Tunde Bello-Ochende

Combined Numerical Optimization and Constructal Theory for the Design of Microchannel Heat Sinks

This study deals with the geometric optimization of a silicon based microchannel heat sink using a combined numerical optimization and constructal theory. The objective is to minimize the wall peak temperature subject to various constraints. The numerical simulations are carried out with fixed volumes ranging from 0.7 mm3 to 0.9 mm3 and pressure drop between 10 kPa to 60 kPa. The effect of pressure drop on the optimized aspect ratio, solid volume fraction, hydraulic diameter, and the minimized peak temperature are reported. Results also show that as the dimensionless pressure drop increases the maximized global thermal conductance also increases.

Optimal Spacings for Mixed Convection

We complete the description of geometry optimization in stacks of parallel plates that generate heat. The spacing between plates, or the number of plates in a fixed volume, has been maximized in two limits: pure natural convection and pure forced convection. The in-between regime of mixed convection is modeled numerically. After simulating the flow and temperature fields in configurations with a variety of spacings, we report the optimal spacings and the dimensionless groups that govern them (Rayleigh number, pressure drop number, mixed convection ratio)

The constructal-law physics of why swimmers must spread their fingers and toes

Here we show theoretically that swimming animals and athletes gain an advantage in force and speed by spreading their fingers and toes optimally. The larger force means larger body mass lifted and greater speed, in accord with the constructal theory of all animal locomotion. The spacing between fingers must be twice the thickness of the boundary layer around one finger. This theoretical prediction is confirmed by computational fluid dynamics simulations of flow across two and four cylinders of diameter D. The optimal spacing is in the range 0.2D-0.4D, and decreases slightly as the Reynolds number (Re) increases from 20 to 100. For example, the total force exerted by two optimally spaced cylinders exceeds by 53% the total force of two cylinders with no spacing when Re=20. These design features hold for both time-dependent and steady-state flows.

Mathematical Optimization: Application to the Design of Optimal Micro-channel Heat Sinks

This paper documents the geometrical optimization of a micro-channel heat sink. The objective is to optimize (minimize) the wall peak temperature of the heat sink subject to various constraints such as manufacturing restraints, fixed pressure drop and total fixed volume. A gradient based 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 for a given fixed volume. 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.

HEAT TRANSFER ENHANCEMENT IN A PARABOLIC TROUGH RECEIVER USING WALL DETACHED TWISTED TAPE INSERTS

In this paper, heat transfer and fluid friction performance of a parabolic trough receiver with twisted tape inserts detached from the absorber tube’s wall is numerically studied. The numerical investigations were conducted for twist ratios in the range 0.30 ≤ y ≤ 2.40, width ratios in the range 0.53 ≤ w ≤ 0.91 and Reynolds numbers in the range 10,260 ≤ Re ≤ 320,000. The numerical simulations were performed using a finite volume method with the realisable k-e turbulence model and Syltherm 800 as the heat transfer fluid. The use of twisted tape inserts shows a significant increase in the heat transfer and fluid friction performance of the receiver. The study also reveals significant reduction in absorber tube’s circumferential temperature difference due to the improved heat transfer performance. For the range of parameters considered, the Nusselt number, fluid friction and thermal enhancement factor are 1.01–3.36, 1.32–21.8, and 0.74–1.25 times those in a receiver with a plain absorber tube respectively. The absorber tube’s circumferential temperature difference reduces between 4–76% compared with a plain absorber tube. Correlations for Nusselt number and fluid friction are also reported for the range of parameters considered.Copyright

Optimum Small-Scale Open and Direct Solar Thermal Brayton Cycle for Pretoria, South Africa

The open and direct solar thermal Brayton cycle is exposed to various weather conditions like changing solar irradiation, wind and surrounding temperature. The geometries of the receiver and recuperator and the turbine operating point as parameter can be optimised in such a way that they accommodate these weather changes and allow for high net power output throughout a typical year. In this paper, a method of obtaining these parameters based on total entropy generation minimisation is presented. A parabolic dish concentrator with a diameter of 4.8 m is used as well as an off-the-shelf turbo-machine. Results show that the net absorbed power at the receiver and maximum allowed receiver surface temperature play important roles in determining the optimum operating point and maximum net power output.Copyright

Geometric Optimisation of Forced Convection in Cooling Channels With Internal Heat Generation

This paper presents a three dimensional geometric optimization of cooling channels in forced convection with internal heat generation within the solid. Three configurations were studied, circular channels, square channels and rectangular channels with different porosities. The configurations were optimized in such a way that the peak temperature is minimum. The optimization is subject to the constraint of fixed volume and solid material. The fluid is forced through the channels by the pressure difference across the channels. The structure has two degrees of freedom as design variables: channel hydraulic diameter and channel-to-channel spacing. The results obtained show the behaviour of the applied pressure drop on the optimized geometry. Results also show that as pressure drop increases the minimized peak temperature decreases.Copyright

Numerical Investigation into the Thermal Performance of Single Microchannels with Varying Axial Length and Different Shapes of Micro Pin-Fin Inserts

ABSTRACT This study investigates numerically the thermal performance of combined microchannel heat sink with micro pin-fins with different cross-sectional shapes. The objective of this study is to investigate the best geometric configuration that maximizes the heat transfer from the heated base when the combined heat sink is subjected to a steady, laminar, incompressible convective fluid flow and heat transfer. The axial length of the solid substrate and microchannel is varied from 1 to 10 mm with fixed total volume of 0.9 mm3 while the number of rows of the different shapes of micro pin-fins was varied between three and seven. It was observed that best performance is obtained with a sixth row of circular-shaped micro pin-fins for the optimized combination of the microchannel and micro pin-fin heat sink. Results of the optimal axial length for fixed pressure drop range are also presented.

Numerical investigation of natural convection of cavity receiver for low power application

ABSTRACT A parabolic dish/cavity receiver configuration is one of the solar thermal systems used for light-heat conversion at high temperature. Such systems are subject to continuous changes in ambient conditions such as wind, solar insolation, and ambient temperature. These environmental variations, as well as changes in receiver inclination angle, affect the overall receiver performance leading to energy loss. Natural convection contributes a significant fraction of the energy loss and hence a thorough understanding of its characteristics is essential to effectively minimize it in order to improve the system efficiency. A three-dimensional numerical investigation was conducted on a modified cavity receiver to quantify the convective components of the total heat loss and to determine the effects of the operating temperature, receiver inclination angle, and aperture size on the heat loss. The effects of the variation of air properties were accounted for by using polynomial relationships for density, specific heat capacity at constant pressure, dynamic viscosity, and thermal conductivity in the simulation. The calculated natural convection heat loss showed a nonlinear dependence on the inclination angle and aperture size. Visualization results such as temperature contours were also presented to gain an insight into the effects of natural convection.

Thermal Performance of a Parabolic Trough Receiver With Perforated Conical Inserts for Heat Transfer Enhancement

Heat transfer enhancement in receivers of parabolic trough collectors offers several benefits including reduction in absorber tube circumferential temperature differences, reduced emissivity of the absorber tube selective coating, thus improved thermal and thermodynamic performance of the receiver. In this work, heat transfer enhancement in a parabolic trough receiver using perforated conical inserts was numerically investigated. The analysis was carried out for dimensionless insert’s cone angles in the range 0.40–0.90, dimensionless insert spacing in the range 0.06–0.18 and dimensionless insert size in the range 0.45–0.91. The flow was considered fully developed turbulent with Reynolds numbers in the range 1.02 × 104 ≤ Re ≤ 7.38 × 105 depending on the temperature of the heat transfer fluid. The heat transfer fluid temperatures used were 400 K, 500 K, 600 K and 650 K. The numerical solution was obtained using the finite volume method together with the realizable k-e model for turbulence modeling. From the study, there is a range of Reynolds numbers and geometrical parameters for which the gain in performance is more than the increase in pumping power due to heat transfer enhancement. The use of perforated conical inserts in the receiver’s absorber tube increases the thermal efficiency in the range 3–8% for some range of geometrical parameters.Copyright