transfer experiments with a vertical plane passage of relatively short length. The working fluid was air. In all three cases the need for the experiments arose in connection with nuclear power plant design and operation. However, the studies are all of a fundamental nature and address an interesting and important aspect of buoyancy-influenced convective heat transfer in vertical passages. The results presented in this first paper show, that with a passage of annular cross-section having a heated inner surface and an adiabatic outer one, the heat transfer behaviour is generally similar to that found with uniformly heated circular tubes but that there are certain important differences. Thermal development is slower and the effects of buoyancy and variable properties are both weaker. As with circular tubes, the mixed convection data for downward flow show systematic enhancement of heat transfer with an increase of buoyancy influence and a fully developed heat transfer situation is readily achieved. The results for such conditions correlate satisfactorily in terms of Nusselt number ratio (mixed to forced) and a buoyancy parameter which combines Grashof number, Reynolds number and Prandtl number in a particular manner designed to characterize the strength of buoyancy influences. The results are fitted satisfactorily by a simple correlation equation based on a semi-empirical model. In the case of mixed convection with upward flow, heat transfer is always less effective than with downward flow. Impairment of heat transfer occurs with the onset of buoyancy influences, but more gradually and at a somewhat higher values of buoyancy parameter than in circular tubes. Non-monotonic axial distributions of Nusselt number are found and the data do not correlate in terms of purely local parameters. A fully developed situation is not readily achieved. With further increase of buoyancy influence, heat transfer recovers and eventually becomes enhanced.

F218 水-空気系泡沫の流れ内に置かれた加熱円筒の熱伝達(オーガナイズドセッション22 : 熱輸送デバイス)

Forced convection heat transfer experiments of a liquid-gas foam were conducted by using a heated circular cylinder. In the temperature range around 373K, the heat flux for the foam flow is greater then that for water. In other temperature ranges, the heat flux for foam flow is lower than that of water. The heat transfer coefficient increases with increasing foam velocity and with decreasing the expansion ratio of the foam. It was indicated that the value of the coefficient greatly increases when the temperature of the heating surface approaches to 373K. Latent heat transfer by water vapor diffusion in the foam is the one of the important factors that increases the heat transfer coefficient near the boiling point of the foam.

Forced convective heat transfer in porous medium of wire screen meshes

The hydrodynamic and heat transfer characteristics of a porous medium consisting of 20 wire screen meshes are examined theoretically and experimentally. The hydrodynamic experiments are conducted for the range of Reynolds number based on mean velocity and wire diameter from 1.5 to 12. The Ergun's constants and thermal dispersion coefficients are calculated in this range. Nusselt number variation is determined in both thermally developing and fully developed flows by the help of forced convection heat transfer experiments conducted for the uniform heat flux boundary condition. Correlation functions of Nusselt number in the range of fully developed and thermally developing, and of thermal entrance length are obtained from experimental data. Solutions of momentum and energy equations simulating the experimental model are obtained numerically with variable porosity and the anticipated thermal dispersion coefficients. The thermal dispersion coefficients well-adjusted to the experimental data are determined by numerical solution of the energy equation.

Heat transfer and hydraulic resistance in evaporative coil condensers made from tubes with bent ribs

we have completed convective heat transfer experiments in which nitrogen is boiled at atmospheric pressure on the ribbed surfaces of tube specimens with bent ribs

Natural convection heat transfer from a vertical plate—II. With gas injection and transverse magnetic field

A natural convection heat transfer experiment was conducted in mercury with gas injection in a vertical enclosure heated on one face at constant heat flux and cooled on the opposite face. Nitrogen gas bubbles were injected from a row of hypodermic tubes facing upwards along the bottom of the heated plate. A transverse magnetic field of magnitude 0.07 ⩽ B ⩽ 0.50 Tesla was imposed perpendicular to the heated surface. The range of the applied heat flux was 370 ⩽ q“ ⩽ 16 000 W m t-2 , corresponding to a modified Boussinesq number range of 10 5 ⩽ Bo ∗ x ⩽ 10 9 . The gas injection rate range was 0.9 ⩽ Q g ⩽ 9.2 cm 3 s −1 . Local heat transfer and void measurements were made with thermocouple and double-conductivity probes. Experimental results showed that for low heat flux rates, a small magnetic field intensity ( B ∼ 0.07 T) significantly reduced the heat transfer coefficient in the presence of gas injection which in the absence of the magnetic field enhanced the heat transfer coefficient two- to three-fold relative to the single-phase result. The decrease was attributed to the suppression of both the laminar convection and the bubble-induced liquid motion. At higher heat fluxes, the decrease in the heat transfer coefficient in the presence of the magnetic field was less significant. Nevertheless, with increasing field intensity the Nusselt number decreased at these higher heat fluxes. An increase in the bubble size and bubble velocity stabilized the continuous decrease in the heat transfer coefficient for field intensities in the range B ∼ 0.25–0.35 T. At higher field intensities ( B ∼ 0.50 T) temperature profiles indicated a conduction-dominated heat transfer mechanism.

Natural convection heat transfer from a vertical plate—I. Enhancement with gas injection

A natural convection heat transfer experiment in mercury was conducted with gas injection in a vertical enclosure heated on one face at constant heat flux and cooled on the opposite face. Nitrogen gas bubbles were injected from a row of hypodermic tubes facing upwards along the face of the heated plate. The range of the applied heat flux was 370 ⩽ q“ ⩽ 16 000 W m −2 , which corresponds to a modified Boussinesq number range of 10 5 ⩽ Bo ∗ x ⩽ 10 9 . The gas injection rate range was 0.9 ⩽ Q g ⩽ 9.2 cm 3 s −1 . Local heat transfer and void fraction measurements were made with thermocouple and double-conductivity probes, respectively. The measured heat transfer coefficient at low heat fluxes with gas injection, where the free convective flow was mostly laminar along the heated plate, was enhanced at least two-fold. In comparison, at higher heat fluxes, where the flow was predominantly turbulent, gas injection enhanced the heat transfer coefficient very little even at the highest injection rate. The overall enhancement in the heat transfer coefficient due to the bubble-induced turbulence is a collective action of both the induced liquid motion by the bubbles and the turbulence created in their trails. Void fraction measurements at both low and high heat fluxes indicated a three layer boundary layer structure distinct from the thermal boundary layer. The layers are: a single-phase inner layer next to the heated wall, a two-component layer where most of the bubbles reside, and an outer single-phase layer extending to the core of the cell. An analysis of our mercury heat transfer data with zero gas injection indicates that the transition to turbulence occurs at a Rayleigh number of about 2 × 10 7 rather than 10 9 , the accepted value.

Heat transfer from polygonal horizontal isothermal surfaces

Each regular surface can be considered as a sum of isosceles triangles of different apex angle (ε). Triangular, square, hexagonal and circular plates consist of three, four, six and an unlimited number of triangles of apex angles ε = π/3, π/4, π/6 and ⇒0°, respectively. Simplified theoretical consideration for extracted repeated fragments of the surface, describing it in the form of triangles, suitable for any polygon, has been performed. Two models of fluid flow over heated surfaces are proposed. The fluid flow direction in the first model was perpendicular to the leading edge and stream lines are parallel to each other. In the second one it has been assumed that fluid flows from the leading edge concentrically towards the apex angle of the considered triangular surface. In both models free boundary layers transform into plumes above the center of the plates. The solutions of these models are presented in the form of dimensionless Nusselt-Rayleigh relations with the function of apex angles as a polygon parameter. The results of experimental investigations of horizontal isothermal triangular, square, hexagonal and circular plates are presented. The free convection heat transfer experiments and visualization were carried out using plates of the same diameter d = 0.07 m of the circle inscribed in polygon and glycerine as the test fluid.

A simple model for estimation of the film coefficients during cooling of certain spherical foodstuffs with water

In this study, a method was proposed and tested for the determination of the convective heat transfer coefficients during cooling of the spherical foodstuffs by means of a water flow in the pool. Experiments were performed in a cold water pool at temperatures of 0.8, 1 and 1.2°C, and at flow velocity of 0.05 m/s. The product temperatures at the center, semi-radial distance and surface were measured and then the experimental convective heat transfer coefficients were obtained by using the proposed model which was the function of dimensionless temperature and the Biot and Fourier numbers. The semitheoretical convective heat transfer coefficients were also calculated by Vliet and Leppert's correlation. Furthermore, the experimental and semitheoretical coefficients were plotted against time at different water temperatures for two products.

An Experimental Convective Heat Transfer Investigation Around a Film-Cooled Gas Turbine Blade

The present paper deals with an experimental convective heat transfer investigation around a film-cooled, high-pressure gas turbine rotor blade mounted in a stationary, linear cascade arrangement. The measurements were performed in the von Karman Institute Isentropic Light Piston Compression Tube facility. The test blade was made of Macor glass ceramic and was instrumented with thin film gages. The coolant flow was ejected simultaneously through the leading edge (three rows of holes), the suction side (two rows of holes), and the pressure side (one row of holes). The effects of overall mass weight ratio, coolant to free-stream temperature ratio, and free-stream turbulence were successively investigated.

Microprocessor-based data-acquisition system for thermo-fluids laboratories

A simple, relatively inexpensive and very flexible microprocessor-based data-acquisition system is described for use in thermo-fluids laboratories. The system, which is currently being used in natural and mixed convection heat transfer experiments, is completely programmable and can be expanded to meet growing needs. It is capable of receiving a variety of inputs, storing and processing the data, as well as controlling the entire data collection procedure. The data-acquisition system described can be assembled at about the same cost as a single, commercially available, one function data-logging device.

Heat Transfer From Horizontal Serrated Finned Tubes in an Air-Fluidized Bed of Uniformly Sized Particles

A primary objective of this work was to gather experimental convective heat transfer coefficient data for extended surfaces. The results were then compared with bare horizontal tubes in a similar flow environment. The finned tube configurations were considered an extension of the bare tube arrangements to enhance heat transfer. Heat transfer coefficients were computed from the heated tube surface temperature, the fluidized bed temperature, and the power input to the tube. Selected serrated finned tubes were used. The experimental heat transfer data were measured as a function of fluidized bed flow parameters and finned tube geometry. Several tests were performed using two different uniformly sized glass particles (.21 mm and .43 mm). Fin efficiency factors were determined and presented as a function of mass velocity and as a function of a fin length parameter. The finned tube demonstrated a better heat transfer capacity over bare tubes for similar fluidization conditions.

Energy transport in high temperature and reacting gases

Methods presently available for estimating convective and conductive heat transfer in high temperature gases are reviewed, emphasizing the role of viscosity and thermal conductivity. Examination of engineering heat transfer correlations shows that large errors in thermal conductivity and viscosity lead to considerably smaller errors in heat transfer coefficient provided transport properties are obtained by techniques derived from rigorous kinetic theory. Because uncertainties in high temperature transport properties arise largely as a result of uncertainties in collision cross-sections, methods of estimating these cross-sections are considered; estimates show encouraging agreement with experiment for both stable and unstable species. Approximate formulas for viscosity and conductivity of mixtures are compared with rigorous formulas and with experiment. Heat conduction in reacting gases is then considered; in such mixtures the diffusional transport of chemical enthalpy may lead to thermal conductivities an order of magnitude greater than in similar, but nonreacting gases. Theoretical predictions of the equilibrium conductivity compare favorably with experimental data for the nitrogen tetroxide and hydrogen fluoride systems. When chemical reaction rates are not high enough to maintain chemical equilibrium locally in a temperature gradient, theory shows that the effective thermal conductivity is related to the ratio of a diffusion time to the chemical relaxation time; low pressure data for the nitrogen tetroxide system are in accord with this analysis. Finally it is shown that experimental convective heat transfer in practical systems can be treated by these methods. It is concluded that methods for calculating high-temperature heat transfer are in many respects well developed, so that rigorous calculations of the transport properties are often justified.

Use of molybdenum sulfide as a lubricant for cold rolling of tubes : I. M. Pavlov, Iu. F. Shevakin, and Iu. S. Seidaliev. Henry Brutcher Translation No. 4678, 4 pp. (From Izv. VUZ, Chern, Met, no. 7, July 1958, p. 191–193.) Henry Brutcher, Altadena, Calif.

The present study demonstrates an alternative approach for describing fluid flow characteristics very close to the wall, using locally resolved convective heat transfer experiments. Heat transfer coefficients on the base surface and around a surface mounted vortex generator of delta-wing shape design, are evaluated with the transient liquid crystal measurement technique and over a range of freestream velocities. Therefore, the local values of exponent m in the equation Nux∼Rexm, which is directly linked to the structure of the boundary layer, can be determined over the complete heat transfer area. The local distributions of exponent m are then directly compared to the footprint of the flow obtained with typical oil and dye surface flow visualisation. The results indicate that a more appropriate interpretation of the flow structures very close to the wall is possible by analysing the spatial variation of exponent m, which approximates better the flow pattern compared to the heat transfer coefficients. As a result, fluid flow topologies can be directly evaluated from the heat transfer experiments since the distributions of oil-flow visualisation and exponent m are qualitatively similar.