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### Research Papers: Energy Systems Analysis

J. Energy Resour. Technol. 2018;140(8):082001-082001-5. doi:10.1115/1.4039617.

The core of the work is the investigation of the possible correlation between the thermodynamics and the hazards of a process. The objective is understanding the role of inefficiency in hazards consequences. To investigate such correlation, a case study from oil and gas sector is developed, where exergy analysis is used to study the thermodynamics of the process and a simplified quantitative risk assessment (QRA) is performed to evaluate the consequences of identified hazards. The thermo-economic approach is then used to correlate the two analyses. Through the analysis, the authors want to identify those components where hazardous consequences may be affected by inefficiency, aiming to reduce the risk of fatalities in processes by operating on the process itself or suggesting possible alternative strategies. The purpose of the paper is also to propose for further investigation on the correlation between inefficiency and process hazards.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(8):082002-082002-8. doi:10.1115/1.4039608.

In this theoretical study, a fully developed laminar convective water flow in a circular tube is “convectively overloaded” toward the microscale, by decreasing the tube diameter below 1 mm. The entropy generation rate ($S˙gen$) is obtained (with and without the viscous dissipation term) for a given rate of heat removal using a fixed rate of coolant (water) flow. The uniform wall heat flux and mass flux in a tube increase toward the micro-scale, which is “thermal and flow overloading,” respectively. The variations of—$S˙gen$ due to fluid friction, fluid conduction heat transfer, and their total ($S˙gen,tot$), toward the micro-scale, are analyzed. Since $S˙gen,tot$ remains more or less the same toward the microscale, it is worth overloading a tube for miniaturization up to the laminar-flow limit.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(8):082003-082003-12. doi:10.1115/1.4039775.

The flow and heat transfer (FHT) in porous volumetric solar receiver was investigated through a double-distributed thermally coupled multiple-relaxation-time (MRT) lattice Boltzmann model (LBM) in this study. The MRT-LBM model was first verified by simulating the FHT in Sierpinski carpet fractal porous media and compared with the results from computational fluid dynamics (CFD). Three typical porous structures in volumetric solar receivers were developed and constructed, and then the FHT in these three porous structures were investigated using the MRT-LBM model. The effects of pore structure, Reynolds (Re) number based on air velocity at inlet, the porosity, and the thermal diffusivity of solid matrix were discussed. It was found that type-III pore structure among the three typical porous structures has the best heat transfer performance because of its lowest maximum temperature of solid particles at the inlet and the highest average temperature of air at the outlet, under the same porosity and heat flux density. Furthermore, increasing the thermal diffusivity of solid particles will lead to higher averaged air temperature at the outlet. It is hoped that the simulation results will be beneficial to the solar thermal community when designing the solar receivers in concentrated solar power (CSP) applications.

Commentary by Dr. Valentin Fuster

### Research Papers: Fuel Combustion

J. Energy Resour. Technol. 2018;140(8):082201-082201-18. doi:10.1115/1.4039548.

A mixed mode combustion strategy with a premixed compression ignition (PCI) combustion event and a mixing controlled load extension injection was investigated in the current study. Computational fluid dynamics (CFD) modeling was used to perform a full factorial design of experiments (DOE) to study the effects of premixed fuel fraction, load extension injection timing, and exhaust gas recirculation (EGR). The goal of the study was to identify a feasible operating space and demonstrate a pathway to enable high-load operation with the mixed mode combustion strategy. The gross-indicated efficiency (GIE) increased with premix fraction, but the maximum premix fraction was constrained by pressure rise rate which confined the feasible operating space to a premix fuel mass range of 70–80%. Injecting part of the premixed fuel as a stratified injection relieved the pressure rise rate constraint considerably through in-cylinder equivalence ratio stratification. This allowed operation with premix fuel mass of 70% and higher and EGR rates less than 40% which resulted in improved GIE of the late cycle injection cases. It was also identified that by targeting the fuel from the stratified injection into the squish region, there is improved oxygen availability in the bowl for the load extension injection, which resulted in reduced soot emissions. This allowed the load extension injection to be brought closer to top dead center while meeting the soot constraint, which further improved the GIE. Finally, the results from the study were used to demonstrate high-load operation at 20 bar and 1300 rev/min.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(8):082202-082202-11. doi:10.1115/1.4039606.

The selective catalytic reduction (SCR) is a promising NOx (a mixture of NO and NO2) reduction technology for various applications. The SCR process entails the conversion of NOx by the use of a reducing agent such as ammonia and a suitable catalyst. Due to increasingly stricter NOx emission regulations, the SCR technology for NOx control needs continuous improvement. The improvement requires better understanding of complex processes occurring in the SCR system. The current study employs a mathematical model to elucidate the effect of key operating and geometric parameters on the performance of SCR systems. The model considers both standard and fast SCR reaction processes. The model was used to investigate the effects of NH3/NOx and NO2/NOx ratios in the exhaust on the SCR performance and the effect of using a dual layer SCR system. Furthermore, the effect of different operating parameters and the interdependence of parameters is analyzed by using a factorial approach. The results show that the SCR performance is very sensitive to NH3/NOx ratio. The SCR performance is also affected by the NO2/NOx ratio particularly at low temperatures. The optimal NOx conversion performance requires a combination of NH3/NOx ratio of 1.0, NO2/NOx ratio of 0.5, low space velocities, and high inlet temperature. The results depict that adding a second catalyzed layer results in increased reaction activity especially when the concentration is still high after the first layer.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(8):082203-082203-7. doi:10.1115/1.4039736.

This study evaluates the outcomes of antioxidants and nanoparticles as additives with biodiesel diesel blends on the engine working characteristics, carried on a single cylinder direct injection (DI) diesel engine, operated at invariable engine speed of 1500 rpm, invariable injection timing of 26 deg before top dead center with invariable injection pressure of 216 bar, under five different engine load conditions (0.08, 0.15, 0.23, 0.30, 0.45, and 0.53 MPa). The antioxidants and nanoparticles blended test fuels are used as fuels in this experimental investigation. The antioxidant as additive in fuel found to be more effective in suppressing the NO emission by disrupting the chain propagating reactions, trapping free radicals, and decomposing peroxides. The high surface area to volume of the nanoparticles acts as fuel borne catalyst by ameliorating the engine working characteristics and downplays the NO emission by buffering the oxygen molecule. The obtained experimental results indicates that B20SNAlCe test fuel enhances engine brake thermal efficiency (BTE) by 13% and reduces level of pollutants such as unburned hydrocarbon (UBHC) by 38%, nitric oxide by 32%, smoke opacity by 21%, and carbon monoxide by 60% in compared with B100.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(8):082204-082204-8. doi:10.1115/1.4039659.

In this report, the influence of pre-oxidation degree and ventilation flow on the parameters of spontaneous combustion of coal (temperature, gas concentration, and exothermic intensity) was studied in six sets of programed temperature experiments. The experimental results showed that the pre-oxidation exerted a positive effect on the spontaneous combustion parameters of coal in the early stage of coal-oxygen recombination reaction, but exerted an inhibitory effect in the later stage of coal-oxygen oxidation reaction. Air supply rate had a positive correlation with the initial oxidation of coal samples and 90 °C pre-oxidation spontaneous combustion parameters. Air supply rate had negative correlation with 140 °C pre-oxidation of coal samples. Meanwhile, secondary oxidation significantly reduced the characteristic temperature of coal. The critical temperature of each coal sample was 83.7 °C (coal sample 1-Y), 68.3 °C (coal sample 1-L), 69.6 °C (coal sample 1-G), 82.1 °C (Coal sample 2-Y), 70.4 °C (coal sample 2-L), and 70.0 °C (coal sample 2-G), and dry cracking temperature was 142.6 °C (coal sample 1-Y), 134.8 °C (coal sample 1-L), 136.2 °C (coal sample 1-G), 147.2 °C (coal sample 2-Y), 136.5 °C (coal sample 2-L), and 134.4 °C (coal sample 2-G). The curves of the characteristic parameters of primary and secondary oxidized coal showed exponential growth. And the oxidation process can be divided into three stages, the first stage (30 °C ∼ critical temperature), the second stage (critical temperature ∼ dry cracking temperature), and the third stage (over the dry temperature).

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(8):082205-082205-7. doi:10.1115/1.4039630.

In an earlier publication (Jupudi et al., 2016, “Application of High Performance Computing for Simulating Cycle-to-Cycle Variation in Dual-Fuel Combustion Engines,” SAE Paper No. 2016-01-0798), the authors compared numerical predictions of the mean cylinder pressure of diesel and dual-fuel combustion, to that of measured pressure data from a medium-speed, large-bore engine. In these earlier comparisons, measured data from a flush-mounted in-cylinder pressure transducer showed notable and repeatable pressure oscillations which were not evident in the mean cylinder pressure predictions from computational fluid dynamics (CFD). In this paper, the authors present a methodology for predicting and reporting the local cylinder pressure consistent with that of a measurement location. Such predictions for large-bore, medium-speed engine operation demonstrate pressure oscillations in accordance with those measured. The temporal occurrences of notable pressure oscillations were during the start of combustion and around the time of maximum cylinder pressure. With appropriate resolutions in time steps and mesh sizes, the local cell static pressure predicted for the transducer location showed oscillations in both diesel and dual-fuel combustion modes which agreed with those observed in the experimental data. Fast Fourier transform (FFT) analysis on both experimental and calculated pressure traces revealed that the CFD predictions successfully captured both the amplitude and frequency range of the oscillations. Resolving propagating pressure waves with the smaller time steps and grid sizes necessary to achieve these results required a significant increase in computer resources.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(8):082206-082206-8. doi:10.1115/1.4039614.

In industrial processes such as chemical looping combustion, single-component spouted beds of monodisperse particles are very rarely used but the spouted beds of polydisperse particles have been widely used. The flow characteristics of polydisperse particles are much more complex than the single particle fraction in a fluidized bed. To investigate the gas–solid two-phase flow characteristics of the particles with different diameters in a spouted bed, the segregation and mixing characteristics, bubble morphology, minimum spouting velocity, and pressure fluctuations of the particles with different sizes under different superficial gas velocities are studied experimentally. The results show that higher the initial bed height and larger the volume fraction of the bigger particles, higher is the minimum spouting velocity. Moreover, the magnitude of the minimum spouting velocity increases exponentially with increase in the volume fraction of the bigger particles. At low superficial gas velocity, there is a clear trend of segregation between the particles of different diameters. At moderate superficial gas velocity, the mixing trend among particles of different diameters is enhanced, and the pressure fluctuations in the bed present some degree of regularity. At high superficial gas velocity, the particles of different diameters tend to separate again, the pressure fluctuations become intense, and the particle flow turns into a turbulent state. Furthermore, when the bed becomes stable, the particles of different diameters distribute within the bed with regular stratification.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(8):082207-082207-8. doi:10.1115/1.4039740.

The low vapor pressure of ethanol and the high latent heat of vaporization at low temperatures cause the difficulties of cold start in a flex fuel vehicle when it is fueled with pure ethanol. Preheating fuel is one of the effective methods to solve the above cold start problem. Thus, it is crucial to obtain the fuel temperature distribution in the heating system for fuel preheating process. For this purpose, the numerical analysis is adopted here to simulate the fuel preheating process at a cold start phase and explore the change of the fuel temperature field under different influence factors. The results indicate that the starting temperature has obvious impact on the temperature field distribution in the heating chamber and preheating time but has little effect on the volume of cold fuel in the connecting line at the end of heating. When the starting temperature is −5 °C, the preheating time is 8.3 s. When the starting temperature increases up to 15 °C, the preheating time will decrease as 4.9 s. Furthermore, the lower the starting temperature is, the higher the overall temperature of the heating chamber is. The installing angle of injectors has some influence on the temperature field distribution, and the cold fuel ratio reduces slightly when the angle increases from 30 deg to 60 deg. The calculation results indicate that the temperature of fuel at the injector inlet is above 20 °C, and the fuel injected during the first three cycles of the engine operation is hot fuel.

Commentary by Dr. Valentin Fuster

### Research Papers: Petroleum Engineering

J. Energy Resour. Technol. 2018;140(8):082901-082901-14. doi:10.1115/1.4039618.

In order to generate a new fracture far away from the original fracture in refracturing and effectively enhancing productivity, the technology of hydraulic refracturing guided by directional boreholes was presented. The effects of induced stress generated by the original hydraulic fracture, fracturing fluid percolation effect, wellbore internal pressure, and in situ stress on stress field distribution around wellbore were considered to obtain a fracture initiation model of hydraulic refracturing guided by two directional boreholes. The variation of maximum principal stress (σmax) under different conditions was investigated. The researches show that the directional boreholes result in a “sudden change region” of maximum principal stress around wellbore, reflecting dual stresses effects from vertical wellbore and directional boreholes on the rock. The width of sudden change region decreases as the distance from wellbore increases. Due to sudden change region, the refracturing fracture tends to initiate around directional boreholes. Whether the new fracture initiates and propagates along directional boreholes depends on comprehensive effect of borehole azimuth, borehole diameter, borehole spacing, horizontal stress difference, height, and net pressure of original fracture. The specific initiation position can be calculated using the theoretical model proposed in this paper. Affected by induced stress of the original fracturing, the rock tends to be compressed during refracturing, i.e., increased fracturing pressure. Sensitivity analysis with “extended Fourier amplitude sensitivity test (EFAST)” method shows the initiation of new fracture is mainly controlled by directional boreholes parameters and has little relation with in situ stress and parameters of original fracture. The influence rank of each parameter is as follows: borehole diameter > borehole spacing > original fracture net stress > borehole azimuth > horizontal stress difference > original fracture height. During design of refracturing, in order to better play the role of directional boreholes, and create a new fracture far away from original fracture, the optimal design is conducted with measures of optimizing boreholes azimuth, increasing borehole diameter and reducing borehole spacing if conditions permit. The research provides the theoretical basis for hydraulic refracturing guided by directional boreholes, which is helpful for the design of fracturing construction programs.

Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(8):082902-082902-12. doi:10.1115/1.4039743.

Theoretically, ultimate water-cut (WCult) defines stabilized well's oil and water production rates for uncontained oil pay underlain with water. However, in a real multiwell reservoir, the well's drainage area is contained by a no-flow boundary (NFB) that would control water coning, so the WCult concept should be qualified and related to the well-spacing size. Also, the presently used WCult formula derives from several simplifying assumptions, so its validity needs to be verified. The study shows that in multiwell bottom-water reservoirs, the production water-cut would never stabilize (after initial rapid increase) but would continue increasing at slow rate dependent on the production rate and well-spacing size. At each production rate, there is a minimum well-spacing size above which water-cut becomes practically constant at the value defined here as pseudoWCult. A new formula—developed in this study—correlates the minimum well-spacing with reservoir properties. Further, formula for pseudoWCult is derived by considering radial flow distortion effects in the oil and water zones. It is found that for well-spacing larger than the minimum well-spacing, the two effects-when combined-do not change the water-cut value. Thus, in practical applications, for sufficiently large well-spacing, the pseudoWCult values can be computed from the presently used WCult formula. The pseudoWCult concept has potential practical use in well-spacing design for ultimate recovery determined by the water cut economic limit, WCec. When the water-cut economic margin (WCec–WCult) is large, well-spacing has little effect on the ultimate recovery, so large well-spacing could be designed. However, when the water-cut economic margin is small, reservoir development decision should consider increase of final recovery by reducing well-spacing below the minimum well-spacing.

Topics: Reservoirs , Water
Commentary by Dr. Valentin Fuster
J. Energy Resour. Technol. 2018;140(8):082903-082903-9. doi:10.1115/1.4039742.

The oriented perforating is the essential technique to guide the refracture reorientation, but the influence of the oriented perforation design on the refracture steering radius is still unclear. In this paper, the factors influencing the refracture reorientation were studied by simulation models and experiments. The effects of initial fracture, well production, and perforations on the refracture initiation and propagation were analyzed. Three-dimensional finite element models were conducted to quantify the impact of perforation depth, density, and azimuth on the refracture. The large-scale three-axis hydraulic fracturing experiments guided by oriented perforations were also carried out to verify the fracture initiation position and propagation pattern of the simulation results. The research results showed that perforations change the near-wellbore induced stress distribution, thus changing the steering radius of the refracture. According to the simulation results, the oriented perforation design has a significant influence on the perforation guidance effect and refracture characteristics. Five hydraulic fracturing experiments proved the influence of perforating parameters on fracture initiation and morphology, which have a right consistency between the simulation results. This paper presents a numerical simulation method for evaluating the influence of the refracture reorientation characteristics under the consideration of multiple prerefracturing induced-stress and put forward the oriented perforation field design suggestions according to the study results.

Commentary by Dr. Valentin Fuster