Showing papers on "Heat transfer coefficient published in 2002"

Dropwise condensation theory and experiment: A review

Abstract: The paper reviews progress in dropwise condensation research from 1930 to the present. Particular attention is given to heat transfer measurements, theory, transition and effects of surface material. Although it has been known since the 1930s that heat transfer coefficients for dropwise condensation of steam are much higher than those for film condensation, there were, until the 1960s, wide discrepancies between the results of different investigators. Subsequently, more accurate measurements have shown good consistency and the mechanism and theory of the dropwise condensation have become better understood. There has been considerable controversy over the magnitude of the so-called ‘constriction resistance’ and the effect of the surface thermal conductivity on the heat transfer coefficient. The balance of evidence suggests that this is only significant at very low heat fluxes and for very small condensing surfaces. Measurements have also been made with sufficiently high cooling intensities to cover...

Convective heat transfer and flow characteristics of Cu-water nanofluid

Abstract: An experimental system is built to investigate convective heat transfer and flow characteristics of the nanofluid in a tube. Both the convective heat transfer coefficient and friction factor of Cu-water nanofluid for the laminar and turbulent flow are measured. The effects of such factors as the volume fraction of suspended nanoparticles and the Reynolds number on the heat transfer and flow characteristics are discussed in detail. The experimental results show that the suspended nanoparticles remarkably increase the convective heat transfer coefficient of the base fluid and show that the friction factor of the sample nanofluid with the low volume fraction of nanoparticles is almost not changed. Compared with the base fluid, for example, the convective heat transfer coefficient is increased about 60% for the nanofluid with 2.0 vol% Cu nanoparticles at the same Reynolds number. Considering the factors affecting the convective heat transfer coefficient of the nanofluid, a new convective heat transfer correlation for nanofluid under single-phase flows in tubes is established. Comparison between the experimental data and the calculated results indicate that the correlation describes correctly the energy transport of the nanofluid.

Measurement of interstitial convective heat transfer and frictional drag for flow across metal foams

Abstract: Convective heat transfer and friction drag in a duct inserted with aluminum foams have been studied experimentally. The combined effects of foam porosity (e=0.7, 0.8, and 0.95) and flow Reynolds number (1900≤Re≤7800) are examined. Frictional drags for flow across the aluminum foam are measured by pressure taps, while interstitial heat transfer coefficients in the aluminum foam are determined using a transient single-blow technique with a thermal non-equilibrium two-equation model. Solid material temperature distribution is further measured for double check of the heat transfer results. To understand the frictional drag mechanisms, smoke-wire flow visualization is conducted in the aluminum-foam ducts. Results show that both the friction factor and the volumetric heat transfer coefficient increase with decreasing the foam porosity at a fixed Reynolds number. In addition, the aluminum foam of e=0.8 has the best thermal performance under the same pumping power constraint among the three aluminum foams investigated. Finally, empirical correlations for pore Nusselt number are developed in terms of pore Reynolds number under various foam porosities

Measurements of Heat Transfer Coefficients From Supercritical Carbon Dioxide Flowing in Horizontal Mini/Micro Channels

Abstract: Heat transfer from supercritical carbon dioxide flowing in horizontal mini/micro circular tubes cooled at a constant temperature has been investigated experimentally. Six stainless steel circular tubes having inside-diameters of 0.50 mm, 0.70 mm, 1.10 mm, 1.40 mm, 1.55 mm, and 2.16 mm were tested. Measurements were carried out for the pressures ranging from 74 to 120 bar, the temperatures ranging from 20 to 110°C, and the mass flow rates ranging from 0.02 to 0.2 kg/min. It is found that the buoyancy effect was still significant, although supercriticalCO2 was in forced motion through the horizontal tubes at Reynolds numbers up to 10 5 . The experimental results also indicate that the existing correlations developed in the previous studies for large tubes deviate significantly from the experimental data for the present mini/micro tubes. Based on the experimental data, a correlation was developed for the axially averaged Nusselt number in terms of appropriate dimensionless parameters for forced convection of supercritical carbon dioxide in horizontal mini/micro tubes cooled at a constant temperature. @DOI: 10.1115/1.1423906#

Condensation of Halogenated Refrigerants Inside Smooth Tubes

Abstract: The paper presents a new predicting model to compute the heat transfer coefficient and pressure drop during condensation inside smooth tubes when operating with pure or blended halogenated refrigerants, including the new high-pressure HFC fluids, for which the existing predicting methods are inadequate. The suggested model is based on a predictive study of the flow patterns occurring during the condensation process. Predictions from this new procedure are compared both with the authors' experimental data and with a wide experimental data bank from independent authors; excellent agreement is demonstrated in almost all cases. Heat transfer coefficients were obtained during condensation of refrigerants R-22, R-134a, R-125, R-32, R-236ea, R-407C, and R-410A in an 8 mm inside diameter plain tube, carried out at a saturation temperature ranging between 30 and 50°C, and mass velocities varying from 100 to 750 kg/(m2·s), over the entire vapor quality range. For zeotropic blended refrigerants, such as R-407C, the ...

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

Abstract: 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.

New correlation to predict the heat transfer coefficient during in-tube cooling of turbulent supercritical CO2

Abstract: The Nusselt number variations of supercritical carbon dioxide during in-tube cooling are presented and discussed. Using data presented in this paper as well as prior publications, a new correlation to predict the heat transfer coefficient of supercritical carbon dioxide during in-tube cooling has been developed. The new correlation is presented in this paper. It is based on mean Nusselt numbers that are calculated using the thermophysical properties at the wall and the bulk temperatures, respectively. It is seen that the majority of the numerical and experimental values are within ±20% of the values predicted by the new correlation.

Heat Transfer Model for Evaporation of Elongated Bubble Flows in Microchannels

Abstract: Reference LTCM-ARTICLE-2005-032View record in Web of Science Record created on 2005-07-06, modified on 2017-05-10

Bubble Behavior and Nucleate Boiling Heat Transfer in Saturated FC-72 Spray Cooling

Abstract: Bubble behavior during saturated FC-72 spray cooling was experimentally investigated. A heater previously used for pool boiling was used to allow direct comparison. The results are analyzed to reveal the interaction between bubbles and impinging droplets. The following are presented: (1) the importance of secondary nuclei entrained by impingement droplets, (2) the role of impinging droplets on bubble parameters such as growth, diameter at puncture, lifetime, life cycle and bubble number density, and (3) the relative contribution of nucleation, especially that of secondary nuclei, to the heat transfer. It is concluded that increasing the droplet flux increases the number of secondary nuclei helps to lower surface temperature for a given heat flux, increases the overall heat transfer coefficient, and increases heat transfer due to both nucleate boiling and enhanced convection. Increasing the droplet flux also shortens the bubble growth time (i.e., resulting in earlier bubble removal) and life cycle. However, increasing the droplet flux (and, therefore, secondary nucleation) for each of the three heat flux values does not affect the percentage of either nucleate or convection heat transfer. This suggests that both the nucleate and convection heat transfer are enhanced, as a result of increased secondary nuclei and turbulent mixing due to the impinging droplets.

Free-Stream Turbulence Effects on Film Cooling With Shaped Holes

Abstract: A comprehensive set of generic experiments has been conducted to investigate the effect of elevated free-stream turbulence on film cooling performance of shaped holes. A row of three cylindrical holes as a reference case, and two rows of holes with expanded exits, a fanshaped (expanded in lateral direction), and a laidback fanshaped hole (expanded in lateral and streamwise direction) have been employed. With an external (hot gas) Mach number of Ma m =0.3 operating conditions are varied in terms of free-stream turbulence intensity (up to 11%), integral length scale at constant turbulence intensity (up to 3.5 hole inlet diameters), and blowing ratio. The temperature ratio is fixed at 0.59 leading to an enginelike density ratio of 1.7. The results indicate that shaped and cylindrical holes exhibit very different reactions to elevated free-stream turbulence levels. For cylindrical holes film cooling effectiveness is reduced with increased turbulence level at low blowing ratios whereas a small gain in effectiveness can be observed at high blowing ratios. For shaped holes, increased turbulence intensity is detrimental even for the largest blowing ratio (M=2.5). In comparison to the impact of turbulence intensity the effect of varying the integral length scale is found to be of minor importance. Finally, the effect of elevated free-stream turbulence in terms of heat transfer coefficients was found to be much more pronounced for the shaped holes.