The Caputo and Caputo-Fabrizio derivative are applied to study a second-grade nanofluid over a vertical plate. A comparative analysis is presented to study the unsteady free convection of a second-grade nanofluid with a new time-space fractional heat conduction. The governing equations with mixed time-space fractional derivatives are non-dimensionalized and solved numerically, and a comparison between the Caputo and the Caputo-Fabrizio models is made. It is found that the temperature is higher for the Caputo-Fabrizio fractional model than the Caputo model, but the higher velocity only exists near the vertical plate for the Caputo-Fabrizio model than the Caputo model. Moreover, the velocity for the Caputo model will exceed the Caputo-Fabrizio model as y evolves.
K E Y W O R D SCaputo fractional derivative, Caputo-Fabrizio fractional derivative, heat transfer, second-grade nanofluid
| INTRODUCTIONThe second-grade fluid is a popular non-Newtonian fluid, and has been used in petroleum industry, chemical industry, light industry, and many other sectors. The second-grade fluid model was first configured by Coleman and Noll, 1 and the heat transfer of the second-grade fluid under various physical conditions has been investigated widely. [2][3][4][5][6] Fractional derivative is suitable for describing some substantial problems than the regular derivative, and it has been applied widely to establish mathematical models of realistic problems. Fractional calculus has been used to establish the constitutive relations of viscoelastic materials and anomalous diffusions. 7-11 Sin et al 12 introduced the fractional K-BKZ model to viscoelastic fluid for the study of unsteady flow between parallel plates, and the fractional Cattaneo-Christov flux model was used by Liu et al 13 to study the diffusion in comb frame. As the nanofluid has better heat transfer efficiency than conventional fluids, 14 the heat transfer of nanofluid flow under various physical conditions has been investigated. [15][16][17] Shen et al 18 proposed a renovated Buongiorno's model with fractional differential equation to study the heat transfer characteristics of Sisko nanofluid. It is clear to note that the researchers mostly utilized Caputo and Riemann-Liouville derivatives with singular kernels.The Caputo-Fabrizio derivative with an unsingular kernel was given by Caputo and Fabrizio, 19 which was based on the exponential function to eliminate the singular kernel. Ali et al 20 used this derivative to investigate magnetohydrodynamics free convection flow on a static vertical plate for generalized Walters'-B fluid, and Asjad et al 6 studied the feature of Caputo and Caputo-Fabrizio fractional derivatives for an unsteady second-grade fluid with Newtonian heating.In this paper, a new time-space fractional heat conduction is introduced into the energy equation to investigate the heat transfer of second-grade nanofluids on a vertical plate by using Tiwari and Das's model. 21 The Caputo and Caputo-Fabrizio derivatives are applied to get numerical solutions of the d...