4. Results

As described in the methodology applied (Figure 1), the first step consists in the validation of the CFD simulations according to the DOE results from where the input data are chosen. Next, from the CFD local Nusselt numbers obtained, correlations are obtained. Then an exergy analysis is applied to the heat exchangers, and finally the simulation in Aspen HYSYS [11] is carried out.

### 4.1 Validation of CFD simulation

For the CFD simulation, the sweep hex elements were used; 473,600 elements were applied to the helical-tube heat exchanger and 949,500 to the finned-tube heat exchanger. The boundary conditions used in the CFD simulations for both types of heat exchangers are inlet mass flow, outlet pressure and insulated external surface. The regime of flow is subsonic. The gas side exhibits turbulent flow and the ethylene glycol a laminar regime.

For the gas side, the conditions at the inlet are mass, temperature, turbulent intensity, and turbulent length scale. The conditions at the outlet are backflow temperature, turbulent intensity, and turbulent length scale. For the ethylene glycol side, the conditions at the inlet are mass and temperature. The CFD solution provides the following results for both exchangers: mean temperature, wall temperature, pressure drop, and heat flux.

For the grid independence, several meshes were tested for each of the 73 configurations; a total of 198 simulations were carried out with the aim of finding the meshes that exhibit less variation in the prediction of results. A finer mesh was used Exhaust Gas Heat Recovery for an ORC: A Case Study DOI: http://dx.doi.org/10.5772/intechopen.86075

at the inner face of the gas cavity, around the fins and at the inner and outer face of ethylene glycol cavity to fulfill the required y + value. Figures 5 and 6 show the refinement for the two types of heat exchangers. The parameters used in the solver of the CFD simulations are shown in Table 3.

The parameters in Table 3 gave the best results regarding mass and energy balance. For turbulent flow, the physical model SIMPLEC is recommended [16]. For gradient, the least squares cell-based method was selected. This method is less expensive in terms of simulation time [17]. For pressure interpolation, the second-order scheme is recommended. Second-order upwind was used to get more accuracy in the solution of the momentum equations [18]. First-order upwind was used to calculate turbulent kinetic energy because it is less time-consuming [17]. First order upwind was used to calculate turbulent dissipation rate because is less time consuming [17]. Second-order upwind was used to get more accuracy in the solution of the energy equations [18].

The results obtained from the CFD simulations were validated using Eqs. (5)–(8). Figures 7–10 show the comparison of Nusselt number. It can be observed that, for both heat exchanger geometries, the numerical results and the ones obtained from the correlation show similar tendency with a good approximation between them.

In the same way, local Nusselt number for both types of heat exchangers is presented in Figures 11–14. The most relevant configurations (experiments) were considered for each type of heat exchanger.

Figure 5. Mesh refinement in the finned-tube geometry.

Figure 6. Mesh refinement in the helical-tube geometry.
