Further study on wall film effects and flame quenching under engine thermodynamic conditions

Abstract

In direct-injection engines, the formation of fuel wall film on piston surface and liner wall is a primary cause for emissions of unburnt hydrocarbons and particulate matter. It is therefore important to investigate the behavior and effect of a wall fuel film under typical engine conditions. Following our previous study of wall film effects on flame propagation and quenching under constant thermodynamic conditions (Tao et al. IJER, 2018), more complexities rooted in real engine conditions have been considered in the current work, including two real engine in-cylinder thermodynamic trajectories occurring at catalyst warming (CW) and low-speed high-load (LSHL) conditions, varying fuel wall film thickness accounting for both vaporization and condensation, and the stagnation boundary layer flow over the wall film. To shed light on the role of wall heat transfer, parameter sweeping of wall temperature is conducted from 303 K to 363 K. Two representative wall film models using empirical vaporization rate and gas-liquid interface heat flux are compared with our numerical simulation. A good correlation between vapor boundary thickness and quenching distance has been found. Competition between enhanced vaporization due to tangential convection and aggravate condensation from elevated pressure is also demonstrated. The results lead to useful insights into the behavior of a wall film in real engine in-cylinder thermodynamic conditions and could be used to construct low dimensional empirical wall film models in three-dimensional engine combustion modeling.

Introduction

Direct injection is very beneficial for modern internal combustion engines, with advantages such as more accurate fuel delivery, higher compression ratio, unthrottled operation, less pumping losses, and modulated pressure rise rate, and it has become a dominant fuel delivery method for modern internal combustion engines [1,2]. However, DI engines tend to have relatively higher particulate matter (PM) and unburnt hydrocarbon (UHC) emissions, primarily from spray-wall interaction [3], [4], [5], [6], [7]. Fuel films on the piston surface or cylinder liner wall are more likely to occur during a cold start with reduced evaporation, or at low-speed high-load conditions with large amounts of fuel injection. As a flame approaches a fuel film, the rich mixture near the film can lead to incomplete combustion and early flame quenching. In addition, there are other scenarios where the wall film becomes a concern for engine emission, such as the non-premixed pool fire forming above the wall film and fuel vapor thermal pyrolysis after flame quenching. To continue meeting increasingly stringent particulate emissions regulations, flame and wall film interaction merits detailed investigation. In addition to exacerbating emission formation, wall wetting can also contribute to oil dilution, causing reduced tribology performance and more frequent needs for maintenance.

Compared with the interaction between a flame and a wall surface, where the quenching distance is typically in the same order of flame thickness [8], the existence of a vaporizing fuel film forms novel upstream boundary conditions of both temperature and fuel vapor concentration when a flame approaches its vicinity. Given that the film thickness in engines is in the order of micrometers, substantial challenges have been placed on direct experimental measurement. Consequently, numerical simulation plays a unique role in the study of flame-wall film interaction under engine conditions. Desoutter et al. [9] computationally investigated premixed flame-film interaction by using one-step overall chemistry and found that quenching distance increases due to chemical effects from local rich mixture instead of heat loss. Recently, Tao et al. [10] investigated the premixed flame-fuel film interaction using one-dimensional (1D) direct numerical simulation (DNS) with detailed chemistry and transport, assuming constant wall film thickness and constant thermodynamic condition. A nearly constant local equivalence ratio at quenching is identified, further confirming the dominant effect of rich mixture on flame quenching [9]. Pressure and boiling point variation is found to dominate the fuel vapor boundary layer development by comparing simulations under constant volume and constant pressure conditions.

However, in real engine conditions, due to piston motion and flow, a more realistic description of the fuel film and engine operating condition needs to be accommodated to determine the lifetime and effect of an impinged fuel film. There are at least four dominant effects that have to be taken into account: (1) pressure and temperature variation due to piston motion; (2) thermal expansion and compression from flame propagation; (3) conjugate heat transfer from the liner wall; (4) convection due to tangential flow over the film. Unfortunately, neither complex engine thermodynamic trajectory nor the fluid mechanics effects on the behavior of a wall film are sufficiently understood. In this work, we have considered these effects strategicially by formulating low-dimensional combustion problems, trying to understand the behavior of a wall film under engine thermodynamic conditions. Furthermore, we recognized that many numerical models for liquid film have been developed, including simple uniform temperature model without consideration of liquid temperature distribution [11], discretized temperature model for thick films [11], wall functions with empirical vaporization rates [12], and no vaporization assumption below boiling point [13]. It will be useful to further compare some of these commonly implemented wall film models with the current DNS approach.

This article is organized as follows. We first introduce the models and results for the 1D unsteady DNS of a laminar premixed flame propagating toward a liquid film with detailed chemistry and transport, subject to two real engine in-cylinder thermodynamic conditions. The results by using the DNS method are further compared with calculation using different empirical evaporation models. Eventually, competing effects between enhanced condensation with elevated pressure and enhanced vaporization due to tangential convection are demonstrated, in a 2D stagnation boundary layer flow over the wall film.