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Accepted Manuscripts

BASIC VIEW  |  EXPANDED VIEW
Technical Brief  
Deify Law and Haden Hinkle
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4037650
Two-phase bubbly flows by gas injection had been shown to enhance heat transfer in channel flows as compared with that of single-phase flows. The present work explores the effects of gas phase distribution such as volume fraction and bubble size on the heat transfer in upward vertical channel flows numerically. A two-dimensional (2D) channel flow of 10 cm wide by 100 cm high at 0.2 and 1.0 m/s inlet water and air superficial velocities, respectively, is simulated. Numerical simulations are performed using the commercial computational fluid dynamics (CFD) code ANSYS FLUENT. The bubble size is characterized by the Eotvos number. The inlet air volume fraction is fixed at 10% whereas the Eotvos number is maintained at 1.0 to perform parametric studies, respectively, in order to investigate their effects on Nusselt number of the two-phase flows. All simulations are compared with a single-phase flow condition. To enhance heat transfer, it is determined that the optimum bubble size for the channel with a 10% inlet air volume fraction has an Eotvos number of 0.2, which is equivalent to a bubble diameter of 1.219 mm. Likewise, it is determined that the optimum volume fraction peaks at 30% inlet air volume fraction using an Eotvos number of 1.0.
TOPICS: Channel flow, Convection, Modeling, Bubbles, Heat transfer, Computational fluid dynamics, Flow (Dynamics), Two-phase flow, Water, Engineering simulation, Computer simulation, Simulation, Bubbly flow
research-article  
Hao-Wei Wu, Hootan Zirakzadeh, Je-Chin Han, Luzeng Zhang and Hee Koo Moon
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4037584
A multi-passage internal cooling test model with an 180° U-bend at the hub was investigated. The flow is radially inward at the inlet passage, while it is radially outward at the trailing edge passage. The aspect ratio of the inlet passage is 2:1 (AR=2), while the trailing edge passage is wedge-shaped and with side wall slot ejections. The squared ribs with P/e = 8, e/Dh = 0.1, a = 45°, were configured on both leading and trailing surfaces along the inlet passage, and also at the inner half of the trailing edge passage. Three rows of cylinder-shaped pin-fins with a diameter of 3 mm were placed at both leading and trailing surfaces at the outer half of the trailing edge passage. For without turning vane case, heat transfer on the leading surface at hub turn region is increased by rotation, while it is decreased on the trailing surface. The presence of turning vane reduces the effect of rotation on hub turn portion. The combination of ribs, pin-fin array and mass loss of cooling air through side wall slot ejection results in the heat transfer coefficient gradually decreased along the trailing edge passage. Correlation between regional heat transfer coefficients and rotation numbers is presented for with and without turning vane cases, and with channel orientation angle ß at 90° & 45°.
research-article  
Renato M. Cotta, Carolina P. Naveira-Cotta and Diego Knupp
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4037576
Application of the Generalized Integral Transform Technique (GITT) in the solution of a class of linear or nonlinear convection-diffusion problems is considered, by fully or partially incorporating the convective effects into the chosen eigenvalue problem that forms the basis of the proposed eigenfunction expansion. The aim is to improve convergence of the eigenfunction expansions, especially for formulations with significant convective effects, by simultaneously accounting for the relative importance of convective and diffusive effects within the eigenfunctions themselves, in comparison against the more traditional GITT solution path, which adopts a purely diffusive eigenvalue problem and fully incorporates the convective effects into the problem source term. After identifying a characteristic convective operator, and through a straightforward algebraic transformation of the original convection-diffusion problem, basically by redefining the coefficients associated with the transient and diffusive terms, the characteristic convective term is merged into a generalized diffusion operator with a space variable diffusion coefficient. The generalized diffusion problem then naturally leads to the eigenvalue problem to be chosen in proposing the eigenfunction expansion for the linear situation, as well as for the appropriate linearized version in the case of a nonlinear application. The resulting eigenvalue problem with space variable coefficients is then solved through the GITT itself, yielding the corresponding algebraic eigenvalue problem, upon selection of a simple auxiliary eigenvalue problem of known analytical solution. The developed methodology is illustrated for linear and nonlinear applications, both in one-dimensional and multidimensional formulations, as represented by a few examples based on Burgers equation.
research-article  
Howard Cheung and Shengwei Wang
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4037543
It is becoming often to measure steady-state heat transfer rate from thermal systems with variable speed and volume equipment and hence with fluctuating properties and mass flow rates. However, it is unclear if the conventional heat transfer rate measurement based on averages of temperature and pressure measurement are representative enough to represent the effect of system dynamics and measure their heat transfer rates accurately. This paper studied the issue by comparing its accuracy and uncertainty to that of alternative data processing methods with theoretically less systematic bias. The comparison was conducted with steady-state data from a variable-speed ductless heat pump system with occasional fluctuation of refrigerant flow and properties. The results show that the accuracy improvement brought by one alternative method is statistically significant albeit small in magnitude, and the other method may reduce uncertainty of the heat transfer rate measurement in tests with large periodic changes of measured variables. Nonetheless, both alternative methods are about 100 times more computationally expensive than the conventional averaging method, and averages of temperature and pressure measurement are still appropriate when computational resources are limited.
TOPICS: Heat transfer, Dynamics (Mechanics), Steady state, Uncertainty, Flow (Dynamics), Temperature, Pressure measurement, System dynamics, Thermal systems, Heat pumps, Refrigerants
research-article  
Dilesh Maharjan, Mustafa Hadj-Nacer, Narayana Rao Chalasani and Miles Greiner
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4037493
Measurements of heat transfer Computational fluid dynamics simulations from of an 8x8 array of vertical heater rods to the walls of within a square cross-section, helium-filled pressure vessel enclosure are performed for a range of enclosure temperatures, helium pressures and rod heat generation rates. This configuration is relevant to a used nuclear fuel assembly within a dry storage canister. The measurements are used to assess the accuracy of computational fluid dynamics/radiation the simulations, the temperature results are compared to measurements made in the same configuration. The simulations employ use the measured enclosure temperatures as boundary conditions, so they essentially calculate and predict the temperature difference between the rods and enclosure. These temperature differences are as large as 72°C for some experiments. The measured temperature of rods near the periphery of the array are sensitive highly dependent on to small, uncontrolled variations in their location. As a result, those temperatures are not as useful for validating the simulations as measurements from rods nearer the array center. The simulated rod temperatures exhibit random differences from the measurements that are as large as 5.7°C, but the systematic (average) error is 1°C or less. The random differences between the simulated and measured maximum array temperature is 2.1°C, which is less than 3% of the maximum rod-to-wall temperature difference.
TOPICS: Heat transfer, Simulation, Helium, Engineering simulation, Temperature, Rods, Computational fluid dynamics, Heat, Radiation (Physics), Manufacturing, Pressure vessels, Storage, Nuclear fuels, Boundary-value problems, Errors
research-article  
Samuel MER, Jean-Paul Thibault and Christophe Corre
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4037449
A cryogenic propellant submitted to heat load during long duration space missions tends to vaporize to such an extent that the resulting pressure rise must be controlled to prevent storage failure. The Thermodynamic Vent System (TVS), one of the possible control strategies, has been investigated using on-ground experiments with NOVEC1230 as substitution fluid. Results obtained for self-pressurization and TVS control phases have been reported in a previous work. The unexpected inverse thermal stratification observed during these experiments is analyzed in the present work and related tothe influence of non-condensable gases. Non-condensablegases, present inside the tank in the form of nitrogen - 10 times lighter than the substitution fluid vapor - generatea concentration stratification in the ullage. Assuming the NOVEC1230 remains at saturation in the whole ullage, the density stratification which results from this concentration stratification can explain the observed inverse thermal stratification.
TOPICS: Gases, Vapors, Fluids, Thermal stratification, Density, Pressure, Heat, Stress, Failure, Nitrogen, Propellants, Storage, Vents
research-article  
Sadia Siddiqa, Naheed Begum, Md Anwar Hossain and A. A. A Al-Rashad
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4037209
In this paper numerical solutions of thermally radiating Marangoni convection of two-phase dusty fluid moving along a vertical wavy surface are established. The results are given with the understanding that the dust particles are of uniform size and dispersed in optically thick gray fluid. The numerical results are obtained by integrating the dimensionless, transformed parabolic equations, through straightforward implicit finite difference scheme. In order to analyze the influence of various controlling parameters, results are displayed in the form of rate of heat transfer, skin friction coefficient, velocity and temperature profiles, streamlines and isotherms. It is observed that the variation of thermal radiation parameter can significantly change the corresponding particle pattern and extensively promotes the heat transfer rate.
TOPICS: Fluids, Thermal radiation, Convection, Heat transfer, Temperature profiles, Particulate matter, Dust, Skin friction (Fluid dynamics)
Review Article  
Jaswinder Singh Mehta, Rajesh Kumar, Harmesh Kumar and Harry Garg
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4037200
Ferrofluids, a distinctive class of nanofluid, consists of suspension of magnetic nanoparticles in a non-magnetic base fluid. Flow and heat transport properties of such a fluid can be manipulated when subjected to external magnetic field and temperature gradient. This unique feature has fascinated researchers across the globe to test its capability as a coolant for miniature electronic devices. The proposed work presents an updated and comprehensive review on ferrofluids with emphasis on heat transfer enhancement of micro-devices. Based on the research findings, a number of important variables that have direct bearing on convective heat transport ability of ferrofluid have been recognized. The paper also identifies the key research challenges and opportunities for future research. By critically resolving these challenges, it is anticipated that ferrofluids can make substantial impact as coolant in miniature heat exchangers.
TOPICS: Ferrofluids, Convection, Coolants, Heat, Fluids, Magnetic fields, Heat transfer, Nanoparticles, Bearings, Heat exchangers, Nanofluids, Temperature gradient, Flow (Dynamics)
research-article  
Mostafa Shojaeian, Masoumeh Nedaei, Mehmet Yildiz and Ali Kosar
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4037199
In this study, two-dimensional numerical simulations of liquid slip flows in parallel-plate microchannels have been performed to obtain heat transfer characteristics and entropy generation rate under asymmetric heating conditions. Heat transfer analysis has been conducted along with Second-Law Analysis through utilizing temperature-dependent thermophysical properties. The results indicate that temperature dependent thermophysical properties have a positive effect on convective heat transfer and entropy generation. Nusselt numbers of the upper and lower plates and global entropy generation rates are significantly affected by slip parameter and heat flux ratio. It is shown that Nusselt number of the lower plate may have very large but finite values at a specific heat flux ratio. This finding resembles to analytical solutions, where singularities leading to an infinite Nusselt number exist.
TOPICS: Flow (Dynamics), Heat transfer, Entropy, Microchannels, Temperature, Computer simulation, Convection, Plates (structures), Heating, Heat flux, Specific heat
research-article  
Mainul Hasan and Latifa Begum
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4037196
A 3-D CFD modeling study has been carried out for the tin bronze (C903) slab of industrial size in a vertical direct chill caster. The melt is delivered from the top across the entire cross-section of the caster. An insulated hot-top is considered above the 80-mm mold to control the melt level in the mold. A porous filter is considered in the hot-top region of the mold to arrest the incoming inclusions and homogenize the flow into the mold. The melt flow through the porous filter is modeled on the basis of the Brinkmann-Forchimier-Extended non-Darcy model. Results are obtained for four casting speeds varying from 40 to 100 mm/min. The metal-mold contact region, as well as, the convective heat transfer coefficient at the mold wall is also varied. In addition to the above, the Darcy number for the porous media is also changed. All parametric studies are performed for a fixed inlet melt superheat of 62oC. The results are presented pictorially in the form of temperature and velocity fields. The sump depth, mushy region thickness, solid shell thickness at the exit of the mold and axial temperature profiles are also presented and correlated with the casting speed through regression analysis.
TOPICS: Slabs, Filters, Bronze, Casting, Flow (Dynamics), Temperature, Metals, Porous materials, Computational fluid dynamics, Convection, Modeling, Regression analysis, Shells, Temperature profiles
research-article  
Zhongran Chi, Haiqing Liu and Shusheng Zang
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4037131
This paper discusses the approach of cooling design optimization of a High Pressure Turbine (HPT) endwall with 3D Conjugate Heat Transfer (CHT) CFD applied. This study involved the optimization of the spacing of impingement jet array and the exit width of shaped holes, which were different for each cooling cavity. The optimization objectives were to reduce the wall temperature level and to increase the aerodynamic performance. The optimization methodology consisted of an in-house parametric design & CFD mesh generation tool, a CHT CFD solver, a database of CFD results, a metamodel, and an algorithm for multi-objective optimization. The CFD tool was validated against experimental data of an endwall at CHT conditions. The metamodel, which could efficiently estimate the optimization objectives of new individuals without CFD runs, was developed and coupled with Non-dominated Sorting Genetic Algorithm II to accelerate the optimization process. Through the optimization search, the Pareto front of the problem was found in each iteration. The accuracy of metamodel with more iterations was improved by enriching database. But optimal designs found by the last iteration are almost identical with those of the first iteration. Through analyzing extra CFD results, it was demonstrated that the design variables in the Pareto front successfully reached the optimal values. The optimal pitches of impingement arrays could be decided accommodating the local thermal load while avoiding jet lift-off of film coolant. It was also suggested that cylindrical film holes near throat should be beneficial to both aerodynamic and cooling performances.
TOPICS: Heat transfer, Cooling, High pressure (Physics), Computational fluid dynamics, Turbines, Pareto optimization, Optimization, Databases, Design, Algorithms, Stress, Coolants, Genetic algorithms, Mesh generation, Cavities, Wall temperature, Parametric design
Guest Editorial  
Qingang Xiong, Jingchao Zhang and Giulio Lorenzini
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4037129
A special issue is launched to serve as a platform for researchers to report their recent activities on heat transfer studies related to processes of developing and applying renewable energies and novel materials. 15 research articles, ranging from nanoscale to macroscale and from theoretical to numerical, have been collected to discuss the fundamental mechanisms and practical applications of heat transfer in processes of developing and applying renewable energies and novel materials. The topics of published papers include computational fluid dynamics simulation of heat transfer in micro mixer and solar cavity receiver, experimental design and measurement of thermal properties for photovoltaic systems and two-dimensional materials, and theoretical analysis of heat transfer in novel nanofluids, etc.
TOPICS: Heat transfer, Simulation, Thermal properties, Computational fluid dynamics, Nanoscale phenomena, Solar energy, Cavities, Experimental design, Nanofluids, Photovoltaic power systems, Theoretical analysis
research-article  
Steven J. Young, D Janssen, Everett Wenzel, Brandon Shadakofsky and Francis Kulacki
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4037130
Onboard liquid cooling of electronic devices is demonstrated with liquid delivered externally to the point of heat removal through a conformal encapsulation. The encapsulation creates a flat microgap above the integrated circuit and is a CFD-enabled design that delivers a uniform inlet coolant flow over the device. The coolant is NovecTM 7200, and the electronics are simulated with a resistance heater on a 1:1 scale. Thermal performance is demonstrated at power densities of ~ 1 KW/cm3 in the microgap. Parameters investigated are pressure drop, average device temperature, heat transfer coefficient and coefficient of performance. Nusselt numbers for gap sizes of 0.25, 0.5 and 0.75 mm are reduced to a dimensionless correlation. With low coolant inlet subcooling, two-phase heat transfer is seen at all mass flows. Device temperatures reach 95 oC for power dissipation of 50 - 80 W (0.67 - 1.08 KW/cm3) depending on coolant flow for a gap of 0.5 mm. Coefficients of performance of ~100 - 70,000 are determined via measured pressure drop and demonstrate a low pumping penalty at the device level within the range of power and coolant flow considered. The encapsulation with microgap flow boiling provides a means for use of higher power CPU and GPU devices and thereby enables higher computing performance, for example, in embedded airborne computers.
TOPICS: Cooling, Coolants, Flow (Dynamics), Temperature, Pressure drop, Subcooling, Electronics, Heat transfer coefficients, Graphics processing units, Heat transfer, Heat, Energy dissipation, Boiling, Computational fluid dynamics, Design, Computers, Integrated circuits
research-article  
Javid Karbalaei Mehdi, Amir Nejat and Masoud M. Shariat Panahi
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4036966
One important safety issue in automotive industry is the efficient cooling of brake system. This research work aims to introduce an optimized cooling vane geometry to enhance heat removal performance of ventilated brake disks. The novel idea of using airfoil vanes is followed as the basis of this investigation. In order to perform the optimization technique efficiently, an integrated shape optimization process is designed. According to the aerodynamic and heat transfer considerations, first an appropriate airfoil is selected as the base profile to be optimized. For the shape modification purpose, a curve parameterization method named Class Shape Transformation (CST) is utilized. The control parameters defined in CST method are then established as the geometrical design variables of an improved Territorial Particle Swarm Optimization (TPSO) algorithm. In order to overcome the potential bottleneck of high computational cost associated with the required CFD-based function evaluations, TPSO algorithm is coupled with a predictive Artificial Neural Networks (ANN), well-trained with an input dataset designed based on the Taguchi method. The obtained profile shows an evident convective heat dissipation improvement accomplished mainly via airflow acceleration over the vanes, avoiding early flow detachment and adjusting the flow separation region at the rear part of the suction sides. The results also reveal the approaches by which such a superior performance is achieved by means of the modified surface curvatures.
TOPICS: Heat transfer, Cooling, Algorithms, Disks, Artificial neural networks, Automotive brakes, Shape optimization, Airfoils, Brakes, Heat, Shapes, Taguchi methods, Flow (Dynamics), Flow separation, Geometry, Particle swarm optimization, Automotive industry, Computational fluid dynamics, Design, Optimization, Safety, Suction, Air flow, Energy dissipation
research-article  
Wang Lingling, Zhu Guihua, Wei Yu, Dahai Zhu, Yingchun Zhang, Liye Zhang and Huaqing Xie
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4036800
Near-spherical gold nanoparticles were synthesized using a facile chemical reduction method. The optical properties, size and morphology of nanofluids were characterized using UV-Vis-NIR spectroscopy and TEM. All the gold nanofluids showed better photothermal conversion characteristics than H2O due to the strong localized surface plasmon resonance effect. The increase in gold nanoparticles diameters resulted in lower photothermal conversion properties, so appropriate reducing agents have great influence on the optical properties of gold nanofluids in our experimental system. Trisodium citrate is the optimum reducing agents compared with NaBH4 and ascorbic acid.
TOPICS: Surface plasmon resonance, Nanofluids, Nanoparticles, Water, Ultraviolet radiation, Spectroscopy
research-article  
Hussein M. Maghrabie, M. Attalla, Hany Fawaz and Mohamed Khalil
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4036788
Numerical study of the effect of jet position on cooling process of an array of heated obstacles simulating electronic components has been investigated based on Realizable k-e model. Jet positions have been changed to impinge each row of obstacles consecutively. The experiments have been achieved at three different values of jet-to-channel Reynolds number ratios, Rej/Rec=1, 2, and 4. In the present study, a comparison between two different cooling processes; cross flow only (CF) and jet impingement with cross flow (JICF) has been achieved. The flow structure, heat transfer characteristics over all array obstacles, and the pumping power have been investigated for different jet positions. The results show that the jet position affects significantly the flow structure, as well as the heat transfer characteristics. Ac-cording to the results of average heat transfer coefficient and the pumping power, the more effective jet position for all values of jet-to-channel Reynolds number ratios (1, 2, and 4) is achieved when the jets impinge the third row of obstacles (JP3).
TOPICS: Cooling, Reynolds number, Flow (Dynamics), Heat transfer, Cross-flow, Jets, Electronic components, Heat transfer coefficients
research-article  
Zhaoxiang Zhang, Huiqing Liu, Xiaohu Dong and Huanli Jiang
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4036789
Steam assisted gravity drainage (SAGD) process has been an optimized method to explore heavy oil reservoirs in the world. The oil viscosity reduction and gravity force near the interface of steam chamber are the main development mechanisms. In classical models, conductive heat transfer plays the only or dominant role in the heat transmission from high-temperature steam to low-temperature oil sands. Although some mathematical studies have paid attention to the convective heat transfer, the role of heat transfer by flowable oil normal to the steam chamber interface has been given little attention. In SAGD, the viscosity of bitumen can be reduced by several orders of magnitude by the release of latent heat from injected steam. In this study, an analytical model is developed for the heat transfer process induced by flowable oil. Also, in order to accurately simulate the oil viscosity characteristics in steam chamber, a correlation between oil viscosity and pressure is proposed. Results indicate that the oil mobility plays an important role on the flow normal to interface when the distance is smaller than 6 m. Even under the most extreme circumstances (µw = 0.1127 cp), the flowing of oil normal to steam chamber interface also cannot be ignored. Comparing to Irani and Ghannadi model, it can be easy to draw the conclusion that new model consists with the UTF field data much better. This new analytical model will benefit to understanding the convective heat transfer mechanism in SAGD process.
TOPICS: Gravity (Force), Drainage, Convection, Steam, Heat transfer, Viscosity, Asphalt, Oil sands, Pitch (Bituminous material), Pressure, Flow (Dynamics), High temperature steam, Hydrocarbon reservoirs, Mechanical admittance, Low temperature, Latent heat, Bituminous materials
research-article  
Hao-Chun Zhang, Yan-Qiang Wei, Cheng-Shuai Su, Gong-Nan Xie and Giulio Lorenzini
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4036791
With the rapid development of the supersonic aircraft technology, the aircraft Mach number continues increasing, but on the other hand, the working condition becomes progressively poor. The photonic crystals (PCs) material could reflect the energy of the thermal radiation effectively and prevent heat transferring into the substrate due to its low thermal conductivity. Consequently, the PCs material could be applied to thermal protection for the supersonic aircraft. In this paper, the aircraft state of Mach 5 is set as the target operating condition, and the PC thermal protection ability is simulated by the method of computational fluid dynamics. Based on the theory of the electromagnetics, the characteristics of the photonic band gaps for three dimensional PCs are calculated and the effects of PCs medium radius, refractive index and lattice constant are fully taken into account. For the three-dimensional diamond PCs structure, two major categories and totally five optimized design schemes are proposed, through combining the condition of supersonic aircraft aerodynamic heating. Results show that the temperature is reduced by 948.431K when the heat passes through thermal protection layer and reduced by 930.4K when the heat passes through PC layer. By the method of “Coupled Optimization Strategy (COS)”, the energy density which enters into substrate material would decrease by 7.99%. In conclusion, the thermal protection capacity for supersonic aircraft could be effectively improved by using the PCs.
TOPICS: Structural optimization, Aircraft, Crystals, Dynamic light scattering, Heat, Temperature, Electromagnetism, Density, Mach number, Thermal radiation, Electromagnetic force, Thermal conductivity, Computational fluid dynamics, Design, Optimization, Refractive index, Diamonds, Energy gap, Heating
research-article  
Jiabin Fang, Nan Tu, Jinjia Wei, Tao Fang and Xuancheng Du
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4036792
The effects of tube layout on the heat losses of solar cavity receiver were numerically investigated. Two typical tube layouts were analyzed. For the first tube layout, only the active surfaces of cavity were covered with tubes. For the second tube layout, both the active cavity walls and the passive cavity walls were covered with tubes. Besides, the effects of water-steam circulation mode on the heat losses were further studied for the second tube layout. The absorber tubes on passive surfaces were considered as the boiling section for one water-steam circulation mode, and as the preheating section for the other one, respectively. The thermal performance of the cavity receiver with each tube layout was evaluated according to the previous calculation model. The results show that the passive surfaces appear to have much lower heat flux than the active ones. However, the temperature of those surfaces can reach a quite high value of about 520 ? in the first tube layout, which causes a large amount of radiative and convective heat losses. By contrast, the temperature of passive surfaces decreases by about 200-300? in the second tube layout, which leads to a 38.2-70.3% drop in convective heat loss and a 67.7-87.7% drop in radiative heat loss of the passive surfaces. The thermal efficiency of the receiver can be raised from 82.9% to 87.7% in the present work.
TOPICS: Solar energy, Cavities, Heat losses, Steam, Water, Temperature, Cavity walls, Boiling, Heat flux, Thermal efficiency
research-article  
Ayoub Gounni and Mustapha El Alami
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4036794
In order to really assess the thermal performance of a wall incorporating Phase Change Material (PCM), a reduced scale cavity has been monitored during two heating cycles. For each cycle, the heat source inside the test cell is switched "on" for 5 hours and its setpoint is 38°C then switched off for 4 hours. The outdoor air temperature is kept constant at a low temperature of 20°C. Two walls are equipped with a PCM layer at different depths in order to study the optimal PCM location. The other walls are wooden and glass to model a real building. The comparison between the four walls is made based on the absorbed heat fluxes and outside surface temperatures. The results show that the location of the PCM close to the heat source reaches its melting temperature and then reduces the surface temperatures. At this location, the PCM layer stores the major part of the inlet heat flux. It takes 10 hours to release the absorbed heat flux. However, the PCM layer, practically, does not have effect on the surface temperatures and absorbed heat fluxes, when it is placed far from the heat source.
TOPICS: Heat transfer, Cavities, Heat, Temperature, Flux (Metallurgy), Cycles, Heat flux, Heating, Melting, Phase change materials, Low temperature, Glass

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