Influence Of Thermal Conditions Research Articles

Influence of thermal conditions on partial oxidation of n-butane over supported Rh catalysts

Abstract Short residence time catalytic reactors for partial oxidation of alkanes are effective for production of H 2 -rich syngas. In this study, partial oxidation of n -butane in O 2 /Ar mixtures at ratios of 1/3.76 have been investigated over Rh catalysts impregnated in γ-Al 2 O 3 washcoat supports coated onto α-Al 2 O 3 ceramic foam monoliths (1.0 cm in length). Experiments in non-adiabatic reactors show that n -butane conversion and H 2 and CO selectivity depend strongly upon the reactor temperature and thereby upon heat lost through external walls of the reactor. For a baseline condition with a C/O atomic ratio of 1.0, inlet flow temperature of 300 °C, and catalyst-contact time of approximately 50 ms, improving reactor adiabaticity dramatically increases n -butane conversion and H 2 and CO selectivity. Raising inlet-flow temperature increases fuel conversion but does not improve H 2 selectivity. However, higher flow rates (with catalyst-contact times below 40 ms) show further increases in fuel conversion and syngas selectivity. These results suggest that under conditions with adequate heat release, partial oxidation occurs rapidly within the first few mm of the reactor. Improved heat confinement in the reactor increases catalyst temperature and thereby results in higher conversion and syngas selectivity. Under these conditions, the reactor effluent has much higher H 2 and CO selectivities than predicted by equilibrium at the exit temperature, which implies that higher internal temperatures determine the effluent composition. These results provide critical insight into the role of thermal processes and indicate a need for detailed mechanisms to model how competitive reaction pathways impact performance in catalytic partial oxidation reactions.

A numerical investigation on effects of ceiling and floor surface temperatures and room dimensions on the Nusselt number for a floor heating system

In this study, the effect of ceiling and floor surface temperatures and room dimensions on the Nusselt number over the floor of a floor heating system has been investigated numerically. The variation of the Nusselt number with Rayleigh number has been analyzed under constant wall temperature condition for different ceiling temperatures (10–25 °C) and room dimensions. It has been seen that when the room dimensions and temperature difference between the ceiling and interior air are increased, the Nusselt number over the floor increases as well. The numerical results have been compared with the correlations given in the literature. It has been seen that the correlations available in the literature are valid only for given thermal conditions and room dimensions. The results calculated from the correlations which do not consider the effects of ceiling and floor surface temperatures deviate up to 35% than the results of this numerical study carried out for different ceiling and floor surface temperatures and room dimensions. Therefore, a new correlation for Nusselt number over the floor, which contain the influence of thermal conditions and all of room dimensions must be discovered.

Influence of thermal conditions on the combustion synthesis of Si 2N 2O phase

This work is a contribution to the study of Si 2N 2O synthesis via Self-Propagating High-temperature Synthesis (SHS). The elaboration of Si 2N 2O was carried out using two kinds of experimental equipments and differences both in reaction temperatures and in products compositions were evidenced. In order to determine the reaction mechanisms, several compositions and thermal conditions of the reactant mixture were studied. Considering our results, two kinds of reaction mechanisms for the combustion synthesis of Si 2N 2O could be suggested. Furthermore, the results previously reported in the literature have been explained in the light of this work. The SHS samples were characterized by X-ray diffraction and scanning electron microscopy (SEM) analysis. Their quantitative compositions were determined by Rietveld refinement.

Self-assembled lead atomic nanowires on pre-structured Sm/Si(1 1 1) interfaces

The (3 × 2) and (7 × 1) chain structures induced by the adsorption of Sm atoms on Si(1 1 1) substrate are used as template to create self-assembled nanowires. Their structure models are based on an appropriate combination of honeycomb chain channel (HCC) and Seiwatz Si chains. In our study, Pb atoms are evaporated on these pre-structured surfaces. Our STM observations reveal the presence of well-aligned Pb nanowires running along the [ 1 1 ¯ 0 ] direction on the (7 × 1)-Sm/Si(1 1 1) structure whereas they are not formed on the (3 × 2)-Sm/Si(1 1 1) phase whatever the growth conditions. A thorough examination of the high resolution STM images reveals that the Pb preferentially grows onto the Seiwatz chains along the Sm rows, suggesting that the Seiwatz chains are the most reactive sites on the (7 × 1)-Sm/Si(1 1 1) surface. The difference of Pb reactivity on these surfaces illustrates the influence of bond energy and strain on the surface induced by the both HCC and Seiwatz Si chains in the growth of nanowires. The influence of thermal conditions on the Pb nanowires formation is also discussed.

Influence of thermal conditions on biomass of fish in the Polish EEZ

Systematic acoustic surveys of fish resources in the Polish EEZ, carried out during the autumn as a part of international ICES programme, begun in 1989. A noticeable increase in fluctuations of fish biomass and water temperature has been observed since 1996. Methods elaborated in Sea Fisheries Institute were applied to correlate acoustic data on fish distribution with relevant environmental factors, and to develop models of trends observed. Relationships between mean temperature at fish depth and total fish biomass in each year were estimated separately for day- and night-time. High positive correlation ( r=0.910 for day-time, r=0.885 for night-time) was found for data from the 1989 to 2000 period. It was shown that fish reaction to temperature was stronger during the night (exponential model), at which time a 3 °C increase of temperature at fish depth could result in a 1.7 multiplier of total biomass in the area. During the day, a 6 °C increase of temperature was necessary to produce a comparable effect. Research was implemented by application of macrosounding methods to confirm direct correlation between temperature and fish distribution along most representative transects in the area. Analysis of all results indicates the role of the surface waters, where temperature seems to be one of the most important factors influencing final geographic fish distributions. Lower temperatures in the Polish EEZ accelerate northeast migration of fish from the zone, reducing a feeding season. It can be concluded also that climate instability can strongly affect temporal distribution of fish. In a longer period such a negative phenomenon can produce a systematic degradation of fish stocks, what could be observed on the basis of our results.

Temperature responses of net nitrogen mineralization and nitrification in conifer forest soils incubated under standard laboratory conditions

Variation between soils in the response to temperature of net nitrogen (N) mineralization and nitrification was investigated in laboratory incubations. The soils, which were sampled from coniferous forests of different regional climates of western Europe, were mixed with 15N-labeled wheat straw and incubated at 4, 10, 16, 23 and 30 °C and constant moisture conditions. The accumulation in soil of the extractable NH 4 +–N and NO 3 −–N was followed for 550 days. First-order rate equations were fitted to data to estimate net mineralization rate constants and the size of the mineralizable part of straw and native soil N (Nmax). This pool was found to be related to temperature indicating an influence of thermal conditions on the degree of microbial exploitation of organic N sources. The Q 10 estimates for the rate constants of mineral N accumulation decreased with increasing temperature but the relationship was less intense for straw than for native soil N results. Optimum temperature for nitrification showed a good correlation with the geographic origin of the soils and it was lower than the highest incubation temperature (30 °C) for the soil of the colder site.

Influence of thermal conditions and presence of additives on fat bloom in chocolate

AbstractThis study focused on the analysis of the effects of thermal history and presence of additives on fat bloom in chocolate. Magnetic resonance data obtained on specific chocolate samples were useful in evaluating the effect of thermal history on the appearance of fat bloom and also in understanding the underlying mechanism. Fat bloom was induced by thermal history such as storage for 3 d at 32 or 28°C. Increasing storage time at 21°C after each thermal treatment also promoted fat bloom. Differential scanning calorimetry experiments confirmed the appearance of polymorphic form VI in the bloomed samples. Also, three different components were added separately to the initial composition of dark chocolate and the appearance of fat bloom was monitored. 1,3‐Dibehenoyl, 2‐oleoylglycerol (BOB) did prevent fat bloom. However, magnetic resonance and differential scanning calorimetry data collected on chocolate samples did not confirm that either added sucrose or milk powder prevented fat bloom. Indeed, fat bloom occurred for these samples, but its characteristics were different from those with pure chocolate.

Synthesis and crystallization of ZnSe by the liquid encapsulation technique

In this paper we present the preliminary results of ZnSe crystal growth by liquid encapsulated Czochralski (LEC) technique. The influence of thermal conditions on the crystal growth process and the properties of the boules were studied. Investigation of physical parameters (resistivity measurements, photoluminescence, infrared transmission) showed the high purity of the obtained material and indicated that the LEC technique can be used for obtaining large single crystals of ZnSe.

Etching and microhardness studies on lead molybdate single crystals

Growth and characterization of lead molybdate (PbMoO4) single crystals have attracted increasing interest due to the variety of applications of these crystals [1–6]. Defect characterization, together with growth-related fundamental studies, were of special importance for many of the device requirements. This material was initially tested for its luminescence properties [7]. Subsequently, the main application has been in the field of acousto-optics because of its high acousto-optic figure of merit, low acoustic loss, low optical loss in the region 420 nm to 3900 nm and good mechanical impedance for acoustic matching [8, 9]. It has also attracted particular interest as one of the possible candidates for use in low temperature scintillator applications [10]. Difficulties associated with the growth of single crystals of lead molybdate have received significant attention due to a greater influence of thermal conditions during and after the growth [1, 11]. Zeng [12] has reported that when an as-grown crystal is held at a temperature close to the melting point for a long time, physical and chemical properties change. In the present investigation, single crystals of lead molybdate were grown by the Czochralski method with high purity starting materials of PbO and MoO3. They were mixed in the stoichiometric ratio and synthesized at around 800 8C with two intermediate grindings before melting. The pre-synthesized material was melted in a platinum crucible at 1063 8C. The detailed growth process has been discussed elsewhere [13]. The crystals for the present work were grown along the c-direction. As soon as the growth was terminated, the crystal was cooled at a rate of 20 8C hy1 to 500 8C and then at a relatively faster rate to room temperature. The crystal was not annealed at any intermediate temperature. The crystal was carefully cut along the aand c-directions using an internal diameter cutting machine so as to obtain a cube of 5 mm3 in size. The cut pieces were finally polished using 0.25 im diamond polishing paste. The cut and polished samples were subjected to chemical and thermal etching studies. Chemical etching was carried out using NaOH 5% solution as etchant at room temperature for 10–20 min. Since the cube was completely immersed in the etching solution, both aand c-faces were etched simultaneously under identical conditions. The etched samples were cleaned and their microstructure was analysed using an optical microscope (Leitz Metallux-II) in the reflection mode. Pits were not seen clearly on the cface whereas the a-face showed imperfection-induced arrow-headed etch pits (Fig. 1). A subgrain boundary was seen as a line of etch pits on the c-face (Fig. 2). This subgrain boundary line was observed on all samples cut perpendicular to the growth direction along the entire length of the crystal. This shows that the general nature of the subgrain boundaries is to run parallel to the pulling direction [14]. The process adopted for thermal etching is as follows. The samples were carefully loaded into a platinum boat and placed inside a resistively heated muffle furnace with temperature controlled by a Eurotherm 818P temperature controller. The furnace was heated to 900 8C at a rate of 30 8C hy1 and held at this temperature for 10 h. The furnace was then cooled at a rate of 10 8C hy1 to 600 8C and then at 25 8C hy1 to room temperature. After the heat treatment, the samples were analysed with an optical microscope. Heat treatment at 900 8C revealed thermal etch

The Influence of Thermal Conditions on Teachers’ Work and Student Performance

Using the Schools Council initiative to construct a Teaching Charter as an example, proposes that the orthodox analysis of the relationships between teachers′ work and student performance is incomplete. To this end, research into climatic conditions that affect human physical, emotional and cognitive performance is reviewed. Argues that Australian educational research and policy have ignored the potential of direct and indirect effects of thermal conditions to influence school outcomes adversely. Concludes that there is a case for educational research and policy to account for climatic factors, especially the interactions between social, psychological, cultural and thermal variables in the formulation of teacher and student performance standards.

The Influence of Thermal Conditions on the Surface Activity of Thirteen‐Lined Ground Squirrels

We examined the relation between surface activity and the surface microclimate in thirteen—lined ground squirrels. We statistically isolated the effects of thermal conditions on squirrel activity and tested the suitability of two optimization models for predicting thermoregulatory behavior. Metabolic and microclimatic data were used to represent thermal conditions as standard operative temperature. The proportion of time spent on the surface was highest at standard operative temperatures between 30° and 40°C. This range approximately matched the squirrel's physiological thermal neutral zone. This pattern was not qualitatively altered by using a multiple—regression model to remove the effects of potential confounding variables (e.g., time of day or time of year). At low temperature, the observed reduction in surface activity resulted in lost foraging benefits °6 times as great as the energy savings in thermoregulatory and activity costs. Therefore, we believe that these squirrels cannot be considered daily energy maximizers. Rather, optimization must be considered in the context of longer time periods. At high temperatures, the observed reduction in surface activity was described by a constrained thermal model of shuttling thermoregulation, which assumed squirrels maximized surface feeding time. The model also explained a systematic decline in body temperature when exiting and entering the burrow as surface operative temperatures increased.

The influence of thermal conditions on rectal temperature, respiration rate and pulse rate of lactating Holstein-Friesian cows in the humid tropics.

The effect of minimum, maximum and mean ambient air temperatures and the temperature-humidity index (THI) of the same and the previous day on morning (a.m.) and afternoon (p.m.) rectal temperatures (RT), respiration rates (RR) and pulse rates (PR) were studied in 17 Holstein-Friesian cows over the first 125 days in the 3rd and 4th lactations. Physiological responses showed a diurnal pattern, being lower in the mornings than the afternoons: 38.6 vs 39.0 degrees C for RT, 52.2 vs 60.7 breaths/min for RR and 58.1 vs 64.1 beats/min for PR. Correlations between RT and RR (r = -0.043 to -0.046) and RT and PR (r = -0.178 to -0.261) were low (P greater than 0.05). Correlations between RR and PR (r = 0.353 to 0.365) were moderate (P less than 0.05). Weather variables, especially ambient temperature of the previous day, were more important and influenced physiological responses to a greater extent than other thermal factors the same day. Generally, physiological responses were influenced to a greater extent by ambient temperature than THI. Weather variables explained variations in RT (5.1-59.6%), in RR (13.0-17.8%) and in PR (22.1-25.4%). Relationships between weather variables the previous day and physiological responses were contradictory, with minimum and maximum values showing a negative relationship in contrast with a positive relationship for mean values.

The influence of thermal conditions on the conception rate of Holstein-Friesian cattle in the humid tropics

Weather data and breeding data for imported Holstein-Friesians and their progeny for the years 1976–1986 were studied in the humid forest zone of Ghana. Average maximum and minimum ambient temperatures over the period were 30.8 and 21.6°C, respectively. The mean temperature, relative humidity, temperature-humidity index ( THI) and total rainfall were 26.3°C, 75.5%, 76.3 units and 1207 mm, respectively. There was no relationship between any thermal variable and the conception rate in the Ghana-born Friesians. There was, however, a negative association between the minimum temperature on the day of or the day after service and conception rate, and THI on the 2nd or 3rd day after service and conception rate, in the imported Friesians.

Experimental study on the influence of thermal conditions upon the human physiological and psychological responses in an air-conditioned room

Experimental study on the influence of thermal conditions upon the human physiological and psychological responses in an air-conditioned room

Influence of thermal conditions on the efficiency of high-performance liquid chromatographic columns

The effect of thermal conditions on efficiency in high-performance liquid chromatography is investigated with special reference to the role of the viscous heat dissipation. In systems thermostatted by a water-bath or jacket it turns out to be advantageous to pre-cool the mobile phase when high flow-rates and small particle sizes are used. This is explained by a compensation of band broadening in the first and second part of the column. The wall region of the band moves faster in the first part, but it moves slower in the second part and it is eluted together with the solute from the core region. Our results indicate that a similar compensation effect occurs when the column is operated in still air. This is due to back-flow of heat through the stainless-steel column wall. For large diameter columns (larger than 3 mm), packed with small particles, a linear temperature profile imposed on the column wall may diminish the detrimental effects of viscous heat dissipation. Results obtained with such a system are promising.

Influence of Thermal Conditions during the Course of Crystal Growth on the Qualities of a Ruby Single Crystal

Ruby crystals were grown by the Verneuil method under three thermal conditions. The temperature distribution in the crystals was determined during the course of crystal growth. The axially steep and radially flat temperature-distribution was observed in the growing crystals. The misorientations of the crystals were investigated by means of an X-ray diffraction method. The optical qualities of the crystals were observed by introducing a gas laser light into the crystals. It was found that axially steep temperature gradient gave rise to misorientations and optical inhomogeneities in the crystals. The properties of the misorientations were interpreted in terms of the thermal stress during crystal growth and the plastic deformation of alumina single crystal.