Q2. What are the future works in this paper?
The purpose of this study was therefore to experimentally investigate the influence of axial position, free convection effects and Prandtl number on the heat transfer characteristics of developing and fully developed flow in smooth tubes in the transitional flow regime.
Q3. How long was the time required to reach steady-state?
In the laminar flow regime, at very low Reynolds numbers, approximately 30 minutes was required to reach steady-state conditions.
Q4. Why is the uncertainty of the Reynolds number indicated by the dotted green lines?
Due to the relatively high uncertainties (due to the small temperature differences) in the turbulent flow regime, an uncertainty of 10% is indicated by the dotted green lines.
Q5. What can cause a heat exchanger to operate in the transitional flow regime?
Design constraints, changes in operating conditions or equipment, corrosion and scaling, can cause heat exchangers to operate in, or close to, the transitional flow regime.
Q6. What was the focus of previous studies?
the focus of previous studies was on the effect of different inlet geometries, enhanced tubes, micro-channels, annular flow, nanofluids, and different Prandtl number fluids.
Q7. What was the effect of free convection on the transitional flow regime?
In the first region, the width of the transitional flow regime decreased significantly with axial position as the thermal boundary layer thickness increased, and free convection effects were negligible.
Q8. How long did the flow stabilize at the critical Reynolds number?
After time (up to 1 hour) the flow stabilised at the critical Reynolds number, and then in the quasi-turbulent flow regime at the next increasing experimental Reynolds number increment.
Q9. How many mm was allowed between the last measuring station and the mixer?
To prevent possible upstream flow effects from influencing the measurements at the last measuring station (station FF), 300 mm was allowed between the last measuring station (at x = 9.5 m) and the mixer (at x = 9.8 m).
Q10. What were the uncertainties of the laminar forced convection nusselt?
The laminar forced convection Nusselt number uncertainties were less than 10%, while the laminar mixed convection uncertainties were less than 5% and decreased with increasing heat flux.
Q11. How much heat loss was calculated in the two test sections?
The maximum heat loss was estimated with one-dimensional conduction heat transfer calculations to be less than 3% in both test sections.
Q12. Why did the Reynolds numbers in Fig. 6(b) increase?
The increasing Reynolds numbers in Fig. 6(b) were only due to the temperature gradient along the tube length, and the decreasing viscosity with increasing temperature (this will be investigated in Fig. 8).
Q13. How long was the flow time required to reach steady-state?
Although the mass flow rates in the transitional flow regime were greater than in the laminar flow regime, up to 1 hour was required to reach steady-state due to the mass flow rate and temperature fluctuations inside the tube.
Q14. How did the transition gradient in the 4 mm test section change with increasing heat flux?
To summarise the effect of free convection on the heat transfer coefficients in the different flow regimes, the Colburn j-factors as a function of Reynolds number are compared in Fig. 13 for different heat fluxes, at x/D = 873 in the 4 mm test section and at x/D = 802 in the 11.5 mm test section.
Q15. What are the boundary values of the different flow regimes?
As the heat transfer results were presented and investigated in terms of Nusselt numbers and Colburn j-factors, it was used to define the boundaries of the different flow regimes, since these parameters are generally available (when the experimental data of other studies are analysed and investigated), while the temperature and mass flow rate measurements are not necessarily available.