Abstract:Taking Sutong Yangtze River Highway Bridge (STB) as an engineering background, a downburst wind and the vibration acceleration responses of the bridge structure were monitored based on the wind-induced vibration response monitoring system of the bridge. Furthermore, the wind speed and direction, vibration responses of the main girder under the thunderstorm were investigated. Firstly, the measured wind speed and wind direction data of the downburst wind at the bridge site were analyzed, the time-varying mean wind and fluctuating wind characteristics of the downburst at the mid-span of the main girder and the pylon tops were obtained. Then, the wind-induced vibration acceleration responses of the main girder under the downburst were analyzed. The research results show that the wind speeds at the main girder level and the pylon tops of the STB changes abruptly under downburst, which lasts approximately 10~24 minutes. The maximum instantaneous wind speeds at the leeward and windward side of the main girder in the middle of the main span of STB are 32.4 m/s and 27.3 m/s, respectively. Meanwhile, the maximum instantaneous wind speeds at the south and north pylon tops are 60.5 m/s and 62.9 m/s, respectively. The turbulence intensity for a time interval of 30 s at the main girder level is about 0.048~0.32, and the turbulence intensity for a time interval of 10 minutes is about 0.43~0.51. The reduced fluctuating wind speed at the downstream of the main girder and the north pylon conforms to Gaussian characteristics, and its power spectrum is in good agreement with the field measurement results by Burlando and other scholars. Obvious short-term vertical and horizontal vibration occurred near the middle of the main girder (that is, the anchorage of NJ26D and NJ32D cables), and the corresponding acceleration response amplitudes were 0.25 m/s2 and 0.10 m/s2, corresponding to the displacement amplitudes of 0.12 m and 0.03 m, respectively. The vertical vibration response of the main girder is obviously larger than the transverse vibration response. The predominant frequency of vertical vibration of the main girder is 0.183 Hz, which is close to the 1st symmetric vertical bending mode frequency 0.174 Hz of the main girder. And the predominant frequency of horizontal vibration is 0.117Hz, which is close to the 1st symmetry lateral bending mode frequency 0.097 5 Hz of the main girder.

Abstract:In order to study the influence of air permeability of wind barrier on vortex-induced vibration performance of the main girder, based on a long-span steel box girder suspension bridge with 808 m main span, wind-induced vibration response and pressure time history data of each measuring point on the surface were obtained by wind tunnel vibration measurement and pressure measurement test. The vortex vibration performance of the original section was measured within the range of ± 5 ° angle of attack after the wind barrier was added. The vortex vibration response performance of the main girder, the mean value and root variance of the fluctuating pressure coefficient of each measuring point on the bridge deck, the correlation and contribution coefficient between local aerodynamic force and overall aerodynamic force were compared and analyzed among the original section as well as three different ventilation rates and five conditions of water adding horizontal splitter plates. The results show that multi-interval vertical bending vortex-induced vibration occurs on the original section at + 5 °angle of attack, and the amplitude of vortex-induced vibration is far beyond the allowable value in the code. The vortex-induced resonance of the main girder is affected by the wind barrier with different air permeability, which eliminates the vortex-induced vibration of the original section of the main beam in the low wind speed range, and the maximum amplitude is also reduced to a certain extent. According to the analysis of the measured pressure data, the pressure fluctuation of the middle and rear part of the main beam is weakened, and the contribution coefficients of the local aerodynamic force and the overall aerodynamic force are reduced, which makes the amplitude of the main beam decrease slightly; after the horizontal splitter plate is added at the wind nozzle of the main beam, the correlation between the local aerodynamic force and the total aerodynamic force is completely destroyed, and the pressure fluctuation is weakened, which effectively suppresses the vortex induced virbration of the main beam in this case.

Abstract:In order to accurately simulate the dynamic process of the cable rupture event, based on the alternate load path (ALP) method, the numerical simulation methods for dynamic analysis of suspension bridges subjected to hanger-breakage event are studied. Taking a prototype self-anchored suspension bridge as the background, the basic principles and characteristics of three methods (e.g., the instantaneous stiffness degradation method, the instantaneous loading method and the equivalent unloading method) are illustrated. In addition, the influencing factors on structural dynamic effect of the collapse responses are quantitatively analyzed. The results indicate that the instantaneous stiffness degradation method is simple and effective to simulate the dynamic process of the hanger-breakage event. The hanger loss induced dynamic responses are closely associated with the influencing factors, such as the broken hanger elements in the finite element model, the duration and time-dependent tension loss function of the breakage process, and hanger loss scenarios.

Abstract:Based on the improved damage algorithm and multi-lane refined traffic flow simulation, a new method for predicting the fatigue life of concrete bridges was proposed, where the damage under each cyclic loading was introduced into the S-N curve in the improved damage algorithm. The S-N curve of materials under fatigue was modified in order that the predicted fatigue life of materials was closer to the real situation. The Markov chain Monte-Carlo simulation method (MCMC) was used to generate multi-lane fine traffic flow considering the correlation between adjacent models and traffic lanes. First, the accuracy of the improved damage algorithm was verified by the multistage variational fatigue tests of a group of reinforced concrete beams and a group of prestressed concrete beams. Then, the refined process of multi-lane random traffic flow simulation was introduced, and the fatigue life prediction process of concrete bridge based on improved damage algorithm and multi-lane stochastic traffic flow simulation was proposed. Finally, the traffic flow data measured on a highway and a simple supported T beam bridge with a span of 20m were used for example analysis. The results show that the prediction error of the five groups of specimens is significantly lower than that of the conventional damage algorithm. Except that the prediction error of two prestressed concrete beams is bigger (53%~56%), the prediction error of the other three groups of specimens is smaller (less than 8%), indicating that the improved damage algorithm can be used to predict the fatigue life of concrete bridges. In case of analysis, the stress spectrums appear the characteristic of multi-peak distribution, which is similar to the vehicle load distribution, illustrating the rationality of the simulation. According to the improved damage algorithm, when the average annual traffic rate (AAGR) is 0, the fatigue life of the bridge is 77.50 years, which didn't meet the requirements of design service life. When AAGR is 3%, the fatigue life of the bridge is 52.49 years, which decreased by 32.27% compared with that when AAGR is 0.

Abstract:The shape coefficient and shielding factor of each member are important in the wind resistant design of the structure for wind sensitive structures such as tubular-angle steel transmission towers. Taking 500 kV and 110 kV transmission towers as the engineering background, the rigid section model of tower body with a scale ratio of 1 ∶ 8.5 and cross-arm with a scale ratio of 1 ∶ 2.5 are designed firstly. Then, through the wind tunnel tests on synchronous pressure measurement of the model under three different wind speeds and different wind incidence angles, the variation rules of shape coefficient and shielding factor of angle and steel pipe members are obtained. On this basis, the shape coefficient distribution of members in the tower body and cross-arm, and the shape coefficients of the section models under different wind incidence angles are summarized. Finally, the results determined by the test are compared with the relevant normative values at home and abroad. The results show that the incoming wind speed has little effect on the shape coefficients of angles in tower body and cross-arm. The shape coefficients of angles in the tower body tend to be small at the top but large at the bottom along the section of tower body under 0° incidence angle. The shielding factors of angles in tower body are consistent with that in the Japanese code JEC. In the test, the difference of the shielding factors of tower body (3.10×104~4.34×104) and cross-arm (5.57×104~8.00×104) under different Reynolds number of main materials is very small. The drag coefficients of tower body and cross-arm are consistent with that in Japanese standard, whereas the values of Chinese standard are 7.2% and 4.5% lower, respectively.

Abstract:To reveal the influence of the wake released from the upstream square cylinder on fluctuating aerodynamic characteristics of the downstream one，wind tunnel tests were carried out to measure the pressure on twin square cylinders，under a uniform inflow condition with a Reynolds number Re of 8.0×104，space ratios P/B of 1.25~5 （where P is the central distance，B is the side length) and various incidence angles of 0° ≤ α ≤ 90°. The variations of fluctuating aerodynamic forces，power spectrums of lift，Strouhal numbers and spanwise aerodynamic correlations with α are investigated considering the multiple space ratios. When P/B < 3，fluctuating forces on the downstream cylinder are roughly smaller than those on a single square cylinder，and they change little at α ≥ 40°. However，when 3 ≤ P/B ≤ 5 and 0° ≤ α ≤ 30°，the fluctuating drag on the downstream cylinder is significantly stronger than that on a single one. The fluctuating forces，power spectrums of lift and spanwise aerodynamic correlations on the downstream cylinder change dramatically when P/B reaches 3. When P/B < 3，vortex shedding from the downstream cylinder is significantly suppressed. When P/B > 3，however，the fluctuating aerodynamic properties of the downstream cylinder become similar to those of a single one.

Abstract:This paper investigates the influences of tall building shape and layout on the pedestrian-level wind (PLW) environment in the urban area by combing the wind tunnel test and Computational Fluids Dynamics (CFD) simulations. The maximum wind speed-up ratio and integrated normalized speed-up area ratio were used to quantify the effects of five different building shapes and four building layouts on the PLW environment. The favored building shape and layout with omnidirectional equal-probability distribution were determined, and the underlying mechanism of building shape and layout that influences the PLW environment in the urban area were elucidated according to the whole flow-domain information attained by CFD simulations. The PLW of groups of Y-shaped tall buildings in the staggered layout was most favored, while the most unfavorable scenario was H-shaped and enclosed layout; In addition, the most unfavorable wind directions of square, H-shaped and X-shaped tall buildings were in the oblique direction, while these of the cross- and Y-shaped buildings were in the normal wind direction. The wind speed-up phenomena of groups of tall buildings at the pedestrian-level were mainly attributed to the flow separation at building corners and channeling effects.

Abstract:In order to establish a post-disaster performance evaluation method for self-tapping screw joints under typhoon, firstly, the time-history wind pressure coefficient data of the roof based on wind tunnel tests were collected. Secondly, typhoon simulation was carried out to get information on wind speed and direction. Then, the coefficient matrix of roof wind pressure during typhoon transit was calculated by rain-flow analysis method. Finally, the node resistance degradation model under different wind speeds was calculated. Combined with the typhoon load model, the time-varying reliability index of self-tapping screw joints in coastal cities was obtained by the Monte Carlo simulation method. Thus, the wind resistance performance evaluation of self-tapping screw joints based on probability is realized. Besides, the analytical solutions of the wind speed and wind direction of the simulated typhoon are given according to the method of simulating typhoon. Simultaneously, the mathematical model of resistance degradation of self-tapping screw joints caused by the typhoon is obtained. Through the analysis of a numerical example, it is concluded that the reliability of self-tapping screw joints is significantly reduced when the typhoon induced resistance degradation is considered. When evaluating the reliability of self-tapping screw joints for light steel roofing, the target reliability indicators in China's codes are relatively conservative.

Abstract:Cold-formed thin-walled rectangular steel tube columns are prone to local buckling failure. In order to study the compression-bending capacity of cold-formed thin-walled rectangular steel tube columns, a nonlinear finite element model of cold-formed thin-walled rectangular steel tube columns was established and verified. This model is used to simulate 884 rectangular steel tube columns with different axial compression ratios, flange width-thickness ratios, and web width-thickness ratios under constant axial pressure and variable horizontal forces, and the failure mode and flexural capacity in the ultimate limit states are examined. The results show that there are mainly two failure modes in the compression-bending process of cold-formed thin-walled rectangular steel tube columns in the strong axial: full-section yield, and compression flange and web buckling. Combined with the stress distribution characteristics of the limit state，a calculation formula for the ultimate flexural capacity of cold-formed thin-walled rectangular steel tube columns considering the relative buckling of the plate group is proposed based on the plastic effective width method. The predicted value is in good agreement with the finite element simulation results.

Abstract:In order to study the distortional buckling behavior of cold-formed steel（CFS） built-up box-section columns，firstly，9 C-section columns，9 U-section columns，and 21 built-up box-section columns composed of C-section and U-section were investigated by the axial compression tests and numerical simulation. The buckling characteristics and mechanical behaviors of the columns were investigated. On this basis，a set of hypothesis models were proposed to study the influence of screw spacing on the deformation，the ultimate capacity，and the relationship between the screw spacing and the half-wavelength（λc） of CFS built-up box-section columns. The results show that：1） The number and half-wavelength of distortion buckling are different for the specimens with different screw spacing. 2） When the screw spacing is less than 0.9λc，the ultimate capacities of built-up box-section columns are greater than the sum of the ultimate capacities of C-section and U-section columns，that is，the splicing effect of 1+1>2. 3） When the screw spacing is greater than 0.9λc，the ultimate capacity of the built-up box-section column is gradually close to the sum of the ultimate capacities of the C-section and U-section columns，i.e. 1+1≈2. In order to calculate the ultimate capacities of CFS built-up box-section columns，an approach to predict the elastic critical load of distortion buckling was proposed based on the direct strength method（DSM） in American code，and then the calculated results were applied to the DSM to obtain the ultimate capacity of CFS built-up box-section columns. The results obtained from the calculation method are in good agreement with the experimental and numerical simulation results，which suggests the accuracy and applicability of the proposed method in this paper.

Abstract:In order to strengthen beam-column joints, avoid the failure mode of "weak joints" in frames and improve the overall seismic and energy consumption capacity of frame structures, a displacement-amplified torsion damper (DATD) with independent intellectual property rights is developed. Numerical analysis and experimental study of DATD were carried out. Firstly, a total of 18 models of DATD with different parameters were designed and their finite element models were established, and then numerical analyses were performed. Then a DATD was designed and manufactured. The results of finite element analysis were verified by performance tests. The results show that the DATD has full hysteretic curve and strong energy dissipation capacity. The hysteretic curves of finite element analysis and performance test are in good agreement, and with the increase of loading displacement, the difference between them decreases. So, the finite element model established in this paper can be used to study the mechanical properties of DATD. Finally, the parameter influence analysis of DATD was performed, the influences of the diameter of the lead core, the distance between the lead core and the central axis, the diameter of the rubber layer, the thickness of the rubber layer and the rubber shear modulus on its characteristic parameters were studied. The results show that the yield shear force, equivalent stiffness, equivalent damping ratio and the energy dissipation coefficient of DATD increase obviously with the increase of the diameter of the lead core, and increase slightly with the increase of the distance between the lead core and the central axis. With the increase of the diameter of the rubber layer and rubber shear modulus, the yield shear force and equivalent stiffness gradually increase, while the energy dissipation coefficient and equivalent damping ratio gradually decrease. The four characteristic parameters all decrease slightly with the increase of the thickness of the rubber layer.

Abstract:In order to study the hysteretic performance and lateral resistance of positive symmetrical damper with corrugated web, two damper specimens, horizontal corrugated web damper and vertical corrugated web damper, were designed, and their quasi-static tests were carried out. The test results show that the designed corrugated web dampers have excellent hysteretic performance and energy dissipation capacity, and the bearing capacity of vertical corrugated web damper is obviously higher than the horizontal corrugated web damper. Meanwhile, the energy dissipation of the damper mainly depends on the shear and buckling of the corrugated web, whereas the contribution of the flange plate on the bearing capacity of the damper is relatively small. The simulation and experimental verification by ABAQUS finite element software show that the simulation results are in good agreement with the experimental results. Based on the vertical corrugated web damper，a total of 22 finite element models are established, with variable parameters including the aspect ratio, thickness and amplitude of the corrugated web. The results show that with the increase of the aspect ratio, the shear bearing capacity decreases greatly; with the increase of the thickness, the shear bearing capacity increases sharply; with the increase of the amplitude, the shear bearing capacity increases slightly. Finally, combined with the finite element models, the shear bearing capacity formula of the corrugated web damper is derived.

Abstract:The seismic intensity of the meizoseismal area reached Ⅷ degrees in Mojiang M5.9 earthquake in Yunnan province，which was an intensive inspection to the results of renovation of the dilapidated rural residences. In order to verify the effectiveness of the renovation of dilapidated rural houses and summarize the seismic damage experience of the fortified farmhouses，the following work is done in this paper. Firstly，the ground motion characteristics are analyzed from the attenuation law of ground peak acceleration and the spectrum characteristics of seismic waves according to the observation records of strong vibration in this region. And then，on the basis of seismic damage investigation，combined with the characteristics of the architecture and structure，the characteristics of the seismic damage of rural houses in the areas with earthquake intensities of Ⅶ and Ⅷ degrees were elaborated，the performance of the reinforced dilapidated rural houses in the earthquake was compared and analyzed，and the experience of seismic damage was summarized. Finally，combined with the investigations of seismic disasters，some suggestions are put forward to improve the earthquake resistance of rural houses from the aspects of strengthening earthquake observation，increasing efforts to promote the renovation of rural dilapidated houses，promoting the use of new technologies and new materials，and attaching importance to seismic damage surveys.

Abstract:In order to study the shear performance of variable cross-section corrugated steel webs, firstly, based on the theory of orthotropic plate and the theory of small deflection of thin plate, the calculation formula of elastic overall shear buckling strength of corrugated steel webs is deduced by Galerkin method. Secondly, the calculation value of the derived formula is compared with those of ANSYS finite element and code formula. Moreover, the derived value of the formula is also compared with the experimental value in the literature. Finally, the influence of different types of corrugations, web thickness and girder height on the elastic shear buckling behavior of variable cross-section corrugated steel web is studied by using the finite element method. The results show that the calculated value of the derived formula is in good agreement with that of the finite element method and test value. Because the contribution of Dxy to the global shear buckling strength of the corrugated steel web is ignored in the specifications formula, the calculation of the value of the specifications is more conservative. With the increase of the corrugated size, the shear buckling strength generally increases first and then decreases, where the shear performance of the 1600 corrugated steel web reaches the maximum. With the increase of the web thickness, the shear buckling strength increases gradually. The shear buckling strength of the variable section corrugated steel web is greater than that of the constant section corrugated steel web. With the increase of the angle β between the girder bottom and the horizontal direction, the shear buckling strength of the variable section corrugated steel web increases. The conclusion can provide a reference for the shear design of the same type of bridge.

Abstract:Incorporating the boundary conditions, initial parameter solutions of vibration differential equations for Timoshenko beam are used to derive the frequency equation of a simply-supported beam. When the natural frequency is less than the critical frequency, the frequency equation can be factorized into the hyperbolic sine function and the trigonometric sine function, while, when the natural frequency is greater than the critical frequency, the frequency equation can be factorized into double trigonometric sine functions, which is the crucial reason for the existence of the second frequency spectrum. Frequency equations for two-span and three-span continuous Timoshenko beams with uniform cross sections and equal spans are derived. Other structures with the second frequency spectrum are forecasted theoretically. The formulas for the first and second frequencies are deduced for simply-supported Timoshenko beam. The existence of the second frequency spectrum is confirmed through the examples. Through solving the differential equation of motion, the critical frequency is proven to be an efficient part of the natural frequencies for the framed structures. The corresponding mode shape of the critical frequency contributes to the displacement mode shape with zero amplitude and rotation mode shape with constant amplitude. Due to the truncation error of the computer, the critical frequency predicted by the finite element method shows error, and the mode shape of the displacement is very irregular.

Abstract:The utilization of Nano-SiO2（NS） in polymer cement based composite can promote its hydration rate, enhance its mechanical property, change its microstructure of hydration products and improve its performance of interface transition zone (ITZ). Hence, mechanical properties, drying shrinkage property, microstructure and composition of hydration products and ITZ related performance of polymer cement based composite modified by various dosages of NS were studied by adopting the combination of macro and micro methods, including electro-hydraulic pressure testing machine, cement mortar comparator, X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray energy spectrometer (EDS). The test results reveal that NS can enhance the strength of the polymer cement mortar (PCM), especially at the early age. With the incorporation of NS, the dry shrinkage rate of PCM increases, which is also more significantly pronounced at the early age. It can be seen from XRD and SEM results that the degree of polymer cement based composite hydration is accelerated because of mixing NS. Pozzolanic reaction of NS can alter the number, microstructure and composition of hydration products of polymer cement based composite, and reduce the Ca/Si atomic fraction ratio of C-S-H gel. ITZ between polymer cement hardened paste and aggregate is improved due to the reaction of NS with Ca(OH)2 rapidly, resulting in a more compact structure without obvious cracks and holes. It tends to be an increased micro-hardness in ITZ as a result of the dense hydration products and the generation of more C-S-H gel.

Abstract:The influence of degradation of shear surface asperity on the load transfer mechanism of the pile-rock interface in the shear process is studied. Firstly, the rough surface between concrete of the pile and bored stratum is abstracted as a series of identical isosceles triangles, and the size of a single rough body is defined by half-wavelength and dilatancy angle. Secondly, the Patton model is introduced to describe the relationship between macroscopic shear responses of the rough body and relative shear displacement. Considering the relative stiffness ratio between the hole wall stratum and the pile body concrete, based on the energy principle, a rough body degradation coefficient is introduced to define the behavior of the rough body surface wear and volume compression generated during the shearing process. Accordingly, the classic Patton model was then improved. On this basis, the vertical load transfer equation of cast-in-place piles considering the degradation of the hole wall roughness is derived. This equation can not only consider the influence of the shear surface roughness (half wave length and dilatancy angle) on the load transfer behavior of piles, but the physical meaning of parameters included in the solution is also clear. Finally, the finite difference method is used to solve the load transfer equation, which is compared and verified by engineering examples. The results show that the theoretical predictions in this paper are in good agreement with the field measured results, and have a certain reference value for the preliminary design of cast-in-place piles.

Abstract:To study the seismic response characteristics of the seamless transfer subway station and improve the understanding of the seismic performances of such station structures, the shaking table test on a scale model of such station structure was carried out for the first time. The design of the shaking table test included the preparation of test models, the arrangement of measuring points, collection of testing data and the design of test working conditions. Then, the processes of the model test were simulated by three-dimensional finite element method. Through comparison between the numerical and measured results, acceleration responses of the model soil, strain and internal force responses of the structure model and soil pressure responses on the sidewall were analyzed. The results showed that the numerical results agreed well with the test data, which validated the rationality of the modeling method in this paper. For the structure model of the seamless exchange subway station, the structural exchange end had an obvious influence on the structural deformation, the structural internal force and its surrounding soils. When the distance between the station exchange ends exceeded 1.5 times station structural width, its effect basically disappeared. These conclusions can provide powerful support to the three-dimensional calculation method for the seismic analysis of complex subway stations and the anti-seismic design of such station structures.

Abstract:In order to study the development and collapse mechanism of the collapsed soil cave in the urban area, the development and failure process of the soil cave were studied through the establishment of a three-dimensional model test. The surface deformation data during the test and the final failure shape of the soil cave were analyzed. Changes in the stress of the covering soil during the development of the cave was also analyzed through the measurement tests of earth pressure. Based on this experiment, a finite element model for calculating the limit radius of the soil cave was established, and the influence of elastic modulus, Poisson's ratio, cohesion and internal friction angle on the limit radius was analyzed through orthogonal experiments. The results show that: for the developed soil cavity, the earth pressure in the failure area is significantly reduced; when the soil cave is damaged, the settlement velocity of the cave top changes suddenly; the law between the height of the damage inside the soil cave and the diameter of the soil cave is consistent with that proposed by Platts theory. The geometric rules are in good agreement; for soil caves with different burial depths, the damage modes are mainly divided into altar-shaped and straight-tube-shaped collapses.

Abstract:To simulate the process of pile installation with a vibratory hammer, this paper presents a theoretical model based on the one-dimensional wave equation and develops a prediction method of the results of the in-situ cone penetration tests (CPT). Computer program is also compiled and verified against an industrial project. Furthermore, the effect of hammer's working frequency, eccentric moment and additional weight are investigated. It shows that, the presented model in this paper simulates the penetration process well for a vibrator driven pile; during installation, the maximum tensile stress in pile shaft is generally smaller than that in compression; the maximum tension stress happens at the end of pile driving. In addition, parameter studies show that increasing the hammer's frequency and eccentric moment improves the hammer's drivability and makes a quicker penetration, e.g. 67% increase in hammer frequency from 30Hz leads to a 170% increase in pile penetration rate, and 50% increase in eccentric moment makes a 240% increase in pile penetration rate; compared with the aforementioned two approaches, a limit positive effect is shown by increasing the additional weight.

Abstract:In order to give a better prediction of the pressure maintaining performance of shield screw conveyor with earth pressure balance, the pressure difference between inlet and outlet of screw conveyor was analyzed on the assumption that the conditioned soil is a viscoplastic fluid, and the pressure maintaining mechanism was explained from the perspective of energy conversion. A steady calculation of the flow state of conditioned soil in screw conveyor was carried out by using computational fluid dynamics (CFD) method, where Herschel-Bulkley model was used to describe the rheological behavior of conditioned soil, and the rotation of screw blade was fully considered by using multi-reference frame (MRF) method. The influence of soil fluid parameters and the working parameters of screw conveyor on pressure maintaining performance was obtained, and a comparison between the simulation results and the indoor test results was carried out. The analysis result shows that the pressure maintaining performance of screw conveyor increases with the increase of yield stress, viscosity index and power-law index, where the influence of viscosity index and power-law index is more obvious. The mechanical energy loss of screw conveyor is basically proportional to the rotating speed of screw and soil removal efficiency. Fast rotating speed and high soil removal efficiency result in excellent pressure maintaining performance. The relationship between the mechanical energy loss, rotating speed and soil removal efficiency can be estimated by using binary linear regression model. The trend of simulation results is consistent with the indoor test results, and the Herschel Bulkley rheological model can be used to calculate the pressure maintaining performance of screw conveyor. However, the prediction of pressure maintaining performance is conservative. Finally, based on the results of numerical calculation, the optimization process of screw conveyor pressure maintaining performance is proposed.

Abstract:In order to explore the calculation method of porosity and pore multi-fractal characteristics of fly ash，the concept of similar dimension is introduced to derive the basic formula of fractal dimension. By processing the sample mold to control the degree of compaction，a fly ash sample with a certain moisture content is configured. Secondly，the diameter and number of pores and particles in the fly ash sample are obtained by using digital microscopic imaging technology，and then the histogram of pores and particles can be obtained. Then 4 different depth sections are selected，whose upper and lower ends are used as the observation surface，and photos at different magnifications are taken with the help of a stereo microscope to analyze the distribution of the pore area ratio in the three-dimensional space；finally，professional image processing is used. The technique gives the porosity of the sample and compares it with the measured results. The research results show that the pore area ratio of the fly ash has a decreasing trend along the depth direction，the variation of which would be about 30%，and its pore distribution has obvious multifractal characteristics.

Abstract:The existing enhancement methods for the heat transfer of phase change material (PCM) in domestic hot water tank, such as adding fins and the expanded graphite, can lead to the reduction in the energy storage density of the encapsulation. Additionally, the graphite may settle during the melting process of PCM. To address this issue, a feasible solution is to promote the PCM's melting performance through improving the structure of cylindrical containers, which have been widely applied in domestic hot water tank, thereby taking advantage of contact melting modes without adding additional materials. In particular, an inverted conical container, which has a relatively lower ratio of the top area to the bottom area when compared to the cylindrical containers, has been proposed to encapsulate the PCM with the goal of establishing the contact melting mode between solid PCM and heated side walls when the PCM drops. To evaluate its performance, a mathematical model was developed and validated by the results obtained from a visualization experiment. Based on this model, the melting performance of the PCM encapsulated in the inverted conical container was analyzed and compared with that encapsulated in cylindrical container. The results show that the total melting time of the PCM encapsulated in the inverted conical container is 2 520 s under the same volume (1.74e-04 m3) and height（0.05 m）. It is decreased by 690 s when compared with that encapsulated in the cylindrical container, which indicates the melting performance has improved 21.5%. Except for the contact melting, the natural convection of the liquid PCM also strongly affects the melting performance of the PCM. It is found that the Rayleigh-Bernard convection in the side region results in the decrease of melting performance of the PCM. In addition, an interesting finding is that higher melting performance (i.e.16.7%) is achieved for the PCM encapsulated in the conical container when compared with that encapsulated in the inverted conical container. In this view, the combined usage of conical enclosure and inverted conical enclosure can be appreciated to improve both the heat storage capacity and the melting performance in practical applications.