Stage Heat Release Research Articles

The comparison between traditional spark ignition and micro flame ignition in gasoline high dilution combustion

Gasoline spark ignition (SI) – Controlled auto-ignition (CAI) hybrid combustion had previously been shown to expanding the operational range of high-efficiency low-temperature combustion and reducing fuel consumption. However, the spark ignition became ineffective when the mixture became highly diluted and the large cyclic variation and even misfire would occur. To achieve high-efficiency combustion in extended engine operational range and overcome the limitation of SI-CAI hybrid combustion, Micro Flame Ignition (MFI) was proposed and researched as a mean to providing multiple auto-ignition sites to initiate the combustion process of the diluted mixture. In this research, both engine experiments and Computational Fluid Dynamics (CFD) simulations were carried out to study the MFI combustion and SI-CAI hybrid combustion in a single-cylinder optical engine. Compared to the SI-CAI hybrid combustion, the flame propagation in MFI hybrid combustion was initiated by a large number of reaction fronts produced by the DME auto-ignition at multiple sites. MFI was found to deliver substantially more heat and ignition energy to the premixed mixture than the single spark ignition, enabling much faster initial heat release. However, the flame front expansion speed of MFI hybrid combustion dropped significantly to a similar value to that of the SI-CAI case because of the slower flame speed of diluted gasoline mixture. The MFI combustion exhibited three phases of autoignition stage, flame propagation stage and fast heat release stage. It is characterized by a higher peak heat release rate and shorter duration of the main combustion than those of the SI-CAI combustion. Besides, the use of spark ignition in the MFI operation promoted the autoignition of DME, leading to a shorter combustion duration and faster combustion than the MFI combustion without spark ignition. As a result, the spark assisted MFI strategy could be used to control the combustion phasing and optimize the MFI combustion process.

Auto-Ignition and Reaction Front Dynamics in Mixtures With Temperature and Concentration Stratification

In-cylinder thermal and concentration stratification is omnipresent in the operation of internal combustion engines, especially for various types of direct injection engines. Meanwhile, mixture stratification and ϕ-sensitivity is frequently adopted in advanced compression ignition strategies to modulate heat release profile and control combustion phasing, such as homogeneous charge compression ignition (HCCI) with partial fuel stratification (PFS), and premixed charge compression ignition (PCCI). Hence, ignition and combustion mode evolution in a stratified charge are of substantial significance in the understanding and prediction of combustion in advanced compression ignition engines. To probe complex combustion in a stratified charge, we have performed one-dimensional direct numerical simulations with detailed chemistry and transport, using a recently developed open source reacting flow CFD platform based on OpenFOAM. N-heptane is adopted in this work as a typical fuel exhibiting low temperature chemistry. Temperature and equivalence ratio gradients are varied among the simulations to observe ignition and the subsequent combustion development. Three stages of heat release are identified, including the low temperature chemistry chain branching, H2O2 chain branching and CO oxidation, consistent with the recent ignition experiment for lean n-heptane air mixture in a rapid compression machine (RCM). Reaction fronts induced by these oxidation stages are found to be unaffected by diffusion process and exhibit unique propagation features. The results provide useful guidance to the understanding of combustion in a stratified charge and shed light on the development of novel low temperature combustion strategy for advanced compression ignition engines.

Multi-stage heat release in lean combustion: Insights from coupled tangential stretching rate (TSR) and computational singular perturbation (CSP) analysis

There is a growing interest in leaner burning internal combustion engines as an enabler for higher thermodynamic efficiency. The extension of knock-limited compression ratio and the increase in specific heat ratio with lean combustion are key factors for boosting efficiency. Under lean burning conditions, there is emerging evidence that certain fuels exhibit unusual heat release characteristics. It has been reported that fuel/air mixtures undergo three-stage heat release or delayed high temperature heat release: starting with an initial low temperature heat release, similar to the one observed in two stage ignition, followed by an intermediate stage where thermal runaway is inhibited, and then advances to a relatively slow third stage of combustion. The focus of this study is to examine the conditions under which various fuels exhibit three stage ignition or delayed high temperature heat release. The auto-ignition of hydrocarbons/air mixtures is simulated in a closed adiabatic homogenous batch reactor where the charge is allowed to auto-ignite at constant volume vessel under predefined initial temperature and pressure. The simulations cover pressures of 10–60 bar, temperatures of 600 K–900 K, and fuel to air ratio from stoichiometry (equivalence ratio) of 0.3–1.0. Tangential stretching rate (TSR) and the computational singular perturbation Slow Importance Indices for temperature are used to identify important reactions contributing to the temperature growth rate at critical time instants of the auto-ignition process. Overall, three-stage ignition or delayed high temperature heat release is found to be present for most fuels under lean fuel/air mixtures, high pressures, and low temperature conditions. The radical termination reactions of H, OH, and HO2 during the high temperature heat release are leading factors for the distinct separation of heat release stages.

Investigation on the dual-fuel active-thermal atmosphere combustion strategy based on optical diagnostics and numerical simulations

The dual-fuel active-thermal atmosphere combustion strategy has the potential to make the homogeneous combustion compression ignition more controllable, achieving low fuel consumption and engine emissions. However, the detailed combustion process and chemical interaction between the high- and low- reactivity fuels have not been fully understood. This work investigated the premixed iso-octane combustion assisted by the two-stage reaction of n-heptane on an optical engine. High-speed imaging technique was applied to visualize the natural flame luminosity and a data-processing method based on Cantera was proposed to evaluate the detailed chemical kinetics process. Results indicate that the high-temperature heat release stage changes from partially premixed combustion with distinct blue flames into the mixing-controlled combustion, showing evident signs of soot radiation when the two-stage ignition transits into three-stage ignition. Heat release from the premixed n-heptane is initially dominated by reaction HCO + O2 = CO + HO2 and turns into hydrogen-oxygen reactions with increasing temperature. In the primary reacting regions for the cases with latter injection timings, reactions C3H5 + H(+M) = C3H6(+M) and C3H3 + H(+M) = C3H4(+M) play the major role, which promotes the formation of soot precursors. Formaldehyde can represent the mixture reactivity. Reaction pathway of premixed n-heptane generated reactive species that promote the consumption of iso-octane. Reactions of iso-octane also enhance the formation of reactive species that further accelerate the combustion process. Retardation of the iso-octane injection timing reduces the local mixture reactivity and weakens the chemical interaction between iso-octane and n-heptane. This lowers the heat release rate, and, objectively, results in the transition of two-stage ignition to three-stage ignition.

Experimental investigation of end-gas autoignition-to-detonation transition for an n-decane/O2/Ar mixture

The transition between different combustion regimes is investigated experimentally for a stoichiometric argon-diluted n-decane/O2 mixture. The focus is put on the influence of initial temperature (T = 420–470 K) and pressure (P = 1.5–3 bar) on the regime transitions. Fast schlieren visualization (≥ 120 kHz) and high-speed pressure and temperature measurements are used to monitor the evolution of the reactive processes inside a combustion chamber of square cross-section (40 mm × 40 mm × 172 mm). Results for P ≥ 2.5 bar and the entire temperature range and for P = 2 bar and T ≥ 440 K show three distinct stages following the adiabatic compression of fresh gases induced by the propagation of a flame into the chamber/test section, namely a cool flame, a main heat release stage, and detonation onset. For P = 1.5 bar, however, only the first two stages of the process are observed in the temperature range studied. A two-stage autoignition phenomenon, typical of large hydrocarbons, occurs systematically in the end gas and generates consecutive reactive fronts. The transition to detonation appears to result from the acceleration of the aforementioned fronts toward the speed of sound in fresh gases. Notably, the compression history plays a key role in setting conditions for detonation onset. Our results are in agreement with classical transition maps available in the literature.

Formation of combustion wave in lean propane-air mixture with a non-uniform chemical reactivity initiated by nanosecond streamer discharges in the HCCI engine

The propagation of combustion wave, which is formed as a result of impact of a filamentary pulsed periodic non-equilibrium discharge on a mixture that is not ignited by only compression in the HCCI engine, has been simulated. The discharge for a short time locally activated the mixture in the engine cylinder at a certain crankshaft rotation angle at the compression stage. The approach to evaluation of the temperature and composition in the region activated by high-frequency corona discharge has been proposed. Multi-pulse and multi-channel nature of discharge was accounted for in our model. The densities of chemically active species produced during streamer propagation were found using two-dimensional axially symmetric fluid model. The obtained composition and temperature in the activated zone were the initial conditions for modeling the propagation of combustion wave along the cylinder radius in 1D approximation. Special terms were included in equations responsible for the compression along the axis of cylinder to mimic the real change in pressure in cylinder. The calculations were performed for a lean C3H8-air mixture, which is characterized by the presence of low-temperature heat release (LTHR) stage, and intermediate and high temperatures heat release (ITHR and HTHR) stages. It was shown that the ignition timing in activated zone depends on the specific energy input into the streamer channel, the radius of activated region coupled with the volume fraction of activated region treated by discharge, and the moment of discharge initiation relative to the top dead center (TDC). The speed of combustion wave, the occurrence of auto-ignition in the end gas and transition to combustion, accompanied by “knocking” or strong pressure fluctuations are largely determined by the moment of discharge initiation and the specific energy input. The significant effect of discharge is explained by the stimulation of oxidation kinetic mechanisms at LTHR and ITHR stages.

Inhibitory effects of three chemical dust suppressants on nitrocellulose dust cloud explosion

AbstractThree types of self‐prepared chemical dust suppressants (CDSs) were investigated for their inhibitory effects on nitrocellulose (NC) cloud dust explosion. The results revealed that NC is extremely sensitive to electric sparks and has a high explosion intensity. CaCl2‐CDS effectively increased the particle size to control fly dust substantially inhibiting dust cloud explosions. However, both Na2SO4‐CDS and MgCl2‐CDS exhibited poor abilities and even promoted explosion. Therefore, neither Na2SO4‐CDS nor MgCl2‐CDS is recommended as a CDS for NC. Inappropriately using CDSs may engender severe explosions. Furthermore, a mechanism underlying NC dust cloud combustion and explosion was proposed. NC has three stages of heat release: autoxidation, thermal decomposition, and combustion. Thermal decomposition, combustion, and explosion were triggered depending on the energy provided from autoxidation. CaCl2‐CDS inhibited only combustion. This study reveals the mechanism underlying NC dust cloud explosions and provides useful information for the development of more optimized CDSs.

Evaluation of thermal performance through development of a PCM-based thermal storage control system integrated unglazed transpired collector in experimental pig barn

The application of unglazed transpired collectors (UTCs) in livestock barns is an excellent solution for air ventilation in winter, for reducing structural energy loss, and for improving indoor air quality. However, thermal energy generated within specific periods may overheat the inside of the livestock barns. Thus, this study developed and applied a PCM-based thermal control system for saving and utilizing excess thermal energy generated by the application of a UTC in a experimental pig barn, through which the corresponding temperature deviation and the thermal performance of the thermal storage system are analyzed. During the experimental period, the UTC plenum temperature rose up to 65 °C, which is approximately 35 °C higher than that of the outdoor air throughout the day. The nonlinear correlation between the amount of solar radiation and UTC efficiency was verified during the experimental period (R2=0.58). Throughout the heat storage stage, the excess heat energy generated at the UTC plenum had a verified heat storage efficiency of up to 85%, a daily average of 22%, and an effect of a temperature difference of the PCM of 22.6 °C. Additionally, an average daily efficiency of 57% and an effect of a temperature increase of 13.8 °C compared to the outdoor temperature were verified through the heat release stage. The average thermal storage time was about 5 h and 50 min and the average release time was about 2 h and 33 min. During the experimental period, the average energy generation in the UTC plenum was 13,548.21 kcal/day and the average storage and release energies were 7739.78 and 1793.47 kcal/day, respectively.

Research on thermal storage and release capacity of freezing sandy soil with phase change

In cold coastal areas, the ground heat exchangers of solar assisted ground source heat pump system were buried in sandy soil and the soil contains water inevitably. When the heat pump supplied heat for buildings, geothermal energy will be extracted, which caused the soil temperature dropping below the freezing point of water. The aqueous sandy soil would be frozen and then the solar heat would be used for thawing. This paper studied the thermal storage and release capacity of freezing sandy soil with phase change by using experimental and numerical methods, respectively. The conclusions could be obtained that the heat transfer per unit length of GHEs and average COPs of heat pump under freezing sandy soil condition were both higher than those under the dry and no freezing sandy soil. The numerical model of the GHEs in soil was built and validated by experimental results. Based on the verified numerical model, the flow rate in pipe, the burial depth of GHEs, the moisture content of soil and the kinds of water in soil were reset to simulate the heat transfer performance of the experimental system for three sunny days. The results showed that the average heat transfer per unit length was calculated as 59.6 W/m in the heat storage stage and 44.4 W/m in the heat release stage, respectively.

Experimental Study on Dynamic Heat Release Performance of Water Tank with Phase Change Materials

Solar heating system has raised much attention due to the global energy shortage and environment pollution. The dynamic heat release performance of water tank (WT) with phase change materials (PCM) plays an important role in the operation of solar heating system, while seldom available literatures focused on this research topic. In this paper, an experimental system for heat release of the WT-PCM was established. The temperature variations of the water and the PCM during heat release of the WT-PCM and the traditional water tank were measured respectively, and their dynamic heat release performance were also analyzed based on the measured data. The results are as follows: the heat release of the WT-PCM can be divided into three phases: sensible heat before phase change, latent heat during phase change and sensible heat after phase change, and the water temperature will be temporarily heated after the sudden drop; the water supply temperature decline rate of both tanks will increase dramatically at the initial stage of heat release, when the PCM temperature is reduced to 50℃, the water supply decline rate of the WT-PCM is less than that of the traditional water tank, while when PCM is completely solidified, the water supply temperature decline rate of the WT-PCM is more than that of the traditional water tank. The heat release performance of the WT-PCM is much better than that of the traditional water tank. The research results can provide useful reference for engineering application of the WT-PCM.

Water adsorption equilibrium and dynamics of LICL/MWCNT/PVA composite for adsorptive heat storage

Recently, a new composite “LiCl inside Multi-Wall Carbon NanoTubes” (LiCl/MWCNT) has been suggested as water sorbent for Adsorption Thermal Energy Storage (ATES), because it has a large thermal storage capacity of 1.7 kJ/g for a daily heat storage cycle. This work addresses the results of the study of water sorption dynamics on the novel composite loaded into representative small scale fragments of a common finned flat-tube HEx. The study consists of four parts: (1) shaping the LiCl/MWCNT composite as grains using polyvinyl alcohol (PVA) as a binder; (2) analysis of sorption equilibrium for the pair “LiCl/ MWCNT/PVA – water”; (3) measuring the thermal storage capacity of the granulated LiCl/ MWCNT/PVA composite; (4) investigation of water sorption dynamics on the LiCl/MWCNT/PVA under typical conditions of the daily storage cycle. It is shown that the thermal storage capacity of the LiCl/MWCNT/PVA composite equals 1.5–1.6 kJ/g. The specific power reaches 4.2 and 9.8 kW/kg of the heat release and thermal storage stages, respectively. The results obtained show that the working pair “LiCl/MWCNT/PVA – water” appears to be advantageous for ATES.

Performance analysis of a solid-gas thermochemical composite sorption system for thermal energy storage and energy upgrade

Solid-gas thermochemical sorption heat storage has attracted considerable interest in recent years due to high energy density and the ability of long-term storage duration. Operation principle of thermochemical sorption heat storage system is based on reversible solid-gas chemical reaction by storing thermal energy in the form of chemical potential. The comparison of thermochemical adsorption and resorption heat storage cycle has been firstly made. Then, the thermochemical composite sorption heat storage cycle is presented for the effective utilization of the low-grade thermal energy. As a novel thermochemical sorption heat storage cycle, thermochemical composite sorption heat storage has integrated the merits of both thermochemical adsorption and resorption heat storage cycle. Furthermore, the performance of a thermochemical composite sorption heat storage system is analyzed by employing MnCl2-SrCl2-NH3 working pair. Theoretical analysis showed that thermochemical composite sorption heat storage cycle is effective for integrating energy storage and energy upgrade of low-grade thermal energy. It can not only decrease the heat input temperature of external heat source during the heat storage stage, but also upgrade significantly the temperature level of stored thermal energy during the heat release stage. As a result, thermal energy temperature can be upgraded from 81 °C to 170 °C via thermochemical composite sorption heat storage cycle. Therefore, it is very beneficial to enlarge the application temperature range and realize the high-efficient utilization of low-grade thermal energy.

Control and optimization of spark ignition–controlled auto-ignition hybrid combustion based on stratified flame ignition

Spark ignition–controlled auto-ignition hybrid combustion, also known as spark assisted compression ignition, is of considerable interest in gasoline engines because of its potential to enlarge the operating range of gasoline diluted combustion. However, it was found that the spark ignition–controlled auto-ignition hybrid combustion process was often characterized with large cycle-to-cycle variations. In this research, a new approach by combining the traditional second-order derivative method and Wiebe function fitting method was proposed to identify different heat release stages of spark ignition–controlled auto-ignition hybrid combustion. The heat release characteristics of the spark ignition–controlled auto-ignition hybrid combustion based on stratified flame ignition strategy and its control methods were investigated in detail. The effect of control parameters, including spark timing, direct injection ratio and dilution strategy, on improving the thermal efficiency and decreasing the variation of heat release trace in spark ignition–controlled auto-ignition hybrid combustion based on stratified flame ignition strategy was analysed. The advance of flame propagation ending point and the increase in the average heat release rate in flame propagation stage benefitted the fuel economy and reduced the variations of heat release in spark ignition–controlled auto-ignition hybrid combustion. Although the increase in direct injection ratio contributed to the stability of heat release in the spark ignition–controlled auto-ignition hybrid combustion based on the stratified flame ignition strategy, the thermal efficiency of spark ignition–controlled auto-ignition hybrid combustion cannot be effectively optimized due to the decrease in combustion efficiency. The application of exhaust gas recirculation and air dilution could decrease the variations of heat release process and increase the thermal efficiency of spark ignition–controlled auto-ignition hybrid combustion based on stratified flame ignition strategy.

Closed-loop combustion phase control for multiple combustion modes by multiple injections in a compression ignition engine fueled by gasoline-diesel mixture

Partially premixed combustion with low octane fuel aims to reduce NOx and soot emission simultaneously without fuel consumption penalty. Cylinder pressure based combustion phase control is an essential technology for partially premixed combustion. A novel closed-loop combustion phase control strategy for multiple combustion modes is proposed in the current study. The combustion modes are classified into three basic categories based on injection patterns and heat release stages: (1) with only one heat release stage; (2) with two separated heat release stages; (3) with two overlapped heat release stages. Crank angle when 50% fuel is consumed (CA50) is chosen as the combustion phase indicator for the first case. Start of combustion (SOC) of each heat release stage is the combustion phase indicator for the second case. Both SOC and CA50 are the combustion phase indicators for the third case. Each combustion phase is closed-loop controlled by a proportional–integral (PI) controller with the timing adjustments of the corresponding injection. The control strategy is verified under different operating conditions in a 1.9 L light duty diesel engine fueled by gasoline-diesel mixture (volumetric 70% gasoline, 30% diesel). The experimental results show that the control strategy is able to control the combustion phase, reduce cylinder to cylinder variations as well as cycle to cycle variations under the operating conditions with exhaust gas circulation (EGR) rates of 10% and 15%.

Three-stage heat release in n-heptane auto-ignition

Abstract Multi-stage heat release is an important feature of hydrocarbon auto-ignition that influences engine operation. This work presents findings of previously unreported three-stage heat release in the auto-ignition of n-heptane/air mixtures at lean equivalence ratios and high pressures. Detailed homogenous gas-phase chemical kinetic simulations were utilized to identify conditions where two-stage and three-stage heat release exist. Temperature and heat release profiles of lean n-heptane/air auto-ignition display three distinct stages of heat release, which is notably different than two-stage heat release typically reported for stoichiometric fuel/air mixtures. Concentration profiles of key radicals (HO2 and OH) and intermediate/product species (CO and CO2) also display unique behavior in the lean auto-ignition case. Rapid compression machine measurements were performed at a lean equivalence ratio to confirm the existence of three-stage heat release in experiments. Laser diagnostic measurements of CO concentrations in the RCM indicate similar concentration-time profiles as those predicted by kinetic modeling. Computational singular perturbation was then used to identify key reactions and species contributing to explosive time scales at various points of the three-stage ignition process. Comparisons with two-stage ignition at stoichiometric conditions indicate that thermal runaway at the second stage of heat release is inhibited under lean conditions. H + O2 chain branching and CO oxidation reactions drive high-temperature heat release under stoichiometric conditions, but these reactions are suppressed by H, OH, and HO2 radical termination reactions at lean conditions, leading to a distinct third stage of heat release.

Theoretical analysis on the exergy destruction mechanisms and reduction under LTC relevant conditions

Low temperature combustion (LTC) is a potential thermodynamic pathway to maximize the thermal efficiency of internal combustion (IC) engines. However, high exergy loss is also observed within this combustion concept. The present study focuses on the homogeneous combustion process and examines the detailed exergy destruction mechanisms under representative LTC engine conditions. By varying both equivalence ratios (φ) and temperatures (T) at initial pressure of 50 bar, it is found that the decreased total exergy destruction fraction (fED) with increasing initial temperature mainly results from the decreased exergy destruction in the high temperature heat release stage, while using rich mixture can significantly reduce the fED in the ignition delay stage, which is dominated by the reactions involving large molecules (C7 species). Reaction pathway analysis reveals that the detailed exergy destruction sources are significantly affected by the reaction pathways. Furthermore, a qualitative exergy loss φ-T map was created to illustrate the exergy loss reduction potential. It is concluded that the combustion pathway that reforming the rich fuel/air mixtures before ignition followed by the low temperature combustion of lean reforming products offers the potential to simultaneously reduce exergy destruction and avoid soot and NOx formation. However, the potential advantages of this exergy reduction combustion concept still require further work.

Influence of a non-equilibrium discharge impact on the low temperature combustion stage in the HCCI engine

A method for the combustion organisation in the cylinder of an HCCI engine is proposed. This method is based on the selective impact of electrical discharge in the form of a high frequency corona. The discharge is switched on at a certain crank angle and affects the mixture for a short time (a few rotation degrees of the crankshaft). The aim of the given study was to clarify how the discharge affects the low temperature combustion (LTC) stage to control the inflammation in an HCCI engine. It was shown that the best discharge impact timing is the beginning of the low temperature heat release (LTHR) or intermediate temperature heat release (ITHR) stages. The discharge does not directly ignite the mixture but only “pushes” the beginning of these stages and promotes their more rapid passing, controlling the heat release. It is mostly due to the dissociation of fuel and oxidiser, instead of heating the area of impact. Consequently, a non-equilibrium discharge can control the combustion timing, which is especially important in the case of a cold start or use of a lean mixture that is characterised by misfiring. It has been shown that the combustion timing after the top dead centre depends on the energy deposited into the injected mass of the fuel–air mixture treated by the discharge and the discharge switching on time. The dependence of the concentrations of NOx, CO, and unburned hydrocarbons on a specific deposited energy and the initial conditions is presented. The chemical kinetic mechanism of a selective discharge effect is revealed. The numerical simulation has been performed for the equivalence ratio ϕ = 0.33–0.5 and the intake temperature Tin = 360–400 K in a propane-air mixture.

Ignition properties of lean DME/H2 mixtures at low temperatures and elevated pressures

Ignition delays of lean dimethyl ether (DME)/hydrogen mixtures with hydrogen mole fraction ranging from 0 to 85% were measured using a rapid compression machine with equivalence ratios from 0.3 to 0.5, temperatures from 621 to 852 K and pressures from 7 to 22 bar. Numerical analysis was performed based on the NUI Aramco Mech 2.0 and, in general, good agreement between measurements and predictions was demonstrated. Both measurements and predictions show that H2 addition increases the ignition delay of DME/H2 mixtures non-linearly. Moreover, the effect of H2 addition on ignition delay becomes more pronounced at H2 mole fraction above 70% and lower pressure. The dilution and chemical effects of H2 addition were quantified, and the dilution effect decreases whereas the chemical effect increases with the increase of H2 mole fraction. It is also found that DME/H2 mixtures show a three-stage heat release behavior at very lean condition (φ = 0.3), namely, a low temperature heat release (LTHR) stage followed by two high temperature heat release (HTHR) stages. The first- and second-stage HTHR are caused by the conversion of HCHO to CO and the production of final reaction products CO2 and H2O respectively. Overall, DME is mainly consumed during the LTHR and first-stage HTHR whereas H2 is mainly consumed during the second-stage HTHR. The two-stage HTHR behavior of DME/H2 mixtures occurs when H2 and HCHO produced during DME’s LTHR stage are present in a proper proportion prior to high-temperature ignition and the temperature after LTHR has reached a proper level.

Composite “LiCl/MWCNT” as advanced water sorbent for thermal energy storage: Sorption dynamics

Sorption heat storage (SHS) is a promising technology towards efficient use of renewable energy sources. Composite materials based on hygroscopic salts have a high potential for SHS in term of the heat storage capacity. Recently, a new sorbent “LiCl confined to Multi-Wall Carbon NanoTubes (MWCNT)” with enhanced storage capacity (1.7kJ/g) has been suggested for SHS. This work addresses the dynamic study of water sorption on this material under operating conditions of a daily heat storage cycle. The study consists of three parts: (1) shaping the LiCl/MWCNT composite as grains (GP) and pellets with (PB) and without a binder (PP); (2) water sorption dynamics under conditions of the selected SHS cycle; (3) evaluation of the specific power achievable. Various configurations of the adsorbent bed were tested, namely, the loose grains GP placed on a metal support, and the pellets PB and PP glued to the support. The dynamic curves of water sorption obey an exponential equation. The PB configuration ensured a maximal specific power of 11.2kW/kg (at 70% conversion) during the heat storage stage. During the heat release stage, the specific power is lower (5.4kW/kg), probably due to a kinetic hindrance attributed to solid-solid transformation during the decomposition of LiCl·H2O complex towards the anhydrous salt. The high specific power along with the large heat storage capacity make the LiCl/MWCNT composite a promising new candidate for SHS.

Investigation on Combustion and Emission Performance of a Common Rail Diesel Engine Fueled with Diesel/Biodiesel/Polyoxymethylene Dimethyl Ethers Blends

Polyoxymethylene dimethyl ethers (PODEs) are an excellent blend for diesel due to their high cetane number, high oxygen content, and low viscosity. Combustion and emission characteristics are investigated based on experimental tests on a turbocharged, in-line 6-cylinder, common rail diesel engine. Results show that combustion starts earlier with PODE blending at low and partial loads. With pilot and main injection, the peak combustion pressures and peak heat release rates of diesel/biodiesel/PODE blend fuels increase due to large amounts of reactive radicals formed in the pilot heat release stage. With an increase in load, a slight decrease in both peak combustion pressure and peak heat release rate is observed. At low loads, the CA10s, CA50s, and CA90s of diesel/biodiesel/PODE blend fuels advance, and both rapid combustion and late combustion phases shorten. At medium and high loads, CA10s advance, CA50s remain unchanged, and CA90s clearly advance. Rapid combustion phases increase very little, whereas th...