Effect of structural conduction and heat loss on combustion in micro-channels

This paper presents a simple analytical model for the effects of heat exchange within the structure of a micro-channel combustor, and heat loss from the structure to the environment. This is accomplished by extending reasoning similar to that employed by Mallard and Le Chatelier in their thermal theory for flame propagation. The model is used to identify some of the basic parameters that must be considered when designing an efficient micro-combustor and its predictions are compared with the results of a numerical simulation of stoichiometric premixed combustion of a hydrogen–air mixture stabilized between two parallel plates. The simulation incorporates a one-dimensional continuity/energy equation solver with full chemistry coupled with a model for thermal exchange in the structure. The results show that heat exchange through the structure of the micro-combustor can lead to a broadening of the reaction zone. Heat loss to the environment decreases the broadening effect and eventually results in flame quenching. This behaviour, which arises from the thermal coupling between the gas and the structure, influences the maximum achievable power density of microscale combustors.     

Pore-resolved simulations of porous media combustion with conjugate heat transfer

Porous media combustion (PMC) is an active field of research with a number of potential advantages over free-flame combustors. A key contributor to these phenomena is the interphase heat exchange and heat recirculation from the products upstream to the reactants. In this paper, we present a network model that captures the conjugate heat transfer in pore-resolved 2D simulations of PMC. A series of simulations are presented with varying solid conduction and inlet velocity to isolate the role of conjugate heat transfer on the salient features of the burner, including flame stability, axial temperature profiles, and flame structure. We show that both the flame stabilization and the propagation behavior are strongly related to the conjugate heat transfer, and the flame stability regime is shifted to higher velocities as the conductivity of the solid material is increased.     

Bifurcation analysis of steady states and limit cycles in a thermal pulse combustor model

Oscillatory behaviour of state variables is desirable in pulse combustors, as properly designed pulsations lead to improved performances, such as higher thermal efficiency and lower emissions compared to steady combustors. In the present work, we perform a systematic investigation of the stability of steady states and limit cycles of a pulse combustor model through numerical continuation. Different bifurcation parameters such as tailpipe friction factor, wall temperature, convective heat transfer coefficient, inlet temperature and inlet fuel mass fraction are varied to identify the complete ranges of those parameters at which limit cycles can be expected. This analysis identifies alternative stable periodic regimes in parameter space (e.g. friction factor). In addition, a few performance indicators such as amplitude of oscillations, cycle-averaged heat transfer and cycle-averaged specific thrust are compared between different ranges of friction factor yielding limit cycle oscillations. The comparison clearly shows that, depending upon the application, friction factor can be chosen from different regimes. The time-integration of the model is also performed to support the bifurcation results obtained from numerical continuation, wherever appropriate. The complete stability margin of limit cycle oscillations for those bifurcation parameters can be useful for improved design of the combustor and for determining the optimal operating conditions of pulse combustors.     

Modeling of a self-excited pulse combustor and stability analysis

The major bottleneck for popularization and utilization of the conventional mechanical valve pulse combustors is the self-priming mode of gas supply. An aerodynamic valve (as against mechanical valve) self-excited pulse combustor of the Helmholtz-type with continuous supply of gas and air was designed and a mathematical model was established in this paper. The theoretical model employed well-stirred reactor model and a single step Arrhenius chemistry, and took those factors which might affect the combustion stability into account. The factors include the variation of the mass rate of the reactants affected by the pressure in the combustion chamber, the convective and radiation heat loss in the combustion chamber, and the heat transfer and wall friction in the tailpipe. The effect of wall temperature of combustion chamber, wall heat transfer coefficient, tailpipe length and friction coefficient on combustionstability were analyzed. The range of combustion oscillations can be predicted. It is theoretically and experimentally shown that combustion oscillations can be produced with a continuous supply of fuel and air without mechanical valves. The experimental data show qualitative agreement with predictions from the theoretical model. The theoretical model could be a tool for designing and optimizing the self-excited pulse combustor.     

底部局部加热多孔介质自然对流传热的格子Boltzmann模拟

运用格子Boltzmann方法研究了底部局部加热多孔介质方腔的自然对流传热.方腔的上壁面为低温热源,下壁面为局部高温热源,左右壁面为绝热条件.重点分析了高温热源位置a及尺寸b对多孔介质方腔自然对流传热性能的影响,提出了平均Nusselt数Nu和位置a及尺寸b的拟合关系式.研究结果表明:高温热源位置及尺寸对多孔介质方腔内自然对流传热性质的影响很大,且存在最佳高温热源位置(a=4/16)和尺寸(b=0.75),以达到最强的对流换热强度(Nu_(max)≈10.35)和最大的对流换热量(Q_(max)≈5.69).     

高功率半导体激光器微通道热沉的方案设计

 对用于高功率半导体激光器的叠片式微通道热沉进行方案设计,利用计算流体力学和数值传热学对各种方案进行数值仿真,研究了微通道的特征尺寸和流量等因素对冷却效果和流动阻力特性的影响,一般情况下,减小微通道的特征尺寸和增加冷却水的流量可以降低传热热阻,但增加了流动压力损失;另外对金刚石热扩散片（次热沉）的效果也进行了数值计算,计算结果表明:金刚石热扩散片在该类型问题中降低温度作用明显。     

Scaling of heat transfer in gas-gas injector combustor

The scaling of heat transfer in gas-gas injector combustor is investigated theoretically, numerically and experimentally based on the previous study on the scaling of gas-gas combustion flowfield. The similarity condition of the gas-gas injector combustor heat transfer is obtained by conducting a formulation analysis of the boundary layer Navier-Stokes equations and a dimensional analysis of the corresponding heat transfer phenomenon. Then, a practicable engineering scaling criterion of the gas-gas injector combustor heat transfer is put forward. The criterion implies that when the similarity conditions of inner flowfield are satisfied, the size and the pressure of gas-gas combustion chamber can be changed, while the heat transfer can still be qualitatively similar to the distribution trend and quantitatively correlates well with the size and pressure as q ∝ pc0 .8d t-0.2. Based on the criterion, single-element injector chambers with different geometric sizes and at different chamber pressures ranging from 1 MPa to 20 MPa are numerically simulated. A single-element injector chamber is designed and hot-fire tested at seven chamber pressures from 0.92 MPa to 6.1 MPa. The inner wall heat flux are obtained and analysed. The numerical and experimental results both verified the scaling criterion in gas-gas injector combustion chambers under different chamber pressures and geometries.     

Influence of wall heat loss on the emission characteristics of premixed ammonia-air swirling flames interacting with the combustor wall

The influence of wall heat loss on the emission characteristics of ammonia-air swirling flames has been investigated employing Planar Laser-Induced Fluorescence imaging of OH radicals and Fourier Transform Infrared spectrometry of the exhaust gases in combustors with insulated and uninsulated walls over a range of equivalence ratios, ?, and pressures up to 0.5 MPa. Strong influence of wall heat loss on the flames led to quenching of the flame front near the combustor wall at 0.1 MPa, resulting in large unburned NH₃ emissions, and inhibited the stabilization of flames in the outer recirculating zone (ORZ). A decrease in heat loss effects with an increase in pressure promoted extension of the fuel-rich stabilization limit owing to increased recirculation of H₂ from NH₃ decomposition in the ORZ. The influence of wall heat loss resulted in emission trends that contradict already reported trends in literature. NO emissions were found to be substantially low while unburned NH₃ and N₂O emissions were high at fuel-lean conditions during single-stage combustion, with values such as 55 ppmv of NO, 580 ppmv of N₂O and 4457 ppmv of NH₃ at ? = 0.8. In addition, the response of the flame to wall heat loss as pressure increased was more important than the effects of pressure on fuel-NO emission, thereby leading to an increase in NO emission with pressure. It was found that a reduction in wall heat loss or a sufficiently long fluid residence time in the primary combustion zone is necessary for efficient control of NH₃ and N₂O emissions in two-stage rich-lean ammonia combustors, the latter being more effective for N₂O in addition to NO control. This study demonstrates that the influence of wall heat loss should not be ignored in emissions measurements in NH₃-air combustion, and also advances the understanding of previous studies on ammonia micro gas turbines.     

竖直微槽道内EHD强化饱和沸腾传热研究

针对微槽内饱和沸腾汽泡建立了简化模型,并利用COMSOL Mu ltiphysics软件对电场中汽泡动力学特性进行了数值模拟,分析了微槽道内EHD(electrohydro dynam ics)强化沸腾传热机理。实验以去离子水为工质,研究了外加直流电场下两种规格的矩形微细槽道内饱和沸腾传热强化特性,电压在0~28kV内,EHD技术对微细槽道内的饱和沸腾传热有明显的强化效果。     

含内热源可燃多孔介质热量传递及影响因素分析

本文模拟了自然对流及恒壁温边界条件下含内热源可燃填充床内最高温度随时间的变化;计算了侧面和端面取不同换热系数时填充床内的温度分布;分析了对流换热系数、堆积状态等对床内最高温度的影响。计算结果表明:各边界取不同换热系数时对最高温度的影响不大;在环境温度低于383．15 K,或壁面温度低于381．15 K的条件下,多孔床不会发生自燃。对流换热系数较大、堆积密度较小时多孔床也不易自燃。     

Scale and geometry effects on heat-recirculating combustors

A simple analysis of linear and spiral counterflow heat-recirculating combustors was conducted to identify the dimensionless parameters expected to quantify the performance of such devices. A three-dimensional (3D) numerical model of spiral counterflow ‘Swiss roll’ combustors was then used to confirm and extend the applicability of the identified parameters. It was found that without property adjustment to maintain constant values of these parameters, at low Reynolds number (Re) smaller-scale combustors actually showed better performance (in terms of having lower lean extinction limits at the same Re) due to lower heat loss and internal wall-to-wall radiation effects, whereas at high Re, larger-scale combustors showed better performance due to longer residence time relative to chemical reaction time. By adjustment of property values, it was confirmed that four dimensionless parameters were sufficient to characterise combustor performance at all scales: Re, a heat loss coefficient (α), a Damköhler number (Da) and a radiative transfer number (R). The effect of diffusive transport effect (i.e. Lewis number) was found to be significant only at low Re. Substantial differences were found between the performance of linear and spiral combustors; these were explained in terms of the effects of the area exposed to heat loss to ambient and the sometimes detrimental effect of increasing heat transfer to adjacent outlet turns of the spiral exchanger. These results provide insight into the optimal design of small-scale combustors and choice of operation conditions.     

Z箍缩驱动混合堆包层瞬态传热特性

Z箍缩驱动聚变-裂变混合能源堆(Z-FFR)以较长周期(10s)脉冲式运行,为实现3000 MW的热功率输出,单个脉冲需要产生的能量较大,包层和第一壁在强热冲击下的瞬态传热和温度特性是决定Z-FFR技术可行性的关键问题之一。通过理论计算,分析了在连续脉冲作用下包层和第一壁温度随时间的变化规律。同时以输出恒定的电功率为目标,提出了展平系统输出功率的简便方法,并分析了出口冷却剂温度的波动特性。结果表明材料最高温度均在安全限值内,第一壁表面瞬时高温层厚度约为0.5mm,系统输出功率波动幅度在-2.84%~+2.05%范围内。     

Catalyzed combustion of hydrogen–oxygen in platinum tubes for micro-propulsion applications

The present investigation addresses the need to understand the physics and chemistry involved in propellant combustion processes in micro-scale combustors for propulsion systems on micro-spacecraft. These spacecraft are planned to have a mass less than 50 kg with attitude control estimated to be in the 1–10 mN thrust class. Micro-propulsion devices behave differently than macro-scale devices because of the differences in magnitude of flow rates and heat transfer. Reducing the combustor size increases the relative surface area, increasing the heat loss, and as combustors are continuously reduced in size, they approach the quenching dimensions of the propellants. Combustors of this size are expected to significantly benefit from surface catalysis processes. A miniature flame tube apparatus is chosen for this study because microtubes can be easily fabricated from known catalyst materials, and their simplicity in geometry can be used in fundamental simulations for validation purposes. Experimentally, we investigated the role of catalytically active surfaces within 0.4 and 0.8 mm internal diameter microtubes, with special emphases on ignition processes in fuel rich gaseous hydrogen and gaseous oxygen. Calculations of flame thickness and reaction zone thickness predict that the diameters of our test apparatus are below the quenching diameter of the propellants in most atmospheric test conditions. The temperature and pressure rise in resistively heated platinum microtubes and the exit hydrogen concentration were used as an indication of exothermic reactions. Data on imposed heat flux/preheat temperature required to achieve ignition versus mass flow rate are presented. With a plug flow model, the experimental conditions were simulated with detailed gas-phase chemistry and surface kinetics. Computational results, in general, support the experimental findings.     

Numerical study of heat transfer enhancement of counter nanofluids flow in rectangular microchannel heat exchanger

This paper reports a numerical analysis of the performance of a counter-flow rectangular shaped microchannel heat exchanger (MCHE) using nanofluids as the working fluids. Finite volume method was used to solve the three-dimensional steady, laminar developing flow and conjugate heat transfer in aluminum MCHE. The nanofluids used were Ag, Al₂O₃, CuO, SiO₂, and TiO₂ and the performance was compared with water. The thermal, flow fields and performance of the MCHE were analyzed using different nanofluids, different Reynolds numbers and different nanoparticle concentrations. Temperature profile, heat transfer coefficient, pressure profile, and wall shear stress were obtained from the simulations and the performance was discussed in terms of heat transfer rate, pumping power, effectiveness, and performance index. Results indicated enhanced performance with the usage of nanofluids, and slight penalty in pressure drop. The increase in Reynolds number caused an increase in the heat transfer rate and a decrease in the overall bulk temperature of the cold fluid. The increase in nanoparticle concentration also yielded better performance at the expense of increased pressure drop.     

压力边界条件下微通道内气体流动换热特性研究

本文用直接模拟蒙特卡罗方法对给定进出口压力边界条件下微通道内气体的流动换热特性进行了数值模拟,给 出了壁面与流体的温差对气体沿程压力、温度及数密度分布的影响。计算结果表明,当壁温高于流体温度时,温差仅出现 在通道进出口处,但其发生机理却不同;流体可压缩性与稀薄性均得到增强,沿程压力分布更加非线性。     

新一代航空发动机燃烧室

针对新一代高油气比(0.051及以上)航空发动机燃烧室,本文提出头部采用化学恰当比直接混合燃烧设计方案.对于新一代高压比(70及以上)低排放民用航空发动机燃烧室,由于其自着火延迟时间极短,因此采用贫油直接混合燃烧,而不能采用预混合预蒸发燃烧.本文提出了一种贫油直接混合低排放燃烧室方案,其燃油空气模由简单的压力雾化喷嘴和...     

The variations of heat transfer and slip velocity of FMWNT-water nano-fluid along the micro-channel in the lack and presence of a magnetic field

Simulation of forced convection of FMWNT-water (functionalized multi-walled carbon nano-tubes) nano-fluid in a micro-channel under a magnetic field in slip flow regime is performed. The micro-channel wall is divided into two portions. The micro-channel entrance is insulated while the rest of length of the micro-channel has constant temperature (T_C). Moreover, the micro-channel domain is exposed to a magnetic field with constant strength of B₀. High temperature nano-fluid (T_H) enters the micro-channel and exposed to its cold walls. Slip velocity boundary condition along the walls of the micro-channel is considered. Governing equations are numerically solved using FORTRAN computer code based on the SIMPLE algorithm. Results are presented as the velocity, temperature, and Nusselt number profiles. Greater Reynolds number, Hartmann number, and volume fraction related to more heat transfer rate; however, the effects of Ha and ϕ are more noteworthy at higher Re.     

Two-phase flow boiling heat transfer of FC-72 in parallel micro-channels

Experimental research is performed on two-phase flow boiling heat transfer in micro-channels. FC-72 is used as the working fluid. In order to analyze the heat transfer mechanism during two-phase flow boiling, the dimensionless parameters, e.g., boiling number and convection number, are used, and the effect of these parameters on the heat transfer can be confirmed during flow boiling in the micro-channel. In addition, the transition criterion from bubbly/slug flow to annular flow is proposed from the modified Weber number. Based on the boiling heat transfer mechanism obtained from the experiments, a new correlation is proposed to predict the heat transfer coefficient. The new correlation predicts well the experimental results within a mean absolute error of 5.2%.     

An improved model to calculate radiative heat transfer in hot combustion gases

Radiative heat transfer plays an important role in the chemical reactions in the combustor. The widely used WSGG model proposed by Smith is established for normal pressure, which shows inevitable computational errors when dealing with radiative heat transfer problems at reduced or elevated pressures. In this paper, an improved global model is established to calculate the radiant energy exchanges between combustion gases and combustor chamber walls. Compared with the Smith model, the new model shows better performance in a wide range of pressure regions. The model accuracy is examined by computing the emissivity, radiative heat flux as well as the radiative source of H₂O–CO₂ gas mixtures at different pressure values. Finally, the radiative heat transfer inside a 3D TBCC(turbine-based combined cycle) engine exhaust system where strong gradients of pressure and temperature exist, is also addressed. The computational results show that the developed model provides approximate results at much less computational costs than the high-precision MSMGFSK-c8 model, which makes it competitive in complicated combustion systems.     

用于高热流密度器件冷却的热管散热器实验研究

针对大型计算机服务器CPU的耗能量,探讨了一种新的热管排布方式的散热器,并对其散热性能进行了实验研究.研究结果表明,采用此种热管散热器,最高热流密度为74.3W/cm2,其冷却风速控制在4m/s即可满足芯片冷却要求.同时根据模拟计算得到的散热器底板温度分布,可有助于对热管排布方式的优化设计.