Experimental and numerical studies of one-directional and bi-directional flow conditions across tube banks heat exchanger

Abstract

Bi-directional flow condition imposes different fluid dynamics and temperature changes compared to that of the usual one-directional flow condition. Bi-directional flow can be found in applications like thermoacoustic systems that offer a green technology for at least two major applications: refrigeration and power production. The technology is appealing as an alternative to traditional systems as it offers the replacement for the use of harmful working media and exhausted resources with the use of inert gaseous with relatively fewer moving mechanisms. As the fluid dynamics and heat transfer of bi-directional flow in a thermoacoustic working environment is less known, it is difficult to estimate losses and gain, especially during the design stage. This paper reveals the differences to be expected in the behaviour of flow and heat transfer through experimental as well as Computational Fluid Dynamics (CFD) results of one-directional and bi-directional flow conditions. Two different drivers were used to create the two different flow conditions: a loudspeaker for the bi-directional flow and a centrifugal blower for the one-directional flow. Both conditions were monitored based on flow amplitude that is calibrated between the two drivers. Results of velocity, temperature and, vorticity are recorded for Reynolds number that ranges between 270 and 1700. Analyses are supplemented with data from validated two-dimensional computational fluid dynamics models that were solved using the Shear-Stress-Transport (SST) k-ω turbulence model with second-order accuracy for all equations. Interesting features of differences in temperature and velocity changes between the one-directional and the bi-directional flows are reported. The temperature and velocity at upstream and downstream locations of the tube banks heat exchanger are almost the same for bi-directional cases but are significantly different when a one-directional flow is flowing over the heated tubes. In addition, the interplay between natural and forced convections is seen to affect the results that were recorded for the two flow conditions. The presence of thermally developing and fully developed regions is also discussed. The results indicate that the heat transfer behaviour of bi-directional flow is not the same as in the one-directional flow and the future calculation for heat transfer for bi-directional flow conditions of thermoacoustic must be carefully done with consideration of changes of flow conditions between the one-directional and the bi-directional flow conditions so that error could be minimized in the evaluation of the system’s performance.