RP-414 -- Investigation of Turbulence Producing Inserts on Heat Transfer in Tubes With Application to Fire Tube Boilers PDF

The enhancement or segmentation of in-tube single-phase forced convective heat transfer in gases by using turbulence promoting bentstrip inserts was investigated. The study included experimental determination of thermal-hydraulic performance data, visualization of flow patterns, and interpretation of the mechanism of heat transfer enhancement by analytical means.

To obtain thermal-hydraulic data, an electrically heated flow facility was used to deliver hot air to a water-cooled steel tube. Tube wall temperatures, fluid bulk temperatures, and flow rates were measured to derive sectional average heat transfer coefficients for four segments of the tube. Reference data for the empty tube were in agreement with the usual correlations. Eleven different geometrical variations of one type of bent-strip insert were tested. Increases in the heat-transfer coefficent of 185% to 285% were recorded at a Reynolds number of 10,000; however, accompanying increases in the friction factor were 400% to 1800%. A study of the insert entrance region was conducted in order to assess the insert length required for developed augmented conditions to be attained. Empirical correlations to predict average heat transfer and pressure drop are given.

Performance evaluation studies based on constant pressure drop and constant pumping power conditions indicate that favorable enhancement ratios are available in specific Reynolds number ranges. To differentiate the effects of the wall and the core regions of an insert, one insert was cut apart to provide core and wall inserts, which were tested separately. The results indicate that the core region of the insert is responsible for the major portion of the heat transfer enhancement. Also conducted were flow visualization experiments, which provided both an understanding of the physical nature of the flow around the bent-strip inserts and further insight into the enhancement mechanisms.

Two additional inserts were developed on the basis of the cut insert tests. Designated as bent-tab inserts, they produced somewhat lower enhancement at considerably less pressure loss. Performance evaluation criteria used to compare their performances with other inserts show improvement over the bent strip insert at low Reynolds numbers.

Finally, basic surface-renewal/penetration theory was utilized to describe, with reasonable accuracy, the trends in heat transfer enhancement in tubes with bent-strip inserts.

Overall improvement in fire-tube boiler efficiency is not necessarily in direct proportion to increases in gas-side heat transfer coefficients due to insert enhancement. Effective use of insert enhancement necessitates an overall system design including the water side coefficient.