Heat Transfer In Smooth Tubes Research Articles

Experimental study on entropic behavior of smooth pipe due to the simultaneous usage of spring turbulator and air bubble injection

Numerous investigations were conducted to enhance heat transfer in smooth tubes. They have used both active and passive methods. However, very rare investigations have reported the simultaneous usage of active and passive methods. Furthermore, the investigations on entropic behaviors are very rare too. Through present paper, effect of air bubble injection (as active method) together with spring wire turbulator (as the passive method) is probed. The pipe was subjected to a constant heat flux. Spring pitch and fluids flow rates were variable parameters of this study. Four flow levels of 2, 3, 4, and 5 were intended for water and air. The studied parameters were Nusselt number, cost-performance coefficient, and dimensionless exergy destruction. The flow patterns were also assessed. To calculate the Nusselt number and exergy destruction, ambient temperature, pipe surface temperature, water inlet, and outlet temperatures were measured. Results showed that reducing the induced rotational flow by the pitch, affects two-phase flow and leads significant changes flow pattern and two-phase flow motion model. Air bubble injection increased Nu by 40% and dimensionless exergy destruction by 36%.

An Experimental Study of In-Tube Condensation and Evaporation Using Enhanced Heat Transfer (EHT) Tubes

A study was carried out to determine in-tube evaporation and condensation performance of enhanced heat transfer tubes (EHT) using R410A, with the results being compared to a plain tube. The test tubes considered in the evaluation include: plain, herringbone (HB) and spiral (HX) microgrooves, herringbone dimple (HB/D), and hydrophobic herringbone (HB/HY). Experiments to evaluate the condensation were conducted at a saturation of 318 K, and at 279 K for evaporation. Mass flux (G) ranged between 40 to 230 kg m−2s−1. Condensed vapor mass decreased from 0.8 to 0.2; and the mass of vaporized vapor increases from 0.2 to 0.8; heat flux increased with G. Inlet and outlet two-phase flow patterns at 200 kg m−2s−1 were recorded and analyzed. Enhanced tube heat transfer condensation performance (compared to a plain tube) increased in the range from 40% to 73%. The largest heat transfer increase is produced by the herringbone–dimple tube (HB/D). In addition to providing drainage, the herringbone groove also helps to lift the accumulated condensate to wet the surrounding wall. Evaporation thermal performance of the enhanced tubes are from 4% to 46% larger than that of smooth tube with the best performance being in the hydrophobic herringbone tube (HB/HY). This enhancement can be attributed to an increase in the number of nucleation sites and a larger heat transfer surface area. Evaporation and condensation correlations for heat transfer in smooth tubes is discussed and compared.

Experimental research of boiling heat transfer of smooth and screwed tube

AbstractIn this paper, based on the analog theory of heat transfer research, we performed an analog experiment on boiling heat transfer in smooth tube and screwed tubes. These are widely used in the high pressure generator of lithium bromide absorption refrigeration. From the experimental research, we obtained a series of results on the boiling heat transfer of a single smooth tube and three screwed tubes. The working condition is near the zone of bubble boiling and the overheat wall temperature ranges from 2–7 °C, with a fluid medium of pure water and salt water solution. These results agreed well with the known results, and are significant for the practical design and application of a high pressure generator of lithium bromide absorption for refrigeration. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(2): 74–84, 2007; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/htj.20145

Flow boiling heat transfer in a vertical spirally internally ribbed tube

Experiments of flow boiling heat transfer and two-phase flow frictional pressure drop in a spirally internally ribbed tube (φ22×5.5 mm) and a smooth tube (φ19×2 mm) were conducted, respectively, under the condition of 6×105 Pa (absolute atmosphere pressure). The available heated length of the test sections was 2500 mm. The mass fluxes were selected, respectively, at 410, 610 and 810 kg/m2 s. The maximum heat flux was controlled according to exit quality, which was no more than 0.3 in each test run. The experimental results in the spirally internally ribbed tube were compared with that in the smooth tube. It shows that flow boiling heat transfer coefficients in the spirally internally ribbed tube are 1.4–2 times that in the smooth tube, and the flow boiling heat transfer under the condition of smaller temperature differences can be achieved in the spirally internally ribbed tube. Also, the two-phase flow frictional pressure drop in the spirally internally ribbed tube increases a factor of 1.6–2 as compared with that in the smooth tube. The effects of mass flux and pressure on the flow boiling heat transfer were presented. The effect of diameters on flow boiling heat transfer in smooth tubes was analyzed. Based on the fits of the experimental data, correlations of flow boiling heat transfer coefficient and two-phase flow frictional factor were proposed, respectively. The mechanisms of enhanced flow boiling heat transfer in the spirally internally ribbed tube were analyzed.

Forced convection heat transfer in smooth tubes roughened by helically coiled ribbons

Forced convection heat transfer in smooth tubes roughened by helically coiled ribbons

Heat Transfer and Fluid Friction for Fully Developed Turbulent Flow of Air and Supercritical Water With Variable Fluid Properties

Abstract A previous analysis for fully developed turbulent flow and heat transfer in smooth tubes with variable fluid properties for a Prandtl number of one is generalized in order to make it applicable to air and supercritical water. Calculated velocity and temperature distributions as well as relations among Nusselt number, Reynolds number, and friction factor for both air and supercritical water with variable fluid properties are presented. Experimental local heat-transfer coefficients, friction factors, and velocity and temperature distributions, which were measured for air flowing turbulently in a smooth tube with heat transfer at large temperature differences, are included and compared with the analysis. Good agreement between analytical and experimental results was obtained for Reynolds numbers above 15,000.

Investigation of Turbulent Flow and Heat Transfer in Smooth Tubes, Including the Effects of Variable Fluid Properties

Abstract Equations are derived for the prediction of radial velocity distributions for fully developed turbulent flow in smooth tubes both with and without heat transfer. The analysis results in an equation which represents both the conventional buffer layer and the laminar layer. In order to check the analysis and determine values for the constants appearing in the equations, tests were conducted to determine fully developed velocity distributions for air flowing without heat transfer in a smooth tube. The results are correlated by using conventional velocity and distance parameters, and agree closely with those of Nikuradse and other investigators. The analysis is extended to include flow with heat transfer, and equations for this case are obtained for Prandtl number of 1 in which the effects of the variation of fluid properties due to temperature variation across the tube were considered. By use of the equations for velocity and temperature distributions, relations are also obtained among Nusselt number, Reynolds number, and friction factor for a fluid with a Prandtl number of 1 for the case where the variation of fluid properties across the tube is large. The trends predicted analytically were found to be similar to those determined experimentally by measurement of average surface heat-transfer coefficients and friction factors of air flowing in tubes.