Non-uniform Velocity Distribution Research Articles

Application of the magnetic tracer-tracking system in solids circulation measurement in a fluidized bed standpipe

In the present study, the application of a magnetic tracer-tracking method in measuring solids circulation in a fluidized bed standpipe is investigated, due to its advantages of little intervention and cost efficiency, especially in pressurized systems. The method only needs to inject one small magnetic tracer to follow the main solid flow in the standpipe, therefore predicting particles’ real-time velocities. The measurement accuracy was thoroughly tested via comparing to conventional descending and accumulation methods. Main tracer properties, including tracer shape, density, and magnet core, were considered. Solids flow patterns in the standpipe were also regulated by changing orifice sizes and adding an inclined pipe, for the purpose of investigating the measurement accuracy in various conditions. The adverse effect of a narrow orifice on measurement was addressed via constructing a model that includes sand particles’ non-uniform velocity distribution across the standpipe cross-section. To interpret behaviors of tracers varied in size and density, a mathematical model was constructed to describe forces exerted on the tracer in the solids bed. The behaviors of the tracer immersed into the solids bed were also examined, providing an insight to that in a standpipe with continuous solids circulation. The solids bed density was also regulated by varying the mixture of olivine sand and carbonaceous particles at different proportions. The magnetic tracer-tracking method has been successfully validated, demonstrating good measurement accuracy of solids discharge flow rates in the standpipe, particularly avoiding cumbersome calibration. Moreover, the method can also determine sand waving and oscillated discharge behaviors, which might be related to solids’ stick–slip phenomena and is unlikely to be accurately determined using conventional descending and accumulation methods.

Thermal recycling analysis in regenerative cooling channels based on liquid rocket engine cycles

For rocket cycles involving electric pumps, gas generators, expanders, and staged combustion, a thermal recycling method is proposed to provide a more realistic simulation of regenerative cooling systems in liquid rocket engines. This method simulates a real situation where the outlet of the cooling channel is thermally recycled through the turbine or preburner to the inlet of the combustion chamber. Supercritical fluid properties are determined using the extended Redlich–Kwong (RK)–Peng–Robinson (PR) real-fluid equation of state. The axisymmetric reacting flows of a thrust chamber with regenerative cooling channels are simulated using the flamelet-based lookup table. To account for the multi-injector effect in the two-dimensional axisymmetric simulation, a non-uniform velocity distribution is implemented, utilizing exponential distributions of each injector’s mass flow rate. For the hot gas temperature, coolant temperature, and cooling mass flowrate, the thermal recycling method is comparatively analyzed with the thermal decoupling method. The regeneration effect of the heated fuel is explored by evaluating the inflow energy, reaction energy, wall heat transfer, and the exit kinetic energy conversion. The thermal recycling method is developed for regeneratively cooled rocket engines with the expander, gas generator, staged combustion, and electric-pump cycles. Through this numerical procedure, the thermal recycling method is successfully applied to a liquid oxygen/methane engine and NASA’s Crew Exploration Vehicle nozzle with two types of cooling. Based on the results of the four types of rocket cycles, it is confirmed that the specific impulse increases by 1.5–2% due to the regenerative heat effect.

An Insight into Perfusion Anisotropy within Solid Murine Lung Cancer Tumors.

Blood vessels are essential for maintaining tumor growth, progression, and metastasis, yet the tumor vasculature is under a constant state of remodeling. Since the tumor vasculature is an attractive therapeutic target, there is a need to predict the dynamic changes in intratumoral fluid pressure and velocity that occur across the tumor microenvironment (TME). The goal of this study was to obtain insight into perfusion anisotropy within lung tumors. To achieve this goal, we used the perfusion marker Hoechst 33342 and vascular endothelial marker CD31 to stain tumor sections from C57BL/6 mice harboring Lewis lung carcinoma tumors on their flank. Vasculature, capillary diameter, and permeability distribution were extracted at different time points along the tumor growth curve. A computational model was generated by applying a unique modeling approach based on the smeared physical fields (Kojic Transport Model, KTM). KTM predicts spatial and temporal changes in intratumoral pressure and fluid velocity within the growing tumor. Anisotropic perfusion occurs within two domains: capillary and extracellular space. Anisotropy in tumor structure causes the nonuniform distribution of pressure and fluid velocity. These results provide insights regarding local vascular distribution for optimal drug dosing and delivery to better predict distribution and duration of retention within the TME.

Investigation of fluid flow during flow boiling inside a horizontal rectangular channel with single-sided heating using particle image velocimetry

Subcooled flow boiling is a highly efficient cooling systems for thermal management systems. This study explores the intricate dynamics of subcooled flow boiling within a horizontal channel, investigating the impact of vapor generation on liquid-phase velocity using Particle Image Velocimetry (PIV) and advanced image processing techniques. Four mass flow rates ranging from 5–20 g/s with subcooled inlet conditions are investigated in a rectangular channel with single-sided heating. Three regions of interest along the heated channel are investigated for instantaneous PIV analysis. The PIV system captures detailed velocity profiles, illustrating the impact of varying mass flow rates and heat flux levels on flow behavior. Vapor masking techniques are introduced to enhance the precision of PIV data by mitigating interference from the vapor phase. Results demonstrate the influence of vapor bubbles on flow resistance, revealing non-uniform velocity distributions and turbulence near the liquid–vapor interface. The study emphasizes the critical role of inertia and buoyancy forces in shaping the velocity profiles. Moreover, the investigation sheds light on the effects of flow rates on the interfacial behaviors, hinting at a transition point between 10 and 15 g/s. In summary, this research contributes valuable insights into the nuanced dynamics of flow boiling, laying the foundation for future studies on turbulence, heat transfer, and phase-change phenomena in two-phase thermal management systems.

“Turbulence effect on total mechanical energy budget and energy loss of turbulent flows with different hydraulic regimes in open-channel transitions”

ABSTRACT Transitions are needed in hydraulic structures which are flumed. In expanding transitions, flow separates from the boundary if the angle of divergence exceeds about 5 degrees. The present paper gives an insight into the turbulence effect and energy loss in expanding transition with small angle of divergence. In wide angle expansion, flow separates from the boundary resulting in highly non-uniform velocity distribution at the exit of expansion resulting in erosion of down stream channel. Author performed experiments to control separation of flow in wide angle expansion. The paper discusses about the devices used by the author to control separation in wide angle expanding transition.

Theoretical performance of a 1.5-µm satellite-borne coherent Doppler wind lidar using a planar waveguide optical amplifier with a demonstrated figure of merit: simulation of signal detection probability, measurement precision, and bias.

We report performance of a satellite-borne coherent Doppler wind lidar (SCDWL), which equips a planer waveguide amplifier (PWA) operating in a wavelength of 1.5 µm. The performance is defined by detection probability, measurement precision, and bias, and is characterized with a Doppler wind lidar (DWL) simulation that considers a realistic wind velocity profile, and instrumental and atmospheric parameters. Among the parameters, we carefully model those related to the PWA whose figure of merit has great impact on the performance of SCDWL and has shown rapid improvement in recent years. Moreover, we introduce three models for a backscattering coefficient (high, moderate, and low) to assess the influence from variation of atmospheric backscattering. Our simulation demonstrates that the SCDWL can work with reasonable performance for the target altitude of 6 km in the case of the high-backscattering model. The simulation also exhibits that the SCDWL can observe wind velocity at the altitude of 12 km if improved instrumental parameters or higher backscattering coefficients are considered. In addition, we reveal that non-uniform wind velocity distribution degrades the performance of the SCDWL and induces a bias between measured and real wind velocity.

Research on optimization technology of excitation coil in downhole annular flow electromagnetic measurement system

External interference and non-uniform velocity distribution influence the downhole annular flow electromagnetic measurement, reducing detection accuracy. Optimizing the structure of the excitation coil can effectively enhance the uniformity of magnetic field distribution, constituting an essential approach for improving measurement accuracy. Aiming at the excitation coil optimization technology of annular flow electromagnetic measurement system, the study first analyzes and obtains the magnetic field model of the excitation coil based on the principle of annular flow electromagnetic measurement, combined with the theory of vector weight function and puts forward the optimization theory of annular excitation coil. Secondly, the simulation models of rectangular and saddle exciting coils are established using finite element software to analyze the magnetic field distribution of annular flow under different coil and core sizes. The optimal structural parameters of the exciting coils in the annular flow electromagnetic measurement system are determined through evaluation indexes for the magnetic field. Finally, an experimental platform was established for optimizing the electromagnetic measurement of annular flow, and this platform was used to conduct comparative experiments. The experimental results show that the maximum reduction of the optimized system is 3.192%, 1.289%, and 1.813% for the single verification relative value error, single-point verification relative value error, and repeatability compared with the rectangular excitation coil annular flow electromagnetic measurement system. In the flow measurement interval of 25–100 m3 h−1, the single-point verification of the optimized system has a maximum relative display value error of 0.378%, and the measurement accuracy reaches 1.0 level and meets the consistency of the transmission of quantity and value. The research findings hold significant guidance for enhancing the measurement accuracy of the downhole annular flow electromagnetic measurement system.

INVESTIGATING THE SUITABILITY OF MULTI-SCROLL VOLUTES FOR IMPROVING SPANWISE INCIDENCE OF MIXED FLOW TURBINE ROTORS WITH VARYING BLADE CONE ANGLES IN AUTOMOTIVE TURBOCHARGING APPLICATIONS

Abstract A Mixed Flow Turbine (MFT) is not constrained to a radial inlet blade angle, providing additional design freedom. As the MFT leading edge varies in radius, the spanwise incidence angle varies, leading to additional separation on the blade suction surface (SS) near the hub because of increasingly positive incidence, most noticeable at off-design conditions. A multi-scroll volute was previously paired with a 45° blade cone angle (Λ) MFT which generated non-uniform spanwise flow that improved off-design efficiency at the cost of peak efficiency. The current study identified the range of blade cone angles that benefitted from a multi-scroll volute to reduce incidence at the hub region. A numerical investigation was conducted which determined the influence a multi-scroll volute can have on MFTs with different blade cone angles. When the MFT with large blade cone angle (Λ=60°) was paired with a multi-scroll volute, the efficiency improved by 2.2%pts at design and 0.5%pts at off-design conditions. The incidence improved and mass flow rate increased at the hub region. The MFT with smaller blade cone angle (Λ=30°) had an efficiency loss of 1.3%pts at design and 0.8%pts at off-design conditions when paired with a multi-scroll volute due to increased incidence within the hub region. The multi-scroll volute concept was shown to offer efficiency improvements for MFTs with larger blade cone angles through better management of the non-uniform spanwise velocity distribution at rotor inlet.

A Simulation and Experiment on the Optimization Design of an Air Outlet Structure for an Air-Assisted Sprayer

In response to the issues of low-velocity zones and non-uniform jet velocity distribution in the airflow field of traditional air-assisted orchard sprayers, an arc-shaped air outlet suitable for axial-flow air-assisted systems is designed. This article employs the method of CFD numerical simulation and experimental verification to compare and analyze the internal flow field of the air-assisted system and validates the reliability of the numerical simulation results through calculation error and chi-square test. The wind speed of the cross-section is measured at different distances from the outlet, and the distribution characteristics of the outflow field wind speed before and after the structural optimization of the air-assisted system are compared. The horizontal distribution of fog droplets is collected using a fog collection chamber. The experimental results show that the design of the arc-shaped outlet increases the average wind speed of the annular outlet from 14.95 m/s to 18.20 m/s and reduces the proportion of low-speed area from 20.83% to 0.71%. When the rounded corner radius of the air outlet is 50 mm, optimal parameters are attained. The maximum error between the simulated and experimental values is 9.52%. At a significance level of 0.05, the χ2 value is 0.252, indicating that the simulated values follow the distribution of the actual measurement values. On the cross-sections located at distances of 0.5, 0.75, 1, 1.25, and 1.5 m from the air outlet, the wind speed distribution with no arc-shaped air outlets exhibits a “low left and high right” type, tending to shift towards the right as a whole. Fog droplets also display a drift tendency towards the right side. The wind speed distribution with arc-shaped air outlets shows a symmetric “high in the middle and low on the sides” type. Fog droplets concentrate in the central position. The optimized air-assisted system can reduce the air field’s low-flow area, increase the airflow distribution uniformity, improve the average wind speed at the outlet, and decrease fog droplet drift. This provides a reference for the structural design of air-assisted systems in current orchard sprayers of the same type.

3D reconstruction of nonuniform and unsteady flow velocity distribution using nonlinear acoustic ray tracing and tomography

The nonuniform and unstable gas flow distribution at the air inlets of the boiler combustion chamber can negatively affect the energy efficiency and safety of the boiler. Accurately reconstructing the flow velocity inside the furnace in three dimensions can help the boiler automatically adjust the air supply to reduce unwanted energy waste and gas emissions.In this study, we propose a novel method for three-dimensional velocity reconstruction based on nonlinear acoustic ray tracing and tomography. The model provides the equation of acoustic ray motion in the flow field, which includes spatial and temporal parameters. During the process of measuring flow velocity, higher precision reconstruction of the three-dimensional velocity distribution was achieved by combining acoustic refractive paths based on derivative-free optimization iteration updates with radial basis functions that use Gaussian kernels.The computational time cost and reconstruction accuracy of the proposed algorithm were analyzed numerically and through simulations. The results indicate that the method incorporating acoustic ray tracing greatly improves the reconstruction accuracy of large gradient nonuniform flow fields. Additionally, we analyzed the impact of different numbers of sound tracing paths on reconstruction quality and computation time, providing a reference for the rational placement of microphones and the selection of the most efficient paths in boilers.Experimental results show that this method has good reconstruction capabilities for nonuniform flow fields and practical applications.

Effects of unstable flow structures on energy transfer mechanism in a centrifugal pump

To reveal the energy loss mechanism of the centrifugal pump, numerical simulation and experimental investigation are conducted to obtain the complex flow field of a single-stage centrifugal pump under various flow conditions. Particular emphasis is focused on the qualitative and quantitative analysis of the distribution and variation characteristics of irreversible loss in the pump model. The results show that the energy loss in the centrifugal pump mainly originates from the entropy generation caused by the turbulent dissipation and wall friction, which are typically generated in the volute and impeller domains. It is worth noticing that the energy loss in the volute is closely associated with non-uniform velocity distribution and the evolution of the shedding vortices from the impeller exit whilst the energy loss in the impeller are greatly affected by unstable flow phenomena such as flow separation, backflow, and jet-wake pattern. At the overload operating conditions, the wall entropy generation possesses a substantial influence on energy loss, which is mainly related to the wall shear stress. Meanwhile, the influence of the rotor-stator interaction and inflow impacting on the energy loss is enhanced with increasing flows. Finally, the omega method captured the vorticity structures near the tongue at partial flow conditions, thereby, revealing the relationship between the high magnitude of flow loss and the evolution of different scales of strong vorticity sheets.

A semi-analytical model for predicting outflow concentration of vented turbidity currents with application in the Xiaolangdi reservoir

Turbidity current venting is an important way to reduce reservoir sedimentation. Existing methods to predict the outflow concentration of vented turbidity currents either depend on three-dimensional (3D) modelling with high computational cost or semi-empirical equation with a simple assumption of uniform velocity distribution. This study presents a semi-analytical model for predicting outflow concentration in turbidity current venting events, which accounts for the non-uniform and asymmetric velocity distribution in the withdrawal layer. Firstly, a 3D numerical model is established to facilitate the determination of withdrawal layer thickness and extraction of flow velocity within it, which is validated by simulating a group of venting experiments in the literature. Then, a velocity distribution equation for turbidity currents insusceptible to downstream disturbance is found to be applicable in the withdrawal layer, after relating the parameter controlling the velocity profile shape with the ratio of the outlet height to the layer thickness. Finally, the equation is derived for the relative concentration of outflow to the turbidity current concentration before the dam, whose solution is dependent on the proposed velocity distribution. Field measured data in the Xiaolangdi Reservoir and operation records of sediment release structures were collected and used to validate the model. A concept of nominal outlet width is proposed in order to apply the model in reservoirs with complex water releasing structures, which is calibrated along with the dimensionless parameter in the withdrawal layer thickness equation. The proposed model can help to optimize the design of sediment release structures and improve the venting operation.

Turbine vane endwall aerothermal and film cooling performance considering realistic swirling inflow conditions

The turbine vane inlet downstream of the lean premixed combustor presents nonuniform distributions of velocity and temperature. For obtaining swirling inflow profiles close to reality, the simulated non-reacting combustor is designed firstly. Applying these profiles as inflow boundary conditions, the effects of realistic swirling inflow on the turbine vane endwall aerothermal characteristics and film cooling performance are numerically investigated through solving the three-dimensional Reynolds-Averaged Navier-Stokes equations coupled with the shear stress transfer ([Formula: see text]) turbulence model. Two swirling orientations (anticlockwise and clockwise) and five swirling core pitch-wise positions (aligned with vane 1 to vane 2) are considered in the current work. The results indicate that the residual vortices in the vane passage are strengthened and move with the swirling core along the pitch-wise direction. The migration of the horseshoe vortex is controlled by this movement. The shrinkage or expansion of the separation line of the horseshoe vortex can be observed under the anticlockwise and clockwise swirling inflow conditions respectively. The anticlockwise swirling inflow results in a larger aerodynamic loss by a 10%–35% increase of the laterally [Formula: see text]. The high Nu region near the pressure side surface enlarges and the area-averaged Nu at [Formula: see text] increases from 2337.9 to 2878.3. For the cases with clockwise swirling inflow, the area of the hot ring is enlarged and the Nu downstream of the row 3 film holes is decreased. As for the film cooling performance, the endwall coverage area shrinks and the phantom cooling area enlarges when the anticlockwise swirling core is aligned with vane 2. The endwall loses the protection from the row 3 film holes and the cooling failure ([Formula: see text]) occurs at [Formula: see text] when the swirling core is aligned with the vane passage. This is an extremely bad phenomenon that should be avoided. Among all cases, the highest endwall area-averaged [Formula: see text] (0.122) is obtained when the clockwise swirling core is aligned with vane 1. The largest endwall coverage area is achieved when the clockwise swirling core is aligned with vane 2.

Examination of effective strategies on changing the amount of heat transfer and entropy during non-Newtonian magneto-nanofluid mixed convection inside a semi-ellipsoidal cavity

Due to the application importance of convection flows under the impact of external fields such as magnetic field for non-Newtonian nanofluids, in cases such as nuclear processes, technologies related to lubrication, production and processing of food products, the modeling of these types of currents is very important. The existing research inquires the influence of heat production/absorption on the characteristics of non-Newtonian flow and entropy formation caused by convection heat transfer. For this purpose, the phenomenon of mixed convection inside the semi-elliptical chamber containing a power-law nanofluid is presumed in such a way that the Brownian motion of nanoparticles is considered. The curved and vertical walls of the enclosure are cold and under three different types of heating, respectively. The cold wall has no speed while the chamber vertical wall has a non-uniform velocity distribution (cosine) with a phase difference. The main goal of the research is to depict the impact of changing the position of applying the magnetic field and changing the phase difference of wall movement on the fluid flow. Flow modeling was accomplished via the utilization of lattice Boltzmann method and the accuracy of the findings was affirmed by comparing previous related studies. According to the research findings, enhancing the mean Nusselt number has a direct relationship with enhancing the phase difference. In addition, by enhancing the phase difference to 180, the mean Nusselt number up to 46% more is acquired, the influence of magnetic field applying becomes more conspicuous. In addition to the fact that the enhancing in Richardson number leads to increment of in the mean Nusselt number value and flow power, the effect of varying the phase difference becomes more noticeable with the enhancing in Richardson number. Enhancing in the Hartmann number, index of power-law and decreasing in the heat production/absorption coefficient lead to decrease in the entropy value and the average Nusselt number. By changing the position of applying the magnetic field, a current with various features can be acquired. Based on the results, it was demonstrated that changing the type of wall heating along with changing the heat absorption/production coefficient are very useful strategies in controlling the system thermal characteristics.

Reconstruction of Three-Dimensional Structures of Electrohydrodynamic Flows Induced by Blade-Plane Geometry

Particle image velocimetry (PIV) method has emerged as a powerful technique for electrohydrodynamic (EHD) flow field measurements. Based on a series of parallel field measurements along the blade electrode, we reconstruct the three-dimensional (3D) structure of EHD flows produced by an actuator in the blade-plane geometry. A non-uniform distribution of injection intensity and velocity is observed along the electrode. With the aid of slice plots from different views of 3D Cartesian, it is understood that the applied voltage, electrode spacing and the electrode surface or tip roughness exert a remarkable effect upon the structure of EHD jets. The findings of this investigation will complement the full view of the 3D structure of the jet and be of broad use to guide the selection of appropriate positions along the blade for profile plane studies.

A novel approach to multi-path ultrasonic transit time flow meter based on measurement model analysis to improve accuracy

Ultrasonic transit time flow meter (UTTFM) is commonly used in a wide range of applications. It is commonly believed amongst researchers, industrialists, and standard committee members that due to nonuniform flow velocity distribution inside the pipe (flow velocity profile) the measured flow velocity using UTTFM needs to be corrected by the flow profile. Mathematical analysis of UTTFM measured quantity shows that UTTFM measures flow correctly when flow is fully developed or laminar. However, the measurement results using flow profile correction factor produces erroneous values. The UTTFM measurement model assessment shows, when flow is not fully developed, there are unknown quantities contributed by flow velocity in the axial and diametrical direction to measurement results. These unknown quantities lead to erroneous measurement results when it is simply corrected by flow profile. Assessment of the UTTFM error model shows that using multi-path UTTFM can significantly reduce the impact of the unaccounted quantities and improve accuracy. A novel approach to UTTFM design utilizing multiple acoustic paths (using different planes and transmitting angles) is proposed to reduce potential error for UTTFM. This approach is consistent with the general measurement modeling method with incomplete information recommended by JCGM GUM-6:2020.

Effect of variable viscosity of electrolytes on mass transport and electrochemical reactions in aqueous organic redox flow battery

In this work, the viscosity of an organic electrolyte solution is quantified and its effect is investigated on the performance of an aqueous organic redox flow battery (AORFB). For a viologen derivative, the solution viscosity is found to be highly dependent on both the state of charge (SOC) and the concentration, which can be attributed to the strong intermolecular interaction in solution. The correlation between the viscosity and the SOC is then developed for different concentrations of the electrolytes. A 2D model with the effect of variable viscosity is developed to elucidate the mass transport and electrochemical reaction processes in the flow battery. It is found that the variable viscosity yields non-uniform distribution of the local velocity and concentration in the porous electrode and lowers the performance of the flow battery. Based on the above findings, the electrode porosity is optimized and a gradient porosity is developed to match the mass transport with the electrochemical reaction in flow batteries. This work not only elucidates the key role of viscosity in the flow battery for the first time, but also provides a rational design of the electrode structure to balance the mass transfer and electrochemical reactions.

Numerical analysis of the flow field and cross section design implications in a multifunctional artificial reef

Nowadays, multifunctional artificial reefs are integrated in coastal areas all around the world. The design of such structures is currently complex and subjective. In this context, it is essential to improve overall design approaches to more effectively relate artificial reef geometry, function and optimal performance to specific deployment sites. Part of the solution to this challenge may lie in the use of in situ data to study the hydrodynamic performance of prospective artificial reefs. This research addresses this issue by performing a numerical investigation of the flow transformation of two similar artificial reef geometries, and the analysis of performance indicators based on (i) artificial reef assembly cross section, (ii) upwelling and (iii) wake regions evaluation, (iv) efficiency indices and (v) streamlines particles. Based on typical data related to wave action, a velocity inlet boundary condition was defined adopting the non-uniform velocity distribution, aimed at simulating the most realistic boundary condition at the chosen deployment location. The results showed that the multifunctional artificial reef assembly with the droplet shape cross section exhibited enhanced function performance when compared to a circular shape cross section by providing significantly higher values of the upwelling velocity, wake region and associated efficiency indices. In addition, the procedure presented in this study, which considers oceanographic data at the deployment site, the geometry of the artificial reef, in situ flow characteristics and boundary conditions, as well as the devised fluid flow performance indicators, can be used for the design of artificial reefs during the concept stage in an objective manner to consider the local design requirements.

Thermal management of GaN HEMT devices using subcooled flow boiling in an embedded manifold microchannel heat sink

Ultra-high heat flux thermal management has been an urgent demand for high electrical performance and reliability of GaN HEMT electronic devices, and direct fabrication of embedded heat sinks have proven to be an effective approach for its thermal management. In this work, a self-developed solver based on OpenFOAM has been employed and verified for numerical investigations of three-dimensional two-phase flow coupled with conjugate heat transfer. An extremely uneven heat source distribution in GaN HEMT is accounted for in the numerical modeling with localized refined mesh. The thermal performance and pressure drop during subcooled flow boiling are investigated and presented considering parametric effects of heat flux, mass velocity, inlet/outlet width ratio, manifold thickness and channel aspect ratio as well as substrate materials. The results indicate that GaN-on-Diamond structure provides a lower thermal resistance and better temperature uniformity, and the superior performance of GaN-on-SiC and GaN-on-Diamond are augmented with increasing heat flux during subcooled flow boiling. Additionally, it can be concluded that maximum temperature and pressure drop decrease with increasing channel height firstly, and then flatten out as the height/width ratio reaches 8 and 6, respectively. The flatten-out relationship between increasing height/width ratio and thermal/hydraulic performance is partly attributed to the balance of decreasing convective heat transfer coefficient and increasing effective area. Besides, flow pattern analysis for high-aspect ratio microchannels shows that hotspots at the bottom of inlet manifold promote the bubble nucleation and bubbles tend to accumulate at the bottom of channels because of nonuniform velocity distribution along the microchannel height.

A flame technique to isolate the detonation/product interface relevant to rotating detonation engines

A novel experimental technique is proposed to study the detonation propagation in a layer of non-reacted gas weakly confined by combustion products. This problem is relevant to rotating detonation engines, where transverse detonations are confined by products of a previous rotation cycle, and other applications such as industrial safety. The experimental technique utilizes a flame ignited along the top wall in a long channel. The preferential growth of the flame along the long direction of the channel creates a finger flame and permits to create a narrow layer of unburned gas. A detonation ignited outside of this layer then propagates through the layer. This permits to conduct accurate observations of the detonation interaction with the inert gas and determine the boundary condition of the interaction. The present paper provides a proof-of-concept demonstration of the technique in a 3.4 m by 0.2 m channel, in which long finger flames were observed in ethylene-oxygen mixtures. The flame is visualized by high-speed direct luminosity over its entire travel, coupled with pressure measurements. A direct simulation of the flame growth served to supplement the experiments and evaluate the role of the induced flow by the flame growth, which gives rise to a non-uniform velocity distribution along the channel length. Detonation experiments were also performed at various layer heights in order to establish the details of the interaction. The structure was visualized using high speed Schlieren video. It was found that an inert shock always runs ahead of the detonation wave, which gives rise to a unique double shock reflection interaction.