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The effect of aiding and opposing buoyancy (−1 ≤ Ri ≤ 1) on flow and heat transfer across cylinders specifically arranged in an equilateral triangular array in a heat exchanger has not previously been studied. Periodic boundary condition in the transverse direction is imposed where porosity of the array ranges from 0.7 to 0.99. The influence of buoyancy and Prandtl number (1 and 50) on the flow structure and its effect on the overall heat transfer is thoroughly elucidated in the present work. The impact of aiding/opposing buoyancy on the thermal performance of the array is predominant at higher porosities. The average drag of the array of cylinders increases with an increase in the aiding buoyancy and decreases with an increase in opposing buoyancy with respect to forced convection. The porosity of the triangular array has an inverse relation with the average drag coefficient, barring few exceptions which are explained in detail in the present work. The heat exchange is assertively higher at opposing‐buoyancy cases for all values of porosity and Prandtl number. The present results are in agreement with the available experimental/numerical studies. [ABSTRACT FROM AUTHOR]

Highlights • Mixed convection characteristics were explored across a periodic array of cylinders. • The dense and curved streamlines and isotherms were obtained. • Drag coefficients were diminished with rise in Reynolds number and fluid volume fractions. • Statistical correlations were developed for drag coefficient and Nusselt number. • The present results displayed an excellent agreement with the available literature. Abstract Aiding buoyancy mixed convection features of Newtonian fluids across a periodic array of heated cylinders have been studied numerically using a commercial CFD solver ANSYS FLUENT. The governing equations have been solved for the following ranges of physical parameters: Reynolds (1 ≤ Re ≤ 40), Prandtl (0.70 ≤ Pr ≤ 50) and Richardson (0 ≤ Ri ≤ 2) numbers and fluid volume fractions of 0.70–0.99. Qualitatively, the dense and curved streamlines and isotherms were seen with the increasing inertial (Re), viscous diffusion (Pr) and buoyancy parameter (Ri) across all the fluid volume fractions. The drag coefficients were observed to be diminished with an upturn in Re and fluid volume fractions, whereas an opposite behavior was noticed with rising in Pr and buoyancy parameter. The Nusselt numbers were found to be enhanced with Re and Pr numbers and moreover with fluid volume fractions also as in contrast to forced convection (Ri = 0) cases. Aiding buoyancy enhances flow as well as heat transfer features and yields unsteady behavior also at the higher fluid volume fractions and Re for all the values of Pr and Ri numbers. Moreover, statistical correlations have been developed for the total drag coefficient and average Nusselt number to gain the more physical insight of the results. Lastly, the findings have been compared with the literature which displayed the good agreement within the ranges of parameters studied herein. [ABSTRACT FROM AUTHOR]

In this research article, we investigated the phenomena of a buoyancy-driven cross-flow impinging on a bulk flow from the inlet for a flow past a pair of side-by-side square cylinders in a confined channel wall (which are kept in an adiabatic condition), a special case of an "internal flow" type problem. The density difference in the flow was achieved through a subtle temperature difference between the ambient fluid and the solid walls present in the domain. The study has been carried out at the Reynolds number Re = 1-40 for a transverse gap ratio s/d = 0.7-10 and the Richardson number Ri = 0-1 at a constant value of the Prandtl number Pr = 50. During a rigorous parametric study, we found that the mixed convection not only brings an early unsteadiness in the flow but also fetches an early formation of different flow regimes at Re = 40. An effort has been made to identify the precise near-wake formations leading to the vortex shedding processes in a mixed convection flow for a various range of s/d values. Different flow regimes can be identified efficiently through a unique combination of occurrences for dominant frequencies in terms of the Strouhal number St of primary, secondary, and harmonic frequencies in the flow. The time variant lift plots indicate that the unsteady periodicity in the flow varies from sinusoidal nature at s/d = 0.7 (single body type flow) to square wave (non-sinusoidal periodic waveform) for s/d = 1.5 (chaotic flow) and finally getting back to sinusoidal at s/d = 6 (non-interacting flow). We have also encountered the drastic pattern changes in the flow which occurred due to the onset of recirculation followed by the transition of steady to unsteady periodic nature with Re = 1-40 for all the values of s/d. This case is analyzed in detail through the study of various flow parameters. [ABSTRACT FROM AUTHOR]

The effects of cross-buoyancy mixed convection from a square cylinder in the proximity of a plane wall are studied for Reynolds number (Re) = 1–100, Richardson number (Ri) = 0–2, and gap ratio (G) = 0.25–1 at Prandtl number (Pr) = 0.7. The flow observed is steady forG = 0.25 and 0.5. The transition from a steady to a time-periodic system is observed forG = 1, and it is found at Re = 56, 60, and 74 for Ri = 0, 1, and 2, respectively. With increasingGand/or Ri, the drag coefficient and average Nusselt number increase for all Re values studied and the lift coefficient decreases with increasing Ri except at Re = 1. Maximum heat transfer augmentation is found about 89% atG = 0.5 (Re = 20, Pr = 0.7, Ri = 0) with respect to the corresponding value atG = 0.25 (Re = 20, Pr = 0.7, Ri = 0). Lastly, the correlations of drag coefficient and heat transfer have been obtained. [ABSTRACT FROM AUTHOR]

The influence of opposing-buoyancy mixed convection from a square cylinder in a vertical channel has been studied at Reynolds numbers (Re) = 1–100, Richardson numbers (Ri) = 0 to −1, and blockage ratios (β) = 10–50% for air as a working fluid. The onset of a steady to a time-periodic regime is found for Ri = 0 (at Re = 35, 65, 74, and 62), Ri = −0.5 (at Re = 12, 39, 48, and 54), and Ri = −1 (at Re = 9, 30, 39, and 50) forβ = 10%, 25%, 30%, and 50%, respectively. The initiation of flow separation is also determined. Finally, the correlations of Strouhal number, drag coefficient, and the Colburn heat transfer factor were obtained. [ABSTRACT FROM AUTHOR]

Two-dimensional numerical simulations are carried out to understand the effects of cross-buoyancy on the confined flow and heat transfer characteristics over an equilateral triangular bluff body at low and intermediate Reynolds numbers (Re). For a fixed Richardson number (Ri), an average cylinder Nusselt number increases with increasingRe. However, the average Nusselt number shows a soft response toward increasingRi. The criticalRefor the onset of periodic unsteady flow increases with increasing cross-buoyancy (Ri). A simple heat transfer correlation is also obtained to relate the values of average cylinder Nusselt number,Re, andRi. [ABSTRACT FROM AUTHOR]

Mixed-convectionflow and heat transfer of non-Newtonian power-law fluids over a pairof side-by-side circular cylinders in aiding buoyancy are investigatednumerically. The calculations are performed in an unconfined domainfor the following range of physical control parameters: Reynolds number(Re) = 1–40, Richardson number (Ri) = 0–1, and power-law index (n) = 0.2–1for a transverse gap ratio of 1.5 at a constant Prandtl number (Pr) of 50. Extensive numerical results are presented, suchas viscous, pressure, and total drag coefficients, average Nusseltnumber, and transition from a steady to a time-periodic regime. Theoverall drag coefficient and its components decrease with the Reynoldsnumber and increase with the Richardson number for all values of thepower-law index considered. The average Nusselt number increases withan increase in both the Reynolds and Richardson numbers but decreaseswith an increase in the power-law index. It is observed that aidingbuoyancy and shear-thinning tendency augment heat-transfer characteristics.The maximum enhancement in heat transfer is found to be approximately85% for Re= 40, n= 0.2, and Ri= 1. However, the maximum enhancement in the value ofthe average Nusselt number for Ri= 0 and 0.5 isfound to be approximately 70% and 81%, respectively. Finally, a simpleheat-transfer correlation is obtained for the values of Re, n, and Ricovered in this study. [ABSTRACT FROM AUTHOR]

The present study focuses on momentum and heat transfer characteristics of the laminar flow of power-law fluids in a vertical channel with a built-in square cylinder under the influence of aiding buoyancy. The simulations are carried out at low Reynolds numbers (1–40) for the range of settings as power-law index = 0.4–1, Richardson number = 0–1, blockage ratio = 25%–50%, and Prandtl number = 50. It is found that the drag coefficient and the Nusselt number increase with the aiding buoyancy, while the shear-thinning behavior reduces the drag coefficient and increases the heat transfer rate. [ABSTRACT FROM AUTHOR]