Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.
The transitional flow regime has been mostly avoided by designers due to uncertainty and perceived chaotic behaviour. However, changes in operating conditions, design constraints or additional equipment can cause that the flow to move into the transitional flow regime. Previous work done in the transitional flow regime focused on fully developed flow or average measurements of developing and fully developed flow across a tube length and developing flow in the transitional flow regime have not been investigated yet. Therefore, the purpose of this study is to investigate the heat transfer characteristics of developing flow in the transitional flow regime of a solar receiver tube and is work in progress. An experimental set-up was designed, built and validated and heat transfer measurements were taken at a heat flux of 6.5 kW/m2 between Reynolds numbers of 500 and 10 000. It was found that the width of the transition region decreased along the tube length and the heat transfer coefficients decreased as the flow approached fully developed flow.