Friction Factor Ratio Research Articles

A comprehensive review of rectangular duct solar air heaters featuring artificial roughness

Abstract Solar air heaters (SAHs) are widely used solar thermal systems with applications in diverse sectors. However, its effectiveness is restrained by low convective heat transfer (HT) coefficients at the absorber plate, leading to inefficient HT, and the elevated temperature of the absorber plate causes significant heat losses, reducing thermal efficiency. This study addresses these challenges by introducing ribs or roughness on the absorber plate creating turbulence in the airflow, resulting in significant improvements. The research investigates various rib configurations, the influence of rib parameters, performance methods, and arrangements to evaluate their HT and friction characteristics. Among these rib configurations, a comparative analysis is done on various factors such as the Nusselt number ratio, thermal enhancement factor, friction factor ratio, and thermal efficiency to optimize distinct roughness parameters and rib arrangement patterns. This study also provides valuable recommendations from existing literature, offering insights into the effective design, prospects, and implementation of SAH systems.

Heat transfer enhancement and pressure drop performance of Al2O3 nanofluid in a laminar flow tube with deep dimples under constant heat flux: An experimental approach

This study examines the heat transfer enhancement and pressure drop of Al2O3 nanofluid in deep dimpled tubes in both longitudinal and circumferential directions. It explores mechanisms that improve the thermal performance of this novel tube geometry. Experiments were conducted using plain and deep dimpled tubes under laminar flow with Reynolds numbers from 500 to 2250, a constant heat flux of 10,000 W/m2, and nanofluid concentrations from 0.1 wt% to 1 wt%. The findings indicate that local velocity enhancement, vortex generation, and flow rotation and mixing are the three main mechanisms that improve the thermal performance of deep dimpled tubes. The results demonstrate that a deep dimpled tube with 1 wt% nanofluid can increase the convective heat transfer coefficient by up to 3.42 times compared to a smooth tube at Re = 2250. At this Reynolds number, the Nusselt number reaches a maximum of 41.80, and the friction factor ratio increases by only 1.82. Additionally, circumferential analysis reveals how dimple-induced vortices enhance heat transfer. The results also indicate that the tube geometry modification changes the flow regime zones, allowing turbulent flow at lower Reynolds numbers near Re = 2000, as identified by Nusselt number and friction factor plots. The deep dimpled tube has a low improvement penalty, with the highest friction factor of 0.38 at Re = 500 and high thermal enhancement, resulting in a performance evaluation criterion (PEC) of up to 2.80 in the studied region. However, the deep dimpled tube is unsuitable for Reynolds numbers below 1000. For higher velocities, replacing simple tubes with deep dimpled tubes in traditional heat exchangers is highly recommended.

Performance optimization of triple tube heat exchanger using aluminum metal foam: A comprehensive numerical investigation

Metal foam is a porous medium with high porosity, convoluted flow pathways, strength-to-weight ratio, and thermal conductivity employed in heat exchanges and other engineering applications because of its properties. Therefore, a novel circular tube filled with aluminum metallic foam is proposed to improve hydraulic thermal performance and reduce pumping power losses and metal foam volume. The current work considers the Brinkman-Forchheimer Extended Darcy model for forced convection flow and the local thermal equilibrium (LTE) model for heat transfer under a constant thermal boundary condition. The governing equations are solved using the finite volume method (FVM). The variable parameters are the pore per inch (PPI) of 10–50, the porosity εrange of 0.79–0.95, and the Reynolds number range of 2500–7000. The results show that the performance index (PI) has its maximum value of 6.8 at 10PPI and ε =0.95 in the heat exchanger; this enhancement at 10PPI is 13% and 34.48% more as compared to the PI value at ε =0.85 and ε =0.79 and at a fixed inner annulus Re of 7000. The average Nusselt number (Nu) increased by 81.63%, 104.3%, and 120% compared to the foamless tube at 10, 20, and 30PPI andε =0.95, respectively, in an intermediate tube. Heat transfer coefficients (HTC) considerably increase compared to foamless tubes nearly 3.2 times, 3.8 times, and 4.2 times at 10, 20, and 30PPI, respectively, at 2500 Re and 144%, 175% and 200% more at 7000 Re. The maximum reduction ratio in normalized friction factor ratio (fP/fNoP) at 10PPI is approximately 50% compared to (fP/fNoP) at 30PPI and 25% at 20PPI; it occurs at 7000 Re. The normalized area goodness factor ratio has the maximum value at 10PPI and ε =0.95, which is 5.8, and for 20 and 30PPI, it is 3.5 and 2.3, respectively. This shows that aluminum metal foam in triple tube heat exchanger's intermediate tube at 10PPI and porosity of 0.95 can be the optimized results for performance enhancement.

Tribological modifications of water flow at liquid–solid interface by nanobubbles

Previous studies investigated on friction reduction at the solid–liquid interface due to the presence of metal nanoparticles and fine bubbles such as microbubbles. This paper experimentally investigated how nanobubbles (ultrafine bubbles) change the tribological nature of water flow at the solid–liquid interface. We flowed air nanobubbles-containing water into a cylindrical cell filled with soda-lime glass, alumina, and high-carbon chromium-bearing steel beads. We then estimated the changes in the ratio of Darcy's friction factor of nanobubbles-containing water flow (fnb) to that of water flow before injecting nanobubbles (fref) with the time of injecting nanobubbles. We found that nanobubbles are capable of reducing the friction in water flow running through the soda glass beads, accounting for up to 6.1% reduction in terms of Darcy's friction factor ratio (fnb/fref) in our experiment. The magnitude of friction reduction by nanobubbles can be greater with a larger total surface area where surface nanobubbles are present. In contrast, nanobubbles encouraged enhancement of the friction of water flow within the high-carbon chromium-bearing steel beads, showing 3.8% enhancement in the friction factor ratio (fnb/fref). The results indicate that nanobubbles play a role in the friction reduction of water flow when the surface of the bead material is rougher than the size of nanobubbles, while nanobubbles enhance the friction of water flow when the bead surface is smooth enough. Therefore, nanobubbles can be a green nanoscopic additive for modifying the friction and lubrication performance of water flow depending on the surface roughness of the flow material.

Experimental investigation of effect of different configurations of semi-circular or half-round ribs on heat transfer enhancement in a rectangular duct

This research study investigated the impact of various configurations of semi-circular or half-round ribs on the thermal performance and frictional factor characteristics of a rectangular duct. During this research study, the ribs were provided on the inner surface of the test section’s bottom wall. Six distinct configurations of semi-circular ribs varying from continuous to hybrid arrangement were used. The range of Reynolds number was varied from 13,500 to 34,800; while the ratio of rib pitch to rib height (P/e ratio) was kept constant at 8. The ratio of the height of rib to hydraulic diameter of duct (also known as blockage ratio) was kept at 0.114. The results of experimental investigation showed that the Nusselt number ratio obtained was ranging from 1.36 to 2.74, while the friction factor ratio ranges from 1.70 to 4.51. The overall enhancement ratio for all configurations obtained was in the range of 1.06–1.80 times to that of plain duct. Among all rib configurations, the hybrid rib configuration showed superior ratio of Nusselt number and intermediate increase in ratio of friction factor as compared to all the other rib configurations. Hybrid rib configuration showed overall enhancement ratio values ranging from 1.45 to 1.80 times as compared to that of plain duct (duct without ribs) within the specified range of Reynolds number. The results of study showed that the hybrid configuration of semi-circular ribs exhibited superior overall thermal enhancement as compared to that of remaining tested rib configurations.

Experimental and numerical investigations of tube inserted with novel perforated rectangular V-shape vortex generators

The utilization of punched vortex generators has been established as an effective means to enhance heat transfer in heat exchangers. However, there are few studies on the use of perforated rectangular winglet pairs in heat exchange tubes and the study of perforation parameters based on this approach in the existing literature. In this paper, three parameters, including three height ratios, four perforation indices, and three pitch ratios, are considered experimentally and numerically. In the Reynolds number range of 7454–13,664, the effects of perforated rectangular vortex generator pairs on fluid flow and heat transfer are obtained by scaling parameters such as Nusselt number ratio, friction factor ratio, thermal enhancement factor, field synergy number, entropy generation, and exergy efficiency. The numerical results indicate that the perforation will lead to a reduction in turbulent kinetic energy in the local multi-longitudinal vortices on the rear side of the vortex generator, resulting in a decrease in local heat transfer as well as friction loss. Meanwhile, the pitch of perforated rectangular vortex generator pairs plays a crucial role in heat transfer and friction loss, enabling an increase in the Nusselt number of smooth tubes by 68–257%. Furthermore, the results are proved using the principle of entropy generation and exergy efficiency analysis. As Reynolds number increases, the thermal enhancement factor gradually decreases. Notably, the maximum thermal enhancement factor of 1.43 is achieved when the Reynolds number is equal to 7454, the height ratio is 0.08, the perforation index is 0.18, and the pitch ratio is 1.04.

Numerical study of parabolic trough solar collector’s thermo-hydraulic performance using CuO and Al2O3 nanofluids

The current study aims to boost the thermal and hydraulic performance of a parabolic trough solar collector by modifying the absorber tube with a porous obstacle insertion. The research investigates how porous media and nanoparticles together influence the system's thermal performance, considering factors like outlet temperature, average Nusselt number, average friction factor, thermal performance factor, and thermal efficiency at different mass flow rates. To achieve better thermal performance, the base fluid, water, is substituted with Copper Oxide (CuO)-water and Aluminum Oxide (Al2O3)-water nanofluids at concentrations ranging from 3% to 7% by volume. The experiments are conducted at 800–1000 W.m-2 heat flux on the outside tube surface. The finite element method discretizes the governing equations for 3-D fluid flow simulations conducted in COMSOL Multiphysics 6.1 software. The simulation is carried out considering four altered rates of mass flow, ranging from 0.015 kg.s−1 to 0.060 kg.s−1. The findings show that at a Reynolds number of 1428 and a 7% volume concentration, the maximum thermal performance factor recorded is 1.97 and 1.99 for Al2O3 and CuO nanofluids, respectively. Additionally, the absorber tube achieves maximum thermal efficiency of up to 77.93% and 79.03%. Also, the augmentation in the average Nusselt number ratio is 26.27% and 36.74% when using Al2O3 and CuO nanofluids, respectively, with the tube height-to-diameter ratio (h/D) set at 0.42. The average friction factor ratio was reduced by 2.9% using CuO nanofluids compared to Al2O3 nanofluids. Thermal efficiency was enhanced by 7.2% using a porous obstacle insert in the parabolic trough solar collector system compared to a non-porous system.

Heat transfer augmentation in a double pipe heat exchanger by tandem utilization of bubble injection and novel magnetic turbulators methods

For the purpose of enhancing the thermal efficiency of double-tube heat exchangers, a new technique has been implemented. By integrating a vibrating turbulator and bubble injection technique, this innovative method produces an added turbulence effect. This research aims to experimentally assess the thermal efficiency of employing two active methods concurrently. Different parameters were analyzed, including water flow rates ranging from 0.5 to 4 kg/min and bubble injection flow rates varying from 2 to 6 l/min, to investigate the effects on heat transfer and pressure drop with and without the magnetic turbulator. The results suggest that by incorporating bubble injection, adopting the magnetic turbulator, and simultaneously utilizing both methods, there is a significant improvement in heat transfer. In optimal scenarios, the heat transfer coefficient is amplified by 150.3% through the injection of bubbles, 328.8% through the implementation of magnetic turbulator, and 586.2% when both techniques are employed in tandem. In comparison to the smooth tube, the friction factor ratios were 6.7, 1.6, and 7.8 times higher in the specified cases. The findings indicated that the concurrent utilization of both techniques can yield an added impact on heat transfer. Ultimately, the integrated approach demonstrated a noteworthy thermal enhancement factor value of 3.49.

An experimental investigation and flow-system simulation about the influencing of silica–magnesium oxide nano-mixture on enhancing the rheological properties of Iraqi crude oil

This study aims to investigate the effects of introducing a 50/50 mixture of silica and magnesium oxide nanoparticles (SNP + MgONP) to the viscosity of Al-Ahdab crude oil (Iraq) at varied concentrations and temperatures. It is observed that the viscosity value drops from 38.49 to 7.8 cP. The highest degree of viscosity reduction is measured to be 56.91% at the maximum temperature of 50 °C and the greatest concentration of 0.4 wt% SM4. The Bingham model can be used to classify the behavior of the crude oil before the Nano-mixture is added. The liquid behavior grew closer to Newtonian behavior once the Nano-mixture was added. Along with a rise in plastic and effective viscosity values, the yield stress value decreases as the concentration of the Nano-mixture increases. The numerical data demonstrate that when the volume proportion of nanoparticles increases, the pressure distribution decreases. Furthermore, as the nanoparticle volume fraction increases, the drag decrease would also increase. SM4 obtains a maximum drag reduction of 53.17%. It is discovered that the sample SM4 has a maximum flow rate increase of 2.408%. Because they reduce the viscosity of crude oil, nanoparticles also reduce the friction factor ratio.

Thermal effectiveness enhancement in heat exchange tube using louver-punched V-baffles

An efficient technique for reducing frictional loss by punching holes on the surface of vortex generators has been extensively researched, particularly in the form of louver-punched holes. This article describes an experimental and numerical examination of thermal effectiveness in a heat exchange tube with louvered baffle vortex generators (LBVG). As air was directed to the LBVG-inserted tube with a consistent heat flux, the flow was measured in a turbulent regime. The LBVGs were set at 30° attack angle (α) and mounted at regular intervals on a two-sided flat tape with one relative baffle height (b/D = RB = 0.25) and pitch (P/D= RP = 1) at the first step. The baffles had five different louver angles (θ = 0 ˗ 90°) and three different relative louver sizes (LR= e/b = 0.4, 0.56, and 0.72). The Nusselt number ratio (NuR), friction factor ratio (fR), and thermal effectiveness factor (TEF) were utilized to quantify the performance of LBVGs. The findings showed that LBVGs had much lower fR values than solid baffles (without holes), whereas NuR values decreased slightly. When θ and LR were reduced, the fR and NuR for LBVGs increased until they resembled those for solid baffles. Through numerical simulations using the realizable k-ε turbulent model based on the finite volume method, the flow and temperature fields of various cases were generated; their results were verified via analysis of the fluid flow patterns. The computational findings revealed that the jet flow from the louver hole could boost heat transfer, and TEF of LBVGs varied depending on the placement of the hole. According to the computations, the optimal TEF at RP = 0.75, LR = 0.4, and θ1 = 20° is roughly 2.64, and the hole should be placed toward the baffle ends rather than in the middle.

Laminar Flow Heat Transfer Studies in a Twisted Square Duct for Constant Wall Temperature

Abstract: Three dimensional analysis of steady fully developed laminar flow inside twisted duct of square cross section flow area is studied for Reynolds number range of 100 – 2000 using a commercially available software. Twist ratio used are 2.5, 5, 10 and 20. Heat transfer results are generated for a uniform wall temperature case for Prandtl number range of 0.7 to 20. Maximum values for the product of friction factor and Reynolds number is observed for a twist ratio of 2.5 and Reynold number of 2000. Maximum Nusselt number is observed for the same values along with Prandtl number of 20. Correlations for friction factor and Nusselt number are developed involving swirl parameter. Local distribution of friction factor ratio and Nusselt number across a cross-section is presented. Heat transfer enhancement for Reynold number of 2000 and Prandtl number of 0.7 for twist ratio of 2.5, 5, 10, and 20 are 33 %, 18 %, 10 % and 7 % respectively. The results are significant because it will contribute to development of compact heat exchanger.

Numerical study of flow and heat transfer in the channel of panel-type radiator with semi-detached inclined trapezoidal wing vortex generators

Abstract To improve heat dissipation performance of panel-type radiator for transformer, this study investigated the flow and heat transfer characteristics of semi-detached inclined trapezoidal wing vortex generator (SDITW) in a closed channel on the air-side of the radiator. The SDITW was compared with the inclined delta wing (IDW) and inclined trapezoidal wing (ITW) channels. The effects of SDITW relative separation height (e 1/e 2), longitudinal pitch (p l), blockage ratio (e/(0.5H)), and inclination angle (α) were analyzed. First, compared with the IDW and ITW channels, the SDITW channel generates stable corner vortices and produces weaker transverse vortices and lower flow resistance due to the semi-detached structure of the wing. For Re = 5,125–15,375, the overall heat transfer performance (performance evaluation criteria; PEC) of the SDITW channel increases by 0.5–8.9 and 1.7–4.9% as compared with IDW and ITW channels, respectively. Furthermore, for the same e/(0.5H) and α, both the Nusselt number ratio and friction factor ratio of SDITW channel increase as e 1/e 2 and p l decrease. For p l = 70 mm, the SDITW channel exhibits a relatively better overall heat transfer performance. For the same e 1/e 2 and p l, the PEC of SDITW channel is maximum and the overall heat transfer performance is best when e/(0.5H) = 0.3 at Re = 10,250 and α = 30°–60°.

Turbulent Flow Heat Transfer and Thermal Stress Improvement of Gas Turbine Blade Trailing Edge Cooling with Diamond-Type TPMS Structure

Additive manufacturing allows the fabrication of relatively complex cooling structures, such as triply periodic minimal surface (TPMS), which offers high heat transfer per unit volume. This study shows the turbulent flow heat transfer and thermal stress of the Diamond-TPMS topology in the gas turbine blade trailing edge channel. The thermal-fluid-solid analysis of the Diamond-TPMS structure, made of directionally solidified GTD111, at the nearly realistic gas turbine condition is executed, and the results are compared with the conventional pin fin array at the Reynolds number of 30,000. Compared to the baseline pin fin structure, the Diamond-TPMS model distributes flow characteristics more uniformly throughout the channel. The overall heat transfer enhancement, friction factor ratio, and thermal performance are increased by 145.3%, 200.9%, and 32.5%, respectively. The temperature, displacement, and thermal stress in the Diamond-TPMS model are also distributed more evenly. The average temperature on the external surface in the Diamond-TPMS model is lower than the baseline pin fin array by 19.9%. The Diamond-TPMS network in the wedge-shaped cooling channel helps reduce the volume displacement due to the material thermal expansion by 29.3%. Moreover, the volume-averaged von Mises stress in the Diamond-TPMS structure is decreased by 28.8%.

Experimental investigation on the heat transfer augmentation and friction factor inside tube enhanced with deep dimples

This study aims to investigate experimentally heat transfer improvement in the longitudinal and circumferential direction and pressure drop of distilled water when flowing inside deep dimpled tubes. Also, the effect of three mechanisms that cause enhancement in the thermal performance of this new tube geometry is observed. Several tests were conducted on plain and deep dimpled tubes in a laminar regime with Reynolds numbers ranging from 500 to 2250 and constant heat flux using an experimental setup. The results indicate that implementing deep dimples on the tube improves the convective heat transfer coefficient up to 3.25 times better than a smooth tube; however, the friction factor ratio increases by just 1.69 in Re = 2250. The circumferential investigation illustrated the influence of exerting dimple on the flow field and revealed how generating vortexes enhanced heat transfer. Besides, results show that modifying tube geometry causes altering the zone of different flow regimes, which can be specified by Nusselt number and friction factor figures. Since the deep dimple tube has a low improvement penalty and appropriate thermal enhancement, increased the performance evaluation criteria (PEC) in the studying region up to 2.71. Also, it has been revealed that a deep dimpled tube cannot be applicable for Reynolds numbers below 1000. However, in higher velocities, replacement of them with simple tubes in conventional heat exchangers is highly suggested.

Heat transfer and pressure drop in a double pipe exchanger equipped with novel perforated magnetic turbulator (PMT): An experimental study

The utilization of a magnetic turbulator presents a novel method for enhancing heat transfer in heat exchangers. This research introduces the use of both simple and perforated magnetic turbulators for the first time, aiming to optimize the thermal enhancement factor. The perforated magnetic turbulator is a composite of a magnet and a perforated oscillator. By inducing an alternating magnetic field around the tube, the magnet and, thus, the oscillator are forced to oscillate and as a result act as a flow disruptor. The oscillator is designed with holes in four varying diameters 1, 2, 3, and 4 mm and five distinct pitches 4, 6, 12, 18, and 24 mm to examine the impact on pressure drop and heat transfer. Also, to ascertain the optimal case, the thermal performance factor has been used. Tests have been carried out in the hot fluid flow range from 0.23 to 1.38 l/min. According to the results, the highest amount of heat transfer has been observed in the presence of a simple magnetic turbulator. The heat transfer and friction factor ratio in the presence of best case were 1.67 to 5.7 and 1.04 to 1.34 times higher than those of the plain tube, respectively. Meanwhile, the maximum value of the thermal performance coefficient (5.35) was observed in the presence of a perforated turbulator with a hole diameter of 2 mm and a pitch of 6 mm.

In situ monitor of superhydrophobic surface degradation to predict its drag reduction in turbulent flow

In situ monitoring is the most insightful technique to examine superhydrophobic surface degradation as it provides real-time information on the liquid–solid interface in a continuous, noninvasive manner. Using reflecting-pixel intensity, we introduced a simple method to characterize in situ the air-plastron over a superhydrophobic surface in a turbulent channel flow. Prior to the turbulent experiments, a no-flow hydrostatic test was carried out to determine a critical absolute pressure under which the surfaces are able to maintain the air layer for a prolonged period of time. Pressure-drop and velocity measurements were conducted in a series of turbulent flow tests. Resulting from the coupling effects of normal and shear stresses over the plastron, the air layer was progressively lost with flow time which caused the drag ratio (i.e., the friction factor ratio between superhydrophobic and smooth surfaces) to increase. Meanwhile, the average pixel intensity also increased with time and exhibited a consistent trend with the drag ratio evolution. At a fixed near-wall y/h location (within the viscous sublayer), the velocity increased with time since the shear stress increased. However, a velocity measurement at the center of the channel exhibited a decrease, consummate with an overall downward shift of the velocity profile. Both pressure-drop and velocity results were observed to be correlated with the average pixel intensities of the images captured over the surfaces, and therefore, this is a suitable proxy measure of the plastron. This technique is confirmed to be valid for monitoring the air layer and, hence, predicting the consequent loss of drag reduction.

Enhancement of concentrator photovoltaic system through convergent-divergent microchannels

Concentrated photovoltaic technology is one of the evolving technologies that converts solar to electrical energy with a high efficiency percentage. Increasing concentrated photovoltaic cell’s temperature significantly reduces the performance of the system. Thus, in order to properly functioning of system, thermal management will be a rudimentary issue. This article focuses on the cooling performance of high-concentration photovoltaic system using microchannels attached to the bottom plate of solar cell. To this end, the effect of convergent-divergent microchannels at three amplitudes and three wave lengths at different Reynolds numbers is investigated, and results are compared with simple microchannels. The output results are presented via temperature, pressure, velocity contours as well as Nusselt number, friction factor, and effectiveness ratio. Compared to the simple microchannels, convergent-divergent microchannel lowers the wall temperature about 0.4–2 Kelvin by making velocity gradients along the channel. This leads to boosting the heat transfer capability up to 3 times in the best case and up to 1.5 times in the worst case. Increasing the wave length enhances the performance of system, while the opposite trend has been seen for greater wave amplitudes. For case A with lowest wave length, Nu increases 1.92 and 2.4 times at lowest and highest Re numbers while friction factor increases due to the higher contact surface area along the fluid stream. Moreover, increasing the wave amplitude leads to higher Nu number, higher friction factor and consequently lower effectiveness ratio. Finally, effectiveness ratio as a criterion to choose best case has shown that a higher wave length alongside with lower wave amplitude offers better heat transfer enhancement and lower pressure drop compared to simple channel. Case C-1 as a best case offers 0.8K lower wall temperature, 1.6 times better Nu number with only 1.3-time higher friction factor.

Effects of liquid viscosity on interfacial and wall friction factors of swirling annular flows in a vertical pipe

Experiments of gas–liquid swirling annular flows in a vertical pipe of diameter D = 30 mm were carried out to obtain interfacial and wall friction factors, fi and fw, based on a three-fluid model. The liquid phase was two kinds of glycerol-water solutions with the kinematic viscosity νL of two or four times larger than that of water. Experimental data were obtained in two axial sections with short and long axial distances from the swirler, which corresponds to swirling and non-swirling annular flow regimes, respectively. Both the azimuthal and axial components of interfacial velocity decayed with increasing νL. The interfacial swirl number si, which is the ratio of the azimuthal component to the axial component of interfacial velocity, was not affected by νL for z*(=z/D) ≤ 3.8, where z is the axial coordinate in the test section. The mean interfacial swirl number si∗, which is the arithmetic mean of si in z* ≤ 3.8, was expressed in terms of dimensionless numbers based on the gas and liquid volumetric fluxes without νL. The interfacial friction factor decreased with increasing νL in swirling flows. The functional form of the Colebrook equation had a prospect of correlating fi in both swirling and non-swirling annular flows. The wall friction factor ratio of fw in the swirling annular flow regime to that in the non-swirling annular flow regime decreased as νL increased for si∗ > 0.33. The wall friction factor ratio was well correlated in terms of si∗ and νL.

Flow and heat transfer profiles in a heat exchanger tube equipped with X-V baffles (XVB): A numerical analysis

Simulations of laminar air flow and thermal configurations through a heat exchanger tube (HXT) equipped with X-V baffles (XVB) are performed. The Reynolds number is between 100 – 2000. The current numerical problem is solved by a numerical method (the finite volume analysis). Effects of the XVB sizes, the XVB distances and the flow directions on the flow and thermal profiles are discussed. The XVB size (b/D) is set between 0.05 – 0.20 and the XVB distance (P/D) is set to 1, 1.5 and 2. X-axis is the flow axis. The positive and negative x-axis are investigated. Two baffle configurations (Type I and II) are compared. The streamlines, vortex flows, temperature profiles and Nusselt number profiles in the HXT installed with these two XVB configurations (Type I and II) are described. The variations for the friction factor ratio (f/f0), Nusselt number ratio (Nu/Nu0) and thermal enhancement factor (TEF) within the HXT equipped with the XVB are presented. In addition, the TEF of 4.07 is found to be the best value for the present investigation.

Heat transfer and pressure drop performance improvement using curved circular spines for flow through circular pipe

The heat transfer augmentation method of inserting curved circular spines increases the dwelling time of the fluid within the system by inducing swirling motion within the fluid. It is a promising passive heat transfer augmentation technique for improving average and local heat transfer coefficients in the flow across uniform & non-uniform cross-sectioned channels at a high Reynolds number. The spines of circular cross-sections can considerably enhance flow passages employed for various purposes. The heat transfer performance study of tubes mounted with curved circular spines is presented for turbulent fluid flow conditions for Reynolds numbers ranging from 10,000 to 50,000. The curved spines are placed in the inline arrangement at the inner wall of the circular tube, disturb the flow, and break the dominant thermal resistance close to the wall, improving the heat transfer to the flowing fluid. The heat transfer enhancement at the cost of a rise in the pressure drop is also studied. The overall performance factor, R3, at the same pumping power, is presented to account for cost-effectiveness. The numerical results show that the enhancement ratio, Nue/Nuo, and friction factor ratio, fe/fo, using curved spines are 1.5 to 3.6, and 2.0 to 20.0, respectively, compared with the smooth tube at similar flow conditions. The Nusselt number ratio, Nue/Nup, at the same pumping power of the equivalent smooth tube, is 0.8 to 1.6.