Numerical modelling of the temperature distribution in a two-phase closed thermosyphon

Interest in the use of heat pipe technology for heat recovery and energy saving in a vast range of engineering applications has been on the rise in recent years. Heat pipes are playing a more important role in many industrial applications, particularly in improving the thermal performance of heat exchangers and increasing energy savings in applications with commercial use. In this paper, a comprehensive CFD modelling was built to simulate the details of the two-phase flow and heat transfer phenomena during the operation of a wickless heat pipe or thermosyphon, that otherwise could not be visualised by empirical or experimental work. Water was used as the working fluid. The volume of the fluid (VOF) model in ANSYS FLUENT was used for the simulation. The evaporation, condensation and phase change processes in a thermosyphon were dealt with by adding a user-defined function (UDF) to the FLUENT code. The simulation results were compared with experimental measurements at the same condition. The simulation was successful in reproducing the heat and mass transfer processes in a thermosyphon. Good agreement was observed between CFD predicted temperature profiles and experimental temperature data.The Saudi Cultural Bureau in London, the Ministry of Higher Education and the Mechanical Engineering Department, Umm Al-Qura University

Two Phase Flow, Phase Change and Numerical Modeling

The heat transfer and analysis on laser beam, evaporator coils, shell-and-tube condenser, two phase flow, nanofluids, complex fluids, and on phase change are significant issues in a design of wide range of industrial processes and devices. This book includes 25 advanced and revised contributions, and it covers mainly (1) numerical modeling of heat transfer, (2) two phase flow, (3) nanofluids, and (4) phase change. The first section introduces numerical modeling of heat transfer on particles in binary gas-solid fluidization bed, solidification phenomena, thermal approaches to laser damage, and temperature and velocity distribution. The second section covers density wave instability phenomena, gas and spray-water quenching, spray cooling, wettability effect, liquid film thickness, and thermosyphon loop. The third section includes nanofluids for heat transfer, nanofluids in minichannels, potential and engineering strategies on nanofluids, and heat transfer at nanoscale. The forth section presents time-dependent melting and deformation processes of phase change material (PCM), thermal energy storage tanks using PCM, phase change in deep CO2 injector, and thermal storage device of solar hot water system. The advanced idea and information described here will be fruitful for the readers to find a sustainable solution in an industrialized society

Building Thermal Envelope

This book results from a Special Issue published in Energies, entitled “Building Thermal Envelope"". Its intent is to identify emerging research areas within the field of building thermal envelope solutions and contribute to the increased use of more energy-efficient solutions in new and refurbished buildings. Its contents are organized in the following sections: Building envelope materials and systems envisaging indoor comfort and energy efficiency; Building thermal and energy modelling and simulation; Lab test procedures and methods of field measurement to assess the performance of materials and building solutions; Smart materials and renewable energy in building envelope; Adaptive and intelligent building envelope; and Integrated building envelope technologies for high performance buildings and cities

Two-phase heat transfer in multi-channel flat heat pipes

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonHeat pipes have recently been introduced as thermal absorbers for photovoltaic panels, with the objective of increasing the performance of Photovoltaic/Thermal (PV/T) technologies, which simultaneously produce electrical and thermal energy. To best fit surface cooling applications, advances in the heat pipe designs have been witnessed with the recent introduction of multi-channel flat heat pipes as efficient heat transfer mediums between photovoltaic cells and heat sink. Despite the promising experimental results observed, the complex two-phase heat transfer mechanisms taking place in multi-channel flat heat pipes are poorly understood and remain to be investigated. In addition to the lack of theory and analytical models considering the flat shape and multi-channel internal geometry, numerical modelling of heat pipes using computational fluid dynamic (CFD) technics is still at an early stage. In this regard, this study investigates thoroughly the two-phase heat transfer in a novel multi-channel flat heat pipe using three approaches: theoretical, numerical, and experimental. The main objectives of this research are as follows: 1) Provide a better understanding of two-phase heat transfer in a multi-channel geometry, 2) Develop an analytical model to predict the performance of a multi-channel flat heat pipe which considers the two-phase heat transfer mechanisms taking place in this new geometry, 3) Simulate the working cycle of multi-channel heat pipes using Computational Fluid Dynamics (CFD) techniques, and 4) Compare the developed analytical model and numerical simulations with experimental data. In this thesis, analytical, numerical, and experimental investigations of two-phase heat transfer in a novel multi-channel flat heat pipe are reported. Based on the two-phase heat transfer theory, a novel analytical model was proposed and used in an iterative tool to predict the performance of the multi-channel heat pipe. In addition, several in-house user-defined functions (UDFs) have been developed and tested to simulate the two-phase heat transfer in multi-channel heat pipes using the Lee model. To develop the analytical and numerical models, a unique three leg multi-channel heat pipe was built and tested. In a second phase, the developed models have been used to predict the thermal performance and simulate the two-phase heat transfer in a novel multi-channel flat heat pipe. The models have been compared to experimental findings from the multi-channel flat heat pipe apparatus for validation. The research demonstrated that the analytical model proposed can predict and describe the two-phase heat transfer that allows the novel multi-channel flat heat pipe to be one of the most efficient Photovoltaic/Thermal systems reported up to date. New opportunities for surface cooling applications using the promising multi-channel flat heat pipes are emerging

Multidisciplinary approach for the sustainable utilization of medium-low temperature geothermal resources

A growing interest in the applications of the medium-low temperature geothermal resources can be observed, but often a reference frame in this field does not exist. Different backgrounds are involved in the design and optimization of geothermal projects: Earth Sciences, Engineering, Economics, Environmental Impact. In the practice one aspect often tends to take priority on the others. In this work firstly the elements for a methodological interdisciplinary framework for geothermal projects analysis are given. A clear frame of the state of the art and possible technical-scientific developments are illustrated. The necessity of an "integrated" interdisciplinary approach is underlined, with reference to several case studies. The geothermal potential evaluation tasks and methods are illustrated, and some outlines for an assessment oriented to the resource utilization are described. The sustainability of geothermal projects is analysed under different perspectives and criteria. The main technological issues of geothermal binary cycle power plants are discussed (mainly for small power size), together with technological solutions. The concept of upper limit to the extraction rate is introduced, with reference to an equilibrium point between power production and resource depletion. Direct heat uses are also briefly described and the common issues related to environmental impact and resource durability are then discussed, with a particular focus on the scaling phenomena and the reinjection strategy. An innovative solution for geothermal power production (SBES) is described: the application of the heat pipe principle, in the CLTPT concept, for power production purposes is proposed. The numerical simulation of geothermal reservoirs is an important instrument for the synthesis between the different backgrounds involved in the geothermal energy study. General aspects and its potentialities (historical data matching and forecast of future utilization scenarios) are illustrated. Several numerical models from scientific literature are reviewed (about 21 geothermal fields and related 24 numerical models). The reservoir models of Momotombo (Nicaragua) and Sabalan (Iran) have been realised from literature data and widely discussed. One original model, Monterotondo Marittimo - Torrente Milia (Italy) has been realised in a very multidisciplinary framework and it is also presented. Different utilization scenarios for the case studies are analysed and their sustainability level is discussed. A purely economic approach is considered to be counter-productive for geothermal utilizations, so the thermoeconomic analysis is here applied to geothermal power plants. Momotombo case study and other small size power plants are analysed in order to estimate their thermoeconomic sustainability (exergy balances and cost items are evaluated). The current Italian geothermal energy market situation is briefly described, in relation to the ORC technology diffusion. The small size plants technological and environmental issues treated in this work are then linked to the current way of diffusion in the Italian market. The outlines coming from the global work of analysis are organised in order to give the main features of the proposed methodological approach. The multidisciplinary perspective is reviewed and extended in order to optimize the sustainability of the projects (environmental impact reduction, resource durability)

Theoretical And Experimental Analysis Of Two-Phase Closed Thermosyphons

Thesis (Ph.D.) University of Alaska Fairbanks, 2008This work presents an analytical and numerical model of a long inclined two-phase closed thermosyphon, known as a hairpin thermosyphon, which is representative of a new configuration for thermosyphons used in arctic applications. A laboratory experiment and a full scale road experiment along with associated modeling are described in detail. The laboratory experiment studies the condensation heat transfer performance of carbon dioxide inside the thermosyphon condenser under conditions of limited heat flux. The operating condition is not far from the critical point for carbon dioxide, which has a significant impact on the condensation heat transfer. An experimental correlation is developed to predict the carbon dioxide condensation heat transfer performance under these specific conditions. The full scale road experiment studies the overall performance of hairpin thermosyphons under actual field conditions. The model is a quasi one-dimensional formulation based on two-dimensional two-phase flow simulations at each cross section. The proposed model is useful for predicting steady state system operating characteristics such as pressure, temperature, liquid film thickness, mass flow rate, heat flow rate, etc., at local positions as well as over the entire system. The comparison of the modeling predictions with both laboratory and field experiments showed a strong correlation between modeling predictions and experimental results

Heat Transfer in Energy Conversion Systems

In recent years, the scientific community’s interest towards efficient energy conversion systems has significantly increased. One of the reasons is certainly related to the change in the temperature of the planet, which appears to have increased by 0.76 °C with respect to pre-industrial levels, according to the Intergovernmental Panel on Climate Change (IPCC), and this trend has not yet been stopped. The European Union considers it vital to prevent global warming from exceeding 2 °C with respect to pre-industrial levels, since this phenomenon has been proven to result in irreversible and potentially catastrophic changes. These climate changes are mainly caused by the emissions of greenhouse gasses related to human activities, and can be drastically reduced by employing energy systems, for both heating and cooling of buildings and for power production, characterized by high efficiency levels and/or based on renewable energy sources. This Special Issue, published in the journal Energies, includes 12 contributions from across the world, including a wide range of applications, such as HT-PEMFC, district heating systems, a thermoelectric generator for industrial waste, artificial ground freezing, nanofluids, and others

Full Analysis of Heat Pipes Including Fins and Nanofluids

Full numerical simulation of heat pipes was performed for heat pipes under various operating conditions with a variety of working fluids including fins and nanofluid. Two and three-dimensional models were developed assuming a laminar compressible vapor core. An advanced thermal resistance network for heat pipes was used along with the numerical model to identify dominant thermal resistances. Simulations were performed on heat pipes with external channel cooling around the condenser with and without external fins to determine the impact of individual heat pipe thermal resistances. It was found that the vapor core thermal resistance is significant as the operating temperature increases. The largest thermal resistances are those corresponding to the external heat sources and sinks. The numerical model was extended for use with nanofluid-filled heat pipes and accounted for flow in the wick; to determine the capillary limit and corresponding optimal nanoparticle concentration. A revised Merit number was proposed for nanofluid-charged heat pipes and used to quantify performance enhancements. Three nanoparticles were explored in this study Al2O3, TiO2 and CuO. The optimal nanoparticle concentration of Al2O3, TiO2 and CuO corresponding to the capillary limit for a conventional nanofluid-filled heat pipe was determined to be 25% by vol. for both Al2O3 and TiO2, and 35% for CuO. Overall, a maximum decrease in total thermal resistance was observed to be 83%, 79% and 76% for Al2O3, CuO and TiO2, respectively. Finally, a homogenous multiphase model was developed for simulation of a thermosyphon and some preliminary results were obtained for the operation of the condenser section

Towards the thermodynamic characterization of an hybrid Pulsating Heat Pipe in micro-gravity conditions through parabolic flight campaigns

In the past years, growing demand of high thermal performances in front of low costs has pushed the development of brilliant two-phase based devices. Among the most promising there is the Pulsating Heat Pipes (PHP), which is a bended evacuated tube partially filled with a working fluid that oscillates thanks to thermally driven pressure differences. PHP opens new possibilities in terms of performances, simplicity, maintenance, cost and gravity independence