Model Validation for the Heat Transfer in Gasket Plate Heat Exchangers Working with Vegetable Oils

Many models for accurately predicting the performance of gasket plate heat exchangers were developed in the last decades, grouped in three categories: empirical, semi-analytical or theoretical/numerical, with the view to saving materials and energy through correct design of industrial equipment. This work addresses one such model, namely Lévêque correlation modified by Martin and by Dović, which is promising due to the correct assumption of the flow in sine duct channels and the consideration of energy losses caused by flow reversal at plate edges and the flow path changing when entering the chevron angle. This model was validated by our own experimental data under industrial conditions for vegetable oils processing, both in laminar flow (Re = 8–42) and fully developed turbulent flow (Re = 446–1137). Moreover, in this study, particular values for constants/parameters of the model were determined for the corrugation inclination angle relative to vertical direction equal to 30°. Through statistical analysis, this study demonstrates that this particularized form of the generalized Lévêque correlation can be used with confidence.

Turbulent flow in curved channels

Fully developed turbulent flow in channels with mild to strong longitudinal curvature is studied by direct numerical simulations. The Reynolds based on the bulk mean velocity and channel half-width $\delta$ is fixed at $20\,000$ , resulting in a friction Reynolds number of approximately 1000. Four cases are considered with curvature varying from $\gamma = 2\delta /r_c = 0.033$ to 0.333, where $r_c$ is the curvature radius at the channel centre. Substantial differences between the mean wall shear stress on the convex and concave walls are already observed for $\gamma = 0.033$ . A log-law region is absent and a region with nearly constant mean angular momentum develops in the channel centre for strong curvatures. Spanwise and wall-normal velocity fluctuations are strongly amplified by curvature in the outer region of the concave channel side. Only near the walls, where curvature effects are relatively weak, do the mean velocity and velocity fluctuation profiles approximately collapse when scaled by wall units based on the local friction velocity. Budgets of the streamwise and wall-normal Reynolds-stress equations are presented and turbulence structures are investigated through visualizations and spectra. In the case with strongest curvature, the flow relaminarizes locally near the convex wall. On the concave channel side, large elongated streamwise vortices reminiscent of Taylor–Görtler vortices develop for all curvatures considered. The maximum in the premultiplied two-dimensional wall-normal energy spectrum and co-spectrum shifts towards larger scales with increasing curvature. The large scales substantially contribute to the wall-normal velocity fluctuations and momentum transport on the concave channel side.

Frictional Pressure Drop and Cost Savings for Graphene Nanoplatelets Nanofluids in Turbulent Flow Environments

Covalent-functionalized graphene nanoplatelets (CF-GNPs) inside a circular heated-pipe and the subsequent pressure decrease loss within a fully developed turbulent flow were discussed in this research. Four samples of nanofluids were prepared and investigated in the ranges of 0.025 wt.%, 0.05 wt.%, 0.075 wt.%, and 0.1 wt.%. Different tools such as field emission scanning electron microscopy (FE-SEM), ultraviolet-visible-spectrophotometer (UV-visible), energy-dispersive X-ray spectroscopy (EDX), zeta potential, and nanoparticle sizing were used for the data preparation. The thermophysical properties of the working fluids were experimentally determined using the testing conditions established via computational fluid dynamic (CFD) simulations that had been designed to solve governing equations involving distilled water (DW) and nanofluidic flows. The average error between the numerical solution and the Blasius formula was ~4.85%. Relative to the DW, the pressure dropped by 27.80% for 0.025 wt.%, 35.69% for 0.05 wt.%, 41.61% for 0.075 wt.%, and 47.04% for 0.1 wt.%. Meanwhile, the pumping power increased by 3.8% for 0.025 wt.%, 5.3% for 0.05 wt.%, 6.6% for 0.075%, and 7.8% for 0.1 wt.%. The research findings on the cost analysis demonstrated that the daily electric costs were USD 214, 350, 416, 482, and 558 for DW of 0.025 wt.%, 0.05 wt.%, 0.075 wt.%, and 0.1 wt.%, respectively.

Benchmark for scale-resolving simulation with curved walls: the Taylor Couette flow

Flow between rotating concentric cylinders, or the Taylor Couette flow, has been studied extensively because of its rich physics, ranging from axisymmetric steady laminar flow, to fully developed turbulent flow. In the present study, we advocate the use of this problem as a benchmark case for scale-resolving simulation, such as large eddy simulation (LES) and direct numerical simulation (DNS). The problem is attractive because of its simple geometry, simple boundary conditions, and complex physics involving wall-shear induced and centrifugal instability. Unlike the well-known fully developed channel flow, this problem has a curved wall boundary, and it is unnecessary to add a source term to the governing equations to sustain the fully developed turbulent flow. A p-refinement study for Re = 4000 is performed first to establish DNS data, including the time history of enstrophy, which can be used as an accuracy and resolution indicator to evaluate numerical methods, and is orders of magnitude faster than using the mean flow quantities and Reynolds stresses to evaluate solution quality. Finally, an hp-refinement study is performed to establish the relative accuracy and efficiency of high-order schemes of various accuracy.

Multi-GPU simulation of turbulent square duct flow under high Reynolds number in lattice Boltzmann method

Using multi-GPU in lattice Boltzmann method (LBM), fully developed turbulent flow in a square duct at the friction Reynolds numbers (Reτ) of 300, 600, 1200 and 1800 are simulated. Through simulation of three-dimensional lid-driven cavity flow under different Reynolds number (Re), the accuracy of lattice Bhatnager-Gross-Krook (LBGK) multi-GPU program is validated. For turbulent flow in a square duct, all mean velocity, secondary flows, root mean square (rms) of pulsating velocity and Reynolds shear stress predicted by LBGK under the lower Reτ agree well with the literature results, which further verified the effectiveness of the LBGK. In addition, fully developed turbulent flow in a square duct with Reτ up to 1800 predicted by LBGK with 600 million grids provides a reference for turbulent flows under high Reτ . Numerical results show that the LBGK model with low accuracy successfully captures turbulent characteristics for flows at high Re by increasing the grid size, indicating the feasibility and practicality of multi-GPU LBM for modeling industrial flows.

Experimental Investigation of Combined Effect of Particle Size and Stability of Al2O3-H2O Nanofluid on Heat Transfer Augmentation Through Horizontal Pipe

The stability of nanofluid plays a rule in heat transfer growth for different engineering systems. The stability and particle size of Al2O3-H2O nanofluid effects on heat transfer are studied experimentally. Two particle sizes (20 and 50 nm) with (0.1, 0.5 and 1%) concentrations were prepared and tested under constant heat flux (1404 W) with fully developed turbulent flow through a horizontal pipe. The results show an increase in Nusselt number by 20.7% and 17.6% with 1 vol.% concentration for 20 and 50 nm, respectively compared to distilled water. Examined nanofluid showed improvement in Nu number by (30.3 and 23.5) % at 1 vol.% concentration compared to water. Obtained results show minor decrease in the pressure drop and friction factor with nanofluid after stability treatment. Different correlations between Nu number and friction factor relating to studied parameters were observed.