Normah Mohd-Ghazali

SIMULTANEOUS OPTIMIZATION OF FOUR PARAMETERS IN THE STACK UNIT OF A THERMOACOUSTIC REFRIGERATOR

Thermoacoustic refrigerators are environmentally friendly cooling systems that use no refrigerants. Optimization of the performance of any cooling system is crucial for an efficient energy management. Most of the optimization techniques in thermoacoustic systems utilized to date involved experimental and numerical parametric studies which are generally limited to the variations of the parameters to be optimized at discrete values. This study reports on the optimization of a thermoacoustic refrigerator using multi-objective genetic algorithm (MOGA). The study introduces the ability of MOGA to optimize four different variables which are length of stack, center position of stack, blockage ratio and drive ratio simultaneously. The four variables are optimized to achieve the two objectives; a maximum cooling and minimum acoustic power required at the stack and provide the optimum coefficient of performance, COP. The results show that the optimum COP = 1.35 with a cooling power of Qc = 6.57 W, acoustic power of Wn = 4.86 W and with the resonator diameter of D = 3.8 cm.

Performance Optimization of a Microchannel Heat Sink Using the Improved Strength Pareto Evolutionary Algorithm (SPEA2)

In this paper, a feasible optimization scheme for rectangular microchannel heat sinks, which incorporates the thermal resistance model and the Improved Strength Pareto Evolutionary Algorithm (SPEA2), is reported. An alternative coolant, namely, ammonia gas, is used to improve the overall thermal and hydrodynamic performances of the considered system. Results from the optimization showed significant reduction in the total thermal resistance compared to the conventional air-cooled systems up to 35% for the same allowable pumping power. The SPEA2 exhibited excellent performance when it was compared to another multiobjective algorithm, NSGA2. The results reported in this study open the door for the incorporation of some other algorithms, which have not been used in the optimization of microchannel heat sinks. Finally, the outcome of this paper predicts a promising future for the usage of ammonia gas in the area of electronics cooling.

Optimization of nanofluid-cooled microchannel heat sink

The optimization of a nanofluid-cooled rectangular microchannel heat sink is reported. Two nanofluids with volume fraction of 1 %, 3 %, 5 %, 7 % and 9 % are employed to enhance the overall performance of the system. An optimization scheme is applied consisting of a systematic thermal resistance model as an analysis method and the elitist non-dominated sorting genetic algorithm (NSGA-II). The optimized results showed that the increase in the particles volume fraction results in a decrease in the total thermal resistance and an increase in the pumping power. For volume fractions of 1 %, 3 %, 5 %, 7 % and 9 %, the thermal resistances were 0.072, 0.07151, 0.07075, 0.07024 and 0.070 [oK W-1] for the SiC-H2O while, they were 0.0705, 0.0697, 0.0694, 0.0692 and 0.069 [oK W-1] for the TiO2-H2O. The associated pumping power were 0.633, 0.638, 0.704, 0.757 and 0.807 [W] for the SiC-H2O while they were 0.645, 0.675, 0.724, 0.755 and 0.798 [W] for the TiO2-H2O. In addition, for the same operating conditions, the nanofluid-cooled system outperformed the water-cooled system in terms of the total thermal resistance (0.069 and 0.11 for nanofluid-cooled and water-cooled systems, respectively). Based on the results observed in this study, nanofluids should be considered as the future coolant for electronic devices cooling systems.

Optimization of the Stack Unit in a Thermoacoustic Refrigerator

ABSTRACT A thermoacoustic refrigerator is a device that uses acoustic power to pump heat in the absence of harmful refrigerants with no or few moving parts. However, the performance of the thermoacoustic refrigerator, particularly the standing wave types, is currently not competitive compared to its counterpart, the conventional vapor-compression refrigerator. Presently, thermoacoustic refrigeration prototypes only achieved 0.1–0.2 relative coefficient of performance, compared with that of 0.33–0.5 for the conventional vapor-compression refrigerators. Past optimization efforts had been completed based on parametric studies where individual parameters are discretely varied and the final optimized outcome was based on the limited series of numerical/experimental tests. This paper discusses the initial investigation of the optimization of the thermoacoustic refrigerator stack parameters using a multi-objective genetic algorithm. The desired outputs, the maximization of the cooling load and the minimization of the acoustic power at the stack, are obtained with the parameters to be optimized set within some range of values. The stack length and center position are then optimized simultaneously. The optimized results showed that the coefficient of performance of the thermoacoustic refrigerator improves from the published value of 1.3 to 1.37.

Multiobjective Optimization of Microchannels with Experimental Convective Heat Transfer Coefficient of Liquid Ammonia

A multi-objective optimization on a system of microchannels with environmentally friendly liquid ammonia is presented. Further, comparative studies were done on two approaches of obtaining the convective heat transfer coefficient necessary for the procedure; from the conventional Nusselt number correlation and that from experimentally measured data. The thermohydrodynamic performance of the coolant agrees well with theory with a higher resistance associated with the experimentally obtained data due to overall contributions from experimental apparatus generally not considered in mathematical representations of actual processes. The study shows that the pairing of a fast and simple evolutionary algorithm method as MOGA with experimental data is a powerful combination when new coolants are being explored for replacements in current systems. The results would be useful in providing the trends and patterns needed to evaluate the potentials of new coolants in microchannels.

Multi-objective optimization algorithms for microchannel heat sink design

This paper investigates the performance of four multi-objective optimization algorithms namely the GAM, MOGA, SPEA2 and NSGA-II on the optimization of a microchannel heat sink based on the total thermal resistance and pumping power. Two case studies with different formulation methodologies were selected for the optimizations. The optimizations results showed that both SPEA2 and NSGA-II algorithms exhibited excellent performance in terms of the number of the optimal solutions, maintaining the desirable diversity and convergence speed toward the Pareto optimal front as compared to GAM and MOGA.

Thermoacoustic Refrigerator’s Stack Optimization

The standing wave thermoacoustic refrigerator, which uses sound generation to transfer heat, was developed rapidly during the past four decades. It was regarded as a new, promising and environmentally benign alternative to conventional compression vapor refrigerators, although it was not competitive regarding the coefficient of performance (COP) yet. Thus the aim of this paper is to enhance thermoacoustic refrigerator’s stack performance through optimization. A computational optimization procedure of thermoacoustic stack design was fully developed. The procedure was designed to achieve optimal coefficient of performance based on most of the design and operating parameters. Cooling load and acoustic power governing equations were set assuming the linear thermoacoustic theory. Lagrange multipliers method was used as an optimization technique tool to solve the governing equations. Numerical analyses results of the developed design procedure are presented. The results showed that the stack design parameters a...

Prediction of the Heat Transfer Coefficient in a Small Channel with the Superposition and Asymptotic Correlations

Heat transfer coefficient as an important characteristic in heat exchanger design is determined by the correlation developed from previous experimental work or accumulation of published data. Although discrepancies still exist between the existing correlations and practical data, several researchers claimed theirs as a generalized heat transfer correlation. Through optimization method, this study predicts the heat transfer coefficient of two-phase flow of propane in a small channel at the saturation temperature of 10∘C using two categories of correlation — superposition and asymptotic. Both methods consist of the contribution of nucleate boiling and forced convective heat transfer, the mechanisms that contribute to the total two-phase heat transfer coefficient, which become as two objective functions to be maximized. The optimization of experimental parameters of heat flux, mass flux, channel diameter and vapor quality is done by using genetic algorithm within a range of 5–20kW/m2, 100–250kg/m2s, 1.5–3mm ...

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. 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 smaller over the larger diameter. A Lagrange multiplier method is used as a technique to optimize the coefficient of performance (COP) of the system. Results show that the resonator small diameter tube dissipates a minimum acoustic power at a diameter ratio of 0.46, which is 17 percent less than the published values. Moreover, the results show that the resonator length increases gradually with the increase of the mean design temperature which leads to the increase of the total acoustic power dissipated by the resonator, reducing the COP of the standing-wave thermoacoustic refrigerator.

Impact of Adjacent Building on Outdoor Ventilation around a Layout of Two Buildings

The outdoor air ventilation impact of a 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-ε 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.