The present paper is a study on the optimum plate to plate spacing for maximum heat transfer rate from a flat plate type heat exchanger. The heat exchanger consists of a number of parallel flat plates. The working fluids are flowed at the same operational conditions, either fixed pressure head or fixed fan power input. Parallel and counter flow directions of the working fluids were considered. While the volume of the heat exchanger is kept constant, plate number was varied. Hence, the spacing between plates as well as heat transfer rate will vary and there exists a maximum heat transfer rate. The objective of this paper is to seek the optimum plate to plate spacing for maximum heat transfer rate. In order to solve the problem, analytical and numerical solutions have been carried out. In the analytical solution, the correlations of the optimum plate to plate spacing as a function of the non-dimensional parameters were developed. Furthermore, the numerical simulation is carried out to evaluate the correlations. The results show that the optimum plate to plate spacing for a counter flow heat exchanger is smaller than parallel flow ones. On the other hand, the maximum heat transfer rate for a counter flow heat exchanger is bigger than parallel flow ones.
While majority of heat transfer occurs by way of the thermal diffusion mechanism and Fourier′s law, at nanoscale, the heat wave thermal transfer takes place at high energy density heating. In this study, we discuss the mechanisms and relations for these two kinds of heat conduction. The solid model in the shape of nanoribbon heated from one end is assumed to consist of particles with the Lennard-Jones potential. The temperature is considered as projections along longitudinal and transverse directions of calculation region exhibiting two kinds of heat waves with different velocities and amplitude. The separation of the temperature on translational component corresponding only to the heat wave and vibrational one showing both types of heat conduction is done. The heat flux evaluations in molecular dynamics have previously been done only in the study of thermal diffusion, but not for propagation of the heat wave. Two kinds of evaluation equations, the external heat flux that shows the heat wave flowing into and out of a small region and the internal heat flux that shows the heat wave passing inside of a small region, has been compared. The internal and external heat flux values of the heat wave differ from the ones for the thermal diffusion. The heat flux between layers normal to the direction of wave propagation shows the layer to layer motion, as opposed to a small region expansion and contraction at thermal diffusion. Force, velocity and displacement of particles at the heat wave propagation are also studied.
We have studied about dust accumulation on a heat sink with fan used for mobile PC. We found the mixture of wool fragments and cotton linter was suitable as a test dust for present study. Experimental result showed rapid reduction of dust accumulation between fin gap of 1.3 mm to 2.0 mm when increased the fin gap. We have found that the structure that set opening above and parallel to the heat sink reduced the dust accumulation effectively instead of increasing thermal resistance of fan heat sink so much. And, the structure could keep the thermal resistance constant for long time.
We have measured the fan performance characteristics and sound pressure level of tandem fans suitable for redundant cooling systems of electronic equipment. Maximum static pressure of tandem fans without flow-regulate-plate is 1.5 time of single fan at no fan spacing. It increases with fan spacing and reaches 1.9 time at fan spacing of 400 mm. Maximum static pressure of tandem fans with flow-regulate-plate which is symmetrical to the axis of fans is about 1.9 to 2.0 times of single fan, and it dose not affected by fan spacing. Improvement of performance characteristics is larger with longer flow-regulate-plate that is slightly shorter than fan spacing. For example, the plate length of 75 mm is found to be almost optimum for fan spacing of 100 mm. When one of the tandem fans is stopped, performance characteristics is about 80 to 90 % of single fan, and rotational speed of stopped fan is almost proportional to the volume flow rate of operating fan. Sound pressure level of tandem fans without load at normal operating condition is reduced by nearly 8 dB at maximum by changing fan spacing from 0 mm to 300 mm.
Parametric study is conducted for optimizing the fan arrangement of the electronic equipment with several fans inthe equipment. Those fans are cooling for central processing unit (CPU), for power supply unit (PSU), and for the other devices in the equipment (system exhaust fan). We have used the Taguchi method of the experimental design for finding the optimal arrangement of those fans. The result shows system fans with high wind capacity does not always mean to enhance the cooling performance of the electronic equipment. When there is a system exhaust fan, it is recommended to set flow direction of CPU fan to up-flow, it is just opposite to flow direction of commercial CPU fan. This setting enables to exhaust the hot wasted air from CPU very efficiently. It is also found that the separation of the CPU from the PSU cooling system is an effective way to cool both components respectively.
A capillary pressure modeling based on the gradient-diffusion assumption is investigated for simulating water saturation processes in the gas diffusion layer in the polymer electrolyte fuel cell. Leverett function which traditionally applied to the gradient-diffusion assumption based on the experimental data can be approximated by the beta function, considering a probability density of pore size distribution in GDL. It indicates a designing method of GDL and also a dependency of Leverett function on the measuring scale or the simulating resolution.