• Energy Research

Abstract This paper investigates the flow structure and thermal performance characteristics of the fluid flow through a heat exchanger tube fitted with perforated hollow cylinders (PHCs) under turbulent flow regime. The effects of the perforated index (0.08 ( 6000 Re 16000 ) on the thermal performance and heat transfer enhancement are investigated. The vortex flow generated near the holes leads to better fluid mixing between the tube wall and the core regions and this recirculating flow enhances the heat transfer rate in comparison with a plain tube. The numerical results indicate that the flow resistance can be reduced up to 86.2% with increasing the perforated index from 0.08 to 0.24. The maximum thermal performance value of 1.456 could be achieved for the case of d/D = 0.74 and PI = 24% at Re = 6000. The results show that the fluid mixing between the core region and the tube walls for the case of 0.7 is higher than the other cases. Therefore, the heat transfer rate is expected to be better for the case of PI = 0.08 and d/D = 0.7 due to the thermal boundary layer destruction caused by the PHCs.

Abstract This paper investigates the flow structure and thermal performance characteristics of the fluid flow through a heat exchanger tube fitted with perforated hollow cylinders (PHCs) under turbulent flow regime. The effects of the perforated index (0.08 ( 6000 Re 16000 ) on the thermal performance and heat transfer enhancement are investigated. The vortex flow generated near the holes leads to better fluid mixing between the tube wall and the core regions and this recirculating flow enhances the heat transfer rate in comparison with a plain tube. The numerical results indicate that the flow resistance can be reduced up to 86.2% with increasing the perforated index from 0.08 to 0.24. The maximum thermal performance value of 1.456 could be achieved for the case of d/D = 0.74 and PI = 24% at Re = 6000. The results show that the fluid mixing between the core region and the tube walls for the case of 0.7 is higher than the other cases. Therefore, the heat transfer rate is expected to be better for the case of PI = 0.08 and d/D = 0.7 due to the thermal boundary layer destruction caused by the PHCs.

Vortex generator is a device that shows promise in generating streamwise vortices that can be utilized for boosting the rate of heat transmission inside a cooling/heating system with a relatively smaller penalty in terms of friction loss. The primary goal of the current research is to maximize the comparative Nusselt number ratio (Nu/Nu0) to be as large as possible to lower the size of the system while keeping thermal performance as high as feasible to save more energy. Thus, in the current study, the impacts of inserting the flapped V-baffle vortex generator (FBVG) on the thermal effectiveness improvement of a round tube were experimentally investigated. At a fixed attack angle (α = 60°) and baffle blockage ratio (BR = b/D = 0.3), the geometrical behaviors of FBVGs placed periodically along two edges of a straight tape were six different flap angles (θ = 0°, 25°, 35°, 45°, 65° and 90°) and three ratios of baffle pitches (P/D = PR = 2.0, 1.5, and 1.0). The current V-baffles, which were positioned on both tape edges, were designed to reduce friction loss caused by interrupting the central core flow when placed on both tape sides. The measurement results focused on the friction loss and thermal behaviors, including exergy and entropy analyses for Reynolds number from 4750 to 29,270. In the findings, the Nusselt number and friction factor of FBVG at θ = 0° and PR = 1 are, respectively, up to 5.6 and 35.24 times larger than those of the smooth tube. The entropy generation (S˙gen′) seems to decline as θ and PR increase, with the smallest S˙gen′ found at θ = 0° and PR = 1 for lower Re. The FBVG has the greatest exergy efficiency (ηEx) at θ = 0° and PR = 1. To find the true benefits of FBVG, its thermal performance is estimated and seen to reach a maximum at about 2.44 with NuR = 4.65 at θ = 45° and PR = 1. The optimal scenario at θ = 25° and PR = 1 was preferred, however, since it yielded the largest NuR = 5.42 at TEF = 2.39.

Vortex generator is a device that shows promise in generating streamwise vortices that can be utilized for boosting the rate of heat transmission inside a cooling/heating system with a relatively smaller penalty in terms of friction loss. The primary goal of the current research is to maximize the comparative Nusselt number ratio (Nu/Nu0) to be as large as possible to lower the size of the system while keeping thermal performance as high as feasible to save more energy. Thus, in the current study, the impacts of inserting the flapped V-baffle vortex generator (FBVG) on the thermal effectiveness improvement of a round tube were experimentally investigated. At a fixed attack angle (α = 60°) and baffle blockage ratio (BR = b/D = 0.3), the geometrical behaviors of FBVGs placed periodically along two edges of a straight tape were six different flap angles (θ = 0°, 25°, 35°, 45°, 65° and 90°) and three ratios of baffle pitches (P/D = PR = 2.0, 1.5, and 1.0). The current V-baffles, which were positioned on both tape edges, were designed to reduce friction loss caused by interrupting the central core flow when placed on both tape sides. The measurement results focused on the friction loss and thermal behaviors, including exergy and entropy analyses for Reynolds number from 4750 to 29,270. In the findings, the Nusselt number and friction factor of FBVG at θ = 0° and PR = 1 are, respectively, up to 5.6 and 35.24 times larger than those of the smooth tube. The entropy generation (S˙gen′) seems to decline as θ and PR increase, with the smallest S˙gen′ found at θ = 0° and PR = 1 for lower Re. The FBVG has the greatest exergy efficiency (ηEx) at θ = 0° and PR = 1. To find the true benefits of FBVG, its thermal performance is estimated and seen to reach a maximum at about 2.44 with NuR = 4.65 at θ = 45° and PR = 1. The optimal scenario at θ = 25° and PR = 1 was preferred, however, since it yielded the largest NuR = 5.42 at TEF = 2.39.

In the present study, numerical simulations have been carried out on thermal characteristics and second-law analysis of turbulent Cu–H2O nanofluid flow with the nanoparticle volume fraction of 0<ϕ<1.5% inside heat exchangers fitted by transverse-cut twisted tapes (TCTTs) with alternate axis. The transverse-cut ratios are in the range of 0.7 < b/c < 0.9 and 2 < s/c < 2.5, and the Reynolds number is varied between 5000 and 15,000. The impacts of the design variables on the turbulent kinetic energy, temperature distribution, thermal and frictional entropy generations and Bejan number have been evaluated. The simulations show that the TCTTs with b/c = 0.7 generate higher turbulent kinetic energy compared to the b/c = 0.9 due to higher swirl generation and flow disturbance. The additional recirculating flow produced near the alternate edges is another main physical factor for heat transfer augmentation. It is found that raising the nanoparticles volume concentration reduces the thermal entropy generation which is attributed to the thermal conductivity enhancement of nanofluids. Besides, raising the nanoparticles volume concentration from 0 to 1.5% reduces the Ng,thermal by 23%.

In the present study, numerical simulations have been carried out on thermal characteristics and second-law analysis of turbulent Cu–H2O nanofluid flow with the nanoparticle volume fraction of 0<ϕ<1.5% inside heat exchangers fitted by transverse-cut twisted tapes (TCTTs) with alternate axis. The transverse-cut ratios are in the range of 0.7 < b/c < 0.9 and 2 < s/c < 2.5, and the Reynolds number is varied between 5000 and 15,000. The impacts of the design variables on the turbulent kinetic energy, temperature distribution, thermal and frictional entropy generations and Bejan number have been evaluated. The simulations show that the TCTTs with b/c = 0.7 generate higher turbulent kinetic energy compared to the b/c = 0.9 due to higher swirl generation and flow disturbance. The additional recirculating flow produced near the alternate edges is another main physical factor for heat transfer augmentation. It is found that raising the nanoparticles volume concentration reduces the thermal entropy generation which is attributed to the thermal conductivity enhancement of nanofluids. Besides, raising the nanoparticles volume concentration from 0 to 1.5% reduces the Ng,thermal by 23%.

In the present work, direct numerical simulation is employed to investigate the unsteady flow characteristics and energy performance of low-pressure turbines (LPT) by considering the blades aeroelastic vibrations and inflow wakes. The effects of inflow disturbance (0 < φ < 0.91) and reduced blade vibration (0 < f < 250 Hz) on the turbulent flow behavior of LPTs are investigated for the first time. The transient governing equations on the vibrating blades are modelled by the high-order spectral/hp element method. The results revealed that by increasing the inflow disturbances, the separated bubbles tend to shrink, which has a noticeable influence on the pressure in the downstream region. The maximum wake loss value is reduced by 16.4% by increasing the φ from 0.31 to 0.91. The flow separation is majorly affected by inflow wakes and blade vibrations. The results revealed that the maximum pressure coefficient in the separated flow region of the vibrating blade has been increased by 108% by raising φ from 0 to 0.91. The blade vibration further intensifies the vortex generation process, adding more energy to the flow and the downstream vortex shedding. The vortex generation and shedding are intensified on the vibrating blade compared to the non-vibrating one that is subject to inflow wakes. The results and findings from this paper are also useful for the design and modeling of turbine blades that are prone to aeroelastic instabilities, such as large offshore wind turbine blades.

In the present work, direct numerical simulation is employed to investigate the unsteady flow characteristics and energy performance of low-pressure turbines (LPT) by considering the blades aeroelastic vibrations and inflow wakes. The effects of inflow disturbance (0 < φ < 0.91) and reduced blade vibration (0 < f < 250 Hz) on the turbulent flow behavior of LPTs are investigated for the first time. The transient governing equations on the vibrating blades are modelled by the high-order spectral/hp element method. The results revealed that by increasing the inflow disturbances, the separated bubbles tend to shrink, which has a noticeable influence on the pressure in the downstream region. The maximum wake loss value is reduced by 16.4% by increasing the φ from 0.31 to 0.91. The flow separation is majorly affected by inflow wakes and blade vibrations. The results revealed that the maximum pressure coefficient in the separated flow region of the vibrating blade has been increased by 108% by raising φ from 0 to 0.91. The blade vibration further intensifies the vortex generation process, adding more energy to the flow and the downstream vortex shedding. The vortex generation and shedding are intensified on the vibrating blade compared to the non-vibrating one that is subject to inflow wakes. The results and findings from this paper are also useful for the design and modeling of turbine blades that are prone to aeroelastic instabilities, such as large offshore wind turbine blades.