Heat transfer enhancement by structured turbulence promoters

The enhancement of heat transfer in pipes is relevant to the improvement of performance of cooling or heating devices. The enhancement of heat transfer can be achieved by inclusion of structured promoters in the inside walls of a pipe. Experiments were carried out on industrial pipes provided by a manufacturing company. The type of structured promoters of these pipes were helicoidal ribs. Velocity profiles were measured for both air and water flow using a Laser Doppler velocimeter. The turbulence intensity of air and water flow was measured close to the pipe wall. The temperature profiles on top of the ribbed surface were determined using liquid crystal thermography. The results from the liquid crystal thermography experiments were used in the computation of local heat transfer coefficients on the internal walls of the pipes. The average heat transfer coefficient was calculated for the pipes through integration of the local heat transfer coefficient profiles. The local heat transfer coefficient profiles were analyzed as a combination of two separate phenomena. One section of these profiles was treated as a developing boundary layer. The other section was treated as a cavity flow. The local heat transfer coefficients corresponding to the boundary layer section were correlated to the theory of boundary layer flow. The results of the cavity section were compared with experimental correlations for cavity flow done by previous researchers. The two correlations for boundary layer and cavity flow were put together to define a correlation for the average heat transfer coefficient. The correlation for the average heat transfer coefficient was compared with the experimental results and with results obtained on different pipe models that were provided by the pipe manufacturers. The distribution of friction factors on the pipe was also calculated using the velocity profiles of the Laser Doppler Velocimetry experiments. The results were analyzed in the same fashion as the local heat transfer coefficients. Two correlations were developed to define the friction factor at the cavity and at the boundary layer region. These two correlations were put together to define the average friction factor in the internal ribbed surface of the pipes. The correlation for friction factor were compared with the smooth pipe correlations and the experimental frictions factors obtained by Laser Doppler velocimetry.