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RESEARCH PAPERS: Applications

Heat Transfer Augmentation: Radiative-Convective Heat Transfer in a Tube With Fiber Array Inserts

[+] Author and Article Information
Andreas Hantsch

Institut für Wärmetechnik und Thermodynamik, TU Bergakademie Freiberg, G.- Zeuner-Str. 7, 09599 Freiberg, Germanyhantscha@mailserver.tu-freiberg.de

Ulrich Gross

Institut für Wärmetechnik und Thermodynamik, TU Bergakademie Freiberg, G.- Zeuner-Str. 7, 09599 Freiberg, Germanygross@iwtt.tu-freiberg.de

Andrew R. Martin1

Department of Energy Technology, KTH, Brinellvägen 68, 10044 Stockholm, Swedenandrew.martin@energy.kth.se

Denoted below as short and long tubes, respectively.

Denoted below as test section.

1

Corresponding author.

J. Heat Transfer 132(2), 023505 (Dec 02, 2009) (6 pages) doi:10.1115/1.4000189 History: Received October 30, 2008; Revised August 22, 2009; Published December 02, 2009; Online December 02, 2009

Abstract

Gas-phase heat transfer plays a critical role in many high temperature applications, such as preheaters, combustors, and other thermal equipment. In such cases common heat transfer augmentation methods rely on the convective component alone to achieve improved internal performance. Radiatively assisted heat transfer augmentation has been suggested as a way to overcome limitations in convective-only enhancement. One example of such a technique is the fiber array insert; thermal radiation emitted by tube walls is captured by a large number of slender fibers, which in turn convect heat to the flowing fluid. Previous numerical studies have indicated that this technique represents a promising enhancement method warranting further investigation. This paper presents results from an experimentally based feasibility study of fiber array inserts for heat transfer augmentation in an externally heated duct. Fibers composed of $140 μm$ silicon carbide and $150 μm$ stainless steel were assembled in arrays with porosities around 0.98, and were tested for empty-tube Reynolds numbers ranging from 17,500 to 112,500 and wall temperatures from ambient up to $750°C$. The arrays cause a significant pressure drop—roughly two orders of magnitude higher than the empty-tube case—but tube-side heat transfer coefficients were improved by up to 100% over the convective-only case in the low flow rate regime. The stainless steel fiber array exhibited similar heat transfer performance as the silicon carbide case, although pressure drop characteristics differed owing to variations in fluid-structure flow phenomena. Pressure drop data were roughly within the range of d’Arcy law predictions for both arrays, and deviations could be explained by inhomogeneities in fiber-to-fiber spacing. Heat transfer was found to depend nonlinearly on wall temperature and flow rate, in contrast to previously reported numerical data.

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Figures

Figure 1

Model of the system with heat fluxes

Figure 2

Test rig and details of short and long tube; (a) Test rig: (1) Exhaust pipe, (2) orifice plate, (3) heaters, (4) heat exchanger tube, (5) settling chamber, (6) flow conditioner, (7) mesh, (8) orifice plate, (9) fan; (b): Positions of the test section in short and long heat exchanger tube

Figure 3

Friction factor depending on Reynolds number with hydraulic diameter as length scale; (a) short tube with SiC fibers and (b) long tube with SiC and steel fibers

Figure 4

Pressure drop depending on Reynolds number based on hydraulic diameter in the long tube with SiC and stainless steel fibers

Figure 5

Nusselt number depending on averaged wall temperature: (a) Short tube with SiC fibers, (b) long tube with SiC fibers, and (c) long tube with stainless steel fibers; with ṁ=0.06 kg/s (○), ṁ=0.04 kg/s (▽), ṁ=0.03 kg/s (△), and ṁ=0.01 kg/s (◻). Empty symbols are used for the empty tube and filled symbols are used for the same mass flow rate in the fiber-filled tube.

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