An overview of the direct and ab initio simulation as well as experimental tools addressing heat conduction in dielectric nanostructures and nanostructured materials is proposed. Recent advancements based on density functional theory to compute phonon properties and thermal conductivity are exposed and the state of the art in metrology is detailed. Finally, new progresses in the experimental demonstration of coherence effects in heat conduction are illustrated, emphasizing the open questions in the still poorly understood field of phononic heat transport.
To elucidate the detailed combustion mechanism of spray flames, experimental and numerical studies were conducted. In the experiment, simultaneous time-series measurements of OH chemiluminescence, CH-band emission, and Mie scattering from droplets by using a Cassegrain optics, spray image illuminated by laser light sheet, the droplet diameter and velocity measured by using PDA were applied to a premixed spray flame of kerosene. In addition, numerical calculations were conducted on combustion processes of n-decane spray entering gaseous flat-flame stabilized in laminar counter flow configuration. With these experimental and numerical results, time evolution of burning processes of fuel spray was discussed in detail.
A numerical investigation of laminar mixed convection flow through a water-alumina nanofluid in a microscale vertical duct preceded with a double-step expansion has been executed. The governing equations are solved by using Lattice Boltzmann equation (LBE) with multiple-relaxation-time (MRT) collision model. The thermal conductivity and effective viscosity of nanofluid have been calculated by Brinkman and Maxwell models, respectively. To examine the effects of nanoparticles concentrations on the heat transfer and the flow behavior, the study has been carried out for the Reynolds number Re=10 to 40, Richardson number Ri=0 to 1.0 and the solid volume fraction 0 to 20%. The results obtained from Lattice Boltzmann modeling clearly show that the inclusion of nanoparticles into the base fluid produces a significant enhancement of the convective heat transfer, especially in the channel entry region. This enhancement increases as function of growing Reynolds number. In addition, the increase in Richardson number leads to decrease the solid concentration effect. Results also show that adding solid particles decreases significantly the fanning friction factor in mixed convection case.
Microwave heating technology has been widely used in both domestic and industrial applications. Temperature control technique is significant in improving the performance of microwave heating. A generalized numerical thermodynamics model associating with the temperature-dependent thermal and physical properties of material for the microwave heating process is proposed in this paper. Experimental data is applied to estimate the microwave power coefficients. Two controllers, sliding mode controller (SMC) and proportional-integral-differential (PID) controller, are presented as the easily implementable and efficient on-line controllers to track the desired temperature profile while acting on the microwave power and the heated material's temperature. The effectiveness of the proposed thermodynamics numerical model is verified by simulations and experiments, which shows that SMC controller has better dynamic control performance than PID controller.
Heat transfer and friction characteristics of the radial slit-finned heat exchangers under wet condition are experimentally investigated. Louver-finned heat exchangers are also tested for comparison purpose. The effect of fin pitch on j and f factor is negligible. Louver fin samples yield higher f factors than slit fin samples. For one row configuration, the average f factor ratio between slit fin sample and plain fin sample is 1.61, which decreases to 1.37 for two row configuration, and to 1.36 for three row configuration. As for the j factor, the ratios are approximately the same. The j factor increases as the number of tube row decreases. The f factor, however, is independent of the number of tube row. The j/f ratios of the slit fin are higher than those of louver fin. A new correlation is developed based on the present data.
Supercritical pressurized water cooled reactor (SCWR), which has a once-through water cooled reactor for supplying supercritical pressure steam at high temperature to a turbine system, is one of the next generation reactors for the purpose of improving economic efficiency and safety. In the SCWR system, water pressure passes through the critical pressure during startup, shutdown and in case of loss of coolant accident (LOCA). Similarly, in fossil-fired power plants, once-through boilers of sliding pressure type operate at supercritical pressure during nominal load, but they work near the critical pressure during partial load. In the high subcritical pressure region slightly below the critical pressure, critical heat flux (CHF) phenomenon readily occurs at relatively low heat flux and in a high subcooled region, and then it causes serious damages to fuel rods or boiler tubes due to the abrupt temperature rise in the rod or tube. In this study, experiments on post-CHF heat transfer in such the high subcritical pressure region near the critical pressure (reduced pressure range of 0.924 up to 0.992) in the range of mass flux 400-1000 kg/(m2・s) with a circular tube of 4.4 mm ID in vertical upward flow. HCFC22 and HFC134a were used as the test fluids. Based on the obtained data, characteristics of the post-CHF heat transfer were examined and found to be classified into two distinctive types mainly according to flow rate conditions. The influences of pressure, flow rate and heat flux on the characteristics were also clarified.
This study was conducted to investigate the transient heat transfer process between the solid surface and the coolant (helium gas) in Very High Temperature Reactor (VHTR). Forced convection transient heat transfer for helium gas flowing over a twisted plate was experimentally and theoretically studied. The heat generation rate of the twisted plate was increased with a function of Q = Q0exp(t/τ)(where t is time, τ is period). Experiment was carried out at various periods ranged from 35 ms to 14 s and gas temperature of 303 K under 500 kPa. The flow velocities ranged from 4 m/s to 10 m/s. Platinum plates with a thickness of 0.1 mm and width of 4 mm were used as the test heaters. The plate was 180° twisted with a pitch of 20 mm. Based on the experimental data, it was found that the average heat transfer coefficient approaches the quasi-steady-state value when the dimensionless period τ* (τ* = τU/L, U is flow velocity, and L is effective length) is larger than about 300 and it becomes higher when τ* is small. Three dimensional numerical study were conducted and the results of three typical turbulence models were compared with experimental data. Numerical simulation results were obtained for average surface temperature difference, heat flux and heat transfer coefficient of the twisted plate and showed reasonable agreement with experimental data. Based on the numerical simulation, mechanism of local heat transfer coefficient distribution was clarified. A comparison of the twisted plate and flat plate was conducted to show the difference in heat transfer coefficient distribution.
Local heat release rate is one of the most concerned parameters in the combustion processes. However, this parameter is hard to be measured directly in the experiments. Therefore local heat release rate indicators have been sought and evaluated, and many works focus on the study in the gaseous laminar or turbulent premixed flames. The motivation of this work is to explore and validate heat release rate indicators for auto-igniting n-heptane droplets. To this end, direct numerical simulation (DNS) is performed with a detailed chemical reacting mechanism. Results show that the product of mass fractions of OH and CH2O is a proper indicator when the local auto-ignition prevails and the temperature rises quickly. The elementary reactions involved are analyzed, which shed light on the construction and performance of heat release rate indicators. Some new definitions of the indicators are proposed and evaluated. The proportional relationship between the indicator and the actual local heat release rate is determined. The heat release associated with different combustion regimes are distinguished, which reveals the dominant role of premixed flames in the droplets auto-ignition processes.
In four stroke internal combustion engines, optimization of engine performance with air-fuel ratio close to stoichiometric condition is still a challenging task specially in transient operation due to cycle-to-cycle coupling of combustion phenomena and gas dynamics in cylinder. In this paper, the cycle-to-cycle in-cylinder gas dynamics coupling model based air-fuel ratio control using the generalized predictive control law has been discussed and validated in which the input parameters of the discrete time model are updated on cyclic event based. With the discrete time model, a Kalman filter-based state variables such as total fuel mass, unreacted air and residual burnt gas are estimated and used to calculated the in-cylinder air-fuel ratio which reflect the cycle-to-cycle coupling effects of residual gas mass. Then based on model, a controller is designed to achieve the air-fuel control. Apart from this, the control performances of generalized predictive controller and PI controller have been compared. Finally, experimental validation results are demonstrated to show the effectiveness of proposed control scheme that is conducted on a full-scaled gasoline engine test bench.
We investigate spinning detonations propagating in a square tube using numerical simulations based on three-dimensional compressible Euler equations and the two-step-reaction model proposed by Korobeinikov et al. The dynamics of the simulation results is used to understand how spinning detonations propagate. The track angles calculated from soot-track images of the square-tube walls are numerically reproduced for various initial pressures and channel length. The results show that the numerically obtained track angles are consistent with those of the previous reports. The spinning detonation maintains its propagation when the rotating transverse detonation completely consumes any unburned gas. Transverse detonation is important for the propagation of spinning detonations. We also discuss acoustic coupling between the transverse detonation and the acoustic wave that propagates along the wall. The rotating transverse detonation propagates with successive reflections off the walls, and acoustic waves are generated at the reflections. When the spinning detonation maintains its propagation, the rotating transverse detonation couples with the acoustic wave traveling between the walls. We analyze the maximum and minimum track angles that allow the coupling and propagation of a spinning detonation in a square tube to be maintained and find that the simulated track angles fall within the estimated maximum and minimum track angles. This result indicates that acoustic coupling is important for maintaining propagation of a spinning detonation in a square tube.
The thin film solar cells have attracted wide attentions for its low implementation cost and representative flexibilities such as selectivity of materials and fabrication conditions. On the thin film solar cells, width of the grooves of pattern 1, 2 and 3 are important factors to determine its efficiency and cost. In this study, as P1 fabrication process, ablation experiments of Transparent Conductive Oxide (TCO) coating on glass substrates of thin film solar cells are conducted using Bessel beam nanosecond laser pulse. The purpose is to assess the capability of the Bessel beam in laser scribing of thin film solar cells from the glass side. In order to fulfill this purpose, a Gaussian beam of 1064 nm nanosecond Nd:YAG was shaped into Bessel beam with few-micrometers beam width, using fluidic optical device. The Bessel beam, is used to backside laser ablation of a thin film. The function of the optical system and products fabricated with Bessel beam are compared with those of Gaussian beam. Experimental results show that: in the single pulse ablation, the vias ablated by Bessel beam have smaller variations in diameter and depth than those of Gausian beam at the same laser spot and peak fluence level. Furthermore, a few micrometer wide, perfectly isolated groove could be fabricated with Bessel beam. The paper concludes with suggestions for future research and potential application.
The effect of the cavity aspect ratio on the flow and heat transfer characteristics is investigated numerically in this paper. The cavity is part of the channel’s lower wall heated at a uniform temperature. The approach is based on the low Reynolds Stress-omega turbulence model. The study is performed at two different Reynolds numbers. Results show that the flow structure depends heavily on the cavity aspect ratio. The increase of the aspect ratio enhances the heat transfer. In addition, the increase of the Reynolds number does not affect the flow structure but improves the heat transfer. To examine the effect of the cavity presence, we compared the local Nusselt number along the cavity walls to the one along the upstream wall. The heat transfer rate was found to be lower within the cavity but superior in the region located just downstream. In addition, the maximum value of the Nusselt number is observed at the reattachment region of the cavities characterized by the shear layer reattachment. At last, we note a clear correlation between the local Nusselt number and the velocity fluctuations profiles. This demonstrates a close relationship between these quantities.
This paper shows the performance evaluation of the variable refrigerant flow (VRF) air-conditioning system subjected to partial and unbalanced thermal (cooling and heating) loadings. The one outdoor unit's and two indoor units' VRF system was used as a test specimen in the controlled outdoor chamber in which temperature and humidity were controlled. The indoor chamber thermal loading was controlled from full to partial thermal loadings and from balanced to unbalanced thermal loadings with respect to the specified rated capacity of the test specimen. The purpose of this study is to determine the real performance of the system when it is operating under actual operating situations. The general results show that the system coefficient of performance much depends on the partial and unbalanced thermal loadings. It is shown that the system maximum coefficient of performance occurred at around 50% of the rated thermal loadings. It is shown that as the load balance ratio decreases, the system coefficient of performance decreases, particularly for the cooling operation, due to the increase of compressor speed to support the refrigerant sub-cooling. In the heating operation, the compressor speed increase is very minimal which results in a very small decrease of the system coefficient of performance as the load balance ratio decreases. Based on this study, the system coefficient of performance becomes much different as the system is operating at partial thermal loadings. Also, during the unbalanced thermal loadings, the system coefficient of performance further decreases, particularly during the cooling operation. With the results of the study and analyses of the system operation, it is important to consider how the VRF system operates in an actual building when estimating the energy consumption of the VRF air-conditioning system to be installed in a new or in a retrofitted building.
Reaction kinetics of coal volatile were numerically investigated based on analyses during reaction mechanism reduction procedure in detail in this paper. Computation on a closed homogeneous reactor model was performed to clarify important chemical species and their reaction pathways with evaluating the capability of prediction of ignition delay time and chemical species concentration under a wide range of equivalence ratios and temperatures. The kinetics of large hydrocarbons such as polycyclic aromatic hydrocarbons (PAH) that comprise a large portion of combustion features for complex hydrocarbon materials was focused on in a coal flame. The data computed here was analyzed through the combined process of the directed relation graph with error propagation and sensitivity analysis (DRGEPSA) and the computational singular perturbation (CSP) that can evaluate the roles of each species and reaction pathway respectively in the chemical system. Results show that hydrocarbons such as aromatics, methyl and ethyl groups play important role in the ignition and flame propagation processes. Finally, the skeletal mechanism including 64 species and 150 reactions was derived from the detailed mechanism that consists of 257 species and 1107 reactions within 30% error in the prediction of the ignition delay time and the mole fractions of major species in the propagating flame.
Savings from heating and cooling energy use while not compromising indoor comfort condition is essential to reduce the energy costs on buildings. In order to accomplish this goal, an alternative is to search for a new insulation material that is an effective insulator and inexpensive at the same time. In the present work, various insulation materials created with successive layers of polystyrene and either of the following inert gases; carbon dioxide, argon, nitrogen, or air were investigated. Radiation effect on the polystyrene surfaces was included. Based on 1.0cm of gas layer size, Rvalue (thermal resistance) of carbon dioxide exceeded Rvalue for argon, nitrogen, or air. This result identified the carbon dioxide as a potential medium to be used inside insulation materials. Insulation effectiveness of carbon dioxide relative to single polystyrene without any radiation effect was calculated for gas layer sizes of 1.0cm, 0.86cm, and 0.6cm on 3cm-thick insulation material, and it was, respectively, 31.8%, 28.6%, and 22.0%. Radiation effect on the polystyrene surfaces needs to be kept at minimum for a better insulation. An insulation material with carbon dioxide layered structure is expected to pay off its implementation cost in regard to energy savings in a reasonable amount of time (2.39yr). Argon was the next alternative to carbon dioxide gas.
This paper presents investigation of optimum insulation thickness, energy savings and payback periods for gas pipeline and liquid pipeline under the heating and cooling modes of a split air conditioning system using R-407C as refrigerant. Analyses are performed for four different insulation materials indicated as extruded polystyrene, foamboard, rockwool and expanded polystyrene. In consequence of the calculations, the optimum insulation thickness of gas pipeline vary from 45 to 50 mm, the energy saving vary between 78 and 83%; and the payback period vary from 0.025 to 0.468 years, whereas, the optimum insulation thickness of liquid pipeline vary from 28 to 45 mm, the energy saving vary between 40 and 79%; and the payback period vary from 0.044 to 0.289 years. Finally, for both gas and liquid pipeline, the expanded polystyrene is a better choice when the optimum insulation thickness is an important consideration.
Heat transfer enhancement with nanofluid appears to be an attractive work in recent years. In present work, a numerical formulation based on the Buongiorno model for convective heat transfer using Al2O3-water nanofluid accounted for the effects of Brownian motions and thermophoresis of nanoparticles, slip velocity and jump temperature at solid-fluid interface. Numerical investigations for laminar forced convection flows in a rectangle channel subjected to a uniform wall heat flux have been conducted. The numerical results show us that, the slip velocity can augment the heat transfer enhancement significantly due to the increase of the convection near the solid-fluid interface. Inversely, the jump temperature is not beneficial to the convective heat transfer because of the increased thermal resistance. The thermophoresis of particles affects heat transfer enhancement by changing local density, local viscosity, and local thermal conductivity. The thermophoresis of particles influences the skin friction coefficient also. The Nusselt number increases with the Reynolds number and particle volume fraction. The impact on the Nusselt number of Reynolds number will be receded in some extent because the thermophoresis velocity will be greater when the Reynolds number increasing. These numerical results help us to design micro-devices and understand the mechanism of heat transfer enhancement by adding nanoparticles in a microchannel.
Vertical ground heat exchanger (GHE) is widely being used to exchange heat into the ground by using the system of ground source heat pump (GSHP). The stability and thermal performance of GHE mainly relate to the grout, the advantages of GHE can be negated if the grout is not backfilled properly. In order to study the influence of grout backfilling in a vertical GHE, this study established a 2D steady-state heat conduction model, which simulated 4 cases as follows: (1) fine backfilling, (2) porous backfilling, (3) hole-wall delaminating and (4) pipe-wall delaminating. The thermal resistances inside borehole in 4 cases were estimated by using numerical and analytical methods. It has been observed that improper backfilling generates heat barriers, prolongs the time required for heat transfer and reduces heat exchange capacity. The pipe-wall delaminating greatly increases the thermal resistance and brings the worst impact to a GHE.
In the present paper, soot formation process was studied in pyrolysis of iso-octane added hydrogen highly diluted with argon in the temperature range of 1800-2600 K and in the pressure of 1.2 ± 0.1 MPa behind a reflected shock wave. As a test gas, 1% iso-C8H18 + 0-1% H2 diluted with Ar was used. Soot formation process was characterized by induction time and soot volume fraction, which was measured from laser light extinction measurement. In addition, time history of soot particle temperature was calculated based on spectral dependence of monochromatic emissive power from thermal radiation from the soot particles. The experimental results show that soot formation has bell-shaped temperature dependence exhibiting maximum at 2000 K with and without hydrogen addition. Adding hydrogen to iso-octane decreases soot volume fraction clearly for an initial ambient temperature T5 = 1800 K. Additionally, hydrogen addition prolongs induction time for T5 = 1800 K and 2000 K. Soot particle temperature TP is higher than T5 by about 200-400 K except for around 1800 K in the case of hydrogen addition. Consequently, soot formation is obviously suppressed for T5 = 1800 K, while no significant effects of hydrogen addition can be seen for T5 = 2000 K and 2500 K.