查看更多>>摘要：The major challenge of multi-energy complementary systems consists of improving the thermal efficiency for latent heat thermal storage pools (LHTSPs). To address this, a new multi-tube LHTSP with tree-shaped fins is designed herein. The enthalpy-porosity approach is employed to model the charging/discharging process in LHTSPs, focusing on the role of inclinations in melting/solidification heat transfer. Moreover, experimental validation is conducted to ensure numerical reliability. The results show that tree-shaped fins effectively improve thermal efficiency and temperature uniformity. Compared with traditional LHTSPs, the innovative LHTSP shortens the total melting/solidification time by up to 29.4% and 22.8%, respectively, and improves the temperature uniformity by 12.3%similar to 19.2%. The difference in the influence regime of natural convection during melting and solidification lies in its onset and duration. Moreover, the thermal performance of LHTSPs is mainly related to the later charging/discharging stage. The inclination significantly affects the thermal charging performance, while it has less influence on the discharging processes. Compared to the horizontal arrangements, the innovative vertical LHTSP has a 46.3% reduction in the melting duration. Interestingly, there is a transition point in the heat storage efficiency of horizontal and vertical LHTSPs, and the LHTSP arrangement with a higher efficiency differs before and after this point.
查看更多>>摘要：To evaluate the potential of inhomogeneous thermoelectric materials, the hybrid system model comprised of a molten carbonate fuel cell, a thermoelectric generator with inhomogeneous heat conduction and a regenerator is established. In particular, thermal conductivity of thermoelectric materials is regarded as a spatial dependence coefficient. Taking electrochemical and thermodynamic irreversible losses into account, the expressions of hybrid system's equivalent output power and efficiency are deduced and optimal operating current density interval of proposed system is determined. The maximum output power density and efficiency of the hybrid system is 35.4% and 6.9% higher than stand-alone molten carbonate fuel cell, respectively. It is proved that inhomogeneous thermoelectric materials are capable of significantly enhancing the hybrid system performance, since the efficiency can be improved by 25.4% and the optimal operating interval is shifted to a lower current density direction compared with that with homogeneous materials. Finally, several critical parameters on the system performance are analyzed through further discussions of the established model. The results obtained can sever as a theoretical guidance for the optimization of thermoelectric integrated system.
查看更多>>摘要：In this research, thermal resistance of the modified inflated aluminium plate that features a closed thermosiphon under natural convection was tested. HFE-7000 dielectric fluid and graphene nanofluid were used as the working fluids for different filling ratios (30%, 50%, and 70%), graphene concentrations (0.3 wt%, 0.5 wt%, 1 wt%), and supplied powers (30 W, 60 W, 90 W). The effect of uniform and non-uniform heat source on the thermal resistance are examined and a separate flow visualization is also conducted to understand the boiling phenomenon. It is found that a 70% filling ratio offers the best heat transfer performance. At a supplied power of 90 W and filing ratio of 70%, the thermal resistance is reduced by around 4% relative to the filling ratio of 30% for the same supplied power of 90 W. In terms of input power, the decrease in thermal resistance was found as the power increases. The uniform heat source has a better heat transfer performance than the non-uniform one. From the visualization experiment, appreciable graphene was entrained from evaporator to condenser due to boiling especially at a higher concentration of 1%, causing the blockage and raising the thermal resistance. Therefore, the best concentration of graphene should be kept to below 0.5 wt% in which the size of the bubbles was sufficiently large enough to facilitate latent heat transfer effectively for supplied powers of 60 and 90 W.
查看更多>>摘要：The temperatures required by ultra-low temperature applications, ranging from -50 degrees C to -100 degrees C, cannot be reached economically with single stage systems because of the limitation of the compression ratio. Different types of solutions such as cascade or two-stage systems could be implemented to achieve the desired working conditions. However, these systems are usually complex or too expensive. The solution might be found in the use of auto-cascade systems working with zeotropic mixtures. The present article proposes two modifications of the auto-cascade system by including an ejector device to improve the COP. The first modification includes the ejector as an expansion device at the outlet of the phase-separator, while the second includes the ejector as a precompression stage. A mixture of the hydrocarbons iso-butane (R600a) and ethylene (R1150) was used as an alternative to conventional refrigerants, which have a very high GWP. The study performed firstly assessed the sensitivity analysis of the free variables on the operating conditions of each cycle. The evaluated variables were the compressor pressure discharge, the mass fraction of the mixture and the phase separator temperature. In the case of the ejector enhanced cycles, the ejector efficiency and the motive pressure were additionally included. Then, the optimal operating conditions were found by means of an optimization process. The results showed a potential improvement in the COP of 12% for the case of the ejector as an expansion device, with an optimal mass fraction of 0.45 of ethylene. On the other hand, the ejector as a pre-compression stage did not show any improvement with regard to the reference case. The present study concludes that the mixture of ethylene and isobutane is a suitable combination for auto-cascade cycles and that the ejector can be implemented to improve the COP without adding excessive complexity and cost.
Mondal, Md Hasan TarekAkhtaruzzaman, MdIslsm, Md AzadulSarker, Md Sazzat Hossain...
15页
查看更多>>摘要：Investigation of exergy transfer is an important aspect for all grain drying industry. Exergy transfer of rice husk fired cyclonic furnace assisted recirculating mixed flow dryer (MFD) equipped with a heat exchanger, was sought in this study. A cyclonic furnace (CF) having capacity 0.5 MW and recirculating MFD were designed and developed to investigate exergy transfer during grain drying. Efficiency of furnace was determined to quantify performance of CF for industrial grain drying system. Exergetic efficiency was also thoroughfare performed for the developed CF, air mixing chamber (heat exchanger) and MFD. Thermal efficiency of the CF varied from 68.96 to 87.01% during drying of high moisture maize grain. Exergy efficiency of the CF, heat exchanger and drying chamber marked between 58.8 and 63.5%, 74.6-93.29%, and 3.03-21.9%, respectively. Exploration of exergy inflow persist between 53.32 and 60.51 kJ/s and 7.58 to 9.81 kJ/s while exergy outflow ranged between 7.58 and 9.81 kJ/s and 1.52 to 3.10 kJ/s, respectively for heat exchanger and drying chamber. Results based on exergy losses indicates that 44.01-51.54 kJ/s and 5.87-7.50 kJ/s of exergy is just loss through heat exchanger and drying chamber, respectively. Exergy improvement potential and sustainability index varied between 1.02 and 1.80 kJ/s and 1.17-1.40 kJ/s, respectively. Quality of final product is in acceptable range as color difference of dehydrated maize kernel varied from 9.19 +/- 0.46 to 17.05 +/- 0.13. Exergy results indicate that CF can aptly be applied for grain drying industry in commercial scale.
查看更多>>摘要：A phenomenon of accidental release from a high-pressure containment into a low-pressure environment is significant in several industrial systems such as chemical reactors and pipeline transportation and storage. The paper aims to investigate the vapor release with condensation, and to develop a vapor release model coupling a blowdown model with a critical flow model. Vapor release tests of the pressurized vessel are performed with micro-cracks, and the fluid pressure and temperature evolutions are recorded. The theory of ideal dropwise condensation is used to describe the interfacial heat transfer, and the wall heat transfer is regarded as the heat conduction in a blowdown model. A six-equation model allowing the vapor leakage simulation with condensation is presented with a bubbly flow regime assumption. The vapor release model is composed of a critical flow model in conjunction with a vessel blowdown model, including the treatment of the vapor condensation, interfacial heat transfer, and interfacial force during the depressurization. The proposed model predictions exhibit strong similarities with the measured evolutions of the fluid pressure, temperature, and the total weight of the released fluid. Finally, the qualitative analysis of the vapor release with a heat transfer/adiabatic process is conducted. The vapor condensation occurs during the release process when the initial pressure is increased, while the condensation is reduced for a heat transfer process. The release time and vapor condensation of the pressurized vessel are significantly affected by the vessel volume and break area.
Sudhan, A. L. SriramSolomon, A. BruslySunder, Shyam
18页
查看更多>>摘要：Grooved Heat Pipes (GHPs) are the essential heat transfer devices for space applications, as they can effectively work with and without the aid of gravity. In the present study, the heat transport limitations of anodized grooved heat pipes were modelled by incorporating an anodised surface coating parameters under gravity and anti-gravity conditions. Variations in the heat transport limitations such as capillary limit, boiling limit, and entrainment limits due to the anodization over non-anodized grooved heat pipes are presented. The heat transfer performance of an internally anodized and non-anodized grooved heat pipe charged with ammonia as a coolant is experimentally investigated and compared. Two sets of grooved heat pipes are fabricated with 40 numbers of rectangular grooves. Semi-spherical pores are formed by carefully adjusting the voltage and current supply between the electrodes during the anodization process. Experiments in grooved heat pipes are conducted at different inclinations (0-90 degrees) under gravity and anti-gravity for various heat inputs of both heat pipes. Due to the anodization, thermal resistance reduction of 51.1% and enhancement in heat transfer coefficient of 36.6% was obtained at an optimum inclination of 45 degrees for the grooved heat pipes with 0.4 mm groove width. Also, a maximum of 35 W heat input was transferred due to the anodization under anti-gravity with an inclination of 30 degrees. Results suggest that the grooved heat pipes with semi-spherical pores charged with ammonia as a coolant was the best combination for enhanced heat transfer, preferably for space applications.
查看更多>>摘要：Three-dimensional swirling impinging jets using the aerodynamic swirl generator are carried out to understand the pure effects of swirl. A shear stress transport model with cross-diffusion correction (SSTCD) is validated for the above flows with the documented experiments. Based on the good performance of the SSTCD model, a detailed flow analysis for Re = 23,000 and swirl numbers of 0 and 0.45 at nozzle-plate spacings of 2 and 6 is presented in terms of the flow structures, mean velocity field, wall shear stress and heat transfer. The results show that the strength of the swirl is reduced with increasing the nozzle-plate spacing near the wall. On the contrary, the effect of swirl on both flow fields and heat transfer is more evident at high nozzle-plate spacing, leading to a broader impact region along the impinging plate. However, downstream, the effect of swirl can be ignored for the above fields at all nozzle-plate spacings. In addition, the swirl motion contributes to high turbulence but the heat transfer rate still decreases with an increase of swirl number. Meanwhile, the uniformity of heat transfer is improved by the swirl motion for both nozzle-plate spacings, except that near the stagnation region at H/D = 2. It is also found that this improvement depends on the Reynolds number.
Sarmadian, A.Dunne, J. F.Jose, J. ThalackottorePirault, J-P...
21页
查看更多>>摘要：A temperature control approach using evaporative spray cooling of vibrating surfaces in the nucleate boiling region is proposed and verified experimentally. This is relevant to temperature control of heat-generating automotive vehicle components. By exploiting an experimentally calibrated dynamic correlation model to represent evaporative spray cooling of a flat test-piece, a PID controller has been adopted with emphasis focused on the choice of gain parameters to ensure both stability of temperature control, and favourable responses in terms of relevant performance measures. Optimum linearisation of the correlation model has been achieved by solving an appropriate Wiener-Hopf equation, mainly to undertake a practical stability assessment of the closedloop temperature control system. To verify the predicted control system performance, experimental measurements have been obtained from an instrumented, and spray-evaporatively-cooled, flat test-piece exposed to displacement vibration from a shaker. Experimental testing, appropriate to automotive vehicle component applications, includes large-amplitude, low frequency vibration at 12 mm and 1.9 Hz, and low amplitude, highfrequency vibration at 0.02 mm and 400 Hz. To assess the effects of different PID controller gains on the thermal performance of the thermal management system, a coefficient of performance (COP) is used, defined as the ratio of heat power removal to the required pumping power. To achieve a reduction in the settling time, and an increase in the rise time of stable control, a PID controller with a negative proportional gain showed most promising results. A 10.5% increase in COP was achieved in comparison to a PID controller with positive gains. This information is useful for the design and optimization of thermal management systems using evaporative spray cooling.
查看更多>>摘要：In-cylinder thermal barrier coatings (TBCs) reduce heat transfer losses and increase thermal efficiency. It has been shown that thick TBCs negatively impact the performance of conventional combustion modes by degrading volumetric efficiency and increasing the propensity for end-gas knock. Low-temperature combustion (LTC) is an advanced combustion strategy that offers high efficiencies and low emissions. Due to the nature of kineticsdriven autoignition, LTC is fundamentally different from the conventional combustion modes, where the benefits and tradeoffs of thick TBCs need to be re-evaluated. Previous experimental studies showed the feasibility and the efficiency gains associated with a 2 mm thick TBC applied to the piston surface, as well as the reduction in the required intake temperature with no observable deterioration on the high load limit. However, the effects of TBCs and their independent thermophysical properties on LTC have not been systematically explored. It is necessary to perform a comprehensive study on the effects of TBC on LTCs from a fundamental thermodynamic perspective, which serves as the motivation for the current study. This study couples a 0D engine thermodynamic model to a 1D transient heat transfer model of the coating and piston. The model was first validated against the metal piston baseline, followed by validation against experimental data of the TBC cases at different engine loads. With confidence established in the model's fidelity, three parameters are investigated independently: thermal conductivity (k), coating thickness, and volumetric heat capacity (s). The results revealed that the volumetric efficiency actually increases by 7.4 percentage points with a thicker coating due to a reduction in heat transfer during the compression stroke, which allows for a lower intake temperature requirement to reach autoignition. However, there is a thickness limit before the intake temperature becomes impractically low. The results show that elevating surface temperature is directly proportional to higher efficiency. Therefore, the optimal coating configuration for kinetically-controlled LTC is a combination of the lowest k, thickest coating before reaching the limit, and the lowest s, where the low k and high thickness contribute the most thermal efficiency gains (4.1 percentage points) and increased exhaust enthalpy (5.7%).
NSTL
Elsevier