Heat Transfer Investigation of Air Flow in Microtubes—Part I: Effects of Heat Loss, Viscous Heating, and Axial Conduction

Abstract

Experiments were conducted to investigate local heat transfer coefficients and flow characteristics of air flow in a 962 μm inner diameter stainless steel microtube (minichannel). The effects of heat loss, axial heat conduction and viscous heating were systematically analyzed. Heat losses during the experiments with gas flow in small diameter tubes vary considerably along the flow length, causing the uncertainties to be very large in the downstream region. Axial heat conduction was found to have a significant effect on heat transfer at low Re. Viscous heating was negligible at low Re, but the effect was found to be significant at higher Re. After accounting for varying heat losses, viscous heating and axial conduction, Nu was found to agree very well with the predictions from conventional heat transfer correlations both in laminar and turbulent flow regions. No early transition to turbulent flow was found in the present study.