M. Charmchi

A stochastic model to simulate the formation of a thermal spray coating

We present a three-dimensional, stochastic model of thermal spray coating. It is capable of predicting coating porosity, thickness, roughness, and the variation of these properties with spray parameters. The model assigns impact properties to molten droplets landing on the substrate by generating random values of process parameters, assuming that these properties follow normal distributions with user-specified means and standard deviations. We prescribed rules to calculate splat sizes after droplet impact and their interaction with each other. Porosity is assumed to be solely due to the curl-up of the splats as a result of thermal stresses. We use a Cartesian grid to define the computational domain and to track the shape and position of the deposited coating. The surface of the coating and the location of pores within it are specified using a variable known as the “volume fraction,” defined as the fraction of the volume of a computational cell occupied by coating material. Results are given for the variation of coating porosity, thickness and roughness with varying particle speed, size, and spraying gun speed. Predicted trends agree with experimental observation.

An Automated Water Vapor Diffusion Test Method for Fabrics, Laminates, and Films

An automated apparatus has been developed to measure the transport of water vapor through coated and uncoated fabrics, fabric laminates, thin foams, and solid films under a variety of conditions. The apparatus is more convenient to use than the traditional test methods for textiles and clothing materials; it allows one to use a wider variety of test conditions to investigate the concentration-dependent and nonlinear transport behavior of many of the semipermeable membrane laminates that are now available. The dynamic moisture permeation cell (DMPS) has been auto mated to permit multiple setpoint testing under computer control and to facilitate in vestigation of transient phenomena. Results generated with the DMPC are in agree ment with and of comparable accuracy to those from the ISO 11092 (sweating guarded hot plate) method of measuring water vapor permeability.

Modeling convection/diffusion processes in porous textiles with inclusion of humidity-dependent air permeability

A set of partial differential equations describing time-dependent heat and mass transfer through porous hygroscopic materials was developed. Water in a hygroscopic porous solid may exist in vapor or liquid form in the pore spaces or in bound form when it has been absorbed by the solid, which is typically some kind of hydrophilic polymer. Factors such as the swelling of the solid due to water imbibition, and the heat of sorption evolved when the water is absorbed by the polymeric matrix, were incorporated into the appropriate conservation and transport equations. A numerical code to solve the set of nonlinear coupled equations was developed, and applied to an experimental apparatus designed to simulate transient and steady-state convection/diffusion conditions for textile materials. Experimental measurements of air permeability and diffusion properties as a function of relative humidity provided fundamental data on the changes in transport properties as hygroscopic textiles fibers swell and decrease the free gas phase volume within the porous structure. When these relations were incorporated into the numerical model, it was possible to directly compare the predictions of the numerical code with results generated in the experimental apparatus. Results are shown for hygroscopic porous textiles under several conditions. Under pure diffusion, with no convective flows across the sample, the temperature changes of hygroscopic textiles subjected to step changes in environmental relative humidity are shown to agree with the numerical predictions. These temperature changes are due to sorption of water vapor from the flows on the two sides of the material, and relate to textile fiber equilibrium sorption isotherms and sorption kinetics, as well as the physical structure and thermal properties of the textile. Under conditions of both a concentration gradient and a pressure gradient across the fabric, which results in combined diffusion and convection, it is shown that the effect of fiber swelling, which results in significant changes in the resistance to convective flow, also has an effect on the resulting total mass flux across the textile layer.

The use of volume‐averaging techniques to predict temperature transients due to water vapor sorption in hygroscopic porous polymer materials

Volume-averaging techniques developed for modeling drying processes in porous materials offer a convenient framework for analyzing vapor sorption in porous hygroscopic polymeric materials. Because of the large temperature changes associated with water vapor sorption in these materials (from 10° to 20°C), sorption/diffusion processes are best characterized through the coupled differential equations describing both the transport of energy and mass through the porous structure. Experimental and numerical results are compared for a variety of natural and man-made porous polymeric materials (textiles) using the volume-averaging technique. Boundary heat and mass transfer coefficients and assumptions about thermal radiative properties of the experimental apparatus are shown to influence results obtained with the numerical solution method.

Effects of Particle Surface Charge, Species, Concentration, and Dispersion Method on the Thermal Conductivity of Nanofluids

The purpose of this experimental study is to evaluate the effects of particle species, surface charge, concentration, preparation technique, and base fluid on thermal transport capability of nanoparticle suspensions (nanofluids). The surface charge was varied by changing the pH value of the fluids. The alumina ( Al 2 O 3 ) and copper oxide (CuO) nanoparticles were dispersed in deionized (DI) water and ethylene glycol (EG), respectively. The nanofluids were prepared using both bath-type and probe sonicator under different power inputs. The experimental results were compared with the available experimental data as well as the predicted values obtained from Maxwell effective medium theory. It was found that ethylene glycol is more suitable for nanofluids applications than DI water in terms of thermal conductivity improvement and stability of nanofluids. Surface charge can effectively improve the dispersion of nanoparticles by reducing the (aggregated) particle size in base fluids. A nanofluid with high surface charge (low pH) has a higher thermal conductivity for a similar particle concentration. The sonication also has a significant impact on thermal conductivity enhancement. All these results suggest that the key to the improvement of thermal conductivity of nanofluids is a uniform and stable dispersion of nanoscale particles in a fluid.

Natural convection experiments in an enclosure between eccentric or concentric vertical cylinders of different height and diameter

Abstract Heat transfer coefficients were determined experimentally for natural convection in the enclosed space between two vertical cylinders maintained at different uniform temperatures. The cylinders were of different height as well as of different diameter. During the course of the experiments, the vertical positioning (i.e. elevation) of the inner cylinder and its eccentricity were varied parametrically. For each enclosure configuration, the inner cylinder Rayleigh number ranged between 1.5×103 and 105. The heat transfer medium was air. To supplement the experimental work, numerical results obtained from finite difference solutions are presented for the concentric case. Comparisons between the experimental and numerical results yielded agreement which, for the most part, was within 1%. In general, the average Nusselt number was nearly independent of both the elevation and eccentricity of the inner cylinder. This finding enabled the Nusselt number to be correlated as a function of the Rayleigh number and the cylinder-to-cylinder diameter ratio, without regard to elevation and eccentricity.

NUMERICAL SOLUTION FOR MELTING OF UNFIXED RECTANGULAR PHASE-CHANGE MATERIAL UNDER LOW-GRAVITY ENVIRONMENT

ABSTRACT An enthalpy method is employed to solve transport processes associated with melting of an unfixed rectangular phase change material (PCM) in a low-gravitational environment. This method permits the phase-change problems to be solved within fixed numerical grids, hence eliminating the need for coordinate transformation. The PCM, initially at its melting temperature, is placed inside a rectangular enclosure. The lower surface of the container is then exposed to a uniform temperature higher than the PCM melting temperature. The difference in densities of solid and liquid causes a force imbalance on the solid in the gravitational and low-gravity environments. In the case where the density of the solid phase exceeds that of the liquid, the solid continually moves downward as melting progresses and hence generates a flow field within the liquid. The problem is formulated as a one-domain problem with the possibility of melting from all the PCM surfaces, and no approximation is mode about the liquid film...

Numerical solution of melting processes for fixed and unfixed phase change material in the presence of magnetic field-simulation of low-gravity environment

Transport processes associated with melting of an electrically conducting phase change material (PCM), placed inside a rectangular enclosure, under a low-gravity environment, and in the presence of a magnetic field, is simulated numerically. Electromagnetic forces damp the natural convection as well as the flow induced by sedimentation and/or floatation, and thereby simulate the low-gravity environment of outer space. Computational experiments are conducted for both side-wall heating and top-wall heating under a horizontal magnetic field. The governing equations are discretized using a control-volume-based finite difference scheme. Numerical solutions are obtained for a true low-gravity environment as well as for the simulated low-gravity conditions that are a result of the presence of a horizontal magnetic field. The effects of magnetic field on the natural convection, solid phase floatation/sedimentation, liquid/solid interface location, solid melting rate, and the flow patterns are investigated. It is found that the melting under a low-gravity environment reasonably can be simulated on earth via applying a strong horizontal magnetic field. However, the flow patterns obtained for the true low-gravity environment are not similar to the corresponding cases solved for the simulated low gravity.

Heat transfer and fluid flow characteristics of spanwise-periodic corrugated ducts

Abstract An analytical study is made of the laminar flow and heat transfer in ducts whose cross section is bounded by a wall with periodic corrugations distributed across the span; the other bounding wall is parallel to the corrugated wall and is plane. The study consists of two parts, the first of which is aimed at providing basic heat transfer and fluid flow results while the second utilizes and illuminates these results by means of performance evaluations and comparisons. The basic results, determined numerically, encompass Nusselt numbers, friction factors, isovels and isotherms, and cross sectional mass flow distributions. For the performance evaluations, comparisons were made between the corrugated-wall duct and the parallel plate channel. It was demonstrated that if the temperature of the duet wall is to be minimized, as in an air-operated solar collector, a corrugated duet can be highly effective, but at the price of additional surface area and greater duet height.

A Test Method to Determine the Relative Humidity Dependence of the Air Permeability of Woven Textile Fabrics

This paper describes a test method used to determine the relative humidity dependence of the air permeability of hygroscopic woven textile fabrics. Changes in fabric structure as hygroscopic fibers swell at high humidities can have a large influence on the measured air permeability of materials such as cotton, wool, silk, and nylon fabrics. The method is sensitive enough to show sorption hysteresis effects in the relative humidity versus air permeability curve. The instrumentation also permits dynamic measurements during a step change in relative humidity. Typical results are shown for seven fabrics, covering a range of fiber hygroscopic properties and air permeabilities.