Delay time of gases's catalytic heterogeneous ignition on various sizes's catalyst particles

In this work, catalytic ignition delay time of combustible gas's small impurities in air on a spherical metal particle of various diameters is analytically determined by the example of gas-air mixtures's flameless combustion with hydrogen impurities on a platinum particle. It is shown that stable flameless combustion is observed after an induction period for particles of a certain range. It has been established that catalytic ignition time of gases is divided into three stages: 1. inert heating, the duration of which still depends on the combustible gas concentration; 2. the stage of self-acceleration and catalyst temperature rise during the course of the catalytic reaction in the transition region; 3. stage of diffusion inhibition and reaching stable catalytic combustion. The characteristic relaxation time was used in a dimensionless form. To determine the duration of the second stage, a modified Frank-Kamenetsky approach is applied. The duration of diffusion inhibition stage in the dimensionless form is practically independent of catalyst particle's diameter, although the catalytic combustion temperature decreases with an increase in the catalyst diameter. Heat transfer by radiation, the role of which increases with the growth of the catalyst size, is included in the effective heat transfer coefficient, which allows maintaining the classical ideology to solving the problem of the induction period.

Effects of Submerged Convective Cooling In The Turning of AZ31 Magnesium Alloy For Tool Temperature And Wear Improvement

Low melting point and material adhesion attributed by the magnesium alloy led to extreme built-up edge (BUE) and built-up layer (BUL) formations. Dry machining is favourable for machining magnesium alloy; however, this strategy inflicts excessive adhesive wear on the cutting tool. Therefore, this current work focuses on innovative cooling technique known as submerged convective cooling (SCC) for the turning of the AZ31 magnesium alloy. Prior to cutting experiment, a computational fluid dynamics (CFD) simulation was conducted to evaluate internal structure of cooling module. Based on the CFD simulation, small inlet/outlet diameter significantly contribute to reduction of tool temperature because of effective heat transfer coefficient of cooling fluid in the SCC. Experimental results revealed that SCC has effectively reduced the tool temperature by 50% and contributed to 37% improvement in surface roughness when compared to dry cutting. Finally, both BUE and BUL were observed in dry and SCC conditions, but the severity of these wear mechanisms improved or decreased remarkably under SCC conditions.

Calculation of temperature and thermoelastic stresses in backups with collars of the unit of combined continuous casting and deformation in steel billets production. Report 1

The article describes the main loads affecting shaped backups of the unit of combined process of continuous casting and deformation in billets production. Importance of determining the temperature fields and thermoelastic stresses in shaped backups with collars is provided at formation of several billets, at slab compression and at idle during water cooling of backups. The authors describe strength and thermophysical properties of steel from which the backups are made. Geometry of backups with collars used for obtaining billets of three different shapes in one pass is shown. Initial data of the temperature field calculation are given for backups with collars of the combined unit. Temperature boundary conditions are considered for calculation of temperature fields of backups with collars. Boundary conditions determining temperature of such backups are described and values of the heat flow and effective heat transfer coefficient are given. The results of calculation of temperature fields are performed in four sections and are given for typical lines and points located on contact surface of backups with collars and in contact layer at depth of 5 mm from the working surface. The sizes of finite elements grid which is used at calculation of temperature field of backups with collars are provided. Temperature field of backups with collars is determined on the basis of solution of unsteady thermal conductivity equation corresponding initial and boundary conditions. Values and regularities of temperature distribution in bases and in tops of the middle and extreme edges of the shaped backups are presented during slab compression and at idle when obtaining billets of three shapes in one pass at the unit of combined continuous casting and deformation.

Influence of airtightness of steel sandwich panel joints on heat losses

Energy saving ordinances requires that buildings must be designed in such a way that the heat transfer surface including the joints is permanently air impermeable. The prefabricated roof and wall panels in lightweight steel constructions are airtight in the area of the steel covering layers. The sealing of the panel joints contributes to fulfil the comprehensive requirements for an airtight building envelope. To improve the airtightness of steel sandwich panels, additional sealing tapes can be installed in the panel joint. The influence of these sealing tapes was evaluated by measurements carried out by the RWTH Aachen University - Sustainable Metal Building Envelopes. Different installation situations were evaluated by carrying out airtightness tests for different joint distances. In addition, the influence on the heat transfer coefficient was also evaluated using the Finite Element Method (FEM). The combination of obtained air volume flow and transmission losses enables to create an "effective heat transfer coefficient" due to transmission and infiltration. This summarizes both effects in one value and is particularly helpful for approximate calculations on energy efficiency.

Experimental Study on Buoyancy-Induced Convection in Open Cell Aluminium Metal Foams

This paper presents the results of an experimental study on buoyancy induced convection in open cell aluminium foams attached to a one sided heated plate. The foam samples had porosity of around 90% and pore densities of 2, 4, 8 and 16 pores per cm. (PPC). The experiments were conducted under different orientations ranging from horizontal (heated from below) to vertical (heated from side). The heat transfer rates from the heater plate was measured and found to be a strong function of pore density and orientation as well as temperature gradient. In all the cases, the heat transfer rate was found to be superior to that of a vertical flat plate. For a given porosity and height the samples with larger pore sizes were found to be more effective. Finally the effective heat transfer coefficient was plotted in the form of Nusselt number as a function of Rayleigh number to capture its dependence on the foam geometry and boundary condition.

Performance and Integration Implications of Addressing Localized Hotspots Through Two Approaches: Clustering of Micro Pin-Fins and Dedicated Microgap Coolers

An evaluation of two approaches to localized hotspot cooling is conducted through both numerical modeling and experimental demonstration, with the advantages and limitations of each approach highlighted. The first approach, locally increasing the density of pins in a micro pin fin heat sink, was shown through numerical modeling to deliver a factor of two enhancement in effective heat transfer coefficient by doubling the pin density near the hotspot. This simpler approach to maintaining temperature uniformity eliminates the need for hotspot specific fluid routing and delivery, and also has minimal impact on the larger flow field. Dedicated hotspot coolers, on the other hand, have the ability to dissipate significantly larger heat fluxes while maintaining manageable pressure drops, because the flow rate to the dedicated cooler can be closely matched to the demands of the hotspot. Dissipation of hotspot heat fluxes in excess of 2 kW/cm2 is demonstrated experimentally using a two phase dedicated hotspot cooler. However, dedicated coolers require additional fluidic routing and manifolding to efficiently deliver the coolant to the hotspot. These integration concerns are considered in concert with the performance of the hotspot cooler itself to enable better informed thermal design for both system level and device level cooling.