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A hybrid nanofluid (HYNF) is an excellent working medium for improved heat transfer performance. To investigate the convective heat transfer performance of an HYNF of various particle mixing ratios (MR), at first samples of MgO-SiO2/water HYNF are prepared by varying the MR of MgO to SiO2 as 0:100, 40:60, 50:50, 60:40, and 100:0 at 0.05 mass % of concentration; then convective heat transfer characteristics are determined experimentally by exposing different mass flows of HYNF from 0.03–0.27 kg/s to a constant heat flux of 5630 W/m2. The results describe that the variation in MR strongly influences the Nusselt number (Nu) of HYNF. The maximum Nu, which is 1.94 times (~ 94.50%) higher than the same with base fluid, is obtained at MR of 60:40 and the mass flow rate of 0.13 kg/s. The variation of MR does not affect the friction factor of HYNF and it is nearly comparable to that of the base fluid. Further, a new concept has been proposed to determine the thermo-economic feasibility of HYNF in heat transfer applications. It was recommended to maintain the MR at 60:40 and 0.13 kg/s mass flow rate for optimum thermo-economic performance. [ABSTRACT FROM AUTHOR]

The electronics in logging tools are susceptible to thermal failure due to the extreme thermal environment. Passive thermal management system (PTMS) has been proven to protect the electronics for several hours. In this paper, a numerical model based on the equivalent thermal conductivity method was developed to determine the transient temperature of downhole electronics. Compared with the traditional analytical model or numerical model considering only one single heat transfer process, the complex heat transfer in the logging tool were comprehensively considered through the proposed model. Briefly, the heat transfer processes in the vacuum bottle were theoretically calculated rather than empirically acquired, and the convective and radiative heat transfer inside the vacuum bottle was equated to the temperature-dependent thermal conductivity of air. In addition, the proposed numerical model was compared with experiments and previous numerical models to verify the accuracy of the model, and the temperature deviation of the electronics was within 5 °C. Moreover, the computational time was greatly reduced compared with the traditional three-dimensional numerical simulation process while ensuring the accuracy of calculation. Therefore, the proposed numerical model achieved both computational accuracy and efficiency, which was expected to be widely used in the development of PTMS for logging tools. • A numerical model based on equivalent thermal conductivity method was proposed. • The complex heat transfer processes in PTMS were comprehensively considered. • The proposed model possessed high computational accuracy and efficiency. [ABSTRACT FROM AUTHOR]