THE EFFECT OF A NON-UNIFORM SURFACE TEMPERATURE ON LAMINAR NATURAL CONVECTION IN A RECTANGULAR ENCLOSURE

Three dimensional solutions were developed for natural convection in a 2 × 1 × 1 rectangular enclosure by finite-difference calculations. Sawtooth temperature distributions with different amplitudes and orientations were imposed on the lower (2×1) surface to simulate the effect of cooling coils or jackets on the collector plate of a flat-plate solar heater. The shorter dimension of the enclosure was progressively inclined.

The circulation patterns were not changed significantly by the surface temperature distributions. The variation of the Nusselt number with inclination was not changed significantly but the magnitude increased with the amplitude of the surface temperature variations for inclinations of less than 90 degrees.

It is concluded that the collecting plate of a solar collector should be maintained as nearly isothermal as possible to minimize heat losses by natural convection through the air gap.     

TRANSIENT BEHAVIOR OF NATURAL CONVECTION INSIDE A VERTICAL CYLINDER

The transient behavior for natural convection heat transfer within a vertical cylindrical enclosure was studied using the SIMPLER numerical algorithm. Initially, the temperature of the internal fluid was uniform. The unsteady state heat transfer was initiated by a temperature step change applied to all boundaries. The boundary temperature was then increased at the same rate as the center temperature until pseudosteady state was achieved. (Pseudosteady state is comparable to steady state convection for a fluid with uniform heat generation.) The transient heat transfer coefficient and the intensity of flow, along with the magnitude and the location of the minimum temperature, mean temperature, and maximum stream function are presented. Temperature and stream function contours are used to show the penetration of heat from the boundary and the development of the flow patterns. The local Nusselt number is also provided as supplementary information. A cylinder with HID = 1 was considered, with the Prandtl number of the fluid equal to 7 and a Rayleigh number of 10⁵. A characteristic length defined as 6 x (volume)/(surface area), which was found to produce the best regression results in our previous studies, was used in defining the Nusselt number and the Rayleigh number. The initial heat transfer rate was found to be correlated by

Nu =0.938 (Dimensionless Time) ^-0567

Oscillatory phenomena were found in support of the prediction made by Patterson and Imberger.     

SOLAR GRAIN DRYING IN HUMID AND HOT-TEMPERATE CLIMATES

A natural solar drying system has been tested with wheat. Simple solar collectors give the small temperature rise needed for low temperature drying. The system, with a capacity of 35 tomes, is placed in a region of humid hot-temperate climate. It has been designed to dry all the locally produced grain (wheat, corn, soybean, sunflower, sorghum, etc.), without a supplemental heat source.

Two different types of solar collectors of the same effective area were tested, with one a storage-collector and the other a bare plate collector

Data from four field tests performed between 1981 and 1983 are given and discussed. The mean mass of water removed from the wheat per hour was of 6.54 kg, under an average air flow of 4.3 m3 /min. tonne     

SOLAR DRYING OF FRUITS IN A MEDITERRANEAN CLIMATE

Two twin forced convection dryers of 1.5 m³ were built in Majorca (Spain). They are of a mixed kind, with solar air collectors and a green house type chamber. A wooden frame supports polycarbonate walls. After two years operation they have been proven weather resistant. Six solar air collectors 2.12 × 1.05 m were used in each dryer.

Apricots were processed in both dryers and at open sun. Three different tray heights were tested 5, 9 and 12 cm. The best results were obtained with 12 cm trays. Recycling part of the exhaust air improves the efficiency of the dryer. Blanching the fruits makes no difference to the dehydration rate. The rate of SO₂ loss during the drying process is higher within the chamber.     

A GRAPHICAL DESIGN METHOD FOR A PARTIAL SOLAR DRYER FOR CORN

This paper presents the application of a design method for a partial solar heating system of polyvalent modular dryers called “GJ-ABAQUE” to the drying of thick layers of grains.

This method is based on the use of charts or polynomial correlations. In the actual case where the drying air is not recycled, we only need one chart which allows one to determine the fraction of the monthly heating load supply by solar energy as a function of two dimensionless parameters. The latter implies the use of monthly average radiation data, the collector surface and estimates of drying loads.

The “GJ-ABAQUE” method was applied for drying 777 kg of corn, corresponding to 1 m³ of fresh product, in a thick layer in each modular dryer.     

Strain Distributions in Adhesive Wood Joints

The influence of overlap length on the strain distribution in a double-lap joint was determined using a computer vision, white light speckle technique. The overlap lengths were 12.7 mm (0.5') and 38.1 mm (1.5'). This range represents a one-tenth scale of overlap lengths in double-lap joints used in adhesive connections in timber trusses. The side members were 50.8 mm (2') long and the center member was 88.9 mm (3.5') long. All joint members were 25.4 mm × 25.4 mm (1' × 1') in cross section. The joint members were yellow-poplar (Liriodendron tulipifera) wood and the adhesive was a resorcinol-formaldehyde.

Even though the connection was loaded in shear, both transverse and shear strains were shown to exist along the gluelines. These strains were significantly intensified at the ends of the overlap. The strain distributions were altered by the degree of overlap and the level of strain concentration was reduced with increased overlap.

The full-field strain distributions as measured with the new computer vision technique revealed progressive failure development, the eventual failure mode and location, and knowledge of the actual distribution of the strains in the entire joint assembly.     

RIBBING INSTABILITY OF A TWO-ROLL COATER: NEWTONIAN FLUIDS

Data are presented for the critical conditions at which the ribbing instability appears on the surface of a Newtonian liquid film emerging from the nip between two rotating rolls. Data are obtained over two decades in Capillary number (Ca = μU/σ) for values of H₀/R in the range 3 × 10^-3 to 5 × 10^-2. The critical Capillary number is found to be given by the expression

$

Data are compared to the predictions that follow from a theory due to Savage, and agreement is found to be poor.     

THERMAL CONDUCTIVITY OF GASEOUS AMMONIA IN THE TEMPERATURE RANGE 358-925 K.

Thermal conductivity of research grade pure ammonia gas has been measured by the column method at pressures of 12.9, 26.5, and 45.0 kN/m² over the temperature range 358 to 925 K. The maximum probable error of measurement is 1.5% at 874 K and it increases in magnitude as the temperature decreases. At 405 K, it is about 4.8%. The experimental data are correlated by the following cubic polynomial in temperature:

k(T)=5.237× 10^-3 + 5.179 × 10^-4 T + 8.404 × 10^-7 T² + 1.557 × 10^-10 T³.

Here k is in mW/cm-K and T is in K.

The experimental values are compared with three kinetic theory expressions for thermal conductivity of polyatomic gases in conjunction with the Stockmayer (12-6-3) potential. It is concluded that none of the theories can accurately predict the thermal conductivity of ammonia and probably of polar gases in general. The diffusion coefficient characterizing the transport of vibrational energy is computed as a function of temperature and on the basis of experimental data and the theory due to Ahtye. It has been long known that the transfer of energy by polyatomic and polar molecules is quite complex because of the presence of internal degrees of freedom and the possibility of resonant energy transfer in the latter (Hirschfelder, Curtiss and Bird, 1964).     

MASS TRANSFER AND DIFFUSION COEFFICIENT DETERMINATION IN THE WET AND DRY SALTING OF MEAT

Dehydrated salted meat is widely used in Brazil as a very important source of animal protein. The main objective of this kind of processing is water removal. initially by osmotic pressure changes and then by drying, resulting in a product with intermediate moisture levels.

In this work, mass transfer and salt diffusion in pieces of meat submitted to wet and dry salting were studied. Slabs of beef m. trapezius with an infinite plate geometry were salted in a NaCl saturated solution or in a dry salt bed, at two temperatures (10 and 20°C) and different time exposures (120 min and 96 hours). Equilibration studies were extended up to six days.

It was observed that water loss increased with salt uptake, for increasing periods of times. At 20°C the moisture loss was higher than it was at 10°C in both salting processes. On the other hand, the kinetics of salt uptake and moisture loss were of greater importance in the process of dry salting than in that of wet salting.

The salt diffusion coefficient for wet salting was 0.26 × 10^-10m²/s at20°C and 0.25 × 10^-10 m²/s at 10°C and for the dry salting the values were 19.37 × 10^-10 m²/s at 20°C and 17.21 × 10^-10 m²/s at 10°C.     

INTERFEROMETRIC STUDIES OF INTERFACIAL MASS TRANSFER IN A LIQUID-LIQUID SYSTEM

Interfacial concentrations were found using a specially designed interferometer and a flowing junction cell. The relationship between refractive index and concentration was established by means of a new refractometer. Both optical instruments had a sensitivity of 1 × 10^-1.

Fringe patterns yielded concentration profiles and total solute transfer at any time. For the system water/acetic acid/carbon tetrachloride equilibrium was never reached at the interface at any solution strength. This indicates the presence of an interfacial resistance. In some cases density driven disturbances were initiated at the surface causing mixing effects.

Interfacial resistance to mass transfer was shown to exist. This resistance was evaluated for different initial concentrations of solution and is time dependent. An empirical relationship is proposed for predicting the value of the interfacial resistance at any exposure time.     

MASS-PRODUCTION OF BIOCATALYST-ENTRAPPING ALGINATE GEL PARTICLES BY A FORCED OSCILLATION METHOD

A forced oscillation method applying reverse-piezo electric effect has been proposed for mass-production of biocatalyst-entrapping polymer gel particles. A jet of aqueous alginate solution flowing out through an orifice was disintegrated into droplets by an oscillating pressure which was exerted by a ceramic plate vibrated by the reverse-piezo electric effect. The droplets formed were immediately solidified into gel particles in calcium chloride solution.

The particle production rate by the method was up to 270 particles s^-1 or 1 × 10^-6m³.s^-1 The size of the particles produced were down to I mm depending on the applied frequency. The smallest matrix size of the gel's polymer network was estimated to be around 5 nm.

The method was applied for immobilization of yeast cells. The diameter of immobilized particles was 2.5 mm. The immobilized yeast grew well in the particle. The damage of yeast cells caused by the immobilization was not remarkable     

GENERALIZED FINITE DIFFERENCE SOLUTION FOR HEAT AND MASS TRANSFER FROM A FINITE CYLINDER DURING QUENCH

A generalized nondimensional solution is presented that describes heat or mass transfer from a finite cylinder during quench. The solution is applicable to three important cases:

Conduction with convection heat transfer at the surface during any single step hot or cold quench.

Conduction with radiation heat transfer at the surface during a single step cold quench with negligible background radiation.

Diffusion with surface desorption of a diatomic gas from a metal specimen during a single step quench in a high vacuum with negligible background pressure.

Application of the generalized solution, which utilizes the numerical method of finite differences with forward stepping, is illustrated by determining a cylinder's transient temperature distribution and surface transfer rate (both instantaneous and cumulative) for an example L/D ratio of 2.0. Selected results are graphed and tabulated for the three cases. The results for the conduction/convection case are verified using the familiar analytical product solution as well as the lumped solution. For the conduction/radiation and diffusion/desorption cases, no analytical solutions are available other than the lumped limit which is in agreement.     

The Anisotropy of the Surface Tension of Polar Liquids: The Case of Liquid Crystals

The contact angle of the liquid crystal (LC) 4-phentyl, 4'-cyano biphenyl (5CB) on glass or polyethylene does not correspond to the value expected from its surface tension measured by the “du Nouy” method (29 × 10^-3 JM^-2). The value deduced from the Young-Dupre law is 40 × 10^-3 JM^-2.

Both these values have already been reported in the literature. Their apparent discrepancy is explained by the LC surface tension anisotropy. We show that 5CB as many other LCs orient perpendicular to the free surface but parallel to glass or polyethylene Thus both values of the surface tension correspond to two different molecular orientations.

The LC oriented perpendicularly has a surface tension of 29 × 10^-3 JM^-2 and 40 × 10^-3 JM when it lies parallel to the surface. We suggest that the anisotropy between the perpendicular and parallel state of LCs also exists between two perpendicular orientations. Furthermore, such anisotropy associated with any polar molecules explains the difference of the interfacial energy of similar polar or non polar (i.e., octanol-octane) compounds and water.     

COMBUSTION OF CARBON/CHAR IN FREE CONVECTION

It has been shown in earlier works that both single and double film char gasification models result in the same burning rate and same char surface temperature for reaction in forced convection, stagnation flows and in quiescent atmospheres. Consequently, comparison of experimental burning rates and/or surface temperatures with analytical results derived from these models cannot identify the appropriate model.

This paper presents some analytical results for burning rates and char surface temperture for a vertical flat plate in free convection flow using the integral method. Results indicate that the two models do result in different burning rates and surface temperatures for the vertical flat plate free convection flow situation offering promise of experimentally identifying the correct mechanism.     

IMPORTANCE OF DISPERSED SOLIDS IN BUBBLES FOR EXOTHERMIC REACTIONS IN FLUIDIZED BEDS

Ignition of activated carbon particles were measured in a vertical tube reactor of 4 cm ID, where single particles fell consecutively through a gas mixture containing oxygen.

A two dimensional fluidized bed reactor 24 cm wide, 51 cm high and 2.5 cm in thickness was used for visual observation through a wide front window 24 cm × 35 cm covered with a silica glass plate 1 cm thick. Activated carbon particles were fluidized incipientiy by air, and a gas mixture containing oxygen was injected upwards into the bed through a nozzle positioned 5 cm above the distributor, forming single bubbles intermittently.

It was observed that carbon particles dispersed in rising bubbles were ignited abruptly at emulsion phase temperatures above 550°C. Experimental findings from the fluidized bed were compared with those from the tube reactor, suggesting that the igniting conditions for particles dispersed in bubbles are nearly the same as for single particles falling in the tube reactor.     

HEAT TRANSFER DURING GLASS FORMING

An important step in the optimization of a glass container production cycle is the determination of the glass temperature distribution during heat treatment. The ideal approach to this problem is to formulate a theoretical model for comparison against experimental data measured in a well-determined system. Discrepancies between theory and experiment may then give further direction for model improvement. This approach, however, is limited because of the difficulties in measuring glass temperature distribution during forming.

Another approach is to use the model to predict glass surface heat fluxes during the forming cycle and test the computed results against published values of glass to metal heat fluxes measured during glass container production on an individual section (I.S.) Blow-and-Blow bottle machine.

A computer model has been developed to calculate the temperature distribution in a glass plate. The model includes the three modes of heat transfer: conduction, convection and radiation. The process is complicated by simultaneous internal emission and reabsorption of radiant energy within the glass in the non-opaque region of the spectrum. With the problem being simplified to a one-dimensional case, the glass plate is divided into several slices. An energy balance on each slice yields a system of integro-partial differential equations which are solved to obtain the temperature distribution.

Results in the form of temperature-time, temperature-distance and surface heat flux-time plots arc presented and compared with published models and with experimental data.     

ACETIC ACID AS AN OXYGEN CARRIER BETWEEN TWO PHASES FOR EPOXIDATION OF OLEIC ACID

Acetic acid was found to be an effective oxygen carrier for epoxidation of oleic acid. The reaction model of oleic acid epoxidation in the two-phase reaction system was systematically analyzed and the rate determining step was experimentally identified.

The results indicated that the rate of oxidation of the unsaturated acid was independent of the concentration of oleic acid and depended on the mixing rate and the rate of formation peracetic acid which in turn depended on the concentration of acetic acid, strength of acid catalyst and the oxygen source, hydrogen peroxide. In the region of reaction control, the rate equation of epoxidation was found to be

$

where k = 2.98 × 10^-2 M^-2 min^-1 at temperature of 35^°C.     

QUANTITATIVE ANALYSIS OF DRYING BEHAVIOUR OF FRUITS AND VEGETABLES

In this study, the drying characteristics of seven vegetables (garlic, potato, bean. ginger, leek, onion, and carrot) and five fruits (avocado, banana, sultana, kiwi fruit and apple) were studied under idealised 'constant' controlled drying conditions using an automatic thermogravimetric analyser. Drying-rate curves were constructed and quantified in a systematic way using the least-squares method. This allowed the drying behaviour of each product to be expressed in terms of three variables; gradient of the “constant rate” stage, gradient of the falling rate stage and critical moisture content (CMC)

The drying curves of fruits and vegetables were found to vary greatly indicating the nature of foodstuffs to have a strong effect on the drying kinetics. The gradient of the constant rate period was not truly constant but had an average gradient of 3.1 × 10^-4 per second with bean having the lowest gradient (1.9 × 10 ^-4/) and garlic having the highest gradient (5.3 × 10^-4/s). This was expected as bean had the highest moisture content (93% wet basis) and was able to feed the surface with sufficient water to maintain a near constant rate of evaporation

The falling-rate period ranged from 10 × 10^-4 per second with avocado and garlic having the lowest gradient while apple had the highest gradient of 30 × 10^-4 per second. The CMC for most of the fruits and vegetables studied was about 1 kg/kg dry mass. This means that, at the CMC, the mass of water was the same as the mass of dry matter but with a spread in values from 0.7 for a fruit like banana to 1.8 for avocado.     

SOLAR ASSISTED DRIER FOR SEEDS

A high performance solar assisted seed dryer has been designed and constructed at a seed processing station in Hungary.

The dryer is arranged in a building specially designed for this purpose, the roof of it serves as solar air collector field. Two drying cells with individual fans of two RPM stages are formed in the building. Drying air preheated by the collector can optionally be distributed between the two drying cells. Material to be dried can be arranged either in a static bed or in containers depending on the requirements. Moist air leaving the bed streams out of the drying cells through openings of the side walls. As auxiliary air heater a natural gas generator is used.     

A CALCULATION METHOD OF OBTAINING DESORPTION ISOTHERMS OF BEECH (FAGUSMOESL4CA) WOOD

This paper presents the specific methods to obtain sorption isotherms. Sorption isotherms were first obtained experimentally for 20°C and 100°C and then, by using the presented method, isotherms were also obtained for 50°C and 80°C. The same treatment can be applied in finding any isotherm sorption in the temperature range between 20°Cand 100°C.

The experiments were carried out on beech (Fagus moesiaca) and sample dimensions were 3×3×3cm. Based on the results of the experiment, by regression analysis, the most suitable analytic formula was proposed. It relsites temperature and air humidity with the equilibrium moisture content in wood. Experimental results were compared with psychrometric tables and Bramhall's formula for desorption curves with good agreement. This proves that the method presented in this paper is precise and efficient to decrease the lime needed to obtain sorption isotherms experimentally.