1 Copyright © 2010 by ASME Proceedings of the International Heat Transfer Conference IHTC14 August 8-13, 2010, Washington, DC, USA IHTC14-22897 HEAT TRANSFER FROM LIQUID NITROGEN FLOWS IN SMOOTH PIPES Luigi De Giorgi Politecnico di Torino Turin, Italy Volfango Bertola * Politecnico di Torino Turin, Italy Emilio Cafaro Politecnico di Torino Turin, Italy Carlo Cima Politecnico di Torino Turin, Italy Mario De Salve Politecnico di Torino Turin, Italy Bruno Panella Politecnico di Torino Turin, Italy * Visiting Professor; on leave from the University of Edinburgh, UK ABSTRACT Convective heat transfer for subcooled liquid nitrogen in a smooth horizontal pipe with internal sources is studied by analytical and numerical methods. For high Reynolds numbers the numerical results are in good agreement with standard heat transfer correlations. At smaller Reynolds numbers (<10,000), large circumferential and longitudinal temperature distributions can observed. The effect of localized heat sources on the heat transfer process is also investigated to simulate insulation failures in cryogenic pipelines. Results show that the presence of constant heat sources is detrimental to the heat transfer from both laminar and turbulent flows. INTRODUCTION Heat transfer processes at cryogenic temperatures are of great importance in many practical applications, in particular in the aerospace industry [1,2]. An interesting perspective application, which however has not received much attention so far, consists in the transport of hydrogen in the liquid state over long distances using twin-tubed pipelines with vacuum cavities [3,4]. Unfortunately, the amount of experimental data on the flow of liquefied gases (either single- or two-phase) at low temperatures available in the open literature is limited. In particular, safety issues make experimentations that involve the use of liquid hydrogen extremely expensive, both in terms of equipment and in terms of human resources. A larger number of data has been obtained from experiments on liquid nitrogen, which has a lighter burden of technical issues to be addressed; however, most of them are relative to micro- and mini-channels [5-7]. One of the issues affecting experiments on larger tubes is thermal insulation of the test section, which is also important during the initial chill- down of the whole system to its working temperature [8,9]. In the recent years, the need for a more in-depth knowledge of the heat transfer mechanism in cryogenic flows has motivated a number of computational works, which investigate convection in subcooled liquid as well as flows with phase change [10-12]. The aim of the present work is to investigate convective heat transfer from laminar and turbulent flows of sub-cooled liquid nitrogen in a smooth pipe, with localized internal sources, by analytical and numerical methods. Understanding the effect of localized heat sources has a practical interest in designing and operating cryogenic pipelines, where one can have unwanted heat sources, for example, because of a local failure of the pipe insulation, or a thermal bridge through one of the pipeline supports [13]. Given constant fluid properties, thermal and hydraulic fully-developed flow conditions, and negligible axial conduction, an analytic expression of the dimensionless heat transfer coefficient is derived from a generalized form of Lyon’s integral, in the Reynolds number range 1000<100000. Numerical simulations of laminar and turbulent flows of liquid nitrogen in a horizontal pipe were performed using a CFD commercial code (Fluent v6.3) to clarify heat transport phenomena in the circumferential and longitudinal directions. Convection heat transfer curves are obtained for different heating densities of the internal heat source and flow velocities.