Theoretical Heat Transfer Research Articles

Preface

The 37th UIT Heat Transfer Conference was organized by the Department of Industrial Engineering at University of Padova (Italy) and held at Centro Linguistico di Ateneo, Padova, on June 24-26, 2019.The annual UIT Conference, which has grown over time, was held in Padova for the first time since the foundation of this conference.The scope of the conference covers a range of many topics in theoretical, numerical and experimental heat transfer and related areas, ranging from energy efficiency to nuclear plants. This time the UIT Conference included an International Symposium on “Refrigerants: heat transfer and applications”. This Symposium was held on June 26, 2019, which was the first World Refrigeration Day, and was organized in collaboration with CNR, Istituto per le Tecnologie della Costruzione.In this conference edition, 99 papers were contributed plus three keynote lectures. The keynote lectures were given by Srinivas Garimella (Georgia Institute of Technology, USA), Alberto Cavallini (Emeritus professor at University of Padova) and Enrico Nobile (University of Trieste). For the Symposium on Refrigerants, invited lectures were given by Pega Hrnjak (University of Illinois at Urbana-Champaign, USA), Bjorn Palm (Royal Institute of Technology, Sweden), Petter Neksa (Norwegian University of Science and Technology, Norway) and José Miguel Corberan (Universidad Politecnica de Valencia, Spain).These proceedings collects two keynote lectures and 58 papers selected from those presented and discussed during the Conference.The conference was an important occasion to stimulate discussion, improve the understanding of heat transfer and related phenomena, present the state of the art of some topics, discuss emerging trends and promote collaborations.A special thank is due to the Organizing and Scientific Committees, to the Sponsors, to the Reviewers of the papers and to all the Participants.Davide Del Col, Luisa Rossetto and Stefano Bortolin - Editors List of Scientific Committee, Organizing Committee and Sponsors are available in this PDF.

Simulation and multi-objective optimization of heat and mass transfer in direct contact membrane distillation by response surface methodology integrated modeling

Membrane distillation (MD) traditionally utilizes low-grade thermal energy to drive vapor transport through hydrophobic membrane pores for water treatment. The concurrent enhancement of water flux, thermal efficiency and water productivity to achieve the global optimization of MD is still a challenge. In this work, a new simulation and optimization approach is proposed by integrating theoretical heat and mass transfer models with response surface methodology to investigate the complicate interaction effects of various influencing factors on heat and mass transfer parameters, including overall mass transfer coefficient, feed/permeate side heat transfer coefficients, membrane surface temperature and temperature polarization coefficient, in direct contact membrane distillation (DCMD). The operating and module configuration variables show important interaction effects on enhancing the heat and mass transfer in the membrane module. The significantly elevated heat transfer coefficient on account of the combined high feed temperature, short module length and high feed velocity leads to a great improvement of water flux and thermal efficiency. The ultimate global optimum conditions for the high water productivity were determined where the high levels of heat transfer coefficient (5905W/(m2·K)), temperature polarization coefficient (0.66), water flux (63.0kg/(m2·h)) and thermal efficiency (0.83) all synchronously approach to their own pinnacles. The results indicate that it is an effective way to take advantages of the interaction effects of operating conditions with module configuration parameters to achieve the overall enhancement of MD performance.

Sustainable Sizing of Geothermal Power Plants: Appropriate Potential Assessment Methods

The paper analyzes the problem of defining the potential of geothermal reservoirs and the definition of a sustainable size of a geothermal power plant in the preliminary design phase. While defining the size of a geothermal plant, the objective is to find a compromise between renewability, technical sustainability, and economic return-related issues. In the first part of the paper the simplified lumped parameter approach based on the First-Order methods and their further evolutions and limitations is proposed. Experimental data available for some geothermal reservoirs are used for critical analysis of the simplified approaches for estimating the renewability and sustainability of the production of geothermal plants. In the second part the authors analyze methods based on theoretical heat transfer analysis supported by experimental data acquired from the geothermal field (thermal properties of the rock, porosity of the reservoir, and natural heat flux) and finally consider the numerical simulation as a method to connect the two approaches discussed before. The sustainability of geothermal power production can be estimated taking into account the energy stored in the reservoir and the thermal and fluid dynamic analysis of the reservoir. From this perspective, the numerical simulation of the reservoir can be considered as an effective method for the estimation of a sustainable mass flow rate extraction. Some specific cases are analyzed and discussed.

Effect of non-condensable gas on thermal characteristics in two-phase closed thermosyphon

With the development of building energy saving technology, low-temperature two-phase closed thermosyphon (TPCT) will be widely used in building energy supply and storage. However, the non-condensable gas (NCG) can affect the performance of TPCT. In this paper, experiment, theoretical heat transfer model and CFD simulation model have been established to comprehensive analyze the operation performance of the TPCT with NCG (NTPCT). The research contents include the variation of TPCT wall temperature, heat transfer efficiency (η), thermal resistance (R) and NCG phase distribution. Experimental results showed that the temperature difference between the evaporator and condenser sections of the NTPCT was greater than that of the TPCT in the steady state, resulting in RexpNTPCT being higher than RexpTPCT, and the difference was widening with the decrease of input temperature (Tb), leading to serious adverse impact on heat transfer. When Tb was higher than 60 °C, the ηtheory difference with different mass of NCG (XNCG=0.0418%~0.0746%) was small, but when Tb was low, the difference became larger, and even whenTb = 27 °C, theηtheory = 44.02% forXNCG = 0.0418%, andηtheory = 0.04% forXNCG = 0.0746%, i.e., the heat pipe failed to work. In addition, with the decrease of inclination angle, NCG diffused downward and occupied more and more length of the condenser section, resulting in worse and worse heat transfer performance.

Comparison of theoretical heat transfer model with results from experimental monitoring installed in a refurbishment with ventilated facade

One of the main points to consider when a building is renovated is the improvement of its energy efficiency, minimizing the heat loss through the enclosures and its heating consumption. Under this scope idea a ventilated facade was designed and incorporated in an educational building located in the city of Burgos (Spain). The main objective of this document is a comparison between the theoretical model of heat transfer across the building envelope separating the environment and the interior space, and the heat intake through a linear regression model with installed experimental monitoring. For this it has been necessary to carry out an exhaustive study of the thermal transmission of each one of the materials that make up the thermal envelope of the building, as well as the linear thermal bridges that can be produced before and after the renovation. In addition, thanks to the monitoring installed in the demonstrator building, the interior and exterior temperatures and the heat consumption of each of the radiators is known. In this way expected and real energy savings have been compared.

A heat transfer model for aluminum droplet/wall impact

The erosive burning in solid rocket motor is partly caused by heat flux increment effect of dense high-temperature droplet jet impact process, of which the basic physical process is single molten droplet impact solid wall. However, due to the complex process in combustion chamber, until now, the erosion mechanism under such conditions has not been scientifically described. In this study, with the combustion chamber condition being simulated, the bouncing process of single molten aluminum droplet/wall impact was obtained experimentally. The theoretical model about droplet contact time as well as the maximum spreading factor were modified based on the experiment data. The theoretical heat transfer model accounting for single aluminum droplet/wall impact process was established, with the assumption of a composite structure consisting of solid material and air cavities on the solid wall surface. The theoretical predicted magnitude order of heat flux between droplet and solid wall surface is approximately 103∼104 W/cm2, and the heat transferred to the wall is approximately 10−6∼10−5 J for single droplet impact.

Assessment of heat transfer correlations in the sub-channels of proposed rod bundle geometry for supercritical water reactor

There are heat transfer correlations for heat transfer analysis in single tube geometries after several experimental and theoretical heat transfer studies in these single tube geometries. This is not the case for heat transfer analysis in rod bundle geometry with regard to proposed square fuel assembly of the Supercritical-Water-Cooled Reactor (SCWR) European Atomic Energy (EURATOM) design. Thus limited heat transfer studies exist on rod bundle geometry at supercritical pressures. Heat transfer correlations with accurate prediction capabilities of coolant and wall temperatures will be helpful in carrying out heat transfer studies at supercritical pressures. This paper presents the performance of twelve selected heat transfer correlations assessed on the 1/8th bare square fuel assembly of the SCWR EURATOM design using Simulation of Turbulent flow in Arbitrary Regions Computational Continuum Mechanics C ++ based code (STAR-CCM + CFD code). The obtained numerical results were compared with the results obtained by Waata numerical experimentation. Overall, the Cheng et al. correlation provided the most satisfying prediction for the wall temperatures in all the sub-channels and captured closely Wataa's Numerical data. The maximum wall temperature was obtained in sub-channel 9, the hottest sub-channel and exceeded the design limit 620 °C by 60 °C for the Cheng correlation. The difference in temperature between the hottest and coldest sub-channels 9 and 1 respectively was approximately 80 °C. It was found that Cheng correlation is best suited for heat transfer prediction in rod bundle geometry at supercritical pressures with regard to the proposed square fuel assembly of the SCWR EURATOM design. It was also found that the different numerical tools adopted for this study and Waata study were able to capture the trends of normal, enhanced and deteriorated heat transfer regimes normally observed at supercritical pressures. Nevertheless, experimental investigations involving rod bundles adopted in this study should be conducted to validate the results obtained numerically and address the inconsistency of the conclusions drawn when compared with Waata data and other similar studies.

Theoretical and Experimental Heat Transfer in Solid Propellant Rocket Engine

Accurate determination of heat flux is an important task not only in the designing aspect, but also in the performance analysis of rocket engines. In this purpose, this work deals with the heat flux determination in a combustion chamber through the inverse method. In this approach, the transient heat flux is determined from the experimental temperature data measured at the outer sidewall of the rocket engine. In this work the physical phenomenon was modeled by the transient one-dimensional heat equation in cylindrical coordinates and the material properties of the chamber were considered constant. Furthermore, the model is solved using the inverse heat conduction problem with least squares modified by the addition of Tikhonov regularization term of zero-order. Moreover, the sensitivity coefficients were obtained by Duhamel’s theorem. Through the regularization parameter, it was able to generate acceptable results even when using data with considerable experimental errors.

Validation of Heat Transfer between Theoretical and Experimental from the Internal Surface of Vertical Tubes with Internal Rings Heated by Electrical Heating Coils

The comparison between experimental and theoretical heat transfer inside heated vertical channels that dissipate heat from the internal surface with and without internal rings is studied. The experimental setup consists of a circular pipe which is heated electrically by providing constant heat flux on the wall. The theoretical and experimental analysis is conducted in several pipes of same diameter but different lengths. The length of the pipe varies from 450 mm to 850 mm. The length to diameter ratios are taken as L/D = 10, 12.22, 15.56, and 18.89. The value of imposed heat flux varies from 250 to 3340 W/m2. The internal ring thickness varies from 4 mm to 8 mm. separation distance between the internal rings varies from 75mm to 300 mm. The theoretical results are compared with experimental data to ascertain numerical accuracy of the method. The effects of L/D ratio, thickness of internal rings and separation distance on the heat transfer performance are studied. The experimental result is compared with theoretical, theoretical results are found by using ANSYS. In this study theoretical result for wall temperature along the height of tube, fluid temperature at exit of tube are compared with experimental data.

The impact of thermal properties on performance of phase change based energy storage systems

Phase change materials (PCMs) are used commonly for thermal energy storage and thermal management. Typically, a PCM utilizes its large latent heat to absorb and store energy from a source. The rate of energy stored (W) and energy storage density (J/m3) over a certain time period are both important performance parameters of a phase change based energy storage system. While significant experimental research has been carried out to improve thermal conductivity of PCMs, there is a lack of theoretical understanding of how thermal conductivity and other thermophysical properties affect these performance parameters. This paper presents a theoretical heat transfer model to predict the rate of energy storage and energy storage density as functions of PCM thermal properties. Using perturbation method based techniques, expressions for these parameters are derived for two geometries, first for a simplified assumption of constant temperature at the source-PCM interface, and then for a more realistic scenario of time-dependent interface temperature. Results indicate that while increasing thermal conductivity results in improvement in rate of energy stored, the energy storage density does not change for a Cartesian system and actually decreases for cylindrical system. This shows that using a high thermal conductivity PCM may not be ideal when energy must be stored compactly because while this increases the total energy absorbed, it also results in greater rate of melting, which reduces the energy storage density. Results also provide guidelines for material selection for phase change based energy storage systems. For example, a trade-off in the choice between materials of disparate thermal properties is identified in terms of whether the rate of energy stored or energy storage density is paramount. Differences in the performance of Cartesian and cylindrical systems is investigated. Theoretical results presented in this work highlight various performance trade-offs related to the thermal properties of the PCM and help understand the impact of thermal conductivity enhancement on phase change energy storage performance.

Heat transfer performance of a porous copper micro-channel heat sink

A novel porous copper micro-channel heat sink based on the high thermal conductivity and structural stability of porous copper micro-channel material is proposed in this study. A circular cross-sectional and a multi-layer staggered arrangement are the main characteristics of the micro-channel heat sink. Additionally, a heat dissipation model is produced for porous copper micro-channel heat sink by analyzing the heat transfer process. Further, the theoretical heat transfer coefficient is calculated using MATLAB, and the experimental heat transfer coefficient is determined based on an experimental platform. The heat dissipation model is verified by comparing the theoretical and experimental heat transfer coefficients. The relation between the porous copper micro-channel heat sink parameters and the heat transfer coefficient that is obtained based on the heat dissipation model of the porous copper micro-channel heat sink is analyzed. The results exhibit that variations in pore diameter, porosity, porous copper length, porous copper width, porous copper height, and volume flow significantly affect the heat transfer performance.

Thermal Fatigue Modelling and Simulation of Flip Chip Component Solder Joints under Cyclic Thermal Loading

Thermal Fatigue of flip chip component solder joints is widely existing in thermal energy systems, which imposes a great challenge to operational safety. In order to investigate the influential factors, this paper develops a model to analyze thermal fatigue, based on the Darveaux energy method. Under cyclic thermal loading, a theoretical heat transfer and thermal stress model is developed for the flip chip components and the thermal fatigue lives of flip chip component solder joints are analyzed. The model based simulation results show the effects of environmental and power parameters on thermal fatigue life. It is indicated that under cyclic thermal loading, the solder joint with the shortest life in a package of flip chip components is located at the outer corner point of the array. Increment in either power density or ambient temperature or the decrease in either power conversion time or ambient pressure will result in short thermal fatigue lives of the key solder joints in the flip chip components. In addition, thermal fatigue life is more sensitive to power density and ambient temperature than to power conversion time and ambient air pressure.

Heat Exchange Structures Based on Copper/CNT Composite

In this study different types of multi-walled carbon nanotubes (MWCNT) were produced by the fixed bed and aerosol chemical vapor deposition (CVD) method. Nanocomposite materials were prepared by incorporation of different MWCNTs in copper matrix using powder metallurgy methods. By using hot pressing in combination with hot extrusion, the orientation of the carbon reinforcement was tuned from 3D to 1D alignment. After a selective etching process the carbon reinforcement is partially free-standing at the composite surface, but still embedded in the metal matrix. The engineered surface acts almost like a black body. The spectral evaluation of the surface functionalization will be shown for wavelengths from 200 nm to 20 µm. These results are compared to bulk copper. The free-standing MWCNT also behave like fins/pins in heat exchanger structures or surface enhancement in pool boiling. The experimental setup will be explained and the measurement described for pure copper. The theoretical heat transfer coefficient of the engineered surface is calculated depending on diameter and length of the free-standing MWCNTs. The results are compared to bulk copper.

Performance of Autonomous Temperature Controlled Photovoltaic Modules with Water Film

In order to reduce the surface temperature of solar cells and reduce the surface dust and improve the power generation efficiency of PV modules, this paper introduces a cooling device for PV modules based on temperature self-regulation, including PV power generation, circulating water and temperature control system. The theoretical heat transfer model of the experimental device during operation is established. Set the temperature control interval of the temperature control system, and compare the test with the same type of panel with the no-surface film flowing at the same inclination angle. The research showed that when the average ambient temperature is 21.61 °C, the average temperature of the panel without water film cooling is about 35.37 °C, and the average temperature of the panel with the water film cooling is about 25.53 °C, which achieves the purpose of cooling and temperature control. At the same time, the output power of the water film cooling panel is relatively increased by about 13.57%. In summary, the water film cooling device in this paper can effectively reduce and stabilize the temperature of the PV module, reduce surface dust and increase its output power.

How Surface Orientation Affects Jumping-Droplet Condensation

Summary The out-of-plane jumping of coalescing droplets on superhydrophobic surfaces is capillary-inertial in nature; therefore, the effects of gravity or surface orientation are generally ignored. Here, we show that surface orientation actually has a profound effect on jumping-droplet condensation. For a horizontally oriented superhydrophobic surface, jumping droplets return to the surface and eventually become stuck. In contrast, vertical orientations can remove stuck droplets by gravitational shedding, which also serves to sweep away neighboring condensate. These differences affect the resulting droplet size distribution, which was captured using time-lapse photography and image analysis for 3 h of jumping-droplet condensation on horizontal, 45° tilted, or vertically oriented surfaces. The maximum droplet size was an order of magnitude smaller for the inclined surfaces compared to the horizontal orientation, which would enhance the theoretical heat transfer coefficient by 40% for the 45° tilt and by 100% for the vertical orientation.

Theoretical and numerical investigation of embedded microfluidic thermal management using gradient distribution micro pin fin arrays

Targeted at improving overall hydro-thermal performance of microfluidic cooling, a novel embedded cooling configuration using gradient distribution micro pin fin arrays (GD-MPFA) is proposed. A moderate porosity range and fin diameter range are recommended by the optimization of theoretical and numerical 3D fluid-solid conjugate heat transfer simulations. To reveal the hydro-thermal characteristics and advantages of the GD-MPFA, the comparisons between the GD-MPFA and the uniform micro pin fin arrays design (U-MPFA) were made with different coolants. For coolant of DI Water and HFE-7100, GD-MPFA design demonstrates a step-rising and periodical-varying trend of temperature respectively, obviously eliminating maximum temperature difference. However, using a dielectric coolant (HFE-7100) exacerbates thermal management challenge with obviously increasing peak temperature. Therefore, a further investigation is still needed to low peak temperature and temperature difference for dielectric coolant at a higher heat flux under an acceptable pumping power.

The development of a thermosiphon photobioreactor and analysis using Computational Fluid Dynamics (CFD)

The energy input into closed photobioreactors (PBRs) for aeration and/or agitation by pneumatic and mechanical devices impacts their material, operational and production costs, accounting for up to circa 80% of the total costs. This work describes the developmental details of a photobioreactor based on the thermosiphon effect, where passive fluid circulation arises from temperature-induced density differences due to light absorption. The prototype reactor’s thermal and hydrodynamic performance was investigated using Computational Fluid Dynamics (CFD) and validated with experimentation. The non-uniform volumetric sensible heating from light absorption, primarily mediated by microbial cells in the reactor, was modeled using a radiative transport equation which incorporates experimentally obtained spectral irradiance and attenuation parameters of Rhodopseudomonas palustris, a photosynthetic bacterium of industrial interest. The thermosiphon photobioreactor’s (TPBR’s) buoyancy driven convection was characterized by the boussinesq approximation as well as experimental and theoretical heat transfer coefficients. Experimental data from thermocouple sensors, light meters, and marker image tracking in the TPBR containing a biomass loading of 0.5 kg/m3R. palustris were used to validate the CFD model at the same operating conditions. The CFD simulation results were generally in very good agreement with experimental data and have indicated that the TPBR demonstrated excellent liquid circulation and turbulence properties, capable of circulating up to 88% of the microbial cells in free suspension, using only passive circulation. The reactor demonstrates an anaerobic photobioreactor system entirely agitated using passive circulation, which has the potential to be used as a low energy-input reactor for photosynthetic organism cultivation.

Investigation of heat transfer performance of various insulating materials for robots

Theoretical and experimental heat transfer studies are conducted for performance assessment and evaluation of various thermal insulation materials for robotic operations in this study. Nineteen different thermal insulation materials are theoretically analysed, and three of these materials (namely stone wool, fiberglass and extruded polyurethane) are tested at both low and high temperatures to obtain their thermal performance under the extreme ambient conditions. According to the experimental results obtained at 40 °C, the heat transfer rates between the inside and outside of the robot are obtained 120 W for stone wool, 126 W for fiberglass, and 173.9 W for extruded polyurethane. On the other hand, at −25 °C, the heat transfer rates are 97 W for stone wool, 107.8 W for fiberglass, and 127.8 W for extruded polyurethane, respectively.

English

English

Modeling of film condensation flow in oval microchannels

A theoretical film condensation heat transfer model was developed for refrigerant flowing through oval-shaped microchannels in annular flow regime with laminar liquid film flowing along the channel walls. The model considered the effects of surface tension, disjoining pressure, interfacial shear stress and interfacial thermal resistance. Study was conducted for mass fluxes in a range of 300–1500 kg/(m2 s) with two refrigerants, R134a and R1234ze(E). The local heat transfer coefficient and circumferential mean heat transfer coefficient along the flow direction were obtained. The liquid film thickness profiles at different locations are presented. Due to the effect of surface tension, the liquid film accumulates towards the semi-circular corner. The circumferential mean heat transfer coefficient is very high after entering the channel inlet, decreases along the flow direction and reaches a constant value. R134a has a slightly higher circumferential mean heat transfer coefficient than that of R1234ze(E) due to its higher liquid conductivity.