Heat transfer and single-phase flow in internally grooved tubes

This study investigates heat transfer and flow characteristics of water flowing through horizontal internally grooved tubes. The test tubes consisted of one smooth tube, one straight grooved tube, and four grooved tubes with different pitches. All test tubes were made from type 304 stainless steel. The length and inner diameter of the test tube were 2 m and 7.1 mm, respectively. Water was used as working fluid, heated by DC power supply under constant heat flux condition. The test runs were performed at average fluid temperature of 25 °C, heat flux of 3.5 kW/m², and Reynolds number range from 4000 to 10,000. The effect of grooved pitch on heat transfer and pressure drop was also investigated. The performance of the grooved tubes was discussed in terms of thermal enhancement factor. The results showed that the thermal enhancement factor obtained from groove tubes is about 1.4 to 2.2 for a pitch of 0.5 in.; 1.1 to 1.3 for pitches of 8, 10, and 12 in., respectively; and 0.8 to 0.9 for a straight groove.     

Heat transfer and hydraulic resistance during condensation of steam in a horizontal tube and in a bundle of tubes

The paper presents an approximate prediction of heat transfer during steam condensation inside a tube on the basis of the analogy between hydraulic resistance and heat transfer in accordance with Reynolds' theory. It describes experimental results obtained by the authors during the condensation of steam inside tubes with diameter of 18 mm and lengths up to 12 m at pressures up to 90 bar. Theoretical and experimental results agree satisfactorily. It also describes original experimental results on tube hydraulic resistance during condensation of steam therein. Finally, it describes experimental results on the conditions of heat transfer during condensatory flow of steam inside tube bundle.     

Heat and mass transfer phenomena for viscous flow in curved circular tubes

Heat transfer to steady viscous flow in curved tubes of circular cross section was studied theoretically for fully developed velocity and temperature fields under the thermal boundary condition of axially uniform wall heat flux with peripherally uniform wall temperature. The fully developed velocity profiles utilized were calculated by means of a rigorous numerical method [1]. The thermal-energy equation was solved numerically by use of a point successive-overrelaxation method. Results are not limited to slightly curved tubes and low Dean numbers, as in previous analyses. Solutions are presented for a Dean-number range from 1 to 1200. Prandtl-number and curvature-ratio parameters varied from 0.005 to 1600 and from 10 to 100, respectively. The peripherally averaged Nusselt number could be correlated as a function of Dean and Prandtl numbers only; but, when peripheral variation of the Nusselt number is considered, the curvature ratio enters as an additional parameter.     

Heat transfer augmentation in developing flow through a ribbed square duct

An experimental study is conducted to investigate the heat transfer augmentation in developing turbulent flowthrough a ribbed square duct.The duct is made of 16mm thick bakelite sheet.The bottom surface of the ribbedwall having rib pitch to height ratio of 10 is heated by passing a c current to the heater placed under it.Theuniform heating is controlled using a digital temperature controller and a variac.The results of ribbed duct arecompared with the results of a smooth duct under the same experimental conditions.It is observed that the heattransfer augmentation in ribbed duct is better than that of the smooth duct.At Re=5.0×10~4,the meantemperature of air flowing through the ribbed duct increases by 2.45 percent over the smooth duct,whereas in theribbed duct Nusselt number increases by 15.14 percent than that of the smooth duct with a 6 percent increase inpressure drop.     

Condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes

This article presents the condensation heat transfer and flow characteristics of R-134a flowing through corrugated tubes experimentally. The test section is a horizontal counter-flow concentric tube-in-tube heat exchanger 2000 mm in length. A smooth copper tube and corrugated copper tubes having inner diameters of 8.7 mm are used as an inner tube. The outer tube is made from smooth copper tube having an inner diameter of 21.2 mm. The corrugation pitches used in this study are 5.08, 6.35, and 8.46 mm. Similarly, the corrugation depths are 1, 1.25, and 1.5 mm, respectively. The test conditions are performed at saturation temperatures of 40–50 °C, heat fluxes of 5–10 kW/m², mass fluxes of 200–700 kg/m² s, and equivalent Reynolds numbers of 30000–120000. The Nusselt number and two-phase friction factor obtained from the corrugated tubes are significantly higher than those obtained from the smooth tube. Finally, new correlations are developed based on the present experimental data for predicting the Nusselt number and two-phase friction factor for corrugated tubes.     

Heat transfer from a tube bundle in a bubble column

Heat transfer coefficient for an immersed five tube bundle in a two-phase (air-water) bubble column is measured as a function of air velocity and other relevant parameters. Average and local air holdup are also measured under identical conditions to characterize the system uniquely.     

Heat transfer enhancement in dimpled tubes

Heat transfer enhancement was investigated in a coaxial-pipe heat exchanger using dimples as the heat transfer modification on the inner tube. Tube-side Reynolds numbers were in the range of 7.5×10³–5.2×10⁴ for water flow. A constant annular mass flow rate was chosen to obtain the highest possible Reynolds number of 1.1×10⁴. Typically, the heating water inlet temperature was 68.1±0.1^∘C.All six variants with inward-facing, raised dimples on the inner tube increased the values of heat transfer coefficient significantly above those for the smooth tube. Heat transfer enhancement ranged from 25% to 137% at constant Reynolds number, and from 15% to 84% at constant pumping power. At a constant Reynolds number, the relative J factor (ratio of heat transfer coefficient to friction factor, relative to smooth tube values), had values from 0.93 to 1.16, with four dimpled tube configurations having values larger than unity. Despite the extremely simple design, this outperforms almost all heat transfer enhancements recommended in the literature. A correlation based on the results of the present work appears to be sufficiently accurate for predicting heat transfer coefficients and friction factors for the design of dimpled-tube heat exchangers.     

Nucleate pool boiling heat transfer over a bundle of vertical tubes

An experimental investigation has been carried out to determine the heat transfer coefficient during pool boiling of water over a bundle of vertical stainless steel heated tubes of 19.0 mm diameter and 850 mm height. The p/D of bundle was 1.66 and was placed inside a glass tube of 100 mm diameter and 900 mm length. The data were acquired for the heat flux range of 2–32 kWm^− 2.     

Heat transfer in a supersonic flow

At present it is commonly assumed that some peculiar surface exists which bounds a viscous region around a body in a supersonic flow. This region is identified with a laminar or turbulent Prandtl layer, the process of transition of visible motion into heat being described by the procedures developed by Prandtl and von Kármán for subsonic flows.

In the present work another approach is used based on the ideas of Osborne Reynolds and the so-called resolution equation where transition of thermal motion into pulsating one is fixed which, in its turn establishes relationship between the Nusselt and Reynolds numbers. This is a power-law relationship in which the coefficients and power exponent may be pre-calculated based on simple considerations. This has been done in the present work.     

Heat transfer and pressure drop for low Reynolds turbulent flow in helically dimpled tubes

Three-dimensional helically dimpled tubes have been experimentally studied in order to obtain their heat transfer and isothermal friction characteristics. Using water and ethylene glycol as test fluids, a wide range of fluid flow conditions was covered: 2000<Re<100,000 and 2.5<Pr<100. An experimental study of 10 tubes with different geometric forms (dimple height h/d ranging from 0.08 to 0.12 and helical pitch p/d, from 0.65 to 1.1) offers insight into the influence of manufacturing parameters on tube thermohydraulic behaviour. The large amount of experimental data have been correlated so as to obtain easy to use expressions for Fanning friction factors and Nusselt numbers as functions of flow and geometry non-dimensional parameters. Performance evaluation criteria, commonly used in the enhanced heat transfer literature, were calculated in order to assess the real benefits offered by dimpled tubes.     

Heat Transfer Characteristics of Laminar Flow in Internally Finned Tubes under Various Boundary Conditions

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Visualization of two-phase flow through microgrooved tubes for understanding enhanced heat transfer

Though widely used, questions remain as to the mechanisms by which micro-fin or microgrooved tubes enhance heat transfer performance. In this work, new experimental liquid film thickness profiles in adiabatic, air-water flow through clear tubes with 20 microgrooves at three different helix angles are presented. These results correlate well with the asymmetrical temperature measurements and heat transfer coefficients seen by previous investigators. Important observations include increased wall wetting for a given flow condition, decreasing influence of the grooves with increasing gas velocity, and a rotational redistribution of the liquid film by helical grooves, though without indication of a swirling behavior.     

Heat transfer in a PEMFC flow channel

A numerical method was applied to the heat transfer performance in the flow channel for a proton exchange membrane fuel cell (PEMFC) using the finite element method (FEM). The heat transfer enhancement has been analyzed by transversely installing a baffle plate and a rectangular cylinder to manage flow pattern in the flow channel of the fuel cell. Case studies include baffle plates (gap ratios from 00.05 to 0.2) and the rectangular cylinder (width-to-height ratios from 0.66 to 1.66 with a constant gap ratio of 0.2; various gap ratios from 0.05 to 0.3 with a constant width-to-height ratio 1.0) at constant Reynolds number. The results show that the transverse installation of a baffle plate and a rectangular cylinder in the flow channel can effectively enhance the local heat transfer performance of a PEMFC. The installation of a rectangular cylinder has a better effective heat transfer performance than a baffle plate; the larger the width of the cylinder is the better effective heat transfer performance becomes.     

Heat transfer performance of aqueous polyacrylamide solutions in turbulent flow through a rectangular channel

A good estimate of the friction factor and dimensionless heat transfer coefficient, j_H, of an aqueous polyacrylamide solution in turbulent flow through a 2:1 rectangular duct may be obtained from the available relation for such solutions in turbulent pipe flow.