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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]

In this study, experimental investigation has been conducted on fluid flow and heat transfer of rough mini-channels with rectangular cross sections. The mini-channel considered consists of twelve identical rectangular channels with 4 mm width, 8 mm height, and 80 mm length. Air is employed as the working fluid with Reynolds number varied from 1500 to 5000. Four mini-channels were examined with different values of relative surface roughness ( ɛ ) 1.6 ∗ 10 −4 , 3.45 ∗ 10 −4 , 6.15 ∗ 10 −4 , and 10.5 ∗ 10 −4 . The friction factor ( f ), average Nusselt number ( Nu ), pressure drop penalty factor ( E f ), heat transfer enhancement factor ( E hn ), and universal evaluation parameter (PEC) were evaluated to study fluid flow and heat transfer of rough mini-channels with rectangular cross sections. The results showed that increasing the Reynolds number ( Re ) of air passing through the rectangular rough mini-channel, increases the average Nusselt number ( Nu ) and the pressure drop penalty factor ( E f ), while decreasing the friction factor ( f ) and the heat transfer enhancement factor ( E hn ) which, has a great effect on the universal evaluation parameter (PEC) rather than the pressure drop penalty factor ( E f ). Moreover, a correlation of the relative surface roughness ( ε ) and the universal evaluation parameter (PEC) was obtained. [ABSTRACT FROM AUTHOR]

The efficiency, availability, and reliability of thermal management for various engineering applications present a crucial challenge facing the designers and engineers. The rapid development and sophisticated technological advancement require innovative devices possessing high operating capacities that are launched into the prodigious growing markets in all engineering sectors. As a result of pronounced advancements, the jet impingement required to transfer high heat fluxes with a target surface becomes one of the essential priorities for researchers. The intensification of jet impingement can be achieved with different techniques such as passive self-exciting jets, active exciting jets, and hybrid exciting jets. The active methods of self-exciting jets include annular, swirling, and sweeping jets, while the active ones contain pulsed and synthetic jets. In the current study, the heat transfer characteristics and the fluid flow behavior of jet impingement with recent modifications of jets are reviewed comprehensively. As well, the critical methods of passive and active exciting jets considering the jet geometrical and operating parameters are introduced. Moreover, the physical phenomena for each technique in comparison to the conventional circular straight jet as well as the published empirical correlations as available, are studied. The present work reviews the published numerical and experimental investigations considering the different modified jets and its application with various installation geometries that provide a higher efficiency with the augmentation of heat transfer rate. The jet excitation causes topological metamorphosis of flow and hence has a remarkable effect on the intensification of heat transfer. • The intensification of jet impingement can be achieved with different techniques. • Active, passive, and hybrid methods of jet excitation were reviewed. • Heat transfer and fluid flow characteristics of each exciting jet were studied. • The geometrical and operating parameters of each exciting jet were introduced. • Jet excitation increased turbulence intensity and hence intensified heat transfer. [ABSTRACT FROM AUTHOR]

• Fluid flow and thermal performances of PHE are investigated experimentally. • The effectiveness and NTU of PHE are improved with increasing Rr. • Pressure drop and pumping power are increased by increasing Rr. • SHR is improved with increasing surface roughness particularly at low Reynolds number. • Empirical correlations are proposed to predict SHR as a function of Rr and Re. In the present paper, an experimental study of the effectiveness and pumping power of plate heat exchanger (PHE) with rough surface was performed. The experiments were developed for different values of relative roughness, Rr, of 2.6 * 10−4 (smooth surface), 3.4 * 10−4, 7.5 * 10−4 and 9.8 * 10−4 for Reynolds number, Re, varied from 500 to 5000. The average heat transfer rate, Q avg , hence the effectiveness of PHE, ε , and number of transfer units, NTU , as well as pressure drop, Δp , consequently pumping power, P, and specfic heat rate, SHR, were estimated. The results show that Q avg and NTU are enhanced with increasing Rr ; while, they are decreased with increasing Re. At Re = 5000, the effectiveness of PHE with relative roughness of 3.4 * 10−4, 7.5 * 10−4 and 9.8 * 10−4 is greater than that of the smooth surface by 11.5%, 16.6% and 18.6%, respectively. Furthermore, Δp and P are increased by increasing Rr and Re. Moreover, at Re = 500, SHR is enhanced with increasing Rr and its improvements compared with the smooth surface are 39.5%, 23.4% and 6.9% for Rr of 3.4 * 10−4, 7.5 * 10−4 and 9.8 * 10−4, respectively. In addition, empirical correlations are proposed to estimate SHR as a function of Rr and Re with a standard deviation of 5.1%. [ABSTRACT FROM AUTHOR]