Faster Heat Release Research Articles

Wood stove material configurations for increased thermal comfort

In this work, a numerical model of the heat transfer through composite wood stove walls has been used to study the effect of material configurations on the heat storage and the heat release profile to the room, to arrive at improved thermal comfort performance. Cast iron and soapstone have been compared, showing that soapstone is a good material from a thermal comfort point of view, while cast iron (for the typical stove weights of today) exhibits a comparably higher peak and faster heat release. Applying a Phase Change Material (PCM) for such a cast iron stove changes this picture. It makes it possible to reduce peak heat release and achieve a more stable as well as an extended heat release period. 53% reduction in peak and 43% reduction in average heat release was demonstrated during the combustion cycle using Erythritol as PCM. However, the selection and use of a PCM must be optimized. Its capacity should be utilized to a maximum extent without overheating the material as it will cause irreversible damage to most PCM.

Front roughening of flames in discrete media.

The morphology of flame fronts propagating in reactive systems composed of randomly positioned, pointlike sources is studied. The solution of the temperature field and the initiation of new sources is implemented using the superposition of the Green's function for the diffusion equation, eliminating the need to use finite-difference approximations. The heat released from triggered sources diffuses outward from each source, activating new sources and enabling a mechanism of flame propagation. Systems of 40000 sources in a 200×200 two-dimensional domain were tracked using computer simulations, and statistical ensembles of 120 realizations of each system were averaged to determine the statistical properties of the flame fronts. The reactive system of sources is parameterized by two nondimensional values: the heat release time (normalized by interparticle diffusion time) and the ignition temperature (normalized by adiabatic flame temperature). These two parameters were systematically varied for different simulations to investigate their influence on front propagation. For sufficiently fast heat release and low ignition temperature, the front roughness [defined as the root mean square deviation of the ignition temperature contour from the average flame position] grew following a power-law dependence that was in excellent agreement with the Kardar-Parisi-Zhang (KPZ) universality class (β=1/3). As the reaction time was increased, lower values of the roughening exponent were observed, and at a sufficiently great value of reaction time, reversion to a steady, constant-width thermal flame was observed that matched the solution from classical combustion theory. Deviation away from KPZ scaling was also observed as the ignition temperature was increased. The features of this system that permit it to exhibit both KPZ and non-KPZ scaling are discussed.

Impact of periodic flow reversal of heat transfer fluid on the melting and solidification processes in a latent heat shell and tube storage system

A numerical study has been conducted on a shell and tube latent heat storage system whereby the inlet heat transfer fluid direction is periodically reversed during charging and discharging. The impact of varying the boundary condition at the interface of the tubes carrying the heat transfer fluid and phase change material (PCM) on the evolution of the phase change front, heat transfer area and heat transfer rate have been evaluated during the charging and discharging processes. Results for the charging processes show a higher heat transfer area develops during the early stages and amplification of natural convection after 40% melt fraction, leading to a higher heat transfer rate. In comparison to the fixed flow condition, periodic flow reversal for the discharge cases results in an increased heat transfer area for a longer period of time, leading to a higher heat transfer rate particularly after 75% solidification. This effect is more important for discharging cases in the absence of convection heat transfer. Periodically reversing the direction of heat transfer fluid, which produced a periodic boundary condition at the tube-PCM interface, also resulted in a lower temperature gradient in space and time and consequently higher exergy recovery, and about a 6% increase in the time-average heat transfer rate in the charging and discharging cases. The novel reversal flow method provides a means to implement a periodic boundary condition without changing the heat source/sink, enhancing the thermal performance and cost effectiveness of latent heat storage systems. Phase change storage systems incorporating periodic flow reversal provide higher energy delivery rates, greater power density and more exergy recovery. This method can support fast heat release to respond to a peak load in a CSP plant or fast heat storage to protect a tubular receiver from high thermal stresses.

The Zeldovich spontaneous reaction wave propagation concept in the fast/modest heating limits

Quantitative mathematical models describe planar, spontaneous, reaction wave propagation (Zeldovich, Combust. Flame, vol. 39, 1980, pp. 211–214) in a finite hot spot volume of reactive gas. The results describe the complete thermomechanical response of the gas to a one-step, high-activation-energy exothermic reaction initiated by a tiny initial temperature non-uniformity in a gas at rest with uniform pressure. Initially, the complete conservation equations, including all transport terms, are non-dimensionalized to identify parameters that quantify the impact of viscosity, conduction and diffusion. The results demonstrate unequivocally that transport terms are tiny relative to all other terms in the equations, given the relevant time and length scales. The asymptotic analyses, based on the reactive Euler equations, describe both induction and post-induction period models for a fast heat release rate (induction time scale short compared to the acoustic time of the spot), as well as a modest heat release rate (induction time scale equivalent to the acoustic time). Analytical results are obtained for the fast heating rate problem and emphasize the physics of near constant-volume heating during the induction period. Weak hot spot expansion is the source of fluid expelled from the original finite volume and is a ‘piston-effect’ source of acoustic mechanical disturbances beyond the spot. The post-induction period is characterized by the explosive appearance of an ephemeral, spatially uniform high-temperature, high-pressure spot embedded in a cold, low-pressure environment. In analogy with a shock tube the subsequent expansion process occurs on the acoustic time scale of the spot and will be the source of shocks propagating beyond the spot. The modest heating rate induction period is characterized by weakly compressible phenomena that can be described by a novel system of linear wave equations for the temperature, pressure and induced velocity perturbations driven by nonlinear chemical heating, which provides physical insights difficult to obtain from the more familiar ‘Clarke equation’. When the heating rate is modest, reaction terms in the post-induction period Euler equations exhibit a form of singular behaviour in the high-activation-energy limit, implying the need to use a nonlinear exponential scaling for time and space, developed originally to describe spatially uniform thermal explosions (Kassoy, Q. J. Mech. Appl. Maths, vol. 30, 1977, pp. 71–89). Here again the result will be the explosive appearance of an ephemeral spatially uniform high-temperature, high-pressure hot spot. These results demonstrate that an initially weak temperature non-uniformity in a finite hot spot can be the source of acoustic and shock wave mechanical disturbances in the gas beyond the spot that may be related to rocket engine instability and engine knock.

Effect of Catalytic Coatings on the Performance, Emission and Combustion Characteristics of Spark Ignition Engines

This study discusses the effect of copper, nickel and chromium coating on the spark ignition engine performance, emission and combustion characteristics. The maximum brake thermal efficiency for copper coated engine is about 5% higher than standard engine at full load and about 4% higher than the standard engine at 2800 rpm. Nitrogen oxides emission for catalytic coated engine is 7% to 20% higher than standard engine at full load. It was observed that carbon monoxide and carbon dioxide emissions of standard engine were higher than catalytic coated engines at all loads. Copper coated engine has the lowest hydro carbon emission. Catalytic coated engines have 6% to 12% higher cyclinder pressure when compared to the standard engine. The crank angle of heat release values and combustion parameters indicate that a faster heat release occured for catalyst coated engines. Similarly combustion duration of standard engine is higher than that of catalytic coated engines. Catalytic coatings increase the pre-flame reactions which lead to better and faster combustion.

The combustion behavior of diesel/CNG mixtures in a constant volume combustion chamber

The stringent emissions and needs to increase fuel efficiency makes controlled auto-ignition (CAI) based combustion an attractive alternative for the new combustion system. However, the combustion control is the main obstacles in its development. Reactivity controlled compression ignition (RCCI) that employs two fuels with significantly different in reactivity proven to be able to control the combustion. The RCCI concept applied in a constant volume chamber fuelled with direct injected diesel and compressed natural gas (CNG) was tested. The mixture composition is varied from 0 - 100% diesel/CNG at lambda 1 with main data collection are pressure profile and combustion images. The results show that diesel-CNG mixture significantly shows better combustion compared to diesel only. It is found that CNG is delaying the diesel combustion and at the same time assisting in diesel distribution inside the chamber. This combination creates a multipoint ignition of diesel throughout the chamber that generate very fast heat release rate and higher maximum pressure. Furthermore, lighter yellow color of the flame indicates lower soot production in compared with diesel combustion.

Every Snowflake is Not Unique

This article discusses various aspects of snowflake architectures. It is certain that every snowflake conforms to only one architecture: a flat star with six fishbones connected at the center. The latent heat of solidification, which is released by the water vapor that becomes solid at the bead surface. There comes a critical time when the spherical bead is no longer an efficient architecture for dissipating heat. The principle calls for design change, toward faster heat release and solidification. The growth of ice morphs abruptly into a ball continued in one plane by needles. Because of the configuration of the water molecule, the needles grow in six directions. The flat star transfers heat to the surroundings more easily than a spherical bead with the same diameter. In order to give credit to the view that every snowflake is unique, the actual configuration depends on many secondary effects, which are of random origin.

Kinetic effects of non-equilibrium plasma-assisted methane oxidation on diffusion flame extinction limits

The kinetic effects of low temperature non-equilibrium plasma assisted CH 4 oxidation on the extinction of partially premixed methane flames was studied at 60 Torr by blending 2% CH 4 by volume into the oxidizer stream of a counterflow system. The experiments showed that non-equilibrium plasma can dramatically accelerate the CH 4 oxidation at low temperature. The rapid CH 4 oxidation via plasma assisted combustion resulted in fast chemical heat release and extended the extinction limits significantly. Furthermore, experimental results showed that partial fuel mixing in the oxidizer stream led to a dramatic decrease of O concentration due to its rapid consumption by CH 4 oxidation at low temperature. The products of plasma assisted CH 4 oxidation were measured using the Two-photon Absorption Laser-Induced Fluorescence (TALIF) method (for atomic oxygen, O), Fourier Transform Infrared (FTIR) spectroscopy, and Gas Chromatography (GC). The product concentrations were used to validate the plasma assisted combustion kinetic model. The comparisons showed the kinetic model over-predicted the CO, H 2O and H 2 concentrations and under-predicted CO 2 concentration. A path flux analysis showed that O generated by the plasma was the critical species for extinction enhancement. In addition, the results showed that O was produced mainly by direct electron impact dissociation reactions and the collisional dissociation reactions of electronically excited molecules with O 2. Moreover, these reactions involving electron impact and excited species collisional dissociation of CH 4 contributed approximately a mole fraction of 0.1 of total radical production. The present experiments produced quantitative species and extinction data of low temperature plasma assisted combustion to constrain the uncertainties in plasma/flame kinetic models.

Molecular relaxations in mixtures of O2 with CO2 observed on laser-induced gratings

Laser-induced gratings in mixtures of O2 with CO2 were formed by excitation of the O2 molecules to the singlet state b 1 Σ + g (v′=0). Density changes from heat release and from electrostriction, and variation of electric polarizability by excitation of molecules contribute to the grating. For modeling the relaxation of the excited O2 molecules, a three-step process is assumed: Fast heat release with respect to the rotational states first, then medium fast electronic de-activation with excitation of vibrational states of the O2 and CO2 molecules, and then the final slow heat release processes. The observed temporal evolution of the diffraction efficiency of the grating agrees rather well with modeling. Average rates of the final relaxations and the value of the polarizability of the singlet state b 1 Σ + g (v′=0) are determined in this way.

Experiments on behaviour of preheated rubber seed oil in a direct injection diesel engine

Rubber seed oil (RSO) is one of the vegetable oils that has potential for use as a fuel for diesel engines. Owing to the high viscosity of RSO, its performance is slightly inferior to diesel fuel. Preheating of RSO reduces its viscosity to the level of diesel fuel that improves the fuel spray and atomisation characteristics, leading to complete combustion. In this experimental work, the effect of various fuel inlet temperatures on the performance, emission and combustion characteristics of a single cylinder diesel engine was evaluated. The experimental results show that the brake thermal efficiency increases from 26˙6 to 28˙4% when the fuel is preheated to a temperature of 150°C. It also indicates that there is a reduction in the CO and smoke emissions with preheated RSO. The reduced ignition delay and combustion duration with RSO at 150°C indicates faster heat release and leads to higher thermal efficiency. It is concluded that the performance, combustion and emission characteristics are improved for preheated RSO compared to raw RSO, but it is still inferior to diesel.

Fast heat release characterization of a diesel engine

Multi-event fuel injection strategies under independently controlled exhaust gas recirculation and intake boost have been applied to produce the heat release patterns that characterize the clean combustion techniques of modern diesel engines. Extensive experimental and analytical comparisons have been performed to better estimate the heat release characteristics from the cylinder pressure traces. A number of heat release models based on the First Principles are compared against a comprehensive heat release model, on the basis of numerical complexity and the ability to characterize the combustion process. Such study indicated that though the estimation from the simplified models is efficient when the heat release is close to the end of the cylinder compression stroke, the simplified models produce shortfalls in estimating the more spread multi-event heat release from the newer combustion systems. A new computationally efficient algorithm, based on the Diesel Pressure Departure Ratio, is proposed to characterize the various heat release patterns with adequate accuracy. The improved heat release analyzing algorithms are further programmed on real-time deterministic devices that process the cylinder pressure data to provide the necessary feedback for the fuel-injection model running on subsequent real-time controllers. The efficacy of the new algorithms has been experimentally demonstrated against selected cases of boost, engine load and exhaust gas recirculation on a modern diesel engine.

Numerical investigations on methods to control the rate of heat release of HCCI combustion using reduced mechanism of n-heptane with a multidimensional CFD code

In this study a semi-reduced reaction scheme developed previously was used to derive a 26 step reduced mechanism, using the sensitivity approach and the steady state approximation (QSS) with Chemkin code. This 26 step model has been implemented in a CFD combustion code (Star-CD/Kinetics) to study combustion process in homogeneous charge compression ignition (HCCI) engines. The first results obtained have confirmed the very rapid combustion phase and fast heat release with completely homogeneous mixtures, for a wide range of operating conditions. This numerical approach has been used first to study the effects of natural thermal stratification when the mixture is initially homogeneous. In a second step, the different possible methods to control the heat release rate have been studied. The stratification with several homogeneous regions of different composition is shown to be very efficient; the limits of this process are discussed.

Study of Multimode Combustion System With Gasoline Direct Injection

In this paper, a multimode combustion system was developed in a gasoline direct injection engine. A two-stage fuel-injection strategy, including flexible injection timings and flexible fuel quantity, is adopted as a main means to form desired mixture in the cylinder. The combustion system can realize five combustion modes. The homogeneous charge spark ignition (HCSI) mode was used at high load to achieve high-power output density; stratified charge spark ignition (SCSI) was adopted at intermediate load to get optimum fuel economy; stratified charge compression ignition (SCCI) was introduced at transient operation between SI and CI mode. Homogeneous charge compression ignition (HCCI) was utilized at part load to obtain ultralow emissions. Reformed charge compression ignition (RCCI) was imposed at low load to extend the HCCI operation range. In SI mode, the stratified concentration is formed by introducing a second fuel injection in the compression stroke. This kind of stratified mixture has a faster heat release than the homogeneous mixture and is primarily optimized to reduce the fuel consumption. In CI mode, the cam phase configurations are switched from positive valve overlap to negative valve overlap (NVO). The test results reveal that the CI combustion is featured with a high gradient pressure after ignition and has advantages in high thermal efficiency and low NOx emissions over SI combustion at part load.

Synthesis of titanium diboride TiB2 and Ti-Al-B metal matrix composites

Titanium diboride TiB2 and TiAl aluminide composites reinforced with in situ borites have been synthesized from the elemental powders of Ti and B, and Ti, Al and B respectively using mechanical alloying technique. No progressive diffusion between Ti and B was observed. The formation of TiB2 was found to be governed by strong and fast exothermic heat release. This indicates that the formation of TiB2 compound in local area of mechanically alloyed powder generated high energy which in turn ignited and promoted the formation of new compound in the rest of the area. Because of the presence of Al in Ti-Al-B system, the concentration of Ti or B was diluted. The exothermic reaction between Ti and B was consequently delayed. However, grain refinement of Ti and Al in this system down to nanometer scale is faster than that in Ti-Al system due to the contribution of B. Using X-ray analysis, strong but broad TiAl, and weak TiB and TiB2 peaks had been detected at 50 h of mechanical alloying indicating the formation of nano TiAl composite reinforced by TiB and TiB2. However, TiB was, however, not a stable phase; it later was transformed into equilibrium phase of TiB2 after annealing at 800 °C.

Effects of combustion on flowfield in a model scramjet combustor

The effects of combustion on the change of the flowfield in a model SCRamjet combustor with a backward step is investigated experimentally and numerically. The main airflow has a Mach number of 2.0. The total temperature is 1000 K for cold flow and 1800 K for hot flow. Hydrogen fuel is injected parallel to the main airflow through a slit on the backward face of the step. The combustion mode is categorized in two modes. One is a weak combustion (WC) mode that is not accompanied by a shock wave, where the flowfield is similar to that in cold flow. Another is an intensive combustion (IC) mode that is accompanied by a shock wave, where the flowfield is much different from that in cold flow. In IC mode, a large separation region is generated behind the step by the shock wave, and the vortex generated at the region rolls the fuel up. The main reacting region is in the shear layer just behind the shock wave, where the main airflow bumps the rolled-up fuel, and the temperature is relatively high due to the shock wave. The flowfield is then controlled by the rate of mixing, leading to fast heat release, which raises the pressure level in the combustor and supports the shock wave. This passive feedback works, and both the mixing efficiency and the combustion efficiency become high. On the other hand, in WC mode, the reacting region spreads over the shear layer downstream of the step, and its heat release rate is lower than that in IC mode. The flowfield is then controlled by the rate of chemical reaction, and the combustion efficiency remains low.

Photothermal probe beam deflection monitoring of photochemical particulate production

The photothermal probe beam deflection method was applied to monitor photochemical particulation induced by a pulsed N 2 laser in a CS 2 vapor. The signal indicates a fast heat release due to collisional relaxation of excited CS 2, and a slow heat release that can be ascribed to the aggregation of carbon monosulfide. The measured signal was modelled by solving the heat conduction equation for the geometry employed. This yields a value for the nucleation and growth time in the microsecond range when literature values for the collisional deactivation and the thermal diffusivity of CS 2 are used.