查看更多>>摘要：This article describes the design, fabrication, and heat transfer characteristics of a novel type of loop thermosyphon with a horizontal porous evaporator (LTHPE) of the kW-class heat transfer performance. The main goal of this investigation is to improve the heat transfer intensity by increasing the fluid circulation in the evaporator annular channel. The Bond number Bo <= 1-2 is typical of this new device. Such thermosyphon with the evaporator annular thickness equal to, or larger than, the capillary limit exhibits some new properties. An additional (to the gravity field) mechanism of liquid flow enhancement with self-exciting oscillation due to the bubble generation in the annular gap is used for intense and efficient fluid circulation through the thermosyphon evaporator in a preferential direction. Experimental data on heat transfer coefficients in boiling and evaporation inside the porous wick were obtained in the evaporator annular horizontal gap flooded and partially saturated with liquid. Due to the two-phase fluid flow along the annular tube, stabilization of heat transfer has been achieved including the subcooled and saturated liquid boiling with evaporation inside the wick. A thin porous coating on the evaporator wall ensures a 2-2.5-fold increase of heat transfer in comparison with the flow boiling heat transfer in a smooth tube. The thermal performance of the LTHPE was investigated at different filling ratios and LTHPE inclination angles to the horizon. The evaporator envelope was made from a copper tube of length 200 mm. A copper sintered powder wick was installed inside the evaporator, and pure water was used as the working fluid. The wick thickness was less than 1 mm. The thermal resistance of the condenser was equal to 0.01.C/W. The LTHPE evaporator and condenser are connected with each other by flexible minipipes for transferring vapor and liquid. The thermal resistance of LTHPE is relatively insensitive to any changes in inclination, when the angle of the latter to the horizon exceeds 18.. The total thermal resistance of LTHPE does not exceed 0.03 K/W (thermal resistance of evaporator is near 0.02 K/W) with the heat load of 100-1750 W. This device guaranties a shortened start-up time, has a decreased evaporator wall temperature, has a small temperature hysteresis on increasing/decreasing the heat load, and suppresses the temperature pulsations inside the evaporator. The determination of the thermal resistance of evaporator and condenser and the analysis of the temperature field along the LTHPE were the main goals of this research.
查看更多>>摘要：This paper concerns the modeling of vertical Borehole Heat Exchangers (BHEs) for Ground Source Heat Pump (GSHP) applications. Focus is devoted to the analyses of Thermal Response Test (TRT) simulations aimed at understanding the main factors that influence the ground thermal conductivity and the effective borehole thermal resistance estimations. The conventional infinite line-source (ILS) model does not include any influence of the external heat transfer rate on the BHE/ground property evaluation. Analyses of numerically simulated TRTs show this omission can sometimes produce an error in the estimate of the ground thermal conductivity. The error may be between +/- 10% and +/- 22% for coaxial boreholes (800 m depth), if the ground has a significant geothermal gradient. On the other hand, for single and double U-pipe BHEs the error is less than +/- 5% under similar conditions. The parameter q(ratio) is identified as an indicator of when the error is significant. This parameter is equal to the external heat rate (injection or extraction) divided by a natural heat rate that is related to the geothermal gradient. Errors greater than +/- 10% tend to occur for coaxial boreholes with a center-pipe fluid inlet when vertical bar q(ratio)vertical bar < 1. Under the same conditions but with the annulus as the fluid inlet, the error is less than +/- 6%.
查看更多>>摘要：Nuclear power plants mostly use the multimodule once-through steam generator (OTSG) parallel system as heat exchange equipment, but the current research mostly focuses on a single OTSG. The huge system brings great challenges to modeling and control system design. In this paper, the ODE method is used to model the main equipment of a multimodule parallel system based on mechanism. The results show that the maximum error is 3.77%. Based on the characteristics of parallel system, a new control strategy is proposed and compared with other control strategies by introducing four typical disturbances. The results show that the proposed feedwater control scheme can effectively solve the coordination between loop valve and module valve of multimodule parallel OTSG system, and the coordination between main feedwater pump and loop valve. Compared with Method 1, the response time of Method 2 in sodium temperature disturbance is shortened by 319 s and the temperature fluctuation is reduced by 1.67 times, but Method 2 has potential safety hazards. Method 1 can improve the system response speed when the sodium flowrate and water flowrate change. Compared with Method 3, the outlet sodium temperature fluctuation of Method 1 is reduced by 10 times and the adjustment time of Method 1 shortens 141.9 s in the case of feedwater flow disturbance.
查看更多>>摘要：Engine downsizing with a higher compression ratio and a higher boosting pressure improves the thermal efficiency of engines. However, with further downsizing of engines, knock intensity is aggravated until super knock occurs which is accompanied by a detonation wave that potentially can destroy engines rapidly. Therefore, it is essential to reveal the parameters that influence the knock intensity and the formation of a super knock. Selfdesigned detonation bomb experiments and corresponding numerical simulations were conducted to explore the end-gas combustion mode as well as the knock intensity problem. The experiment was conducted at three typical in-cylinder pressures for representing different mixture energy densities, which classify the knock phenomenon into subcritical (lower pressure), critical (medium pressure) and supercritical (higher pressure) conditions. Spark ignitions with low and high intensity realized by adjusting the spark ignition energy were given in each condition, respectively. The in-cylinder combustion process and pressure oscillation process were monitored by the synchronous acquisition of three pressure sensors that were installed in different positions of the chamber. It is found that irrespective of the low or high ignition intensity, the super knock as well as the detonation would not occur at the lower pressure, and only mild knock occurs. At the medium pressure, the high ignition intensity would result in a detonation wave, while the low ignition intensity would not. At the higher pressure, despite the intensity of ignition, a detonation wave as well as the super knock would always occur. Therefore, the ignition intensity and the in-cylinder pressure are two essential factors that affect the knock intensity of engines.
查看更多>>摘要：The supercooling of phase change material (PCM) significantly affects the heat release characteristics of heat storage systems. Therefore, accurate numerical modeling of the solidification process is key for studying heat storage. Few models considering the crystal growth of supercooling PCMs have been established; however, their accuracy under slight supercooling conditions is unsatisfactory. In this study, the implicit finite difference method was used to establish a two-dimensional PCM heat transfer model that considers the crystal growth process in detail. The growth rate of the crystallization front was used to control the crystallization start time at each node, and the solidification speed was used to manage the heating process once the crystallization was triggered. The accuracy of the model was verified by the experimental results obtained by melting, cooling, and triggering crystallization in a stable supercooled state and during cooling. Based on the simulations of a concentric tube PCM heat exchanger, the effects of several parameters on the heat release rate were investigated, such as the supercooling degree, the PCM initial temperature, and the inlet temperature of Heat transfer fluid (HTF). The results show that a large supercooling degree will accelerate the heat release rate of the PCM heat exchanger after the crystallization is triggered. An increase in the initial PCM temperature reduces the sharp increase in HTF outlet temperature caused by crystallization; a decrease in HTF inlet temperature also has the same effect. A comparison of various models demonstrated that the use of the crystallization front to calculate the PCM temperature directly causes up to 3% of the heat to be released earlier or later. Moreover, ignoring the crystal growth process causes up to 9% of the heat to be released in advance, and ignoring the supercooling causes an even greater error.
查看更多>>摘要：Experimental and numerical investigations of a brazed plate heat exchanger are performed in this study. Three approaches are considered in numerical modeling of brazed plate heat exchanger in order to realize the difference amid simulations and experimental data. The departures between simulations and experiments are strongly associated with the complicated configurations of the plate geometry. Hence, modeling regarding the modified geometry, modeling of original geometry, and modeling of original geometry by considering brazing joints are conducted in numerical simulations for elaborations. Results show that the third geometry yields the best agreement with the experimental data, indicating the importance of considering brazing joints in the modeling of brazed plate heat exchangers. However, the modified geometry has the worst agreement with the experimental data. This approach has been adopted by many existing literatures. Flow field for all approaches illustrates that a zig-zag pattern from the inlet to outlet nozzles exists. This is because of large chevron angle of plates (75 degrees). Comparison of Nusselt number and friction factor of this brazed plate heat exchanger with the existing correlations demonstrates the necessity of developing new correlations for brazed plate heat exchangers. Correlations for Nusselt number and friction factor of the modeled heat exchangers are developed in a wide range of Reynolds number (50-10,000) with mean deviations of 5.1% and 3.4%, respectively.
Bilen, KadirTokgoz, NehirSolmaz, IsmailBalta, Tuba...
14页
查看更多>>摘要：In this study, the influences of swirl generators fixed at the pipe inlet and having different swirl angles (0 degrees, 22.5 degrees, 41 degrees and 50 degrees) on the heat transfer and fluid flow characteristics in a pipe flow were experimentally and numerically studied under a constant heat flux condition in the range of Reynolds (Re) number 2400-23000. The axial type passive swirl generator aims to increase the turbulence effect and gives tangential acceleration while increasing the total flow length in the pipe. This study is novel with the aspects of; (i) the number of helical air passage channels employed in the investigated swirl generators and, (ii) 360 degrees rotation of each helical channel at different swirl angles, i.e., lengths of considered swirl generators are different. Experiments were carried out in a smooth aluminum pipe having an inner diameter of 15 mm and a length of 675 mm with and without swirl generators for turbulent airflows. Empirical correlations for the local and mean Nusselt (Nu) numbers, and friction factor (f) were developed as a function of Re number and swirl angle as well as smooth pipe flow. The obtained experimental results for the heat transfer and friction factor at different swirl angles were compared with the smooth pipe results. The findings showed that the maximum heat transfer rate is achieved at a swirl angle of theta = 50 degrees, and the heat transfer rate increases as the swirl angle and Re number are increased. The decaying swirl flow leads to around 1.1-1.41 times enhancement of Nu and 2.3-6.76 times increase of f compared to the smooth pipe. The maximum value of performance evaluation criteria (PEC), around 0.83, is achieved by the decaying swirl flow when the theta = 0 degrees and Re = 2400. The experimental and numerical Nu and f values for swirl flow are in good agreement within the mean deviation +/- 7.1% and +/- 8.4%.
查看更多>>摘要：A multi-cylinder-type liquid-piston Stirling engine (MCLPSE) is an external combustion engine characterized by a low operation temperature difference below 100 K, a simple structure consisting of only a few parts, and the use of harmless working fluids, namely, air and water at atmospheric pressure. Although the structure is simple, the design method has not been established to date. This study proposes a systematic design guideline for the MCLPSE to attain a target output power under a given temperature condition. The guideline is based on the analysis of the natural mode oscillations of the system, the results of a thermoacoustic theory, and a simple heat transfer model of heat exchangers. The designed MCLPSE was numerically verified using a simulation code (DeltaEC). We introduced a linear alternator based on electromagnetic induction to extract the output power from the designed engine. The results showed that the designed engine achieved the target output power of 102 W under the heat source and sink temperatures of 130 degrees C and 23 degrees C, respectively.
查看更多>>摘要：Noise fluctuation is the most common issue of small heat flux measurement and it disturbs the measurement precision. In the previous studies, how to select an appropriate filter and determine its optimal operating parameter was not given systematically. In this paper, the aim is to explore an effective filter selection and optimal operating parameter determination method for the small heat flux signal. Firstly, six filters with the large range operating parameters were chosen to remove the noise of a set of 10,000 numbers time-series heat flux in the range of 5-10 W/m(2). Then, signal-to-noise ratio, standard deviation, mean absolute deviation and correlation coefficient were introduced to comprehensively assess the filtering results. Lastly, the intersection region method is firstly proposed to determine the optimal operating parameters based on the tradeoff between repeatability and fidelity. The numerical results indicated that the signal-to-noise ratio of the sampled heat flux was ranged from 13.310 db to 20.703 db. The standard deviation and correlation coefficient was decreased from 1.111 to 0.093-0.672 and from 1 to 0.640-0.883, while the mean absolute deviation was increased from 0 to 0.498-1.056. When the filter is fixed, the repeatability and fidelity levels of filtered data are contradictory. When comparing the filtering effects of filters, some filters simultaneously showed a high repeatability and fidelity levels. For the heat flux signal in this study, the moving average filter with 51 window width and Savitzky-Golay filters with 101 window width and three highest order are the best choice. The proposed intersection region method with four assessment metrics is effectively to find an appropriate filter with high repeatability and fidelity.
Chang, MengzhaoYu, Young SooPark, SungwookPark, Suhan...
12页
查看更多>>摘要：In the gasoline direct injection (GDI) engine, spray atomization contributes significantly to the formation of a uniform mixture and the effective combustion of the spray. In this study, a phase Doppler particle analyzer (PDPA) was used to measure the spray atomization characteristics of two five-hole GDI injectors with different hole arrangements under subcooled and mild flash-boiling conditions (superheated degree = 20 degrees C). Based on the spray patterns of the two injectors on a plane 30 mm below the injector tip, the two injectors were called triangle-patterned and pentagon-patterned injectors. A total of 100 measuring points were selected on a plane 30 mm below the injector tip, and the spray droplet diameter and velocity at these points were measured to understand the overall atomization characteristics of the spray in space. The experimental results indicated that as the spray transitioned from subcooled to flash boiling conditions, a polarization phenomenon occurred in which the velocity in the center of the plume increased and the velocity in the peripheral region of the plume decreased. However, the spray droplet diameter significantly decreased under flash-boiling conditions. The average Sauter mean diameter (SMD) of the two injectors on a plane 30 mm below the injector tip decreased by approximately 4 mu m, and the uniformity of the spatial distribution of the SMD also increased. Compared with the triangle patterned injector (injector A), the average SMD of the pentagon-patterned injector (injector B) was slightly smaller; this was attributed to the stronger air entrainment inside the spray and the enhanced air flow caused by the larger plume spacing of the pentagon-patterned injector.
NSTL
Elsevier