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This paper summarizes an experimental study of three popular “turbulator” inserts for fire tube boilers. An electrically heated flow facility was developed to deliver hot air to a water-cooled steel tube instrumented to derive sectional average heat transfer coefficients for four regions of the tube. Reference data for the empty tube are in excellent agreement with the accepted correlation. Two commercial turbulators, consisting of narrow, thin metal strips bent and twisted in zig-zag fashion to allow periodic contact with the tube wall, displayed 135 and 175 percent increases in heat transfer coefficient at a Reynolds number of 10,000. A third commercial turbulator, consisting of a twisted strip with width slightly less than tube diameter, provided a 65 percent increase in heat transfer coefficient. The friction factor increases accompanying these heat transfer coefficient increases were 1110, 1000, and 160 percent, respectively, for the same Reynolds number. These data should be useful in assessing overall performance gains to be expected when turbulators are used in actual boilers.

Experimental data indicating some effects of free-stream turbulence intensity on time-average boundary-layer velocity profiles and on heat transfer from a constant-temperature flat plate with a favorable pressure gradient are presented for local Reynolds numbers ranging from 4 × 104 to 4 × 105 and for free-stream turbulence intensities from 0.4 to 8.3 percent. It is concluded that, for the range of variables covered by the experiments: (a) The effect of free-stream turbulence intensity on heat transfer through the laminar boundary layer with a zero pressure gradient is negligible; (b) for a given Reynolds number, the local Nusselt number increases with increasing free-stream turbulence intensity when a pressure gradient is present, the boundary-layer profiles for these conditions changing with a variation in free-stream turbulence intensity; and (c) no increase in Nusselt number with increase in free-stream turbulence intensity occurs for turbulent boundary layers with a favorable pressure gradient.

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An experimental investigation was conducted into augmentation of forced convection heat transfer in air by mechanical removal of the boundary layer. A rotating blade element passing in close proximity to a flat plate convective surface was found to increase the rate of convective heat transfer by up to eleven times in certain situations. The blade element effectively scrapes away the boundary layer, thus reducing the resistance to heat flow. Parameters investigated include scraping frequency, scraper clearance, and type of boundary layer. Increased coefficients were found for higher scraping frequencies. Significant augmentation was obtained with clearance as large as 0.15 in. (0.0038 m) between the moving blade element and the convective surface. The technique appears most useful for laminar and transitional boundary layers, although some improvement was obtained for the turbulent boundary layers investigated. The simple surface renewal theory developed for scraped surface augmentation in liquids was found to approximately predict the coefficients obtained. A new relation is proposed which gives a better prediction and includes the effect of scraper clearance.