Numerical Investigation of Thermal-Hydraulic Characteristics for Turbulent Nanofluid Flow

What is it about?

In this paper, thermal-hydraulic characteristics and performance of turbulent flow of various Nanofluids in a various conical double pipe heat exchanger (fourteen different combinations of flow direction and conical tubes geometries) are numerically investigated. Intensification of heat transfer rate with geometrical modification of double-pipe heat exchanger by altering the traditional straight cylindrical form of the tubes into conical tubes as a passive heat transfer enhancement technique with a combination with various Nanofluids as another passive technique of heat transfer enhancement is numerically studied. Effects of the combination between various straight/conical tube arrangements with different flow directions (parallel/counter) and various Nanofluids (DW/SiO2, DW/Al2O3, and DW/GNP-SDBS) are studied and compared with water as a base-fluid within Reynolds number range from 7000 to 35,000. Validation and grid independence study were performed. Structured, non-uniform grid generation is applied. Continuity, momentum, and energy equations were treated by means of a finite volume method (FVM) using the SIMPLE pressure-velocity coupling algorithm with the k–ε turbulence model and enhanced wall function as a wall treatment using Ansys-Fluent package. Heat transfer in terms of Nusselt number (Nu), pressure drop in terms of friction factor (f), and overall thermal efficiency in terms of performance evaluation criteria (PEC) have been carried out as the main parameters of the study. In addition, for deep-studying of the thermal/flow structure, contours of velocity, temperature, and turbulent kinetic energy (TKE) are presented. The results reveal that the combination between the straight/conical shapes and Nanofluids have noticeable effects upon the Nusselt number (Nu), the friction factor (ƒ) and thermal evaluation criteria (PEC) compared with the traditional straight tubes or without Nanofluids. Case (g) provides the highest values of heat transfer rate in the form of Nusselt number (Nu), and the highest values of pressure drop in the form of friction factor (f) is presented by Case (j) and Case (m) compared with the rest of studied cases. For case (g), the average percentages of enhancement of Nu compared with the traditional straight tube with parallel flow are 55.2%, 48.1%, 48.8%, and 28.8% for DW, DW/Al2O3, DW/SiO2, and DW/GNP-SDBS. Furthermore, for case (g), the average values of PEC are 1.13, 1.065, 1.06, and 0.87 for DW, DW/SiO2, DW/Al2O3, and DW/GNP-SDBS. Moreover, the maximum values of PEC are 1.209, 1.138, 1.133, and 0.92 for DW, DW/SiO2, DW/Al2O3, and DW/GNP-SDBS. It can be concluded that the conical double pipe heat exchangers are considered as a promising technique in heat transfer enhancement (percentage of augmentation in Nu is 55.2 % compared with the traditional straight double pipe heat exchanger) with taking in consideration energy saving (PEC ≥ 1).

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Photo by Karsten Würth on Unsplash

Photo by Karsten Würth on Unsplash

Why is it important?

In this paper, thermal-hydraulic characteristics and performance of turbulent flow of various Nanofluids in a various conical double pipe heat exchanger (fourteen different combinations of flow direction and conical tubes geometries) are numerically investigated. Intensification of heat transfer rate with geometrical modification of double-pipe heat exchanger by altering the traditional straight cylindrical form of the tubes into conical tubes as a passive heat transfer enhancement technique with a combination with various Nanofluids as another passive technique of heat transfer enhancement is numerically studied. Effects of the combination between various straight/conical tube arrangements with different flow directions (parallel/counter) and various Nanofluids (DW/SiO2, DW/Al2O3, and DW/GNP-SDBS) are studied and compared with water as a base-fluid within Reynolds number range from 7000 to 35,000. Validation and grid independence study were performed. Structured, non-uniform grid generation is applied. Continuity, momentum, and energy equations were treated by means of a finite volume method (FVM) using the SIMPLE pressure-velocity coupling algorithm with the k–ε turbulence model and enhanced wall function as a wall treatment using Ansys-Fluent package. Heat transfer in terms of Nusselt number (Nu), pressure drop in terms of friction factor (f), and overall thermal efficiency in terms of performance evaluation criteria (PEC) have been carried out as the main parameters of the study. In addition, for deep-studying of the thermal/flow structure, contours of velocity, temperature, and turbulent kinetic energy (TKE) are presented. The results reveal that the combination between the straight/conical shapes and Nanofluids have noticeable effects upon the Nusselt number (Nu), the friction factor (ƒ) and thermal evaluation criteria (PEC) compared with the traditional straight tubes or without Nanofluids. Case (g) provides the highest values of heat transfer rate in the form of Nusselt number (Nu), and the highest values of pressure drop in the form of friction factor (f) is presented by Case (j) and Case (m) compared with the rest of studied cases. For case (g), the average percentages of enhancement of Nu compared with the traditional straight tube with parallel flow are 55.2%, 48.1%, 48.8%, and 28.8% for DW, DW/Al2O3, DW/SiO2, and DW/GNP-SDBS. Furthermore, for case (g), the average values of PEC are 1.13, 1.065, 1.06, and 0.87 for DW, DW/SiO2, DW/Al2O3, and DW/GNP-SDBS. Moreover, the maximum values of PEC are 1.209, 1.138, 1.133, and 0.92 for DW, DW/SiO2, DW/Al2O3, and DW/GNP-SDBS. It can be concluded that the conical double pipe heat exchangers are considered as a promising technique in heat transfer enhancement (percentage of augmentation in Nu is 55.2 % compared with the traditional straight double pipe heat exchanger) with taking in consideration energy saving (PEC ≥ 1).

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