Hydraulic fracturing is a widely employed well stimulation technique in which a synthesized fracking fluid is pumped into the well bore at high pressures to propagate fractures in rock formation matrix for hydrocarbon production. Proppant being one of the essential component of fracturing fluid is used to keep the hydraulically induced fractures open and conductive by acting as a mechanically strong support particle. Hydraulic fracturing operations are gradually shifting into deeper and low permeability (tight) formation at elevated temperature and pressure conditions at which conventionally used silica sand is becoming unviable as proppant due to its crushing. For resin-coated sand (RCS), temperatures greater than 60°C under high pressures, the glass transition temperature (Tg) threshold limit of polyurethane-based resin coating is reached at which this applied coating tends to soften that results reduction in the mechanical strength of the coated sand pack. Glass beads are also considered as good contestant for proppant that were selected in this study due to their high roundness and sphericity. They were drip coated with urethane resin incorporated with three different concentrations of 0.1 wt%, 0.5 wt% and 1 wt% carbon nanotubes (CNTs) and reduced graphene oxide (rGO) nanofillers, respectively. Crush test analysis by means of Universal Testing Machine (UTM) revealed significant improvement in the mechanical strength of coated glass bead proppants. 0.5 wt% loading of CNTs into urethane resin proved to be the best optimum concentration at which the mechanical strength of the coated glass bead improved by 84% along with 40% reduction in fines generation due to its containment within the applied coating. Crush test results also further revealed that nanofillers concentration loadings greater than the optimum threshold limit into urethane resin promoted their mutual agglomeration that resulted in profound reduction in mechanical strength of coated glass beads therefore resulting in more crushing and higher generation of undesired and detrimental fine particles.
Abstract. Since the pressure development in a combustion chamber is uniquely related to the combustion process, substantial variations in the combustion process on a cycle-by-cycle basis are occurring. To this end, an experimental study of cycle-by-cycle variation in a direct injection spark ignition engine fueled with natural gashydrogen blends combined with exhaust gas recirculation at relative air-fuel ratios was conducted. The impacts of relative air-fuel ratios (i.e. λ = 1.0, 1.2, 1.3 and 1.4 which represent stoichiometric, moderately lean, lean and very lean mixtures respectively), hydrogen fractions and EGR rates were studied. The results showed that increasing the relative air-fuel ratio increases the COVIMEP. The behavior is more pronounced at the larger relative air-fuel ratios. More so, for a specified EGR rate; increasing the hydrogen fractions decreases the maximum COVIMEP value just as increasing in EGR rates increases the maximum COVIMEP value. (i.e. When percentage EGR rates is increased from 0% to 17% and 20% respectively. The maximum COVIMEP value increases from 6.25% to 6.56% and 8.30% respectively). Since the introduction of hydrogen gas reduces the cycle-by-cycle combustion variation in engine cylinder; thus it can be concluded that addition of hydrogen into direct injection compressed natural gas engine employing EGR at various relative air-fuel ratios is a viable approach to obtain an improved combustion quality which correspond to lower coefficient of variation in imep, (COVIMEP ) in a direct injection compressed natural gas engine employing EGR at relative air-fuel ratios.Keywords-Coefficient of variation in indicated mean effective pressure (COVIMEP); Direct injection compressed natural gas (DI-CNG); Exhaust gas recirculation (EGR); Hydrogen fractions; Relative air-fuel ratios.
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