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AbstractJet impingement is a heat transfer technique utilized in different applications in industry. It is used for cooling the turbine blade and the combustor wall because of its high cooling rate. The heat transfer (HT) and flow structure of a circular air-impinging jet over a flat target plate with a circular row of 48 roughness elements are numerically investigated using renormalization group k–ε turbulence model. Different shapes of roughness elements, namely cylindrical roughness elements (CREs), droplet roughness elements (DREs), and hemispherical roughness elements (HREs), are examined. The effects of Reynolds number changed from 7,000 to 35,000, and the roughness elements location relative to jet diameter (s/d) of 1.5, 2, 2.5, and 3 at a constant jet-to-target plate distance (H/d) of 2 are studied. Temperature distribution, local Nusselt number (Nu), average Nusselt number (Nuavg), average Nusselt number ratio (Nuavg,r), velocity distribution, turbulence kinetic energy, streamline contours, and static pressure (p) over the roughness elements are evaluated and discussed. The results showed that the existence of roughness elements has a significant effect on the thermal and hydraulic characteristics. The Nuavg,r for the CREs, DREs, and HREs is up to 1.61, 2.02, and 1.2, respectively. All roughness element shapes increase the flow velocity in the narrow area between the elements and increase the wetted area. CREs augment the HT rates by increasing the turbulence intensity, while DREs reduce the drag force effects. [ABSTRACT FROM AUTHOR]

In the present study, an experimental investigation of performance evaluation of plate heat exchanger (PHE) with the rough surface using AL2O3/water nanofluid was achieved. The experiments were developed for different surface roughnesses of 0.978 µm (smooth surface), 1.276 µm, 2.825 µm and 3.657 µm with different volume fractions of nanoparticles of 1.2 vol.%, 1.9 vol.% and 2.6 vol.% at Reynolds number of 500 ≤ Re ≤5000. The Nusselt number (Nu), the friction factor (f) and the heat transfer enhancement factor (HTEF) were estimated. The results show that increasing the volume fraction of nanoparticles and surface roughness enhances the heat transfer rate as well as increases the pressure drop in PHE. Hence, the influence of surface roughness is more considerable than that of volume fraction of nanoparticles on PHE performance. Furthermore, for smooth surface, the HTEF is improved by 2%, 3.7% and 5.4% for volume fraction of nanoparticles of 1.2 vol.%, 1.9 vol.% and 2.6 vol.%, respectively; while these values for the highest value of surface roughness, i.e., 3.657 µm, are 9.4%, 11.8% and 13.7%. Based on the experimental results, a new empirical correlation to predict Nu for base fluid is obtained with a maximum deviation of 2.87%. [ABSTRACT FROM AUTHOR]

An experimental investigation is performed to study the effect of the inclination angle of air jets on heat transfer from a horizontal surface. A pair of inclined circular jets issuing from the straight pipes of length to diameter ratio of 40 is considered. The inclination angles of 0°, 10°, 20°, 30° 45°, and 60° are selected while the Reynolds number is varied from 10,000 to 40,000 based on a jet exit diameter. Jets to impingement plate distance (L/D) as well as the spacing distance (S/D) are changed from 2 to 8. The results of the temperature contours as well as the Nusselt number contours are illustrated to study the characteristics of heat transfer from a flat surface cooled by a pair of oblique jets. The experimental results show that the maximum magnitude of average heat transfer coefficient is achieved in the range of inclination angle from 10° to 20°. Empirical correlations are presented to predict the value of average Nusselt number as a function of Re , Pr , and θ. Moreover, a comparison between the values of average Nusslet number of the present study and those available in the literature is carried out. [ABSTRACT FROM AUTHOR]

The present study investigates experimentally the influence of a movable air jet impingement on the characteristics of heat transfer over a flat plate. The experiments are conducted with a nozzle translation velocity ( V n ) of 200 mm min−1, a Reynolds number (Re) of 24,000, and a nozzle-to-plate distance ratio (H/D) of 1, 3, 5, and 7. The characteristics of heat transfer on a flat surface due to the use of a movable air nozzle are compared with those of a fixed one using an IR camera. The local Nusselt number (Nu), average Nusselt number ( Nu ¯ ), heat transfer uniformity index (HTU), and heat transfer uniformity (U) are investigated. The results demonstrate that the movable nozzle is more effective than the fixed one for reducing the temperature variation over the heating surface, leading to more heat transfer uniformity. The maximum values of both the Nu and Nu ¯ are obtained at a H/D of 1. The U that is improved for the movable nozzle when compared to the fixed nozzle is significantly influenced by the H/D. The maximum values of U are 85 and 72% at a H/D of 7 for the movable and fixed nozzles, respectively. The movable nozzle improves the uniformity of heat transfer more than the fixed one by 75, 42, 39, and 43% at a H/D of 1, 3, 5, and 7, respectively. Moreover, a comparison between the results of the Nu ¯ of the present work and those of previous studies is conducted. [ABSTRACT FROM AUTHOR]

In this study, the cooling of a gas turbine (GT) blade leading edge using a single row of five impinging air jets was numerically and experimentally investigated. The inner surface of the blade leading edge was depicted as a semicircle profile. The numerical study was performed using a shear stress transport (SST) k-ω turbulence model. On the other side, the experimental work was conducted using the infrared (IR) thermal camera. Additionally, the heat transfer characteristics were investigated with changing the Reynolds number (⁠Re ⁠) from 5000 to 40,000. The study was carried out for different cases based on the jet diameter (⁠d ⁠), named as fixed diameter (FD) and variable jet diameter in the streamwise flow direction, i.e., ascending diameter (AD) and descending diameter (DD). The local and average Nusselt numbers and heat transfer uniformity were studied at different operating and design parameters. The results revealed that for FD, increasing the jet diameter decreased the average Nusselt number while it enhanced the heat transfer uniformity. Furthermore, for all values of Re ⁠, the thermal performance of the AD was better than the DD, which has the worst heat transfer uniformity. Finally, predicted developed correlations to estimate the average Nusselt number, surface heat transfer uniformity, and performance evaluation factor (PEF) for all cases of jet diameter were introduced. [ABSTRACT FROM AUTHOR]