THE EFFECT OF A MAGNETIC FIELD ON NATURAL CONVECTION IN A SHALLOW CAVITY HEATED FROM BELOW

A numerical study is presented for magnetohydrodynamic free convection of an electrically conducting fluid in a shallow cavity heated from below and cooled from above. The side walls are maintained adiabatic. A uniform magnetic field, inclined with an angle 0 with respect to the horizontal plane, is externally imposed. The investigation covers the range of the Rayleigh number, Ra, from 1.8 × 10₃ to 3 x 10₄the Hartmann number, Ha, from 0 to 35, the Prandtl number, Pr, from 0.005 to 1 and aspect ratio of the cavity, A = 6. The effect of the magnetic field on the flow structure is presented. For supercritical convection it is found that, upon increasing Ha, the number of roll cells in the cavity increases when it is perpendicular to it. The imposition of an inclined magnetic field gives rise to new flow patterns with tilted lateral cell walls. The effect of both strength and orientation of the magnetic field on the overall heat transfer is found to be significant.     

Correlation for estimation of the chemical availability (exergy) from ultimate analysis of pyrolytic oils obtained from fast pyrolysis of biomass

The purpose of this study is to evaluate the chemical exergy (E^CH) of liquid products obtained from fast pyrolysis of biomass. I have calculated the chemical exergy values from a formula in literature and have developed a formula for estimating the chemical exergy of biomass from the higher heating value and their ultimate analysis values. The mean differences between these values range from –0.391% to 0.460%. The formula developed for estimating the chemical exergy of biomass from the higher heating value and their ultimate analyses had a correlation coefficient (R² = 0.9999), and the prediction of this formula is good. The goal is to identify desirable attributes that may serve as the basis for decision-making for future biofuel options. Studies on the pyrolytic oils showed that the oils obtained from chestnut cupulae and maple fruit can be used as a renewable fuel and chemical feedstock.     

Wavelet transform-based strain estimator for elastography

A new signal processing algorithm based on a wavelet transform (WT) is proposed for instantaneous strain estimation in acoustic elastography. The proposed estimator locally weighs ultrasonic echo signals acquired before tissue compression by a Gaussian window function and uses the resulting waveform as a mother wavelet to calculate the WT of the postcompression signal. From the location of the WT peak, strain is estimated in the time-frequency domain. Because of the additive noise in signals and the discrete sampling, errors are commonly made in estimating the strain. Statistics of these errors are analyzed theoretically to evaluate the performance of the proposed estimator. The strain estimates are found to be unbiased, but error variances depend on the signal properties (echo signal-to-noise ratio and bandwidth), signal processing parameter (time-bandwidth product), and the applied strain. The results are compared with those obtained from the conventional strain estimator based on time-delay estimates. The proposed estimator is shown to offer strain estimates with greater precision and potentially higher spatial resolution, dynamic range, and sensitivity at the expense of increased computation time.     

Weibull renewal equation solution

The existing solution methods for the Weibull Renewal Equation suffer from a lack of sufficient accuracy due to the singularity at the origin for some parameter values of the weibull density. The proposed method of solution provides accuracy to any desired degree of precision for all parameter values particularly in the singular range. The method utilizes a cubic spline approximation of the unknown renewal function and applies the Galerkin technique of integral equation solution. Gaussian quadratures are used to evaluate integrals. The singular nature of the integrand is handled by the Gauss-Jacobi quadrature. Results are compared with those obtained by simulation.     

The nonstationary strain filter in elastography: Part II. Lateral and elevational decorrelation

The nonstationary evolution of the strain filter due to lateral and elevational motion of the tissue scatterers across the ultrasound beam is analyzed for the 1-D cross-correlation-based strain estimator. The effective correlation coefficient that includes the contributions due to lateral and elevational signal decorrelation is used to derate the upper bound of the signal-to-noise ratio in the elastogram (SNRe) predicted by the ideal strain filter. In the case of an elastically homogeneous target, if the transducer is on the axis of symmetry of such target in the elevational direction, the motion of the scatterers out the imaging plane is minimized. In addition, the ultrasound beam along the elevational direction is broader, allowing scatterers to stay longer within the beam during tissue compression. Under these conditions, lateral signal decorrelation becomes the primary contributor to the nonstationary behavior of the strain filter. Both the elastographic SNRe and the dynamic range are reduced, with an increase in lateral decorrelation. Finite element simulations and phantom experiments are presented in this paper to corroborate the theoretical strain filter. The nonstationary behavior of the strain filter is reduced by confining the tissue in the lateral direction (minimizing motion of tissue scatterers), thereby improving the quality of the elastogram.     

Natural convection in composite wall collectors with porous absorber

Steady natural convection heat transfer has been studied in a modified Trombe wall solar collector with a porous medium used as an absorber. The boundary conditions were: Two isothermal walls at different temperatures, two horizontal bounding adiabatic walls and either uniform or nonuniform heat generating porous layer with orifices. The aspect ratio A was from 5 to 10. The influence of orifice opening and position as well as the nonuniform heat generation within the porous medium have been studied in detail with the Darcy number varying from 10⁻⁸ to 10⁻². The results are presented in terms of practical parameters (θ, θ_max, q) as a function of Ra, Da and other nondimensional geometrical parameters. The isotherms and stream lines within the cavity are also produced. The overall results indicate that Da and geometrical parameters are the most important parameters affecting system performance.     

Error analysis in acoustic elastography. I. Displacement estimation

Correlation between acoustic echo signals obtained before and after application of an external compressional force provides information about the internal deformation of an elastic medium. In this paper, the variance for displacement estimated from an echo data segment and the covariance between two windowed segments that may overlap are derived. The signal and noise spectra are Gaussian and independent. The dependence of the displacement variance on input signal-to-noise ratio (SNRi), time-bandwidth product W, fractional bandwidth Y-1, and the rate of displacement variation with depth a is investigated. The relationship between a and the other experimental parameters is crucial for understanding how signal decorrelation affects displacement error. The expression for displacement variance reduces to the Cramer-Rao lower bound result when a = 0 and W > > 1 for both bandpass and base-band signals. When a not equal to 0 displacement variance increases, and there is an optimal window length at W = square root of 20/a square root of 1 + Y2 for which the displacement variance is minimum. Narrow-band signals produce larger errors than broadband signals for long observation windows when a not equal to 0 and just the opposite when a = 0. Errors are greatest for displacements estimated from the envelope of narrow-band signals. Finally, a general expression for the minimum displacement variance for arbitrary signal and noise spectra is derived as a function of the experimental parameters. These results form a framework for analyzing strain estimates in elastography, the subject of a companion paper.     

Domestic hydrogen production using renewable energy

A brief review shows that domestic production of hydrogen to fuel a car is feasible by using various means. Among these, the solar––photovoltaic electricity––electrolysis process seems to be the most practical if a renewable energy source is to be used. A simplified model has been developed to determine and optimize the thermal and economical performance of domestic photovoltaic-electrolyzer systems, either with fixed or sun tracking panels using annual total solar radiation on a horizontal surface and climatic data. Twelve locations in the United Sates from four climatic zones (tropical-sub tropical, dry, temperate, cool snow-forest) have been selected. Simulations have been carried out to produce data for hydrogen production for these various locations and the resulting data have been correlated to obtain hydrogen production in kg/kW_p/year photovoltaic system as a function of total annual solar radiation on horizontal surface. The economical feasibility has been studied by taking the photovoltaic and electrolyzer systems' price as variable parameters. It is assumed that the necessary capital is 100% borrowed from a financial institution to pay back in monthly installments. It has been found that the hydrogen production with fixed photovoltaic panels varies from 26 to 42 kg/kW_p/year and the cost from 25 to 268 $/GJ.     

An assessment on hydrogen production using central receiver solar systems

An assessment is presented on hydrogen production using a dedicated central receiver solar system concept coupled to two types of hydrogen producing processes, electrolysis and thermochemical. The study on solar electrolytic hydrogen was carried out using solar electricity and four different electrolytic technologies, namely industrial unipolar 1980 and 1983 technologies, industrial bipolar and solid polymer electrolyte technology. The thermochemical process was the sulphur/iodine cycle which is being developed by General Atomic Co. Systems which is capable of producing about 10⁶ GJ hydrogen per year were developed at the conceptual level and site specific computations were carried out. A general mathematical model was developed to predict the optical and thermal performance of the central receiver system coupled directly to the chemical plant. Cost models were developed for each sub-system based on the database published in the literature. Levelized and delevelized costs of solar hydrogen were then computed.     

Horizontal concrete slabs as passive solar collectors

Natural convection, radiation and conduction heat transfer on horizontal concrete slab systems is experimentally studied. The system was a 0.78 m long, 0.40 m wide and 0.10 m thick concrete slab. A heat source was used to impose a constant heat flux which could be varied from about 200 to 700 W/m². Temperatures at various points and heat flux by natural convection at the horizontal surface were measured. Using various assumptions, the system was also analyzed theoretically. It is found that the mathematical model to study the transient heat transfer in the slab system was satisfactory to predict its thermal behavior in various conditions. An empirical correlation for natural convection on the horizontal concrete slab was derived and used in the analysis. The results showed that the incident energy on the concrete slab was not a parameter affecting strongly the absorbed heat by the slab, the radiation heat losses made about 60% while those by natural convection 40%, and the major energy storage–restitution was taking place during the first 3 to 4 h. The optimization of energy storage density and the thermal performance was also discussed and various parameters affecting them were defined.