MATHEMATICAL OPTIMIZATION: APPLICATION TO THE DESIGN OF OPTIMAL MICRO-CHANNEL HEAT SINKS

This paper documents the geometrical optimization of a micro-channel heatsink embedded inside a highly conductive solid, with the intent of developing optimal solutions for thermal management in microelectronic devices. The objective is to minimize the peak wall temperature of the heat sink subject to various constraints such as manufacturing restraints, fixed pressure drop and total fixed volume. A gradient based multi-variable optimization algorithm is used as it adequately handles the numerical objective function obtained from the computational fluid dynamics simulation. Optimal geometric parameters defining the micro-channel were obtained for a pressure drop ranging from 10 kPa to 60 kPa corresponding to a dimensionless pressure drop of 6.5 × 107 to 4 × 108 for fixed volumes ranging from 0.7 mm3 of 0.9 mm3. The effect of pressure drop on the aspect ratio, solid volume fraction, channel hydraulic diameter and the minimized peak temperature are reported. Results also show that as the dimensionless pressure drop increases the maximised dimensionless global thermal conductance also increases. These results are in agreement with previous work found in literature

Designed optimum geometry for micro-channel and micro pin fins heat sinks cooling technology

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.dc201

Numerical modelling and optimisation of natural convection heat loss suppression in a solar cavity receiver with plate fins

This study details the numerical modelling and optimization of natural convection heat suppression in a solar cavity receiver with plate fins. The use of plate fins attached to the inner aperture surface is presented as a possible low cost means of suppressing natural convection heat loss in a cavity receiver. In the first part of the study a three-dimensional numerical model that captures the heat transfer and flow processes in the cavity receiver is analyzed, and the possibilities of optimization were then established. The model is laminar in the range of Rayleigh number, inclination angle, plate height and thickness considered. In the second part of the study, the geometric parameters considered were optimized using optimization programme with search algorithm. The results indicate that significant reduction on the natural convection heat loss can be achieved from cavity receivers by using plate fins, and an optimal plate fins configuration exit for minimal natural convection heat loss for a given range of Rayleigh number. Reduction of up to a maximum of 20% at 0 receiver inclination was observed. The results obtained provide a novel approach for improving design of cavity receiver for optimal performance.Department of Mechanical and Aeronautical Engineering, University of Pretoria and the National Research Foundation.http://www.elsevier.com/locate/renene2016-02-28hb201

Thermodynamic optimisation and computational analysis of irreversibilities in a small-scale wood-fired circulating fluidised bed adiabatic combustor

An analysis of irreversibilities generated due to combustion in an adiabatic combustor burning wood was conducted. This was done for a reactant mixture varying from a rich to a lean mixture. A non-adiabatic non-premixed combustion model of a numerical code was used to simulate the combustion process where the solid fuel was modelled by using the ultimate analysis data. The entropy generation rates due to the combustion and frictional pressure drop processes were computed to eventually arrive at the irreversibilities generated. It was found that the entropy generation rate due to frictional pressure drop was negligible when compared to that due to combustion. It was also found that a minimum in irreversibilities generated was achieved when the AireFuel mass ratio was 4.9, which corresponds to an equivalence ratio of 1.64, which are lower than the respective AireFuel mass ratio and equivalence ratio for complete combustion with theoretical amount of air of 8.02 and 1.National Research Foundation (NRF), University of Pretoria and the Council for Scientific and Industrial Research (CSIR)http://www.elsevier.com/locate/energyhb201

Numerical analysis of thermal performance of an externally longitudinally finned receiver for parabolic trough solar collector

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.A receiver is a central component of the parabolic trough collector system. Its design and state greatly affects performance of the entire collector system. In this paper a three dimensional numerical model of a receiver with an externally longitudinally finned absorber tube was developed to study the effect of fin dimensions on heat transfer and fluid flow performance characteristics. Results show that external longitudinal fins improve the performance of the receiver. Results further show that collector efficiency and useful heat gain will increase as the fin thickness and height increase and so do heat losses, glass temperature and maximum absorber temperature. A 6% increase in efficiency and a pressure drop of 370.65Pa/m are obtained for a finned absorber tube with lower dimensions 1.0 cm x 0.2 cm when compared to a non-finned tube larger in diameter by 1 cm. An increase in efficiency of 8% is obtained when the same finned absorber tube is compared with a non-finned tube of the same size.dc201

Heat transfer and thermodynamic performance of a parabolic trough receiver with centrally placed perforated plate inserts

In this paper, a numerical investigation of thermal and thermodynamic performance of a receiver for a parabolic trough solar collector with perforated plate inserts is presented. The analysis was carried out for different perforated plate geometrical parameters including dimensionless plate orientation angle, the dimensionless plate spacing, and the dimensionless plate diameter. The Reynolds number varies in the range 1.02×104 ≤ Re ≤ 7.38 × 105 depending on the heat transfer fluid temperature. The fluid temperatures used are 400 K, 500 K, 600 K and 650 K. The porosity of the plate was fixed at 0.65. The study shows that, for a given value of insert orientation, insert spacing and insert size, there is a range of Reynolds numbers for which the thermal performance of the receiver improves with the use of perforated plate inserts. In this range, the modified thermal efficiency increases between 1.2 – 8 %. The thermodynamic performance of the receiver due to inclusion of perforated plate inserts is shown to improve for flow rates lower than 0.01205 m3/s. Receiver temperature gradients are shown to reduce with the use of inserts. Correlations for Nusselt number and friction factor were also derived and presented.NRF, TESP, and Stellenbosch University/University of Pretoria, SANERI/SANEDI, CSIR, EEDSM Hub and NAC.http://www.elsevier.com/locate/apenergyhb201

Numerical investigation of the effect of slope errors and specularity errors on the thermal performance of a solar parabolic trough collector system

Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.The optical efficiency of a parabolic trough collector is one of the most important parameters that affect the performance of the entire collector system. In this study, an optical and thermal analysis of a parabolic trough collector system for different slope errors and specularity errors is presented. For optical analysis, the influence of slope errors, specularity errors and system geometry on heat flux distribution on the receiver's absorber tube is determined using Monte-Carlo ray tracing in SolTrace. The ray tracing results are then coupled with a computational fluid dynamics code to investigate the thermal performance of the receiver. Results show that the slope and specularity errors influence the heat flux distribution on the receiver's absorber tube. The optical efficiency of the collector is shown to reduce significantly as the slope errors increase and slightly as specularity errors increase. The influence of slope errors and specularity errors on the thermal performance of the entire system is investigated numerically and presented.dc201

Maximum heat transfer rate density from ducts with wrinkled entrances

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.This paper reports an increase in the heat transfer rate density by using wrinkled entrance regions in ducts with laminar flow. The heat transfer rate density is increased by taking advantage of the presence of relatively isothermal fluid in the entrance regions. In order to stimulate a more complete thermal interaction between walls and fluid, the square entrances are wrinkled on the perimeter, at the one-third and the two-third positions. The new structure has two degrees of freedom. The fluid flow through the ducts is forced by the imposed pressure difference across the duct. Numerical simulations document the effects of the dimensionless pressure drop on the optimized configurations and show a fifteen percent enhancement in heat transfer rate density.vk201

Three-dimensional analysis and numerical optimization of combined natural convection and radiation heat loss in solar cavity receiver with plate fins insert

The numerical study and optimization of combined laminar natural convection and surface radiation heat transfer in solar cavity receiver with plate fins is presented in this paper. Minimizing heat loss in cavity receivers is seen as an effective way to enhance the thermal performance and the use of plate fins has been proposed as a low cost means to minimize heat loss. Firstly, the influence of operating temperature, emissivity of the surface, orientation and the geometric parameters on the total heat loss from the receiver was investigated. It was observed that convective heat loss is largely affected by the angle of inclination of the receiver, the presence of fins and the number of fins in the receiver. As for the radiation heat loss it was observed that it is mainly influenced by the properties of the cavity receiver surface. The radiation heat loss was found to be constant at all the angles of the receiver. Significant reduction in natural convection heat loss from the cavity receiver was accomplished by using the plate fins whereas radiation heat loss was marginally reduced by about 5%. Secondly, the optimization was conducted to obtain the optimal fin geometry and lastly, the overall thermal efficiency of the receiver was presented at different operating temperatures. The overall cavity efficiency marginally increased by approximately 2% with the insertion of fin plates although the convective heat loss was suppressed by about 20%. This is due to the fact that radiation heat loss dominates at high operating temperatures compared to convective heat loss.National Research Foundation and University of Pretoriahttp://www.elsevier.com/locate/enconman2016-09-30hb201