Dynamical energy limits in traditional and work-driven operations II.: Systems with heat and mass transfer

This is the second part of the work that analyses dynamical energy limits for diverse operations with finite rates important in engineering. Our position is that a dynamic limit of a sufficiently high hierarchy may be helpful in modelling and design of a prescribed operation. In particular, we treat active systems with coupled heat and mass transfer important in separation and biological systems. The operations considered occur in separation units, heat and mass exchangers, energy converters and chemical reactors. The energy limits are expressed in terms of classical exergy and a residual minimum of entropy generated in equipment of a fixed dimension. To ensure physical limits we treat sequential work-driven operations, in particular those of dissolving or evaporation which run jointly with thermal machines (e.g. heat pumps). We also compare structures of optimization criteria describing these limits (in particular “endoreversible limits”) in traditional and work-driven operations. Through quantitative analyses we extend to the realm of mass transfer operations the method initiated in Part I that applies “Carnot variables” as suitable controls. Functions of extremum work, which apply a residual minimum entropy production, are found in terms of initial and final states, duration and (in discrete processes) number of stages. Mathematical analogies between entropy production expressions in traditional and work-driven operations are helpful to formulate optimization criteria in both cases.     

Energy Deposition in a Model of Man: Frequency Effects

A computer-controlled scanning system and implantable, nonperturbing electric field probes were used to measure spatial distributions of the electric field in a full scale homogeneous model of a human body. The measurements were performed at three frequencies (160, 350, and 915 MHz) in the far-field and in the near-field of resonant dipoles. The specific absorption rate (SAR) distributions and the averages for body parts and the whole body are analyzed as functions of frequency. In the far-field, the SAR decreases exponentially in the direction of wave propagation in the torso at all frequencies, and large gradients of the SAR are observed along the body main axis, particularly for the E polarization. At 160 and 350 MHz high local SAR's are produced in the neck. It appears that for plane wave exposures the ratio of the peak SAR to the whole-body average SAR does not exceed 20. In the near-field, large SAR gradients are also produced, and the ratios of the peak spatial SAR to the whole-body average SAR vary from about 30 to 250 depending on the frequency and polarization. It is suggested that for near-field exposures the whole-body average SAR is not a proper dosimetric measure, and the SAR averaged over any 0.1 of the tissue volume is recommended instead.     

Manufacturing and physico-mechanical characterization of carbon nanohorns/polyacrylonitrile nanocomposites

The use of carbon nanohorns (SWCNHs) as a modifying filler in a polyacrylonitrile (PAN) matrix is studied with the goal of elaborating nanocomposites. The study deals with assessment of the dispersity of SWCNHs in a PAN polymer suspension. The SWCNHs were introduced into the PAN-based suspension using different methods, including mechanical stirring, ultrasonification and the combination of ultrasonification with addition of a surfactant. Agglomeration and dispersion processes of SWCNH in the polymer suspensions were studied using DLS technique and turbidimetry. The resulting properties of nanocomposite foils after solidification in water ambient were verified in various tests. The mechanical tensile properties (tensile strength, modulus and strain to fracture) of the nanocomposites before and after the dispersion process were compared. The nanocomposites obtained under optimally prepared suspension perform the highest strain to fracture in tensile test. Electrical resistivity and thermal conductivity of nanocomposites samples after appropriate dispersion of SWCNHs in the PAN suspension were also determined. The presence of SWCNH in the PAN suspension affects the structure of nanocomposites after solidification through changing structural ordering of the polymer. The study revealed that the polymeric suspensions prepared in optimum processing conditions contain the carbon aggregates the size of which correspond almost to the mean size of a dahlia flower-like structured particle, i.e., 50–250 nm and it was not possible to separate such particles into a single form of carbon nanohorn by the techniques used.     

RF Energy Deposition in a Heterogeneous Model of Man: Near-Field Exposures

The electric field strength was measured in a full-scale heterogeneous model of man exposed in the near field of resonant dipoles. The model was comprised of skull, spinal cord, rib cage, all other major bones, brain, lung, and muscle tissue. Electrical properties of these simulated tissues were the same as respective live tissue properties at test frequencies of 160, 350, and 915 MHz. The rates of energy absorption were calculated on the basis of the measured field strengths and tissue conductivities. Patterns of the energy deposition are compared for two orientations of the antennas with respect to the body. Also the results for the heterogeneous model are compared to data for homogeneous model having average tissue electrical properties.     

On the three-dimensional problem of an interface crack under uniform heat flow in a bimaterial periodically-layered space

This paper deals with the study of thermal stresses due to a plane crack lying on an interface in a microperiodic two-layered composite under a uniform perpendicular heat flow. An approximate analysis is carried out within the framework of linear stationary thermoelasticity with microlocal parameters. Taking this approach, a general method of solving the resulting boundary-value problem is presented. Effective results have been achieved through the use of suitable potential solutions and reducing the thermal crack problem to its skew-symmetric isothermal mechanical counterpart. An illustrative example is given in the paper by considering an insulated stress-free penny-shaped crack. For this problem the solution is obtained in terms of elementary functions and discussed from the point of view of fracture theory.     

Secure Distributed Key Generation for Discrete-Log Based Cryptosystems

A Distributed Key Generation (DKG) protocol is an essential component of threshold cryptosystems required to initialize the cryptosystem securely and generate its private and public keys. In the case of discrete-log-based (dlog-based) threshold signature schemes (ElGamal and its derivatives), the DKG protocol is further used in the distributed signature generation phase to generate one-time signature randomizers (r = g^k). In this paper we show that a widely used dlog-based DKG protocol suggested by Pedersen does not guarantee a uniformly random distribution of generated keys: we describe an efficient active attacker controlling a small number of parties which successfully biases the values of the generated keys away from uniform. We then present a new DKG protocol for the setting of dlog-based cryptosystems which we prove to satisfy the security requirements from DKG protocols and, in particular, it ensures a uniform distribution of the generated keys. The new protocol can be used as a secure replacement for the many applications of Pedersen's protocol. Motivated by the fact that the new DKG protocol incurs additional communication cost relative to Pedersen's original protocol, we investigate whether the latter can be used in specific applications which require relaxed security properties from the DKG protocol. We answer this question affirmatively by showing that Pedersen's protocol suffices for the secure implementation of certain threshold cryptosystems whose security can be reduced to the hardness of the discrete logarithm problem. In particular, we show Pedersen's DKG to be sufficient for the construction of a threshold Schnorr signature scheme. Finally, we observe an interesting trade-off between security (reductions), computation, and communication that arises when comparing Pedersen's DKG protocol with ours.     

Rational admittance function realisation using two operational amplifiers

The technique of the realisation of nth-order transfer functions with only two operational amplifiers is presented.     

Study of a plate modelling a dock gate

Computer aided finite element analysis has been applied to evaluate the structural behavior of a solid plate representing statically certain box-type construction of a dry dock gate. Possible comparison of particular results following from different procedures, valid for constant plate stiffness and rigid supports, has been performed and the distribution of reactions for variable plate stiffness, within two different numerical accuracies, has been studied. For practical requirements the nonlinear elastic flexibility of dock gate packing has been taken into account and an adequate numerical step-by-step approach, satisfying the considered material characteristics, developed. For last case the deflections, reactions, and internal forces have been examined in detail and a comparison with those for rigid supports presented.     

Analysis of active ladder network by using coates flow graphs and continuants

A simple method for solving the matrix equation describing the general active ladder network, is presented. Using Coates flow graphs and continuants the expressions for current, voltage, and also example of application are given.