Showing papers in "Experimental Heat Transfer in 1994"

Frictional flow characteristics of water flowing through rectangular microchannels

Abstract: Experiments were conducted to investigate the flow characteristics of water flowing through rectangular microchannels having hydraulic diameters of 0.133-0.367 mm and H/W ratios of 0.333-1. Experimental results indicated that the laminar flow transition occurred at Reynolds numbers of 200-700. This critical Re for the laminar transition was strongly affected by the hydraulic diameter, decreasing with corresponding decreases in the microchannel. In addition, the size of the transition range was diminished and fully developed turbulent flow also occurred at much lower Re. The friction behavior of both the laminar and turbulent flow was found to depart from the classical thermqfluid correlations. lite friction factor, f, was found to be proportional to Re−1.98 rather than Re for the laminar condition, and proportional to Re−1.72i for turbulent flow. The geometric parameters, hydraulic diameter, and H/W were found to be the most important parameters and had a critical effect on the flow. Generally, increasing...

Heat transfer characteristics of water flowing through microchannels

Abstract: The forced-flow convection of water through rectangular microchannels having hydraulic diameters of 0.133-0.367 mm and aspect ratios of H/W = 0.333-1 was investigated experimentally. The flow friction was measured to analyze the heat transfer regimes and to explore the physical aspects of the connective heat transfer. The experimental measurements indicated that the upper bound of the laminar heat transfer regime occurred at a Reynolds number of 200-700, and fully turbulent connective heat transfer was reached at Reynolds numbers of 400-1,500. The transition Reynolds number diminished with the reduction of the microchannel dimension, and the transition range was observed to become smaller in magnitude. For the laminar heat transfer regime, the Nusselt number was found to be proportional to Re0.62while the turbulent heat transfer case exhibited a typical relationship between Nu and Re, but with a different empirical coefficient, CH,t. The geometric parameters were found to be important variables that could...

An experimental study on the initiation and growth of frost formation on a horizontal plate

Abstract: An experimental study was made to clarify the fundamental nature of the early stage of crystal growth period of frost formation phenomena. The effect of four dominant parameters on frost formation, namely, plate temperature, air temperature, air humidity ratio, and Reynolds number, are demonstrated through several frost formation properties such as frost height, frost deposition rate, and frost density. The thickness of the frost layer is affected primarily by the air humidity ratio, plate temperature, and air temperature, while the effect of Reynolds number is less significant. High humidity ratio, Reynolds number, and temperature difference between the air stream and the plate all yield high mass deposition rates. Frost density depends primarily on frost surface temperature, besides the other parameters. Vapor diffusion in the frost layer during the crystal growth period was found to be insignificant,

Correlations for predicting the air-side nusselt numbers and friction factors in chilled-water cooling coils

Abstract: An experimental study was conducted to determine Nusselt numbers and friction factors on the air side of wavy-finned, chilled-water cooling coils. General correlations of the dry-surface Nusselt numbers and friction factors were developed from the data obtained from five different cooling coils. A comparison of the Nusselt number correlation to data from the literature revealed that the correlation was generally able to predict the reported Nusselt numbers within 20%, Under wet-surface conditions, the measured Nusselt numbers showed considerable scatter, with some of the results being higher than the corresponding dry-surface values, while others were lower than the dry-surface values. Friction factors were substantially higher under wet-surface conditions. A correlation to predict Ike friction factors for wet surfaces was also developed.

Solidification of molten metal droplets impinging on a cold surface

Abstract: Two experiments are conducted to study flow and heat transfer behavior in a molten metal droplet on a cold surface following impingement. Laser holographic interferometry is employed to visualize molten metal-solid contact state in real time over the entire solidification process by means of a high-speed video camera. The formation of two distinct solidified layers is disclosed. One, formed in the early stage of solidification, adheres to the cooling surface and is latent heat controlled. The other, formed in the later stage, is nonadhesive and radial-shaped, and is dominated by a slip flow of the molten phase accompanied by phase change. The latter is verified by a separate experiment using thermocouple measurements and inverse conduction theory. These two different heat transfer mechanisms may cause transient thermal and residual stress problems, which would affect the quality of solidified metal products. Results for solidification of a single molten metal drop on quartz and copper surfaces, representi...

Mixed-convection heat transfer to sodium in a vertical pipe

Abstract: Experimental turbulent mixed-convection data are presented for sodium in a uniformly heated vertical pipe. The behavior is found to be opposite to that established for fluids of higher Prandtl number, including mercury, with heat transfer enhanced in upward flow and impaired in downward flow. Velocity and temperature profiles in the sodium have been measured using miniature magnet probes. Velocity profiles are similar to those observed under comparable conditions in other fluids; temperature profiles are somewhat different, with significant variation of temperature extending almost to the pipe centerline. Experimental measurements are compared with predictions obtained using a low-Reynolds-number κ-e turbulence model. Reasonable agreement is achieved for upward flow, but the model overpredicts heal transfer rates for downward flow, as in the case of conventional fluids.

Heat transfer characteristics of an axisymmetric jet impinging on a wall with concentric roughness elements

Abstract: An impinging jet produces a high heal transfer coefficient in the stagnation region by a conventional procedure; an axisymmetric impinging jet has practical applications and produces a higher heat transfer coefficient than does a two-dimensional impinging jet. This high heat transfer coefficient decreases rapidly with the streamwise distance from the stagnation region. Since most of the heat transfer area is located in the downstream region, the heal transfer augmentation of the broad downstream region is a prime target through which to achieve high heat transfer performance. Large-scale concentric repeated roughness on heat transfer surfaces was created to carry out the augmentation for an axisymmetric, submerged, impinging air jet. The investigation was made to measure the local heat transfer coefficients over rough surfaces by varying jet velocity as well as to investigate of the flow structure using the visualization technique.

Dynamics of Cutting Tool Temperatures during Cutting Process

Abstract: An experimental study is performed to measure temperature variation of a carbide insert during metal cutting by means of fine thermocouples connected to a data acquisition system. A semiempiriad equation is developed based on transient conduction solutions combined with experimental data to predict temperature-time history in the cutting tool during operation. The governing parameters in the equation are derived, using AISI 1045 steel as the workpiece, to be the cutting speed, feed rate, and depth of cut at fixed rake and lead angles. Emphasis is placed on analyzing effects of the important factors on the cutting process, such as frictional heat generated at the tool-workpiece interface and wear of the tool. Dynamic behavior of the metal cutting process as predicted by the direct thermocouple measurement agreed with phenomenological observations reported in the existing literature.

Heat transfer regimes in the coolant passages of a diesel engine cylinder head

Abstract: An investigation aimed at identifying the heat transfer regimes present in the valve bridge coolant passages of a diesel engine cylinder head is reported. An inverse heat conduction method utilizing experimental results along with a finite-element model was developed and applied. The sensitivity of the results to uncertain boundary conditions is discussed and found to be minimal. Locations where local boiling occurs were identified.

AN EXPERIMENTAL INVESTIGATION OF THERMAL CHARACTERISTICS IN A 12-MWth CFB BOILER

Abstract: Temperature distributions and beat heat transfer coefficients were measured with shielded thermocouple probes and a heat flux meter, respectively. The measurements were carried out in the combustion chamber of a l2-MWth, circulating fluidized bed (CFB) boiler. Effects of the bed bulk temperature, the suspension density, the superficial gas velocity, and the shape of the heat transfer surfaces on the temperature distribution and heat transfer coefficient were investigated. The results indicate that the heat transfer coefficient increases with increasing bulk bed temperature, superficial gas velocity, and suspension density. The thermal boundary-layer thickness decreases with increasing gas velocity, while the bulk bed temperature has only a small influence on it. The shape of the heat transfer surfaces affects particle motion near the wall, as a result of which differences appear in the heat transfer coefficient and thermal boundary layer. The results were compared with available experimental data in the l...

Heat transfer and fluid flow of turbulent natural convection along a vertical flat plate with a backward-facing step

Abstract: Heat transfer and fluid flow of turbulent natural convection along a vertical flat plate with a backward-facing step were investigated experimentally. The effect of step height on heat transfer was classified through a series of heat transfer measurements. The wall temperature and fluid flow were then visualized using a liquid-crystal sheet and water-soluble fluorescent paint in order to investigate the separation and reattachment of fluid flow. It was revealed that the characteristic low-temperature patterns appear on the backward heat transfer plate and that they play a significant role in heat transfer enhancement. These patterns are generated by the penetration of the low-temperature fluid lumps having three-dimensional and unstable structures near the wall. It was further found that the maximum heat transfer point is not consistent with the reattachment point of the separated flow and that these points shift downstream with increasing step height.

Visualization of Convective Instability Phenomena in the Entrance Region of a Horizontal Rectangular Channel Heated from Below And/or Cooled from above

Abstract: The developing secondary flow patterns caused by buoyancy forces in the simultaneous hydrodynamic and thermal entrance region of a horizontal rectangular channel (aspect ratio, a/b = 2) with isothermally heated lower wall and/or isothermally cooled upper wall are studied using the smoke injection method. The entrance air is at room temperature (around 23°C), and the temperature ranges for the lower and upper walls are 23-51°C and 15-23°C, respectively. The mean air velocities are 0.1, 0.2, 0.3, and 0.4 m/s. The Reynolds number varies from Re = 2.10 × 10 2 to 1.05 × 10 3 and the Grashof number ranges from Gr = 1.56 × 10 5 to 5.04 × 10 5 . Photographic results reveal mixed convection, convective instability, and chaotic phenomena. Many new secondary flow patterns are revealed by this flow visualization study, and the photographic results should be useful for future theoretical study and measurements. The experiments were designed to understand flow physics

Thermal and mechanical responses of gold films during nanosecond laser-pulse heating

Abstract: Free electrons in metal films absorb laser light and then transfer the absorbed photon energy to the metal lattice through electron-lattice collisions, which can result in lattice heating, thermal stress, melting, and evaporation. This work studies nanosecond laser heating of gold films both theoretically and experimentally. A two-step radiation heating model is utilized to characterize transient temperatures of the electron system and the lattice system. Results show that in the nanosecond regime electrons and the lattice are in thermal equilibrium and the classical Fourier heat conduction model is applicable. Microstructures and morphology of films before and after laser pulse heating are characterized with optical and electron microscopes. Two different types of thermal and mechanical responses of gold films are observed. For thin films, thermal stress plays a significant role in laser-film interactions, which can lead to structure changes of films at a temperature much lower than the melting point. Fo...

Enhancement and control of local heat transfer coefficients in a gas flow containing soft magnetic particles

Abstract: A novel technique of enhancing and controlling the local heat transfer coefficients from the hot watt to cooling gas flow has been proposed using soft magnetic solid particles in order to reduce die disadvantage of inefficient gas-solid two-phase flow. The characteristics of the flow and local heat transfer were examined in a vertical rectangular channel flow involving fertile particles 145 pm in diameter. The Reynolds number of the channel flow was set on the order of 104, and the particle mass loading ratio was varied up to 1.6. The result indicated 60% enhancement of heat transfer downstream from the location of the magnet at a mass loading ratio of about 1.6. A farther enhancement in heat transfer is provided by increasing the particle mass loading ratio and the magnetic field intensity. The mechanism of heat transfer enhancement can be clarified by considering the increase in the local heat capacity near the heat plate.

Interfacial Waves with Condensation of a Vapor Flowing Inside a Horizontal, Rectangular Duct

Abstract: Interfacial waves on condensate films inside ducts increase heat transfer rates and lead to changes inflow regimes. We report the results of an experimental study of interfacial waves with condensation of a vapor on the cooled bottom surface of horizontal duct and with air-glycerine-water mixture flow in an identical duct. R-I13 and FC-72 (developed by the 3M Company) were used in the condensation experiments. Wavy flow patterns with two- and three-dimensional waves were observed. Interfacial waves with condensation appeared when the vapor velocity was in the range 1.24-1.56 m/sfor R-I13 and 0.81-1.13 m/sfor FC-72, corresponding to vapor Reynolds numbers in the range 20,000-30,000—the value depends on the difference in the temperatures of the condensing surface and saturated vapor. The ratio of the distance from the leading edge (at which the waves appeared) to the liquid film thickness was found to be a Junction of the liquid Reynolds number. Condensation stabilizes the interface. None of the several ava...

Two-dimensional jet impinging on a wall with roughness elements

Abstract: Since a very high heat transfer coefficient can be obtained by an impinging jet in its stagnation region, this flow geometry has been used for heating, cooling, and drying in a variety of industrial applications. This high heat transfer coefficient, however, deteriorates rapidly with the streamwise distance downstream of the stagnation line, and hence the broad downstream region is targeted by the present study on heat transfer augmentation; large-scale roughness elements of a transverse repeated-rib type are mounted over the heat transfer surface. The experimental results obtained at various jet velocities indicate effective heat transfer augmentation.

Effect of Fluid Properties on Heat Transfer in Channels with Offset Strip Fins

Abstract: A set of experiments was devised to investigate the effect of Prandtl number on the thermal performance of offset strip fins and to verify the extension of Kays and London air test data to liquids. An offset strip fin with a cross section of 6.25 cm × 0.938 cm (2.5 in × 0.375 in) and a flow length of 25.4 cm (10 in) was tested for water and ethylene glycol at Reynolds numbers from 30 to 3,000, and Prandtl numbers from 4 to 80. Results correlate according to Nu = 0.287 Re0.58 Pr0.5. For the conditions tested in the present work, the fin efficiencies were in the range of 5-25%. It is suggested that for liquid coolants or for asymmetric boundary conditions, a different fin geometry needs to be designed for higher efficiencies.

Simple apparatus for thermal conductivity measurements of unconsolidated powders

Abstract: A transient technique for measuring the thermal conductivities of unconsolidated powders is described. The apparatus is fully automated and allows measurements to be made conveniently from room temperature to 800 K and in various void-fitting gases at pressures up to several atmospheres. Results are presented for the effective thermal conductivity of powdered iron carbide in several gases and the data are compared with the predictions of several simple models.

Study of Heat Transfer Augmentation in a High-Temperature Field by a Radiation Promoter Generating a Secondary Flow

Abstract: Characteristics of flow and heat transfer have been investigated for a circular pipe in which a pair of twisted tapes were inserted to enhance the convective and radiative heat transfer. From the measurement of the three-dimensional velocity field, it was clarified that a secondary flow was induced by the present promoter, heat transfer performance was enhanced about three times compared with that of a smooth pipe, and a remarkable increase of the local heat transfer rate was observed near the impinging region of the secondary flow. The performance of the present promoter was evaluated based on the total surface area for heat transfer under the same conditions of flow rate, pressure loss, and heat load. It was found that, at room temperature, the total heat transfer area could be reduced to about 30% less than for a smooth pipe. In the case of high temperature, above 450°C, the heat transfer rate was further increased 50% due to the effect of radiation between the pipe watt and the promoter. By using a ve...

Heat Transfer Enhancement due to Turbulence Induced by Particle Motion in Gas-Solid Fluidized Beds

Abstract: Heat transfer augmentation due to turbulence in the gaseous flow of a gas-solid fluidized bed is analyzed. Since the heat transfer in such beds is enhanced by various mechanisms, each mechanism's contribution can only be separately evaluated using special means. Therefore, we employed a new mass transfer measurement technique to measure the contribution of the turbulence induced by particle motion on the total heat transfer occurring around a horizontal test cylinder immersed in a fluidized bed. Results indicate that the mass transfer, i.e., analogous to convective heat transfer to or from the gaseous flow, is enhanced by the turbulence produced from particle motion on the front side of the cylinder surface, but that other heat transfer mechanisms besides turbulence contribute to the heat transfer augmentation taking place on the cylinder side walls and back-side surface.

Measurements of local heat transfer for forced convection and flow boiling in horizontal, uniformly heated smooth tubes

Abstract: This work deals with improved ways to characterize the local heat transfer inside smooth circular channels. The objectives art to: (1) measure local (axial) channel wall temperatures, and (2) make comparative predictions of local subcooled flow-boiling heat transfer for the case of the a uniformly heated channel. Flow boiling experiments were conducted with Freon-II in horizontal, copper, uniformly heated coolant channels with smooth walls. Temperature measurements were made on the outside surface of the copper channel at four circumferential and seven axial locations. Comparisons were made for inside computed wall temperature and the axial mean circumferential heat transfer coefficient with a subcooled flow-boiling model based on the assumption of large Froude number. The predictions served as a comparative baseline and a qualitative check for the data.