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research-article  
Peiqi Liu, Kehan WU, Sheng Liu, Wenhu Tan, Che Zhu and Dapeng HU
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4040895
Pressure oscillating tubes are core components of the gas wave refrigerator. The reverse compression waves have limited refrigeration efficiency by re-heat cool gas. The promotion from traditional rotation gas wave refrigerator with single opened pressure oscillating tubes to the streamlined pressure exchange gas wave refrigerator with double opened pressure oscillating tubes is mainly in term of this issue. However, through weakened, reverse compression waves are still inevitable. For further development, the concept of a wave attenuator has been proposed, installed at the high-temperature (HT) port. Numerical simulation has been utilized to analyze mechanism of wave attenuator and the practical effect has been proved by experiment research. The conclusions are as follows: due to structure of wave attenuator, the intensity of reverse compression waves has been weakened; the optimal structure of wave attenuator has been obtained; the refrigeration efficiency of the refrigerator has been significantly increased because of wave attenuator in HT port.
TOPICS: Waves, Compression, Pressure, Refrigeration, High temperature, Rotation, Heat, Computer simulation
research-article  
Mustafa Koz and H. Ezzat Khalifa
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4040896
An experimentally validated finite element model was developed to analyze the design parameters of a latent heat storage device (LHSD) for a micro environmental control system (µX). The µX provides local cooling to an office worker in a room whose thermostat setpoint has been elevated from 23.9°C (75°F) to 26.1°C (79°F) in order to reduce HVAC energy consumption. For this application, the LHSD is designed to provide =50W of cooling for a full, 8.5h workday to restore thermal comfort in the warm, 26.1°C room. The LHSD comprises several parallel slabs of encased phase change material (PCM) with interposed airflow channels. The airflow rate is selected to obtain =50W of cooling at the end of the 8.5h operation. The LHSD exhibits a decreasing cooling rate over the 8.5h period when a constant airflow is passed through it, indicating that more cooling is supplied during the day than the minimum 50W required for thermal comfort. The parametric analysis explores the effects of PCM thermal conductivity, slab thickness, air channel width, and number of slabs on LHSD performance. Parametric cases are compared against each other on the basis of their required PCM mass and energy consumption.
TOPICS: Control systems, Melting, Phase change materials, Energy storage, Cooling, Air flow, Slabs, Energy consumption, Finite element model, Latent heat, Storage, HVAC equipment, Temperature controls, Design, Thermal conductivity
research-article  
H. Ezzat Khalifa and Mustafa Koz
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4040697
This study analyzes phase change material (PCM) freezing process in a novel latent heat storage device. Heat is removed from the PCM with an embedded evaporator. A mathematical model of freezing in a finite-thickness PCM slab is presented. An experimentally-validated reduced order model (ROM) based on the mathematical model was developed to analyze the heat transfer between the freezing PCM and an evaporating refrigerant flowing inside a flat, microchannel tube coil embedded in the PCM. A detailed finite element model of the same device was also developed and employed to verify the validity of the ROM over a wider range of conditions. The freezing times and total "cooling" stored in the PCM computed by the ROM agree very well with those computed by the detailed FEM. The ROM executes in ~1 minute for a full heat exchanger, compared with more than 10 h for the FEM, making the former much more practical for use in parametric analysis and optimization of design alternatives.
TOPICS: Freezing, Control systems, Energy storage, Phase change materials, Finite element model, Latent heat, Refrigerants, Storage, Microchannels, Design, Heat, Evaporation, Heat exchangers, Optimization, Slabs, Heat transfer, Cooling
research-article  
Naris Pattanaprates, Ekachai Juntasaro and Varangrat Juntasaro
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4040654
The present work is aimed to investigate whether the modification to the bend geometry of a multi-pass internal cooling passage in a gas turbine blade can enhance heat transfer and reduce pressure drop. The two-pass channel and the four-pass channel are modified at the bend from the U shape to the bulb and bow shape. The first objective of the work is to investigate whether the modified design will still improve heat transfer with reduced pressure drop in a four-pass channel as in the case of a two-pass channel. It is found out that, unlike the two-pass channel, the heat transfer is not improved but the pressure drop is still reduced for the four-pass channel. The second objective is to investigate the rotating effect on heat transfer and pressure drop in the cases of two-pass and four-pass channels for both original and modified designs. It is found out that heat transfer is improved with reduced pressure drop for all cases. However, the modified design results in the less improvement on heat transfer and lower reduced pressure drop as the rotation number increases. It can be concluded from the present work that the modification can solve the problem of pressure drop without causing the degradation of heat transfer for all cases. The two-pass channel with modified bend results in the highest heat transfer and the lowest pressure drop for rotating cases.
TOPICS: Cooling, Gas turbines, Blades, Geometry, Pressure drop, Heat transfer, Design, Shapes, Rotation
research-article  
Ojas Satbhai, Subhransu Roy and Sudipto Ghosh
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4040655
Abstract In this work, numerical experiments were performed to compare the heat transfer and thermodynamic performance of melting process inside the square-shaped thermal energy storage system with three different heating configurations: an isothermal heating from left side-wall or bottom-wall or top-wall and with three adiabatic walls. The hot wall is maintained at a temperature higher than the melting temperature of the PCM, while all other walls are perfectly insulated. The transient numerical simulations were performed for melting Gallium, (a low Prandtl number Pr = 0.0216, low Stefan number, Ste = 0.014, PCM with high latent heat to density ratio) at moderate Rayleigh number (Ra\approxeq10^{5}). The transient numerical simulations consist of solving coupled continuity, momentum, and energy equation in the unstructured formulation using the PISO algorithm. In this work, the fixed grid, a source-based enthalpy-porosity approach has been adopted. The heat transfer performance of the melting process was analysed by studying the time evolution of global fluid fraction, Nusselt number at the hot wall, volume averaged normalised flow-kinetic-energy. The thermodynamic performance was analysed by calculating the local volumetric entropy generation rates and absolute entropy generation considering both irreversibilities due to the finite temperature gradient and viscous dissipation. The bottom-heating configuration yielded the maximum Nusselt number but has a slightly higher total change in entropy generation compared to other heating configurations.
TOPICS: Melting, Natural convection, Thermal energy storage, Heating, Entropy, Temperature, Heat transfer, Transients (Dynamics), Computer simulation, Algorithms, Temperature gradient, Enthalpy, Gallium, Latent heat, Porosity, Prandtl number, Fluids, Density, Momentum, Flow (Dynamics), Energy dissipation, Rayleigh number
research-article  
Vikrant Khullar, Prashant Mahendra and Madhup Kumar Mittal
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4040653
In the present work, a novel parabolic trough receiver design has been proposed. The proposed design is similar to the conventional receiver design except for the envelope and the annulus part. Here, a certain portion of the conventional glass envelope is coated with Sn-In2O3 and also Sn-In2O3 coated glass baffles are provided in the annulus part to reduce the radiative losses. The optical properties of the coated glass are such that it allows most of the solar irradiance to pass through, but reflects the emitted long wavelength radiations back to the absorber tube (Sn-In2O3 coated glass is referred to as 'transparent heat mirror'.) Thus, effectively reducing the heat loss area and improving the thermal efficiency of the solar collector A detailed one dimensional steady state heat transfer model has been developed to predict the performance of the proposed receiver design. It was observed that while maintaining the same external conditions (such as ambient/initial temperatures, wind speed, solar insolation, flow rate, concentration ratio etc.) the heat mirror-based parabolic trough receiver design has about 3-5% higher thermal efficiency as compared to the conventional receiver design. Furthermore, the heat transfer analysis reveals that depending on the spatial incident solar flux distribution, there is an optimum circumferential angle (? = ?optimum, where ? is the heat mirror circumferential angle) up to which the glass envelope should be coated with Sn-In2O3. For angles higher than the optimum angle, the collector efficiency tends to decrease owing to increase in optical losses.
TOPICS: Heat, Solar collectors, Mirrors, Design, Glass, Tin, Annulus, Solar energy, Parabolic troughs, Thermal efficiency, Heat transfer, Wavelength, Solar radiation, Temperature, Wind velocity, Flow (Dynamics), Steady state, Transparency, Heat losses
research-article  
Tonny Tabassum, Mainul Hasan and Latifa Begum
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4040645
A two-dimensional numerical study is carried out to investigate the thermal performance of an impure phase-change material (PCM) in an equilateral triangular-shaped double pipe heat exchanger. To tackle the irregular boundaries a non-orthogonal body-fitted coordinate (BFC) transformation technique is employed. The non-dimensional transformed curvilinear conservation equations for mass, momentum, and energy are written in terms of physical variables and they are solved using a control-volume based finite difference method on a staggered grid arrangement. The developed model is then used to study the effects of the inner tube wall temperature, the initial temperature of the solid PCM, and the shape, as well as the position of the inner tube in the annulus on the melting characteristics, and cumulatively stored energy. Various quantities such as, average Nusselt numbers over the inner tube surface, the total and complete melt fractions, the latent and total stored energies all as a function of the melting time are reported. A correlation for the average Nusselt number on the inner tube wall is also provided. The numerical results show that the shape and the placement of the inner tube are crucial for the efficient design of a latent heat thermal energy storage system. The storage of energy is greatly influenced by the change of the inner tube wall temperature compared to the change of initial solid PCM temperature.
TOPICS: Heat transfer, Space, Latent heat, Storage, Shapes, Wall temperature, Melting, Temperature, Momentum, Phase change materials, Design, Heat exchangers, Pipes, Annulus, Finite difference methods, Thermal energy storage
research-article  
Raj Nandkeolyar, Mahesha Narayana, Sandile S Motsa and Precious Sibanda
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4040644
The steady hydromagnetic flow of a viscous, incompressible, perfectly conducting and heat absorbing fluid past a vertical flat plate under the influence of an aligned magnetic field is studied. The flow is subject to mixed convective heat transfer. The fluid is assumed to have a reasonably high magnetic Prandtl number which causes significant induced magnetic field effects. Such fluid flows find application in many magnetohydrodynamic devices including MHD power-generation. The effects of viscous dissipation and heat absorption by the fluid are investigated. The governing non-linear partial differential equations are converted into a set of non-similar partial differential equations which are then solved using a Spectral quasi-linearization method (SQLM). The effects of the important parameters on the fluid velocity, induced magnetic field, fluid temperature and as well as on the coefficient of skin-friction and the Nusselt number are discussed qualitatively.
TOPICS: Flow (Dynamics), Magnetohydrodynamics, Magnetic fields, Vertical plates, Fluids, Partial differential equations, Heat, Temperature, Energy dissipation, Skin friction (Fluid dynamics), Convection, Energy generation, Flat plates, Prandtl number, Absorption, Fluid dynamics
research-article  
Mirza Mohammed Shah
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4040652
A general correlation is presented for heat transfer during flow of gas-liquid mixtures flowing in vertical channels prior to dryout. It has been verified with a wide range of data that include upwards and downwards flow in heated and cooled tubes, annuli, and rectangular channels. The data are from 19 studies and include 14 gas-liquid mixtures with a very wide range of properties. The parameters include pressure 1 to 6.9 bars, temperature 16 to 115 oC, liquid Reynolds number from 2 to 127231, superficial gas and liquid velocities up to 87 and 13 m/s respectively, and ratio of superficial gas and liquid velocities 0.03 to 1630. The 1022 data points are predicted by the new correlation with mean absolute deviation (MAD) of 18.1 %. Several other correlations were also compared to the same data and had MAD of 28.6 to 45.5 percent.
TOPICS: Flow (Dynamics), Heat transfer, Reynolds number, Annulus, Temperature, Pressure
research-article  
Roberto Bubbico, Francesco D'Annibale, Barbara Mazzarotta and Carla Menale
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4040643
Temperature control is one of the most significant factors to improve the performance and extend the cycle life of a battery. It is therefore important to design and implement an effective battery thermal management system. Phase Change Materials (PCM) can be used as a cooling means for batteries. In the present paper a preliminary analysis of the thermal behavior of PCMs used to cool down a heated metal surface, was carried out. Tests have shown that pure PCMs are able to limit the temperature increase, but only for relatively low heat fluxes. At higher values of the heat produced, the thermal conductivity of the PCM was increased by using solid foams characterized by higher thermal conductivity; it was thus possible to keep the surface temperature within safe limits for longer times. A CFD model of the composite material (PCM+solid foam), was also developed, which allowed to accurately predict the temperature trend within the system under different boundary conditions. However, the average thermal conductivity of the composite system which best fitted the experimental results was found to be much lower than predicted by using common semi-empirical correlations.
TOPICS: Cooling, Temperature control, Temperature, Thermal conductivity, Phase change materials, Heat, Composite materials, Batteries, Flux (Metallurgy), Metal surfaces, Foams (Chemistry), Computational fluid dynamics, Design, Boundary-value problems, Cycles, Thermal management
research-article  
Faisal Al-Malki
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4040657
We study in this paper the combined effect of heat loss and reversibility on the propagation of planar flames formed within the counterflow configura- tion. The problem has been formulated first using the thermo-diffusive model with constant density and then solved numerically using finite elements. The impact of four main parameters, namely the reversibility r, the heat loss ?, the strain rate ? and the activation energy ß, on the propagation of planar flames has been discussed in details. The study has shown that planar flames under reversible conditions behave qualitatively similar to those observed for irreversible reactions, which agree with the asymptotic findings. In the pres- ence of heat loss, the problem exhibits multiplicity of solutions whose number and stability were found to vary according to the strain rate ?. In addition, the study has predicted the existence of a certain value of the reversibility pa- rameter r beyond which the impact of reversibility becomes negligible. Finally, we have examine the stability of the solutions and determined the domain of stability of solutions and their multiplicity for this problem.
TOPICS: Flames, Heat losses, Stability, Finite element analysis, Density
Technical Brief  
Subhashish Dasgupta, Anurag Nandwana and K RaviKumar
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4039927
Most oil-cooled equipment like transformers are provided with radiators or heat exchangers, for the heated oil to exchange heat with the surrounding air by natural convection cooling, assisting the overall cooling process. While such radiators are effective accessories in controlling equipment temperature rise, it is ever desirable to further enhance the cooling capacity by design modifications or incorporating simplistic and cost effective cooling technologies. In this study, computational fluid dynamic (CFD) analysis has been performed to evaluate the possibility of improving radiator performance by flow channelizing structures. Significant benefits (up to 17% increase in heat transfer coefficient) of imposing such structures, like a top chimney and an enclosure surrounding the radiator, were obtained. Although several past studies have confirmed that natural convection cooling effect can be intensified by flow channelization, the phenomenon is unique to a particular application. Given the wide variety in applications, in terms of shape, size and structural features, it is necessary to study the effect in a given application of interest. This study points to a new direction in enhancing cooling capacity of transformer radiators, inducing flow channelization, an easy to implement and cost effective technology. Further, the study offers interesting learnings regarding flow channelization effects, which are invaluable guidelines for designers of future radiators.
TOPICS: Cooling, Flow (Dynamics), Natural convection, Computational fluid dynamics, Design, Heat exchangers, Shapes, Heat transfer coefficients, Heat, Temperature
Errata  
Mohsen Ali Mandegari, Somayeh Farzad and Hassan Pahlavanzadeh
J. Thermal Sci. Eng. Appl   doi: 10.1115/1.4032332
TOPICS: Exergy, Optimization, Wheels

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