Single-phase heat transfer characteristics of concentric-tube thermosyphon

An experimental study has been conducted to evaluate the influence of the presence of inner tube and the Rayleigh number on free convective heat transfer in an open thermosyphon. Water and fluorocarbon R-11 refrigerant as the working fluids were utilized. Heat transfer results using the concentric geometry were given for modified Rayleigh number from 3.6×10² to 4.1 × 10⁷ which encompasses the regions of similarity, impeded and boundary layer flow conditions. It was found that the presence of the inner tube markedly increases the overall heat transfer coefficient of open thermosyphon by a factor as large as 2 to 10 in the turbulent impeded and boundary layer regimes.

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Wärmeübergang in einem Thermosyphon aus konzentrischem Rohr bei einphasiger Strömung

Zusammenfassung Es wurde experimentell untersucht, wie der Einbau eines Innenrohres und wie die Rayleigh-Zahl auf die freie Konvektion in einem offenen Thermosyphon, gefüllt mit Wasser oder dem Kältemittel R 11, einwirkt. Der untersuchte Bereich bei konzentrischer Geometrie lag bei modifizierten Rayleigh-Zahlen von 3,6 · 10² bis 4,1 · 10⁷ und umfaßte damit die Regionen der Grenzschichtströmung. Es ergab sich, daß der Einbau eines Innenrohres den Gesamtwärmeübergang eines offenen Thermosyphons im Bereich der behinderten turbulenten Strömungen und Grenzschichtströmungen um den Faktor 2 bis 10 steigert.

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Nomenclature a inner radius of heated tube, D/2 - A_in sectional space area of inner tube, d_i²/4 - A_an sectional space area of annulus, (D^2-d₀²)/4 - C_p specific heat - d_i inner diameter of inner tube - d₀ outer diameter of inner tube - D inner diameter of heated tube - g gravitational acceleration - L tube length of thermosyphon - Nu_a Nusselt number based on inner radius of heated tube - Nu_r Nusselt number based on equivalent heattransfer radius - Nu_x Nusselt number, defined in equation (1) - Pr Prandtl number, defined in equation (3) - q heat flux from heated tube - r equivalent heat-transfer radius, defined in equation (4) - Ra_a modified Rayleigh number based on inner radius of heated tube - Ra_r modified Rayleigh number based on equivalent heat-transfer radius - Ra_x modified Rayleigh number, defined in equation (2) - T_e temperature of entrance-fluid - T_w temperature of heated surface - T temperature difference between heated wall and entrance-fluid, T_w-T_eGreek Symbols coefficient of volumetric expansion - thermal diffusivity - thermal conductivity - viscosity - kinematic viscosity