FLOW ORIENTATION IN CONJUGATE COOLING CHANNELS WITH INTERNAL HEAT GENERATION

ABSTRACT This work presents a three-dimensional geometric optimisation of conjugate cooling channels in forced convection with internal heat generation within the solid for an array of circular cooling channels with different flow orientations based on constructal theory. Three flow orientations were studied: Firstly, an array of channels with parallel flow; secondly, an array of channels in which flow of the every second row is in a counter direction to one another and thirdly, with the every flow in the array of channels in counter direction to one another. The geometric configurations and the flow orientations were optimised in such a way that the peak temperature was minimised subject to the constraint of fixed global volume of solid material. The cooling fluid was driven through the channels by the pressure difference across the channel. The system had hydraulic diameter and channel to channel spacing as degrees of freedom of the design variables. A gradient-based optimisation algorithm was applied to search for the best optimal geometric configurations that improve thermal performance by minimising thermal resistance for a wide range of dimensionless pressure differences. This optimiser adequately handles the numerical objective function obtained from numerical simulations. The effect of porosities, applied pressure difference, flow orientation and heat generation rate on the optimal hydraulic diameter and channel to channel spacing were reported. Results obtained show that the effects of dimensionless pressure drop on minimum thermal resistance were consistent with those obtained in the open literature

Natural Convection Heat Transfer and Entropy Generation Analysis in Saltbox Roof under Summer Conditions

This study investigates numerically the 2D laminar natural convection in a Saltbox roof type geometry under summer climate conditions as obtained in Africa, particularly Nigeria using ANSYS FLUENT to model the boundary conditions. The effects of Rayleigh number (Ra) within the range of 103-107 and pitch angles (top and base) on heat transfer, the flow structure, temperature distribution and entropy generation within the geometry were analysed. Results show that the flow is nearly symmetric at lower Ra, while for higher Ra, the flow becomes asymmetric. The Nusselt number (Nu) has a proportional relationship with the top pitch angle and an inverse relationship with the base pitch angle when the Rayleigh number is fixed. The effect of the Ra on the Nu is insignificant at lower Ra, but becomes noticeable at higher Ra. The total entropy generation increases with an increase in top pitch angle and a decrease in base pitch angles, at fixed Rayleigh numbers. The physical implication is that, for a Saltbox roof type geometry, at fixed Ra, the best convective heat transfer process is achieved by lowering the base pitch angle and increasing the top pitch angle

Thermal energy processes in direct steam generation solar systems : boiling, condensation and energy storage

Direct steam generation coupled is a promising solar-energy technology, which can reduce the growing dependency on fossil fuels. It has the potential to impact the power-generation sector as well as industrial sectors where significant quantities of process steam are required. Compared to conventional concentrated solar power systems, which use synthetic oils or molten salts as the heat transfer fluid, direct steam generation offers an opportunity to achieve higher steam temperatures in the Rankine power cycle and to reduce parasitic losses, thereby enabling improved thermal efficiencies. However, its practical implementation is associated with non-trivial challenges, which need to be addressed before such systems can become more economically competitive. Specifically, important thermal-energy processes take place during flow boiling, flow condensation and thermal-energy storage, which are highly complex, multi-scale and multi-physics in nature, and which involve phase-change, unsteady and turbulent multiphase flows in the presence of conjugate heat transfer. This paper reviews our current understanding and ability to predict these processes, and the knowledge that has been gained from experimental and computational efforts in the literature. In addition to conventional steam-Rankine cycles, the possibility of implementing organic Rankine cycle power blocks, which are relevant to lower operating temperature conditions, are also considered. This expands the focus beyond water as the working fluid, to include refrigerants also. In general, significant progress has been achieved in this space, yet there remain challenges in our capability to design and to operate high-performance and low-cost systems effectively and with confidence. Of interest are the flow regimes, heat transfer coefficients and pressure drops that are experienced during the thermal processes present in direct steam generation systems, including those occurring in the solar collectors, evaporators, condensers and relevant energy storage schemes during thermal charging and discharging. A brief overview of some energy storage options are also presented to motivate the inclusion of thermal energy storage into direct steam generation systems

Constructal flow orientation in conjugate cooling channels with internal heat generation

WOS:000313466900026International audienceThis paper presents the development of the three-dimensional flow architecture of conjugate cooling channels in forced convection with internal heat generation within the solid for an array of circular cooling channels with different flow orientation. Three flow orientations were studied: array of channels with parallel flow; array of channels in which the flow in every second row is in a counter direction with its neighbours, and flows in all the arrays of channels are in counter flow relative to each other. The geometric configurations were determined in such a way that the peak temperature was minimised subject to the constraint of fixed global volume of solid material. The degrees of freedom of the design were hydraulic diameter and channel to channel spacing. A gradient-based optimisation algorithm was applied to search for the best optimal geometric configurations that improve thermal performance by minimising thermal resistance for a wide range of dimensionless pressure differences. The effect of porosities, applied pressure difference, flow orientation and heat generation rate on the optimal hydraulic diameter and channel to channel spacing is reported. The results show that the effects of dimensionless pressure drop on minimum thermal resistance were consistent with those obtained in the open literature. (C) 2012 Elsevier Ltd. All rights reserved

The influence of media properties, geometric and operational parameters on the thermal performance of bilayered composite cylinder

This paper presents the investigation of the conjugate heat transfer in a fluid conveying bilayered composite cylindrical pipe subjected to external convection. The problem is considered a two-dimensional problem in each of the three media. A finite difference scheme is employed to discretize the differential equations for the three media and conjugated at the interfaces. Codes were written in C-language which provided a fast solution to the models. The effects of the operational parameters (Peclet and Biot numbers), the media properties (pipe-to-fluid and laminate-to-pipe thermal conductivity ratios) and the geometric parameter (laminate-to-pipe thickness ratio) were investigated on the thermal performance – the Nusselt number of the composite pipe. These parameters and properties were found to influence the thermal performance to varying degrees significantly.10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, Chinahttp://www.elsevier.com/locate/procediaam2020Mechanical and Aeronautical Engineerin

Numerical Forced Convection Heat Transfer, Fluid Flow and Entropy Generation Analyses of Al2O3-Water Nanofluid in Elliptical Channels

This study investigates a three-dimensional elliptical microchannel heat sink for heat dissipation in laminar forced convection. The study seeks to improve thermal performance and overcome overheating associated with excessive temperature commonly experienced in heat-generating equipment, which is beyond the temperature usually specified by the manufacturer. The objective of the study is to evaluate the heat transfer, fluid flow, and entropy generation characteristics of Al2O3-water nanofluid in an elliptical cooling channel. The numerical analysis is investigated on the structure experiencing constant volumetric heat generation. The parameters considered are Reynolds number of 100 ≤ Re ≤ 500, nanoparticle concentration ϕ, from 0% to 4% with channel aspect ratio Ar from 1 to 3. The impacts of these parameters on the maximum temperature, heat transfer coefficient, friction factor, and volumetric entropy generation are reported. The study demonstrates that heat transfer is enhanced in the elliptical cooling channel at different aspect ratios, nanoparticle concentrations, and Reynold numbers. The results showed that as the nanoparticle concentration, channel aspect ratio, and Reynolds number (Re) increase, the maximum temperature, and total entropy generation decrease. As the channel aspect ratio increases at a specified Re = 200 and nanofluid concentration, ɸ = 3%, the maximum temperature, and total entropy generation decrease by up to 62% while the heat transfer coefficient increases by up to 78% and the friction factor increase by less than 2% with aspect ratio. However, the friction factor is not sensitive to the nanofluid concentration as a coolant

Thermal energy processes in direct steam generation solar systems : boiling, condensation and energy storage

Direct steam generation coupled is a promising solar-energy technology, which can reduce the growing dependency on fossil fuels. It has the potential to impact the power-generation sector as well as industrial sectors where significant quantities of process steam are required. Compared to conventional concentrated solar power systems, which use synthetic oils or molten salts as the heat transfer fluid, direct steam generation offers an opportunity to achieve higher steam temperatures in the Rankine power cycle and to reduce parasitic losses, thereby enabling improved thermal efficiencies. However, its practical implementation is associated with non-trivial challenges, which need to be addressed before such systems can become more economically competitive. Specifically, important thermal-energy processes take place during flow boiling, flow condensation and thermal-energy storage, which are highly complex, multi-scale and multi-physics in nature, and which involve phase-change, unsteady and turbulent multiphase flows in the presence of conjugate heat transfer. This paper reviews our current understanding and ability to predict these processes, and the knowledge that has been gained from experimental and computational efforts in the literature. In addition to conventional steam-Rankine cycles, the possibility of implementing organic Rankine cycle power blocks, which are relevant to lower operating temperature conditions, are also considered. This expands the focus beyond water as the working fluid, to include refrigerants also. In general, significant progress has been achieved in this space, yet there remain challenges in our capability to design and to operate high-performance and low-cost systems effectively and with confidence. Of interest are the flow regimes, heat transfer coefficients and pressure drops that are experienced during the thermal processes present in direct steam generation systems, including those occurring in the solar collectors, evaporators, condensers and relevant energy storage schemes during thermal charging and discharging. A brief overview of some energy storage options are also presented to motivate the inclusion of thermal energy storage into direct steam generation systems.Data supporting this publication can be obtained on request from ceplab@ imperial.ac.uk.The Department for International Development (DFID) through the Royal Society-DFID Africa Capacity Building Initiative and by the UK Engineering and Physical Sciences Research Council (EPSRC).http://www.frontiersin.org/Energy_Researcham2020Mechanical and Aeronautical Engineerin