Heat transfer and pressure drop investigation for prescribed heat fluxes on both the inner and outer wall of the annular duct

Abstract

Heat exchangers are used in industrial processes to recover heat between two processes fluids and are widely used. Although the equations for heat transfer and pressure drop characteristics in a double pipe heat exchangers are available, there is still need to completely understand how these characteristics interact which geometrical factors like annular diameter ratio or some thermal boundaries conditions which have not yet drawn more attention from the research community. The purpose of this study was to experimentally measure the heat transfer and pressure drop characteristics of a concentric annular duct of ratio 0.593 for different heat fluxes simultaneously on the inner and outer tube in the turbulent flow regime and to describe or discuss the impact or interaction of heat flux ratios on the flow and heat transfer behaviour. An experimental set]up was designed to achieve this goal. It consisted of an overall facility and a removable test section. The test section allowed for the measurement of the temperature along the length of the test section, the pressure drop, the heat flux inputs and the flow rate. These quantities were used to determine the heat transfer coefficients and friction factors of the!system. The concentric duct was an annulus formed of a single (15.88]mm]outer diameter and 14.46]mm]inner diameter) copper tube inserted inside a 0.91mm] thick] copper tube of 26.76 mm of inner diameter. The overall length of the annular duct was 4.84 m. To transfer heat, a heating element made of constantan wire was wrapped around each heat transfer area. Heat transfer and pressure drop data were obtained on heating the inner and the outer wall separately with four different heat flux densities and eight heat flux ratio were used for the case of simultaneously heating both walls. Reynolds numbers for unilateral heating range from 5800 to 12 000 while bilateral heating were focus around two Reynolds numbers, 6 500 and 9 500. Satisfactory results were found between the measurements of this experiment and currently available literature for the case of unilateral heating. An estimate of the accuracy of the experimental setup showed the maximum relative error was about 5 % in the determination of the Nusselt number and 1.8 % for the friction factor. Diabatic friction factors have been presented using adiabatic friction factors with a correction term which considered the effect of temperature difference between the fluid and walls. Heat flux density ratio showed to have an impact on the heat transfer characteristics. The Nusselt number on the inner wall could be enhanced by 19% with increasing the heat flux ratio up to 2.3 times.