Theoretical investigation of operation modes of MHD generators for energy-bypass engines

A MHD generator with different arrangements of electromagnetic fields will lead the generator working in three modes.A quasi-one-dimensional approximation is used for the model of the MHD generator to analyze the inner mechanism of operation modes.For the MHD generator with a uniform constant magnetic field,a specific critical electric field E_（cr） is required to decelerate a supersonic entrance flow into a subsonic exit flow.Otherwise,the generator works in a steady mode with a larger electric field than E_（cr） in which a steady supersonic flow is provided at the exit,or the generator works in a choked mode with a smaller electric field than E_（cr） in which the supersonic entrance flow is choked in the channel.The detailed flow field characteristics in different operation modes are discussed,demonstrating the relationship of operation modes with electromagnetic fields.     

Shock train and pseudo-shock phenomena in supersonic internal flows

When a normal shock wave interacts with a boundary layer along a wall surface in supersonic internal flows and the shock is strong enough to separate the boundary layer, the shock is bifurcated and a series of shocks called "shock train" is formed. The flow is decelerated from supersonic to subsonic through the whole interaction region that is referred to as "pseudo-shock". In the present paper some characteristics of the shock train and pseudo-shock and some examples of the pseudo-shocks in some flow devices are described.     

Combustion stabilization based on a center flame strut in a liquid kerosene fueled supersonic combustor

A newly designed strut is proposed in this paper for fuel injection and flame holding in a liquid-kerosene-fueled supersonic combustor.The thickness of the strut is 8mm and the front blockage is about 8%.The characteristic of this strut is that extra oxygen can be injected through a set of orifices at the back of the strut,which can change the local flow field structure and ER(Equivalence Ratio).Based on the above mentioned strut,a stable local flame is generated at the back of the strut and the main combustion can be organized around this local fire.Numerical simulation is conducted to compare the local flow field distribution at the back of the strut with/without extra oxygen injection.Experiments are conducted to test the combustion characteristics based on this fuel injection and flame holding strategy.The temperature distribution which can reflect the local flame characteristic has been measured in the experiments conducted under cold incoming supersonic air flow condition.In addition,the overall combustion performance in a full-scale supersonic combustor has been evaluated in the experiments conducted under hot incoming supersonic air flow condition.Results show that this strut strategy is very promising since it can organize stable supersonic combustion at the center of the combustor without any cavity or rearward facing step.Besides that,even with the 8mm thick strut,the combustion can be stable in a wide range of ER from 0.25-1 by using liquid room-temperature kerosene.     

Characteristics of the Mach Disk in the Underexpanded Jet in which the Back Pressure Continuously Changes with Time

When the high-pressure gas is exhausted to the vacuum chamber from the nozzle, the underexpanded supersonic jet contained with the Mach disk is generally formed. The eventual purpose of this study is to clarify the unsteady phenomenon of the underexpanded free jet when the back pressure continuously changes with time. The characteristic of the Mach disk has been clarified in consideration of the diameter and position of it by the numerical analysis in this paper. The sonic jet of the exit Mach number Me=1 is assumed and the axisymmetric conservational equation is solved by the TVD method in the numerical calculation. The diameter and position of the Mach disk differs with the results of a steady jet and the influence on the continuously changing of the back pressure is evidenced from the comparison with the case of steady supersonic jet.     

Numerical Computation of Compressible Viscous Internal Flows

Some implicit time-marching finite-difference solutions of time-averaged Navier-Stokes equations for two-dimensional compressible internal flows are presented.Five numerical examples including subsonic,transonic,supersonic and hypersonic flow fields with steady and unsteady phenomena show validity and flexibility of thepresent calculation code.The TVD scheme suggested by Harten et al is used to improve the shock resolution,and an algebraic turbulence model suggested by Baldwin and Lomax is introduced to evaluate the viscous effectin the turbulent flows.It is found that the computational results show fairly good agreement with the experi-mental data.     

Preliminary Investigation of Full Model of Two—Mode Scramjet

IntroductionThe latest Studies of the ovens characteristics ofhypersonic fifor vehicles with ~ engineshave shown that these vehicles are fairly prondsing.Some addihonal. PbolelnS arise, how~ that arerelated tO the dedrihon Of the geneal shape of theaircraft, the choice Of Propulsion type and operationmpbo, the enghe size and POsition on the body.The ~ version under consideration boltesthe use of a COmbined PtOPulsion act with a twO-modecombustor (subsonic or supersonic combustion). sucha…     

Heat Transfer Augmentation in Developing Flow Through a Ribbed Square Duct

An experimental study is conducted to investigate the heat transfer augmentation in developing turbulent flowthrough a ribbed square duct.The duct is made of 16mm thick bakelite sheet.The bottom surface of the ribbedwall having rib pitch to height ratio of 10 is heated by passing a c current to the heater placed under it.Theuniform heating is controlled using a digital temperature controller and a variac.The results of ribbed duct arecompared with the results of a smooth duct under the same experimental conditions.It is observed that the heattransfer augmentation in ribbed duct is better than that of the smooth duct.At Re=5.0×10~4,the meantemperature of air flowing through the ribbed duct increases by 2.45 percent over the smooth duct,whereas in theribbed duct Nusselt number increases by 15.14 percent than that of the smooth duct with a 6 percent increase inpressure drop.     

A new cascade-less engine operated from subsonic to hypersonic conditions: designed by computational fluid dynamics of compressible turbulence with chemical reactions

By using our computational fluid dynamic models, a new type of single engine capable of operating over a wide range of Mach numbers from subsonic to hypersonic regimes is proposed for airplanes, whereas traditional piston engines, turbojet engines, and scram engines work only under a narrower range of operating conditions. The new engine has no compressors or turbines such as those used in conventional turbojet engines. An important point is its system of super multijets that collide to compress gas for the transonic regime. Computational fluid dynamics is applied to clarify the potential of this engine. The peak pressure at the combustion center is over 2.5 MPa, while that just before ignition is over 1.0 MPa. The maximum power of this engine will be sufficient for actual use. Under the conditions of higher Mach numbers, the main intake passage located in front of the super multijet nozzles, takes in air more. That results in a ram or scramjet engine for supersonic and hypersonic conditions.     

Experimental and Numerical Evaluation of the Performance of Supersonic Two-Stage High-Velocity Oxy-Fuel Thermal Spray （Warm Spray） Gun

The water-cooled supersonic two-stage high-velocity oxy-fuel (HVOF) thermal spray gun was developed to make a coating of temperature-sensitive material,such as titanium,on a substrate.The gun has a combustion chamber (CC) followed by a mixing chamber (MC),in which the combustion gas is mixed with the nitrogen gas at room temperature.The mixed gas is accelerated to supersonic speed through a converging-diverging (C-D) nozzle followed by a straight passage called the barrel.This paper proposes an experimental procedure to estimate the cooling rate of CC,MC and barrel separately.Then,the mathematical model is presented to predict the pressure and temperature in the MC for the specific mass flow rates of fuel,oxygen and nitrogen by assuming chemical equilibrium with water-cooling in the CC and MC,and frozen flow with constant specific heat from stagnant condition to the throat in the CC and MC.Finally,the present mathematical model was validated by comparing the calculated and measured stagnant pressures of the CC of the two-stage HVOF gun.     

Numerical Analysis of Thermal Convections in a Three-dimensional Cavity: Influence of Difference in Approximation Models on Numerical Solutions

In order to develop the practical approximation models suitable to flow fields at low Mach number with large temperature difference, the influence of difference in approximation models on numerical solutions was investigated by solving the natural convection in the 3-D enclosures with vertical sidewalls differentially heated and the heated bottom wall using 3 approximation models, that is Boussinesq approximation, low Mach Number approximation and approximation model proposed by Mlaouah. As results of the simulation, the effects of the differences in the three approximation models on the numerical solutions become clear.     

Numerical investigations of mixed supersonic and subsonic combustion modes in a model combustor

Flame dynamics and statistics of mixed supersonic and subsonic combustion modes under different air inflow and global equivalence ratio conditions in a hydrogen-fueled model combustor are numerically studied. Three methods including spanwise-averaged Mach number, spanwise-averaged Mach number conditioning on the local heat release, and fraction of heat release are proposed to identify supersonic and subsonic combustion modes. The probability distributions of supersonic and subsonic combustion modes are also analyzed based on the statistics on multiple instantaneous snapshots of the numerical results. The critical global equivalence ratio for thermal choking in a range of supersonic inflow conditions is derived theoretically based on a one-dimensional duct flow with heat addition. Furthermore, it is found that the flame lift-off distance in both supersonic and subsonic flows decreases with increased air inflow velocity, but increases with global equivalence ratio. The fraction of supersonic heat release and its oscillation increase with increased air inflow velocity.     

Numerical and Experimental Studies of 3D Hypersonic Inlet

The results of numerical and experimental studies of a new configuration of 3D hypersonic inlet with the minimum throat area, which was called a convergent inlet, are presented in this paper. It is shown that the use of this inlet configuration allows one to reduce the drag and thermal protection of surfaces of a hypersonic engine within the entire range of flight velocities. The calculations were performed within the framework of inviscid gas model by the method of finite volumes. The flow and inlet characteristics, taking account of viscosity, were also calculated using the boundary layer equations. The experimental studies were performed within the Mach number range from 2 to 10.7 and Reynolds number based on the model inlet height of Re=1-5×106. The results included the flow parameters on the external compression surface and in the inlet duct, the Mach number in the throat, the air flow rate, the total pressure recovery coefficient, the inlet drag, and the boundary layer characteristics on compressi     

Modeling of the non-uniform combustion in a scramjet engine

Due to the high-speed of the air stream, the scramjet combustion is neither uniform nor complete. An empirical fuel distribution model is developed to describe non-uniform combustion for scramjet engines. The combustor is subdivided into different regions by radius with different local equivalence ratios. The conservation equations of the regions for the combustion and exhaust expansion are computed independently. The results indicate that scramjet thrust is more related to the fuel equivalence ratio and combustion efficiency. If the combustion efficiency is 100% and the fuel equivalence ratio is constant, there is no obvious effect of different fuel distribution to the engine except for the exhaust parameter distribution. It is also revealed that the sum of isolator shock loss and combustor Rayleigh loss is nearly constant under the same isolate cross-sectional area. Lower isolator inlet Mach is benefit to the thrust performance and the best thrust performance is at the thermal choking boundary. When the isolator inlet Mach increases, the thrust decreases.     

Heat flow choking in carbon nanotubes

Based on Einstein’s mass–energy relation, the equivalent mass of thermal energy or heat is identified and referred to as thermomass. Hence, heat conduction in carbon nanotubes (CNTs) can be regarded as the motion of the weighty phonon gas governed by its mass and momentum conservation equations. The momentum conservation equation of phonon gas is a damped wave equation, which is essentially the general heat conduction law since it reduces to Fourier’s heat conduction law as the heat flux is not very high and the consequent inertial force of phonon gas is negligible. The ratio of the phonon gas velocity to the thermal sound speed (the propagation speed of thermal wave) can be defined as the thermal Mach number. For a CNT electrically heated by high-bias current flows, the phonon gas velocity increases along the heat flow direction, just like the gas flow in a converging nozzle. The heat flow in the CNT is governed by the electrode temperature until the thermal Mach numbers of phonon gas at the tube ends reach unity, and the further reduction of the electrode temperature has no effect on the heat flow in the CNT. Under this condition, the heat flow is said to be choked and temperature jumps will be observed at the tube ends. In this case the predicted temperature profile of the CNT based on Fourier’s law is much lower than that based on the general heat conduction law. The thermal conductivity which is determined by the measured heat flux over the temperature gradient of the CNT will be underestimated, and this thermal conductivity is actually the apparent thermal conductivity. In addition, the heat flow choking should be avoided in engineering situations to prevent the thermal failure of materials.     

Evaluation and analysis of evaporation cooling on thermodynamic and pressure characteristics of intake air in a precooled turbine engine

The precooled turbine engine is applied to overcome the limitation of Mach number due to high temperature inlet air. This paper aims to investigate the effect of water injection cooling on the high-temperature intake air. Then, the theory evaluation and Eulerian-Lagrangian multiphase flow method are conducted to explore the thermodynamic process and resistance characteristics of the pre-cooling section built-in even spray apparatus with a drag reduction. Results show that larger amount of the injection flow rate at higher Mach number will deteriorate total pressure loss and flow field uniformity. Evaporation cooling can decrease flow loss by 9.4%–60.7%. Within 27 ms, total-temperature drop is in 14–144 K range with a low total-pressure drop coefficient of 0.56%–1.29%. Especially, mass flow will increase by 1.15%–18.50%. Thus, water injection cooling is conducive to a higher acceleration, as well as for improving the thermodynamics characteristics of inlet air for a turbine engine at a high Mach number.     

Flow maldistribution and thermal performance deterioration in a cross-flow air to air heat exchanger with plate-fin cores

The inlet and outlet duct geometry in an air to air compact heat exchanger is always irregular. A skewed Z-type arrangement is popular between the impinging flow and the core. Such duct placements usually lead to a non-uniform flow distribution on core surface. In this research, the flow maldistribution and thermal performance deterioration in cross-flow air to air heat exchangers are investigated. The inlet duct, the core and the outlet duct are combined together to calculate the flow distribution on core inlet face. First, a CFD code is used to calculate the flow distribution, by treating the plate-fin core as a porous media. Then a heat transfer model between the two air flows in the plate-fin channels is set up. Using the flow distribution data predicted, the heat exchange effectiveness and the thermal performance deterioration factor are calculated with finite difference scheme. Experiments are performed to validate the flow distribution and heat transfer model. The results indicate that when the channel pitch is below 2.0 mm, the flow distribution is quite homogeneous and the thermal deterioration due to flow maldistribution can be neglected. However, when the channel pitch is larger than 2 mm, the maldistribution is quite large and a 10–20% thermal deterioration factor could be found. The study proves that the inlet duct, the outlet duct, and the core should be coupled together to clarify flow maldistribution problems.     

Effect of the angle of inclination of a plate shield on the thermal and hydraulic performance of a plate-fin heat sink

Using CFD software FLUENT, we investigated the effect of the angle of inclination of a plate heat shield on the thermal and hydraulic performance of a plate-fin heat sink. The variation of this angle causes a substantial and complicated variation of the flow field in space both upstream and downstream near such a heat sink. This distinctive behavior modifies the pressure drop between the inlet and outlet of the investigated duct, but that variation influences only slightly the flow field in the space from fin to fin, and thus the thermal resistance of the heat sink. This trend is further smoothed with increasing Reynolds number and height of the heat sink. As a compromise between the demands of small thermal resistance and a small pressure drop, the angle of inclination of a plate heat shield must be chosen carefully.     

The effects of outlet boundary conditions on simulating supersonic microchannel flows using DSMC

High speed gas flows through two-dimensional microchannels have been investigated using the Direct Simulation Monte Carlo (DSMC) method, where the pressure boundary condition has been implemented using the theory of characteristics as an alternative to the vacuum boundary. Two species, nitrogen and helium, have been used to conduct the flow simulations. It was found that the pressure boundary condition cannot only predict the flow with exit-plane pressure equal to the back pressure, which the vacuum boundary condition fails to do, but can also simulate the flow with expansion waves outside the channel. Therefore, it is considered to be more appropriate. Two inlet Mach numbers, 4.15 and 3.39, have been employed for the nitrogen flow cases with an inlet Knudsen number (Kn) of 0.062. It has been shown that for cases with an inlet Mach number equal to 4.15, the back pressure only has an effect on flow in the latter half of the microchannel, where the wall heat flux can be enhanced by increasing the back pressure. At an inlet Mach number of 3.39, the wall heat flux has the same trend as that in the higher Mach number case, though its magnitude is considerably lower. In addition, no significant effect of a step change in wall temperature distribution on the total heat exchange between the wall and the bulk flow was detected for the same inlet Mach number and back pressure.     

The effect of heat transfer on the pressure drop through a heat exchanger for aero engine applications

This work is focused on the experimental study of the performance of a heat exchanger designed for aero engine applications. The heat exchanger is operating as a heat recuperator by taking advantage of the thermal energy of the exhaust gas of the aero engine in order to obtain a better combustion with less pollutant emissions. The experimental study has been performed in a wind-tunnel by taking detailed flow and thermal measurements on a 1:1 model of the heat exchanger under various operating conditions described by the hot gas inlet mass flow rates and its spatial direction (different angles of attack and inclination) towards the heat exchanger. The hot gas has been modeled with preheated air. Six sets of measurements have been carried-out for different hot gas inlet and outlet temperatures, including also isothermal measurements without any heat transfer in order to have a reference point for the pressure drop of the flow through the device. The experimental results showed that the effect of the angle of attack on the pressure drop is significant while the effect of the angle of inclination is negligible. Additionally, the pressure drop through the heat exchanger is greatly affected by the heat transfer.     

Numerical investigation of the effects of ITD length on low pressure nozzle

The advantage of high efficiency,low SFC (Specific Fuel Consumption),ultra-high bypass ratio turbofan engine attracts more and more attention in modem commercial engine.The intermediate turbine duct (ITD),which connects high pressure turbine (HPT) with low pressure turbine (LPT),has a critical impact on the overall performance of turbine by guiding flow coming from HPT to LPT without too much loss.Therefore,it becomes more and more urgent to master the technique of designing aggressive,even super-aggressive ITD.Much more concerns have been concentrated on the duct.However,in order to further improve turbine,LPT nozzle is arranged into ITD to shorten low pressure axle.With such design concept,it is obvious that LPT nozzle flow field is easily influenced by upstream duct structure,but receives much less interests on the contrary.In this paper,numerical method is used to investigate the effects of length of ITD with upstream swirl blades on LPT nozzle.Nine models with the same swirl and nozzle blades,while the length of ITD is the only parameter to be changed,will be discussed.Finally,several conclusions and advices for designers are summarized.After changing axial length of ducts,inlet and outlet flow field of nozzle differs,correspondingly.On the other hand,the shearing stress on nozzle blade (suction and pressure) surface presents individual feature under various inlet flow.In addition to that,"Clocking-like effect" is described in this paper,which will contribute much to the pressure loss and should be paid enough attention.