A 20-step reduced kinetic mechanism of ethanol, a potential sustainable energy source as a biofuel, was developed based on the detailed reaction mechanism proposed by Saxena and Williams using the Computational Singular Perturbation (CSP) method based on the Quasi-steady State Assumption (QSSA). Feasibility evaluation of the reduced kinetic mechanism for multi-dimensional flame analysis, i.e., the difference in numerical results and convergence time between the detailed reaction mechanism and the reduced kinetic mechanism, was also performed to investigate the applicability of the ethanol reduced kinetic mechanism to the development of practical combustors. To consider further industrial applications, the reduced kinetic mechanism was incorporated into the commercial computational fluid dynamics (CFD) code FLUENT 6.3.26 using the User Defined Function (UDF) code developed in the present study. Numerical results calculated with the detailed reaction mechanism and the reduced kinetic mechanism, i.e., temperature profiles, chemical species profiles and laminar burning velocities, were in good agreement for both two-dimensional premixed and non-premixed flame calculations. Convergence time using the reduced kinetic mechanism was considerably reduced compared to that using the detailed reaction mechanism, indicating the applicability and advantage of a reduced kinetic mechanism based on QSSA for multi-dimensional flame analysis. An additional reduction of the computational time was achieved by using both the reduced kinetic mechanism and In Situ Adaptive Tabulation (ISAT) solver by Pope et al.
The viscosity of 2,3,3,3-tetrafluoropropene (HFO-1234yf) has been successfully correlated with the extended corresponding states (ECS) model for transport properties. The thermodynamic equation of state developed in our earlier work forms the basis of this work. The viscosity correlation presented here is valid for temperatures from 260 K to 340 K and for pressures up to 20 MPa. The correlation can be applied to both liquid and gas phases in a single procedure. The estimated uncertainties are 2 % for liquid phase and 2.5 % for gas phase, which roughly correspond to experimental uncertainties.
The direct transition boiling process from non-boiling to film boiling at critical heat flux (CHF) by exponentially increasing heat input, Q0exp(t/τ), was investigated in a pool of FC-72. Investigations were made on a 1.0 mm diameter gold horizontal cylinder heater under a wide range of system pressures for saturated condition. Direct transition predominantly occurs from heat conduction process in the non-boiling regime by rapid increasing heat input and then followed by incipient boiling and CHF simultaneously. However, during quasi-steady-state heat transfer, it was observed that the direct transition also occurs from single phase natural convection at around atmospheric pressure and by an extent of pre-pressure, which was given to the cylinder surface at atmospheric pressure. Direct transition phenomena were confirmed to exist due to the explosive-like heterogeneous spontaneous nucleation (HSN) in originally flooded cavities on surface. The predictions of direct transition phenomena were also derived from typical incipient boiling superheat and CHF.
Lotus-type porous metal with many straight pores is attractive as a heat sink because larger heat transfer capacity is obtained due to the small diameter of the pores. The heat transfer capacity of the lotus-type porous copper heat sink was calculated using the model with the pores of uniform diameters. However, actual lotus-type porous metals have a distribution of pore diameter. In the present work, we investigated the lotus-type porous copper fin model (non-uniform pore model) by considering size distribution of measured on the actual measurement of the pore diameters on a cross-section of the lotus-type porous copper fin. Prediction of the heat transfer characteristics for the lotus-type porous copper heat sink shows a good agreement with the experimental data. In addition, the predicted heat transfer coefficient of non-uniform pore model also shows a good agreement with the uniform pore models. Thus, it is clarified that the heat transfer characteristic of the lotus-type porous copper heat sink can be predicted by the uniform pore model.
The interaction of spray and combustion processes forms a complex system of physical phenomena undergoing in IC engines. Studying this interaction is important to determine strategies for simultaneously reducing soot and NOx emissions from diesel engines. Spray combustion interactions are evaluated by the flame lift-off length - the distance from the injector orifice to the location of hydroxyl luminescence closest to the injector in the flame jet. Various works have been dedicated to successful simulations of lifted flames of a diesel jet by use of various combustion modeling approaches. In this work, flame surface density and flamelet concepts were used to model the diesel lift-off length. Numerical studies have been performed with the ECFM3Z model and n-Heptane fuel to determine the flame lift-off length under quiescent conditions. The numerical results showed good agreement with experimental data, which were obtained from an optically accessible constant volume chamber and presented at the Engine Combustion Network (ECN) of Sandia National Laboratories. It was shown that at a certain distance downstream from the injector orifice, stoichiometric scalar dissipation rate matched the extinction scalar dissipation rate. This computed extinction scalar dissipation rate correlated well with the flame lift-off length. For the range of conditions investigated, adequate quantitative agreement was obtained with the experimental measurements of lift-off length under various ambient gas O2 concentrations and ambient gas densities.
This article presents the transient modeling and performance of waste heat driven pressurized adsorption chillers for refrigeration at subzero applications. This innovative adsorption chiller employs pitch-based activated carbon of type Maxsorb III (adsorbent) with refrigerant R134a as the adsorbent-adsorbate pair. It consists of an evaporator, a condenser and two adsorber/desorber beds, and it utilizes a low-grade heat source to power the batch-operated cycle. The ranges of heat source temperatures are between 55 to 90 °C whilst the cooling water temperature needed to reject heat is at 30 °C. A parametric analysis is presented in the study where the effects of inlet temperature, adsorption/desorption cycle time and switching time on the system performance are reported in terms of cooling capacity and coefficient of performance.
To investigate the effects of flow rate, diameter and offset of secondary fuel injection on combustor noise level, pressure fluctuation and NOx emission, four types of injectors were examined in a swirl-stabilized combustor for overall equivalence ratio (φ) of 0.7 ∼ 0.9 and flow rate of secondary fuel (Qsec) from 0.6 to 4.2 L/min. As for the reference injector used in previous related studies, secondary fuel injection of 3.0 L/min is the best condition for the reduction of pressure fluctuation and combustion noise with tolerable NOx emission. For lower secondary fuel rate of 1.8 L/min, reduction of the injection diameter of reference injector results in a better performance in terms of combustion noise and pressure fluctuation reduction. By the secondary fuel injection with offset, NOx emission shows lower values than those of the injections without offset for the whole range of equivalence ratios. Spectral analysis of pressure fluctuations revealed that larger amount of secondary fuel (Qsec ≥ 3.0 L/min) by the injectors with injection offset induce a lot of new modes in the wide range of frequency. For near stoichiometric operating condition (φ ≥ 0.85) by higher amount of secondary fuel injection with offset (Qsec = 4.2 L/min), the flame lifts over the swirl injector, NOx emission decreases and instability modes appear in a frequency range lower than those of the injection without offset. The results also suggest that the emission index is deeply related to flame lift-off and the dominant mode of instability.
The concentration of unburned fuel in the quenching layer on the single surface of a combustion chamber is measured by the infrared laser absorption method. The combustion chamber is divided into two compartments by a partition that has two holes covered with fine-meshed stainless gauze. After a propane-air mixture is ignited by a spark plug, a flame develops and quenches at the quenching block on the partition in the upper compartment. The mixture in the lower compartment remains unburned, but both compartments are at the same pressure. Two laser beams are introduced into the chamber; one is used to measure the fuel concentration near the quenching block (quenching wall) in the upper compartment and the other is used to measure the mixture concentration in the lower compartment. The fuel concentration near the surface of the quenching wall is obtained by analyzing the intensities of the two laser beams. A quenching layer is observed immediately after the flame reaches the quenching wall that is so thin that the thickness of the preheat zone of the flame overlaps each other and the fuel concentration in the quenching layer is very low. A little fuel then diffuses into the burned gas, but almost all the fuel stays near the quenching plate surface. The quantity of fuel remaining in the quenching layer at the time of flame extinction was lowest with an equivalence ratio of about 1.2, which is inconsistent with the characteristics exhibited by the quenching layer thickness.
The thermal evolution of Los Humeros geothermal fluids was estimated by using liquid and gas chemical geothermometers and results indicate two separated fluid entries to the wells: a (∼250°C) liquid produced from a shallower reservoir and a (>300°C) steam produced by a deeper stratum. Both entries were identified by Na/K and SiO2 geothermometers for the liquid and H2/Ar for the steam. CO2/Ar geothermometer seems to provide temperatures of mixed fluids entering the wells. Based on thermal evolution of fluids, multiple fluid entries in wells were characterized while reservoir exploitation-related processes like cooler water entries and condensation phenomena, were also identified.
An effective design method of the infrared heating furnace has not yet established, because there are the complexity and variety in the products and the heating form. We focused on the heating system of the emission subject for a thin substrate such as film, and expressed it in a concise unsteady numerical model within the three parallel plates. As a result, it was shown that the temperature of the intermediate surface “substrate” changes depending upon the combination of the emissivity of each surface, even with the constant injection energy. Especially, in the case of vacuum furnaces, a combination of “high emissivity of heating surface and low emissivity of non heating surface” is desirable to raise the temperature of substrate more. In addition, the lower emissivity of substrate surface is, the higher arrival temperature of substrate becomes, in the case that the heating surface and non heating surface have the same emissivity, and both walls have the same thickness, thermal conductivity and outer heat transfer coefficient. Since this tendency is extremely difficult to grasp intuitively, it is very worthy in the efficient design of furnaces to calculate a large number of patterns using the numerical model proposed in this article, and to conduct careful comparisons and evaluations.
To point out the dominant factor of mass, heat and electric charge transfer phenomena in a single cell of PEFC for the promotion of power generation performance, the impact of gas channel pitch on in-plane distribution of mass concentration and temperature on reaction surface is investigated by simulation and experiment. Two-dimensional model of single cell which has the different gas channel pitch is simulated by CFD software. In the experiment using the separators which have different gas channel pitches, the data of power generation and in-plane distribution of temperature measured by thermograph are acquired to verify the simulation result. As a result, the in-plane distribution of gas, water and temperature on the reaction surface becomes more even with decreasing gas channel pitch under the condition investigated in this study, resulting that the power generation performance is promoted. With decreasing gas channel pitch, the temperature in observation area is dropped and total voltage is elevated. To enhance the power generation performance of PEFC under the conditions investigated in this study, the flow rate of supply gas of stoichiometric ratio of 1.00 and the gas channel pitch of 0.5 mm can be proposed.
Edited and published by : The Japan Society of Mechanical Engineers and The Heat Transfer Society of Japan Produced and listed by : Showa Joho Process Co., Ltd.(Vol.8 No.3-) Sanbi Printing Co., Ltd.(Vol.1 No.1-Vol.8 No.2)