چكيده به لاتين
Simultaneous use of nanofluid and porous media in a double-pipe heat exchanger is studied in order to enhance its heat transfer with minimum power increase. Both channels of the heat exchanger are partially/completely filled with porous media, and different configurations of the thickness of the porous layers with various Darcy number are analyzed in different Reynolds numbers. Nanofluid flow is simulated using two-phase mixture model and flow through porous media obeys from Darcy–Brinkman–Forchheimer rule. Results are presented in terms of the performance evaluation criterion (Performance numbers). First, the problem are investigated in condition that nanofluid flow has the same volume flow rates in two channels. Depending on Darcy number, three optimal regions exist:
-partial filling of the inner pipe ,
-partial filling of the outer pipe and
-complete filling of the both pipes.
For instance at {Re}_i = 500 and {Da}_o = 10^{-4}, when a very low (10^{-4}), moderate (10^{-2}) and high (10^{-1}) permeable porous medium is used in the inner pipe, the optimal situations are, respectively, the first, third and second situation. The effect of Darcy number of porous media on heat transfer, pressure drop and performance number was studied. The results indicated that by increasing the permeability of porous media, the heat transfer is reduced to a very small extent, the pressure drop is dramatically increased, and the performance number that is a proportion of these two parameters will increase accordingly. The effect of Reynolds number is always positive in this situation, that is, the performance number will always increase by increasing the Reynolds number of inner pipe flow. In the following, the problem is studied in conditions that nanofluid flow has different volume flow rates in two channels. The results show the high impact of Reynolds number of fluid flow on Performance number. In this case, there's an optimal Reynolds number for different configurations of heat exchange; that is, the performance number of heat exchange will not always increase by increasing the Reynolds number in both pipes. Based on the working conditions of the heat exchanger, proper porous layer thickness and Darcy number configuration have to be selected to meet the maximum performance. Useful information on optimizing the heat exchanger is provided to help designers.
