Characteristics of a Non-Premixed Rotating Detonation Combustor Using Natural Gas-Hydrogen Blend at Elevated Air-Preheat Temperature and Backpressure

Operational characteristics of an air breathing Rotating Detonation Combustor (RDC) fueled by natural gas-hydrogen blends are discussed in this paper. Experiments were performed on a 152 mm diameter uncooled RDC with a combustor to inlet area ratio of 0.2 at elevated inlet temperature and combustor pressure while varying the fuel split between natural gas and hydrogen over a range of equivalence ratios. Experimental data from short-duration (∼6sec) tests are presented with an emphasis on identifying detonability limits and exploring detonation stability with the addition of natural gas. Although the nominal combustor used in this experiment was not specifically designed for natural gas-air mixtures, significant advances in understanding conditions necessary for sustaining a stable, continuous detonation wave in a natural gas-hydrogen blended fuel were achieved. Data from the experimental study suggests that at elevated combustor pressures (2–3bar), only a small amount of natural gas added to the hydrogen is needed to alter the detonation wave operational mode. Additional observations indicate that an increase in air inlet temperature (up to 204°C) at atmospheric conditions significantly affects RDC performance by increasing deflagration losses through an increase in the number of combustion (detonation/Deflagration) regions present in the combustor. At higher backpressure levels the RDC exhibited the ability to achieve stable detonation with increasing concentrations of natural gas (with natural gas / hydrogen-air blend). However, losses tend to increase at intermediate air preheat levels (∼120°C). It was observed that combustor pressure had a first order influence on RDC stability in the presence of natural gas. Combining the results from this limited experimental study with our theoretical understanding of detonation wave fundamentals provides a pathway for developing an advanced combustor capable of replacing conventional constant pressure combustors typical of most power generation processes with one that produces a pressure gain.

Multiscale Investigation of Thickness Dependent Melting Thresholds of Nickel Film Under Femtosecond Laser Heating

A multiscale modeling that integrates electronic scale ab initio quantum mechanical calculation, atomic scale molecular dynamics simulation, and continuum scale two-temperature model description of the femtosecond laser processing of nickel film at different thicknesses is carried out in this paper. The electron thermophysical parameters (heat capacity, thermal conductivity, and electron-phonon coupling factor) are calculated from first principles modeling, which are further substituted into molecular dynamics and two-temperature model coupled energy equations of electrons and phonons. The melting thresholds for nickel films of different thicknesses are determined from multiscale simulation. Excellent agreement between results from simulation and experiment is achieved, which demonstrates the validity of modeled multiscale framework and its promising potential to predict more complicate cases of femtosecond laser material processing. When it comes to process nickel film via femtosecond laser, the quantitatively calculated maximum thermal diffusion length provides helpful information on choosing the film thickness.

Automated Engine Calibration Optimization Using Online Extremum Seeking

Emissions compliance during engine start-up conditions is a major obstacle for current automotive manufacturers across global markets. The challenges to meeting emissions targets are both due to increasingly stringent regulations and the difficulty in developing control strategies for a high degree-of-freedom and highly non-linear system. Online extremum-seeking (ES) methods offer a promising alternative to traditional optimization based on design-of-experiment based automotive calibration. With extremum-seeking methods, results from all prior experiments are used to intelligently and efficiently generate the next iteration of the control parameter(s). In this work, the applicability of the online extremum-seeking method is explored to optimize five performance variables (injection timing for two injection events, the injection fuel mass divided between the first and second injection events, air-fuel equivalence ratio and exhaust cam timing) to minimize brake specific fuel consumption while imposing different constraints on NOx emissions. The experiments were conducted using a production turbocharged four-cylinder gasoline engine with an advanced fuel injection system. The results show the utility of the ES strategy to quickly identify optimal control parameter combinations and the emissions and engine performance improvements during the calibration process. The results also demonstrate the dramatic benefit of the ES calibration strategy in terms of test time required.

Micro Combustor Development Using Hydrogen As Fuel

Swiss Roll Combustor of 1 W capacity for micro gas turbine engine is operated with premixed hydrogen air mixture at ultra-lean equivalence ratio. The reaction zone under consideration has volume of 60 mm3. The reactant passage and product passage are coiled around the reaction zone facilitating the recovery of heat loss, by preheating the reactants. The reactants are entered through an increasing cross-sectional area passage from 0.6mm × 5mm inlet to 1.5mm × 5mm outlet, thereby reducing the velocity levels in the reactant passage, facilitating stable combustor operation. The combustor performance parameters, temperature levels, pattern factor and emissions are measured is operated at ultra-lean equivalence ratio of 0.12 to lean equivalence ratio of 0.43, for premixed hydrogen air combustion. The results are compared at similar (not same) equivalence ratios for constant area reactant passage. The visual inspection shows stable flame front in the combustion zone for variable area passage and extended flame front in the constant area passage combustor. However, the combustor performance deteriorates drastically above equivalence ratio of 0.43, leading to hot spots generation on walls.

Efficiency Improvement in Small Internal Combustion Engine Using Exhaust Heat Recovery System

Strict regulations are set up in various parts of the world with respect to vehicular emissions by their respective government bodies forcing automakers to design fuel-efficient vehicles. Fuel economy and carbon emission are the main factors affecting these regulations. In this competitive industry to make fuel efficient vehicles and reduce Green House Gas (GHG) emissions in internal combustions has led to various developments. Exhaust Heat Recovery System (EHRS) plays a vital role in improving powertrain efficiency. In this system, heat rejected by the engine is reused to heat the vehicle fluids faster (like engine coolant, engine oil, etc) also reducing harmful gases emitted. In internal combustion engines, generally only 25% of the fuel energy is converted into useful power output and approximately 40% of it is lost in exhaust heat. Certain studies show that by using the EHRS, the power output can be increased to 40% and the heat loss can be reduced to as much as 25%. The purpose of this study is to make use of this lost energy and convert most of it into useful energy. The thermodynamic properties and fuel consumed during the warmup period were analyzed to measure the improvement in the engine efficiency. The design was implemented on a Briggs and Stratton Junior 206cc engine. This system includes the use of heat exchangers. The main goal of this study is to develop a robust EHRS design and compare it with the baseline engine configuration to see the thermal and fuel economy improvement.

Simulation of Nucleate Boiling With Interfacial Temperature Gradients and Sharp Interface

Effective heat transfer techniques benefit the development of nuclear and fossil fuel powered steam generators, high power electronic devices, and industrial refrigeration systems. Boiling dissipates large heat fluxes while keeping a low and a constant surface temperature. However, studies of the fluid behavior surrounding the bubble and the heat transfer near the contact-line are scare due to difficulties of flow visualization, chaotic conditions, and small length scales. The preset study shows the simulation of bubble growth over a heated surface from conception to departure. The computation of mass transfer with interfacial temperature gradients leads to proper bubble growth rates. Models to include the interface sharpness uncover the dynamic and thermal interaction between the interface and the fluid. Results indicate that the nucleation of a bubble (in water at 1 atm with 6.2 K wall superheat) has an influence region of 2Db (where Db is the departure bubble diameter). In addition, results reveal a thin thermal film near the interface that increases the heat transfer at the contact-line region. Numerical bubble growth rates compare well with experimental data on single bubble nucleation.

Preheat Limits in Practical Combustor Design: Experiments and Simulations

Autoignition in commercial and residential gas appliances is typically a phenomenon to be avoided. The autoignition temperature for a particular fuel, defined as the minimum temperature at which spontaneous ignition will occur without an external source of energy, is often used to characterize this phenomenon. In the design of combustion systems, it is used to demarcate conditions where autoignition may occur. In an emerging class of residential and commercial heating, cooling, and power generation appliances, preheating air and fuel can provide an effective means of boosting the overall energy efficiency by recuperating residual energy in the exhaust and reinvesting it back into the thermodynamic process. In such applications, the design question to answer is: How much can the air and fuel be preheated without autoignition? The autoignition temperature, often determined experimentally and can vary as much as 100°C for methane, may not be the most useful metric in this context. This work describes the results of a recent experimental investigation into the preheat limits for autoignition of air and natural gas with the aim of recuperating as much heat as possible in a heat pump. The experimental apparatus consisted of an air-fuel mixer supplying preheated mixture to a radiant burner. The air was first heated in excess of 750°C, cool natural gas was injected into and mixed with the hot-air stream, and all while avoiding autoignition. The current capability to predict autoignition in such applications a priori was also assessed using available chemical kinetic models and numerical simulations.

Calibration of External Heat Transfer Coefficients During Cooling of a Partially-Filled Water Tank Using Measured Temperature-Time Data

A combined Computational Fluid Dynamics (CFD) and experimental approach is presented to determine (calibrate) the external convective heat transfer coefficients (h) around a partially-filled water tank cooled in a climactic chamber. A CFD analysis that includes natural convection in both phases (water and air) was performed using a 2D-axisymmetric tank model with three prescribed average heat transfer coefficients for the top, side and bottom walls of the tank. The commercial CFD code ANSYS-Fluent™, along with User-Defined Functions (UDFs), were utilized to compute and extract temperature vs. time curves at five different thermocouple locations within the tank. The prescribed h values were then altered to match experimentally obtained temperature-time data at the same locations. The calibration was deemed successful when results from the simulations exhibited match with experimental data within ±2°C for all thermocouples. The calibrated h values were finally used in full-scale 3D simulations and compared to the experimental data to test their accuracy. Predicted 3D results were found to agree with experimental results within the error of the calibration, thereby lending credibility to the overall approach.

Uniquely Designed Solar Tube in a Natural/Forced Mini-Water Heating System

Solar power is a clean source of energy, i.e. it does not generate carbon dioxide or other air pollutants. In 2017, solar power produced only 0.6 percent of the energy used in the United States, according to the Energy Information Administration. Consequently, more solar energy should be implemented, such as in solar water heaters. This research took place in Riverside, Southern California where there is an abundance of solar energy. In house uniquely designed and assembled solar tubes were used in designing a mini solar water heating system. The mini solar water heating system was set to operate under either natural or forced convection. The results of running the system under forced convection then under natural convection conditions are reported and discussed in the article. In addition, comparison of using two different solar water storage systems are reported: the first was water; the second storage medium was a combination of water and gravel. Since water heaters are extensively used for residential purposes, this research mimicked the inefficiencies in residential use and is compared with ideal operating conditions. The performance of the different cases studied is evaluated.

CFD Modelling of NOx and Soot Formation in Aluminum Anode Baking Furnace

The cost and quality of aluminum produced by the reduction process are strongly dependent on heat treated (baked) carbon anodes. A typical aluminum smelter requires more than half a million tons of carbon anodes for producing one million ton of aluminum. The anode baking process is very energy intensive, approximately requires 2GJ of energy per ton of carbon anodes. Moreover, pollutant emissions such as NOx and soot formation are of major concern in the aluminum anode baking furnace. The current study aims at developing an accurate numerical platform for predicting the combustion and emissions characteristics of an anode baking furnace. The Brookes and Moss model, and the extended Zeldovich mechanism are employed to estimate soot and NOx concentration, respectively. Considering a fire group of three burner bridges, one after the other in the fire direction, combustion and emissions features of these three firing sections are interrelated in terms of oxidizer’s concentration and temperature. In the present study, considering this interconnection, the effect of diluted oxygen concentration at elevated oxidizer’s temperature (∼1200°C), which are the key features of the moderate or intense low oxygen dilution (MILD) combustion are analyzed. It is observed that by circulating some of the exhaust gases through the ABF crossovers, oxygen dilution occurs which results in higher fuel efficiency, lower pollutant emissions, and more homogeneous flow and temperature fields.