Forced convection heat transfer with nanofluids in flat plate solar collector

Abstract

Nanofluids are fluids with immersed nanoparticles in it. It demonstrates much higher convective heat transfer coefficient than conventional working fluids. Single-phase and mixture-phase models are the two models that are being used for the nanofluid study. In this study, forced convection heat transfer of nanofluids is done using both single-phase and mixture-phase models and the results are compared with experimental results. The nanofluid is then used as the working fluid in a flat plate solar collector to investigate the performance improvement of the collector. The governing equations of various studies here are discretised using the finite volume method. The models are assumed axisymmetric and at steady state. Hybrid differencing scheme is used during discretization. A code is written using SIMPLER algorithm and then solved using the MATLAB engine. The mixture-phase model studied here, considers two slip mechanisms between nanoparticle and base-fluid, namely Brownian diffusion and thermophoresis. Al2O3-water nanofluid is used for the study of nanofluid and the study shows significant increase in convective heat transfer coefficient while the mixture-phase model demonstrates slightly lower values than the single-phase model. The study is done with various nanoparticle concentrations and Reynolds numbers. With increasing particle concentration and Reynolds number, the convective heat transfer coefficient increases and as well as the shear stress. For low concentrations of the nanoparticle, Nusselt number is slightly lower than the base fluid and as the concentration increases, the Nusselt number also increases than the base fluid. The study also investigates the effect of particle size but the mixturephase model didn’t provide any variation with particle size. The Al2O3-water nanofluid is then used as the working fluid of a flat plate solar collector. The study is done with various nanoparticle concentration and flow rates and the efficiency increases with increase of both nanoparticle concentration and flow rates. The efficiency found by Al2O3-water nanofluid is then compared with CuO-water nanofluid in the flat plate collector and the CuO-water provides better results. The efficiency increases by 6.6% and 8.6% for Al2O3- water and CuO-water nanofluid respectively for 0.025 mL/s of flow rate. There is also a significant efficiency increase of 10.3% found by using internal fins to the riser tube.