Heat Transport Limitations Research Articles

Experimental Investigation of the Maximum Heat Transport in Triangular Grooves

An experimental investigation was conducted and a test facility constructed to measure the capillary heat transport limit in small triangular grooves, similar to those used in micro heat pipes. Using methanol as the working fluid, the maximum heat transport and unit effective area heat transport were experimentally determined for ten grooved plates with varying groove widths, but identical apex angles. The experimental results indicate that there exists an optimum groove configuration, which maximizes the capillary pumping capacity while minimizing the combined effects of the capillary pumping pressure and the liquid viscous pressure losses. When compared with a previously developed analytical model, the experimental results indicate that the model can be used accurately to predict the heat transport capacity and maximum unit area heat transport when given the physical characteristics of the working fluid and the groove geometry, provided the proper heat flux distribution is known. The results of this investigation will assist in the development of micro heat pipes capable of operating at increased power levels with greater reliability.

Analysis of mass and heat transfer in transient experiments over heterogeneous catalysts

This article discusses the effects of extra- and intraparticle mass and heat transfer in transient experiments over heterogeneous catalysts. To this purpose, the time-dependent transport in a catalyst particle when introducing a reactant in an inert gas phase has been analyzed. The analysis has led to one criterion assuring the absence of both extra- and intraparticle mass transfer limitations during steady-state experiments. Furthermore, criteria for the absence of mass and heat transport limitations during transient experiments have been developed by extending the criteria for the absence of mass and heat transport limitations during steady-state experiments. As will be shown, transient kinetics are not affected by heat transfer as long as the steady-state criteria for the absence of mass and heat transport limitations are fulfilled.

Intrinsic kinetics of the oxidation of methane over an industrial copper(II) oxide catalyst on a γ-alumina support

The kinetic rate of the heterogeneously catalysed methane oxidation reaction was studied at temperatures ranging from 723 to 923 K and atmospheric pressure. A commercially available CuO catalyst supported by thermally stable γ-Al 2O 3 was used as the catalyst. This kinetic study was carried out in order to evaluate whether this reaction can be suitably handled in a novel type of membrane reactor (J.W. Veldsink et al., Chem. Eng. Sci., 47 (9–11) (1992) 2939–2944). The equipment in which the experiments were conducted consisted of a differentially operated fixed-bed reactor. For this set-up heat and mass transport limitations have been checked and shown to be absent. The influence of the concentrations of the products and reactants on the reaction rate was studied over a wide range of conditions. Fractions of methane ranged from 1 to 6 vol.%, oxygen between 600 ppm and 21 vol.%, carbon dioxide between 0 and 20 vol.%, and water from 0 to 8 vol.%. All experimentally observed reaction rates could be represented within 30% by an Eley-Rideal type reaction rate expression.

The effective length of a flat plate heat pipe covered partially by a strip heater on the evaporator section

An approximate method is presented for calculating the effective length of a flat heat pipe when a strip heater is partially covering the evaporator section. For a specific width of the strip heater, the optimal position to achieve a minimum effective length is found when the heater is placed symmetrically at the centre of the evaporator section. In this position, a higher value of the capillary heat transport limit can be achieved as compared to the case where the heater is placed on one side of the evaporator section.

Heat transport limitations and reaction scheme of partial oxidation of methane to synthesis gas over supported rhodium catalysts

The partial oxidation of methane to synthesis gas over supported Rh catalysts is investigated, paying particular attention to removing heat transport limitations and identifying the reaction conditions within the kinetic-controlling regime. The results obtained suggest that the reaction follows the sequence of total oxidation to CO2 and H2O, followed by reforming reactions to synthesis gas.

Reaction rate oscillations during CO oxidation over Pt/ λ-Al 2O3: Isothermal, intermediate pressure conditions

In this paper we present experimental observations of reaction rate oscillations during CO oxidation over Pt/λ-Al 2O 3 under isothermal conditions, in the intermediate total pressure (1–10 Torr) region. To our knowledge this is the first time that reaction rate oscillations have been reported, for a supported catalyst, under conditions completely free from internal/external mass and heat transport limitations. It is our opinion that these observations prove that the oscillatory behavior, of this system, is the result of intrinsic surface kinetic phenomena and not of extraneous transport limitations. The implications of these observations for reactor design, for this and other catalytic reaction system, are obvious.

Coupled heat and mass transport and chemical kinetic rate limitations during coal rapid pyrolysis

Devolatilization of two eastern bituminous coals was studied under rapid heating conditions (∼104 K/s) in an entrained flow furnace. Experiments were conduced overa range of thermal flux conditions by varying the composition of the coal entraining, medium. Observed pyrolysis rates were dominated by the that transfer characteristics of the entraining gases. Experimental and numerical evidence presented illustrates the role of thermal transport during devolatilization. Chemical kinetic and certain and certain mass ransport mechanisms are intrinsically coupled with the thermal transport process. Coal particle temperatures therefore are a function of the heat flux and the instantaneous pyrolysis rate, with the rate being exponentially coupled to the temperature. This coupling obscures the resolution of coal particle-temperature history and complicates the interpretation of devolatilization behavior. It is suggested that relatively large discrepancies in reported devolatilization rates arise because, in general, no provisions are made to account for coupled chemical, mass, and thermal transport processes.

General diagnostic criteria for transport limitation in porous solid chemical reactions

Theoretical criteria for establishing the absence of heat and mass transport limitations are of fundamental importance in chemical reactor engineering. Many criteria have been proposed in the past, but they cover only particular situations. This work shows that only two general expressions are needed to establish the absence of either intraparticle or interphase mass and heat transport. They encompass all previous theoretical findings and cover new cases. Moreover, when properly applied, the procedure can be used to generate useful criteria for systems with more than one reaction.

Effect of channel restrictions on the axial heat transport in subcooled superfluid helium

The present study investigates the influence of partial restrictions on the axial heat transport and critical heat flux limits in subcooled superfluid helium (helium II) channels. Different size orifices are used to simulate partial plugging of superconducting magnets cooling channels by frozen oxygen, nitrogen, hydrogen, neon or moisture during the cool down process. Thin stainless steel sharp edged orifices of sizes 0.5, 1, 2 and 5 mm id are mounted between stainless steel flanges attached to 9 mm diameter (helium II) channel. The helium II channel is heated at one end with a copper block heater while the other end heat sinks to an atmospheric superfluid helium heat exchange. Temperature drop across the restriction is measured by two calibrated carbon resistors. Measurements are carried out at both temperatures ranging from 1.8 to 2.2 K. As the orifice/channel area ratio decreases, data show a considerable decrease in the axial heat transported by internal convection process resulting in lower critical heat flux at the phase transition from helium II to helium I by the destruction of superfluidity and inititation of boiling. A linear correlation between critical channel heat flux and orificeI channel area ratio gives a good fit to the experimental data. For heat fluxes higher than the critical heat flux, transient temperature measurements for a step heat input are correlated with the time required to reach the phase transition.

Rational Approximations of Effectiveness Factor and General Diagnostic Criteria for Heat and Mass Transport Limitations

Many efforts have been made to predict the effect of diffusion on the observed rate of reaction and its role in modifying the activity and selectivity of porous catalysts. The discussion of rational approximation predicts the effect of diffusional phenomena on the overall rate of reaction under a great variety of circumstances and shows how some part of the theoretical development can be used to deduce two general criteria to establish the conditions where diffusional phenomena can be safely neglected. The reviewed approximations give accurate results with minimal computational effort as long as multiplicity is absent. The expression is given that accurately predicts effectiveness factor values under isothermal conditions provided the apparent reaction order is greater than 0.5. Expressions have been previously reported that are applicable under nonisothermal conditions. The review of the 54 references was devoted to the single reaction case because not much work has been done on complex reaction systems. (BLM)

Kinetics of the potassium carbonate-catalyzed steam gasification of carbon and coal

A study of the atmospheric p pressure kinetics of the K/sub 2/CO/sub 3/-catalyzed steam gasification of activated carbon and Illinois No. 6 coal char is presented. The data were obtained using an electrobalance and experimental conditions which excluded mass and heat transport limitations. A model which assumes that the reaction rate is proportional to the catalyst-solid contact area was used to analyze the data. The porosity was related to solid surface area by using the grain model to describe pore structure. The model fits well the data in the complete range of operating conditions used. Activation energies of 58 and 62 kcal/mol obtained for coal and activated carbon, respectively, are independent of conversion and suggest a unique reaction path. 17 refs.

Entrainment or flooding limit in a closed two-phase thermosyphon

Closed two-phase thermosyphons and gravity-supported heat pipes are highly efficient heat transfer elements which become increasingly applied in terrestrial heat transport and heat recovery systems. Their performance is limited by various heat transport limitations. In the present paper the entrainment or flooding limit is investigated in more detail. Experiments with a copper-water thermosyphon (2.5 m long, 20 mm outer dia.) have been carried out. The steady-state operation has been studied and the maximum performance due to flooding has been measured. The influence of liquid fill charge, inclination angle and operating temperature on the maximum performance has been determined. The experimental results have been assessed and compared with existing theories. Good agreement with the theories of Wallis and Sakhuja has been found.

Uniqueness criteria for first order catalytic reactions with external transport limitations

Uniqueness and multiplicity criteria are developed for a non-isothermal first order reaction occurring in infinite slab or spherically shaped catalyst pellets, with external heat and mass transport limitations. The analysis is done for the analytically resolvable model wherein the catalyst pellet is assumed to be internally isothermal, but with internal gradient of concentration. The results are also compared with those obtained from numerical integration of the complete heat and mass balance equations, including internal temperature gradient as well. The general features of the results will carry over to pellets of any shape.

Intrareactor heat transfer criterion—I. Zero order catalytic reactions

A lumped parameter model is used to describe temperature gradients in bench-scale, fixed-bed catalytic reactors cooled at the wall. A dimensionless criterion is derived for evaluating the significance of intrareactor heat transport limitations in cases of integral conversion and zero order reactions. The new criterion is of nearly the same form as an earlier “hot spot” criterion, but is less conservative. Application of the criterion to published data on methylcyclohexane dehydrogenation indicates significant influence of intrareactor heat transport on the kinetics.

CATALYTIC OXIDATION OF CO ON CuO/Al2O3. II. MEASUREMENT AND DESCRIPTION OF HYSTERESIS AND OSCILLATIONS IN A LABORATORY CATALYTIC RECYCLE REACTOR†

Abstract Under certain experimental conditions sustained oscillations of catalyst bed temperature and CO concentration in the reactor described in Part I were observed. Ignition-extinction phenomena with associated hysteresis were also measured in the regions of experimental conditions where observed reaction rate was substantially effected by heat and mass transport limitations. The application of a two-phase model for explanation and description of oscillations and ignition-extinction phenomena is discussed in detail

Diagnostic criteria for heat transport limitations in fixed bed reactors

Dimensionless criteria are derived for evaluating the significance of interphase and interparticle temperature gradients in fixed bed catalytic reactors. Comparison of the criteria reveals that the magnitudes of the heat transport resistances are generally in the order: interparticle > interphase > intraparticle. The criteria show the critical importance of decreasing the diameters of the reactor and the catalyst particles to minimize transport limitations. Dilution of the bed with inert solids is shown to be advantageous only at Reynolds numbers sufficiently low that the effective thermal conductivity of the bed is insensitive to the flow rate.