# Thermal Performance Analysis of a Double-Pass Solar Air Collector: A CFD Approach
## 4 Conclusions
**Acknowledgments:** The authors wish to thank the Mexican National Science Council (CONACYT) for the financial support through scholarship 698432. To Durango Science and Technology Council (COCYTED) for the financing of the annual ANSYS software license. Also, to the Laboratorio de Computo Cientifico of FCQ-UJED and the Academic Group UJEDCA129 for the use of software and computational facilities. **Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results. **Nomenclature** \begin{tabular}{l l} \(<\)\(n\)\(>\) & Average reflections number, dimensionless \\ \(A\) & Area, m\({}^{2}\) \\ \(C\) & Constant \\ \(C_{p}\) & Specific heat, J/(kg K) \\ \(D_{h}\) & Hydraulic diameter, m \\ \(E\) & Relaminarization source \\ \(F\) & Empirical correlation of length of the transition region, m \\ \(\rightarrow\) & Force vector, N \\ \(\rightarrow\) & Gravitational acceleration, m/s\({}^{2}\) \\ \(G_{k}\) & Generation of turbulence kinetic energy due to mean velocity gradients, J/kg \\ \(G_{w}\) & Generation of specific dissipation rate, 1/s \\ \(h\) & Height, m \\ \(h^{\prime}\) & Species enthalpy, J/kg \\ \(I\) & Solar irradiance, W/m\({}^{2}\) \\ \(J\) & Mass flux, kg/m\({}^{2}\) \\ \(k\) & Turbulent kinetic energy, J \\ \(L\) & Length, m \\ \(\dot{m}\) & Mass flow rate, kg/s \\ \(Nu\) & Nusselt number, dimensionless \\ \(P\) & Pressure, Pa \\ \(P_{g}\) & Gap loss factor, dimensionless \\ \(q\) & Heat generation, W/m\({}^{3}\) \\ \(Q_{u}\) & Useful heat gain, W \\ \(Re\) & Reynolds number, dimensionless \\ \(S\) & Strain rate magnitude \\ \(t\) & Time, s \\ \(T\) & Temperature, K \\ \(u\) & Velocity magnitude, m/s \\ \(U\) & Local velocity, m/s \\ \end{tabular} \begin{tabular}{l l} \(u_{T}\) & Friction velocity, m/s \\ \(V\) & Free-stream speed, m/s \\ \(v\) & Wind velocity above cover, m/s \\ \(w\) & Width, m \\ \(x\) & Velocity field coordinate \\ \(y\) & Wall-normal distance, m \\ \(Y\) & Destruction term of SST turbulence model, m\({}^{2}\)/s\({}^{3}\) \\ y\({}^{+}\) & Dimensionless distance in wall coordinates \\ Greek letters \\ \(\alpha\) & Absorptance, dimensionless \\ \(\gamma\) & Intermittency \\ \(\Gamma\) & Effective diffusivity, dimensionless \\ \(\varepsilon\) & Emittance, dimensionless \\ \(\eta\) & Thermal efficiency, dimensionless \\ \(\mu\) & Molecular viscosity of air, kg/(m s) \\ \(\rho\) & Air density, kg/m\({}^{3}\) \\ \(\rho\) & Reflectance, dimensionless \\ \(\tau\) & Transmittance, dimensionless \\ \(\overline{\tau}\) & Stress tensor, Pa \\ \(v\) & Eddy viscosity, m\({}^{2}\)/s \\ \(\Omega\) & Vorticity magnitude, 1/s \\ Subscripts \\ \(a\) & Ambient \\ \(a1\), \(a2\) & Turbulence damping constants \\ \(ap\) & Aperture \\ \(avg\) & Average \\ \(b\) & Bottom duct \\ \(c\) & Cover \\ \(c\) & Cavity \\ \(collector\) & Collector \\ \(duct\) & Duct \\ \(elb\) & Elbow \\ \(e1\), \(e2\) & Dissipation \\ \(h\) & Hydrodynamic \\ \(i\), \(j\) & Orthogonal coordinate \\ \(in\) & Inlet \\ \(k\) & Turbulence kinetic energy \\ \(length\) & Length \\ \(out\) & Outlet \\ \(p\) & Flat plate receiver \\ \(ps1\) & Pass 1 (Through the cavity) \\ \(ps2\) & Pass 2 (Through the duct) \\ \(r\) & Reflectors \\ \(s\) & Sky \\ \(turb\) & Turbulent \\ \(\gamma 1\), \(\gamma 2\) & Intermittence \\ Acronyms \\ CFD & Computational fluid dynamics \\ CPC & Compound parabolic concentrator \\ EPS & Expanded polystyrene \\ ETFE & Ethylene tetrafluoroethylene (fluorine-based plastic) \\ HTC & Heat transfer coefficient, W/(m\({}^{2}\) K) \\ PV/T & Photovoltaic/Thermal \\ \end{tabular}