查看更多>>摘要：In this study, a design of an ice bank heat exchanger with pipes arranged in a column inside a volume-limited water bath, was investigated by a numerical simulation. The aim of the research is to determine the design of heat exchanger which would result in the highest quantity of ice formed in a given time period. Prior to the consideration, the accuracy of the numerical model was validated by comparison with experimental measurements. The boundary conditions are chosen to simulate the real process during the regeneration of the ice bank in the draft beverage cooler. The study is carried out for the condition of natural convection in a water bath and constant temperatures of a secondary coolant flowing through the pipes. The base design of the water bath and the heat exchanger used in the experimental measurements was further considered by CFD simulations through the variation of number of pipes in a column i.e. pipe row spacing. The optimal distance between the axes of the adjacent pipes and consequently the optimal distance gap between the pipes (x) are found as a function of the pipe diameter (d) as x/d ~ 1.5. This finding is further confirmed by CFD simulations which are performed for cases of different pipe diameters, with different water bath height as well as by heat flow analysis for individual pipes in the bundle.
查看更多>>摘要：Combustion instability has been a common problem accompanied with the low NOx emission technologies for the industrial premixed burner. The effects of flue gas recirculation ratio (FR) on the characteristics of combustion instability and NOx emissions of a premixed flame in a 350 kW industrial boiler are investigated by experimental tests and numerical simulations. The cavity acoustic mode of the whole chamber is computed by a Helmholtz solver combined with CFD simulations. As the FR increased from 0 to 20%, the NOx emission is reduced by around 85% and self-excited combustion instability is trigged when the FR exceeds 10%. Four modes -combustion noise, limited cycle, intermittence and breathing -of combustion instability with the different FRs are recorded in the experiment. Two dominant pressure oscillation frequencies ranging from 21 to 25 Hz and from 1 to 2 Hz are observed when the FR is 10-20%. The measured pressure oscillation modes are compared with the acoustic simulation results. The results show that these two dominant frequencies are consistent with the first and second acoustic modes of the whole boiler cavity. The difference between the dominant oscillation frequency and the cavity acoustic eigenfrequency may be caused by the time delay of the response of the flame. The high amplitude pressure oscillations featured as ultra-low frequency (1-2 Hz) are caused by the periodic extinction and ignition of the flame under a high FR (> 18%).
查看更多>>摘要：An electrodynamic tether (EDT) system that utilizes geomagnetic field and space plasma in the Earth's ionosphere to generate thrust has been studied. In this study, a bare EDT deorbiting model, including the realistic discharge characteristics of a hollow cathode, is introduced. (1) The effect of tether dimensions on the deorbiting performance of the EDT system was systematically studied using this model. A longer and wider tape cannot always effectively promote the EDT performance. It significantly increases the mass of the system and thus the launch cost. A tape with a length of 18 km and width of 1.5 cm was chosen as optimal in this study. (2) The effect of seasonal variations on the EDT performance was studied using the model. The four seasons in 2005 were selected as reference starting dates for the Earth's ionosphere and atmospheric density models. Spring was recommended as the starting date for a better deorbiting performance of the EDT. The effect of seasonal variations on the different position distributions of high-power current in the tether was observed in the numerical simulations.
查看更多>>摘要：The flame kernel propagation characteristics stemmed from the spark ignition of low -Re, non-premixed propane (C3H8) under varying air co-flow conditions were studied using DFCD (Digital Flame Colour Discrimination) incorporated stereoscopic image processing technique. The current work focused on the viability of obtaining the combustion initiating kernel propagation speed (S-P), equivalence ratio (phi) and kernel depth (z) characteristics simultaneously as these parameters are important in the fundamental combustion theories and highly desired in the field of practical burner monitoring and diagnostics. Through the superimposition of schlieren and DFCD- enhanced colour images, the observed spatial location of kernel spearhead was found to conform to the disturbance front observed from the schlieren visualisation. Flame stretch principle was applied to experimentally determine the un-stretched kernel propagation speeds (SP,O). Concurrently, the estimation of the temporal state of reactant mixture equivalence ratio (phi(e)) variation was made using DFCD-segregated spatial colour signal characteristics. Further comparison were made against the CHEMKIN simulated results of un-stretched laminar flame speed (SL,O) at different phi. The experimentally determined SP,O values were found to overlap the SL,O range corresponding to the measured phi(e). The steady occurrence of digital colour fidelity observed for all considered cases suggest that the most reactive mixture fraction (xi(MR)) path followed by the kernel occurred in the fuel-rich premixed phi condition (approximately phi(e) = 1.34 to 1.40). Furthermore, stereoscopic reconstruction of DFCD- enhanced flame colour images provided additional dimensional linkage to correlate the dimensional-spectral variation along with changes in combustion flow conditions. The dimensional-spectral property, through the computation of local colour ratio and stereoscopically reconstructed z, illustrated linearity in both the time domain and in temporal cross-correlation with phi(e). The observed shift of the cross-correlated cluster towards the fuel-richer phi(e) condition is coherent to the corresponding increase in Re and exemplifies the observed conformance of the derived SL,O and phi(e) to properties of premixed combustion along the propagation front potentially denoting the path of xi(MR).
查看更多>>摘要：The increasingly popular use of screw conveyors for thermo(chemical) processing of food or waste requires modeling the temperature distribution inside such a device. The purpose of this work is to demonstrate the temperature distribution around an electrically heated rotating helix inside a circular cylindrical channel of infinite length filled with a homogeneous biomass moving at a constant velocity in the axial direction. The non stationary problem of temperature distribution is solved by replacing the real heat source with continuously distributed point sources, expansion of the given and required functions into a Fourier series over angular coordinate and applying integral Fourier and Laplace transforms over axial coordinate and time, respectively. The exact solution of the problem is represented as the product of the energy source determined by the Joule heat and the influence function in the form of the Fourier-Bessel series, which depends on the thermodynamic characteristics of substance and its velocity, radius, pitch and angular velocity of the helix. The temperature field consists of two components, one proportional to the time and the other forming the micro-structure of the temperature profile. On the basis of numerical calculations the analysis of space-time microstructure of the temperature field in the channel is provided. It is shown that in time the temperature undergoes low-amplitude quasi-monochromatic oscillations, the period of which is determined by the angular velocity of the spiral. The spatial temperature distribution has a plateau-like character formed by the geometry of the helix. In particular, the conditions under which the resonant amplification of the amplitude of temperature fluctuations occurs are established.
查看更多>>摘要：The experiments were carried out to investigate the influence of different post injection timing and post injection rate on the performance of combustion and unregulated hydrocarbons species emissions from a common-rail diesel engine. Fourier transform infrared spectroscopy was used to measure and analyze the unregulated hydrocarbons emissions which include light hydrocarbons (HCs), aromatic hydrocarbon (AHC), and n-octane (NC8). The results showed that the emissions of individual hydrocarbon species were the similar to that of single injection when the post injection time was at 20 ?A and 40 ?A. In the measured hydrocarbons components, CH4 and C4H6 species maintained a low level and weak sensitivity to the different post injection conditions. The overall variation of NC8 species emission was similar to that of THC emission and accounted for most of the THC, which indicated that most of the fuel had no chemical change and entered the exhaust directly under different post injection conditions. It should be noticed that the olefin species decreased and the alkane species increased gradually as the HCs species raised from C1 to C4. Furthermore, the distribution of HC species at 60 ?A was less changed than that of 40 ?A under different post injection rates and was closer to the required after-treatment gas atmosphere due to its larger proportion of highly reactive components.
查看更多>>摘要：With the advancement of industries and the trend towards miniaturization, sustainability and higher energy efficiency of devices, compact heat exchangers become an urgent need in the wide range of heat transfer applications. However, the requirements for high heat energy dissipation due to extreme thermal loads may limit their development, mainly because of the inherently poor thermophysical properties of the thermal fluid commonly used. Nanofluids with an optimal design considering their stability, heat transfer enhancement and a lower increase in the pressure drop, are suggested as a promising solution to increase the energy efficiency of thermal energy systems. Here, a thorough and updated review is presented about the heat transfer and fluid flow of these promising nanofluids in compact heat exchangers with mini and micro-passes (channels) for several operation conditions, considering the several factors of nanofluids such as nanoparticles type and shape, base fluid type, preparation method, stability, thermal conductivity and rheology. Moreover, this article presents the experimental and numerical methods employed to investigate nanofluids' thermal performance, analysing the results in terms of heat transfer enhancement and suitability in such heat exchangers. The latter will enable to define the lack of knowledge, the contradictions that may be found between previous studies and the best options for further investigations in this developing field for sustainable thermal energy and better utilization, mainly selecting compact heat exchanger geometry and nanofluids structures (nanoparticle, base fluid and preparation method) and investigation methods (experimental and numerical).
查看更多>>摘要：The design of shell-and-tube heat exchangers is typically done based on standards, best practices and experience. This is however a time consuming process that does not necessarily yield the optimal design. Recently, several methods have been proposed to optimize this process to minimize the combined investment and pumping costs. While most publications use only a fixed fouling factor, some studies apply a more elaborate fouling model and take cleaning or maintenance into account. Besides fouling, also corrosion of the heat transfer surface is possible. This causes the wall thickness to change, which is typically not considered in heat exchanger design models. Therefore, this paper proposes a method where the total cost of ownership of a shell-and-tube heat exchanger is evaluated in case the heat transfer surface is subject to uniform corrosion and corrosion fouling. The novelty of this model lies in the fact that the wall thickness reduction and possible heat exchanger replacements are considered, while typically the performance of the device is assumed constant. Both tube-and shell-side performance are determined, a corrosion model is implemented, and investment, cleaning and pumping costs are calculated. The presented methodology is demonstrated with the design of a heat exchanger for a geothermal power plant, where the cost of using an expensive, non-corroding material for the construction of a heat exchanger is compared to the cost of using a more economic, but corroding metal. The results indicate that a lower cost is possible by using a corroding material, as long as the corrosion rate does exceed a certain threshold.
查看更多>>摘要：Thermoacoustic heat pumps (TAHPs) can be used for heating and/or cooling purposes. Current designs of travelling-wave TAHPs normally employ a single acoustic driver and rely on a looped pipe to establish and maintain the required acoustic field. The resultant system is thereby bulky and expensive, detracting from the structure simplicity and low-cost advantages of thermoacoustic technology. To address this issue, this paper investigates the dual-acoustic-driver concept that can significantly increase the system's compactness. Theoretical analyses are conducted for the dual-acoustic-driver TAHP, and the acoustic and temperature fields in the thermoacoustic core are examined. Parametric studies are undertaken to investigate the effects of acoustic drivers on the acoustic and thermal characteristics of the TAHP. It is found that, as the frequency of acoustic drivers changes, the acoustic field in the thermoacoustic (TA) core could be dominated by a standing, traveling, or hybrid standing-traveling wave. The temperature distribution within the TA core and temperature difference between the core ends will change accordingly. Results show that the temperature difference is non-zero only when the acoustic field contains a traveling-wave component. To obtain a large temperature difference, the acoustic drivers should be driven near resonance frequencies at which the acoustic field is hybrid and the pressure amplitude is large. This study gains new insights into the working mechanisms behind the dual-acoustic driver TAHP concept, paving the way for developing compact, efficient, and high-power-density TAHPs for industrial waste heat recovery.
Ayub, IqraNasir, Muhammad SalmanLiu, YangMunir, Anjum...
13页
查看更多>>摘要：The novelty of this research work is the exergo-economic analysis (including the cost of exergy destruction and exergy loss) of metal hydride based thermal energy storage system coupled with a solar bakery unit for the screening of metal hydride pairs (Case 1: pure MgH2/LaNi5 & Case 2: V2O5 doped MgH2/LaNi5) for thermo-chemical energy storage. Firstly, a numerical simulation is performed by using COMSOL Multiphysics 5.3a software. Secondly, an economic and exergo-economic model is developed to calculate the annual levelized cost of the thermal energy storage system. The life-cycle economic assessment findings indicate that the Levelized thermal energy storage cost of the pure MgH2 based system (32.28 $ /kWh(th)) is 8.2 times higher than that of the V2O5 doped MgH2 system (3.954 $/kWh(th)). Moreover, an 87.75% decrease in cost was observed in Case 2 (V2O5 doped MgH2). Furthermore, Case 2 (V2O5 doped MgH2) can save 92.58 % of hydrogenation reaction time as compared to Case 1 (Pure MgH2). Ultimately, the selection of V2O5 based MgH2 as a thermal heat-storing me-dium is then assessed as a better option for the MHTES for the solar bakery unit (SBU). The findings of this research provide a clear insight into the mechanism of cost formation in the system.
NSTL
Elsevier