Experimental investigation of a water/nanofluid jacket performance in stack heat recovery

The effects of nanofluids (Al2O3-water) on the overall thermal performance of an annular enclosure (or jacket) are experimentally investigated which is used for recovering waste heat from a typical stack of a gas heater. In the initial stages of the heating process, the inner cylindrical wall becomes hotter, while the bulk fluid is nearly at the preceding uniform temperature; hence, the wall heat flux is strongly enhanced at the beginning. Afterward a decline in the wall heat flux is observed due to increasing Rayleigh number and correspondingly generating cellular flows in the annulus that leads to temperature enhancement of the liquid. Using nanofluids has the advantage of improving key parameters such as Nusselt number. Nanofluids with higher nanoparticle concentrations need less response time to react to any changes in thermal environment, and consequently they have smaller time constant. Higher convective heat transfer coefficient as well as greater temperature uniformity in the enclosure is achieved by selecting nanofluids with larger values of nanoparticle concentration. The results also reveal that convective heat transfer coefficient and Nusselt number of nanofluids are comparatively enhanced with time, since hotter base fluid results in higher effective thermal conductivity.