Magnetic Resonance Velocimetry Research Articles

Investigation on the Flow and Heat Transfer Characteristics in a Multi-Pass Channel Using Magnetic Resonance Velocimetry and Transient Thermochromic Liquid Crystal

Abstract The three-dimensional flow field in multi-pass channels with and without ribs was measured by magnetic resonance velocimetry (MRV), while heat transfer performance on the endwall of channels in the same geometry was investigated using transient thermochromic liquid crystal (TLC) technique. The evolution of comprehensive three-dimensional flow field and their correlation with local heat transfer enhancement on end wall of multi-pass channels with and without ribs were revealed as a whole picture. Results show that the flow characteristics in the right-angle bend as well as the second pass are dramatically different for the smooth and ribbed channels, resulting in totally different features of heat transfer distribution on the end wall in those two channels. For the smooth channel, strong dean vortices form around the bend region near the outer wall where heat transfer is enhanced substantially. For the ribbed channel, no dean vortex but complex three-dimensional flow presents around bends. Heat transfer downstream of ribs close to the reattachment regions is strengthened noticeably. Comparison between velocity and heat transfer results suggest that one of the principle mechanisms driving heat transfer enhancement is both endwall directed velocity for smooth and ribbed channels, even though they are induced by different flow structures.

Magnetic resonance velocimetry reveals secondary flow in falling films at the microscale

To design better falling film microreactors, concrete knowledge of the flow and the ability to produce complex channel structures are necessary. The efficiency of falling film microreactors is most often determined by studying the composition of the liquid after it has exited the system. In the present study, Magnetic Resonance Velocimetry is used to obtain three-dimensional, three-component velocity fields inside of falling films flowing in open microchannels. The microchannels were made by two-photon polymerization. It was possible to resolve the complex shape of the liquid–gas interface and to observe, for the first time, secondary flow inside of a falling film at the microscale.

Analytical solution for laminar entrance flow in circular pipes

This study introduces an analytical solution for the laminar entrance flow in circular pipes, aiming to confirm the occurrence of velocity overshoot. Velocity overshoot is characterised by the maximum axial velocity appearing near the pipe wall instead of the central axis. Similar to the previous studies, the analytical solution is derived from the parabolised Navier–Stokes equation; however, the specific approach used in linearising the momentum equation has not been attempted before. The accuracy of this analytical solution has been verified through a comprehensive comparison with various published experimental data. The existence of velocity overshoot at a short distance from the inlet, which is evident in numerous numerical calculations based on the full Navier–Stokes equations and corroborated by recent magnetic resonance (MR) velocimetry experiments, is identified analytically for the first time. The parabolised Navier–Stokes equation has inherent self-similarity with respect to the Reynolds number, implying that $Re$ is incorporated into the dimensionless variables rather than serving as an independent flow parameter. According to both MR velocimetry measurements and the present analytical solution, the self-similarity is not valid immediately following the pipe inlet, and this becomes more evident as $Re$ decreases; hence, the analytical solution derived from the parabolised Navier–Stokes equation cannot accurately predict the evolution of the velocity profile within this region near the pipe inlet.

Investigation of the effects of nasal surgery on nasal cavity flow using magnetic resonance velocimetry and computational fluid dynamics

The nasal cavity has the function of conditioning the air inhaled into the lungs by heating, humidifying, and filtering dust and virus-borne aerosols. Analyzing the flow field in the nasal cavity is vital because its function is strongly related to flow dynamics. Due to experimental limitations posed by the complex internal geometry of the nasal cavity, most previous studies have utilized Reynolds averaged Navier–Stokes based computational fluid dynamics (CFD) simulations. In this study, the flow field in a post-operative nasal cavity was evaluated using not only CFD simulations but also four-dimensional magnetic resonance velocimetry. The study was conducted under resting breathing conditions in pre- and post-operative models of a patient who received septoplasty and turbinoplasty. The experimental results confirmed balanced flow rates in the left and right nasal cavities after septoplasty and a decrease in velocity after turbinoplasty with a reduction in regions with vortices and reverse flow. Upon comparison, CFD results using the laminar, k–ω, and shear stress transport models were deemed to be consistent with the experimental results. However, there was a relatively large deviation observed with the k–ε model. Using the validated laminar CFD model, it was shown that the pressure and wall shear stress decreased after surgery.

Magnetic resonance imaging and velocimetry of ethane

This study investigates the experimental conditions required for magnetic resonance imaging (MRI) of thermally polarized hydrocarbon gas, focusing on ethane. The nuclear magnetic resonance (NMR) spectra and relaxation properties of ethane were analysed at different pressures in the range from 1.5 to 6 bar at 7 T using 1H NMR spectroscopy. The spin-lattice relaxation time (T1) and spin-spin relaxation time (T2) were measured, and their dependence on the pressure was determined, showing that both relaxation times increase with pressure. Using the estimated relaxation times, we adjusted parameters for imaging of static ethane using rapid acquisition with relaxation enhancement (RARE) and fast low-angle shot (FLASH). The signal-to-noise ratio (SNR) of ethane images was evaluated and compared to the calculation for the given range of pressures. Then, we imaged flowing gas using a 2D velocity-encoded pulse sequence, which is usually used for liquid flow studies. The MRI-measured flow rates are compared to those pre-set with a pump, showing good agreement in the slow flow range. Overall, the results provide insights into the feasibility of 1H MRI for imaging and flow measurements of thermally polarized ethane.

Quantitative analysis of colloid retention and pore fluid flow in monolithic, open-porous media by X-ray computed microtomography and magnetic resonance velocimetry

X-ray computed microtomography (μCT) is a state-of-the-art technique for the experimental investigation of colloidal transport and retention in porous media. However, quantitative analysis of the amount of retained particles in laboratory CT scanners is problematic due to the polychromatic spectrum of the X-ray sources used, so such investigations are predominantly carried out in synchrotron based scanners. Synchrotron sources are expensive to operate and the measurement time at these facilities is severely limited, so that extensive parameter studies cannot usually take place here.In this work, we present an experimental investigation of deep bed filtration processes in a laboratory scale, conducted in porous, monolithic ceramics, including the quantification of retained particles. Experimental insights were given by an innovative combination of μCT and magnetic resonance velocimetry (MRV). Image analysis was carried using a novel approach for detection and quantification of even very low concentrated colloidal depositions, hereby correcting for beamhardening artifacts. Axial deposition profiles are presented, revealing a hyper-exponential behavior, thus indicating unfavorable attachment conditions and a transport-length dependent distribution of the filter efficiency kf. Due to the monolithic, porous media, further evaluation of colloid deposition is based on individual pore objects. It was found that not all pores contributed equally to colloid deposition, numerous pores even remained empty. It is very likely, that those pores are not well incorporated in the porous network and thus cannot be reached by fluid flow. The structural data of the porous media and colloidal depositions was correlated with 3D velocity information. It could be shown that the filter efficiency for particle retention is a function of time and axial position of the individual pore, and changes over the duration of the process. At a certain point in the filtration process, the rate of change of the volumetric particle fraction per pore becomes negative, predominantly at the media's inlet, while it remains positive throughout deeper pore layers. Thus, the rate of change is governed by a complex relocation process along the porous medium.

Large amplitude oscillatory shear rheo-NMR velocimetry

Complex fluids display flow heterogeneities and a variety of nonlinearities under large amplitude oscillatory shear (LAOS). LAOS has developed as a means of characterizing nonlinear fluid rheology. In this study, the spatial velocity distribution during LAOS is measured by rheo-nuclear magnetic resonance (Rheo-NMR) velocimetry. Rheo-NMR is frequently used to measure responses of complex fluids to steady shear deformations by imaging local velocity across a fluid gap noninvasively. Rheo-NMR analysis of oscillatory flow has been more limited due to hardware limitations. Recently developed Rheo-NMR equipment makes LAOS Rheo-NMR possible. LAOS Rheo-NMR provides measurement of spatial velocity profiles as a function of oscillatory period, providing time dependent spatially resolved local velocity time series, which show distinct features dependent on material response.

Comparative full-field velocimetry of liquid flow within monolithic catalyst carriers via CFD simulations and MRV measurements

In reaction engineering, reactor performance can be improved in many cases by overcoming transport limitations. This requires detailed analyses of transport phenomena in the catalytic beds. Nuclear magnetic resonance (NMR) velocimetry measurements have been utilized for analyzing mass transport of gas flows within opaque monoliths. Comparisons to full-field computational fluid dynamics (CFD) simulations, however, show significant deviations. In this study, polyethylene glycol (PEG) and 3D-printed monoliths including one open-cell foam (OCF) and one honeycomb were used to demonstrate that both operating fluid and monolith morphology influence the achievable signal-to-noise ratio and resolution of NMR data. The velocity profiles measured by NMR in OCF agreed well with full-field CFD simulations with ± 5% deviation. In addition, the similarity between the simulated and experimental velocity fields was quantified by the similarity index, which is 1 for identical images. A mean value of 0.83 was determined for a 10 PPI OCF. Thus, using PEG as the operating fluid and a 10 PPI OCF allows to improve both spatial resolution by 34% and the quality of agreement by 13 percentage points compared to the published results of gas velocimetry within 20 PPI OCF. We further identified and quantified possible sources of deviation between CFD and MRV velocity fields. By limiting our analysis to velocities higher than 45% of the maximum velocity, we could achieve similarity indices of 0.95–0.99.

Investigation of flow behavior in a refrigerator machine room using magnetic resonance velocimetry

This study aims to improve the energy efficiency and reduce flow noise in a refrigerator machine room by investigating the flow behavior using magnetic resonance velocimetry (MRV). A 1/2 scale model of the machine room was fabricated using stereolithography 3D printing, and the 3-dimensional mean velocity and turbulent kinetic energy (TKE) field were measured. Results from the reference model showed highly non-uniform flow distribution and high TKE regions. An improved machine room geometry was proposed, which led to an increase in mean flowrate through the condenser by 17.3% and a reduction in high TKE regions by over 77%, potentially resulting in improved energy efficiency and reduced turbulence-induced noise. These findings demonstrate the effectiveness of MRV in analyzing the flow behavior of complex 3D geometry such as the refrigerator machine rooms. The proposed improvements, including repositioning of flow-blocking pillars, relocating refrigerant pipes, changing inlet configurations, and increasing the area of inlet and outlet grilles, were validated using MRV. The results also confirm the accuracy of the MRV measurements, which were validated using an electromagnetic flowmeter. These findings have implications for the practical application of cooling systems and highlight the potential of MRV for future research in analyzing fan noise and applying MRV to models with built-in fans.

Magnetic Resonance Velocimetry for porous media: sources and reduction of measurement errors for improved accuracy

Nuclear Magnetic Resonance (NMR) is a powerful tool for noninvasively investigating static and dynamic properties of fluids inside complex and opaque structures. Thus, Magnetic Resonance Velocimetry (MRV) allows for the in situ analysis of the local flow velocity of fluids. Such an analysis characterises mass transport properties and helps to validate or improve numerical predictions for porous media. The benefit of such validations is lowered by several problems worsening the measurement accuracy. This publication addresses systematic errors and the influence of noise, which may reduce the accuracy of MRV measurements. Techniques are described to minimise or correct displacement and phase errors. A new multi-echo MRV-sequence is proposed as a compromise between Velocity-to-Noise Ratio (VNR), total measurement time, spatial resolution and displacement errors. For improved VNR, a dual-VENC encoding scheme was used and repetition time T_{text{R}} was optimised. As an example, the proposed MRV sequence was used to measure the three-directional flow velocity of water in an Open Cell Foam (OCF) with 10 pores per inch with three-dimensional isotropic spatial resolution.

Resolving the three-dimensional flow field within commercial metal additive manufacturing machines: Application of experimental Magnetic Resonance Velocimetry

Resolving the three-dimensional flow field within commercial metal additive manufacturing machines: Application of experimental Magnetic Resonance Velocimetry

Magnetic Resonance Velocimetry Measurements of Internal Blade Cooling Flow and Computational Fluid Dynamic Validation by Data Matching With the Experimental Data

Abstract The optimal Reynolds-averaged Navier–Stokes (RANS) turbulence model to be used in a Computational Fluid Dynamics (CFD) simulation varies depending on the application. Conventionally, the model is selected from benchmark tests and experience, but its performance is difficult to predict. For this reason, this study presents a cost-effective CFD validation routine, which uses three-dimensional experimental velocity data obtained in replicas of the specific flow system. Magnetic Resonance Velocimetry is used as the measurement technique. Since the objective is only the validation of the turbulence model, the experiment and the simulation are performed with simplified flow conditions, hence stationary isothermal isovolumetric flow without inertial forces. The routine applies a data-matching routine to align the two three-dimensional data sets before they are interpolated on a common grid. Various error metrics are presented, which provide the degree of the CFD modeling error and indicate its source. For demonstration, the validation routine is used to evaluate RANS-CFD results of a three-pass internal cooling system of a high-pressure turbine airfoil used in a small industrial gas turbine. The simulations are performed with the eddy-viscosity-based turbulence model k–ω shear stress transport (SST), the Reynolds-stress Speziale, Sarkar and Gatski (SSG), and baseline-Explicit algebraic Reynolds stress model turbulence (BSL-EARSM) models. The results indicate strong local errors in the examined turbulence models. None of the models performed well enough, underlining that every RANS-CFD application needs to be validated.

Investigation of the complex 3D flow structure within a selective catalytic reduction (SCR) reactor of a coal-fired power plant

A selective catalytic reduction (SCR) reactor is commonly used to remove nitrogen oxides (NOx) from coal-fired boilers. Uniformity of the flow passing through the catalyst layer is important for increasing denitrification (de-NOx) efficiency. In order to examine flow uniformity, this study conducted an experimental and numerical analysis of the complex internal flow within a realistic SCR model. Magnetic resonance velocimetry (MRV) was utilized to obtain non-invasive measurements of three-dimensional three-component average velocity and validate Reynolds-averaged Navier-Stokes (RANS) numerical simulations. The computational results showed similar overall flow structure compared with the MRV results. Parameters representing flow quality such as relative standard deviation (RSD) and recirculation zone strength (RZS) were calculated by integrating the flow field. These parameters have the largest value after the straightener area and decrease towards the catalyst reactor, and are not significantly affected by Reynolds number upstream of the catalyst layer. The recirculation zone size was analyzed using spanwise uniformity and skewness indicators. As the recirculation zone induces biased flow, the non-reacted NOx concentration was more prominent in the outlet zone opposite of the recirculating area in the corresponding actual on-site SCR reactor. Based on this finding, a meaningful correlation between flow maldistribution and de-NOx reaction could be deduced.

Optimal control flow encoding for time-efficient magnetic resonance velocimetry

Phase contrast velocimetry relies on bipolar gradients to establish a direct and linear relationship between the phase of the magnetic resonance signal, and the corresponding fluid motion. Despite its utility, several limitations and drawbacks have been reported, the most important being the extended echo time due to the encoding after the excitation. In this study, we elucidate a new approach based on optimal control theory that circumvents some of these disadvantages. An excitation pulse, termed FAUCET (flow analysis under controlled encoding transients), is designed to encode velocity into phase already during the radiofrequency excitation. As a result of concurrent excitation and flow encoding, and hence elimination of post-excitation flow encoding, FAUCET achieves a shorter echo time than the conventional method. This achievement is a matter of significance not only because it decreases the loss of signal due to spin–spin relaxation and B0 inhomogeneity, but also because a shorter echo time is always preferred in order to reduce the dimensionless dephasing parameter and the required residence time of the flowing sample in the detection coil. The method is able to establish a non-linear bijective relationship between phase and velocity, which can be employed to enhance the resolution over a specific range of velocities, for example along flow boundaries. A computational comparison between the phase contrast and optimal control methods reveals that the latter’s encoding is more robust against remnant higher-order-moment terms of the Taylor expansion for faster voxels, such as acceleration, jerk, and snap.

Characterization of unsteady flow in a 3D‐printed Schwarz Diamond monolith using magnetic resonance velocimetry

AbstractProcess engineering applications such as heat transfer, reactions, and separations involve passing fluid through a porous medium. Historically, random‐channel porous media have been used for these operations. Such systems do not represent optimal configurations for process performance because of poor flow distribution and high‐pressure drop. It is now possible to fabricate porous monoliths with tailored morphology and regular channel structure using 3D‐printing. In this work, we use magnetic resonance imaging to study flow through a Schwarz Diamond triply periodic minimal surface (TPMS) monolith for Reynolds numbers up to 350. A transition to unsteady flow was observed experimentally for the first time. The channel structure diverts flow such that free shear layers form in the channel centers that contribute to flow instability. These measurements serve to inform the design of novel transport processes with enhanced performance.

MRV challenge 2: phase locked turbulent measurements in a roughness array

Magnetic resonance velocimetry (MRV) provides capabilities to measure three velocity components across three-dimensional fields of view without the need for optical accessibility. Predominant usage of the technique in engineering applications centers on steady flows and the discussion of a single time-averaged set of data. In this second MRV Challenge comparison, phase-locked MRV measurements of a pulsatile flow are conducted by five teams from across the globe. A geometry is explored which consists of turbulent flow past six identical cubic elements placed in the center of a square water channel, with partial elements along each side of periodically varying heights. A pulsatile injection flow through the floor between the second and third cubic roughness elements creates a three-dimensional jet that interacts with the complex cube wakes. The details of each scan sequence are summarized for the teams. Results are compared for three different time windows—one pre-injection, a second while the injection flow accelerates, and a third during a quasi-steady condition with the injector fully on. Finally, the influence of the temporal resolution selection for the phase locked MRV is discussed. The results are remarkably similar despite the complex flow configuration selection and showcase a relatively underutilized capability to obtain time-dependent, volumetric data for periodic flows through three-dimensional geometries using MRI. Recommendations for best practices are also provided.

3D magnetic resonance velocimetry for the characterization of hydrodynamics in microdevices: Application to micromixers and comparison with CFD simulations

3D velocity field measurements in an arrow-type micromixer are performed using nuclear magnetic resonance (NMR) imaging methods. A specific device has been set up including a home-made radiofrequency milli-coil matching the geometry of the manufactured micromixer, allowing an improvement of the signal-to-noise ratio by a factor 7 comparing to a standard commercial hardware. The NMR images obtained are compared to computational fluid dynamics (CFD) simulations. The very good agreement obtained between numerical data and experimental measurements revealed, on the one hand, the capacity of the developed method to perform a detailed and accurate study of phenomena occurring in a microstructured system and on the other hand, the great impact of small manufacturing defects on the hydrodynamics in a micromixer and thus on the mixing performances.

Magnetic Resonance Imaging and Velocimetry of Ethane

Magnetic Resonance Imaging and Velocimetry of Ethane

Assessment of the Flow Field and Heat Transfer in a Vane Cooling System Using Magnetic Resonance Velocimetry, Thermochromic Liquid Crystals, and Computational Fluid Dynamics

AbstractComputational fluid dynamics (CFD) is the standard tool in the turbomachinery industry to analyze and optimize internal cooling systems of turbine components, but the code applied has to be validated. This paper presents a combined experimental and numerical study on the flow field and heat transfer in a cooling system consisting of a three-pass serpentine with rib turbulators and trailing edge ejection. The cooling geometry is taken from a stator vane currently used in an industrial gas turbine and operates at a coolant inlet Reynolds number of 45,000. As an experimental technique, magnetic resonance velocimetry (MRV) was used to obtain the three-dimensional time-averaged velocity field of the isothermal flow. The measurements were conducted in a large-scale model and resulted in 3.2 million velocity vectors and measurement uncertainty of 6.1% of the bulk inlet velocity. The local wall heat transfer was measured in a separate experiment using thermochromic liquid crystals (TLC). These measurements yielded the distribution of the heat transfer coefficient on both the pressure and the suction side internal walls with a measurement uncertainty of 12%. The experimental data are used as a reference for the numerical study. In total, eight turbulence models are evaluated here, including one-equation, two-equation, algebraic and differential Reynolds stress models, and a scale adaptive simulation. The results show the differences between the velocity fields and the heat transfer coefficient distribution, allowing for the identification of the optimum turbulence model for this particular type of flow.

Magnetic Resonance Velocimetry of a Turbine Blade With Engine-Representative Internal and Film Cooling Structures

Abstract As a key technology to ensure turbine survival, blade cooling encompasses a whole range of strategies with ever-increasing geometric complexities. Flow measurement for turbine blades with such intricate internal and external cooling structures is very challenging and calls for non-intrusive, three-dimensional measuring techniques. As a response, this work utilizes magnetic resonance velocimetry (MRV) to measure the velocity field in a scaled turbine blade with engine-representative internal and film cooling structures. Internal cooling structures include leading edge impingement cooling, U-shaped serpentine passage with/without turbulence ribs at blade mid-chord, and trailing edge pin fins. External cooling structures include film holes near the leading edge stagnation point, at the blade tip, and on the trailing edge. Experiments were performed in water and the Reynolds number local to the leading edge, mid-chord, and trailing edge cooling channels falls within the range typically reported in the literature. This is the first time that MRV is used to measure the flow field of a turbine blade with all the typical internal and external cooling geometries combined. The results show that MRV has great capacity in measuring the complex fluid flow associated with blade cooling designs. Vortical flow features in leading edge impingement cooling, and at the U-bends of mid-chord serpentine channel are captured. Interestingly, internal flow around pin fins redistributes the velocity of external flow ejected from trailing edge slots and film holes, indicating strong coupling between the internal and film cooling flow of the turbine blade.