Air-Gap Thermal Insulation for Large Geothermal Pipelines

Abstract

A single layer air-gap thermal insulation model with atmospheric-pressure air between horizontal concentric pipes was studied experimentally. The inner pipe was an isothermal large diameter geothermal steam pipeline, and the gap/ diameter ratios were small. The outer aluminium cylinder was convectively cooled by the atmospheric air. Dimensional analysis showed that a transition regime existed in the air-gap model. In this region, heat transfer changed from pseudo conduction to a fully developed laminar convection. The transition region identified was larger than that categorized by previous investigators. It was found that large diameter annuli (i.e. small radius of curvature) led to high Rayleigh numbers that defined the different regimes, while small gap/diameter ratios resulted in lower Rayleigh numbers. The air-gap insulation for geothermal pipelines would be mainly in the transition region. The existing correlations were invalid either because they were based on the fully developed convection or the definition of the regimes was different to what was found in this study. Four regional correlations were established for determining the Nusselt number in each of the specified regimes. By using these correlations, heat losses can be predicted for the single-layer air-gap insulation. The heat transfer regimes identified in this study are of theoretical importance and substantiate experimentally the hypothesis predicted by some previous investigators.