Inlet Reynolds Number Research Articles

Analyzing tube arrangements of a finned-tube heat exchanger to optimize overall efficiency using artificial neural network and response surface methodology

Temperature is a critical factor in numerous equipment, industries, and daily life applications. Heat exchangers are essential devices that help regulate and optimize this factor. The finned-tube heat exchanger (FTHE) is widely favored due to its high efficiency in facilitating heat transfer between liquids and gases. Improving the performance of FTHE can significantly provide thermal requirements in industrial and engineering processes and reduce energy consumption. The present research includes a numerical study of an FTHE and the optimization of the performances based on the input variables by response surface methodology (RSM) and artificial neural network (ANN) methods. The tubes' transverse and longitudinal pitches and inlet Reynolds number were selected as input variables. Also, the examined responses were the Colburn and friction factors. Changing tubes' pitches makes it possible to significantly affect the thermo-hydraulic performance without incurring additional costs and processes. The acquired results showed that the responses predicted by the models are very close to the numerical results, which indicates the high accuracy and validity of these models. According to the results, the optimum heat exchanger's efficiency index was obtained at Pt=26.128mm, Pl=28mm, and Re=800. It was also observed that the overall performance of the optimal design is 185% higher than the weakest FTHE.

Numerical investigation on the heat transfer and pressure loss characteristics of precooler under negative pressure

Precoolers have garnered considerable attention owing to their applications in aerospace and industrial fields. Nonetheless, there is a paucity of research concerning the performance of precoolers under conditions of extremely low pressure. This study conducts a numerical analysis of the heat transfer and pressure loss characteristics of a plate-fin precooler operating under negative pressure, considering the coupled heat transfer interactions among the hot fluid, solid wall, and cooling fluid. The comprehensive performance and flow field on each side of the precooler are investigated. Moreover, the impact of negative pressure on the heat transfer and pressure recovery performance of the hot fluid in the rectangle channel is evaluated. The results show that, even under operating conditions with pressures below 30 kPa and inlet temperatures exceeding 1100 K, the precooler attains a pressure recovery coefficient greater than 95.6 % and achieves a temperature drop exceeding 43.2 % for the hot fluid as the mass flow rate ranges from 0.205 kg/s to 0.564 kg/s. Additionally, the heat transfer coefficient of the hot fluid is less than 1/40 that of the cooling fluid, resulting in increased thermal resistance between the hot fluid and the wall. Consequently, a distinct thermal boundary layer develops between the hot fluid and the wall, which is not present between the cooling fluid and the wall. Furthermore, the performance of heat transfer and pressure recovery for the hot fluid exhibits considerable variability with changes in static pressure at a constant inlet velocity. However, a turning point is observed when these parameters are evaluated in terms of the inlet Reynolds number. At low Reynolds numbers, both the heat transfer coefficient and the friction factor remain relatively stable, notwithstanding fluctuations in static pressure. Conversely, at higher Reynolds numbers, a reduction in static pressure negatively influences these factors. For instance, when the static pressure is 10 kPa, the associated Reynolds number is 500. Nevertheless, the Reynolds number at which this turning point occurs escalates with an increase in static pressure.

Comparison of theoretical methods for describing the heat transfer in vertical tube condensers under conditions corresponding to the condensation of flue gas from a biomass boiler

Abstract A theoretical and experimental study was conducted on water vapor condensation in a vertical tube condenser under conditions corresponding to the condensation of flue gas from biomass boiler where flue gas is considered as a mixture of water vapor and a high content of noncondensable gas (NCG). Four fundamental theoretical methods were identified to determine the heat transfer coefficient and condenser heat output - Empirical correlations, Heat and mass transfer analogy, Diffusion layer theory, Boundary layer theory. These methods were compared with respect to usability in the design of flue gas condensers in energy systems and experimentally verified. Experiments were carried out in a 1.5 m long vertical double-pipe condenser with a condensing gas flowing downward in an inner tube with a diameter of 25 mm. The mass concentration of NCG in a mixture with water vapor ranged from 11 to 86 vol.% and the inlet Reynolds number of the condensing gas ranged from 1936 to 14408, which corresponds with the conditions of condensing flue gas from biomass boilers. The boundary layer theory is very complex and impractical for the calculation of heat exchangers. Empirical correlations have a wide dispersion of the result, because they take into account only the basic parameters of the process. However, Heat and mass transfer analogy and Diffusion layer theory seem to be the most suitable for flue gas condensers since they capture the physical essence of the phenomena.

Thermal Frictional Analysis of a Novel Vibrating Cylindrical Turbulator in Double Tube Heat Exchangers for Engine Cooling

This study focuses on enhancing the thermal efficiency of a double-tube heat exchanger used in cooling systems for large-scale internal combustion engines by incorporating a novel vibrating cylindrical turbulator. The experimental investigation examined inlet Reynolds numbers ranging from 1052 to 8430 and evaluated various turbulator configurations, including fixed close-ended, fixed open-ended, vibrating close-ended, and vibrating open-ended cylindrical turbulators. Additionally, the impact of turbulator length, varying from 10 to 30 cm, on thermal-frictional characteristics was analyzed. The optimal configuration was determined using the thermal enhancement factor (TEF). Results showed that close-ended cylindrical turbulators significantly outperformed the open-ended configurations in terms of heat transfer and TEF. Although the vibrating turbulator produced a higher pressure drop compared to the fixed turbulator, it achieved a much higher heat transfer rate and TEF, making it a viable option for heat exchangers. The study also found that increasing the turbulator length leads to increased heat transfer, TEF, and pressure drop. The maximum TEF was recorded with the vibrating close-ended cylindrical turbulator, where heat transfer and pressure drop were up to 385% and 95% greater than those of a plain tube heat exchanger, respectively, resulting in a perfect TEF value of 3.45.

Numerical study on fluid flow pattern regulation and heat transfer characteristics in oval-pit spiral tube

Spiral tube heat exchangers are widely applied in daily industrial production. In order to improve the comprehensive heat transfer performance of the spiral heat exchanger to meet industrial production and policy demands, improvements to spiral heat exchangers are necessary. In this study, a new type of oval-pit spiral tube is proposed. Ansys Fluent software was used to develop the numerical models and simulate the heat exchange conditions in smooth spiral tube and oval-pit spiral tube. The development and evolution of secondary flow and heat transfer characteristics in spiral tube were analyzed by theoretical analysis, and the heat transfer mechanism was explored. The dimensionless temperature parameter Θ and dimensionless acceleration number Ω were defined to explore the influence of secondary flow on heat transfer characteristics, which provided a new approach for improving spiral heat exchangers. The results show that the secondary flow in the smooth spiral tube has a symmetric double-vortex structure, and it has an asymmetric double-vortex structure in the oval-pit spiral tube. Compared with smooth spiral tube, the local dimensionless temperature parameter Θ in oval-pit spiral tube fluctuates more sharply, the heat transfer non-uniformity is greater, and the local Nu number is higher. Additionally, the fluid in the oval-pit tube is fully developed at the smaller axial angle, and the flow and heat transfer stability are better. In the working range of inlet Reynolds number of 1000∼5000, compared with smooth spiral tube, the heat transfer coefficient of oval-pit spiral tube increases by 11.58∼44.73 %, the pressure drop increases by 33.56∼119.27 %, and the comprehensive heat transfer performance increases by 1.23∼14.65 %.

Laminar swirling slender pipe flows

Swirling flows are extensively used to enhance mixing in chemical and physical processes. Although many fundamental studies involve decaying laminar swirls in circular ducts with constant area—for which several theoretical descriptions exist—converging and diverging pipe sections are often employed, demanding the extension of the analysis to swirling laminar flows in pipes with varying areas. This work analytically investigates axial swirl decay in laminar axisymmetric flows in converging and diverging pipes at moderate Reynolds numbers. The theoretical model extends the classical theory for laminar low swirling flows in straight pipes with impermeable walls to pipes with slowly varying areas. Inlet Reynolds number and converging/diverging angle effects are analyzed for incompressible axisymmetric laminar flows in slender pipes (lengths much greater than the radius). Results show that diverging pipes enhance the swirl decay rate, whereas converging systems have a weakening effect for flows with the same inlet Reynolds number. Conversely, increasing the inlet angle in converging flows decreases the decay rate, whereas larger diverging pipe inlet angles promote swirl decay. A simplified eigenvalue analysis shows that the relationship between the swirl decay lengths in diverging (ℓd,div), straight (ℓd,st), and converging (ℓd,conv) pipes is given by ℓd,div<ℓd,st<ℓd,conv for flows with similar inlet Reynolds numbers. Comparisons with numerical simulations show that the model predictions are almost exact for nondimensional pipe lengths up to 5×10−2 times the inlet Reynolds number. Approximating the axial velocity profile through a fully developed function representative of laminar flows with heat transfer through the walls shows that the narrower axial velocity profile in diverging flows increases viscous effects, enhancing the swirl decay rate. On the contrary, the flatter axial velocity profile in converging pipes further extends the decay length.

Investigation on the effect of vertical baffle plate on droplet size distribution and mixing efficiency in Taylor Reactor

The Taylor reactor is a dynamic mixer based on the principles of Taylor vortex flow. However, traditional Taylor reactors feature smooth inner and outer cylinder surfaces, presenting challenges in achieving uniform mixing and efficient mass transfer. These limitations do not meet the requirements of modern chemical processes for highly efficient mixing equipment. This article introduces a novel Taylor reactor equipped with vertical baffle plates, designed to overcome these challenges. A comprehensive methodology combining CFD-PBM and experimental techniques was employed to investigate both droplet size distribution and mixing performance in the annular gap. Compared to traditional designs, the new device demonstrates significantly lower COV at 0.018 and Sauter mean diameter at 38.3 μm. Optimal mixing efficiency is achieved at rotating Reynolds number (Re) of 1.17 × 105 and inlet Reynolds number (Rei) of 2.65 × 103. Numerical simulation results suggest that vertical baffles effectively enhance liquid-liquid interactions including collision, interface contact, and droplet residence time within the annular gap. Moreover, increased turbulent stress in the flow field contributes to greater vorticity, turbulence intensity, and turbulent kinetic energy resulting in smaller droplet sizes and improved mixing efficiency. These findings provide a theoretical foundation for hydraulic structural optimization design as well as widespread application of Taylor reactors.

Numerical investigation of heat transfer performance and flows characteristics in turbine blade internal cooling using Pin-Fin arrays coupled with discontinuous ribbed endwall

In the scientific domain of cooling techniques research utilizing pin-fins, a number of studies have concentrated on the configurations of pin-fins. However, recent investigations have shifted their focus towards the optimization of endwalls. The objective of this optimization is to better control and maintain vortices, which in turn leads to an increase in heat transfer near the endwall. Further research has taken this a step further by optimizing the lower and upper walls of the unadorned heated channel, resulting in a significant boost in heat transfer efficiency. These studies have also led to the discovery of new heat transfer properties and alterations in the flow structure. This research unveils the findings from an examination into the flow field and heat transfer properties of pin–fin arrays featuring a ribbed endwall, specifically referred to as a Discontinuous Ribbed Endwall (DRE). The investigations are executed using Reynolds-Averaged Navier-Stokes (RANS) equations with the k-ω turbulence model at the mesh parameter of the 20.4 million mesh model is used throughout the work. The study involves a numerical investigation of the heat transfer and pressure drop characteristics of the channel, comparing them with the case of flat endwall across a range of inlet Reynolds numbers, spanning from 7400 to 36000. The entire section of the heated channel is divided into 7 upper surfaces, 7 lower surfaces, and cylindrical surfaces to comprehensively investigate the heat transfer characteristics of both pin-fins and endwalls. The results reveal that the heat transfer regions at the pin-fins and endwalls are expanded and significantly enhanced, particularly causing notable alterations in the flow structure and velocity field. However, the coefficient of friction also increases. The Area-averaged Nusselt Number (Nu¯) and the Heat Transfer Efficiency Index (HTEI) improves from 42.99% to 88.65% and from 36.81% to 73.66% for the DRE compared to the case of flat endwall across the entire range of Reynolds numbers. With Reynolds number 21500, when varying the height parameter of the DRE, the maximum value of the HTEI improves by 84.13%. Other geometric parameters of the DRE, including forward width, behind width, left width, streamwise position, and left position, also undergo changes, with the maximum values of HTEI improving by 73.76%, 75.35%, 80.60%, 75.41% and 74.16%, respectively.

Experimental investigation of humidified air condensation in different rows of serpantine heat exchanger – Cooling water flow rate effect

Condensing economizers are used in the industry as they enable the acquisition of additional heat during vapor condensation. In condensing heat exchangers, water is the usual cooling agent due to its high specific heat capacity and thus efficient heat removal. Therefore, experiments of hot humid air condensation in a serpentine economizer being cooled by water were performed to reveal the effect the cooling water flow rate has on humidified air condensation in separate rows of the serpentine type economizer (with vertical tubes). The results have shown that the cooling ratio (mass flow ratio between the coolant and the humid air) had a minor effect on the distribution of the humidified air temperature along test section for inlet Reynolds number of 3000–10000. The effects on the condensation flux distribution in different rows were much stronger. The most optimal cooling ratio was determined to be 3. The results have shown that the biggest condensation efficiency is up to 35%. It was revealed that in some cases the convection was prevailing in the first rows of the economizer and this resulted in a decreased efficiency/performance of the exchanger.The obtained results from the practical point of view will provide an extended basis for the optimisation of the design of economizers for waste heat recovery in the cases of different cooling water flow rates. It could also be applied to validate computational models developed for condensation heat transfer and condensate flux numerical modelling along economizer.

An experimental study of thermoacoustic instabilities in a RP-3 fueled multi-swirl stabilized lean direct injection combustor

An investigation of self-excited combustion instabilities in a RP-3 fueled, multi-swirl stabilized, lean direct injection (LDI) combustor was performed and investigated with different operating conditions (inlet Reynolds number and fuel–air ratio (FAR)). Pressure fluctuation and heat release data were simultaneously collected using a 3-channel synchronous acquisition system. The results show that as the FAR increases, combustion transitions from a low oscillation mode to a high oscillation mode. During the modal transition, the oscillating flame structure, formed by vortex shedding in the shear layer and the recirculation zone, is then transformed into contraction and expansion movements from the front of the central recirculation zone towards the downstream and radial directions of the combustor. When combustion is in a low oscillation mode, the driving capability of the flame is relatively low. Upon entering a high oscillation mode, the overall driving energy of the flame can maintain combustion in a limit cycle oscillation state.

A coupled modeling framework for investigating flow maldistribution in cross-flow-corrugated plate heat exchangers

Accurate prediction of the impact of flow maldistribution on the thermo-hydraulic behavior of plate heat exchangers is important for thermal design and optimization in a number of practical applications. In this paper, a coupled modeling framework, consisting of a heat transfer model and a hydraulic model, is proposed for quantitatively evaluating the impact of maldistribution on the thermo-hydraulic performance of a cross-flow-corrugated plate heat exchanger. The numerical results of the proposed approach are validated by comparison with experimental data. Using the coupled model, the flow distribution nonuniformity, heat exchanger effectiveness deteriorations, and pressure drop deviations are analyzed. Furthermore, the combined effects of side stream maldistribution are investigated for different inlet Reynolds numbers and inlet temperatures. The results demonstrate that when the inlet gas Reynolds number is in the range of 1100–2700, nonuniform distribution of the gas side will cause air maldistribution, and a 10%–30% effectiveness deterioration rate. It is found that air maldistribution will not result in gas maldistribution when the inlet air Reynolds number is in the range of 6000–18 000, but increasing air nonuniformity. Will lead to increasing deterioration in heat exchanger effectiveness. In the presence of air and gas flow maldistribution, the pressure drops on the gas and air sides increase by 5% and less than 12.4%, respectively. This work provides a computational framework for optimization of cross-flow-corrugated plate heat exchangers to achieve desired thermo-hydraulic performance.

Experimental study on the influence of the Buoyancy number on heat transfer in a pin-fins channel under rotational conditions

The trailing edge of turbine blades poses a significant challenge for research in internal cooling technology. In previously published studies on heat transfer coefficient (HTC), the limitations of experimental testing techniques have hindered the ability to scale up the rotational radius ratio in a similar manner, resulting in significantly lower Buoyancy number (Buo) values tested compared to the actual operating conditions of turbine blades, leading to biased HTC results.This article delves into experimental studies to explore the HTC of a pin-fin channel. The experimental testing encompassed a broad spectrum of conditions, including varying inlet Reynolds numbers ranging from 5000 to 70,000, rotation numbers (Ro) from 0 to 1.0, temperature ratio from 0.04 to 0.22, rotational radius ratio (r/D) from 20 to 52.33, and Buo from 0 to 2.55. For the same Buo, there is a significant difference in HTC, with the high Ro and low TR scenario demonstrating a distinct advantage in HTC compared to the low Ro and high TR condition. Furthermore, heat transfer enhancement was observed at approximately 30 % of the dimensionless positions. Therefore, to achieve more accurate HTC results, it is necessary to precisely match the values of Re, Ro, Buo, TR, and r/D.

Influence of PCHE size and types on thermodynamic and economic performance of supercritical carbon dioxide Brayton cycle for small modular reactors and its optimization

The supercritical CO2 Brayton cycle is a potential technology in small modular reactors (SMRs), and the overall system efficiency can be significantly improved by optimizing the PCHE design parameters. This study develops a thermodynamic-economic analysis model for a supercritical CO2 simple Brayton cycle cooled SMR, with consideration of the detailed parameters of PCHEs. The impact of the length, channel number and channel type for the recuperator and the precooler on system thermal efficiency and levelized cost of electricity (LCOE) are analyzed. A dual-objective optimization study is conducted at the system level to identify the optimal design of size parameters and types for the heat exchangers. The results indicate that the influence of PCHE size parameters variations on system performance varies across different channel types. Additionally, the selection of the recommended recuperator type is influenced by the recuperator inlet Reynolds number. At the optimum design, the recommended channel type for recuperator and precooler are both S-shaped, resulting in a system efficiency of 39.12% and LCOE of 0.0456 $/kWe. The findings are valuable for enhancing energy utilization and reducing the power generation cost of SMRs.

Flow regime prediction of a self-oscillatory flow induced by a vertical jet in a heated cavity: Computational and analytical approach

Interaction effects between the inlet Reynolds number (250≤Rei≤3000) and the heat flux of the impingement wall (0.5≤q″≤20kW/m2) was investigated in a vertical self-excited oscillating jet confined by a heated cavity. The results revealed that some high amounts of q″ stop oscillating flow. Thus, this study developed a new prediction method based on an analytical approach to identify the threshold of flow regime transition from oscillatory to non-oscillatory based on Rei. To this aim, a new dimensionless number named Non-dimensional Effective Number (NEN) was introduced. The proposed NEN index refers to the ratio of the thermal power applied on the impingement wall to the effective momentum power of the incoming jet. The performance of the proposed index was examined and compared in vertical upward and downward-facing chambers for different heat flux values. Investigations indicated that the proposed analytical method and the new NEN index have a much lower computational cost, despite the very high accuracy (about 95 %) compared to the numerical results extracted from the OpenFOAM code. The results also demonstrated that vertical downward self-excited jets maintain oscillatory behavior regardless of the amount of q″. In this case, the frequency of oscillations varied linearly with Rei for Rei >2000.

Investigating the impact of cooling configurations on heat transfer coefficient and friction coefficient under rotating state

A comprehensive study was conducted to compare the heat transfer coefficients (HTC) of four different pin–fin structures in a rotating state. The HTC were evaluated within specific conditions: the inlet Reynolds number (Re) varied from 10,000 to 70,000, the rotation number (Ro) ranged from 0 to 1.0, the temperature ratio (TR) spanned 0.04 to 0.22, and the Buoyancy number (Buo) ranged 0 to 2.55. The diamond pin-fins channel exhibited optimal HTC performance, whereas the elliptical pin-fins channel exhibited the least HTC. Remarkably, the droplet pin-fins channel demonstrated enhanced cooling effects near the channel entrance compared to the circular pin-fins channel, but in the latter part of the channel, the circular pin-fins channel exhibited higher HTC. Overall, the circular and droplet pin-fins channels had lower HTC than the diamond pin-fins channel but surpassed the elliptical pin-fins channel. The incorporation of pin-fins increased the interaction area between the coolant and the surface, leading to enhanced turbulence and coolant blending, thereby improving HTC. The pin-fins structure design was instrumental in boosting HTC. Research findings indicated a gradual enhancement in heat transfer effectiveness as the pin-fins structure transitioned from elliptical to diamond. Moreover, the study analyzed the influence of TR and Re on HTC, unveiling the complex interplay among pin-fins structures, fluid dynamics, and heat transfer efficiency.

Thermal enhancement of a heat exchanger of engine via U-cut and V-cut zigzag strip turbulator combined with bubble injection method

This experimental study proposes a new approach to enhance heat transfer performance inside heat exchangers of engine by tandem utilization of bubble injection and a punched zigzag strip turbulator. The side edges of the zigzag turbulator underwent two distinct geometry modifications, involving V and U shape cuts. These cuts were made at varying zigzag angles, ranging from 60° to 150° and four diffirent surface area. The potential improvement in thermal efficiency of the heat exchanger was evaluated at different inlet Reynolds number ranging from 5390 to 10,860. Based on survey results, it has been demonstrated that at a Reynolds number of 5390, the Nusselt number and friction factor increase by 48% and 154% for U-Cut with a bending angle of 150°, and by 61% and 170% for V-Cut, respectively. Hence, V-cut boasts a TEF of 1.15, representing a 6.5% improvement over U-cut. Additionally, to assess further enhancement, air bubbles with flow rate ranging 2–6 LPM were introduced to the central tube while maintaining the superior geometry. The data suggests that incorporating air bubbles can enhance heat transfer up to 142%, achieving a TEF of 1.17 at an air flow rate of 6 LPM.

Enhancement in thermo-hydraulic performance of ceramic-based disk heat sink with fractal micro-channels via double layering, hybrid nanofluids, and cross-sectional convergence

The focus of this paper is to further improve the thermo-hydraulic performance of a disk heat sink with fractal-like microchannels by introducing four distinct modifications: incorporating convergent cross-sections in microchannels, implementing double layering, utilizing a high-conductivity material (AlN ceramic) for the heat sink, and integrating hybrid nanofluids. Hybrid nanofluids, encompassing Aluminum Oxide-Graphene, Aluminum Oxide-Copper, and Copper-Graphene, each with varying volume fractions and an overall volume fraction of 2 % and 4 %, were systematically evaluated. The cumulative impact of these modifications on the four levels fractal-like microchannel heat sink's thermo-hydraulic performance was rigorously assessed through numerical simulations. The flow rate ranges from 300 to 600 ml/min, corresponding to an inlet Reynolds number in the range of 3014 to 6018.The findings reveal that the increase around 10 % in Nusselt number can be achieved through implementing converging cross-sections at levels 3 and 4. Substituting silicon with AlN ceramic leads to a reduction in maximum temperature from 1.85 to 2.58 ℃, depending on the flow rate. Under a heat flux of 40 W/cm2, double layering can reduce thermal resistance by 0.2. Most effective configuration involves a double-layered fractal microchannel with convergent cross-sections at levels 3 and 4, employing AlN ceramic as the material, and utilizing a hybrid water-based nanofluid with a volume fraction of Cu 3 %-Al2O3 1 %. This configuration achieves a notable reduction in maximum temperature by 11.1 °C and 17.97 °C under heat fluxes of 40 and 80 W/cm2, respectively, at a volumetric flow rate of 300 ml/min.

Effects of non-uniform wavy ribs on flow and heat transfer characteristics of turbine blade cooling channel

Aiming at the problem of reduced heat transfer performance and uneven distribution along the ribbed channel in aero-engine turbine blades, the influence mechanism of rib height, rib angle and fillet radius on the flow and heat transfer characteristics of wavy rib cooling channel under different inlet Reynolds numbers is analyzed by experimental and numerical methods. The research illustrates that the non-uniform change of the rib height has the most profound influence on the heat transfer performance of the ribbed channel. Schemes featuring larger initial and final rib heights demonstrate superior heat transfer performance and uniformity, with the friction factor primarily determined by the maximum rib height. The optimal rib height variation scheme shows a 22.6 % increase in overall Nu/Nu0, with only a 6 % reduction in heat transfer evenness. The non-uniform variation of rib angle and fillet radius mainly affects the local heat transfer and friction factor. Increasing Reynolds number, the distribution of friction factor of the large rib height scheme changes more drastically. At a large Reynolds number (Re = 40000), the enhanced heat transfer in the middle and rear sections of the channel due to the larger rib angle and fillet radius comes at the cost of increased pressure loss.

Temperature uniformity analysis and multi-objective optimization of the microchannel heat sink with cavities under longitudinal vortex flow

With the development of electronic devices toward a more centralized direction, there is an urgent need to introduce new technologies in the microchannel heat sink to address thermal deformation or failure caused by uneven temperature distribution. Therefore, microchannels with longitudinal vortex generators (LVGs) and different shapes of cavity structures combination are designed innovatively to address this issue. The laminar flow solver solves for single-phase, steady, and fully developed liquid flows through microchannels in the inlet Reynolds number range of 168–793. Study finds that the longitudinal vortex flow not only mixes the fluid in the mainstream region but also effectively weakens the retention effect in the cavities. Among all microchannels, the microchannel with LVGs and triangular cavities combination (MC-LTC) shows the best performance. Subsequently, multi-objective optimization of the MC-LTC is carried out using Non-dominated Sorting Genetic II Algorithm with the pumping power (Pp) and the temperature uniformity factor (E) as objective functions, and the relative heights (α, β, γ) of the three pairs of LVGs as the three design variables. The optimal solutions set for Pp and E range from 0.706 mW to 1.018 mW and 0.547 to 0.716, respectively. Among the five selected Pareto optimal solution parameters, MC-LTC (α = 0.6001, β = 1.3528, γ = 1.3707) and MC-LTC (α = 0.6001, β = 1.0725, γ = 1.3017) show the best temperature uniformity performance.

Reduction of Disk Friction Loss by Applying a Fin to the Back of a Centrifugal Impeller

Abstract It is well known that the ratio of disk friction loss of low specific speed pumps is large. When the specific speed is 100 min−1, m3/min, m or less, the disk friction loss increases remarkably. In this study, focusing on the clearance flow from the diffuser outlet to the impeller outlet (This is called the outward flow) behind the centrifugal pump, a fin was installed on outer diameter side of the rotating wall on the back side of a centrifugal pump as a countermeasure, and the influence of changes in velocity distribution near the wall on disk friction loss was investigated by torque measurements, velocity measurement, and computational fluid dynamics (CFD) analysis. As a result, it was clarified that disk friction loss is decreasing by installing the fin in both torque measurements and CFD results, because the clearance flow is separated by fin and circumferential velocity is increased in the wake region of the fin in both measurements and CFD. In addition, it was clarified that disk friction coefficient (normalized torque) can be expressed as a function of the inlet swirl ratio and Reynolds number. Also, prediction equation is derived for each shape (with and without fin). According to the equation, it was found that disk friction loss reduction effect by installing a fin becomes larger when Reynolds number and the inlet swirl ratio are small.