OPTIMUM DESIGN OF PLATE HEAT EXCHANGER WITH STAGGERED PIN ARRAYS

The optimization of the plate heat exchanger with staggered pin arrays for a fixed volume is performed numerically. The flow and thermal fields are assumed to be periodic fully developed flow and heat transfer with constant wall temperature and they are solved using the finite-volume method. The optimization is carried out by using the sequential linear programming (SLP) method, and the weighting method is adopted for solving the multiobjective problem. The results show that the optimal design variables for the weighting coefficient of 0.5 are as follows: S = 6.497 mm, P = 5.496 mm, D ₁ = 0.689 mm, and D ₂ = 2.396 mm. The Pareto optimal solutions are presented also.     

Fourier and non-Fourier heat conduction analysis in the absorber plates of a flat-plate solar collector

This paper presents an analytical analysis of both Fourier and non-Fourier heat conduction in the absorber plates of a flat-plate solar collector. Separation of variables was employed to develop the model. For the analysis, a repetitive heat transfer module was used for the solution of parabolic and hyperbolic equations. From the practical point of view, two types of boundary conditions were separately chosen. A numerical technique based on the finite difference method was employed to determine the temperature for validation purposes. A comparative investigation was carried out to understand the requirements for use of the non-Fourier heat conduction model easily. A significant difference in the temperatures obtained from the Fourier and non-Fourier models was observed for lower values of the Fourier number and higher values of the Vernotte number. Finally, the effect of the boundary conditions on the Fourier and non-Fourier heat transfer was demonstrated.     

Rheological studies of disulfonated poly(arylene ether sulfone) plasticized with poly(ethylene glycol) for membrane formation

Disulfonated poly(arylene ether sulfone) (BPS) random copolymers, prepared from a sulfonated monomer, have been considered for use as membrane materials for various applications in water purification and power generation. These membranes can be melt-processed to avoid the use of hazardous solvent-based processes with the aid of a plasticizer, a low molecular weight poly(ethylene glycol) (PEG). PEG was used to modify the glass transition temperature and melt rheology of BPS to enable coextrusion with polypropylene (PP). Our previous paper discussed the miscibility of BPS with PEG and the influence of PEG on the glass transition of BPS. In this study, the rheological properties of disulfonated poly(arylene ether sulfone)s plasticized with poly(ethylene glycol) (PEG) are investigated to identify coextrusion processing conditions with candidate PPs. The effects of various factors including PEG molecular weight, PEG concentration, temperature and BPS molecular weight on blend viscosity were studied. The rheological data effectively lie on the same master curve developed by Bueche and Harding for non-associating polymers such as poly(methyl methacrylate) (PMMA) and polystyrene (PS). Although sulfonated polysulfone contains ionic groups, the form of its viscosity versus shear rate (or frequency) behavior appears to be dominated by the relaxation of polymer entanglements.     

The structural variation of the gas diffusion layer and a performance evaluation of polymer electrolyte fuel cells as a function of clamping pressure

Interfacial contact resistance between gas diffusion layers (GDLs) and bipolar plates (BPs) has a substantial effect on the performance loss of polymer electrolyte fuel cells (PEFCs). Particularly during the final manufacturing process of a fuel cell stack, an externally applied clamping load determines the extent of electrical contact between those two solid components. In order to have the least electrical contact loss, it is highly necessary to keep all PEFC components close each other without causing structural failure of fuel cell stacks. In the present work, we investigated the effect of the clamping pressure on extrinsic properties such as porosity and permeability, which is closely related to mass transfer of reactants. Also, the variance of interfacial electrical resistance was analyzed as a function of the stack clamping pressure or the compressed GDL thickness, which reflects the external clamping load. Then with these experimentally obtained material properties of GDL, computational efforts were made to account for the effect of the clamping pressure on the fuel cell performance.     

A numerical study of flow distribution effect on a parallel flow heat exchanger

The effect of flow distribution on thermal and flow performance of a parallel How heat exchanger has been numerically investigated. The flow distribution has been altered by varying the geometrical parameters that included the locations of the separators, and the inlet/outlet of the heat exchanger. Flow nonuniformities along paths of the heat exchanger, which were believed to be dominantly influential to the thermal performance, have been observed to eventually optimize the design of the heat exchanger. The optimization has been accomplished by minimizing the flow nonuniformity that served as an object function when the Newton’s searching method was applied. It was found that the heat transfer of the optimized model increased approximately 7.6%, and the pressure drop decreased 4.7%, compared to those of the base model of the heat exchanger.     

Exact and Approximate Analytic Methods to Calculate Maximum Heat Flow in Annular Fin Arrays with a Rectangular Step Profile

Exact and approximate techniques to determine the performance of annular fins with a step rectangular profile are developed. A simple approximate method is proposed to analyze the prevalent heat transfer characteristics of the fin array. An algebraic expression based on the mean value approach is used as an approximation tool, and the temperature distribution in the fins is determined using an exponential function. A method based on a modified Bessel function formulation is employed for exact analysis. The analyses are extended to optimize fins based on the principle of maximizing heat transfer rate for a given volume. The results obtained from the exact and approximate analyses are presented in a one-to-one comparative manner to allow for a wide range of practical design variables. The error in the approximate analysis calculations is investigated, and it is found to be well within engineering accuracy requirements. It is expected that the approximate analytical tool designed here will be extremely useful for designers who want to easily determine design parameters. Because the approximate methodology is so simple, all calculations can easily be done.     

Thermo-acoustic random response of temperature-dependent functionally graded material panels

A nonlinear finite element model is provided for the nonlinear random response of functionally graded material panels subject to combined thermal and random acoustic loads. Material properties are assumed to be temperature-dependent, and graded in the thickness direction according to a simple power law distribution in terms of the volume fractions of the constituents. The governing equations are derived using the first-order shear-deformable plate theory with von Karman geometric nonlinearity and the principle of virtual work. The thermal load is assumed to be steady state constant temperature distribution, and the acoustic excitation is considered to be a stationary white-Gaussian random pressure with zero mean and uniform magnitude over the plate surface. The governing equations are transformed to modal coordinates to reduce the computational efforts. Newton–Raphson iteration method is employed to obtain the dynamic response at each time step of the Newmark implicit scheme for numerical integration. Finally, numerical results are provided to study the effects of volume fraction exponent, temperature rise, and the sound pressure level on the panel response.     

Frost formation on a cold surface under turbulent flow

This study presents a mathematical model to predict the frosting behavior on a cold surface under turbulent flow. The model consists of the standard κ–ε model for turbulent flow and the diffusion equation for the frost layer. The numerical results show that turbulent flow promotes the growth of the frost layer on the cold surface, compared to the laminar flow. Increase in air velocity has little effect on mass transfer under turbulent flow, while frost growth under laminar flow is influenced by the air velocity. With constant air humidity, the frost layer thickness increases with decreasing air temperature, while the relationship for the frost density is reversed. The effect of the air temperature on the mass flux is negligible, compared to the other frosting parameters.