Numerical Studies of Methanol PPC Engines and Diesel Sprays

The global environment suffers from utilizing fossil fuels to powering internal combustion engines (ICE), due to the massive amounts of released CO2. Besides the global impact, the local environments experience high concentrations of harmful pollutants such as NOx, CO, soot and particulate matter (PM). The automotive industry is continuously striving to find new solutions to decrease fuel consumption and also to develop cleaner and more advanced combustion systems, i.e., low-temperature combustion (LTC) engines.The goal of this thesis is to employ computational fluid dynamics (CFD) simulations to investigate methanol under the partially premixed combustion (PPC) regime, which is one of the advanced LTC concepts alongside HCCI and RCCI. The benefit of PPC engines is the reduced average combustion temperature, which results in optimized emission rates of UHC/CO and NOx, maintaining high thermal efficiency. Interesting properties of methanol, such as a low stoichiometric air to fuel ratio and high latent heat of vaporization as well as non-sooting combustion, may enable further improvement of the PPC concept. Studies have been carried out by employing RANS and LES models to simulate mixing and ignition processes. It was found that methanol PPC can be achieved at relatively later injection timings (similar to those in diesel engines), in comparison to gasoline. Late injection timings can ease injection targeting into the piston bowl and utilize strong wall-spray interaction to help control the in-cylinder flow and therefore reduce the wall heat losses. The well-stirred-reactor (WSR) approach fails to predict pressure traces at highly stratified mixture compositions, such as $0.3 2$) can be accumulated in the near-wall region until the impingement vortices are developed, which then accelerates the mixing rate. Both wall jets resulted in more entrained air after the end of injection, which is considered to be the main reason for the faster oxidation of soot, with comparison to the free jet, which is in agreement with experimental measurements of the optical soot thickness KL

Cold Moderator Hydrogen Flow and Cooling

In this master thesis, cold moderator flow concepts have been developed on behalf of the European Spallation Source, ESS AB in Lund. The function of the cold moderator is to slow down neutrons, generated in the spallation process. The moderation is done in liquid para-hydrogen at 15 bar and 17 K. Moderating neutrons is the primary function of the liquid hydrogen, but it also removes nuclear heat load, which is deposited in the hydrogen and the moderator aluminum vessel. The flow in the cold moderator has to therefore meet requirements for the cooling performance and limit pressure losses. In order to analyze and develop the flow in the cold moderator, commercial CFD and structural analysis codes has been used

Cryogenic hydrogen cooling of heated moderator vessel

In ESS, a pulsed proton beam of 5 MW mean power will hit a tungsten target to generate neutrons by spallation. The pulses are 2.86 ms long and occur with 14 Hz; the power within a pulse is 125 MW. Only centimeters from the target, the neutrons are moderated by liquid hydrogen in aluminium vessels. The deposited power heats the surrounding structures and fluids which are circulated and cooled. The hydrogen is operating at 15 bar and average temperature between 17 and 21 K, i.e. above the critical pressure 12.8 bar, but below the critical temperature 32.9 K. During the pulses, the peak heat deposition in the aluminium is 15 W/cm3 and in the hydrogen 4 W/cm3. If the cooling of the aluminium is neglected during one pulse, the temperature increases to 34 K. That is above the critical temperature, where physical properties change strongly with temperature. Therefore the conjugate heat transfer has to be investigated in detail. This work includes 1D principal transient calculations of a general configuration as well as CFD simulations of the heating and cooling of a specific design. The 1D calculations are performed using GNU/Octave and the CFD using ANSYS/CFX. It is concluded that with an inlet temperature of 17 K, the wall temperature can be kept below the critical temperature in the general configuration and sufficient cooling can be ensured in the investigated specific design

Heat Loss Analysis for Various Piston Geometries in a Heavy-Duty Methanol PPC Engine

Partially premixed combustion (PPC) in internal combustion engine as a low temperature combustion strategy has shown great potential to achieve high thermodynamic efficiency. Methanol due to its unique properties is considered as a preferable PPC engine fuel. The injection timing to achieve methanol PPC conditions should be set very close to TDC, allowing to utilize spray-bowl interaction to further improve combustion process in terms of emissions and heat losses. In this study CFD simulations are performed to investigate spray-bowl interaction for a number of different piston designs and its impact on the heat transfer and the overall piston performance. The validation case is based on a single cylinder heavy-duty Scania D13 engine with a compression ratio 15. The operation point is set to low load 5.42 IMEPg bar with SOI -3 aTDC. After satisfactory agreement with experiments in terms of combustion phasing, in-cylinder pressure and heat release rate, the effect of piston bowl geometry is investigated by performing several CFD simulations with modified piston bowl geometry while keeping the compression ratio, CA50 and injection conditions the same as the baseline case. The influence of the wall temperature gradient, the near wall effective conductivity and the piston bowl area on the heat transfer is studied. It was observed that the flow structures that re-direct the hot vapor away from the in-cylinder walls will reduce the wall area that actively transfer the heat. The final piston performance comparison showed that piston bowl designs with a reduced area to volume ratio does not guarantee lower heat loss. Therefore, the mixing process as the result of the spray-bowl interaction and the resulting fuel distribution are considered as the main mechanisms to minimize the total heat losses

LES study of diesel flame/wall interaction and mixing mechanisms at different wall distances

In this paper, the flame-wall interaction of reacting diesel spray under engine like conditions is investigated using large eddy simulations. The aim of this study is to understand the influence of the distance between the wall and the spray nozzle on the air entrainment rate, which is a key variable in formation/oxidation process of soot. Three experimental cases are investigated, a free jet case and two wall impingement cases with a distance from nozzle to wall of 30 mm and 50 mm, which are considered as characteristic wall impingement distances for light- and heavy-duty bores in diesel engines, respectively. The optical soot measurements imply a positive influence of wall on the rate of soot oxidation. Numerical simulations are employed to elucidate importance of different mechanisms for the air entrainment, i.e., air entrainment prior to flame lift-off position, enhanced mixing due to the wall impingement and enhanced mixing by the entrainment wave. The results show that oxidation process after the end of injection is driven by a different mixing mechanism depending on the distance to the wall. The 30 mm case resulted in a "mixing boost", where the dominant mixing mechanism is the wall impingement vortex mixing, which gives rise to the fastest soot decay among the cases. The mixing in the 50 mm case is governed by a late wall impingement vortex mixing, giving rise to a low, but a constant air entrainment rate, i.e., a "mixing plateau". The free jet case resulted in mixing governed by the entrainment wave mechanism. Both wall impingement cases have faster soot oxidation rate compared with the free jet case, but due to a different underlying mixing process. LES is shown to be able to replicate the line-of-sight measurements of natural OH* chemiluminescence and distribution of soot region from the optical soot diagnostics

Effect of Start of Injection on the Combustion Characteristics in a Heavy-Duty DICI Engine Running on Methanol

Methanol as an alternative fuel in internal combustion engines has an advantage in decreasing emissions of greenhouse gases and soot. Hence, developing of a high performance internal combustion engine operating with methanol has attracted the attention in industry and academic research community. This paper presents a numerical study of methanol combustion at different start-of-injection (SOI) in a direct injection compression ignition (DICI) engine supported by experimental studies. The aim is to investigate the combustion behavior of methanol with single and double injection at close to top-dead-center (TDC) conditions. The experimental engine is a modified version of a heavy duty D13 Scania engine. URANS simulations are performed for various injection timings with delayed SOI towards TDC, aiming at analyzing the characteristics of partially premixed combustion (PPC). The simulations are based on a relatively detailed chemical kinetic mechanism and a well-stirred reactor (WSR) approach, accelerated using a so-called chemistry coordinate mapping (CCM). The injection of the fuel is treated with Lagrangian Particle Tracking (LPT) method. A baseline case with SOI of -20 after TDC (ATDC) was studied experimentally; this case was chosen to validate the model and a good agreement between the experiments and the simulation is found after adjustment of the initial pressure and temperature condition. In all injection conditions the combustion phasing is kept the same, i.e. with the 50-percentage heat release at the same crank angle (CA50) by adjusting the intake temperature. It is shown that as SOI is delayed the combustion characteristics changes significantly leading to a high maximal pressure-rise-rate (MPRR). The SOIs between -20 and -7 ATDC results in a combustion process governed by auto-ignition with propagating ignition fronts. The MPRR increases with SOI due to the rapid heat release caused by ignition at lean but increasingly richer conditions towards stoichiometry. The diffusion controlled, diesel like combustion (CDC), starts to occur around SOI -3 ATDC. The first portion of injected fuel ignites with a delay at leaner conditions, and then forms a diffusion flame. The amount of fuel consumed in the ignition process is larger than the amount of fuel consumed in the diffusion flame. Thus, contribution to the total heat release from the ignition process is larger and more rapid from that when using diesel or gasoline in the same CDC injection. Such behavior is attributed to a longer ignition delay time, large latent heat value and higher stoichiometric mixture fraction for methanol than hydrocarbon fossil fuels. It is concluded that a single main injection strategy of methanol may not be preferable due to the high MPRR thus other injection strategies, e.g., multiple injections should be used

Comparison of kinetic mechanisms for numerical simulation of methanol combustion in dici heavy-duty engine

The combustion process in a homogeneous charge compression ignition (HCCI) engine is mainly governed by ignition wave propagation. The in-cylinder pressure, heat release rate, and the emission characteristics are thus largely driven by the chemical kinetics of the fuel. As a result, CFD simulation of such combustion process is very sensitive to the employed reaction mechanism, which model the real chemical kinetics of the fuel. In order to perform engine simulation with a range of operating conditions and cylinder-piston geometry for the design and optimization purpose, it is essential to have a chemical kinetic mechanism that is both accurate and computational inexpensive. In this paper, we report on the evaluation of several chemical kinetic mechanisms for methanol combustion, including large mechanisms and skeletal/reduced mechanisms. These mechanisms are evaluated in terms of homogeneous ignition delay time, laminar flame speed, and multi-phase simulations of HCCI heavy-duty engine. The results are compared with experimental data and evaluated in terms of the accuracy and computational cost. It was found that scattering of ignition delay time predicted from different chemical kinetic mechanisms reported in the literature under homogeneous mixture ignition conditions give rise to a high sensitivity of the engine in-cylinder pressure prediction to the selected mechanism

Numerical Investigation of Methanol Ignition Sequence in an Optical PPC Engine with Multiple Injection Strategies

Methanol is a genuine candidate on the alternative fuel market for internal combustion engines, especially within the heavy-duty transportation sector. Partially premixed combustion (PPC) engine concept, known for its high efficiency and low emission rates, can be promoted further with methanol fuel due to its unique thermo-physical properties. The low stoichiometric air to fuel ratio allows to utilize late injection timings, which reduces the wall-wetting effects, and thus can lead to less unburned hydrocarbons. Moreover, combustion of methanol as an alcohol fuel, is free from soot emissions, which allows to extend the operation range of the engine. However, due to the high latent heat of vaporization, the ignition event requires a high inlet temperature to achieve ignition event. In this paper LES simulations together with experimental measurements on an heavy-duty optical engine are used to study methanol PPC engine. After a successful calibration of the pressure trace in terms of required intake temperature and combustion model, the optical natural luminosity data is used to validate prediction of ignition kernel and vapor penetration length. Moreover, it is shown that the inlet temperature requirement is reduced by 47 K degrees when applying multiple injection strategy. Changing the injection strategy also affects the average temperature of combustion and thus the emissions rates. Additionally, an ignition sequence analysis is performed to identify the mode of combustion and the heat release (HR) distribution depending on the local equivalence ratio, recognizing characteristics of PPC regime. Based on this analysis, a conceptual heat distribution model for PPC engine and other low temperature combustion (LTC) engine concepts is proposed