Typical Heat Transfer Research Articles

Comparison of ground coupled heat transfer models for predicting underground building energy consumption

Underground buildings have gained attentions to alleviate urban land-scarcity. Evaluation of energy performance remains complex and uncertain for underground buildings, where the heat transfer through building envelopes is a typical three-dimensional unsteady heat transfer process. A few models have been proposed to calculate the ground coupled heat transfer (GCHT) through underground building envelopes. This paper evaluates and analyzes the influence of different GCHT models on energy consumption prediction of underground buildings. The models tested include: Ground Domain Xing (EnergyPlus), Basement Preprocessor (EnergyPlus), Foundation Kiva (EnergyPlus), and Frequency Response Analysis (DeST), which are compared under the criteria of consistency of model outputs and computational time. The study reveals that the Ground Domain Xing model can be considered as the relatively accurate GCHT model with low computational cost. One critical factor that differentiates these GCHT models is the ground surface boundary condition used. The study further explores the influence of thermal characteristics of underground building envelopes on annual energy consumption for five climate zones of China using the Ground Domain Xing model. The results show that the better thermal performance of underground building envelopes may increase the annual energy consumption in different climate regions.

APPLICATION OF «MICRO-PROCESSES» METHOD FOR MODELING HEAT CONDUCTION AND DIFFUSION PROCESSES IN CANONICAL BODIES

This paper shows that, in many technological processes, raw materials are subjected to high-temperature heat treatment and, in most cases, they have a geometric shape of the canonical form: a plate, a cylinder and a sphere. The convection drying process is considered as a typical heat and mass transfer process. The processes occurring under heat treatment conditions are reduced to transfer problems for an unbounded plate, cylinder, and ball with boundary conditions of the first kind, when the transfer potential (temperature, moisture content) is set on the surface of a solid. A number of expressions for calculations in the context of arbitrary distribution of initial values of transfer potentials as well as for uniform distributions are presented. It is shown that, when modeling heat and mass transfer processes in which the thermophysical characteristics of a solid body change significantly in the course of thermal treatment thereof, the use of already known solutions that have been previously developed becomes problematic. The «zonal» method and the «micro-processes» method are considered herein. It is shown that, for both methods, on the basis of experimental data referring to the dynamics of temperature and mass (moisture) content of the material over the course of the process, their dependences on the average (for the «zone» or «micro - process») temperatures and mass contents are determined. The next stage for calculations using the «zonal» method is formalization of the results obtained in the form of histograms of the values of mass conductivity coefficients from the average values of mass contents. For the «micro-processes» method, the kinetic curve can be used in calculations simultaneously. The smaller the range of measured values of temperatures and mass contents is the greater is the adequacy of calculated experimental data. It is emphasized that, under uneven initial conditions, analytical solutions to the heat transfer problem are usually presented in the form of infinite Fourier series. The convergence of the Fourier series deteriorates with decreasing time intervals. The great relevance of the application of the considered methods can be traced when modeling heat and mass transfer with intensive processes of phase transitions.

Experimental Investigation on Heating Performance of Newly Designed Air Source Heat Pump System for Electric Vehicles

Abstract Utilizing the heat from air source with heat pump system in electric vehicles shows a significant advantage from thermoelectric heat source for heat supply in cold climate. It could improve the driving range of electric vehicles considerably in winter and replace the positive temperature coefficient (PTC) heater with an acceptable cost and reliability. In this work, a newly designed heat pump system was first introduced with less components and cost. Second, experiments were conducted to investigate its cooling performance, and subsequent heating performance from −10 to 10 °C. The typical heat transfer and flow characteristics of refrigerant were recorded, and the behavior of each component including compressor, evaporator, condenser, and outside heat exchanger were analyzed and interpreted. The results showed that the heating and cooling performance of the new heat pump system could almost remain the same with traditional air-conditioning system in automobile and surely satisfy with the heat requirement of electric vehicles. In the heating mode, the maximum heating capacity increases by 13% at 400 m3/h air volume from 300 m3/h at the ambient temperature −10 °C, while the outlet air temperature decreases by 4–6%. In addition, using a heat pump system showed an increase in the driving range of electric vehicles by 25–31% as compared to PTC heaters.

Improved modeling of heat transfer in dropwise condensation

Dropwise condensation has drawn significant attention due to its efficient heat transfer performance compared to filmwise condensation. In this paper, typical experimental data for dropwise condensation on smooth hydrophobic surfaces were collected as well as the typical available heat transfer models. The comparisons between the prediction results and the experimental data indicated that the existing models were not generally applicable to various conditions. A new model for a vertical smooth surface was developed to predict the heat transfer characteristics of dropwise condensation. The new model was based on the nucleation condensation mechanism, and the total heat transfer on the surface includes the heat through all the droplets and the heat through the surface between the droplets. For the latent heat through the droplets the effect of the contact angle was taken into consideration on the basis of the nucleation condensation mechanism. The surface area between the droplets on the surface was thought to be the bare surface, and sensible heat transferred on the bare surface and the droplets surface was viewed as forced convection heat transfer. The calculation results from the model show that, although the heat transferred by forced convection is greatly dependent on the experimental parameters, it is three orders of magnitude smaller than the latent heat through the droplets. Comparisons show that the present model has better prediction precision, with an error range of -35–20% for 87.39% of the data and an error range of -35–25% for 90.37% of the data. The findings obtained from the model suggest that the heat transfer rate and the critical nucleation radius for a single droplet and the droplet size distribution are remarkably affected by the contact angle. In fact, a smaller contact angle enhances the condensation heat transfer and increases the nucleation density. In addition, the thickness of the promoter layer weakens the condensation heat transfer and decreases the nucleation density.

Novel WS2-Based Nanofluids for Concentrating Solar Power: Performance Characterization and Molecular-Level Insights.

Nano-colloidal suspensions of nanomaterials in a fluid, nanofluids, are appealing because of their interesting properties related to heat transfer processes. While nanomaterials based on transition metal chalcogenides (TMCs) have been widely studied in catalysis, sensing, and energy storage applications, there are few studies of nanofluids based on TMCs for heat transfer applications. In this study, the preparation and analysis of nanofluids based on 2D-WS2 in a typical heat transfer fluid (HTF) used in concentrating solar power (CSP) plants are reported. Nanofluids prepared using an exfoliation process exhibited well-defined nanosheets and were highly stable. The nanofluids were characterized in terms of properties related to their application in CSP. The presence of WS2 nanosheets did not modify significantly the surface tension, the viscosity, or the isobaric specific heat, but the thermal conductivity was improved by up to 30%. The Ur factor, which characterizes the thermal efficiency of the fluid in the solar collector, shows an enhancement of up to 22% in the nanofluid, demonstrating great promise for CSP applications. The Reynolds number and friction factor of the fluid were not significantly modified by the addition of the nanomaterial to the HTF, which is also positive for practical applications in CSP plants. Ab initio molecular dynamics simulations of the nanoparticle/fluid interface showed an irreversible dissociative adsorption of diphenyl oxide molecules on the WS2 edge, with very low kinetic barrier. The resulting "decoration" of the WS2 edge dramatically affects the nature of the interface interactions and is therefore expected to affect significantly the rheological and transport properties of the nanofluids.

A general and rapid method for performance evaluation of enhanced heat transfer techniques

Evaluation of the comprehensive performance of different heat transfer technologies and selection the best enhanced surface to meet the requirements are of great significance for efficient energy recovery. In this paper, based on the previous research results from our group, a rapid multi-criterion comprehensive evaluation method of enhanced heat transfer surfaces was first proposed; it realizes the rapid evaluation of heat transfer performance under the condition of equal flow rate, equal pressure drop and equal pump power constraints. Three baselines divide evaluation plot into six zones, namely zone 1 (non-energy-saving zone), zone 2 (relative energy-saving zone), zone 3 (value-added energy-saving zone), zone 4 (ideal energy-saving zone), zone 5 (pure energy-saving zone), and zone 6 (simple energy-saving zone). A simplified evaluation plot was then developed to overcome the dependence on the correlations of the reference surface. Next, the prediction results of the evaluation plot under the condition of variable minimum flow cross-section area, variable total heat transfer area, variable equivalent diameter, and variable tube bundle arrangement were compared with the traditional method to verify the applicability and accuracy of the evaluation plot. Finally, numerical simulations and engineering application of typical heat transfer surfaces were conducted to compare the energy-saving effectiveness of different enhanced surfaces and to analyze the effect of geometry and operation parameters on comprehensive performance. The results showed that the evaluation chart established in this paper can be successfully applied to the comprehensive comparison of multiple evaluation criteria and has a high prediction accuracy. Compared with the common evaluation methods, this method has excellent practicability and visualization, and contributes to the rapid development of new enhanced surfaces and efficient utilization of energy.

Database for Research Projects to Solve the Inverse Heat Conduction Problem

To achieve the optimal performance of an object to be heat treated, it is necessary to know the value of the Heat Transfer Coefficient (HTC) describing the amount of heat exchange between the work piece and the cooling medium. The prediction of the HTC is a typical Inverse Heat Transfer Problem (IHCP), which cannot be solved by direct numerical methods. Numerous techniques are used to solve the IHCP based on heuristic search algorithms having very high computational demand. As another approach, it would be possible to use machine-learning methods for the same purpose, which are capable of giving prompt estimations about the main characteristics of the HTC function. As known, a key requirement for all successful machine-learning projects is the availability of high quality training data. In this case, the amount of real-world measurements is far from satisfactory because of the high cost of these tests. As an alternative, it is possible to generate the necessary databases using simulations. This paper presents a novel model for random HTC function generation based on control points and additional parameters defining the shape of curve segments. As an additional step, a GPU accelerated finite-element method was used to simulate the cooling process resulting in the required temporary data records. These datasets make it possible for researchers to develop and test their IHCP solver algorithms.

An experimental and theoretical investigation of refrigerant charge on a secondary loop air‐conditioning heat pump system in electric vehicles

Automotive air-conditioning heat pump systems are particular interest worldwide in energy conservation and emission reduction for electric vehicles, hybrid electric vehicles, and fuel cell electric vehicles. Refrigerant charge amount is a key factor for the air-conditioning heat pump system optimization affecting the condensing pressure and subcooling in both heating and cooling modes. In this paper, the influence of the refrigerant charge on system performances was investigated using the experiment method on a secondary loop air-conditioning heat pump system. The typical heat transfer and flow parameters were recorded, and both cooling and heating performances of the system were investigated and illustrated by pressure-enthalpy diagrams. The critical refrigerant charges were determined in both heating and cooling modes. Three typical void fraction correlation models were also applied for the refrigerant charge determination modeling as a system off-design method. Results show that the Hughmark void fraction correlation method has the best prediction of the critical refrigerant charge in both cooling and heating modes.

Thermal Design of Double Pipe Heat Exchanger Used as an Oil Cooler in Ships: A Comparative Case Study

Double-pipe heat exchangers are devices that provide the transfer of thermal energy between two fluids at different temperatures. The major use of these heat exchangers is the sensible heating or cooling process of fluids where small heat transfer areas are required. The oil cooler is an example of these processes. In this study, fouled finned, clean finned, fouled unfinned, and clean unfinned double-pipe heat exchangers used as an oil cooler in ships have been compared. In thermal design, suggested different Nusselt number equations have been used, and the results of these equations have been shown in tables and figures. As a result, it is evaluated that using fouled finned double-pipe heat exchanger as oil cooler in ships is the most appropriate selection. 1. Introduction A double-pipe heat transfer exchanger consists of one or more pipes placed concentrically inside another pipe of a larger diameter with appropriate fittings to direct the flow from one section to the next. One fluid flows through the inner pipe (tube side), and the other flows through the annular space (annulus). The inner pipe is connected by U-shaped return bends enclosed in a return-bend housing. A typical double-pipe heat transfer exchanger is shown in Fig. 1. Double-pipe heat exchangers can be arranged in various series and parallel arrangements to meet pressure drop. The major use of the double-pipe heat exchanger is the sensible heating or cooling process of fluids where small heat transfer areas (up to 50 m2) are required. This configuration is also very suitable for one or both of the fluids at high pressure because of the smaller diameter of the pipes. The major disadvantage is that they are bulky and expensive per unit of heat transfer surface area. These double-pipe heat exchangers are also called hairpin heat exchangers, and they can be used when one stream is a gas, viscous liquid, or small in volume. These heat exchangers can be used under severe fouling conditions because of the ease of cleaning and maintenance. The outer surface of the inner tube can be finned, and then, the tube can be placed concentrically inside a large pipe, as shown in Fig. 2. In another type, there are multi tubes, finned or bare, inside a larger pipe. The fins increase the heat transfer surface per unit length and reduce the size and, therefore, the number of hairpins required for a given heat duty. Integrally, resistance-welded longitudinal fins have proven to be the most efficient for double-pipe heat exchangers. Fins are most efficient when the film coefficient is low.

Streamlining the Power Generation Profile of Concentrating Solar Power Plants

A scheme to streamline the electric power generation profile of concentrating solar power (CSP) plants of the parabolic trough collector (PTC) type is suggested. The scheme seeks to even out heat transfer rates from the solar field (SF) to the power block (PB) by splitting the typical heat transfer fluid (HTF) loop into two loops using an extra vessel and an extra pump. In the first loop, cold HTF is pumped by the cold pump from the cold vessel to the SF to collect heat before accumulating in the newly introduced hot vessel. In the second loop, hot HTF is pumped by the hot pump from the hot vessel to a heat exchanger train (HXT) to supply the PB with its heat load before accumulating in the cold vessel. The new scheme moderately decouples heat supply from heat sink allowing for more control of heat delivery rates thereby evening out power generation.

A numerical study of a liquid drop solidifying on a vertical cold wall

Solidification of a liquid drop on a vertical wall is a typical phase change heat transfer problem that exists widely in natural and engineering situations. In this study, we present the fully resolved two-dimensional simulations of such a problem by a front-tracking/finite difference method. Because of gravity, the liquid drop assumed stick to the cold wall shifts to the bottom during solidification. Numerical results show that the conical tip at the solidified drop top is still available with the presence of volume expansion, but the tip location is shifted downward, resulting in an asymmetric drop after complete solidification. The tip shift, height and shape of the solidified drop are investigated under the influences of various parameters such as the Prandtl number Pr, the Stefan number St, the Bond number, the Ohnesorge number Oh, and the density ratio of the solid to liquid phases ρsl. We also consider the effects of the growth angle ϕgr (at the triple point) and the initial drop shape (in terms of the contact angle ϕ0) on the solidification process. The most influent parameter is Bo whose increase in the range of 0.1–3.16 makes the drop more deformed with the tip shift linearly increasing with Bo. The tip shift also increases with an increase in ϕ0. However, increasing Oh (from 0.001 to 0.316), St (from 0.01 to 1.0) or ρsl (from 0.8 to 1.2) leads to a decrease in the tip shift. Concerning time for completing solidification, an increase in Bo, ϕgr or ϕ0, or a decrease in St, or ρsl results in an increase in the solidification time. The effects of these parameters on the drop height are also investigated.

Towards the improvement of the global efficiency of concentrating solar power plants by using Pt-based nanofluids: The internal molecular structure effect

Nanofluids are a promising alternative to the typical heat transfer fluid (HTF) used in concentrating solar power (CSP) plants, possibly improving their global efficiency and leading to the increase the use of renewable clean energy. This study analyses nanofluids based on a typical HTF used in CSP and Pt nanoparticles. Pt nanoparticles were synthesized and dispersed in the base fluid. Dodecylamine (DDA) was used as a phase transfer and as a surfactant. Also, 1-octadecanethiol (ODT) was added as a surfactant and pulsed ultrasonication was used to disperse the nanoparticles. As the base fluid, the eutectic mixture of diphenyl oxide (73.5%) and biphenyl (26.5%) was used. This fluid is typically used in CSP plants based on parabolic through collectors. The stability of the nanofluids was analysed according to the kind of surfactant and ultrasonication process. Furthermore, to analyse the efficiency of the nanofluids, several properties were measured, including density, dynamic viscosity, isobaric specific heat and thermal conductivity. We found an increase in thermal conductivity of up to 37%, and the heat transfer coefficient also improved by up to 20%. Molecular dynamics calculations were performed to determine how the inclusion of ODT affected the system. ODT competes with DDA to interact with the Pt, forming a lattice around the Pt. The base fluid molecules, and in particular the diphenyl oxide molecules, take advantage of this competition to move closer to the Pt. This movement of molecules as the temperature rise must be Brownian in nature and enhances the heat transfer processes, improving the thermal properties of the nanofluids with both ODT and DDA compared with those prepared only with DDA. Thus, nanofluids with ODT and DDA would appear to be of interest for use in CSP.

An artificial immune system algorithm applied to the solution of an inverse problem in unsteady inward solidification

The numerical simulation and optimization in solidification involving geometries beyond one dimension normally requires expensive computational approaches in memory and data processing, generating consequently high cost associated with runtime execution. When boundary conditions are unknown, such as how heat is extracted from the casting surface, which is a typical inverse heat transfer problem (IHTP), the search for such conditions by trial-and-error makes the whole process less feasible. In this sense, this work introduces the application of an artificial immune system (AIS) algorithm as a possible meta-heuristic alternative, with a view to returning acceptable objective values to converge on a set of acceptable solutions. In the present work, the search process of the heat transfer coefficient (hg) at the casting surface during two-dimensional inward solidification of Al-1.5 wt.%Fe alloy castings of Cartesian and Cylindrical geometries in chilled molds is optimized. Two water-cooled solidification apparatuses were designed to in situ melt the alloy by a set of electrical resistances, in which heat is extracted only through the chilled faces of the molds. The thermal history during solidification was obtained via thermocouples placed at different positions in the castings. A Finite Difference heat transfer model integrated to an optimized version of the artificial immune network algorithm, which uses the experimental thermal profiles as inputs, has been applied to solve the IHTP through the search for acceptable values of heat transfer coefficient. It is shown that fast convergence of the developed algorithm can be achieved for a relatively small number of iterations. The mean relative errors associated with differences between simulated and experimental temperatures are shown to be 1% and 0.07% for Cartesian and Cylindrical geometries, respectively and expressions relating hg to time have been determined for both geometries.

Estimation of Efficiency Potentials Generated by Innovative Equipment Technologies

AbstractThe integration of innovative equipment technologies in production processes offers efficiency potentials with regards to energy and resource demand as well as investment and operating costs. For the estimation of these potentials, a sufficiently detailed database is necessary as well as the possibility to consider particular boundary conditions of the production process, infrastructure of the production site, and the overall plant design. A structured concept for the selection of suitable equipment technologies and sizing rules for the design of the equipment for a desired process task are presented in this review. The combination with an appropriate modeling approach allows a holistic assessment of technically feasible solutions to quantify economic and ecological efficiency potentials of the equipment technologies. The application of this concept is demonstrated on the task of a typical heat transfer problem in the special chemicals sector.

Heat Transfer Characteristics in Oscillatory Hydromagnetic Channel Flow of Maxwell Fluid Using Cattaneo–Christov Model

The aim of this investigation is to discuss the hydromagnetic flow and heat transfer of Maxwell fluid in a channel with oscillatory stretching walls. Unlike typical heat transfer studies, here Cattaneo–Christov heat flux model is used. The transformed dimensionless nonlinear partial differential equations are solved by means of the homotopy analysis method. The convergent series solutions are utilized to discuss the main effects of emerging parameters on velocity and temperature. The obtained results illustrate that Hartmann and Deborah numbers suppress the velocity profiles. However, an increase in ratio of oscillation frequency to stretching rate increases the velocity profiles. A reverse flow occurs in the central region of the channel which is found to decrease by increasing Hartmann and Deborah numbers. Moreover, for the Cattaneo–Christove model fluid temperature inside the channel is less when compared with temperature obtained using heat flux based on Fourier law.

A New Model of and Insight into Hydrate Film Lateral Growth along the Gas–Liquid Interface Considering Natural Convection Heat Transfer

Modeling of hydrate growth at the interface between gas and aqueous phases is critical for evaluating the growth rate of hydrate film and initial thickness in the oil and natural gas industries. However, most existing heat transfer models in presumption and hypothesis possess logical or physical deficiencies, which may lead to wide deviations when predicting results. In this work, a new model of hydrate film lateral growth along a planar gas–liquid boundary is developed, which considers natural convection heat transfer. This model not only correlates subcooling, which is generally regarded as the main controlling factor in the process of hydrate growth, but also describes a quantitative relation with experimental temperature that has never been embodied in previous models. In combination with the experimental data in this paper and the previous literature, a comparison of the simulation between the proposed model and typical heat transfer models is provided. The simulated results show that this model agre...

Investigation on the Variability of Existing Models for Simulation of Local Temperature Field during a Single Discharge for Electrical Discharge Machining (EDM)

In electrical discharge machining (EDM) material removal and rim zone modifications are resulted by generated heat during numerous discharges. By estimating the temperature field applied on the workpiece as material loading, removal rate and induced modifications in the rim zone can be predicted. Different scientists tried to simulate the induced temperature field on the workpiece during a single or multiple discharges. However, due to the complex removal mechanism in EDM, these models are mostly incomplete and contain considerable uncertainties. In this study, the strengths and weaknesses of typical heat transfer models for a single discharge are discussed. For this propose a finite element model is constructed and influences of different modeling parameters on the obtained results have been analyzed. Moreover, potentials of reverse microstructure simulation as an assistant for modification of heat source models discussed.

Investigation of enhanced thermal properties in NiO-based nanofluids for concentrating solar power applications: A molecular dynamics and experimental analysis

Nanofluids could be a promising alternative to the typical heat transfer fluids (HTF) used in concentrating solar power. This study analyses nanofluids based on a typical HTF for concentrating solar power (CSP) applications and NiO nanoparticles. The optimum nanoparticle concentration was determined by analysing the stability of the nanofluids. Some of their properties, such as density, viscosity, isobaric specific heat and thermal conductivity, were characterized to evaluate their performance. Their thermal conductivity increased by up to 96% and the heat transfer coefficient by 50%. Molecular dynamics calculations were performed to explain from a molecular perspective how the presence of equal proportions of two surfactants, benzalkonium chloride (BAC) and 1-Octadecanethiol (ODT), enhanced the thermal properties of the NiO nanofluid. The isobaric specific heat and thermal conductivity values followed the same experimental tendency. The analysis of the radial distribution functions (RDFs) and spatial distribution functions (SDFs) revealed an inner layer of base fluid and surfactant molecules around the NiO cell. This first layer contained BAC molecules at all the temperatures, while ODT was only incorporated at higher temperatures. The exchange of surfactant and base fluid molecules around the NiO as the temperature increases may play an important role in the enhancement of the thermal properties.

Large eddy simulation on turbulent heat transfer in reactor vessel lower head corium pools

Due to the high-Rayleigh-number natural convection and strong turbulence in the volumetrically heated corium melt pools, it is difficult to capture the heat transfer characteristics in detail. The Large Eddy Simulation (LES) methods are more and more applicable to nuclear industry and have become powerful tools to analyze the complicated turbulent and multi-phase flows. In this paper, the Wall-Modeled LES (WMLES) method was employed for the simulation of three typical corium pool heat transfer experiments: BALI, LIVE and COPRA experiments. The melt pool temperature and heat flux from numerical simulations were in good comparison with experimental data. Then numerical simulation for the transient formation of two-layer corium pool was performed with prototypical corium to obtain the heat transfer data in reactor situation. There was a distinctive gap between the temperatures in two layers. The heat flux showed a decrease at the two-layer interface and a huge jump at the metal layer side. The thin metal layer on the top at the early stage of the two-layer formation will threaten the vessel integrity.

Development of Moving Bed Temperature Swing Adsorption (MBTSA) Process for Post-combustion CO2 Capture: Initial Benchmarking in a NGCC Context

A moving-bed temperature-swing adsorption (MBTSA) process has been evaluated for post-combustion CO2 capture in a natural-gas combined cycle (NGCC) context using Zeolite 13X as adsorbent. The performance of the different sections of the reactor (adsorption, regeneration and heat exchange) were modelled in gPROMS using real equilibrium and kinetic adsorption data and typical heat transfer parameters. As a consequence of the internal heat transfer between the hot adsorbent powder leaving the regenerator with the cold powder entering the regenerator, a low specific heat requirement for regeneration of 2.3 GJth/ton CO2 captured is estimated. When also including the electrical energy needed (also compression of CO2) a total energy penalty of 6.5%-points is obtained. For regeneration, flue gas at 222°C was used for heating and it was assumed that the flue gas was heated after the power plant by using saturated steam condensing at 230°C. A major reduction in the heat requirement for the process can be gained if the heat required for regeneration is provided from more optimal integration with the steam cycle of the power plant.