Thermo-hydraulic Performance Analysis of the Solar air Heater Duct with Various Rib-Turbulators

Abstract

The maximum heat transfer with a minimal friction penalty through the solar air heater duct (SAHD) has been challenging throughout the years. However, SAHD with a ribbed absorber plate dramatically augments the heat transfer by creating turbulence in the flow. The disruptions due to turbulence are made very close to the absorber surface to keep the friction penalty at a minimum level. In the present research, the thermo-hydraulic characteristics of the SAHD with various rib-turbulators are studied. Also, the optimal roughness parameters for the maximum thermal enhancement ratio are calculated for the effective design of the SAHD. For the numerical analysis, different governing equations (Navier-Stokes equation, turbulence equation (k-ϵ equation), and energy equation) are solved using the finite volume method (FVM) based solver of ANSYS Fluent. Initially, the preliminary research is performed on the heat transfer analysis for the SAHD with a smooth absorber plate. The result shows that the thermo-hydraulic performance of the SAHD with a smooth absorber plate is not more due to the lower thermal conductivity of the air. The research is further extended for the absorber plate with quarter-circular rib-turbulators to analyze heat transfer enhancement and friction penalty through SAHD. The quarter circular rib-turbulators are placed on the absorber plate in three ways (type-1, type-2, and type-3). For the analysis, different roughness (relative roughness pitch (P/e) and relative roughness height (e/Dh)) and flow parameters (Reynolds number (Re)) are varied to study the thermo hydraulic performance. The result depicts a maximum thermal enhancement ratio of 1.88 for type-1 quarter-circular ribbed rectangular SAHD with P/e of 7.14 and e/Dh of 0.042 (fixed) at Re of 15000. A maximum enhancement in heat transfer is 1.78 times that of smooth SAHD for P/e of 7.14 and e/Dh of 0.042 at Re of 15000. Also, a maximum frictional penalty of 3.43 times that of smooth SAHD is encountered for the P/e of 7.17 and e/Dh of 0.042 at Re of 3800. A numerical investigation is also done using the inverted-T rib turbulators on the surface of the absorber plate to analyze the heat transfer improvement and frictional penalty through the rectangular SAHD. The analysis for the inverted-T rib-tubulators is performed by keeping the same range of parameters as that of quarter-circular rib-turbulators. The analysis shows a maximum enhancement in the heat transfer of 2.74 times the smooth SAHD is achieved for the P/e of 7.14 and e/Dh (fixed) of 0.042 at Re of 15000. The maximum thermal enhancement ratio of 1.87 is observed in the parametric range. The study is extended by considering the triangular SAHD with pentagonal rib-turbulators. The various heat transfer parameters, such as P/e, e/Dh, and Re are kept in the range of 6 to 12, 0.03 to 0.05, and 4000 to 18000, respectively. A maximum enhancement in heat transfer of 2.01 times the smooth duct is observed for the pentagonal rib-turbulators with P/e of 10 and e/Dh of 0.05 at Re of 15000. Within the parametric range, a maximum frictional penalty of 2.97 is encountered for P/e of 10 and e/Dh of 0.05 at Re of 4000. Flow behavior and heat transfer characteristics are highlighted for each case with various contours, vectors, and streamline plots. In the subsequent stage of the work, an experimental investigation was carried out to explore the influence of roughness and flow parameters on the thermo-hydraulic performance analysis of a triangular SAHD. The transverse circular rib-turbulators with gaps are placed on the surface of the absorber plate. The roughness parameters, P/e and e/Dh are kept in the range of 4.88 to 20 and 0.021 to 0.044, respectively. Re is kept in the range of 4000 to 18000. Two and three gaps of each of 0.01 m are provided to each odd and even number rib, respectively. Non-dimensional primary width (𝑤1/𝑊) and non-dimensional secondary width (𝑤2/𝑊) are kept constant at 0.29 and 0.4, respectively. A maximum heat transfer of 3.14 times that of the base model is achieved for the transverse ribs with gaps having P/e and e/Dh of 9.76 and 0.044, respectively, at 𝑅𝑒 = 18000. Again the experimentation is done to evaluate the heat transfer and friction factor characteristics of the triangular SAHD with transverse broken rib-turbulators. For the study, e/Dh is kept constant at 0.044, and P/e is varied in the range of 4.88 to 19.51. In the parametric range, the maximum enhancement in heat transfer is found to be 2.44 times the smooth duct for P/e of 9.76 at Re of 15000, and the maximum friction factor is found to be 3.19 times the smooth duct for P/e of 4.88 at Re of 4000. For each of the studied cases, the Nusselt number and friction factor correlations are developed with minimum error.