Xili Duan

Performance study of mode-pursuing sampling method

Since the publication of the authors’ recently developed mode-pursing sampling method, questions have been asked about its performance as compared with traditional global optimization methods such as the genetic algorithm and when to use mode-pursing sampling as opposed to the genetic algorithm. This work aims to provide an answer to these questions. Similarities and distinctions between mode-pursing sampling and the genetic algorithm are presented. Then mode-pursing sampling and the genetic algorithm are compared via testing with benchmark functions and practical engineering design problems. These problems can be categorized from different perspectives such as dimensionality, continuous/discrete variables or the amount of computational time for evaluating the objective function. It is found that both mode-pursing sampling and the genetic algorithm demonstrate great effectiveness in identifying the global optimum. In general, mode-pursing sampling needs much fewer function evaluations and iterations than the genetic algorithm, which makes mode-pursing sampling suitable for expensive functions. However, the genetic algorithm is more efficient than mode-pursing sampling for inexpensive functions. In addition, mode-pursing sampling is limited by the computer memory when the total number of sample points reaches a certain extent. This work serves the purpose of positioning the new mode-pursing sampling method in the context of direct optimization and provides guidelines for users of mode-pursing sampling. It is also anticipated that the similarities in concepts, distinctions in philosophy and methodology and effectiveness as direct search methods for both mode-pursing sampling and the genetic algorithm will inspire the development of new direct optimization methods.

Thermal Protection of a Ground Layer With Phase Change Materials

Conventional ground surface insulation can be used to protect power line foundations in permafrost regions from the adverse effects of seasonal freezing and thawing cycles. But previous studies have shown ineffective thermal protection against the receding permafrost with conventional insulation. In this paper, an alternative thermal protection method (phase change materials (PCMs)) is analyzed and studied experimentally. Seasonal ground temperature variations are estimated by an analytical conduction model, with a sinusoidal ground surface temperature variation. A compensation function is introduced to predict temperature variations in the foundation, when the ground surface reaches a certain temperature profile. Measured data are acquired from an experimental test cell to simulate the tower foundation. With thermal energy storage in the PCM layer, the surface temperature of the soil was modified, leading to changes in temperature in the foundation. Measured temperature data show that the PCM thermal barrier effectively reduces the temperature variation amplitude in the foundation, thereby alleviating the seasonal freezing and thawing cycles. Different thermal effects of the PCM thermal barrier were obtained under different air temperature conditions. These are analyzed via melting degree hours and freezing degree hours, compared with a critical number of degree hours.

Nanosecond Laser Fabrication of Hydrophobic Stainless Steel Surfaces: The Impact on Microstructure and Corrosion Resistance

Creation of hydrophobic and superhydrophobic surfaces has attracted broad attention as a promising solution for protection of metal surfaces from corrosive environments. This work investigates the capability of nanosecond fiber laser surface texturing followed by a low energy coating in the fabrication of hydrophobic 17-4 PH stainless steel surfaces as an alternative to the ultrashort lasers previously utilized for hydrophobic surfaces production. Laser texturing of the surface followed by applying the hydrophobic coating resulted in steady-state contact angles of up to 145°, while the non-textured coated base metal exhibited the contact angle of 121°. The microstructure and compositional analysis results confirmed that the laser texturing process neither affects the microstructure of the base metal nor causes elemental loss from the melted regions during the ultrafast melting process. However, the electrochemical measurements demonstrated that the water-repelling property of the surface did not contribute to the anticorrosion capability of the substrate. The resultant higher corrosion current density, lower corrosion potential, and higher corrosion rate of the laser textured surfaces were ascribed to the size of fabricated surface micro-grooves, which cannot retain the entrapped air inside the hierarchical structure when fully immersed in a corrosive medium, thus degrading the material’s corrosion performance.

Dispersing different nanoparticles in paraffin wax as enhanced phase change materials

AbstractHighly conductive nanoparticles were proposed to be dispersed into phase change materials (PCMs) such as paraffin wax for heat transfer enhancement. The mixture, often referred to as nanoparticle-enhanced phase change material (NePCM), has been studied extensively for latent heat energy storage but with conflicting results. This study attempts to understand this problem by investigating the stability of NePCMs under multiple thermal (melting–solidification) cycles, which has not been well explained in previous studies. We believe that stability of a NePCM is prerequisite for any experimental investigation of its thermal properties or application. In this study, paraffin wax was chosen as the base material. Three different types of nanoparticles were tested, i.e., multi-walled carbon nanotubes, graphene nanoplatelets, and aluminum oxide nanoparticles (Al2O3). The nanoparticles were dispersed into paraffin wax at varying mass fractions using mechanical dispersion methods (sonication, stirring) with and without different surfactants. Stability of different mixtures was investigated after consecutive thermal cycles performed in an environmental chamber. Significant coagulation and deposition of nanoparticles were found after a few thermal cycles regardless of the nanoparticle type, concentration, or dispersion method. Different boundary conditions in heating were also examined for their effects. None of these methods led to long-term stable NePCMs. The “negative” results from this study indicate that long-term stability of NePCM (at least for the paraffin wax and nanoparticles tested) remains a major challenge and requires further research with a multidisciplinary approach.

Numerical study on ocean thermal energy conversion system

A thermodynamic model is developed for ocean thermal energy conversion (OTEC) systems. Considering the narrow temperature range in the evaporator, different system capacities were analyzed and compared in the sub-critical state with practical ocean thermal conditions. The results show that higher evaporation pressure will lead to less thermal load in the evaporator and the condenser and lower mass flow rates of working fluid. The thermal efficiency of different systems and the power generation for unit heat transfer area were found to be closely related to evaporation pressure, with a positive linear relationship. It was also found that increasing the capacity of the system can increase the thermal efficiency and its power generation for unit heat transfer area. This study provides useful insights into the design and equipment selection of OTEC systems.

New mechanism and correlation for degradation of drag-reducing agents in turbulent flow with measured data from a double-gap rheometer

Flow drag reduction with polymer additives has been studied and applied for many years. But degradation of the drag-reducing agent (DRA) is still not well understood. In this study, a new theory for the mechanism of DRA degradation is proposed: the degradation of polymers in flow drag reduction process is a first-order chemical reaction based on the molecular weight. A modified Arrhenius equation can be used to predict the chemical reaction constant. This brings physicochemical meaning to the previously empirical degradation coefficient k in existing correlations. We then conduct a series of flow drag reduction experiments with PEO (polyethylene oxide) of three different molecular weights in a double-gap rheometer to investigate the degradation phenomena. A new correlation is developed to predict the lifespan of DRAs. Predictions with this correlation agree with measured data with an average relative error of 15%, better than previous correlations. The results indicate validity of the proposed new mechanism of DRA degradation.

A New Calculation Method of Dynamic Kill Fluid Density Variation during Deep Water Drilling

There are plenty of uncertainties and enormous challenges in deep water drilling due to complicated shallow flow and deep strata of high temperature and pressure. This paper investigates density of dynamic kill fluid and optimum density during the kill operation process in which dynamic kill process can be divided into two stages, that is, dynamic stable stage and static stable stage. The dynamic kill fluid consists of a single liquid phase and different solid phases. In addition, liquid phase is a mixture of water and oil. Therefore, a new method in calculating the temperature and pressure field of deep water wellbore is proposed. The paper calculates the changing trend of kill fluid density under different temperature and pressure by means of superposition method, nonlinear regression, and segment processing technique. By employing the improved model of kill fluid density, deep water kill operation in a well is investigated. By comparison, the calculated density results are in line with the field data. The model proposed in this paper proves to be satisfactory in optimizing dynamic kill operations to ensure the safety in deep water.

Thermal Insulation With Latent Energy Storage for Flow Assurance in Subsea Pipelines

Flow assurance is critical in offshore oil and gas production. Thermal insulation is an effective way to reduce heat loss from subsea pipelines and avoid the formation of hydrates or wax deposits that could block the flowlines. This paper presents heat transfer analysis from a subsea flowline with different insulation materials, particularly with nano-enhanced phase change materials (NPCMs) that allow thermal energy storage in the pipeline system. The phase change materials (PCMs) can effectively regulate fluid temperature during production fluctuations or increase the cool-down time during production shutdown. This paper considers a pipe in pipe configuration with different insulation methods; the cool-down times are calculated and compared. The results show that thermal insulation can greatly delay the fluid cool-down process. A significant improvement of cool-down time can be achieved with PCM energy storage under a good conventional insulation layer. Moreover, with nanoparticles in a PCM, the latent energy storage is enhanced thus it takes even longer time for the internal fluid to reach its hydrate formation temperature.Copyright