在內(nèi)燃機設(shè)計領(lǐng)域中，企業(yè)不僅需要在同行競爭中保持領(lǐng)先，而且還需積極探索，努力遵循新的排放法規(guī)要求。
計算流體動力學(xué)（CFD）工具可協(xié)助發(fā)動機設(shè)計師不斷努力，設(shè)計出更高性能、更低排放的發(fā)動機，且期間無需打造成本高昂的實體原型。在CFD工具的協(xié)助下，設(shè)計師可以直觀地觀察、測試虛擬燃燒室中的模擬燃料燃燒與點火情況，打造更加輕松、清潔、高效的發(fā)動機。
顯而易見，只有當(dāng)模擬結(jié)果能夠真實反映客觀事實時，CFD技術(shù)才有其存在的價值。因此，為了準(zhǔn)確預(yù)測發(fā)動機的真實性能與排放情況，設(shè)計師的模擬必須精準(zhǔn)反映燃燒過程中的化學(xué)動力學(xué)變化。需要指出的是，經(jīng)過簡化的燃料化學(xué)模型，以及哪怕最精細的網(wǎng)格劃分技術(shù)，均無法實現(xiàn)這一目標(biāo)，而如果模擬手段不能發(fā)揮作用，設(shè)計師們就不得不重返成本昂貴且耗時久遠的物理測試手段。
CFD技術(shù)應(yīng)對“燃燒模擬挑戰(zhàn)”
具體來說，利用CFD技術(shù)模擬燃燒過程非常復(fù)雜，需要進行大量的計算，特別是在一些煙炱形成或發(fā)動機震爆等過程中尤其如此。傳統(tǒng)的多層CFD模擬需要設(shè)定幾千個變量，計算過程可能需要數(shù)天才能完成。此外，為了追求最優(yōu)設(shè)計，設(shè)計師通常會一點點調(diào)整發(fā)動機的幾何尺寸和燃料種類，進行大量嘗試，這個過程很有可能需要持續(xù)數(shù)周，甚至數(shù)月的時間。
為了加快設(shè)計速度，許多CFD解決方案均選擇了簡化的燃燒化學(xué)模型，并寄希望于異常精細的網(wǎng)格劃分技術(shù)來彌補因此損失的精確度。然而，由于這些簡化模型通常來源于一些未經(jīng)縝密驗證的第三方資源，因此很容易出現(xiàn)來源不一致或互不兼容的問題。在模擬過程中，由于不同來源的模型可能會重復(fù)反映某些物質(zhì)或某些反應(yīng)過程，因此會給燃料的配比與定制帶來很大困難。
現(xiàn)如今，發(fā)動機燃油的配方經(jīng)常隨季節(jié)（例如，美國夏季汽油配方中的丁烷含量高于冬季配方）、地區(qū)（例如，美國地區(qū)柴油的屬性與歐洲柴油不同）及具體應(yīng)用場景的不同而變化。此外，乙醇和生物柴油等替代燃料也已成為化石燃料的有力補充。在此背景之下，設(shè)計師必須借助更強大的模擬工具才能處理成分日益復(fù)雜的發(fā)動機燃料。
為了探究這部分燃料的性能，我們必須確保模擬中采用的燃料模型能夠反映真實的化學(xué)過程。但遺憾的是，傳統(tǒng)CFD軟件包中自帶的燃料模型都采用了非常復(fù)雜的算法，且計算時間很容易成倍增加，特別是在涉及多成分燃料的情況下。
對煙炱形成和發(fā)動機爆震進行模擬
目前，最新的顆粒物（PM）排放法規(guī)已經(jīng)對發(fā)動機設(shè)計師提出了特別挑戰(zhàn)。眾所周知，由于物理化學(xué)反應(yīng)過程復(fù)雜，傳統(tǒng)的CFD工具很難模擬煙炱的形成過程。因此，在傳統(tǒng)的內(nèi)燃發(fā)動機設(shè)計中，設(shè)計師為了改善煙炱的生成情況，通常需要進行為期多年的原型制造與測試。
發(fā)動機震爆是指燃燒室內(nèi)的高壓燃料與空氣混合物的自燃，通常發(fā)生在駕駛員點火之前或之后的時段。為了準(zhǔn)確預(yù)測發(fā)動機的震爆，設(shè)計師必須準(zhǔn)確模擬火焰頭在進入燃燒室時的位置與結(jié)構(gòu)。不過，傳統(tǒng)的CFD工具主要基于簡化的化學(xué)過程與精細的網(wǎng)格劃分技術(shù)，因此通常很難準(zhǔn)確模擬自燃的過程。
由于火焰頭的厚度遠遠小于模擬工具中的網(wǎng)格尺寸（哪怕是最精密的網(wǎng)格劃分），為了確定火焰頭的位置，傳統(tǒng)CFD模擬工具需要借助數(shù)量非常龐大的網(wǎng)格才能解決火焰拓撲的難題。然而，由于需要借助的網(wǎng)格單元數(shù)量過于龐大，為了保證穩(wěn)定性，這種CFD模擬通常需要劃分成極細的步驟分步進行，因此耗時非常漫長。
作為發(fā)動機設(shè)計者需要怎么做呢？
從化學(xué)動力學(xué)入手
燃燒的核心就是化學(xué)反應(yīng)。因此，為了準(zhǔn)確預(yù)測發(fā)動機的真實性能表現(xiàn)，CFD模擬必須抓住真實化學(xué)反應(yīng)的核心--化學(xué)動力學(xué)。
舉例而言，ANSYS公司的內(nèi)燃發(fā)動機模擬工具ANSYS Forte并未采用公認(rèn)的尺度公式，而是轉(zhuǎn)而采用時間尺度。這樣一來，設(shè)計師可以根據(jù)真實需求盡可能多地加入各種反應(yīng)，整個過程并不會增加模擬時間。因此，即使一些規(guī)模更大且準(zhǔn)確度要求更高的燃料模型, 也能達到與“簡化版”模型相仿的計算速度。在這種情況下，設(shè)計人員可以快速、準(zhǔn)確地預(yù)測發(fā)動機的排放情況，且大幅減少實體硬件原型的使用需求。
ANSYS Forte工具支持行業(yè)標(biāo)準(zhǔn)版燃料模型庫（Model Fuel Library），可直接使用其中的大量燃料模型，其中包括很多新型混合燃料模型。
在精確化學(xué)燃料模型的協(xié)助下，設(shè)計師得以大幅提升燃燒過程的模擬質(zhì)量，進而更加敏捷、高效地應(yīng)對日益嚴(yán)苛的監(jiān)管條例要求，并打造真正先進的清潔發(fā)動機與燃料技術(shù)。
For internal-combustion (IC) engine design, companies need to stay ahead of not only the competition but also of new emissions mandates.
Computational fluid dynamics (CFD) can help engine designers create higher performance, lower emissions IC engines without costly physical prototyping. CFD lets engine designers visualize and test fuel and ignition behaviors within a virtual combustion chamber, providing a faster way to design cleaner, more efficient engines by simulating ignition and fuel dynamics.
But combustion CFD can only be of value if the results predict real-life behaviors. To predict actual performance and pollutant emissions, simulations need to accurately account for the chemical kinetics of the combustion process. Simulations that rely on drastically simplified fuel chemistries and ever-finer meshing technologies fall far short of this goal. And when they fall short, designers must fall back on expensive, time-consuming physical testing for answers.
Combustion CFD challenges
Combustion CFD is complex and computation-intensive, especially for applications such as soot formation and engine knock. Times can easily stretch into days for traditional CFD multi-stage simulations with thousands of variables. Incremental changes to engine geometries and fuel models can stretch the total time to weeks or months before an optimized design is realized.
To speed design time, many CFD solutions simplify combustion chemistry, trusting that severe mesh refinements can make up in detail what they lack in precise chemistry. These simplified fuel models rely on weakly validated, third-party mechanisms from disparate and incompatible sources. Using models from multiple sources makes it very difficult to blend or customize fuels in simulations because species and reactions may be duplicated—perhaps in a contradictory way—in different sources.
Better models are needed now because motor fuels have become more complex. Fuels vary by seasonal formulation (U.S. summer gasoline contains less butane than winter formulations), by region and by application (U.S. diesel has different properties than European diesel). Alternative fuels such as ethanol and biodiesel now supplement petroleum-derived fuels.
To understand the effects of these diverse fuel types, chemically correct fuel models are required. Unfortunately, fuel model algorithms in conventional CFD packages are complex and compute time can multiply exponentially when they are combined to represent multicomponent fuels.
Simulating soot formation and engine knock
New particulate matter (PM) regulations present particular challenges for engine designers. Soot phenomena are notoriously difficult to simulate and too complex to run in conventional CFD software, due to the physics and chemical reactions leading up to soot formation. As a result, optimization of soot in conventional combustion-engine design typically requires years of building and testing prototypes.
Knocking occurs when the highly compressed fuel and air mixture in the combustion chamber auto-ignites, either before or after the spark that is meant to trigger ignition. Accurately modeling the location and structure of the flame front as it expands into the combustion chamber is extremely important for predicting knock. But simulating auto-ignition is very difficult with conventional CFD approaches that rely on mesh refinement and simplified chemistry.
Since the scale of the flame front thickness is significantly smaller than computational mesh—even with severe grid refinement—CFD simulations that rely on mesh to resolve the flame location will require an inordinately large number of tiny cells to resolve the flame topology sufficiently. Simulations with large numbers of small cells can easily get bogged down by the tiny time steps needed to maintain simulation stability, and require an impractical amount of computation time.
What’s an engine designer to do?
Follow the chemistry
Chemistry is crucial. It’s at the heart of combustion, and for internal-combustion CFD to accurately predict real-world engine behavior, it must precisely account for real chemical kinetics.
ANSYS Forte, for example, changes the well-established scaling equation: instead of scaling with the cube of the number of species, the simulation time scales linearly. This allows an engine designer to include as many reactions as he or she requires for accurate simulations, without incurring a compute time penalty. Even larger, more accurate fuel models achieve compute times comparable to those with severely reduced, less accurate models. As a result, designers can quickly and accurately predict emissions that translate reliably to actual engine designs—with far less trial-and-error hardware prototyping.
Fuel components derived from the industry-validated Model Fuel Library enable ANSYS Forte to simulate combustion for a large variety of new or existing fuel blends and foresee what emissions will occur for a wide range of operating conditions.
By using accurate fuel models based on precise chemistry, engine designers greatly increase the predictive quality of combustion simulations, to more quickly and effectively meet strict regulatory guidelines and create advanced clean engine and fuel technologies.
Author: Bill Kulp
Source: SAE Truck & Off-highway Engineering Magazine
在SI扇形網(wǎng)格模擬中，燃料噴射器啟動時的可視化噴霧液滴。
ANSYS Forte模擬的煙炱形成過程。
ANSYS Forte可通過精確參考化學(xué)反應(yīng)中的真實化學(xué)動力學(xué)表現(xiàn)，準(zhǔn)確模擬內(nèi)燃發(fā)動機的燃燒過程。
