Inlet Reynolds Number Research Articles

Experimental exergy analysis of a printed circuit heat exchanger for supercritical carbon dioxide Brayton cycles

Abstract Exergy analysis could improve the quality of energy in the printed circuit heat exchanger. However, most of the current researches were carried out based on the first law of thermodynamics, which only focus on heat transfer, pressure drop, and heat transfer rate of the heat exchanger. There are no reports about experimental analysis of the printed circuit heat exchanger using the exergy analysis. Therefore, in this study, experimental exergy analysis of a printed circuit heat exchanger used as a recuperator for the supercritical carbon dioxide Brayton cycle is introduced. In the tests, the zigzag channels are adopted for the current heat exchanger to achieve higher heat transfer capability. The maximum running temperature and pressure in the experiments are 715.2 K and 22.5 MPa, respectively. The Reynolds number varies from 3212 to 23888. Pressure drops and temperatures are measured at the inlet and outlet of both sides. The effects of the inlet temperatures and Reynolds numbers on the exergy loss and efficiency of the printed circuit heat exchanger are investigated. The results show that the low Reynolds number and high inlet temperature on the cold side have positive effects on the exergy efficiency. New Nusselt number and friction factor correlations are developed according to the test results.

Experimental Investigation on the Flow Resistance of a Staged Fuel Injector with Thermal Protection

The multistaged rotating swirling usually adopts the staged combustion and multiple point fuel supply, which has the advantages of high efficiency and low emissions. However, the high inlet temperature rise will cause autooxidation and fuel coking. Multiple fuel circuit mutual cooling technology by using different flight conditions is an effective measure for the thermal protection of the nozzle. But the complicated fuel circuit configuration inevitably increase the flow resistance in the fuel jet. This paper presents an experimental investigation of flow resistance of a triple-stage rotating swirling nozzle, concerning the effects of various factors on total pressure loss and friction factor in nozzle fuel circuit. The factors include fuel velocity V f , i , inlet temperature T f , i , and inlet Reynolds number Re f , i . The results show that the complicated fuel-cooled structure made flow resistance of the pilot fuel circuit much bigger than the main one. Meanwhile, the empirical correlations between the friction factor and the inlet Reynolds number have been fitted, which can be the reference for engineering design.

Numerical study on swirl cooling flow, heat transfer and stress characteristics based on fluid-structure coupling method under different swirl chamber heights and Reynolds numbers

In this study, a three-dimensional swirl cooling model coupled with high temperature mainstream cascade channel and blade leading edge solid region is established to simulate the flow, heat transfer and stress characteristics. The fluid-structure coupling model is utilized to analyze the interaction of external mainstream, solid blades and swirl cooling air. Six different swirl chamber heights are selected to explore the optimal height which is the most beneficial to gas turbine blade operation. Finally, influences of coolant chamber inlet Reynolds number on swirl cooling performance are discussed in detail. Results show that the temperature distribution of blade solid region is highly related to swirl cooling effect. Compared with aerodynamic load, the thermal load behaves more significant effects for blade stress conditions. As the swirl chamber height increases, the swirl cooling heat transfer performance gradually decreases, and the flow resistance also gradually decreases. The comprehensive heat transfer factor is introduced to evaluate the swirl cooling overall performance. When swirl chamber height is 19 mm, it has higher heat transfer capacity and lower flow resistance, and the comprehensive heat transfer factor is the highest. The Reynolds number has a strong influence on heat transfer performance. In order to avoid high aerodynamic losses, the Reynolds number should be selected appropriately.

Effect of Surface Roughness and Inlet Turbulence Intensity on a Turbine Nozzle Guide Vane External Heat Transfer: Experimental Investigation on a Literature Test Case

Abstract Surface roughness is well known to significantly influence turbine aerodynamics and heat transfer; different studies have been undertaken in the last decades, in order to precisely characterize its effects and pursue a reliable and unified computational fluid dynamics modeling approach. Despite the effort, further research is still required to completely fulfill the goal, due to the complexity of the considered environment, with many other aspects and flow characteristics factoring into the final behavior. In this work, an experimental campaign was carried out to evaluate the heat transfer coefficient on a linear nozzle guide vane geometry. The adopted geometry has been developed and tested, at different inlet turbulence intensity, Reynolds and Mach number, at Von Kármán Institute. The results achieved on a test article with smooth surface were made available. In the present work, the effect of increased turbulence level and surface roughness was taken into account, respectively, using passive grids and conditioning the test sample surface finishing. Experiments were conducted using a transient technique by measuring the surface temperature evolution by IR thermography. The collected results integrate the existing database available in the open literature in order to support development and benchmarking of numerical approaches aimed at a reliable characterization of these aspects.

Investigation of the Effects of Different Working Fluids on Compressor Cascade Performance

The compressor of closed Brayton cycle (CBC) plant operating with working fluid other than air is a vital element of the energy conversion unit. However, due to insufficient understanding of the influence of the physical properties of working fluids on the performance of the compressor, the actual working conditions and design conditions of the compressor’s performance deviate greatly. In this paper, the objective is to analyze the influence mechanism of the physical properties on the performance of the cascade of compressor (static pressure ratio and total pressure loss coefficient). Therefore, the impact of a specific heat ratio on the performance of the compressor cascade is studied utilizing carbon dioxide (γ = 1.29), air and carbon monoxide (γ = 1.4), argon and helium (γ = 1.667). Moreover, the relationships of static pressure ratio and total pressure loss coefficient with physical properties of the working fluids are analyzed in the compressor cascade. It is established that a higher specific heat ratio fluid gives a higher coefficient of total pressure loss and static pressure ratio in contrast to smaller specific heat ratio at matching inlet Reynolds number and Mach number.

Experimental and numerical investigation of pulsed flows in asevere aortic stenosed model

Steady and pulsatile aortic stenotic flows through stenosis tubes were experimentally and numerically investigated. The objective was the understanding of the fluid dynamics in arterial geometries most relevant in the context of atherosclerosis. Axisymmetric phantoms corresponding to significant artery stenosis of 50% in diameter and severe aortic stenosis of 75% were respectively machined from silicon. A water flow circuit was established, a steady flow was provided by gravity and a pulsed flow by a pulsatile pump. At inlet Reynolds numbers in the range of 85 to 1125, flows at the stenosis region were investigated using two-component Particle Image Velocimetry (PIV). For the unsteady flow, three different heartbeats (60, 69 and 90 beats per minute) were considered. The k-ω shear-stress-transport first-order turbulence model in Computational Fluid Dynamics (CFD) commercial software was adopted for simulations. Experimental measurements of the velocity fields show good agreements with CFD for both steady and pulsed flows. Recirculation regions were found near the stenosis in both cases. Reverse flow through the stenosis was also observed in pulsatile flow during the end diastolic phase of the cycle. CFD simulations allowed us to accurately assess wall shear stress in the stenotic region where the optical measurements are very noisy. High values of wall shear stress (with high variations both in space and time), are observed, which are indicators of possible future aortic wall damage.

The effects of wall heat fluxes and tube diameters on laminar heat transfer rates to supercritical CO2

Nusselt number (Nu) correlations associated with constant property fluids have been deemed to be invalid when quantifying heat transfer rates to supercritical CO2 (sCO2) in the vicinity of its pseudocritical temperature (TPC). For laminar sCO2 flow, under constant heat flux conditions, previous studies have demonstrated a sudden deterioration in heat transfer near TPC, followed by a recovery phase where Nu approaches a value greater than 4.36 (associated with fully developed flow conditions for constant property fluids). Through numerical simulations of sCO2 heat transfer in horizontal circular tubes of diameters (D) from 0.2 mm to 2 mm, subject to constant wall heat fluxes (Q) in the range 0.25 – 25 kW/m2 and for inlet Reynolds (Re) numbers of 40, 100 and 400, this study demonstrates for the first time that the extent of the heat transfer deterioration and its subsequent recovery is critically dependent on the magnitude of the imposed heat flux. However, the Nu variations were independent of inlet Re across the different simulation scenarios as long as the product QD was held constant. Therefore, a modified Nu correlation in terms of thermophysical properties evaluated at both the bulk (TBULK) and wall temperatures (TWALL) is proposed to represent the observed variations.

Effect of misalignment at 2nd vane endwall on heat transfer with purge flow

Since the domestic power plants are typically operated at a partial load rather than the design point, the thermal expansion decreases, resulting in misalignment between first stage rotor and second stage stator. In this study, we investigate the heat transfer characteristics of a misalignment under purge flow at the second stage vane endwall through heat/mass transfer experiments using the naphthalene sublimation method and CFD simulations. Experiments are conducted at 4 vanes in a linear cascade with an inlet Reynolds number of 120,000, based on the vane axial chord length. In the absence of purge flow, a high heat transfer occurs in upstream of the vane endwall via horseshoe vortices. When a misalignment occurs, a more severe thermal load is observed as the mainstream attaches to the endwall due to recirculation flow upstream of the leading edge. Additionally, two high heat transfer regions are observed when a step-induced vortex occurs in the vane flow path. In case of the purge flow, the mixing of the mainstream and purge flow increases the intensity of the vortex, which increases heat transfer in the region upstream of the both flat and stepped endwall. In conclusion, we have found that the step difference has a crucial effect on thermal damage upstream of the second stage vane endwall as a little misalignment occurrence. The area-averaged heat transfer of the stepped endwall is increased by about 11.4% and 13.7% without and with purge flow, respectively, as compared to that of the flat endwall.

A micro impinging jet with supercritical carbon dioxide

A micro device with a 206 µm nozzle diameter and a standoff of 7.3 was fabricated and used to study the characteristics of micro jet impinging heat transfer with supercritical CO2. The issuing jet impinged on a 500 µm thick fused silica substrate measuring 30.5 × 1.9 mm2, onto which a heater, three temperature sensors (RTDs), and their respective leads were sputtered. The RTDs were arranged in a concentric manner with r/d values of 0.21, 1.44, and 2.89. Four holes were drilled into the device to allow passage of spent fluid, which were collected in a single channel out of the medium. The effects of heat flux, radial positions, mass flux and inlet temperature were studied for a pressure of 7.45 MPa — higher than the supercritical pressure of CO2. The highest local heat transfer coefficient of ~40,000 W/m2K was recorded at the RTD closest to the stagnation region at a heat flux of 54.5 W/cm2 and an inlet Reynolds number of approximately 15,500, corresponding to conditions near the pseudocritical temperature. Deviations between experiments and available correlations were noted especially at higher Reynolds number. A Nusselt number correlation for micro jet impingement with sCO2 was proposed.

Large eddy simulations and analyses of hydrocarbon fuel heat transfer in vertical upward flows at supercritical pressures

Gravity and/or flight acceleration would lead to significant buoyancy effect on heat transfer of a hydrocarbon fuel in the propulsion system. Large eddy simulations have been conducted to study upward flows and heat transfer of n-decane in a vertical tube at supercritical pressures. Detailed data analyses indicate that the fluid flow and heat transfer process with buoyancy effect can be divided into three stages. In the first stage, owing to the relatively low inlet Reynolds number, flow and heat transfer are initially in the laminar state. Large density reduction under strong heating at a supercritical pressure leads to flow acceleration, which is mainly due to buoyancy and helps improve heat transfer. Further downstream, buoyant acceleration results in an M shaped velocity profile, which promotes the shear production of turbulence. Turbulence is also strongly generated by buoyant production at this stage. In addition, viscous dissipation decreases at high temperature. These factors eventually cause turbulence transition. Heat transfer is thus significantly enhanced, and the wall temperature is drastically reduced. Comparing to the cases of forced convection, buoyancy could promote or delay turbulence transition under different surface heat fluxes. In the third stage, fluid flows and heat transfer enter the fully developed turbulence region, which is maintained mainly by the shear production mechanism. Numerical analyses herein would provide fundamental understanding of buoyancy effect on supercritical-pressure heat transfer of hydrocarbon fuel in the regenerative engine cooling application.

Numerical Investigation of Active Flow Control on Laminar Forced Convection over a Backward Facing Step Surrounded by Multiple Jets

Laminar, transient forced convection problem over a 2D backward facing step (BFS) at an inlet Reynolds number (Re) of 400 is investigated numerically using OpenFOAM. To increase the Nusselt number (Nu) along the bottom wall, active flow control is applied by zero-net-mass-flux (ZNMF) combinations of suction and injection through three thin slits which are placed on the top, step and the bottom walls in the vicinity of the BFS. The combinations of each jet velocity is determined by jet to inlet mean velocity ratios which are limited to integer numbers between -2 and 2 and satisfying ZNMF condition where negative and positive values indicate suction and injection, respectively. All 19 cases which satisfy these rules are investigated. Average Nusselt number, friction coefficient and recirculation zone lengths are calculated along the bottom wall from time averaged flow fields. Among 19 cases with each having different jet configuration, some cases converged to steady state solution while others indicated temporal effects and converged to periodic solutions. To understand these transient effects, velocity oscillation magnitude and Strouhal number which are monitored at a selected critical point are evaluated. It is shown that temporal interaction of chosen active flow control methodology has significant effect on enhancing mixing which results in an increase of Nusselt number. Among all cases, the best case concerning thermal improvement has an increase of 78.5% in Nu number while the best aerodynamic improvement is achieved for another case with a decrease of 81% in total recirculation zone length compared to the reference case where no control is applied.

Film Cooling Performance of a Fully Cooled Vane at High Subsonic Conditions

This work focuses on the experimental study of the heat transfer and film cooling characteristics of a fully cooled vane under high subsonic conditions; the transient heat transfer measurements with thermocouples were employed. The inlet Reynolds numbers of the cascade ranged from to and the exit Mach number was 0.8. Two freestream turbulence intensities (1.3% and 14.7%) and three mass flow ratios (5.5%, 8.4%, and 12.5%) were tested. The averaged film cooling effectiveness of the current vane with shaped holes was found to be increased by 15%–43% more than that of the vane only with cylindrical holes, and elevated turbulence intensity reduced the averaged film cooling effectiveness by 3%–26%. The heat transfer augmentation of the current vane was lower than that of the vane only with cylindrical holes. With the mass flow ratio increasing, the heat transfer augmentation gradually increased on the pressure side, whereas it was nearly kept invariable on the suction side. The net heat flux reduction results showed that the vane with shaped holes significantly improved the cooling performance, whereas elevated turbulence intensity significantly deteriorated the cooling performance.

Turbulent convective heat transfer in helical tube with twisted tape insert

Enhancement of heat transfer has been a main focus in thermal-fluid engineering field among the other aspects in industrial for decades. Despite numerous studies have been done on turbulent flows, helical tubes, and twisted tapes related to enhancement, no study has been done on the combination of all three components in augmentation of heat transfer. The objective of this study is to numerically investigate heat transfer of turbulent flow in helical tube with twisted tape insert subjected to constant wall temperature. A three-dimensional computational fluid dynamic model was developed and validated against experimental measured data and correlations. The model was then used to evaluate the turbulent convective heat transfer in straight and helical tube with and without twisted tape insert. To obtain comprehensive view on the heat transfer performance, the effect of twist ratio and inlet Reynolds number are evaluated as well. The insertion of twisted tape is found to increase the intensity of secondary flow in the radial direction. Helical tube with twisted tape insert was found to have higher heat transfer performance. The results show that for every case, the higher the inlet Reynolds number, the heat transfer increases proportionally while friction coefficient drops with decreasing amount of decrement. It is found that helical coiled tube with twisted tape at twist ratio of 7.86 has the highest performance among the configuration studied after taking consideration of heat transfer, friction coefficient and entropy generation. Finally, correlations of Nu and f are developed to predict the heat transfer and pressure drop within considered range of parameters in this study. This study serves as a foundation on the development of a high-performance heat exchanger for industrial application.

Investigation of impingement cooling on a heat sink using CFD simulation

The aim of this study is to investigate heat transfer characteristics of a circular array of air jets impinging on to a heat sink under a thermal load through CFD simulation. This is necessary to maintain the optimum temperature for the component to function as efficiently as possible. The simulation is done using a conjugate heat transfer model along with an SST k-ω model to model the turbulence in ANSYS FLUENT. The dependent parameters are the inlet Reynolds number and the non-dimensional quantity Z/d representing the ratio of distance between the inlet and the imping surface, to the inlet jet diameter. Multiple values of Z/d and Inlet Reynolds number are taken for each iteration. It is seen that the steady state temperature drops from 85 °C to 60 °C when the inlet Reynolds number goes from Re = 6000 to Re = 12000. The temperature goes from 61 °C to 69 °C when the Z/d ratio is changed from 4 to 10.

Grid-point and time-step requirements for direct numerical simulation and large-eddy simulation

We revisit the grid-point requirement estimates in Choi and Moin [“Grid-point requirements for large eddy simulation: Chapman’s estimates revisited,” Phys. Fluids 24, 011702 (2012)] and establish more general grid-point requirements for direct numerical simulations (DNS) and large-eddy simulations (LES) of a spatially developing turbulent boundary layer. We show that by allowing the local grid spacing to scale with the local Kolmogorov length scale, the grid-point requirement for DNS of a spatially developing turbulent boundary layer is N∼ReLx2.05 rather than N∼ReLx2.64, as suggested by Choi and Moin, where N is the number of grid points and Lx is the length of the plate. In addition to the grid-point requirement, we estimate the time-step requirement for DNS and LES. We show that for a code that treats the convective term explicitly, the time steps required to get converged statistics are Nt∼ReLx/Rex06/7 for wall-modeled LES and Nt∼ReLx/Rex01/7 for wall-resolved LES and DNS (with different prefactors), where Rex0 is the inlet Reynolds number. The grid-point and time-step requirement estimates allow us to estimate the overall cost of DNS and LES. According to the present estimates, the costs of DNS, wall-resolved LES, and wall-modeled LES scale as ReLx2.91, ReLx2.72, and ReLx1.14, respectively.

FLUID FLOW AND HEAT TRANSFER BEHAVIOR IN DISTINCT ARRAY OF STEPPED MICRO-PIN FIN HEAT SINK

In this paper, numerical analysis has been performed on micro-pin fin heat sinks with novel arrangements of the fins. Varying the fin height in an array of two, three, and four fins evolves stepped-type pin fin heat sink. Geometry selection of all seven configurations of the heat sinks is made on the basis of equal footprint area as well as net convective surface area. Single-phase liquid water has been used as a cooling medium where inlet Reynolds number (Re) varied from 200-600. Comparative numerical results are presented through the illustration of temperature distribution, Nusselt number (Nu), pressure drop, velocity contour, and thermal performance factor. Estimated findings clearly indicate that the overall performance of the stepped configurations is better than uniform fin height. Moreover, arrays of increasing fin heights facilitate favorable flow behavior compared to decreasing fin height; consequently, they support higher heat transfer rate from the heat sink. Among the stepped heat sinks, THSI (three-stepped increasing) configuration exhibits superior performance due to distinct coolant flow and favorable heat dissipation circumstances originated by constructive fin arrangements. Similarly, the thermal performance factor of THSI is found to be consistently higher compared to all other heat sinks.

On the use of tapered channels gas distributors to promote convection in PEM Fuel Cells

Polymeric Exchange Membrane Fuel Cells (PEMFC) are promising power propulsion systems for the decarbonization of the transportation sector. Despite being a well-known method for the direct production of electric current from the reactants chemical energy, one of the major limitations to their large-scale industrial development are fluid dynamics and mass transport aspects, crucially limiting the electrochemistry rate under critical conditions. This is especially verified in PEMFC with serpentine-type gas distributors, for which such areas are identified in proximity of the gas channel bends where the dominant mechanism for species transport shifts from a convection-enhanced to a diffusion-limited one. An engineering method to enhance the convective transport in such deficient areas is the use of gas distributors with tapered channels, effectively forcing the flow in diffusive media and improving the reactants delivery rate and products removal. A numerical analysis is presented on a limited domain representing a section of a serpentine gas distributor. A multi-dimensional CFD study is carried out comparing conventional-type and tapered channel distributors, evaluating the combined effect of pressure losses, catalyst layers utilization, flow regime in anisotropic diffusion media and convection/diffusion balance via a non-dimensional analysis. The study covers various inlet Reynolds numbers and in-plane permeability of porous materials for two diffusion media thicknesses, with the aim to extend the generality of the study. Conclusions based on the simulation results outline channel tapering as a very effective way to improve the power density of PEMFC, although an energetic cost/benefit analysis indicates a reduced cell efficiency.

Performance of Supercritical CO2 Power Cycle and Its Turbomachinery with the Printed Circuit Heat Exchanger with Straight and Zigzag Channels

Since printed circuit heat exchangers (PCHE) are the largest modules of a supercritical carbon dioxide Brayton cycle, they can considerably affect the whole system’s performance and layout. Straight-channel and zigzag-channel printed circuit heat exchangers have frequently been analyzed in the standalone mode and repeatedly proposed for sCO2−BC. However, the impact of heat exchanger designs with straight and zigzag-channel configurations on the performance of the cycle and its components, i.e., the turbine and compressor, has not been studied. In this context, this study evaluates the effect of different heat exchanger designs with various values of effectiveness (ϵ), inlet Reynolds number (Re), and channel configuration (zigzag and straight channel) on the overall performance of the sCO2−BC and its components. For the design and analysis of PCHEs, an in-house PCHE design and analysis code (PCHE-DAC) was developed in the MATLAB environment. The sCO2−BC performance was evaluated utilizing an in-house cycle simulation and analysis code (CSAC) that employs the heat exchanger design code as a subroutine. The results suggest that pressure drop in PCHEs with straight-channel configuration is up to 3.0 times larger than in PCHEs with zigzag-channel configuration. It was found that a higher pressure drop in the PCHEs with straight channels can be attributed to substantially longer channel lengths required for these designs (up to 4.1 times than zigzag-channels) based on the poor heat transfer characteristics associated with these channel geometries. Thus, cycle layouts using PCHEs with a straight-channel configuration impart a much higher load (up to 1.13 times) on the recompression compressor, this in turn, results in a lower pressure ratio across the turbine. Therefore, the overall performance of the sCO2−BC using PCHEs with straight-channel configurations is found to be substantially inferior to that of layouts using PCHEs with zigzag-channel configurations. Finally, optimization results suggest that heat exchanger’s design with inlet Reynolds number and heat exchanger effectiveness ranging from 32 k to 42 k and 0.94>ϵ>0.87, respectively, are optimal for sCO2−BC and present a good bargain between cycle efficiency and its layout size.

Investigation of the flow and heat transfer characteristics of helium gas in printed circuit heat exchangers with asymmetrical airfoil fins

This work presents a numerical investigation on the flow and heat transfer performance of printed circuit heat exchangers with asymmetrical airfoil fins, and the influence of the arrangements, attack angles, horizontal spacings, and staggered spacings of fins are studied. Then an orthogonal experiment is designed to optimize the comprehensive performance of these printed circuit heat exchangers. Finally, empirical correlations of Nusselt number and friction factor for these heat exchangers with the inlet Reynolds number ranging from 2000 to 10,000 are proposed based on our numerical results. The results show that the printed circuit heat exchangers with a consistent layout of fins show better comprehensive performance than those with a reverse layout. The comprehensive performance of these printed circuit heat exchangers with asymmetrical airfoil fins increases as the horizontal fin spacing and fin attack angle decrease. The staggered spacing has little influence on the comprehensive performance of printed circuit heat exchangers. In the present work, the printed circuit heat exchanger with the values of the normalized horizontal spacing, staggered spacing, and attack angles of 4, 1.0, and 1.269, respectively, exhibits the best comprehensive performances.

Numerical study of mixing of cavitating flows in a Venturi tube

AbstractThis work is devoted to a computational investigation of the cavitation and mixing characteristics of two miscible (Schmidt number Sc = 103) turbulent water flows with different viscosity ratios in a Venturi tube. The 2D axis‐symmetric RANS and 3D LES turbulence models are used for the prediction of flow characteristics. The numerical results reveal that the RANS solutions underpredict the rate of vapour production and overestimate the mixing rate of two miscible fluids in comparison to LES simulations for the inlet Reynolds number of 19 738 (Case‐1) and 19 286 (Case‐2). In particular, there is a larger cavitation zone in the Venturi tube with LES in comparison to RANS calculations.