Self-compacting concrete: the role of the particle size distribution

This paper addresses experiments and theories on Self-Compacting Concrete. The\ud “Chinese Method”, as developed by Su et al. [1] and Su and Miao [2] and adapted to European circumstances, serves as a basis for the development of new concrete mixes. Mixes, consisting of slag blended cement, gravel (4-16mm), three types of sand (0-1, 0-2 and 0- 4mm) and a polycarboxylic ether type superplasticizer, were developed [3]. These mixes are extensively tested, both in fresh and hardened states, and meet all practical and technical requirements such as a low cement and powder content (medium strength and low cost). It follows that the particle size distribution of all solids in the mix should follow the grading line of the modified Andreasen and Andersen [4] model

Analytical solutions for non-linear conversion of a porous solid particle in a gas–I. Isothermal conversion

Analytical description are presented for non-linear heterogeneous conversion of a porous solid particle reacting with a surrounding gas. Account has been taken of a reaction rate of general order with respect to gas concentration, intrinsic reaction surface area and pore diffusion, which change with solid conversion and external film transport. Results include expressions for the concentration distributions of the solid and gaseous reactant, the propagation velocity of the conversion zone inside the particle, the conversion time and the conversion rate. The complete analytical description of the non-linear conversion process is based on a combination of two asymptotic solutions. The asymptotic solutions are derived in closed form from the governing non-linear coupled partial differential equations pertaining to conservation of mass of solid and gaseous reactant, considering the limiting cases of a small and large Thiele modulus, respectively. For a small Thiele modulus, the solutions correspond to conversion dominated by reaction kinetics. For a large Thiele modulus, conversion is strongly influenced by internal and external transport processes and takes place in a narrow zone near the outer surface of the particle: solutions are derived by employing boundary layer theory. In Part II of this paper the analytical solutions are extended to non-isothermal conversion and are compared with results of numerical simulations

Analytical solutions for non-linear conversion of a porous solid particle in a gas–II. Non-isothermal conversion and numerical verification

In Part I, analytical solutions were given for the non-linear isothermal heterogeneous conversion of a porous solid particle. Account was taken of a reaction rate of general order with respect to the gas reactant, intrinsic reaction surface area and effective pore diffusion, which change with solid conversion and external film transport. In this part, the analytical solutions are extended to non-isothermal conversion. Analytical solutions for the particle overshoot temperature due to heat of reaction are derived from the governing differential equation pertaining to conservation of energy, considering the limiting cases of small and large Thiele moduli. The solutions are used to assess the effect of interaction between chemical reaction rate and particle overshoot temperature on particle conversion. The analytical solutions are shown to compare favourably with numerical simulation results

Analytical methods for predicting the response of marine risers, communicated by W.T. Koiter

A Marine riser, which links a floating oil or gas production system to a sea-bed manifold, can be modelled as a tensioned beam, the hydrodynamic transverse forces being described by the relative velocity form of Morison's equation. To analyse the response of the riser to random waves and floater motions, a number of characteristic regions has been identified along the riser. For each of these regions, the riser differential equation is reduced to an approximate form and analytical solutions, in terms of known time- and position-dependent functions, are given. The solutions hold asymptotically for slender (tension-dominated) risers in deep water and compare favourably with numerical results for a typical rise

Stochastic processes in mechanical engineering

Stochastic or random vibrations occur in a variety of applications of mechanicalengineering. Examples are: the dynamics of a vehicle on an irregular roadsurface; the variation in time of thermodynamic variables in municipal wasteincinerators due to fluctuations in heating value of the waste; the vibrationsof an airplane flying through turbulence; the fluctuating wind loads acting oncivil structures; the response of off-shore structures to random wave loading.Attention will be focussed on problems of external noise. That is, we shall consider models of mechanical engineering structures where the source of random behavior comes from outside: e.g. a prescribed random force or a prescribed random displacement. The structures inhibit inertia, damping and restoring, linear and non-linear. The main questions we intend to answer are: how do these structures respond to random excitation, and how can we quantify the random behavior of response variables in a manner that an engineer is able to make rational design decisions

Rotational particle separator : an efficient method to separate micron-sized droplets and particles from fluids

The rotational particle separator (RPS) has a cyclone type house within which a rotating cylinder is placed. The rotating cylinder is an assembly of a large number of axially oriented channels, e.g. small diameter pipes. Micron-sized particles entrained in the fluid flowing through the channels are centrifugated towards the walls of the channels. Here they form a layer or film of particles material which is removed by applying pressure pulses or by flowing of the film itself. Compared to conventional cyclones the RPS is an order of magnitude smaller in size at equal separation performance, while at equal size it separates particles ten times smaller. Applications of the RPS considered are: ash removal from hot flue gases in small scale combustion installations, product recovery in stainless environment for pharma/food, oil water separation and demisting of gases. Elementary formulae for separation performance are presented and compared with measurements performed with various RPS design

Condensed rotational separation

Condensed Rotational Separation is based on partial condensation of components of gas-gas mixtures. Condensation is induced by flash evaporation or pressure distillation. The rotational particle separator removes the micron sized particles formed by condensation from the gas. Yields and purities are enhanced by adding a next stage of liquid flash and relooping the gas to the first stage. A great improvement in separation performance, in both yields and purities, is achieved by allowance for operation of CRS within the vapour-liquid-solid region. Condensed rotational separation is shown to be an economically attractive process for upgrading sour gas fields contaminated with CO2 and/or H2S

Asymptotic solutions for Mathieu instability under random parametric excitation and nonlinear damping

A theoretical analysis is presented of the response of a lightly and nonlinearly damped mass–spring system in which the spring constant contains a small randomly fluctuating component. Damping is represented by a combination of linear and nonlinear power-law damping. System response to some initial disturbance at time zero is described by a sinusoidal wave whose amplitude and phase vary slowly and randomly with time. Leading order formulations for the equations of amplitude and phase are obtained through the application of methods of stochastic averaging of Stratonovich. The equations of amplitude and phase are given in two versions: Fokker–Planck equations for transient probability and Langevin equations for response in the time-domain. Solutions in closed-form of these equations are derived by methods of mathematical and theoretical physics involving higher transcendental functions. They are used to study the behavior of system response for ever increasing time applying asymptotic methods of analysis such as the method of steepest descent or saddle-point method. It is found that system behavior depends on the power density of the parametric excitation at twice the natural frequency and on the magnitude and form of the damping. Depending on these parameters different types of system behavior are found to be possible: response which decays exponentially to zero, response which leads to a stationary state of random behavior, and response which can either grow unboundedly or which approaches zero in a finite time