Numerical simulation of acoustic waves in a rectangular chamber

Numerical simulation of acoustic waves in a closed two-dimensional rectangular chamber is considered. The waves are generated by a membrane or piston boundary condition on one wall. The simulations are performed through many acoustic cycles. A stack of heat exchanger plates are sometimes included. The study is motivated by thermoacoustic refrigeration, a phenomena which uses soundwaves in a chamber to achieve a cooling effect. The present study treats the flow numerically. The governing equations are the viscous compressible Navier-Stokes system, assuming a perfect gas. The numerical method employs a finite difference spatial discretization and semi-implicit time-marching procedure. Verification is accomplished by propagating linear acoustic waves. The desired result of forcing is a standing wave. However, the results show a significantly more complex flow than the expected standing wave, including beating, crosswaves, and streaming. Vortex flow also appears near the wavemaker and in the area surrounding the plate

Thermal analysis of Malaysian double storey housing - low/medium cost unit

Almost half of the total energy used today is consumed in buildings. In the tropical climate, air-conditioning a housing unit takes much of the energy bill. Malaysia is no exception. Malaysian double storey terrace housing is popular among developers and buyers. Surveys have shown that housing occupants are much dissatisfied with the thermal comfort and artificial cooling is often sought. The objective of this study is to assess the thermal comfort of the low and medium-cost double storey housing in the area surrounding Universiti Teknologi Malaysia. A simulation program using the Weighting Factor Method calculates the heat transfer interaction, temperature distribution, and PMV level in three types of housing units in relation to the size. Fanger's PMV model based on ISO Standard 7730 is used here because it accounts for all parameters that affect the thermal sensation of a human within its equation. Results showed that both the low and medium-cost housing units studied are out of the comfortable range described by ASHRAE Standard 55 with the units all complied with the local bylaws. In view of the uncertainties in energy supply, future housing units should consider natural ventilation as part of the passive energy management

Optimization of a thermoacoustic refrigerator with an evolutionary algorithm approach

Current non-environmentally friendly refrigerants released into our environment have caused serious concern over reports of the depleting of the ozone layer and global warming. Alternative technologies and efficient energy-related systems are being investigated to perhaps reduce if not stop the environmental degradation. This paper reports the outcomes of an optimization procedure performed on an environmentally friendly standing wave thermoacoustic refrigerator. A typical system to date has a low coefficient of performance (COP) and thus is not attractive to the general public. Optimization is completed using genetic algorithm over four design variables; the stack length and center position within a thermoacoustic resonator, the blockage ratio, and drive ratio. Optimization results show a maximum COP obtainable at 1.64. The outcomes indicate a potential for better thermoacoustic refrigerators in future

CFD simulation of a plate heat exchanger with trapezoidal chevron

In this research, the trapezoidal shaped chevron plate heat exchanger (PHE) is simulated using computational fluid dynamics (CFD) software to determine its heat transfer capacity and friction factor. The PHE is modelled with chevron angles from 30° to 60°, and also the performances are compared with the plain PHE. The validation is done by comparing simulation result with published references using 30° trapezoidal chevron PHE. The Nusselt number and friction factor obtained from simulation model is plotted against different chevron angles. The Nusselt number and friction factor is also compared with available references, which some of the References used sinusoidal chevron PHE. The general pattern of Nusselt number and friction factor with increasing chevron angle agrees with the references. The heat transfer capacity found in current study is higher than the References used, and at the same time, the friction factor also increased. Besides this, it is also found that the counter flow configuration has better heat transfer capacity performance than the parallel flow configuration

Performance of a multi-stack microchannel heat sink

The usage of a very large scale integrated circuits generate high heat fluxes and require an effective cooling system. A microchannel heat sink (MCHS) is one of the reliable cooling systems that had been applied. In terms of performance, a MCHS can be appraised by obtaining low total thermal resistance and pumping power. However, as the total thermal resistance decreases, the pumping power will increase. A few studies have been focused on the minimization of the thermal resistance and pumping power of a multi-stack MCHS. Optimization of two objective functions which are the total thermal resistance and pumping power has been done by using genetic algorithm. It is demonstrated that both objective functions can be minimized by optimizing two design variables which are the channel aspect ratio, α, and wall width ratio,β. It was found that the usage of a stacked configuration for the MCHS is able to reduce the total thermal resistance. From the optimization, it was found that the optimum number of stacks that can be implemented is three. With the three-stack configuration, the total thermal resistance found is 0.1180 K/W which is 21.8% less compared to the single-stack MCHS. However, the pumping power needed for the three-stack MCHS is increased by 0.17 % compared to single-stack which is 0.7535 W

Multi-layer micro channels system: interpretation and developments

During the last three decades the concept of the traditional cooling systems was modified to include single, double, and multi-layer micro channels. The new studies, applications, fabrication, and research focus on four main areas: the geometrical shape of the micro channels, the number of stacked layers, the type of the coolants, and the heat performance optimization. The previous studies have shown a significant reduction in the power consumption as the optimization is accomplished. In this paper, a semi-review for the previous works is provided, an attempt to interpret the nature of the work done, and show another trial for optimization. In this study, water was used as a coolant agent, stacked multi-channel was adopted, and thermal resistance network was calculated.The heat sink under consideration is a rectangle of width ?? and length ??. The thickness ???????? of the base of the micro-channel is 100 [?m] while the depth ????of the micro-channel is 500[?m], both kept constant for all future optimization cases

Optimization of multilayer micro channels heat sinks cooling system using genetic algorithm

Cooling of electronic devices is problematic by its nature simply because of the space restriction. Recent advances in high powered miniaturized electronic systems have come at the expanse of very high heat fluxes that pose challenges to thermal management research. Uncontrolled excessive heat may cause thermal fatigue and stresses and the current micro electro-mechanical cooling systems (MEMS) which utilize the single layer micro channel heat sink, introduced a decade ago, may no longer be an adequate solution. Possible extension of the layer of parallel micro channels into a stacked system, by developing two, three, and multi-layer channel systems are being investigated. The design of all these systems depends on several parameters; coolant type, channel geometry, channel dimensions, and the number of the channels. This paper reports a new model for optimizing the thermal resistance, developed based on specific parameters of the dimensions of the channel, the wall width between the channels, and using water as a coolant at 27°C. The outcomes of the model were compared with other published studies. The results showed that the model is valid and reliable for further studies

Effects of the optimized resonator dimensions on the performance of the standing-wave thermoacoustic refrigerator

Thermoacoustic refrigerator is an alternative cooling system, which is environmentally safe due to the absence of any refrigerants. The resonator tube of the system is of great importance; its design and dimensions influence the design and performance of the entire refrigerator. The central component of the resonator is the stack. So this work describes the design of the stack and the resonator along with the influence of its dimensions on the performance of the standing-wave thermoacoustic refrigerator. The resonator consists of two tubes, one larger than the other, characterized by the diameter ratio of the small over the larger diameter. A Lagrange multiplier method is used as a technique to optimize the coefficient of performance (COP) of the system. The computational analyses show that the resonator small diameter tube dissipates minimum acoustic power at a diameter ratio of 0.46, which is about 17 percent (at least) less than the published values. Moreover, the results show that the resonator length increases gradually with the increase of the mean design temperature. The increase of the resonator length leads to increase of the total acoustic power dissipated by the resonator, which reduces the COP of the standing-wave thermoacoustic refrigerator

Single-objective optimization of a thermoacoustic refrigerator

Optimization of energy-related systems with by-products that involve environmental degradation has never been so crucial today with depleting resources and global concerns over negative impacts on our environment. This paper reports the results of an optimization scheme on the coefficient of performance (COP) of a standing wave thermoacoustic refrigerator based on genetic algorithm. The environmentally friendly refrigerator operates without any CFCs, which has been associated with the depletion of ozone, a substance that prevents uv light from reaching the earth’s atmosphere. A single- objective optimization to maximize the COP of a thermoacoustic refrigerator has been completed. The variables investigated include the length of the stack, Lsn, center position of the stack, xsn, blockage ratio, B and drive ratio, DR. The results show that a COP of up to 1.64 is achievable which provides promise for future improvements in the present systems

Impact of adjacent building on outdoor ventilation around a layout of two buildings

The outdoor air ventilation impact ofa taller building in different configurations of a layout of two adjacent buildings is presented in this paper. The critical parameters investigated are the separation distance (S) between the buildings and the ratio of height of downwind building to that of the building upwind, herein referred to as building height ratio (HR). The aim is to explore intermediate spacing distances which may engender acceptable ventilation around the buildings.A three-dimensional (3-D) numerical simulation employing the Computational Fluid Dynamics technique which adopts the Reynolds-Averaged Navier-Stokes equation and the realizable k-e turbulence model was used to study the turbulent flow field around the full-scale two-building configurations.Results show that velocity ratio generally increases with height ratio, indicating that more air motion is induced at the pedestrian level as the height of the downwind building increases. For each of the height ratios, there is a spacing distance at which the velocity ratio is highest. The spacing distances at which the maximum velocity ratio occurs for the various height ratios are proposed. The dimensionless air exchange rate generally increases with height ratio, indicating that greater quantity of air from within the cavity between the buildings is exchanged with air from outside the cavity, which should result in better air quality. The findings of the study demonstrate the importance of incorporating wind data of an urban area in formulating guidelines for layout of buildings in the area