Heat Transfer

Over the past few decades there has been a prolific increase in research and development in area of heat transfer, heat exchangers and their associated technologies. This book is a collection of current research in the above mentioned areas and describes modelling, numerical methods, simulation and information technology with modern ideas and methods to analyse and enhance heat transfer for single and multiphase systems. The topics considered include various basic concepts of heat transfer, the fundamental modes of heat transfer (namely conduction, convection and radiation), thermophysical properties, computational methodologies, control, stabilization and optimization problems, condensation, boiling and freezing, with many real-world problems and important modern applications. The book is divided in four sections : "Inverse, Stabilization and Optimization Problems", "Numerical Methods and Calculations", "Heat Transfer in Mini/Micro Systems", "Energy Transfer and Solid Materials", and each section discusses various issues, methods and applications in accordance with the subjects. The combination of fundamental approach with many important practical applications of current interest will make this book of interest to researchers, scientists, engineers and graduate students in many disciplines, who make use of mathematical modelling, inverse problems, implementation of recently developed numerical methods in this multidisciplinary field as well as to experimental and theoretical researchers in the field of heat and mass transfer

THE EFFECTS OF FLOW MALDISTRIBUTION ON THE THERMAL PERFORMANCE DEGRADATION OF FIN-TUBE HEAT EXCHANGERS

Maldistribution of flow in a heat exchanger has an adverse effect on its thermal and hydraulic performance. Not only does the heat duty reduce but the fluid pressure drop across the exchanger increases too. The characteristics of a maldistribution profile are described by its four statistical moments of probability density function, viz. mean, standard deviation, skew and kurtosis. A novel mathematical analysis technique has been developed to demonstrate the influence of these statistical moments on the heat transfer and pressure drop performance of an exchanger. The analysis has shown that both the mean and standard deviation have the highest degradation effect on the heat exchanger performance while subsequent higher moments have declining effects until the fourth moment kurtosis, which has no significant effect. A discretized numerical method was then used on the fin-tube heat exchanger coil to calculate the magnitudes of thermal degradation as the statistical moments of the air inlet velocity distribution and geometrical parameters of the exchanger are systematically changed. The results show that the degradation is not only dependent on the moments but also on the exchanger NTU, ratio of external to internal heat transfer coefficients, R, and the number of tube rows in the coil. Consequently, new correlation equations have been developed to predict the magnitude of deterioration from a known air velocity maldistribution profile, for a given heat exchanger geometry. An experimental test rig was fabricated to validate the correlation equations. The same experimental data were then used to validate the Computational Fluid Dynamics (CFD) model of the fin-tube heat exchanger. With the same modelling technique, simulations with various exchanger geometry and layout designs can be performed to extract the statistical moments of the maldistribution and predict the heat exchanger performance. By doing so, the design could be optimized to find the lowest possible degradation effects. Maldistribution with low standard deviation and high positive skew is seen to give low thermal performance deteriorations

Efficient modeling of latent thermal energy storage systems for optimal design and operational research

As a booming economy drives the need for more electricity, demands on freshwater for thermoelectric power generation also grow. Facing the limited freshwater resources, alternative dry cooling technologies such as air cooled condensers (ACCs) that reduce water consumption are becoming more prevalent. However, the performance of air ACCs is seriously deteriorated at ambient temperature. To address this challenge, a novel application of a Phase Change Material (PCM) based cooling system for supplementary cooling of ACCs is proposed. One of the engineering challenges that prevent the commercial application of latent thermal energy storage (LTES) systems is the lack of computationally efficient methods to model the transient and nonlinear behavior of the system for design and operational research. In this dissertation, efficient modeling approaches of an LTES system are proposed for optimal design of the PCM based cooling system. Also large scale modeling of such a LTES system is proposed for operational research

Aeronautical engineering: A continuing bibliography with indexes (supplement 253)

This bibliography lists 637 reports, articles, and other documents introduced into the NASA scientific and technical information system in May, 1990. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

Computational Investigation of Swirling Jet Impingement in a Concentrated Solar Tower Receiver

With growing concern of climate change and environmental pollution the need for better renewable technologies is a necessity. Solar energy shows the most promise in meeting global energy needs and competing with fossil fuels economically. Currently solar power is generated with photovoltaic (PV) panels and stored in batteries. The disadvantages of PV are expensive batteries, limitations on panel efficiencies and electrical grid considerations to balance electricity generation. Concentrated solar power (CSP) is an alternative that addresses PV limitations and shows potential in a hybrid power generation mix, especially because of its thermal storage capabilities and ability to provide process heat directly. CSP consists of a variety of systems. Of all available CSP technologies, solar power towers (SPT) show potential to reach high temperatures and effectively store thermal energy. For SPT the central receiver shows promise for improvement in effectively capturing heat. Of the many methods available to improve heat transfer, jet impingement with swirl can improve heat transfer for the receiver fluid. Jet impingement heat transfer is well known to enhance local heat transfer because of the local increase in the heat transfer coefficient and Nusselt number. Swirling flows have also shown to enhance heat transfer for internal pipe flow arrangements and other heat transfer applications. The effect of swirl and jet impingement are not often considered cumulatively as in the current study. For a proposed solar receiver design, a swirling impinging jet is proposed to enhance heat transfer. The flow behaviour is investigated numerically using computational fluid dynamics (CFD). Ansys Fluent is used to model the flow behaviour and to validate the model with available experimental results. From the validation study the Transition Shear-Stress-Transport turbulence model is shown to predict jet impingement the best. A 2D axisymmetric assumption is however shown to not predict the heat transfer well while a costly full 3D transient Large Eddy simulation does. As LES is too expensive for use in a parametric investigation, both 2D and 3D RANS simulations were used as an engineering tool to improve and optimise heat transfer, keeping in mind their shortcomings. Swirling jet impingement is further investigated for a curved impingement surface. This is the first investigation of its kind where swirl, jet impingement and a curved impingement surface are considered. From the validation study, a CFD model is used to investigate how curvature affects heat transfer. The parameters show that surface curvature has a large effect on heat transfer and it is shown that a potential optimal curvature exists for the unique flow arrangement. A surrogate optimisation model is used from the numerical results to improve the design. To provide a realistic heat source on the solar receiver, Monte Carlo ray tracing (MCRT) is used to model the heliostat field. The MCRT model can better predict the solar flux distribution on the receiver absorbing surface. The solar flux distribution is an important consideration for the receiver design. The CFD model of the receiver showed that while swirling jet impingement did not increase the outlet temperature of the heat transfer fluid, it did however show potential to reduce the receiver’s maximum surface temperature and as well as radiation losses. The thermal enhancements made do however come at the cost of an increased pressure drop.Dissertation (MEng)--University of Pretoria, 2021.Mechanical and Aeronautical EngineeringMEngUnrestricte

Enhancing the performance of concentrating photovoltaics through multi-layered microchannel heat sink and phase change materials

Concentrating Photovoltaic technology is considered now as a promising option for solar electricity generation along with the conventional flat plate PV technology especially in high direct normal irradiance areas. However, the concentrating photovoltaic industry sector still struggles to gain market share and to achieve adequate economic returns due to challenges such as the high temperature of the solar cell which causes a reduction its efficiency. The work presented in this thesis is targeted to influence the overall performance of a high concentrated photovoltaic system by integrating both the multi-layered microchannel heat sink technique and a phase change material storage system. The proposed integrated system is composed of a multi-layered microchannel heat sink attached to a single solar cell high concentrated photovoltaic module for thermal regulation purposes. This is expected to reduce the solar cell temperature hence increasing the electrical output power. The high concentrated photovoltaic and multi-layered microchannel heat sink system is then connected to a phase change material thermal storage system to store efficiently the thermal energy discharged by the high concentrated photovoltaic and multi-layered microchannel heat sink system. The first part of the thesis discusses the influence of the multi-layered microchannel heat sink on the high concentrated photovoltaic module using both the numerical and experimental approaches. The multi-layered microchannel heat sink has been integrated for the first time with the single cell receiver and tested successfully. A numerical analysis of the high concentrated photovoltaic and multi-layered microchannel heat sink system shows the potential of the heat sink to reduce the solar cell maximum temperature and its uniformity. The thermal behaviour of the multi-layered microchannel heat sink under non-uniform heat source was experimentally investigated. The results show that in extreme heating load of 30W/cm² and in heat transfer fluid flow rate of 30ml/min, increasing the number of layers from 1-layer to 4-layers reduced the heat source temperature from 88.55°C to 73.57°C, respectively. In addition, the single layer multi-layered microchannel heat sink suffers of the most heat source temperature non-uniform compared to the heat sinks with higher number of layers. Also, the results show that increasing the number of layers from 1-layer to 4-layers reduced the pressure drop from 16.6mm H2O to 3.34 mm H2O. The indoor characterization of the high concentrated photovoltaic and multi-layered microchannel heat sink system investigated the effect of the number of layers, the homogeniser materials, and the heat transfer fluid flow rate and inlet temperature on the electrical and thermal performance of the system. The results show that the maximum power of the high concentrated photovoltaic module with glass homogeniser is 3.46W compared to 2.49W when using the crystal resin homogeniser for the 2-layers multi-layered microchannel heat sink and 30ml/min under 1000W/m² irradiance intensity. Increasing the number of layers from 1-layer to 3-layers on the high concentrated photovoltaic and multi-layered microchannel heat sink system increased the maximum electrical power by 10% and decreased the solar cell temperature 3.15°C for the heat transfer fluid flow rate of 30ml/min. This gives an increase in the maximum electrical power of 98.4mW/°C. The outdoor characterisation of the high concentrated photovoltaic and multi-layered microchannel heat sink system performance was evaluated at the University of Exeter, Penryn Campus, UK. The achieved maximum output electrical power of the system was 4.59W, filling factor of 75.1%, short circuit current of 1.96A and extracted heat of 12.84W which represents of 74.9% of the maximum solar irradiance of 881W/m². In addition, the maximum solar cell temperature reached to 60.25°C. Secondly, the experimental studies were carried out in order to investigate the performance of the phase change material storage system using paraffin wax as the PCM materials. The thermal storage system performance was evaluated in various conditions. The results show that inclination of the phase change material storage influences the melting behaviour of the phase change material where the phase change material storage of 45º inclination position melts faster than the phase change material storages in the 0º and 90º inclination positions. The phase change material melting time is reduced in the PCM storage of 45º inclination position by 13% compared to the 0º inclination position. The last part of the thesis discusses the integration of the phase change material storage with the high concentrated photovoltaic and multi-layered microchannel heat sink system. A 3D numerical model was developed to predict the behaviour of the integrated high concentrated photovoltaic and multi-layered microchannel heat sink system with the phase change material storage system using variable source conditions. The results show a higher heat absorption rate on phase change material storage that uses a lower melting temperature phase change material compared to the higher phase change material melting temperature. The multi-stages storage with different phase change materials melting temperature showed a lower heat absorption compared to the phase change material arrangement with the lower melting temperature. Also, the rate of the absorbed heat fluctuation is less affected by the phase change material arrangement with higher melting temperature

Training Manual on Advances in Marine Fisheries in India

Training Manual on Advances in Marine Fisheries in Indi

Proceedings of the First International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

1st International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Kruger Park, 8-10 April 2002.This lecture is a principle-based review of a growing body of fundamental work stimulated by multiple opportunities to optimize geometric form (shape, structure, configuration, rhythm, topology, architecture, geography) in systems for heat and fluid flow. Currents flow against resistances, and by generating entropy (irreversibility) they force the system global performance to levels lower than the theoretical limit. The system design is destined to remain imperfect because of constraints (finite sizes, costs, times). Improvements can be achieved by properly balancing the resistances, i.e., by spreading the imperfections through the system. Optimal spreading means to endow the system with geometric form. The system construction springs out of the constrained maximization of global performance. This 'constructal' design principle is reviewed by highlighting applications from heat transfer engineering. Several examples illustrate the optimized internal structure of convection cooled packages of electronics. The origin of optimal geometric features lies in the global effort to use every volume element to the maximum, i.e., to pack the element not only with the most heat generating components, but also with the most flow, in such a way that every fluid packet is effectively engaged in cooling. In flows that connect a point to a volume or an area, the resulting structure is a tree with high conductivity branches and low-conductivity interstices.tm201

Supramolecular functionalization of graphene related materials for heat transfer applications and devices

L'abstract è presente nell'allegato / the abstract is in the attachmen