Experimental investigations of the heat transfer by combined free and forced convection to water in a horizontal tube with a uniform heat flux have revealed many interesting features of the transition from laminar to turbulent flow. Fluctuations in fluid temperature and overall pressure drop variations facilitated the study of the transition regime.
The first phase of the experiments studied the extreme cases of combined forced- and free-convection transition regimes. For forced convection without heat transfer (isothermal flow) the results compared well with the existing data, predicting a transitional Reynolds number of about 2300. For free convection with no flow, the records of temperature fluctuations during transient heating indicated the transition regime.
In the second phase, extensive measurements were made for the combined forced- and free-convection case. A plot of the Reynolds number against the Rayleigh number Rag enabled the present results (L/D = 150) to be compared with those of Petukhov & Polyakov (for X/D ratios 40 and 100).X = length from the start of flow; L = effective length of heated test section; D = inner diameter of tube. The results show that beyond transition two types of turbulence - hydrodynamic and thermal- can be distinguished depending on whether the fluctuations are pre- dominantly in velocity or in temperature. For large vaIues of Re and Ruq, these two regions merge into turbulent, combined free and forced convection.
In the transition regime two types of flow are encountered: unstable flow characterized by pulses and a stable flow with stable fluctuations. While a curve in the plot of heat input vs. flow velocity demarcates the two regimes, the main features of transition are described by means of an intermittency factor defined as the ratio of thermal fluctuation at any point to a corresponding magnitude of thermal fluctuation on the demarcating curve.
The iso-intermittency lines drawn in the transition envelope assist in describing the structure of transition. The concentration of these lines at the central area of the envelope indicates the region of maximum coupling between thermal and hydrodynamic effects.
A qualitative study of inlet turbulence confirmed the prediction of Mori et al. for air. The results presented here along with those of Mori et al. and Petukhov & Polyakov indicate that thermally induced secondary flows attenuate the fluctuations in low inlet turbulence flows, while they restabilize the flow as the inlet turbulence is increased. Also, the effect of inlet turbulence on transition to an unstable flow regime was studied. In general, an increase in the inlet turbulence brought about an increase in the critical Rayleigh number.