Pulsation Amplitude Research Articles

Effect of coolant pulsation on film cooling performance on flat plate

• Large eddy simulation was used for investigating square-wave pulsating film cooling on flat plate. • The experiment of pulsating film cooling was carried out. • Evolution of coherent structures in pulsating film cooling was analyzed. Combined with infrared thermography tests, large eddy simulation was used to study the square-wave pulsating film cooling on flat plate at the time-averaged blowing ratios of 0.5 and 1.5. The effects of two pulsating parameters including frequency and amplitude were discussed in detail. At low blowing ratio, the introduction of coolant pulsation leads to the decrease of cooling effectiveness, and the effect of pulsating amplitude is much more pronounced than frequency. At high blowing ratio, cooling effectiveness can be enhanced by coolant pulsation at low amplitude, but reduced at high amplitude. As the typical vortex structure in pulsating film cooling, the starting vortex is formed as the jet is turned on. The interaction between hairpin vortex and starting vortex shows that, at high pulsating amplitude, the starting vortex can help coolant detach from the wall, and has an adverse influence on cooling performance. The statistical characteristics and power spectrum density of instantaneous velocity were investigated. It was found that the introduction of coolant pulsation can result in the increases of eddy energy and turbulent kinetic energy in the near filed region, but this effect becomes weaker and weaker as the streamwise distance increases.

Spectral pulsation dynamics of soliton molecules in ultrafast fiber lasers based on pump intensity modulation

<sec>This study employs real-time Fourier transform spectroscopy to investigate the pulsation dynamics of soliton molecules in a mode-locked erbium-doped fiber laser, by modulating pump intensity. By controlling the driving voltage of the pump source, we systematically observe and characterize the influence of external modulation signals on the amplitude, period, pulsation frequency, and the relative phase evolution among the pulsating soliton molecules in their spectra.</sec><sec>The results demonstrate that under specific conditions of pump intensity modulation, the pulsation period of soliton molecule spectra can be precisely regulated by the pump modulation frequency. At the same time, the amplitude of soliton molecule pulsations and the evolution of relative phase among the solitons are intricately tied to the pump modulation frequency. At lower modulation frequencies, such as 1 kHz, the relative phase among the pulses in the soliton molecule exhibits a sliding-type dynamics as a function of propagation time.</sec><sec>As the modulation frequency gradually increases to 5 kHz, a scenario emerges where three soliton molecules are generated. Notably, both the soliton spacing and relative phase undergo synchronous adjustments influenced by the pump modulation. With the modulation frequency further increasing, say, to 20 kHz, the relative phase evolution among the pulses within the soliton molecule gradually descends into chaos. This observation suggests the plausible existence of an inherent resonant frequency associated with pulsating soliton molecules, which has direct implications for their stability.</sec><sec>The findings of this research are of significance in advancing our comprehension of soliton molecule generation and enhancing their stability. In addition, they provide valuable insights into the broader domain of all-optical manipulation and applications of soliton molecules, and their application in pulse encoding in mode-locked laser systems.</sec>

ОЦЕНКА ГОМОГЕННОСТИ ТОПЛИВОВОЗДУШНОЙ СМЕСИ В МАЛОЭМИССИОННОЙ КАМЕРЕ СГОРАНИЯ ГАЗОТУРБИННОГО ДВИГАТЕЛЯ С ПОМОЩЬЮ КОРРЕЛЯЦИИ ПУЛЬСАЦИЙ ДАВЛЕНИЯ И РАСХОДА ПИЛОТНОГО ТОПЛИВА

Connection between pilot fuel flow and pressure pulsations in dry low emission combustor is established on base of experimental data. Determining quantitative criteria of homogeneous or diffusion flame is formulated. The criteria give chance to evaluate quality of mixing fuel and air and combustion without emission measuring. The root-mean-square deviation of the dependence of the distribution of the air-fuel mixture concentration field over the combustion chamber cross section on the correlation coefficient between the pilot fuel ratio and the double amplitude of pressure pulsations in the flame tubes for a homogeneous (technically mixed air-fuel mixture) circuit was approximated based on the data obtained. The results obtained make it possible to significantly improve the quality of emission prediction by refining the semi-empirical mathematical model of the nitrogen oxides' generation, built on the basis of the Zeldovich thermal mechanism for low-emission operating modes of dry low emission combustor of the gas turbine engines. The account of heterogeneity of the process in the flame tubes makes it possible to move from the assumption of the geometric and gas-dynamic identity of the flame tubes to their actual heterogeneity. This conclusion allows refining the model. The developed mathematical model can be used as a virtual emission sensor in advanced tracking control systems of gas turbine engines.

Multi-scale analysis of concentration distribution in unsteady Couette–Poiseuille flows through a porous channel

A multiple-scale perturbation analysis is presented to analyse the two-dimensional concentration distribution of passive contaminant released in an incompressible viscous fluid flowing between two parallel plates filled with a porous medium. The flow is driven by the combined effect of the upper plate oscillation in its own plane moving with a constant velocity, and the periodic pressure gradient. Mei’s homogenization technique is used to find the concentration distribution up to third order, complemented with the dispersion coefficients for four different situations, namely, steady, pulsatile, oscillatory and the combined effect of all these. We observe that when the flow is under the combined effect of wall oscillation and pressure pulsation, then the respective frequency (Womersley number) and amplitude parameters oppose each other while influencing the dispersion coefficient. Our analysis reveals that for a fixed amplitude of oscillation and pulsation, the frequency of pressure pulsation has a stronger effect on the dispersion coefficient compared with the wall oscillation. On the other hand, when the Womersley number is kept fixed, amplitude of the wall oscillation dominates the pressure pulsation. This behaviour is more prominent for higher values of the Darcy number. The transverse concentration distribution and its dependency on porous medium parameters are also discussed in detail.

Spectral pulsation dynamics of soliton molecules in ultrafast fiber lasers based on pump intensity modulation

This study employs real-time Fourier transform spectroscopy to investigate the pulsation dynamics of soliton molecules in a mode-locked erbium-doped fiber laser, utilizing pump intensity modulation. By manipulating the driving voltage of the pump source, we systematically observe and characterize the impact of external modulation signals on the amplitude, period, pulsation frequency, and the relative phase evolution among the pulsating soliton molecules within their spectra.The results demonstrate that, under specific conditions of pump intensity modulation, the pulsation period of soliton molecule spectra can be precisely regulated by the pump modulation frequency. Concurrently, the amplitude of soliton molecule pulsations and the evolution of relative phase among the solitons are intricately tied to the pump modulation frequency. At lower modulation frequencies, such as 1 kHz, the relative phase among the pulses within the soliton molecule exhibits a sliding-type dynamics as a function of propagation time.As the modulation frequency gradually increases, e.g., to 5 kHz, a scenario emerges where three soliton molecules are generated. Notably, both the soliton spacing and relative phase undergo synchronous adjustments influenced by the pump modulation. With further escalation of the modulation frequency, such as to 20 kHz, the relative phase evolution among the pulses within the soliton molecule gradually descends into chaos. This observation suggests the plausible existence of an inherent resonant frequency associated with pulsating soliton molecules, which has direct implications for their stability.The findings of this research hold significant relevance for advancing our comprehension of soliton molecule generation and enhancing their stability. Furthermore, they offer valuable insights into the broader domain of all-optical manipulation and applications of soliton molecules, as well as their utilization in pulse encoding within mode-locked laser systems.

Numerical study of local heat transfer in tube bundle in pulsating flow

Forced pulsating flows enhance the heat transfer of various heat exchange equipment; however, such flows remain poorly understood. A numerical simulation was used to study how the position of a cylinder in a tube bundle affects the heat transfer during pulsating flow. The tubes of the tube bundle were arranged in an in-line order with the same relative pitch of 1.4. The Reynolds number and Prandtl number had constant values of 1500 and 4.03, respectively. The pulsating flow exhibits a reciprocating asymmetrical character. The pulsation amplitude related to the cylinder diameter was in the range from 0.1 to 0.4, and the pulsation frequency was in the range from 0.2 to 0.8 Hz. The numerical simulation results indicated that the heat transfer of the cylinder in the tube bundle was affected by the row number. With pulsating flow, the heat transfer of the cylinder in the first row had a minimum value, whereas the maximum heat transfer was observed in the last row. The effect of the position of the cylinder along the flow in the tube bundle decreases with increasing pulsation intensity. The maximum heat transfer enhancement of 51% was observed in the first row at a frequency of 0.8 Hz and amplitude of 0.4.

Thrust Control Method and Technology of Variable-Thrust Liquid Engine for Reusable Launch Rocket

A high-precision variable-thrust control method based on real-time measurement of pintle displacement and closed-loop feedback control is proposed to solve the technical problems of deep throttling variable-thrust regulation and control of pintle liquid rocket engines (LRE). By optimizing the system structure and control parameters, the closed-loop control of displacement with high precision and a fast response under a wide range of variable thrust can be realized, and thus the large-range, fast-response, and high-precision control of the chamber pressure, equivalent to thrust, can be indirectly realized. The chamber pressure response time is not more than 0.3 s, the overshoot is not more than ±3%, and the pulsation amplitude is not more than ±5%, which can meet the technical requirements of the large-range thrust adjustment and control of variable-thrust LRE of reusable launch rockets. The proposed variable-thrust LRE thrust control system is simple, reliable, and easy to use and maintain, which solves the problem of the large range, high precision, and fast response of thrust adjustment and control. The proposed system can provide important technical support for carrier rocket recycling and launch cost reduction. This is the first time a closed-loop control method of displacement of an integrated gas generator/flow regulator to achieve a 5:1 large-range continuous-variable-thrust control for the LRE of a reusable launch rocket has been proposed.

Pulsatile gas-liquid flow resembling Decompression Sickness: Computational Fluid Dynamics simulation and experimental validation.

This work performs two-dimensional Computational Fluid Dynamics (CFD) simulations of pulsatile bubbly flow in a column resembling the flow inside human vena cava during Decompression Sickness (DCS), aiming to illustrate the effect of certain parameters in bubbly blood flow and so facilitate the design of the: a) corresponding in-vitro bubbly flow experiments under pulsatile flow conditions inside a flow loop and b) in-vivo trials on swines for assessing a novel electrical impedance spectroscopy technique on the detection of bubbles (as those found during DCS) in their bloodstream. The commercially available ANSYS 2019-R3 CFD code was employed to simulate the pulsatile bubbly flow that resembled DCS. Simulations were validated against experiments conducted in a vertical co-current upward pulsatile bubbly flow provided by a flow loop equipped with electrical, optical and pressure diagnostics. CFD simulations under pulsatile conditions were initially validated by oscillatory in-vitro bubbly flow experiments. Then, the influence of pulsation parameters on void fraction, α, and flow velocity, U, profiles was computationally investigated. Intense periodic fluctuations of void fraction were observed along the column and their intensity increases with pulsation amplitude. Moreover, U and α radial profiles were uniform for bubbles 30 μm but showed a core-peaking profile for bubbles 300 μm. CFD simulations of pulsatile bubbly flow resembling DCS provided unconventional information about the influence of different-sized sub-millimetre bubbles on the flow velocity and void fraction profiles, which are expected to improve the design of in-vitro and in-vivo trials for the detection of bubbles such as those found in DCS.

Thermal performance analysis of artificially roughened solar air heater under turbulent pulsating flow with various wave shapes

This paper presents a numerical investigation of the operation of a solar air heater (SAH) with cylindrical shape ribs under turbulent pulsating flow based on thermal and frictional considerations. Turbulence and thermal natures of flow physics are modeled by taking the relevant governing equations into the account using RNG k−ε turbulence model and transient state Reynolds-Averaged Navier-Stokes (RANS) equations. The pulsating flow is modeled with four different wave shapes including rectangular, sinusoidal, triangular, and sawtooth functions. The influence of various factors such as wave shape, Reynolds number (Re), frequency, and amplitude of pulsating flow on the thermal and hydraulic performance of SAH has been evaluated. Results indicated that the SAH with ribbed absorber section under pulsed flow has a larger Nu number compared to the air heater with smooth duct under steady flow. Moreover, the pulsating flow with a sawtooth velocity profile leads to the greatest value of the thermo-hydraulic performance parameter (THPP). The growth of pulsation frequency up to 200 enhances the heat transfer coefficient of the air heater while for frequencies larger than 200, the thermal performance declines. In addition, the increase in pulsation amplitude has a negative effect on the THPP. It was proved that the increment of the Re number enhances the THPP. Up to 53.3% improvement of THPP can be achieved by applying pulsed flow with sawtooth wave shape and ribbed absorber section compared to air heat with smooth duct under steady flow.

Coupling immersed boundary and lattice Boltzmann method for modeling multi‐body interactions subjected to pulsatile flow

This paper numerically investigates the effect of pulsating flow on the settling dynamics of rigid circular particles. This is an interdisciplinary subject and spans several areas ranging from mathematical and numerical modeling to fluid mechanics. For this purpose, pulsatile flow characteristics are embedded in the combination of the direct‐forcing immersed boundary method and the split‐forcing lattice Boltzmann method. Inter‐collision forces between the solid boundaries (particles and boundaries) and the added mass force due to acceleration are considered. Adequate verification tests are done to ensure the credibility of the findings. The critical parameters of pulsating flow, such as amplitude and frequency of pulsation, are investigated in detail. The paper especially puts emphasis on the interaction between particles and studies the well‐known drafting, kissing, and tumbling (DKT) phenomena. Two different scenarios are taken into account and also compared with the stationary flow. The first case is when the pulsating flow is in the direction of gravity (co‐flow), while in the latter, there is an opposing flow (counterflow). The sedimentation manners of 12 particles in a vertical channel are also presented. The findings shed light on the importance of pulsating flow and the extension of the proposed computational method for such problems. It is also revealed that pulsation and its variables can alter DKT by either postponing or speeding up the process. Also, in some cases, the cycle of DKT can be maintained incompletely, and particles would just stick together. The results can be useful for various engineering problems like filtration and particle sorting.

Pulsating multiple nano-jet impingement cooling system design by using different nanofluids for photovoltaic (PV) thermal management

The present work proposes a new cooling system for thermal management and cooling of photovoltaic (PV) systems. Pulsating flow with multiple jet impingement is considered by using different fluid types. Hybrid nanofluid and alumina–water nanofluid having cylindrical ans spherical shaped nanoparticles are used as the cooling medium. The study is conducted by using finite volume method for various values of pulsating amplitude (between 0 and 1), Strouhal number (between 0.01 and 1), solid volume fraction of nanoparticles (between 0 and 2%) and slot number of the impinging jet (between 1 and 13). It is observed that pulsating amplitude is more effective on the cooling performance enhancement as compared to frequency while average Nusselt number (Nu) rises by about 63.5% while temperature drop of 2.16 °C can be achieved when pulsation amplitude is increased from 0 to 1. Nanofluid with cylindrical shaped nanoparticles and hybrid nanofluid show very similar trends while temperature drop of 2.6 °C is achieved when cooling system with nanofluid-cylinder in pulsating flow case is compared with pure-fluid in non-pulsating flow configuration. When nanoparticles loading amount on the thermal improvement is compared, the most favorable cases are obtained for nanofluid-cylinder and hybrid nanofluid case. The average Nu increments become 3.5%, 22.8% and 22.9% for nanofluid-spherical, nanofluid-cylinder and hybrid nanofluid when lowest and highest nanoparticle loading amount cases are compared. Increasing the slot number in pulsating flow case significantly rises the Nu and drops the average panel surface temperature. When different systems are compared pulsating nano-jets cooling system using alumina–water nanofluid with cylindrical shaped nanoparticles provides the most effective cooling system while while temperature drop of ΔT=37.30 °C is achieved at the highest amplitude and highest loading of nanoparticles in the pure fluid as compared to uncooled PV system.

Research on the Vortex Rope Control Techniques in Draft Tube of Francis Turbines

Francis turbines are most widely used in hydropower due to their characteristics which include a fast response and wide time-scale operation. The vortex rope inside Francis turbines is a common flow phenomenon, which always causes strong vibration, pressure pulsations, fatigue load, and even serious failure of the components. Vortex suppression methods can effectively change the velocity and pressure distribution of the flow field in the draft tube, reduce the volume of vortex rope and the amplitude of pressure pulsation, inhibit the development of cavitation erosion, and improve the operation stability of the hydro turbine. However, the vortex suppression method is not suitable for all working conditions, and the vortex suppression effect is also different. There are still many problems with how to analyze the vortex suppression effect and practicability of the turbine from multi-dimensions. It is of great significance to analyze the vortex suppression techniques and their practicability in hydraulic turbines from various aspects. The primary focus of the present study is to analyze the hazards of vortex rope in draft tubes and summarize the methods of suppressing vortex rope and pressure pulsation. This review article provides a basis for controlling the vortex rope in the draft tube, which can help the designers choose the suitable control method to mitigate it. Future research directions are also briefly discussed.

Limited driving characteristics of combustion instability in a swirling flame: An experimental study

Regarding combustion instability, heat-release pulsation leads to pressure pulsation. Conversely, the driving force of the pulsating flame restricts the oscillation equilibrium point of combustion instability. Thus, in this paper, we developed a flame limited driving model explaining the finiteness of the driving energy of flame pulsation. Our research indicates that the limit cycle state of combustion instability was affected by the limitations of heat-release pulsation and Rayleigh integration. The coupling intensity of the heat-release pulsation and oscillating pressure decreases gradually with the increase in the oscillating pressure amplitude. In addition, an increase in flame pulsation amplitude presents finiteness similarly. When the acoustic amplitude exceeds a certain threshold, the amplitude of heat-release pulsation stops increasing until the flame approaches the oscillation blow-out limit. The further strengthening of the oscillation intensity of combustion instability is simultaneously restricted by the limitations of heat-release pulsation and Rayleigh integration, and thus the combustion instability finally stabilizes at the oscillation equilibrium point.

Anomalous Cepheids: Updated Theoretical Period–Luminosity–Color and Period–Wesenheit Relations

AbstractIn the context of a project aimed to provide an updated theoretical scenario for various classes of radially pulsating stars, we present the first results obtained for anomalous Cepheids. By adopting reliable and updated evolutionary prescriptions concerning the luminosity levels for various core He-burning stellar models with masses suitable for entering the instability strip, we have computed nonlinear convective pulsation models for both fundamental and first-overtone mode anomalous Cepheids by exploring the impact of varying the metal abundance as well as the efficiency of super-adiabatic convection. These numerical simulations have allowed us to retrieve the boundaries of the instability strip and all relevant pulsation properties, namely period, amplitude, bolometric light and radial velocity curves. This theoretical scenario has been transformed into the Gaia photometric system to derive the first theoretical Period–Luminosity–Colour and Period–Wesenheit relations in the Gaia bands.

Photometric and Spectroscopic Analysis of the SX Phe Star BL Cam

In the present paper, we report the photometric and spectroscopic observations obtained by the 1.88 m telescope at the Kottamia astronomical observatory of the pulsating star BL Cam. Fourier analysis of the light curves reveals that the fundamental mode has two harmonics. The O-C method is used to establish the period changes. So far, the analysis has been very successful in mapping the pulsation amplitude of the star across the instability strip. By using the formalism of Eddington and Plakidis (1929), we found significant results and strong indications of the evolutionary period change. A total of 55 new maximum light timings are reported. New values of (1/P) dP/dt are estimated using the O-C diagram based on all newly obtained times of maximum light combined with those taken from the literature, assuming the periods are decreasing and changing smoothly. To compute the effective temperature and surface gravity of the star, we performed model atmosphere analysis on its spectra. The physical parameters of the star are calculated and compared with the evolutionary models.

Dopamine is involved in regulating vascular tone in retinal arterioles

AbstractPurpose: There is clinical evidence suggesting that the neurotransmitter dopamine may play a role in regulating retinal vascular function. Patients with Parkinson's disease tend to have a narrower retinal vessel diameter in fundus images and reduced perfusion density1. Healthy volunteers given dopamine IV show increased retinal blood cell velocity (by 37%) and fundus pulsation amplitude (by 24%)2. However, it is not clear if these observed effects may be due to direct actions of dopamine on retinal vessels, independent of any systemic actions, and if so through which subtype of dopamine receptors. We therefore measured the effects of dopamine ligands on isolated segments of porcine retinal arterioles by small wire myography.Methods: Dissected segments of porcine retinal arterioles and surrounding retinal tissue were mounted in a DMT630MA small wire myograph system for measurement of contractile activity, and pre‐contracted with 10−6 MU‐46619, a thromboxane analog, added to the organ bath. With the vascular tone stabilized, all dopamine ligands tested were applied separately to the bath with the vessel, and the effects of each on the wall tension recorded. Results are presented as wall tension (in mN/mm) normalized for vessel length.Results: In isolated vessel segments pre‐contracted with U‐46619, dopamine induced a vasodilation in a dose‐dependent manner, which was complete at 2 mM. The dopamine D1 receptor agonist SKF38393 induced dose‐dependent vasodilation, complete at 0.7 mM. The D2 receptor agonist sumanirole (10−4 M) had no effect on wall tension in pre‐contracted retinal arterioles. The D2 receptor antagonist S‐sulpiride (10−3 M) had no effect on wall tension and did not block the vasodilation induced by either dopamine or SKF38393.Conclusions: Dopamine induces vasodilation in pre‐contracted retinal arterioles, primarily via D1 receptors.

Experimental characterization and mechanism of hydraulic pulsation waves driving microscopic residual oil

To clarify microscopic mechanisms of residual oil displacement by hydraulic pulsation wave, microscopic visualization experiments of hydraulic pulsation wave driving residual oil were carried out by using the microscopic visualization device of pulsating water drive. For the four types of residual oil left in the reservoir after water flooding, i.e. membrane, column, cluster, and blind end residual oils, hydraulic pulsation waves broke the micro-equilibrium of the interface by disturbing the oil-water interface, so that the injected water invaded into and contacted with the remaining oil in small pores and blind holes, and the remaining oil was pushed or stripped to the mainstream channel by deformation superposition effect and then carried out by the injected water. In the displacement, the pulsation frequency mainly affected the cluster and blind end remaining oil, and the hydraulic pulsation wave with a frequency of about 1 Hz had the best effect in improving the recovery. The pulsation amplitude value mainly affected the membrane and column residual oil, and the larger the amplitude value, the more remaining oil the hydraulic pulsation wave would displace. The presence of low intensity continuous flow pressure and holding pressure end pressure promoted the concentration of pulsating energy and greatly improve the recovery of cluster residual oil. The rise in temperature made the hydraulic pulsation wave work better in displacing remaining oil, improving the efficiency of oil flooding.

AlphaTg: GUI application for flux and limb derivatives of UBVRI, Kepler and TESS pass-bands for asteroseismology applications

The flux derivatives, limb darkening integrals and limb darkening derivatives have been used in many astronomical applications. Among these applications, the flux and limb darkening derivatives are mandatory parameters in asteroseismology whenever determining the theoretical amplitudes of stellar pulsations. The amplitude ratio method, which compares the observed amplitudes of UBVRI color bands with the theoretical amplitudes is widely used in asteroseismology. The computation of theoretical amplitudes is very complex and time-consuming due to the dependency on several parameters, including flux and limb darkening derivatives. Besides, more theoretical amplitude models are needed to compare with the observations for an ideal conclusion. In addition, the computation of synthetic line profiles in spectroscopy is also used the flux and limb darkening derivatives; therefore, we introduce the AlphaTg code to determine flux and limb darkening derivatives in a single application to minimize the complexity of computation. The facility enables to compute flux derivatives for any effective temperature (Teff) within the range of 3500 K ≤ Teff≤ 50000 K and any gravity value within the range of 0.0 ≤ log (g) ≤ 5.0 for UBVRI, Kepler and TESS photometric systems. The limb darkening derivatives can be determined for a spherical degree, l = 0, 1, and 2 for linear, quadratic and non-linear limb darkening laws for the effective temperature range of 3500 K ≤ Teff≤ 50000 K and gravity range of 1.0 ≤ log (g) ≤ 5.0. A case study is also discussed to explain the usage and limitations of the application.

Heat Transfer in 3D Laguerre–Voronoi Open-Cell Foams under Pulsating Flow

Open-cell foams are attractive for heat transfer enhancement in many engineering applications. Forced pulsations can lead to additional heat transfer enhancement in porous media. Studies of heat transfer in open-cell foams under forced pulsation conditions are limited. Therefore, in this work, the possibility of heat transfer enhancement in porous media with flow pulsations is studied by a numerical simulation. To generate the 3D open-cell foams, the Laguerre–Voronoi tessellation method was used. The foam porosity was 0.743, 0.864, and 0.954. The Reynolds numbers ranged from 10 to 55, and the products of the relative amplitude and the Strouhal numbers ranged from 0.114 to 0.344. Heat transfer was studied under the conditions of symmetric and asymmetric pulsations. The results of numerical simulation showed that an increase in the amplitude of pulsations led to an augmentation of heat transfer for all studied porosities. The maximum intensification of heat transfer was 43%. Symmetric pulsations were more efficient than asymmetric pulsations, with Reynolds numbers less than 25. The Thermal Performance Factor was always higher for asymmetric pulsations, due to the friction factor for symmetrical pulsations being much higher than for asymmetric pulsations. Based on the results of a numerical simulation, empirical correlations were obtained to predict the heat transfer intensification in porous media for a steady and pulsating flow.

Vortex-Pressure Fluctuation Interaction in the Outlet Duct of Centrifugal Pump as Turbines (PATs)

The outlet duct is the only outlet flow component of the pump as turbines (PATs). The flow state within it not only affects its operation stability but also influences the safe operation of downstream process equipment. The undesirable flow phenomenon of the vortex is the main reason for pressure pulsations and vibrations; therefore, it is important to adopt simulation and experimental methods to reveal the vortex-pressure fluctuation interaction characteristics in the outlet duct of PATs. Firstly, the spatial and temporal evolution of the vortex in the outlet duct under different operation conditions were compared based on the Q-vortex identification criterion. Subsequently, the frequency components corresponding to local high amplitude vorticity and pressure pulsations were clarified. Finally, the relationship between vortex evolution and the pressure pulsations in the outlet duct was established. The results showed that the flow rates of the turbine significantly affected the spatial and temporal evolution of the vortex rope in the outlet duct. The front chamber leakage flow and vortex shedding from the blade trailing edge also influenced the vortex distribution characteristics in the outlet duct. The dominant frequency of the pressure pulsation in the outlet duct was 6 fn under different operating conditions, and the amplitude of the pressure pulsation increased with the flow rates. The effect of vortex evolution on the local pressure pulsation characteristics decreased with increasing flow rates. The results can be used to improve and stabilize the operation and further optimization of PATs.