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The paper describes a computational study of heat transfer enhancement inside a square duct with V-shaped flapped baffles located repeatedly on the bottom and top walls for fluid flowing with Reynolds numbers (Re) from 3000 to 21,000. The basic goal of this work is to attain the largest relative Nusselt number (Nu / Nu 0) whilst maintaining the highest thermal performance to improve energy savings. A finite volume method was used in the computations, along with the Realizable k ‒ ε turbulent model. The variable baffle parameters considered first in the current simulation were the relative height/blockade ratio (B R = 0.05−0.2) and the flap angle of the baffle hole (β = 0° − 90°), while the fixed parameters included the attack angle (α = 60°), hole diameter ratio (d R = 0.5), and pitch ratio (P R = 0.5). To accomplish this goal, the previously mentioned parameters providing the best thermal performance were investigated further by extending the values of B R to 0.25−0.3, d R to 0.8 and α to 45°−30°. The simulation results indicate that the jet flowing from the flapped hole, as well as the vortices created by the baffle, can boost heat transfer and friction loss in comparison to the plain duct. In comparison, using a flapped baffle with β > 0° results in less friction loss, a greater thermal enhancement factor (TEF), and a higher Nusselt number than using a baffle with no flap. The first investigation disclosed that for B R = 0.2 and β = 20°, the greatest TEF of 2.19 with Nu / Nu 0 of 7.9 times are obtained. The extended study, on the other hand, showed that the highest TEF of roughly 2.49 with Nu / Nu 0 of 8.4 times are seen for α = 45°, d R = 0.8, B R = 0.25 and β = 20° at lowest Re. Thus, the flapped baffle provides a significant increase in Nu / Nu 0 and TEF over the baffle alone. [ABSTRACT FROM AUTHOR]

An experimental and numerical work has been carried out to study the heat transfer enhancement in a heat exchanger square-duct fitted with 30° oblique horseshoe baffles (HB). In the current work, air is passed through the HB-inserted duct having a constant surface heat-flux. The air flow and heat transfer behaviors are presented for turbulent flow region, Reynolds number ranging from 4000 to 25,000. The pertinent parameters of the 30° HB elements include three relative baffle-pitches ( P R = P / H = 0.5, 1 and 2) and five relative baffle heights ( B R = b / H = 0.05, 0.1, 0.15, 0.2 and 0.25). Influences of those parameters on heat transfer and energy loss due to friction in terms of Nusselt number and friction factor, respectively are studied. The experimental result shows that at a given B R , the smallest pitch spacing ( P R = 0.5) provides the highest heat transfer and friction factor. The HB at B R = 0.25 and P R = 0.5 yields the highest heat transfer and friction factor but the one at B R = 0.2 and P R = 1 gives the maximum thermal performance. In addition, the thermal performance of using the HB is much higher than that of the wire coil insert, in comparison with other turbulators. To understand the heat transfer mechanism, a numerical inserted-duct flow simulation is also conducted and the obtained numerical results are in good agreement with measurements. Numerical flow and heat transfer behaviors such as streamlines, temperature and Nusselt number contours of the duct flow model are also reported. [ABSTRACT FROM AUTHOR]

Abstract: Experimental investigations have been carried out to study the effect of combined wedge ribs and winglet type vortex generators (WVGs) on heat transfer and friction loss behaviors for turbulent airflow through a constant heat flux channel. To create a reverse flow in the channel, two types of wedge (right-triangle) ribs are introduced: wedge ribs pointing downstream and pointing upstream. The arrangements of both rib types placed inside the opposite channel walls are in-line and staggered arrays. To generate longitudinal vortex flows through the tested section, two pairs of the WVGs with the attack angle of 60° are mounted on the test channel entrance. The test channel has an aspect ratio, AR=10 and height, H =30mm with a rib height, e/H =0.2 and rib pitch, P/H =1.33. The flow rate in terms of Reynolds numbers is based on the inlet hydraulic diameter of the channel ranging from 5000 to 22,000. The presence of the combined ribs and the WVGs shows the significant increase in heat transfer rate and friction loss over the smooth channel. The Nusselt number and friction factor values obtained from combined the ribs and the WVGs are found to be much higher than those from the ribs/WVGs alone. In conjunction with the WVGs, the in-line wedge pointing downstream provides the highest increase in both the heat transfer rate and the friction factor while the staggered wedge pointing upstream yields the best thermal performance. [Copyright &y& Elsevier]

The article deals with thermal and flow resistance characteristics in a tubular heat exchanger fitted with staggered-winglet perforated tapes (WPT). The experiment was conducted in the test tube having a constant wall heat-flux for turbulent airflow, Reynolds number (Re) from 4180 to 26,000. The aim of using the WPT is to generate longitudinal vortex flows to disrupt thermal boundary layer on the tube wall and to provide stronger fluid mixing. The WPT having an winglet inclination angle of 30° was inserted into the test tube at five different winglet blockage ratios ( B R = 0.1, 0.15, 0.2, 0.25 and 0.3) and three winglet pitch ratios ( P R = 0.5, 1.0 and 1.5). To find an optimum thermal performance condition, the effect of B R and P R on the heat transfer and pressure loss due to flow friction in the tube is examined. The experimental results reveal that Nusselt number (Nu) and friction factor ( f ) for the WPT increase with the increment of B R but the reduction of P R . The highest thermal enhancement factor (TEF) of 1.71 is achieved by utilizing the WPT with B R = 0.15, P R = 1.0 at Re = 4180. Compared to staggered-winglet typical non-perforated tape (WTT), the WPT yields the TEF of about 1.2 times higher than the WTT. Correlations of Nu, f and TEF for the WPT and the WTT are also proposed. [ABSTRACT FROM AUTHOR]

Abstract: The paper presents an experimental study on turbulent flow and heat transfer characteristics in a solar air heater channel fitted with combined wavy-rib and groove turbulators. The experiments are performed by controlling the airflow rate to obtain Reynolds numbers in the range of 4000 to 21,000. To produce recirculation flow in the tested channel having a constant heat-flux on the upper wall only, the triangular wavy ribs are placed repeatedly on the tested grooved channel walls. Three test cases of different rib-pitch to channel-height ratios (PR= P/H =0.5, 1 and 2) with a single rib-to-channel height ratio (BR= b/H =0.25) are introduced in the present work. The wavy ribs are placed with the attack angle of 45° relative to main flow direction. There are three types of rib arrangements, namely, rib-groove on the upper wall only, inline rib-groove, and staggered rib-inline groove on two principal walls. The experimental result reveals that the combined rib-groove on both the upper and lower walls of the test channel provides the highest heat transfer rate and friction factor in comparison with the smooth channel with/without ribs. However, the ribbed-grooved upper wall at PR=0.5 yields the highest thermal performance. The combined rib-groove turbulator is found to be considerably higher thermal performance than the groove alone. [Copyright &y& Elsevier]

An experimental and numerical study of turbulent convective heat transfer in a solar air heater duct with winglet-type vortex generators (WVGs) placed on the absorber plate is presented. Air as the test fluid enters the duct having a uniform wall heat-flux applied on the upper wall or the absorber plate with Reynolds number from 4100 to 25,500. Two types of WVGs are introduced: rectangular (RWVG) and trapezoidal (TWVG) WVGs, in order to create multiple vortex flows along the duct. The WVG parameters in the present study include two relative height (B R  =  e / H  = 0.2 and 0.48), three longitudinal pitch ratios (P R  =  P l / H  = 1, 1.5 and 2) and a single attack angle, α  = 30°. The experimental result reveals that the RWVG with B R  = 0.48 and P R  = 1 provides the highest heat transfer and friction factor at about 7.1 and 109.5 times above the flat duct, respectively while the TWVG with B R  = 0.2 and P R  = 1.5 yields the maximum thermal performance around 1.84. Then, to improve the performance by reducing the substantial pressure loss, both the WVGs with B R  = 0.48 and P R  = 1.5 are modified to be perforated rectangular and trapezoidal winglet-type vortex generators (P-RWVG and P-TWVG) with four different punched hole/pore diameters ( d  = 1, 3, 5 and 7 mm) on their central area. The investigation indicates that among the perforated WVGs, the P-RWVG at d  = 1 mm yields the highest heat transfer and friction factor up to 6.78 and 84.32 times higher than the smooth duct but the best thermal performance of about 2.01 is found for the P-TWVG with d  = 5 mm. To explore the flow and heat transfer pattern, a 3D numerical flow simulation is performed and validated with available measurements where both the numerical and measured results are in good agreement. [ABSTRACT FROM AUTHOR]

Effects of insertion of a straight tape with double-sided delta wing pairs (called “delta-wing tape”, DWT) used as a longitudinal vortex generator (LVG) on forced convective heat transfer and flow friction characteristics in a uniform heat-flux heat exchanger tube are investigated experimentally and numerically in the present work. The experiment is conducted for turbulent airflow with the Reynolds number ( Re ) from 4200 to 25,500. The delta wings are in a forward-wing arrangement with three wing inclination angles (α = 30°, 45° and 60°) and with five ratios of wing-pitch to tube-diameter ( P / D = PR = 0.5, 1.0, 1.5, 2.0 and 2.5). Influences of the DWT insert on heat transfer in terms of Nusselt number (Nu) and pressure drop in the form of friction factor ( f ) are examined. The results indicate that the DWT provides the increases in Nu and f up to 505% and 69 times above the plain tube, respectively and the maximum thermal enhancement factor (TEF) is at 1.49. The 60° DWT with PR = 0.5 yields the highest Nu and f but the 30° one with PR = 1.0 gives the best TEF. To understand the flow structure and heat transfer mechanism, a three dimensional CFD simulation of the inserted tube is also performed. The simulated results are validated and are in good agreement with the current measurements. A simulated heat transfer and fluid flow structure is also reported. [ABSTRACT FROM AUTHOR]

Abstract: A numerical work has been conducted to examine turbulent periodic flow and heat transfer characteristics in a three dimensional square-duct with inline 60° V-shaped discrete thin ribs placed on two opposite heated walls. The isothermal-flux condition is applied only to the upper and lower duct walls while the two sidewalls are insulated, similar to internal passage cooling of gas turbine blades. The computations are based on the finite volume method with the SIMPLE algorithm for handling the pressure–velocity coupling. Air is the working fluid with the flow rate in terms of Reynolds numbers ranging from 10,000 to 25,000. The numerical result is validated with available square-rib measured data and found to agree well with measurement. The computation reveals that the ribbed duct flow is fully developed periodic flow and heat transfer profiles at about x/D =7–11 downstream of the inlet. Effects of different rib height to duct diameter ratios, BR, on thermal characteristics for a periodic ribbed duct flow are investigated. It is found that a pair of counter-rotating vortices (P-vortex) caused by the rib can induce impingement/attachment flows on the walls leading to greater increase in heat transfer over the test duct. In addition, the rise of BR values leads to the increase in heat transfer and friction loss. The maximum thermal performance is around 1.8 for the rib with BR =0.0725 where the heat transfer rate is about 4.0 times above the smooth duct at lower Reynolds number. [Copyright &y& Elsevier]