Enhancement of Heat Transfer and the Forced Convection

Abstract

Forced Convection of air in horizontal tubes with conical inserts has been studied experimentally. The experiments are performed in turbulent regime. Different arrangement of conical ring have been employed for the experiments by different researchers. It was found that insertion of rings have significantly increased the heat transfer rate of the pipe as compared to the smooth pipe but at the same time pressure drop also got increased which lead to high frictional losses. The configuration with perforated conical ring (PCR) array minimizes frictional losses and thus, the thermal performance factor reaches maximum as compared to other arrangements

Introduction

The use of passive methods for enhancing heat transfer rate has got strong attention by the researchers over past few decades. These techniques are used to design a compact heat exchanger which can be used in various engineering applications. In passive methods, no extra energy is provided from outside to increase the heat transfer rate unlike the active methods, where external source of energy is used. In passive methods treated surfaces, rough surfaces, extended surfaces, etc. are used. These are called turbulator devices as they increase the mixing of fluid by increasing the turbulence and diminishing the growth of fluid boundary layer near the surface. This process results in increase in the effective surface area and residence time of the fluid and consequently heat transfer coefficient in the system increases. This also leads to increase in pressure drop which further increases frictional losses, which is undesirable. Hence, an optimal condition is necessary where frictional losses increase less with significant increase in heat transfer rate.

Literature Review

Heat exchangers are required to transfer heat from one fluid to another. Earlier, heat exchangers with plain tubes with less heat transfer rate was used. So, it was compulsory to make big heat exchangers to obtain more heat transfer rate but big heat exchanger comes with many disadvantages, like it requires more space and energy, the whole system becomes bulky and the operational costs are high. To resolve this problem, researchers developed few methods, namely active and passive but in this review, passive techniques are documented in detail. Passive methods uses various types of turbulators like conical rings, twisted tapes, helical screw tapes, etc. They diminishes the growth of boundary layer by disturbing the laminar sublayer and thus, effective area for heat transfer increases. Many studies have been conducted on Passive techniques and conical turbulator is discussed here. Yakut and Sahin (2004) analysed flow induced vibrations of conical rings used for heat transfer enhancement in heat exchanger. They used three different pitch arrangement. Their study showed significant rise in Nusselt number (Nu) with increase in Reynolds number (Re) and the best result was obtained for the lowest pitch arrangement. Durmus (2004) studied the effect of heat transfer rate on cut out conical turbulators with four different conical angles. Heat transfer rate increased but at the same time frictional losses also increased. Although, the configuration with highest conical angle showed maximum increase in heat transfer rate but the pressure drop was also very high. Therefore, the upper limit of the conical angle should be less than the highest conical angle. Satisfactory results were not obtained. P. Promvonge (2008) came up with a new idea for conical turbulator arrangement. He investigated three different arrangements (Converging Conical Ring, Diverging Conical Ring, and Converging-Diverging Ring). The results obtained were better than that of a plain tube but increase in friction factor for the same Re was much higher compared to Nu. In another attempt, V. Kongkaitpaiboon et al. (2008) studied conical rings having three different pitch ratio with perforated holes. PCR diminished the frictional losses with increasing perforation. The results obtained from this experiments were better than the experiments performed earlier as the thermal performance factor of PCR was better than CR. Pokhriyal S. et al. (2018) performed their results on solid circular ring, perforated circular ring and slotted circular ring. As the number of holes and slots increased, the friction factor reduced for all value of Reynolds number. Again, the conical ring with perforation showed best result.

Conclusion

From all the above experiments, it is evident that use of turbulators in heat exchanger tubes results in better heat transfer rate than that of plain tube. As friction factor also increases with increase in Nu, it is necessary to reach an optimal condition where increase in Nu is more significant than the friction factor. From the initial experiments, DR array was found to be better than the CR array. But, after introduction of perforated conical ring by V. Kongkaitpaiboon, the performance of PCR was found to be far better than the CR array. Latest research (Nakhchi & Esfahani, 2019) in this field was done by Nachchi M.E. et al. (2019). The work of V. Kongkaitpaiboon (2008) was used as base for this experiment. They increased the number of perforated holes in conical ring and it was observed that with increase in perforation, the friction factor decreased significantly.