Effect of heat transfer space non-uniformity of combustion chamber components on in-cylinder heat transfer in diesel engine

Combustion chamber components (cylinder head-cylinder liner-piston assembly-oil film) were treated as a coupled body. Based on the three-dimensional numerical simulation of the heat transfer of the coupled body, a coupled three-dimensional calculation model for the in-cylinder working process and the combustion chamber components was built with domain decomposition and boundary coupling method, which adopts the coupled three-dimensional simulation of in-cylinder working process and the combustion chamber components. The model was applied in the investigation of the influence of space non-uniformity in heat transfer among combustion chamber components on in-cylinder heat transfer. The results show that the effect of wall temperature space non-uniform distribution of combustion chamber components on heat transfer happens mainly at the end of the compression stroke and expansion stroke. Therefore, it can be concluded that wall temperature space non-uniform distribution of combustion chamber components would influence heat transfer during the intake and exhaust stroke obviously.     

Ignition and boost effects on large-bore engine in-cylinder heat transfer

One cylinder of the Colorado State University large-bore test engine was instrumented with fast-response surface thermocouples for heat-transfer analysis. Probes were installed at several locations progressively farther from the ignition source and their outputs were recorded along with combustion pressure using a high-speed data-acquisition system. The engine was operated with two different ignition methods and the manifold boost pressure and cylinder jacket water temperature (JWT) were varied. The recorded surface temperature data were processed to calculate in-cylinder heat transfer.Combustion initiated with a screw-in type pre-combustion chamber resulted in significantly different characteristics than that initiated by a conventional spark plug. The differences in peak heat-flux value could likely be attributed to flame quench distance. Differences in other portions of the cycle could have been caused by significantly increased flame velocities associated with the pre-chamber jet. Increasing boost pressure from 25 to 54 kPa decreased peak heat-flux values about 20–30% and steady-state values about 13%. Increasing JWT 14 K had an insignificant effect on heat flux and combustion pressure.     

A highly efficient six-stroke internal combustion engine cycle with water injection for in-cylinder exhaust heat recovery

A concept adding two strokes to the Otto or Diesel engine cycle to increase fuel efficiency is presented here. It can be thought of as a four-stroke Otto or Diesel cycle followed by a two-stroke heat recovery steam cycle. A partial exhaust event coupled with water injection adds an additional power stroke. Waste heat from two sources is effectively converted into usable work: engine coolant and exhaust gas. An ideal thermodynamics model of the exhaust gas compression, water injection and expansion was used to investigate this modification. By changing the exhaust valve closing timing during the exhaust stroke, the optimum amount of exhaust can be recompressed, maximizing the net mean effective pressure of the steam expansion stroke (MEP_steam). The valve closing timing for maximum MEP_steam is limited by either 1 bar or the dew point temperature of the expansion gas/moisture mixture when the exhaust valve opens. The range of MEP_steam calculated for the geometry of a conventional gasoline engine and is from 0.75 to 2.5 bars. Typical combustion mean effective pressures (MEP_combustion) of naturally aspirated gasoline engines are up to 10 bar, thus this concept has the potential to significantly increase the engine efficiency and fuel economy.     

Evaluation of heat transfer correlations for HCCI engine modeling

Combustion in HCCI engines is a controlled auto-ignition of well-mixed fuel, air and residual gas. The thermal conditions of the combustion chamber are governed by chemical kinetics strongly coupled with heat transfer from the hot gas to the walls. The heat losses have a critical effect on HCCI ignition timing and burning rate, so it is essential to understand heat transfer process in the combustion chamber in the modeling of HCCI engines. In the present paper, a comparative analysis is performed to investigate the performance of well-known heat transfer correlations in an HCCI engine. The results from the existing correlations are compared with the experimental results obtained in a single-cylinder engine. Significant differences are observed between the heat transfer results obtained by using Woschni, Assanis and Hohenberg correlations.     

Numerical prediction of the instantaneous regenerator and in-cylinder heat transfer of a stirling engine

The paper is concerned with the study of the effects of the in-cylinder and regenerator heat transfer characteristics of a single-acting opposed-piston Stirling engine, with heater and cooler both omitted, for which a simulation model has been developed. The engine thermodynamic cycle is divided into a number of time-steps, and a system of nonlinear ordinary differential equations, which describe the energy balances over the three basic control volumes (hot and cold cylinders and regenerator), is solved numerically. Empirical correlations are used to determine the instantaneous heat transfer coefficients in the regenerator (flow across a porous medium) and inside the cylinder space (gas confined in a cylindrical volume with a moving boundary). Numerical results from the model are presented.     

Numerical technique for modeling conjugate heat transfer in an electronic device heat sink

A fast running computational algorithm based on the volume averaging technique (VAT) is developed to simulate conjugate heat transfer process in an electronic device heat sink. The goal is to improve computational capability in the area of heat exchangers and to help eliminate some of empiricism that leads to overly constrained designs with resulting economic penalties.VAT is tested and applied to the transport equations of airflow through an aluminum (Al) chip heat sink. The equations are discretized using the finite volume method (FVM). Such computational algorithm is fast running, but still able to present a detailed picture of temperature fields in the airflow as well as in the solid structure of the heat sink. The calculated whole-section drag coefficient, Nusselt number and thermal effectiveness are compared with experimental data to verify the computational model and validate numerical code. The comparison also shows a good agreement between FVM results and experimental data.The constructed computational algorithm enables prediction of cooling capabilities for the selected geometry. It also offers possibilities for geometry improvements and optimization, to achieve higher thermal effectiveness.     

段法模型在柴油机缸内辐射传热的应用

从段法基本概念出发,推得了柴油机缸内火焰段对整个缸壁段的辐射全交换面积计算式,并以此为基础给出了缸内辐射传热的单区段法模型。与实验结果比较表明,该模型能更准确地模拟缸内辐射传热量。将该模型与传统的单区模型比较,可得到壁面有效吸收系数的表达式,它是壁面黑度和火焰辐射系数的函数,并随壁面黑度的增大而增大,随火焰辐射系数的增大而减小。而传统的方法认为壁面有效吸收系数仅为壁面的黑度的函数。     

An estimation of quench-gas bias in internal combustion engine in-cylinder sampling

A simplified empirical-analytic model of flame quenching and subsequent removal of probe boundary-layer gases during in-cylinder sampling was used to study the quench-bias problem. For conditions similar to those of a diesel cylinder the model showed that the flame temperature has the greatest influence on the mass fraction of quench gas in a sample; cylinder pressure, orifice size, and sampling interval have less influence. For flame temperatures greater than 1900°K, the quench-mass fraction was computed to be relatively small—about 4% of the total mass. Even when the hydrodynamic boundary layer is neglected, the quench gas assumed at a uniform temperature equal to the wall temperature and the flame temperature taken at 2222°K, the quench-mass fraction is calculated to be only 9 to 16%. Extension of the model to the case where the bulk gas is flowing did not alter the conclusion that quench bias appears to be relatively unimportant in high temperature regions.     

冲程缸径比对汽油机缸内传热影响研究

针对中小排量缸内直喷单缸汽油机,通过台架试验数据与一维工作过程仿真相结合的手段,分析了冲程缸径比(S/B)变化对缸内传热功率、传热损失以及指示热效率的影响.研究表明:S/B变化对传热功率的影响因素不仅有面容比优化带来的收益,还有活塞运动速度增加导致的缸内对流强度的变化,两者叠加的结果是S/B并非越大越好,试验结果表明缸内直喷汽油燃烧系统,在常用的中等转速工况范围内,S/B的推荐值为1.2左右.     

Numerical modeling of conjugate heat transfer through concrete hollow bricks

The purpose of this study is to determine the thermal conductance of concrete hollow bricks, which is necessary for the evaluation of the energy efficiency of a building. The three varieties of hollow concrete bricks that are often used to build walls in Morocco are the subject of this study. A computational model created using the finite volume method is used to evaluate the conjugate heat transfer through concrete hollow bricks. According to the results, the use of hollow brick type A_h3 reduces the heat flux by approximately 86% compared with type A_h1. It is undeniable that hollow bricks type A_h3 with a thermal conductivity of 1 W/m K can improve the thermal characteristics of building walls.     

Analysis and modeling of char particle combustion with heat and multicomponent mass transfer

A char combustion model suitable for a large-scale boiler/gasifier simulation, which considers the variation of physical quantities in the radial direction of char particles, is developed and examined. The structural evolution within particles is formulated using the basic concept of the random pore model while simultaneously considering particle shrinkage. To reduce the computational cost, a new approximate analytical boundary condition is applied to the particle surface, which is approximately derived from the Stefan–Maxwell equations. The boundary condition showed reasonably good agreement with direct numerical integration with a fine grid resolution by the finite difference method under arbitrary conditions. The model was applied to combustion in a drop tube furnace and showed qualitatively good agreement with experiments, including for the burnout behavior in the late stages. It is revealed that the profiles of the oxygen mole fraction, conversion, and combustion rate have considerably different characteristics in small and large particles. This means that a model that considers one total conversion for each particle is insufficient to describe the state of particles. Since our char combustion model requires only one fitting parameter, which is determined from information on the internal geometry of char particles, it is useful for performing numerical simulations.     

补燃型不可逆联合循环热机的性能分析

在原有的不可逆联合动力循环模型的基础上,建立了一个存在热阻、热漏、补燃、内不可逆性的定常流联合卡诺热机循环模型。研究其在补燃作用下的功率和效率特性并对其进行优化,导出功率、效率的基本优化关系,分析了补燃对最优性能的影响。     

Detailed transient numerical simulation of H2/air hetero-/homogeneous combustion in platinum-coated channels with conjugate heat transfer

Transient two-dimensional simulations of fuel-lean H₂/air combustion were performed in a 2-mm-height planar channel coated with platinum, using detailed hetero-/homogeneous chemistry and transport as well as heat conduction in the solid wall. The developed model resolved, for the first time, all relevant spatiotemporal scales in a practical channel-flow reactor configuration. A parametric study was carried out to investigate the effects of wall material, inlet velocity, and inlet temperature on the fundamental catalytic and gas-phase combustion processes. Computational singular perturbation (CSP) analysis identified the key catalytic reactions affecting light-off and homogeneous ignition. Homogeneous ignition crucially depended on the OH desorbing fluxes from the catalyst, while flame propagation and stabilization involved time scales of a few milliseconds. During the short duration of the light-off event, the ensuing Stefan velocity appreciably altered the flow field. Predictions of time accurate numerical simulations were further compared against those of a code relying on the quasisteady state assumption, and the specific conditions under which the latter was invalidated were identified. Finally, CSP analysis unraveled the reasons for the high computational cost of the fully transient 2-D simulations. The surface reaction mechanism exhibited a high stiffness with fastest time scales of the order of 10^-12${10}^{-12}$ s, pertaining to the hydrogen adsorption and to the H(s) + O(s) = OH(s) + Pt(s) reactions. These time scales were in turn six orders of magnitude shorter than the ones associated with gas-phase chemistry or with a simplified single-step catalytic reaction.     

基于缸压的船用柴油机燃烧闭环控制策略研究

针对柴油机全寿命过程中动力性下降及各缸工作不均匀的问题,以MAN 6L16/24型船用柴油机为研究对象,模拟因各缸喷油器老化而引起柴油机动力性下降和各缸工作不均匀,对比分析开环、转速闭环和燃烧闭环等不同控制策略改善柴油机动力性、工作均匀性的效果及动态控制性能,针对燃烧闭环控制转速滞后较大的问题,提出转速-燃烧闭环协同控制策略,分析了该控制策略对轨压波动和进气流道阻塞干扰因素的鲁棒性。仿真结果表明,协同控制策略可有效改善柴油机各缸工作不均匀现象,提升转速响应速度。     

Numerical simulation of conjugate,turbulent mixed convection heat transfer in a vertical channel with discrete heat sources

This paper presents the results of a comprehensive numerical study to analyze turbulent mixed convection in a vertical channel with a flush-mounted discrete heat source in each channel wall. The conjugate heat transfer problem is solved to study the effect of various parameters like the thermal conductivity of the wall material (k_s), the thermal conductivity of the flush-mounted discrete heat source (k_c), Reynolds number (Re_s), modified Richardson number (Ri⁎) and the aspect ratio of the channel (AR). The standard k–ε turbulence model, modified by including buoyancy effects, without wall functions, has been used for the analysis. The two-dimensional governing equations are discretised on a semi-staggered, non-uniform grid, using the finite volume method. The asymptotic computational fluid dynamics (ACFD) technique has been then applied to obtain a correlation for the non-dimensional maximum temperature θ¯_max, which can be used for a wide range of parameters.     

Numerical analysis of combustion and transient heat transfer processes in a two-stroke SI engine

A zero-dimensional model is presented to simulate the transient processes occurring within a two-stroke SI engine. A two zone combustion model, with a spherically expanding flame front originating from the spark location, is applied. The model is numerically solved using the network simulation model which allows coupling the combustion model with a heat transfer model where both radiant and convective heat contributions have been taken into account for the in-cylinder gases. The boundary conditions for this model are the convective heat transferred to the cooling medium. A gas mixture model has been used to obtain the influence of working fluid properties on combustion development.     

Numerical heat transfer and thermal engineering of AdBlue (SCR) tanks for combustion engine emission reduction

In the automotive industry the selective catalytic reduction (SCR), which converts nitrogen oxides into nitrogen and water in the presence of a reducing agent, namely the so-called AdBlue urea–water solution, becomes major importance as a powerful emission reduction technique. Thermal engineering of SCR-tanks is a great challenge due to the melting and freezing behaviour of the AdBlue-fluid. In this paper, an efficient use of computational fluid dynamics and numerical heat transfer methods is presented for developing and optimising automotive SCR-tank systems. These thermal simulations include the phase change (melting and freezing), heat conduction, power generation, and natural convection effects within the liquid. The accuracy of the numerical scheme is assessed by comparisons with benchmark cases and experimental data. Typical industrial application examples are presented.     

Methods for in-cylinder EGR stratification and its effects on combustion and emission characteristics in a diesel engine

The effects of in-cylinder EGR stratification on combustion and emission characteristics are investigated in a single cylinder direct injection diesel engine. To achieve in-cylinder EGR stratification, external EGR rates of two intake ports are varied by supplying EGR asymmetrically using a separated intake runner. The EGR stratification pattern is improved using a 2-step bowl piston and an offset chamfer at the tangential intake port. When high EGR gas is supplied to the left (tangential) port, a high EGR region is formed at the central upper region of the combustion chamber. Consequently, combustion is initiated in the low EGR region, and PM is reduced significantly. When high EGR gas is supplied to the right (helical) port, a high EGR region is formed at the lower periphery of the combustion chamber. Therefore, combustion is initiated in the high EGR region, and NO_x is reduced without PM penalty. Stratified EGR potentially reduces NO_x by maximum 45%, without penalties of performance and other emissions. A proper in-cylinder swirl with stratified EGR maximizes the effects and achieves simultaneous reduction of NO_x by 7% and PM by 23%. Moreover, the robustness of stratified EGR is evaluated under various operating conditions and injection strategies.     

Non‐gray modeling of radiative heat transfer in hydrogen combustion scenarios

New weighted‐sum‐of‐gray‐gases model (WSGGM) parameters for H₂O vapor are derived from emissivity correlations in the open literature and presented for use in hydrogen combustion simulations. Predictions employing the new WSGGM are seen to compare favorably against the spectral‐line‐based WSGGM on benchmark problems as well as in media conditions representative of turbulent hydrogen diffusion flames (Sandia Flame A and a model hydrogen gas‐turbine combustor). The Sandia Flame A calculations were performed in a decoupled manner employing experimental measurements of temperature and gas compositions as inputs. The measured temperature variance data were employed to model the turbulence–radiation interactions. A coupled computational fluid dynamic (CFD) calculation was performed to obtain the conditions within the gas‐turbine combustor geometry. The observed accuracies of the proposed set of WSGGM parameters in conditions encompassing a wide range of H₂O vapor concentrations and temperature non‐homogeneities encountered in combustion media make them amenable to implementation in CFD codes. Copyright © 2011 John Wiley & Sons, Ltd.