Thermal Mixing Efficiency Research Articles

Influence of a double-mixer configuration on the mixing and combustion characteristics of a multibypass combustor for a turbine-based combined cycle engine

The changes in the flow field and combustion characteristics induced by the mode transition process of a turbine-based combined cycle (TBCC) engine may lead to failure of the mode transition. In this paper, the cold mixing characteristics, combustion efficiency, and cold and thermal flow losses of a TBCC multibypass combustor with a double-mixer configuration are investigated through numerical simulations, and the correlations between the thermal mixing efficiency and combustion efficiency is established. The results show that the thermal mixing efficiency of double-lobed mixer combustors is better at low ram inlet Mach numbers, but the ring-plus-lobed mixer combustor has better mixing and combustor characteristics at relatively high Mach numbers. However, the effect of the turbo core inlet Mach number on the combustion efficiency is not regular. The spacing between the double mixers that maximizes the mixing and combustion characteristics of the combustor is 75 mm. With increasing of expansion angle of the lobed mixer, the thermal mixing efficiency and combustion efficiency increase by 31.9 % and 23.3 %, respectively. In addition, the positive correlation coefficient between combustion efficiency and thermal mixing efficiency caused by changes in the inlet Mach number is greater than that caused by changes in structural parameters.

A case study of thermal mixing behavior of hot and cold fluid in T-junction with/without mixing jets

Three-dimensional numerical simulations are performed to study the turbulent mixing of hot and cold fluids at a T-junction with unique branch pipe extended mixing jets. The major aim of the current research is to compare the performance of two distinct designs for thermal mixing and flow characteristics of a T-junction. We investigate the effects of the momentum ratio and temperature difference of mixing fluids on temperature gradients for proposed configurations of T-junctions. The finding demonstrated that a T-junction with jets appears to mix hot and cold fluids more homogeneously. The addition of the mixing jets significantly lessens the internal wall's temperature gradients and improves the efficacy of thermal mixing. The thermal mixing efficiency for T-junctions with mixing jets increased by 25%, 42%, and 50%, respectively. At the same momentum ratio, the length of the thermal mixing in the T-junction with mixing jets is dramatically reduced by 77%, 79%, and 83%. The stress distribution is also determined using thermo-mechanical coupled analysis. The result shows the reduction in peak stresses substantially with the mixing jet T-junction. The current study will help the researcher to evaluate safety issues in high-temperature applications and better understand thermal mixing, flow characteristics, and thermal cracks.

Experimental study on flow field characteristics of TBCC multibypass combustor

The turbine-based combined cycle (TBCC) engine, as a hypersonic air-breathing propulsion system, involves the transition between multiple bypasses during flight. During the transition process, the flow field of the combustor is affected. In this paper, the flow field of a TBCC multibypass combustor with typical structures is investigated experimentally. The effect of the combined structure on the mixing performance, total pressure loss and flow field distribution downstream of the TBCC multibypass combustor is investigated by changing the inlet aerodynamic parameters and structural parameters of the combustor. The results show that the diffusion ratio increased from 1.0 to 1.4, the thermal mixing efficiency decreased by 24.7%, and the total pressure recovery coefficient increased by 1.86%. Meanwhile, the recirculation zone is wider, and the velocity distribution behind the stabilizer is more uniform. The thermal mixing efficiency increases by 55.8% with the increase in the down inclination (35°∼45°) of the lobe mixer. The mixing characteristic of the triple-bypass combustor is greater than that of the double-bypass combustor. There is an obvious “phase difference” of the vortex shedding in the double-bypass combustor at different spanwise sections, while it does not exist in the triple-bypass combustor. The turbulence disturbance degree in the recirculation zone behind the stabilizer in the triple-bypass combustor is slightly higher than that in the double-bypass combustor, but the velocity distribution is more uniform. The increase in inlet Mach number increases the flow losses and decreases the mixing performance. However, the thermal mixing efficiency is positively impacted with respect to the inlet temperature. In addition, the turbulent kinetic energy of the shear layer behind the stabilizer is large, while it is relatively small in the recirculation zone. There is not only spanwise vortices but also radial vortices behind the stabilizer.

Study of the mixing characteristics of a multibypass combustor of a turbine-based combined cycle

When the multimode and multibypass turbine-based combined cycle (TBCC) engine performs mode transition, it involves switching between multiple bypasses. During the transition process, the flow field of the combustor is adjusted rapidly, and the mixing performance is affected. However, superior mixing is the premise of the superior combustion performance of the combustor. In this paper, the mixing characteristics of the TBCC triple-bypass combustor during the mode transition process are studied by numerical simulation. Different structural and aerodynamic parameters, including equivalent expansion angle (5.5°, 9.2°, 12.8°), lobe mixer width (20, 25, 30 mm), bypass ratio (0.44, 0.56, 0.66) and inlet temperature, were studied to discuss the trending of flow field, vorticity field, temperature field, and oxygen content distribution. The results show that the performance of triple-bypass combustor is worse than that of double-bypass combustor under the condition of turbo core inlet Mach number 0.3. The total pressure loss is increased by 0.3% and the thermal mixing efficiency is decreased by 12.4%. The increase in the equivalent expansion angle reduces the mixing efficiency (12.5%) and improves the total pressure recovery coefficient (1%). However, increases in lobe width, bypass ratio, and ram inlet temperature increased mixing characteristics but increased flow losses. The increase in turbo bypass inlet temperature (300–500 K) improved the total pressure recovery coefficient by 0.17% and the thermal mixing efficiency by 7.0%. Conversely, the increase of the turbo core inlet temperature (800–1200 K) not only reduces the total pressure recovery coefficient, but also reduces the mixing performance. In addition, the studies show that the mixing performance of the lobe mixer is mainly affected by the velocity difference between the primary and secondary flow. Besides the streamwise vortex and normal vortex, the normal vortex dissipation rate controlled by the velocity difference is also one of the factors affecting the mixing performance.

Optimization of temperature distribution in the novel power-to-heat matrix-in-batch OnePot© reactor

In this work, we assess the optimal temperature distribution inside a new automated, stand-alone, matrix-in-batch patented reactor, named OnePot©. This novel reactor is equipped with seven rotating hot rotating cylinders—here referred to as spots—which make it possible a precise tuning of fluid temperature. To conduct this investigation, we consider two radial layout of spots, here indicated as uniform configuration and alternate one, respectively. The former characterised by a single uniform equilateral triangular pitch, whereas the latter by two different equilateral triangular pitches alternated to form a double-triangle star. We consider two different fluids, water and argon, as representative of the behaviour of liquids and gases, respectively. Furthermore, the effect of viscosity is also taken into account by forcefully increasing that of water by 100 and 1,000 times. The optimization of the temperature distribution is performed obtaining velocity and temperature fields using a computational fluid dynamic (CFD) approach. As a sort of objective function to maximise, we defined a thermal mixing efficiency to provide a quantitative measure of the temperature distribution uniformity. As a remarkable result, we find an optimal value of pitch approximately equal to 36% of the vessel diameter for both liquid water and argon gas. As for the alternate configuration, we found that it provides a better temperature distribution than the uniform one, especially at high viscosity values. This is because the inner spots are able to prevent the formation of large colder “islands” around the centre. Furthermore, we estimate the overall heat transfer coefficient between thermal spots and fluid bulk, whose values are perfectly in line with the literature ones. The modularity of our novel fully-electric reactor allows for applications in a number of industrial contexts, especially pharmaceutical ones.

Enhancement of thermal mixing under supercritical condition by increasing shear rate

The dynamical characteristics of the shear-driven mixing layer can be inhibited due to the variations in fluid density. In this work we expect to address this issue by increasing shear rate. Direct numerical simulations of mixing layer between two streams with different temperatures at supercritical pressure are conducted. We confirm that the turbulent characteristics of mixing layer are inhibited with increasing density ratio between the lower and upper streams, resulting in the destruction of large-scale vortex, especially at the high-density side. By increasing the velocity ratio between the two streams, however, the mixing performance is enhanced with regeneration of the hairpin vortex due to increase in mean shear. The enstrophy production thus increases due to enhancement of vortex stretching and the thermal mixing efficiency is enhanced due to the entrainment of vortical structures. Further, the streamwise spectra of the temperature fluctuations follow a slope of −3 in the viscous-diffusive range.

A Preliminary Numerical Investigation of Thermal Mixing Efficiency in T-Junctions with Different Flow Configurations

When hot and cold fluids flow through a converging T-junction, rapid temperature fluctuations occur in the mixing region due to the thermal mixing of fluids. This temperature fluctuation causes thermal fatigue, which is responsible for the shortening of service life in a T-junction. Hence, the design of T-junction for thermal mixing requires not only superior mixing performance but minimize thermal fluctuation during mixing is also desirable. The objective of the present paper is to determine the thermal mixing performance at the mixing region of T-junction with two different flow configurations. Water, at different inlet temperatures, is used as a working medium and is assumed incompressible. Two types of flow configurations, namely intersecting and colliding regular T-junction with a sidearm pointing at 12 o’clock position have been evaluated in this paper. Realizable k-epsilon turbulence model was assumed, and its validity benchmarked against RANS and RSM-EB turbulence models. The thermal mixing efficiency of both flow configurations is calculated and compared. The results show that the thermal mixing efficiency of both intersecting and colliding mixing tee increases with the increase of distance and time. Intersecting tee shows higher temperature fluctuation than colliding tee at the mixing outlet, but colliding tee shows higher thermal mixing efficiency than intersecting mixing tee.

Configuration of the lobed nozzle in an ejector mixer for extreme mixing efficiency

An alternating-lobe nozzle with nail-like crests and sword deep valleys (NSALN-1) was modified to achieve the ultimate mixing efficiency by enhancing the mixing in the core region. The main modifications included extending the sword deep valleys, adding a central plug between the sword deep valleys, and connecting the sword deep valleys to form arched deep valleys. The jet mixing of NSALN-1 and its modified configurations was numerically simulated, and the efficiency of the modifications was investigated. It was found that each modification can improve the thermal mixing efficiency, but also reduce the total pressure recovery coefficient. In particular, adding a central plug of an appropriate size can increase the thermal mixing efficiency to an extreme value while the reduction in the total pressure recovery coefficient remained within an acceptable range. The thermal mixing efficiency can reach 0.98 at 1.5d and exceed 0.99 at 2.0d. Furthermore, the shape and position of the longitudinal vortex determine the length of the path by which the cold stream is transported to mix with the hot stream by the longitudinal vortex. Regulating the mixing in each region to achieve balanced mixing speeds is thus helpful to achieve the ultimate mixing efficiency of the circularly lobe nozzle.

Effects of Bypass Flow Distribution on Cold Flow Characteristics of Integrated Afterburner

Integrated design is a trend in the development of afterburners, and the distribution of cold flow is directly related to their flow field characteristics, combustion organization, and the cooling effect of components. Numerical simulations were performed to illustrate the effects of bypass flow distribution on the flow distribution, mixing characteristics, and cooling efficiency of the components by varying the cooling flow path structure parameters. Within the range of parameters in this study, it can be indicated that with the increase of heat shield inlet height and afterburner annulus height, the total pressure recovery coefficient along the path increased accordingly, and the increasing rate at the afterburner outlet is 1.12% and 1.19%, respectively. The average cooling efficiency of radial flameholder, circumferential flameholder, and fuel injector all decrease, but the rate of decrease varies slightly depending on the location of the components. The increase of heat shield inlet height would reduce thermal mixing efficiency by approximately 5.4% at the afterburner outlet, and the increase of afterburner annular height would increase about 2.9%.

Predicting the Behaviour of a Vortex Shedding-Based Passive Mechanical Micro Heat Exchanger

In the recent years, microscale applications are gaining increasing importance. Despite their advances, the technology and resources needed to develop new designs may be a drawback for reduced scale engineering testing. To overcome this, computational methods are an efficient tool to predict how a real-life system may behave prior physical construction. The present work aims to investigate numerically effective models to predict the conditions at which a micro heat exchanger (MHE) is able to promote mixing by vortex shedding mechanics. In spite of vortex-shedding is a well-known mechanism in flow physics, it is not possible to know a priori whether a configuration (for a given geometry and flow velocity) may or may not lead to this desired vortex detachment to enhace mixing. Thus, Machine Learning methods are used for prediction, trained with finite-volume numerical simulations of different MHE devices selected based on their performance. A classification model is used to predict which configurations lead to vortex shedding. Also, a correlation regression model is developed to predict the critical Reynolds number. When the critical Reynolds number is surpassed for a given geometry, vortex shedding appears and its intensity controls the thermal mixing efficiency of the microdevice. These predictors could be useful in the search of optimal configurations by optimisation algorithms, since in the sampling process could be used to define constrains.

Characterization of natural convection in a PCM-based heat sink with novel conductive structures

This paper numerically investigated the effect of natural convection on the thermal behaviour of a PCM-based heat sink with novel tree shape conductive structures and conventional plate fin structures. Simulations were realized using a CFD model based on the enthalpy-porosity and VOF methods. Results showed that the influence of natural convection was related to the applied metal structure, orientation and metal volume fraction. Among them, orientation expressed the most significant impact on the natural convection heat transfer by affecting the flow pattern and in return the fin efficiency. Increasing metal volume fraction tended to weaken the influence of natural convection ascribed to the enhanced heat conduction and restricted convection flow. The optimized tree shape structure with a better heat conduction path and curvilinear interface promoted natural convection, by which the better thermal mixing and higher fin efficiency were achieved, and thus enhanced heat transfer. When natural convection was negligible, the tree shape structure design initially performed a better but subsequently worse heat transfer due to its higher fin efficiency but shorter contact lines. It is suggested that the optimized tree shape structure is more superior in applications where minimum conductive material is necessary and natural convection is significant.

Investigation on the thermal mixing enhancement in a T-junction pipe

In this paper, three-dimensional numerical simulations are conducted to study the thermal mixing of turbulent flow in a T-junction with hot fluid flowing in the main pipe and cold fluid in the branch. The main objective is to investigate the effects of the axial and cross-injection angles to the main pipe and the branch-to-main flow rate ratio on the turbulent thermal mixing. In this regard, an appropriate mixing index is proposed to evaluate the thermal mixing efficiency. The turbulence model employed in this research is the SST k-ω model that can properly capture the mixing behavior of the flow in the main pipe. The results show that the cross-injection angle can considerably affect the thermal mixing enhancement. However, the effects of injection angles on the thermal mixing at lower flow rate ratios are more significant.

Effect of notch width-ratio of core lobe to bypass lobe on performance of two-dimensional lobed mixer

Geometrical and numerical simulation models of 2-D lobed mixer with differences of bh/bc were established to study the effects of bh/bc and bypass lobe to thermal mixing efficiency and total pressure recovery coefficient of the mixer. The results show that when the bh/bc range from 0.444 to 1.364, the maximum average non-dimensional streamwise vorticity along flow direction rises as bh/bc increases. The mixer of which thermal mixing efficiency is maximum varies over stream wise and bh/bc of the mixer of which thermal mixing efficiency is maximum increases along the flow direction. Besides, the total pressure recovery coefficient of the lobed mixer ascends as bh/bc increases.

The Effects of Spoilers on Jet Mixing of Lobed Nozzles

In this study, spoilers were installed on the lobed nozzle principal model. Keeping axial position unchanged, the different widths of spoilers were mounted along the circumferential direction of a mixer tube. The effects of spoilers on the jet mixing of lobed nozzle were investigated by numerical method, and the results showed that, before or after the installation, the flow ratio did not change significantly. The value of thermal mixing efficiency and total pressure recovery coefficient had a corresponding improvement. The spoilers increased the borderline area of the primary and secondary streams, thus the mixing was accelerated. Spoilers can avoid high-temperature primary stream impinging the wall of the tube, and the temperature of mixing tube wall decreased significantly.

Effect of Core Flow Inlet Swirl Angle on Performance of Lobed Mixing Exhaust System

AbstractGeometric model of a lobed mixing exhaust system is created and its flow field is simulated by using the steady Reynolds Averaged Navier-Stokes (RANS) equations under the condition of different core flow inlet swirl angles. According to the numerical simulation results, due to the guidance effect of the lobe parallel side wall, the structure and vorticity of streamwise vortices change little near the lobe exit with inlet swirl angle, and it is the same with the thermal mixing efficiency. As the flow develops, although the inlet swirl angle has limited influence on the streamwise vorticity, it greatly affects the structure of streamwise vortices. It causes the thermal mixing efficiency to increase with the swirl angle. As for the total pressure recovery coefficient, it falls slightly when the inlet swirl strengthens. At the nozzle exit, the total pressure recovery coefficient of CFISA = 30° model is 0.5% lower than CFISA = 0° model. Moreover, as the inlet swirl strengthens, the thrust fall of lobed mixing exhaust system gradually accelerates, especially when the inlet swirl angle is over 15°.

Numerical Predictions of Scarfing on Performance of S-Shaped Nozzle with Asymmetric Lobe

Numerical predictions of a lobed jet mixing flow within an S-shaped nozzle were conducted with respect to the influence of lobe scarfing on performance of the lobed S-shaped nozzle. The simulation results presented in this paper were all obtained by solving compressible steady three-dimensional Reynolds-averaged Navier–Stokes equations with a realizable turbulence model. The effect of scarfing optimization was mainly addressed by both the flowfield downstream of the lobed mixer and the interested variables characterizing the performance of the lobed S-shaped nozzle. Due to the studied S-shaped nozzle, a sharp decrease of total pressure recovery coefficient, as well as a reduced thermal mixing efficiency, within the flowfield close to the outlet is shown. The model with a bigger scarfing angle is suggested to induce the forced mixing process ahead of schedule, but with less mixing strength as compared with the baseline model. However, the model with a more aggressive scarfing angle is shown to be the case with both a decreased thermal mixing efficiency and an augmented total pressure recovery coefficient within the flowfield downstream of the cross section, except the 20 deg scarfing model. Both the thrust coefficient and thrust mixing efficiency are all found to decrease as a result of the tested model with a bigger scarfing angle.

Numerical Investigation of Performance of the Air Tab in the Turbofan Nozzle

The air tab is a new technique with better effects on mixing but minimum penalty on thrust. The present numerical investigation aims to study the mixing effect in the turbofan nozzle. Meanwhile, the parametric studies of the air-tab flow rate and the tube inner diameter of the air tab are also carried out. The thermal mixing efficiency , the total pressure recovery coefficient , and the thrust coefficient are compared to find out the quantitative differences among the models researched. The results reveal that the air-tab technique would increase the thrust a little further rather than create a great penalty on thrust by the jet itself. The thermal mixing efficiency increases greatly from 0.1756 to 0.3425, whereas the high-temperature region (where the temperature is higher than 650 K) is reduced from 29.2 to 5.2% and the thrust coefficient is nearly unchanged, with the air-tab flow rate only about 0.58%. The results also reveal that the depth of penetration cannot determine the thermal mixing efficiency. In addition, there is little difference for total pressure loss among the air-tabbed models at the lobe exit.

Parametric Effects on Internal Aerodynamics of Lobed Mixer-Ejector With Curved Mixing Duct

The internal flow characteristics inside lobed mixer-ejector with curved mixing duct and the parametric effects on the lobed mixer-ejector performance are investigated numerically and validated by experimental test. The curved mixing duct affects the development of the streamwise vortices induced by the lobed mixer. When the mixing process undergoes the transition from the straight section to the bent section, the flow inside the curved mixing duct is dominated by the impinging and centrifugal effects. In general, the pumping ratio is decreased approximately 20%–30% once the bent section is mounted on the straight duct. The mixer-ejector performance could by improved by increasing the straight section length, due to more fully momentum utilization of primary jet and weaker influence of bent section on the back pressure near nozzle exit. The mixer-ejector pumping capacity is also augmented with the increase of mixing duct area ratio until the area ratio is reached to 3.5. And the fully-utilization of primary jet momentum inside mixing duct with big area ratio needs long mixing distance. The pumping ratio is decreased as the increase of bent angle of curved mixing duct in approximately linear relationship. When the bent angle exceeds 45 deg, the thermal mixing efficiency is decreased rapidly as the increase of bent angle.

Numerical Investigation of Lobe Structure on Performance of Lobed Forced Mixing Exhaust System

Geometric models of lobed mixer nozzle with different lobe shapes are created. The Reynolds Averaged Navier-Stokes (RANS) equations with standard k-e turbulence model are selected as controlling equations to simulate the corresponding flow field, and they are discretized by the Arbitrary Lagrange Euler method. The simulation results are compared with the splitter mixer nozzle, and the comparison outcomes indicate that lobed mixer can not only greatly increase the mixing between the core and fan flow, but also reduce the noise of exhaust. Besides, although energy loss of lobed mixer is bigger than splitter mixer, the difference of total pressure recovery coefficient and thrust coefficient between lobed mixer nozzle and splitter mixer nozzle is small. Among different lobe shape models, the straight cut and elliptical cut at the lobe exit can enhance the mixing, and their energy loss is slightly bigger than the corresponding model without cut. When the guide lines of lobed mixer is modified, thermal mixing efficiency declines, but the total pressure recovery coefficient increases. In addition, as lobe shape changes, although thrust coefficient varies, the difference between maximum and minimum thrust coefficient is less than 1%.

Analyzing of thermal mixing phenomena in a rectangular channel with twin jets by using artificial neural network

In this study, an experimental investigation was carried out to analyze the thermal mixing phenomena in a rectangular cross-section narrow channel. The channel has two circular water jet inlets, in different temperatures and a circular exit hole to supply continuity of mass. Several parameters were used in the experiments to control thermal mixing efficiency such as inclination angle of the channel, ratio of flow rate of inlet fluid, temperature difference between hot and cold jets and jet inlets diameters. Thermal mixing index was calculated from the measured temperatures. In order to reduce experimental time, an artificial neural network (ANN) model was built with limited number of measurements for a forward model. The obtained results indicated that estimation data of ANN shows that the ANN model can predict the output parameters without carry out any experiment.